Process Simulation and Control Using Aspen

astern

PROCESS SIMULATION AND CONTROL USING

METHANOl

BUTENES

RDCOLUMN

CCS

AMIYA K. JANA

This Thispage pageintentionally intentionallyleft leftblank blank

Rs. 295.00

PROCESS SIMULATION AND CONTROL USING ASPEN

Amiya K. Jana

@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9

The export rights of this book are vested solely with the publisher.

Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.

Rs. 295.00


Amiya K. Jana





Rs. 295.00


Amiya K. Jana





Rs. 295.00


Amiya K. Jana




Contents

Preface Acknowledgements

Part I

vii ix

Steady State Simulation and Optimization

using Aspen Plus 1

.

Introduction and Stepwise Aspen Plus

Simulation:

Flash Drum Examples 1 1 .

3-53

Aspen: An Introduction

3

2 Getting Started with Aspen Plus Simulation 1 3 Stepwise Aspen Plus Simulation of Flash Drums 1

4 7

.

.

13 1

Built-in Flash Drum Models

13 2

Simulation nf a Flash nmm

.

.

7 ,

1 33 .

.

1 3

.

,

Computation of Bubble Point Temperature

.

Summary and Conclusions

50

,

,

,

,

Reference 2

,

Aspen Plus 2 1 .

50

53

Simulation of Reactor Models

Built-in Rpartor Models

54-106 54

2 Aspen Plus Simulation of a RStoic Model 2 3 Aspen Plus Simulation of a RCSTR Model 2 4 Aspen Plus Simulation of a RPlug Model 2

8

35 42

.

Prnhlpms

_

28

4 Computation of Dew Point Temperature 1 3 5 T-xy and P-xy Diagrams of a Binary Mixture .

,

.

.

.

25

Aspen Plus Simulation of a RPlug Model using LHHW Kinetics Summary and Conclusions .

55 65 78 93 104

Prohlpms

704

Reference

106 v

Copyrighted maierlal

VI

3

.

CONTENTS

Aspen Plus

Sinmlation of Distillation Models

107-185

3 1

Rnilt-in nistillntinn Mndols

107

32

Aspen Plus Simulation of the Binary Distillation Columns

108

.

3

.

3 2 1

Simulation of a DSTWTT Mnripl

IQfl

3 9. 9

Simulation of a RaHFrnr MoHpI

122

3 Aspen Plus Simulation of the Multicomponcnt Distillation Columns Simnlnt.ion of a RaHFrar MoHpI

13fi

332

Simulation of a PetroFrac Model

148

.

.

3

.

3

.

.

4 Simulation and Analysis of an Absorption Column

164

5 Optimization using Aspen Plus

178

Part II .

Chemical Plant Simulation using Aspen Plus

Aspen Plus 4 1

4

.

4

.

181 l2

f

Summary and Conclusions Problems

4

136

3 3 1

Simulation of Chemical Plants

189-226

TntrnHnrtion

2 Aspen Plus Simulation of a Distillation Train

189

3 Aspen Plus Simulation of a Vinyl Chloride Monomer (VCM) Production Unit

203


220

Prnhlpms

;

,

220

-

References

Part III 5

.

226

Dynamics and Control using Aspen Dynamics

Dynamics and Control of Flow-driven Processes 5J 52 .

229-284

Tnt.roHiirt.ion Dynamics and Control of a Continuous Stirred

229

Tank Reactor (CSTR) 5

.

230

3 Dynamics and Control of a Binary Distillation Column

255


279

Prnhlpms ,

,

References .

Dynamics and Control of Pressure-driven Processes il

Tnt.rndnrtinn

6 2

Dynamics and Control of a Reactive Distillation (RD) Column

f

6

.

,..

279

284

285-313 285

286


310

Problems References

31J 313

Index

315-317

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r

Preface

"

The future success of the chemical process industries mostly depends on the ability to design and operate complex, highly interconnected plants that are profitable and that meet quality, safety, environmental and other standards To achieve this goal, the software "

.

tools for process simulation and optimization are increasingly being used in industry.

By developing a computer program, it may be manageable to solve a model structure of a chemical process with a small number of equations. But as the complexity of a plant integrated with several process units increases, the solution becomes a challenge. Under this circumstance, in recent years, we motivate to use the process flowsheet simulator to

solve the problems faster and more reliably. In this book, the Aspen

software package

has been used for steady state simulation, process optimization, dynamics and closedloop control. To improve the design, operability, safety, and productivity of a chemical process

with minimizing capital and operating costs, the engineers concerned must have a solid knowledge of the process behaviour. The process dynamics can be predicted by solving the mathematical model equations. Within a short time period, this can be achieved

f

f

quite accurately and eficiently by using Aspen lowsheet simulator. This software tool is not only useful for plant simulation but can also automatically generate several control structures, suitable for the used process flow diagram. In addition, the control parameters, including the constraints imposed on the controlled as well as manipulated variables. are also provided by Aspen to start the simulation run. However, we have the option to modify or even replace them.

This well organized book is divided into three parts. Part I (Steady State Simulation

and Optimization using Aspen Plus

) includes three chapters. Chapter 1 presents the f

introductory concepts with solving the lash chambers. The computation of bubble point and dew point temperatures is also focused. Chapters 2 and 3 are devoted to simulation of several reactor models and separating column models, respectively.

Part II (Chemical Plant Simulation using Aspen Plus

) consists of only one chapter

(Chapter 4). It addresses the steady state simulation of large chemical plants. Several

individual processes are interconnected to form the chemical plants. The Aspen Plus simulator is used in both Part I and Part II. vii


viii

PREFACE

The Aspen Dynamics package is employed in Part III (Dynamics and Control using Aspen Dynamics ) that comprises Chapters 5 and 6. Chapter 5 is concerned with the f

dynamics and control of low-driven chemical processes. In the closed-loop control study

,

the servo as well as regulatory tests have been conducted. Dynamics and control of pressure-driven processes have been discussed in Chapter 6. The target readers for this book are undergraduate and postgraduate students of chemical engineering. It will be also helpful to research scientists and practising engineers. Amiya K. -Jana


Acknowledgements

It is a great pleasure to acknowledge the valuable contributions provided by many of my well-wishers. 1 wish to express my heartfelt gratitude and indebtedness to Prof. A.N.

Samanta, Prof. S. Ganguly and Prof. S. Ray, Department of Chemical Engineering, IIT Kharagpur. I am also grateful to Prof. D. Mukherjee, Head, Department of Chemical Engineering, IIT Kharagpur. My special thanks go to all of my colleagues for having

created a stimulating atmosphere of academic excellence. The chemical engineering students at IIT Kharagpur also provided valuable suggestions that helped to improve the presentations of this material.

I am greatly indebted to the editorial staff of PHI Learning Private Limited, for their constant encouragement and unstinted efforts in bringing the book in its present form.

No list would be complete without expressing my thanks to two most important people in my life-my mother and my wife. I have received their consistent encouragement and support throughout the development of this manuscript.

Any further comments and suggestions for improvement of the book would be gratefully acknowledged.

rial

Part I

Steady State Simulation and Optimization using Aspen Plus

Copyrigf


Rs. 295.00


Amiya K. Jana




CHAPTER

i

Introduction and Stepwise Aspen Plus Simulation: Flash Drum Examples

11 .

ASPEN: AN INTRODUCTION

By developing a computer program, it may be manageable to solve a model structure of

a chemical process with a small number of equations. However, as the complexity of a plant integrated with several process units increases, solving a large equation set f

becomes a challenge. In this situation, we usually use the process lowsheet simulator,

such as Aspen Plus

and PRO/II

(AspenTech). ChemCad

(Chemstations), HYSYS

(Hyprotech)

(SimSci-Esscor). In 2002, Hyprotech was acquired by AspenTech.

However, most widely used commercial process simulation software is the Aspen software.

During the 1970s, the researchers have developed a novel technology at the Massachusetts Institute of Technology (MIT) with United States Department of Energy funding. The undertaking, known as the Advanced System for Process Engineering (ASPEN) Project, was originally intended to design nonlinear simulation software that could aid in the development of synthetic fuels. In 1981, AspenTech, a publicly traded company, was founded to commercialize the simulation software package.

AspenTech went public in October 1994 and has acquired 19 industry-leading companies as part of its mission to offer a complete, integrated solution to the process industries (http://www.aspentech.eom/corporate/careers/faqs.cfm#whenAT).

The sophisticated Aspen software tool can simulate large processes with a high degree of accuracy. It has a model library that includes mixers, splitters, phase separators, heat exchangers, distillation columns, reactors, pressure changers, manipulators, etc. By interconnecting several unit operations, we are able to develop a

f

process low diagram (PFD) for a complete plant. To solve the model structure of either a

Copynghled material

4


a single unit or a chemical plant, required Fortran codes are built-in in the Aspen simulator. Additionally, we can also use our own subroutine in the Aspen package. The Aspen simulation package has a large experimental databank for thermodynamic and physical parameters. Therefore, we need to give limited input data for solving even a process plant having a large number of units with avoiding human errors and spending a minimum time.

Aspen simulator has been developed for the simulation of a wide variety of processes, such as chemical and petrochemical, petroleum refining, polymer, and coalbased processes. Previously, this flowsheet simulator was used with limited

applications. Nowadays, different Aspen packages are available for simulations with promising performance. Briefly, some of them are presented below. Aspen Plus-This process simulation tool is mainly used for steady state simulation of

chemicals, petrochemicals and petroleum industries. It is also used for performance monitoring, design, optimization and business planning. Aspen Dynamics-This powerful tool is extensively used for dynamics study and closed-

loop control of several process industries. Remember that Aspen Dynamics is integrated with Aspen Plus.

Aspen BatchCAD-This simulator is typically used for batch processing, reactions and distillations. It allows us to derive reaction and kinetic information from experimental data to create a process simulation. Aspen Chromatography-This is a dynamic simulation software package used for both batch chromatography and chromatographic simulated moving bed processes. Aspen Properties-It is useful for thermophysical properties calculation. Aspen Polymers Plus-It is a modelling tool for steady state and dynamic simulation, and optimization of polymer processes. This package is available within Aspen Plus or Aspen Properties rather than via an external menu.

Aspen HYSYS-This process modelling package is typically used for steady state simulation, performance monitoring, design, optimization and business planning for petroleum refining, and oil and gas industries.

It is clear that Aspen simulates the performance of the designed process. A solid understanding of the underlying chemical engineering principles is needed to supply reasonable values of input parameters and to analyze the results obtained. For example, a user must have good idea of the distillation column behaviour before attempting to use

Aspen for simulating that column. In addition to the process flow diagram, required input information to simulate a process are: setup, components properties, streams and blocks. ,

12 .

GETTING STARTED WITH ASPEN PLUS SIMULATION

Aspen Plus is a user-friendly steady state process flowsheet simulator. It is extensively used both in the educational arena and industry to predict the behaviour of a process by using material balance equations, equilibrium relationships, reaction kinetics, etc.

Using Aspen Plus, which is a part of Aspen software package, we will mainly perform in this book the steady state simulation and optimization. For process dynamics and

INTRODUCTION AND STEPWISE ASPEN PLUS

SIMULATION

5

f

closed-loop control, Aspen Dynamics (formerly DynaPLUS) will be used in several subsequent chapters. The standard Aspen notation is used throughout this book. For example, distillation column stages are counted from the top of the column: the condenser is Stage 1 and the reboiler is the last stage. As we start Aspen Plus rom the Start menu or by double-clicking the Aspen Plus icon on our desktop, the Aspen Plus Startup dialog appears. There are three choices and we can create our work from scratch using a Blank Simulation, start from a Template or Open an Existing Simulation. Let us select the Blank Simulation option and click OK (see Figure 1.1). MM

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FIGURE 1.1

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The simulation engine of Aspen Plus is independent rom its Graphical User Interface (GUI). We can create our simulations using the GUI at one computer and run them connecting to the simulation engine at another computer. Here, we will use the simulation engine at Local PC'. Default values are OK. Hit OK in the Connect to Engine dialog (Figure 1.2). Notice that this step is specific '

to the installation.

f

f

The next screen shows a blank Process Flowsheet Window. The irst step in developing a simulation is to create the process lowsheet. Process flowsheet is simply defined as a blueprint of a plant or part of it. It includes all input streams, unit operations, streams that interconnect the unit operations and the output streams. Several process units are listed by category at the bottom of the main window in a toolbar known as the Model Library. If we want to know about a model, we can use the Help menu from the menu bar. In the following, different useful items are highlighted briefly (Figure 1.3). Copyrighted material

6


Connect to Engine Serve« type

Local PC

Liter Into Node name:

Uset name Password

Working dfedory:

Q Save as Default Cormeciion OK

Exit

Help

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INTRODUCTION AND STKPWISK ASPEN PI.US

SIMULATION

7

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To develop a lowsheet, irst choose a unit operation available in the Model Library.

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Proprietary models can also be included in the lowsheet window using User Models option. Excel workbook or Fortran subroutine is required to define the user model. In the subsequent step, using Material STREAMS icon, connect the inlet and outlet streams

with the process. A process is called as a block in Aspen terminology. Notice that clicking f

on Material STREAMS, when we move the cursor into the lowsheet area red and blue

arrows appear around the model block. These arrows indicate places to attach streams f

to the block. Red arrows indicate required streams and blue arrows are optional. When the lowsheet is completed, the status message changes from Flowsheet Not

Complete to Required Input Incomplete. After providing all required input data using input forms, the status bar shows Required Input Complete and then only the simulation results are obtained. In the Data Browsery we have to enter information at locations where there are red semicircles. When one has finished a section, a blue checkmark

appears. In subsection 1.3.2. a simple problem has been solved, presenting a detailed stepwise simulation procedure in Aspen Plus. In addition, three more problems have

also been discussed with their solution approaches subsequently. 13

STEPWISE ASPEN PLUS SIMULATION OF FLASH DRUMS

.

1 3 1 Built-in Flash Drum Models .

.

f

In the Model Library, there are ive built-in separators. A brief description of these models is given below.

f

Flash 2: It is used for equilibrium calculations of two-phase (vapour-liquid) and threephase (vapour-liquid-liquid) systems. In addition to inlet stream(s), this separator can include three product streams: one liquid stream, one vapour stream and an optional water decant stream. It can be used to model evaporators, lash chambers and other single-stage separation columns.

Flash 3: It is used for equilibrium calculations of a three-phase (vapour-liquid-liquid) system. This separator can handle maximum three outlet streams: two liquid streams and one vapour stream. It can be used to model single-stage separation columns. f

Decanter: It is typically used for liquid-liquid distribution coeficient calculations of a two-phase (liquid-liquid) system. This separator includes two outlet liquid streams along

with inlet stream(s). It can be used as the separation columns. If there is any tendency of vapour formation with two liquid phases, it is recommended to use Flash3 instead of Decanter.

f

Sep 1: It is a multi-outlet component separator since two or more outlet streams can be produced rom this process unit. It can be used as the component separation columns. Sep 2: It is a two-outlet component separator since two outlet streams can be withdrawn from this process unit. It is also used as the component separation columns.

At this point it is important to mention that for additional information regarding a built-in model, select that model icon in the Model Library toolbar and then press Fl on the keyboard.

8


132 .

.

Simulation of a Flash Drum

Problem statement

r

A 100 kmol/hr feed consisting of 10, 20, 30, and 40 mole% of propane, c-butane, n-pentane and n-hexane, respectively, enters a lash chamber at 15 psia and 50oF. The lash drum (Flash2) is shown in Figure 1.4 and it operates at 100 psia and 200oF. Applying the SYSOP0 property method, compute the composition of the exit streams. f

f

,

3

-

FLASH

f

A lowsheet of a lash drum. f

FIGURE 1.4

Simulation approach

From the desktop, select Start button followed by Programs, AspenTech, Aspen Engineering Suite, Aspen Plus Version and Aspen Plus User Interface. Then choose Template option in the Aspen Plus Startup dialog (Figure 1.5).

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FIGURE 1.5

As the next window appears after hitting OK in the above screen, select General with English Units (Figure 1.6). Copyrighted material

INTRODUCTION AND STEPVV1SE ASPEN PLUSIM SIMULATION -Hi

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Then click OK. Again, hit OK when the Aspen Plus engine window pops up and

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subsequently, proceed to create the lowsheet. Creating flowsheet

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Select the Separators tab from the Model Library toolbar. As discussed earlier, there are ive built-in models. Among them, select Flash2 and place this model in the window. Now the Process Flowsheet Window includes the lash drum as shown in Figure 1.7. By

default, the separator is named as Bl. '

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PROCESS SIMULATION AND CONTROL USING ASPEN1

To add the input and output streams with the block, click on Streams section (lower left-hand comer). There are three different stream categories (Material, Heat and Work), as shown in Figure 1.8.

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FIGURE 1.8

Block Bl includes three red arrows and one blue arrow as we approach the block

after selecting the Material STREAMS icon. Now we need to connect the streams with f

the lash chamber using red arrows and the blue arrow is optional. The connection procedure is presented in Figure 1.9.

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INTRODUCTION AND STFPWISK ASPEN PLUS

SIMULATION

11

Clicking on Material STREAMS, move the mouse pointer over the red arrow at the

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inlet of the lash chamber. Click once when the arrow is highlighted and move the cursor so that the stream is in the position we want. Then click once more. We should see a stream labelled 1 entering the drum as a feed stream. Next, click the red arrow

coming out at the bottom of the unit and drag the stream away and click. This stream is marked as 2. The same approach has been followed to add the product stream at the f

top as Stream 3. Now the lowsheet looks like Figure 1.10. Note that in the present

case, only the red arrows have been utilized. ..

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.

We can rename the stream(s) and block(s). To do that highlight the object we want to rename and click the right mouse button. Select Rename Block and then give a new name, as shown in Figure 1.11 for Block Bl.

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Alternatively, highlight the object, press Ctrl + M on the keyboard, change the name, and finally hit Enter or OK. After renaming Stream 1 to F, Stream 2 to L,

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Stream 3 to V and Block Bl to FLASH, the lowsheet inally resembles Figure 1.12.

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FIGURE 1.12

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In order to inspect completeness for the entire process lowsheet, look at the status f

indicator. If the message includes Flowsheet Not Complete, click on Material STREAMS. If any red arrow(s) still exists in the lowsheet window, it indicates that the process is

not precisely connected with the stream(s). Then we need to try again for proper f

connection. To ind out why the connectivity is not complete, hit the Next button on the Data Browser toolbar. However, if we made a mistake and want to remove a stream

f

f

f

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(or block) from the lowsheet, highlight it. right click on it. hit Delete Stream (or Delete Block), and inally click OK. Anyway, suppose that the lowsheet connectivity is complete. Accordingly, the status message changes rom Flowsheet Not Complete to Required Input Incomplete.

We have defined the unit operation to be simulated and set up the streams into and out of the process. Next we need to enter the rest of the information using several input forms required to complete the simulation. Within Aspen Plus, the easiest way to

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ind the next step is to use one of the followings: .

2

.

3

.

click the Next button ind Next in the Tools menu

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1

use shortcut key F4

As a consequence. Figure 1.13 appears.

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INTRODUCTION AND STKPWISK ASPEN PLUS

SIMULATION

13 i

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Configuring settings As we click OiC on the message. Aspen Plus opens the Data Browser window containing

the Data Browser menu tree and Setup/Specifications/Global sheet. Alternatively, clicking on Solver Settings and then choosing Setup /Specifications in the left pane of the Data Browser window, we can also obtain this screen (Figure 1.14). JUS.'

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14

PROCESS SIMUIvVTION AND CONTROL USING ASPEN

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Although optional, it is a good practice to ill up the above form for our project giving the Title (Flash Calculations) and keeping the other items unchanged (Figure 1.15). .

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FIGURE 1.15

In the next step (Figure 1.16), we may provide the Aspen Plus accounting information (required at some installations). In this regard, a sample copy is given with the followings: User name: AKJANA

Account number: 1

Project ID: ANYTHING Project name: AS YOU WISH

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FIGURE 1.16



SIMULATION

15

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We may wish to have streams results summarized with mole ractions or some other basis

that is not set by default. For this, we can use the Report Options under Setup folder. In the f

subsequent step, select Stream sheet and then choose Mole raction basis, ...

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As illed out, the form shown in Figure 1.17, inal results related to all inlet and product streams will be shown additionally in terms of mole raction. Remember that all values in the inal results sheet should be given in the British unit as chosen it previously. Specifying components

Clicking on Next button or double-clicking on Components in the column at the left side and then selecting Specifications, we get the following opening screen (Figure 1.18).

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PROCESS SIMULATION AND CONTROL USING ASPEN1"

Next, we need to fill up the table as suggested in Figure 1 18. A Component ID is essentially an alias for a component It is enough to enter the formulas or names of the components as their IDs Based on these component IDs, Aspen Plus fills out the Type .

.

.

,

Component name and Formula columns But sometimes Aspen Plus does not find an exact match in its library. Like in the present simulation, we have the following screen (Figure 1.19) after inserting chemical formulas of the components in the Component ID .

,

column. 3513 I

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Obviously, only for Component ID C3H8, Aspen Plus provided the Component name (PROPANE) and Formula (C3H8). This simulator does not recognize the other three components by their IDs. Therefore, we have to search in the following way (Figure 1.20) to obtain their names and formulas. Click on a component ID (say, N-C4H10), then hit Find button.

Now, we have to give a hint with Component name or formula (butane) and then

hit Enter or Find now button (Figure 1.21). Apart from component name or formula, we can also search a component by giving component class or molecular weight (range) or boiling point (range) or CAS (Chemical Abstracts Service) number. Click on Advanced button in the following screen to get these options.

INTRODUCTION AND STKPWISE ASPEN PLUS

23

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i

-i 'Ipi

i

m\

I Hl .i-i

i

i

35Ji

.

e?*-'aTtyr '

t

I

l'Tl S!J "SI -I

u'tt WMM

'

I'tM* |M.| fiiM

Mirr?

1X414

tOD O* 114 2n

-VJ J-'

'

* .

tuhtn

J 1 'ttrVM r C4M»4

runrn

PURE 11

J

.

Si K

FIGURE 1.21

I

*MM2 SrM O-' '

wj

III I

I

ll itL f

r

aKlftl-l

18


f

Aspen Plus suggests a number of possibilities. Among them, select a suitable component name (N-BUTANE) and then click on Add. Automatically, the Component name and Formula for Component ID N-C4H10 enter into their respective columns. For last two components, we follow the same approach. When all the components are completely defined, the illed component input form looks like Figure 1.22. m

u

-

m:

rai-«-l«»|««i|-4| *»-| »"l

vr

,

I

r-

I

"

i

I

I

M -leal

:

!

.) !

i"!

-I vj

ttl "

let >.Si

- ~

8

j

s- I

n

tt-

FIGURE 1.22

f

The Type is a specification of how Aspen calculates the thermodynamic properties. For luid processing of organic chemicals, it is usually suitable to use 'Conventional* option. Notice that if we make a mistake adding a component, right click on the row and then hit Delete Row or Clear.

Specifying property method

Press Next button or choose Properties I Specifications from the Data Browser. Then if we click on the down arrow under Base method option, a list of choices appears. Set the SYSOPO' method as shown in Figure 1.23. A Property method defines the methods and models used to describe pure component and mixture behaviour. The chemical plant simulation requires property data. A wide variety of methods are available in Aspen Plus package for computing the properties. Each Process type has a list of recommended property methods. Therefore, the Process type narrows down the choices for base property methods. If there is any confusion, we may select All' option as Process type. '

Specifying stream information In the list on the left, double click on Streams folder or simply use Next button. Inside that folder, there are three subfolders, one for each stream. Click on inlet stream F, and

f

enter the temperature, pressure, low rate and mole fractions. No need to provide any data for product streams L and V because those data are asked to compute in the present problem (see Figure 1.24). This property method assumes ideal behaviour for vapour as well as liquid phase.

C

ll

INTRODUCTION AND STEI'WISK ASPEN PLUS

SIMULATION

19 cina

Tiers r "

3

i 0

samii (Ham

UVUM

.

par-

-

r

AFU

I t4 -I - I . -

-

|M

Co

f>

.

FBI

P j mi«D»

a-

-

HO-e-o-i-it.

! FIGURE 1.23

Ha

0]t*lMI rmr

'ssH

I

_

i~i-..t>-rv

f5~

Im«7V= f,

nns

.rilll

,

ri.ttn

31 Dt it:

'

I

JIU-*"- I'M-

.1.

-.. .11.

:

.

*

...

h o e czd- @ - it. FIGURE 1.24

Specifying block information

Hitting Next button or selecting Blocks/FLASH in the column at the left side, we get the block input form. After inserting the operating temperature and pressure, one obtains Figure 1.25.

20

PROCESS SIMULATION AND CONTROL USING ASPRN U3SE Toob

Ron

Piol

Lfciaiy

Wmdow

Help

did -J a ~

~

i :r

D - I

u>i"i-

.-

I

'I -isil

I

M

lai alS*l

UNIFAC Gioup 3 /Sp«c>rioalion>{ Floih.Ophwn | ErJ )

UN1FAC G
_

. __

Cl

J

0ot ,

J

A sJyBJ

_

-

PMP>SMi

-

*

.

O

K>OE5I
0

tMCPMAL

(#>

TXPOftT

O

VIE

ilj

Advanced

Lifl

>=

gp-n=-3 -

i

JQ -

-

Input EO varial

IS |

FLASH Be

i

Conv Op«noj EO Conv Option*

O

Setup DMOBasK

49

DMOAdv

Input CompteK

[1

Mbcwt/SpBtsit

: STREAMS

Sopjuato.. j HmI Exciwigsi t Columni | FtMclnt | Pfonuio Chonoe

H 0 - 9 -CD'

Fl«h2

FlaihS

Deca/Kei

5ep2

Sep

FIGURE 1.25

Now the Status message (Required Input Complete) implies that all necessary information have been inserted adequately. Moreover, all the icons on the left are blue. It reveals that all the menus are completely filled out. If any menu is still red, carefully enter the required information to make it blue. Running the simulation

Click on Next button and get the following screen (see Figure 1 26). To run the simulation, press OK on the message. We can also perform the simulation selecting Run from the Run pulldown menu or using shortcut key F5 .

.

-

r

Zl

"

Tl SJ b li""

1

1 ] all*3

-l±j

cjJ

_

I

- * I .IPI

. I > in

rnim

Cl ~-T

£=1 3

8

! 7.1

33

.TfUAMt

CarrvOpllam

to Conv Option*

'

FWttJ

SgM

L -«o>i

S p

"fJ

FIGURE 1.26

The Control Panel, as shown in Figure 1.27, shows the progress of the simulation. It presents all warnings, errors, and status messages.

jNIRODUCTlON AND STEPW1SE ASPEN PLUS SIMULATION Q rtm eai vw« DM* roota

1"! _=J

3?) -

21

Lih..i..

QhrjAj*i j-j an

,

I

4

H -iroh

.| ihi .j

M

L_jih

3 I

"

3 r

"

3 r

. loch:

Pt.iofva and Po«U<**» Soipti

p"

l t*«i * *S'«-

f" '.. i ' r..:.

Command Lr» | AI bkK+» h«v» bean .

STREAMS

0

6 -ciD

FU>»K3

Fl
D«canl-

Sup

S»p2

FIGURE 1.27

Viewing results

f

Hitting ATex button and then clicking OK, the Run Status screen appears irst (see Figure 1.28). yil l .i.l.lJIII«.II..IIHIII.I.IMItMIIIIH.HI II.Wl'ltlll.Ml.llltHW I

-

I Ffe

Edt

VKm

ItflHI

Data

Tools

Rvxi

Mot

Lbtoty

Window

htetp

-I v| daHal -

S i

QU

3 m I _iJ_iMi_L

3 sQg r

B Ru-i Slatut Streams

RaMiU Swranarv -

Run Statu* Streams

Convergence

Atpen Plui Vetswn Lite

prrr [fLash CALCULATIONS "

Dale and lime

[JUNE 5. 2007

Uminam» S*»\D

[AOMIN IS TRATOB |TEAM_EAT [WIN32

Machnelypo

1 23621 Pm

Hott

iCONTROLLAB

Use << and >> robiowie testitt

MBW./Scfcie..

S*

.

) H»al E-changst | CcWa | Be«clor.

e Chang**

i

Man«

j Sobd. | U>«Mo4* |

(0-9 o 8 . FIGURE 1.28

From the Data Browser, choose Results Summary /Streams and get the following 1 29). .

f

screen that includes the inal results of the given problem (see Figure

We can Backup i le (*.bkp) takes name the ile whatever we want. Note that an Aspen Plus

f

f

Save the work by choosing File/Save As/...from the menu list on the top. f

much less space than a normal Aspen Plus Documents ile

( .apw).

22

PROCESS SIMULATION AND CONTROL USING ASPEN 1j Fto

'. ,-V

. -

Took

Run

P

JSbd JMSj-d HIP

:

»

J/l

I

Block*

I

£1 fo. * r

"

Vapo* Froc

3 5l<»amT»blf[

rjj 1

r

- 3

50 0

2000

200 0

i f. nr i

ion oo

ion on

0016

1 000

0 000

220 462

1 Tf 971

42 492

15906 41*

13312.698

2593 716

1039 561

382.439

3008 065

16 583

1243?

2 236

C3H8

22 046

9 275

12 771

NC4H10

44092

30124

13 969

f

Mote Flow cmot/hi Mas* Ftow b/h.

. l:...- Flow culler

v

lE.Hh»lpy

jsJ . j . i

MMBtu/hi

Mole Flow bmolVIv

N.C6H14

(V

Mixw

pMto!

'

FVwh2 "

56 242

9 896

eeies

82 329

5856

SoiMralof* { Heal Enchangon | Column* | Re»cto.. \ P-eume Chongeij \ Mo puMw* | So§*. | Um. Models )

HO 0 cd

STREAMS

66139

Flaih3

D>came>

3ep

Sep2

""

C.\-

For H*te, press Fi *

Start}}

.g Pol
!

NUM i

- .

.

..

Av,4,>:-

Aspen Pkn - Simulatl_

FIGURE 1.29

If we click on Stream Table button, the results table takes a place in the Process Flowsheet Window, as shown in Figure 1.30. Fie Edt

View Data

Tocfc Run Ffevaheet Librvy Whdow Help

1 global j

|£e.|

. I

lai 1

F

Temptntuit

F

Pttiiun

pri

L

200.0

15 00

100 00

100.00

0 018

0 000

1000

177 971

42 492

15906 414

13312 698

2593 716

1639 561

382439

3008 065

.

r

f

cnoVhi f

f

ctu

VokuntFlw

EnlhJpy

200 0

220462

V*poi Fnc HoUFtow

V

50 0

MMBtu/hi

16583

-

-

12.499

-

2236

Hole Flw C3H8

22 046

9 275

12 771

H-C4H10

44 092

30 124

13569

K-C5H12

66139

56 242

9996

H-C6H14

88 185

82.329

5 856

C3K8

0 100

0053

0 301

HX4HI0

0 200

0 169

0329

H-C5H12

OJOO

0 316

0233

H-C6H14

0 400

0 463

0 138

-

Mok Trie .

.

.

Mm/Spitlan Sflprntms { Heat Eicchangeit { Cdum | Reactori | PrMtue Chmgeii j Mmpdalai | Soldi j Use. Models j D-»

-

<0-8-o C:V.oF<*lefs\A»penMu»n.l

?1 1 FIGURE 1.30

.

INTRODUCTION AND STKPWISK ASPKN PI.US,M SIMUI.ATION

23

Viewing input summary

To obtain the input information, press Ctrl + Alt + I on the keyboard or select Input

f

Summary rom the View pulldown menu. The supervisor may ask to include the results, f

shown in Figure 1.30, along with the input summary in the inal report on the present project (see Figure 1.31). Fl»

Ml

ft

>W

He»

linput swimtrf crvaccd bv upen Plus "el. U.l tt tiiMtiS rrf jun a, 2007

;

Dlr»ctory CtSproarur 11 TBc\Aspanrai:n\WDrklng Polaei ' j' iveft Plus 11.1

~

tllnnm*

mMPuis DPLUS RCSULTS-ON TITLE

'PlHh Calculations '

IN-UNir»

lii.

DEC-STRESS CONVtlt ALl

CCOUNI-tKEO KC0UNT>1 PROJECT-ID»*MtTHING 4 ff>0)6C'OU WISH 0SE('-H**S-"«J/f«' DGKRIPriON '

General Sl*u1al1e*< w
Ib/hf,

lEf«ol/»». oiu/hr, eirft/hr,

Propariy Haihooi wona eln* M*l» for "

i r j -

Incur: NOll

report

PUBCII

:

. Mola *lo»

'.

/ AQUCOUS

/ SOLIDS

/ UttROANIC

/

t

tOASPENPCO

PROP-IOURCES CUBEll

/ MJUCOUS

/ SOCIOl

/ INC>Ra»"IC

CJm8 C3h8 / N-camo caxio-z N-cenW

/

CftHH-l

"lOWSHEET

bicc>

flash

ih-e

aut-v

l

PROPERTIES SY5OP0

SUOSTRCAH -EO TEWB.lo. xe-fb»c ana o.i / w

PBE5-11, »MLE-PLOB-i00. -ktcVr>-> kio o.j / n-cihi? o. t / »

.*

N-c6nl4

-

0 .4

plash Flash; kabah rtwp- ao.

"sr.-ic-j

.

FIGURE 1.31

Creating report file

f

To create a detailed report of the work we have done, including input summary, stream information, etc., select Export (Ctrl + E) from the File dropdown menu. Then save the work as a report ile (e.g. C/Program Files/AspenTech/Working Folders/Aspen Plus ,

f

Version/ Flash.rep). Subsequently, we may open the saved report ile (Flash.rep) going

f

through My Computer with using a program, such as the Microsoft Office Word or WordPad or Notepad. A report ile for the present problem is opened below. ASPEN PLUS IS A TRADEMARK OF

HOTLINE:

ASPEN TECHNOLOGY. INC.

U.S.A. 888/996-7001

TEN CANAL PARK

EUROPE (32) 2/724-0100

CAMBRIDGE. MASSACHUSETTS 02141 617/949-1000

24


PLATFORM: WIN32

JUNE 10, 2007

VERSION: 11.1 Build 192

SUNDAY

11:23:23 A.M

INSTALLATION: TEAM EAT

.

_

06/10/2007 PAGE I

ASPEN PLUS PLAT: WIN32 VER: 11.1 FLASH CALCULATIONS

ASPEN PLUS (R) IS A PROPRIETARY PRODUCT OF ASPEN TECHNOLOGY, INC. (ASPENTECH), AND MAY BE USED ONLY UNDER AGREEMENT WITH ASPENTECH. RESTRICTED RIGHTS LEGEND: USE, REPRODUCTION, OR DISCLOSURE BY THE U S GOVERNMENT IS SUBJECT TO RESTRICTIONS SET FORTH IN .

.

(i) FAR 52.227-14, Alt. Ill, (ii) FAR 52.227-19, (iii) DEARS 252.227-7013(c)(l)(ii), or (iv) THE ACCOMPANYING LICENSE AGREEMENT, AS APPLICABLE. FOR PURPOSES OF THE FAR, THIS SOFTWARE SHALL BE DEEMED TO BE "UNPUBLISHED" AND LICENSED WITH DISCLOSURE PROHIBITIONS. CONTRACTOR/SUBCONTRACTOR: ASPEN TECHNOLOGY, INC. TEN CANAL PARK, CAMBRIDGE, MA 02141.

TABLE OF CONTENTS

RUN CONTROL SECTION RUN CONTROL INFORMATION DESCRIPTION

1 1 1

FLOWSHEET SECTION FLOWSHEET CONNECTIVITY BY STREAMS FLOWSHEET CONNECTIVITY BY BLOCKS

2 2 2

COMPUTATIONAL SEQUENCE

2

OVERALL FLOWSHEET BALANCE

2

PHYSICAL PROPERTIES SECTION

3

COMPONENTS

3

U-O-S BLOCK SECTION

4

BLOCK: FLASH MODEL: FLASH2

4

STREAM SECTION

5

F L V

5

PROBLEM STATUS SECTION

6

BLOCK STATUS

6

ASPEN PLUS PLAT: WIN32 VER: 11.1 06/10/2007 FLASH CALCULATIONS RUN CONTROL SECTION RUN

CONTROL

INFORMATION

THIS COPY OF ASPEN PLUS LICENSED TO TYPE OF RUN: NEW INPUT FILE NAME:

1437xbh.inm

_

OUTPUT PROBLEM DATA FILE NAME:

_

1437xbh VERSION NO. 1

PAGE 1


SIMULATION

25

LOCATED IN:

PDF SIZE USED FOR INPUT TRANSLATION:

NUMBER OF FILE RECORDS (PSIZE) = 0 NUMBER OF IN-CORE RECORDS - 256 PSIZE NEEDED FOR SIMULATION - 256

CALLING PROGRAM NAME: LOCATED IN:

apmain C:\PROGRA~ I\ASPENT~-1 \ASPENP~1.1 \Engine\xeq

SIMULATION REQUESTED FOR ENTIRE FLOWSHEET DESCRIPTION

GENERAL SIMULATION WITH ENGLISH UNITS : F, PSI, LB/HR, LBMOL/HR, BTU/HR, CUFT/HR. PROPERTY METHOD: NONE FLOW BASIS FOR INPUT: MOLE STREAM REPORT COMPOSITION: MOLE FLOW

ASPEN PLUS

PLAT: WIN32

VER: 11.1

06/10/2007

PAGE 2

FLASH CALCULATIONS FLOWSHEET SECTION FLOWSHEET STREAM

CONNECTIVITY SOURCE

F L

BY

STREAMS

DEST

STREAM

SOURCE

FLASH

V

FLASH

DEST

FLASH

FLOWSHEET

CONNECTIVITY

BY

BLOCKS

BLOCK

INLETS

OUTLETS

FLASH

F

V L

COMPUTATIONAL SEQUENCE SEQUENCE USED WAS: FLASH

OVERALL

FLOWSHEET

BALANCE

MASS AND ENERGY BALANCE

IN

RELATIVE DIFF.

COMPONENTS

OUT (LBMOL/HR)

C3H8 N-C4H10

22.0462

22.0462

0 101867E-09

44.0925

44.0925

0 326964E-10

N-C5H12

66.1387

66.1387

N-C6H14

88.1849

88.1849

CONVENTIONAL

.

.

0 113614E-10

-

.

0 332941E-10

-

.

26

PROCESS SIMULATION AND CONTROL USING ASPEN TOTAL BALANCE

MOLE( LBMOL/HR) 220.462 220.462 0.000000E+00 MASS(LB/HR) 15906.4 15906.4 -0.782159E-11 ENTHALPY(BTU/HR) -0.165833E+08 -0.147349E+08-0.111463

ASPEN PLUS PLAT: WIN32

06/10/2007

VER: 11.1

PAGE 3

FLASH CALCULATIONS PHYSICAL PROPERTIES SECTION COMPONENTS

ID

N-C5H12

TYPE C C C

N-C6H14

C

C3H8 N-C4H10

FORMULA

NAME OR ALIAS

REPORT NAME

C3H8 C4H10-1 C5H12-1

C3H8

C3H8

C4H10-1

N-C4H10

C5H12-1

N-C5H12

C6H14-1

C6H14-1

N-C6H14


06/10/2007

VER: 11.1 FLASH CALCULATIONS

PAGE 4

U-O-S BLOCK SECTION BLOCK:

FLASH

MODEL:

FLASH2

INLET STREAM: OUTLET

VAPOR

OUTLET

LIQUID

PROPERTY

F STREAM:

STREAM:

OPTION ***

V

SET:

MASS

L SYSOP0

AND

IDEAL

ENERGY

IN

LIQUID /

BALANCE

OUT

IDEAL

GAS

***

RELATIVE DIFF.

TOTAL BALANCE

MOLE(LBMOL/HR) 220.462 MASS(LB/HR) 15906.4 ENTHALPY(BTU/HR) -0.165833E+08

220.462 15906.4

-0.147349E+08

INPUT DATA

TWO PHASE TP FLASH SPECIFIED TEMPERATURE

F

SPECIFIED PRESSURE

PSI

MAXIMUM NO. ITERATIONS CONVERGENCE TOLERANCE

100.000 30 0 000100000 .

***

OUTLET TEMPERATURE

200.000

F

RESULTS ***

200.00

0 000000E+00 .

-

0 782136E-11 .

-0.111463

INTRODUCTION AND STEPWISE ASPEN PLUS OUTLET PRESSURE HEAT DUTY

PSI

100.00

BTU/HR

0 18484E+07

PHASE

0 19274 .

EQUILIBRIUM:

COMP

F{I)

X(I)

Yd)

K(I)

C3H8

0 10000

0 52117E-01

0 30055

5 7668

N-C4H10 N-C5H12

0 20000

0 16926

0 32874

1 9422

0 30000

0 31602

0 23290

0 73697

N-C6H14

0 40000

0 46260

0 13781

0 29790

.

.

.

.

.

.

.

.

.

.

.

.


VER: 11.1

.

.

.

.

06/10/2007

FLASH CALCULATIONS STREAM SECTION F L V

STREAM

FROM

ID

:

TO

L

V

FLASH

FLASH

FLASH

SUBSTREAM:

MIXED

PHASE:

MIXED

COMPONENTS:

LIQUID

VAPOR

LBMOL/HR

C3H8

22.0462

9 2754

12.7709

N-C4H10

44 0925

30.1237

13.9688

N-C5H12

66 1387

56.2424

9 8963

N-C6H14

88 1849

82.3291

5 8558

COMPONENTS:

.

.

.

.

MOLE

.

.

FRAC

C3H8

0.1000

5 2117-02

0 3005

N-C4H10

0 2000

0 1693

0 3287

N-C5H12

0 3000

0 3160

0 2329

N-C6H14

0 4000

0 4626

0 1378

TOTAL

.

.

.

.

.

.

.

.

.

.

.

FLOW:

220.4623

177.9706

42.4917

LB/HR

1.5906+04

1 3313+04 .

2593.7158

CUFT/HR

1839.5613

382.4385

3008.0650

LBMOL/HR

STATE

27

.

VAPOR FRACTION

V-L

SIMULATION

VARIABLES:

TEMP

F

50.0000

200.0000

200.0000

PRES

PSI

15.0000

100.0000

100.0000

VFRAC

1.8002-02

0 0

1 0000

LFRAC

0.9820

1 0000

0 0

S FRAC

0.0

00

0 0

.

.

.

.

.

.

PAGE 5

28

PROCESS SIMULATION AND CONTROL USING ASPEN1 ENTHALPY:

BTU/LBMOL BTU/LB BTU/HR

-7.5221+04 -1042.5543 -1.6583+07

-7.0232+04 -938.9019 -1.2499+07

-5.2612+04 -861.9118 -2.2356+06

-130.1235 -1.8035

-123.3349 -1.6488

-87.8846

0.1198 8.6469 72.1503

0.4654 34.8100 74.8028

1.4126 02 0.8623 61.0406

ENTROPY:

BTU/LBMOL-R BTU/LB-R

-

1 4398 .

DENSITY:

LBMOL/CUFT LB/CUFT AVG MW

ASPEN PLUS PLAT: WIN32 VER: 11.1

-

06/10/2007

PAGE 6

FLASH CALCULATIONS PROBLEM STATUS SECTION BLOCK

STATUS

********************************************************

**************

*

*

Calculations were completed normally

*

* *

*

All Unit Operation blocks were completed normally

*

* *

* *

*

f

All streams were lashed normally

*

*

************************************************************************:!:;!=

13 .

.

3

Computation of Bubble Point Temperature

Problem statement

Compute the bubble point temperature at 18 bar of the following hydrocarbon mixture (see Table 1.1) using the RK-Soave property method. TABLE 1.1

Component

Mole fraction

Ci c2 C3

0 1

i-Ci

0 1

n-Ci

02

0 05 .

.

0 15 .

.

.

i-C5

0 25

n-C5

0 15

.

.

f

Assume the mixture inlet temperature of 250C, pressure of 5 bar and low rate of 120 kmol/hr.

S,MULA' noN

29

Simulation approach

After starting the Aspen Plus simulator, the Aspen Plus Stnrt

Among the three choices, select Template option and then S

,.,

v i e

BlMtt i ~| S!| -j j jj

L L J.-i..'i- I iM

t ,J;'&9'lr.lrtoi\Ait«r.leI:MV l,1gffj

g

F Tl 3 j

,Asinwi

Ptft.,..- "" TTrTtrtilVfnrt.i0ritliiiV>iWnrfca 11

C 'Pi09'*T>F'f'''-!CW"lecl-AW1>t»>jFc«eii'A Mr!rt,: n :

H

!i

j

FIGURE 1.32

When the next window pops up (see Figure 1.33)

,

select General with Metric Units

and then hit OK.

3 -II

...d..ji:;L:

i

1 1

raliH

FIGURE 1.33

In the next

,

press OK in the Connect to Engine dialog. Once Aspen Plus connects to

the simulation engine, we are ready to begin entering the process system.

30


Creating flowsheet

Using the Flash2 separator available in the equipment Model Library, develop the

f

following process low diagram (see Figure 1.34) in the Flowsheet Window by connecting f

the input and output streams with the lash drum. Recall that red arrows are required ports and blue arrows are optional ports. To continue the simulation, we need to click either on Next button or Solver Settings as discussed earlier. Note that whenever we have doubts on what to do next, the simplest way is to click the Next button.

rjafn ..|-|..|. {k

jl .15)1

I

gl *w

.

0 o

mm

1

o-e-oi-ir2£

_

£S-| »... >

FIGURE 1.34

Configuring settings

From the Data Browser, choose Setup I Specifications. The Title of the present problem is given as 'Bubble Point Calculations'. Other items in the following sheet remain untouched (see Figure 1.35). However, we can also change those items (e.g., Units of measurement. Input mode, etc).

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3

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FIGURE 1.35

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INTKODUCTION AND STHPWISE ASFKN PLUSIM SIMULVTION

31

'

In the next, the Aspen Plus accounting information are given (see Figure 1.36). _

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FIGURE 1.36

Specifying components

Click on TVex button or choose Components /Specifications in the list on the left. Then define all components and obtain the following window (see Figure 1.37). rfc

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mm

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FIGURE 1.37

Copyrlqhted material

32



Hit Next button or select Properties / Specifications in the column at the left side. In Property method, scroll down to get RK-Soave. This equation of state model is chosen for thermodynamic property predictions for the hydrocarbon mixture (see Figure 1.38).

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=1 3

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.

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FIGURE 1.38

Hitting ATex/ button twice, we have the following picture (see Figure 1.39). The binary parameters are tabulated below. When we close this window or cbck OK on the message. it implies that we approve the parameter values. However, we have the opportunity to

edit or enter the parameter values in the table. In blank spaces of the table, zeros are there. It does not reveal that the ideal mixture assumption is used because many

thermodynamic models predict non-ideal behaviour with parameter values of zero.

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FIGURE 1.39

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INTRODUCTION AND STEPWISR ASI-KNJ>LU

sim

33

Specifying stream information

Click OK. Alternatively, use the Data Browser menu tree to navigate to the Streams/1/ Input/Specifications sheet. Then insert all specifications for Stream 1 as shown in Figure 1 40 J

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FIGURE 1.40

Specifying block information r

Hit Afot or select Blocks/BUBBLE from the Data Browser. After getting the blank input

form, enter the required inputs (Pressure = 18 bar and Vapour fraction = 0) for block BUBBLE (see Figure 1.41). -

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FIGURE 1.41

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34


Running the simulation

Press Next button and then hit OK to run the simulation The following Control Panel .

demonstrates the status of our simulation work (see Figure 1 42). .

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FIGURE 1.42

Viewing results

Clearly, Figure 1.42 includes the Status message: Results Available. As the simulation calculations completed, click on Solver Settings and then double-chck on Blocks to obtain the following screen (see Figure 1.43). -

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INTKODUCTION AND STKPWISK ASI'KN PLUS

SIMULATION

35

Choosing Blocks/BUBBLE/Results in the column at the left side, we get the

following results summary for the present problem (see Figure 1.44).

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FIGURE 1.44

From the results sheet, we obtain the bubble point temperature = 42.75411960C. 13 .

4

.

Computation of Dew Point Temperature

Problem statement

Compute the dew point temperature at 1.5 bar of the hydrocarbon mixture, shown in Table 1.2, using the RK-Soavc property method. TABLE 1.2

Component

Ci C2 Ca

Mole fraction 0 05 .

0 1 .

0 15 .

<-c4

0 1

n-CA

0 2

.

.

M3a

0 25

(>

0 15

"

-

,

.

.

f

Assume the mixture inlet temperature of 250C, pressure of 5 bar and low rate of 120 kmol/hr.

36


Simulation approach f

As we start Aspen Plus rom the Start menu or by double-clicking the Aspen Plus icon on our desktop theAspe?i Plus Startup dialog appears (see Figure 1.45). Select Template option ,

.

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FIGURE 1.45

As Aspen Plus presents the window after clicking OK as shown Figure 1.45, choose General with Metric Units. Then press OK (see Figure 1.46).

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FIGURE 1.46

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SIMULATION

37

Subsequently, dick OK when the Aspen Plus engine window pops up. Creating flowsheet

f

In the next, we obtain a blank Process Flowsheet Window. Then we start to develop the process lowsheet by adding the Flash2 separator from the Model Library toolbar and joining the inlet and product streams by the help of Material STREAMS (Figure 1.47).

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FIGURE 1.47

f

Now the process low diagram is complete. The Status bar in the bottom right of the above window (see Figure 1.47) reveals Required Input Incomplete indicating that input data are required to continue the simulation. Configuring settings

Hitting Next button and then clicking OK, we get the setup input form. The present problem is titled as Dew Point Calculations' (see Figure 1.48). In Figure 1.49, the Aspen Plus accounting information are provided. '


f

Here we have to enter all the components we are using in the simulation. In the list on the left, choose Components /Specifications and ill up the table following the procedure explained earlier (see Figure 1.50).

Copyrighted malenal

38

PROCESS SIMULATION AND CONTROL USING ASPEN1 J9J »i

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SIMULATION

39

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FIGURE 1.50

Specifying property method From the Data Browser, select Properties /Specifications to obtain a blank property input form. From the Property method pulldown menu, select RK-Soave (see Figure 1.51). !

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FIGURE 1.51


40


Specifying stream information In the column at the left side choose Streams/1. As a result, a stream input form opens ,

Entering all required information one obtains the screen as shown in Figure 1.52 ,

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FIGURE 1.52


f

The inal area that requires input is the Blocks tab. In the list on the left, double-click on Blocks and then select DEW. Filling up the input form, we have Figure 1.53. too**

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FIGURE 1.53

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INTRODUCTION AND STEPWISK ASPEN PLUS

3!MUU\TION

41


Running the simulation, the following progress report is obtained (see Figure 1.54).

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FIGURE 1.54

Viewing results First click on Solver Settings. From the Data Browser, choose Blocks/DEW/Results (see Figure 1.55) to get the dew point temperature = 22.19453840C.

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42


13 .

.

5 T-xy and P-xy Diagrams of a Binary Mixture

Problem statement

A binary mixture consisting of 60 mole% ethanol and 40 mole% water, is introduced into a lash chamber (Flash2) with a flow rate of 120 kmol/hr at 3 bar and 250C f

,

.

(a) Produce T-xy plot at a constant pressure (1.013 bar) (b) Produce xy plot based on the data obtained in part (a) (c) Produce P-xy plot at a constant temperature (90oC) Use the Wilson activity coefficient model as a property method. Simulation approach

As usual, start Aspen Plus and select Template. Click OK to get the next screen and choose General with Metric Units. Then again hit OK. In the subsequent step, click OK in the Connect to Engine window to obtain a blank Process Flowsheet Window. Creating flowsheet

f

From the equipment Model Library at the bottom of the Aspen Plus process lowsheet window, select the Separators tab and insert the Flash2 separator. Then connect the separation unit with the incoming and outgoing streams. The complete process is shown in Figure 1.56.

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FIGURE 1.56


After clicking on Solver Settings, select Setup /Specifications in the list on the left. The Title of the present problem is given as TXY and PXY Diagrams'. Subsequently, the '

Aspen Plus accounting information are also provided [see Figures 1.57(a) and (b)].

INTKOIHTTION AND STKI'WISK ASl'liN I'l.l'S ' SIMULATION

43

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FIGURE 1.57(b)

Specifying components Hitting Next button and defining the components (ethanol and water) in the input form, one obtains Figure 1.58. Specifying property method

The user input under the Properties tab is probably the most critical input required to run a successful simulation. Clicking Next button we obtain the property input form. For this problem, choose the Wilson model by scrolling down (see Figure 1.59). ,

44

PROCESS SIMULATION AND CONTROL USING i £«*

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FIGURE 1.59

Once the base property method has been selected and we click the Next button, a window pops up asking whether to continue to the next step or to modify the properties (see Figure 1.60).

INTRODUCTION AND STEPWiSK ASPEN PLUS

Required Properties Input Complete

SIMULATION

45

i

Go to the Next requiied step, or supply additional properties information,

Go to Next required input step '

Modify required property specifications E nter property parameters

Enter raw properly data

OK

Cancel

FIGURE 1.60

Specifying stream information f

The next window includes a stream input form. Specifying temperature, pressure, low rate and components mole fraction, one obtains Figure 1.61 as shown.

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(a) Creating T-xy plot:

Selecting ToolslAnalysis I Property I Binary, we have

Figure 1.62.

Copyrighled material

46


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ni lMl -I U

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FIGURE 1.62

We must note that this option can be used to generate T-xy, P-xy or Gibbs energy of mixing diagrams. Select Txy for the present problem. We aim to do an analysis on the mixture of ethanol and water; so select these components accordingly. The user has the option of specifying, which component will be used for the x-axis (which component s mole raction will be diagrammed). The default is whichever component is indicated as component 1. Make sure that we are creating the diagram for the mole raction of ethanol. Entering required information Figure 1.62 takes the following form (see Figure 1.63). '

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f

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FIGURE 1.63

Click on Go and get the T-xy plot at a constant pressure (1 013 .

bar) as shown in

Figure 1.64. Although the Status bar shows Required Input Incomplete

,

problem to get the plot based on the given information.

but there is no

INTRODUCTION AND STEPWISE ASPEN PLUS1 M SIMUL.\TION

5J r3ii-|*i*i
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r-i-i-i>nr

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FIGURE 1.64

f

It should be noted that if we move the T-xy plot slightly or close it, we ind Figure 1.65 having a databank. Some of these values have been used to make the plot (Figure 1.64). n3K|fc!»|-qM!!H 3i -

i

Mil*

i

i

.i .m-

i

m\

I MMI

rwrrc

Wtfm :

:

-

lM«4Ml-

FIGURE 1.65


46

PROCESS SIMULATION AND CONTROL USING ASPEN ru

tut

M*

"

.

r

-

:;

-

I**

REAMS

'

H>«>>

Vrt

Ji-

j

i Mf n: .

j tRLAMb

BiW

i i

'i mo;

i

i"i SM

.

ri«h2

fLwM

C'mjtj-

'if

'

f-

FIGURE 1.62

We must note that this option can be used to generate T-xy, P-xy or Gibbs energy of mixing diagrams. Select Txy' for the present problem. We aim to do an analysis on the mixture of ethanol and water; so select these components accordingly. The user has the option of specifying, which component will be used for the x-axis (which component s '

'

mole fraction will be diagrammed). The default is whichever component is indicated as component 1. Make sure that we are creating the diagram for the mole fraction of ethanol. Entering required information. Figure 1.62 takes the following form (see Figure 1.63).

M il SI M SI

~

2

|W*TER

3

fETHANOL

3 (\oflm
-

o-

FIGURE 1.63

Click on Go and get the T-xy plot at a constant pressure (1 013 .

bar) as shown in

Figure 1.64. Although the Status bar shows Required Input Incomplete problem to get the plot based on the given information.

,

but there is no


SIMULATION

49

Clicking on Go button, we have the following P-xy plot |see Figure 1.68(a)| at a constant temperature (90oC) and respective databank produced (Figure 1.68(b)|.

I

I- I-I.-IkU-

-

L

i

*

:

1

1

1

DM

-

.

. ..

. 11

FIGURE 1.68(a)

3/11 uoitnuc

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UOUO

ETWMOl

mti

K

ni

QAMHt

'' IBi.-l ' I

UOUD

UOUO cum

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i ma

urci

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J

UMH on

nr*'

n na

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THS

raro

nryn

-

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I

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T7ZB5 ms

an

nnss

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nan

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r

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r

.

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B

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www

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awn

r

na!

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i-

)i

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r

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B

171 KB

HO-Q-o-i-iIIkvil

.H«i)

Ohm

Sh>

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IU

MM

FIGURE 1.68(b)

Copyrighled material

50

PROCESS SIMULATION AND CONTROL USING ASPEN r

Notice that the plot window can be edited by ight clicking on that window and

selecting Properties

In the properties window, the user can modify the title axis scale colour of the plot, etc. Alternatively, double-click on the different elements of the plot and modify them as we like to improve the presentation and clarity .

,

,

font

,

.

SUMMARY AND CONCLUSIONS

In this chapter a brief introduction of the Aspen simulator is presented first. It is well recognized that the Aspen software is an extremely powerful simulation tool in which a large number of parameter values are stored in the databank and the calculations are ,

,

,

pre-programmed. At the preliminary stage of this software course, this chapter may help to accustom with several items and stepwise simulation procedures. Here four simple problems (flash calculation, bubble point calculation, dew point calculation and T-xy as well as P-xy plot generation) have been solved showing all simulation steps ,

.

PROBLEMS | 1

.

1 A liquid mixture, consisting of 60 mole% benzene and 40 mole% toluene, is fed f

with a low rate of 100 kmol/hr at 3 bar and 250C to a flash chamber (Flash2) operated at 1.2 atm and 100oC Applying the SYSOP0 method, compute the .

amounts of liquid and vapour products and their compositions. 1 2 A liquid mixture, consisting of 60 mole% benzene, 30 mole% toluene and 10 mole% o-xylene, is lashed at 1 atm and 110oC. The feed mixture with a low rate of 100 kmol/hr enters the lash drum (Flash2) at 1 atm and 80oC Using the SYSOP0 property method, f

f

.

f

.

(a) Compute the amounts of liquid and vapour outlets and their compositions (b) Repeat the calculation at 1.5 atm and 120oC (operating conditions) 1

3 A hydrocarbon mixture with the composition, shown in Table 1.3, is fed to a

.

f

lash drum at 50oF and 20 psia. TABLE 1.3

Component i-C4 n

-C4(LK)

i-C

5(HK)

Ce C7

Flow rate (lb moiyhr) 12 448

36 23 39.1 272.2

c9

31 876.3

f

The lash chamber (Flash2) operates at 180oF and 80 psia. Applying the SYSOP0

thermodynamic model, determine the amounts of liquid and vapour products and their compositions.

INTRODUCTION AND STEPWISK ASPEN PLUS 1

.

SIMULATION

51

4 Find the bubble point and dew point temperatures of a mixture of 0.4 mole fraction toluene and 0.6 mole fraction rso-butanol at 101.3 kPa. Assume ideal mixture

f

and inlet temperature of 50oC, pressure of 1.5 atm, and low rate of 100 kmol/hr. 5 Find the bubble point and dew point temperatures and corresponding vapour and liquid compositions for a mixture of 33 mole% n-hexane, 33 mole% n-heptane and 34 mole% n-octane at 1 atm pressure. The feed mixture with a low rate of 100 kmol/hr enters at 50oC and 1 atm. Consider ideality in both liquid and vapour phases. 1 6 Compute the bubble point and dew point temperatures of a solution of 1

f

.

.

hydrocarbons with the following composition at 345 kN/m2(see Table 1.4). TABLE 1.4

Component

Mole fraction

c3

0 05

n-C4

0 25

n-C5

04

Ce

03

.

.

.

.

f

The ideal solution with a low rate of 100 kmol/hr enters at 50oC and 1 atm. 1

.

7 Calculate the bubble point pressure at 40oC of the following hydrocarbon stream (see Table 1.5). TABLE 1.6

Component

Mole fraction

c,

0 05

c2 Ca

0 1

i-C4

0 1

n-C4

02

.

.

0 15 .

.

.

i-Cs

0 15

n-C5

0 15

.

.

c6

0 1 .

Use the SRK thermodynamic model and consider the inlet temperature of 30oC, pressure of 4.5 bar and low rate of 100 kmol/hr. 8 A binary mixture, consisting of 50 mole% ethanol and 50 mole% 1-propanol, is f

1

.

f

f

fed to a lash drum (Flash2) with a low rate of 120 kmol/hr at 3.5 bar and 30oC.

(a) Produce T-xy plot at a constant pressure (1.013 bar) (b) Produce P-xy plot at a constant temperature (750C) (c) Produce xy plot based on the data obtained in part (b)

.

Consider the RK-Soave thermodynamic model as a base property method. 9 A ternary mixture with the following component-wise low rates is introduced f

1

into a decanter model run at 341.1 K and 308.9 kPa. To identify the second

liquid phase, select n-pentane as a key component (see Table 1.6).

52

PROCESS SIMULATION AND CONTROL

USING ASPEN

TABLE 1.6

Component n

Flow rate (kmol/hr)

-pentane

10

ethanol

3

water

75 .

Applying the NRTL property method simulate the decanter block to compute the flow rates of two product streams 10 A ternary mixture having the following low rates is fed to a separator (Sep2) at ,

.

.

f

1

50oC and 5 bar (see Table 1.7). TABLE 1.7

Flow rate (kmol/hr)

Component n

33.623

-pentane

ethanol

0 476

water

3 705

.

.

To solve the present problem using Aspen Plus, the following specifications are provided along with a T/F ratio of 0.905478 (see Table 1.8 and Figure 1.69). TABLE 1.8

Component n

-pentane

Split fraction in stream T 0 999 .

ethanol

09

water

(calculated by Aspen)

.

B -O

FIGURE 1.69

A flowsheet of a separator.

f

Applying the SRK property method, simulate the lowsheet, shown in Figure 1.69, and determine the product compositions. 1

.

11 Repeat the above problem with replacing the separator Sep2 by Sep and using split fraction of water 0.4 in Stream T.

1

12 A dryer, as specified in Figure 1.70, operates at 200oF and 1 atm. Apply the

.

SOLIDS base property method and simulate the dryer model (Flash2) to compute

the recovery of water in the top product.

INTRODUCTION AND STKPWISE ASPEN PLUS

SIMULATION

53

Wet

Temperature = 75DC Pressure = 1 aim

AiROur;

Flow rates

S(02 = 800 Ib/hr H20 = 5 Ib/hr

Air

WET

0

AIR

Temperature = 200oC Pressure = 1 atm dry;

Flow rates = 50 Ibmol/hr

N2 = 80 mole%

O

DRYER

O, b 20 mole%

A lowsheet of a dryer f

FIGURE 1.70

REFERENCE

AspenTech Official Site, When was the Company Founded?, http://www.aspentech.com/ corporate/careers/faqs.cfm#whenAT.

C H A PT E R

2

Aspen Plus Simulation of Reactor Models

2 1 .

BUILT-IN REACTOR MODELS

In the Aspen Plus

model library, seven built-in reactor models are available. They

are RStoic, RYield, REquil, RGibbs, RCSTR, RPlug and RBatch. The stoichiometric reactor, RStoic, is used when the stoichiometry is known but the reaction kinetics is either unknown or unimportant. The yield reactor, RYield, is employed in those cases where both the reactions-kinetics and stoichiometry-are unknown but the product yields Eire known to us. For single-phase chemical equilibrium or simultaneous phase and chemical equilibrium calculations, we choose either REquil or RGibbs. REquil model solves stoichiometric chemical and phase equilibrium equations. On the other hand,

RGibbs solves its model by minimizing Gibbs free energy, subject to atom balance constraints. RCSTR, RPlug and RBatch are rigorous models of continuous stirred tank reactor (CSTR), plug low reactor (PER) and batch (or semi-batch) reactor respectively. Eor these three reactor models, kinetics is known. RPlug and RBatch handle ratef

,

based kinetic reactions, whereas RCSTR simultaneously handles equilibrium and ratebased reactions. It should be noted that the rigorous models in Aspen Plus can use built-in Power law or Langmuir-Hinshelwood-Hougen-Watson (LHHW) or user defined kinetics. The user can define the reaction kinetics in Fortran subroutine or in excel worksheet.

One of the most important things to remember when using a computer simulation program, in any application, is that incorrect input data or programming can lead to solutions that are correct based on the program's specifications but unrealistic with "

"

,

regard to real-life applications. For this reason, a good knowledge is must on the reaction engineering. In the following, we will simulate several reactor models using the Aspen Plus software package. Apart from these solved examples, interested reader may simulate the reactor models given in the exercise at the end of this chapter. 54

ASPEN PLUS

22 .

SIMULATION OF REACTOR MODELS

55

ASPEN PLUS SIMULATION OF A RStolc MODEL

Problem statement

Styrene is produced by dehydrogenation of ethylbenzene. Here we consider an irreversible reaction given as: CgHs-C2H5 -> CgHs-CH - CH2 + H2 ethylbenzene

styrene

hydrogen f

Pure ethylbenzene enters the RStoic reactor with a low rate of 100 kmol/hr at 260oC and 1.5 bar. The reactor operates at 250oC and 1.2 bar. We can use the fractional conversion of ethylbenzene equals 0.8. Using the Peng-Robinson thermodynamic method, simulate the reactor model.

Simulation approach

As we start Aspen Plus from the Start menu or by double-clicking the Aspen Plus icon on our desktop, first the Aspen Plus Startup dialog appears (see Figure 2.1). Choose Template option and then click OK.

iaj _1_J __J *j rv.Mft, I-Hid 3

I I l-J±]-J _J

_

J

FIGURE 2.1

As the next window pops up (see Figure 2.2), select General with Metric Units and hit OK button.

Copyrighted materia

56

4

PROCESS SIMULATION AND CONTROL USING ASPEN jzj

I M I I I lAl

I

I

I- I

[5'.f»**-«i "v.* (Ma j" *-** /.r JV--- *.j m ,

' j

W

jJ.

r .

mo; Mil E-v v 'Mi

3

'th B«M

.

-

-

. ,

( 'to-.

_

jw*-N«ta»et« SkwtrM

j .j--jc-r;

.

] f.-S- -.r 3 C j

n-V;

j «'

if!: VV.

FIGURE 2.2

Here we use the simulation engine at 'Local PC. Click OK when the Connect to Engine dialog is displayed (see Figure 2.3). Note that this step is specific to the installation .

Connect to Engine Server type:

Local PC

User Info

Node name :

User name: Password:

Working directory:

O Save as Default Connection OK

Exit

Help

FIGURE 2.3

Creating flowsheet

f

We are now ready to develop the process low diagram. Select the Reactors tab from f

the Model Library toolbar, then choose RStoic icon and inally place this unit in the

f

blank Process Flowsheet Window. In order to connect the feed and efluent streams

MODELS

with the reactor block, click on Material STREAMS tab in th As we move the cursor, now a crosshair, onto the proces lnw

1

57

fui ,

s

COriier

f

two red arrows and one blue arrow. Remember that red aarr0WSfare 're(luired rrow

blue arrows are optional ports.

Click once on the starting point, expand the feed li

-

ts and

ne and click a~Hn tv f a stream is labelled as 1. Addmg the outlet stream to the reactort tJXwa WW

we make the image as shown in Figure 2.4.

I .lal

I

-

,

n

y'

UIiaiiy

Ml

03-

=

-

Q

a

In

i . i . S -O-M-i -

o

a

Ri

astt.

tb

pfvjj

FIGURE 2.4

f

After renaming Stream 1 to F, Stream 2 to P and Block Bl to REACTOR, the lowsheet looks like Figure 2.5. » l«IVl -

. r'

C«J

'

Kf!

Pin

tr-Kl«-

f

c*

DltflBI «BI

LI'-TV

iWoc,-.

i

Id iff! GN-|e>IM
I IH

-i

-

at

rsms

Eh-

acs'R

FIGURE 2.5

Obviously, the Sia s md/cator in the bottom right of the mam window h

changed

the message rom Flowsheet Not Complete to Required Input /ncom ff . fsimulation. ation lete the f

to enter th* remaining data using several input forms required to comp

58



Hitting Next icon and clicking OK on the message sheet displayed we get the setup input form. First the title of the present problem is given as 'Simulation of the RStoic Reactor' In the next, the Aspen Plus accounting information (required at some installations) ,

are provided.

User name: AKJANA Account number: 5

Project ID: ANYTHING Project name: YOUR CHOICE

Finally, select Report Options under Setup folder choose 'Mole' as well as 'Mass' fraction item under Stream tab (see Figure 2.6(a) (b) and (c)). ,

,

MM ±S i «

i r- i - i- i jv

_

i

-

_

iai

UMsi

[jjttiEjjft L-

J

.

- .1

lU -

I- S . S . § -Q-M-OB.BM

Bi

u.

.

'-.C---

KC TIi

PFtjj

Rfem.

FIGURE 2.6(a)

Jl-T -

i I- fV

I -M

I

lal fifj

FIGURE 2.6(b)

ASPEN PLUS

SIMULATION OF REACTOR MODEI S

59

Mil

: r-i-hi r»

,

-

! .|gi

.

Dm

dm

r _

utM

i

ip' h-i

it I

-

i i I M>l Umomm I

O

-

tifc

f

'

waw

«

FIGURE 2.6(c)


f

In the Data Browser window, choose Components /Specifications to obtain the component input form. Now ill out the table for three components, ethylbenzene, styrene and hydrogen (see Figure 2.7). If Aspen Plus does not recognize the components by their IDs as defined by the user, use the Find button to search them. Select the components from the lists and then Add them. A detailed procedure is presented in Chapter 1.

I?!

1

-1-

i "" TH III

sr-l© 8 18 0IIU FIGURE 2.7

fd materic

60



Choosing Properties /Specifications in the column at the left side one obtains the ,

property input form. Use the Peng-Robinson thermodynamic package by selecting PENG

-

ROB under the Base method tab (see Figure 2 8). .

ol lBj

_

J

J

w]

_

KW«>|
m -1 H JpJjJ J

3 r

"

3 3 3 3

""

"

1 3

f

J * ew Proc*li«t

"

U **-

fn'«M

BE

RSbte

RCSTR

iPH)

f

RSldc

STREAMS

BB*

_

'

-

3M| #

FIGURE 2.8


f

f

f

The Streams IFIInput I Specifications sheet appears with the Data Browser menu tree in the left pane. Entering the values for state variables (temperature, pressure and total low) and composition (mole raction), we inally have the following screen (see Figure 2.9). DZSMSSEGSSSD

I r

i*

f

:

Hot

Utorr

Wrdo*

H-fc

I -1 "I T»

'

I -Ml

I . 131

A|>Mdiedtio
i] t

o'*** "

Ware 2mu«

3

~

3 "

p3"

(i a ******

1

3

3

'wr-S-

0 D

ur

J -

'

su -1 - Bli. - BO i q m eS f"" SSiS '

r

ttBUH,

1

.

FIGURE 2.9

,.JCT»J

.

:

ASPEN PLUS

Specifying block


61

information

From the Data Browser, select Blocks/REACTOR. Specifying operating conditions for the reactor model, the form looks like Figure 2.10.

Efb

»|-.| ..IB

q .>| ol,,!

|

3

-

PCStB

To

C

F tc.

l Qactg

Mvg-

.

Mom. I » Vsm"

FIGURE 2.10

Specifying reaction information In the next, either hit Next button or Reactions tab under Blocks /REACTOR Chck iVeiy, .

to choose the reactants and products using the dropdown list input the stoichiometric ,

coefBcients and specify the fractional conversion In the Aspen Plus simulator, coefficients .

should be negative for reactants and positive for products (see Figure 2 11). .

**

b*

>

bo

"e*

'-'

J

RiACTQR

Wt
BCSTR

BtVn

FIGURE 2.11

62


Running the simulation In Figure 2.12 Status message includes Required Input Complete. It implies that all required input information have been inserted by the user. There are a few ways to ,

run the simulation. We could select either the Next button in the toolbar which will tell us that all of the required inputs are complete and ask if we would like to run the simulation. We can also run the simulation by selecting the Run button in the toolbar

(this is the button with a block arrow pointing to the right). Alternatively, we can go to Run on the menu bar and select 'Run' (F5). MM.|8W«'!i ,l|Hllir

DMll I /Sp«£tfeahont /Re-

A«s8V.'Bend

M

ill

Elfb Imeicbah

""

,

1 Contujlion | HMHiResclion | Setacli«ly | PSO | EowmrtAm |

Rcadicxs

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Specilicaiun type Stochiotneby

IttrCanpi

ETHYL-01

I

> STYREHE . KrtiflOGEN

UNIFAC Group*

-

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'

*

1 1

Cperty Methods tstrfi ficn

Jj > p

Moiecua- Sbuctm ParameJers

':

D S

At tequfed npd u ocmpHe Y j can rui the MnuMlon nitw. w iiu can erttr more input To er4er more f-pj. Bated Cared th«n

t

S

seled t e ooUont yoj mM tnyn Ihe Dais poldOAT-, menu

a

Rui ir-e sirxilatiwi now?

Advanced &reanS

_

-

Jfl : (1 EOVsraH« CJ P Bocks

3

P Rwchom occu r ien«

RECTOR

.

Inpu C«nplete

[H " -

Mnwii/SpWer;

STREAMS F

-

Hea
i9Pt;

J.,,

Rucloi

Chsnga, | MM>t ( 5c«> j U»Mo*b |

CH '

RStdc

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r H«o press F1

,

'

,

-

Stall *

Boot .

_

Aww.RaocDdr | « Awr.Mcd

I

FIGURE 2.12

Viewing results

As we click OK on the above message the Control Panel appears showing the progress of the simulation. After the simulation is run and converged we notice that the Results Summary tab on the Data Browser window has a blue checkmark Clicking on that tab ,

,

.

will open up the Run Status. If the simulation has converged it should state ,

"

Calculations were completed normally" (see Figure 2 13). Pressing Next button and then OK, we get the Run Status screen In the subsequent .

.

step, select Results Summary /Streams in the list on the left and obtain the final results (see Figure 2.14). Save the work done by choosing File/Save As/...in the menu list on the top.

If we click on Stream Table knob just above the results table, the results are recorded

in the Process Flowsheet Window, as shown in Figure 2.15

ASPEN PLUS

>k

'

r«

An

[Mi

Tot

\r

Um

<.>«.

MO

tnut iulmtiu imi." ui

um

-

CH

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-

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FIGURE 2.13

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-

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FIGURE 2.14

63


Ffc

'

-

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Vfew

0«a

lup

Into

I

,

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tjn

f

64

nowheri

Lfc fy

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|

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gIBl«l|Oi.

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S 0 U 31U

,

1 BSinc

BEoii

HGMis

RCSTB

BPItg

RBaM. \ s FoWen JJswn Ru» H 1

HUM lfloAi Artahie f

STREAMS

FIGURE 2.15


f

For input information, press Ctrl + Alt + I on the keyboard or select Input Summary rom the View pulldown menu (see Figure 2.16). CBSES Fie

£*

Forw*

>Atw

input Sugary created by Aspen Plus K«1. 11.1 at 12:U:CM Thu jul 5, 300?

Oirecrory C: Proqr-5R Pi les'AspenTech .norfcing Pol ders'.Aspen Plus 11.1

'

e Ci

Users-.akjana.AppMtaMocal Terep -ape906.tK}

.

'SlmUllon of the iStolc Reactor" f

title

Fllep

"

IN-UNITS KET VOLU> E-FLOS<- cuB hr HCAT-TRAHS-C-

ENTM*LPV-Fl.O-'*»lkcal/hr' A kcal/hr-sqn-K" PRESSURE"bar TEMPERATURE-C

'

'

f

VOLUHE-CUIT OELTA-T-C HEAD-neter httLE-DENSin'- cisol/cuni' fASS-DENSITVt-ASS-EWTM

.P-

'

kg.'CUH" W)LE-£NTHALP- kcal,'noV

'

& &

t,

kcal/kg' HE*T-MMkcal t'OLE-CONC-'mol

T

' .

&

POBOP-bar OCF-STREAt'S COMVEN AIL

DESCRIPTIOH " General SlHllailoi) mith Metric units :

C, bar, kg/hr, knclhr. MMKcal/hr, c\m/hr. property Method: Mone

Flow basis for Input: Kole Stream report cooposltlon: Kole flow

ROP-SOURCES PUBEll

- AQUEOUS

/ SOLIDS

f INORGANIC

COMPONENTS

ETHYL-01 C8H10-4

/

STVRENE C8H8 ,' HVOfiOGEN H2

PBOPERTIFS PENG-ROB 5THCAH

F

SUBSTBEAf KIXEO TCHP-J60. W>LE-FMC ETHYL-01 1.

'

B i

PRE5-1.S MOLE-FLOW-100.

I vjnwi-*

|- la»«Jtol |

FIGURE 2.16

lto.»,-s ||

-WEME1

:« jpCittU

y

-

f

If one may wish to generate a report ile (* rep)

or the nrp

f

instructions as presented in Chapter 1

f

wkjusu,jO

t f

65 DO

u,

P eSent Problem, follow the

.

2

.

3 ASPEN PLUS SIMULATION OF A RCSTR MODEL

Problem statement

The hydrogenation of aniline produces cyclohexylamine in following reaction:

a CSTR accord f ' accor(lirig to the

C6H5NH2 + 3H2

CeHnNHa aniline hydrogen cyclohexylamine

The reactor operates at 40 bar and 120oC

and its volume is 1200 t3 (75% liquid) For f

,

the liquid-phase reaction, the inlet streams have the specifications shown in Table 2 1 ,

.

.

TABLE 2.1 Reactant

Temperature (0C)

Pressure (bar)

43

41

45

230

41

160

Pure aniline

Pure hydrogen

Flow rate (kmol/hr)

Fake reaction kinetics data for the Arrhenius law are given as:

Pre-exponential factor = 5 x 105 m3/kmol s Activation energy = 20,000 Btu/lbmol

[CJ basis = Molarity

f

Use the SYSOP0 base property method in the simulation. The reaction is irst-order in

aniline and hydrogen. The reaction rate constant is defined with respect to aniline. Simulate the CSTR model and compute the component mole fractions in both the liquid as well as vapour product. Simulation approach

Start with the General with Metric Units Template, as shown in Figures 2.17(a) and (b). Click OK in the above screen. When the Connect to Engine dialog appears, again hit OK knob to obtain a blank Process Flowsheet Window.

Select the Reactors tab from the Model Litwy RCSTR

models available. Among them, choose

Adding inlet and product streams and renaming look like Figure 2.18.

t

P cetheit process in tn low magr them, f

Creating flowsheet

"


Q|a|B|

JJ J_J

_

nMfel I 1 :1 si 21 __1_L.J ni M M ®l

A1 ] c 8lor+. SmuWen

r OMUsnE.ulr.lSim.jl.j'i-

"

Aap«n Plus

FIGURE 2.17(a)

g *apen IP= Strean Prx&hts mnz Lines.

I Beetle, «|fa Enshh ijrit |aklnt«ill wth Medic IMi

nitpwi wi mmi

MMtajJ-V arvtr ,

Propetty I lhod; None t

Bow oss crinpiif 'tee

fAs

Unfa

Strtom eaai r

Procws g

f

cwrpcttEfi: Mote low

SpNtft/Chmic*

'

SUrti

FIGURE 2.17(b)

«

Vf

# i VSJ6

ASPEN PLUS1" SIMUIATION OF REACTOR MODELS h

W

..>

3a

Hi*

o|rf|y|giai

_

.<-»-»

67

MMa

lg|g rj|twi
I}-

-

u

tmuHt 1 Igj

gMij

gM

Wii*.

.Hi*

->

FIGURE 2.18


Hit Afe as

button and then OK and get the setup input form. The present project is titled

'

Simulation of the RCSTR Reactor' and the accounting information are given as AKJANA/6/ANYTHING/YOUR CHOICE* (see Figures 2.19(a) and (b)).

'

*! El &iMiid

3

I i"l

3 *I

-

FIGURE 2.19(a)

II

-». w

.

t -.-.

f

Jim _iJ

68

PROCESS SIMULATION AND CONTROL USING ASPEN1 M '

Fie

E«

On

TmH

PU

Lfrvy

Wilder-

*k>

0 Spiicfcii

l li«< MBW .

jfl IM-SHi

.

Rovci ID

CuHsflUnli f

O

kfUCdRfMi

. '

STREAMS

RSioc

RYwId

11 -y-Bo REgnl

RGMw

RCSTfl

RWjg

REafch

O

* $3 17 1'.

FIGURE 2.19(b)

f

In the subsequent step, choose Setup/Report Options / Stream rom the Data Browser Mole' as well as 'Mass' fraction basis (see Figure 2.20). B*

E*

i

Mxr

'

CM*

Todi

tr

f

window and select

PW

Uorv

AWow

h«b

ajJJ iBJ J al-rlfeKKI I n>i ij J |h| a|

1 M

Cereal | Ftowiho* | Bbcf Ali j Roperty j AW | 0

SkW* Qnl t

turn U be ndmM r, iiMm itpoii

Jfl Ml S«t»

.

!

P MtJa

P Mcta

TFF [gENJ

r Uau

P Mm

|S Standard fa0cdm>i

t

P Componerti t h (wo o-. 01 H itDon

.:abh

t

-

M- Sc*-.. | S».*n | HME

StfltW

(Bill BV

fvuc

f

f "

P S.>-

T]

RE.M-

RGte.

RCS1R

RPI

m j,

1 " -

FIGURE 2 20 .

ASPEN PLUS

SIMULATION OP REACTOR MODELS

-f

69

Specifying components The example reaction system includes three components. They are aniline, hydrogen and cyclohexylamine. Defining all these species in the component input form, one obtains Figure 2.21. eu To* FU.

f

V nt Eik 4n

Pla

Uh

~

3 Mdiilfs-3ij bj rl

AMIUNE

C6H7I11

WyMOGEN

K1T1R0G H CYCLO H EWLAMICSH13W -01

Ffesctons

'

Eire V/cw)

UtwCMnd

Rtttdei:

""'""

D

in

I I Sotd. | U«>M«Mt t

Ml RGfcb)

lCStft

RFtifl

f

tEqai

f

Brttj r

RSac

Rflaieh

FIGURE 2.21


We know that a property method is a bank of methods and models used to compute

physical properties. For the sample reactor model, select SYSOP0 base property method (see Figure 2.22) after clicking on Next icon in the above screen. eu

f

Fk

VW*

D«»

liA

Fj,

li

-f

V,Wfe/.

hefc

i I

3

urvac

_

j F rm

I*

si*

| .>j3l*J<>TtQ('«W -i.d"°°«*''",l''fi'

FIGURE 2.22

Aipcn rim - Sani

70



As we hit Next followed by OK, a stream input form appears. For Stream A (pure aniline) and Stream H (pure hydrogen), values of state variables and composition are

inserted in the following two forms, shown in Figures 2.23(a) and (b). m mr.i .

ffe

*

'-Am

mm

D«i

T«ol«

An

Fix

Uc**y

Wnfe*

'k

>. Ittieiwj nH-clalsKM!sJ 31

! HiJ21«) »)

J -

3

i«*f

j PiAiW

_

Strunu

fj EOVar-ittai

BGMw

SIBEAMS

BCSTH

FIGURE 2.23(a)

'

::

St Edi Mw* 0«« To* An a* ifc,. whd*. Htfc

10 3

Owerti

i Jy MIXED

~

3

3 :iu*f..

-

3 -

O

UMFACQtsun

3

« Zj EMMbn

ra;

ToW IT

BCSIB

«fl,

m»

FIGURE 2.23(b)


In the next, there is a block input form. Providing required information for the CSTR block, we have the screen as shown in Figure 2.24.

ASPEN PI-US lim

-Vii.l.-l!,!

E«

.

9

Sm

-

'mm

d

.

Lbw>

« «

II

MrtM

I

M

SIMULATION OF REACTOR MODEIiJ .

i -I

'

f

..

|.if.|.iu«-.| ne

I'

'-

B

---

4a

71

I..

i

-I

p=-31 -r-3 '

i -

ff .

1,--.

-

J

1

J

j .

~

F 3

I

.

-

s

Siiir

__-__

r |®- 9 . S . 9 Q U O

ITXUK

Mm

fJte

>-. -'«

»-

FIGURE 2.24

Product streams have been defined with their phases (see Figure 2.25). Ifflll

I

r-M-|r |T

I .ICI

'

I

Ml

71

I. li: .

I

-

0

uj

llji lli*! i j I o-

-

I1XJM

t XMUtavn I UMa III Hi | tm*mammu | »

mi I Ma I iMHwk j

m 0 . 8 . o y JE D IMMI

Ptmt

hm.

unit

TVl

ggg

0»>W

a<

t

»

-p

-

<

FIGURE 2.25

Press Afexf button or click on Reactions and get the window (as shown in Figure 2.26).


PROCESS SIMULATION AND CONTROL USING ASPEN" ««»

DKB

UtatCSTRflCCTn i

Tooit

tr

Pta

e|ej

_

Hdp

i 1 i HT

-

JJ ,

IJfewy Wirvis

rgklaKKM

f

Ed*

f

I

«b

f

72

"I

! I"l J JJ J ®l

I leal; I M Hi

Setup

Solsd .e

ion Mlt lo be nciideii n ihs

Arabia i««clw wU rWft

i SriBctedttwc'cn'-

af Studio

MvanMd

a h .

11

L

Bocta C5TR

-

e s-up

(J EOVsnai>« O EOhpu O Sp«c GfWJpt

-

Pott

leschon E3 ID

Peaili -

ii

[1 " -

MiMMiyS{«ter9 | S«p«aU»i ] He*E hangefi j Coluwij Heoclou j Preiwe Changeii ] MfloipUaloft | Sf** | UiwH&Wi |

a->

Mated \

RGtb!

iCSTR

f

REeril

STREAMS

RPbg

f

C \ l FtAJerj'Aweo Piu) 11 i

f

i Ofttce Woni j

f-toggft Pcwergjrt . l . j

MjCe toX>< frofett f [

'

HUM

~

FIGURE 2.26

Right click on Available reaction sets, hit New button, then either accept default name R-l or give a name as we want for the reaction set and inally click on OK Subsequently, select POWERLAW in the Enter Type list and hit OK to get the screen

f

.

as shown in Figure 2.27. Ffc

&»

*w

tWa

Toe*

fU

Fte

Ubnr,

Vttyfcw

Hdp

MHl

r .-l-i- PT

-.1 M- I

Ml jW

/Spccft hm j/S««atm/Ba ljont] PSD j CwvwMAm [

.

£j Pwwt/Henccs

S dect (sacbcn tw to t« ndudsd r

nwdel

LJ "

1

J _

:

-

j Data

V) tco-S*: f

i

Mdecwer Su-me

_

+

'

Jfl H

i o l -

csrn

O

'

9 Sp«Gto*i

H SbeenRMub ,

<0 s s s o

O LJ.

FIGURE 2.27

ASPEN PLUS


73

Specifying reaction information Hitting Next knob, we obtain the screen, shown in Figure 2.28. ul.i cm

**

<%*

S

fw

»

-.>=-

**

.

| SiMMt | HewE h-vjpr | a m- Rm om { Praiiu,Charge.! | H««a«» |

*

I UmiH
i - s - § .©.mof.W

RtajJ

RGttK

lCSTR

RPl

f

RStoc

f

STfttAKS

RBWtf,

FIGURE 2.28

As we click on New; button a form is displayed as shown in Figure 2.29. In this form, we need to enter the stoichiometric coefficient as well as exponent for all components. The exponents represent the order of the reaction with respect to each ,

component. Note that there are two types of reactions [kinetic (rate-controlled reactions) and equilibrium] permitted under Power law reaction ID .

Dli*lBj J J eiej *l nrMfcl-NM '»! f~

1

_

f

_

I

l-.l. li IT

Caw**

! -lEI

1 CMtft** 1 f.t

*

Cow«rt

I H -I l?l I

Co o*

1 ®|

|gl

[ r.i.-' |

*

_

*

M.

|

|

j

-

J

iifitw4i

hB- 0 . i . 0

f

t j A J» "

f.4M

.»

Bf»J

1

1 1 Km

wifl

»*r**:««w«i-Lij<* * -*,»1 1 i '

FIGURE 2.29

. ||

5-i

- «ft ' 11"

74

PROCESS SIMULATION AND CONTROL USING ASPENT As stated, the reaction

C6H5NH2 + 3H2

C6HnNH2

f

is irst-order in aniline and hydrogen. Also, the reaction rate constant is defined with respect to aniline. Accordingly, we may use the following information to specify the reaction (see Table 2.2). TABLE 2.2

Component

Coefficient

Exponent

1

1

aniline

-

hydrogen cyclohexylamine

3

1

1

0

-

f

Recall that in Aspen Plus terminology, coefficients must be negative for reactants and positive for products. As we ill up the form, it looks like Figure 2.30. lltiliiii ESS

' i

aj} f*, Fe>

iw tup Tcotr

.

"

BoacMrNo.: |7i RuctMi

3

Product!

-

Coeficient

-

IYDR0GEN

f

Comnonent ANILINE

3

Reaction type:

Enponent

.

CompafieW

1

j ;

1 3

Coelficient

CYCLO-01

Ej
j *

*

1 i

Ctote

Bock,

-

y Reactiom r J Chemolry B

Peacuons

Edt

Delete

R-I

f

t

1

Convefgcnce

fj Rowaheetng Onions

1

.r

Reojrad tipul hcowMe

IT

Mam pKen | Sepaators | HeatEndiangen | Cokams Haachm | PtenueOiaven ]

SoUt

UnModeb

1.0 .y-U-U-

KWariel RSIoic

STREAMS

RYieU

HMj

RStb.

RCStfl

BPIm

Rieldi "

ForHefc weMfl .

«

!C\i,fi*ta.vW«iHi.111 ,

HUH

-

ReuMtnO

« b3

FIGURE 2.30

If we do not specify the exponent for a species, Aspen Plus takes a default value of zero. In Figure 2.31, the resulting relation is displayed in the stoichiometry sheet. In the subsequent step (see Figure 2.32), we move on to Kinetic tab.

ASPEN PLUS


PHPI Liasigl -

.

-

.

3aft l"-"

if?:.

JSldilJP

BiiJfllalfil

j am

I» -

j

0-.

1 <

)

- .IW I

I

I Ihmt

KiMWiingwr r ~ »

!

FIGURE 2.31

Irl |x|

IM .Q C3WA\*\
"I I"! -I vl -I 9|

*

r

.

"

3alt:

»l*l <
»| Gh-t ml

1.-.,.

-

'

jfl -

9 9 .

I

3

f

t into mn*&*B**n*

t

.

ta

3 .

.

jfl . -

*

P '

*

jfl

.m*

Vm

m t>

mam

>ew* r

FIGURE 2.32

.

.

KIT

75

PROCESS SIMULATION AND CONTROL USING SPEN]

76

'

As directed in the problem statement, we use

'Molarity

basis. Accordingly, the

Power law is expressed as: E n

n

r= k

[T0;

exp

1

(2.1)

R

where r is the rate of reaction, K the reaction rate constant (kinetic factor in Aspen Plus terminology), k the pre-exponential or frequency factor, T the temperature m degree K Tn the datum temperature in degree K, n the temperature exponent S the activation energy R the universal gas constant, C the molarity in kmol/m a the concentration exponent, i the component index, and 0 the product operator. If To is ignored, the Power law expression has the following form: ,

r= kTnexp

E

n(G)

(2.2)

RT

where,

K = kTn exp

E

(2.3)

RT

In most of our simple cases, the reaction rate constant is represented by the Arrhenius law, that is

K - k exp

E N (2.4)

RT)

Notice that when the Arrhenius formula is used

we put zero for n and nothing for T0 in the Aspen Plus window. Also, the units of the pre-exponential factor are identical to those of the rate constant and vary depending on the order of the reaction As we ,

.

know, the dimensions of the rate constant for an nth order reaction are:

(time)-1 (concentration)1-'1 Next come back to the problem The kinetic data are required to provide in the above sheet. Here we use the Arrhenius law to represent the reaction rate constant. It is .

important to mention that the pre-exponential factor must be specified in SI unit. For

the example CSTR problem

,

the pre-exponential factor and activation energy are given

as 5 x 105 m3/kmol s and 20 000 Btu/lbmol respectively (see Figure 2.33). ,


In the window shown in Figure 2 33, the Status bar clearly indicates that all required .

mputs are now complete Hitting Next Control Panel (see Figure 2 34). .

.

knob and clicking on OK

,

we have the foUowing

ASPEN PLUS


77

mam ._

..

-

-J »«. Pte Un* OMn m,

QMIHI -I .1

gJ al-i-|«>l*l
I r l-'l-'l-JV

|

M

l .lalr : I: Ml .

"

3 ANIUNE . 3 HYDROGEN -i CYaO-Cl

f

s

LMMMto twite

csm

3

ll

-

;

US

a-i *

Kdlarai." stream

farHsfe.pnMn

.. | g a»»ita

11

.aroe. || S

fSTI- « 45.}

,

t

afc. .| gdifcT,«fi C

| igiM

«.s

FIGURE 2.33

:

Ffc

Dm Taofai Run Lfesry Wirdiw KHp

DMB| al M -H x?! nklaKI I I »>| IS -I H g|-|3| @|; 1 1- i,JV -Hal JJ iLWjilSla) "

,

-

5 @ CSTPJ

oxputatich carsB

Bi«ck.- csra

fV

.

rsi

uc tai.

rcstr

j Sep«a(«> i HwlE-changer. [ Cokm* Hb«1o« | Pte eChsr rt | M npuWw: | StJd; 1 UiwModel: |

MitoJ

RStM

fffxM

REquJ

RCiibOi

BC5TR

B Jg

tSalch

f

SIflEAMS fo K o

..

..

FIGURE 2.34

Viewing results ,

select Solver Settings, choose igsuto Summary/Sf ms in the list on the f

In the next

left and finally get the results shown in Figure 2.35 in a tabulated form.

78

PROCESS SIMULATION AND CONTROL USING ASPEN1 B» Ebl V«- D*. TMi. Hun fW ijt

I f

J4J«J

MiiM m

I I i PT

! .leal I

i

"

I

I - Ml tM

i

i

I

I "

3 '-"

l

il

il

i

-

nil 1

am

0541

1000

0«5

MUM

0 001

0J30

nooo

tso 601

-

mmmi

1

sm

'

ITTre

0«J

MPPM

DOM

0 98)

| HuiE«*w> ! C<*jwi l-ctet. | FYB.M.Change..

»

i | UisrWodeU |

f

nr

i -1 .QMi-O' RVWd

t

o*

SEgJ

HQtei

j

RCSIR

RPfaa

RftWi

3 ecofQB.c .jjJ Hereto P yP j Jatwlpd

|

t

R&tac

.Ei wprf [{ AwenPkw-S

-

«

1*35

FIGURE 2.35

Save the simulation work in a folder giving a suitable file name. 2

.

4

ASPEN PLUS SIMULATION OF A RPlug MODEL

Problem statement

f

The combination of two benzene molecules forms one molecule of diphenyl and one of hydrogen (Fogler, 2005). The elementary reversible vapour-phase reaction occurs in a plug low reactor (PER). 2CqHq <-> C12H40 + H2 benzene diphenyl hydrogen

The forward and reverse reaction rate constants are defined with respect to benzene. f

The vaporized benzene (pure) with a low rate of 0.02 Ibmol/hr enters the reactor at

1250oF and 15 Psi. The data for the Arrhenius law are given below

.

Forward reaction: A; = 3.2 x lO-6 kmol/s . m3 . (N/m2)2 E = 30200 cal/mol

Reverse reaction: k = 1.0x lO-5 kmol/s . m3 . (N/m2)2 E = 30200 cal/mol

[C,] basis = Partial pressure

The reactor length is 36 in and diameter is 0.6 in. It operates at inlet temperature.

Applying the SYSOP0 thermodynamic model, (a) compute the component mole fraction in the product stream, and (b) produce a plot ofreactor molar composition (mole raction) vs i-eactor length' (in). f

'

ASPEN PLUS

SIMULATION OF REACTOR MODEI S

79

Simulation approach Select Aspen Plus User Interface. When the Aspen Plus window pops up, choose Template and click on OK (see Figure 2.36).

i

-

...

.

...

-

iwmmmlt

mm

FIGURE 2.36

In the next step (see Figure 2.37), select General with English Units and hit OK button.

1 V-

I -

-

-

FIGURE 2.37

Click O/C when the Aspen Plus engine window appears.


80


TM

Creating flowsheet

In the Model Library, select the Reactors tab. Expanding the RPlug icon, the following screen is obtained (see Figure 2.38).

-

li,-1?:- ?-- IM

Uj

jS's - s - § o '

RStoc

lY»fc)

f

SIftEAMS

_

W»J

RCte

RCSTR

RBtfd<

FIGURE 2.38

Inserting the left bottom symbol in the Process Flowsheet Window adding the feed and ,

product streams, and renaming the block as well as streams, finally we see Figure 2.39. &M ro* tj>

Uonn WnSo* H*

f

Be £* *>

r|ttF..U|-. -nr

Nsi|--..| -MBi

>|[T>
I*

h~o

-****** | f«M». t hmI- mw | c*-« iu««« I rM..1,o,

iS- SSI Gj tc

nstfa.

HCSTB

lfy

f

FTiMd

f

' BS*»

,

i _

q.

-

S'W

IN

,

Tftj T

FIGURE 2 39 .

1

'

ASPEN PLUS


81

Configuring settings f

At this moment, we are sure that the process low diagram is drawn correctly message

.

The Status

directs us to provide the input information. Hitting Next knob and clicking on

OK, we obtain a form for setup specifications. First we input the Title of the present

nroject (Simulation of the RPlug Model), followed by the accounting information

(AKJANA/7/ANYTHING/AS YOU WANT) and Report Options [see Figures 2.40(a) to (c)]

I >>i liai g f

3Sif*r-~3 *m si ISrolWoneilheHPVjgMocW

Vdd|*MMC

o-> '

HSteic

tVM

i s u -= u

myt

RG|te. t

-

SIKAMS

|

RC?tR

Rptq

RBtuh

FIGURE 2.40(A) UaTSil

>

nt

Mm OKk TMIp An W L±>»v WWo* H*p

arsi aiobdj-/Deicnmn >/Acciwnlina| 0>agr>o«(«ci {

MMi- pdim I Smmnc I HulE«chv4«i | Cot-mi. lo«'«" | Pimm«C»W f

[T

hB- 1 -1 - 8 Q l5ia

f

SIRLWi

__

R.'*

RfrMl

RGfaU;

W Iff

O Hf''-.

FIGURE 2.40(b)

gMdiM-AiMf But " I

"

.

'

U

.

82


dmbl

Melm mbhjsM«!] 21 g

r m«i »

K C

r SM

twwrH »4i , »« flow W 'IK

»O-S-0 y FIGURE 2.40(c)


From the Data Browser, select Specifications under the Components folder. As we provide the chemical formula of the components in the Component ID column, the other columns of the table are automatically filled up (see Figure 2.41). f

Fit E* Htw D«i Tat* tji RsT Ihwf fntotr Hife f

<

IMM

FvmU,

cia
_

j «r
SET r*

"

rg '. Bin

k«W iooxi Id*

-J

d '

rvJ.cnttm

«*h

OIR

T

Mwl

FIGURE 2.41


In the list on the left, choose Properties /Specifications to obtain the property input form. Then choose SYSOPO by scrolling down (see Figure 2.42).

ASPEN PLUS


83

tmum

~

3

1 I

is

I I I

.

»

3

"~

d 3

r.

r

u

FTV dd

SEtMl

RCSTS

RPljg

lgateh

f

tStac

f

ETREAfce

FIGURE 2.42

Specifying stream information In the left pane of the Data Browser window select Streams IF and enter the values ,

for all state variables and composition as shown in Figure 2 43. .

i r

_

_

.

IF

~

3

{y MIXED State vsmUm "

|12SJ

|f

3 3

1-5

|p.

d

Toid lwr

(m.,-

|0 02

jbmot/N

3 3

"

"

3r

3

H2

f

UH**C i3rtu»

Miinii

Tdat IT

"

Hoi 'jmvUf t .

i-1 . § . § u-i j

'

i*

Rfrfi

be j

note

ncsm

npijg

m»a

FIGURE 2.43

84



In the next, select PFR by opening the Blocks folder The reactor is specified in the .

window, shown in Figure 2.44 tl f

Wl

f

i

P*l

.

Lb try Wn

Jill r-.i:i

-

J Ntw QuwclWMldn '

* -

J/j

IMMM '

i nr

3

Prcf n«

-

J

> -a f aock-

_

_

J Readx J C -wssxe

I Sold* { UnrMixM. j

BE**

ftGMn

FIGURE 2.44

Open the Configuration sheet and enter the reactor dimensions in the next form (see Figure 2.45). F«* Edt tm 043 Tuafc flun fVK tbwy Wndm H*

Ma

t

DlcglBl

_

j

.

1 M iteial *l uW\&\**\<\vi n>| Hi

! |Mi H i?i :H

/- itarj

36

DwtmUt

j

_

.

-J

PAttwn

?»cp(rtif Veered

a

:

| Sohk | Um> Modab |

a §is q-u '

RStot

Rrail

lE
f

S

06

Wite'

BCS'R

Bp

HUM

FIGURE 2.45

ASPEN PLUS


85

T the subsequent step, we define a reaction set for the simulation. The default name R-l

has been accepted. Then select Power law kinetics and obtain the picture, shown in

Figure 2.46. M

I

'

**

"rw*

r

22

H®-1 j . I y ' HStac

fiW

RCqU

RGtti

RCSTR

iB

a BB*J>

f

SIR£>M$

D*«

f

'

i)

*"

FIGURE 2.46

Specifying reaction information

Hitting Afof button and clicking on New we have the following forms (see Figures 2.47(a) and (b)) for reaction number 1 (2C6H6 -> C12H10 + H2). Since the reaction rate constants are defined with respect to benzene we convert the stoichiometric coefficient of benzene r

,

,

to unity for both the reactions Obviously the reactions are second-order. .

,

Jala_jj iiei wj nHM'.teM ».| m .| |h| .| pi 1

r..l..|,.l it

1 .ibi-

.

®|

1 / ial

Rmmm

I1-I'

A

v

R 1

ii

-

o

-

0 S 0

1J -

FIGURE 2.47(a)

a .a

#

1

86

PROCESS SIMULATION AND CONTROL USING ASPEN HI »l

D|tf|y|

I I

<«l aHM-KM

~

l

! |h| -i ~ij j

05

*

J

j i .

_2=J

:

3-1-0 o =u -

RE

R6ife

RnSTR

f ue

HB** '

j y-

Bi-KOT- W j K Mmrst* Moiod j r

jj

Aver. Plu< - 5M

"

ij.} 30» '

FIGURE 2.47(b)

As mentioned previously, when we do not specify the exponent for a component Aspen Plus uses a default value of zero. As the message on the screen, shown in Figure 2.47(b) reveals, it is true that the forward reaction rate does not depend on the ,

QHe

3| |B| JJ Mgl jgl nklaNUI I n-i 3 _

l

-

.

ff alaltfrfi

C H + H2) and click OK (see Figure 2 48). .

LliiJ

El

_

.1 ilBl: I

-

.

f

-

.

f

f

product components. After completing the irst reaction, select 'New' rom the Reaction

No. list. Enter '2; for the reverse reaction

si

J «|-

r

a

1

Oeate a nm Redcton No

.

PR

R-t

nS>«

tMM

BCtM

tI"..-.., Gbb.

Cir.-,.;

f

i'R£aUS

-.n RCSTf t n

""

Fa-Htfc mm FI

RFVp

IS

" "~

-

'-

---

II

*p»f\«-a»i

«

FIGURE 2 48 .

f

Subsequently provide the stoichiometric coeficients along with ex ponents,

the screen,

shown in Figure 2 49. .

and get

ASPEN PLUS

SIMUIATION OF REACTOR MODELS

87

iViirtiT.r

433 1

n-i

1.1

nr

-

i ,ieii

i

.

mi *m

i;

i 71

am*. | CJk-< | [(on

'

CI.X'6

>

1

J

jjWM

[1

1

REaJ

_i5Lj

gg»]

HCs

lt

nawcft

ff

.

wi

.

FIGURE 2.49

Hit A exf knob and obtain two stoichiometric relations as shown in Figure 2 50. .

-

.

DMBI

y.

To* An fV

Lirwy (fntjrw

Mai

1

i-nr

.1 w - i

-

-

HmNo

:

3>>J qLJniJ

Stuctimttry

Kn«c

j MHnnd

_

,

I Sehdt I Ui*M«Jrt )

1r§

c

' fif.ioc

ff/ id

he j*

new-

tCMn

lrv

f

,.

61 bio's

*

f

u

E .11

Rn»th

c v e (BiiTffiirr ft* n-i

" "

FIGURE 2.50

In the simulation of the present problem we use partial pressure basis (applicable for vapour only) and therefore, the Power law expression has the following form: ,

,

( f > r = k

exp

R

1

To

(2.5)

,

P represents the partial pressure (N/m2). If fo is not specified, the above equation

where 18 replaced by: ,

E ri

n

88

PROCESS SIMULATION AND CONTROL USING ASPEN r= kTn exp

RT,

mPif1

2 6) .

For the prescribed reactions, values of the pre-exponential factor and activation energy 2 are provided in the two forms, shown in Figures 51(a) and (b). To apply the Arrhenius law, we put zero for temperature exponent n and left the box, allotted for datum .

temperature T0, empty. i»f.i

I r

mi r»

! .isi; I - IB!

as

±1 "

[i) cfwe-. sciwio.

1

3

d

E

§

ill

*

ai F

* a ?

Si Bacfa i

PR

0 R-1

REqui

Rtjfco

RCSTFI

t

fteM r

RSI

f

StflEfiMS

RPVJ5

RBVch

FIGURE 2.51(a) .

». Ea »«, 0«, r i, a .

i

'

-

.

r u>i-«i» rr

'i-.joii

"

HMfcl""

»|-»l «l|Ii

(31 50*10. 5m;

a i

.

3 >>l Dj J n.|

C6M6

KiMtel«daNUT/T>>|"*'(E
i a *******

SfBtMK

' BStet

R>wto

8 i 0

Rt

fjfl

I Mill

a

,

"

11

FIGURE 2 51(b) .

Lin

i

ASPKN PLUS


89


r

Hitting Afet button and running the simulation, we obtain the Control Panel (Figure 2.52) showing the progress of the present simulation.

i r-i I ! f»

-i-igi

'

_

_

1

w aisd

(0 9 S 8 O = U M

t<<

Of*

FIGURE 2.52

f

(a) Viewing results: Click on Solver Settings knob, choose Results Summary/ Streams in the column at the left side and inally obtain the results for all streams, shown in Figure 2.53.

I r-i-i -rf7

'i -Hi i in i*l

J

"

a 1 -m

IUWi

-

"

4

-

(stcsss: rwm

-

*m

no-

sub-

-

ve-

aaraocc-

tm-1

"

TW

1-

d

S -

im

1

ROT

L

Ml Mr

-

oi TWI M

I -

inuA

-I *

mm

mm

mik

I M»l

--

I-

FIGURE 2.53

C

ll

90


(b) Producing a plot of mole fraction vs length: Use the Data Browser menu tree to navigate to the Blocks IPFRI Profiles sheet (see Figure 2.54). MJi HillLlim-WPMlMli »in: «. Fte

Wi &*l

V*» V*t

Dloi|y|

C#» C#»

roe* Toe*

..l»1.T71. pw PW

Ltmv L*f«7

>

"«

I -.1 EtelBl

ifl

.

it

-. Hji H i

-

«d H H I"!

li

U

P.OC..I Sbe«. I

Pt >wt«

f

t aa 9»um

e v.

-

-

a pf pn

F

z

_

Utt Sutra.

bt

!S

f] nwdb

fariHo TZZf&VS

IS

"

15

;

sSiTFW

r?55

4

.

!5

s

5

m

lb

i55S

Z'

i*

-

-

15

IS

last

1

[ri!DK4IIt f

a 9fM- p« A m RNdm QniMgra

00001 u*?

g LSiQFM, j

it.

I Maroiato-s | So** ) UtaHvkk |

C :., a fciJen'j'jsei Pin v

FIGURE 2.54

In the next, select Plot Wizard from the Plot pulldown menu Alternatively, press Ctrl+Alt+W on the keyboard and obtain Figure 2 55. .

.

a S5 I :

PlOCCUtilMnKtXEflM

1 a

f

t

Wercome ta Aspen Plus Plat WU.rdl

; IJ

9 EM if/

t** Ocw

jlE E ;

f

t Fa*

ii 24

IE

ir -

9 a EOCor-Ortcm

fj

51 REAMS

;

LSSOPBu

1

1 HSbe

HVMJ

-

L

J

J

i . i y=o

REcU

ROttx

FIGURE 2 55 .

Click on Next icon and get a variety of plots (see

Figure 2 56). .

ASI'KN PLUS


91

MM

pi-eniajaaisi

=i r. -i ht

3fif* I

3 4321 iiB1

t

a ---

a3f j

3iil 3t J Id I

a tfmm

m

9 -

?

-

i

N 1 1

n

17

f

n ri n

R

<

H

-

1 ( 1

J

iTmao

Nftj

-

<

-

i

w "

-

.. -_;=_

f

ind*

mfc

I

FIGURE 2.56

Among the available options, select one plot type that is titled as 'Composition' and press Next button (see Figure 2.57).

r-l |..l'fT

'i-lci

-

-

1

fi

ita.l

3 i±d «JP-3a -''ail -i -

. -

-

-

i

a -

io -

r

j m a Bin

3S

v- I

3 ''

-

i-*

4

-

{© 9 . i 0 Q -O'

w

>

mm

"tj~

mm

m>

mm

FIGURE 2.57

Again click on Next and get the form, shown in Figure 2.58.

J

92

PROCESS SIMULATION AND CONTROL USING ASPEN1

I mim 1?! r3l-<-lfcl<.UM "-I H _jLH jd JEl

V)

I

PlOCBU SUUM j

- ,

I

i iJ PHI

fi

f''V.I

.

-1

,.

I

5r

.

i .

{

i

CIS-

t t

ri

! _

Cvitl

si y

CH

-

RYaM

STREWS

TlSitd

mt*»

ir

'Sack

i

u

SCS?fl

BWug

BflWch

9 -B-

1« j-

«M

FIGURE 2.58

Check whether the information displayed in the window, shown in Figure 2.58 are ok or not. Hitting Finish knob. Figure 2.59 is obtained by plotting 'reactor molar composition (mole fraction) as ordinate against 'reactor length' (in) as abscissa. ,

'

t-

<\<-

Dtfa

Tooa

Put

trv

.

Wnsmr

H(*>

t

Dl lHl am oivj ipi al-nal-KI I"»! Its

I M .l lal yj Block PFfi Cemmin

| Xnxan. | Sou | u>Mod> |

si u=u 111 *

HTot)

Qg

RGtfc,

tCSIR

tFy

f

RS'jc

f

STROIMS '

"

.

'

8M,

FIGURE 2.59

Note that the plot window can be edited by right clicking on that window and selecting Properties In the properties window .

,

the user can modify the title, axis scale,

font and colour of the plot Alternatively, double-click on the different elements of the .

plot and modify them as we like to improve the presentation and clarity.

ASPEN PLUS

2

.

5

SIMULATION OK KKACTOR MOOEI

93

ASPEN PLUS SIMULATION OF A RPlug MODEL USING LHHW KINETICS

Problem statement

In acetic anhydride manufacturing, the cracking of acetone produces ketene and methane according to the following irreversible vapour-phase reaction:

CH3COCH3 -> CH2CO + CH4 acetone

ketene

methane

f

f

This reaction is irst-order with respect to acetone. Pure acetone feed with a low rate of 130 kmol/hr enters a PFR at 7250C and 1.5 atm. The kinetic data for the Aspen Plus simulation are given below. k = 1.1 s"1

E = 28.5 x 107 J/kmol n=0

T0 = 980 K The unit of pre-exponential factor clearly indicates the |C 1 basis. To use the LangmuirHinshelwood-Hougen-Watson (LHHW) kinetic model, set zero for all coeficients under Term 1 and that for all coeficients except A under Term 2. Take a very large negative value for coeficient A. The sample adiabatic PFR is 3 m in length and 0.6 m in diameter. Applying the SYSOP0 base method, compute the component mole fraction in the product stream. f

f

f

,

Simulation approach

As we select Aspen Plus User Interface, first the Aspen Plus Startup window appears, as shown in Figure 2.60. Choose Template option and press OK.

f

2I=flHJ-J-Lag Pl-W i-H=J Tl

I I I 'IW *l

1

1

-I

I

**mmm*mH

MM

FIGURE 2.60

94


In the next, select General with Metric Units and again hit OK button (see Figure 2.61)

.

pea

M

IPE a-wm tcpwl* f

An

<*-Sxar Mair>

'

Penmen

1

"

'

11

'C*

'

FIGURE 2.61

As the Connect to Engine dialog pops up

,

click OK.

Creating flowsheet

From the Model Library toolbar we have selected RPlug reactor and developed the ,

f

process low diagram as displayed in Figure 2.62 He &

3an Tocfc fir FW mI Jy»r, WnSe* Htfc

Qi lHI aiai

|a| yj nl-i-iaKKi i w.| 3

rlttF-I l- l PT

s,flt M

.

I

Mi

_

I

i ii
j 3 _j

_

igl

H8- S . 8 -
awif-

~

-

FIGURE 2.62

|

_

ASPEN PLUS"1 SIMULATION OK REACTOR MODELS

95


In the list on the left, choose Setup /Specifications. For the present problem, we wish to give the Title as Simulation of the PFR'. and accounting information as 'AKJANA/8/ '

ANYTHING/AS WE LIKE'. In addition, choose 'Mole' and 'Mass' fraction basis for the

streams under Report Options [see Figures 2.63(a), (b) and (c)l.

r

'

i

LU.

o . § 6 onu

Ml

-

m»t

«>>.

FIGURE 2.63(a)

I'HIM

'

-

XM

-i..

FIGURE 2.63(b)

Gopyngt-

96

PROCESS SIMULATION AND CONTROL USING ASPEN1 ttn

fci

VV*

CMi

teds

FLn

Pw

lirat,

3ip

VAmtow

o|a!|ai I I tfeiel t?! phlftltl l'-l n) _L_L!iJ iJ 21 j2J i r- i-i pt | -|m i - imi I

Cor i j now***- I etod. /StaM»| p'««"y i W

t

ti-n» o hi NAKtad

FkMbM

hi--- 'i

P Hde

PM*

T Mm.

P

TFf, IGEN M .

3

P Cwowit nih IWO ib- «I'KUjn

t6tU

RCSIR

RFV)

BSatc ~

f

f

CV lFoldenXAaDerPlB 1- 1

NUH :

-r irt- t.- r tr.-arpt-i r

REqal

f

frriii r

SI REAMS PStoc efHeb pcufl

FIGURE 2.63(c)


Select Specifications under Components folder in the Data Browser window As we out the Component ID column Aspen Plus provides the rest of the information in component input form shown in Figure 2.64. .

,

,

~

1 f

-

t

f

f

le EJI Wen On tim» t* Put Utray Vfrifcw o«

i i-i- r» 3 M£ i "

J

jiAm \ m nJ -3 »l Qj -.1 «*!

3 S-L* O

SfamOM*

Tim

SottTSe

tCEIO-JE

KEIENE

itENE

)3
seths -pn

$ a«pm)
|1 i Bk«k>

-

wiac

f

f

stficwi

.

8 . 1 -y-lE-U

Pfcu

'

ns tu

ncsTp

FIGURE 2.64

Specifying property method r

Hit Afot button and in property method (see Figure 2.65), scroll down to get SYSOPO

ASPEN PLUS

SIMULATION OF REACTOR MODEI.S

97

JtUI

l r Ll_L_F

-iCI

_

_

I

! !

9 "w-cwr.

3

I

j»

.

-

la

mr'iiir

.

(0- 0 I

: I jn U-

t»*

ia

).

FIGURE 2.65


f

In the left pane of the Data Browser window, select Streams IF. Inputting the values for temperature, pressure, total low and mole fraction, we have the picture as displayed in Figure 2.66. I.UH -

I

,

,

3 I

'

3

.

I--... I '-

figs?

i---

;

r --

-

3

3

I'*

g M

f|7 .

»

1

i iT

I- 0 . i . 8 OMU »»««

gjfc

«ani

ggi

FIGURE 2.66


98



In the subsequent step (see Figure 2.67), select PFR under Blocks folder. Specify the reactor as an adiabatic one.

i

ffc

Ed»

We*

OUIHI

Oto

Ta*

Rn

Rot

lisrary

I I itelal i?l rsK

VAnJo*

i KSS

3

_

UjiJ _J

3 »hJiii(s

aft I -

313

He*

NKiH »'| 3

-il SI

£i»i

ulwCcr/gi/Wcr. |wRtK(nni| Pitt*** j

J AM H

_

J

.

y

_

c

9MM

-

Jfl F

-

O

BUM*: 3;-* Ctro'

~

|

|

= 41111) Mta

O EOrw

-J

J

RVaM

8 i US IJ

BEqut

RGhta

BCSTfl .

SPlup

HBatet f

.

STREAMS

.

C ' g Foktect- apen FVi il l :

j - jj

.

'

tii

ftoM } }LladuTtfi5f

r;;a«cfefZ-M!S

60*

j

HUM

.'debt AgttK Pr |{

fW Ki rtsi muc

Plus - S

.

«

.

s

1107

FIGURE 2.67

In the Configuration sheet reactor length as well as diameter are given (see Figure 2.68). ,

Rfe

E«

'rim

QNB|

_

'tieo

An

PW

u ie|jg] n|-<-|fcN M Hi a »l |n| .la l i|

I "

a SM

StMrtl .

3i

t

36

3

J6

"

3

f

rj f

f

& tdta

8

PnAn

f a 0

EOVw-b*.

EOmpJ

-

D

SWMlttf I HtHEttl -

D-*

StflE*MS

iisi ( RSlac

frtM

i -1 -o-n-o-

M*A

RBto

PCSTR

FIGURE 2.68

In the next we define a reaction set for the present simulation The default name is ok Then select LHHW kinetics and obtain the screen exhibited in Figure 2.69. ,

R-l

.

.

,

ASPEN PUJSTM SIMULATION OF REACTOR MODELS

1

-

99

W-l-K-l..

a .

(@- 0 g g u a u 1

.

-

WW

-G-

Hi-'

WS.

FIGURE 2.69

Specifying reaction information Press Nex/ knob and then click on New. Under Reactants, select 'ACETONE' from the

Component dropdown menu and set the coefficient to -1. Similarly under Products, select KETENE' and 'METHANE', and set both coefficients to 1 (see Figure 2.70). '

i

r.:i..i-u rr

. «-

d

1

*.Ml'

mi-*ifc|

d

-

2 roK-.

t

i

.

I* Q

-

-

in«Mt

<@ S 8 § Q»0 *a>.

'mh

gi»

FIGURE 2.70

PROCESS SIMULATION ANnjWQlOLUSING

100

ASPEN'

Hitting on Next and clicking Kinetic button, we get Aeldn ics input form. A littl description is given below to understand the use of LHHW kmetxc model m Aspen

e

simulator.

The LHHW rate expression is represented by: (kinetic factor) (driving force) r =

(2 1)

(adsorption expression)

.

The kinetic factor (reaction rate constant) has the following form: '

\

1 >

T

To)

(2.8)

If Tq is ignored, Eq. (2.3) replaces the above expression.

Note that all the notations

n

K = k

E exp

R

k

used in Eq. (2.8) have been defined earlier. The driving force is expressed by:

f N

A

n c?

and the adsorption is modelled as: M

N

nc"J Li=i where,

In (Ki) =Ai + Bi/T + Ci IniT) + D.T

(2.9)

Here, m is the adsorption expression exponent M the number of terms in the adsorption expression, N the number of components a the concentration exponent, K2, K, the equilibrium constants [Eq (2.9)], A,, it, Q, the coefficients and I Notice that the ,

,

f

.

concentration term C used in the above discussion is dependent on the [CJ basis Say when [CJ basis is selected as molarity the concentration term represents the component molar concentration (kmol/m3); similarly when [CJ basis is partial .

for example

,

,

pressure, the concentration term represents the component partial pressure (N/m2). Providing required data we have the illed kinetic sheet shown in Figure 2.71. Click on Driving Force to obtain a blank form as shown in Figure 2.72. f

,

,

Select 'Term 1' and then 'Molarity' as [CJ basis Under Concentration exponents for set acetone exponent to 1. Similarly for products set ketene and methane exponents to 0 Also enter zero for all four driving force constants as mentioned in the .

reactants,

,

.

problem statement (see Figure 2 73). In the subsequent step (see Figure 2.74), select Term 2' rom the pulldown Enter f

.

term menu Since the given reaction is irst-order with respect to acetone no second term enter zero for all exponents and coefficients Owing to

and there is the method Aspen Plus uses to specify a reaction, we should insert a very large negative value for f

.

,

.

coetticient A (say

on Next icon

.

,

-106)

to make Term 2 essentially zero [see Eq (2 9)1 Finally click n-

.

,

ASPEN PLUS

0m

f

t*

-

L T

Om

Ta«i

.av*

VMw

I I 'i r-i

-


-I -lei

I

!«!

3 -~

.

j

.

3

,

ii w

j9 O -

:

'm
i

Zj

i

am i(t/T«f

lUlllll cuM. f

*

j

J .

Zj

a *-

j f

. -

i

D -

.

-.

Ml

fF

ItWMn

II

| IWBMM | MlMnpn | Man W

om.

T

.

ggjl

gjj

MM.

I | .W.w.Oa n | Mwwl-i | MB | IMMM |

WWI 1IWJ'

<5

FIGURE 2.71

I Mk |

WB.

B'Mt

'CM

Wto.

"lac*-

FIGURE 2.72

- --

l-B

101

102


1

j -

[-EETEEXSC Idlbsw

j

_

I

'.D- o-xr'

I

-

ep

eis

f

p

.

1

.

BEoJ

AG**!

FIGURE 2.73

Rwcaigthsai

jVapm

ErteHtrm

[l«rm2

3 Hi .

a _

j t(*clartr

Expowii

0 )

.

lJ

Q

u _

)

Data

a .

SbMM F p

lj

r'. u

.

PFS

-

Ml

T

co(W,c*»1t A

21 - i f/fT tewrddning low* J«m Ln(ccr.;fanl .

"

SeeHfle t

.

Omttry

_

G

Ci ytw tyw »fival«n enerw 'a t«

.

_

j

u

l»

j

Jn

poww law wpittWin

Mom

BE(M

RSfca f

STRCAHS

RCSTR FUin

llQi
HUM

ig ito w J

FIGURE 2.74


r

The Stoins bar displays a message of Required Input Complete in the bottom ight comer of the window shown in Figure 2 74. Subsequently run the simulation and obtain the status report as displayed in Figure 2 75 .

,

ASPEN PLUS

j r i -

_

-

i-i'

'

_

nr

-i

\

SIMUUVTION OF REACTOR MODELS

103

m -sw

-

i --»-

tii

-

»**"' I

'

II I -*« -- I

.

*"l »->

£r <@ 6 S 0 O H U IIIMH

HlK

«*

Mm

".iril

A*

Mar

FIGURE 2.75

Viewing results

Pressing Solver Settings knob and selecting i?esw/ s Summary /Streams, we obtain the final results as reported in Figure 2.76.

i i-liisialiil: "

1

1 I -i

1 r[.-

3

, ....

fami

____

_

el

ST

-

1

r

iffan

ur

rzw

i

'

BTTiW

I'M

Tivi '

ii

M- Q . S . § U S U Wii

TM-

l»-

FIGURE 2.76

Copynghied material

104

PROCESS SIMULATION AND CONTROL USING ASPF.N

SUMMARY AND CONCLUSIONS | This chapter presents the simulation of several reactor models. Here, we have considered a variety of chemical reactions in the Aspen Plus simulator. Probably the most useful kinetic models. Power law and Langmuir-Hinshelwood-Hougen-Watson (LHHW). have been used in the solved examples. A number of problems are given in the exercise for extensive practice.

PROBLEMS | 2

.

1 Ethyl acetate is produced in an esterification reaction between acetic acid and ethyl alcohol. acetic acid + ethyl alcohol <-> ethyl acetate + water The feed mixture, consisting of 52.5 mole% acetic acid, 45 mole% ethyl alcohol f

and 2.5 mole% water, enters the RCSTR model with a low rate of 400 kmol/hr at

750C and 1.1 atm. The reactor operates at 70oC and 1 atm. Both the reactions are first-order with respect to each of the reactants (i.e., overall second-order). For these liquid-phase reactions, the kinetic data for the Arrhenius law are given below: Forward reaction: k = 2.0 x 108 m3/kmol - s E= 6 0 x 107 J/kmol .

Reverse reaction: k = 5.0 x 107 m3/kmol . s E= 6 0 x 107 J/kmol .

[C,l basis = Molarity Perform the Aspen Plus simulation using the NRTL thermodynamic model and reactor volume of 0.15 m3 2

.

.

2 Repeat the above problem replacing RCSTR model by RStoic model with 80% conversion of ethyl alcohol.

2 3 Simulate the reactor (Problem 2.1) for the case of an RGibbs model. .

4 An input stream, consisting of 90 aole% di-tert-huty\ peroxide, 5 mole% ethane and 5 mole% acetone, is introduced in a CSTR at 10 atm and 1250C and a low f

.

r

2

rate of 0.2 kmol/hr. The following elementary irreversible vapour-phase reaction is performed isothermally with no pressure drop.

(CH3)3COOC(CH3)3

C2H6 + 2CH3COCH3

Fake kinetic data for the Arrhenius formula are given as: k = 1.67 x 104 kmol/m3 s (N/m2) £ = 85 x 103 kJ/kmol

LCJ basis = Partial pressure The reactor operates at 50oC and its volume is 6 m3. Using the SYSOP0 thermodynamic method, simulate the CSTR model and compute the component mole fractions in the product stream.

ASPEN PLUS

.

105

5 A feed stream, consisting of di-tert-buty\ peroxide, ethane and acetone, enters a RYield model at 10 atm and 1250C. The reactor operates at 10 atm and 50oC. Use the SYSOP0 property method and assume the following component-wise low rates in the feed and product streams (see Table 2.3).

f

2

SIMULATION OK REACTOR MODELS

TABLE 2.3

Component di-tert-hntyl peroxide

Feed flow rate (kg/hr)

Product flow rate (kg/hr)

26.321

1 949

ethane

0 301

5 314

acetone

0 581

.

.

.

19.94

.

Simulate the RYield reactor and compare the results (mole fractions in the product) with those obtained for Problem 2.4. 2 6 As stated in Problem 2.1, the reaction between acetic acid and ethanol gives ethyl acetate and water. .

CH3COOH + C2H5OH (-> CH3COOC2H5 + H20 The inlet stream, consisting of 50 mole% acetic acid, 45 mole% ethanol and 5 mole% water, is fed to a REquil model with a flow rate of 400 kmol/hr at 750C and 1.1 atm. The reactor operates at 80oC and 1 atm. Using the NRTL property method, simulate the reactor model and report the compositions of the product streams. .

7 Ethylene is produced by cracking of ethane in a plug low reactor. The irreversible elementary vapour-phase reaction is given as: f

2

C2H6 - C2H4 + Hg ethane ethylene hydrogen f

Pure ethane feed is introduced with a low rate of 750 kmol/hr at 800CC and

5 atm. The reactor is operated isothermally at inlet temperature. The kinetic data for the LHHW model are given below (Fogler, 2005). 5

.

k = 0.072 s"1 £ = 82 x 103 cal/mol

Tq = 1000 K

|C,] basis = Molarity The reactor length is 3 m and diameter is 0.8 m. Using the SYSOP0 thermodynamic model, simulate the reactor. 2 8 Repeat the above problem replacing the PFR by a stoichiometric reactor with 80% conversion of ethane. If require, make the necessary assumptions. 2 9 In acetic anhydride manufacturing, the cracking of acetone occurs and produces ketene and methane according to the following irreversible vapour-phase reaction: .

.

CH3COCH3 i CHoCO + CH3

1

106


In the CSTR model, ketene is decomposed producing carbon monoxide and ethylene gas. K

'

CH2CO-> CO + 0.5 C2H4 where, 15

,

rk = K

.

'

-

K=

26586

exp 22.8-

K' = exp 19.62-

mol/lit s . atm15

T 25589

mol/lit . s

[C,] basis = Partial pressure

Here, -rA is the rate of disappearance of acetone (A), -rk the rate of disappearance of ketene ik), PA the partial pressure of A, and K and K the reaction rate '

constants. Pure acetone feed with a flow rate of 130 kmol/hr enters the reactor at 7250C and 1.5 atm. The reactor with a volume of 1

.

4 m3 operates at 700oC

and 1.5 atm. Applying the SYSOPO base method compute the component mole fractions in the product stream ,

.

REFERENCE | Fogler

,

H. Scott (2005), Elements of Chemical Reaction Engineering

3rd ed.. New Delhi

.

,

Prentice-Hall of India

CHAPTER

Aspen Plus Simulation of Distillation Models

31 .

BUILT-IN DISTILLATION MODELS

An Aspen simulation package has nine built-in unit operation models for the separating column. In the Aspen terminology, these packages are named as DSTWU, Distl, RadFrac. Extract. MultiFrac, SCFrac, PetroFrac, RateFrac and BatchFrac. Under these categories,

several model configurations are available. Note that Extract model is used for liquidliquid extraction. Among the built-in column models, DSTWU, Distl and SCFrac

r

represent the shortcut distillation and the rest of the distillation models perform igorous calculations.

DSTWU model uses Winn-Underwood-Gilliland method for a single-feed two-product fractionating column having either a partial or total condenser. It estimates minimum number of stages using Winn method and minimum reflux ratio using Underwood method. Moreover, it determines the actual reflux ratio for the specified number of

stages or the actual number of stages for the specified reflux ratio, depending on which is entered using Gilliland correlation. It also calculates the optimal feed tray and reboiler as well as condenser duty. Remember that this model assumes constant molar overflow and relative volatilities.

Distl model includes a single feed and two products, and assumes constant molar

overflow and relative volatilities. It uses Edmister approach to calculate product composition. We need to specify a number of stages, e.g. feed location, reflux ratio,

pressure profile and distillate to feed iD/F) ratio. Actually, when all the data are provided, we can use this column model to verify the product results. RadFrac is a rigorous fractionating column model that can handle any number of feeds as well as side draws. It has a wide variety of appUcations, such as absorption,

stripping, ordinary distillation, extractive and azeotropic distillation, reactive distillation, etc. MultiFrac is usually employed for any number of fractionating columns and any number of connections between the columns or within the columns. It has the ability to simulate the distillation columns integrated with flash towers, feed furnaces, side 107


108

PROCESS SIMUKATION AND CONTROL USING ASPEN

strippers, pumparrounds, etc. This rigorous column model can be used as an alternative of PetroFrac, especially when the configuration is beyond the capabilities of PetroFrac As mentioned earlier, SCFrac is a shortcut column model. It simulates a distillation .

unit connected with a single feed, multiple products and one optional stripping steam

.

It is used to model refinery columns, such as atmospheric distillation unit (ADU) and vacuum distillation unit (VDU).

f

PetroFrac is commonly employed to fractionate a petroleum feed. This rigorous model simulates the refinery columns, such as ADU, VDU, luidized-bed catalytic cracking (FCC) fractionator, etc., equipped with a feed furnace, side strippers, pumparounds and so on. RateFrac is a rate-based nonequilibrium column model employed to simulate all

types of vapour-liquid separation operations, such as absorption, desorption and distillation. It simulates single and interlinked columns with tray type as well as packed type arrangement.

BatchFrac is a rigorous model used for simulating the batch distillation columns. It also includes the reactions occurred in any stage of the separator. BatchFrac model does not consider column hydraulics, and there is negligible vapour holdup and constant liquid holdup. It is worthy to mention that for detailed information regarding any built-in Aspen

Plus model, select that model icon in the Model Library toolbar and press Fl. In this chapter, we will simulate different distillation models, including a petroleum refining column, using the Aspen Plus software. Moreover, an absorption column will be analyzed. In addition to the steady state simulation the process optimization will ,

also be covered in the present study. 3

2

.

ASPEN PLUS SIMULATION OF THE BINARY DISTILLATION COLUMNS

32 1 .

.

Simulation of a DSTWU Model

Problem statement

A feed stream, consisting of 60 mole% ethane and 40 mole% ethylene enters a DSTWU column having a low rate of 200 Ibmol/hr at 750F and 15 psia This feed is required to ,

f

.

fractionate in a distillation column capable of recovering at least 99 6% of the light key .

component in the distillate and 99 9% of the heavy key component in the bottoms. The sample process operates at 300 psia with zero tray-to-tray pressure drop The pressure .

.

in the reboiler as well as condenser is also 300 psia In the simulation, use total .

30 theoretical stages (including condenser and reboiler) and a total condenser Applying the RK-Soave property method simulate the column and calculate the minimum reflux ratio, actual reflux ratio minimum number of stages actual number of stages, and .

,

,

,

feed location.

Simulation approach

From the desktop select Start button and then click on Programs, AspenTech, Aspen ,

Engineering Suite

,

Aspen Plus Version and Aspen Plus User Interface. Then choose Template option in the Aspen Plus Startup dialog and hit OK (see Figure 3 1). ,

.

ASPEN PLUS

Q\a\m -I -I

SIMULATION OF DISTILLATION MODKUS

|r|
I ' l-l-l I- 1 1

I

I

-

I I l !

gj J

109

-

1-1

.

i

FIGURE 3.1

Select General with English Units as the next window appears (see Figure 3.2).

.

MM -

Hi .

4./

-

...

mm

...

;

l

__

FIGURE 3.2

C

aterial

110


Again press OK to see the Connect to Engine dialog (see Figure 3.3). Here we choose PC by scrolling down. Hit OK knob and move on to develop the process low diagram f

T,ocal

.

a

Connect to Engine Server type: User Info Node name:

User name:

Password:

Working directory:

Save as Default Connection

(

OK

1

Exit

Help

FIGURE 3.3

Creating flowsheet

As we select Columns tab in the bottom Model Library toolbar (Figure 3 4), Aspen Plus .

shows all built-in column models.

«a 6t Mr- 0*s

locii Rfi Rewhart

Ltrary Wxto- H«fc>

Model Library toolbar StftEAMS

1 DiTVU

Ci-J

R»fEjJikI

M tfug

Sffru

PWtrf.te

Rurf-

FIGURE 3.4

Bwctfi -

ASPEN PLUS

SIMULATION OK DISTILLATION MODELS

111

f

In the next, select DSTWU icon to represent the short-cut distillation process. Once we have selected the icon, place the icon on the lowsheet by clicking with the

f

f

cross-hair somewhere on the lowsheet background. When inished, click on K | symbol f

r

or ight-click on the lowsheet background. By default, the column is named as Bl (see Figure 3.5). i\n

Hi

tM

'amiami I

Dn

'hr

nrann

lw

-«

Dfagyai aial id g] aififci K!--! "i r|rrFf,.|..|..h HT

'MPl

i |m| ! v\ *\ I

Bl -fW

UJ

iT

_

CH

-

"SAW.

i

' DIIMI

Out

"*l
1M

MtfMi

IW

c-.i

l*.

..

C- a'aMAcwi a IM

AM

ru- MMC*

FIGURE 3.5

In the screen, shown in Figure 3.5. only the block is displayed; there are no incoming and outgoing streams connected with the block. Therefore, the Status message in the

bottom right of the window includes Flowsheet Not Complete. Interestingly, after connecting all required streams with the unit, this message sometime may also be retained. This happens because of improper flowsheet connectivity.

To add a single feed stream and two product outlets (distillate and bottom), click on Material STREAMS tab in the lower left-hand corner. As we move the cursor

f

(a crosshair) onto the process lowsheet, suddenly three red arrows and one blue arrow appear around the block. These arrows indicate places to attach streams to the block.

As we know, red arrows are required ports and blue arrows are optional ports. Click once on the connection point between the feed stream and the DSTWU block, enlarge the feed line and finally click again. By default, this stream is labelled as 1. In the similar fashion, we can add the two product streams, namely 2 and 3, to the distillation unit (see Figure 3.6).

Copyrighted malarial

112

PROCESS SIMULATION AND CONTROL USING ASPEN He

EA

V«*

data

Tooti

Rr. FtowtfiM

Ut-av

Wtxfaw

H«to

rlRFi-|...httt lT

1

irol

I - lal

1 ~

3

0

-

&

Ul

J

. Mewt/SpUeu 1 Stpaatai | HwlEMhangwt Criumn* j ReKloit | PrwawOiangsi | MsripuWwi | SoW« | UMtMwWt j

f

ilf STREAMS

' DSTWU

Dntl

Rrfisc

EntisO

Mutftw;

SCFiac

PeOoFi

Ratrf.ac

BWchFiac

flOflCfcrJ-

r

Book

FIGURE 3.6

f

After renaming Stream 1 to F, Stream 2 to D, Stream 3 to B and Block Bl to DSTWU, the lowsheet finally looks like Figure 3.7. fte

EiJI

«ew

OKa

MiBl alal

Tocti f**i

lei

fte«h«i

Uxsy

Wndm.

H-t

1 ni-rlftl Nkl H li! -

-

0

O

E

-

'

.

'

Rrf(»e

DSVM

Vn

co-

f

STREAMS

E**d .. , -

MtAffc

I Qw i

SCfttc

W l- O

PMcfiae

il

RjuF.ic ~r

Eatctfr IR Wfc.

W l UN T

FIGURE 3.7

.

*

nR*!!!

|| A

MUM

TW*o

-a

.

ASPEN PLUS

SIMULATION OF DISTILLATION MODELS

113

Now the Status bar in the window, shown in Figure 3.7, says Required Input f

Incomplete indicating that the lowsheet is complete and the input specifications are required to provide using available input forms for running the Aspen simulator. Configuring settings

Recall that within the Aspen simulation software, the simplest way to find the next step is to use one of the following equivalent commands: (a) press the Next button f

(b) ind 'Next' in the Tools menu

(c) use shortcut key F4 and obtain Figure 3.8.

mF-M-i-i nr

'i -ici

\-\m

-

0

-

c-Q-

D«pr, rce rout to**'

3

HmnnUBmt | VwMdi | HMUOwpn

STRUMS

' DSrwU

Pit

Wi we

1*1

fil

I

| Rmckb | Pm*j*0««Bi | Mar«i«Mn | iota | UwMolM

tOit

| *,0-H-» Wot I tJO

WtwT

Warfwe

jtjjjg

t ttoM I <]OmH»1 Mnw |aj
AvxfV

S~

Ql'f. t*9t

FIGURE 3.8

Hitting OK on the above message, we obtain the setup input form. Alternatively, select Solver Settings knob and choose Setup /Specifications in the list on the left f

(see Figure 3.9). Although optional, it is a good practice to ill out the above form with a title and to

provide the accounting information subsequently. The present project is titled as Simulation of a Shortcut Distillation Column' (see Figure 3.10).

'

114

PROCESS SIMULATION AND CONTROL USING ASPEN Sid

t

oltflBl A t Nel fl rahclfc l lwl n J 21 JiJilJ zJ 2l J ©1 rv 'I -ipi i w L r i-i _

_

I "'M'.

WIMAJ

Cxi

Brf.K

MiJtfue

5Ct'«c

Pk. i

tttrfii

f

'

SintMC

Brf.»f«:

FIGURE 3.9

3Mbl

JMlJiilF

Bid LJ

J W«w

_

i (Mi

"

I

-

3

= 3 EOCsn.C«n

'* ,

7 '*"'

I

""I

1 -TO- I Mrow
FIGURE 3 10 .

In the next (see Figure 3 11) the Aspen Plus accounting information (required .

some installations) are given in the following way. User name: AKJANA Account number: 9

Project ID: ANY ID Project name: YOU CHOOSE

ASPEN PLUS'" SIMUI-ATION OF DISTILLATION MODELS

.

9

l\

115

I r",

-

'

3f a)

-.

a mi

5! ) MB I

1 nlVI

M

(M*

fMa

WlB

- - -

I

Ir- .

FIGURE 3.11

We may wish to have stream results summarized with mole fractions and/or some other basis that is not set by default. For this, we can use Report Options under Setup folder. In the subsequent step, open Stream sheet and then choose 'Mole' fraction basis.

In this regard, a sample copy is shown in Figure 3.12. although this is not essential for the present problem.

i-d 3-J-itiiJa.ii

0

-»

P <*

1

' i PBS

.* luiTi

3

.«*>*>

FIGURE 3.12

116

PROCESS SIMULATION AND CONTRQLUSING ASPEN


Use the Data Browser menu tree to navigate to the Components/Specifications/ Selection sheet (see Figure 3.13). .

«t W

DM T«ii t*. PW tto¥

i

f

.

1

HHP

:

J ComiKifun" WTO

Caww*iD .

.

-

.

FamO)

5 Drill-id!

Uyr-End Pmpoti rHudocariiMW

ti

aeehi

C
iO II SiiKi ait to tr rMneved Itom dsiatw* J. erte<

O

Mtw SpUen | S«p»*« | HealEttJiWBen Cohmnt } lwcto« 1 P-essueO owt | MwpuWWi | Sate | UraM«W. j f

[if

,

Drti

ErtaO

'

Mutfac

U *

-

r

STREAMS 1 OSTWJ Sr«*,B«.ft

PatftFi

? Rahjiac

-

BalchFrae

r:siHa-Ai>i,ivini1

FIGURE 3.13

f

In the window, shown in Figure 3.13, the table has four columns; they are under the headings oi Component ID, Type, Component name and Formula. Among them, the Type is a specification of how an Aspen software calculates the thermodynamic properties. For fluid processing of organic chemicals, it is generally suitable to use Conventional optiom Remember that component ID column should be illed out by the user. A Component ID is essentially an alias for a component It is sufficient to use the .

chemical formulas or names of the components as their IDs On the basis of these .

component IDs, Aspen Plus may spontaneously ill up the Type Component name and

f T haPPen' * that AsPen Plu« to ind an eXaCt A«Pen Plus does not recognize the components by

lt}lhrATyin °*er words

0 0 Search the components Select the components rom '

T86 fj!?

f

mateh in

f

f

,

.

detaiIs See the solution aPProach Subsectiri 3 COmPonent hane and ethylene, as thefr 6 0f (see lfir (see Figure 3.14). ThefootherT three columns have been automatically filled out.

in

'

IDs

i


tZSlTjiT

j11'1!!?68 I?0118 meth0ds *** mod to compute the phy 0ht th? Pr0Perty input f0rm er hit Next icon or choos

Pron l i2cations in the left pane of the Data Browser window Propernes/Specif '

.

property method by scrolling down (see Figure 3.15).

.

e

Set RK-Soave

ASPEN PLUS

SIMULATION OK DISTILLATION MODELS ..i-j

117

;

3 .

FIGURE 3.14

3S3

Plata l I wi Qb3Mslllid5d 3 I r-l I..|-f7 .: .id ! zsjti » 9

D»ld

'« -It -3l

alig|g|

BS aJ -

i

!

r

-

FIGURE 3.15


The Streams /F/Input / Specifications sheet appears with the Data Browser menu tree in the left pane (see Figure 3.16). Here, we have to provide the values for all state variables (temperature, pressure and total flow) and composition (component mole fractions).


118

PROCESS SIMULATION ANnCO TROLJ-JSING Ffe til '.

Tut. Teal, r-m FW limy Wr*«

i r - I .i-l rv

ASPEN'

H*

. j

J-Igl

M.

mm m

.

21

J PltKTMM

r

a) enjr,h«»caa

-

THWIE

|pmtu«

:

RK£6'J1

f

;

-

-

l

nrtxu i

ri

UNKK Owe-

rj um(KO j>( m

i

if

111'

1

1

1 DSTWU

5TR£JWS

-

MJflK

< -

SCFlK

PWoF

E.- --"

B-r*f,»-

FIGURE 3.16

Filling out the form, shown in Figure 3.16, with the data given in the problem statement, one obtains the data, shown in Figure 3.17. Took

e>

FM

f

Vc« ata t

He Ed!

.

Ltrary

M*km

-i*

3Mi ] EOOpbora |

J/) sT»*r. M«hoC

.

g

-

Conmotnon

, - .-

| Mole f-*:

h |Pini«o RXSBU-1

[is

2

3r

Corrconan -

IS 04

RKTKUI

Q E*anM*To-is

1J «.

j

.

_

«.w

lew [T

ll

-

_

o

~*

SIB6W6

i

: bStWU

J- «

.

!

»2l

FIGURE 3 17 .


f

ia wDianKtblock in Sfn t under Blocks folder. As a result. put form is displayed (see Figure 3.18).

ASPEN PLUS


119

FIGURE 3.18

Under Column specifications option, here we enter the number of stages that is 30. It is fairly true that we can alternatively specify the reflux ratio when the number of stages is asked to compute. Note that ethylene is the light key and naturally ethane is the heavy key. As mentioned in the problem statement, recovery of the light key component in the distillate (= moles of light key in the distillate/moles of light key in the feed) is 0.996 and recovery of the heavy key component in the distillate (= moles of heavy key in the distillate/moles of heavy key in the feed) is 0.001. In addition, the pressure of the total condenser and reboiler is given as 300 psia. Entering all these information, one obtains the result, shown in Figure 3.19.

-

l-l

-

t'

-

f

i f

,,

--

o

-

1

I

|

-

FIGURE 3.19

CopynghlGd material

120

PROCESS SIMULATION AND CONTROL USING ASPKN'


The Status message includes Required Input Complete indicating that we are in a positio to run the simulation. Simply press Next button and receive a message regarding th

n

present status (see Figure 3.20).

TTJ

f

iT

e

[30

Jj UkifIC Gkc

jgi i Cm* f

ioW

I ]300

KiHictoipooert

s*.

.

Com

[ETMYLEHE

Smm

t

itjucioMtiTCienpu Totftw- x*npu «fccC*wM t*n

flBMy: [0 0C1 E0V»Mblw _

-

33 Dsmni

a Q

Be-* C-pua-i Mb

H

STRWMS

FIGURE 3.20

Click OK on the above message and obtain the Control Panel window that shows the progress of the simulation (see Figure 3 21). .

F»» Efe ««

£to T«* An iMy -AWto*

H*

-

]aj®iJ i£ll w| KHIMKI h>\ 0 >Nh| *i lacal -

*bs

atrsitrro rkx sot iabli

taw

EH

-

HUM

f

it -. I

FIGURE 3.21

ASPEN PLUS SIMU1.ATI0N OF DISTILLATION MgggUj

121

Hitting Next followed by OK, we have the Run Status screen (see Figure 3.22).

i r -i _

rr .

HMtf mutt

WIWW

(M

.i.ipi

i

ibi

f

fMF(«

iMMO

*fik

SOik

i*rfi«

f

_

HH-'Ifci

'*.

.

FIGURE 3.22

Viewing results

f

In the next, select Blocks/DSTWU/Results rom the Data Browser. In the following (Figure 3.23), we get the answers as: Minimum reflux ratio = 7.724 Actual reflux ratio = 8.751

Minimum number of stages = 33.943 Actual number of stages = 67.887 Feed location = 40.417

f

f

Save the work by choosing File I Save As /... in the menu list on the top. We can name the ile whatever we like. Remember that a backup ile (*.bkp) takes much less space than f

a normal Aspen Plus documents ile (*.apw). Viewing input summary

If we wish to have the input information, press Ctrl + Alt + I on the keyboard or select Input Summary from the View pulldown menu (see Figure 3.24).

Copyrtghtod material

122

PROCESS SIMULATION AND CONTROL USING ASPEN dim -

I

-i .iai,

r..|-M= IT

I

i 'j-i .

ibi

i a SH

r -

1S?497652 »3*J12.;3 -

Br.il.

ifM IM|NMM

f

0399 HE IP

STREAMS

DSmj

sti| s

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OaK

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FIGURE 3.23

lalxt '

i

Edi

Font

\kB

'

irpuc SuwMry creic«d by Aspen Plus Bel. 11

.

1

at 10:15:40 Tho

Jol

12,

Directory c:\Pr09ran f nes'.Aspenrechvworklng foIi working FoldersVupen plus 11.1 .

[TITLE

2007

Fllenw c :\users\4kjana\AppMt«\Local\Te«p- ap6336. trt ~

'SinulatiorL of 3 Shortcut Cist Illation column'

I-UNITS

EPXC

Ikf-STREjWS COMVEW ALL

bescfiiPTiON Central simulation with Eoallih units : F. psl, Ib/hr Ibool/hr, Btu/hr. coft/hr. ,

property Method: nort

Flow basis for Input: Mole Strea* report composition: HoU flow

PSOP-SDUSCES PUHEll C0KPOMEKTS

ETHANE C2H5 / ElKfLEKE C2H>

PROPERTIES Pk-SOAVE

PROP-OATA RirSKD-l IH-W.ITS ENC PROP-LIST BKSKI3

BPVAL ETHANE ETHYLENE . OlOOMOfriJO BPVAL ETMYLENE ETHANE iTREW

.0100000000

F

S085TRE»t fIXEO TEKP"' 5. PRES-1S t«0LE-FLOW-200 M016-FRAC ETHANE 0.6 - ETWlENt 0 4 .

.

,

-

»±

| <1A .

.1I.». |Hn««»|.« | Mj

ynn |

|

FIGURE 3.24

Creating report file To create a detailed report on the complete work we have done including input ,

f

summary, stream information, etc., select Export rom the File pulldown menu. Then f

save the work as a report ile (e.g., C/Program Files/AspenTech/Working Folders/Aspen

Plus Version/ DSTWU.rep). In the next, open the saved report file (DSTWU.rep) goingd

f

through My Computer and inally using a program, such as the Microsoft Office Wor f

or WordPad or Notepad. For the present problem, the inal report is shown

below.

ASPEN PLUS


ASPEN PLUS IS A TRADEMARK OF

HOTLINE:

ASPEN TECHNOLOGY, INC.

U S A 888/996-7001

TEN CANAL PARK

EUROPE (32) 2/724-0100

.

.

123

.

CAMBRIDGE, MASSACHUSETTS 02141 617/949-1000 PLATFORM: WIN32 VERSION: 11.1 Buiid 192

JULY 12. 2007 THURSDAY 12:07:22 P.M.

INSTALLATION: TEAM EAT .

ASPEN PLUS PLAT:

WIN32

VER: 11.1

07/12/2007

PAGE I

SIMULATION OF A SHORTCUT DISTILLATION COLUMN

ASPEN PLUS (R) IS A PROPRIETARY PRODUCT OF ASPEN TECHNOLOGY. INC. (ASPENTECH). AND MAYBE USED ONLY UNDERAGREEMENT WITH ASPENTECH

RESTRICTED RIGHTS LEGEND: USE, REPRODUCTION. OR DISCLOSURE BY THE U S GOVERNMENT IS SUBJECT TO RESTRICTIONS SET FORTH IN .

.

(i) FAR 52.227-14. Alt. Ill, (ii) FAR 52.227-19. (iii) DEARS 252.227-7013(cMl)(ii). or (iv) THE ACCOMPANYING LICENSE AGREEMENT, AS APPLICABLE. FOR PURPOSES OF THE FAR, THIS SOFTWARE SHALL BE DEEMED

TO BE "UNPUBLISHED" AND LICENSED WITH DISCLOSURE PROHIBITIONS. CONTRACTOR/SUBCONTRACTOR; ASPEN TECHNOLOGY. INC. TEN CANAL PARK. CAMBRIDGE. MA 02141. TABLE OF CONTENTS RUN CONTROL SECTION RUN CONTROL INFORMATION DESCRIPTION FLOWSHEET SECTION FLOWSHEET CONNECTIVITY BY STREAMS FLOWSHEET CONNECTIVITY BY BLOCKS..

2

2 2


2

OVERALL FLOWSHEET BALANCE

2

PHYSICAL PROPERTIES SECTION COMPONENTS

3 3

U-O-S BLOCK SECTION

i

BLOCK: DSTWU MODEL: DSTWU STREAM SECTION EOF

4

5 5

PRORT.RM STATUS RfTnTION ninr,K STATUS

ASPEN PLUS PLAT-WIN32

VER- 11 1

07/19/9007

SIMULATION OF A SHORTniTT DISTTT.T.ATION COLUMN

RUN CONTROL SECTION

PAGF/1

124


RUN CONTROL INFORMATION

THIS COPY OF ASPEN PLUS LICENSED TO TYPE OF RUN: NEW INPUT FILE NAME:

00341ji.inm

_

OUTPUT PROBLEM DATA FILE NAME: _

00341ji VERSION NO

.

1

LOCATED IN:

PDF SIZE USED FOR INPUT TRANSLATION:

NUMBER OF FILE RECORDS (PSIZE) = 0 NUMBER OF IN-CORE RECORDS

= 256

PSIZE NEEDED FOR SIMULATION

= 256

CALLING PROGRAM NAME: apmain LOCATED IN:

C:\PROGRA~l\ASPENT~l\ASPENP-l l\Engine\xeq .

SIMULATION REQUESTED FOR ENTIRE FLOWSHEET DESCRIPTION

GENERAL SIMULATION WITH ENGLISH UNITS : F PSI, LB/HR, LBMOL/HR, BTU/HR, CUFT/HR. PROPERTY METHOD: NONE FLOW BASIS FOR INPUT: MOLE STREAM REPORT COMPOSITION: MOLE FLOW ,

ASPEN PLUS PLAT:

WIN32

VER: 11.1

07/12/2007

PAGE 2

SIMULATION OF A SHORTCUT DISTILLATION COLUMN FLOWSHEET SECTION

FLOWSHEET CONNECTIVITY BY STREAMS

STREAM

SOURCE

F B

DSTWU

DEST

STREAM

SOURCE

DSTWU

D

DSTWU

FLOWSHEET CONNECTIVITY BY BLOCKS

BLOCK DSTWU

INLETS F

OUTLETS DB


SEQUENCE USED WAS: DSTWU

BEST

ASPEN PLUS

SIMUIAT10N OF DISTILLATION MODELS

125

OVERALL FLOWSHEET BALANCE MASS AND ENERGY BALANCE

IN

CONVENTIONAL

OUT

COMPONENTS

ETHANE

0 000000E+00

120.000

120.000

ETHYLENE

RELATIVE DIFF.

(LBMOIVHR)

80.0000

.

0 000000E+00

80.0000

.

TOTAL BALANCE 200.000 MOLE( LBMOIVHR) 5852.66 MASS(LB/HR) ENTHALPY(BTU/HR) -0.252753E+07

0 000000E+00

200.000

.

0 155399E-15

5852.66

-

.

-0.363687E+07

0 305025 .

ASPEN PLUS PLAT: WIN32 VER: 11.1 07/12/2007 PAGE SIMULATION OF A SHORTCUT DISTILLATION COLUMN PHYSICAL PROPERTIES SECTION

COMPONENTS ID ETHANE

TYPE C

FORMULA C2H6

NAME OR ALIAS C2H6

ETHANE

C

C2H4

C2H4

ETHYLENE

ETHYLENE

ASPEN PLUS PLAT:

WIN32

VER: 11.1

REPORT NAME

07/12/2007

PAGE 4

SIMULATION OF A SHORTCUT DISTILLATION COLUMN U-O-S BLOCK SECTION BLOCK: DSTWU MODEL: DSTWU INLET STREAM:

F

CONDENSER OUTLET:

D

REBOILER OUTLET:

B

PROPERTY OPTION SET:

RK-SOAVE STANDARD RKS EQUATION OF STATE

MASS AND ENERGY BALANCE

IN

OUT

RELATIVE DIFF.

TOTAL BALANCE MOLE( LBMOIVHR)

200.000

200.000

MASS( LB/HR)

5852.66

5852.66

ENTHALPY(BTU/HR)

-0.252753E+07

-0.363687E+07

*

*

INPUT DATA ***

.

HEAVY KEY COMPONENT

ETHANE

RECOVERY FOR HEAVY KEY

0.00100000

LIGHT KEY COMPONENT RECOVERY FOR LIGHT KEY

ETHYLENE 0.99600

TOP STAGE PRESSURE (PSI) BOTTOM STAGE PRESSURE (PSI)

300.000 300.000

0 000000E+00 .

-

0 155399E-15 .

0.305025

1"

126


NO. OF EQUILIBRIUM STAGES

30.0000

DISTILLATE VAPOUR FRACTION

00

***

.

RESULTS ***

DISTILLATE TEMP. (F) BOTTOM TEMP. (F) MINIMUM REFLUX RATIO ACTUAL REFLUX RATIO

-18.3114 20.4654 7.72431 8.75092

MINIMUM STAGES

33.9434

ACTUAL EQUILIBRIUM STAGES

67.8868

NUMBER OF ACTUAL STAGES ABOVE FEED

39.4169

DIST. VS FEED

0.39900

CONDENSER COOLING REQUIRED (BTU/HR)

3,034,310.

NET CONDENSER DUTY (BTU/HR)

-3,034,310.

REBOILER HEATING REQUIRED (BTU/HR)

1,924,980.

NET REBOILER DUTY (BTU/HR)

1,924,980.

ASPEN PLUS PLAT: WIN32 VER: 11.1 07/12/2007 PAGE SIMULATION OF A SHORTCUT DISTILLATION COLUMN STREAM SECTION BDF STREAM ID FROM:

B

D

DSTWU

DSTWU

TO :

F

DSTWU

SUBSTREAM: MIXED PHASE:

LIQUID

LIQUID

VAPOUR

COMPONENTS: LBMOL/HR ETHANE

119.8800

0 1200

120.0000

ETHYLENE

0 3200

79.6800

80.0000

.

.

COMPONENTS: MOLE FRAC ETHANE

0 9973

1 5038-03

0 6000

ETHYLENE

2 6622-03

0 9985

0 4000

TOTAL FLOW: LBMOL/HR LB/HR CUFT/HR

.

.

.

120.2000

79.8000

200.0000

2238.9320

5852.6576

140.3489

82.0590

7 5963+04 .

18.3114

75.0000

300.0000

15.0000

-

00

00

1 0000

1 0000

1 0000

00

00

00

.

.

.

00 .

.

.

ENTHALPY: BTU/LBMOL

.

.

3613.7256

STATE VARIABLES: TEMP (F) 20.4654 PRES (PSI) 300.0000 VFRAC LFRAC SFRAC

.

4 1532+04

-

.

1 6983+04 .

.

.

.

1 2638+04

-

.

ASPEN PLUS BTU/LB

1381.4403

605.3231

4 9921+06

1 3553+06

-

BTU/HR ENTROPY:

-

.

BTU/T.RMOL-R BTU/LB-R


-

58.6713 1 9515

-

30.5758

-

.

431.8608

-

2 5275+06

.

1 0898

-

-

127

.

-

-

.

28.8269 0 9851 .

DENSITY: LBMOiyCUFT

0 8564

0 9725

2 6329-03

LB/CUFT

25.7482

27.2844

7 7046-02

AVGMW

30.0643

28.0568

29.2633

.

ASPEN PLUS PLAT:

.

WIN32

.

.

VER: 11.1

07/12/2007

PAGE 6

SIMULATION OF A SHORTCUT DISTILLATION COLUMN PROBLEM STATUS SECTION BLOCK STATUS

*

*

*

Calculations were completed normally

*

*

All Unit Operation blocks were completed normally

*

«

*

*

*

f

All streams were lashed normally

«

#

322 .

.

Simulation of a RadFrac Model

Problem statement

We will continue the above problem with few modifications. A hydrocarbon stream, consisting of 60 mole% ethane and 40 mole% ethylene, enters a RadFrac column having

f

a low rate of 200 Ibmol/hr at 750F and 15 psia. The distillation process that has total 68 theoretical stages (including condenser and reboiler) and a total condenser operates at 300 psia with zero pressure drop throughout. The distillate rate, reflux ratio and feed tray location are given as 79.8 Ibmol/hr, 8.75 (mole basis) and 41 (above-stage),

respectively. Consider the RK-Soave property method. (a) Simulate the column and compute the compositions of top as well as bottom

products. f

(b) Is there any discrepancy in product compositions obtained rom RadFrac and DSTWU columns? If yes, what is the main reason?

Note: In the comparative study (for part b), consider total 68 theoretical stages (including condenser and reboiler) keeping other entered data unchanged for the DSTWU column (see Subsection 3.2.1).

Simulation approach (a) Start with the General with English Units Template, as shown in Figures 3.25(a) and 3.25(b).

128


I

I

FIGURE 3.25(a)

Click OiiTin the screen, shown in Figure 3.25(b). When the Connect to Engine dialog pops up, again press OK button to obtain a blank Process Flowsheet Window.

FIGURE 3.25(b)

Creating flowsheet

Among the built-in columns in the Model Library of Aspen Simulator select RadFrac ,

Connecting feed, distillate and bottom product and changing the default names of the block and

f

and place it in the lowsheet window streams with the distillation column

,

.

f

all streams, inally we get Figure 3 26. .

ASPEN PLUS

r|gf7-| .|..|' pr

.


1 -tCi

i

t

sne»fi

-

muu

am

im

mj**

iw

IH

13

0

129

ujw

J r-

-

<

FIGURE 3.26


r

In the subsequent step, simply hit Afet button followed by OK to open a setup input form. These two windows, shown in Figures 3.27(a) and (b), include the Global and Accounting information for the present project. .

%

fa

M»

Dm

tM»

*

imh

n m

n*

9 i"-

.

9

0

ITOf Mr.

i

3 >- w.

-

11

- I

ii

-

'

MIUU

Da

oM

t

-

I

IM*a

«0
Narfw

..McfiAB

FIGURE 3.27(a) Copyrighted material

130


:| Mftl yJ nWkfoKM 1 til .]J_nJ juJ

JaglHl

1

1 ill

Skv

iwi-s«i

MET METCfiAR

l

METCMGCM

f>

5V-C6AH

f

O O

US YOU UKE

Sttwrnf

v a *

Flo»*e«bftg Opbon* MoM

-

,1-! Took

f

t1 a

4 Gfl

2Jil

i

STREAMS

DSTWU

Dim

Rrft«c

E- ad

Muffnc

SCFfac

PaBcfiae

Ratftac

Batetfrac

farHaip.pMn

| .si) Chapter 3 ttowJWcri | 4]Q>«pMf2-lto«rfi Wtrt H

A xn Pkj. - SM«i

. fi'"

FIGURE 3.27(b)

In the Setup/Report Options / Stream sheet select basis as shown in Figure 3.28. ,

ib b«

on To*

r Uhv

n« u»v nvhw Hdp r

:

f

.

R

1 >IB3|- . { lal

UMbl'1

0 Becwrt Opdcro

i

-I

-3>>JialajNil

G«mi4 I Ftatt««t j Btaek Vsi»m} Proper j ADA | H«MtebancUWinsD««nMpwl

;

Ffaubwit

FiMjonbMa

j f? Mote

P Moio

r mm

' P S»S

'

T SUHovcAjw

i jjj Bock.

ShWDfamM

Iff Si«ndsd(BOoctant

r SUi viAm ;

_

K ConpoAMvAweftMoftKlnn

-

; tff- [geTTe r WUaPBcoUw)

1 P SoK-rewatfwwKw*

Wo«tWu.iartwiw* MOM

tWSrten | S Mtatt | HMEwhangm t

*

Arf

E*ao.

SCf

>1>rf

l

f

' DSTMl DSTMJ

f

SIRCMC

.ac

FIGURE 3.28

e

IK

ole' as well as 'Mass' fraction

ASPEN PLUS


131

Specifying components In the list on the left, choose Components /Specifications to define the components. f

Using the component names, ethane and ethylene, as their IDs, we obtain the illed table as shown in Figure 3.29. =W

54

*»

:«>

T

FV«

itun

Wr*s-

r-i-i-i

i .w

-

-

i -igi *m

3a !« Mn* ;lrM*N£ .

cats

IHYUNC

>

O

o

,.

si

-

r

Pttfnc

R«rfW::

B«ct#.*;

FIGURE 3.29

Specifying property method From the Data Browser select Specifications under Properties folder and then set RK,

Soave base method to compute the physical properties (see Figure 3 30). .

Fit

'

CM

0|<*|H| - ; >I>'.|--|T .

»MBJ|v

mrai

B«i«»a(tw6 .

OwoCTylO

J

fflr.SOAVt - r

«J|a.

3 1

J

j

1

IJ

.

It 5T««)J

' OSTWU

FIGURE 3.30

132



Use the Data Browser menu tree to navigate to the Streams /F/Input / Specifications

sheet. Inserting the given values for the feed stream, Figure 3.31 is obtained. r

3EfSiF -q

l-li Fi-Hid QUIH

3

|m*

dr it

(200

itfi>t|

-

,

jbrt*.

1j

1 Hm(t jr r:

Ejiki

CMlma | FtMcicn | RMMfOungett | Htrie-Mm \ SiteJ UtftKoM |

Mulfia:

SCFlK

FmfiK

PmfiiK

9*0*1*:

FIGURE 3.31


In the left pane of the Data Browser window select Blocks/RADFRAC/Setup. Fill up the Configuration sheet as shown in Figure 3 32. ,

.

Sa To* Rn Pa tfea/ WMw Help

I r.-.|-.i-l fT

Nv i

11] isN

3 g BO VMbki

a «*

H*o4erHcuv*i

O Dwt

7]F

1 ?98

(trrotA,

i

Ha

-

FIGURE 3.32

ASPEN PLUS


133

Under Setup subfolder, the filled Streams sheet looks like Figure 3.33.

i 'to

.

1

«

«!«.;

>

|

| MB I M»» |

it r

-

FIGURE 3.33

In the next, simply input 300 psi under Stage 1/Condenser pressure. Aspen simulator assumes that the column operates isobarically if no additional pressure information is provided (see Figure 3.34).

IB I' W tl*)

i:.,ir.ir.ii.0.ii'.fi..#.s .j-. FIGURE 3.34


To run the simulation, hit Next and then OK to observe the progress of the simulation in the Control Panel window, shown in Figure 3.35.

PROCESS SIMULATION AND CONTROL USING ASPEN n* E#

M«m

Dal* Toch t* Utrvy WMdaw M*. f

134

I ~

_.

iJ

-

l i-.i T -

*

l"l "li-dail 4;|

ai Jfilp

Hao«Miafl input «p«cl
...

Ml JSTftllVtO fSON 3 Or tABLt

IAELE tIAJa - KJWSTD

owpotatiom owata ro> tmi jiowshmt;

-

>C«lcaJ.«ttQn» t»«in . . .

Block: aAcrajkC

«
IS LOHM THM( JTXGJ

raofsac

*1 MKMVM

D.7O*i«*07 (H/SSW1

f

Coif/«cg*nfi» iearttlOK*: 01

IL

Sce/Tol

ill

KL

Eo.oas

a

i

J 4

13 1 «

i :

5

.

j

U.m 7.oc«a i.73*fr

i

o.i6a»7i-ai

.

lli>ra»l

STREAMS

PSTWU

Dirf

Radfw , lAaci

MUtfxic;

SCFuw

fahoFrac

Ratrffac

Batetfrac

__

_

FIGURE 3.35

Viewing results

Click on Solver Settings followed by Results Summary and Streams, we have the table, shown in Figure 3.36, accompanying the results of all individual streams. Save the f

work in a folder as a ile.

j He Ed) Urn Curt TooU Ron

Plot Ifinry WndoM l-<*

JjlJ _

3Mbl

121

1 1*1

da)

±l±l«jPi-H JMaljjj ~

3 Sii.i»I«e| 1

J CcnvOotcm

i "

l»316

8 078

7*1154

J982

13i5

752(1

OS*

Um

0517

QOQi

0995

0383

|e.fr«:

i

1

t "

TTfhUNE

ComOnta

tiHiilt

11371!

120«(«

'

~

CTHYIENC

~

0498

73512

man)

EIHAHE

0 336

bou

oeao

EIHYUNE

00M

03»

-

-

FIGURE 3.36

cx.magttgicffTr-wi

ASPEN PLUS


135


f

Select Input Summary rom the View dropdown menu to obtain all input information of the present problem (see Figure 3.37).

ire«.i uiu-y itHin bv up*- »lia

Olr.ciory I

Pro m (tin

Tin* 'SII»l1»T*("i .*

I-IIUVI

.>. It.I « ll.'S'l* Sal ).i 11,

00'

-Oin! .s't-t .ic.r Hut II. 1

UlM'M ts'w*

CVfllD ILL

.ccolvi.ii>o

xcow-io

cavit 'i-.

ill-

.Mie'-io-un 10

- '- i-.'iM' ..

t

:,

;

a. B»«. fV*r> ll-al Kr. Hh.Vt , c H.O, Ml> 'w live ">!. 'nrari

(oiMitilax: Mil flo"

Miiauas main

unimi

yxio-

ikkmiu

4

tiHu« ei-» -

rmkM C!"

W'OITl Ultll'l

HWii ill wii:i

CfWuM EIMK .oio»Xo<»: MCKMI VMB VMIK fi "IWl, ««ll "iw-.'OC. 1

.

..

FIGURE 3.37

Results of the RadFrac column TABLE 3.1

Composition (mole fraction) Component

B

D

ethane

0 996

0 004

ethylene

0 004

0 99G

.

.

.

.

Results of the DSTWU column TABLE 3.2

f

Composition (mole raction) B

D

ethane

0 997

0 002

ethylene

0 003

0 998

Component

.

.

.

.

From Tables 3.1 and 3.2, it is obvious that there is a little difference between the

product compositions. However, the main reason behind this fact is that the RadFrac

performs rigorous calculations, whereas the DSTWU is a shortcut model. Another possibility is the round-off error associated in the reflux ratio and feed tray position. Copyrighled malarial

136 3


3

'

ASPEN PLUS SIMULATION OF THE MULTICOMPONENT

.

DISTILLATION COLUMNS 33 .

1

Simulation of a RadFrac Model

.

Problem statement

A multicomponent distillation column, specified in Figure 3.38, has total 20 stages (including condenser and reboiler) with 60% Murphree efficiency. A hydrocarbon feed mixture enters above tray 10 of the RadFrac column. Apply the Peng-Robinson correlation and consider 120 psia pressure throughout the column. (a) Simulate the model and calculate the product compositions, and (b) Produce a Temperature' (0F) vs. 'Stage' plot. Feed Specifications

<

Vapour Distillate Specifications

Flow rate = 100 Ibmol/hr


Temperature = 120F Pressure = 120 psia

Reflux rate = 125 Ibmol/hr

Component

Mole% 5

C3 /-C

15

n-C4

20

C5 A?-C5

25

4

'

-

35

FIGURE 3.38

A flowsheet of a distillation column

.

Simulation approach

(a) As we start Aspen Plus from the Start menu or by double-clicking the Aspen Plus icon on our desktop the Aspen Plus Startup dialog appears (see Figure 3 39). ,

.

Select Template option

.

FIGURE 3.39

VSI'KN I'll 'S

SIMl'LATION OF DISTOIATIOM MHHKl.S

137

As Aspen Plus presents the window after clicking OK in Figure 3.39, choose General

with English Units. Then hit OK (see Figure 3.40).

FIGURE 3.40

Click OK when the Aspen Plus engine window is displayed (see Figure 3.40). Remember that this step is specific to the installation. Creating flowsheet

r

f

f

At present, we have a blank Process Flowsheet Window. So, we start to develop the process low diagram by adding a RadFrac column from the Model Library toolbar and drawing the inlet and product streams by the help of Material STREAMS. Now the process lowsheet is complete. The Status bar in the bottom ight of the screen, shown in Figure 3.41, displays a message of Required Input Incomplete indicating that input data are required to enter to continue the simulation. Configuring settings

Hitting Next knob and then clicking OK, we get the setup input form. In Figures 3.42(a) and (b), the Title of the problem ( Simulation of a Multicomponent Column') followed by the Aspen Plus accounting information (AKJANA/ll/ANY ID/FINE) are provided. Include the additional items in Report Options/Stream sheet under Setup folder (see Figure 3.43). '

C

aterial

PROCESS SIMULATION AND CONTROL USING ASPEN fi, 6* v«« rwa To* IVi rk**M ll»»v wn*~

H*

J

-

CM

STREAMS

' DSm)

Diiii

Kvfiec

E«liacl

Muffiac

SCFw

PelroFiK

Hattfims '

i

FsrIMs.miFI

|

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Met* Acicla ftcfewl

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FIGURE 3.41

|a
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.

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FIGURE 3.42(a)

ASPEN PLUS

I

f

I ! c(PJ S fJ- >l»l !-H

3 'I 1*1 i| TJ

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illJIBI ~ -

l -I-1 fV


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139

j J

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FIGURE 3.42(b)

.

i -iff]

i

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I

-

5

r

.

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aim

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FIGURE 3.43

Specifying components In the left pane of the Data Browser window, select Components /Specifications. Filling out the Component ID column, we obtain the table as shown in Figure 3.44.

Copynghied material

140

PROCESS SIMULATION AND CONTROL USING ASPEN mas. m

Do*

lock

?j 'I

:

Br. PU

Lbtwy

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lai

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it

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PROPANE CuBjn Uh*l

C4H10-2

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wet*

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0 Papc" JPCItTWIE

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r

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Prtcaties

far Help p-eai M

FIGURE 3.44


In order to define the base property method press Next icon or select Properties/ Specifications in the column at the left side (Figure 3 45). From the Property method pulldown menu, select PENG-ROB. This equation of state model is chosen for thermodynamic property predictions ,

.

.

Ffe E* W

Pa

das

W

0|e?|B|

I

W si «lStl

Jt Specficabom

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tw-waioi method j

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.

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-

STREAMS

P

fwt

FIGURE 3.45

B#etf.K

ASPEN PLUS


141


In the next, use the Data Browser menu tree to navigate to the Streams /F/Input / Specifications sheet. Entering the values of all state variables and component mole fractions, we get this picture (see Figure 3.46). .

I rH-H-F

" -IB I'll IW Ml

I2S

II- P=!J

f»

3

k

'U*w

5?

3

,

&

3

I

3

-

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3

~

I-

-P--

....

-

!?-

Mftg

«!Ei

M=

-*.

-~

»

'O--------

FIGURE 3.46


f

Open the Configuration sheet choosing Blocks /RADFRAC in the list on the left. In the problem statement, the information on number of stages, condenser type, vapour distillate low rate and reflux rate are given (see Figure 3.47).

r£Z3_

3=

'

:

.

3

mam

t

ea .

rauv

s-.rv,

Ma

Cm

mm

«.

FIGURE 3.47

.*>>

142

4 PROCESS SIMULATION AND CONTROL USING ASPEN

In the subsequent step, specify the feed tray location in the Streams sheet as shown in Figure 3.48. D«i

T«* t* PW Uom* VMcm N* f

fl. Ml

I

CJ

31 P«p 3«.

.

Q flow

i rj dv 20

Um4

S .

t . . 9

,

' RADFRAC

0-

f

Hi j j lndxra Cj

T3<*

SinDWS

! OSTW

OiW

t

K

f

*

tm*t

MJJ.tc

Sgt*e

fWuc

.

Hwfi»e

B*<(W-<«

FIGURE 3.48

Enter the coliunn pressure of 120 psi and get Figure 3 49 .

as shown in the screen.

1*1 xl

-

5*

f--3

:

pfboi

Cj b**<**t Ci 'jruJACCw Cj

uwff«&o

t cj «~* Cj E ( J *

CJ earfiMngCkUn f

_

Vt. j xr-

CJ . cj w

'

f*M*

Mfr

!-

FIGURE 3.49

The Blocks/RADFRAC/Efficiencies/Options sheet appears with the Data Browser menu tree in the left pane To input the Murphree eficiency value for all trays (excluding the condenser and reboiler) we have the screen, shown in Figure 3.50 irst. ,

f

f

.

ASPEN PLUS

f

T He S« <*- Ma Tn* t*

Ft* Un,

wal _U


143

WMdw 4*

a|t1«>hNIH!£l 2l jiiiiii J ©i I .W I - W jgfel

r.>i..i..|. fT -

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0 :j

9 0

p

llhMWHcu Trr. Smg 9

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0

0

Gsmaigra

® 0 "

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T'

4

Km)

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PdtoFiac

Ra
:

SalcbFiK

t

Aspen Phis - 9b.. /- AOeb« ooba

j « l:

i fM 1S<6

FIGURE 3.50

Press the knob to open the Vapour-Liquid sheet (see Figure 3 51). .

tj He

&t Uew

Teob

Rji

Pie!

liw/

VAvfcw

±Jr~--l h.b HT

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a-»

MeMTiel '

.

.

-'

5

SEi

.

n.

....

Dj.kT.

FIGURE 3.51

Assume the rectifying along with the stripping zone as Section 1 and fill up the table

,

shown in Figure 3.52.

144

-f

PROCESS SIMULATION AND CONTROL USING ASPEN1 mmmamm RU>

&H

\Aew

Dats

Tec*

i r-i y i nr . -

am i m

J <

'

joc*w.

St«V9

Ugp 19

1

PAOFRAC

1

*

Mm CM

C vientm hi

Pock Sa j

EMMm

Report User Suooj-

LiJ_

f -r. STREAM? NUM

o

Mobetuot* It

FIGURE 3.52


Hit Afcci button followed by OK and observe the progress of the simulation in Control Panel window as shown in Figure 3.53. Fte

E*

Vtew

0«e

To .

Run

Jbnrf VHvhw

±Jjij < Loading SlmXat cn Jngtn* lS:61r33 i

D RADFRAC »;rDC«8»ing Input .p»cif tc»ti =Ti« .

, . .

IHTOUUTTflH

eitJAfly PUJutfOTiRS paru

can* set d

mie muivn from scf tasli .

f

slock: tirraxc

roa hoo«l SEParrD

xuu kms - lsptste

maids a*cnuc

«

esM

1 2195 .

c

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f " "

M«.iJSra»> vrM«ii<$«aM> II S.O.*.. SeMJtJ. II IMiM H«<£«»«w

CM fill

OSIVAJ OSTWU

MJf«

Pa-nOwvn I| (Won Rajrtai || IWnCh n ||

MnuM i

V J

STREAMS

'

Dfl«

Rrfwc

iCfiK

FWrfnc

R ft*: J'j

gaafwe

jT«-tfc,

FIGURE 3.53

i- *

ASPEN PLUS

SINfULATION OF DISTILLATION MODELS

145

Viewing results

Click on Solver Settings knob and then choose Results Summary /Streams to obtain the product compositions (see Figure 3.54).

r-i .i.i>fv

jci

-

rl

3'-f

13 "--l 9

J

_

IM Mi

mmf ~

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1 T»

Hi "

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ue

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ntn

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TM III

» hihi

ti>«

rmt»

I "i

|

Km*~ .

rw »

-.«»«* I- .> '..

FIGURE 3.54

f

It is a good habit to save the work done at least at this moment. If we wish to see the tabulated results with the process low diagram in a single sheet, simply hit Stream Table button just above the results table (see Figure 3.55).

JMMI .til _!.) a nH-|»l%l
-

I i-l .IglJi £j

i -id

urn

«-C3-

FIGURE 3.55

146



As stated previously, to obtain the input information, press Ctrl+Alt+I or select Input Summary from the View pulldown menu (see Figure 3.56). imut Su«Hry' cr«««d by Ajp«r Clul R«l. 11-1 it VMM Sun )ul ;

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f, psl. Ib-tv. Itaol/hr. Btu hr. cuft/hr. property method: none

fIoh basis for Input: Nola Strcan report cowposlclon: t»ol« fl« AOUEOUS PROP-SOURCES POREll

/ 50LIOS

aflUEOUS

.

/ INOBGANIC

,' SOLIDS

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FIGURE 3.56

(b) First, choose Blocks /RADFRAC /Profiles in the column at the left side

Accordingly, we have the stage-wise data as shown in Figure 3 57. .

mm 3fl *«-

J >>j am w

rmi I c

1

Vapofto.

*j 1

3

9TIJ3K?SI

T- t c I

i

20

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171 385017

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195 3*a»6

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aj

t

r

oawnd

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20

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3ra

20

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201354309

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;:

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fir

STntAMS

||

.

.

- i

:iT:;nr-si

I HME

| Mh | M.UoM |

jgwc

tW-«

H«rf.>i

*mtr»C

FIGURE 3.57

J

ASPEN PLUS"" SIMULATION OF DISTILLATION MODELS

147

f

In the next, select Plot Wizard rom the Plot dropdown menu or press Ctrl + Alt + W

on the keyboard to get Figure 3.58.

I ... '. 1H|_U*1«*) nKW*haMH n

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71

_

j

.1

. «Cj owlWelcome to Aspen P!b» net Wlzwdl

1 am

r

|

a 'mm ijte>

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1

15" -

CH MnrtJ

D,>

tuf
MJfac

Sg-«c

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Ralrfrac

Bl£tfi»c

FIGURE 3.58

Click on Next button in the Plot Wizard Step 1 dialog and get a variety of plot types shown in Figure 3 59. .

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ift

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g

f -

3

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d»| olaii n>i

IPFQ | CwvoMora j KVAjW j

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.

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,

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ac.-A»»H.MI f

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FIGURE 3.59

,

148


Select the plot type under the heading of Temp and press Finish button to obtain a '

plot of Temperature (0F) vs. 'Stage' (see Figure 3.60).

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I

I

J

s

<

i

io

u

£

15

i«

:

k

w

tr

tt

is-a

it

FIGURE 3.60

r

Recall that the above plot window can be edited by ight clicking on that window and selecting Properties. Then the user can easily modify the title, axis scale, font and colour of the plot. 332 .

.

Simulation of a PetroFrac Model

Problem statement

An artificial petroleum refining column (PRC), shown in Figure 3.61, consists of a feed f

furnace and a distillation tower. The tower has two pumparound circuits, a partial condenser and three side strippers. The furnace (single stage lash type) operates at

25 psia and provides a fractional overflash of 40% (StdVol basis) in the tower. The outlet stream of the furnace goes to the tower on Stage 22. The tower has 26 stages f

f

with a Murphree stage eficiency equal to 90%. A steam stream, STEAM, is introduced at the bottom of the ractionator (26th stage with on-stage convention). There are another three steam streams, STM1, STM2 and STM3, used in the side strippers. The condenser

runs at 15.7 psia with a pressure drop of 5 psi. The tower pressure drop is equal to 4 psi. The distillate rate is 10000 bbl/day and the distillate vapour fraction in the condenser is 0.2 (StdVol basis).


ASPEN PLUS

SIMULATION OF EHSTILLATIQM MODELS

149

<

LIGHTS

WATER

STMl sir,STM2

BOT

A lowsheet of a petroleum refining column. f

FIGURE 3.61

f

A hydrocarbon mixture with the following component-wise low rates enters the furnace at 1170F and 44.7 psia (see Table 3.3). TABLE 3.3

Flow rate (bbl/day)

Component

Ci c2 C3

3 65 575

i-C4

1820

«-c4

7500

i-C5

30000

n-C5

42000

H2O

250

In Table 3.4, two pumparound circuits and three side strippers are specified. TABLE 3.4

Loeatum

Pumparound (drawoff type)

Draw stage

1 (partial) 2 (partial)

Specifications

Return stage

Flow rate

Heat duty

(bbl/day)

(MMBtu/hr)

8

6

49000

1

12

1000

40 (for cooling) 17 (for cooling)

-

-

Location

Stripper

No. of

Stripper

Draw

Return

Stripping

stages

product

stage

stage

steam

Bottom product flow rate (bbl/day)

1

5

SID1

6

5

STMl

11000

2

4

SID2

12

11

STM2

15000

3

3

SID3

19

18

STM3

8000

1

150


Four steam streams used in the column model are described in Table 3.5. TABLE 3.5

Specifications

1

Steam stream

Location

Temperature (0F)

Pressure (psia)

Flow rate (lb/hr)

STEAM

Main tower

350

50

11500

STM1

SID1 stripper

350

50

4000

STM2

SID2 stripper

350

50

1500

STM3

SID3 stripper

350

50

1000

Considering the 'BK10' base method under 'REFINERT process type, simulate the f

PetroFrac column and report the low rates (bbl/day) of all product streams. Simulation approach

Select Aspen Plus User Interface. When the Aspen Plus Startup dialog appears choose Template and click on OK (see Figure 3.62).

I

I

I

I

I

I

It

Cioata a Utm SmMan Using A+

f

A+

Blank Smiiabon

Temptale

"

OpendnEwiingSimulatior

e*r\ADU BOOK apw

DAeoolAChaptesVAOU OWN xm D:V0ook\Chap(«sViDU t*{> .

0«

|

I

Adobe Ao-dia P,d«»« I Chace.s

W \ 4]Q*i** 2

HmcHW ||

Av

Pka

FIGURE 3.62

As the next window pops up (see Figure 3.63), select Petroleum with English Units

and press OK knob

.

,

ASPEN PLUS SIMULATION OF DISTILLATION MODELS

mm

151

'Mm. 1

linn

v-

.

..

L

EJ J-J

-

.

.

..

FIGURE 3.63

Click OK when the Connect to Engine dialog appears. The next screen presents a f

blank process lowsheet. Creating flowsheet

Select the Columns tab from the Model Library toolbar. As we expand the PetroFrac block icon, a variety of models is displayed as shown in Figure 3.64. Select a model icon and press Fl to know more about that.

rinF. I- I-h HI

I .Ml

I

lal

iN

J?

it Jb

[1 ir .

f

tr ffc* | turn* t

i> B p

#

lb

c

J

1

a

.

# I .

Jt

ir Op

t

.

IT

if

& ir

4

if"

-

W 11 1

FIGURE 3.64

-

152

PROCKSS SIMULATION AND CONTROL USING ASPEN

As the distillation tower described in the problem statement, it is appropriate to choose CDUIOF PetroFrac model. Then place it in the flowsheet window. Adding all

incoming and outgoing streams and renaming the streams as well as block, the process f

low diagram takes the shape as shown in Figure 3.65.

L'Mi-

111" A

KM*

1W

mm m

31*

1Mb

-jM«.

FIGURE 3.65

Configuring settings Click Next to continue the simulation (see Figure 3.66). In the Title field, enter Simulation of a Petroleum Refining Column'. Open the Accounting sheet keeping untouched the other global defaults set by Aspen Plus. '

t.

r

k

-

.

M

-

-o-

Ut-

W»

i.-r

W.

.

FIGURE 3.66

Copyrlghtf

ASPEN PLUS


153

In the form, shown in Figure 3.67, the Aspen Plus accounting information are included. .

| i l"! 1 1 F

MI

9 -i*---

1

I

,1'5-l-Mi-'

9 M Mjri

-

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s 9i<>>

»1

.

ITBUM

:-1TMj

ba

1m

.kMnc

I"- 1

tOM

MXai

Mai

FIGURE 3.67


In the subsequent step, use the Data Browser menu tree to navigate to the Components / Specifications sheet. Filling out the component input form, we have Figure 3.68.

U>

ZlgjFi

aTJ*

3 >id
3 gr

1 -

1

3 T

Lj

tntUM

..

i

-i;

7 -

-

,

-

it!

T

r?

f

KJETTH

-

tSruT

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' wraij

a-

B

i.

wo.,

ip i

tw..

*****

FIGURE 3.68

154



We know that the thermodynamic models calculate the properties, such as vapour

liquid equilibrium coefficient, enthalpy and density. In the list on the left shown " Figure 3.69, choose Properties/Specifications to open the property input form In th 6 ,

.

and in the Base method ield, select 'BKlO' (Brau

'

f

f

Process type ield, select 'REFINERY K-10 method).

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tr

f

Ea

NBI

HO

th*>

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t

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< i-j :

1 aj c v-lu

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KTVM

OM

ikfiK

PtftaPut

f

STRUMS

RatoFiv

8«=rfi«

FIGURE 3.69


Next the Streams /FEED /Input/Specifications sheet appears with the Data Browser menu tree in the left pane Entering the feed data, Figure 3.70 is obtained. .

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iJ e|jgJ njKjfti i-g -i ».i

{ |h| Miigj .}

_

Pn

zi

bJiiif

_

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3 »J QN h

n

| n b iei

i

0.1

cs

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no

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1

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wwi

»

.,

m,,,

s

,

«

t«,«

FIGURE 3.70

i

i*

ASPEN PLUS


155

f

As we hit Next icon, an input form for Stream STEAM opens up. After illing out, it

looks like Figure 3.71

.

t*

*

V««

D*»

T«w

fVn PW

Marwt VMem

M-i rv -

-

[ft

3

HuTorCoew

| EOOptan |

31 1*0 .

F/traBy

£

Mol(Qior Stioam

i:

FtranKn

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ict

d

ia 1

d

H 11500

g ST

:

E S S

LGHTS

3 SE>1 3 O SlOS a

Total NlSOO

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itiputCMeM*

fV

MMd iAw. } SwoWort 1 HutEttharpei' Columw j Rwawt j h w O* '

STREAMS

DSIWU ntnjl

r. .1

Raifitc

EntlKl

MJtf

t)_-.r... Pt fwFrac

SCfiac

.

| M*v niE | S<*t> | U«Modes | njj Bat efiae -

._

Bwctjj.. fiac

b

;

-

S . * AwnWM

NUVI

cj/Ki

i Tote O

*

t'

FIGURE 3.71

In Figures 3,72(a) (b) and (c), three illed input forms are shown for STM1, STM2

f

,

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Although the Status bar says Required Input Complete, we have to specify the two pumparound circuits connected with the main fractionator. Select Blocks/PRC/ Pumparounds in the list on the left. Click on New as the object manager appears. We may accept the default Pumparound ID T-l Then specify the first pumparound circuit (see Figure 3.80). '

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To obtain the input information (see Figure 3.84), select Input Summary from the View pulldown menu.

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164 34 .


ASPEN

SIMULATION AND ANALYSIS OF AN ABSORPTION COLUMN

Problem statement

A hydrocarbon vapour enters an absorption column below the bottom stage and the absorbent enters above the top stage. The column operates at 75 psia with no pressure

drop and it has four equilibrium stages. The absorber is specified in Figure 3.85. GAS-PDT

Absorbent

Pure n-C10 Temperature = 90oF Pressure = 75 psia Flow rate = 1000 Ibmol/hr ABSORBENT

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Pressure = 75 psia Component

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FIGURE 3.85 A Tlowsheet of an absorption column.

Apply the Peng-Robinson equation of state

model in the simulation.

(a) Simulate the absorber model (ABSBR2 under RadFrac) and compute the product compositions.

f

(b) Perform the sensitivity analysis by examining the effect of absorbent low rate on the exiting C3 concentration in the top product (c) Compute the absorbent low rate to keep 15 mole% of C, in the gas product (GAS-PDT). 3

f

.

Simulation approach

(a) Double-click Aspen Plus User Interface icon on the desktop. When Aspen Plus window pops up, select General with English Units Template as shown m Figures 3.86(a) and (b).

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outlet streams and changing the all default labels

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FIGURE 3.88(c)

Specifying components From the Data Browser, choose Components/Specifications. In the input form, shown in Figure 3.89, all components are defined. Specifying property method

In the list on the left, shown in Figure 3.90. select Properties /Specifications to obtain the property input form. Set PENG-ROB property method.


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Specifying stream information In the next data entry step, press Next button and click on OK. Enter the feed information for both the gas stream and absorbent in two forms as shown in Figures 3.91(a) and (b). Specifying block information

Use the Data Browser menu tree to navigate to the Blocks/ABSORBER/Setup/Configuration sheet (see Figure 3.92).

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169

170


Select the Streams tab to specify stream location. Under Convention, there are two

feeding options: On-Stage and Above-Stage. In the present problem, the top stage is the first stage and the bottom stage is the fourth one. Therefore the absorbent is fed above Stage 1 and the gas feed is introduced above Stage 5 (see Figure 3.93). -

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In the next step (see Figure 3 94), select Pressure tab to specify the pressure profile across the absorption column In this case, the column is operated isobarically at 75 psia. Under Top stage / Condenser pressure enter 75 psi. Aspen software assumes that the column operates isobarically if no additional information is provided. .

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Hit Next foUowed by OK to run the simulation. The Control Panel window is

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Viewing results

Choosing Results Summary /Streams in the left pane of the Data Browser window get the results as shown in Figure 3.96.

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TiB

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r

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a

i iaajgLg

FIGURE 3.96

| .

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we

172


(b) In the sensitivity analysis we will manipulate the absorbent low rate and examine its effect on the exiting propane concentration In the column at the f

,

.

left side, double-click on Model Analysis Tools folder and then select Sensitivity

.

As the Object manager is displayed, choose New. On the next window shown in Figure 3.97, Aspen prompts us for an ID. Enter 'C3' as ID, and click OK ,

.

t*

idl

f

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I C-.**

i j Sohh | Utti

FIGURE 3.97

In the next step (see Figure 3.98), select New under Define tab. Then we are prompted to enter a variable name. Enter C3' and press OK. Subsequently, the following information are required to provide: '

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.

1 .lEI

1

,

|B| 1

StaM

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FIGURE 3.98

't

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ASPEN PLUS*" SIMULATION OF DISTILLATION MODELS

173

Type: Mole-Frac Stream: GAS-PDT Substream: MIXED

Component: C3 Hit Next and select the Vary tab (see Figure 3.99) The manipulated variable iis

specified with the following

data:

Type: Mole-Flow Stream: ABSORBEN Substream: MIXED

Component: NC10 Overall range Lower: 500

Upper: 1500 Increment: 50

>

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aa

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-

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FIGURE 3.99

In the subsequent step (see Figure 3 100), .

select the Tabulate tab. This screen is

used by Aspen to set up tables. Insert T under Column No. Then right click on the adjacent cell under Tabulated variable or expression. Select Variable List and drag and drop the variable name (C3) into the cell. We may also directly type 03' in the cell. '

Then run the simulation and get the screen, shown in Figure 3.101.

174

PROCESS SlMULVriON AND CONTROL USING ASPEN

»

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(.

<« . Mi

10 >

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FIGURE 3.100

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t Mai

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Umo

uut

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IOai

DMTw

FIGURE 3.101

taXw

\SPEN PLUS

SIMULATION OF DISTIUJVTION MODELS

175

From the Data Browser, select Model Analysis Tools/Sensitivity/C3/Results to

display the tabulated data (see Figure 3.102).

5 -

-

I »

- "

.

-

I rijo- - 11;

.--

FIGURE 3.102

In order to represent the results graphically, highlight a column in the table and select X-Axis Variable (Ctrl + Alt + X) from the Plot pulldown menu. By the similar way. select Y Axis Variable (Ctrl + Alt + Y) for the next column. Then select Display Plot (Ctrl + Alt + P) from the Plot menu and obtain Figure 3.103.

J T -

I-1 l- iv

i-g

I

i»< siW

I --. I - l - I

J FIGURE 3 103

176


(c) In the left pane of the Data Browser window {see Figure 3.104), open Flowsheeting Options folder and then select Design Spec. We need to provide

this design spec a name in the same manner that we did for the sensitivity analysis. Press New, enter DSC3* and click on OK. '

38 2i£B_uaas

'

3

-'-i ~ _u-j.ii2a

~--

5=.

o

-

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AH

.>

Mfc.

lw

Mto

W.

W«

FIGURE 3.104

Select ihTettJ under Define tab. Then enter 'CS' as a variable name and press OK. In the next step (see Figure 3.105), the following information are required to input: Type: Mole-Frac Stream: GAS-PDT

Substream: MIXED

Component: C3

v . .

K-MJ-B-M-iM-fFIGURE 3.105

Copyrlghiod material

ASPEN PLUS1M SIMULATION OF DISTILLATION MODELS

177

In the subsequent step (see Figure 3.106), select the Spec tab. Design specification data are noted below:

Spec: C3 Target: 0.15 Tolerance: 0.001

DSTVU

Dag

R»Jf>:

Erftaci

M frac

SCF»c

PihoF-JC

flyrf.K

cj'ctfix

f

;Ti=E4Mi

FIGURE 3.106

Finally (see Figure 3 107), select the Vary tab and enter the following information: .

4. Lfc

_

Ar-

Cm« to-Ji Hpi Ptoi

Ubr,

AWtow

»H>

MHj_U jff) nkiaiaKiH n.| P >|'l"l imi i ibi j r iii nr _

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FIGURE 3.107

?

:

1

178

PROCESS SIMULATION AND CONTROL USING ASPKN"

Type: Mole-Flow Stream: ABSORBEN

Substream: MIXED

Component: NC10

Manipulated variable limits Lower: 500

Upper: 1500

As we run the simulation, we get the screen, shown in Figure 3.108. Tw, fu.

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FIGURE 3.108

f

As we choose Streams subfolder under Results Summary folder in the list on the left (see Figure 3.109), we get the absorbent low rate of 1179.467 Ibmol/hr to keep 15 mole of C3 in the gas product. This answer we can also obtain from the sensitivity plot. 35 .

OPTIMIZATION USING ASPEN PLUS

It is well known that Aspen Plus is capable to optimize a function Here, we will continue the above absorption problem (Section 3 4) for optimization. In the present study, we wish to maximize C3 mole fraction in the gas product (GAS-PDT) with respect to absorbent inlet temperature (lower limit = 50oF and upper limit = 300oF) .

.

.

Simulation approach

f

First solve part (a) of the previous absorption problem It means, ill up the input forms for setup components, properties, streams and blocks. In the next, simulate tne optimization problem as described in the following .

,

.

ASPEN PLUS


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lit KB r

r

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FIGURE 3.109

In the column at the left side (see Figure 3.110), choose Model Analysis Tools/ Optimization. As the Object manager is displayed, hit New button and accept the default ID 0-1' Press OK and then New. Entenng variable name 'CS'. again click OK Provide the following information to maximize C3 mole fraction in the gas product.

N t) I'M*

FIGURE 3.110

180

PROCESS SIMULATION ANT) CONTROL USING ASPEN1

Hit Next knob twice and get the screen, shown in Figure 3.111. T-Birry m.v.-.T'-iB

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FIGURE 3.111

Right-click in the empty cell with selecting Maximize option. Then select Variable List and drag and drop the variable name (C3) into the cell (see Figure 3 112). We can also simply type C3 in the field. .

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fwe ~ada Z,r.

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FIGURE 3.112

In the subsequent step select the Vary tab. Under Variable number, as we choose New', automatically the number T appears FillinR out the form, we have the window ,

'

.

as shown in Figure 3

.

113.

ASPEN PLUS'" SIMULATION OF DISTILLATION MODELS

!5)|

I

\£\

181

j l

21 Cw j

_

f

fi

-

rw-m

nw aw i

D

-

FIGURE 3.113

Pressing iVex symbol and running the simulation we get the answer (see Figure 3 114). ,

.

The maximum C3 mole raction of 0.259 is obtained at absorbent inlet temperature of 179 80F f

.

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vi>*

C*i

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fa

.

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FIGURE 3.114

SUMMARY AND CONCLUSIONS | At the beginning of this chapter a brief of all built-in column models of Aspen software has been presented Several separating columns, including a petroleum refining column and an absorber have been simulated using Aspen Plus. The process optimization has ,

.

,

also been discussed with an example. The present study covers both the binary as well as multicomponent systems Interested readers may try to simulate the models given .

in the exercise

.

182

PROCESS SIMULATION AND CONTROL USING ASPKN

PROBLEMS| 3

.

1 A feed mixture, consisting of 60 mole% ethanol and 40 niole% water, is to be

f

f

separated by using a DSTWU model having a low rate of 100 kmol/hr at 40oC and 1 atm so as to recover at least 85% of the light key component in the liquid distillate and 80% of the heavy key component in the bottoms. The column operates at 1 atm with no pressure drop throughout. In the simulation, consider the reflux ratio of 1.5 and a total condenser. Applying the Wilson property method, simulate the column and ind out the minimum number of stages, actual number of stages, and feed position. .

2 A feed stream, consisting of 50 mole% ethane and 50 mole% ethylene. enters a Distl column having a low rate of 200 Ibmol/hr at 750F and 15 psia. This separator f

3

runs at 300 psia with no tray-to-tray pressure drop. The pressure in the reboiler as well as condenser is also 300 psia. The feed enters the model at 6th stage and the column has total 15 theoretical stages (including condenser and reboiler) and a total condenser. If the reflux ratio is 7 and the distillate to feed ratio is 08

compute the mole fraction of ethane in both the product streams with applying the RK-Soave equation of state model. 3 3 A feed mixture specified in Figure 3.115 is to be distilled by a rigorous RadFrac model (FRACT2). The column consists of total 24 equilibrium stages (including condenser and reboiler) with a stage pressure drop of 2 kPa. Consider the .

.

.

condenser (total) pressure of 125 kPa and the top stage (Stage no. 2) pressure of f

130 kPa. The distillate low rate is 120 kmol/hr and the reflux ratio (mole basis)

is 2. A side product (vapour) is withdrawn from 14th stage. Applying the SoaveRedlich-Kwong (SRK) property method, simulate the column model and report the product compositions. Feed

Temperature = 110nF Pressure = 175 kPa

Dj

O

Sj

cC.

B\

$

Feed stage = 10 (above stage) Component benzene

Flow rate (Ibmol/hr) 250

toluene

80

diphenyl

10 FIGURE 3.115

.

4 A reboiled stripper is to be employed to remove mainly propane and lighter

components from a feed stream, shown in Figure 3.116. It has total 6 stages (including condenser and reboiler) and no condenser. The bottoms rate is 100 Ibmol/hr and the column pressure is 150 psia throughout. Using the PengRobinson thermodynamic method, simulate the RadFrac model (STRIP2) and ind out the product compositions.

f

3

A flowsheet of a distillation column.


ASPEN PLUS


183

Feed

D -

Temperature = 40oF Pressure = 300 psia

Feed stage = 1 (above stage) Component

Flow rate

(Ibmol/hr) c,

60

c2 c3

150

75

n-C

4

175

n-C5

60

n-C s

35

FIGURE 3.116 3

.

A flowsheet of a stripping column.

5 A feed mixture of cyclopentane and cyclohexane is to be separated employing a liquid-liquid extraction unit at 250C and 1 atm with the use of methanol as a

f

solvent. The schematic diagram of the process with feed specifications is given in Figure 3.17. The process unit, having toted ive stages, is operated adiabatically. Applying the UNIQUAC property method, simulate the extraction model (ICON1) and note down the product compositions. Feed

Temperature = 30oC Pressure = 1 atm

Feed stage =

1 Flow rate

Component

(Ibmol/hr) 250

cyclopentane cyclohexane

750

EXTRACT

FEED

Solvent

SOLVENT'

-

RAFFINAT

,

Temperature - 30DC Pressure = 1 atm

Feed stage = Component

5

Flow rate

(Ibmol/hr) 1000

FIGURE 3.117 .

6 A gas consisting of 40 mole% ammonia, 60 mole% air at 20CC, 25 psia, flowing at the rate 120 kmol/hr, is to be scrubbed counter-currently with water (pure) entering at 60oC and 30 psia at a rate 100 kmol/hr. The column operates at 1 atm throughout

and it has four stages. Using the UNIFAC thermodynamic model, (a) simulate the RadFrac absorber (ABSBR2) and determine the exiting ammonia concentration in

the gas product, (b) Perform the sensitivity analysis by examining the effect of absorbent low rate on the exiting ammonia concentration in the top product. f

3

A lowsheet of an extraction column. f

methanol

Copyrlghled malarial

184

7 An artificial petroleum refining column (PRC) shown in Figure 3.118 consists of a feed furnace and a fractionation tower. The tower includes one pumparound circuit, a partial condenser and one side stripper. The furnace (single stage flash type) operates at 20 psia and provides a fractional overflash of 50% (StdVol basis) in the tower. The outlet stream of the furnace enters the tower on stage 18

.

.

The column has total 20 stages. A steam stream, STEAM, is fed at the bottom of the fractionator (20th stage with on-stage convention). There is another steam stream, STEM1, used in the side stripper. The condenser runs at 15 psia with a

pressure drop of 5 psi. The tower pressure drop is equal to 5 psi. The distillate rate is 12000 bbl/day and the distillate vapour fraction in the condenser is 0.25 (StdVol basis). The liquid product, SID1, is withdrawn from 5th stage with a flow rate of 2000 bbl/day.

A hydrocarbon mixture with the given component-wise low rates (Table 3.6) f

3


enters the furnace at 120oF and 45 psia.

LIGHTS

WATER

IS

o

-

SID1

STEM1 -O FEED

STEM

FIGURE 3.118 A flowsheet of a petroleum refining column TABLE 3.6

Component

Flow rate (bbl/day)

c2

100

10

C

3

600 1800

n-C

4

7500 30000

1-0,

42000

nrCt

250

H 0

250

2

SID2

C>

BOT

O

ASHEN PLUS


185

The pomp around circuit (for cooling) and the side stnpper are specified with the following information (see Table 3.7). TABLE 3.7

Location

Specifications i

Pumparound

Draw

Return

Flaw rate

Temperature

idrauoff type)

stage

stage

(bbl/day)

feF,

8

6

40000

20

I (partial)

Location

Stnpper

Stnpper product

Draw

Return

Stripping

stages

stage

stage

steam

5

SID1

12

10

STEM1

No. of

1

Bottom product flow rate (bbl/day; 15000

Two steam streams, used in the column model, are described in Table 3.8. TABLE 3

Specifications Steam stream

Location

STEAM

STEM!

Temperature (8F)

Pressure (psia)

Flow rate Ob/hr)

Main tower

350

50

12000

Stnpper

350

50

5000

Selecting the PENG-ROB base method under RE FINE RV process type simulate the model using a PetroFrac column and report the flow rates (bbl/day > of all ,

product streams.


Rs. 295.00


Amiya K. Jana




Part II Chemical Plant Simulation

using Aspen Plus


Rs. 295.00


Amiya K. Jana




Aspen Plus Simulation of Chemical Plants

4 1 .

INTRODUCTION

In the last three chapters, we have studied in detail the simulation of individual processes, such as flash drum, dryer, chemical reactor, distillation column including petroleum refining process, absorber, stripper and liquid-liquid extraction unit, using

Aspen Plus

software. Here, by a 'chemical plant' we mean a chemical process

f

integrated with several single process units. The chemical process industries usually include flash chamber, mixer, splitter, heat exchanger, pump, compressor, reactor, fractionator, ilter and so on. It is easy to simulate even a large chemical plant by the use of Aspen software package. In the present chapter, the simulation of two chemical process flowsheets is discussed. They are a distillation train and a vinyl chloride monomer (VCM) manufacturing unit. After thoroughly reading this chapter and simulating the solved

examples in hand, we will be able to use Aspen Plus flowsheet simulator for solving a wide variety of chemical plants. To improve the flowsheet simulation skills, it is recommended to solve the problems given in the exercise. 4 2 .

ASPEN PLUS SIMULATION OF A DISTILLATION TRAIN

Problem statement

f

A hydrocarbon stream H is supplied at 50C and 2.5 atm. The pump Pi discharges the feed F at 10 atm. In Table 4.1 the component-wise low rates are tabulated for stream H.

The schematic representation of the complete process integrated with a pump and f

ive DSTWU column models (Cl, C2, C3, C4 and C5) is shown in Figure 4.1.

189

Copyrk

190

PROCESS SIMULATION AND CONTROL USING ASPEN TABLE 4.1 F/ouj rate (kmol/hr)

Component

10 35

50 130 200 180 200 5

n-C

.

pi

C1

C3

cs

C4

A lowsheet of a distillation train. f

FIGURE 4.1

C2

For Aspen Plus simulation of the distillation train, required information are given in Table 4.2. TABLE 4.2 Column

Condenser

Reboiler

(abbreviation)

pressure (aim)

pressure (atm)

Deethanizer (CD

9

9

Depropanizer (C2)

5

6

Deisobutanizer (03) Debutanizer (04)

4

4

3

3

Deisopentanizer (05)

2

2

All distillation models have total 20 theoretical stages (including condenser and reboiler) and a total condenser. For the light key (LK) and heavy key (HK), we expect 99.9% and 0.1% recovery, respectively, in the distillate of all columns. Using the PengRobinson property method, simulate the distillation train and report the compositions

of all distillation products. Simulation approach From the desktop, select Start button followed by Programs, AspenTech, Aspen Engineering Suite, Aspen Plus Version and finally Aspen Plus User Interface. Then

choose Template option in the Aspen Plus Startup dialog (see Figure 4.2).

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\SI'KN PWB

SIMULATION OP CHKMK \l PLANTS

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191

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1

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FIGURE 4.2

As wo hit OK button, the following window appears (sec Figure 13). Based on the units used in the problem statement we select General with Metric Uliits, ,

in-i

.

1»

1-

.

l-.

<.-

.

I

r< J

mil I-'

- .

J I"-

_

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FIGURE 4 3

192

PROCESS SIMULATION AND CONTROL USING ASPEN '

Press OK and obtain the Connect to Engine dialog. Select Local PC as Server type and click OK. Actually, this step is specific to our installation (see Figure 4.4). Connect to Engine Server type:

User Info Node name:

User name;

Password;

Working directory:

Save as Delaull Connection OK

]

ExB

Help

FIGURE 4.4

Creating flowsheet

The next screen represents a Process Flowsheet Window. Add a pump by selecting the Pressure Changers tab from the Model Library toolbar. Moreover, in the library, select the Columns tab and then choose DSTWU model to include five such columns

consecutively on the flowsheet. Notice that to incorporate a block click on the appropriate icon and then place the block on the process lowsheet by clicking with the

f

,

f

cross hairs somewhere on the lowsheet background. Right click to de-select the block. Now we need to interconnect the blocks and add the inlet as well as outlet streams. Select Material STREAMS on the left of the toolbar at the bottom In the next, as we .

move the cursor to the process lowsheet window several red and blue arrows appear around the blocks. The red arrows indicate required streams and the blue arrows are f

,

optional. In the previous chapters, we have learned how to connect the feed and product streams with a single block. Let us observe Figure 4.5 to know how to interconnect the two blocks by a stream.

f

Here, irst we wish to interconnect the pump PI with the column Cl using the feed stream F. Right-click with highlighting feed block select Reconnect Destination and ,

then move the cursor to click on an arrow that is fed to the column Cl.

m

QD-o

o

-

SOD-*

r PI

Cl

FIGURE 4.5

]-o

ASPEN PLUS

SIMU1.ATION OF CHEMICAL PLANTS

193

We can select Reconnect Source instead of Reconnect Destination if we modify Figure 4.5 to Figure 4.6.

j3L

D1

B1

c>

C1

FIGURE 4.6

f

f

By the same way, interconnect remaining blocks. Renaming all blocks as well as incoming and outgoing streams, finally we have the screen shown in Figure 4.7. To rename a particular stream (or block), irst select it, then right-click, next select Rename Stream (or Rename Block) and inally enter the appropriate name. Re

E«

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On

Took

tin

Uban

Whfen

Help

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kl

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o ai ,

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C

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FIGURE 4.7

The status indicator in the bottom right of the window, shown in Figure 4.7, says

Required Input Incomplete indicating that the process flowsheet is complete and input data are required to enter for running the simulation.

194


Configuring settings As we hit Next icon and then click on OK, the following window pops up (see Figure 4.8).

Remember that in the Data Browser, we need to enter information using data input f

forms at locations where there are red semicircles. As we inish a section, a blue

checkmark appears.

» r-

-i'iv -

life

,

FIGURE 4.8

It is always a good practice to represent a simulation problem with entering a title. In the Tattle field, enter 'Simulation of a Distillation Train'. Note that we may change the input/output data units under Units of measurement (see Figure 4.9).

I r

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i

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figur:

m 9

The next window (see Figure 4.10) includes the Aspen Plus accounting information as given below, required at some installations.

,

ASPEN PLUS

SIMULATION OF CHEMICAL PLANTS

195

User name: AKJANA Account number: IIT-KGP

Project ID CHEMICAL Project name: DT

1

,

o-

-

*

i S

MM

-

FIGURE 4.10

If we want the streams results summary sheet to display mole fractions select Report Options under Setup folder to the left Under the Stream tab select 'Mole' as Fraction basis (see Figure 4 11) ,

,

.

n fT7

-

i

i

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FIGURE 4.11

.

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196

PROCESS SIMULATION AND CONTROL

USING ASPEN


In the subsequent step, use the Data Browser menu tree to navigate to the Components/

Specifications sheet. It is shown in Chapter 1 how to define components in the component input form. Here, we have this table as shown tw

f

ffc E* *p.

in Figure 4.12.

To* R»

i r l-l .l- fT

>i -"Pi

g :.r.:

I

iai

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.

.

J O pOa**

_

.

i i r

id

5 _

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| > M
-J

FIGURE 4.12

Specifying property method In the list on the left

,

choose Properties/Specifications to obtain the property input form. f

A property method includes the models and methods to calculate the physical properties, such as vapour-liquid equilibrium coeficient enthalpy and density. For the example ,

plant, set PENG-ROB base property method by scrolling down (Figure 4 13). .

5*

C*, T**

P«

U

,

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I

tt*

3

-

-

.

J DM

3

PT

FIGURE 4.13

ASPEN PLUS


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197

Note that there is no compulsion to use only a single thermodynamic property method for all processes in a chemical plant. Aspen software provides an option to choose different property methods for different processes. To do so select Block Options! ,

Properties under a particular model of Blocks folder in the list on the left and then choose the suitable property method.

Specifying stream Information The Streams/H/Input/Specifications sheet appears with the Data Browser menu tree in the

left pane. Entering the given data for stream H, we obtain the sheet as shown in Figure 4.14. j T-i i-i r>

i-w

'

-

I'M

-

3 "

3

j. a.

-j],. ,.

j

t

ti»» .rtl.m '-if

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r w

sit.

FIGURE 4.14

Specifying block information As we hit Next button the block input form appears. The deethanizer column is specified ,

with the given data as shown in Figure 4.15. arn
& V- tm,

r

,

m

31i£iai _U *iB| 51 nl-mi*!
. _

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FIGURE 4.15

198


Subsequently, the filled input forms are shown in Figures 4.16(a), (b), (c) and (d) for other four DSTWU columns.

fir.

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Click on Afet and specify the pump (PI) outlet by providing the discharge pressure

of 10 atm (see Figure 4.17).

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FIGURE 4.17

The status bar in the window, shown in Figure 4.17, includes a message of Required Input Complete; it reveals that to run the simulator, sufficient data have been provided. But there is no such restriction that we cannot specify the process with more input information. Again, as we click on Next, Aspen Plus shows a message under the heading of Required Input Complete as shown in Figure 4.18.

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wni D>38

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9 -m *

ASPEN PLUS"' SIMULATION OF CHEMICAL PLANTS

201

Notice that if there are no red semicircles in the left it can be said that the data ,

entry for running the Aspen simulator is complete. Running the simulation

As we approve the simulation run, the Control Panel, displayed in Figure 4 19, .

shows

the progress of the flowsheet simulation in addition to a message o[ Results Available

.

.

1

.

-

B a B O B w Z

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93

us states; ~. Jt

n

tjxll huz - jjtkt:

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v. ex.

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FIGURE 4.19

Viewing results

f

Choose Results Summary /Streams in the column at the left side and obtain the compositions of all distillation products as shown in Figure 4.20. We may save the work by choosing File/Save As/...using the menu list on the top. W< tan give a name of the ile whatever we like. Note that if we click on Stream Table,

the results summary table is incorporated in the Process Flowsheet Window, as shown in Figure 4 21. .


If we wish to have the systematic input information, press Ctrl + Alt + I on the keyboard or select Input Summary from the View pulldown menu (see Figure 4*22). In order to create a report file (*.rep) for the present problem, we may follow the approach presented in Chapter 1 It is worthy to mention that the report file contains all necessary information on the solved Aspen Plus problem including given process .

,

data and computed results

202


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FIGURE 4.21

Copyhghied malarial

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203

|t«i1«t<«i of a DUinUtlon TrtU

'

voilM-CM otir*-!

MCW-aetrr i
for Inpm: Ho1«

Stream report ct

xnltiwi: «o1t fli SOLIDS

W-SDUKCS

CI C2M ( 1 ClHt

Wll

/ -QOWWS

SOLIDS

1MMCAMIC

INOtMMC

I

1C4 C4H10-2 i '

'

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/

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ukk ci

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u

FIGURE 4.22

43 .

ASPEN PLUS SIMULATION OF A VINYL CHLORIDE MONOMER

(VCM) PRODUCTION UNIT Problem statement

f

The process low diagram for Aspen Plus simulation of the vinyl chloride monomer manufacturing plant is shown in Figure 4 23. The lowsheet has been developed based on the VCM production technology reported in a book by Seider et al (1998). f

.

.

O-fcmi

O-feu

BB

66

B7

F10

9 [purge]-o

FIGURE 4.23 A flowsheet of a vinyl chloride monomer production unit

.

204

PROCESS SIMULATION AND CONTROL IISINO ASRKN

Pure ethylene, stored as a gas at 70oF and 1000 psia, with a flow rate of 20 tons/hr and pure chlorine, stored as a liquid at 70oF and 150 psia, with a flow rate of 50 tons/hr enter the mixer block Bl operated at 2 atm. The mixer outlet Fl then goes to the ,

reactor B2 run at 363 K and 1.5 atm. In this stoichiometric reactor (RStoic), the following chlorination reaction occurs with 98% conversion of ethylene to 1, 2-dichloroethane:

C2H4 + Cl2 -> C2H4C12 ethylene chlorine dichloroethane

In the next, mixer B3 operated at 1.4 atm allows the mixing of the recycled stream F12 with the reactor product F2. The outlet stream F3 is then condensed fully to liquid phase in block B4 at 298 K before being pumped to an evaporator. The pump B5 has discharged the liquid at 26 atm. The evaporator B6 performs the phase change operation and then the vapour temperature is increased in the same unit to 515 K. In the subsequent step, stream F6 is introduced in the reactor B7 (RStoic) in which the following pyrolysis reaction occurs: C2H4C12 -> C2H3C1 + HC1 dichloroethane

VCM

hydrogen chloride

The dichloroethane is converted to VCM and it takes place spontaneously at 773 K and 25 atm with 65% conversion. To reduce carbon deposition in the heat exchanger, the hot vapour stream leaving the reactor is quenched in block B8 yielding a saturated vapour stream at 443 K. Quencher effluent stream F8 is condensed to liquid phase in block B9 at 279 K and then fed to a DSTWU column B10 as stream F9. In the next

,

Stream F10 is introduced in another DSTWU column Bll. The first column mainly separates HC1 from other components, while the second column purifies VCM from the rests. Both the distillation columns have 10 theoretical stages (including condenser and reboiler) and a total condenser along with the specifications shown in Table 4.3. ,

TABLE 4.3

% Recovery of LK/HK in distillate Block

Light key (LK)

Heavy key (HK)

B10

99.9% of HC1

0.1% of VCM

Bll

99.9% of VCM

0.1% of dichloroethane

Pressure (atm) Condenser 20

Reboiler 22

75 .

8

Finally block B12 (FSplit) splits stream Fll to ensure the recycling of 99.999% of Fll as F12 stream to mixer B3. A purge stream is introduced to prevent accumulation of unreacted components. Using the POLYSRK property method simulate the complete plant to compute the ,

composition of all streams.

Simulation approach To start Aspen Plus package select Aspen Plus User Interface under Programs. When the Aspen Plus window pops up choose Template and click on OK. In the next, select Polymers with Metric Units (see Figure 4.24) and press OK button. ,

,

ASPEN PLUS

-

.

.

r .ii.-i.*.

Ti 3 i

:

;


. i i .rj 3 :

205

a

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FIGURE 4.24

Click OA" when the Aspen Plus engine window appears to obtain a blank Process Flowsheet Window.

Creating flowsheet

f

We can develop the process low diagram (see Figure 4.25) by incorporating the following

IpF -I- I

-

I- IT

MCI

I

m

ti

ED

413

? 4f

FIGURE 4.25

206


built-in process units available in the Aspen Plus Model Library: two mixers (Bl and B3)

two Stoic' type reactors (B2 and B7) four 'Heater1 type heat exchangers (B4, B6, B8 and B9) one Tump type pressure changer (B5) two T TWIT type columns (BIO and BID one TSplit type splitter (B12) 1

'

All the blocks and streams are renamed according to the problem definition. The status message directs us to provide the input information required to run the

f

complete Aspen Plus simulation program. In the subsequent sections, we will ill up several input forms one by one. Configuring settings

After creating the lowsheet for the VCM manufacturing unit hit Next button followed by OK to open the Setup /Specifications / Global sheet. In the following the irst screen shown in Figure 4.26(a), includes the Title of the present project as Simulation of a VCM Production Unit' and the next screen displayed in Figure 4.26(b), shows the Aspen Plus accounting information as given below ,

,

.

User name: AKJANA

Account number: SAY X

Project ID: ANYTHING Project name: AS YOU LIKE f

t*

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SIMUI.ATION OF CHEMICAL PLANTS

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FIGURE 4.26(b)

We wish to have streams results summarized with mass fraction basis that is not

set by default. Accordingly, we choose Mass' fraction basis in the Report Options/ Stream sheet under Setup folder (see Figure 4.27). '

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208



The components that are involved in the monomer manufacturing process are ethylene (C2H4), chlorine (CI2), 1,2-dichloroethane (C2H4C12), vinyl chloride (C2H:iCl} and hydrogen chloride (HC1). In order to get a blank component input form, choose Components/ Speciftcatiojis in the left pane of the Data Browser window. Defining all species in the Selection sheet, we have Figure 4.28. 1 Data

ma

(ComponaO Spooftoiioni . Dal a Bnnrasr)

Tods

fkn

Plot

Library

Window

Hdp

510*101

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Selection] Pelidftum | Noraiorivenliona! j-/Dalabanl s |

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Smjiaion Options Slrean- Class

Define comp«*nli

(

Conventtonal

Custom Unrts

CL2

Report C ions

C2H4CL2

Cemporwrti HCI

Lg l-End Propert.es

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Foirtiula

Comconent name

Type

Componerit ID ::h4

J/) UrUs-S«S Q

Tii.J J

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ETHYLENE

C2H4

Conventional

IHLORINE

ri;

Conventional

1 2-DICHL0R0ET

r :h4

Conventional

VINYL-CHLORIDE C2H3CL

Conventional

HYDROGEN CHLC HCI

.

.

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Petrc Cha cterzatoi FieudoccmocneiTts

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NUM

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FIGURE 4.28


In the subsequent step choose Properties/Specifications to set the property method. As mentioned in the problem statement accordingly select POLYSRK base method under POLYMER process type (see Figure 4.29). ,

,

Specifying stream information From the Data Browser

,

choose Streams folder and see the name of all input, output

and intermediate streams. However, we have to provide information for only two input streams, C2H4 and CL2 which are fed to the mixer block Bl. Figures 4.30(a) and (b) ,

show the filled stream input forms.

ASPEN PLUS

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SlMULVnON OF CHEMICAL PLANTS

.

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209

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.

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209

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PROCESS SIMULATION AND CONTROL USING ASI KN

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FIGURE 4.30(b)

Specifying block information Unlike stream information

,

we need to input required data for all blocks of the process

f

,

f

low diagram. As stated earlier the lowsheet of the VCM plant consists of two mixers, two reactors, four heat exchangers one pump, two distillation columns and one splitter. ,

f

Although discussed during the Aspen Plus simulation of different single process units in the preceding chapters, we must remember the following points when we ill up the block input forms.

To simulate a mixer model, at least provide the pressure data and valid phases. In the simulation of the reactor model coefficients should be negative for ,

reactants and positive for products In the Vapour fraction ield of a heater model put 0 to indicate bubble point f

.

,

and 1 to indicate dew point. For subcooled liquid and superheated vapour, use Flash specifications.

The windows, shown in Figures 4.31(a) to 4.31(n) display the block-wise information using the input forms.

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Viewing results

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220


SUMMARY AND CONCLUSIONS | In the previous chapters, we have studied the steady state simulation of a large variety of individual process units using Aspen Plus package. In the present chapter, several chemical

processes have been assembled to develop the chemical plants and those plants have

f

f

f

been simulated subsequently. The solved examples include a distillation train and a vinyl chloride monomer unit. In the second example, the loop is closed by a recycle stream, whether in the irst unit, there is no such complicacy. However, the straightforward approach to simulate a lowsheet is that after developing the process low diagram in the

f

lowsheet window of Aspen Plus, we can simply use Next button for data entry. As we receive the message of Required Inpu t Completey we can move on to run the simulation.

In the next two chapters, we will study the process dynamics and closed-loop control of f

low-driven as well as pressure-driven processes using Aspen Dynamics package.

PROBLEMS| f

1 A hydrocarbon stream with component-wise low rates, shown in Table 4.4, enters the isentropic compressor at 120oF and 1 atm. The compressor has discharged the vapour stream at 3 atm. TABLE 4.4

Flow rate (kmol/hr)

Component

10

95 150 n

-C4

25

/i-C3

10

n-C 6

100

f

The complete process lowsheet for lashing and stripping operation is shown in Figure 4.36. The lash drum (Flash2) runs at 1250F and 2.8 atm. The stripper (STRIP2) has total 6 stages (including condenser and reboiler) and bottoms to feed ratio (mole basis) is 0.8. The feed stream to the stripper is introduced above f

.

f

4

the top stage and the pressure throughout the column is 2 atm. V24

I

0

1

COMPRESS

FIGURE 4.36

FLASH

STHiPPER

A flowsheet for flashing and stripping operation

.

CopyHghlod material

ASPEN PLUS

SIMUIJUION OF CHKMICAL PLANTS

221

Using the UNIQUAC property method, simulate the plant to compute the product compositions and flow rates. 4

2 A ternary mixture, as shown in Table 4.5, is fed as stream H at 100oF and 290

.

psia to a pump Pi employed to increase 20 psi pressure. TABU 4.5

Component

Flow rale (Ibmol/hn 500 300

"

r 11,

10

The stripper (STRIP2) has total 100 stages (including condenser and reboilen with a reboiler duty of 107 Btu/hr Stream F enters above 70th stage and Stream R) mien above 1st stage. The top stage pressure of the stripper is 280 psia with a stage pressure drop of 0.5 psi The intercolumn pump P2 has increased 25 psi pressure The RECT column has total 120 stages (including condenser and reboileri with a reflux ratio (mole basis) of 10 and a bottoms to feed ratio

mole basis) of 0.6. Stream Dl enters below 120th stage. In the simulation, consider condenser pressure of 275 psia with a pressure drop of 5 psi and a stage pressure drop of 0.1 psi (see Figure 4.37). '

.

-

0

0

PI

FIGURE 4

37

.

8TRIP2

'

w"

A flowsheet of a propylene-propane mixture separation process

Applying the RK-Soavc thormodynamit mod* I

(a; simulate the above propylene propane mixer Beparation plant and report the product compositions, and (by perform the seneitivity aaalysifl to observe the effect of the second column

f

f

eficiency varied rom 20'/. to 10091 on the propylene mole fraction in the

distillate

222 4


3 The hydrogenation of aniline produces cyclohexylamine in a CSTR according to the following reaction:

.

CgHgNH;, + 3H2 -) CgHnNHs aniline

hydrogen

cyclohexylamine

To simulate the aniline hydrogenation process using Aspen Plus, we develop the process flow diagram as exhibited in Figure 4.38.

S-o

FA

C>-I ANILINE

El

PUMP

C>-| HYDROGEN I-I

CD-

-

CSTR

COMPRESS

FIGURE 4.38

A flowsheet for aniline hydrogenation.

f

The reactor model (RCSTR) operates at 580 psia and 2480F, and its volume is 1200 t3 (75% liquid). For the liquid-phase reaction, the inlet streams have the specifications, shown in Table 4.6. TABLE 4.6

Stream

Temperature (0F)

Pressure (psia)

Flow rate (Ibmol/hr)

ANILINE (pure aniline)

95

100

150

HYDROGEN (pure hydrogen)

12

100

600

Both pump and compressor (isentropic) have discharged the fluids at 585 psia. Data for the Arrhenius law are given as:

Pre-exponential factor = 5x 105 m3/kmol . s Activation energy = 20,000 Btu/lbmol [CJ basis = Molarity

f

Use the SYSOPO base property method in the simulation. The reaction is irstorder in aniline and hydrogen. The reaction rate constant is defined with respect to aniline. Simulate the process and compute the component mole fractions in the liquid product and the vent stream. 4

.

4 The process flow diagram for an azeotropic distillation process is shown in Figure 4.39. The technique involves separating close boiling components (acetic acid and water) by adding a third component (vinyl acetate), called an


ASPEN PLUS


223

entrainer, to form a minimum boiling azeotrope which carries the water overhead and leaves dry product (acetic acid) in the bottom. The overhead vapour is condensed and then separated in the decanter into two liquid phases: the organic phase and aqueous phase. DECANTER

1 VA-RICHh DIST VA

1 FEEDf

HW-RICHh AA

RADFRAC

FIGURE 4.39

A flowsheet of an azeotropic distillation process

.

A feed stream, namely FEED, enters above 15th stage of the azeotropic distillation and 90 psia in addition to the low rates, shown in Table 4.7. f

column at 330oF

TABLE 4.7

Component

Flow rate (Ibmol/hr) 2700

acetic acid

500

water

f

The entrainer, VA (vinyl acetate), with a low rate of 455 Ibmol/hr enters above 12th stage of the column at 200oF and 100 psia. The azeotropic column (RadFrac) has the following specifications:

Number of stages (including condenser and reboiler): 55 Condenser type: total Valid phases: vapour-liquid-liquid Reflux ratio (mole basis): 4 Bottoms rate: 2700 Ibmol/hr

Condenser pressure: 66 psia Column pressure drop: 12 psi Key component in the second liquid-phase: water Stages to be tested for two liquid-phases: 1 to 55 The specifications for the decanter model are noted below:

Pressure: 50 psia Temperature: 110oC

Key component in the second liquid-phase: water Using the NRTL RK thermodynamic model simulate the process to compute the -

,

f

component-wise product low rates.

4

.


5 A hydrocarbon stream H is at 50C and 2.5 atm. The pump has discharged the liquid feed F at 5 atm. The component-wise low rates are shown in Table 4.8 for f

224

stream H. TABLE 4.8

Component

Flow rate (kmol/hr) 35

C3

50

i-C, n-C

130 200

4

c5

180

n-C5

200

'

-

n-C

5

6

In Figure 4.40 the schematic representation of a hydrocarbon separation process integrated with a Pump, three DSTWU columns (Cl, C2 and C3) and two RadFrac (RECT) columns (CR1 and CR2) is shown.

DRl

DR2

CR2

CRT

BRI

G1

PUMP

BR2|-0

h C2

C3

B3 |

FIGURE 4.40

B2}<>

C1

A flowsheet of a hydrocarbon separation process

.

AH DSTWU fractionators have total 20 stages (including condenser and reboiler) and two RECT models have 10 stages (including condenser and reboiler) with no reboiler. The specifications, shown in Tables 4.9(a) and (b) are required for simulating the process. ,

«PEN PLUS

SIMULXTION OF CHKMICAL PLANTS

225

TABLE 4.9(a) *

Block

Recovery of LK/HK in distillate

Lighi key

Heavy kev

IK

(HK)

Cl

99

of r»-C4

C2

99* of t-C4

C3

99* of 1-C5

Pressure (atm)

Condenser (type)

Reboiler

1% of i-CB

4 (partial condenser with all vapour distillate)

21 of n-C4 4* of n-C5

1 5 (total condenser)

15

3 (total condenser)

3

4

.

.

TABLE 4.9(b) Condenser

Distillate to feed ratio

Pressure

(type)

(mole basis)

(atm)

CR1

Partial vapour

02

2

CR2

Total

05

15

Block

.

.

Applying the Peng-Robinson property method, simulate the separation process to compute the flow rates and compositions of all product streams. 6 An inlet Stream H supplied at SOT and 300 psia is compressed to 4000 psia by the use of an isentropic compressor Bl. Stream H has component-wise flow rates,

.

shown m Table 4.10. TABLE 4.10

Flow rate (Ibmol/hr)

Component mtrogen

100

hydrogen

300

ammonia

0

carbon dioxide

1

A low diagram for the ammonia process (Finlayson 2006) is shown in Figure 4.41. ,

f

B '

m-< H

-

0 ED-0 Bl

B2

FIGURE 4 41

B3

84

A lowsheet of an ammonia process f

4

H"

226


f

Stream Fl is mixed with the recycle stream F8 in a mixer block B2 operated at 4000 psia. Before introducing into the reactor, the mixer efluent F2 is heated in

block B3 to 900oF at 4000 psia. Note that the reactor (RGibbs) B4 runs at 900oF and 3970 psia. In the next, the reactor outlet F4 is cooled in a heat exchanger

B5 operated at 80oF and 3970 psia. The flash drum (FIash2) B6 produces Streams

f

Bl and F6 at 80oF and 3970 psia. In the subsequent step, Stream F6 enters the splitter (FSplit) B7 and 0.01% of it is used as purge. Finally, an isentropic compressor B8 has discharged Stream F8 to the mixer block B2 at 4000 psia. Using the NRTL thermodynamic model and the Newton's iteration method (from the Data Browser, choose Convergence/Conu Options), simulate the ammonia process to compute the component-wise low rates and compositions of all streams.

REFERENCES | Finlayson, B.A. (2006), Introduction to Chemical Engineering Computing, 1st ed., Wiley Interscience, New Jersey.

Seider, W.D., J.D. Seider and D.R. Lewin (1998), Process Design Principles: Synthesis, Analysis, and Evaluation, 1st ed., John Wiley & Sons, New York.


Part III

Dynamics and Control using Aspen Dynamics

1

CHAPTER

Dynamics and Control of Flow-driven Processes

51

INTRODUCTION

.

Dynamic -imulation of a chemical process greatly helps to understand the transient , which is tightly integrated with Aspen Plus is widely used or process design and control. This powerful simulator can automatically initialize the dynamic simulation using the steady state esults of the Aspen Plus simulation Interestingly, when the ile containing the lowsheet is opened in Aspen Dynamics the default control structures are already installed on some loops Usually, level, pressure and temperature controllers are included where appropriate However these default control schemes can be modified or even eplaced with other suitable control loops available in Aspen Dynamic- package Note that there is a scope to include some additional controllers for the used process Moreover this simulation tool provides a graphical environment to show the process response. To convert a steady state simulation into a dynamic simulation there are several items that should be taken care of For example the size of all equipments must be specified and the control structures must be devised For steady state simulation using Aspen Plus the size of the equipment is not needed, except for eactors. On the other

behaviour Aspen Dynamics f

,

,

f

f

r

.

.

r

,

,

,

,

r

,

hand

,

for dynamic simulation using Aspen Dynamics, the inventories of material

contained in all the piece* of equipment affect the dynamic response Therefore, the .

f

physical dimensions of all process units must be known. When the steady state Aspen Plus simulation is exported into Aspen Dynamics, we need to choose either simpler low-driven dynamic simulation or more rigorous pressuredriven dynamic simulation Pres ure-driven simulations include pumps and compressors where needed to provide the required pressure drop for material low Control valves must be installed where needed and their pressure drops selected For flow-driven

f

.

-

,

simulations

,

however, no such arrangements are required. 229

230


f

In the present chapter, we wish to study the dynamics and control of the lowdriven processes. For this intention, we choose a reactor (RCSTR) as well as a distillation

f

column (RadFrac) example rom the model library of Aspen simulator. 52 .

DYNAMICS AND CONTROL OF A CONTINUOUS STIRRED TANK

REACTOR (CSTR) Problem statement

f

Ethyl acetate is produced in an esteriication reaction between acetic acid and ethyl alcohol. acetic acid + ethyl alcohol

ethyl acetate + water

A feed mixture, consisting of 52.5 mole% acetic acid, 45 mole% ethyl alcohol and 2 5 mole% water, enters the RCSTR model with a low rate of 400 kmol/hr at 750C and 1

.

f

.

1 atm. The reactor, as shown in Figure 5.1, operates at 70oC and 1 atm.

FIGURE 5.1

A flowsheet of a CSTR

Both the reactions are first-order with respect to each of the reactants {i.e., overall second-order). For these liquid-phase reactions, the kinetic data for the Arrhenius law are given below: Forward reaction: A = 2.0 x 108 m3/kmol s S = 6.0 x 107 J/kmol Reverse reaction: k = 5.0 x 107 m3/kmol . s £ = 6.0 x 107 J/kmol

Composition basis = Molarity Here, k is the pre-exponential factor and E represents the activation energy. The reactor

geometry data are reported below.

f

Vessel type: vertical Head type: lat Diameter: 0,45711 m

Volume: 0.15 m3

(a) Simulate the reactor model using the SYSOP0 thermodynamic model to compute the product compositions.

Copyrighied malerial

)YNAMICS AND CONTROL OF PI.OW-DHrVKN PROCKSSKS

231

(b) Report the default controllers tuning parameters and control actions used, and constraints imposed on variables. (c) Investigate the servo performance of the default liquid level and temperature control algorithms and discuss the effect of loop interaction.

(d) Show the regulatory behaviour of both the controllers in presence of disturbance in feed temperature. Simulation approach (a) To open the Aspen Plus Startup dialog box. click the desktop Start button, then point to Programs, AspenTech, Aspen Engineering Suite, Aspen Plus Version and then click the Aspen Plus User Interface. Let s select the option with Template and then click OK (see Figure 5.2). '

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1 I

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bHft mm

rt m mots; am -

1

;

:_ tm

FIGURE 5.2

As the next window appears (see Figure 5.3), it is appropriate to select General with Metric Units and hit OK button.

Here we use the simulation engine at 'Local PC When the Connect to Engine dialog pops up (see Figure 5.4), press OK. Note that this step is specific to the installation. Creating flowsheet f

The process low diagram, shown in Figure 5.5, includes a reactor, namely RCSTR, with an incoming FEED stream and an outgoing PRODUCT stream.

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Hitting Next button, we get Global sheet of the Specifications form under Setup folder in the left pane of the Data Browser window. Enter the Title of the present problemDynamic Simulation of a CSTR'. change the Input mode from 'Steady-State' to "Dynamic" and leave the remaining items at their defaults. The window looks like Figure 5.6. "

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In the next window, as shown in Figure 5.7, the Aspen Plus accounting information required at some installations are provided.

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In the subsequent step (see Figure 5.8), select Stream sheet with opening the Report Options form under Setup folder and include Mole fraction item. E*

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Specifying components In the Data Browser window, choose Components /Specifications to obtain the component input form. Filling out the table with the components (acetic acid, ethanol, ethyl acetate and involved in the present reaction system, the screen looks like Figure 5.9.

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Use the Data Browser menu tree to navigate to the Streams/FEED I Input I Specifications sheet. Specifying the FEED stream by its temperature, pressure, flow rate and composition, we have this window, shown in Figure 5.11.

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In the next step (Figure 5.13), select RCSTR I Dynamic! Vessel sheet under Blocks

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The status indicator in the above window reveals by the message Required Input Complete that no more input specifications are required to run the simulation

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dialog box appears Hitting OK on the message, we are displayed the Control Panel where the simulation messages during the run are recorded (see Figure 5.16). .

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Viewing steady state results

In the next, select Solver Settings, choose Results Summary /Streams in the list on the left and inally get the steady state results as shown in Figure 5 17. f

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(b) Exporting dynamic simulation: In the subsequent stage (see Figure 5.18), we wish to carry out the simulation dynamically. Accordingly, at this moment, we have to follow the sequential steps noted below:

Click on Export from the pulldown File menu or simply press Ctrl+E on the keyboard.

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Open the Drive and then Folder where we want to save the work as a ile. .

RCSTR' in the File name ield. f

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Choose 'Flow Driven Dyn Simulation (*.dynf & *dyn.appdf)' rom the options available in the Save as type box. Finally, hit Save button.

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the same folder within which the exported dynamic simulation ile is saved. Originally f

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many iles are saved along with the backup or dynamic ile. Anyway, we are now ready to run Aspen Dynamics and we may quit Aspen Plus.


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Starting Aspen Dynamics As we click the Start button

point to Programs, then AspenTech, then Aspen Engineering Suite, then Aspen Dynamics Version and then click on Aspen Dynamics a blank dynamic simulation window appears as shown in Figure 5.19 ,

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Opening existing simulation

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DYNAMICS AND CONTROL OF FLOW DRIven PROCESSES

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Details of the two control loops, to be used inally, are given below

.

Loop 1 Controller: LCI

Type of controller: proportional (P) only Controlled variable: reactor liquid level Manipulated variable: product flow rate Controller action: direct

Loop 2 Controller: TC2

Type of Controller: proportional integral (PI) Controlled variable: reactor temperature Manipulated variable: heat duty (cooling operation) Controller action: reverse

Note that the direct acting control system increases the output signal as the input signal to the controller increases. On the other hand, as the input signal to the control structure increases, the output signal from the controller must decrease for the case of

reverse acting control strategy. The direct acting control law has negative gain and increase/increase (or decrease/decrease) term is commonly used to represent it For the reverse action, increase/decrease (or decrease/increase) term is used and controller gain .

has positive sign.

The reactor flowsheet includes two (LCI and TC2) single-input/single-output (SISO) control loops. Therefore we can say that this is a multi-input/multi-output (MIMO) or ,

simply a multivariable closed-loop system In Aspen terminology the process variable or controlled variable is denoted by PV, .

,

the set point is represented by SP and the controller output or control variable or manipulated variable is abbreviated by OP For the example CSTR system level and temperature controllers are automatically implemented when the Aspen Dynamics simulation is created The default values for .

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SP

,

PV and OP are computed from the steady state simulation. To achieve better closed-

loop process response the Aspen-generated control structures can be modified or even replaced by the suitable control schemes available in the control library of Aspen ,

software

.

In addition, the default values for controller tuning parameters, such as gain,

integral time derivative time and so on, can also be changed. ,

,

Most of the control strategies are easily tuned by simply using heuristics. As suggested by Luyben (2004) all liquid levels should use P-only controllers with a gain of 2. All flow controllers should use a gain of 0.5 and an integral time of 0.3 minute also enable iltering with a ilter time of 0.1 minute). The author also mentioned that ,

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the default values in Aspen Dynamics for most pressure controllers seem reasonably well But temperature controllers often need some adjustments.

to work

.

Viewing default values of variables In Aspen Dynamics, the steady state values of process variable and controller output ar displayed in a table At this stage the set point value, displayed in table, shown in .

,

244


Figure 5.22, is same with the value of process variable. To show the results table of r

loop I, highlight the controller block LCI, press the ight mouse button, go to Forms and then select Results. ' -

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We can have the same information in a faceplate, shown in Figure 5.23, simply by

double-clicking on the block LCI. But as a difference, the units are not mentioned here with the values of SP. PV and OP. ;i J

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DYNAMICS AND CONTROL OF FLOW DRIVEN PROCESSES -

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Similarly, we have the results table, shown in Figure 5 24 for the temperature loop 2. .

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Modifying controller tuning properties

First we need to open the sheet that contains the controller tuning information. To do so for the level controller, highlight the controller block LCI, press the right mouse button, point to Forms and then select Configure (see Figure 5.25).

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PROCESS SIMULATION AND CONTROL USING ASPEN"1

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Alternatively, to obtain the Configure dialog box, irst double-click on the controller block LCI and then click on Configure symbol (yellow colour) in the faceplate as shown in Figure 5.26. I

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By the similar way, we obtain the tuning data sheet, shown in Figure 5.27, for the temperature controller TC2.

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Note that the default Operator set point value is the steady state value of the process The reactor liquid level is the PV for loop 1 and reactor temperature for loop 2. Bias signal is the output rom the controller when the error (= SP-PV) is zero. From the results tables shown earlier, it is obvious that the error is zero for both loops. f

variable (PV).

Therefore, Aspen Dynamics has set the value of OP as the bias value. The proportional integral (PI) control methodology is automatically installed with default values for the controller gain (= 10 %/%), integral time (= 60000 minutes) and derivative time (= 0 minute) to monitor the reactor level. However, as mentioned

previously, the proportional-only controller with a gain of 2 is sufficient to effectively control the liquid level. Remember that to make the integral action inactive, we can

use a very large value, for example 105 minutes (even the default value of 6 x 104 minutes may also be accepted), for the integral term. For loop 1, the controller action should be Direct' as set by default (see Figure 5.28). '

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In loop 2

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reactor temperature.

The TC2 is truly controller tuning

In data sheet, shown in Figure 5.29, the default values are g

a reverse acting controller. However, we may adjust the

parameters (gain and integral time) during the

the control performance is not satisfactory.

values of closed-loop study if

Modifying ranges for process variables and controller outputs In the Configure dialog box, hit the Ranges tab and get Figure 5.30 for level control loop.

PROCESS SIMULATION AND CONTROL USING ASPENIM

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As shown in Figure 5.30, the default ranges for both the process and output variables

are too large (± 100% of the steady state values) It may be practical to consider the following constraints. .

Process variable

Range minimum: 0.6855 m (25% subtracted from steady state value of PV) Range maximum: 1.1425 m (25% added with steady state value of PV) Output

Range minimum: 15812.7 kg/hr (25% subtracted from steady state value of OP) Range maximum: 26354.5 kg/hr (25% added with steady state value of OP) Entering these upper and lower bounds, we have the window, shown in Figure 5.31, for the level controller. Ul«l Ut»

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FIGURE 5.31

Again the typical ranges for the temperature control loop are noted here. Process variable

Range minimum: 52 50C (25% subtracted) Range maximum: 87 50C (25% added) .

.

Output

Range minimum: 1 1447 MMkcal/hr (25% subtracted) Range maximum: 0 6868 MMkcal/hr (25% added) .

-

.

The corresponding Aspen Dynamics window is shown in Figure 5 32. It is worthy to mention that the negative value of heat duty reveals the cooling operation (heat removal). .

250


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FIGURE 5.32

Both the control algorithms are completely specified above. In the next, the controller performance will be examined in terms of set point tracking (servo) and disturbance rejection (regulatory). (c) Starting the Run: Before running the program, we must be accustomed with some frequently used items of the toolbar as described in Figure 5.33. Step *c

601

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Re-start Simulation

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FIGURE 5.33

Rewind to a saved Snapshot


251

We wish to carry out the simulation for a certain time, say 5 hours To ix up this f

.

time period, select Pause At from the Run pulldown menu or simply press Ctrl+F5 on f

the keyboard. Then select Pause at time, type 5 in the ield or whatever we want and click on OK (see Figure 5.34). yrr&ntmsmfmsssi a

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Viewing servo performance of LC1

f

As we double-click on LCI block in the lowsheet, first the faceplate appears. In the

next, press on Configure and Plot symbols in the faceplate. Alternatively, to open the faceplate Configure dialog box and ResultsPlot dialog box, first select LCI block, then choose Forms and subsequently press one-by-one on faceplate Configure and ResultsPlot, .

,

respectively Judiciously arrange all three items within the Aspen (see Figure 5 35) so that we can properly observe them together. .

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Dynamics window

252

4- PROCESS SIMULATION AND CONTROL USING ASPEN

First make sure that all the items in the Configure dialog box and faceplate are correct. In order to execute the dynamic closed-loop simulation click on Run button in ,

the toolbar. During the simulation run, give a step change in the set point value

of

reactor liquid level from 0.914029 to 1.1 metre at time = 1 5 hours. Typing the new set .

point value in the faceplate, press Enter button on the keyboard so that the Operator set point value in the Configure dialog box also changes automatically to 1 1 .

meter

Note that the new set point must be within the specified ranges of PV In Figure 5 36 the servo performance of the level controller is depicted for 5 hours as selected earlier Obviously, the plot also includes the manipulated input profile ,

.

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FIGURE 5.36

Figure 5.36 represents an excellent set point tracking performance of the level controller (P-only). Obviously, the LCI provides process responses with almost no

deviation from the desired set point value and with very fast approach to reach the target liquid level.

Notice that the above plot can be edited by right clicking on that plot and selecting

Properties option or by clicking on that plot and pressing Alt+Enter on the keyboard. In the properties window, user can modify the title, axis scale, font and colour of the plot. Alternatively, double-click on the different elements of the plot and modify them as we like to improve the clarity and overall presentation.

Now, we will discuss the interaction of two control loops. When we introduce a set point step change in the reactor liquid level, the LCI scheme attempts to compensate ll

f

f

for the changes through the manipulation of the efluent low rate. This, in turn, wi disturb the reactor temperature and loop 2 will compensate by manipulating the hea

t

DYNAMICS AND CONTROL OF FLOW DRIVEN PROCESSES

253

-

removal of the CSTR appropriately. Thus we can say that loop 1 affects loop 2. In Figure 5.37, Aspen Dynamics window demonstrates the loop interaction under the same set point step change (0.914029 to 1.1 metre at time = 1.5 hours) as considered previously .

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Viewing servo performance of TC2

As described in Figure 5.38, open the faceplate along with Configure dialog box and a

blank plot sheet. Before starting the simulation run, carefully check all entries in the faceplate as well as Configure dialog box. In the next, choose Initialization run mode in the toolbar and then run the program once. After completion, go back to Dynamic mode from Initialization mode (see Figure 5.38).

ehj

f

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FIGURE 5.38

Now we wish to conduct the servo performance study for the TC2 controller with two consecutive set

point step (pulse input) changes in reactor temperature (70 -) 750C at

time = 1 2 hours and then 75 -> 70oC at time = 3 hours). .

254

PROCESS SIMULATION AND CONTROL USING AS PEN

Clearly, the proportional integral controller with default tuning parameters values shows a high-quality temperature tracking performance. As stated if the performance of any controller is not satisfactory, we have the option to tune the parameters simply by trial-and-error method. If we introduce a set point change in the reactor temperature the TC2 controller ,

,

takes necessary action with adjusting the heat duty to compensate for the changes

.

But interestingly, the liquid level remains undisturbed. Figure 5.38 confirms this fact

.

At this point we can conclude that loop 1 affects loop 2, but loop 2 does not affect loop 1 Actually here the interaction is in a single direction. (d) Viewing regulatory performance of LCI and TC2:

.

To perform the

regulatory study, we need to introduce at least a single change in the input disturbance. However, here we consider two subsequent step changes in the feed temperature. Initially, the feed temperature changes from 75 to 80oC at time = 2 hours and then the temperature (80oC) returns to 750C after 1.2 hours To change the feed temperature twice as prescribed above, irst we need to open the feed data sheet by double-clicking on the FEED block in the process flowsheet (see Figure 5.39).

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In the subsequent step, run the program with Initialization run mode. As it is inished, go back to Dynamic mode. Then, open the plot sheets for both the controllers. The regulatory behaviour is illustrated in Figure 5.40 giving changes in feed temperature

255

DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES in the feed data sheet. For brevity, the faceplate and configure dialog box included in the Aspen Dynamics window, shown in Figure 5.40. 2

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It is obvious that the reactor liquid level remains unchanged with a change in feed temperature since there is no interaction involved On the other hand, the reactor temperature is disturbed However the TC2 controller provides satisfactory disturbance .

.

,

rejection performance under this situation. So far we have studied mainly the closed-loop behaviour of a reactor system coupled

with Aspen-generated control schemes. We did not include any additional controller with the CSTR model In Section 5.3 we consider a distillation example to elaborate this point. .

,

5 3 DYNAMICS AND CONTROL OF A BINARY DISTILLATION COLUMN Problem statement

A partially vaporized binary mixture of benzene and toluene enters a RadFrac distillation model as displayed in Figure 5 41. .

he column has total 25 theoretical stages (including condenser and reboiler) and

operates at a pressure in the reflux drum of 18 psia and reboiler of 21 psia. The ow rate is 285 Ibmol/hr and reflux ratio is 2 2 (mole basis) .

.

256

PROCESS SIMULATION AND CONTROL USING ASPEN Feed Specifications o

TOP


Temperature = 225° F

Pressure = 21 psia Feed stage = 13 (above stage)

FEED

Mole %

Component benzene

45

toluene

55

FIGURE 5.41

BOTTOM

A flowsheet of a distillation column

.

In Table 5.1, the reflux drum and the base of the column (the 'sump' in Aspen terminology) are specified. It is fair to use an aspect ratio (length to diameter ratio) of 2 (Luyben, 2004). TABLE 5.1 Item

Vessel type

Head type

Height / Length (ft)

Reflux drum

horizontal

elliptical elliptical

5

25

5

25

Sump

-

Diameter (ft) .

.

The column diameter is 5 ft. Use default values for other tray hydraulic parameters (e.g., tray spacing, weir height and weir length to column diameter ratio). Consider logmean temperature difference (LMTD) assumptions for the total condenser. Actually the LMTD is calculated using the temperatures of process luid and coolant In the simulation. assume constant reboiler heat duty and apply the UNIFAC base property method. ,

f

.

Simulate the column model to obtain the products mole fractions. Keeping the default level and pressure control algorithms unaltered, inspect the servo as well as regulatory performance of a proportional integral (PI1 controller that is required to insert to control the benzene composition in the distillate by manipulating the reflux low rate. (0 Devising an another PI control scheme to maintain the benzene composition in the bottom product with the adjustment of heat input to the reboiler, observe the interaction effect between the top and bottom composition loops. f

(a) (b)

Simulation approach

(a) Select Aspen Plus User Interface and when the Aspen Plus window pops up. choose Template and press OK. In the subsequent step, select General with

f

English Units and hit OK button. To open the process lowsheet window, click OK when the Aspen Plus engine window appears.

Creating flowsheet

From the Model Library toolbar, select the Columns tab. Place the RadFrac model on f

the lowsheet window and add the feed as well as two product streams. Renaming all the streams along with distillation block, we have Figure 5.42.

DYNAMICH AND rnNTKOI, OP KI-OW DIUVKN I'lfOCKHHKH

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Ah we hit Nt'xf followed by OK button, the ROtUp input Corni appcarH (h«m* Kitfun!

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258

PROCESS SIMULATION AND CONTKOI, IISINC ASPEN

Figure 5.44 includes the Aspen I'lus (iccon/ilin Accounting sheet with any name, number and ID.

infornuition We can fill up the .

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We like to see the composition of all incoming and outgoing streams in mole raction basis in the inal results table. Accordingly, we use Stream sheet under the Report Options of Setup folder (see Figure 5.45). rif

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259

Specifying components '

From (In- Data Brow.scr, hoIoc! ( oniponcnta/Speeipcctions to open the componont input lorm In ill'' lahlc. shown in l- i mc S 1 (> Ihc Ivvo species are dclincd '

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choose Properties/Specifications and gel the property Inpul form. a property method originall} Includes several models for calculating the physical properties For the distillation example, set the UNIPAC base method b> in Aspen simulation

.

,

scrolling down (see Figure 5.47). Specifying stream Information ,"

next, (.pen Streama IFEED IInput ISpecifications sheel Entering the given

Values lor all State variables and teed eompo Figure 6 'IH

ion Ihe slream mpnl lorm looks like .

Specifying block information ,"

lefl pane ol the Data Browser window select Blocks IRADFRACI Setup to open Configuration sheet and then insorl the required datn (see Figure 5.49) ,

PROCESS SIMULATION AND CONTROL USING ASPEN >

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DYNAMICS AND CONTROL OK KLOW-DKiVEN PROCESSES

261

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In the subsequent step (see Figure 5.50), fill up Streams sheet with informing t\ location 113th tray (above stage)!. M

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262

PROCESS SIMULATION AND CONTROL USINKi ASPEN

In Figure 5.51, the column pressure profile is defined. Re

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Entering heat transfer data for condenser and reboiler Next select Dynamic under RADFRAC of Blocks folder. There are three heat transfer options: constant duty, constant medium temperature and LMTD. As mentioned in the

f

problem statement, the condenser heat duty depends on the log-mean temperature differential between the process luid and the coolant. The coolant inlet temperature is set constant. Here the temperature approach represents the difference between the process temperature and the coolant outlet temperature at the initial steady state Note that among the heat transfer specifications the coolant inlet temperature and temperature approach may vary during a dynamic simulation whereas the specific heat capacity of the coolant is ixed during a dynamic run (see Figure 5.52). ,

f

,

For the reboiler

,

simulation. However

it is fair to use constant heat duty computed in the Aspen Plus the reboiler duty may be changed at dynamic state either by

,

manually or automatically with employing a controller (see Figure 5.53).

Entering geometry data for reflux drum and sump

The reflux drum and sump are specified in Figures 5.54(a) and (b) with their given geometry data. The information on vessel orientation, head type, length (or height) and diameter are used to compute the vessel holdup

.

DYNAMICS AND CONTROL OF FLOW DmVKN PROCKSSP S -

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Entering tray geometry

The example column has total 25 stages-Stage 1 being the condenser and Stage 25 the reboiler. We already have inserted the necessary information for stages 1 and 25. Now, we need to inform the simulator the tray geometry specifications for stages 2 through 24. Note that the tray holdups are computed using these geometry data (see Figure 5.55).

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Hit Afecf button and press OK to run the steady state simulation. Finally, the result." table

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Open a blank dynamic simulation window for the example column, following a similar procedure as previously shown for the CSTR problem. In the next, simply open the flow-driven dynamic ile 'Ch5 5 3 RadFrac.dynf. As a result, the Aspen Dynamics window appears (see Figure 5.57) accompanying with the closed-loop process low f

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diagram. The flowsheet actually includes the three default control schemes LCI, PC2 and LC3 to monitor the reflux drum liquid level, top stage pressure and column base liquid level, respectively.

In the present discussion, we do not want to change anything of the three automatically inserted control strategies. All data, including timing parameters, ranges,

bias values and controller actions, remain untouched. A little detail of these control structures is given below. Loop 1 Controller: LCI

Type of controUer: P-only (since integral time is very large (60000 minutes)) f

Controlled variable: liquid level in the reflux drum Manipulated variable: distillate low rate Controller action: direct


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f

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Adding a new PI controller for top composition loop

Now we wish to include a proportional integral (PI) law to control the benzene composition in the top distillation product by manipulating the reflux rate. In the top left of the window the Dynamics library is included within Simulation folder of Ml Items pane Click on expand (+) button of Dynamics subfolder. Consequently, the expand ,

.

button changes to collapse (-) button as shown in Figure 5.58.

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Again hit expand button next to the ControlModels icon Then select PID controller .

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drag it to the low diagram and drop the control block near to the top product stream Renaming the top composition controller as CCT we have Figure 5.59. ,

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Ixpand Stream Types under Dynamics subfolder and hold down the mouse button he ControlSignal icon. As we drag it onto the lowsheet window, many blue an

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Hit OK button and obtain Figure 5.62. Obviously the CCT controller is partially configured. To complete the top composition loop, the controller output should be ,

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Connecting controller with control variable (Manipulated variable)

f

Again hold the ControlSignal icon, drag it onto the process lowsheet and drop it on

the blue outgoing arrow marked OutputSignal from the CCT block. As Select the

Control Variable dialog box appears (see Figure 5.63), choose 'CCT.OP' by name and press OK.

f

Immediately, a solid black arrow representing the controller output signal is automatically generated. Move the mouse pointer to reflux stream and make a connection to InputSignal2 port. To use the reflux low rate as control variable, select BLOCKSC'RADFRAC). Reflux.FmR' in the dialog box and click OK (see Figure 5.64). '

Now the binary distillation column is coupled with four control schemes, LCI, PC2, LC3 and CCT, and the closed-loop process looks like Figure 5.65. The subsequent discussion includes the modification of different tuning properties of the CCT controller.

DYNAMICS AND CONTROL OK FLOW DRIVKn PROCESSES

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First we wish to see the default tuning properties. So double-click on the CCT block and then hit Configure symbol in the faceplate to open the Configure dialog box (see Figure 5.66). ,

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Obviously, some of the default values set by Aspen Dynamics are not acceptable. For example, the operator set point value of process variable (benzene composition in distillate) should not be greater than L Secondly, the CCT controller action must be

Reverse'. In addition, the value of control variable (reflux flow rate) at steady state is

'

usually used as

bias value.

We have two options in our hand to correct the default values. Either manually we can do it or Aspen Dynamics can automatically initialize the values of set point process variable, control variable, bias and ranges. Note that the controller action is ,

changed only manually. It is wise to initialize the values by the help of Aspen Dynamics For this, press Initialize Values button in the Configure dialog box and use 'Reverse' .

controller action. It is obvious in the window, shown in Figure 5.67, that the values of SP, PV and OP in the faceplate change automatically to their steady state values. If

this approach fails to initialize the simulation of controller model with the steady state data, check and replace, if necessary, the values of PV and OP with their steady state values by double clicking on signal transmission lines (input to the controller and output from the controller).

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Process variable

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It is important to mention that it is a good idea to carry out Initialization as well

as Dynamic run after each new change in the control scheme so that any error in controller installation can be detected individually.

We have now completed all required control specifications for the top composition loop In the ongoing study, we prefer to conduct the simulation experiment to observe the designed controller performance continuously for 5 hours. As done for the previous f

CSTR problem, similarly either simply press Ctrl+F5 on the keyboard or select Pause At rom the Run menu and put 5 hours as Pause at time.

In the next, we will inspect the CCT controller performance first dealing with the servo problem followed by the regulatory problem. Viewing servo performance of CCT f

As we double-click on the CCT controller block in the lowsheet window the faceplate appears. Then open the Configure as well as ResultsPlot dialog box The second one is ,

.

basically a blank graph sheet that presents the variations of process variable set point and controller output with respect to time. Before running the program make sure that all the items in the Configure dialog box ,

,

and faceplate are correct. In the next hit Run button to start the dynamic simulation. The plots, shown in Figure 5 70, illustrate the servo behaviour of the PI control algorithm with a step increase (0 9437 - 0 97 at time = 1.51 hours) followed by a step decrease (0 97 - 0 9 at time = 3 hours) in the set point value of the distillate composition of benzene. To achieve an improved closed-loop performance we have used the values of proportional ,

.

.

,

gain of 10 %/% and integral time of 10 minutes. These values have been chosen based on a pulse input test in the distillate composition of benzene and using the trial-anderror approach It should be kept in mind that the objective at this point is not to come

up with the best control structure or the optimum controller tuning. We only need a control scheme and tunings that provide a reasonably good tracking performance to drive the simulation to a new steady state.

Remember that to edit the plots, shown in Figure 5 70, double-click on different elements of the plots and modify them as we like. Viewing regulatory performance of CCT

In order to investigate the regulatory performance of the CCT controller, we give a step

input change in the feed pressure (21 -» 23 psia) at time = 1.48 hours and that in the feed temperature (225 -» 230oF) at time = 3 hours. The PI controller tuning set provides good

disturbance rejection performance (see Figure 5.71) although the tuning parameter values . gain and integral time) have been chosen based on a pulse set point input change.

276

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(c) Adding a new PI controller for bottom composition loop: We have to devise another PI control scheme to monitor the bottoms composition of benzene

by adjusting the heat input to the reboiler. As developed, the CCT controller for the top loop, similarly we can configure the CCB controller for the bottom loop as shown in Figure 5.72.

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In addition, the used constraints are reported below: Process variable

Range minimum: 0.0 Range maximum: 0.1 Output

Range minimum: 6000000 Btu/hr Range maximum: 18000000 Btu/hr Viewing interaction effect between two composition loops

To observe the effect of interaction between two composition loops, the set point value of bottoms composition of benzene has been changed twice. The simulation result is depicted in Figure 5.74 for a step increase (0.0033 -> 0.0045 at time = 1.5 hours) followed by a step decrease (0.0045 -> 0.0025 at time = 3 hours). EC

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Clearly, the CCB controller shows satisfactory set point tracking performance against a pulse input change It is observed rom Figure 5.74 that owing to strong f

.

interaction between the two composition loops of the distillation column the set point changes in bottom loop affect the top product composition Similarly, when any set point change is introduced in the top composition loop, the bottom product composition ,

.

will also be affected.

DYNAMICS IND CONTROL OF FLOW-DRIN KN I'HOrKSSKS

279

SUMMARY AND CONCLUSIONS | This chapter has investigated the closed-loop process dynamic characteristics using

Aspen Dynamic- package. To observ e the controller performance in terms of set point tracking and disturbance rejection, a CSTR in addition to a distillation column have been illustrated The default control strategies have been tested for the reactor example, whereas the two additional composition control loops have been included along with the default control laws for the distillation example. Several simulation experiments have been executed for both the processes under flow-driven dynamic simulation. Note

that Chapter 6 presents the dynamic simulation and control of more rigorous pressuredriven dynamic process.

PROBLEMS| 5 1 A feed mixture of benzene and toluene is fed to a flash drum (Flash2). The .

separator operates at 1.2 atm and 100oC For dynamic simulation, required feed specifications are provided in Figure 5.75. .

Feed

Temperature = 25°C Pressure = 3 bar FLASH

Flow rate = 100 kmol/hr

Component

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benzene

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FIGURE 5.75 '

A flowsheet of a flash drum.

a) Use the SYSOP0 property method to compute the amounts of liquid and vapour products and their compositions

.

.

b) As shown in Figure 5 75, employ a PI control scheme to monitor the .

temperature in the flash drum by manipulating the heat duty

.

(c) Show the closed-loop servo performance with +10% and then -10% step changes in the flash temperature

.

(d) Report the tuning parameters obtained by trial-and-error method, controller action and ranges imposed 2 A vapour mixture of toluene, methane and hydrogen is heated using a shell and tube heat exchanger (HeatX) The superheated steam is used as a heating medium. Complete specifications required for closed-loop dynamic simulation are shown in .

5

.

.

Figure 5 76. .

280

PROCRSS SIMULATION AND CONTROL USING ASPEN Hot Stream Out

Pressure = 14 psia

Cold Stream In

I

Temperature = 2780F Pressure = 500 psia

HOT-OUT

Cold Stream Out Flow rate

Component

(kmol/hr)

j cold-out hoi Temperature = HOOT

ICOLD-INf

Pressure = 498 psia

200

toluene

Dead time

2300

methane

HOT-INK

1000

hydrogen

pi

>AT

>o

ii Hot Stream In

Temperature = 1160oF Pressure = 14.7 psia Flow rate = 5110 kmol/hr

FIGURE 5.76

A flowsheet of a heat exchanger.

f

(a) Simulate the heat exchanger model using the shortcut method, countercurrent low direction and NRTL-RK property method. (b) Include a PI control structure to observe the set point (cold stream outlet temperature) tracking performance and the manipulated input (steam inflow rate) profile. In the closed-loop simulation experiment, assume that the temperature sensor takes 1 minute time (dead time) to measure the controlled variable. Report the used tuning properties. (c) Examine the regulatory performance by introducing + 10% and subsequently 10% step changes in the inlet temperature of the cold stream. -

3 Device a cascade control scheme for the above heat exchanger and investigate the controller performance. 5 4 A liquid mixer model with a typical ratio controller (Seborg et al. 2003) is shown in Figure 5.77. The low rates for both the disturbance or wild stream (Fw) and

5

.

f

.

the manipulated stream (FE) are measured, and the measured ratio, R = FE/Fw is calculated. The output of the ratio element is sent to a ratio controller (PI) that compares the calculated ratio Rm to the desired ratio Rd (set point) and adjusts the manipulated low rate accordingly.

f

m

Input 2

>o Ratio

PI

>

Input 1

FIGURE 5.77

A flowsheet of a mixer

| POT >

,

DYNAMICS AND CONTROL OF KLOW-DRIVKN PROCESSES

281

The input data are shown in Table 5.2 for simulation. TABLE 5.2 Stream

Temperature CO

Pressure (aim)

E

50

1

W

60

1

Flow rate (kmol/hr)

Pe

Composition

= 100

pure ethanol

= 150

pure water

Process variable at steady state = 0.667 (FE/FW = 100/150) Controller output at steady state = 100 kmol/hr Proportional gain = 4 %/% Integral time = 20 minutes Controller action = reverse

(a) Appljang the SYSOPO base property method, simulate the mixer model operated at 1 atm. (b) Using the given controller properties and default ranges, report the ratio controller performance with two consecutive set point step changes (0.667 -> 0.72

Double-click on Input 1 transmission line and ill up Tables 5.3(a) and (b). f

Hint:

0.65) in the ratio.

TABLE 5.3(a) Value *,

Spec

>STREAMS("E ) Fcn("ETHANOL")

100.0

Free

100.0

Free

Value

Spec

>STREAMS("W ) Fcn("WATER")

150.0

Free

150.0

Free

.

Similar table for Input 2 is obtained as: TABLE 5.3(b)

w

.

In the next, double-click on Ratio element and get Table 5.4. TABLE 5.4

Description Inputl Input2 Output

Input signal 1 Input signal 2 Output signal, Inputl/lnput2

Value

Units

100.0

kmol/hr

150.0

kmol/hr

0 667 .

Use Initialize Values button and incorporate the given tuning properties before running the program.

282

,

5 A reboiled stripper is used to remove mainly propane and lighter species from a feed stream, shown in Figure 5.78. It has total 6 stages (including condenser and reboiler) and no condenser.

The bottoms rate is 100 Ibmol/hr and the column top stage pressure is 150 psia with a column pressure drop of 8 psi. The diameter of the stripper (Stages 1 to 5) is 6.5 ft. The reboiler heat duty is assumed constant, although it changes at

dynamic state. The sump has elliptical head with a height of 5 ft and diameter of 2.5 ft.

For the closed-loop simulation, use the following data: Dead time = 2 minutes

Magnitude of noise (standard deviation) = 0.01 Ibmol/lbmol Proportional gain of PI = 1 %/% Integral time of PI = 20 minutes Controller action = Reverse

PCI

Feed

Temperature = 40oF Pressure = 160 psia Feed stage = 1 (above stage) Component

Dead time

Flow rate

Pi

Noise

(Ibmol/hr) c,

60

c2 C3

150

n-C4

175

75

n-C

n-C

>o

5

60

8

35

FIGURE 5.78

A flowsheet of a stripping column

.

(a) Using the Peng-Robinson thermodynamic method simulate the RadFrac (STRIP2) model and compute the product compositions. (b) Keeping the default controllers (PCI and LC2) unaltered configure a composition control scheme (PI) coupling with a 'Dead time' and 'Noise elements to maintain the propane mole fraction in the distillate by manipulating the reboiler heat duty as shown in Figure 5 79. Use the given ,

,

'

_

.

closed-loop data and execute the dynamic simulations to test the developed composition controller performance

.

5

.

6 Ethylene is produced by cracking of ethane in a stoichiometric reactor. The irreversible elementary vapour-phase reaction is given as .

C2H6 -i C2H4 + H2 ethane

ethylene

hydrogen

shown in Figure 5.79, with a low 5 atm. The reactor operates at inlet

Pure ethane feed enters the reactor model rate of 750 kmol/hr at 800oC and 5

,

f

5

PROCESS SIMULATION AND CONTROL USING ASI'RN""

.

temperature and pressure with 80% conversion of ethane

.

DYNAMICS AND CONTROL OF FTOW-DRIVKN PROCESSES

283

pi

>o

> M

A lowsheet of a reactor f

FIGURE 5.79

f

U) Using the SYSOPO thermodynamic method, simulate the reactor model. (b) Develop a control loop as configured in the low diagram to maintain the desired reactor temperature by the adjustment of heat duty. Considering the measurement lag of 1 minute, inspect the servo as well as regulatory control performance. Report the tuning properties used to achieve a satisfactory closed-loop performance. 7 A binary feed mixture consisting of methylcyclohexane fMCH) and toluene is introduced above tray number 14 of a RadFrac distillation model, shown in Figure 5.80.

O

1 phenol [

O

1 FEED h

FIGURE 5.80

A lowsheet of a distillation column f

.

It is dificult to separate this close-boiling system (MCH-toluene) by simple binary distillation Therefore, phenol is used as an extractant and introduced above tray number 7 of the column The two input streams have the following f

5

.

.

specifications shown in Table 5.5. ,

TABLE 5.5

Stream

Temperature (*C)

PHENOL FEED

Pressure 'bar)

Flow rate

Mole fraction

105

14

100 nrVhr

10

105

14

181.44 kmol/hr

0 5/0.5

.

.

.

.

(MCH/toluene)

The column has 22 theoretical stages (including condenser and reboiler) with a total condenser The distillate rate and reflux ratio are given as 90.72 kmol/hr and 8 (mole basisrespectively. The pressure profile is defined with Stage 1 .

pressure of 1 10316 bar and Stage 22 pressure of 1 39274 .

bar. Use LMTD

assumptions for the condenser The reboiler heat duty is assumed constant. The reflux drum and sump are specified in Table 5.6.

284

PROCESS SIMULATION AND CONTROL USING ASPEN TABLE 5.6 Item

Reflux drum

Sump

Head type

Vessel type horizontal -

Height /Length (m)

Diameter (m)

elliptical

15

0 75

elliptical

15

0 75

.

.

.

.

The column diameter and tray spacing are given as 2 m and 0.6 m respectively, ,

(a) Simulate the distillation column using the UNIFAC property method to compute the composition of MCH in the distillate and that of phenol in the bottom product. (b) In addition to the default level and pressure controllers, insert a PID structure to control the MCH composition in the top product by manipulating the flow rate of PHENOL stream.

(c) Produce the plots to show the closed-loop control responses, and report the tuning parameters, control actions and operating ranges for controlled as well as manipulated variables used.

REFERENCES| Luyben, W.L., (2004), "Use of Dynamic Simulation to Converge Complex Process Flowsheets", Chemical Engineering Education, pp. 142-149. Seborg, D.E., T.F. Edgar and D.A. Mellichamp, (2003), Process Dynamics and Control, 2nd ed., John Wiley & Sons, Inc.

CHAPTER

6

Dynamics and Control of Pressure-driven Processes

61 .

INTRODUCTION

To know the transient characteristics of a complicated chemical plant, we need a dynamic

process simulator. It is well-recognized that Aspen Dynamics is such an efficient flowsheet simulator used for dynamic process simulation. As we have seen in Chapter 5,

Aspen Dynamics simulator can be employed to design a process as well as its associated control strategies.

Aspen Dynamics extends an Aspen Plus

steady-state model into a dynamic process

model. If the steady state Aspen Plus simulation is exported to Aspen Dynamics, there is a necessity to choose either flow-driven dynamic simulation or pressure-driven dynamic simulation In a rigorous pressure-driven simulation, pumps and compressors are inserted where needed, to provide the required pressure drop for material low. Control valves are installed where needed, and their pressure drops selected. For good .

f

,

,

control

,

the pressure drop across a control valve should be greater than 0.1 bar. The

f

f

luid that lows through a valve should normally be liquid-only or vapour-only because the two-phase low through a control valve is unusual f

.

It should be pointed out that for a pressure-driven case, we must not insert a valve in the suction of a pump or at the discharge of a compressor (compressor speed or its equivalent compressor work is manipulated). The control valves are positioned on the

f

luid streams such that the controllers can manipulate the valve positions.

f

The simple low-driven dynamic simulations have been discussed in detail in the

previous chapter. Therefore simulation

.

,

here we are intended to study the pressure-driven

A reactive or catalytic distillation column is exampled for the rigorous

pressure-driven Aspen Dynamics simulation as well as control.

285

286 6

.

2


DYNAMICS AND CONTROL OF A REACTIVE DISTILLATION (RD) COLUMN

Problem statement

The methyl tertiary butyl ether (MTBE) column configuration (Jacobs and Krishna, 1993) chosen for the simulation is shown in Figure 6.1. Pure methanol (MeOH) feed (liquid) Temperature = 320 K Pressure = 1 aim Flow rate = 711.30 kmol/hr

Feed stage = 10 (above-stage) 0| METHANOL

Jy

Ft

tCl-fpisT-Q CV2

PUMP

Butenes feed (vapour)

CH butenes]-1 "

Temperature = 350 K

1-H l-IbotI-<> '

CV3

Pressure = 1 aim Flow rale = 1965.18 kmol/hr

RDCOLUUN

Feed stage = 11 (above-stage) COMPRESS

Mol fracl

Component /so-butene (IB)

0 36

-butene (NB)

0 64

.

.

FIGURE 6.1

A lowsheet for the production of MTBE. f

n

The RD column (RadFrac) consists of 17 theoretical stages, including a total condenser and a partial reboiler. Reactive stages are located in the middle of the column, Stage 4 down to and including Stage 11. In Aspen terminology, the numbering of the stages is top downward; the condenser is Stage 1 and the reboiler is the last stage. MTBE is produced by reaction of IB and MeOH: (CH3)2C = CHa + CH3OH «-»(CHgk COCH3 IB

MeOH

MTBE

The liquid-phase reaction is catalyzed by a strong acidic macroreticular ion exchange

resin, for example Amberlyst 15. and n-butene does not take part in the reaction (inert). The forward and backward rate laws (Seader and Henley, 1998; Rehfinger and Hoffmann, 1990) are derived in terms of mole fractions, instead of activities (products of activity coefficient and mole fraction): '

Forward rate: rf= 3 67 x 1012 exp

9244(M

-

.

IB

.

Backward rate: r,, = 2 67 .

x 1017 exp

RT -

134454> RT

xMeOH , VMTB!'

DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES

287

Here, z represents the liquid-phase mole fraction. The pre-exponential factors, including the activation energy (kJ/kmol), are given in SI units. The catalyst is provided only for reactive stages (8 stages total), with 204.1 kg of catalyst per stage (Seader and Henley. 1998). The used catalyst is a strong-acid ion-exchange resin with 4.9 equivalents of acid groups per kg of catalyst. So, the equivalents per stage are 1000 or 8000 for the

8 stages. In some references, the equivalents per stage are directly given.

f

The column, starting from Stages 2 to 16, is packed with 'MELLAPAK' (vendor: SULZER) having a size of 250Y. Use 'Simple packing' hydraulics and the height equivalent to a theoretical plate (HETP) may be considered as 1 m. The distillation column diameter is 6 m. Stage 1 (condenser) pressure is 11 atm with a column pressure drop of 0.5 atm. The reflux ratio is set to 7 (mole basis) and the bottoms low rate is 640.8 kmol/hr. In the MTBE synthesis process, it is desirable to obtain a bottom product

containing high-purity MTBE and a distillate containing high-purity NB. In Table 6.1 the reflux drum and the sump (the next-to-last stage in the column) are specified. TABLE 6.1 Item Reflux drum

Sump

Vessel type

Head type

Height/Length (m)

Diameter (m)

horizontal

elliptical

2

1

elliptical

22

-

.

1 1 .

The pump delivers the liquid stream POUT at 11.7 atm. The compressor (isentropic) has discharged the vapour feed FV at 11.5 atm. The three control valves (adiabatic

f

lash) CV1, CV2 and CV3 have the outlet pressures of 11.5 atm, 10.8 atm and 11.3 atm respectively. Using the UNIFAC base property method, f

(a) simulate the process lowsheet to obtain the distillation product summary, and

(b) develop the control configurations to achieve the desired product purity under disturbance input. Simulation approach

(a) Start the Aspen program by double-clicking the Aspen Plus User Interface icon

on the desktop. Then select Template option and press Oif (see Figure 6.2).

:

aM

;

FIGURE 6.2


288

1

PROCESS SIMULATION AND CONTROL USING ASPEN"

We choose General with Metric Units option and hit OK button (see Figure 6.3). 016*11 l

l

I

_

I

L I _!_) *d U-i-lfcl I M l

I

3

J J J

HI

_

_

,

I I I J jU J J J

U ali

.

mi in >..

-

I

.

I

0*. I ,.J h 1

r P.*

FIGURE 6.3

When the Connect to Engine window appears, use the default Server type (Local PC). Creating flowsheet

The process low diagram includes a feed pump a feed compressor, a distillation column and three control valves. The complete process lowsheet drawn in an Aspen window should somewhat resemble the one shown in Figure 6 4 Recall that Aspen has a tool in the toolbar that automatically takes the user through the required data input in a

f

f

,

.

stepwise fashion. The blue Next button does this.

o-

-

"

if

- *x

FIGURE 6.4

.

DYNAMICS AND CONTROL OF PRKSSIIRE-DRI\T N PROCESSES ,

289


f

At the beginning of data entry, ill up Global sheet followed by Accounting sheet under Specifications of Setup folder. Moreover, select 'Mole' fraction along with 'Std.liq.volume' low basis in Stream sheet under Report Options [see Figures 6.5(a)

f

,

(b) and (c)]. 41

&

-

Sim

|

Pi bw HUB

.

MMH

.

fMM, r 3 EC

ir

S FIGURE 6.5(a)

:

>-B|

*ifi »] atmnn-at

.

-

T i-l h r*

-

a*-=- =

.! 3

.1 -IE;

I 1"! jiJ 3

:

II

aa-2-

-

FIGURE 6.5(b)

290


j r

_

I I -I 1

_

; ,

nafhi

| .imi

I

M MgJ

>bJiiiF-a

Jj

0 P lr«tmd«.CTvtlci.i

Optioni

j

[V

.

STREAMS FaH«te

.

Pmp

SUWlflHllH

f7 Mod

P Mod

Iff [.''.>.

r mwi

r mm

P S-dlQrt***.

r Stdklvcim.

P Conconni

RosrMru

_

fmctmUm

...m .

3 .

.

2«o Hnw or hKbm

| ., , v . | --vr. w Conif

r

MCow

| CdUHM | RtKicni Pimtw*ClMngnt | MwdMrt | Scfcc | UiaMotWi | f

CtiHomUtii

FV>-.bM>

fV»

(wun

gFoUeiiVwenButllV

HUM

FUntiwIlrpj r

cWc

FIGURE 6.5(c)


The components involved in the example system are MeOH (CH40) IB (C4H8-5), NB (C4H8-1) and MTBE (C5H120-D2). Within the parentheses, the chemical formulas used in Aspen terminology are mentioned (see Figxire 6.6). ,

fh

I* v** Date

1Mb Rui Pw

itnuy Wndew

|

r

Iglg) _U *le) £1 aRI&Kl l-l n.| I r

M I IT

I

W!

I

[a) -«l»«l

3MS MEOH

KTHANOL

h

Hti

EQWmfMi

NB

i-BUIEHE

C4HB'1

MIBE

. _

.

.

J PwoCh.

J Con - .

PA

J Si 35! I u'*

tdMAriMi | SwMka. ) KMEKtw«p> |

4 . | iMcm Pimm Ounv" | f

|»

i »tr»iiiiin

FIGURE 6.6

|


291


The user input under the Properties tab is probably the most critical input required to run a successful simulation. This has been discussed in much greater detail in the previous chapters. This key input is the Base method found in Global sheet under Specifications option. Set UNIFAC for the present project (see Figure 6 7). .

1

d

1 1

J 3

1 1

J d

1

J

1 I 1

3 J J

'

1

l f

| HatfE****

i CAM | iuucf

tfclj

r

* -Ol«inii j

1 id* | UhWoMt |

FIGURE 6.7


Under the Streams tab, we have used Specifications sheets to input the data for both the feed streams, BUTENES and METHANOL [see Figures 6.8(a) and (b)]. «k

U

DM

9 S3HS

I*M

a

m

lMm>

Vnfc.

rtw

r i j-j-i ..if;

3 ») qU I 1I

I

wo

-

ass

.

.

1

FIGURE 6.8(a)

292

PROCESS SIMULATION AND CONTROL USING ASPEN E« V*. D«j

i r

1b.

m i nr

i ei

-

DgrfcR

i

m mi

FH »| |..|[Tr ) mum* i

i

HPS la

J r:

!.

3

"lur

-.

1

il

Pi9

mibe

3

i

'

SJIENIS

1 E0v« . _

j

j

Fl

li

FV

_

*

-

ICIHMBI Res*;

Evripn | C ,,...-. | c«bk | UisModn |

iKall-V

HUM

RtMndkwJfraOk

FIGURE 6.8(b)


f

In Figures 6.9(a) to (d), irst the feed compressor details are giver.. Subsequently, the three control valves, CVl, CV2 and CV3, are specified.

i r- I -1 I- IT

_

I lal

_

-

fir-.

afbRTr

H

I

JulF

la

321) QLJ!!!!

q hm

3

J tUIENES

r

1 S PI J POUI

_

COMffltSi

Q

f f

i

Salup

COVmUk tO Ira'

I iaomMt | He Ertw vi | Coimi | ne*clait Ple»i»« Ch»»«< | MmWcn | St*df ) U « «(«** | -

CH

.

FIGURE 6.9(a)

;

MM

DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES Ffc {« Am l "** If* "

fV LtMV

r Or.

ir LUJV

'I'N

_

Mai gH

0 -

a, n A.

-

F\

ij M(t>«nOL

.

ai n

.

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0

a a a a

Bo.* O(*ora

s

t'J in"

3 MmnMtatm p Spk BMW J

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_

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a

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5TREAKS

fSi*

SSc*

_

f o Mtfc sen R

D4M* AOMiM] .

A.per. flu,

MUM

t.Pbl

FIGURE 6.9(b)

Ed|

Pfc, Liv y

nmm

i i

ABE

K-

ibi "Pi

»h r

1 hl J J J ill »i Dial n.i

| - 'MM ni»pMii|tH r c

w**,

I

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I-

1

I

1

r

o

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:

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m ,

:

.

r nti

uai D ttoA

.

OiWMit

HUM 1107

FIGURE 6.9(c)

294

PROCESS SIMl'UVnON AND CONTROL USING ASPEN d *

E*

(BfacK-VlrVJ.rllnpui

0

C«* Tadb Rrf, Plo i±1Mr wnx*. h*

"

3

r CAii»

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la tMtodoUM Peru-Mr

,

.:>-:

J -

I

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H EO-M.,

,

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Mm

Ffp*

SSt*

essr;

"

'

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.HIM

FIGURE 6.9(d)

When the data entry for the feed pump is complete Figure 6.10.

,

l r _

_

L

_

a i

LLJV

the window should look like

'I -isi

-'

.

-r -

y

m

r

RiMP

f

:

O Sim v

_J

t

I MMMta. I ekk I J«M-M« I

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FIGURE 6.10

J


295

f

In the list on the left, choose Blocks I RDCOLUMN I Setup to ill up Configuration sheet (see Figure 6.11).

Wal _LJ «m *) fJMfcl'.KM -I -il - I M -I 71 -I _J i T I I I' I - Wl I Ml

'

-

-

3

3|.. ji :.

-5j

1

I

f

i | IWMMi | ttaMlKf***, | CAM | txtin -

Pmm Omw

D-

FIGURE 6.11

Streams sheet defines both the feed streams (see Figure 6.12), FL and FV, as well as product streams, PI and P2. FTS C« vW. Dm r«* fW> fta

-! i-

PWO| Uaelgl ral-rlai-l-al l »l r I M PT \ -m _

i

I I"! .! wi

.«B

1

'I J

ftH"

Ml

It

-

ia

Ese K J . -»...- >. " I

MUM

FIGURE 6.12

The pressure profile of the sample RD column is described in window shown in Figure 6 13. .

296

PROCESS SIMULATION AND CONTROL USING ASPEN p l|yi|yTff!l ?lWTfffF!ffBIHTi]f!Bl;I K | l] E*

Vwm

OVs

Took

Run

PM

Lb«v Window

-

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DltflHI _

_

_

_

.

3 _

(|-/C0M»»ni« i

J Pw,

|

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-

B«nl.

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r j PM

_

S'iMnRtnJii

PieiM*

a n»

r SUflep-eiuwAop P Column pt 'Me <*op

.

O

lor f*tl ol CoUm (oprwiBll

| ffllT

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0 EO Irvul 0 Spec Grtt Pa»i

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-

lei "

J

Scwci

r

Varv

7 5l*jte OPurcwo***!)

Sep«alo(s [ H»al E'-changen ] Coktfrrtt | Re-scloit Pie*ii«e Channeli j MaMpUaloc. | ScW: | UswMotW; |

CV 0Foltas\AipennAlt.1

| -UCKy

6 Mno-.oliVoftl |[

Atpen Pkrt - Simutali..

FIGURE 6.13

In the left pane of the Data Browser window select Blocks/RDCOLUMN/Reactions. Filling out Specifications and Holdups sheets we have two windows as shown in ,

,

Figures 6.14(a) and (b). -

fit

EcW

Rin

PW

Lfcraiy

Wrrfow

Help

mil 1*1 nl-rl&hHJidr l M.

i i i

Haft !HtTr8.sd

i*7

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Endng

_

Reactor. 10

leaCton

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JSOe

aape

9 SpccGicmm j PW: Q S 'team RejJt

f

(§ EOVanafcist

Rl

4

*

ja pump O 0 0 0

Se«» Peiiair.M e Cu Um Suboine bkK Opborc Hen*.

(9 COVmMei 0 EOlnpti Q Sc«c G-cmk Seeen.Ren*i

) RDCOLUMN 9 Sew OeivSoect V«y HeefenCoctoi

PtJTOWOUKll Decertefi Enoercc

fF STREAMS

ItotAplMi | Smaat I M»£«l»g.i | Ctoi I R-om P-.o-eCh. " | M '

caw

ypgi

.. I S
v**

_

C1-

FIGURE 6.14(a)

BFob»: iO«PV* 1'

1

if**!

DYNAMICS AND CONTROL OK PRESSURE DRIVEN PROCESSES -

-

-

i -

--

nu

Todi

nw

_

J

.)

"

LT -I t-'h f

l

297

1

I >im!

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FIGURE 6.14(b)

Select Pack Rating under RDCOLUMN of Blocks folder. Creating a new ID, T, and specifying the packing section as well as packing characteristics, we obtain Figure 6.15. f »

.

Vmm Da*

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FIGURE 6.15

298


f

Choose Blocks/RDCOLUMN/Convergence and ix up the maximum iterations to 200 (see Figure 6.16). : F*s

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FIGURE 6.16

In the next, dick Dynamic tab wader Blocks/RDCOLUMN The design specifications of the reflux drum and sump are reported in Figures 6 17(a) and (b). .

.

l»lxl

-

Fir EA View Data Took Run PW Lbmy WMm H«t>

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1619

DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES IjtfiBj _U Mej J l n?Mftl*HH«!l I 1 1"! H 1 I r i -i.i IT -himi ( iiai «g!N

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FIGURE 6.17(b)

Hydraulics sheet incorporates the information displayed in Figure 6

.

18.

-

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FIGURE 6.18

5>

299

300

PROCESS SIMULATION AND CONTROL USINd ASRKN1

Hit Next icon to open the Reactions folder. For the forward reaction (Reaction No 1) and the backward reaction (Reaction No. 2), the stoichiometric coefficients and exponents are defined under Kinetic' Reaction type in the two sheets as shown in .

'

Figures 6.19(a) and (b).

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SI REAMS

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FIGURE 6.19(b)

HUM

DYNAMICS AND CONTROL OF PRESSURE DRIVEN PROCESSES -

301

The Power law kinetdc data for both the reactions are provided in Figures 6 20(a) and (b). .

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l r LL i _

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MEOH. iB

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FIGURE 6.20(b)

Running steady state simulation and viewing results As we hit Next knob followed by OK, Control Panel window pops up. Under Summary/Streams the results are displayed in Figure 6.21. ,

Results

302

PROCESS SIMULATION AND CONTROL USINd ASI'KN I y I'M i hi- Mojiin i) fie

-

Edi

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Data

To<*

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97 974

4053

10 396

41 770

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29 549

10 396

41 770

40150

711 300

.

711,300

MEOH

100 007

0 605

100 607

0605

IB

707 465

93 059

4 517

93 059

4 517

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1257,715

1231 290

26 425

1231 290

26 425

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MTBE

0635

609 253

0 635

609 253

.

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j FlomhBeting Options

1000

MEOH

0071

344 FfM

0071

944 PPM

0 065

01X17

0 065

0 007

0 664

0 041

0864

0 041

446 PPM

0 951

446 PPM

0 951

_

j

ModelAnalysrs Tooli

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146 743

Mete Flow kmcWn

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zi I

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STREAMS

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1126

FIGURE 6.21

f

The mole fraction of MTBE in BOT stream is computed as 0.951. (b) Exporting dynamic simulation: In order to conduct the dynamic process simulations, export the steady-state Aspen Plus simulation into Aspen Dynamics with saving as a pressure-driven dynamic ile. Opening existing simulation

As we press the Start knob, point to Programs, then AspenTech, then Aspen Engineering Suite, then Aspen Dynamics Version and then select Aspen Dynamics, a blank dynamic

f

simulation window appears. In the next, open the pressure-driven dynamic ile saved earlier. The screen looks like Figure 6.22.

f

It is obvious that the process lowsheet includes the automatically inserted two level controllers (LCI and LC3) and one pressure controller (PC2). Each of these controllers has an operator set point (SP), a process variable (PV), also known as controlled variable, and a controller output (OP), also called as manipulated variable, whose values are obtained from the Aspen Plus simulation. These control structures

also have their own tuning parameters, and so on, suggested by Aspen

Dynamics.

However, there is a scope to modify (or remove) the controller and its related items.

The Aspen generated control loops defined below should be used in the closed-loop

study of the prescribed catalytic distillation column.

J

SAND QNTROL OF PRF.SSURE-Drivkn PRnrPgg,Q

t 303

HSJ-Jl-(»>. 1 nr-
teTH

J 1 ,

-,.,

FIGURE 6.22

Loop 1 Controller: LCI

Type of controller: proportional (P)-only (since reset time is very large) Controlled variable, liquid level in the reflux drum

Manipulated variable: distillate (DIS) flow rate (percentage opening of valve CV2) Controller action: direct

Use all default data, except proportional gain of 2 (suggested by Luyben, 2004) Loop 2 Controller: PC2

Type of controller: proportional integral (PI) Controlled variable: top stage pressure

Manipulated variable: condenser heat removal Controller action: reverse

Use all default data (suggested by Luyben, 2004)

the condenser heat removal and P denotes the pressure to be controlled.

Assuming itten for Aspen Dynamics direct control action the controlle r equation can be rewr "s Qr = - 47 48 - Kc {PSP - P), where 47.48 is the bias signal (Vr.s' dy state position, e from stea gn indicates heat removal (cooling operation). If we mov lue becomes negahv.. < is dear that when pressure (P) increases, the error (PS/. P) va ,

**

.

-'

'

d ultimately

.

,

the neRative vain., ofQc decreases. Originally, the negative value should

304


increase because if pressure increases, there is a need to increase the heat removal

rate. Therefore, our assumption is wrong and it should be reverse action in Aspen Dynamics.) Loop 3 Controller: LC3

Type of controller: P-only Controlled variable: liquid level in the column base

f

Manipulated variable: bottoms (BOT) low rate (percentage opening of valve CV3) Controller action: direct

Use all default data, except proportional gain of 2 (suggested by Luyben 2004) ,

Configuring new control loops The primary objective of the example process is to produce a bottom MTBE product of high purity. To achieve the desired product purity in presence of disturbance and uncertainty, several control algorithms need to be employed with the reactive distillation, It should be noted that in the control system of a RD process, the liquid level and column pressure controls constitute inventory control, maintaining the basic operation of the column. Thus, here emphasis is placed on the response of composition control methodologies to maintain product quality as well as correct stoichiometric ratio between the feed streams. In the following, different control schemes have been discussed for three distillation sections, namely feed section, top section and bottom section. Feed section

For a chemical reaction with two reactants

the type of lowsheet depends on whether we want to operate the catalytic distillation column with no-excess of either reactant or excess reactant (Kaymak and Luyben, 2005). For a double-feed RD column if there is any imbalance in the inflow of the two reactants ('excess reactant' case) the product purity drops. This is because one of the reactants becomes excess and exits with the f

,

,

,

product stream, and this stream would have to be further processed to purify the product and recover the reactant for recycle. Obviously the 'excess reactant' lowsheet requires at least two separating columns and is therefore more expensive. However it is easier f

,

,

f

to control. On the other hand, the 'no-excess reactant' lowsheet has better steady state

economics but presents challenging control problems because of the need to precisely balance the stoichiometry of the reaction. Several control structures used to maintain the correct stoichiometric ratio of the

reactants have been proposed by researchers (e g Al-Arfaj and Luyben, 2000; 2002; Wang et al., 2003). To meet this control objective the controller requires some type of .,

.

,

f

feedback of information rom within the process to indicate the accumulation or depletion

f

f

of at least one of the reactants. This can simply be done by the use of an internal composition controller by manipulating the low rate of one of the resh feeds. There are also other efficient control techniques (e.g. cascade control, inferential control) reported for stoichiometric balancing (Wang et al. 2003). However, it is not practical ,

,

to simply ratio the two feed streams as has been proposed in some of the literature ,

papers. Flow measurement inaccuracies and feed composition changes doom to failure


305

any ratio controller that does not somehow incorporate information about compositions inside the reactive system and feed this information back to adjust fresh feed. For the concerned distillation column the methanol composition is controlled on 10* stage by the adjustment of the methanol fresh feed. The butene feed rate is low controlled. It is worthy to mention that manipulating the methanol feed to control an internal methanol composition is preferred when the butene feed coming rom the upstream units is not ree to be adjusted. If this is not the case then alternatively the

f

f

,

,

f

f

so-butene concentration, instead of methanol concentration may be controlled on a

.

,

reactive stage by adjusting the butene feed rate. We are now moving on to configure the composition controller for methanol feed.

f

To do this, click on expand symbol (+) of Dynamics subfolder. Then again hit expand button of ControlModels icon. Subsequently, select the PID object, drag it to the low diagram, place the control block near to CVl block and rename it as CC4. In the next, expand Stream Types and use ControlSignal icon to complete the CC4 configuration, shown in Figure 6.23. Chapter 5 presents a detail of how to configure a control structure in Aspen Dynamics.

1

i

a

Urn

. . -

B-

FIGURE 6.23

A little detail of the composition control loop for methanol feed is demonstrated below

.

Loop 4 Controller CC4 .

Type of controller: PI

CVl) Controller action: reverse

f

f

Controlled variable: liquid phase mole fraction of MeOH on Stage 10 Manipulated variable: resh methanol (FL) low rate (percentage opening of v alve

306

PROCESS SIMULATION AND CONTROL USING ASPLN

Before executing the simulation run, it is customary to have a look on the data

sheet. For this, double-click on CC4 control block and then press Configure knob in the faceplate to open the Configure dialog box. As mentioned in Chapter 5 it is wise to click on Initialize Values button. Still one doubt is there: is the value of process variable ,

(PV) displayed same with the steady state liquid phase concentration of MeOH on Stage 10 obtained in the Aspen Plus simulation? Be sure about it choose Blocks/ ,

RDCOLUMN/Profiles with opening the Aspen Plus simulation file. Then select 'Liquid' in the View ield in Compositions sheet and obtain the table shown in Figure 6 24, f

.

with

liquid mole fraction of MeOH on 10th stage of 0.04886022. This value is identical with that of PV in the Configure dialog box. od*

Run

PV)

MBI : 1: 1 m|»l *l

ii

Jfl PUMP RDCOLUMN MEOH

MB

0

HTBE )a;o**55i

0 Ennnoei Jj

Con*

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STREAMS 0

w* ?y3>Wl« .

I[lr53

FIGURE 6.24

The controller CC4 is tuned by trial-and-error approach and the parameter values have been chosen as:

Proportional gain = 5 %/% Integral time = 5 min Use default values for other items including bias signal, ranges, etc.

f

Notice that by the similar way, we can design the low controller for butene feed of the RD column.

Top section

In addition to the LCI and PC2 control structures, the distillate composition can be

f

controlled by manipulating the reflux low rate. In an alternative approach, along withf the pressure control (PC2), we can control the reflux drum level by the manipulation o f

the reflux rate and the distillate low rate can be adjusted by a ratio control law to give a constant reflux ratio. In the present case, the former control scheme has been incorporated for performance study.

DYNAMICS AND CONTROL OF PRESSURE-DRIVKN PROCESSES

307

Bottom section

In the bottom section of a distillation column, it is a common practice that either the bottom product purity or the tray temperature near the bottom of the column, which has a strong correlation with the product purity, is controlled at its desired value by

the manipulation of the reboiler heat duty. For the sample process, we have implemented a composition control structure for product quality control. As the CC4 control block has been connected, similarly we can incorporate the other control structures discussed above with the distillation flowsheet. The window,

shown in Figure 6.25, includes a closed-loop scheme in which the MTBE purity is controlled in the bottoms by adjusting the reboiler heat input and the methanol impurity in the top is controlled by manipulating the reflux flow rate. As stated earlier, the concentration of methanol on the reactive stage it is being fed to (Stage 10) is measured f

and controlled by the manipulation of the fresh methanol feed rate. The butene flow rate is low-controlled. The liquid levels in the reflux drum and the base of the column

are maintained by the distillate flow rate and the bottoms flow rate, respectively. The condenser heat removal is manipulated to control the column pressure. All of the structures are single-input/single-output (SISO) structures with PI controllers (P-only on levels).

c 0 a *q -

M

-

m

5

JC»r.Lt»*v

TV)-HM*

m m r«uv(.-,

m

.

uMflr*ti»f nm ?i li
t

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119 16 t'

tiu|*tion ttuwH 10 lit* L> I ll

Bool ;
Jus" T«»» dwnl ,

.

FIGURE 6.25

The details of control Loops 5 6 and 7 are presented below. ,

Loop 5 Controller: FC5

Type of controller: PI

1

308


f

Controlled variable: molar low rate of butene feed (FV)

Manipulated variable: brake power (shaft power or brake power of motor or engine

required to drive a compressor) Controller action: reverse

Proportional gain = 0.5 %/% Integral time = 0.3 min Use default values for other terms

Loop 6 Controller: CC6

Type of controller: PI Controlled variable: MTBE mole fraction in the bottoms

Manipulated variable: reboiler heat input Controller action: reverse

Proportional gain = 5 %/% Integral time = 5 min Use default values for other terms

Loop 7 Controller: CC7

Type of controller: PI Controlled variable: MeOH mole fraction in the distillate

f

Manipulated variable: reflux rate (mass low) Controller action: reverse

Proportional gain = 5 %/% Integral time = 5 min Use default values for other terms

Now the flowsheet is ready for closed-loop performance study Start the program as usual. It is important to mention that to restart a dynamic simulation click 'Restart' .

,

f

(F7) rom the Run menu or press 'Re-start Simulation' button on the Run Control toolbar.

Performance of the closed-loop RD process

In the present study, two consecutive step changes in methanol feed temperature (46.85 -

» 40oC at time = 1.7 hours and then 40 -> 460C at time = 3.9 hours) have been

introduced to examine the performance of the closed-loop RD process. A change in feed temperature affects the internal composition in the reactive zone. This, in turn, may deteriorate the product quality. The system responses to temperature disturbance are illustrated in Figure 6.26. It is obvious that the proposed structure is able to maintain the MTBE purity in the bottoms under the influence of disturbance variable. It can

f

f

also prevent excessive losses of both methanol and iso-butene in the products. Each Aspen Dynamics model includes different plots and tables from which we can easily access the simulation inputs as well as results. For this, irst highlight a block or stream, then right-click to point Forms and inally select the item that we want to access.


309

EBB

A

-

-

1

1 r»5fl

ill 1

A

jf5

i

4--

FIGURE 6.26

Performance of the closed-loop RD process with Measurement lags Aspen Dynamics screen, shown in Figure 6.27 includes three dead time blocks (DTI, ,

DT2 and DT3) connected with three composition controllers (CC4 CC6 and CC7). ,

Ot-

[ZtK-

o5 3

F

4 x-i

-

-

mf

Lii 13 iJ

FIGURE 6.27

310


The measurement lag of 15 sec (0 25 min) is used in all composition loops. To incorporate a dead time for a measured variable say methanol mole fraction on Stage 10, highlight DTI block right-click on the block, point to Forms and then select Configure to open the configure table. In the Value cell enter 0.25 min as a sensor dead time. Follow the same approach for other two dead time blocks .

,

,

,

.

or

f

Here, we have used the proportional gain of 1 %/% and integral time of 20 min

all composition controllers. The effects of disturbance in butene feed temperature have been depicted in Figure 6.28. mm View

Took

Q b: B SQi SfnUahon

Wtxjow

Heb

IS

.iV IDynamic

r tt Tf Gi«i|oo5

3

j i; a* v»

_

h «

'

IB » l f

Fte

b? t!

Cl B

K

I phut !-:-»&-

-

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step =i=e- 5 0000e-002

to 23 to 23 to 23

83 84 85

to

86

23

-

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gl«

| jjDlWS-MiCCToll.. I

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FIGURE 6.28

f

Initially a step decrease (76.85 -> 650C at time = 8 hours) and subsequently a step increase (65 -> 760C at time = 15 hours) have been considered in the simulation study. The developed closed-loop process lowsheet responds satisfactorily under load variable

change and measurement lag.

SUMMARY AND CONCLUSIONS |

f

f

In Chapter 5, we have studied the dynamics and control of the low-driven chemical processes. Here, a case study has been conducted on a MTBE catalytic distillation column using the pressure-driven dynamics. The complete process low diagram includes a distillation column, a feed compressor, a feed pump and three control valves. In the

MTBE synthesis process, a bottom product containing high-purity MTBE and a top product enriched with n-butene are obtained. To maintain the MTBE purity in the f

bottoms stream, several control structures have been configured with the lowsheet in

DYNAMICS AND CONTROL OF PKKSSURK-DRIVKN PROCKSSKS

.*J 1 1

Aspen Dynamics. All of the structures are SISO schemes with PI controllers (P-only on levels). The controllers have been tuned by simply using heuristics. The proposed closed-loop process provides satisfactory results under disturbance input and measurement lag.

PROBLEMS 6

.

1 A binary mixture of ethanol and l-propanol enters a flash drum (Flash2) The

f

feed specifications are shown in Figure 6.29 with the process low diagram

.

Liquid mixture

(UQ-MIX)

CV2

Temperature = 90X Pressure = 1,4 bar Flow rate = 120 kmol/hr

cCH liq-mix f»B-[fgiE CV1

Mol fract

Component ethanol

06

1-propanol

04

(pF]->t'i-|pdt-uq1-o

-

.

CV3

FIGURE 6.29

A flowsheet of a flash drum

f

The lash chamber operates at 90oC and 1.2 bar. The vertically placed separator with a length of 2 m and diameter of 1 m has elliptical head type. All the control valves have a pressure drop of 0.2 bar. Applying the RK-Soave thermodynamic model as a base property method, f

(a) simulate the lowsheet to obtain the product compositions, (b) design the two control schemes to maintain the pressure and liquid level in f

the lash chamber, and

(c) examine the performance of the designed controllers.

2 Styrene is produced by dehydrogenation of ethylbenzene Here we consider an

.

.

irreversible reaction: -

C2H5 -> CgHs - CH = CH2 + H2

ethylbenzene

styrene

hydrogen

The process low diagram that consists of a reactor (RSTOIC) a feed compressor ,

f

(COMPRESS) and a control valve (CV) is shown in Figure 6 30 .

An isentropic compressor discharges the FEED stream that enters the RStoic reactor at 2 bar The reactor runs at 260oC and 2 bar The control valve involves .

a pressure drop of 0 2 bar Use the fractional conversion of ethylbenzene equals .

08 .

.

Applying the Peng-Robinson thermodynamic method.

(a) simulate the lowsheet and ' b) observe the closed loop process response employing the flow controllers. f

6

,

-

312

PROCESS SIMULATION AND CONTROL USING ASPECT Pure ethylbenzene

Temperature = 260oC Pressure = 1 bar

Flow rate = 100 kmol/hr

"

M !

[pptI

-

o

cv

|feed|-1

-

COMPRESS

FIGURE 6.30 .

A flowsheet for the production of styrene.

3 The hydrogenation of aniline produces cyclohexylamine in a CSTR according to the following reaction:

C6H5NH2 + 3H2 -> CeHnNHa aniline hydrogen cyclohexylamine

The complete process flowsheet is provided in Figure 6.31. It includes a pump having a discharge pressure of 41.2 bar, an isentropic compressor having a discharge pressure of 41 bar, an elliptical head-type vertically placed reactor having a length of 1 m and three control valves with a pressure drop of 0.2 bar in each.

FEED

FL

F1

P1

CV1

-

u

CV2

PUMP

>

<

F2

>ff J

1 PDT-LIQ \-0

CV3

COMPRESS

RCSTR

FIGURE 6.31

A flowsheet for aniline hydrogenation

The reactor operates at 41 bar and 120oC and its volume is 1200 t3 (75% liquid). For the liquid-phase reaction the inlet streams Fl and F2, are specified in Table 6.2. ,

,

f

6

RSTOIC

,

TABLE 6.2

Reactant

Pure aniline (Fl)

Pure hydrogen (F2)

Temperature (°C) 40 -

12

Pressure (bar)

Flow rate (kmol/hr)

7

45

7

160

DYNAMICS AND CONTROL OF PKKSSURE DRIVEN PROCESSES

313

Data for the Arrhemus law:

Pre-exponentiaJ factor = 5 x lO8 m3/kmol s Activation energ>' = 20 000 .

Btu/lbmol

ICJ basis = Molanty Use the SYSOP0 base property method in the simulation. The reaction is firstorder in aniline and hydrogen, and the reaction rate constant is defined with respect to aniline.

(a) Simulate the lowsheet to compute the product compositions ibi configure the control schemes for maintaining the liquid level pressure and f

,

,

temperature in the CSTR. and

(c) investigate the closed-loop process response under any disturbance input 6

.

6

4 Repeat the above problem with adding a time lag of 0.2 min in temperature measurement and carry out the closed-loop process simulation to report the disturbance rejection performance of the developed scheme 5 In addition to the level, pressure and temperature controllers, include the flow controllers with the flowsheet, shown in Problem 6.3. and inspect the closed-loop

.

process response.

REFERENCES | Al-Arfaj. M A. and W L Luyben (2000) "Comparison of Alternative Control Structures for an Ideal Two-product Reactive Distillation Column Ind. Eng. Chem. Res., 39, .

"

,

pp 3298-3307.

Al-Arfaj. M A and W L. Luyben (2002) "Control Study of Ethyl fert Butyl Ether Reactive Di-tillation." Ind. Eng Chem Res., 41, pp. 3784 -3796. ,

.

Jacobs. R. and R Krishna (1993) "Multiple Solutions in Reactive Distillation for Methyl .

ot-Butyl Ether Synthesis Ind. Eng. Chem. Res., 32. pp 1706-1709. Kaymak D B and W L. Luyben (2005) "Comparison of Two Types of Two-temperature "

t

.

,

,

Control Structures for Reactive Distillation Columns pp 4625-4640.

"

,

Luyben

Ind. Eng. Chem. Res , 44,

W L. i2004i "Use of Dynamic Simulation to Converge Complex Process Chemical Engineering Education pp. 142-149

,

Flowsheets

"

.

,

Rehfinger A and U Hoffmann (1990) .

,

"Kinetics

of Methyl Tertiary Butyl Ether Liquid

Phase Synthesis Catalyzed by Ion Exchange Resin-I Intrinsic Rate Expression in Liquid Phase Activities Chem Eng. Set.. 45. pp. 1605-1617. .

"

.

Seader J D and E J Henley Sons In< . New York .

11998)

.

"Separation

'

Process Principles, John Wiley &

.

W Bng, S J I) s H WonK and E K Lee (2003) "Control of a Reactive Distillation Column m the Kinetic Regime for the Synthesis of n Butvl Acetate Ind Eng. Chem Re* . ,

.

"

.

42

.

pp B182-5194.


Rs. 295.00


Amiya K. Jana




Index

ABSBR2. 164

('hmmnil

AbNorplittn cnliunn, UM AnounlinK mformnhon. I I. 'M. 58

Compoaonl ))>, I Ml

Acetone, 93

('onfiguro dialog bosp li'io

Activation energy, (>r>

Control pnncl, 20

Adsorption, 100

Control vnlvi'M. 22!)

Aniline, M

(iontrol modali icon, 2(18 ( outI'ol Mi mil icon, 2(18

ArrhrniUH Inw, Bf*. 70 ASPEN. :J

Aopen Aapen Aapen Ahpimi Aapen Aapen Aapen

(

'

phtnt, 180

omponpnt tijuiii<, I it.

'

(!yclohoxylamina, (ir»

batchCAD, 1 chroniHloKmphv. I Dynamica, 1 22!) Dynaniica HYSYS. 1 Plus, I polymers pliiH 1

I

cnniei', 7, 51

1 )(«(>( lianisuir column, n>7

,

DcHi n ipac, I7(i Di'w point, 35

Direcl acting control, 243 Diaplay plot, I7i>

.

Anpcn prnpcrl ich I

Diatillation, l()7 Diatillation train, 180, 100 Diatl, 107

Hhmc method )H Bati hKrac I0H ,

,

Binary mixture

,

\2

Mth

'

Binary diatillation column

,

I )nvirin tor i r, 100 I )i mn modela, 7

Dryer, r>2

BK10 tr>i Block 7

D8TWII. 107. 108

,

Block inftirmation 33 ,

''

'-Me

"

'

point

28

,

I dynamic mode, 253 i kynumicN library, 2(i7 Dvnn I'M IS. r>

'

t'tnlvtir dialillation 28fi ,

T

OIJIOK 152 .

flbemCad

,

.

1

Mi hyll'onMtne, 56 rixpOlll'lltN, 2il7

316

INDKX

Flash 2, 3, 7

Peng-Robinson 60 PetroFrac 108 PetroFrac model 1 48 ,

Flow-driven, 229

Flow-driven simulation, 229

,

,

Formula, 116

Plot wizard 48. 90, 147 POLYSRK 204 Power law 54, 87 ,

Fraction basis, 195

,

FSpht, 204

,

Pre-exponential factor 65 ,

Pressure-driven simulations 229, 285 ,

Geometrv data, 237

PRO/1ITM 3 ,

Process flowsheet window 9 ,

Process variable 249 ,

HETP, 287

Hvdraulics sheet, 299

Property method 18, 32. 39 Pulse input 253 Pumparound circuits 149 ,

,

HYSYSTM, 3

,

Initialization mode, 253

Initialize values button, 273

Input summary, 23, 64

RadFrac, 107

RadFrac model, 127

Ranges tab, 247 RateFrac, 108

Ketene, 93

RBatch, 54

Kinetic, 74

RCSTR, 54

Kinetic factor, 100

RCSTR model, 230

Kinetic reaction type, 300

Reconnect destination, 192

Kinetic sheets, 238

Reconnect source, 193 REFINERY, 154

Regulatory performance, 254, 275 Rename block, 11, 193

LMTD, 256

Rename stream, 193

MTBE column, 286

Measurement lags, 309, 310

Report ile, 23, 122 Report options, 15 Requil, 54 Results plot dialog box, 251 Reverse acting control, 243

MELLAPAK, 287

RGibbs, 54

Methane, 93

RK-Soave, 28. 32

Model library, 5 Molarity, 76 Multi-input/multi-output, 243

RPlug. 54, 78 RStoic, 54, 55 Run status, 62

MultiFrac, 107

RYield, 54

NRTL, 52

SCFrac, 108

f

LHHW, 54, 93

Material STREAMS, 7

Pause at time, 251

Sensitivity analysis 172 Sep 1, 2, 7 Separators, 42 Servo performance 275 Setup, 15 Side strippers 149 Single-inputysingle-output, 243 Solver settings, 13

PENG-ROB, 140

SRK, 52

,

Object manager, 179

Operator set point, 247 Optimization, 178

,

,

INDEX

f

Stepwise, 7 Stoichiometric coeficients, 237

UNIFAC. 287

User Models, 7

Stream information. 18. 33 Stream table, 22

Styrene, 55

Vapour raction 210

SULZER, 287 SYSOPO*. 18

Variable number 180

f

,

.

Vinyl chloride monomer 189, 203 ,

Temperature approach, 262

Wilson model 43

Template. 5

Winn-Underwood-Gilliland method 107

,

,

317


ASPEN

AMIYA K. JANA

As Ihe complexilv of a plant integrated with several process units increases solving Ihe model structure with a large equation ,

set becomes a challenging task. To overcome this situation, various process flowsheet simulators are used. This book describes the simulation, optimisation, dynamics and closed-loop control of a wide variety of chemical processes using the most popular commercial flowsheet simulator Aspen '"

.

The book presents the Aspen simulation of a large variety of chemical units, including flash drum, continuous stirred tank reactor (CSTR), plug flow reactor (PFR), petroleum refining column, heat exchanger, absorption lower, reactive dislittation, disiillation

train, and monomer production unit. It also discusses the dynamics and control of flow-driven as well as pressure-driven chemical processes using Ihe Aspen Dynamics package. KEY FEATURES

Acquaints Ihe students with the simulation of large chemical plants with several single process units.

Includes a large number of worked out examples ittustrated in step ay-step format for easy understanding of the concepts. f

*

Provides chaptered problems lor extensive practice.

This book is suitable for the undergraduate and postgraduate students of chemical engineering. It will also be helpful to research scientists and practising engineers. THE AUTHOR

Amiya K. Jana received his B.E. degree in chemical engineering in 1998 from Jadavpur University, M.Tech. in chemical engineering

in 2000 from IIT Kharagpur, and Ph.D. in chemical engineering in 2004 from IIT Kharagpur. Presently. Or. Jana is Assistant Professor at IIT Kharagpur. His areas of research include control system process intensification, ,

and modelling and simulation. He is also the author of ChemiesJ Process Mode/ting and Computer Simukuon published by PHI learning.

You may also be interested in Process Control: Concepts. Dynamics and Applications, S.K. Singh Heat Transfer: Principles and Applications Binay K. Dutta ,

Principles of Mass Transfer and Separation Processes, Binay K. Dutta A Textbook of Chemical Engineering Thermodynamics, K.V. Narayanan lSBN:')7fl-flWD3-3l.S1-,1

Rs. 295.00 www.phlndia.com

Process Simulation and Control Using Aspen

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