Reactors

Introduction to Aspen Plus

Reactor Models

Reactor Models Introduction to Aspen Plus

©2003 AspenTech. All Rights Reserved.

Lesson Objectives • Intro Introduce duce the various various classes classes of reactor models models available available • Examine in some detail detail at least least one reacto reactorr from each each class

Aspen Plus References Unit Operation Models Reference Manual , Chapter 5, Reactors ©2003 AspenTech. All Rights Reserved.

© 2003 AspenTech. AspenTech. All Rights Rights Reserved. Reserved.

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Aspen Technology, Inc.


Reactor Models

Reactor Overview Reactors

Balance Based RYield RStoic

Equilibrium Based REquil RGibbs

Kinetics Based RCSTR RPlug RBatch


Balanced Based Reactors (1) • RYield – Requires a mass balance only, not an atom balance – Is used to simulate reactors in which inlets to the reactor are

not completely known but outlets are known (e.g., to simulate a furnace) RYield

1000 lb/hr Coal

70 lb/hr H2O 20 lb/hr CO2 60 lb/hr CO 250 lb/hr tar 600 lb/hr char

IN

OUT


© 2003 AspenTech. All Rights Reserved.

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Reactor Models

Balanced Based Reactors (2) • RStoic – Requires both an atom and a mass balance – Used in situations where both the equilibrium data and the

kinetics are either unknown or unimportant – Can specify or calculate heat of reaction at a reference

temperature and pressure RStoic

C, O2

2 CO + O2 2 CO2 C + O2 CO2 2 C + O2 2 CO →

IN

→

→

C, O2, CO, CO2 OUT


Equilibrium Based Reactors (1) • These reactors: – Do not take reaction kinetics into account – Solve similar problems, but specifications are different – Allow individual reactions to be at a restricted equilibrium

• REquil – Computes combined chemical and phase equilibrium by

solving reaction equilibrium equations – Cannot do a three-phase flash – Useful when there are many components, a few known reactions, and when relatively few components take part in the reactions



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Reactor Models

Equilibrium Based Reactors (2) • RGibbs – Useful when reactions occurring are not known or are high in

number due to many components participating in the reactions. – A Gibbs free energy minimization is done to determine the

product composition at which the Gibbs free energy of the products is at a minimum. – This is the only Aspen Plus block that will deal with solid-liquid-

gas phase equilibrium.


Kinetic Reactors (1) • Kinetic reactors are RCSTR, RPlug and RBatch. • Reaction kinetics are taken into account, and hence must be specified. • Kinetics can be specified using one of the following builtin models, or with a user subroutine: – Power Law – Langmuir-Hinshelwood-Hougen-Watson (LHHW)

• A catalyst for a reaction can have a reaction coefficient of zero. • Reactions are specified using a Reaction ID. ©2003 AspenTech. All Rights Reserved.


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Reactor Models

Kinetic Reactors (2) • RCSTR – Use when reaction kinetics are known and when the reactor

contents have same properties as outlet stream – Allows for any number of feeds, which are mixed internally – Up to three product streams are allowed – vapor, liquid1,

liquid2 or vapor, liquid, free water – Will calculate duty given temperature or temperature given duty – Can model equilibrium reactions simultaneously with rate-

based reactions


Kinetic Reactors (3) • RPlug – Handles only rate-based reactions – A cooling stream is allowed – You must provide reactor length and diameter

• RBatch – Handles rate-based kinetics reactions only – Any number of continuous or delayed feeds are allowed – Must provide one of the following: stop criteria, cycle time, or

result time – Holding tanks are used to interface with steady-state streams of Aspen Plus



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Reactor Models

Using a Reaction ID • Reaction IDs are setup as objects, separate from the reactor, and then referenced within the reactor(s). • A single Reaction ID can be referenced in any number of kinetic reactors (RCSTR, RPlug and RBatch). • To set up a Reaction ID, go to the Reactions Object Manager.

Reactions


Power-Law Rate Expression rate

= k × ∏[concentrat ion ] exponent

i

i

i

n

 T   Activation   exp − k = ( Pre - exponentia l Factor)  T   R  0  

1   −  T T    0   

Energy  1

If reference temperature, T0 , is not specifed, k is expressed as: k

= (Pre - exponentia l Factor)× T

n

 

exp −

Activation Energy  RT

 



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Reactor Models

Power-Law Rate Expression Example 1   → C+ 2 D 2 A+ 3 B ←  2 k

k

Forward reaction: (Assuming the reaction is 2nd order in A) coefficients: exponents:

A: A:

B: B:

C: C:

D: D:

Reverse reaction: (Assuming the reaction is 1st order in C and D) coefficients: exponents:

C: C:

D: D:

A: A:

B: B:


Heats of Reaction • Heats of reaction need not be provided for reactions. • Heats of reaction are typically calculated as the difference between inlet and outlet enthalpies for the reactor (see Appendix A). • If you have a heat of reaction value that does not match the value calculated by Aspen Plus, you can adjust the heats of formation (DHFORM) of one or more components to make the heats of reaction match. • Heats of reaction can also be calculated or specified at a reference temperature and pressure in an RStoic reactor.



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Reactor Models

Reactor Workshop (1) • Objective: Compare the use of different reactor types to model one reaction. 70% conversion of ethanol P-STOIC

F-STOIC RSTOIC F-GIBBS

FEED

Feed: Temp = 70°C DUPL Pres = 1 atm Water: 8.892 kmol/hr Ethanol: 186.59 kmol/hr Acetic Acid: 192.6 kmol/hr

P-GIBBS

RGIBBS F-PLUG

Length = 2 m P-PLUG

RPLUG F-CSTR

Diameter = 0.3 m P-CSTR

Use the NRTL-HOC property method Filename: REACTORS.BKP

RCSTR

Volume = 0.14 m 3


Reactor Workshop (2) • Reactor Conditions: • Stoichiometry:

Temperature = 70°C, Pressure = 1 atm

Ethanol + Acetic Acid ↔ Ethyl Acetate + Water

• Kinetic Parameters: – Reactions are first order with respect to each of the reactants

in the reaction (second order overall). – Forward Reaction: k = 1.9 x 10 8, E = 5.95 x 10 7 J/kmol – Reverse Reaction: k = 5.0 x 107, E = 5.95 x 10 7 J/kmol – Reactions occur in the liquid phase. – Composition basis is Molarity. Hint: Check that each reactor is considering both Vapor and Liquid as Valid phases. ©2003 AspenTech. All Rights Reserved.


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Reactors

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