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Azeotropic Distillation with Aspen HYSYS® V8.0 Production of Anhydrous Ethanol Using an Entrainer 1. Lesson Objectives
Design a separation train for anhydrous ethanol production using cyclohexane cyclohexane as an entrainer Include recycle of cyclohexan cyclohex ane e and the azeotropi c mixture so that the recovery of ethanol is >99.5% >99.5% and the recovery of cyclohex ane is nearly nearl y 100 100 %
Successfull y converge a flowsheet with multiple recycle recycle streams
Configure a three phase distillation column column
2. Prerequisites
Aspen HYSYS V8.0 V 8.0
Understanding of azeotropes
3. Background Ethanol production via fe rmentation occurs occurs in water, which must later late r be separated to make anhydrous ethanol (99.95% (99.95% ethanol) . There is an azeotrope in the ethanol -water -water system at approximately 95 mol-% mol- % ethanol, which is a barrier to separation. Cyclohexane is one of the solvents used for the production production of anhydrous ethanol ethanol for food and pharmaceutical usage. It is used as an e ntrainer: the ternary mix mixture ture forms a ternary azeotrope with a different dif ferent ethanol concentration, concentration, which whi ch allows ethanol to enrich in the other stream. The azeotropic liqui d is separated to recover the entrainer and the ethanol that ex its the column in the azeotropic mixture. The examples example s presented are are solely intended to illustrat ill ustrate e specific concepts and pri nciples. They may may not reflect refle ct an industrial application application or real si tuation.
4. Problem Statement and Aspen HYSYS Solution Problem Statement The feed to the separation train is a stream at at 100 kgmole/h kgmol e/h with 87 mol-% ethanol and 13 mol-% mol -% water. Cyclohexane Cyclohex ane is added to the column, column, and > 99.95 99.95 mol-% mol -% ethanol exits the bottom of the column. The distillate distil late is then separated in three phase condenser. The cyclohexane-rich stream is recycled directly to the the first fi rst column, while whil e the water- and ethanol-rich stream is sent to a second column from which almost- pure water exits in the bottoms. The distill ate of the second column is recycled to the first column. Design the separation se paration train so that the ethanol product stream meets the puri ty specification and the water efflue eff luent nt stream has a purity of 99mol-%. 99mol-%.
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Aspen HYSYS Solution This model is bui lt using a specifi c path. The order in whi ch things are are done is important for successful convergence convergence of the model. Do not reinitialize the run unless asked to, and and if steps are skip ped or done out of order you may need to be start at the be ginnin ginni ng or from a previousl y saved version. version. 4.01.
V8.0. Start a new simulation in Aspen HYSYS V8.0.
4.02.
Lists folder Add. Add Ethanol, Ethanol , Water, Water, and Create a component list. li st. In the Component Lists fol der click click Add. Cyclohexane to Cyclohexane to the component list.
4.03.
Packages fol Add. Select PRSV as PRSV as the property package. Select Sel ect property package. In the Fluid Packages folder der click click Add.
4.04.
Simulation button in the bottom left of the screen. Go to the simulati on environment environment by cli clicking cking the Simulation button screen.
4.05.
Streams and We will wi ll begi n by adding adding the feed fee d and recycle streams to the flowsheet. Add four Material Streams and Mixer to a Mixer t o the flowsheet. f lowsheet. Name them as shown below.
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4.06.
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Double click on the mixer (MIX-100). Select streams Make Up and SolventRecycle as Inlets and create an Outlet called Solvent.
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4.07.
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In order for the mixer to solve, we must define the i nlet streams. Double click on stream Make Up. This stream will add a small amount of solvent to the system to account for any sol vent losses to product streams. We will l ater implement an adjust block to solve for the correct flow rate of solvent, but for now we will enter a small number as a guess. In the Worksheet tab enter a Temperature of 25°C, a Pressure of 1 bar, and a Molar Flow of 0.01 kgmole/h. In the Composition form enter a Mole Fraction of 1 for cyclohexane.
4.08.
Double click on the Solvent Recycle stream. This stream will be the solvent that exits the condenser of the first column and wi ll be recycled and fed back into the column. We will add a recycle block that wil l calculate the correct fl owrate and composition, but for now we will ente r an initial guess. In the Worksheet tab enter a Temperature of 25°C, a Pressure of 1 bar, and a Molar Flow of 400 kgmole/h. In the Composition form enter Mole Fractions of 0.5 for cyclohexane and ethanol.
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4.09.
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The mixer should now solve.
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4.10.
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We wil l now define the Feed and Feed Recycle streams. Double click on the Feed stream. This is the stream that pumps the ethanol-water mixture into the process. Enter a Vapour Fraction of 0.3, a Pressure of 1 bar, and a Molar Flow of 100 kgmole/h. In the Composition form enter Mole Fractions of 0.87 for Ethanol and 0.13 for Water.
4.11.
Lastly we wil l defi ne the Feed Recycle stream. This stream will be the ethanol- water mixture that exi ts the condenser of the second column. This stream will be fed back to the first column to prevent losses of ethanol. Later on we will implement a recycle block to calculate the actual specifications for this stream, but for now we wi ll e nter an initial guess. Double click on the Feed Recycle stream. In the Worksheet tab enter a Vapour Fraction of 0, a Pressure of 1 bar, and a Molar Flow of 25 kgmole/hr. In the Composition form enter Mole Fractions of 0.7 for Ethanol, and 0.3 for Water.
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4.12.
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Remember to frequently save your progress as you are creating this simulation. Save this file as Dist011_Azeotropic_Distillation.hsc.
4.13.
We are now ready to insert a Three Phase Distill ation Column to the flowsheet.
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4.14.
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Double click on the column (T-100) to open the Three Phase Column Input Expert window. In the first window that appears select the Distillation radio button. Click Next.
4.15.
In the next window, change the Number of Stages to 62. Make sure that the Condenser is selected to check for two liquid phases. Cl ick Next when complete.
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4.16.
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In the third window, select the Total radio button for Condenser Type. Create a Light Outlet stream called Sol-Rec, a Heavy Outlet stream called C2-Feed, and an Energy stream called Q-Cond. Click Next when complete.
4.17.
In the fourth window, leave all fields blank and click Next.
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4.18.
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The Distillation Column Input Expert window will now appear. Select streams Feed, Feed Recycle, and Solvent as Inlet Streams. Specify streams Feed and Feed Recycle to enter on stage 20, and stream Solvent to enter on stage 1. Create a Bottoms Liquid Outlet stream called ETOH. Click Next when complete.
4.19.
On Page 2 of the Distillation Column Input Expert click Next.
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4.20.
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On Page 3 of the Distillation Column Input Expert enter Condenser and Reboiler Pressures of 1 bar. Click Next when complete.
4.21.
On Page 4 of the Distillation Column Input Expert leave all fields for temperature estimates blank. Click Next.
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4.22.
4.23.
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On the fi nal page of the Distillation Column Input Expert click Done to configure the column.
The Column: T-100 window should automatically open. We must defi ne the design specifications for this column. Go to the Specs Summary form under the Design tab. For this column we will specify the Heavy Reflux Ratio, the Light Reflux Ratio, and the Mole Fraction of Ethanol in the bottoms. Enter a value of 3.5 for the Heavy Reflux Ratio and a value of 1 for the Light Reflux Ratio. First uncheck the active box for Bot Product Rate and check the active boxes for Light Reflux Ratio and Heavy Reflux Ratio.
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4.24.
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We must create a specification for the mole fraction of ethanol in the bottoms stream. Go to the Specs form under the Design tab. Click Add and select Column Component Fraction. Select Stream for Target Type, ETOH for Draw, enter 0.9995 for Spec Value, and Ethanol for Component. The column should automatically solve.
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4.25. 4.26.
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Again, be sure to pe riodically save your simulation as you make progress. Before we construct the second column, we wi ll add an Adjust block and a Spreadsheet to find the correct flowrate for the Make Up stream.
4.27.
Double click on the spreadsheet ( SPRDSHT-1). Go to the Spreadsheet tab. Enter the following text in cells A1 and A2.
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4.28.
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Right click on cell B1 and select Import Variable. Select the Master Comp Molar Flow of Cyclohexane in stream ETOH.
4.29.
Right click on cell B2 and select Import Variable. Select the Molar Flow of the Make Up stream. Having these two flow rates side by side wil l easily allow you to check that the amount of sol vent leaving the system is equal to the amount of sol vent ente ring the system.
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4.30.
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As you can see from the spreadsheet there is more solvent leaving the system than is entering. This will cause convergence issues when we attempt to close the recycle streams. This is where we will use the adjust block. Double click on the adjust block (ADJ-1). Select the Adjusted Variable to be the Molar Flow of the Make Up stream, select the Target Variable to be the Master Comp Molar Flow (Cyclohexane) of stream ETOH, and set the Target Value to cell B2 in the spreadsheet.
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4.31.
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The adjust block will vary the Make Up stream flowrate until the amount of solvent l eaving the system equals the amount entering the system. Go to the Parameters tab. Change the Tolerance to 0.001 kgmole/h and change the Step Size to 0.01 kgmole/h. Click the Start button to begin calculations. After a few moments the flowsheet will converge.
4.32.
Open the spreadsheet and you will see that the solvent leaving the system is now equal to the solvent entering the system.
4.33.
Save the simulation.
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4.34.
We will now add a Recycle block to close the recycle loop for the solvent.
4.35.
Double click on the recycle block (RCY-1). Select the Inlet stream to be Sol-Rec and select the Outlet stream to be Solvent Recycle. The flowsheet should converge after a few moments.
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4.36.
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We are now ready to add the second column. Add a Distillation Column Sub-Flowsheet from the Model Palette.
4.37.
Double click on the column (T-101) to open the Distillation Column Input Expert window. Change # Stages to 50 and sel ect stream C2 Feed as the Inlet stream entering on stage 35. Select Total for Condenser and create an Ovhd Liquid Outlet called Feed Rec, a Bottoms Liquid Outlet called Water, and a Condenser Energy Stream called Q-Cond2. Click Next when complete.
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4.38.
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On Page 2 of the Distillation Column Input Expert leave the default selections for a Once-through, Regular Hysys reboiler. Click Next.
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4.39.
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On Page 3 of the Distillation Column Input Expert enter Condenser and Reboiler Pressures of 1 bar. Click Next when complete.
4.40.
On Page 4 of the Distillation Column Input Expert leave all fields blank for temperature estimates. Click Next.
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4.41.
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On the fi nal page of the Distillation Column Input Expert enter a Reflux Ratio of 0.5. Click Done when complete to configure the column.
4.42.
The Column: T-101 window should automatically open. We need to define another design specification in order for the column to solve. Go to the Specs Summary form under the Design tab and make sure that the Reflux Ratio is the only active specif ication.
4.43.
We must now create a specif ication for the mole fraction of water in the bottoms stream. Go to the Specs form under the Design tab. Click Add and select Column Component Fraction. Select Stream for Target Type, Water for Draw, enter 0.99 for Spec Value, and select H2O for Component.
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4.44.
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The Degrees of Freedom for the column should now be 0. Click Run to begin calculations. The column should solve.
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4.45.
Save the simulation.
4.46.
The last step is to connect the Feed Recycle loop. Add a Recycle block to the flowsheet.
4.47.
Double click on the recycle block (RCY-2). Select streamFeed Rec as the Inlet and stream Feed Recycle as the Outlet. The flowsheet will begin to solve. After a minute or two the flowsheet will solve. Be patient as there are many variables attempting to converge. At each iteration both recycle loops must converge, both columns must converge, and the adjust block must converge.
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4.48.
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The flowsheet i s now complete and should l ook similar to the following.
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5. Conclusions In this example, cyclohexane is used as the entrainer to separate water and ethanol to produce anhydrous ethanol. By using the proper amount of solvent, we obtain pure ethanol from the bottom of the first column. The stream from the top of the first column is separated into two streams usi ng a three phase condenser: One stream is solvent rich and is recycled back to the first column as sol vent; the other stream is well wi thin another distillation region so that we can use the se cond column to obtain pure water. The top stream of the second column is recycled back to the first column as feed.
6. Copyright Copyright © 2012 by Aspen Technology, Inc. (“AspenTech”). All rights reserved. This work may not be
reproduced or distributed in any form or by any means without the prior written conse nt of AspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS WORK and assumes no liabi lity for any errors or omissions. In no event will AspenTech be liable to you for damages, including any loss of prof its, lost savings, or other incidental or consequential damages arising out of the use of the information contained in, or the digital fi les supplied with or for use with, this work. This work and its contents are provided for educational purposes only. AspenTech®, aspenONE®, and the Aspen leaf l ogo, are trademarks of Aspen Technology, Inc.. Brands and product names mentioned in thi s documentation are trademarks or service marks of thei r respective companies.
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