149504061 azeotropic distillation

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Dist-011H Revised: Nov 20, 2012

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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 as an entrainer Include recycle of cyclohexane and the azeotropic mixture so that the recovery of ethanol is >99.5%

and the recovery of cyclohexane is nearly 100 %

Successfully converge a flowsheet with multiple recycle streams Configure a three phase distillation column

2. Prerequisites Aspen HYSYS V8.0 Understanding of azeotropes

3. BackgroundEthanol production via fermentation occurs in water, which must later be separated to make anhydrous ethanol

(99.95% ethanol). There is an azeotrope in the ethanol-water system at approximately 95 mol-% ethanol, which

is a barrier to separation. Cyclohexane is one of the solvents used for the production of anhydrous ethanol for

food and pharmaceutical usage. It is used as an entrainer: the ternary mixture forms a ternary azeotrope with a

different ethanol concentration, which allows ethanol to enrich in the other stream. The azeotropic liquid is

separated to recover the entrainer and the ethanol that exits the column in the azeotropic mixture.

The examples presented are solely intended to illustrate specific concepts and principles. They may not

reflect an industrial application or real situation.

4. Problem Statement and Aspen HYSYS SolutionProblem Statement

The feed to the separation train is a stream at 100 kgmole/h with 87 mol-% ethanol and 13 mol-% water.

Cyclohexane is added to the column, and > 99.95 mol-% ethanol exits the bottom of the column. The distillate isthen separated in three phase condenser. The cyclohexane-rich stream is recycled directly to the first column,

while the water- and ethanol-rich stream is sent to a second column from which almost-pure water exits in the

bottoms. The distillate of the second column is recycled to the first column.

Design the separation train so that the ethanol product stream meets the purity specification and the water

effluent stream has a purity of 99mol-%.


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Aspen HYSYS Solution

This model is built using a specific path. The order in which things are done is important for successful

convergence of the model. Do not reinitialize the run unless asked to, and if steps are skipped or done out of

order you may need to be start at the beginning or from a previously saved version.

4.01. Start a new simulation in Aspen HYSYS V8.0.4.02. Create a component list. In the Component Lists folder click Add. Add Ethanol, Water, and

Cyclohexane to the component list.

4.03. Select property package. In the Fluid Packages folder click Add. Select PRSV as the property package.

4.04. Go to the simulation environment by clicking the Simulation button in the bottom left of the screen.

4.05. We will begin by adding the feed and recycle streams to the flowsheet. Add four Material Streams anda Mixer to the flowsheet. Name them as shown below.


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4.06. Double click on the mixer (MIX-100). Select streams Make Up and SolventRecycle as Inlets and createan Outlet called Solvent.


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4.07. In order for the mixer to solve, we must define the inlet streams. Double click on stream Make Up. Thisstream will add a small amount of solvent to the system to account for any solvent losses to product

streams. We will later 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 Worksheettab enter a Temperature of25C, a

Pressure of1 bar, and a Molar Flow of0.01 kgmole/h. In the Composition form enter a Mole Fraction

of1 for cyclohexane.

4.08. Double click on the Solvent Recycle stream. This stream will be the solvent that exits the condenser ofthe 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 flowrate and composition, but for now we will enter an initial guess. In the

Worksheettab enter a Temperature of25C, a Pressure of1 bar, and a Molar Flow of400 kgmole/h. In

the Composition form enter Mole Fractions of0.5 for cyclohexane and ethanol.


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4.09. The mixer should now solve.


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4.10. We wil l now define the Feed and Feed Recycle streams. Double click on the Feed stream. This is thestream that pumps the ethanol-water mixture into the process. Enter aVapour Fraction of0.3, a

Pressure of1 bar, and a Molar Flow of100 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 define the Feed Recycle stream. This stream will be the ethanol-water mixture that exitsthe 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 will enter an initial guess. Double click on the Feed Recycle stream. In the

Worksheettab enter a Vapour Fraction of0, a Pressure of1 bar, and a Molar Flow of25 kgmole/hr. In

the Composition form enter Mole Fractions of0.7 for Ethanol, and 0.3 for Water.


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4.12. Remember to frequently save your progress as you are creating this simulation. Save this fi le as Dist-011_Azeotropic_Distillation.hsc.

4.13. We are now ready to insert a Three Phase Distillation Column to the flowsheet.


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4.14. Double click on the column (T-100) to open theThree Phase Column Input Expert window. In the firstwindow 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 tocheck for two liquid phases. Click Next when complete.


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4.16. In the third window, select the Total radio button for Condenser Type. Create a Light Outlet streamcalled 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. The Distillation Column Input Expert window will now appear. Select streams Feed, Feed Recycle, andSolventas Inlet Streams. Specify streams Feed and Feed Recycle to enter on stage 20, and stream

Solventto 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. On Page 3 of the Distillation Column Input Expert enter Condenserand ReboilerPressures of1 bar.Click Next when complete.

4.21. On Page 4 of the Distillation Column Input Expert leave all fields for temperature estimates blank. ClickNext.


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4.22. On the final page of the Distillation Column Input Expert click Done to configure the column.

4.23. The Column: T-100 window should automatically open. We must define the design specifications forthis 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 ofEthanol in the bottoms. Enter a

value of3.5 for the Heavy Reflux Ratio and a value of1 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. We must create a specification for the mole fraction of ethanol in the bottoms stream. Go to the Specsform 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. Again, be sure to periodically save your simulation as you make progress.4.26. Before we construct the second column, we will 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 incells A1 and A2.


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4.28. Right click on cell B1 and select Import Variable. Select the Master Comp Molar Flow ofCyclohexane instream ETOH.

4.29. Right click on cell B2 and select ImportVariable. Select the Molar Flow of the Make Up stream. Havingthese two flow rates side by side wil l easily allow you to check that the amount of solvent leaving the

system is equal to the amount of solvent entering the system.


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4.30. As you can see from the spreadsheet there is more solvent leaving the system than is entering. This willcause 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. The adjust block will vary the Make Up stream flowrate until the amount of solvent leaving the systemequals 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 wil l converge.

4.32. Open the spreadsheet and you will see that the solvent leaving the system is now equal to the solvententering the system.

4.33. Save the simulation.


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4.34. We wil l 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-Recand select the Outletstream to be Solvent Recycle. The flowsheet should converge after a few moments.


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4.36. We are now ready to add the second column. Add a Distillation Column Sub-Flowsheet from the ModelPalette.

4.37. Double click on the column (T-101) to open theDistillation Column Input Expert window. Change #Stages to 50 and select stream C2 Feed as the Inlet stream entering on stage 35. Select Total for

Condenserand 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. 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. On Page 3 of the Distillation Column Input Expert enter Condenserand Reboiler Pressures of1 bar.Click Next when complete.

4.40. On Page 4 of the Distillation Column Input Expert leave all fields blank for temperature estimates. ClickNext.


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4.41. On the final page of the Distillation Column Input Expert enter a Reflux Ratio of0.5. Click Done whencomplete to configure the column.

4.42. The Column: T-101 window should automatically open. We need to define another design specificationin 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 specification.

4.43. We must now create a specif ication for the mole fraction of water in the bottoms stream. Go to theSpecs 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. The Degrees of Freedom for the column should now be 0. Click Run to begin calculations. The columnshould 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 f lowsheet.

4.47. Double click on the recycle block (RCY-2). Select stream Feed Rec as the Inlet and stream Feed Recycleas 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. The flowsheet is now complete and should look similar to the following.


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5. ConclusionsIn 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 using a three phase condenser: One

stream is solvent rich and is recycled back to the first column as solvent; the other stream is well within another

distillation region so that we can use the second column to obtain pure water. The top stream of the second

column is recycled back to the first column as feed.

6. CopyrightCopyright 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 liability for any errors or omissions. In no event will AspenTech be

liable to you for damages, including any loss of profits, lost savings, or other incidental or consequential

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this work. This work and its contents are provided for educational purposes only.

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149504061 azeotropic distillation

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