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Pressure Swing to Overcome Azeotropes with Aspen HYSYS V8.0

Separation of Ethanol and Benzene

1. Lesson Objectives Learn how to use pressure swing to separate a binary mixture that forms an azeotrope into two

pure components

2. Prerequisites Aspen HYSYS V8.0 Introduction to azeotropic mixtures Introduction to distillation

3. BackgroundEthanol and benzene form an azeotrope and the azeotropic composition is sensitive to pressure. Therefore, it is

possible to use pressure swing to separate this binary mixture into pure components.

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 first column operates under a pressure of 3 bars and the second one at 0.1 bar. A compressor is used to

pressurize the recycle stream from 0.1 bar to 3 bars before it is recycled back to the first column.

Since the relative volatility is large except for the azeotrope point, there is no need to add a third component (as

a solvent).

Aspen HYSYS Solution

4.01. Start a new simulation in Aspen HYSYS V8.0.4.02. Create a component list. In the Component Listsfolder select Add. Add Ethanoland Benzeneto the

component list.

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4.03. Define property package. In the Fluid Packagesfolder select Add. Select PRSVas the property package.

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

4.05. We will first add twoMaterial Streamsto the flowsheet. Name one of the streams Feedand the otherRecycle.

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4.06. Double click on the Feedstream. This will be the ethanol-benzene feed to the process. In theWorksheettab enter a Vapour Fractionof 0.5, a Pressureof 3 bar, and a Molar Flowof 100 kgmole/h.

In the Compositionform enter Mole Fractionsof 0.5for both ethanoland benzene.

4.07. We will now define the streamRecycle. This stream will consist of ethanol and benzene vapors thatcome off the top of the second column, which we will recycleso we dont throw away any product.

Double click on the Recyclestream. In the Worksheettab enter a Vapour Fractionof 1, a Pressureof 3

bar, and a Molar Flowof 200 kgmole/h. In the Compositiontab enter Mole Fractionsof 0.5for both

ethanoland benzene. These values are initial estimates. They will eventually be replaced by the actual

recycled product.

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4.08. We are now ready to add a Distillation Column Sub-Flowsheetto the flowsheet from the ModelPalette.

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4.09. Double click the distillation column (T-100). This will launch the Distillation Column Input Expert. OnPage 1specify the following information. Change the # Stagesto 30and select streamsFeedand

Recycleto enter on stage 10. Select Full Refluxfor Condenser, create an Ovhd Vapour Outletstream

called Vap, a Bottoms Liquid Outletcalled Benzene, and a CondenserEnergy Streamcalled Q-Cond1.

When complete click Next.

4.10. On Page 2of the Distillation Column Input Expertselect aOnce-through, Regular Hysys reboiler. ClickNext.

4.11. On Page 3of the Distillation Column Input Expert enter Condenser andReboiler Pressuresof 3 bar.For this simulation we will assume no pressure drop through the column. In real life this wouldnt be

the case. Click Next.

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4.12. Page 4asks for Temperatureestimates. These are optional values that will help the column solverconverge. For this column we will leave all estimates blank. Click Next.

4.13. On the f inal page of the expert enter a Reflux Ratioof 3. Click Done.

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4.14. The Column: T-100window will now appear. We must define the design specifications for the column.We have already specified the reflux ratio, but we still need to specify the mole fraction of benzene in

the bottoms stream. First go to the Specs Summaryform and make sure that only the Reflux Ratio

specifi cation is checked as active.

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4.15. Now we will create a specification for the mole fraction of benzene in the bottoms stream. Go to theSpecsform under the Designtab. Click Addand select Column Component Fraction. Select Streamfor

Target Type, Benzenefor Draw, enter 0.999 for Spec Value, and select Benzenefor Component.

4.16. The Degrees of Freedomfor the column should now be 0. Click the Runbutton to begin calculations.The column should quickly converge.

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4.17. We will now insert a secondDistillation Column Sub-Flowsheet.

4.18. Double click on the second column (T-101) to open the Distillation Column Input Expert. On the firstpage change the # Stagesto 30and select the streamVapto enter on stage 10. Select Full Refluxfor

the Condenser, create an Ovhd Vapour Outletcalled Rec, a Bottoms Liquid Outletcalled Ethanol, and a

Condenser Energy Streamcalled Q-Cond2. Click Nextwhen complete.

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4.19. On Page 2of the Distillation Column Input Expert leave the default selections for aOnce-through,Regular Hysys reboiler. Click Next.

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4.20. On Page 3of the Distillation Column Input Expertenter Condenserand Reboiler Pressuresof 0.1 bar.Click Nextwhen complete.

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

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4.22. On the final page of the Distillation Column Input Expertenter a Reflux Ratioof 1. Click Donewhencomplete to configure the column.

4.23. Go to the Specs Summaryform under the Designtab. We wish to specify the reflux ratio and the molefraction of ethanol in the bottoms stream. We have already specified the reflux ratio, but we still need

to create a specification for the mole fraction of ethanol in the bottoms. First, make sure that the only

current active specification is the Reflux Ratio.

4.24. Now we will create a specification for the mole fraction of ethanol in the bottoms stream. Go to theSpecsform under the Designtab. Click Addand select Column Component Fraction. Select Streamfor

Target Type, Ethanolfor Draw, enter 0.999 for Spec Value, and select Ethanolfor Component.

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4.25. The Degrees of Freedomfor the column should now be 0. Click the Runbutton to begin calculations.The column should converge.

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4.26. Before we connect the recycle loop we must first add a compressor to raise the pressure of stream Rec.Add a Compressorto the flowsheet from the Model Palette.

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4.27. Double click on the compressor (K-100). Select stream Recas the Inlet. Create an Outletcalled Rec-HighP and an Energystream called Q-Comp.

4.28. In the Worksheettab enter an outlet Pressureof 3 bar. The compressor should solve.

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4.29. We are now ready to connect the recycle loop. Add a Recycleblock to the flowsheet from the ModelPalette.

4.30. Double click on the recycle block ( RCY-1). Select streamRec-HighPas the Inletand select streamRecycleas the Outlet. The flowsheet should solve after a few moments.

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4.31. The flowsheet is now complete. Check results. Double click on stream Benzeneand stream Ethanol.You will find that the flowrate of each stream is roughly 50 kgmole/hwith a mole fraction of 0.999of

each respective product.

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5. ConclusionsPressure swing distillation can be a good method for separating a binary mixture that forms an azeotrope when:

The azeotropic composition is sensitive to a pressure change The relative volatility of the two components is large except at the azeotropic point

6. CopyrightCopyright 2012 by Aspen Technology, Inc. (AspenTech). All rights reserved. This work may not be

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