dist 002h pressureswing

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Dist-002H Revised: Nov 5, 2012

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Pressure Swing Distillation with Aspen HYSYS® V8.0

1. Lesson Objectives

Configure distillation columns in Aspen HYSYS

Learn to use pressure swing to overcome azeotropic mixture

2. Prerequisites

Aspen HYSYS V8.0

Working knowledge of vapor-liquid equilibrium and distillation

3.

Background

Basics on Azeotropic Distillation

An azeotrope occurs when the liquid and vapor mole fractions of each component are the same. On a y-x plot,

an azeotrope is shown by a line which passes through the x = y line. Azeotropes present challenges to

separation processes and need to be accounted for in process design and operation.




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No further enrichment can occur in either phase when the system reaches the azeotrope constraint because the

driving force is eliminated. A mixture will separate towards a pure component and the azeotropic mixture. The

component which is purified depends on which side of the crossover the initial mixture is. To purify the minority

component, you must first cross the azeotrope. This can be done by adding an entrainer, another chemicalwhich breaks the azeotrope. This creates the need for additional separation and usually material recycle with a

purge stream. Alternatively, the composition of the azeotrope is dependent on pressure, which can be

exploited to get the mixture across the azeotrope. This is called pressure swing distillation.

Ethanol and water form an azeotrope at approximately 95.5mol-% ethanol at 1 atm. This is a low-boiling point

(or positive) azeotrope. The boiling point of the mixture is lower than either of the pure components, so the

azeotropic mixture exit from the top of the column regardless of which compound is being enriched in the

bottoms.

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

A feed of 24,000 kg/h of 20mol-% ethanol and 80 mol-% water must be separated. The required product stream

is 99 mol-% ethanol at a flowrate of at least 7,500 kg/h. This separation will be achieved by using pressure swing

distillation.

We begin by creating a technically feasible design for a two-column separation train. We will report for each

column: operating pressure, number of stages, reflux ratio, and the purity and recovery specifications. Also

report a stream table with the flowrates and compositions of relevant streams. Material recycle will be

necessary to achieve these results. We will use an operating pressure of 0.1 bar for the first column, and an

operating pressure of 20 bar for the second column.

Aspen HYSYS Solution

4.01. Create a new simulation in Aspen HYSYS V8.0.

4.02. Create a component list. In the Component Lists folder selectAdd. Add ethanol and water to the

component l ist.




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4.03. Define property package. In the Fluid Packages folder select Add. Select NRTL as the property package.

Select RK as the Vapour Model. The non-random, two liquid (NRTL) model works well for very non ideal

liquid systems which is important because of the hydrogen bonding present. The Redlich-Kwong

equation model works much better at high pressures than the ideal gas assumption in the vapour phase.

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

4.05. Add a Distillation Column Sub-Flowsheet to the flowsheet from the Model Palette.




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4.06. Double click the column (T-100). This will open the Distillation Column Input Expert window. Enter the

following information and click Next when complete.




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4.11. Go to the Composition form under the Worksheet tab to define the composition of the feed stream.

Enter Mole Fractions of 0.2 for ethanol and 0.8 for water. When complete, the feed stream should be

fully defined and will solve.

4.12. Double click the column (T-100) to finish specifying the operating conditions. Go to the Specs form

under the Design tab. First, we would like to recover99% of ethanol from the feed stream. To do this

we will click the Add button and select Column Component Recovery. Select stream D1 as the Draw,

enter a Spec Value of 0.99, and select Ethanol as the Component.




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4.13. Second, we would like the distillate stream (D1) to have a mole fraction of 0.90 for ethanol. The

azeotrope prevents us from the reaching the desired product purity of 99% with a single column, but we

would still want the distillate from the first column to be very pure while losing as little product to the

bottoms stream as possible. Click Add and select Column Comp Fraction. Select Stream for Target

Type, D1 for Draw, 0.90 for Spec Value, and Ethanol for Component.




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4.14. Go to the Specs Summary form under the Design tab. Make sure that the only specifications marked as

active are Comp Recovery and Comp Fraction. The column will attempt to solve once these specs are

active.

4.15.

If the solver fails to converge, we may have to add some parameter estimates. For example, we canprovide an estimate for the distillate stream flow rate based on a simple mass balance. We know that

the feed contains approximately 200 kgmole/h of ethanol. We also know that we want to recover 99%

of ethanol with an ethanol mole fraction of 0.9 in the distillate. We can then provide an estimate of 220

kgmole/hr for the distil late stream. In the Specs Summary grid, enter 220 kgmole/h for Distillate Rate

which will serve as an estimate as long as i t is not an active specification. Click Run when complete. The

column should now converge. Also note that the bottoms stream is over 99% water, which means that

we are throwing away very little ethanol. A pure water stream is also desirable because we can now

repurpose or dispose of this stream with minimal further processing.




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4.16. We are now ready to construct the second column, which will operate on the other side of the

azeotrope at a pressure of 20 bar. We first need to insert a pump to increase the pressure of the

distillate stream leaving the first column. Add a pump to the flowsheet from the Model Palette.




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4.17. Double click the pump (P-100). Select stream D1 as the Inlet, create an Outlet stream called Feed2, and

create an Energy stream called Q-Pump.

4.18. In the Worksheet tab, enter a Pressure of 20 bar (operating pressure of the second column) for stream

Feed2. The pump should solve.




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4.19. Next we wi ll insert the second Distillation Column Sub-Flowsheet to the flowsheet, after the pump.

4.20. Double click the second column (T-101). This will open the Distillation Column Input Expert. On the

first page, enter the following information and click Next when complete.

4.21. On page 2 of the input expert, leave the default selections for a Once-through, Regular Hysys reboiler.

Click Next.

4.22. On page 3 of the input expert, enter Condenser and Reboiler Pressures of 20 bar. Click Next when

complete.




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4.23. On page 4 leave all fields for temperature estimates blank. Click Next. On the final page, also leave all

fields blank. Click Done to configure the column.

4.24. We must now enter the operating specifications for the column. In the column window for T-101 go to

the Specs form under the Design tab. Add a column specification for the mole fraction of ethanol in the

bottoms stream. Click Add and select Column Component Fraction. Select Stream for Target Type,

Ethanol for Draw, 0.99 for Spec Value, and Ethanol for Component.




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4.25. We will now add a specification for the mole recovery of ethanol in the bottoms stream. In the Specs

Summary form under the Design tab, double click the specification for Btms Prod Rate. We know that

approximately 203 kgmole/h of ethanol are entering the process in the original feed stream. Since we

are recovering 99% of the ethanol in the first column, we expect the final product stream to have aflowrate of approximately 200 kgmole/h.

4.26.

In the Specs Summary form, make sure that the only active specifications are Btms Prod Rate and Comp

Fraction. The column will attempt to solve, but you should find that it will not be able to converge. This

is because we need to add a recycle stream to the flowsheet.




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4.27. Double click on the first column (T-100) and add a second Inlet Stream called Recycle. This stream will

enter on the same stage as the Feed stream (stage 25).

4.28. We will define the recycle stream with a guess of what the actual recycle stream will consist of. We will

use this “dummy” recycle stream to allow both columns to converge, and then we will add a recycle

block to find the actual recycle stream conditions. Double click the Recycle stream on the flowsheet. In

the Worksheet tab enter a Vapour Fraction of 0, a Pressure of 1.2 bar, and a Molar Flow of 400

kgmole/h. In the Composition form enter Mole Fractions of 0.9 for ethanol, and 0.1 for water. Again,

these values are just guesses that will be used to converge both columns. The actual value s for the

recycle stream will be determined later through the use of a recycle block. Make sure that you specify

Recycle in its own window, rather than in the Worksheet tab of Column T-100, as this will cause a

consistency error due to overspecification when you try to complete the recycle loop.




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4.29. Once the recycle stream is fully specified, double click on each column and click Run to converge the

columns. Both columns should now successfully converge.

4.30.

Now we can close the recycle loop to determine the actual values for the Recycle stream. First we must

lower the pressure of stream D2 through the use of a valve. Add a Valve to the flowsheet from the

Model Palette.




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4.31. Double click on the valve (VLV-100). Select streamD2 as the Inlet and create an Outlet called Rec.

4.32. Go to the Worksheet tab and specify an outlet Pressure of 1.2 bar. The valve should solve.




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4.33.

We will now add a Recycle block to the flowsheet from the Model Palette.




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4.34.

Double click on the recycle block (RCY-1). Select stream Rec as the Inlet and stream Recycle as theOutlet. The recycle block should automatically solve.




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4.35. Go to the Worksheet tab to view recycle convergence results. The two streams should be equal to each

other within a certain tolerance.

4.36. If you are not satisfied with the recycle convergence, go to the Parameters tab and lower the

sensitivities. For example if we change the sensitivity value to 1 for Flow and Composition we get the

following results.




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4.37. The flowsheet is now complete. Further analysis can be performed to optimize the column size, feed

location, and energy requirements, but that analysis is not covered in this lesson.




5. Conclusions

The azeotrope in the ethanol-water system presents a barrier to separation, but pressure swing distillation can

be used to purify ethanol. A technically feasible design for purifying ethanol to 99mol-% with pressure swing

distillation can be constructed using Aspen HYSYS. A column with 30 equilibrium stages and operating at 0.1 bar

with a reflux ratio of 6.9 increases the ethanol composition to 90mol-%. A second column with 75 equilibrium

stages and operating at 20 bar with a reflux ratio of 10.8 increases the purity to 99mol-%.

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 consent 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.

AspenTech®, aspenONE®, and the Aspen leaf logo, are trademarks of Aspen Technology, Inc.. Brands and

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dist 002h pressureswing

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