dist-012h extractive distillation

Dist-012H Revised: Nov 19, 2012

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Extractive Distillation for Heptane-Toluene Separation using Aspen HYSYS V8.0

1. Lesson Objectives Essentials of extractive distillation

How to compare design alternatives

2. Prerequisites Aspen HYSYS V8.0

Introduction to distillation

3. Background When the two components in a binary mixture have very close normal boiling points, their relative volatility is

likely to be small if they do not form an azeotrope. For such cases, it may be more efficient to use extractive

distillation with a solvent than normal distillation. In extractive distillation, a less volatile solvent is used to

increase the relative volatilities of the original mixtures, allowing for easier separation. In this example, phenol

is used as the solvent for the separation of n-heptane and toluene.

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 Solution

Problem Statement

Determine whether conventional distillation or extractive distillation with phenol as a solvent is a more efficient

method to separate n-heptane and toluene.

Aspen HYSYS Solution

4.01. We will build models to simulate the separation of n-heptane and toluene. One model has a single

distillation column and the other uses the extractive distillation approach with two columns. First we

will build a simulation for a single distillation column. Start a new simulation using in Aspen HYSYS V8.0.

4.02. Create a component list. In the Component Lists folder select Add. Add n-Heptane and Toluene to the

component list.


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

and select RK as the Vapour Model.

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


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4.05. Place a Distillation Column Sub-Flowsheet on the main flowsheet from the Model Palette.

4.06. Double click on the column (T-100) to open the Distillation Column Input Expert. On Page 1 enter the

following information and click Next when complete.


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4.07. On Page 2 of the Input Expert leave the default selections for a Once-through, Regular Hysys reboiler.

Click Next.

4.08. On Page 3 of the Input Expert enter Condenser and Reboiler Pressures of 1 bar. Click Next when

complete.


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4.09. On Page 4 and 5 of the Input Expert leave all fields empty. Click Done on the final page to configure the

column.

4.10. In the Column: T-100 window go to the Worksheet tab to specify the feed stream. For the Feed stream

enter a Vapour Fraction of 0.5, a Pressure of 1 bar, and a Molar Flow of 100 kgmole/h.

4.11. In the Composition form under the Worksheet tab enter Mole Fractions of 0.5 for both components.

This stream should solve.

4.12. Now we must define the column design specifications. Go to the Specs Summary form under the

Design tab. Uncheck the Active boxes so that there are no active specifications.


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4.13. Go to the Specs form under the Design tab. We want to add a specification for the mole purity of both

product streams. Click Add and select Column Component Fraction. Select Stream for Target Type,

Heptane for Draw, enter 0.99 for Spec Value, and select n-Heptane for Component.

4.14. Add a similar specification for the mole fraction of toluene in the bottoms product stream.


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4.15. After entering both design specifications the Degrees of Freedom should now be 0. Click Run to begin

calculations. The column should converge.

4.16. Go to the Cond./Reboiler form under the Performance tab. Make a note of both the Condenser and

Reboiler duties. The Condenser Duty is 5.390e+006 kcal/h and the Reboiler Duty is 5.388e+006 kcal/h.


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4.17. Save this file as Dist-012H-Single_Column.hsc.

4.18. We will now create a second simulation, this time using extractive distillation. Create a new file in

Aspen HYSYS V8.0.

4.19. Create a component list. In the Component Lists folder select Add. Add n-Heptane, Toluene, and

Phenol to the component list.

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

and select RK as the Vapour Model.


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

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


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4.23. Double click on the column (T-100) to open the Distillation Column Input Expert. Enter the following

information on Page 1 and click Next when complete.


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4.24. On Page 2 of the Input Expert leave the default selections for a Once-through, Regular Hysys reboiler.

Click Next when complete.


complete.

4.26. On Page 4 and 5 leave all fields empty. Click Done on the final page to configure the column.

4.27. First we must define the feed streams. In the Column: T-100 window go to the Worksheet tab. For the

Feed stream enter a Vapour Fraction of 0.5, a Pressure of 1 bar, and a Molar Flow of 100 kgmole/h.

For the Solvent stream enter a Temperature of 181C, a Pressure of 1 bar, and a Molar Flow of 60

kgmole/h.


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4.28. In the Composition form under the Worksheet tab enter Mole Fractions of 0.5 for n-Heptane and

Toluene in the Feed stream, and a Mole Fraction of 1 for Phenol in the Solvent stream. Both streams

should solve.

4.29. We must now define our design specifications for the column. Go to the Specs Summary sheet under

the Design tab. We want to specify a distillate product rate of 50 kgmole/h with a mole fraction of 0.99

n-Heptane. Enter 50 kgmole/h in the field for Vent Rate and uncheck the active box for Reflux Ratio.


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4.30. Go to the Specs form under the Design tab. Here we will add a specification for the mole fraction of

heptane in the distillate stream. Click Add and select Column Component Fraction. Select Stream for

Target Type, Heptane for Draw, enter 0.99 for Spec Value, and select n-Heptane for Component.

4.31. The Degrees of Freedom for the column should now be 0. Click Run to begin calculations. The column

should solve.

4.32. We must now add a second column to separate the solvent from the toluene in the Rich-Solvent

stream. Insert a second Distillation Column Sub-Flowsheet from the Model Palette.


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4.33. Double click on the second column (T-101) to open the Distillation Column Input Expert. On Page 1

enter the following information and click Next when complete.

4.34. On Page 2 of the Input Expert leave the default selections for Once-through, Regular Hysys reboiler.

Click Next.


complete.


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4.36. On Page 4 and 5 of the Input Expert leave all fields blank and click Done on the final page to configure

the column.

4.37. We must define the design specifications for this second column. Go to the Spec Summary form under

the Design tab. Uncheck the active boxes so that there are no active specifications.

4.38. Go to the Specs form under the Design tab. Here we will create two specifications for the mole

fractions of toluene and phenol in the product streams. Click Add and select Column Component

Fraction. Select Stream for Target Type, Toluene for Draw, enter 0.99 for Spec Value, and select

Toluene for Component.


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4.39. Add a similar specification for the mole fraction of phenol in the bottoms product stream. Enter .99999

for Spec Value.

4.40. The Degrees of Freedom for the column should now be 0. Click Run to begin calculations. The column

should converge.


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4.41. We will now recycle the Lean-Solvent stream back to the first column. Add a Recycle block to the

flowsheet from the Model Palette.


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4.42. Double click on the recycle block (RCY-1). Select stream Lean-Solvent as the Inlet and stream Solvent as

the Outlet. The recycle block should solve.

4.43. Check results. Double click on the first column (T-100) and go to the Cond./Reboiler form under the

Performance tab. Make note of the Condenser and Reboiler Duties.


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4.44. Double click the second column (T-101) and go to the Cond./Reboiler form under the Performance tab.

Make a note of the Condenser and Reboiler Duties.

4.45. The following table will summarize the energy requirements from the case with 1 column versus the

case using extractive distillation.

Single Column Distillation Extractive Distillation

Total Heating Duty (kcal/h) 5,388,000 1,803,000 Total Cooling Duty (kcal/h) 5,390,000 1,410,000

5. Conclusions For the separation of n-heptane and toluene, extractive distillation has a significant advantage in total energy

requirements. Adding phenol as a solvent increased the relative volatilities of n-heptane and toluene in the

mixture and allowed for a much easier separation. However, extractive distillation required more equipment in

this case. Therefore a further analysis on capital versus operational costs would have to be performed in order

to make a decision as to which design is the better option.


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dist-012h extractive distillation

Documents

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normal distillation

conventional distillation

column t

heptanetoluene separation

column design specifications

separation of n

prerequisites aspen