5. Reactor (CSTR)

Isomerization in a CSTR with Aspen HYSYS® V8.0

1. Lesson Objectives Use component mass balances to calculate the time required to reach a desired conversion in a

continuous stirred tank reactor.

Use Aspen HYSYS to confirm the analytical solution

2. Prerequisites Aspen HYSYS V8.0

Basic knowledge of reaction rate laws and mass balances

3. Background 2-Butene is a four carbon alkene that exists as two geometric isomers: cis-2-butene and trans-2-butene. The

irreversible liquid phase isomerization reaction with 1st

order reaction kinetics is shown below. It is desired to

determine the residence time required to reach 90% reaction conversion in a continuous stirred tank reactor.

Assume steady state.

Homogeneous reaction

1st order reaction kinetics

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

reflect an industrial application or real situation.

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4. Solution

Analytic Solution:

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Aspen HYSYS Solution:

4.01. Start Aspen HYSYS V8.0. Select New to create a new simulation.

4.02. Begin by creating a Component List. In the properties navigation pane, go to Component Lists and

select Add. Change the Search by criteria to Formula and search for C4H8. Select cis2-Butene and tr2-

Butene and add them to the component list.

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4.03. Add a Fluid Package. Go to Fluid Packages in the navigation pane and select Add. Select NRTL as the

property package.

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4.04. Define reaction. Go to Reactions in the navigation pane and click Add to add a new reaction set. In

Reaction Set 1, click Add Reaction and select a HYSYS, Kinetic reaction.

4.05. Double click on Rxn-1 to define kinetic reaction. In the Kinetic Reaction: Rxn-1 window, Add cis2-

Butene and tr2-Butene to the component column, and assign Stoich Coeffs of -1 and 1, respectively. In

the Forward Reaction section, set A to be .23000 and both E and B to 0.00000. Make sure that the Base

Units and Rate Units are lbmole/ft3 and lbmole/ft3-min, respectively.

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4.06. Attach the reaction to a fluid package. In the Set-1 (Reaction Set) form, click Add to FP and select Basis-

1.

4.07. Move to the simulation environment by clicking the Simulation button on the bottom left of the screen.

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4.08. Press F-12 to open the UnitOps window. Select the Reactors radio button and add a Cont. Stirred Tank

Reactor to the flowsheet.

4.09. Upon clicking Add, the Cont. Stirred Tank Reactor: CSTR-100 window will appear. Enter an Inlet stream

called Feed, a Vapour Outlet stream called VAP-Product, and a Liquid Outlet stream called LIQ-Product.

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4.10. Go to the Reactions tab and select Set-1 for Reaction Set.

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4.11. Specify the feed stream. Go to the Worksheet tab. For the Feed stream enter a Temperature of 25°C, a

Pressure of 10 bar (1000 kPa), and a Molar Flow of 1 kgmole/h.

4.12. Go to the Composition form and enter a Mole Fraction of 1 for cis2-Butene.

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In the Design | Parameters form, enter a volume of 0.005 m3 and specify a Liquid Volume of 100%. This

is just a random volume, we will soon add an adjust block to determine the volume required to achieve

90% reaction conversion.

4.13.

4.14. Add an Adjust block to the flowsheet from the Model Palette.

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4.15. Double click the adjust block (ADJ-1). We would like adjust the reactor volume in order to achieve a

reaction conversion of 90%. For the Adjusted Variable select the Tank Volume of CSTR-100. For the

Targeted Variable select Act. % Cvn. of CSTR-100. Enter a Specified Target Value of 90.

4.16. In the Parameters tab, change the Maximum Iterations to 1000.

block should solve.

Press Start to begin calculations. The

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4.17. Create a spreadsheet to calculate the residence time. Add a Spreadsheet to the flowsheet from the

Model Palette.

4.18. Double click the spreadsheet (SPRDSHT-1). In the Spreadsheet tab, enter the following text in cells A1,

A2, and A3.

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4.19. Right click on cell B1 and select Import Variable. Select the Tank Volume of CSTR-100. Right click on

cell B2 and select Import Variable. Select the Actual Volume Flow of stream LIQ-Product.

4.20. In cell B3 enter the following formula: = (B1/B2)*60. This will display the residence time in minutes.

4.21. The residence time is ? minutes, identical to the analytical solution.

5. Conclusion Both the analytical solution and design spec in Aspen HYSYS produced the same required residence time of

? min. to achieve 90% reaction conversion in a CSTR. The residence time for a CSTR is longer than for a

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batch reactor or PFR because of the back-mixing: product is mixed in with the feed, slowing the reaction.