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Isomerization in CSTRs in Series with Aspen HYSYS® V8.0 1. Lesson Objectives Use component mass balances to calculate the reaction conversion achieved with two continuous
stirred tank reactors reactors in series. serie s. Use Aspen Aspe n HYSYS HYSYS to confirm the analytical solution sol ution
2. Prerequisites
Aspen HYSYS V8.0 V 8.0
Basic knowl edge of reaction rate laws and mass mass balances
3. Background 2-Butene is a four carbon alkene that ex ists as two geometric isomers: cis-2-butene -2-but ene and trans-2-butene. -2-butene. The st
irreversi ble liquid phase isomerizatio isomerization n reaction wi th 1 order reaction kinetics is i s shown below.
Homogeneous reaction
st
1 order orde r reaction kinetics kine tics
The examples example s presented are are solely intended to illustrat ill ustrate e specific concepts and principles. They may may not reflect refle ct an industrial application application or real si tuation.
4. Problem Statement and Solutions Problem #1 Determine the conversion achieved achieved if two CSTRs are are used in serie s. Each Each CSTR has a resi dence time of 20 min. Assume steady state.
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Analytic Solution:
First Reactor Component A Balance
Second Reactor Component A Balance
Conversion
( )
Aspen HYSYS Solution: 4.01.
Start Aspen HYSYS V8.0. Create a new simulation.
4.02.
Create a component list. In the Component Lists folder 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.
Define property package. In the Fluid Packages folder select Add. Select NRTL as the property package.
4.04.
Define reaction. In the Reactions folder select Add to create a new reaction set. In the newly created reaction set select Add Reaction and select Kinetic. Close the Reactions window.
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4.05.
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Rxn-1 will be created. Double click on Rxn-1 to define the kinetic reaction. Add cis2- Butene and tr2Butene 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.
4.06.
Attach reaction to a fl uid package. Click Add to FP and select Basis-1.
4.07.
Go to the simulation environment. Sel ect the Simulation button in the bottom left of the screen.
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4.08.
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Add two CSTR blocks to the flowsheet. Press F12 to open the UnitOps window. Select the Reactors radio button and add 2 Cont. Stirred Tank Reactors to the flowsheet.
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4.09.
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Double click on the first reactor ( CSTR-100). Create an Inlet stream called Feed, a Vapour Outlet called
Vap1, and a Liquid Outlet called Liq1.
4.10.
In the Reactions tab select Set-1 for Reaction Set.
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4.11.
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Specify the feed stream. Go to the Worksheet tab and enter a Temperature of 25°C, a Pressure of 10
bar, and a Molar Flow of 1 kgmole/h.
4.12.
In the Composition form under the Worksheet tab, enter a Mole Fraction of 1 for cis-2-butene in the feed stream.
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4.13.
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In the Design | Parameters form, enter a Volume of 0.005 m and a Liquid Volume % of 100%. We will soon create an Adjust block and a S preadsheet to find the volume required for the desired residence time of 20 minutes.
4.14.
Add a Spreadsheet to the flowsheet from the Model Palette .
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4.15.
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Double click the spreadsheet ( SPRDSHT-1). In the Spreadsheet tab enter the following text in cell s A1,
A2, and A3.
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4.16.
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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 Liq1. Click on cell B3 and enter the f ollowing: = (B1/B2)*60. This wil l display the residence time in minutes of CSTR-100.
4.17.
We will now create an adjust block to vary the tank volume of CSTR- 100 to achieve a residence time of 20 minutes. Add an Adjust block to the flowsheet from the Model Palette .
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4.18.
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Double click on the adjust block (ADJ-1). Specify the Adjusted Variable to be the Tank Volume of CSTR-
100. Specify the Target Variable to be cell B3 of SPRDSHT-1. Enter a Target Value of 20.
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4.19.
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In the Parameters tab, change the Maximum Iterations to 1000. Click Start to begin calculations. The block should solve.
4.20.
The first CSTR is now full y specified and has residence time of 20 minutes. Note that the vapor outlet stream has a flowrate of zero.
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4.21.
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Double click the second reactor ( CSTR-101). Select Liq1 as the Inlet stream and create Outlet streams called Vap2 and Liq2.
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4.22.
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In the Reactions tab select Set-1 as the Reaction Set.
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4.23.
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Repeat steps 4.13 to 4.20 for the second reactor. When finished the second reactor should solve and have a residence time of 20 minutes.
4.24.
Check the results of stream Liq2. Double click stream Liq2 and go to the Composition form under the
Worksheet tab.
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4.25.
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You can see that the mole fraction of trans-2-butene in the outlet stream is 0.9681. You can add the reaction extents of each reaction to achieve the total reaction conversion. To find the reaction extent, double click a reactor and go to the Reactions | Results page. In this case the reaction extent of the first CSTR is 0.8214 and 0.1467 for the second CSTR. This totals to 0.9681, identical to the analytic solution.
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Problem #2 st
Consider the same 1 order reaction, ex cept this time using two CSTRs of diffe rent sizes. Calculate the conversion achieved if the first reactor has a residence time of 30 min and the second reactor has a resi dence time of 10 min. Assume steady state.
Analytic Solution:
First Reactor Component A Balance
Second Reactor Component A Balance
Conversion
()( )
Aspen HYSYS Solution: 4.26.
The same procedure described in the case of two equal volume CSTRs in serie s should be followed. The only difference being the first CSTR has a residence time of 30 min and the second CSTR has a residence time of 10 min.
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4.27.
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Open the file you created for the previous problem. In the Adjust blocks change the Target Value to 30 for the first reactor and 10 for the second reactor.
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4.28.
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Check results. Add up the reaction extent for both reactors. The first reactor has a reaction extent of
0.8734, and the second reactor has an extent of 0.08822. This totals to 0.9616, which is identical to the analytic solution.
5. Conclusion The conversion is sli ghtly higher when the residence times are the same. When both are 20 min., the conversion is 96.81%, and it is only 96.16% when they are 30 and 10 min. respectively. This is a result of the decreasing dependence of conversion on residence time: the second derivative of conversion with respect to residence time is negative.
Total residence time is not sufficient to describe a series system of CSTRs. Multiple CSTRs in series yield higher conversion than a single CSTR that has a resi dence time equal to the sum of the seri es arrangement.
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