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Aspen Plus Tutorial 5_Preparing and Configuring a Distillation Column using RADFRAC

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Aspen Plus Tutorial 5: Preparing and Configuring a Distillation Column using RADFRAC
We have been asked to design a C2 splitter that will give 95% recovery of the ethylene feed, as well as a purity of 99% ethylene in the distillate. The feed conditions are as follows: Component Hydrogen- H2 Methane-CH4 Ethylene-C2H4 Ethane-C2H6 Propylene- C3H6-2 Mole Fraction 0.00014 0.00162 0.75746 0.24003 0.00075

We will use the DSTWU model to estimate the operating conditions. Run the DSTWU at P=18 bar, RR=3.1, and basis=100 lbmol/hr. DSTWU performs a Winn-Underwood-Gilliland shortcut design calculation for a single-feed, two-product distillation column, with a partial or total condenser. Then we will use these results to design a RADFRAC analysis. RadFrac is a rigorous tray to tray model for simulating all types of multistage vapor-liquid fractionation operations. Make sure that you have a total condenser, and that you have a spec. which sets the purity of ethylene in the Distillate to 99%. Next week we will use a sensitivity analysis to determine the optimum feed location and number of stages. Vary number of stages from 23-35 and feed location from 12-21. Output reflux ratio, reboiler duty, and condenser duty.

Using the DSTWU model
1. Inset a DSTWU column, one feed to the right of the column and two product streams two the left of the column in the flowsheet space. 2. Connect the feed arrow to the Feed on the DSTWU block. 3. Connect the top product arrow to the Distillate arrow on top of the DSTWU block. 4. Connect the bottom product arrow to the Bottoms arrow on the bottom of the DSTWU block. Note: The stream numbers become important later, so make sure that they appear the same as the example picture.

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5. Hit the Next button and enter "DSTWU Distillation Example" as the title. 6. Hit the Next button and you should be at the Components-Specifications form. 7. Enter the components which are in the feed stream, as defined at the beginning of the worksheet.

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8. Hit the Next button, and the Properties-Specifications form should appear. For Property method choose PENG-ROB. 9. Hit Next twice, and then OK and the Streams-1-Input-Specifications form should appear. 10. Enter the following values: Pressure Vapor Fraction Composition Basis HYDROGEN METHANE ETHANE ETHYLENE PROPYLEN Total Mole Flow 18 bar 0 Mole Fraction 0.00014 0.00162 0.24003 0.75746 0.00075 100 lbmol/hr

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11. Hit the Next button, and the Blocks-B1-Input-Specifications form should appear. 12. Enter the following values: Reflux Ratio Light Key: Component Recovery Heavy Key: Component Recovery Condenser Pressure Reboiler Pressure 3.1 Ethylene 0.95 Ethane 0.03 17.8 bar 18.2 bar

Note: The Light Key Recov of .95 is to meet the 95% recovery of ethylene specification. The Heavy Key Recov of .03 is to make the purity of the distillate stream approximately 99% ethylene. This value can be calculated by doing a material balance around the column. The value of .03 does not account for any of the other components going into the distillate product stream, and is therefore just an estimate.

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13. Hit the Next button, and ASPEN should indicate that the required input is complete. You should save the program before pressing OK and running it. 14. You may want to turn on the mole fraction output in the Setup-Report Options-Stream form prior to running the simulation. 15. Run the simulation, and check the results. We will use the results from both the StreamSum.Main form and the Blocks B1 (Dstwu.Results). The most important values to note are: a) Total Molar Flow Rate of the distillate product stream. This is one of the necessary inputs for the RADFRAC model. This number is not completely correct since the .03 value for the Recovh specification is just an estimate. b) Actual Equilibrium Stages, this will be used as the first guess for our RADFRAC column. c) Feed Stage Location, this will also be used as the first guess for the RADFRAC column. The rest of the information could also be useful as an initial estimate of the results from the RADFRAC model. This information will be calculated more rigorously when we run the RADFRAC model simulation.

Using the RADFRAC model
We can start from the DSTWU model 1. Save the last model and the save it again under a new name. Delete the DSTWU block. Then add a RADFRAC block in its place 2. Connect the feed line to the center Feeds arrow on the RADFRAC block.

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3. Connect the top product line to the Liquid Distillate arrow in the upper right hand corner of the RADFRAC block. 4. Connect the bottom product line to the Bottoms arrow on the bottom of the RADRAC block. Note: The stream numbers become important later, so make sure that they appear the same as the example picture.

5. The Components-Specifications-Selection, Properties-Specifications-Global, and the Streams-1Specifications forms should be filled out exactly as was done for the DSTWU model. 6. After pressing NEXT you should reach the Blocks-B1-Setup-Configuration form. 7. Enter the following values: Setup Options Number of Stages: 32 (This value was taken from the results of the DSTWU model.) Condenser: Total (The difference between total and partial condensors should be discussed in class) Reboiler: Kettle (The difference between kettle and thermosyphon reboilers should be discussed in class) Operating Specifications Distillate rate (Mole): 72.68 (This is the total molar flow rate of the distillate product. This value is close to the value in the DSTWU model, but has been calculated from the material balance.) Reflux ratio (Mole): 3.1 (This is the same RR in the DSTWU model.)

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2005-2-24

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8. Hit the Next button, and the Blocks-B1-Setup-Streams form appears. The feed stream 1 should be set to enter Above-Stage 18. (this was also taken from the DSTWU model). 9. Be sure that product stream 2 is coming from stage 1 and that it is a liquid and that product stream 3 is coming from stage 32 and that it is a liquid.

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10. Hit the Next button, and the Blocks-B1-Setup-Pressure form appears. Enter Top Stage pressure as 17.8 BAR.

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11. Open the Blocks-B1-DesignSpecs-Object Manger form. 12. Click on the New… button. 13. Click OK to accept the default design specification number. 14. Enter the following design specification in the Blocks-B1-DesignSpecs-1-Specifications form. Design specification type: Mole purity Specification Target: .99 Stream Type: Product Note: This tells Aspen that you want the molar purity in one of the product streams to be 99%.

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15. Hit the Next button and the Blocks-B1-DesignSpecs-1-Components form will open. Select Ethylene as the component to specify.

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16. Hit the Next button and the Blocks-B1-DesignSpecs-1-Feed/Product Streams form will open. Select stream 2 (the distillate product stream) as the specified stream.

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17. Hit the Next button and the Blocks-B1-Vary-Object Manager will appear. Click New… and accept the default Vary ID number. 18. Enter the following information in the Blocks-B1-Vary-Specifications form. Adjusted variable type: Reflux Ratio Lower Bound: 2 Upper Bound: 20 Maximum step size (optional): 1

http://www.che.utexas.edu/~che473k/aspen/aspentutor5.htm

2005-2-24

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19. Save the simulation, then run it (don't forget to turn on the mole fraction output in the SetupReport Options-Stream form prior to running the simulation). Check the results. You should have a 95% ethylene recovery, and 99% purity of ethylene in the distillate product. Make sure that the RR seems reasonable.

Assignment
How close are the RADFRAC results to the values estimated by DSTWU? On one page, give relevant information to allow someone to design your final column and summarize its performance for separating ethane from ethylene. Some important things to show might be (but is not restricted to) number of trays, heat duties on condenser and reboiler, reflux rate and ratio.

http://www.che.utexas.edu/~che473k/aspen/aspentutor5.htm

2005-2-24


				
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