ASPEN Tutorial #2ChE 473K

DSTWU and RADFRAC Distillation Models

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 Mole FractionHydrogen- H2 0.00014Methane-CH4 0.00162Ethylene-C2H4 0.75746Ethane-C2H6 0.24003

Propylene- C3H6-2 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%.

Later we will use a sensitivity analysis to determine the optimum feed location and number of stages. This will be accomplished by varying the number of stages from 23-35 and the feed location from 12-21, and examining the reflux ratio, reboiler duty, and condenser duty.

1. Open a new simulation, and you should be taken to the Components-Specifications form. Enter the components in the feed stream as defined at the beginning of the worksheet.

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2. Hit the Next button, and the Methods-Specifications form should appear. Under “Method Name,” select PENG-ROB.

3. Hit Next twice, and select Go to Simulation environment.

Using the DSTWU model

4. Insert a DSTWU column, one feed to the left of the column and two product streams to the right of the column in the main flowsheet space.

5. Connect the feed arrow to the Feed on the DSTWU block.6. Connect the top product arrow to the Distillate arrow on top of the DSTWU block.7. 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.

The flowsheet should look like this:

8. Click Next and the Streams-S-1-Input-Specifications form should appear.9. Enter the following values for the feed stream:

Flash Type: Vapor Fraction and Pressure Composition Basis: Mole-FracPressure: 18 bar Comp ID (mole fraction)Vapor Fraction: 0 HYDROGEN: 0.00014Total flow rate: 100 lbmol/hr METHANE: 0.00162

ETHYLENE: 0.75746ETHANE: 0.24003PROPYLEN: 0.00075

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The Streams-S-1-Input-Specifications form should look like this:

10. Hit the Next button, and the Blocks-B1-Input-Specifications form should appear.11. Enter the following values:

Reflux ratio: 3.1Light key

Comp: ETHYLENERecov: 0.95

Heavy keyComp: ETHANERecov: 0.03

PressureCondenser: 17.8 barReboiler: 18.2 bar

Note: The recovery of 0.95 is to meet the 95% recovery of ethylene specification. The recovery of 0.03 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 0.03 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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The Blocks-B1-Input-Specifications form should look like this:

12. Select the Calculation Options tab and check the Generate Table block. Set the Initial Number of Stages at a value above the minimum stage count (18 will work) and the Final Number of Stages at 40.

The Blocks-B1-Input-Calcualtion Options form should look like this:

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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 Blocks-B1-Stream Results and the Blocks-B1-Results forms. 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 0.03 value for the Recovery 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. d) Select an appropriate trade-off between reflux ratio and theoretical stages from the

Calculation Option tabulation / plot (shown below). A reflux ratio = 3.1 and 32 theoretical stages appears to be an appropriate starting point.

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.

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The Blocks-B1-Stream Results form should look like this:

The Blocks-B1-Results form should look like this:

Before starting the next section, save your current file. Start a new file by clicking “Save As” and change the name of the file to indicate RADFRAC

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Using the RADFRAC model

1. Insert a RADFRAC column, one feed and two product streams as in the figure below.2. Connect the feed arrow to the center Feeds arrow on the RADFRAC block.3. Connect the top product arrow to the Liquid Distillate arrow in the upper right hand corner

of the RADFRAC block.4. Connect the bottom product arrow 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.

The flowsheet should look like this:

5. The Components-Specifications-Selection, Methods-Specifications-Global, and the Streams-S-1-Input-Specifications forms should be filled out exactly as was done for the DSTWU model.

6. After these forms are filled out, you should reach the Blocks-B1-Setup-Configuration form.7. Enter the following values:

Setup OptionsNumber of Stages: 32 (This value was taken from the DSTWU model.)Condenser: Total Reboiler: Kettle

Operating Specifications

Distillate rate (Mole): 72.85 lbmol/hr (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.)

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Reflux ratio (Mole): 3.1 (This is the same RR as in the DSTWU model.)

The Blocks-B1-Specifications-Setup-Configuration form should look like this:

8. Hit the Next button, and the Blocks-B1-Specifications-Setup-Streams form appears. The feed stream (S-1) should be set to enter Above-Stage 19. (This was also taken from the DSTWU model.)

Note: For the RadFrac model, “Above-Stage N” means that the liquid portion of the feed will enter Stage N, and the vapor portion of the feed will enter Stage N-1. See the figure below.

9. Be sure that the distillate (S-2) is coming from stage 1 and is a liquid, and that the bottoms (S-3) is coming from stage 32 and is also a liquid.

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The Blocks-B1-Specifications-Setup-Streams form should now look like this:

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

11. Open the Blocks-B1-Specifications-DesignSpecs form and click on the New… button.12. Enter the following design specification in the Blocks-B1-Specifications-DesignSpecs-1

form.Design specification

Type: Mole puritySpecification

Target: 0.99Stream Type

ProductNote: This tells Aspen that you want the molar purity of a product stream to be 99%.

The Blocks-B1-Specifications-DesignSpecs-1-Specifications form should look like this:

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

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

15. Hit the Next button and the Blocks-B1-Specifications-Vary form will appear. Click New… and accept the default Vary ID number.

16. Enter the following information in the Blocks-B1-Specifications-Vary-Specifications form.Adjusted variable

Type: Reflux RatioUpper and lower bounds

Lower Bound: 2Upper Bound: 20

OptionalMaximum step size: 1

The Blocks-B1-Specifications-Vary-Specifications form should look like this:

17. Save the simulation, then run it. (Don’t forget to turn on the mole fraction output in the Setup-Report 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. How close are these results to the values estimated by DSTWU?

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The Results Summary-Streams-Material form should look like this:

The Blocks-B1-Results-Summary form should look like this:

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You can plot the mole fraction of each of the components at each stage in the column to get an idea of how the separation occurs. Go to the Blocks-B1-Profiles form, and then click on Composition in the Plot tab of the ribbon above. A popup window will appear and allow you to select the vapor or liquid compositions of any component.

ASPEN HW Assignment

Complete the Tutorial Design Problem using the RadFrac simulation and turn in the following information:

1. Number of stages2. Feed stage3. Reflux ratio4. Ethylene recovery5. Ethylene purity in the liquid distillate6. Reboiler duty7. Condenser duty

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Also turn in the Stream Table for both the DSTWU run and the RADFRAC run, and a plot of the vapor mole fraction profiles in the column (see above for an example).

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