Absorption Column Simulation Tutorial 1. Insert a RADFRAC column model in the workspace. 2. Attach two streams to the feed of the RADFRAC model. One should be labeled 1 and the other labeled 2. Note: The order that you attach the feed blocks is very important. If they are not numbered as seen in the illustration, you will have problems later.
3. Attach the Vapor Distillate output from the top of the RADFRAC model to one of the product output arrows. This arrow should be labeled 3. 4. Attach the Bottoms output from the bottom of the RADFRAC model to one of the product output arrows. This arrow should be labeled 4. The simulation should look like this.
5. Hit the Next button, enter the title of “Example Adsorption Problem”. 6. Hit the Next button again. Enter the following in the Comp ID field: METHANE ETHANE PROPANE N-BUTANE N-DECANE
The Components-Specifications form should look like this.
7. Hit the Next button, and the Properties-Specifications window should appear. 8. Choose NRTL for the Property Method. 9. Use the Next button to get to the Stream-1-Input form. 10. In this simulation Stream 1 is the lean oil absorbing medium. This is because it is feeding into the top, or first stage, of the RADFRAC model. Enter the following values: Temp: 100 F Pres: 5 ATM Composition Basis: MOLE-FRAC N-BUTANE: .005 N-DECANE: .995 Total MOLE-Flow: 10000 LBMOL/HR Note: Make sure that you have changed the pressure units to ATM and the you have changed the composition basis to Mole-Frac. The Stream-1-Input form should look like this:
11. Use the Next button to get to the Stream-2-Input form. 12. Enter the following values: Temp: 100 F Pres: 5 ATM Composition Basis: MOLE-FRAC METHANE: .88 ETHANE: .04 PROPANE: .05 N-BUTANE: .03 Total MOLE-Flow: 5500 LBMOL/HR Note: Make sure that you have changed the pressure units to ATM and the you have changed the composition basis to Mole-Frac. Note: Do not enter zero in the other component MOLE-FRAC fields, just leave them blank. Stream-2-Input should look like this:
13. Hit Next and the Blocks-B1-Setup-Configuration form will appear. 14. Define the number of stages as 5. 15. We do not have a condenser or a reboiler, so we enter none in the condenser field and none in the reboiler. Blocks-B1-Setup-Configuration should look like this:
16. Hit Next and the Blocks-B1-Setup-Streams form appears. 17. Make the feed location for Stream 1, Stage 1 (the top of the column) and the feed location for Stream 2, Stage 5 (the bottom of the column). 18. Change the Convention for Stream 2 to ON-STAGE, by clicking on the field and using the list function. 19. Product stream 3 (the exit gas) should be coming from stage 1 (the top of the column) and should be labeled as vapor phase. Product stream 4 (the exit liquid) should be coming from stage 5 (the bottom of the column) and should be labeled as a liquid phase.
Blocks-B1-Convergance-Advanced form should look like:
24. Next select open the Blocks-B1-DesignSpecs form. 25. Click New in the Object Manager. Hit OK and a default Spec. number of 1 will be set. 26. This will open the Blocks-B1-DesignSpecs-Specifications form. Under Design specification type enter Mole purity. 27. Under Specification Target enter .01. 28. The stream type should be set to Product. Note: This sets the total removal of propane in the vapor stream to about 80%.
Blocks-B1-DesignSpecs-Specifications form should look like:
29. Hit the Next button and the Blocks-B1-DesignSpecs-Components form will appear. 30. Under Available components select PROPANE and move it to Selected components using the > button.
Blocks-B1-DesignSpecs-Components form should look like this:
31. Hit the next button and the Blocks-B1-DesignSpecs-Feed/Product Streams form will appear. 32. Under available product streams select 3 and move it to selected stream using the > button. The completed design specification states that you wish for the mole fraction of propane in the product stream number 3 to be equal to .01.
Blocks-B1-DesignSpecs-Feed/Product Streams form should look like this:
33. Click the Next button and the Blocks-B1-Vary-Object Manager will appear. Hit the New… button and hit OK to accept the default vary ID of 1. 34. This will open the Blocks-B1-Vary-1-Specifications form. Set the Adjusted variable type to Feed rate. 35. Set the stream name to 1. This means that you are going to vary the feed rate of the lean oil absorbing medium. 36. Set the following values: Lower bound: 5000 lbmol/hr Upper bound: 20000 lbmol/hr Maximum step size: 1000
Blocks-B1-Vary-1-Specifications form should look like this:
37. Save the simulation and then run the simulation. Before running the simulation you may want to go to the Setup-Report Options-Stream form and the Mole Fraction Basis output. This will make interpreting the results easier. 38. You will notice that the mole fraction of propane is stream 3 is .01 as we specified, and that the molar flow basis of stream 1 is larger than the 10000 lbmol/hr value that we initially specified. This indicates that the vary block had to increase the feed rate to meet the desired specification.
