Aspen Plus Tutorial Separations

Aspen Plus Tutorial, adapted from You Tube Series: https://www.youtube.com/watch?v=xC4SGlVn4HI&list=PLbTw6Y7MopQVwoM7pAMN5oJifKiJgakyH&in dex=2 Separations 1. There are many different separation blocks to choose from in AspenPlus. The three listed below are most commonly used: Flash Drums These can model surge/knockout drums, single stage liquid-liquid extractors, and partial condensers/heaters. There are two types on flash drums available to choose from. Flash2 has 2 outlets (1 vapor and 1 liquid product) and Flash3 has 3 outlets (1 vapor and 2 liquid products). DSTWU This is used to model a simple distillation column and uses the methods of Winn, Underwood, and Gilliand to estimate the minimum number of stages, minimum reflux ratio, the appropriate number of stages for a given reflux, or appropriate reflux ratio for a given number of stages. The model can take a single feed and 2 products (distillate and bottoms). Other assumptions made by this model are constant molar overflow and constant relative volatility. This model is used to get a general sense for the distillation parameters needed to achieve a desired separation. Those parameters then should be refined using a more rigorous model such as RadFrac. RadFrac This is a rigorous model used to model processes such as absorption, stripping, extractive distillation, azeotropic distillation, and reactive distillation. The components can be highly non-ideal and multiple feed and product streams can be considered. The model can differentially simulate a column of trays, random packing, or structural packing. Further, the size and profile of the column can be calculated by specifying information such as the type of trays for example.

2. Let’s try working with flash separators. Open a new simulation with English units. 3. Define the following components:

4. Define PENG-ROB as your property method under “Properties” -> “Methods” folder. 5. Set up the flowsheet below using 2 Flash2 separators:

6. Define the Feed stream as follows:

7. Define the flash drums as shown below. Note that for FL2 we assume adiabatic operation:

8. Run the simulation. The results show look as shown below. Note that most hydrogen and methane come out of Flash 1 as vapor and that Flash 2 does not do a great job of separating benzene and toluene.

9. The results of the simulation for the flash drums are shown below. Note the heat duty for FL1 and the outlet temperature for FL2.

10. Let’s try working with the DSTWU model. Open a new simulation with English units.

11. Define the components below:

12. Define NRTL-RK as your property method under “Properties” -> “Methods” folder. 13. Go to the simulation environment and create the following flowsheet using a DSTWU column.

14. Define the Feed stream as shown below:

15. Define the column as shown below. Note that the “-1.3” means 1.3 times the minimum reflux ratio. Recovery represents the amount of the component in the distillate as compared to the feed.

16. Run the simulation. The Stream Summary is shown below. This will accurately model the desired composition of the distillate.

17. Locate the results for the column shown below. Especially note the minimum reflux ration, the number of stages, the feed stage, and the reboiler and condenser duties.

18. We should now refine our distillation parameters with a more rigorous model like RadFrac. On the flowsheet delete the DSTWU column and replace it with a RadFrac model. Reconnect the streams in order to have the flowsheet shown below:

19. Define the column using the parameters developed in the DSTWU simulation. For the feed tray, round down since trays are numbered from top to bottom.

20. With a RadFrac model we can specify information about the trays so that Aspen can compute the column diameter and profile. Under Blocks -> COLUMN -> Sizing and Rating -> Tray Sizing, click “New” and select “OK”. Specify the trays as shown below, remembering that tray 1 is the condenser and 8 is the reboiler.

21. Check the assumptions on the “Design” tab. The fractional approach to flooding can be between 0.75 – 0.85, the minimum downcomer area should be between 0.1 – 0.2, and the foaming factor should be 1 for distillation columns. Therefore, we shouldn’t have to change anything.

22. Run the simulation. Note the results below, especially the condenser and reboiler heat duties. This simulation also gives us data on the mass balance and the split fraction. This model predicts less recovery of both components in the distillate because of the non-ideal vapor-liquid equilibrium behavior between the two components.

23. Column sizing results are available under Blocks -> COLUMN -> Sizing and Rating -> Tray Sizing -> 1 and by clicking on the “Results” tab. Note the column diameter. Click on the “Profiles” tab to see how the diameter changes with tray (the cone shape would be what the column would look like on the inside, with the outside of course being @ constant diameter for structural integrity).