Appendix A. Computer Simulation of Flash Distillation

Multicomponent flash distillation is a good place to start learning how to use a process simulator. The problems can easily become so complicated that you don’t want to do them by hand, but are not so complicated that the working of the simulator is a mystery. In addition, the simulator is unlikely to have convergence problems. Although the directions in this appendix are specific to Aspen Plus, the

procedures and problems are adaptable to any process simulator. The directions were written for Aspen Plus V 7.2, 2010 but will probably apply with little change to newer versions when they are released.

Additional details on operation of process simulators are available in the book by Seider et al. (2009) and in the manual and help for your process simulator.

As you use the simulator take notes on what you do and what works. If someone shows you how to do something, insist on doing it yourself—and then make a note of how to do it. Without notes, you may find it difficult to repeat some of the steps even if they were done just 15 minutes earlier. If difficulties persist, see the Appendix A, “Aspen Plus Separations Trouble Shooting Guide,” at the end of the book.

Lab 1.

The purposes of this lab are to become familiar with your simulator and to explore flash distillation. This includes drawing and specifying flowsheets and choosing the appropriate physical properties packages.

1. Start-up of Aspen Plus

First, log into your computer. Once you are logged in, use the specific steps for your computer to log into the simulator and ask for a blank simulation. This should give you an Aspen Plus (or other

simulator) blank screen.

2. Drawing a Flowsheet in Aspen Plus

Go to the bottom menu and left-click Separators (flash drums). (If you can’t see a bottom menu, go to VIEW and click on Model Library.) After clicking on Separators, left-click Flash2 (That is Flash drums with two outlets). Drag your cursor to the center of the blank space and left-click. This gives the basic module for a flash drum. You can deselect the Flash2 option (to avoid getting extra copies by accident) by clicking on the arrow in the lower-left corner. This is the Cancel Insert Mode button—it is a good idea to click it after completing each step of setting up the flowsheet.

Try left-clicking on the module (in the working space) to select it and right-clicking the mouse to see the menu of possibilities. Rename the block by left-clicking Rename Block, “typing in a name such as

“FLASH” and clicking OK.

The basic flash drum needs to have a feed line and two outlets. Left-click on the icon labeled Material Streams in the bottom menu to get possible ports (after you move the cursor into the white drawing screen). Move cursor to one of the arrows until it lights up. Left-click (take your finger off the button) and move cursor away from flash drum. Then left-click again to obtain a labeled material stream.

Additional streams can be obtained the same way. Put the feed, vapor product, and liquid product streams on your flowsheet. After clicking the Cancel Insert Mode button, highlight the stream names by clicking the left mouse button, and use the right button to obtain a menu. Rename the streams as desired.

An Aspen Plus screen shot is shown in Figure 2-A1.

Figure 2-A1. Screenshot of Aspen Plus for flash distillation

Note that Aspen Plus will happily create mass streams and modules that you do not want (and can hide one behind another), and thus some deletion may be necessary. If a stream is created in error, highlight it with the left button. Then click the right button to obtain a menu that allows you to delete the stream.

Click OK with the left button. Play with the functions until you determine how everything works. If things appear that you don’t want, click the Cancel Insert Mode arrow and then delete.

Once you are happy with your flowsheet, click the Next button (blue N with an arrow in the menu bar).

This will tell you if the flowsheet connectivity is OK. If it is, click OK. If not OK, either complete connectivity or delete blocks and streams that are not being used.

3. Input Data

You will now obtain a Setup Specifications Data Browser. Set the units you want (e.g., MET). Run should be set at Flowsheet input mode at steady state, and stream class at CONVEN. Use mole units,

Ambient pressure = 1 bar, Valid phases = Vapor – liquid, and Free water = No. When happy with this page, click on the Next button. The next page lets you select the components. First, pick a binary mixture of interest (use ethanol and water). Component ID is whatever you want to call it. Type should be

conventional. Then click on the component name box or hit ENTER. If you used ethanol and water as the component IDs, Aspen will complete that row. If you used a different ID, such as E or W, give the appropriate component name (ethanol or water). Then do the next component. Aspen Plus will

recognize these two components. If Aspen Plus does not fill in the formula, click on the Find button and proceed. When done with components, click on Next. Note that Aspen Plus can be picky about the names or the way you write formulas. Note: If you use W as component ID, Aspen will think this is tungsten. Type water as component name instead.

Click Next. You will probably now see the Properties Specifications-Data Browser screen. You must select the appropriate physical properties package to predict the equilibrium for your chemical system.

There is no choice that is always best. The choice is made through a menu item on the right side

labeled “Property Method.” You may need to click twice to get the complete menu. This choice is very important (Carlson, 1996; O’Connell et al., 2009; Schad, 1998). If you pick the wrong model, your results are garbage. A brief selection guide is given in Table 2-4.

We will try different models and compare them to data. [Note that data also needs to be checked for consistency (Barnicki, 2002; O’Connel et al., 2009; Van Ness and Abbott, 1982).] First, try the IDEAL model. Before allowing you to check the VLE data, Aspen (in flowsheet mode) requires you to

complete the input information. Thus, we need to continue. Left-click on the Next button after you have selected a VLE model (IDEAL). If you get a data bank with binary parameters, left-click Next again.

When you get a box that says, “Required Properties Input Complete,” click either “go to next required input step” or “modify required property specifications.” Assuming you are happy with what you have, left-click on OK to go to next required input step.

You should now get a data browser for your input stream. Fill this out. Try a pressure of 1.0 bar, vapor fraction of 0.4 (click on arrow next to Temperature and select vapor fraction), total flow of 100.0 kmol/h, ethanol mole fraction of 0.9, and water mole fraction of 0.1 (use menu under Composition to select mole fraction). Then left-click on the Next button.

Note, always use mole fraction or mass fractions for the units for the composition of the stream (use the menu). Other choices will often lead to inadvertent errors.

Fill out the conditions for the flash drum block (same pressure and vapor fraction as in the feed) and click on the Next button. At this point you will probably get a screen that says input is complete and asks if you want to run the simulation now. Don’t. Click Cancel.

4. Analysis of VLE Data

Aspen will now let us look at the VLE so that we can determine if it makes sense. To do this, go to the menu bar to tools. Choose, in order, Analysis-Property-Binary and left-click. Click OK on box that says you will disable the Interactive Load. On the menu under Analysis Type, pick Txy plot, make sure Valid Phases menu is Vapor-Liquid, change pressure unit to bar, and then click on GO. Cancel or minimize this plot to reveal the data table below it. Compare the vapor and liquid mole fractions to the ethanol water data in Table 2-1. To generate a y-x plot, click on Plot Wizard and follow the instructions.

Compare the y-x plot to Figure 2-2. Note that ideal is very far off for systems such as ethanol and water that have azeotropes. Since Aspen Plus is quite willing to let you be stupid in picking the wrong

properties package, it is your responsibility to check that the equilibrium data make sense. Later, we will find that some VLE packages closely match the actual data.

5. Doing a Flash Run with Ideal Model

Cancel the screens from the analysis. Click the Next button, and when the dialogue box asks if you want to do a run, click OK, and watch Aspen Plus as it calculates (this takes very little time). When it says

“Simulation calculations completed” or “Generating results” you can either click on the blue check box on the menu or go to Run on the menu bar and click Check Results or the Next button.

The most important item in the Results Summary is the line that hopefully says, “Calculations were completed normally.” If it says anything else, you may have a problem. To scroll through the results, use the << or >> buttons near the top of the screen, bracketing the word “results.” If you want to see the input, use the menu to scroll to input. Another useful way to look at results is to go to the tool bar and click on View, then scroll to the bottom and click on Report, then in the window click on the box next to the block you want to see the report for and then click on OK. This report can be printed using the file column in Notepad. By scrolling in the window labeled “Display report for,” you can obtain other reports—Streams is very useful.

Note that Aspen Plus gives a huge amount of results. Spend some time exploring these. Write down the values for vapor and liquid mole flow rates and drum temperature. Also look at the phase equilibrium and record the x and y values or print the xy graph. Of course, all these numbers are wrong, since we used the wrong VLE model.

6. Rerun with Better VLE Model

Now, cancel screens until you get back to the flowsheet (simulators run faster if there are not a large number of open screens). Go to Data in the menu and click on properties. In the Global screen, change the base method for VLE to NRTL-2. Click on Next. Continue clicking OK and redo the run. Check the results. Write these results down or print the plots. Compare the vapor and liquid products with this equilibrium data to the previous run. Go to Analysis and look at the T-x-y and the x-y plots. Compare to the VLE data in the textbook (the most accurate comparison is with Table 2-1).

7. Try Different Inputs

Once you are happy with the previous runs, change the input conditions (using the same flowsheet) to look at different feed compositions and different fraction vaporized. There are at least two ways to input new data. (1). Left-click on the block for the flash. Go to Data in the menu and left-click on input.

Put in the desired numbers using the << or >> buttons (not Next button) to move to different screens. (2) Go to Data in the menu and click on Streams. Click on the << or >> button until you get the Stream (Material) Input-Data Browser. Change the data as desired. Try feeds that are 10, 30, 50, and 70 mol%

ethanol and vary vapor fraction.

8. Adiabatic Flash

The most common way to operate a flash system is to feed a hot liquid at elevated pressure through a valve into an insulated (adiabatic) flash chamber that operates at lower pressure. Try a feed of 100 kmol/h that is 30 mol% ethanol and 70 mol%. The flash chamber operates at 1.0 atm and is adiabatic (set Heat duty = 0).

a. The feed is at 110°C and a pressure of 50 psia. Repeat for 100 psia.

b. The feed is at 130°C and a pressure of 100 psia.

c. The feed is at 150°C and a pressure of 100 psia.

d. The feed is at 151.5°C and a pressure of 100 psia.

e. The feed is at 151.5°C and a pressure of 200 psia.

For all of these cases, look at the feed stream and the two product streams (e.g.,

View→Report→Streams→All). Is the feed stream entirely liquid? When the feed is not entirely liquid, what happens to the liquid and vapor product flow rates? What is the effect of the feed pressure? Why

are the two runs for part a essentially identical, but runs d and e give very different results?

9. Switch to a Ternary Problem

Remove any leftover dialog boxes or screens. Go to Data in the menu and click on components. Use Edit to delete the two rows. You may need to click on the row to obtain the Delete Row command in edit. Then add propane, n-butane, and n-pentane as the three components. Using the menu, go to Data Properties and change the choice of VLE model. Peng-Robinson is a good choice for hydrocarbons. Use the Next button and input the mole fractions (propane 0.2, n-butane 0.3 and n-pentane 0.5). Keep

fraction vaporized at 0.4 and pressure of 1.0 bar for now. Click on the Next button and do the run when ready. Record your results (component flow rates, T, y and x) or print out the report. If you have time, try different fraction vaporized, different feed compositions, and different temperatures.

10. What Does It All Mean?

Reflect on the meaning of your results for both the binary and the ternary flash systems.

a. Binary: How are the compositions of the vapor and liquid streams from the flash system related?

What is the role of the fraction vaporized? How can you do the calculation by hand?

b. Ternary: How are the compositions of the vapor and liquid streams from the flash system related?

What is the role of the fraction vaporized? How can you do the calculation by hand?

Note that the calculation methods used for hand calculations will be different for the binary and ternary systems since the equilibrium data are available in different forms (graphically for the binary and DePriester chart for the ternary).

11. Finish. Exit Aspen Plus and log out. There is no formal lab report for this lab.