Creating a Reaction Engineering Process Model

Now that you have gained access to ASPEN PLUS^TM, you are ready to begin creating a process model. The following series of steps will create a process model for the tubular reactor (PFR) example problem 4-3 taken from the 4th Edition of Elements of Chemical Reaction Engineering by H. Scott Fogler.

Here is a summarized version of the problem:

Example 4-3

Determine the plug-flow reactor volume necessary to produce 300 million pounds of ethylene a year from cracking a feed stream of pure ethane. The reaction is irreversible and elementary. We want to achieve 80% conversion of ethane, operating the reactorisothermally at 1100K at a pressure of 6 atm.

C₂H₆ (g) C₂H₄ (g) + H₂ (g)

A B + C

Where A is gaseous ethane, B is gaseous ethylene, and C is gaseous hydrogen.

Other information:

• Fa_o = 0.425 lbmol/s (calculated from 300 million pounds of ethylene at 80% conversion)

• k = 0.072s^-1 at 1000K

• Activation Energy, E = 82 kcal/gmol

Building the Process Flowsheet

The first step in creating a process model is drawing the flowsheet in ASPEN PLUS^TM, much like you would on paper. Note that while you're constructing the flowsheet, text in the lower right corner will state "Flowsheet Not Complete." This will change to "Required Input Incomplete" when the flowsheet is finished.

Unit Operations

The easiest way to create the flowsheet is to start with the Unit Operation and add the streams to it.

1. Along the bottom of the window there are several tabs with different types of equipment as labels. Click on the Reactors tab.

2. There are seven different types of reactor models in ASPEN PLUS™ 12.1. For this example we are going to use RPlug

3. Click on the RPlug button once and then click again on the blank process flowsheet. You will notice on the left of the different reactor buttons there is a down arrow wich brings up a pulldown menu. You can change the icon that appears on the process flow sheet from this menu. There will be no difference in the calculations ASPEN PLUS™ performs. The icon affects only the appearence of the flowsheet.

   
   
   

4. You should now see an icon for a plug flow reactor on the process flow sheet. It will arbitrarily be named B1. (you can change the name later)

Inlet (Feed) Streams

Next we need to add the inlet stream.

1. There is a button labled Material STREAMS at the lower left of the window. Click on this button.

2. When you move the cursor onto the process flowsheet you will see two red arrows and two blue arrows appear on the reactor. These arrows indicate places to attach streams to the reactor.

   
   
   

3. Move the cursor over the red arrow at the top of the reactor. This is the feed stream. Click once when the arrow is highlighted and move your cursor so that the stream is in the position you want. Then click once more. You should see a stream labled 1 entering the top of the reactor icon.

You have finished adding the inlet stream. Note this example only has one arrow (representing the ethane feed). More than one inlet stream can be drawn. Note also that if you have more than one reactant in the feed, you do not need more than one inlet stream in ASPEN PLUS™. You can specify multiple components in one stream.

At this point your screen should look something like this:

To finish our flowsheet we need an outlet stream

Outlet (Product) Streams

You are now ready to add the outlet stream (containing both ethylene and hydrogen) to your flowsheet. Repeat the steps described above for the inlet stream, however instead of highlighting the red arrow at the top of the reactor, highlight the red arrow ar the bottom. Of course, for other examples, there could be more than one outlet stream.

Your screen should now look like this:

Connecting the Streams to the Unit Operation

If you did not connect the stream to the unit when you created the stream, you can still attatch it to the unit.

1. To attatch a stream already on the flowsheet, first click on the arrow button, directly abovee the Material STREAMS button.

2. Double click on the end of the stream you wish to connect to the unit. Your mouse will now control the end of the stream.

3. To attach the stream simply move the end of the stream over one of the red or blue arrows on the unit. While the arrow is highlighted click once more and the stream will be attatched to the unit.

Entering Process Conditions

Now that the process flowsheet is complete, it is time to enter the process conditions. In the bottom right of the screen the warngin message has changed from "Flowsheet Not Complete" to "Required Input Incomplete". This means that entering conditions, operating conditions or kinetic information is missing. ASPEN PLUS^TM will guide you through the required input windows, simply click on the next button near the middle of the main toolbar. When each input is complete, a will appear next to the completed section in the explorer window on the left.

1. Click Next with the left mouse button. A window will prompt you that the flowsheet is complete and asks whether the next input form should be displayed. Click OK (At the University of Michigan, a Windows Installer window appears again. As before you should cancel the Installer (it may take several times before the Installer finally quits).

2. The first input window will be called Setup Specifications. With the left mouse button, click once on the Title box to highlight it, enter the title of your process model.

3. Check that the desired units are correct. You can change the units in the pull-down menu under Units of measurement. Click on the desired units.

4. A should appear next to Specifications in the navigation window on the left. Click Next.

The next input window is Components Specifications. Here is where all of the chemical species in your process model are specified.

5. For this particular example, the components are: ethane, ethylene, and hydrogen. Start with the second to last column called Component name. Click on the first row in the column and type in: ethane. Hit Enter.

   ***Note: If you are unsure of how to spell the chemical name, or do not know whether it's in the ASPEN PLUS^TM library, simply click the Find button below the components table. From here you can search for the correct chemical.

6. Ethane is in the ASPEN PLUS^TM chemical library. Notice how the molecular formula automatically appears after typing it in. Now click under Component ID. Enter an id name (to show up in results) for ethane, perhaps ETHA. Hit Enter.

7. Repeat steps 5 and 6 for ethylene and hydrogen. In this example, the component id names used were ETHY and H2, respectively.

8. appears. Click Next.

The next window to appear is Properties Specifications. Here is where you specify the solving engine used to simulate your process model.

9. From the Property method pulldown menu select SYSOP0
   Hit Enter.

10. appears. Click Next.

Once the required properties information has been input and you click the next button, a window will pop up asking weather to continue to the next step or to modify the properties. Check the circle next to Go to Next required input step.

The next window to appear is Streams. Here is where you specify the components, temperature and pressure of input streams.

11. Under "State variables" go to the menu that says Temperature. Enter 1100 and from the pulldown menu to the right of the entry select K for Kelvin.

12. In the Pressure entry type 6 and select atm from the pulldown menu.

13. Now look to the right and find the Composition table. From the first pulldown menu select Mole-flow and change its units to lbmol/sec from the right pulldown menu.

14. Since ethane is the only component of the inlet stream, click in the box next to ETHA and enter 0.425 for the molar flowrate.

15. appears. Click Next.

The next input window is titled B1. This window is where you will specify the operating conditions of the PFR. This is the window you will come back to upon running the simulation to change any operating conditions as well.

16. You start at the Specifications tab. From the Reactor type pulldown menu select Reactor with specified temperature.

17. In the Operating conditions section mark the first choice: Constant at inlet temperature

18. Click Next

19. Click in the Length box and enter a value. 10 feet is a good starting point. Be sure the units are ft. Change the units accordingly via the pull down menu method if necessary.

20. Do the same for the Diameter box. 3 feet is a good guess for the diameter. Be sure the units are in ft.

    ***Note. You are solving this problem by guessing a volume. When you run the simulation you will see what conversion is achieved with the guessed volume. You will keep changing the volume (increasing/decreasing the length while keeping the diameter constant) and rerunning the simulation until the desired conversion is achieved. Remember that a PFR is a cylinder with a volume of V = (Pi/4)D²L.

21. Hit the next button. (The will not show up yet)

22. The next window asks you to select a reaction set. However, you have not created a reaction set yet. From then navigation menu on the left, double click on the Reactions folder. Two subfolders will appear. Click on the Reactions subfolder.

This is where you will specify the stoichiometry of the reaction occurring in the PFR.

22. In this window, selct New

23. Enter a name for the reaction ID (for this example we used R-1) and then selcet LHHWH from the Select type pulldown menu. Hit ok.

24. This takes you to the Stoichiometry tab. From here select New....

25. Under Reactants, select ETHA from the Component pulldown menu.

26. Now look at the balanced reaction equation in the problem statement. The coefficient of ethane is 1 so enter 1 in the Coefficient box. Notice that ASPEN PLUS™ automatically changes coefficients of reactants to negative numbers.

27. Under Products, select ETHY from the Component pulldown menu. Enter a 1 in the coefficient box. Then select H2 from the Component pulldown menu and enter 1 for the coefficient. Click on the Next button to close the window and again to advance to the next input.

The next input is under the Kinetic tab. Here you will describe the rate law of the reaction in the PFR.

28. The reaction you just entered should be shown in the first pulldown menu. From the Reacting Phase pulldown menu select Vapor

29. Then enter the information in the Kinetic Factor section.

30. Enter 0.072 for k, 82 for E, and 1000 for To. Make sure the units for E are kcal/mol and K for To.

31. Lastly, the rate law must be entered. To do this click on the Driving Force button to the right.

32. This example has an elementary rate law, hence, -rA= kC_A. Therefore, under the Reactants, enter 1 in the box next to ethane (ETHA). Type 1 for the Exponent.

33. Under Products, Enter 0 for the Exponents of both ethylene and hydrogen (ETHY and H2).

34. Enter 0 for all four driving force constants (A, B, C, D) at the bottom of the window.

35. from the Enter term pulldown menu, select Term 2. Since this was an elementary reaction and there is no second term enter 0 for all exponents and B, C, and D coefficients.

36. Due to the method ASPEN PLUS™ uses to specify reactions, you must enter a very large negative number for coefficient A. -10000000 should work. This will make term 2 essentially 0, leaving us with the elementary rate law. Click Next to close the window.

37. Click Next

38. This takes you back to the B1 block. If you remember, we did not specify a reaction set for the PFR becasue we did not have one created. Now that we are done creating our reaction set, we need to add it to blcok 1. Select R-1 (or whatever you titled the reaction) and select the button.