Design-Specification Manual:
A How-to on Aspen Plus

If this screen appears, enter in a title for the simulation and click on the Next button.

If it doesn’t you can find it by clicking on Setup, which is found on the left hand side of the input screen. However, completing the Setup form isn’t manditory.

Once finished inputting a title, Click Next.

In the Component Name box enter the two components for this problem: isobutane and isobutylene (isobutene). Note that the data bank doesn’t recognize the name isobutene, hence isobutylene is entered because it is the same compound and it is in the data bank. Enter these names in the component name column. The names in the component ID column are aliases.

Click Next.

At this screen, enter in NRTL for the "base method” as we have done before in other simulations. This screen is under the Specifications sub-directory of the Properties section.

Click Next.

Aspen will now ask you for the FEED stream information. (Note how the title bar indicates which input form you are in.) Enter a temperature and a pressure of your choice. Next, enter the known flow rate of isobutane in the Total Flow box (make sure that you are using the right units). Also, Aspen Plus requires some composition info for the feed stream. For this example, in the Composition box, select the Mole-Frac and input 1 in the Value box for isobutane. Click on the Next button.

The next screen will be the input screen for the Recycle stream. (Look at the title bar if you are unsure of which form you are in.) Once again enter in a temperature and pressure of your choice as well as the known mole fractions.
Since we don't know the flow rate of this stream, just enter in a guess as was done here. The guess does not matter because we will design-spec will iterate to find this value later.
Enter in a guess.

After all the information is entered, click the Next button

Continue entering information and clicking the Next button until you arrive at this screen. Here is where Aspen will ask you if you want to run a simulation. At this point, we don't want to run the simulation so click the Cancel button.

The next step is to click on the Data button from the toolbar. Go to Flowsheeting options and then Design-Spec (shown below). This will take you to the Design-Spec mode in Aspen Plus.

This is the first window of the Design-Spec mode in Aspen. We want to create a new design-spec so click on the New button.

Aspen will now ask you to name a variable. You can give a name of your choice or choose the default name that Aspen has provided.

Click OK after choosing a name.

The next step is to enter in the type of variable. If you click on the arrow you will see a list of choices. The manipulated variable is given the arbitrary name, “RECYCL” It represents the unknown flow rate of the recycle stream. The type of variable is inputted as
“Stream-Var” which stands for stream variable. Use this choice when your variable is a property of the stream, i.e. temperature, pressure, flow rate, etc.

After entering the type of variable, Aspen will then ask for the location of the variable. In this case, we need to define the name of the stream associated with this variable. Enter “Recycle” for the stream location. The Substream cell will then appear. The default is “mixed” and should be left unchanged. Finally, if we define the type of variable as a Stream-Var, we also need to specify the property variable. In this case the variable is a molar flow rate, so we enter MOLE-FLOW for the last cell. At this point, the screen should be complete and we can move on to the next screen.

Before moving on, we need to define the specified variable that is the mole-fraction of isobutane in the product stream. Do this by creating a new variable the same way as the RECYCL variable was created.

At left is the input screen for the mole fraction variable.

After both variables are entered, go to the Spec folder.

The screen at left is where the value of the “specified variable” is stated. Aspen asks for the name of the variable you wish to specify. At the “Spec” cell, enter the name of the variable that you want to specify., i.e., ANEP. Now enter the target value for the variable which is 0.95. This is the mole-fraction of isobutane in the product stream. The mole-fraction in the product stream is 0.95. Finally, we select the tolerance, an arbitrary value indicating the accuracy of Aspen’s result. Click the Next button.

The last screen of the design-spec mode is where we set the limits of the manipulated variable. The left-hand side is similar to the definition screen. The input is much the same. All we need to do is set the upper and lower boundaries for the manipulated variable. These values are educated guesses.
For our example, the limits were set from 1 to 200.

After all the information is entered, click the next button.

The simulation is ready to run so go ahead and click the OK button.

Here are the results. The Recycle stream flow rate came out to be 33.78 Kmol/hr. One can also notice that Aspen calculated the product stream flow rate as well. This was originally an unknown. Even though we didn’t define this variable, Aspen still calculates the mass balance simultaneously along side the Design-spec. Click on the “>>” button at the top.

If you create a Design-Spec and run a simulation, you should always check this screen. This will tell you if Aspen converged on a value or not.
If convergence was not achieved, it usually means you need to change the limits of your manipulated variable.