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Figure 3.34 Control panel. 3.6 USING "DESIGN SPEC/VARY" FUNCTION

The specifications for product impurities are 1 mol% propane in the bottoms and 2 mol% isobutane in the distillate. To achieve these precise specifications, Aspen Plus uses the

Figure 3.35 Stream results for column with RR = 2.
Figure 3.36 Stream results for column with RR = 3.

"Design Spec/Vary" function. A desired value of some "controlled" variable is specified, and the variable to be manipulated is specified. The simulation attempts to adjust the manipulated variable in such a way that the specified value of the controlled variable is achieved.

In the example under study, we want to find the values of distillate flowrate and reflux ratio that drive the distillate composition to 2 mol% isobutane and the bottoms composition to 1 mol% propane.

A word of caution might be useful at this point. The solution of a large set of simultaneous nonlinear algebraic equations is very difficult. There is no guarantee that a solution will be found because of numerical problems. In addition, if good engineering judgment is not used in selecting the target values, there may be no physically realizable solution. For example, if the specified number of stages is less than the minimum required for the specified separation, there is no value of the adjusted variable that can produce the desired result.

Another possible complication is multiplicity. Because the equations are nonlinear, there may be multiple solutions. Sometimes the program will converge to one solution and at other times it will converge to another solution, depending on the initial conditions.

It is usually a good idea to start by converging only one variable at a time instead of trying to handle several simultaneously. In our example, we will converge the distillate specification first by adjusting distillate flowrate. Then, with this specification active, we will converge the bottoms specification by adjusting the reflux ratio. The order of this sequential approach is deliberately selected to use the distillate first because the effect of distillate flow-rate on compositions throughout the column is much larger than the effect of the reflux ratio.

To set up the Design Spec/Vary function, click on Design Spec under the C1 block in the Data Browser window. The window shown in Figure 3.37a opens up. Clicking the New button opens the window shown in Figure 3.37b. Click OK and another window opens

Figure 3.37 Setting up the design spec (a) and the design spec number (b).

(Fig. 3.38a), which has several page tabs. On the first one, Specifications, you can specify the type of variable and what its desired value is. Clicking the dropdown menu under Design specification and Type opens a long list of possible types of specifications (Fig. 3.38b). Select mole purity. Go down to Target and type in "0.02." This is the desired mole fraction of isobutane in the distillate. Then click the second page tab Components. Click the IC4 in the left column under Available components. Clicking the ">" button moves IC4 over to the right Selected components column (Fig. 3.39b).

Click the third page tab Feed/Product Streams, select D1 in the left column, and click the ">" button to move it to the right column. The Design Spec is now completed. Note that the number "1" in Figure 3.39c is blue. Now we must specify what variable to adjust. Clicking the Vary item under the C1 block opens the window shown in Figure 3.40a. Clicking the New button and specifying the number as "1" opens the window shown in Figure 3.40b, where the manipulated variable is defined.

Opening the dropdown menu under Adjusted variable and Type produces a long list of possible variables. We select Distillate rate, which opens several boxes (Fig. 3.40c) in which the range of changes in the distillate flowrate can be restricted. We set the lower bound at 0.2 and the upper bound at 0.6 kmol/s.

Figure 3.38 Specifying the controlled variable (a) and selecting the type of variable (b).

Note that all the items in the Data Browser window are blue, so the simulation is ready to run. We click the blue N button and run the program. The Control Panel window opens and tells us that it has taken three iterations to converge (Fig. 3.41a). Going down to Stream Results at the bottom of the list under the C1 block lets us look at the new values of the stream properties. Figure 3.41b shows that mole fraction of IC4 in D1 is 0.01999713, which is within the error tolerance of the 0.02 mole fraction (mf) desired. Note that the flowrate of D1 has changed to 0.39753743 kmol/s.

The second Design Spec/Vary is set up in the same way. Clicking Design Spec opens a window on which you specify a new design spec ("2"). Then the mole purity of the bottoms B1 is specified to be 0.01 mf propane. See Figures 3.42a-3.42c for the three steps on the three page tabs.

Next a second Vary ("2") is set up as shown in Figure 3.43 with reflux ratio selected. The upper and lower bounds are set at 1 and 5, respectively, since we know a reflux ratio of ~3 gives results that are close to the desired.

Everything is ready to run again. Clicking the blue N button executes the program. The simulation converges in three iterations.

Figure 3.44 shows the new stream results. The mole fraction of iC4 in the distillate is 0.0200027, and the mole fraction of the C3 in the bottoms is 0.0100008. Both are now very close to their specified values. Of course, the distillate flowrate and the reflux ratio have

Figure 3.39 (a) Select components; (b) select IC4; (c) specify stream.
Figure 3.40 (a) Opening the Vary window; (b) defining manipulated variable; (c) setting limits on distillate flowrate.

Type:

- Specifica Target:

- Stream Pro I

H Control Panel

B-l Calculation Sequence