Performance Evaluation

We want to see how well the three alternative control structures developed above perform in the face of disturbances, specifically, how close to the desired values of temperature and composition these variables are maintained, both at steady state and dynamically. A disturbance is made and the transient responses are plotted.

7.6.1 Installing a Plot

To see what is going on, the first thing to do is to set up a plot or a stripchart. This will show how the variables of interest change dynamically with time. To open a plot, go to Tools on the top toolbar and click New Plot. A plot window opens, as shown on the left in Figure 7.35a. A number of variables will be plotted: flowrates of the feed, distillate, bottoms, and reflux; stage 7 temperature; reboiler heat input; and distillate impurity (mf isobutane) and bottoms impurity (mf propane). We "drop and drag" each one of these variables onto the plot window. The table on the right of Figure 7.35a is opened by clicking the icon for the F1 stream, right-clicking, and selecting Forms and then Results. Place the cursor just to the right of F (total molar flow) in this table and click once. Then click and hold the left mouse button, drag the cursor to the plot window, and release. This procedure is repeated for each variable. The plot with all variables added is shown in Figure 7.35b. Now we need to define the scales of each variable, simplify the labels, and make other "beautification" changes to the plot. This is done by right-clicking on the plot and selecting Properties. The window shown in Figure 7.36 opens, which contains a number of page tabs. On the Axis Map page tab, click the box One for Each to get different scales, which are then set on the Axis page tab. If you want fixed scales, uncheck the box to the left of Reset axis range to data for each of the variables and define minimum and maximum values for each.

Figure 7.37 illustrates the type of plot that results from making a dynamic run. At time equal 0.1 h, the setpoint of the feedflow controller is changed from 1 to 1.2 kmol/s. To

(a)

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