Figure 7.40 Manipulate window.

Figure 7.40 Manipulate window.

The window shown in Figure 7.40 opens, on which the mole fractions of propane and isobutane are changed from 0.40/0.60 to 0.50/0.50.

The data are loaded into Matlab for plotting. Figure 7.41 compares the responses for a 20% increase in feedflow. The solid lines indicate when only a tray temperature is controlled. The dashed lines show when the composition/temperature cascade control structure is used. The direct composition control structure results are not shown because the responses are very poor. This is as we would expect because of the very low controller gain and the very large integral time.

The cascade control structure gives tighter control with smaller peak transient disturbances and a shorter settling time. Both control structures return the product purities to their desired values because of the R/F ratio used in both.

Figure 7.42 gives results for feed composition disturbance. At time equal 0.1 h, the feed composition is changed from 40 mol% propane to 50 mol% propane. The temperature control structure holds the temperature on stage 9 constant. Both distillate and bottoms

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