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Control Structure CS2 Figure 8.8 gives results using control structure CS2. Condenser heat removal is fixed, pressure is controlled by distillate flowrate, and level is controlled by reflux. Figure 8.8a illustrates the large-distillate case and Figure 8.8b, the small-distillate case.

The pressure deviations are much less than with the previous control structure because condenser heat removal is constant and does not contribute to the changes in pressure. The variability in the flowrate of the vapor distillate is also significantly reduced. This is true for both the large- and small-distillate flowrate cases. Therefore, from the perspective of plantwide control, this structure is dynamically better than the previous one.

Control Structure CS3 Figure 8.9 gives results using control structure CS3. Pressure is controlled by condenser heat removal, level is controlled by reflux, and distillate flow-rate is ratioed to the reflux flowrate. Figure 8.9a gives results for the large-distillate case and Figure 8.9b, the small-distillate case.

The deviations in pressure are even smaller than in either of the previous control structures. In addition, the changes in the vapor distillate flowrate are more gradual.

However, note that stage 8 temperatures take longer to return to the setpoint value and that the changes in reboiler heat input are larger. This occurs because both the distillate and the reflux flowrates change for a change in feed flowrate, which necessitates larger changes in vapor boilup.

Thus this control structure is better from a plantwide control perspective, but it may not be as good from an individual column control perspective.

Comparisons Figure 8.10 provides direct comparisons among the three alternative control structures for the two cases. The solid lines represent the small-distillate case; the dashed lines, the large-distillate case.

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