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The required changes in the reflux flowrate are less than those required in the reflux ratio. Therefore a fixed reflux-to-feed structure should do a better job in maintaining, at steady state, the desired product purities than a fixed reflux-ratio structure for this column.

Control structure CS3 may have a steady-state disadvantage, but it may provide dynamic advantages because of lower variability in the vapor distillate flowrate. The dynamic simulation results presented in the next section illustrate these effects.

8.2.3 Dynamic Performance

The three control structures are simulated in Aspen Dynamics, controllers are tuned, and feed flowrate disturbances are imposed on the system. At time equal to 0.2 h, the feed flowrate is increased from 100 to 120lb.mol/h. At time equal to 4 h, the feed is dropped to 80lb.mol/h. Finally, at time equal to 7 h, the feed is increased to 120 lb.mol/h. These very large disturbances are handled with different degrees of effectiveness by the three control structures.

Control Structure CS1 Figure 8.7 gives results using control structure CS1. Reflux is ratioed to the feed flowrate, pressure is controlled by distillate flowrate, and level is controlled by condenser heat removal.

Figure 8.7a gives dynamic responses for the large-distillate, low-reflux-ratio case. This control structure produces very large changes in the distillate flowrate as well as fairly large deviations in pressure. When feed is increased and reflux is increased, the level starts to drop, which increases condenser heat removal. Note that heat removal is

220 "a 210

Ol 200 1

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