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shown as a negative number, using Aspen notation, so the lower the QC curve, the more heat removal. This tends to reduce pressure. However, at the same time, the temperature controller sees a drop in stage 8 temperature caused by the increase in feed flowrate, so it increases reboiler heat input. The net effect is a small initial drop in pressure followed by a large increase.

The disturbance that produces the largest deviations from normal is the 50% step decrease from 120 to 80 lb.mol/h in feed flowrate at time equal to 4 h. This produces a drop in pressure of over 10psi and an increase in temperature of 15°F. The distillate flowrate goes all the way to zero, which would make life very difficult for a downstream unit.

Figure 8.7b gives dynamic responses for the small-distillate, high-reflux-ratio case. The changes in the distillate flowrate are even larger (on a percentage basis). In fact, the distillate is completely shut off for about 20 min following the large drop in feed flowrate at time equal to 4 h. The deviations in pressure are much larger than those seen in the previous case.

These results show that this control structure has poor dynamic performance, particularly when the distillate is fed to a downstream unit.

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