Dynamic Control

The control of this purge sidestream column is much more complex than one might expect. Since the sidestream is very small, we might assume that simply flow-controlling the sidestream at a rate high enough to remove all the intermediate in the worst case (highest feed concentration) might do the job.

For example, suppose that we set the sidestream flowrate at 3 kmol/h instead of the design 1.21 kmol/h. Unfortunately this reduces the concentration of the methanol in the sidestream from 81.7 mol% to 33.3 mol% under design conditions where the feed

Stage

Figure 10.30 Purge sidestream column; RR = 0.5.

Stage

Figure 10.30 Purge sidestream column; RR = 0.5.

composition is 1 mol% MeOH. Let us consider a control structure in which the temperature on stage 17 is controlled by manipulating reboiler heat input and reflux flowrate is fixed. Now, if the feed composition is increased to 2 mol% MeOH, the sidestream composition changes to only 42 mol% MeOH. This is not enough to remove all the additional MeOH in the feed, so the distillate purity is severely affected (it increases to 1.55 mol% MeOH). So a simple control structure with a fixed sidestream flowrate does not provide effective product quality control. The control structure must be able to adjust the sidestream flowrate in some manner so that methanol cannot drop out the bottom or go overhead.

An apparently straightforward control structure is to measure the methanol composition of the sidestream and to control this composition by manipulating sidestream flowrate. As we demonstrate below, this scheme does not work well.

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