This control scheme is straightforward. First put a flow controller on the feed. Since it is high purity overheads the composition is regulated by adjusting the distillate rate. This is achieved with temperature in the top half of the column but some distance down as an inner loop cascaded with an analyser. Bottoms temperature is controlled with the steam rate. Finally the inventory loops - reflux drum level controlled by the reflux rate and the reboiler level on the bottoms rate.
Distillate rate is set by flow control so reflux must be used for drum level. Overheads composition can be regulated by boilup rate which is quite satisfactory because of the low vapour inventory in the column. But how to do bottoms composition? This could be done on bottoms flow, but reboiler level would then have to be by feed adjustment. Unfortunately this has a long series of hydraulic delays from tray to tray. It might be better to do reboiler level with bottoms and bottoms composition by feed, although this is an unusual scheme!
Both product rates by flow control. The level in the reflux drum can be controlled by the reflux rate. The bottoms composition is controlled by the reboiler steam rate. This leaves the level in the reboiler either floating or unsatisfactorily controlled by the feed rate.
Distillate composition controlled conventionally either by distillate or reflux rate depending on purity, the other adjustment used for reflux drum level. Bottoms composition by reboiler steam rate leaving the same difficulty with reboiler lever as before.
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