Reflux Drum and Base Level Control

If the distillate is fed to a downstream unit, the variability in flowrate will be a disturbance. So what can we do We can make use of feedforward control to anticipate the required changes in reflux and distillate flowrates (ratio distillate to feed with the ratio reset by the composition contoller). 

In the second case (manipulate distillate to control reflux ratio), the variability of the distillate flow would be greatly reduced. The reflux drum level controller, manipulating reflux flowrate, is made P-only to get slow changes in reflux flowrate, and this gives slow changes in distillate flowrrate in the reflux-ratio control structure. 

So from a plantwide control perspective, setting distillate flowrate to control reflux ratio is a better strategy than using distillate to control composition. Of course similar arguments can be made about bottoms flowrate in the case of a column with a high boilup ratio. 

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