Instrumentation and Control of Distillation Columns with Sharp Temperature Profiles

Abstract

Distillation columns separating components of high relative volatility are characterised by sharp temperature profiles and a highly non-linear relationship between temperature and reboil rate. This paper presents a stability analysis of the problem in a typical case which shows that both linearisation and dynamic compensation techniques are necessary to prevent limit cycling. Linearisation using an optimal linear combination of temperatures on different trays is compared with that of a diode function generator. The dynamic compensation of a band notch filter is compared with that of a standard three-term controller. The paper concludes that a diode function generator should be tried first and if limit cycling still persists, then a band notch filter should also be introduced into the control loop.

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