ON USING SUMS AND DIFFERENCES TO CONTROL DISTILLATION

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Abstract
The idea to use sums and differences of compositions (or temperatures) to improve dual-composition control of distillation has been suggested a number of times in the literature, at least since 1962. The main advantage of the scheme has been claimed to be that it results in noninteraction, i.e. the two composition control loops act as noninteracting SISO-Ioops. Most of the schemes suggested have used distillate flow D and vapor boilup V as manipulators for the composition loops, although also other manipulators have been suggested, Some new theoretical results (Haggblom and Waller, 1987) show that noninteraction (triangular system matrix) at steady state is obtained for separations where the distillate flow D is equal to the bottoms flow B when the sum of product composition is controlled by D or by B. Dynamic simulations for one such case show that, compared to the usual approach of using D to control top composition and vapor boilup V to control bottoms composition, an improvement in control may be obtained when controlling the composition sum by D and the difference by V. Also for another case with D + B an improvement is obtained although the controlled variables are not the product compositions but the compositions on plates 3 and 17. When temperatures are the controlled variables or when D + B, noninteraction is not generally obtained. An experimental example illustrates that controlling the temperature sum and difference can degrade control quality. The hypothesis that the controlling of sums and differences would always result in noninteraction and improve control is thus falsified.

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