A Systematic Approach To Plantwide Control

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This paper summarizes Sigurd Skogestad's struggles in the plantwide control field.

A chemical plant may have thousands of measurements and control loops. By the term plantwide control it is not meant the tuning and behavior of each of these loops, but rather the control philosophy of the overall plant with emphasis on the structural decisions. In practice, the control system is usually divided into several layers, separated by time scale.

My interest in this field of plantwide control dates back to 1983 when I started my PhD work at Caltech. As an application, I worked on distillation column control, which is excellent example of a plantwide control problem. I was inspired by Greg Shinskey's book on Distillation Control, which came out with a second edition in 1984 (Shinskey, 1984). In particular, I liked his systematic procedure, which involved computing the steady-state relative gain array (RGA) for 12 different control structures ("configurations"); the DV-configuration, LV-configuration, ratio configuration, and so on. However, when I looked in more detail on the procedure I discovered that its theoretical basis was weak. First, it did not actually include all structures, and it even eliminated the DB-configuration as "impossible" even through it is workable in practise (Luyben, 1989). Second, controllability theory tells that the steady-state RGA by itself is actually not useful, except that one should avoid pairing on negative gains. Third, the procedure focused on dual composition control, while one in practise uses only single end control, for example, because it may be optimal economically to use maximum heating to maximize the recovery of the valuable product.

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