The control of column AP by throttling steam to the reboiler was once very popular in the chemical industry, particularly for small columns. The usual practice was to run at a boilup that would give considerably more reflux than called for by design. This would usually provide a product purer than specification. In an era when it was common practice to overdesign columns (low f factors, bubble-cap trays, and extra trays) and there was litde concern about saving energy, this approach to control did have the advantage of usually providing a good-quality product with simple instrumentation. For today's tighdy designed columns, it is technically less satisfactory, and with the rapidly rising energy costs its wastage of steam is economically unattractive. Nevertheless we still have an interest in this control technique for override purposes; an override controller is now commonly used to keep column A P from exceeding the maximum value specified by the column designer or determined by plant tests.
Before looking at the overall control scheme, let us discuss what is meant by the column impedance, Zcoi(s).
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