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Distillation column dual-temperature control—Part 2

04.01.2010  |  Friedman, Y. Z. ,  Petrocontrol, New York, New York

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Part 1 of this editorial1 has considered a distillation dual temperature control structure and concluded that such strategy is problematic and not likely to work. If you accept that dual-temperature control is not the ideal basic structure—how then should we configure DCS for distillation? In my experience, either with heat balance or mass balance, only one of the tray temperature controllers can work. Many considerations affect the selection of rectifying versus stripping tray, and it goes without saying that this temperature controller must ultimately affect the yield and, hence, it isn't desirable to choose a stripping tray temperature for a mass-balance control structure.

Say we have selected the stripping tray temperature on a heat-balanced column, and that is a simplified inference of bottom purity. What should we do to control top purity? The simplest way is to set the reflux-to-feed ratio to a reasonable value. I prefer reflux-to-feed ratio over reflux-to-product because the latter introduces a mass-balance component into a heat-balance structure. I.e., the ratio to product flow makes reflux proportional to the drum level controller output.

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