Distillation columns are inherently multi-variable. As discussed in Chapter 1, even the most simple single-feed, two-product column is a 5 × 5 system: five manipulated variables and five controlled variables. The column can normally be reduced to a 2 × 2 system because the control loops for the two liquid levels and the pressure do not interact significantly in many columns. Thus we have in the simplest of distillation columns a system with two compositions or two temperatures that can be controlled and two manipulated variables to achieve this control.
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