Control of High-Purity Distillation Columns from a Robustness Viewpoint

A realistic study of the LV-control of a high-purity distillation column is presented. Linear controllers designed based on a linearized model of the plant are found to yield acceptable performance also when there is model-plant mismatch. The mismatch can be caused by uncertainty on the manipulated inputs, nonlinearity and variations in reboiler and condenser holdup. The presence of input uncertainty makes the use of a steady-state decoupler unacceptable. The effect of nonlinearity is strongly reduced by using the logarithm of the compositions. A simple diagonal PI-controller is not sensitive to model-plant mismatch, but yields a response with a sluggish return to steady-state.