A nearly optimal on-off control strategy, previously applied to a rectification column, has been successfully implemented on a full distillation column. The controller employs a linear combination of temperature measurements to adjust the reflux flow rate in order to maintain a desired overhead product composition. A simple relationship between the control coefficients is shown to be characteristic of countercurrent systems. This relationship allows the controller to be tuned on-line by the adjustment of a single parameter, thereby eliminating the need for a dynamic model of the process and for lengthy design calculations.
The performance of the tuned on-off controller is compared with that of a tuned proportional plus integral controller and a single measuremnt on-off controller for feed flow disturbances and set point changes. Results show that the multi-measurement on-off control system is superior for both types of disturbances.