CONTROL OF A BINARY SIDESTREAM DISTILLATION COLUMN

This paper presents results of a comprehensive study of the dynamics and control of a large industrial distillation column that separates a binary mixture of propylene and propane into three products: polymer-grade propylene distillate, chemical-grade propylene liquid sidestream, and propane bottoms. The column is a superfractionator with 190 trays and a high reflux ratio. A two-stage vapor recompression system is used for energy conservation. Plant data, both steady state and dynamic, were used to validate steady-state and dynamic models of the column. Then steady-state rating programs were used to calculate steady-state gains for various alternative control structures, and several steady-state indices were used for initial screening of alternatives. The most promising structures were evaluated on the rigorous dynamic model. Use of sidestream location as a manipulative variable was included in the analysis. This strategy yielded an unusual dynamic response: a naturally occurring derivative action in the blending of liquid sidestreams. Several control objectives were studied, ranging from control of only one composition to control of all three product compositions plus the sidestream flow rate.