Self-Tuning Control of Distillation Columns: Theory and Practice

An experimental evaluation of multivariable self-tuning control algorithms for the simultaneous control of both top and bottom product compositions of a binary distillation column is presented. The study was carried out using an 8-tray, 30.5 cm diameter methanol-water pilot scale column. The column is controlled using an LSI 11/03 microcomputer with continuous capacitance analysis of top product composition and with bottom composition analyzed on a 3 min cycle, by gas chromatograph. Column control is studied for 25% step increases and decreases in feed flow rate. Terminal composition regulation using `positional´ and `incremental´ forms of both single and multiple rate formulations of the algorithms is compared with that achieved using conventional digital PI/PID control strategies. In multirate self-tuning top and bottom composition loops are controlled at different sample rates.