Profitable and Stable Control for Alcohol Separation in the Presence of Water

Old pneumatic instruments were replaced on a get of alcohol distillation columns with new microprocessor-based distributed controls. The objectives of the control system were to provide safe operation, stable control, increase throughput, save energy, and control product composition as close as possible to desired values. All of the objectives above were achieved with the control strategy described in this article. The control system, with the appropriate composition measurements, increased throughput by 16 percent, reduced energy by more than 10 percent, and at the same time considerably reduced the variation of the composition on both the top and bottom products. Combined reflux and distillate control was used to control top composition, while heat input was manipulated to control bottom composition. Moreover, constraint control was included to operate the columns against equipment limits for optimization. Safety override control was also included to shut down feed and/or reboiler steam when certain conditions occurred, to assure safe operation. A single stream chromatograph was used to measure the percent water and percent isopropyl alcohol in the top product, with the sample taken from the overhead vapor line. A separate analyzer was used to measure methanol and percent isopropyl alcohol in the bottom of the column using a vapor sample below the bottom tray. These composition measurements were used in the control strategies. Under some process conditions changing relative volatilities between methanol, isopropyl alcohol, and water created reverse response in some composition loops, i.e., positive feedback. This problem was resolved by process analysis and added controls.