Effect of Feed Composition on the Selection of Control Structures for High-Purity Binary Distillation

Abstract

This paper discusses the selection of an effective control structure for a binary distillation column producing high-purity products. Results show that this selection depends on the feed composition. If the concentration of the light component is large, the reflux ratio is small. Control of a single appropriate tray temperature by manipulating the reboiler heat input and using a fixed feed-to-reflux ratio provides effective control for both feed-rate and feed-composition disturbances. The reflux-drum level is controlled by the distillate flow rate. For intermediate concentrations of the light component and for high-purity products, this study shows that a two-temperature control structure with reflux and reboiler heat input manipulated is required to handle feed-composition changes. Small concentrations of the light component yield small distillate flow rates and high reflux ratios. Conventional distillation control wisdom advises that the reflux-drum level should be controlled by the reflux flow rate when the reflux ratio is larger than ∼2. A control structure is frequently recommended in which the flow rate of the distillate is ratioed to the reflux. However, in many columns, a constant reflux-ratio strategy is not as effective as a constant reflux-to-feed strategy for maintaining product purity at both ends of the column in the face of feed-composition disturbances when a single tray temperature is controlled. An alternative control structure is proposed in this paper that achieves the preferred constant reflux-to-feed strategy by controlling the reflux-drum level with reboiler heat input and manipulating the small distillate flow rate to control a tray temperature.