13.5. BATCH DISTILLATION

A batch distillation plant consists of a still or reboiler, a column with several trays, and provisions for reflux and for product collection. Figure 13.9 (c) is a typical equipment arrangement with controls. The process is applied most often to the separation of mixtures of several components at production rates that are too small for a continuous plant of several columns equipped with individual reboilers, condensers, pumps, and control equipment. The number of continuous columns required is one less than the number of components or fractions to be separated. Operating conditions of a typical batch distillation making five cuts on an 8-hr cycle are in Figure 13.10.

Figure 13.9: Batch distillation McCabe-Thiele constructions and control modes. (a) Construction for constant overhead composition with continuously adjusted reflux rate. (b) Construction at constant reflux at a series of overhead compositions with an objective of specified average overhead composition. (c) Instrumentation for constant vaporization rate and constant overhead composition. For constant reflux rate, the temperature or composition controller is replaced by a flow controller.

Figure 13.10: Operation of a batch distillation with five cuts.

Operation of a batch distillation is an unsteady state process whose mathematical formulation is in terms of differential equations since the compositions in the still and of the holdups on individual trays change with time. This problem and methods of solution are treated at length in the literature, for instance, by Holland and Liapis ( Computer Methods for Solving Dynamic Separation Problems, 1983, pp. 177 213).