HOLDUP ISSUES IN BATCH DISTILLATION-BINARY MIXTURES

The role of column holdup on the performance of batch distillation has been dealt with by many researchers in the past. It was found that in creasing holdup in some cases improved column performance and that the effect was in other cases detrimental. However, none of the studies w ere done systematically to explain either the positive or the negative effects of holdup. Here we examine the role of column holdup for bina ry mixtures in a systematic way and explain both effects on physical g rounds. It is found that the characterization of the column separation task using 'the degree of difficulty of separation' [Christensen and Jorgensen (1987) Chem. Engng J. 34, 57] measure and the evaluation of column performance using 'the minimum time' measure are very useful in explaining the positive and negative effects of column holdup. We use two different but fairly detailed dynamic batch distillation models. For the purpose of direct comparison with the previous work we retaine d the assumption of constant molar holdup (CMH) in one of the models, and replace this by a more realistic assumption of constant volume hol dup (CVH) in the other model. The study allows identifying at the desi gn stage an optimum holdup required to achieve the best performance fo r a given column configuration and separation task. (C) 1998 Elsevier Science Ltd. All lights reserved.