Investigating the processing capacity of batch distillation by applying a second, smaller column

Abstract

Increasing the processing capacity is important, especially in the pharmaceutical industry, where a given amount of waste solvent must be processed in a specific time period, else the remaining amount of waste solvent must be incinerated, which results in organic solvent loss and considerable environmental impact. In this paper, the possibility of increasing the processing capacity of batch distillation by using a second, already available, smaller batch column in addition to the original one is studied. The size of the columns is not changed during the calculations performed. Two case studies are presented. In the first one (Mixture 1), acetone, in the second one (Mixture 2), acetic acid must be recovered in high purity from their aqueous mixtures by operating either the larger column only or both columns simultaneously. The smaller column serves for preliminary separation, and its main-cut (Mixture 1) or residue (Mixture 2) is further processed in the larger one. The goal of this work is to maximise the processing capacity of the two-column process with the same recovery as in the single-column process. Both processes are modelled with a professional flow-sheet simulator. For the single-column process, the only independent variable is the reflux ratio. For the two-column process, the independent variables are the reflux ratios of both columns and the amount of intermediate product (distillate or residue) of the first column transferred to the second one. The ranges of the independent variables are determined by sensitivity studies for different charge compositions (20–50-80 mass% acetone or acetic acid), and each case is optimised by a genetic algorithm coupled to the flow-sheet simulator. It is also shown in this work that the processing capacity maximum of the two-column process is not always at the equality of the processing times of the two columns.