Insights into batch extractive distillation using a middle vessel column

Abstract: "Researchers have begun to study a batch column with simultaneous top and bottom products, sometimes called a middle vessel column. The column is similar to a continuous column in that it has both rectifying and stripping sections. However, instead of a feed tray, the middle vessel column has a tray with a large holdup that acts like the still pot. Meski et al. (1993) showed that this column has several advantages over the rectifying and stripping batch columns including faster processing times and increased separation flexibility. We show that one can identify the feasible product and possible column profile regions for the batch rectifier, stripper and middle vessel columns using methods developed for continuous distillation. Extractive distillation has been used for continuous and rectifying batch columns in separating azeotropic mixtures by feeding an extractive agent, normally a high boiler, near the top of the column. Using insights developed for continuous distillation, we compare extractive distillation using the batch rectifier and middle vessel column and show that these columns can theoretically recover all of the pure distillate product from an azeotropic feed. However, the batch rectifier requires a still pot of infinite size. It is possible to 'steer' the still pot composition in the middle vessel column by adjusting column parameters such as the product and extractive agent flow rates. This steering enables the middle vessel theoretically to recover all of the distillate product without the need for an infinite still pot."