Design and synthesis of multicomponent batch distillation

Interest in batch processes has increased with the growing importance of specialty chemicals, characterized by high-value, low-capacity, short-term production and strongly nonideal mixtures. While separation is an important step in these processes, few publications deal with the feasibility and design aspects of batch distillation for azeotropic mixtures. In this dissertation, a simple dynamic model is developed to describe composition changes in batch distillation by decoupling the variations in flows and compositions through a dimensionless warped time. The model is based on constant molar overflow and quasi-steady state assumptions in the column and on a differential model for the batch tank. Two types of batch distillation columns are presented: the batch rectifier, where products are withdrawn at the distillate, and the batch stripper, where products are withdrawn at the bottom. Composition changes for azeotropic mixtures are complex because the sequence of cuts collected depends on the relative amount of each component in the feed. However, in the limiting case of large number of stages and large reflux or reboil ratio, a method is presented to predict the variation of the batch and product compositions as a function of time. As a result, the composition space is divided into regions leading to the same set of fractions. The method is geometrical and requires little or no computation. A systematic procedure to devise a feasible sequence is proposed. This procedure includes determining suitable entrainers to break binary azeotropes. It is shown that the use of a batch stripper is essential in breaking minimum boiling binary azeotropes. The techniques developed are illustrated on the separation of methyl acetate from a quaternary azeotropic feed coming from a transesterification reactor. A simple, tractable, design method is presented to estimate flows, equipment sizes, utility loads and costs for any batch column. This method provides a rapid estimate of the design targets for a variable reflux (or reboil) policy without the need of integrating the column model numerically. This policy, which approximates the constant distillate (or bottom) composition policy, shows significant cost savings over the constant reflux (or reboil) policy