Minimum Reflux for Batch Distillations of Ideal and Nonideal Mixtures at Constant Reflux

Abstract

This paper presents a short-cut procedure for estimating the instantaneous separation performance in batch columns having an infinite number of stages that it is applicable to nonideal mixtures with any number of components. The model allows the estimation of the instantaneous top composition without resorting to stage-by-stage computation and therefore can be used both as a method for rapid simulation of the batch operation and as a procedure for calculating the minimum-energy demand. A key aspect of the method is the determination of the different regimes that can be found during the operation of batch columns. Each one of the different regimes is controlled by a pinch that determines the geometry of the internal profiles. For systems with constant relative volatilities, the presented approach is compared with the use of the Underwood equation to determine the instantaneous separation column performance. We show that both methods are equivalent. This analysis also gives an interpretation of the application of the Underwood equation in the context of batch rectification. Finally, we analyze the application of the model to highly nonideal mixtures with azeotropes and distillation boundaries and briefly discuss the modifications that have to be introduced to extend the method.