Operation of a Batch Stripping Distillation Column

Abstract

Batch process is becoming more popular as chemical process industries move toward manufacturing fine and specialty chemicals, where flexibility is a key issue due to the frequent change of product demand[ 1 ]. Batch distillation columns are inherently flexible, as a single column can separate many different components from a multicomponent feed. Thus the use of batch distillation is becoming more important for the seperation and purification of high-value chemicals in many chemical, food, and pharmaceutical industries. Traditionally, the most popular kind of batch distillation column is the so-called rectifying column, which has a large reboiler, to which all the feed is charged, and different products are removed from the top. There are three ways to operate a rectifying column[ 2,3 ]. They are: (1) constant reflux and variable distillate composition, (2) variable reflux and constant distillate composition of key component, (3) optimal reflux policy which trades off (1) and (2) and is based on the most profitable operation. Batch stripping column is opposed to a batch rectifier. It has its storage vessel at the top and the products leave the column at the bottom. A stripping batch distillation column is preferred when the amount of the light component in the feed is small and the products are to be recovered at high purity[ 4,5 ] . The operation modes of a batch stripping are believed to be the same as those of a rectifier. However, the control system of a stripper is different. In this paper, we explore three different control methods with Hysys (Hyprotech Ltd. 1997) for a batch stripper. The control schemes are shown in Fig. 1. The main difference is the control scheme for reboiler liquid level: (a) controlled by reflux flow, (b) controlled by reboiler heat duty, (c) controlled by bottom product flow. The main characterics of operating a batch stripper with different control scheme are presented in this paper. Guidlines are provided for the startup and the effect of some control tuning parameters on the column performance are discussed.