Semicontinuous, middle-vessel, extractive distillation

A decentralized control configuration is proposed for the semicontinuous, extractive distillation of a low-boiling azeotropic mixture of acetone and methanol. The separation is performed in a middle-vessel column (MVC), having a large external middle vessel, from which the column is fed, and to which a full-liquid sidedraw from the stage above the feed tray is sent. The extractive agent, water, feeds the column on a scheduled basis to facilitate the separation. Based upon the simplicity of a decentralized configuration and the desire to avoid controller overrides, a DB-control configuration is selected. This configuration, in which the distillate flow rate is manipulated to control the composition of the distillate, and the bottoms flow rate is manipulated to control the composition of the bottoms product, has been widely labeled as inoperable. Herein, the configuration is shown to perform satisfactorily when a middle vessel with a full-liquid sidedraw is utilized. A cyclic campaign is simulated showing the satisfactory performance of the DB-MVC control configuration. The advantages of the control configuration are discussed and design specifics are provided. An analysis of the campaign, highlighting the problems overcome, is given. (C) 2000 Elsevier Science Ltd. All rights reserved.