Semicontinuous, middle-vessel, extractive distillation

A decentralized control configuration is proposed for the semicontinuous, e xtractive distillation of a low-boiling azeotropic mixture of acetone and m ethanol. The separation is performed in a middle-vessel column (MVC), havin g a large external middle vessel, from which the column is fed, and to whic h a full-liquid sidedraw from the stage above the feed tray is sent. The ex tractive 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 manipul ated to control the composition of the distillate, and the bottoms flow rat e is manipulated to control the composition of the bottoms product, has bee n widely labeled as inoperable. Herein, the configuration is shown to perfo rm satisfactorily when a middle vessel with a full-liquid sidedraw is utili zed. A cyclic campaign is simulated showing the satisfactory performance of the DB-MVC control configuration. The advantages of the control configurat ion are discussed and design specifics are provided. An analysis of the cam paign, highlighting the problems overcome, is given. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.