DYNAMICS AND CONTROL OF DISTILLATION COLUMNS A tutorial introduction

Distillation is the most common unit operation in the chemical industry and understanding its behavior has been a defining characteristic of a good chemical engineer. Yet, distillation research has repeatedly been proclaimed to be a dead area, and some universities have even considered to stop teaching the basics of McCabe-Thiele diagrams. However, there has been renewed interest the last years, especially since distillation columns has become a favorite subject in the process systems engineering field, including the areas of process synthesis, process dynamics and process control. The reason is that distillation columns are themselves a system; a distillation columns may be viewed as a set of integrated, mostly cascaded, flash tanks. However, this integration gives rise to a complex and non-intuitive behavior, and it is difficult to understand the system (the column) based on the knowledge about the behaviorof the individualpieces (the flash tanks). In this paper I want to present, in a simple manner, some of the important issues for understanding the dynamics, operation and control of distillationcolumns, includingsome useful tools for controllabilityanalysis in the frequency domain. The goal is to develop insight and intuition. It is hoped that, when the reader has understood the essentials, then the details can easily be obtained from the literature. Five years ago, I wrote a quite detailed literature survey on distillationdynamics and control (Skogestad, 1992), concentrating on the the period 1985-1991, and I had the ambition to update that survey, but I have not had the capacity to keep up with my ambition. In any case, the 1992 survey paper was in 1997 reprinted in the Norwegian journal Modeling, Identification and Control, so it should be easily available. The reader should consult it for more detailed and appropriate references. However, I would like to mention at least a few of the important books. In terms of design and steadystate behavior there are many books, but let me here only mention King (1971) which gives a comprehensive and insightful treatment. In terms of distillation dynamics and control, the book by Rademaker et al. (1975) contains a lot of excellent material, but the exposition is rather lengthy and hard to follow. Furthermore, since most of the work was completed around 1959, the book is somewhat outdated. It includes a good treatment of the detailed material and energy balances for each tray, including the flow dynamics, but discusses only briefly the overall response of the column. The discussion on control configuration selection is interesting, but somewhat outdated. The books by Shinskey (1977, 1984) on distillation control contain many excellent practical recommendations which reflect the authors vast experience in the field. There is a detailed treatment on the issue of composition control and various configuration alternatives. However, the explanations are often lacking or difficult to follow. Buckley et al. (1985) give a detailed discussion of the design of level and pressure control systems, but the issue of composition control (configuration selection) is only briefly discussed. There is a lot of good material in the book based on the extensive experience of Page Buckley, but it could be argued that the book was published about 20 years too late. The book by