Reactive Distillation

Most chemical processes involve two important operations (reaction and separation) that are typically carried out in different sections of the plant and use different equipment. The reaction section of the process can use several types of reactors [continuous stirred-tank reactor (CSTR), tubular, or batch] and operate under a wide variety of conditions (catalyzed, adiabatic, cooled or heated, single phase, multiple phases, etc.). The separation section can have several types of operations (distillation, extraction, crystallization, adsorption, etc.), with distillation being by far the most commonly used method. Recycle streams between the two sections of these conventional multiunit flowsheets are often incorporated in the process for a variety of reasons: to improve conversion and yield, to minimize the production of undesirable byproducts, to improve energy efficiency, and to improve dynamic controllability.

Instead of conducting the reaction and separation in separate units and vessels, it is sometime possible to combine these operations in a single vessel. This is called reactive distillation or catalytic distillation, which is the subject of this book.

Economic and environmental considerations have encouraged industry to focus on technologies based on process "intensification." This is an area of growing interest that is defined as any chemical engineering development that leads to smaller inventories of chemical materials and higher energy efficiency. Reactive distillation is an excellent example of process intensification. It can provide an economically and environmentally attractive alternative to conventional multiunit flowsheets in some systems.

One important inherent advantage of reactive distillation is the feature of simultaneous production and removal of products. For reversible chemical reactions, the removal of the product components drives the reaction toward the product side. Thus, the chemical equilibrium constraint on conversion can be overcome and high conversions can be achieved, even in cases with small chemical equilibrium constants. Of course, the relative volatilities among the reactants and the products must be such that the products can be fairly easily removed from the region in the column where the reaction is occurring and reactants are not lost from this region.

Reactive Distillation Images
Figure P.1 The Eastman methyl acetate reactive distillation column. (Courtesy of Jeff Siirola.)

An important limitation of reactive distillation is the need for a match between the temperature favorable for reaction and the temperature favorable for separation. Because both operations occur in a single vessel operating at a single pressure, the temperatures in a reactive distillation column are set by vapor-liquid equilibrium and tray compositions. If these temperatures are low and produce low specific reaction rates for the reaction kinetics involved, very large holdups (or large amounts of catalyst) will be required. If these temperatures are high and correspond to very small chemical equilibrium constants (as can occur with exothermic reversible reactions), it may be difficult to achieve the desired conversion. High temperatures may also promote undesirable side reactions. In either the low- or high-temperature case, reactive distillation may not be economical. As a result, the design of reactive distillation columns is much more sensitive to pressure than a conventional distillation column.

A small number of industrial applications of reactive distillation have been around for many decades. One of the earliest was a DuPont process in which dimethyl terephthalate was reacted with ethylene glycol in a distillation column to produce methanol and ethylene terephthalate. The methanol was removed from the top of the column. The ethylene terephthalate, which was used for polyester production, was removed from the bottom.

However, there were few applications of reactive distillation until about two decades ago. The publication of a very influential paper by engineers from Eastman Chemical1 produced a surge of interest in reactive distillation in both industry and academia. The Eastman reactive distillation column (see Fig. P.1.) produces methyl acetate out the top and water out the bottom, with methanol fed into the lower part of the column and acetic acid fed in the upper part. Jeff Siirola reports that this single reactive column replaced a conventional multiunit process that consumed 5 times more energy and whose capital investment was 5 times that of the reactive column.2 The methyl acetate reactive distillation column provides an outstanding example of innovative chemical engineering.

Several hundred papers and patents have appeared in the area of reactive distillation, which are too numerous to discuss. A number of books have dealt with the subject such as (1) Distillation, Principles and Practice by Stichlmair and Fair,3 (2) Conceptual Design of Distillation Systems by Doherty and Malone,4 and (3) Reactive Distillation— Status and Future Directions by Sundmacher and Kienle.5 These books deal primarily with the steady-state design of reactive distillation columns. Conceptual approximate design approaches are emphasized, but there is little treatment of rigorous design approaches using commercial simulators. The issues of dynamics and control structure development are not covered. Few quantitative economic comparisons of conventional multiunit processes with reactive distillation are provided.

The purpose of this book is to present a comprehensive treatment of both steady-state design and dynamic control of reactive distillation systems using rigorous nonlinear models. Both generic ideal chemical systems and actual chemical systems are studied. Economic comparisons between conventional multiunit processes and reactive distillation are presented. Reactive distillation columns in isolation and in plantwide systems are considered. There are many parameters that affect the design of a reactive distillation column. Some of these effects are counterintuitive because they are different than in conventional distillation. This is one of the reasons reactive distillation is such a fascinating subject.

We hope this book will be useful for both students and practitioners. We have attempted to deal with many of the design and control challenges in reactive distillation systems in a quantitative way.

William L. Luyben Cheng-Ching Yu

'V. H. Agreda, L. R. Partin, and W. H. Heise, High-purity methyl acetate via reactive distillation, Chem. Eng. Prog. 86(2), 40-46 (1990).

2J. J. Siirola, Industrial applications of chemical process synthesis, Adv. Chem. Eng. 23, 1062 (1996). 3J. G. Stichlmair and J. R. Fair, Distillation, Principles and Practice, Wiley-VCH, New York, 1998, p. 252. 4M. F. Doherty and M. F. Malone, Conceptual Design of Distillation Systems, McGraw-Hill, New York, 2001, Chapter 10.

5K. Sundmacher and A. Kienle, Editors, Reactive Distillation—Status and Future Directions, Wiley-VCH, New York, 2003.

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