Development of reactive distillation technology

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In the previous article, a brief introduction to the concept and applications of reactive distillation was provided; those interested can review that article: Reactive Distillation: Principles and Applications [HaiChuan Chemical Engineering Documentation]. Reactive Distillation: Principles – https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719575 (Source: HaiChuan Chemical Forum (HuaHaiChuanLiu hcbbs)). As the name suggests, reactive distillation is essentially the combination of reaction and distillation processes. However, although it may seem simple and easy to understand, in reality, this type of coupling presents many scientific and engineering challenges. This article provides an introduction to typical reactive distillation and its application examples, as well as the new technologies and equipment that have emerged in recent years through the integration of reaction and distillation, in order to enhance the understanding of this reactive distillation process. The figure above shows the three main configurations of the classical reactive distillation process: the configuration on the left is that of a conventional reactive distillation column, suitable for four-component reaction systems (where both reactants and products consist of two components with certain differences in volatility); a typical example of this is the synthesis process of methyl tert-butyl ether (MTBE) ； In the middle is the azeotropic reaction distillation configuration, with a decanter added at the top of the tower to separate the heterogeneous azeotropes formed during the reaction. This configuration is commonly used in esterification reactions; for example, in the esterification of acetic acid with butanol to produce butyl acetate, the water generated as a byproduct forms a heterogeneous azeotrope with the ester, and the water can be removed through phase separation ； On the right is the configuration with a prereactor and/or side-reactor, where the dashed lines indicate optional configurations. This configuration increases the reaction residence time by adding a reactor outside the tower; it is suitable for systems with slow reaction rates. While ensuring high reaction conversion, it utilizes the distillation tower to separate the products and recycle the reactants. From the perspective of the catalytic mechanism in the reaction process, reactive distillation can be divided into three categories: catalyst-free (relying on thermal reactions), homogeneous catalysis, and heterogeneous catalysis. In homogeneous catalytic systems, the catalyst moves along with the liquid phase within the tower, and it can ultimately be removed from the system in various ways: by being taken out along with the products at the top or bottom of the tower, separated as salt residue after neutralization, recovered through additional separation units, or without any special separation process as long as the product quality is not affected. Heterogeneous catalytic reactive distillation (i.e., catalytic distillation) involves fixing a solid catalyst in a specific area within the tower to form a catalytic reaction zone. Catalysts are typically filled into the tower in the form of bulk particle beds, structured catalytic packing, or catalytic packs. Structural catalytic packing (such as Sulzer Katapak, Koch-KT) combines the functions of catalytic reaction and gas-liquid contact mass transfer, and represents a catalytic internal component that is widely used in industry at present. With the development of biotechnology, a special branch has emerged in the field of catalytic distillation in recent years – Enzymatic Reactive Distillation (ERD). Enzyme-catalyzed reaction distillation uses enzymes fixed on the inner surface of the tower as biocatalysts, providing a new technical approach for biocatalytic applications in organic synthesis, and exhibiting unique advantages especially in the preparation of chiral compounds. Enzymes are typically fixed to the surface of structured fillers via hydrophobic biocatalytic coatings, thereby forming a catalytically active filler layer. However, the application of enzyme-catalyzed reaction distillation faces stringent operational temperature limitations. Enzymes are prone to irreversible inactivation at temperatures above 60–80°C, while the boiling points of most organic reactants are higher than this range, making it difficult to align the operating windows for reactions and distillation. Therefore, the temperature and pressure operating ranges for such processes are narrow, which limits their industrial application to a certain extent.