Process Technology: An Introduction - Haan A.B. 2015
4 Chemical reactors and their industrial applications
4.1 Introduction
The reactor in which the chemical reaction takes place occupies a central position in the chemical process. This chapter provides an overview of commonly encountered reactor types, illustrated with various examples of their industrial applications. However, first the different ways to classify chemical reactors will be discussed.
4.1.1 Classification of reactors
Chemical reactors exist in such a wide range of forms and types that a complete systematic classification is impossible. Two main categories which can be distinguished are homogeneous and heterogeneous reactors. In homogeneous reactors only one phase, usually a gas or a liquid, is present. If more than one reactant is involved, provision must be made for mixing them together to form a homogeneous mixture.
Another kind of classification, which cuts across the homogeneous-heterogeneous division, is the mode of operation, batchwise or continuous. Homogeneous batch reactions are carried out in vessels, tanks, or autoclaves, in which the reaction mixture is agitated and mixed in a suitable manner. This operation is familiar to anyone who has carried out small-scale preparative reactions in the laboratory. Continuous flow reactors for homogeneous reaction systems already show a much greater variety. The predominant forms are the tubular reactor and the mixed tank reactor, which have essentially different characteristics.
In heterogeneous reactors, two or more phases are present. The classification of reactors for heterogeneous systems has a great number of possibilities. The dominant factor is the contact between the different phases. This leads to a classification of reactors as a contact apparatus. Some common examples are gas-liquid, gas-solid, liquid-solid, liquid-liquid, and gas-liquid-solid systems. In many cases the solid phase is present as a catalyst. Gas-solid catalytic reactors comprise an important class of heterogeneous chemical reaction systems. Generally, heterogeneous reactors exhibit a greater variety of configurations and contacting patterns than homogeneous reactors.
4.1.2 Influence of the heat of reaction on the reactor type
Associated with every chemical change there is a heat of reaction, which is small enough to be neglected in only a few cases. The magnitude of the heat of reaction often has a major influence on the design of a reactor. With a highly exothermic reaction, for example, a substantial rise in temperature of the reaction mixture will take place unless provision is made for heat to be removed as the reaction proceeds.
When feasible, adiabatic operation of a reactor without provision for heating or cooling is preferred for simplicity of design. If the reactor cannot operate adiabatically, its design must include provision for heat transfer. Commonly used ways to heat or cool a reactor are external jackets, internal coils, or an external heat exchanger through which the reactor contents is circulated. If one of the constituents of the reaction mixture is volatile, the external heat exchanger may be a reflux condenser, just as in the laboratory.