Process Technology: An Introduction - Haan A.B. 2015

3 Principles of chemical reaction engineering
3.2 Classification of reactions

There are many ways of classifying chemical reactions. In chemical reaction engineering probably the most useful scheme is a breakdown according to the number and types of phases involved, the big division being between homogeneous and heterogeneous systems. A reaction is homogeneous if it takes place in one phase alone. A reaction is heterogeneous if it requires the presence of at least two phases to proceed at the rate that it does. It does not matter whether the reaction takes place in one, two, or more phases, or at an interface, or whether the reactants and products are distributed among the phases or are all contained within a single phase. All that counts is that at least two phases are necessary for the reaction to proceed as it does.

Sometimes this classification is not as clear cut as with the large class of biological reactions, the enzyme substrate reactions. Since enzymes themselves are highly complicated proteins of large molecular weight and colloidal size, enzyme-containing solutions represent a gray region between homogeneous and heterogeneous systems. Another example for which the distinction is not sharp is a very rapid chemical reaction, such as a burning gas flame, where large nonhomogeneity in composition and temperature may exist.

Cutting across this classification is the catalytic reaction whose rate is altered by materials which are neither reactants nor products. These foreign materials, called catalysts, need not be present in large amounts. Catalysts act somehow as gobetweens, either hindering or accelerating the reaction process while being modified relatively slowly if at all.

Tab. 3.1: Classification of chemical reactions useful in reactor design.