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Reaction Kinetics & Chemical Reaction Models

The fundamentals of chemical reaction kinetics will be presented with the purpose of building, starting with elementary reactions, complex mechanisms. These mechanisms consisting of many elementary reactions can be used with existing software (discussed later) to make predictions on the performance of chemical reactors with special consideration to the formation of trace species. The focus will be on homogeneous processes taking place in the gas phase. The majority of the material presented in this manuscript is based on the author’s own research (Gargurevich, 1997).

Both in past and present literature dealing with the design of chemical reactors, there is an oversimplification of the chemical reaction models, with the use of global mechanisms consisting of a few reactions with empirically determined reaction rates (Worstell, 2001; Arakawa et al., 1998). For example, the rate of consumption of reactant A by B to form product C, represented by the overall reaction (1) below, is presented in the form of Equation (2),

(1)

(2)

where A_e is the pre-exponential factor, E an empirically determined activation energy, and a and b, the exponents of the reactant concentrations that are able to represent the concentration dependence over a range of conditions also empirically determined.

Unfortunately, these oversimplified mechanisms, to give an example, may not be able to accurately predict the formation of toxic products present in very small concentrations because depending on conditions their formation is dependent on complex chemistry involving stable and radical species, as well as reaction temperature. There is a need then to arrive at more complex mechanisms consisting of elementary reactions that are relevant to the consumption of the reactants, formation of intermediate species and products, and any other chemical species of interest (Senkan, 1992).

General Features of Reaction Mechanisms