Multiple and Nonelementary Reactions

Chapter 1 treated single, elementary reactions in ideal reactors. Chapter 2 broadens the kinetics to include multiple and nonelementary reactions. Attention is restricted to batch reactors, but the method for formulating the kinetics of complex reactions will also be used for the flow reactors of Chapters 3 and 4 and for the nonisothermal reactors of Chapter 5. 

The most important characteristic of an ideal batch reactor is that the contents are perfectly mixed. Corresponding to this assumption, the component balances are ordinary differential equations. The reactor operates at constant mass between filling and discharge steps that are assumed to be fast compared with reaction half-lives and the batch reaction times. Chapter 1 made the further assumption of constant mass density, so that the working volume of the reactor was constant, but Chapter 2 relaxes this assumption. 

Multiple reactions involve two or more stoichiometric equations, each with its own rate expression. They are often classified as consecutive as in 

Note that the Roman numeral subscripts refer to numbered reactions and have nothing to do with iodine. All these examples have involved elementary reactions. Multiple reactions and apparently single but nonelementary reactions are called complex. Complex reactions, even when apparently single, consist of a number of elementary steps. These steps, some of which may be quite fast, constitute the mechanism of the observed, complex reaction. As an example, suppose that 

This reaction is complex even though it has a stoichiometric equation and rate expression that could correspond to an elementary reaction. Recall the convention used in this text when a rate constant is written above the reaction arrow, the reaction is assumed to be elementary with a rate that is consistent with the stoichiometry according to Equation (1.14). The reactions in Equations (2.5) are examples. When the rate constant is missing, the reaction rate must be explicitly specihed. The reaction in Equation (2.6) is an example. This reaction is complex since the mechanism involves a short-lived intermediate, B. 

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component C is formed by reaction I and consumed by reaction II. An example of a competitive reaction is 

Component A is consumed by both reactions but the products are different. Typically, only one of the products is desired. The fraction unreacted for component A, Ya = a(l)/ao, provides no measure of selectivity. Instead, measures such as yield, 

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If the forward and reverse reactions are nonelementary, perhaps involving the formation of chemical intermediates in multiple steps, then the form of the reaction rate equations can be more complex than Equations 5.33 to 5.36. 

The multiple reactions involve parallel, series, and mixed reactions. They consist of complex reactions in which the specific rates of each reaction should be determined. They often occur in industrial processes. These reactions can be simple, elementary, irreversible and reversible, or even nonelementary. Some cases are ... 

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