Finding the Mechanism of Irreversible Heterogeneous Reactions

The simple ideas of the previous section are the benchmark of this entire chapter, the standard against which more complex concepts are compared. Our basic strategy has been to assume a simple mechanism and calculate the overall reaction rate for this situation. The result is a close parallel to the results for interfacial mass transfer. 

However, in some ways, the strategy of the previous section is misleading, for it implies that the mechanism is known. This is often not the case. In many situations, we already have experimental results, and we want to find which mechanisms are consistent with these results. In other words, the arguments of the previous section are backwards. 

In this section, we want to explore how the mechanism of an irreversible reaction can be inferred from the overall reaction rate, instead of the other way around. This exploration can be complicated, hampered by elaborate algebra. As a result, we consider only special cases of the two types of heterogeneous reactions shown in Fig. 16.4-1. These types differ in the products produced by the reaction. In some cases, these products are fluid, and hence diffuse away. More commonly, the products form a layer of ash around an unreacted core this second case is sometimes called a top-ochemical model. 

To reduce algebraic complexity, we consider only limits in which the overall rate is controlled by a single diffusion or reaction step. The five limits we consider are tabulated in Table 16.4-1. In cases A and C, we assume that the surface reaction 

Physical situation Rate-controlling step Size R =/(time, reagent) Size =y(teiiiperature) Size = / (flow) Remarks 

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