Heterogeneous Reactions of Unusual Stoichiometries

Solution The variation of reaction rate with flow indicates that the process is controlled by diffusion outside of particle. By inference, the resistance of any ash formed is apparently negligible. At the same time, the variation of particle radius with time is in the range suggested by case B in Table 16.4-1 checking this point further requires using a mass transfer correlation in fluid beds parallel to Eq. 16.4-6. Although no dependence on temperature is mentioned, we would expect this dependence to be small. 

In the previous sections we discussed how the overall rate of reaction was affected by the rates of diffusion and reaction. The rate of diffusion could be altered by changes in factors like fluid flow or diffusion coefficient or ash thickness. The overall rate of reaction was always assumed to be first order, always doubling when the reagent 

In this section we want to consider two examples of other chemistries that can alter the simple combinations of diffusion and reaction developed earlier. The first example is an irreversible second-order reaction. The second involves fast reactions of concentrated reagents and products. 

The case considered here, shown schematically in Fig. 16.5-1, involves two sequential steps. The first of these steps is simple mass transfer, but the second step is a second-order irreversible chemical reaction. As before, we want to calculate the overall rate rj of this heterogeneous reaction. In mathematical terms, this rate is 

We can combine these two equations to find the unknown interfacial concentration  

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