Effect of interparticle mass transfer on catalytic selectivity

2 EFFECT OF INTERPARTICLE MASS TRANSFER ON CATALYTIC SELECTIVITY 

Mass and heat transfer between the bulk fluid phase and the external catalyst surface can have an affect on reaction rates, and hence the selectivity, because of modified concentration and temperature driving forces. Such effects are unimportant for porous catalysts, but are significant for catalysis by non-porous metallic gauzes (for example, in NH3 oxidation referred to in Sect. 6.1.1). 

These interfacial concentrations may be written in terms of known bulk gas phase concentrations by invoking steady state conditions between interface and gas phase. Thus, the rate of formation of B at the surface is balanced by its rate of mass transfer between fluid and solid 

Eliminating the interfacial concentrations of A and B from eqn. (19) by employing eqns. (20) and (21), the selectivity becomes 

Reactions described by other kinetic routes may be treated in similar fashions. Although, for reasons already explained in Sect. 4.1, mass transfer effects will not influence the selectivity of two concurrent reactions arising from the same reactant, heat transfer between fluid and solid does have an affect. Thus for the first-order reactions 

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