Diffusion and Heterogeneous Chemical Reactions in Isothermal Catalytic Pellets

15-4 DIFFUSION AND HETEROGENEOUS CHEMICAL REACTIONS IN ISOTHERMAL CATALYTIC PELLETS 

As indicated by equation (15-12), the simplified homogeneous mass transfer model for diffusion and one chemical reaction within the internal pores of an isolated catalytic pellet is written in dimensionless form for reactant A as 

The dimensionless kinetic rate law in the mass balance is defined by 

The dimensionless spatial coordinate rj is measured in the thinnest dimension of rectangular catalysts. For cylindrical and spherical catalysts, r] is measured in the radial direction. The characteristic length L which appears in the intrapellet Damkohler number and is required to make the spatial coordinate dimensionless (i.e., rj = spatial coordinate/L) is one-half the thickness of catalysts with rectangular symmetry, measured in the thinnest dimension the radius of long cylindrical catalysts or the radius of spherical catalysts. q A is the molar density of reactant A divided by its value in the vicinity of the external surface of the catalyst, CAsurf- Hence, by definition, q A = 1 at r = 1. 

The following irreversible heterogeneous catalytic reaction is exothermic and it occurs on active sites within the internal pores of a thin wafer  

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