Effect of External Resistances on Selectivity

10-5 Effect of External Resistances on Selectivity Suppose two parallel reactions 

If the system is isothermal, only external-diffusion resistance is involved. However, for parallel reactions involving the same reactant, diffusion has no effect on the selectivity. This is seen by using Equation (10-2) to express the rate of each reaction. Taking the global (or pellet) selectivity as Sp 

Sp is equal to the ratio of the intrinsic rate constants just the same as though diffusion was not involved. For consecutive reactions, such as A B C, diffusion has an adverse effect on the selectivity of B with respect to C. This is because diffusion resistance reduces the surface concentration of A (from which B is produced) and increases the surface concentration of B (from which C is produced), with respect to bulk concentrations. 

For nonisothermal conditions the situation is more complicated, but it can be analyzed conveniently in terms of global and bulk rates. For the parallel reaction case, the relation between bulk selectivity and y may be written 

Equation (10-30) is not applicable for the y values, because and are determined by both reactions 1 and 2 that is, Eq. (10-27) is not valid. However, 7i and yj may be formulated directly by taking ratios of the rates at surface and bulk conditions  

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