Influence of Mass Transport on Selectivity

A deficient design of a catalyst pellet for a single reaction leads to a poor utilization of the often expensive catalyst mass. Fortunately, we can usually adjust some operating conditions, such as the temperature or residence time, or install additional catalyst material to compensate for a design error. This section discusses the influence of diffusion limitations on the yield of the desired product in a multipath reaction. Here, when the diffusion intrusions are not properly accounted for, it is often impossible to counterbalance the impact of the design error by a change in the operating conditions. 

Wheeler [8] presented the first theoretical analysis of the influence of diffusion on the yield of a desired product. Many additional investigations have been reported since then, and an excellent survey is available [9]. Only one simple case is described here, to illustrates the influence of diffusion limitations on the selectivity. Consider a first-order isothermal catalytic sequence 

The total amounts of A and B converted inside the catalyst pellet per unit surface area and per unit time can be found as the fluxes of A and B to the pellet  

The selectivity oB (the ratio between the amount of B obtained and the amount of reactant A converted) can be computed from 

This selectivity may be called a point selectivity, because it depends on the concentrations in the fluid phase, which may be different at different points in the reactor. If CBj = 0, Equation 8.26 predicts 

---