Effects of nonuniform activity distributions on the performance of porous catalytic pellets

This work investigates the effect of the distribution of catalytic activity within a porous pellet on the activity and selectivity, as well as the poisoning and regeneration rates. It is shown that a considerable improvement in the performance of a porous catalyst may be realized by carefully controlling the distribution of active material within the pellet. It is shown that for a single, first-order irreversible reaction, A B, the reaction rate per pellet is always increased by concentrating the catalytic activity close to the surface of the pellet. Likewise, for a single reversible reaction, A<->B, the rates of both the forward and the reverse reactions are increased by this activity distribution; and this results in a more rapid approach toward equilibrium. For two consecutive first-order reactions, A—>-B—>-C, concentrating active sites close to the surface of a pellet always enhances the rate of conversion of component A and, in most cases, the net rate of production of component B is increased as well. It is concluded that for these simple reaction networks it is usually desirable to concentrate catalytic activity at the surface of the pellet. Unfortunately, in reaction systems in which poisoning occurs, this distribution of catalytic activity is not always desired since it can enhance the rate of poisoning. When poisoning occurs, the optimal activity profile is not clearly defined, but is a complex function of the mechanism and kinetics of the particular reaction system considered. Concentrating active sites at the surface of a pellet may also lead to detrimental effects during catalyst regeneration, since it may cause large internal temperature gradients to develop inside the pellet during an exothermic regeneration reaction. It is concluded that the incentives for using a nonuniform activity distribution in any given application must be derived from a careful analysis of the particular reaction system. The discussion in this study is restricted to rather simple systems. However, the techniques presented here can easily be extended to an analysis of more general types of catalytic systems.