The influence of non-uniform catalytic activity on the performance of a single spherical pellet

This work investigates the influence of a non-uniform distribution of the intrinsic catalytic activity of a single spherical pellet on its effectiveness factor, selectivity and deactivation. In most cases the performance of non-uniformly active catalysts is superior to that of uniformly active catalysts which have the same volume averaged activity. The specific deactivation mechanisms and rate as well as the diffusional resistance have an important influence on the most desired activity profile. Concentrating the catalytic activity in the exterior shell most often yields the maximal selectivity but it also yields minimal resistance to deactivation when compared with catalysts with the same volume averaged activity and different activity profiles. For this reason, when it is important to maintain the catalytic activity for long periods it may be desirable not to use catalysts in which the activity is concentrated in the exterior shell.It is also shown that the maximal temperature rise during regeneration of a catalyst, in which coke is deposited non-uniformly, may largely exceed that obtained for a uniformly coked pellet.