Catalyst deactivation bimodal catalysts

As discussed in Section IV, Agrawal and Wei (1984) and Ware and Wei (1985b) have successfully modeled experimental deposit profiles by using the theory of coupled, multicomponent first-order reaction and diffusion. Wei and Wei (1982) employed this theory to evaluate the influence of catalyst properties on the shape of the deposit profile. Agrawal (1980) developed a model for the deactivation of unimodal and bimodal catalysts based on the consecutive reaction path. These approaches represent a more realistic consideration of the HDM reaction mechanism than first-order kinetics and will, accordingly, be discussed in more detail. 

Agrawal (1980) adopted the grain model of Sohn and Szekely (1972) to model the deactivation of a bimodal catalyst for the HDM reaction. The schematic in Fig. 59 illustrates the proposed physical structure of the catalyst pellet. The macrospherical pellet of radius Ra is composed of numerous microspheres of radius / , where the number of microspheres per unit volume is given by... 

This value is considerably higher than the experimental value (0.17) obtained from rate measurements on different size particles, but several factors may be invoked to explain the inconsistency. There will be a distribution of both pore radii and pore lengths present in the actual catalyst rather than uniquely specified values. Alumina catalysts often have a bimodal pore-size distribution. Our estimate of an apparent first-order rate constant using the method outlined above will be somewhat in error. The catalyst surface may not be equally active throughout if selective deactivation has taken place and the peripheral region is less active than the catalyst core. Other sources of error are the... 

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