Effect of mass transfer on solid-catalyzed reactions: The dehydrogenation of cyclohexane to benzene



The reaction of gaseous components at a solid catalytic surface has long been a subject of prime engineering interest. Generally speaking one expects the reaction velocity constant to follow the Arrhenius exponential temperature dependence. However as the reaction temperature increases, the intrinsic reactivity of the surface will increase and mass transfer begins to limit the rate of the reaction. In the case of a porous solid catalyst, where most of the active surface is on the catalyst pellet interior, the rate-limiting process will frequently be internal diffusion, and in such a case the milder effect that temperature has on the diffusion process is the one observed in the gross kinetics, rather than the exponential Arrhenius dependence.

The study reported here has examined the kinetics of the dehydrogenation of cyclohexane to benzene over a platinum-on-alumina pelleted catalyst. This is a notorious problem system, and in seeking to describe the observed rate data the authors provided a stern test for an analytical model treating the coordinate diffusion and reaction mechanisms. In particular the parameter of particle size was studied over a temperature range of from 640° to 910°F. For these runs reactor pressure was held constant at essentially 200 lb./sq. in. gauge (14.7 atm.), und the feed was maintained at 20 mole % cyclohexane, 80 mole % hydrogen.