Silica-alumina catalysts inhibitors

Regeneration of Silica-Alumina Catalysts by Desorption of Chemisorbed Inhibitors... 

In the elucidation of the kinetics of the cracking of cumene on silica-alumina catalyst, the actions of inhibitors (poisons) on the reaction were studied. These inhibitors compete with cumene for cracking sites. Theoretical analysis leads to an expression from which the equilibrium constant for adsorption of inhibitors on cracking sites can be calculated. 

Studies (4) made on the cracking of cumene by silica-alumina catalyst show that the kinetics is represented in the temperature range 300-500° by scheme I on top of the following page, where S is cumene, A is a catalytic site, SA is adsorbed cumene, m is benzene, mA is adsorbed benzene, n is propylene, P is an inhibitor, and PA is inhibitor adsorbed on a cracking site. 

Ks are found. Nitrogen bases are very good electron donors and yield values of Kp at 420° as much as 10 times as large as Ks. Some of the nitrogen compounds decompose on silica-alumina catalyst ( ). If, however, the decomposition rate is negligible in comparison with the desorption rate of the adsorbed inhibitor, correct values of Kp are still obtained. 

Adsorption Equilibrium Constants for Various Inhibitors on Silica-Alumina Cracking Catalysts... 

These catalysts are silica-aluminas whose cracking and disproportionation power has been altered by steam treatment, the use of an inhibitor, or of aluminas containing a halogenated compound or fluorine. They are very rugged, are employed without hydrogen and hence cannot isomerize ethylbenzene, which is therefore cracked or transformed by a disproportionation reaction into benzene and C10 aromatics. Consequently, they can only be used with feeds poor in ethylbenzene. However, no naphthenic hydrocarbons are formed. 

For example, consider the smallest particles of silica-alumina beads (14-mesh Tyler, r = 0.058 cm.) used in the styrene experiments at 800° F (426°). The initial rate dn /dt calculated as outlined above was 7 X 10 mole/cc. sec. The diffusivity of cumene in the pore structure of catalysts of this type having the same surface area was found by the porous plug method (5) to be 7 X 10 cm. /sec. The left side of Equation (22) yields the value 2. Thus the criterion for absence of significant diffusion transport is violated. Using the curves of Weisz and Prater (5) and the fact that (dur/dt) X (l/c)rVD ff = the value of 2 obtained above means that the observed rate in the front part of the catalyst bed was only 85 % of the rate in absence of diffusion transport effects. Since the addition of inhibitor reduces the rate and therefore reduces the diffusion transport effect, the value of Kp will be too small when it is determined under conditions in which either dn/dt for pure cumene or both dn/dt for pure cumene and inhibited cumene are affected by diffusion transport. 

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