Deactivating catalysts pore diffusion effects

Instead, they proposed a time on stream theory to model the catalyst deactivation. However, in an earlier work by Voorhies (2), a linear correlation between conversion and coke on catalyst for fixed-bed catalytic cracking was derived. Rudershausen and Watson (3) also observed the similar behavior. Coke on catalyst can reduce the activity by covering the active sites and blocking the pores. The effects of pore size on catalyst performance during hydrotreating coal oils in trickle-bed reactors have been studied experimentally by Ahmed and Crynes (4) and by Sooter (5). The pore size effects in other studies are also reported 7, 8). Prasher et al. (9) observed that the effective diffusivities of oils in aged catalysts were severely reduced by coke deposition. 

When gum formation proceeds, the minimum temperature in the catalyst bed decreases with time. This could be explained by a shift in the reaction mechanism so more endothermic reaction steps are prevailing. The decrease in the bed temperature speeds up the deactivation by gum formation. This aspect of gum formation is also seen on the temperature profiles in Figure 9. Calculations with a heterogenous reactor model have shown that the decreasing minimum catalyst bed temperature could also be explained by a change of the effectiveness factors for the reactions. The radial poisoning profiles in the catalyst pellets influence the complex interaction between pore diffusion and reaction rates and this results in a shift in the overall balance between endothermic and exothermic reactions. 

Another deactivation factor is the deposited substances that hinder intraparticle diffusion of reactants. The vanadium deposition is supposed to have a significant effect on the effective diffusivity in the catalyst pores. The decline of the effective diffusivity is expressed by the following equations for sulfur Ds and vanadium Dy, respectively ... 

A series of CoMo/Alumina-Aluminum Phosphate catalysts with various pore diameters was prepared. These catalysts have a narrow pore size distribution and, therefore, are suitable for studying the effect of pore structure on the deactivation of reaction. Hydrodesulfurization of res id oils over these catalysts was carried out in a trickle bed reactor- The results show that the deactivation of reaction can be masked by pore diffusion in catalyst particle leading to erro neous measurements of deactivation rate constants from experimental data. A theoretical model is developed to calculate the intrinsic rate constant of major reaction. A method developed by Nojcik (1986) was then used to determine the intrinsic deactivation rate constant and deactivation effectiveness factor- The results indicate that the deactivation effectiveness factor is decreased with decreasing pore diameter of the catalyst, indicating that the pore diffusion plays a dominant role in deactivation of catalyst. 

Wood, J. and Gladden, L.F., Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts, AppL Catal. A Gen., 249, 241-253, 2003. Wheeler, A., Reaction rates and selectivity in catalyst pores, Adv. Catal., 3, 249-327, 1951. 

The catalyst particles cannot be very small. The intraparticle diffusion effects can be significant. The catalyst pore-mouth plugging can cause rapid deactivation. 

S.T. Sie, and J.B. Wiffels, Catalyst Deactivation through Pore Mouth Plugging during Residue Desulphurization, in "Chemical Reaction Engineering - Houston" (eds. V. W. Weekman and P. Luss), p. 254-267, ACS Symposium Series 55, 1978. Shah, Y. T. and J. A. Paraskos, Intraparticle Diffusion Effects in Residue... 

The hydrogenation of para-substituted anilines over rhodium catalysts has been investigated. An antipathetic metal crystallite size effect was observed for the hydrogenation of /Moluidinc suggesting that terrace sites favour the reaction. Limited evidence was found for catalyst deactivation by the product amines. Catalysts with pore diameters less than 13.2 nm showed evidence of diffusion control on the rate of reaction but not the cis trans ratio of the product. 

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