Pore-mouth plugging

Fig. 10. Catalyst macropores showing D noble metal sites and (a) narrowed micropores after exposure to high temperatures where H represents thermally damaged noble metal sites and (b) pore mouth plugging from poisons where A, if aUowed, diffuses in to be converted to B.

The accumulation of matter on the surface of the catalyst restricts gas passage into the catalyst by a mechanism known as pore mouth plugging (see Fig. 10b). It takes only a smaU amount of material on the surface of the catalyst to restrict the free passage of gases into and out of the active porous... 

Carbon-based catalysts have also been considered for the methane decomposition reaction. Yoon and co-workers have recently investigated the kinetics of methane decomposition on activated carbons as well as on carbon blacks.In case of activated carbons the authors observed mass transport effects in the catalyst particles and also significant pore mouth plugging. The reaction order was found to be 0.5 and the activation energy was found to be 200 kJ/mol for the different activated carbon samples. On the other hand, for... 

Dautzenberg et al. (1978) considered a pore mouth plugging model in which the local pore radius (initially uniform) changes with time according to... 

The model formulated by Ahn and Smith (1984) considered partial surface poisoning for HDS and pore mouth plugging for HDM reactions. The conservation equations with first-order reactions for metal-bearing and sulfur-bearing species were based on spherical pellet geometry rather than on single pores. Hence, a restricted effective diffusivity was employed... 

Catalyst Deposits Fouling and Pore Mouth Plugging... 

With the deposits of catalyst poisons as coke and heavy metals, fouling and pore mouth plugging phenomena can be observed [18, 19]. 

Dautzenberg, F.M. van Klinken, J. Pronk, K.M.A. Sie, S.T. Wijffels, J.-B. Catalyst Deactivation through Pore Mouth Plugging during Residue Desulfurization presented at the 5th International Symposium on Chemical Reaction Engineering Houston, March 13-15,1978. 

The catalyst pore-mouth plugging can cause rapid deactivation. 

The percentage reduction in pore volume and surface area for the four used catalysts are in the order S > R > P > Q. The pore volume loss is generally caused by the deposition of coke and metals in the pore. In catalysts with a large amount of small pores the deposited metals can cause pore mouth plugging leading to a large loss in pore volume and surface area. 

The model was basically developed for hydrodesulfurization of residual oil with a high level of catalyst contaminants causing pore mouth plugging of catalyst. 

The catalyst crystal sizes It has been observed that the best ZSM-22 catalysts are those consisting of small crystals (largest dimension <0.5 ju.m). For these unidimensional pore systems, the smaller the crystal size, the lower will be the aging rate by pore mouth plugging. 

The relatively uniform profiles of nickel and vanadium arc desirable, because this pattern results in maximum metah storage capacity without pore mouth plugging. The strong rinding tendency of iron can be a problem in commercial usc because it can accelerate pore mouth plugging and lead to degradation of the catalyst support... 

Throughout this work was aasumed that the coke is uniformly distributed over the catalyst particles on a macroscopic scale. This implies that we assumed no significant coke gradients over the particles to be present and, moreover, that pore mouth plugging by coke had occurred. With the email particles (30—30 mesh) used in this work both assumptions seem Justified. The absence of pore mouth plugging has been substantiated by pore volume measurements of the fresh and spent catalysts, which led to derived densities for the coke deposits in the range 1.2-1.5 g/mlh... 

The results of the mechanism described above are (1) that V and Ni concentrate on the surfaces of catalyst particles (as shown by EDS), (2) a high degree of metal sintering occurs on the catalysts particles (as observed by SEM) and (3) active metals are very sensitive to oxidation-reduction treatments. We interpret the lower MAT catalytic activity (w.r.t. the zeolite surface area) as due to a high coke-making tendency of the catalyst leading to pore mouth plugging and increased intraparticle diffiisional constraints. 

Ca/S equal to 2. Sorbent conversion is often limited to about 25% or less because of pore mouth plugging of the particles and sintering at the high temperatures. 

By integrating optimized acid sites with superior mass transport characteristics and a pore architecture that reduces pore-mouth plugging, a catalyst with enhanced performance can be created. Figure 5 demonstrates that both the catalyst selectivity and lifetime are significantly improved. As shown in figure 6, which compares the performance of Exelus solid-acid catalyst with other commercially available systems, the new catalyst system is easily able to achieve a step-change in performance over other solid-acid catalysts. 

Sahebdelfar, S., et al.. Deactivation behavior of the catalyst in solid acid catalyzed alkylation effect of pore mouth plugging. Chemical Engineering Science, 2002. 57 p. 3611-3620. 

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