Catalyst deactivation pore structure effect

The area-temperature curves and the isotherms make clear the accelerated sintering produced by steam and the profound reorganization of pore structure effected in the case of silica-alumina catalysts. This reorganization or intimate interaction between steam or adsorbed water and the catalyst is not too surprising when it is considered that the catalysts are prepared in aqueous media, that surface water is necessary for activity (Hansford, 26) and that steam deactivates catalysts. This... 

J. Wood, L. F. Gladden 2003, (Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts), Appl.Cat. A General, 249, 241. 

Early efforts to model catalyst deactivation either utilized simplified models of the catalyst s porous structure, such as a bundle of nonintersecting parallel pores, or pseudo-homogeneous descriptions in terms of effective diffusivities and tortuosity... 

Pore size. The pore size distribution of the catalyst matrix plays a key role in the catalytic performance of the catalyst. An optimum pore size distribution usually helps in a balanced distribution of smaller and larger pores, and depends on feedstock type and cracking conditions. The pore size distribution of the matrix changes when another component is added e.g. by adding 35-40% kaolin to a silica-alumina gel, a pore structure with a significant amount of micropores can be obtained. Figure 27.9 Pore volume. Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

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. 

To determine the effect of zeolite pore structure on coke removal by SCF regeneration a series of wide pore zeolites, which included acidic Ys, betas, and mordenite, were fully deactivated under a flowing isobutane/butene mixture and then regenerated under flowing SC isobutane for 60 min (77). The spent catalysts were examined ex-situ by TPO, DRIFTS, and UV-Vis spectroscopy both before and after regeneration. These analyses demonstrated that although most adsorbed hydrocarbons were removed in some catalysts none of the catalysts were completely free of hydrocarbon deposits after SC isobutane regeneration. 

Although addition of alkali metal reduced the catalytic activity of Fe UFP for FT synthesis, catalyst deactivation was suppressed by alkali promotion. Figure 2 shows the average STY s of hydrocarbons, oxygenates, and CO2 over the Fe UFP catalysts promoted by various kinds of alkali metals in a comparison with the precipitated catalyst. These data were taken for the products in the initial 6 hr of run. The activities of UFP catalysts were higher than that of the ordinary K-promoted Fe precipitation catalyst, in spite of comparable surface areas. This is interpreted as due to an effect of surface structure of catalyst. In the case of the precipitated catalyst having a rather porous structure compared with UFP, the reactant diffuses into the pores and reacts on the catalyst surface. If the reaction is faster than diffusion processes, the concentration of reactant falls along with the distance from the pore mouth. Thus, a limited portion of the surface of the precipitated catalyst can be used for reaction (ref. 7). 

The effects of temperature and steam on the deterioration of catalysts employed in cracking processes are under continuous surveillance, and these effects may be studied in a rather straightforward manner by physical property measurements independent of the chemistry of the surface. For example, as will be shown later, if a significant increase in pore radius accompanies loss in area, steam deactivation is probably indicated. Moreover the pore structure as determined by adsorption techniques is undoubtedly related to the ease of admission of reactant molecules and the diffusion out of product molecules as well as to the regeneration properties when carbonaceous deposits must be removed. 

The phosphorus deactivation curve is typical type C, and, according to the Wheeler model, this is associated with selective poisoning of pore mouths. Phosphorus distribution on the poisoned catalyst is near the gas-solid interface, i.e. at pore mouths, which confirms the Wheeler model of pore mouth poisoning for type C deactivation curves. Thus we may propose that in the fast oxidative reactions with which we are dealing, transport processes within pores will control the effectiveness of the catalyst. Active sites at the gas-solid interface will be controlled by relatively fast bulk diffusional processes, whereas active sites within pores of 20-100 A present in the washcoat aluminas on which the platinum is deposited will be controlled by the slower Knudsen diffusion process. Thus phosphorus poisoning of active sites at pore mouths will result in a serious loss in catalyst activity since reactant molecules must diffuse deeper into the pore structure by the slower Knudsen mass transport process to find progressively fewer active sites. 

The effects of pore structure on catalyst deactivation by coke formation... 

There were no effects of pore dimension on the deactivation rates (due to coke formation) of fresh catalyst over the range investigated. Further studies of deactivation rates after one and two hours of utilization at 205 C also revealed no influence on pore structure. This would rule out intraparticle mass transport as controlling deactivation rates as well as the occurrence of any pore blockage resulting from coking in this reaction. 

As indicated by Cases 8 and 9, some parameter sets result in multiple steady states. These are characterized by large 63 Yg (60 in these examples) and (3s/Bp) < 1 such results itldicate Wat multiplicity can be induced in a deactivating particle even when the fresh catalyst shows no such possibility. However, (Ps/3p) < 1 seems physically improbable. If deactivation by cok-ing closes off a portion of the pore structure, then (3s/Pp) >1 while poisoning should have little effect on the ratio. Tnis multiplicity would require a type of deactivation leading to an increase in porosity as decay proceeds. 

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