Nickel catalyst diffusion rates

Hydrodemetallation reactions require the diffusion of multiringed aromatic molecules into the pore structure of the catalyst prior to initiation of the sequential conversion mechanism. The observed diffusion rate may be influenced by adsorption interactions with the surface and a contribution from surface diffusion. Experiments with nickel and vanadyl porphyrins at typical hydroprocessing conditions have shown that the reaction rates are independent of particle diameter only for catalysts on the order of 100 /im and smaller (R < 50/im). Thus the kinetic-controlled regime, that is, where the diffusion rate DeU/R2 is larger than the intrinsic reaction rate k, is limited to small particles. This necessitates an understanding of the molecular diffusion process in porous material to interpret the diffusion-disguised kinetics observed with full-size (i -in.) commercial catalysts. 

Both catalysts were activated at the optimum conditions determined using TPR. The rates at the maximum selectivity to benzyl alcohol were compared. In the presence of particulate catalyst the rate amounted to 0.0009S mol/(gNi./ min), while for monolithic catalyst the rate was approximately 0.00175 mol/(gNi min), i.e., about two times more. The diffusion length in the nickel monolith is much shorter than in the 3.2effectiveness factor for the nickel pellets, and hence in a lower reaction rate. Selectivity of both catalysts with respect to benzyl alcohol was nearly the same, at least within the precision of analytical methods used 94.9% for pelleted catalyst and 95.1% for monoliAic catalyst. We may therefore conclude that the selectivity is not controlled by internal diffusion but by the surface properties of the catalysts. 

At the high gas velocities required for measuring intrinsic rates at low eonversions away from equilibrium, the lack of back-diffusion of product gases may lead to oxidation of the nickel catalyst, as methane behaves as an inert in the Ni/NiO equilibrium (refer to Section 4.1). The problem is solved by addition of hydrogen to the feed. Axial dispersion also plays a signifieant role in determining the stability of carbide catalysts (refer to Section 4.1). 

When the formation of nickel aluminium spinel has taken place, temperatures above 800 C [389] may be required for complete reduction. Even without the presence of a spinel phase, alumina-supported nickel catalysts may show less reducibility, probably due to penetration of aluminium ions in the nickel oxide surface layers during the impregnation [364], These efforts may be aggravated by oxide additions (La203, MgO) [366], Counter diffusion of nickel and aluminium appear to be rate-determining for reduction of nickel present in spinel phases [389],... 

Although the above simple illustration of the concept for a homogeneous isothermal lumped system is applicable to other more complicated systems, the situation for catalytic reactors is much more involved because of the complexity of the intrinsic kinetics as well as the complex interaction among reactions, heat release (or absorption), and mass and heat diffusion inside the reactor. For example, many catalytic and biocatalytic reactions show nonmonotonic dependence of the rate of reaction on reactant concentrations. For example, the hydrogenation of benzene to cyclohexane over different types of nickel catalyst has an intrinsic rate of reaction of the form... 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

An effect of pore diffusion in residuum demetallation is illustrated in Figure 9, which shows nickel and vanadium concentration profiles measured through a catalyst pill after residuum desulfurizing service. The catalyst originally contained neither of these metals. These profiles confirm that the rate of reaction of the metal-containing molecules in the feed (particularly the vanadium compounds) is high compared with their rate of diffusion. 

In Section IV, the kinetics and mechanisms of catalytic HDM reactions are presented. Reaction pathways and the interplay of kinetic rate processes and molecular diffusion processes are discussed and compared for demetallation of nickel and vanadium species. Model compound HDM studies are reviewed first to provide fundamental insight into the complex processes occurring with petroleum residua. The effects of feed composition, competitive reactions, and reaction conditions are discussed. Since development of an understanding of the kinetics of metal removal is important from the standpoint of catalyst lifetime, the effect of catalyst properties on reaction kinetics and on the resulting metal deposition profiles in hydroprocessing catalysts are discussed. 

Weisz (2) carried experiments on silica- alumina beads (many times the size of an average FCC particle). Heobservedthattheintrinsic cokebuming rate was independent of the coke composition and the catalyst characteristics but dependent on initial coke level and the diffusivity. Weisz (3) inanotherstudy, found that the CO /CO ratio during intrinsic coke burning is only a function of temperature. He also observed that this ratio i s affected by the presence oftrace metals like iron and nickel etc. Even though this study was elaborate, it was limited to only silica-alumina catalysts in the form of beads. 

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