In hydrodemetallization

Agrawal, R., Kinetics Diffusion in Hydrodemetallation of Nickel and Vanadium Porphyrins. Sc.D. thesis, MIT, 1980. 

Restricted transport or, by a different name, configurational diffusion [13] occurs when the diffusing molecules are comparable in size to the pores within which they diffuse. This happens, for example, in hydrodemetallation over alumina-supported Co-Ni catalysts [14]. The observation that the effective diffusivity depends on the fourth power of the molecule-to-pore size ratio is important, but it is not yet evident how to correlate complex pore size distributions with effective diffusivities in the configurational... 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Trickle-bed reactors are used in catalytic hydrotreating (reaction with H2) of petroleum fractions to remove sulfur (hydrodesulfurization), nitrogen (hydrodenitrogena-tion), and metals (hydrodemetallization), as well as in catalytic hydrocracking of petroleum fractions, and other catalytic hydrogenation and oxidation processes. An example of the first is the reaction in which a sulfur compound is represented by diben-zothiophene (Ring and Missen, 1989), and a molybdate catalyst, based, for example, on cobalt molybdate, is used ... 

Wei, J. 1987. Towards the design of hydrodemetallation catalysts. In Catalyst Design Progress and Perspectives, L. L. Hegedus, ed.. New York Wiley. 

In Section III, commercial residuum hydroprocessing technology is discussed to establish the role and requirements of hydroprocessing in the overall refinery residuum conversion scheme. Commercial residuum hydroprocessing catalysts and residuum hydrodesulfurization (RDS)-hydrodemetallation (HDM) technology are reviewed briefly. 

In the last section, future perspectives for the study of residuum hydroprocessing and the rational design of hydrodemetallation catalysts and processes are offered. 

Despite the importance of hydrodemetallation reactions and their intimate relationship to HDS, HDN, and HDO, relatively little is understood of the underlying fundamentals. Only recently have model compounds been used to explore the intrinsic reactivity of metal-bearing compounds and the nature of the catalytic sites responsible for these reactions. This is in sharp contrast to the wealth of model compound information existing in the literature on HDS (Gates et al., 1979 Mitchell, 1980 Vrinat, 1983), HDN (Katzer and Sivasubramanian, 1979 Satterfield and Yang, 1984 Ho, 1988) and HDO (Furimsky, 1983 Satterfield and Yang, 1983). 

Hydrodemetallation pathways for Ni-etioporphyrin and Ni-tetra(3-methylphenyl)porphyrin are shown in Fig. 20. Both are characterized by a sequential hydrogenation-hydrogenolysis global mechanism, but important differences are apparent. Ware and Wei (1985a) rationalized the differences in porphyrin reactivity on the basis of porphyrin molecular structure. Structural differences on the periphery of the metalloporphyrin, in particular the substituent groups at the /3-pyrrolic and methine bridge... 

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. 

Studies undertaken with petroleum feedstocks to elucidate an understanding of hydrodemetallation reactions have yielded ambiguous and in some cases conflicting results. Comparison of kinetic phenomena from one study to the next is often complicated. Formulation of a generalized kinetic and mechanistic theory of residuum demetallation requires consideration of competitive rate processes which may be unique to a particular feedstock. Catalyst activity is affected by catalyst size, shape, and pore size distribution and intrinsic activity of the catalytic metals. Feedstock reactivity reflects the composition of the crude source and the molecular size distribution of the metal-bearing species. 

A summary of hydrodemetallation kinetic studies is presented in Table XXVI. The list is not exhaustive but does include a diversity of feedstocks and catalysts. It is apparent that a discrepancy in reaction order rt with respect to total metal (Ni or V) concentration has been observed. Riley (1978) reported first-order kinetics for both nickel and vanadium removal when hydrotreating a Safaniya atmospheric residuum. Demetallation kinetic order of 1.0 to 1.5 depending on reactor configuration has been reported by van Dongen et al. (1980) for vanadium removal. Oleck and Sherry (1977) report a better description of the reaction system is obtained with second-order kinetics for nickel and vanadium removal from Lago-medio (Venezuelan) atmospheric residuum. All studies were conducted on CoMo/A1203 catalysts. 

Hydrodemetallation reactions are revealed to be diffusion limited by examination of metal deposition profiles in catalysts obtained from commercial hydroprocessing reactors. Intrapellet radial metal profiles measured by scanning electron x-ray microanalysis show that vanadium tends to be deposited in sharp, U-shaped profiles (Inoguchi et al, 1971 Oxenrei-ter etal., 1972 Sato et al., 1971 Todo et al., 1971) whereas nickel has been observed in both U-shaped (Inoguchi et al., 1971 Todo et al., 1971) and... 

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