Catalysts hydrodemetallation

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

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

Coke Versus Metal Deactivation of Residue Hydrodemetallization Catalysts... 

Samples of used residue hydrodemetallization catalysts prepared by hydrotreating a Safanyia atmospheric residue have been characterized and tested using model compounds in order to investigate the initial deactivation of the catalyst Samples containing 4 to 10 wt % carbon and less than 200 wt ppm V or 10 to 15 wt % carbon and 1.3 wt % V have been obtained from tests in batch and continuous flow reactors respectively. It is shown that in the early stage of the catalyst deactivation a small amount of vanadium is more deactivating than a large amount of carbon. 

The study of residue hydrodemetallization catalyst deactivation using real feedstocks is of evident interest but also very complex due to the influence of many variables (29). One difficulty is to determine separately the influence of the carbon deposit from the influence of the metal deposit. 

The present study has shown that during the initial deactivation of a residue hydrodemetallization catalyst, coke formation is very rapid but is far less poisoning than small amount of vanadium well dispersed in the grain volume. This suggests that metal deactivation predominates over coke deactivation at the beginning of a run of a resid hydrotreater. 

Reactors and Catalysts. The reactor configuration consists of two reactor trains of two reactors in series each with two beds (8). Since the reactors are adiabatic, each bed temperature, except the first bed, is controlled by recycle hydrogen gas. However, as shown in Figure 1, the bed temperatures increase with reactor depth. A maximum reactor inlet temperature is set to avoid coking in the furnace coil and the first bed. A maximum reactor outlet temperature is limited by reactor shell metallurgy. The hydrodesulfurization catalyst, which occupies about 70% of a total catalyst volume, is packed in a lower half of the second bed and the whole third and fourth beds. The rest is the hydrodemetallation catalyst. We have used Orient Catalyst HOP-802 as the hydrodesulfurization catalyst, which contains about 2% Ni and 8% Mo on an alumina support. 

Summary. Optimization of the three-dimensional pore structure of a hydrodemetallation catalyst will be described. A random network model with different connectivities has been used. The influence of connectivity, diffusion coefficient, outer dimension of pellet and operating time on optimcd pore structure has been investigated. Numerical methods employed will be discussed. 

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. 

Ying, Z.-S. Gevert, B. Otterstedt, J.-E., and Sterte, J., Hydrodemetalation of residual oil with catalysts using fibrillar alumina as carrier material. Applied Catalysis, A General, 1997. 153(1-2) pp. 69-82. 

Ancheyta-Iuarez, J. Maity, S. K. Betancourt-Rivera, G., et al., Comparison of different Ni-Mo/alumina catalysts on hydrodemetallization of Maya crude oil. Applied Catalysis, A General, 2001. 216(1-2) pp. 195-208. 

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 ... 

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. 

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... 

The optimum catalyst and the optimum processing conditions for hydrodemetallation will depend upon the feedstock and the process application. To the extent that metal deposits determine the operating lifetime of the catalyst, knowledge of the intrinsic metal removal chemistry and molecular transport processes will enable prediction of metal deposition location within catalysts and will provide criteria for optimum catalyst design. 

Webster, I. A., "Catalytic Hydrodemetallation of Nickel Porphyrins Reactivity and Catalyst Surface Studies." Sc.D. thesis, M.I.T., 1984. 

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