Hydrotreating catalysts vanadium deposition

The equivalent nickel content of the feed to the FCCU can vary from <0.05 ppm for a weU-hydrotreated VGO to >20 ppm for a feed containing a high resid content. The nickel and vanadium deposit essentially quantitatively on the cracking catalyst and, depending on catalyst addition rates to the FCCU, result in total metals concentrations on the equiUbrium catalyst from 100 to 10,000 ppm. 

Figs. 25 to 28 show that the metal deposition in CoMo/A1203 hydrotreating catalysts is a function of the radial position within the catalyst and the axial location of the catalyst sample within the fixed-bed reactor. Nickel and vanadium both exhibit radial profiles with internal maxima, termed M-shaped profiles, at the reactor entrance. These maxima shift to the pellets edge at the reactor outlet, generating the classic U-shaped profile. 

The question arises as to how a 1.3 wt % vanadium amount can strongly poison the catalyst. A rough calculation of the number of vanadium atom deposited per molybdenum sulfide particle can be made using the geometrical model of hydrotreating catalyst (31), Two different sizes of M0S2 slabs have been... 

Vanadium deposition profiles in catalyst pellets have been determined by various researchers (14-30). For this purpose porphyrinic model compounds and industrial feedstocks are used. The used catalyst are mainly conventional hydrotreating catalysts with narrow pores. Therefore, metal deposition profiles show mainly deposition in the outer shells of the catalyst pellets (M- or U-shaped profiles), indicating that the metal deposition process is diffusion limited. 

Assuming that the vanadium deposits consist mainly of vanadium sulphides, it is then possible to extend the arguments of Tamm et al [20] to calculate possible effects of deposition of coke and metals on pore closure. Accepting that 200 mV is a typical surface area for hydrotreating catalysts, it is known that ca 50% of the surface area is lost during the initial deactivation [19-21], It is also known that V deposition occurs in the outermost part of the pellet [20,31,37,38]. As a result, the amount of V needed to provide a monolayer over the surface area available can be calculated to be between 0.034 and 0.079g cm ... 

A number of refinery processes require the use of a fixed-bed catalyst These processes include catalytic reforming, hydrodesulfurization, hydrotreating, hydro-cracking, and others. These catalysts become inactive in six months to three years and are eventually replaced in the reactors with fresh catalyst during a unit shutdown. Many of these catalysts contain valuable metals which can be recovered economically. Some of these metals, such as platinum and palladium, represent the active catalytic component other metals such as nickel and vanadium are contaminants in the feed which are deposited on the catalyst during use. After valuable metals are recovered (a service usually performed by the outside companies), the residuals are expected to be disposed of as solid waste. 

In hydrotreating processes heteroatoms, such as sulphur, nitrogen, oxygen and metals, are catalytically removed from heavy oil residua. These metals, mainly vanadium and nickel, remain in the reactor as solid deposits accumulating on the catalyst surface after decomposition of the organo-metallic compounds. 

A catalytic reaction involving the deposit as the catalyst seems more likely. Hydrotreating reactions have been suggested to be favoured by Ni and V sulphides, albeit at a lower rate of reaction [35]. Welters et al [40] have shown that nickel sulphides in or on a zeolite are active hydroprocessing catalysts and vanadium sulphides have also been found to promote hydrotreating [27], as least to some extent. 

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