Hydroprocessing deposition

Reclamation, Disposal, and Toxicity. Removal of poisons and inorganic deposits from used catalysts is typically difficult and usually uneconomical. Thus some catalysts are used without regeneration, although they may be processed to reclaim expensive metal components. Used precious metal catalysts, including automobile exhaust conversion catalysts, are treated (often by the suppHers) to extract the metals, and recovery efficiencies are high. Some spent hydroprocessing catalysts may be used as sources of molybdenum and other valuable metals. 

It has been mentioned in an earlier chapter that nickel deposits are basically of two types sulfidic and lateritic (oxide). The scenario of nickel extraction from nickel sulfide concentrates and nickeliferrous pyrrho tite (these two are the two products of physical beneficiation of nickel sulfide ores), and from limonitics and gamieritics (these are the common lateritic ores) has been presented in Figure 5.6. It can be seen that nickel is extracted from its various sources by pyro, pyro-hydro and hydroprocessing. The account given here pertains to the latter two processes applied to the various nickel sources. 

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. 

Although hydroprocessing will remove most of the naturally occurring antioxidants contained in fuel, other less stable, more reactive components will also be reduced. As a whole, ultra-low sulfur diesel fuel will be much less prone to color degradation and deposit formation than earlier-era diesel fuels. 

A better understanding of the chemical nature of the metal compounds, the mechanisms of HDM reactions, and metal deposition phenomena would establish a basis for developing improved hydroprocessing catalysts and reactors. A goal of research in this area is to develop catalysts with greater metals tolerance and operational life in reactors. 

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. 

In Section V, deactivation of catalyst pellets and reactor beds during residuum hydroprocessing is considered. The chemical nature of the metal deposits is described, including a discussion of the physical distribution of these poisons in aged catalysts and reactor beds. Models to predict... 

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... 

Based on elemental analyses and microprobe tracing (Dautzenberg et al., 1978), metal deposits appear to be present in sulfide forms and not as adsorbed porphyrin-type compounds or as metals in the elemental or metallic state. Takatsuka et al. (1979) and Rankel and Rollmann (1983) have reported direct linear correlations of the spent catalyst sulfur content with the deposited metal content. The sulfide forms of nickel and vanadium are consistent with expectations based on thermodynamics for the conditions typically encountered in residuum hydroprocessing units (600-800°F, 1000-2200 psig, H2/H2S environment). 

When catalytic processes are employed, complex molecules (such as those that may be found in the original asphaltene fraction) or those that are formed during the process, are not sufficiently mobile. They are also too strongly adsorbed by the catalyst to be saturated by the hydrogenation component and, hence, continue to react and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the hydroprocess. 

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

Hereafter we focus on a detailed understanding and model description of coke formation on catalysts in a trickle-bed reactor during hydroprocessing of VGO under the severe conditions mentioned above. Firstly, we will address the nature of the coke deposits in relation to that of the catalyst. A distinction between catalytic and thermal coke is made, based on information obtained from analytical techniques as well as from re-testing of the spent catalysts. Secondly, the extent of coke formation is dealt with on the basis of both experimental and modelling work. In this part the impact of vapour liquid equilibria is shown to be of prime importance. 

Vanadium is known to exist in petroleum and shale deposits as a porphyrin complex. Many catalytic processes that take place in oil production or advanced coal conversion may be critically affected by the presence of trace amounts of vanadium. Also it is known that vanadium, in combination with S compounds, can cause problems in oil hydroprocessing. Investigation of the manner in which V is present in coal, oil, and bitumen has resulted in many studies on porphyrin complexes ofV and V. ... 

LHSV Effect Figure 3 illustrates the influence of feed space velocity on the deposition of carbon and vanadium on the catalyst during hydroprocessing of Kuwait vacuum residue. It is seen that the vanadium on the catalyst increases while the amount of carbon decreases with increasing feed space velocity. The loss in catalyst surface area is substantially high at low feed flow rates (Figure 3b), presumably due to increased carbon deposition. Considering these results, it is reasonable to conclude... 

Technical interest has concentrated on hydroprocessing heavy and residual oils and coal. The catalyst is required to be effective in hds and also in hydrocracking, hdn and hdo (Section 6), and demetallization. A particular problem in these applications is catalyst by carbon and metals deposition. 

The hydroprocessing of heavy oils is associated with extensive catalyst deactivation mainly as a result of the deposition of coke and of metal sulphides. Deactivation is fast when the catalyst is first brought on line subsequent loss of activity is much slower. The importance of various effects occurring during deactivation has been re-assessed. 

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