Mechanism of hydroprocessing

Kinetics and Mechanism of Hydroprocessing Reactions over Carbon and Carbon-Supported Catalysts... 

The information in previous sections indicates some fundamental differences between the overall mechanisms of hydroprocessing reactions involving... 

Hensen et al observed that the morphology of M0S2 crystallites can influence the mechanism of hydroprocessing reactions. The morphology can be tailor-made by selecting a proper support and the catalyst preparation... 

Mathur, K. N. Sarbak, Z. Islam, N., Kwart, H., and Katzer, J. R., Kinetics and Mechanism of Catalytic Hydroprocessing of Components of Coal-Derived Liquids, DOE, Office of Fossil Energy. Washington, DC. X and XI Quarterly 1982 August 16, 1981 to February 15, 1982. 

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. 

Figure 13 A shows some calculated radial temperatures within catalyst beds for some typical cases of hydroprocessing of oils in either a gas phase or a trickle-flow process. In these calculations, effective radial thermal conductivities were used that have been determined from existing correlations involving both static and convective mechanisms of heat transfer. It can be seen that whereas deviations from true iso-thermicity are reasonably small for a bed diameter of 1 cm, isothermal operation is hardly possible at the diameters in the range of pilot-plants, especially if the reactor is relatively short and operated with gas only. Under the latter circumstances, the reactor may even be unstable and temperature may run away.

P. C. H. Mitchell reported on sulphide catalysts in Volume 1 and now gives a review of the extensive literature appearing from July 1976 to December 1979. He reports on the work on the characterization and the improvement by promoters and dopants of sulphide catalysts and he summarizes the new information about the nature of the active sites, about intermediates, and about mechanisms on such catalysts. He also refers to the increasing interest in other forms of hydroprocessing for which sulphide catalysts are being tested in addition to their main application in hydrosulphurization. D. L. Trimm in the final chapter deals with a complex and broad subject of carbon as a catalyst and the associated field of reactions of carbon with gases. His review is drawn from a wide range of journals and prefaced by an account of the nature of the carbon surfaces, their impurities, and the complexes found on them. 

Just like any catalytic process in the crude oil industry, the big problem in hydroprocessing is the deactivation of the catalyst during the process. There two main mechanisms of catalyst deactivation during hydroprocessing ... 

Galiano-Roth, N., and Page, M. (1994). Effect of hydroprocessing on lubricant base stock composition and product p>erformance. Lubrication Engineering. 50(8) 659 -664. Holder, G. A., and Winkler, J. (1965). Wax crystallization from distillate fuels. Part III. Effect of wax compwsition on response to pour depressant and further development of the mechanism of pour depression. /. Inst. Pet. 51(499) 243- 252. 

The comprehensive review on deep HDS published by Whitehurst et provides detailed accounts of the HDS mechanism of a wide range of the model S-containing compounds. This review deserves attention in spite of the fact that it deals predominantly with the y-Al203-supported hydroprocessing catalysts. 

Delmon and coworkers established the database on the HDO of model compounds that are typical of those present in biocrude, i.e. 4-MA, DES and GUA. The tentative mechanisms of HDO are shown in Figures 42 and 51. Kinetic parameters in Table 27 indicate the effect of supports such as 7-AI2O3 and Si02 in comparison with AC. There is little information suggesting that the carbon-supported hydroprocessing catalysts were used to study the HDO of the 0-containing model compounds that are present in petroleum fractions and CDL (e.g., phenols and furanic heterorings). 

The new specifications not only limit the concentration of sulfur to 0.05% but also specify that the fuel should have the combustion properties of a 10% or lower aromatics-containing fuel and have a minimum cetane number of 40. Fuels that have more than 10% aromatics are now able to meet these specifications through additives (22). However, as smoke emission standards become more restrictive, the aromatics content of diesel fuels may have to be lowered to a true value of 10% or less. A very thorough review of the consequences of this potential problem has recently been written by Stanislaus and Cooper, which covers in detail aromatic hydrocarbon hydroprocessing kinetics, thermodynamics, catalyst compositions, and mechanisms (109). There is little need to repeat the details of that report... 

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. 

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

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