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Deactivation of Hydro Processing Catalysts

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Deactivation of hydroprocessing catalystsEdward Furimskya,*, Franklin E. MassothbaIMAF Group 184 Marlborough Avenue, Ottawa, Ont., Canada K1N 8G4bDepartment of Fuels Engineering, University of Utah, Salt Lake City 84112, UT, USA1. IntroductionHydroprocessing of commercial feeds is exten-sively practised in the petroleum industry, and to someextent in coal liquefaction and in upgrading of syn-thetic fuels. The process, employing a molybdenumcatalyst supported on a high surface area transitionalumina and promoted by cobalt or nickel, is carriedout in a trickle-bed reactor or ebullating-bed reactor atelevated temperature and hydrogen pressure.Hydroprocessing catalysts are quite versatile, exhi-biting activity for a number of important reactions.Those of major interest in hydroprocessing areremoval of heteroatoms, viz hydrodesulfurization(HDS), hydrodenitrogenation (HDN), hydrodemetal-lation (HDM), and for coal-derived liquids, hydro-deoxygenation (HDO). These reactions involvehydrogenolysis of C-heteroatom bonds. An importantattendant reaction is hydrogenation of aromatics(HYD). Typical classes of these reactants are shownin Fig. 1 Hydrogenolysis of CC bonds is generallyminor, except when hydrocracking catalysts areemployed.For relatively light feeds, deactivation of the cata-lyst is minimal and the process can operate for longperiods of time before replacement of the catalyst.However, in hydroprocessing heavy residues, catalystdeactivation can be severe, having an important com-mercial economic consideration with respect to cata-lyst lifetime. Similar deactivation is experienced inhydroprocessing coal liquids.Hydroprocessing reactions occur on the active sitesof the catalysts. Also, a suitable pore size distributionis required to ensure the access of reactant moleculesto the active sites. A main reason for deactivation ofthe catalysts involves loss of active sites. A number ofbasic causes for this loss are listed in Table 1. Block-ing of pore mouths, of course, render still active sitesunavailable to reactants, while pore mouth restrictioncould accentuate diffusional limitations on reactionrates. Irreversible site poisoning would reduce thenumber of sites available for reaction, and may bemore severe on promotional sites. Sintering of theactive slabs, comprising a plane of Mo(W) atomssandwiched between the two hexagonal planes ofsulphur atoms, would reduce the total number ofsurface vacancies. Also, rearrangement of the struc-ture might disproportionally reduce certain site centercongurations more than others, affecting catalystselectivity as well as activity.In commercial operation, hydroprocessing catalystsinvariably experience some degree of deactivation,depending on the feed source. Under commercialoperating conditions, catalyst activity, for exampleHDS conversion, is maintained by constantly raisingthe temperature. Deactivation is then manifested bythe temperature-rise prole as a function of time onstream, as illustrated by the typical S-curve of Fig. 2for a resid feed [1]. Initial deactivation is caused bycoke, which appears to rapidly reach a pseudo steady-Catalysis Today 52 (1999) 381495*Corresponding author. Tel.: +1-613-565-5604; fax: +1-613-565-5618E-mail address: [email protected] (E. Furimsky)0920-5861/99/$ see front matter # 1999 Elsevier Science B.V. All rights reserved.PII: S0 9 2 0 - 5 8 6 1 ( 9 9 ) 0 0 0 9 6 - 6state level. Continued deactivation over a longer timeperiod is due to metal deposits, whose rate of deac-tivation depends on the metals level in the feed. Thenal, catastrophic loss in activity is attributed to poreconstriction and ultimate pore blockage. At this stage,Fig. 1. Typical heteroatom and aromatic compounds found in petroleum.Table 1Basic deactivation factorsActive site poisoning by strongly adsorbed speciesActive site coverage by deposits (coke, metals)Pore mouth constriction/blockageSintering of active phaseFig. 2. Typical S-shaped deactivation curve [1].382 E. Furimsky, F.E. Massoth / Catalysis Today 52 (1999) 381495the temperature cannot be raised sufciently to keepup with deactivation, and the run has to be terminated.The molecular structure of reactants in the feed hasan indirect bearing on catalyst deactivation. Sincerates are affected by adsorbed species, stronglyadsorbed species can lower reaction rates consider-ably. This requires increase in temperature to maintaincatalyst activity, which is usually accompanied byincreased deactivation rates. Adsorbed species reducethe number of active sites via competition with thereactant. Even a simple molecule as indole has beenfound to have a long-time adsorption effect on HDSand HYD. Table 2 shows that even after two days afterits removal, the HDS of dibenzothiophene has notbeen entirely recovered, and HYD of naphthalene wasstill appreciably lower than before the addition ofindole. This may be explained on the basis of relativereaction rates to desorption rates. A strongly adsorbedcompound, having a slow rate of desorption comparedto the given reaction, will experience only very slowrecovery. This is especially marked in the case ofpolyaromatic nitrogen compounds, whose rates ofdesorption are exceedingly slow.Many studies have been performed to understandthe underlying phenomena leading to deactivation inorder to develop longer life catalysts [110]. Most ofthese studies have been of a semi-empirical nature, inwhich various deactivating factors have been com-pounded, making difcult assessment of the individualfactors responsible. There have been few attempts tocomprehensively review published information rele-vant to deactivation of hydroprocessing catalysts.Deactivation of HDS catalysts was part of a reviewby Bartholomew [2] on deactivation occurring duringvarious reactions. The information on deactivationduring direct coal liquefaction was reviewed by Tho-mas and Thakur [3]. Subsequently, the same authorshave published a summary of more than 250 publishedworks on deactivation during hydroprocessing ofheavy feeds and synthetic crudes [4]. Deactivationby coke was reviewed by Menon [5] and Absi-Halabiet al. [6]. It is to be noted that the latter two reviewsdealt mainly with the chemical aspects of coke for-mation. Thus, little attention was paid to catalystporosity and associated restrictive diffusion phenom-ena. These aspects of deactivation were discussed inthe review published by Gualda and Toulhoat [7].Tamm et al. [1] discussed in detail phenomena occur-ring during deactivation by metals. Various modelsapplied to HDM and deactivation were reviewed byDautzenberg et al. [8] and Wei [9]. The catalystdeactivation during hydroprocessing of residues wasthe primary focus of another reviewpublished recentlyby Bartholomew [10].In this review, we draw upon material from theliterature in an attempt to elucidate various basicfactors responsible for catalyst deactivation. After abrief background discussion of catalyst structure andactive sites, we concentrate on deactivation phenom-ena with poisons, coke, metals deposits and changes incatalyst active phase structure. A number of remediesrelevant to practical situations are also presented.2. Hydroprocessing catalystsThe catalysts of concern in hydroprocessing consistof molybdenum supported on a high surface areacarrier, most commonly alumina, promoted by cobaltor nickel. These catalysts are active in the suldedstate, being either presulded or sulded on streamwith a sulfur containing feed. CoMo/Al2O3 catalystsare usually employed for HDS, HDM catalysts gen-erally have large pores and lower metal contents.Extensive characterization studies of these catalystshave been reviewed by a number of authors [1115],and we will only summarize here current opinionrelative to structure and catalytic sites, without refer-ence to original papers on the subject.The supports employed usually consist of highsurface area (~200 m2/g) transition-aluminas (orsilica-alumina or zeolite for hydrocracking). The aver-age pore size is generally between 75 and 300 A,although a distribution of pore sizes is prevalent. Somesupports, especially those used for demetallation, mayalso sustain macropores, i.e., a bimodal pore structure.Table 2Effect of indole on HDS and HYD conversion% ConversionHDS HYDBefore indole 87 69After indole 64 21After 2 days with DBT 80 39E. Furimsky, F.E. Massoth / Catalysis Today 52 (1999) 381495 383The micropore dimensions can have a signicantbearing on diffusion of reactants to the active sites,particularly for heavy feeds. The literature on pre-paration of the hydroprocessing catalysts is quiteextensive [1619]. For the purpose of this review, abrief summary of the key issues on the subject willonly be given, without reference to the original works.Research has shown that sulded catalysts contain-ing Mo consist of essentially monolayer slabs orclusters of sla

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