Al2O3hydrodesulphurization catalysts

© igc)^ Elsevier Science B. V. All rights reserved. Hydrotreatment and hydrocracking of oil fractions G.F. Froment, B. Delmon and P. Grange, editors

307

Deactivation studies on NiO-MoOs/AliOs and C0O-M0O3/AI2O3 hydrodesulphurization catalysts R.Ma^mkovic-Neducin^ E.Kis", M.I>juric', J.Kiurski^ D.Z.Obadovic*' ,P.Pavlovic^ KMcic' ^University of Novi Sad, Faculty of Technology, 21000 Novi Sad, Bul.Cara Lazara 1, YU ^ University of Novi Sad, Faculty of Sciences, 21000 Novi Sad, Trg D.Obradovica 4, YU ''NIS-Rafinery Novi Sad, 21000 Novi Sad, Put Sajkaskog odreda bb., Yugoslavia The results of comparative investigation of NiO-Mo03/y-Al203 hydrodesulphurization (HDS) catalyst deactivation in industrial plant and laboratory conditions are presented. Structural (XRD, DRS, XPS), textural (LTNA) and morphological (SEM) characteristics were followed depending on temperature, time of treatment and the atmosphere of regeneration. Based on the mathematical models the of textural changes intensity depending on two independent variables (time, temperature) the role of these variables in catalyst sintering were estimated. The mechanism of deactivation process is proposed, pointing out critical conditions in processing and/or regeneration. Parallel investigation of C0O-M0O3/Y-AI2O3 catalyst was the basis for relative stabiHty estimation depending on the precursor type.

1. INTRODUCTION Broad appHcation of NiO-Mo03/Y-Al203 or CoO-Mo03/y-Al203 catalysts in hydrodesulphurization (HDS) processes generate permanent interest in investigation of different aspects of these standard industrial catalysts. Due to long-term activity of these catalysts, their deactivation received less attention, the investigations being mostiy oriented to the chemistry, structure and reaction mechanisms [1-2]. The scientific hterature in the field of HDS catalyst deactivation is mainly concemed with coldng and poisoning [3-6]. New environmental regulations impose advanced HDS/HDN processes, with improved catalysts being able to operate under more severe conditions, which together with increased interest for heavier fraction processing in more rigorous process conditions, bring about the new stimulus for catalyst aging investigations. Our previous investigations on aging of HDS catalyst and corresponding model systems have shown that sintering is one of the main processes that could cause activity decline in unpreferable regeneration conditions[7-9]. Oxidizing atmosphere in regeneration process is identified as specially critical for rapid sintering of the catalyst, the mechanism of sintering process being based on previously structurally changed active phase under the unsuitable operating conditions This study deals with comparative investigations of industrially and laboratory deactivated HDS catalyst, both based on Ni and Co as the promoters.

308 2. EXPERIMENTAL 2.1. Catalyst samples Two types of standard commercial HDS catalysts were investigated, NiO-Mo03/Y-Al203 and CoO-Mo03/y-Al203. The samples of NiO-Mo03/y-Al203 from industrial hydrotreating plant, from different reactor layers, were taken after regeneration process in steam atmosphere. Comparative investigation of the samples of both catalyst types after laboratory simulation of catalyst aging were investigated. 2.2. Laboratory simulation of catalyst aging Laboratory simulation of catalyst aging in oxidation atmosphere was reahzed in mufiQe ftimace in static air, nitrogen and steam conditions. Temperature of treatment was 500, 600, 700 and SOO^'C and treatment duration 1, 3, 6 and 9 h. 2.3. Methods Catalyst structure was characterized by: XRD (Phil^s, PW 1050 CuKa); SEM (JEOL, ISM 35); DRS (SPM-2 monochromator Veb Zeiss, Jena) with a reflection cell of the R-45/0 type; XPS (Surface Science SSX-100, small spot). Low temperature nitrogen adsorption (Micromeritics, ASAP 2000) was performed for textural properties' investigations. 2.4. Mathematical modeling Mathematical modehng was appHed m interpreting textural properties (surface area, pore volume, average pore diameter) as the ftmctions of temperature and time as two independent variables. Based on an analysis of shapes of the surfaces, Sp(T, t), Vp(T, t) and R(T, t), different polynomials models have been tested and the simplest (linear in terms of time but quadratic in terms of heating temperature) was accepted: y(T,t)=bi+b2T+b3t+b/

(1)

3. RESULTS AND DISCUSSION The activity decrease was observed after restarting hydrotreating unit with steam regenerated catalyst. The process investigation with variation of feed quahty (total sulphur level, end of distillation) confirmed pronounced activity decline in processing higher suJ^hur level, i.e. more thiophenic feeds, being still in plant limits. The problem could not be solved by increasing reactor temperature eider/or decreasmg LSHV, indicating partial deactivation of the catalyst. The characterization of regenerated catalyst samples from different reactor layers confirmed that only a part of catalyst loading was changed concerning structure and texture. The upper layer contained a considerable fraction of the sintered catalyst grains (surface area decrease of 79% in comparison to fresh sample). XRD and SEM results showed the nicrease of the crystallinity of catalyst support in that fraction, followed by formation of aluminium molybdate and M0O3 phase. The SEM investigations of the samples taken from reactor nozzle indicated formation of free M0O3 crystals, extracted from the catalyst. In the deeper catalyst layers the major part of catalyst loading did not show considerable changes concerning structure and texture. Some changes of a fraction of catalysts loading was observed, but not to the extent as

309 in upper layer, concerning both the intensity of the smtering and the ratio of the fraction changed in structure. The XPS analysis (Figure 1) of the average sample from the deeper reactor layer, with no considerable bulk properties changes, indicated fine restructuring of the active phase. The monolayer structure of molybdenum Mo(VI) phase is partially changed to multilayer structure [10, 11], while the reduced Mo(IV)/Mo(V) phase remahied stable [12]. An indication of nucleation of aluminum molybdate on the surface reveals that a part of molybdenum phase is strongly bonded to the alumina surface. These changes could be considered as an initial step in catalyst bulk restructuring, stepwise leading to loss of active component, catalyst smtering and partial hindering of active component by chemical interaction with alumina support. The changes begin on the catalyst surface and later on these processes spread through the catalyst grain causing pronounced deactivation of the catalyst.

Energy

%

235.79 232.59 234.10 230.90

30.9 44.9 9.9 14.3

242.6

Used catalyst

Fresh catalyst

224.4 243.3

225.1

Binding Energy (eV)

Figure 1. Deconvolution of Mo 3d XPS signals of NiO-Mo03/y-Al203 catalyst samples Laboratory simulation of catalyst aging in different atmospheres confirmed critical role of oxidizing atmosphere in intensive smtering and bulk restructuring. Textural and structural changes similar to that in industrially deactivated sample were observed at 800 ^C treatment in both air and steam atmosphere. A gradual shift of dominant pore size to higher values, approaching at critical temperature that of deactivated referent sample, illustrates the phenomena (Figure 2).

0.8 0.6 0.4

0.2-J

10 100 Pore diameter, (nm)

fW\ 'jM 10

— fresh — N2 air — - steam j

'300"C,9'' 100

Figure 2. Pore size distribution of MO-MOOB/Y-AIIOS catalyst samples: a) Reactor samples; b) Laboratory simulation in air atmosphere; c) Laboratory simulation in different atmospheres

310 DRS spectra indicated the change of promoter structure, with spinel formation at temperatures exceeding 700 ®C. Comparative investigation of Ni- and Co-based catalysts confirmed sHght preference of C0M0/AI2O3 concerning thermal stability. Mathematical modeling confirmed critical role of temperature concerning the intensity of smtering, indicating the change of sintering mechanism depending on temperature. The formation of fi*ee M0O3 phase, characterized by relatively low melting point (795''C), could be considered as critical for rapid sintering in oxidizmg conditions. In the same time, the formation of less reactive molybdate in the interaction of the active phase and catalyst support contribute to activity dechne. Mtensive loss of active component as an additional reason, with crystals of molybdenum oxide formed at the laboratory reactor outlet is specially pronounced in steam atmosphere.

4. CONCLUSIONS The restructuring of the original monolayer structure of active phase of the catalyst in the upper part of the reactor could be caused by formation of hot spots in reactor bed. The reducing process conditions are not critical concerning smtering, but active phase restructuring in hot spots brings about the precursors of the critical phase that initiates sintering in oxidizing conditions during regeneration. The formation of fi-ee M0O3 phase could be considered as critical for rapid sintering during regeneration. The strong interaction of the active phase and catalyst support and loss of active component contribute to activity dechne.

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