Ind. Eng. Chem. Res. 1994,33, 1692-1699
1692
Activation and Regeneration of a NiMo/A1203 Hydrotreatment Catalyst V. L. S. Teixeira da Silva,t R. Frety? and Martin Schmal'J NUCATIPEQICOPPE-Universidade Federal do Rio de Janeiro, C.P. 68502, Rio de Janeiro, Brazil, and Institute de Reserches sur la CatalyselCNRS, 2, Avenue Albert Einstein, 69626, Villeurbanne, France
Activation and regeneration procedures applied t o a nickel-molybdenum on alumina catalyst, both fresh and spent, were tested by the hydrodesulfurization of thiophene. Characterization techniques used included temperature programmed reduction and oxidation (TPR, TPO), diffuse reflectance spectroscopy (DRS), and X-ray diffraction (XRD). The fresh catalyst was treated by sulfiding, reoxidation, and resulfiding. This sequence was found to be more effective than one sulfiding step, possibly because of the formation of a nickel molybdate phase during reoxidation. The spent catalyst could not be regenerated completely although its original surface properties were attained. The loss of activity of the spent catalysts was alluded by TPO to result from nickel-molybdenum segregation which probably happened because of the excessive heat from burning the coke present on the catalyst.
Introduction In the past few years, hydrotreating catalysts has undergone progress, at both the fundamental and applied levels. In particular, better understanding of the activation processes has enabled the obtaining of highly divided and more efficiently sulfided active phases. It was recognized that, during the sulfiding of molybdenum (tungsten)based catalysts, the nature of the sulfiding compound, its partial pressure, the final temperature of sulfiding, the temperature program used to reach the final temperature, and the pretreatments of the oxide precursor can lead to different ratios of S/(Mo(W) + Co(Ni)) corresponding to different degrees of interaction between the various components and, consequently, to different catalytic properties. Hydrotreating catalysts are mainly deactivated by coke deposition and poisons present in the feedstocks. In a general study of the regeneration of cobalt-molybdenum/ alumina catalysts, Arteaga et al. (1984, 1986, 1987a,b) showed that catalysts deactivated by carbon deposition using a butadiene/argon mixture can lead, after carbon burnoff and sulfiding of the catalyst, to a better activity in both HDS (hydrodesulfurization)of thiophene and HYD (hydrogenation) of cyclohexene than the freshly sulfided catalyst. A simulation of the regeneration process using the freshly sulfided catalyst, free of carbon deposits, also led to a catalyst more active than the starting sulfided material. The preceding results suggestthat the precursors in the starting oxide catalyst are not in an optimal state to obtain an active catalyst by sulfiding. However, most of the regenerations performed on catalysts submitted to a forced deactivation in laboratory are not representative of industrial catalysts (Delmon, 1985). Industrial catalysts are submitted to more severe conditions that laboratory catalysts, and several operational factors such as the time scale,temperature of reaction, and nature of the feed stock can lead to coke formation, sintering, and adsorption of metals and poisons, the latter being difficult to remove. The present work is part of a study of the regeneration of nickel-molybdenum catalyst used in a pilot plant for shale oil hydrotreatment (Souzaet ai., 1992),and the main objectives were (1)to study the modifications occurring on the fresh catalysts during an activation process simulating a regeneration cycle and (ii) to evaluate the t t
NUCAT/PEQ/COPPE. CNRS. 0080-5005/94/2633-1692$04.50/0
efficiencyof the regeneration of spent catalysts containing 6 and 11 wt % of carbon.
Experimental Section Catalyst. The fresh sulfided catalyst was commercial NiMo/AlzOa (Shell S324) supplied in extrudate form (3 mm X 1mm) with nominal composition of 17.0 w t 7'% (Mo) and 2.16 wt % (Ni). Prior to use, the extrudates were crushed and sieved to 100 mesh Tyler. The spent catalyst (NiMo/AlzOa,Shell 5324) originated from the hydrotreatment of shale oil in a fluidized trickle bed reactor operated at 573 K, 15 MPa, and reaction time longer than 72 h (Souza et al., 1992). After reaction in the trickle bed reactor, the catalyst was deactivated and had carbon and sulfur contents of 11and 6 wt % ,respectively. Of the total carbon content, 5 wt % was due to shale oil impregnated on the catalyst surface, and for this reason, part of the catalyst was submitted to an extraction with cyclohexane in a Soxhlet system, which resulted in a spent catalyst with 6 wt 5% carbon content. Activation and Catalytic Test. The reoxidation, regeneration, activation, and catalytic tests were carried out in a U-shaped Pyrex microreactor with a Pyrex frit for holding the catalyst. Activation of HDS catalysts consists of transforming metallic oxides to the corresponding sulfided phases, by a gas- or liquid-phase sulfiding agent. Such transformations lead to the formation of MoS2, Ni3S2, and a so-called NiMoS phase which is suggested as the possible active phase in HDS reactions (Topsrae et al., 1981a,b). In this study, sulfiding of 0.2 g of the oxide catalyst was carried out by passing a 5% (v/v) H2S/H2 mixture (Oxigbnio do Brasil, 99.99%) through the reactor. A local thermocouple monitored the temperature of the sample, and a furnace coupled to a controller/programmer (GEMD) heated the reactor from 298 K up to Tsulf (Tsulf = 573,673, or 773 K) at a heating rate of 4 K/min. Once the desired value of Tsdf was reached, the temperature was held constant for 2 h in the flow of the sulfiding gas mixture. After this period, the gas was switched from the 5% (v/v) H&H2 mixture to pure H2 flow (1.2 L/h; White Martins UP, 99.99% ), while the temperature was decreased to 553 K. Immediately after the sulfiding step, the reaction was started by passing a 10/1molar ratio of HdCdHdS reactant mixture through the reactor. This ratio was obtained by flowing pure Hz (1.2 L/h) through two saturators connected 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1693 in series maintained at room temperature by a water bath and filled with thiophene (Merck, 99%). The reaction conditions were 553 K and 0.1 MPa, with the HdC4H4S mixture flow rate adjusted to maintain low conversions of thiophene (
