Hydrodesulfurization of Cold Lake Diesel Fraction Using Dispersed

The hydrodesulfurization (HDS) of a straight-run diesel fraction of a Cold Lake crude was investigated using ammonium tetrathiomolybdate (ATTM) and ...
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Energy & Fuels 1998, 12, 598-606

Hydrodesulfurization of Cold Lake Diesel Fraction Using Dispersed Catalysts: Influence of Hydroprocessing Medium and Sources of H2 Ce´cile N. Siewe and Flora T. T. Ng* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Received September 30, 1997

The hydrodesulfurization (HDS) of a straight-run diesel fraction of a Cold Lake crude was investigated using ammonium tetrathiomolybdate (ATTM) and phosphomolybdic acid (PMA) as dispersed catalysts. The experiments were performed in an autoclave at 340 °C using different hydrotreating media such as H2, H2/H2O, and H2 generated in situ from CO/H2O via the water gas shift reaction (WGSR). The external supply of molecular hydrogen proved to be the most effective medium for HDS with both catalysts, while similar levels of sulfur removal were achieved using either the H2/H2O or the CO/H2O medium. Desulfurization using molecular H2 was thus apparently inhibited by the addition of water to the feed, while H2 generated in situ proved to be at least as efficient at removing sulfur from diesel as using molecular H2 in the presence of water. Under our reaction conditions, ATTM exhibited greater activity for HDS and for the WGSR than PMA. This work demonstrates that Mo-based dispersed catalysts and partial oxidation flue gases containing CO, H2O, CO2, and H2 can be used to achieve satisfactory levels of sulfur removal from an industrial-type feedstock, as well as the feasibility of using dispersed catalysts and recycle streams for the treatment of heavy oil emulsions.

Introduction The decline in the quality and quantity of existing crude reserves has resulted in refineries being increasingly supplied with lower-quality feedstocks while at the same time being required to perform to higher compositional specifications that limit the content of sulfur and aromatics, as mandated by progressively stringent environmental legislation. Consequently, both deep desulfurization and a reduction in the costs associated with hydrogen production are becoming ever more important in refinery operations. Considerable research effort has been devoted to the development of dispersed catalysts as suitable alternatives to the conventional supported catalysts currently used for upgrading the quality of heavy, “dirtier” feedstocks.1-8 Dispersed catalyst systems have a number of advantages over the supported ones:2 they are less prone to deactivation and are hence more suitable * To whom correspondence should be addressed. E-mail: fttng@ cape.uwaterloo.ca. (1) Bearden, R.; Aldridge, C. L. Energy Prog. 1981, 1, 44-48. Bearden, R.; Aldridge, C. L. U.S. Patent 4,313,818, 1980. (2) Del Bianco, A.; Panariti, N.; Di Carlo, S.; Elmouchnino, J.; Fixari, B.; Le Perchec, P. Appl. Catal. A 1993, 94, 1-16. (3) Cebolla, V. L.; Membado, L.; Vela, J.; Bacaud, R.; Rouleau, L. Energy Fuels 1995, 9, 901-905. (4) Kim, H.; Curtis, C. W. Energy Fuels 1990, 4, 206-214. (5) Kim, H.; Curtis, C. W. Energy Fuels 1990, 4, 214-219. (6) Ting, P.-S.; Curtis, C. W.; Cronauer, D. C. Energy Fuels 1992, 6, 511-518. (7) Lee, D. K.; Park, S. K.; Yoon, W. L.; Lee, I. C.; Woo, S. I. Energy Fuels 1995, 9, 2-9. (8) Sandford, E. C.; Steer, J. G.; Muehlenbachs, K.; Gray, M. R. Energy Fuels 1995, 9, 928-935.

for processing heavier feeds; their microsizes ensure a high degree of catalytic metal utilization; diffusional limitations of reactants are greatly reduced; the interaction of oil and hydrogen on the high surface area of the small particles is maximized, leading to more efficient activation of molecular hydrogen and, hence, high suppressibility of coke formation. The metal constituents of dispersed catalysts are usually transition metals, notably Mo, Co, W, and Fe. Mo-based precursors appear to provide best overall effectiveness in promoting conversion in terms of boiling point reduction, Conradson carbon conversion, and hydrodesulfurization (HDS).2 They have also been reported to achieve better results than precursors based on the other metals for the hydroconversion of a deasphalted vacuum residue3 and heavy oil residues7,9 and the HDS of thiophenic model compounds.10 The requirement to increase hydrogenation processes in refineries, coupled with the need to process heavier crudes and residua, which require substantial quantities of hydrogen for upgrading, has resulted in increased demand for this gas,11 and the cost of its production has thus taken on additional significance. Apart from hydrogen recovered from catalytic reformer processes, H2 is mainly produced today from steam reforming or partial oxidation of hydrocarbons.11 The in situ genera(9) Fixari, B.; Peureux, S.; Elmouchnino, J.; Perchec, P.; Vrinat, M.; Morel, F. Energy Fuels 1994, 8, 588-592. (10) Curtis, C. W.; Chen, J. H.; Tang, Y. Energy Fuels 1995, 9, 195203. (11) Speight, J. G. The Chemistry and Technology of Petroleum: Chemical Industries; Marcel Dekker: New York, 1991; Vol. 3, pp 588592.

S0887-0624(97)00188-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998

Cold Lake Diesel

tion of hydrogen via the water gas shift reaction (WGSR) in refinery processes would be highly beneficial to the industry. Since the water gas shift reaction (WGSR) is already employed as a secondary reaction in these processes to reduce CO levels and to produce additional hydrogen, the development of single-stage hydrotreatment processes for which hydrogen is generated in situ via the WGSR deserves closer attention. If successful, such a process would obviate the need for the separation and purification stages in hydrogen production; partial oxidation flue gases could be fed directly to the hydrotreater and, if necessary, additional steam could be supplied to consume the CO.12 The supported transition metal oxides widely used in hydroprocessing have been shown to catalyze the WSGR in studies investigating the in situ generation of hydrogen for subsequent use in the hydrogenation and hydrogenolyses of model compounds.13-15 Sulfided forms of these metal oxides, which are reportedly more active in hydrotreatment processes,16-18 have also be shown to catalyze the WGSR as well as HDS.19-22 Hook and Akgerman12 used hydrogen generated in situ via the WGSR to study the desulfurization of dibenzothiophene (DBT), which is reported to be one of the polycyclic thiophenes most resistant to sulfur removal.23-26 The observed HDS rate constants for DBT were an order of magnitude greater than those reported from work using H2 feed. They suggested that the hydrogen formed on the catalyst surface during the water gas shift reaction is nascent and possibly more active than diatomic H2. Fewer workers have examined the efficacy of dispersed catalysts for concurrent WGS and hydroprocessing reactions. Ng and Rintjema27 studied the HDS of benzothiophene in an emulsion using dispersed molybdic acid, for which they reported that hydrogen generated in situ was apparently more active than externally supplied molecular H2. Up to 96% sulfur removal was achieved using hydrogen generated in situ, whereas H2 supplied to the reactor could only effect 30% sulfur removal. Ng and Tsakiri28 reported similar results for (12) Hook, B. D.; Akgerman, A. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 278-284. (13) Takemura, Y.; Itoh, H.; Ouchi, K. J. Jpn. Pet. Inst. 1981, 24, 357-362. (14) Takemura, Y.; Itoh, H.; Ouchi, K. Ind. Eng. Chem. Fundam. 1981, 20, 94-96. (15) Takemura, Y.; Onodera, K.; Ouchi, K. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 539-542. (16) Gissy, H.; Bartsch, R.; Tanielian, C. J. Catal. 1980, 65, 150159. (17) De Beer, V. H. J.; Bevelander, C.; Van Sin Fiet, T. H. M.; Werter, P. G. A.; Amberg, C. H. J. Catal. 1976, 43, 68-76. (18) Massoth, F. E.; Kim, C.-S.; Cui, J.-N. Appl. Catal. 1990, 58, 199-208. (19) Newsome, D. S. Catal. Rev. Sci. Eng. 1980, 21 (2), 275-318. (20) Hou, P.; Meeker, D.; Wise, H. J. Catal. 1983, 80, 280-285. (21) Kumar, M.; Akgerman, A.; Anthony, R. G. Ind. Eng. Chem. Res. 1984, 23, 88-93. (22) Fu, Y. C.; Ishikuro, K.; Fueta, T.; Akiyoshi, M. Energy Fuels 1995, 9, 6-412. (23) Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, G. C. J. Catal. 1979, 57, 509-512. (24) Singhal, G. H.; Espino, R. L.; Sobel, J. E. J. Catal. 1981, 67, 446-456. (25) Girgis, X. X.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058. (26) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Ind. Eng. Chem. Res. 1995, 34, 748- 754. (27) Ng, F. T. T.; Rintjema, R. T. In Studies in Surface Science and Catalysis; Smith, K. J., Sandford, E. C., Eds.; Elsevier: New York, 1992; Vol. 73, pp 51-58. (28) Ng, F. T. T.; Tsakiri, S. K. Fuel 1992, 71, 1309-1314.

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the HDS of benzothiophene in an emulsion using phosphomolybdic acid (PMA) as the catalytic precursor. The objective of this work was to examine the efficiency of Mo-based dispersed catalysts for the desulfurization of a diesel fraction from a Cold Lake feedstock. We will present results from preliminary investigations using different reaction media and sources of hydrogen: an external supply of molecular H2, hydrogen generated in situ from CO and H2O via the water gas shift reaction (WGSR), and a mixture of H2/H2O for the purposes of comparison with the CO/H2O mixture. This work is of particular significance for two reasons: (a) the novel approach of using dispersed catalysts as well as in situ generation of hydrogen to investigate the desulfurization of an industrial-type feed; (b) the use of an H2/H2O reaction medium both to determine the effect of H2O on the HDS activity of the catalysts and to evaluate the feasibility of using a recycle stream containing appreciable amounts of steam in commercial hydrotreatments. We have recently developed a novel single-stage process for the upgrading of simulated emulsions based on model compounds and using dispersed catalysts with H2 generated in situ from the WGSR.27-30 This study provides another model for our process based on a more complex feed. Experimental Section Feedstock and Catalysts. The feed used was a Cold Lake crude straight-run diesel fraction (boiling range of 161-343 °C). The density of the diesel fraction was determined to be 0.867 g/mL, and the sulfur content was determined to be 1.7 wt % via X-ray fluorescence (XRF) analysis. The Mo-based precursors used were ammonium tetrathiomolybdate (ATTM, i.e., (NH4)2MoS4, containing 36.9 wt % Mo) and phosphomolybdic acid (PMA, i.e., 12MoO3‚H3PO4‚xH2O, which contained 63.1 wt % Mo theoretically, since PMA is hygroscopic). Both catalysts were obtained from Aldrich Chemical Co. Reaction Conditions. HDS of the diesel fraction was investigated using different hydrotreating media: (a) an external supply of molecular H2; (b) an external supply of molecular H2 in the presence of H2O; (c) CO/H2O for in situ generation of hydrogen via the WGSR. The standard feed composition was 60 mL of diesel and a total gas loading of 4.14 MPa, with 21 mL of deionized water added when required. The typical amounts of catalyst used were 1.06 g of PMA and 0.41 g of ATTM, corresponding to a theoretical Mo content in the reactor of 0.67 g and 0.15 g, respectively. Apparatus and Reaction Procedures. The experiments were carried out in a 300 mL stainless steel, bolted-closure batch reactor (Autoclave Engineers) fitted with a magnetically driven impeller (MagneDriveII). This reactor is located in a special reactor room with separate vents for the reactor off gases. The reactor room is fitted with a gas detection system for H2S and CO (Dra¨ger model 4001 B). Previous work in our group29,30 using this reactor system for concurrent WGSR and HDS had determined the optimum reaction parameters to be as follows: a batch time of 4 h, reaction temperature of 340 °C, an impeller speed of 550 rpm, and an H2O/CO molar feed ratio of 4 for experiments involving the WGSR. At the end of each run the reactor was allowed to cool overnight. The gases were then evacuated into a gas bag before the reactor was disassembled and the liquid and solid products recovered and separated. (29) Rintjema, R. T. MASc. Thesis, University of Waterloo, Ontario, Canada, 1992. (30) Milad, I. K. MASc. Thesis, University of Waterloo, Ontario, Canada, 1994.

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Table 1. Dependence of Diesel HDS on Hydrotreating Medium (Source of H2) H2a

sulfur removal (%)d HDS activity (×104)e XRF H2S (mmol)f GC H2S (mmol)g total H2 uptake (mmol)h HDS H2 (mmol)i

H2/H2Oa

CO/H2Oa

ATTMb

PMAb

ATTMb

PMAb

ATTMb

run 13c

run 9c

run 7c

run 5c

run 11c

PMAb run 3c

100 4.06 27.6 6.99 74 56

90.5 0.83 25.0 5.06 189 50

74.4 3.02 20.6 2.58 98 42

59.8 0.51 15.2 2.88 59 30

73.5 2.99 20.3 7.24 58 40.6

53.0 0.46 13.7 2.31 33 27.4

a Hydrotreating medium (4.14 MPa of H or CO, 60 mL of diesel, 21 mL of H O when used). b Catalyst (Mo content of 0.15 g for ATTM 2 2 and 0.67 g for PMA). c 4 h batch time for all experiments. d Percent difference in sulfur content between untreated and treated diesel, determined from XRF analysis. e Weight of sulfur removed per gram of Mo per second (g g-1 s-1). f Expected amount of H2S generated if S removal as determined by XRF were entirely in the form of H2S. g Actual amount of H2S in gaseous product determined by GC analysis. h Difference between initial and final H content, or between CO and H in final gaseous product, for experiments involving WGSR. 2 2 2 i Amount of H that would be consumed in desulfurization if all sulfur were removed exclusively in the form of H S. 2 2

Product Analysis and Definitions. Although entirely operated in a batch mode as far as the liquid-phase components were concerned, the reactor was modified to enable periodic sampling of the gas phase, via a 150 mL stainless steel bomb into which small aliquots of the gas-phase components could be sampled. GC analysis of the gas samples indicated the presence of CO, CO2, H2, and H2S. It was thus possible to determine the kinetics of the WGSR by monitoring the changes in the CO concentration in the gas phase with time. The thermal conductivity detector (TCD) of a Perkin-Elmer gas chromatograph (model 8500) was used for analysis of the gaseous products, with a carrier gas of 8.5% H2 in He. The sulfur content of the treated and untreated diesel samples was determined by X-ray fluorescence (XRF), using the 55Fe source of an OXFORD Lab-X 1000 XRF equipment. At the end of the experiment, the reactor was allowed to cool overnight (12 h on average) and the gaseous products were collected in a gas bag and analyzed as described above. Hence, the CO conversion refers to the percent difference in the amount of CO (mmoles) fed to the reactor and the amount of CO detected in the final gaseous product. Since the production of equimolar amounts of CO2 and H2 is expected from the stoichiometry of the WGSR, the amount of hydrogen consumed in the reactor or H2 uptake is defined as the difference between the mmoles of CO2 and H2 in the final gaseous product of each experiment. For a typical experiment with 60 mL of diesel, 21 mL of water, and 1.06 g of PMA or 0.41 g of ATTM, about 55-57 mL of liquid hydrocarbon, 14-17 mL of water, and 1-2 g of black solids were obtained. The decrease in water content is attributed largely to the consumption of water via the water gas shift reaction. The weight of the black solid suggested the presence of coke. However, owing to the fact that we sampled the gas phase during the run, no mass balance was carried out. For the liquid products, sulfur removal is expressed as the percent difference of sulfur in the untreated and treated liquids. For the purposes of this work, the HDS activity of each catalytic precursor is defined as the amount of S removed per gram of Mo per second, i.e., g g-1 s-1. This number provides a basis for a comparison of the relative activity for the catalysts under similar reaction conditions, and as defined here, it assumes a linear dependence of activity on the Mo content for each catalyst. The reproducibility of the sulfurremoval data are within 2%. Catalyst Characterization. X-ray diffraction (XRD) and thermogravimetric analyses (TGA/DTA) were performed on the spent catalysts mainly to obtain qualitative information on bulk-phase composition. At the end of each run, the solids recovered from the reactor were dried overnight at 110 °C in flowing N2 to remove lightly adsorbed material. Powder XRD patterns were obtained using a Siemens D500 X-ray diffractometer with the Cu KR radiation (λ ) 1.543 24) as the X-ray source, operating at 1.2 kW (40 kV × 30 mA). Thermal analysis was performed using equipment from TA Instruments

(TA Instruments 2100 thermal analysis, TGA/DTA, 1500 °C). The fresh catalyst precursors and commercial grade samples of MoS2, MoO2, and MoO3 were analyzed by both XRD and thermogravimetry to facilitate the identification of the dominant species in the spent catalysts and enable comparison of the catalyst before and after use.

Results and Discussion Desulfurization Using External Supply of H2. These experiments were performed to determine the activity of the dispersed catalysts for HDS under typical industrial conditions. The catalytic precursors were used under the standard conditions detailed in the preceding section. Although the untreated diesel fraction was almost black, the treated samples were very slightly tinged with a yellowish color. The solids recovered from the reactor were in the form of a finely divided, black powder. The results obtained from these experiments are presented in Table 1. The catalysts showed appreciable sulfur-removal capability; complete desulfurization of the diesel was obtained with ATTM and 90.5% with PMA. Since the Mo content in the amount of PMA used was about 4 times that of ATTM, the sulfur-removal data indicated that ATTM was more active than PMA. This was reflected in the “HDS activity” data, defined as the amount of S removed per unit gram of Mo per second (g g-1 s-1), which was found to be 4.06 × 10-4 and 0.83 × 10-4 g g-1 s-1 for ATTM and PMA, respectively. The HDS activity defined in this work assumed a linear relationship between Mo content and sulfur removal for each catalyst. Hence, the data obtained should be interpreted primarily as an indication of the relative activity of the catalysts under similar reaction conditions. Even though H2S was detected the gas phase, owing to the low concentrations of H2S and the possible consumption of H2S to sulfide the catalyst precusor, the changes of the H2S concentration detected in the gas phase would not give reliable kinetic data. To establish a more absolute activity series for the catalysts used, further work would be required to establish the kinetics of the desulfurization based on analysis of the sulfur content of the liquid products, as well as to characterize the active species. It is generally accepted in the literature that sulfidic species are more active in desulfurization processes than oxidic species.16-18 Therefore, it is reasonable to expect ATTM, which is already in a sulfide form (MoS4), to exhibit greater HDS activity than PMA, in which the oxide form of the Mo precursor

Cold Lake Diesel

would necessitate prior reduction/sulfidation to produce the active MoSx species. The results from TGA and XRD of the spent catalysts (presented in a later section) provided further evidence of a greater presence of MoSx species in ATTM and a greater presence of MoOx species in PMA. It can also be seen from Table 1 that in all these experiments, the amount of H2S determined to be present in the final gaseous product via GC analysis (GC H2S) was markedly smaller than expected from the amount of sulfur removal determined from the XRF data (XRF H2S), assuming sulfur was removed from diesel exclusively in the form of H2S. This was true of all the experiments performed, regardless of the hydrotreating medium. This was attributed to some of the H2S generated dissolving in the reactor liquids and to sulfur removed from the diesel feed being used up in converting the oxidic form of the catalytic precursors into the active sulfided species. This is in accordance with reports in the literature,31,32 which showed that during the initial reaction period of thiophene HDS using MoO3, the sulfur removed was consumed in sulfidation of the oxide, transforming the MoO3 into a mixture of MoO2 and MoS2. The amount of hydrogen apparently consumed during reaction (“total H2 uptake”, defined as the difference in the amount of H2 fed to the reactor and the H2 content of the final gaseous product) did not indicate a direct relationship with sulfur removal (Table 1). In each case, the overall H2 uptake was greater than the amount of H2 required for desulfurization (“HDS H2”), assuming that all the sulfur (determined by XRF) was removed in the form of H2S. Some of the excess hydrogen apparently consumed during the reaction could be due to utilization in hydrogenation of the unsaturated species present in the oil, although loss of H2 due to sampling cannot be discounted. In every case, there was no apparent correlation between HDS and the H2 consumption. Desulfurization Using H2 in the Presence of H2O. The effect of H2O on the diesel desulfurization using H2 was studied using PMA and ATTM under the standard reaction conditions. For the purposes of comparison, 21 mL of water was added to 60 mL of diesel so that the quantity of H2O was the same as that subsequently used in the WGSR/HDS experiments; the total sulfur content remained constant in each case. The addition of water had no apparent effect on the visible physical attributes of the reaction products. The final liquids retrieved from the reactor separated naturally into an aqueous phase and an organic phase with a very slight yellow tinge. Similarly, the solids recovered were in the form of a finely divided, black powder. The results obtained showed that the presence of water apparently inhibited the HDS activity of both catalysts (Table 1). Sulfur removal decreased from 100% to 74.4% with ATTM and from 90.5% to 59.8% with PMA. There was a corresponding decrease in the HDS activities of the catalysts from 4.06 × 10-4 to 3.02 × 10-4 g g-1 s-1 for ATTM and from 8.3 × 10-5 to 5.1 × 10-5 g g-1 s-1 for PMA, suggesting that PMA was more (31) Sotani, N.; Hasegawa, M. Chem. Lett. 1975, 12, 1039-1043. (32) Zabala, J. M.; Mainil, M.; Grange, P.; Delmon, B. Acad. Sci., C.R., Ser. C 1975, 280, 1129-1137.

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susceptible to the presence of water. The addition of water could affect the activity of the catalysts by a number of mechanisms, e.g., by altering the phase equilibria in the reactor, creating additional mass transfer and diffusional resistance, or directly modifying the nature of the active catalytic species. We postulate that the inhibition observed was partly due to the fact that water could hinder the creation of the active sulfide species or facilitate their reoxidation. The greater susceptibility of PMA to the presence of water was attributed to the different chemical compositions of PMA and ATTM. Under the reaction conditions, PMA [12MoO3‚H3PO4‚xH2O] is likely to undergo rapid thermal decomposition to MoO3 followed by subsequent reduction/sulfidation into MoO2 and MoSx species by H2 and the sulfur-containing species in the diesel. Hence, when present, water could affect the activity of PMA directly by inhibiting the formation of the MoSx species or indirectly by promoting the conversion of the sulfided species back to the oxide species. ATTM [(NH4)2MoS4], on the other hand, does not require an external source of sulfur for activation. Its thermal decomposition to MoS2 proceeds via the MoS3 intermediate,2,33 and it is hence less likely to be affected by the presence of water, as indicated by the sulfur-removal data. Subsequent XRD analysis of the reactor solids (results presented in a later section) showed the presence of a well-defined MoO2 phase for PMA used with H2/H2O, whereas no such phase was observed for the corresponding spent ATTM. In fact, Fixari et al.9 have reported that PMA thermally liberated water and MoO3 particles, which were rapidly reduced to MoO2 in the presence of H2, in their study of the deep hydroconversion of heavy oil residues with dispersed catalysts. They also reported that the sulfidation of the MoO2 thus created was low and sensitive to the presence of water on the basis of elemental and XRD analyses of the spent solids. A number of researchers34-37 have also reported inhibition of the activity of sulfided supported CoMo catalysts by steam for the hydroprocessing of model compounds. This was attributed to the dissociative adsorption of H2O onto the catalysts, which resulted in the conversion of the active sulfide species into less active oxide forms. The inhibitive effect of water on the HDS of an industrial-type feed under typical hydrotreating conditions was also reported by Takemura et al.13 who studied the desulfurization of an atmospheric residual oil of a Khafji crude containing 4.39 wt % sulfur, using alumina-supported CoMo catalysts in a batch reactor. At 350 °C, 53.1% sulfur removal was achieved using 192 mmol of feed H2, but on the addition of 278 mmol of H2O the desulfurization dropped to 5.1%. This degree of inhibition was more severe than was observed in our work using either ATTM or PMA, even though the H2O/H2 ratio of 1.45 used in their work was smaller than the ratio of 4.17 employed in this work. It therefore appears that the inhibiting effect of water on (33) Zhang, F.; Vasudevan, P. T. J. Catal. 1995, 157, 536-544. (34) Lipsch, J. M. J. G.; Schuit, G. C. A. J. Catal. 1969, 15, 179189. (35) Weiser, O.; Landa, S. Sulfide Catalysts: Their Properties and Applications; Pergamon: New York, 1973. (36) Krishnamurthy, S.; Shah, Y. T. Chem. Eng. Commun. 1982, 16, 109-117. (37) Siewe, C. N. Ph.D. Thesis, University of London, England, 1994.

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Table 2. WGSRa in the Presence of Diesel Using PMA and ATTM run 2 run 11 run 3 8 h batch time 4 h batch time 4 h batch time ATTMb ATTMb PMAb CO conversion (%)c k1′ × 104 (s-1)d H2 generated (mmol)e H2 uptake (mmol)f sulfur removal (%)g XRF H2S (mmol)h HDS H2 (mmol)i

77.6 3.34 152 62 77.5 19.9 39.8

98.7 3.75 118 58 73.5 20.3 40

67.3 2.46 146 33 53.0 13.7 27.4

a Standard feed for WGSR: 60 mL of diesel (27.63 mmol S), 21 mL of H2O (1.17 mol), 4.14 MPa of CO. b Catalyst (Mo content of 0.15 g for ATTM and 0.67 g for PMA). c Percent difference in initial and final CO content of reactor (determined after reactor was cooled to room temperature). d Pseudo-first-order rate constant for WGSR. e Based on extent of WGSR and assumed to be equivalent to amount of CO2 in final gaseous product. f Assumed to be equal to the difference between CO2 and H2 in final gaseous product. g Difference in sulfur content between treated and untreated diesel, determined from XRF analysis. h Expected amount of H2S generated if S removal as determined by XRF were entirely in the form of H2S. i Amount of H2 required to remove S in the form of H2S.

the activity for HDS was more severe with a supported catalyst than with a dispersed catalyst. Desulfurization Using H2 Generated in Situ from WGSR. In these experiments, both CO and H2O were added to the diesel feed to generate hydrogen in situ via the water gas shift reaction. The results obtained are presented in Table 2. ATTM was used to study the dependence of desulfurization on batch time, and it was found that 73.5% sulfur removal was achieved after 4 h at reaction temperature (run 11) compared with 77.5% achieved after 8 h (run 2), suggesting that most of the sulfur-containing compounds in the diesel could be easily desulfurized while some were more resistant to desulfurization. This observation is in agreement with the results of Ma et al.38 who determined that the sulfur-containing compounds in a diesel fuel were almost exclusively alkylsubstituted benzothiophenes and dibenzothiophenes and that although most of the alkylbenzothiophenes exhibited high reactivities at temperatures as low as at 280 °C and were completely desulfurized at 360 °C, the alkyldibenzothiophenes were more difficult to desulfurize. The composition of the WGSR products in the gas phase was determined by GC analysis of samples periodically removed for the duration of the experiment. Figure 1 shows a typical plot of the product distribution with time for the WGSR. Equilibrium conversion of CO was generally attained within 3 h at reaction temperature, but although equimolar amounts of H2 and CO2 were expected from the conversion of CO, it can be seen that the amount of CO2 detected was consistently greater than that of H2. This difference was primarily attributed to some of the hydrogen generated being subsequently consumed in the HDS process. The data in Table 2 show that more than sufficient hydrogen was generated in the reactor to effect complete sulfur removal in the form of H2S, since the sulfur content in the feed was approximately 27.6 mmol and the amount of H2 generated was always greater than (38) Ma, X.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1994, 33, 218-222.

Figure 1. Product distribution with time for the WGSR at 340 °C over 4 h with ATTM, with hydrogen generated in situ for HDS of diesel.

the stoichiometric amount of hydrogen (55 mmol) required for complete desulfurization. “HDS H2” in Table 2 represents the stoichiometric amount of hydrogen required to remove S as H2S on the basis of the sulfurremoval data obtained from XRF, and these values were consistently smaller than the overall amount of hydrogen apparently consumed in the reactor. As discussed before, other processes by which hydrogen could be consumed in the reactor include saturation of aromatics in the feed, reaction with CO2, or surface coverage of the catalytic species. The WGSR is an equilibrium reaction k1

CO + H2O y\ z CO2 + H2 k -1

(1)

but in our experiments, owing to the presence of excess H2O in the reactor (H2O/CO ≈ 4) coupled with the expectancy that some of the hydrogen generated would be subsequently used up in the HDS of diesel, it was assumed that the reaction is irreversible and follows pseudo-first-order kinetics:

rate ) -

d[CO] ) k1[H2O][CO] dt

) k′1[CO]

(2) (3)

where k′1 is the pseudo-first-order rate constant equal to k1[H2O]. Figure 2 shows the pseudo-first-order rate plots for experiments performed with PMA and ATTM, and it can be seen that the data obtained provided a good fit to the kinetic plots. Very similar rate constants, k1′, were obtained with ATTM for 4 and 8 h batch time, 3.34 × 10-4 and 3.75 × 10-4 s-1, respectively (Table 2), suggesting that the assumption of a pseudo-first-order kinetics for the WGSR was reasonable. A value of 2.46 × 10-4 s-1 was obtained for k1′ with PMA (run 3), which was in good agreement with results from previous work in our laboratory30 for which a value of 2.20 × 10-4 s-1 was obtained under similar conditions with the same reactor, albeit in the presence of benzothiophene instead of diesel. As with sulfur removal, the ATTM precursor was found to have a greater activity for the WGSR than the PMA precursor probably because of the greater presence of the active MoSx species in the former. The activity of sulfur-tolerant catalysts for the WGSR is well

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Energy & Fuels, Vol. 12, No. 3, 1998 603

Figure 3. XRD spectra of the fresh catalysts, ATTM and PMA, employed in the experiments.

Figure 2. Pseudo-first-order rate plots for the WGSR on the basis of the disappearance of CO, using ATTM (run 11) and PMA (run 3). [CO]o represents the initial concentration of CO (mmol/L) at reaction temperature, and [CO]t represents the concentration of CO at batch time t.

documented in the literature.13,19,20,22,39-42 It is postulated in these reports that the unique ability of MoS2 to catalyze the WGSR was based on a redox cycle involving Mo4+/Mo5+ species, since the sulfided catalyst was alternately oxidized by reaction with water (dissociative adsorption followed by the evolution of H2) and reduced by reaction with CO, which was converted to CO2. Table 1 also shows that the quantity of H2S in the final gas-phase product (GC H2S) was always less than expected on the basis of the amount of sulfur removed from the diesel (XRF H2S). As previously discussed, this was attributed to some of the H2S generated in the reactor being utilized in sulfiding the catalytic precursors in the reaction medium, in addition to dissolution of H2S in the liquid phase. We believe that the active species thus created in the PMA system were responsible for catalyzing the WGSR as well as HDS. Stenberg et al.43 reported that hydrogen sulfide in the gas phase could promote the WGSR according to eqs 4 and 5, although no mechanisms or kinetics were proposed.

CO + H2S f COS + H2

(4)

COS + H2O f CO2 + H2S

(5)

A number of supported transition metal catalysts have reportedly been used for the in situ generation of hydrogen via the WGSR in the hydroprocessing of model and nonmodel feeds.13-15,21,22,42 Takemura et al.14 reported that hydroprocessing using H2 generated in situ greatly reduced the tendency for cracking and coking (39) Hakkarainen, R.; Salmi, T.; Kesiki, R. L. Catal. Today 1994, 395-408. (40) Millar, G. J.; Rochester, C. H.; Waugh, K. C. Catal. Lett. 1992, 14, 289. (41) Massoth, F. E. J. Catal. 1975, 36, 164-184. (42) Lund, C. R. F. Ind. Eng. Chem. Res. 1996, 35, 3067-3075. (43) Stenberg, V. I.; Raman, K.; Srinivas, V. R.; Baltisberger, R. J.; Woolsey, N. F. Angew. Chem., Int. Ed. Engl. 1982, 21, 619-620. (44) Yoshimura, Y.; Furimsky, E. Appl. Catal. 1986, 23, 157-171. (45) Furimsky, E.; Yoshimura, Y. Ind. Eng. Chem. Res. 1987, 26, 657-662. (46) Yoshimura, Y.; Yokokawa, H.; Sato, T.; Shimada, H.; Matsubayashi, N.; Nishijima, A. Appl. Catal. 1991, 73, 39-53. (47) Bartholdy, J.; Zeuthen, P.; Massoth, F. E. Appl. Catal. 1995, 129, 33-42.

with alumina-supported MoO3 compared with using an external supply of H2. The authors postulated that the occurrence of the shift reaction on the Mo catalyst resulted in surface coverage by nascent hydrogen that subsequently participated in the hydrogenolysis reaction. In this work, we have shown that dispersed catalysts can also be used to generate sufficient hydrogen via the WGSR and effect appreciable levels of sulfur removal from a diesel feed. Dependence of HDS on Source of Hydrogen. The dependence of the HDS activity of both PMA and ATTM on the source of H2 was evaluated by comparing the results from experiments performed using the different hydrotreating media: H2, H2/H2O, CO/H2O. The data used in this discussion are presented in Table 1, and as can be seen, the hydrotreating medium of H2 was found to be the most efficient for sulfur removal; of the other two, H2/H2O was only slightly better than the CO/H2O medium. With ATTM, the degree of desulfurization decreased from 100% to 74.4% and 73.5% for H2, H2/H2O, and CO/H2O, respectively, while with PMA, desulfurization decreased from 90.5% to 59.8% and 53.0% for the corresponding media. It can also be seen from the HDS activity data (g S removed per g Mo per s) that ATTM was more active for diesel desulfurization than PMA, regardless of the hydrotreating medium or source of hydrogen, even though the Mo content of the PMA used was greater than that used in experiments with ATTM by a factor of 4. As stated earlier, the higher activity of ATTM was attributed to the greater presence of sulfide species, and this was subsequently confirmed by both XRD and TGA of the spent catalysts. Our previous work and that of Hook and Akgerman showed that higher levels of desulfurization of model compounds such as benzothiophene and dibenzothiophene were achieved using hydrogen generated in situ from CO and H2O than by using an external supply of H2.12,27,28 We feel the difference between these reports and our present result may be related to the types of sulfur-containing compounds in the diesel, as well as the nitrogen-containing compounds and polynuclear aromatics that may affect HDS. Characterization of Spent Catalysts by XRD. As described in the Experimental Section, the spent catalysts were analyzed using X-ray diffraction (XRD) in order to obtain qualitative information on their bulk composition. For reference purposes samples of the catalyst precursors (PMA, ATTM) were also analyzed, as well as commercial grade samples of MoS2, MoO2, and MoO3, which were the phases most likely to be present in the spent catalysts. Figure 3 shows the XRD

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Figure 4. XRD spectra of commercial grade Mo oxides and sulfide. Figure 6. XRD spectra of spent ATTM catalysts from reactions with the different media: H2 (13); CO/H2O (11); H2/H2O (7).

Figure 5. XRD spectra of spent PMA catalysts from reactions with the different media: H2 (9); CO/H2O (3); H2/H2O (5).

spectra of the catalytic precursors, while Figure 4 shows the XRD spectra of commercial grade MoO3, MoO2, and MoS2. Comparison of Figures 3 and 4 shows that the fresh PMA was predominantly MoO3. The spectra from spent PMA catalysts are presented in Figure 5, and comparison with the diffraction pattern for the fresh PMA in Figure 3 shows that there was a distinct change in the bulk structure of the precursor with reaction for every hydrotreating medium used. The spent PMA catalysts were far less crystalline than the fresh catalyst, with the exception of the spent catalyst from Run 5. The typical XRD pattern of MoO3 (characteristic signal at 2θ of ∼9.6), which was evident in the fresh catalyst, was absent from the diffractograms of all the spent PMA catalysts. Instead, there was some evidence of the presence of MoO2 (characteristic signal at 2θ of ∼26) and little evidence of the presence of MoS2 species, supporting our postulation that the PMA precursor was converted into a predominantly MoO2 species in the reactor. However, the possibility of MoS2-type species being present in the spent catalysts in the form of XRD amorphous microcrystallites cannot be discounted. Figure 5 shows that although the spent catalyst from run 9 (H2) was largely amorphous (and hence possibly a combination of oxide and sulfide species), the spent solids from run 5 (H2/H2O) was predominantly well-defined MoO2. The solids from run 3 derived from the CO/H2O medium also exhibited stronger evidence of the presence of the MoO2 phase compared with that from run 9 but less so than that observed with run 5. These XRD spectra suggest that PMA was predominantly converted into MoO2 in the presence of water, and this is in accordance with the

observed reduction in HDS activity when water was added to the feed. Figure 6 shows the diffraction patterns obtained from the spent ATTM used in the different hydrotreating media. It can be seen that in each case, the bulk structure of the fresh catalyst was also completely altered, as evidenced by the disappearance of the characteristic MoS4 peaks shown in Figure 6 (2θ of ∼18) and the appearance of the characteristic MoS2 peak at 2θ of ∼14.7. This supports our suggestion that the higher activity of the ATTM is due to the generation of MoS2 and to less conversion to MoO2 than was observed with the PMA catalysts. Characterization of Spent Catalysts by Thermal Analyses (TGA/DTA). The spent catalysts were also subjected to simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) under conditions described in the Experimental Section. As shown in the XRD data, PMA and ATTM were converted into a mixture of MoO2 and MoS2 under the reaction conditions employed, and it was expected that in an oxidizing atmosphere (5% O2 in He) used in the thermal analyses, both species would be converted into MoO3. Accordingly, assuming that there was little coke formation under our reaction conditions, a net decrease in weight would suggest a predominantly sulfidic sample being converted into MoO3 accompanied by the evolution of SO2, while an overall increase in weight would indicate that the sample under examination was predominantly oxidic:

MoS2 + 31/2O2 f MoO3 + 2SO2

(6)

MoO2 + 1/2O2 f MoO3

(7)

There are a number of reports in the literature on the oxidation of sulfide catalysts.44-47 Yoshimura et al.44-46 studied the oxidative regeneration of supported Co-Mo and Ni-Mo systems from which they ascribe peaks around 250 °C to the production of SO2 from the conversion of sulfide species to the oxide forms, and peaks around 450 °C to the production of SO2 from the burnoff of organic sulfur. To provide a basis for comparison with our spent catalysts, commercial grade MoO2 and MoS2 were subjected to the thermal treatment. It can be seen in Figure 7 that treatment of MoO2

Cold Lake Diesel

Figure 7. TGA plots from the temperature programmed oxidation of reference materials (MoS2 and MoO2) showing changes in weight with increasing temperature in an oxidizing atmosphere (5% O2 in He).

Figure 8. Differential thermal gravimetry (DTG): plots of differential change in weight with temperature (% change per °C change in temperature) from thermal analysis of the reference materials MoO2 and MoS2.

was accompanied by the an increase in weight ascribed to the formation of MoO3, while treatment of MoS2 was accompanied by loss in weight indicating the formation of MoO3. Figure 8 shows the differential thermal gravimetry (DTG) defined as a unit change in weight per unit temperature d(wt %)/d(°C), plotted against increasing temperature. On the scale used, peaks above the X-axis denote weight loss while peaks below the axis denote weight gain. For MoO2, the gain in weight was resolved into a small initial weight loss at 300 °C, which was possibly due to loss of impurities. The main region of weight gain contained a slight shoulder at ∼410 °C, and the main peak had a maximum at ∼580 °C. With MoS2, the overall loss in weight was resolved into a number of peaks, the main one of which had a maximum at ∼540 °C. It is possible that these different peaks represent the various stages in the conversion of MoS2 to MoO3 via MoxSy and MoOz species. Differential thermal analysis (DTA) for these samples showed that the weight changes for both MoO2 and MoS2 resulted from exothermic reactions, as expected. Thermal treatment of the spent catalysts generally resulted in an overall loss in weight for both catalysts. Figures 9 and 10 show typical weight loss plots for the spent ATTM and PMA catalysts, respectively. It can be seen that with both catalysts, there was greater weight loss with solids from the H2/H2O and CO/H2O media than with those from the H2 medium. The DTG and DTA plots for samples from experiments with H2/ H2O and CO/H2O were always very similar. Figures 11 and 12 show the DTG plots obtained for spent ATTM and PMA catalysts, respectively, while Figures 13 and

Energy & Fuels, Vol. 12, No. 3, 1998 605

Figure 9. Plots of weight loss with increasing temperature for spent ATTM catalysts: H2 (13); CO/H2O (11); H2/H2O (7).

Figure 10. Plots of weight loss with increasing temperature for spent PMA catalyst: H2 (9); CO/H2O (3); H2/H2O (5).

Figure 11. Plots of differential change in weight (DTG) with temperature for spent ATTM catalysts: H2 (13); CO/H2O (11); H2/H2O (7).

Figure 12. Plots of differential change in weight (DTG) with temperature for spent PMA catalysts: H2 (9); CO/H2O (3); H2/ H2O (5).

14 show the DTA plots obtained for the spent ATTM and PMA catalysts, respectively. As can be seen from the DTG plots (Figures 11 and 12) for both spent catalysts, resolution of the weight loss data revealed that generally there was an initial significant loss in weight in the range 150-250 °C with a maximum at ∼180 °C, except when the hydrotreating

606 Energy & Fuels, Vol. 12, No. 3, 1998

Figure 13. Plots showing the temperature difference between the spent ATTM samples and the alumina reference material with increasing temperature: H2 (13); CO/H2O (11); H2/H2O (7).

Siewe and Ng

tions could effectively mask any increase in weight from the conversion of MoO2 to MoO3. Despite these limitations, the TGA of the spent catalysts from run 5 (PMA with H2/H2O) proved to be noteworthy when compared with those of run 9 (PMA with H2). It can be seen from the DTG plots in Figure 12 that after the initial weight loss up to ∼300 °C for both catalysts, there was further weight loss with temperature for the catalyst from run 9 whereas with the catalyst from run 5, a further increase in temperature resulted in a weight increase. This was ascribed to the conversion of MoO2 to MoO3, confirming that the spent catalyst from this run was predominantly oxidic as had been observed from XRD analysis (see Figure 5). These observations suggest that for run 9, the catalyst was in a less oxidic form than in run 5 and was hence more active for HDS as shown in the data in Table 1. Conclusion

Figure 14. Plots showing the temperature difference between the spent PMA samples and the alumina reference material with increasing temperature: H2 (9); CO/H2O (3); H2/H2O (5).

medium was H2 alone. However, the DTA plots (Figures 13 and 14) showed little or no accompanying heat of reaction, suggesting that the weight loss in this region was due to the desorption of adsorbed material such as H2O and light hydrocarbons. There was apparently greater deposition of material on the catalysts when water was present in the reactor, and this may have contributed to the loss in HDS activity reported earlier. The DTG plots also show that there was further weight loss with increasing temperature resulting from a number of transitions in the region 300-600 °C, with corresponding exothermic peaks as shown in the DTA plots. Interpretation of these observations was limited by the fact that there was no accompanying analysis of the evolved gas(es). The peaks depicting weight loss could be a combination of the oxidation of sulfur from MoSx species, from the burnoff of sulfur-containing compounds adsorbed on the catalyst or the burnoff of either coke or organic carbon. In addition, these transi-

In this study, we have shown Mo-based dispersed catalysts (ammonium tetrathiomolybdate and phosphomolybdic acid) to be effective in the desulfurization of a straight-run diesel fraction at 340 °C. Three hydrotreating media were employed with the different catalysts: an external supply of H2, H2/H2O, and CO/H2O from which hydrogen was generated in situ via the water gas shift reaction. The degree of desulfurization of the diesel feed was found to be in the order H2 > H2/ H2O ∼ CO/H2O for all the catalysts. Bulk analysis of the spent catalysts via XRD and TGA confirmed that the catalytic precursors were converted into a mixture of oxide and sulfide species on reaction with the diesel. The results from this work showed that partial oxidation flue gases containing a mixture of H2, CO, CO2, and H2O could be used to achieve satisfactory levels of sulfur removal from an industrial-type feed. Furthermore, this process can be applied to the simultaneous de-emulsification and upgrading of heavy oil emulsions. Acknowledgment. The authors are grateful for financial support from the Imperial Oil Research Grant program and the Natural Sciences and Engineering Research Council of Canada for an operating grant and an Industrial Oriented Research grant. We acknowledge assistance from S. Ganeshalingham for carrying out some of the autoclave runs. EF970188G