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Performances of Co-W/γ-Al2O3 Catalysts on Hydrotreatment of Light Gas Oil Derived from Athabasca Bitumen Santosh K. Vishwakarma, V. Sundaramurthy, and Ajay K. Dalai* Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, UniVersity of Saskatchewan, Saskatoon, SK, S7N 5A9, Canada
John Adjaye Syncrude Edmonton Research Centre, Edmonton, AB, T6N 1H4, Canada
γ-Al2O3-supported Co-W-based catalysts with varying cobalt (1-3 wt %) and tungsten (7-13 wt %) loadings were prepared using impregnation and sonochemical methods. Brunauer-Emmett-Teller (BET) analysis indicated that the sonochemical method of preparation resulted a larger reduction in surface area of the γ-Al2O3 support than the impregnation method for all the prepared catalysts. X-ray photoelectron spectroscopy (XPS) showed that most of tungsten metal segregated on the support surface of sonochemically prepared catalysts, whereas catalysts prepared via the impregnation method showed uniform metal dispersion on the support. The performances of all the synthesized catalysts were tested at a pressure of 8.9 MPa, a liquid hourly space velocity (LHSV) of 2 h-1, and temperatures of 340, 350, and 360 °C in a trickle-bed microreactor for the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of light gas oil (LGO) derived from Athabasca bitumen. The initial screening tests indicated that an impregnated catalyst with 3 wt % cobalt and 10 wt % tungsten and a sonochemical catalyst with 3 wt % cobalt and 13 wt % tungsten are the most active catalysts for the HDN and HDS of LGO. These two catalysts were selected for detail performance, optimization, and kinetic studies. The effects of reaction temperature (340-380 °C), pressure (7.6-10.3 MPa), LHSV (1.52.0 h-1), and H2/gas oil ratio (400-800 mL/mL) were examined in the HDS and HDN of LGO with these catalysts. The impregnated catalyst showed higher nitrogen and sulfur conversion than the sonochemical catalyst under all reaction conditions. The reaction kinetics for HDS was best-fitted with a power-law model, whereas the same for HDN was determined to be best represented by a Langmuir-Hinshelwood model with a reasonable accuracy (0.90 10 wt % tungsten) leads to the formation of multiple layers and leads to a slight drop in activity.1 The catalytically active phase of Co-W/Al2O3 is Co-WS, and its formation is dependent on the W/Co ratio. The greater the amount of Co-W-S phases on the support, the higher the HDN and HDS activities. Catalysts such as Imp 1/13, Imp 2/13, and Imp 3/13 have similar tungsten loading (13 wt %) and a cobalt loading of 1.0, 2.0, and 3.1 wt %, respectively. The observed difference in the hydrotreatment activity of these catalysts provides evidence on the effect of cobalt on CoMo/ Al2O3 catalysts. As shown in Table 4, at all temperatures studied, as the cobalt concentration increases, both nitrogen and sulfur conversions are increased gradually. It is noted that the cobalt addition promotes the sulfur conversion much more than the nitrogen conversion. The HDS activity of both series of catalysts is much higher than their HDN activity within the temperature range studied, indicating the ease in conversion of sulfur compounds, compared to nitrogen compounds, present in LGO derived from oil sands. For comparison, the activity of a CoMo/Al2O3 catalyst that contains 3 and 13 wt % of Co and Mo, respectively, prepared by conventional impregnation method is given in Table 4 for comparison. The HDN and HDS activities of Co-Mo/Al2O3 are higher than the corresponding Co-W/ Al2O3 catalysts with LGO. The initial screening studies of Co-W/Al2O3 catalysts showed that Imp 3/10 and Sono 3/13 are the most active catalysts for HDN and HDS of LGO from respective series. These two catalysts were selected for optimization and kinetic studies. The studies were conducted at three different values of each of the parameters, including reaction pressure (7.6-10.3 MPa), temperature (340-380 °C), and LHSV (1.5-2.5 h-1).
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Figure 7. Effects of temperature and liquid hourly space velocity (LHSV) on sulfur conversion of light gas oil (LGO) over Sono 3/13 and Imp 3/10. Legend: (s) Imp 3/10, (- - -) Sono 3/13, (9) LHSV ) 1.5 h-1, (0) LHSV ) 2.0 h-1, and (×) LHSV ) 2.5 h-1. Total pressure ) 8.9 MPa and G/L ) 600 mL/mL.
Figure 9. Effects of pressure on sulfur and nitrogen conversions of LGO over Sono 3/13 and Imp 3/10. Legend: (s) Imp 3/10, (- - -) Sono 3/13, (b) nitrogen conversion, and (O) sulfur conversion. Temperature ) 380 °C, LHSV ) 1.5 h-1, and G/L ) 600 mL/mL. Table 5. Calculated Dimensionless Moduli for HDS of LGO with Sono 3/13 at Temperature ) 340 °C, Pressure ) 8.9 MPa, and a Hydrogen/Gas Oil Ratio of G/L ) 600 mL/mL Moduli Sono 3/13
Imp 3/10
LHSV (h-1)
temperature (°C)
ΦS
ΦN
Φs
ΦN
1.5 2.0 2.5 1.5 1.9 2.5 1.4 2.0 2.5
340 340 340 360 360 360 380 380 380
5.0 3.4 4.5 12.8 7.4 9.6 37.9 32.9 22.6
0.2 0.3 0.3 0.8 0.8 0.7 3.9 1.8 1.3
7.5 5.6 6.3 21.1 14.8 14.1 71.7 47.5 43.9
0.6 0.5 0.4 1.6 1.1 0.9 6.1 3.8 2.7
Table 6. Hydrodesulfurization (HDS) Rate Constants and Activation Energies for Imp 3/10 and Sono 3/13 Apparent Rate Constant, khds (g-mol0.43 m1.71)/(s kg-cat.) Figure 8. Effects of temperature and LHSV on nitrogen conversion of LGO over Sono 3/13 and Imp 3/10. Legend: (s) Imp 3/10, (- - -) Sono 3/13, (9) LHSV ) 1.5 h-1, (0) LHSV ) 2.0 h-1, and (×) LHSV ) 2.5 h-1. Total pressure ) 8.9 MPa and G/L ) 600 mL/mL.
The HDN and HDS of selected catalysts were examined for 23 days by changing the above-mentioned reaction parameters. To confirm the deactivation of the catalysts, check-back experiments at a reaction temperature of 370 °C, LHSV ) 2 h-1, a pressure of 8.9 MPa, and G/L ) 600 mL/mL were done. The check back experiments showed that there was no significant catalysts deactivation for 23 days of run. Figures 7 and 8 showed the effects of temperature on sulfur and nitrogen conversions, respectively, on Sono 3/13 and Imp 3/10 catalysts. The effect of temperature at 340, 360, and 380 °C was studied at LHSV ) 1.5, 2.0, and 2.5 h-1. The total pressure and G/L values were kept constant, at 8.9 MPa and 600 mL/mL, respectively. Both catalysts showed increase in the sulfur and nitrogen conversion with an increase in temperature from 340 °C to 380 °C. As shown in the figures, the Imp 3/10 shows better sulfur and nitrogen conversion than Sono3/ 13 at all the temperatures and LHSV investigated. The effect of reaction pressure on the HDS and HDN activities of both impregnated and sonochemically synthesized catalysts were studied in a range of 7.6-10.3 MPa at a temperature of 380 °C and LHSV ) 1.5 h-1. The effect of pressure on sulfur and nitrogen conversion are shown in Figure 9. The sulfur conversion reached optimum value (>95 wt %) under the studied
temperature (°C) 340 360 380
Imp 3/10
Sono 3/13
(2.32 ( 0.136) × 10-5 (4.67 ( 0.249) × 10-5 (9.48 ( 0.929) × 10-5
(2.00 ( 0.137) × 10-5 (3.70 ( 0.249) × 10-5 (7.04 ( 0.549) × 10-5
HDS Energy of Activation Sono 3/13 Imp 3/10
12.6 kJ/mol 14.1 kJ/mol
reaction conditions. Hence, the effect of pressure in HDS is not clearly observed. As shown in Figure 9, both catalysts showed a significant increase in the nitrogen conversion with an increase in pressure. For example, Sono 3/13 shows a nitrogen conversion of 74.2 wt % at 7.6 MPa, which increases to 84.1 wt % at 10.3 MPa. Similarly, Imp 3/10 shows a nitrogen conversion of 77 wt % at 7.6 MPa, which increases to 91.3 wt % at 10.3 MPa. This might be because HDN proceeds through hydrogenation of the aromatic ring, which is strong function of hydrogen partial pressure.1 The study of the variation in process parameters showed that the impregnated catalysts showed higher nitrogen and sulfur conversion than the corresponding sonochemical catalysts under all reaction conditions. TPR plots of the sonochemically prepared catalysts indicate the presence of a greater amount of Co3O8 bulk type of species than the impregnated catalysts. The Co3O8 is a catalytically inactive species.1 This showed that the added Co metals get wasted as Co3O8, instead of decorating the edge of WS2. The Co-decorated WS2 is the active site for HDN and HDS reactions. This might be the reason for inferior activity of sonochemical catalysts,
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Table 7. Hydrodenitrogenation (HDN) Rate Constants, Activation Energy, and Heat of Adsorptions for Imp 3/10 and Sono 3/13 Adsorption Equilibrium Constant, Khds (m3/g-mol)
Rate Constant, khdn temperature ( °C)
Imp 3/10 ((mol)0.5/s m1.5 kg-cat)
Sono 3/13 ((mol)0.6/s m1.8 kg-cat)
Imp 3/10
Sono 3/13
340 360 380
(2.62 ( 0.177) × 10-6 (3.80 ( 0.294) × 10-6 (6.46 ( 0.534) × 10-6
(1.37 ( 0.941) × 10-6 (1.70 ( 0.240) × 10-6 (3.21e ( 0.0931) × 10-6
0.09325 ( 0.007 0.0734 ( 0.010 0.0664 ( 0.018
(4.78 ( 1.09) × 10-3 (1.85 ( 1.08) × 10-3 (1.50 ( 0.10) × 10-3
n m HDN energy of activation HDS heat of adsorption
0.5 2 9 kJ/mol 3.4 kJ/mol
relative to that of the impregnated catalysts. This result is further supported by XPS study, which showed poor W dispersion with sonochemical catalysts. HDS and HDN of Light Gas Oil: Kinetic Study. The Imp 3/10 and Sono 3/13 catalysts were investigated in detail for kinetic analysis. The data obtained were used to examine the pore diffusion resistances and to fit the power-law and Langmuir-Hinshelwood models to derive the kinetic expression for the HDS and HDN reactions. The rate equations were based on the following assumptions: (i) the HDS and HDN are irreversible, and (ii) the effect of hydrocracking on HDS and HDN reactions is negligible. The rate expressions for HDS and HDN were fitted using nonlinear regression analysis. The values of activation energy and pre-exponential constant for HDS and HDN reactions with both the catalysts were evaluated using linear regression analysis. Pore diffusion resistances were evaluated for the HDS and HDN reactions under different reaction conditions. A calculation for β (defined as the ratio of the maximum temperature difference that could exist between the catalyst pellet core and the catalyst pellet surface temperature18) was done to estimate the temperature increase in the catalyst pellet due to the HDS reaction:
β)
∆Tmax ∆HRxDeCAS ) TS ktTS
(1)
where ∆HRx is the heat of HDS reaction, De the effective diffusivity for sulfur-containing molecules; CAS the catalyst surface concentration of sulfur species; kt the thermal conductivity of the catalyst pellet; and TS the catalyst pellet surface temperature. A value of β ) 0 indicates that the pellet core temperature is equal to the surface temperature and no temperature gradient exists in the catalyst pellet. The calculation showed a β value of 0.00009, which indicated that the catalyst pellet core was 0.06 °C hotter than its external surface. This implied that the catalyst pellet could be assumed to be isothermal. To further ensure the isothermality of the catalyst pellet, Anderson’s criterion,19 which compares the rate of heat generation from the reaction inside the catalyst pellet with the rate of heat removal by conduction and convection, was also used:
|∆HRx|〈RA〉dp2 0.75TSR < 4ktTS Ea
(2)
where 〈RA〉 is the rate of reaction per unit volume of catalyst, kt the thermal conductivity of the catalyst, R the universal gas constant, Ea the energy of activation, dp the catalyst pellet diameter, and Ts the catalyst pellet surface diameter. The left side of the criterion indicated a value of 8.9 × 10-5, whereas
0.4 2 8.5 kJ/mol 11.6 kJ/mol
the right side indicated a value of 0.05. These values indicated that the catalyst pellet could be assumed to be isothermal for analysis. The Thiele modulus could not be used in the pore diffusion resistance analysis, because it required information about intrinsic rates of reactions and the values of the reaction rates obtained were overall in nature. Therefore, another dimensionless modulus was used, which was based on the global rate of reaction,20 as shown by eq 3:
Φ≡
(
)
R2 1 dn 1 De Vc dt CAS
(3)
where Φ is the dimensionless modulus, based on the global rate of reaction; R the catalyst particle radius; and (-1/Vc) dn/ dt the rate of reaction per unit volume of catalyst. Table 5 shows the calculated values of dimensionless modulus (Φs) for HDS reactions with Sono 3/13 and Imp 3/10. The table indicates lower values of Φs at lower reaction temperatures and shows increased values with an increase in reaction temperature. For example, Sono 3/13 shows a Φs value of 3.4 at 340 °C, which increases to 37.9 as the temperature is increased to 380 °C. Similarly, Imp 3/10 shows a Φs value as 5.6 at 340 °C, which increases to 71.7 with an increase in reaction temperature to 380 °C. The values of the effectiveness factors are inversely related to Φs in the higher modulus zone, which indicates a decrease in the effectiveness factor with an increase in reaction temperature.20 For a first-order isothermal reaction in spherical particles, the value of the effectiveness factor ranges from 0.7 (for Φs ≈ 5) to 0.2 (for Φs ≈ 45).20 This indicates that, at lower reaction temperatures (340-360 °C), the global rate of reaction is governed by both diffusional resistance and surface reaction;20 however, as the reaction temperature increases, the rate of surface reaction increases faster (following the Arrhenius rate law) than the rate of diffusion (following linear behavior with temperature, as shown by Wilke-Chang equation21) and the overall rate of the HDS of LGO becomes more controlled by pore diffusion resistance. Table 5 also shows the values of the dimensional modulus (ΦN) for the HDN of LGO with Sono 3/13 and Imp 3/10. Note that the values of ΦN are much lower for HDN than those for HDS. For example, Sono 3/13 shows a ΦN value of 0.2 at 340 °C, which increases to 3.9 as the temperature is increased to 380 °C. Similarly, Imp 3/10 shows a ΦN value of 0.4 at 340 °C, which increases to 6.1 as the temperature is increased to 380 °C. This indicates that the HDN of LGO is not limited by pore diffusion resistance and the rate expression for HDN can be assumed to be intrinsic. A similar result was obtained by Van Zoonen and Douwes,22 who studied the HDS and HDN of straight-run gas oil on 3 mm × 3 mm pellets of Co (3 wt %)-Mo (10.4 wt %)/γ-Al2O3 catalyst at a pressure of 3.4 MPa and temperature of 375 °C.
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The kinetics of the removal of the sulfur compounds and other impurities has a very important effect on optimizing process variables and selection of catalyst for the HDS and HDN processes. Traditionally, the kinetics of removal of sulfur compounds has been studies in two ways, namely, the powerlaw model and the Langmuir-Hinshelwood Hougen Watson model.23 The power-law model is overall in nature and does not account for the individual steps taking part in the reaction, whereas the Langmuir model incorporates the detail mechanism in rate equation. The kinetic studies with sulfur or nitrogen model compounds typically follow first-order kinetics.24 The HDN of gas oils also have been observed to follow first-order kinetics.25 For moderate HDS of gas oils, the orders of reactions have been well-documented and reported to be in the range of 1-2. The reaction order is dependent on the type and distribution of the sulfur and nitrogen compounds in the feed and the type of the catalyst used. The HDS reaction path can be represented as khds
R-S 98 Product + H2S Applying the power-law model gives
Ratehds ) -
dCS ) (CSF - CSP)nLHSV dt
(4)
where Ratehds is the rate of HDS reaction, CSF and CSP are the feed and product concentrations of S species, and LHSV is the inverse of the residence time in the reactor. Nonlinear analysis gave the best fit for a value of n ) 0.57 for HDS over two catalysts. The HDN process can represented as shown below: khdn
R-N 98 Product + NH3 and the HDN rate expression using the Langmuir-Hinshelwood model can be written as
Ratehdn ) -
khdnCnN dCN ) dt (1 + KhdsCS)m
(5)
where khdn is the rate constant and Khds is the adsorption equilibrium constant for sulfur species in LGO. Tables 6 and 7 show the kinetic data for HDS and HDN. The value of m in eq 5 for both catalysts is observed to be 2.0, indicating that the HDN proceeds through a dual sites mechanism and is inhibited by adsorption of the sulfur species. The HDS and HDN rate constants at all temperatures with Sono 3/13 catalyst are lower than those with the Imp 3/10 catalyst. The similar apparent HDS and HDN activation energy with both catalysts showed that HDS and HDN reactions follows similar pathways on both catalysts. The HDN and HDS rate constants with impregnated catalyst, which are higher than those of the sonochemical catalyst, can be due to the greater number of actives sites (Co-promoted WS2) and also a fine dispersion in the former catalyst than the latter. This is consistent with the TPR and XPS results. It can be observed that the HDS activation energies are on a lower side, in comparison to those reported in the literature. It could be due to the lower inherent activity of the prepared catalysts,10 in addition to the diffusion resistances. The HDN activation energies also are shown to be comparable for both
catalysts, but heat of adsorption for the sulfur species is greater for Sono 3/13 than Imp 3/10. Conclusions Co-W/Al2O3 catalysts were prepared using conventional wet impregnation and sonochemical methods, characterized, and tested for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of light gas oil (LGO). The sonochemical method of preparation of CoMo on an Al2O3 support has been observed to lead to a great reduction in surface area and crystallinity of support than the conventional impregnation method. The sonochemically prepared catalysts showed lower sulfur and nitrogen conversions than the impregnated catalysts, which may be due to the segregation of tungsten species on the support surface in the case of former catalysts, as indicated by X-ray photoelectron spectroscopy (XPS) studies. The HDS could be best-represented with reasonable accuracy with the power-law model and a reaction order of 0.57 over the selected two catalysts. The HDN could be best-expressed with the Langmuir-Hinshelwood model with an inhibition term for adsorption of sulfur species. The values of dimensionless moduli indicate that the HDS of LGO and over both the catalysts is limited by pore diffusion resistance and the fitted rate expression is apparent. A similar analysis for HDN of LGO over the two catalysts indicated that it was not inhibited by pore diffusion and fitted rate expressions were apparent. Acknowledgment Financial support from NSERC Collaborative Research and Development Grant and Syncrude Canada, Ltd., is acknowledged. Nomenclature CAS ) catalyst surface concentration of sulfur-containing molecules (mol/cm3) CF ) concentration of aromatic compounds in light gas oil (mol/ cm3) CN ) concentration of nitrogen species in light gas oil (mol/ cm3) CS ) concentration of sulfur species in light gas oil (mol/cm3) DiL ) diffusivity of sulfur species in gas oil (cm2/s) dp ) catalyst pellet diameter (cm) Ea ) energy of activation (kJ/mol) khds ) rate constant for HDS of light gas oil (mol0.43 m1.71/(s kg-cat.)) Khds ) adsorption equilibrium constant for sulfur species in light gas oil (m3/mol) kt ) thermal conductivity of catalyst pellet (J/(cm K)) LHSV ) liquid hourly space velocity (h-1) m ) exponent in Langmuir-Hinshelwood model n ) exponent in power-law and Langmuir-Hinshelwood models R ) catalyst particle radius (cm) 〈RA〉 ) global rate of reaction (mol/(s cm3-cat.)) rhdn ) rate of conversion of nitrogen species in light gas oil (mol/(s kg-cat.)) rhds ) rate of conversion of sulfur species in light gas oil (mol/ (s kg-cat)) TS ) catalyst pellet surface temperature (K) Vc ) catalyst volume loaded in the rector (cm3) B ) factor defined as the ratio of the maximum temperature difference that can exist between the catalyst pellet external surface to its core
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∆HRx ) HDS heat of reaction (kJ/mol) ∆Tmax ) Maximum temperature difference that could exist between a catalyst pellet core and its surface (K) ΦS ) modulus for HDS of LGO ΦN ) modulus for HDN of LGO Literature Cited (1) Clausen, B. S.; Massoth, F. E.; Topsoe, H. CatalysissScience and Technology; PRODUserv Springer: Berlin, 1996; p 77. (2) Daage, M.; Chianelli, R. R. Structure-function relations in Molybdenum sulfide catalysts: The rim edge model. J. Catal. 1994, 149, 414. (3) Dhas, N. A.; Ekhtiarzadeh, A.; Suslick, K. S. Sonochemical preparation of supported hydrodesulfurization catalysts. J. Am. Chem. Soc. 2001, 123 (34), 8310. (4) Lee, J. J.; Heeyeon, K.; Sang, H. M. Preparation of highly loaded, dispersed MoS2/Al2O3 catalysts for the deep desulphurization for dibenzothiophenes. Appl. Catal., B 2003, 41, 171. (5) Mahajan, D.; Marshall, C. L.; Castagnola, N.; Hanson, J. C. Sono synthesis and characterization of nano-phase molybdenum-based materials for catalytic hydrodesulfurization. Appl. Catal., A 2004, 258 (1), 83. (6) Gusta, E.; Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Hydrotreating of heavy gas oil derived from Athabasca bitumen over Co-Mo/γ-Al2O3 catalyst prepared by sonochemical method. Top. Catal. 2006, 37, 147. (7) Mauchausse, C.; Kural, E.; Trimm, D. L.; Cant, N. W. Optimization of tungsten-based catalysts for the hydrotreatment of coal-derived liquids. Fuel 1992, 71 (2), 203. (8) Suvanto, M.; Raty, J.; Pakkanen, A. T. Catalytic activity of carbonyl precursor based CoW/Al2O3 catalysts in hydrodesulfurization of thiophene. Appl. Catal., A 1999, 181, 189. (9) Kishan, G.; Coulier, L.; Van Veen, J. A. R.; Niemantsverdriet, J. W. Promoting synergy in CoW hydrotreating catalysts by chelating agents. J. Catal. 2001, 200, 194. (10) Kubota, T.; Sato, K.; Kato, A.; Usman, Ebihara, T.; Fujikawa, T.; Araki, Y.; Ishida, K.; Okamoto, Y. Edge dispersion of supported MoS2 and WS2 catalysts as evaluated by using Co(CO)3NO as a probe molecule. Appl. Catal., A 2005, 17, 290. (11) Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Effect of EDTA on hydrotreating activity of CoMo/γ-Al2O3 catalyst. Catal. Lett. 2005, 102, 299.
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ReceiVed for reView January 29, 2007 ReVised manuscript receiVed March 20, 2007 Accepted April 25, 2007 IE070169M
