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Ind. Eng. Chem. Res. 2006, 45, 544-552
Hydrodenitrogenation and Hydrodesulfurization of Heavy Gas Oil Using NiMo/ Al2O3 Catalyst Containing Boron: Experimental and Kinetic Studies D. Ferdous,† A. K. Dalai,*,† and J. Adjaye‡ Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, UniVersity of Saskatchewan, Saskatoon, 57 Campus DriVe, Saskatoon, Saskatchewan S7N 5A9, Canada, and Syncrude Canada Ltd. Edmonton Research Center, 9421 17th AVenue, Edmonton, Alberta T6N 1H4, Canada
In this work, a systematic study has been conducted to optimize the process conditions and to evaluate kinetic parameters for hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) of heavy gas oil derived from Athabasca bitumen using NiMo/Al2O3 catalysts containing boron (B). In the catalyst, the concentrations of boron were varied from 0 to 1.7 wt %. Experiments were performed in a trickle-bed reactor at the temperatures, pressures, and liquid hourly space velocities (LHSVs) of 340-420 °C, 6.1-10.2 MPa, and 0.5-2 h-1, respectively. H2 flow rate and catalyst weight were maintained constant at 50 mL/min and 4 g, respectively, in all cases. Statistical analysis of all experimental data was carried out using ANOVA to optimize the process conditions for HDN and HDS reactions. Kinetic studies for HDN and HDS reactions were studied within the temperature range of 340-400 °C using a power law model as well as the Langmuir-Hinshelwood model. The power law model showed that HDN of heavy gas oil follows first-order kinetics while the HDS process follows 1.5-order kinetics. The activation energies for HDN and HDS reactions from power law and LangmuirHinshelwood models were 75 and 87 kJ/mol and 110 and 159 kJ/mol, respectively. 1. Introduction Oil sand bitumen and bitumen derived gas oil contain high levels of nitrogen and sulfur compounds. Thus, products such as heavy gas oil obtained from the processing of bitumen also contain very high levels of nitrogen. Catalytic hydrodenitrogenation (HDN) is the only process used commercially for reducing the level of nitrogen content in those types of feedstocks. Because of high nitrogen content hydrodenitrogenation (HDN) of this heavy gas oil is more difficult and less effective with conventional NiMo/Al2O3 catalyst. In the past, HDN and hydrodesulfurization (HDS) studies of heavy gas oil derived from Athabasca bitumen have been performed using NiMo/Al2O3 and CoMo/Al2O3 catalysts.1-3 Yui4 studied HDN, HDS, and mild hydrocracking of bitumen derived coker gas oil. Hydrotreatment was carried out at 340400 °C, 7-11 MPa, 0.7-1.5 h-1 liquid hourly space velocity (LHSV) and 60:1 vol/vol H2/feed ratio in a pilot-scale tricklebed reactor, over presulfided commercial Ni-Mo/Al2O3 catalysts. HDN, HDS, and mild hydrocracking were investigated using a modified power law kinetic model, which incorporated power terms for LHSV and pressure. It was found that HDN and mild hydrocracking (MHC) had first-order kinetics whereas HDS had 1.5-order kinetics. Diaz-Real et al.1 also studied the HDN of Athabasca bitumen derived heavy gas oil over Ni-Mo, Ni-W, and Co-Mo catalysts supported on zeolite. Maximum nitrogen and sulfur conversions of 75 and 99 wt % were obtained at the temperature, pressure, LHSV, and H2/feed ratio of 425 °C, 6.89 MPa, 2 h-1, and 890 mL/mL, respectively. They reported that data from their HDN studies fitted a pseudo-first-order kinetic model satisfactorily. The activation energies for HDN reactions were 25.1, 18.9, and 16.6 kcal/mol, respectively, over Ni-Mo, Ni-W, and Co-Mo catalysts. * Corresponding author. E-mail:
[email protected]. Tel.: (306) 966-4771. Fax: (306) 966-4777. † University of Saskatchewan. ‡ Syncrude Canada Ltd.
Bej et al.5,6 studied the HDN and HDS of oil sand derived heavy gas oil over a commercial NiMo/Al2O3 catalyst. The experiments were performed at the temperature, LHSV, and H2/ feed ratio of 365-415°C, 0.5-1.9 h-1, and 400-1000 mL/mL. The maximum HDN and HDS conversions of ∼80 and ∼96 wt % were obtained at the temperature, pressure, LHSV, and H2/feed ratio of 385 °C, 8.8 MPa, 0.5 h-1, and 600 mL/mL, respectively. The kinetics of nitrogen removal was described by a 1.5-order rate equation, whereas sulfur removal was described by a second-order rate equation. Activation energies for HDN and HDS reactions were 80 and 28 kJ/mol, respectively. In the past, attempts have been made to modify conventional NiMo/Al2O3 catalyst using boron and other additives such as phosphorus and tungsten to improve HDN and HDS of model nitrogen compounds. Borate ion incorporated on alumina forms a system exhibiting acidic properties.7,8 Moreover, borate ions bring about an increase in the activity of CoMo/Al2O3 and NiMo/Al2O3 catalysts in the reaction of HDS.9,10 Lulic11 investigated the performance of NiMo/Al2O3-B2O3 catalyst on the HDN of a mixture of light and heavy gas oils (3:1) containing 390 ppm nitrogen and concluded that the catalysts containing boron are more active than commercial HDN catalyst. DeCanio and Weissman12 studied the characterization and activity of boron (0.3-1.8 wt %) modified commercial NiMo/ Al2O3 catalyst. At higher boron loadings of over 1.8 wt % boron, poor catalytic performance was observed due to the presence of bulk borate phase. Lewandowski and Sarbak13 studied the effect of boron addition on the texture and structure of NiMo/ Al2O3 catalysts. They found that addition of boron did not change the pore volume significantly; however, it increased the pore radius to 2, 2.5, 3.0, and 4.0 nm in comparison with the unmodified catalyst. Lewandowski and Sarbak14 studied the HDS and HDN activities of boron-modified NiMo/Al2O3 catalyst using quinoline, carbazole, and coal liquid. The promotion with borate ions increased the acidity of Ni-Mo catalyst, in particular, the amount of acid centers of intermediate strength. The HDN activities of the catalysts containing 9.1 and 11.7 wt
10.1021/ie050094r CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 545 Table 1. Feed Characteristics of Heavy Gas Oil Derived from Athabasca Bitumen boiling range, °C sulfur content, ppm nitrogen content, ppm basic nitrogen content, ppm nonbasic nitrogen content, ppm aromatic content, ppm density, g/mL
185-576 40 370 2986 1043 1943 3720 0.98
% boron were considerably higher than that of NiMo catalyst. Lewandowski and Sarbak concluded that the HDN of carbazole indeed depends on the number of acid centers present on the catalyst surface. On the other hand, in the case of quinoline, the acid centers also play a certain role; however, reaction may proceed on centers of weak strength. They also concluded that a support modification with borate ions led to a decrease in deactivation of the Ni-Mo catalyst in the HDN of coal liquid. It is observed from the literature that most of the research has been performed by modifying the support with boron. However, little information is available on the modification of NiMo/Al2O3 itself with boron. To our knowledge, information on the performance of boron-promoted NiMo/Al2O3 on the hydroprocessing of actual feed, especially from Athabasca bitumen, is not available. Also, no kinetic data are available for HDN and HDS of actual feed using boron-modified NiMo/ Al2O3 catalyst. In the present work, a statistical design has been performed to optimize the process conditions for HDN and HDS of heavy gas oil derived from Athabasca bitumen using an NiMo/Al2O3 catalyst containing boron. A kinetic study was also performed for HDN and HDS reactions using power law and Langmuir-Hinshelwood models. 2. Experimental Section A series of NiMoB/Al2O3 catalysts was prepared using the incipient wetness impregnation method using different concentrations of boron. Boron concentrations were varied from 0.5 to 1.7 wt %. In all catalysts, Ni and Mo concentrations were maintained at 2.9 and 13.2 wt %, respectively. In this method, an ammoniacal solution (28 wt % concentrated NH3) was prepared using the required amounts of ammonium heptamolybdate [(NH4)6Mo7O24‚4H2O] and nickel nitrate [Ni(NO3)2‚ 6H2O]. NiMoB/Al2O3 catalysts were prepared by impregnating appropriate solutions of boric acid (H3BO3) onto a NiMo/Al2O3 catalyst. The details of the preparation procedures are given elsewhere.15 A central composite design was applied with three factors, namely temperature, LHSV, and pressure, to design the experiments. Experiments were performed at the temperatures, pressures, and LHSVs of 340-420 °C, 6.1-10.2 MPa, and 0.5-2 h-1, respectively. H2 flow rate was maintained constant at 50 mL/min, while catalyst volume was maintained at 5 cm3. All experiments were performed in a micro-trickle-bed reactor using heavy gas oil derived from Athabasca bitumen. The properties of the feed material are given in Table 1. The reaction system consisted of liquid and gas feeding sections, a highpressure reactor, a heater with a temperature controller for precisely controlling the temperature of the catalyst bed, a scrubber for removing the ammonium sulfide from the reaction products, and a high-pressure gas-liquid separator. The length and internal diameter of the reactor were 240 and 14 mm, respectively. The catalyst bed inside the reactor was diluted with 9 mL of 90 mesh silicon carbide. Before loading, the catalyst was dried for 3 h at 150 °C. The details of the experimental procedures are given elsewhere.16
Figure 1. HDN and HDS activities of heavy gas oil using NiMo/Al2O3 catalyst containing boron at the temperature, pressure, LHSV, and H2/feed ratio of 385 °C, 8.7 MPa, 1 h-1, and 600 mL/mL, respectively.
The liquid product sample was collected after a 24 h interval. The products were stripped with nitrogen for removing the dissolved ammonia and hydrogen sulfide and then were analyzed for their total nitrogen and sulfur contents by a combustion/ chemiluminescence technique (using Antek 9000 analyzer) following the ASTM D4629 method. 3. Results and Discussion In the literature, most of the studies were conducted using a NiMo catalyst supported on boron-modified alumina.11,13,14 Lewandowski and Sarbak14 studied the HDS and HDN activities of NiMo catalyst supported on boron-modified alumina using quinoline, carbazole, and coal liquid. The HDN activities of the catalysts containing 9.1 and 11.7 wt % boron was considerably higher than that of NiMo catalyst. They concluded that the HDN of carbazole increased due to the increase in the number of acid centers. On the other hand, in the case of quinoline, the reaction proceeded on the acid centers of weak strength. However, DeCanio and Weisman12 have indicated the decrease in activity of commercial NiMo on alumina support modified by boron (>1.2%) for HDN of light atmospheric gas oil. The have reported an increase in nitrogen and sulfur removal activity of this catalyst from 10.2 to 18.0 h-1 and from 17.4 to 19.2 h-1, respectively, when the boron concentrations were increased from 0 to 1.2 and from 0 to 0.5 wt %. These studies indicate that the catalyst activity is generally increased when boron is added either to alumina or to NiMo/Al2O3. Therefore, in the present study boron was added to NiMo/Al2O3. Our previous studies16 indicated that addition of boron to NiMo/Al2O3 caused a significant increase in HDN activity of the catalyst. For example, the total nitrogen conversion increased from 62.0 to 78.0 wt %, but no significant change in sulfur conversion was observed with the increase in boron conversion from 0 to 1.7 wt % (see Figure 1) when the experiments were performed at the temperature, pressure, LHSV, and H2/feed ratio of 385 °C, 8.7 MPa, 1 h-1, and 600 mL/mL. Therefore, further studies were conducted using catalyst containing 1.7 wt % boron. A central composite inscribed method was used to design the experiments. Different operating variables have different impacts on the reaction mechanism, which is discussed in Bej et al.6 However, the central composite inscribed method itself will not have any impact on the reaction mechanism. The purpose of using this method is to optimize the process conditions with the minimum number of experiments. Three
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Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 Table 2. Experimental Results for Nitrogen and Sulfur Conversion at Different Operating Conditions for Optimizing Process Conditions
Figure 2. Effect of time on stream on the stability of the catalyst during hydroprocessing of heavy gas oil at the temperature, pressure, LHSV, and H2/feed ratio of 375 °C, 8.7 MPa, 1 h-1, and 600 mL/mL. White bars, nitrogen; gray bars, sulfur.
important response factors such as temperature, pressure, and LHSV were chosen in this work which have significant effects on HDN and HDS reactions. Temperature, pressure, and LHSV were varied in the range of 340-420 °C, 6.1-10.2 MPa, and 0.5-2 h-1, respectively. Before the experiments, catalyst was activated in situ by sulfiding at 193 °C for 24 h and then at 343 °C for another 24 h. The objectives for sulfiding the catalyst for 48 h were to obtain complete conversion of MoO3 to MoS2 and to saturate the catalyst surface with MoS2. Before collecting experimental data, the catalyst surface was stabilized at the temperature, pressure, LHSV, and H2/feed ratio of 375 °C, 8.7 MPa, 1 h-1, and 600 mL/mL, respectively, and the experiment was continued for 5 days. The activity decreased after 24 h of time on stream (TOS) and then remained constant for the next 4 days (see Figure 2). The conversions during HDN and HDS reactions are calculated based on eq 1.
conversion ) [total N or S content in feed total N or S content in product %]/ [total N or S content in feed] × 100 (1) where N ) nitrogen and S ) sulfur. Statistical analysis of experimental data, process optimization, and kinetic studies for HDN and HDS reactions using NiMo/ Al2O3 catalyst containing 1.7 wt % boron are described below. 3.1. Statistical Analysis of Experimental Data. The experimental results obtained from hydrotreating reaction at different operating conditions that were selected by experimental design are given in Table 2. Run number 2 was repeated five times. The nitrogen and sulfur conversions were in the range of 41.247.2 and 81.1-88.1 wt %, respectively, which is within the range of experimental error. The reproducibility of the nitrogen and sulfur values obtained from the nitrogen and sulfur analyzer falls in (3 and (3.5 wt %, respectively. Experimental results were analyzed using DESIGN-EXPERT 6.0.1 software for optimizing the process conditions with respect to nitrogen and sulfur conversions. Regression analysis of experimental data generated the following regression equation for total nitrogen conversion:
(nitrogen conversion + 0.07)1/2 ) -11.31069 + 0.054205 (temperature) - 6.38681(LHSV) + 2.21007 × 10-3 (pressure) + 1.70813(LHSV2) (2)
run no.
temp, °C
LHSV, h-1
press., MPa
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
356 380 380 380 404 420 356 404 356 380 340 404 380 380 356 380 404 380 380 380
0.8 1.3 1.3 1.3 0.8 1.3 0.8 1.7 1.7 1.3 1.3 0.8 1.3 1.3 1.7 1.3 1.7 0.5 2.0 1.3
9.4 8.3 8.3 8.3 9.4 8.3 7.2 9.4 9.4 8.3 8.3 7.2 8.3 10.2 7.2 6.1 7.2 8.3 8.3 8.3
conversion, wt % nitrogen sulfur 49.3 45.2 46.2 41.2 93.3 71.0 42.8 69.7 28.9 43.2 18.1 81.6 42.2 51.0 20.3 36.2 50.2 83.7 31.5 47.2
85.4 81.1 82.1 85.1 98.2 97.0 85.8 95.1 72.6 85.1 64.3 97.4 84.1 88.0 69.6 86.6 91.5 96.8 80.3 88.1
Table 3. ANOVA Analysis for Response Surface Reduced Quadratic Model for Nitrogen and Sulfur Conversion HDN source
sum of squares
DFa
model temperature LHSV pressure LHSV × LHSV residual lack of fit pure error total
38.72 22.91 12.17 1.92 1.69 1.15 0.99 0.16 39.87
4 1 1 1 1 15 10 5 19
mean square
F value
prob>F
9.68 22.91 12.17 1.92 1.69 0.077 0.099 0.032
126.06 298.39 158.45 25.04 22.05
