Energy & Fuels 2001, 15, 1103-1109
1103
Effect of Hydrotreating Conditions on the Conversion of Residual Fraction and Microcarbon Residue Present in Oil Sands Derived Heavy Gas Oil Shyamal K. Bej,† Ajay K. Dalai,*,‡ and John Adjaye§ Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, STN 5C9, Canada, and Syncrude Canada Ltd., Edmonton Research Centre, 9421-17 Avenue, Edmonton, Alberta T6N 1H4, Canada Received December 29, 2000. Revised Manuscript Received June 12, 2001
Oil sands derived heavy crude oil also known as synthetic crude is emerging as an alternative source of feedstock to the refiners for producing transportation fuels.The presence of very high levels of sulfur and nitrogen in these heavy gas oils needs to be reduced before they are used for further processing. This at present is accomplished by hydrotreating these stocks over Ni-Mo based alumina catalysts at high temperature and pressure. During hydrotreatment of oil sand derived heavy gas oil, residual fraction (500 °C+) is converted into more valuable gasoline and middle distillate range products. The effect of different process variables on the conversion of residual fraction (500 °C+) of heavy gas oil has been studied in a trickle bed reactor using 5 mL of alumina supported Ni-Mo based commercial catalyst. The process variables studied were temperature (365-415 °C), liquid hourly space velocity (0.5-1.9 h-1), pressure (65-88 bar), and hydrogen/heavy gas oil volumetric ratio (400-1000 mL/mL). The effect of all these variables on the conversion of microcarbon residue (MCR) has also been investigated. It was found that significant reduction in the residual fraction and MCR content of the heavy gas oil can be achieved by selecting appropriate hydrotreatment conditions. The kinetics of the removal of residual fraction and MCR content as well as the generation of more valuable distillate products has also been studied in this work. Power law models having order of reaction equal to 2.0 can be used to describe the kinetics of the removal of residual fraction and the MCR content.
Introduction As the supply of conventional crude oil is decreasing day by day, efforts for finding alternative sources of crude oils are growing. In this regard, oil sands derived synthetic crude is emerging as an alternative source of feedstock to the refiners for producing transportation fuels. Syncrude Canada Ltd. operates a surface mining oil sands in northern Alberta and produces synthetic crude oil. The bitumen after being extracted from the oil sand is processed either through a fluid coker or an LC-Fining hydrocracker.1 The liquid products obtained from these operations still contained very high levels of nitrogen and sulfur. The presence of very high sulfur and nitrogen contents in these stocks needs to be reduced before they are used for further processing. This at present is accomplished by hydrotreating these stocks over Ni-Mo based alumina catalysts at high temperature and pressure.2-4 * Corresponding author. Tel: 306-966-4771. Fax: 306-966-4777. E-mail:
[email protected]. † Present address: Department of Chemical Engineering, University of Michigan, H. H. Dow Building, Ann Arbor, MI 48109. ‡ University of Saskatchewan. § Syncrude Canada Ltd. (1) Yui, S.; Chung, K. K. Paper presented at the ITIT International Symposium on Utilization of Super-Heavy Hydrocarbons Resources at Toranomon Pastoral, Tokyo, September 18-19, 2000.
During the removal of the sulfur and nitrogen atoms through hydrotreating operations some conversion in the boiling range of the feedstock also takes place because the heteroatom-containing molecules are broken down to lighter fractions thus shifting molecular weight as well as the boiling range of the feedstock. Besides this, depending on the severity of the process some mild hydrocracking of the feedstock may also take place during hydrotreatment. All these reactions bring some conversion in the residual fraction of the heavy gas oil into valuable distillate products. Since the downstream use of the hydrotreated heavy gas oil is mainly as synthetic crude for blending with conventional crude oil, understanding its potential to yield various types of distillate products is required. The knowledge about this will also help to optimize the hydrotreating process conditions, which at present is done keeping nitrogen and sulfur removal as the sole objective in mind. Besides this, microcarbon residue (MCR) is another critically important parameter influencing the downstream processing of heavy gas oil particularly for the coke lay down on the catalyst used in the downstream processes. During hydrotreatment of heavy gas oil, some reduction of MCR also takes place. Kim et al.5 have (2) Bej, S. K.; Dalai, A. K.; Adjaye, J. Energy Fuel 2001, 15, 377.
10.1021/ef000287l CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
1104
Energy & Fuels, Vol. 15, No. 5, 2001
Bej et al.
Table 1. Different Physico-chemical Properties of Heavy Gas Oil boiling range density at 20 °C, g/cm3 sulfur content, wppm total nitrogen content, wppm microcarbon residue content, wt % asphaltene content, wt %
210-655 °C 0.9859 41000 3900 1.98 1.55
suggested that conradson carbon residue (CCR) conversion and residuum conversion take place through the same mechanism. This may be because of the significant overlap between what is considered CCR and residuum. Though there is some literature6-9 available in the area of mild hydrocracking of heavy gas oil produced from conventional crude, systematic studies on the effect of different process variables of hydrotreating operation on the conversion of residual fractions and MCR of the oil sand derived heavy gas oil into different ranges of distillate products are but few.10,11 In our earlier work,2,12 we have studied the effect of different process variables on the conversion of nitrogen and sulfur compounds from oil sand derived heavy gas oil. The present article reports on the effect of various process variables such as temperature, pressure, liquid hourly space velocity (LHSV), and hydrogen/heavy gas oil volumetric ratio on the conversion of different residual fractions to distillate products. The kinetics of the conversion of the residual fraction and MCR removal as well as the generation of more valuable distillate products has also been discussed in this article.
Figure 1. Boiling point curve of feed heavy gas oil.
Experimental Setup and Procedure The oil sand derived heavy gas oil used for the present experiment was supplied by Syncrude Canada Ltd. The various physicochemical properties of the heavy gas oil are given in Table 1. The boiling point curve of the feed heavy gas oil is shown in Figure 1. The sulfur content of the heavy gas oil was measured following ASTM 5463 procedure and using the technique of combustion/fluorescence. Similarly, ASTM D4629 procedure was used to determine the total nitrogen content of the heavy gas oil. The density of the heavy gas oil was measured in a digital precision density meter. The MCR content of the heavy gas oil was measured following ASTM D4530 procedure. The asphaltene content of the oil was determined from its pentane insolubility. Hydrogen of very high purity (99.99 vol %) was used for the experiments. One commercial grade Ni-Mo based alumina catalyst was used for the study. The catalyst was trilobe in shape and had a diameter of 1.5 mm. Silicon carbide of 80 (3) Mann, R. S.; Sambi, I. S.; Khulbe, K. C. Ind. Eng. Chem. Res. 1987, 26, 410. (4) Diaz-Real, R. A.; Mann, R. S.; Sambi, I. S. Ind. Eng. Chem. Res. 1993, 32, 1354. (5) Kim, J. W.; Longstaff, D. C.; Hanson, F. V. Fuel 1998, 77, 1815. (6) Trasobares, S.; Callejas, M. A.; Benito, A. M.; Martinez, M. A.; Severin, D.; Brouwer, L. Ind. Eng. Chem. Res. 1998, 37, 11. (7) Desai, P. H.; Asim, M. Y.; van Houtert, F. W.; Nat, P. J. Oil Gas J. 1985, (July 22), 106. (8) Elkes, G. J.; Page, T. H.; Thomas, M. E. Prepr. Pap.s AIChE (Spring Meeting, Houston, TX), 1987, paper 62a. (9) Gembicki, V. A.; Andermann, R. E.; Tajbal, D. G. Oil Gas J. 1983 (Feb 21), 116. (10) Gray, M. R.; Jokuty, P.; Yeniova, H.; Nazarewycz, L.; Wanke, S. E.; Achia, U.; Sanford, E. C.; Yui, S. M. Can. J. Chem. Eng. 1991, 69, 833. (11) Yui, S. M.; Sanford, E. C. Ind. Eng. Chem. Res. 1989, 28, 1278. (12) Bej, S. K.; Dalai, A. K.; Adjaye, J. Proceedings of 4th International Petroleum Conference & Exhibition; New Delhi, India, 2001; Vol. 2, p 278.
Figure 2. Schematic diagram of the experimental setup. mesh was used as diluent in the catalyst bed. The surface area and pore volume of the catalyst were 160 m2/g and 0.5 mL/g, respectively. Figure 2 shows the schematic diagram of the experimental set up used in the present study. The system consisted of liquid and gas feeding sections, a high-pressure microscale trickle bed reactor, a heater with 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 internal diameter and length of the reactor were 14 and 240 mm, respectively. A 5 mL amount of catalyst was used in the present study. The catalyst bed inside the reactor was diluted with 10 mL of 80 mesh silicon carbide to remove all the problems related to the testing of commercial catalyst in a microreactor.13,14 For loading the catalyst, the reactor was packed from bottom to top in three parts. The bottom part was loaded first with 3.5 mL of 3 mm size glass beads followed by 3.5 mL of 16 mesh silicon carbide, and another 3.5 mL of 60 mesh silicon carbide. The middle part of the bed was packed with a mixture of 5 (13) Bej, S. K.; Dabral, R. P.; Gupta, P. C.; Mittal, K. K.; Sen, G. S.; Kapoor, V. K.; Dalai, A. K. Energy Fuels 2000, 14, 701. (14) Bej, S. K.; Dalai, A. K.; Maity, S. K. Catal. Today 2000, 64, 333.
Oil Sands Derived Heavy Gas Oil mL of catalyst and 10 mL of 80 mesh silicon carbide. Both catalyst and silicon carbide were divided into four equal parts, and a small quantity of catalyst and silicon carbide were alternately loaded in the reactor with intermittent vibration to the reactor. The top part was also loaded with 3.5 mL each of 80, 46, and 16 mesh silicon carbide. After loading the reactor with catalyst and diluent, it was placed inside the tubular furnace and ∼50 mL of distilled water was injected into the scrubber. The unit was then pressurized to 88 bar using helium. The temperature of the reactor was then raised to 100 °C while passing helium at the rate of 50 mL/h. After the reactor had reached 100 °C, the sulfidation of the catalyst was started. The sulfidation of the catalyst was carried out by passing a sulfiding solution containing 2.9 vol % of butanethiol in straight run atmospheric gas oil. Initially, the flow rate of the sulfiding solution was kept high (at 2.5 mL/min) to wet the catalyst bed. This flow rate was maintained for 1 h after which the flow rate was reduced and adjusted to maintain a LHSV of 1.0 h-1. Hydrogen flow was then started corresponding to a hydrogen/sulfiding solution volumetric ratio of 600 mL/mL while reducing helium flow to zero. The temperature of the reactor was then slowly increased from 100 to 193 °C. The sulfidation of the catalyst was carried out in two stages. At the first stage, it was carried out at 193 °C for 24 h. After this, the sulfidation was carried out at 343 °C for another 24 h. After the sulfidation was over, the catalyst was then precoked by passing heavy gas oil at the rate of 5 mL/h. The temperature of the reactor was then increased slowly to 375 °C and the precoking of the catalyst was done at this temperature for 7 days. After precoking the catalyst, the hydrotreatment of the heavy gas oil was carried out. The temperature of the reactor was set to the desired level and the product was collected every 24 h interval. The collected samples were then analyzed through simulated distillation for determining the percentage of various fractions.
Results and Discussions As discussed earlier, the present study has been carried out with a view to determine the effect of different process variables such as temperature, pressure, LHSV, and hydrogen/heavy gas oil volumetric ratio on the conversion and generation of various fractions having different boiling ranges. In this respect, three fractions having boiling ranges, respectively, of 200-370 °C, 370-500 °C, and 500 °C+ have been considered. The reasons for considering these fractions are discussed below. The 200-370 °C fraction is considered for the present analysis because it represents the middle distillate fraction consisting of diesel and jet fuel, which will be in high demand in the future. The 370-500 °C cut is of importance because of its use as a feedstock for fluid catalytic cracking and hydrocracking for the production of transportation fuels. The conversion of last fraction having boiling range of 500 °C+ is of importance because of its very low demand and its deteriorating effects on the performance of the catalysts used in the downstream processing such as fluid catalytic cracking and hydrocracking. The conversion of any fraction is defined as % conversion ) (wt % of the fraction in feed - wt % of the fraction in product) (100) wt % of the fraction in feed
A negative value in the conversion indicates the generation of material. The results are discussed below.
Energy & Fuels, Vol. 15, No. 5, 2001 1105
Mass Transfer Analysis and Significance of the Generated Data. In a trickle bed hydroprocessing reactor, the transport of the reacting molecules from the bulk phase to the active surface of the catalyst faces various mass transfer resistances, viz., from bulk gas to gas-liquid interface, from gas-liquid interface to bulk liquid, from bulk liquid to external catalyst surface, and finally intraparticle diffusion. Sie and Krishna15 have reported that out of all these resistances, intraparticle diffusion is the predominant one (accounting for ∼77% of the total mass transfer resistances for a typical case of hydrotreating). According to them, as the flow-related mass transfer limitations do not play an overriding role in these cases, the design of industrial trickle bed reactor on the basis of reliable data obtained with actual feeds and catalysts in proper laboratory process development reactors is relatively straightforward. As mentioned earlier, the resistance of predominance is the diffusional resistance within the catalyst particles. This can be reduced while generating data in the laboratory scale reactor by using small particles obtained by crushing the larger catalyst particle. However, the practical relevance of data generated in this way may not be very meaningful for direct commercial application in which the larger size of catalyst particles is used to avoid excessive pressure drop problems. Hence, this technique of testing catalysts in their crushed form in a small-scale reactor though often used for academic research but has seldom been suitable to obtain reliable data for industrial application.16 The approach, which is experimentally verified and hence most often recommended for the generation of practical data in a microreactor for commercial application, is to use the catalysts in their commercially applied shape and size but dilute the catalyst bed of the microreactor using a proper size of diluent.13,16,17 In such a diluted bed, the hydrodynamics and kinetics are decoupled by the use of fine size of diluent.16 The hydrodynamics of the reactor is mainly controlled by the fine diluents and the kinetics is mainly by the catalyst particles. Also, it is known that the flow of liquid closely approaches plug flow type and the wetting of catalyst is also almost complete when fine size of diluent is used. In addition, as the catalyst is tested in the form as used in the commercial reactor, lots of modeling could be avoided to account for the intraparticle diffusion.18 Because of the above-mentioned reasons, the data in the present study has been generated using a microreactor in which the catalyst bed was diluted with 80 mesh silicon carbide. Hence, the data generated in this study could be used directly for scale-up purposes using a very simple model. Effect of Temperature. The effect of temperature on the conversion of 500 °C+ fraction of heavy gas oil into gasoline and diesel was studied by varying the temperature from 365 to 415 °C keeping LHSV, pressure, and hydrogen/heavy gas oil ratio constant, respectively, at 1.0 h-1, 88 bar, and 600 mL/mL. The results (15) Sie, S. T.; Krishna, R. Rev. Chem. Eng. 1998, 14 (3), 203. (16) Sie, S. T. Rev. Inst. Fr. Pet. 1991, 46, 501. (17) Sie, S. T. Paper presented at the International Symposium on the Deactivation and Testing of Hydrocarbon Conversion Catalysts, Chicago, IL, August, 1995. (18) Al-Dahhan, M. H.; Dudukovic, M. P. AIChE J. 1996, 42, 2594.
1106
Energy & Fuels, Vol. 15, No. 5, 2001
Figure 3. Effect of temperature on the conversion and generation of different fractions at a pressure of 88 bar, LHSV of 1.0 h-1, and a hydrogen/heavy gas oil ratio of 600 mL/mL.
are shown in Figure 3 in which the conversion and generation of 200-370 °C, 370-500, and 500 °C+ fractions are plotted as a function of temperature. In this figure, the positive value in the y-axis represents the conversion while generation or production of any fraction is represented by the negative value of the axis. The figure shows that as the reaction temperature is increased the conversion of the 500 °C+ fraction increases. Thus, at a temperature of about 415 °C almost complete conversion (about 93.5 wt %) of this residual fraction can be achieved. Moreover, it is also seen that the rate of increase in conversion of residual fraction is higher in the temperature range of 390-415 °C as compared to that in the lower range of temperature (365-390 °C). This trend of conversion of 500 °C+ fraction may possibly be explained as that in the lower temperature zone, the conversion in the residual fraction is mainly associated with that caused due to the removal of the heteroatoms, while in the higher range of temperatures, besides this, mild hydrocracking plays a significant role. The other fraction having a boiling range of 370-500 °C shows somewhat different behavior. In the lower range of temperature, however, there is little generation of this fraction, but it starts getting converted to lighter ones after a temperature of 380 °C. It is also evident from the figure that a substantial quantity of 200-370 °C fraction is produced during the hydrotreatment of the heavy gas oil. The production of this fraction increases with an increase in temperature. The results of the figure also shows that the increase in the production of this fraction is faster in the range of higher temperature (390-415 °C) as compared to its generation in the lower temperature zone (365-385 °C). This is due to the higher conversion of 500 °C+ and 370-500 °C fraction into the 200-370 °C fraction in the temperature range of 390-415 °C. Thus, carrying out the hydrotreatment operation at higher temperature, namely, at 400-415 °C provides the additional benefit of producing an appreciable amount of middle distillate having a boiling range of 200-370 °C. However, this may accelerate the deactivation of the catalyst, which needs to be taken care of by increasing hydrogen pressure. The effect of temperature on the conversion of MCR is shown in Figure 4. It is observed from Figure 4 that
Bej et al.
Figure 4. Effect of temperature on the conversion of MCR at a pressure of 88 bar, LHSV of 1.0 h-1 and a hydrogen/heavy gas oil ratio of 600 mL/mL.
during hydrotreatment reactions, substantial reduction in the MCR content of the feed takes place. It is also seen from the Figure that the conversion of MCR increases at a very sharp rate from 87.5 to 94.3 wt % for an increase in temperature from 365 to 385 °C, beyond which no further reduction in the MCR content is observed. It is reported in the literature 5 that the conversion of residual fraction and MCR are closely associated. But from the comparison of Figures 3 and 4, it is observed that the trend of conversions of residual fraction and MCR with temperature is quite different. The possible explanation for this may be as follows. The high boiling molecules, which are responsible for the high values of MCR, get converted in the temperature range of 365 to 390 °C. Thus with the fragmentation of these molecules, the percent conversion of MCR is higher. But with further increase in temperature, though the conversion of the residual fraction increases at a faster rate, the molecules, which get converted in this higher range of temperature, may not be very high boiling or high molecular weight and hence does not contribute to MCR. Thus, with the conversion of these molecules there is no effect on the conversion of MCR and hence MCR conversion remains more or less constant in the higher zone of temperature (i.e., 390415 °C). Sanford and Chung19 have also considered two classes of residue molecules; one of which forms residue in the test CCR and the other, which does not form residue in the CCR test. Effect of Pressure. The effect of pressure on the conversion and generation of different fractions have been studied at three levels of pressure, viz., 65, 74, and 88 bar. The other process variables, namely, temperature, LHSV, and hydrogen/heavy gas oil volumetric ratio, were kept constant, respectively, at 385 °C, 1.0 h-1, and 600, respectively. The results are given in Table 2. It is observed from Table 2 that as the pressure increased, the conversion of 500 °C + fraction is enhanced. Similarly, a slight increase in the production of the 200-370 °C fraction is observed when pressure is increased from 65 to 88 bar. However, it does not have any significant effect on the conversion of 500 °C+ fraction within the range of pressures studied. On the (19) Sanford, E. C.; Chung, K. H. AOSTRA J. Res. 1991, 7, 37.
Oil Sands Derived Heavy Gas Oil
Energy & Fuels, Vol. 15, No. 5, 2001 1107
Table 2. Effect of Pressure on the Conversion and Generation of Different Fractions and MCR of Heavy Gas Oil reaction pressure of conversion or generation (wt %) of fraction fraction
65 bar
74 bar
88 bar
200-370 °C 370-500 °C 500 °C+ MCR
-33.5 2.4 66.5 86.9
-36.9 1.0 68.4 88.8
-40.0 4.5 71.9 94.4
Figure 6. Effect of hydrogen/heavy gas oil ratio on the conversion of MCR at a temperature of 385 °C, LHSV of 1.0 h-1, and pressure of 88 bar.
Figure 5. Effect of hydrogen/heavy gas oil ratio on the conversion and generation of different fractions at a temperature of 385 °C, LHSV of 1.0 h-1, and pressure of 88 bar.
other hand, the removal of MCR increases steadily as pressure is increased. Effect of the Hydrogen/Heavy Gas Oil Ratio. The effect of the hydrogen/heavy gas oil volumetric ratio on the conversion or production of different fractions has been studied by changing it from 400 to 1000 mL/mL keeping other process parameters such as temperature, pressure, and LHSV constant, respectively, at 385 °C, 88 bar, and 1.0 h-1. The results are shown in Figure 5. It is observed from the results of the Figure that hydrogen/heavy gas oil volumetric ratio has some influence on the conversion of the 500 °C+ fraction and production of the 200-370 °C fraction. For example, as the hydrogen/heavy gas oil volumetric ratio is increased from 400 to 800 mL/mL, the conversion of the 500 °C+ fraction is enhanced from 69 to 81 wt %, whereas the production of 200-370 °C fraction was increased from 33 to 41 wt %. However, this ratio does not have any effect on conversion beyond 800 mL/mL. Beyond this range, the generation of the 200-370 °C fraction is more or less constant around 41 wt %. The effect of hydrogen/heavy gas oil volumetric ratio on the conversion of MCR is shown in Figure 6. The results show that similar to the trend of conversion of 500 °C+ (as shown in Figure 5), the conversion of MCR increases at a sharp rate (from 89 to 94 wt %) for the change in hydrogen/heavy gas oil volumetric ratio from 400 to 600 mL/mL, beyond which it does not have any significant influence on MCR conversion. This indicates that increasing the hydrogen/heavy gas oil ratio beyond 600 mL/ml is of no help with respect to the reduction of MCR content of the heavy gas oil. In our previous study 2 it was also observed that hydrogen/heavy gas oil volumetric ratio beyond a value of 800 mL/mL does not have any significant effect on the removal of nitrogen from the heavy gas oil.
Figure 7. Effect of LHSV on the conversion and generation of different fractions at a temperature of 385 °C, pressure of 88 bar, and a hydrogen/heavy gas oil ratio of 600 mL/mL.
Effect of Liquid Hourly Space Velocity. The effect of liquid hourly space velocity (LHSV) has been studied within the range of 0.5-1.9 h-1 keeping temperature, pressure, and hydrogen/heavy gas oil ratio constant, respectively, at 385 °C, 88 bar, and 600 mL/mL. The results are shown in Figure 7. It is seen from the results of Figure 7 that if we decrease the LHSV from 0.5 to 1.0 h-1, the conversion of the 500 °C+ fraction decreases from ∼84 to ∼72 wt %. Beyond this value of LHSV, there is no significant effect of decreasing LHSV on the conversion of this fraction. This is because of the availability of more time for reaction of the heavier molecules. Similarly, LHSV showed some significant effect also on the generation of the 200-370 °C fraction. As more contact time is provided by decreasing the LHSV of the heavy gas oil, some appreciable quantity of 200-370 °C fraction is generated. For example, the production of this fraction can be increased steadily by decreasing the LHSV from 1.9 to 0.5 h-1. On the other hand, LHSV had very little effect on the conversion or generation of 370-500 °C fraction. The effect of LHSV on the removal of MCR from heavy gas oil at a constant pressure of 88 bar, temperature of 385 °C and a hydrogen/heavy gas oil volumetric ratio of 600 mL/ml is shown in Figure 8. It is evident from the figure that if the LHSV is decreased from 1.9 to 1.0 h-1, the removal of MCR increases from ∼86 to ∼94 wt %. Gray et al.10 have also found that as the liquid flow
1108
Energy & Fuels, Vol. 15, No. 5, 2001
Bej et al.
Figure 8. Effect of LHSV on the conversion of MCR at a temperature of 385 °C, pressure of 88 bar, and a hydrogen/ heavy gas oil ratio of 600 mL/mL.
decreased, the CCR conversion during the hydrocracking of residues from Alberta bitumen also increased. It is also seen from Figure 8 that if the LHSV is decreased further from 1.0 to 0.5 h-1, there is no substantial increase in the conversion of MCR (only from 94 to 95 wt %) These results indicate that the larger high boiling molecules responsible for MCR are broken down when the hydrotreatment is carried out in the LHSV range of 1.9 to 1.0 h-1 corresponding to a reaction temperature of 385 °C, pressure of 88 bar, and a hydrogen/heavy gas oil volumetric ratio of 600 mL/ mL. But when the LHSV is decreased to 0.5 h-1, the molecules, which get converted in this lower range of LHSV, may not contribute to the MCR content. Kinetics of Conversion of Residual Fraction (500 °C+). The kinetics of conversion of the residual fraction of heavy gas oil having a boiling range higher than 500 °C can be described by a simple power law model and the values of apparent rate constant can be calculated using the following equation:
k1 )
[
]
1 1 1 LHSV n - C n-1 C n-1 p f
[Cp1 - Cf1 ] LHSV
Table 3. Values of Apparent Rate Constants for the Conversion of Residual Fraction and MCR Present in Heavy Gas Oil values of apparent rate constants k1 and k2 apparent for different temperatures rate constants, unit, h-1 (wt %)-1 365 °C 385 °C 400 °C 415 °C k1 k2
0.10 4.38
0.138 10.54
0.232 11.88
0.778 14.31
(1)
where, k1 ) apparent rate constant for conversion of residual fraction, n ) order of reaction, Cp ) residual fraction (500 °C+) present in the product (wt %), Cf ) residual fraction (500 °C+) present in feed (wt %), and LHSV ) liquid hourly space velocity (h-1). It was attempted to find out the best fit of experimental data for the conversion of residual fraction for different values of order of reaction and it was found that the conversion rate data gave a best fit for a secondorder reaction. The fit of experimental data for the conversion of residual fraction for n ) 2 is shown in Figure 9. Hence, using n ) 2, eq 1 then takes the following form (eq 2):
k1 )
Figure 9. Fit of experimental data generated at 385 °C, 88 bar, LHSV of 1.0 h-1, and a hydrogen/heavy gas oil of 600 mL/ mL to the power law model for an order of reaction equals to 2 (n ) 2) for the conversion of residual fraction and removal of MCR.
(2)
Hence, the apparent rate constants for the removal of residual fraction have been calculated from eq 2. The values of apparent rate constants for four temperatures (365, 385, 400, and 415 °C) are given in Table 3. The Arrhenius plots for these rate constants (k1) are shown in Figure 10. It is interesting to note that two different levels of activation energy are obtained for the conver-
Figure 10. Arrhenius plots of rate data.
sion of the residual fraction. In the lower range of temperature (365-390 °C), the activation energy was found to be 87 kJ/mol. On the other hand, as the reaction is conducted in the higher temperature zone (400-415 °C), a much higher value of activation energy (333 kJ/mol) was obtained. This change in activation energy from lower to higher values for an increase in temperature indicates the significant contribution of the thermal hydrocracking reaction to the overall conversion of the residual fraction in the higher temperature range. Also, these values indicate that the reaction was not diffusion-limited over the process conditions studied. Kinetics of Removal of MCR. The rate data for the kinetics of removal of MCR has also been tried to fit a power law model. It has been found that our experimental data for the removal of MCR also fit best for an order of reaction, n ) 2. The fit of experimental data for n ) 2 for the removal of MCR is shown in Figure 9. Hence, the apparent rate constants for this were calculated using the following second-order reaction (eq 3):
Oil Sands Derived Heavy Gas Oil
k2 )
[
Energy & Fuels, Vol. 15, No. 5, 2001 1109
]
1 1 LHSV MCRp MCRf
(3)
and
ka ) kao exp(-Ea/RT)
where, k2 is the apparent rate constant for the removal of MCR, and MCRp and MCRf are the MCR content of the hydrotreated product and feed heavy gas oil, respectively. The values of k2 for four different temperatures, calculated using eq 3, are given in Table 3. The Arrhenius plot for the rate constant k2 is shown in Figure 10. The apparent activation energy for the removal of MCR was calculated to be 91 kJ/mol. Kinetics of Distillate Production. The production of distillate fraction (having boiling range less that 370 °C) from residual fraction of the feed (having boiling range of 370 °C +) can be described by a reaction scheme as shown below:
(6)
where, CBo and CAo are the initial concentrations of the 370 °C- fraction and 370 °C+ fraction, respectively, in the feed, and CB is the concentration of the 370 °Cfraction in the product. The values of ka, kao, m, and Ea were determined from our experimental data. It was found that the production of distillates shows a very loworder dependency on hydrogen pressure (m ) 0.18). With the experimentally determined values, eq 6 takes the following form:
ka ) 6.14 × 10-06 exp(-11,120/RT)
(7)
Conclusion ka
A 98 B
(4)
where, A and B represent the 370 °C+ and 370 °Cfractions, respectively, and ka is the rate constant for the formation of B from A. It is evident from the earlier section that the conversion of residual fraction (having a boiling range of 500 °C+) shows a best fit for a reaction order of 2. Thus, it is expected that the production of the distillate fraction B from the 370 °C+ fraction will also most likely follow a second-order kinetics. Hence, we have assumed a second-order kinetics for deriving a rate expression for the production of fraction B from fraction A (as shown in eq 4). The derived rate expression including the mth power dependency on hydrogen pressure (pH) is given below (eq 5):
[
CB ) CBo + CAo 1 -
1 1 + (kaCAopHm/LHSV)
]
(5)
The effect of different hydrotreatment process variables such as temperature, pressure, hydrogen/heavy gas oil volumetric ratio, and LHSV on the conversion and generation of different fractions of oil sand derived heavy gas oil has been studied. It has been observed that during hydrotreatment,a substantial portion (∼92 wt %) of the residual fraction (500 °C+) of the heavy gas gets converted into valuable light distillate products. Similarly, the MCR of heavy gas oil also gets reduced during the hydrotreatment operation. The temperature has a significant effect on the conversion of residual fraction as well as on the removal of MCR from heavy gas oil. The thermal cracking reactions seem to be playing a significant role in the conversion of residual fraction beyond a temperature of 390 °C. The conversion of the residual fraction as well as MCR present in heavy gas oil can be described by a power law model having an order of reaction equal to 2. EF000287L
