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A Comparative Study of the Effect of Catalyst Type on Hydrotreating Kinetics of Kuwaiti Atmospheric Residue A. Marafi,*,† S. Fukase,‡ M. Al-Marri,† and A. Stanislaus† Petroleum Refining Department, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, 13109, Kuwait, and Kuwait Branch Laboratory, Japan Cooperation Center, Petroleum, C/O Kuwait Institute for Scientific Research Received August 12, 2002
In the upgrading of heavy petroleum oils and residues by hydrotreating, multiple-reactor fixedbed units loaded with different types of catalysts are extensively used. Catalysts for such hydrotreating processes are chosen on the basis of activity, selectivity, and life. The performance of the overall hydrotreating process with regard to various reactions, such as hydrodemetallation (HDM), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), asphaltenes cracking (HDAsph), and conversion to distillate and catalyst life-on-stream, is clearly linked to the performance of the catalyst in different reactors. Information regarding the activity, selectivity, kinetics parameters, and deactivation of the individual catalysts are, therefore, highly desirable for optimizing reactor loading in a multiple catalyst system. In the present work, a comparative study was conducted on the kinetics of various reactions such as HDS, HDV, HDNi, HDN, CCR reduction, and asphaltenes conversion in hydrotreating Kuwaiti atmospheric residue (KU-AR) over three types of catalysts. The results showed diverse kinetics behavior of different hydrotreating reactions. The diverse kinetics behavior of the different reactions and the strong dependence of the kinetics parameters on catalyst type are consistent with the kinetic aggregation theory. The results are discussed in comparison with those available in published literature.
1. Introduction In recent years, demand for heavy fuel oil has decreased significantly while the need for light and middle distillates has moved in the opposite direction. At the same time, environmental concerns have increased, resulting in more rigorous specifications for petroleum products including fuel oils. These trends have emphasized the importance of processes that convert the heavier oil fractions into lighter and more valuable clean products.1 A number of technologies have been developed over the years for residual oil upgrading, which include processes based on the carbon rejection route and hydrogen addition route.2,3 The hydrogen addition route has the advantage over the carbon rejection route of producing high-quality, environmentally friendly distillate products.4 Consequently, heavy oil upgrading by hydrogen addition, in particular the atmospheric residue desulfurization (ARDS) process, has gained considerable importance in recent years.5 In the upgrading of heavy petroleum oils and residues by hydrotreating, multiple-reactor fixed-bed units loaded * Author to whom correspondence should be addressed. Fax (+965)3980445. E-mail:
[email protected]. † Petroleum Refining Department. ‡ Kuwait Branch Laboratory, Japan Cooperation Center, Petroleum. (1) Absi-Halabi, M.; Stanislaus, A.; Qabazard, H. Hydrocarbon Process. 1997, 2, 45-55. (2) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (3) Le Page, J. F. Resid and Heavy Oil Processing; Editions Technip: Paris, 1992. (4) Speight, J. Petroleum Chemistry and Refining; Taylor and Francis: London, 1998.
with different types of catalysts are extensively used.7,8 Catalysts for such hydrotreating processes are selected on the basis of activity, selectivity, and life. Catalyst life is dependent on the rate of deactivation by coke and metal deposits and sintering of the active phase. The performance of the overall hydrotreating process with regard to various reactions, such as hydrodemetallation (HDM), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), asphaltenes cracking (HDAsph), and conversion to distillate and life-on-stream, is clearly linked to the performance of the catalyst in different reactors.6,8 Information regarding the activity, selectivity, and deactivation of the individual catalysts are, therefore, highly desirable for optimizing reactor loading in the multiple catalyst systems. Information on the hydrotreatment reaction kinetics for different types of catalysts used in the composite catalyst beds of residual hydrotreaters will be useful for catalyst selection and development, composite catalyst bed design, and catalyst deactivation modeling in residual hydrotreatment processes. Among the factors that influence the reaction kinetics, feedstock quality and catalyst type play a major role. (5) Kent Murry, J. Impact of technology on future trends in petroleum refining: Catalysis in Petroleum Refining and Petrochemical Industries; Besharah, J.; Absi-Halabi, M.; Qabazard, H.; Stanislaus, A. Kuwait 1996, 45-59. (6) Kressmann, S.; Morel, F.; Harle, V.; Kasztellan, S. Catal. Today 1998, 43, 203-215. (7) Furimsky, E. Appl. Catal. A: General 1998, 171, 177-206. (8) Adams, C. T.; Del Peggio, A. A.; Schaper, H.; Stock, W. H. J.; Shiflett, W. K. Hydrocarbon Process. 1989, September, 57-61.
10.1021/ef020177+ CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003
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Table 1. KU-AR Feed Characteristics property
value
API total sulfur wt % asphaltene wt % total nitrogen wt ppm kinematic viscosity @ 50 °C cSt CCR metal content Ni wt ppm V wt ppm
12.27 4.30 3.75 2670 871.2 12.20 21 69
The kinetics of petroleum residue hydrotreating is highly complicated due to the complex composition of the residues; these residues contain a wide variety of sulfur, nitrogen, and organometallic species together with large molecular materials such as asphaltenes. Conflicting results have been reported in the literature for the kinetics of various reactions such as HDS, HDM, HDN, asphaltenes conversion, and hydro-Conradson Carbon Residue (HDCCR) in residual hydrotreating. Most of the literature cited in this area is fragmented and does not provide complete and in-depth information about the roles of different catalysts typically used in a multi-reactor atmospheric residue desulfurization unit. Hence, we have embarked on an extensive investigation of a catalyst system consisting of three types of catalysts with the aim of providing a complete account of the kinetics of various reactions as well as the deactivation behavior of the catalyst. In this first paper, we have made a comparative study of the kinetics of some important reactions such as HDS, HDV, HDNi, HDN, HDCCR, and asphaltenes cracking in hydrotreating Kuwaiti atmospheric residue (KU-AR) over three types of catalysts that are designed to promote HDM, HDM/ HDS, and HDS/HDN, and are used in the first, second, and third reactors of an industrial hydrotreating unit, respectively. The performances of the three catalysts in removing various heteroatoms such as S, V, Ni, N, some carbon reduction, and asphaltenes from the feed are also compared. 2. Experimental Section 2.1. Materials. Kuwaiti atmospheric residue (KU-AR) obtained from the Kuwait National Petroleum Company refinery was used as feedstock in our experiments. The feedstock is a typical feed being currently used in the ARDS units at Kuwait’s refineries. The characteristics of KU-AR are shown in Table 1. Three industrial hydroprocessing catalysts, namely, A, B, and C, used in the front, middle, and back-end of a multiple reactor hydrotreating unit were examined to conduct performance comparison and kinetic studies. The composition and the main characteristics of the three catalysts are shown in Table 2. 2.2. Experimental Techniques. The hydrotreating experiments were conducted in a fixed-bed reactor unit (manufactured by Vinci Technologies, France) in the up-flow mode using Kuwaiti atmospheric residue (KU-AR) as feed. A 50 mL volume of catalyst charge diluted with an equal amount of carborundum was used for each run. The performance test was carried out for catalysts A, B, and C individually. The catalyst was presulfided using straight run gas-oil containing 3 wt % dimethyl disulfide (DMDS) under the following conditions: H2 flow rate ) 57 L/h (H2/oil ) 570 mL/mL); pressure ) 120 bar; and presulfiding feed flow rate ) 100 mL/h, liquid hourly space velocity (LHSV) ) 2 h-1). During presulfiding, the reactor temperature was increased from 150 °C to 350 °C by ramping
Table 2. Characteristics of Catalysts A, B, and C Used in the Present Study property catalyst type bulk density surface area average pore diameter type of active metals metal content Mo Ni P metal capacity
unit
B
C
HDM g/mL 0.4-0.5 m2/gm 150-200 Å 150-200 Mo
A
HDS 0.6-0.7 200-250 80-100 NiMo
HDS/HDN 0.7-0.8 170-200 80-100 NiMoP
wt % wt % wt %
2-3
7-9 2-3
high
medium
9-11 2-4 2-4 low
Table 3. Run Conditions for Temperature and LHSV Effect Studies process parameter temperature effect study LHSV effect study temperature (°C) pressure (bar) LHSV (h-1) H2/Oil (mL/mL) time on stream (h)
360, 380, 400, 420 120 1 570 120
380 120 0.5, 1.0, 2.0, 4.0 570 120
at a rate of 15 °C/h. After reaching 350 °C, presulfiding was continued for 7 h under the stated conditions. After presulfiding, conditions for each run were adjusted to the desired operating temperature, pressure, hydrogen flow rate, and LHSV using KU-AR as feedstock. Table 3 shows the various operating conditions for the temperature and LHSV studies. The catalyst activity declined slightly during the initial 48 h and then reached the steady state. The samples for analysis were taken at steady-state conditions. During the course of each run, product samples were collected every 12 h and analyzed for sulfur, metals (V and Ni), nitrogen, asphaltenes, CCR, viscosity, density, and distillate yield using ASTM and/ or IP standard methods for testing petroleum products.
3. Results and Discussions The main reactions that occur during catalytic hydrotreating of petroleum residue are HDS, HDV, HDNi, HDCCR, HDN and HDAsp. In the present work, tests conducted at various liquid hourly space velocities (LHSV) were used for kinetics data analysis and to determine the apparent reaction order for the above reactions using the following nth order or first-order kinetics expression:
k ) LHSV (n - 1)-1 {Cp1-n - Cf1-n} for n > 1 (1) k ) LHSV ln(Cf/Cp) for n ) 1
(2)
where Cf is the concentration of the selected component in the feed and Cp is the concentration in the product in wt % or wt ppm. Similarly, using the results of tests conducted at different temperatures, the activation energies for the different reactions were calculated from the Arrhenius equation:
k ) Ae-EA/RT
(3)
where A is frequency factor, EA is the activation energy (kcal g-1 mol-1), R is the gas constant (1.987 cal g-1 mol-1 K-1), and T is the reaction temperature (K). The results are presented and discussed below. 3.1. Hydrodesulfurization. Plots of the kinetics data for HDS reactions are shown in Figure 1. The results clearly indicate that the data fit second-order kinetics for all three catalysts. The information available
Catalyst Type and Hydrotreating Kinetics of KU-AR
in the literature on the reaction order for HDS reaction in residual oil hydrotreating shows significant variation. Thus, while some researchers reported first-order kinetics for the desulfurization of residues in a trickle bed reactor,9,10 several other studies have found secondorder kinetics.11-14 A reaction order in the range 1.9 to 2.3 for the desulfurization of atmospheric residue in a continuous stirrer tank reactor (CSTR) unit was found.15 The reaction orders for removal of sulfur in the asphaltenic and non-asphaltenic fractions of residual oil were found to be 3 and 2, respectively.16 Second-order kinetics for hydrodesulfurization of Kuwait atmospheric over a Co-Mo/Al2O3 catalyst in a fixed bed reactor was found by Beutter and Schmid.14 Several others also reported second-order kinetics for HDS of Kuwait atmospheric residue over Co-Mo/Al2O3 catalyst using a fixed bed reactor.17,18 Our results clearly indicate that the desulfurization of Kuwaiti atmospheric residue over the three catalysts obeys second-order kinetics, in agreement with the results of others who have used a similar feedstock and reactor.14,17 The apparent overall reaction orders >1 observed for HDS reaction in residual oil hydrotreating are in contrast to the first-order behavior of individual sulfur compounds.19,20 This has been explained on the basis of the wide variation in the reactivity of individual sulfur-bearing species present in the residues. Mathematical analysis of residual oil HDS kinetic behavior on the basis of a number of first-order reactions of different sulfur compounds has shown that an overall order between 1 and 2 could be obtained depending on the activity distribution of the sulfur compounds.21 In our experiments, hydrotreating of Kuwait atmospheric residue was carried out in a fixed bed reactor using three different types of catalysts, which had different properties. Catalyst A contained 400 °C. For nickel, the activation energies are 12.5 kcal g-1 mol-1 in the temperature range of 360400 °C and 28.9 kcal g-1 mol-1 at a temperatures above 400 °C. Apparent activation energy values reported in the literature for vanadium removal from residues by HDM reaction range from 10 to 38 kcal g-1 mol-1, depending on the reaction order.25 For first-order kinetics, these vary from 10 to 18 kcal/mol. An activation energy of 18 kcal/mol for HDM of deasphalted oils using first-order kinetics was observed.37 For second-order kinetics, an activation energy of 38.2 kcal g-1 mol-1 was found.33 Recently, other workers reported activation energies for vanadium and nickel removal from Kuwaiti atmospheric residue of 36.1 kcal g-1 mol-1 and 27.3 kcal g-1 mol-1, respectively, based on 1.5-order kinetics.38 The discrepancies observed in the activation energy values may reflect differences in crude source resulting in different reactivities of metal-containing species and different rate-limiting steps. Furthermore, metal removal reac(37) Reyes, L.; Zerpa, C.; Krasuk, J. H. Stud. Surf. Sci. Catal. 1994, 88, 85-94. (38) Bhan, O. K.; George, S. E. Stud. Surf. Sci. Catal. 1996, 100, 135-145.
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Figure 8. Second-order plots of kinetic data for the HDAsph of Kuwait atmospheric over catalysts A, B, and C.
Figure 9. Arrhenius plots for HDAsph over catalysts A, B, and C.
tions in conventional hydrotreating catalysts are diffusion-limited. Diffusion-disguised first-order kinetics would also lead to lower apparent activation energy values. A combination of factors, such as improved diffusion and a change in reaction mechanism, may be responsible for a sharp increase in the activation energy for vanadium and nickel removal at temperatures above 400 °C observed in this study. At temperatures above 400 °C, the vanadium porphyrin structures in the complex asphaltene micelles may dissociate and become free due to the depolymerization and cracking, and destruction of asphaltene micelles. Diffusion limitations on such dissociated metalloporphyrin structures are relatively low, and temperature sensitivity for the demetallation reactions are high. 3.3. Asphaltenes Conversion. The kinetics data plotted in Figure 8 show that the conversion of asphaltenes of Kuwaiti atmospheric residue fits second-order kinetics for the three types of catalysts. Reaction orders varying between 0.5 and 2.0 have been reported in the literature for asphaltenes cracking in residual oil. A 1.5 order of kinetics for asphaltenes removal reaction in the hydrotreating of a Maya residue was found.36 On the other hand, second-order kinetics has been reported for asphaltenes conversion in the hydrotreating of Boscan and Greek residues.16,30 The activation energies calculated from the Arrhenius plots in Figure 9 were 23.6 kcal g-1 mol-1, 31.5 kcal g-1 mol-1, and 15.1 kcal g-1 mol-1 with second-order reactions for asphaltenes removal for A, B, and C catalysts, respectively. Such differences reflect the different tem-
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Figure 12. Arrhenius plots for HDN over catalysts B and C. Figure 10. First-order plots of kinetic data for the HDN of Kuwait atmospheric over catalyst B.
Figure 11. 1.5-order plot of kinetic data for HDN of Kuwait atmospheric over catalyst C.
perature sensitivities of asphaltenes cracking reactions over different catalysts. Activation energy values reported in the literature for asphaltenes cracking also vary considerably. While other workers reported activation energies in the range of 41.1 kcal g-1 mol-1,34 a much lower value for activation energy was found by Philippopoulos and Papayannakos.16 These authors used two types of CoMo/Al2O3 catalysts for hydrotreating Greek atmospheric residue and found two different activation energies, namely 14 kcal g-1 mol-1 for one catalyst and 21 kcal g-1 mol-1 for anther catalyst. 3.4. Denitrogenation. The results presented in Figures 10 and 11 show that the kinetics of hydrodenitrogenation is different for different catalysts. For catalyst B, the data fit first-order kinetics, whereas for catalyst C, the HDN reaction obeys 1.5-order kinetics. Catalyst A showed a very poor activity for HDN, and data for kinetic plots were not available for this catalyst. The apparent reaction order for HDN was reported to be 1 with respect to total nitrogen content.39,40 For HDN of single nitrogen-containing model compounds, fractional orders with values less than unity have been reported.39 The higher HDN reaction order observed for the real feeds could be interpreted as arising from (39) Ho, T. C. Catal. Rev.: Sci. Eng. 1988, 30, 117-160. (40) Christensen, H.; Cooper, B. H. The influence of catalyst and feedstock properties in FCC feed pretreatment. Paper presented at the American Institute of Chemical Engineers Spring National Meeting, 1990, pp 220-234.
Figure 13. Second-order plots of kinetic data for HDCCR of Kuwait atmospheric over catalysts A, B, and C.
lumping a large number of nitrogen compounds with reactivities less than unity into a pseudo species (i.e., total nitrogen content). However, in a recent study on hydroprocessing of a Maya residue, 0.5-order for the HDN reaction with respect to total nitrogen content was found.36 Similar 0.5-order kinetics was also observed for HDN of asphaltenic nitrogen from a Maya residue.36 Figure 12 shows the Arrhenius plots for the HDN reaction in Kuwaiti atmospheric residue hydrotreating over catalysts B and C. The HDN activation energies calculated from the plots are 29.2 and 31 kcal g-1 mol-1 for catalysts B and C, respectively. The values are significantly lower than that obtained for HDN in Maya residue hydrotreating.36 3.5. Hydro-Conradson Carbon Residue Conversion (HDCCR). The results presented in Figure 13 show that the Conradson Carbon Residue (HDCCR) conversion kinetics in Kuwaiti AR hydrotreating obeys second order for all the three catalysts. Information on CCR conversion kinetics is scarce in published literature. Kinetics on CCR conversion in the catalytic hydrotreating of a Maya crude was examined using a commercial catalyst in a CSTR.41 The results revealed that the data of CCR conversion fit 0.5-order kinetics with an activation energy of 66.09 kcal g-1 mol-1. The activation energy values found in the present study (2024.5 kcal g-1 mol-1) from the Arrhenius plots (Figure (41) Trasobares, S.; Masria, A.; Callejas, A.; Benito, M.; Marinaz, M. T. Ind. Eng. Chem. Res. 1998, 37, 11-17.
Catalyst Type and Hydrotreating Kinetics of KU-AR
Figure 14. Arrhenius plots for HDCCR over catalysts A, B, and C.
14) are remarkably lower than the values reported in the literature. The differences observed between the three catalysts in the kinetics parameters of various hydrotreating reactions can be attributed to the differences in their catalytic activity in promoting different reactions. The three catalysts used in the present study have different compositions (Mo, and promoter content) and pore sizes. Catalyst A, which contained solely Mo on a larger pore alumina support, is more active for demetallization and asphaltenes conversion reactions than the other two catalysts. Catalysts B and C had similar pore sizes and both of them contained Mo and Ni; however, catalyst C had higher concentrations of the active metals than catalyst B. In addition, catalyst C contained an additional promoter. Catalyst C was more active for HDS, HDN, and CCR conversion reactions than catalyst A and B due to its higher hydrogenation function. In summary, the results of the present study and those reported in the literature indicate diverse kinetics behavior of residual oil hydrotreating reactions. The overall reaction order for different reactions such as HDS, HDV, HDNi, HDN, and other reactions was higher than that for the individual sulfur, nitrogen, organometallic compounds. The results are consistent with the kinetics aggregation theory proposed in the literature.21 In aggregated kinetics,21,42 the mixture of components in a feed oil is treated as a continuum, with a distribution function to characterize reaction rates. The approach showed mathematically that reaction orders up to second-order kinetics would arise if a fraction of the feed were essentially unreactive, or if the concentration-reaction rate distribution followed an exponential distribution. The dependence of the overall reaction order on the activity of the catalyst, conversion level, and reactor type has been explained using the continuum theory and aggregated kinetics. A strong dependence of the activation energy values on catalyst type is also observed in the present work. However, no clear relationship between catalyst properties and the activation energy for different hydrotreating reactions is noticed. Catalyst pore size and active site distribution are generally known to influence the activation energy of a chemical reaction. In diffusion-limited reactions, catalyst pore size can have a strong influence on the apparent activation energy. If the intraparticle (42) Ho, T. C. Stud. Surf. Sci. Catal. 1999, 127, 179-186.
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mass transfer strongly influences the reaction rate, the apparent activation energy for the reaction could be approximately one-half the true activation energy. However, in our studies no significant reduction in activation energy is noticed for catalysts B and C, which have narrow mesopores with pore diameters in the range 80-100 Å. Diffusion limitations appear to be negligible in all the catalysts. This is probably due to the absence of large molecular species in the feedstock under the hydrotreating conditions used in our experiments. It is suggested that asphaltenes which are held together by weak forces to form large micelles at room temperature are easily broken by shear or thermal motion at the processing temperatures of more than 370 °C, so that the individual asphaltene molecules and other organometallic compounds may be not larger than 1000 in molecular weight and 12 Å in diameter.43,44 Diffusion limitation for various hydrotreating reactions could be expected to be negligible under such conditions. The other catalyst parameter that can have an effect on the activation energy is the nature of the active sites and their distribution. Massoth proposed that a distribution of sites with different activation energies existed in the hydrotreating catalysts and the sites with lowest activation energy (18 to 22 kcal/mol) contained the highest activities despite relatively smaller number of sites in this region.45 A majority of sites had higher activation energies (>25 kcal/mol). In our experiments no clear trends or relation between the catalyst properties and activation energy for various hydrotreating reaction are noticed. Similar results have been reported by several others who studied hydrotreating kinetics using different catalysts with a single feedstock.22,23 Perhaps the difference between the various catalysts in the active site distribution is more complex. This complexity coupled with the complex nature of the feedstock with respect to distribution of various types of heteroatom-containing species could be responsible for the unclear trend. Conclusions A comparative study of the kinetics of various reactions such as HDS, HDV, HDNi, HDN, asphaltenes conversion, and HDCCR in hydrotreating Kuwaiti atmospheric residue (KU-AR) was carried out using three types of individual catalysts, namely, A, B, and C, that are used in the front, middle, and backend of an industrial residue hydrotreating unit. The results showed that catalyst C possessed remarkably high activity for HDS, HDN, and hydrogenation reactions, whereas catalyst A showed more selectivity for metals removal and asphaltenes conversion than the other two catalysts. Kinetics data analysis showed second-order for HDS HDCCR, and asphaltenes cracking reactions for all the three catalysts, but for metals removal different kinetics behavior was observed with catalyst A showing 1.5 order, whereas second order, for both catalysts B (43) Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 263-270. (44) Wei, J. Modeling demetallation catalyst deactivation; Bartholomew, C. H., Butt, J. B., Eds.; Elsevier: Amsterdam, 1991. (45) Massoth, F. E. Catalyst Deactivation; Bartholomew, C. H., Fuentes, G. A., Eds.; Elsevier: Amsterdam, 1997.
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and C. The catalyst type influenced the activation energies of different reactions differently. However, no clear relationship between catalyst properties and the activation energy for different hydrotreating reactions were noticed. The diverse kinetics behavior of the different reactions and the dependence of the kinetics parameters on catalyst type are consistent with the kinetic aggregation theory. Acknowledgment. This work was carried out under a joint research project between the Japan Cooperation
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Center, Petroleum (JCCP) funded by the Ministry of Economy Trade and Industry (METI), Japan, and the Kuwait Institute for Scientific Research, Kuwait (KISR). It bears the KISR Project No. PF010C. Note Added after ASAP Posting. This article was released ASAP on 4/16/2003 with an incorrect value of the gas constant, R. The correct version was posted on 4/29/2003. EF020177+
