Residual-Oil Hydrotreating Kinetics for Graded Catalyst Systems

However, while the feed entering the front-end catalyst bed is the original .... After the system was loaded, it was purged with nitrogen at a flow ra...
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Energy & Fuels 2003, 17, 1191-1197

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Residual-Oil Hydrotreating Kinetics for Graded Catalyst Systems: Effect of Original and Treated Feedstocks A. Marafi,*,† H. Al-Bazzaz,† M. Al-Marri,† F. Maruyama,‡ M. Absi-Halabi,† 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 November 19, 2002

In the upgrading of heavy petroleum oils and residues by hydrotreatment, multiple-reactor fixed-bed units loaded with different types of catalysts are used extensively. Catalysts for such hydrotreatment processes are chosen on the basis of activity, selectivity, and life. The performance of the overall hydrotreatment process, with regard to various reactions, such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallization (HDM), asphaltenes cracking (HDAsph), and conversion to distillates, as well as catalyst life-on-stream, are clearly linked to the performance of the catalyst in different reactors. Information regarding the activity, selectivity, kinetic parameters, and deactivation of the individual catalysts are, therefore, highly desirable for optimizing reactor loading in the multiple-catalyst system. This paper presents the performance tests for various reactions on two types of industrial hydrotreating catalysts: those used at the midsection and the tail-end of a graded catalyst system designed to hydrotreat atmospheric residual oils. The tests were conducted using straight-run Kuwait atmospheric residue, a demetallized residue, and a demetallized/desulfurized residue. The activity and kinetic parameters for different reactions that are typically occurring during the hydroprocessing of these feedstocks were determined. The results revealed significant changes in activity, depending on the feedstock used for the tests. Furthermore, apparent rate orders and rate constants for some reactions were significantly changed. The study demonstrates the importance of proper selection of the feedstocks used in the performance evaluation and screening of candidate catalysts for graded catalyst systems for residual-oil hydrotreatment.

1. Introduction Demand for heavy fuel oil in the industrialized countries has been declining over the past two decades, whereas the demand for transportation fuel has been increasing.1,2 Simultaneously, stricter environmental regulations limiting the sulfur content to ultralow levels in petroleum products have been introduced.3-5 Furthermore, crude oil that is available to refineries has * Author to whom correspondence should be addressed. E-mail: [email protected]. † Petroleum Refining Department. ‡ Kuwait Branch Laboratory, Japan Cooperation Center, Petroleum. (1) Absi Halabi, M.; Marafi, M.; Qabazard, H.; Akashah, S. The Future of the Arab and International Oil Refining Industry and the Role of Research in Its Development. Oil Arab Coop. 2002, 28, (102), 9-53. (2) Absi-Halabi, M.; Stanislaus, A.; Qabazard, H. Trends in Catalysis Research to Meet Future Refining Needs. Hydrocarbon Process. 1997, 76 (2), 45-55. (3) Song, C. Catalyst and Chemistry for Deep Desulfurization of Gasoline and Diesel Fuels: An Overview. Presented at the American Institute of Chemical Engineers (AIChE) National Meeting, 5th International Conference on Refinery Processing, New Orleans, LA, March 11-14, 2002. (4) Peri, B.; Sartori, R.; Coccia, M. The Refinery and the Incoming European Specifications for the Transportation Fuels. Presented at the 16th World Petroleum Congress, Forum No. RFP6, Calgary, Canada, June 12-17, 2000. (5) Prevot, C.; Valais, M. Impact of Refining Structures and Regional Capacity Balances. Presented at the 16th World Petroleum Congress, Forum No. RFP6, Calgary, Canada, June 12-17, 2000.

generally decreased in American Petroleum Institute (API) gravity and, simultaneously, increased in sulfur, metal, nitrogen, and asphaltenes contents. These trends left clear marks on the petroleum refining industry, converting refineries to a more complex industry with residual-oil conversion processes as cornerstones of any economically viable modern refinery.6 Numerous residual-oil conversion processes have been developed, on the basis of either hydrogen addition or carbon rejection.7,8 However, fixed-bed atmosphericresidue hydrotreatment processes have gained significant importance.9 This is largely due to the ability of these processes to fully utilize the heavy hydrocarbon feedstock by partially converting it to clean distillates and, simultaneously, producing low-sulfur fuel oil and hydrotreated feedstocks (with low sulfur, nitrogen, and metals) for deep conversion in downstream units, such (6) Khan, M. R.; Patmore, D. J. Heavy Oil Upgrading Processes. In Petroleum Chemistry and Refining; Speight, J. G., Ed.; Marcel Dekker: New York, 1998; pp 149-173. (7) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (8) Le Page, J. F. Resid and Heavy Oil Processing; Paris: Editions Technip: Paris, 1992. (9) Kent Murry, J. Impact of Technology on Future Trends in Petroleum Refining. In Petroleum and Petrochemical Industries: Present Status and Future Outlook; Besharah, J., Absi-Halabi, M., Qabazard, H., Stanislaus, A., Eds. Kuwait Institute for Scientific Research: Safat, Kuwait, 1996; pp 45-59.

10.1021/ef020282j CCC: $25.00 © 2003 American Chemical Society Published on Web 07/02/2003

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as fluidized catalytic cracking (FCC) and delayed coker. Typical industrial units that are based on these processes consist of multiple reactors operated in series and loaded with a catalyst or a catalyst system to achieve the desired levels of hydrodesulfurization (HDS), hydrodemetalation (HDM), hydrodenitrogenation (HDN), and conversion of the residual oil to distillates.10,11 The initial main drawback of fixed-bed residual-oil hydroprocessing units is the rapid deactivation of the catalyst, which reduced the time-on-stream factor of the industrial units. However, research efforts have led to significant advancements in catalyst technology and have resulted in the development of graded catalyst systems that consist of three or more types of catalysts, which enabled the extension of the time-on-stream from less than six months to over fifteen months.10,12 Current typical catalyst systems consist of a front-end HDM catalyst for metal removal, a catalyst with balanced HDM and HDS activities in the midsection, and a very active HDS/HDN catalyst in the tail-end.12 Research for further improvements of the catalyst system is still in progress in numerous laboratories, with pilot plant testing, using residual oil as feedstock, being the most important tool for evaluating the performance of new catalysts. However, while the feed entering the front-end catalyst bed is the original atmospheric residue, the feed to the second catalyst bed is actually the partially hydrotreated product (i.e., demetallized atmospheric residue) of the front-end catalyst. Furthermore, the feed entering the third catalyst bed is demetallized and desulfurized, with low asphaltene content. The quality of the feedstocks, with respect to their contamination with hetroatoms (H/C), and the degree of aromaticity and alkyl substitution entering the second and third catalyst beds are significantly different from the original atmospheric residue. These differences are strongly dependent on the operating severity of the first and second catalyst beds and can have a strong influence on the performance of the catalysts in the following catalyst bed, with regard to various conversions and deactivation rates.13,14 Therefore, although the use of straight-run atmospheric residue as feedstock in evaluating the front-end HDM catalyst is scientifically sound, their usage for evaluating the performance of subsequent catalysts in a catalyst system is likely to lead to erroneous conclusions. Hence, we embarked on an extensive investigation of a catalyst system that consisted 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 a previous study, a complete evaluation of the catalyst system (10) Adams, C. T.; Del Peggio, A. A.; Schaper, H.; Stock, W. H. J.; Shiflett, W. K. Hydroprocessing Catalyst Selection. Hydrocarbon Process. 1989, 68, (9), 57-61. (11) Kressmann, S.; Morel, F.; Harle, V.; Kasztellan, S. Recent Developments in Fixed-Bed Catalytic Residue Upgrading. Catal. Today 1998, 43, 203-215. (12) Furimsky, E. Selection of Catalysts and Reactors for Hydroprocessing. Appl. Catal. A 1998, 171, 177-206. (13) Seki, H.; Kumata, F. Deactivation of Hydrodesulfurization Catalysts: Effect of Hydrodemetalation Operation Conditions. Stud. Surf. Sci. Catal. 1999, 357-364 (Proceedings of the 8th International Symposium, Brugge, Belgium). (14) Higashi, H.; Takashi, T.; Kai, T. The Effect of Start-Up Conditions on Deactivation of Hydrotreating Catalyst for Heavy Residue with High Asphaltene Content. Catal. Surv. Jpn. 2002, 5, (2), 111-119.

Marafi et al. Table 1. Characteristics of Catalysts A, B, and C Used in the Present Study property

A

B

C

catalyst type bulk density (g/mL) surface area (m2/g) average pore diameter (Å) type of active metals metal content (wt %) Mo Ni P metal capacity

HDM 0.4-0.5 150-200 150-200 Mo

HDS 0.6-0.7 200-250 80-100 Ni, Mo

HDS/HDN 0.7-0.8 170-200 80-100 Ni, Mo, P

2-3

7-9 2-3

high

medium

9-11 2-4 2-4 low

using straight-run Kuwait atmospheric residue was conducted, including the determination of the orders, rate constants, and apparent activation energies of various reactions, such as desulfurization, demetallization, etc.15 In this work, a comparative study is presented on the activity and kinetics of the hydrotreating reactions using straight-run atmospheric residue (KU-AR), demetallized atmospheric residue (DM-AR), and demetallized desulfurized atmospheric residue (DMDS-AR). These feedstocks are considered to be typical feed entering the first, second, and third catalyst beds, respectively, in a commercial atmospheric residue desulfurization (ARDS) unit that is processing Kuwait atmospheric residue. The study focuses on two types of catalysts (B and C), which comprise the midsection and tail-end of the catalyst system under investigation. 2. Experimental Section 2.1. Materials. Three industrial hydroprocessing catalysts, in the form of extrudates, were acquired from a catalyst manufacturer. The three catalysts, which are labeled as catalysts A, B, and C, are used as one system in commercial ARDS units, as front-end, midsection, and tail-end catalysts, respectively. The major characteristics of the three catalysts, including their composition, are presented in Table 1. KU-AR was acquired from one of the refineries of the Kuwait National Petroleum Company. DM-AR and DMDS-AR were prepared by hydrotreating KU-AR, using a fixed-bed pilot plant. The main characteristics of KU-AR, DM-AR, and DMDS-AR feedstocks are presented in Table 2. 2.2. Experimental Techniques. 2.2.1. DM-AR Feedstock Preparation. DM-AR feedstock was prepared using a fixed-bed reactor system (manufactured by Vinci Technologies, France). The reactor has a total volume of 242 mL, with an internal diameter of 1.9 cm; 50 mL of catalyst A, diluted with an equal amount of carborundum, was charged into the midsection of the reactor. Coarse carborundum and inert R-alumina balls were placed above and below the catalyst bed. Thermocouples inserted into a thermowell at the center of the catalyst bed were used to monitor the reactor temperature at different points. After the system was loaded, it was purged with nitrogen at a flow rate of 100 L/h at 5 bar and 150 °C, and then hydrogen was introduced into the reactor. The catalyst was then presulfided with straight-run gas oil that contained 3 wt % dimethyl disulfide (DMDS), using a standard procedure.15 After the catalyst was presulfided, Kuwait atmospheric residue feed was introduced and the operating conditions were adjusted to obtain 60%-70% demetallized and 40%-50% desulfurized feedstock: H2 flow rate, 57 L/h; residue feed flow rate, 100 mL/h (liquid hourly space velocity (LHSV) ) 2 h-1; (15) Marafi, A.; Fukase, S.; Al-Marri, M.; Stanislaus, A. A Comparative Study of the Effect of Catalyst Type on Hydrotreating Kinetics of Kuwaiti Atmospheric Residue. Energy Fuels 2002, 17 (3), 661-668.

Performance on Industrial HDM, HDS, and HDS/HDN Table 2. Feedstock Characteristics property density @ 15 °C (g/mL) API gravity kinematic viscosity @ 50 °C (cSt) Conradson carbon residue, CCR (wt %) carbon content (wt %) hydrogen content (wt %) molar H/C total sulfur (wt %) total nitrogen (wt ppm) metals content (wt ppm) V Ni asphaltenes (wt %) Distillation Simdist ASTM D-2887 IBP 5% 10% 20% 30% 40% 50% FBP volume recovery (%)

Energy & Fuels, Vol. 17, No. 5, 2003 1193 Table 3. Operating Conditions for Pilot Plant Tests

KU-AR DM-AR DMDS-AR 0.9774 12.84 765.1 12.20

0.9619 15.61 208.5 8.48

0.9240 21.60 112.9 5.02

83.4 11.1 1.58 4.30 2740

85.5 12.0 1.68 2.30 2312

86.5 12.7 1.76 0.67 1540

69 21 3.78

21 12 2.52

15 8 0.90

237 335 372 419 454 488 522 738 57.1

200 302 341 391 427 459 493 736 63.7

158 279 325 380 419 455 485 736 67.7

H2/oil ) 570 mL/mL); pressure, 120 bar; and temperature, 390 °C. A total of 100 L of DM-AR was collected and thoroughly characterized, in accordance with Table 2. 2.2.2. DMDS-AR Feedstock Preparation. The DMDSAR feedstock was prepared using a four-reactor fixed-bed pilot plant (manufactured by ZETON Technology). The reactors are identical, with an internal diameter of 2.8 cm and a reactor volume of 658 mL. Two reactors were used in this experiment. The first reactor was charged with 175.5 mL of catalyst A and the second was charged with 214 mL of catalyst B. The catalysts were diluted with equal volumes of medium-grade carborundum and were placed in the midsection of each reactor. Coarse carborundum and inert R-alumina balls were placed above and below the catalyst beds. Thermocouples inserted into thermowells at the center of the catalyst beds were used to monitor the reactor temperature at different points. The system was started up and the catalysts were presulfided, in accordance with standard procedures.15 Kuwait atmospheric residue feed was introduced and the operation conditions were adjusted as follows: H2 flow rate, 85.7 L/h; feed flow rate, 126 mL/h (H2/oil ) 680 L/L; LHSV ) 0.32 h-1); temperature, 360-380 °C; and pressure, 120 bar. A total of 100 L of DMDS-AR was collected and thoroughly characterized, in accordance with Table 2. 2.2.3. Kinetic Studies of Catalysts B and C. The hydrotreating experiments were conducted in a fixed-bed reactor unit (manufactured by Vinci Technologies) in the up-flow mode, using AR, DM-AR, and DMDS-AR feedstocks. The reactor aspect ratio (L/d) is 58. Separate experiments were conducted with each feedstock using fresh catalyst, namely, B and C. For catalyst B, straight-run AR and DM-AR were used in the first and second experiments, respectively. Similarly, for catalyst C, two separate experiments, using straightrun AR and DMDS-AR, were conducted. A 50-mL volume of catalyst charge diluted with an equal amount of carborundum with 20 mesh size was used for each run. The catalyst dilution will be useful to minimize the wall effects, back mixing, and channeling in the reactor. The catalyst was presulfided with straight-run gas oil containing 3 wt % DMDS, using standard procedure.15 After the catalysts were presulfided, conditions for each run were adjusted to the desired operating temperature, pressure, hydrogen flow rate, and LHSV (Table 3). The catalyst activity declined slightly during the initial 48 h and then reached the steady state. The samples for analysis were taken under steady-state conditions. During the course of each run, product samples were collected every 12 h and analyzed

process parameters temperature (°C) pressure (bar) LHSV (h-1) H2/oil (mL/mL) time-on-stream (h)

LHSV effect study 380 120 0.5, 1.0, 2.0, 4,0, 8.0 680 120

for the presence of sulfur, metals (vanadium and nickel), nitrogen, asphaltenes, and Conradson carbon residue (CCR). Viscosity, density, and distillate yield analyses, using ASTM and/or IP standard methods for testing petroleum products, were also applied. 2.2.4. Feedstocks and Products Analysis. The petroleum feedstocks and products of the kinetic studies were analyzed using standard ASTM/IP methods. Sulfur content was determined using an Oxford model 3000 XRF instrument. The presence of metals was determined using a Varian model Liberty Series II ICP spectrophotometer. Nitrogen content was determined using an Antek analyzer (model 7000). Asphaltenes were analyzed using a Cosmo asphaltenes analyzer. Distillation and CCR tests were performed using standards methods D-1160 and D-189, respectively.

3. Results and Discussion The group of catalysts investigated in this study are typical industrial hydrotreating catalysts which are used as a system to process atmospheric residues (ARDS processes). Catalyst A is a front-end hydrodemetallization (HDM) catalyst with high capacity for the foulant metals, namely, vanadium and nickel, that are typically found in atmospheric residue. Catalyst B is a hydrodesulfurization (HDS) catalyst designed to be loaded in the midsection of an ARDS unit. In addition, the catalyst has moderate metal capacity and hydrogenation function. Under industrial conditions, catalyst B encounters a demetallized feedstock (typically similar to DM-AR), which is the product of the hydrotreatment of AR over catalyst A. Finally, catalyst C is an effective finishing HDS catalyst with strong hydrogenation and denitrogenation functions. This catalyst encounters, in a typical industrial situation, a partially demetallized/ desulfurized residue (similar to DMDS-AR) produced through the hydrotreatment of atmospheric residue over catalysts A and B. The three catalysts are typically used in ratios that will effectively achieve the targets of the ARDS process, in terms of product quality and life-onstream. For the purposes of the current study, this paper is focused on catalysts B and C. Results for catalyst A performance have been reported earlier.15 3.1. Performance of Catalysts B and C. The initial activities of catalysts B and C were first studied using Kuwait atmospheric residue; catalyst B then was tested with DM-AR, and catalyst C was tested with DMDSAR at 380 °C and different LHSVs. Conversions at 120 h for different reactions commonly occurring during the hydrotreatment of atmospheric residuesnamely, hydrodesulfurization (HDS), vanadium removal (HDV), nickel removal (HDNi), hydrodenitrogenation (HDN), asphaltenes reduction (HDAsph), and Conradson carbon reduction (HDCCR)swere determined. All the conversion data was collected under steady-state conditions during which the catalyst activity remained relatively stable. Figures 1 and 2 show the results of the performance of catalyst B for different reactions, as a function of

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Marafi et al.

Figure 1. Comparison of catalyst B activity for different reactions in the hydrotreatment of AR feed at different LHSVs.

Figure 3. Comparison of catalyst C activity for different reactions in the hydrotreatment of AR feed at different LHSVs.

Figure 2. Comparison of catalyst B activity for different reactions in the hydrotreatment of DM-AR feed at different LHSVs.

LHSV, using AR and DM-AR as feedstock. The reactivity toward the various reactions investigated is different for the two feedstocks. The results reveal that the catalyst’s activity for AR at low LHSV is in the following order:

HDS > HDV > HDNi > HDCCR >

HDAsph > HDN

diffusion into the catalyst more effectively and allow them to undergo hydrogenation and hydrocracking reactions. The trend for HDN somewhat parallels that of HDAsph, because the nitrogen content in residues is closely linked with asphaltenic components.16 Finally, CCR conversion is observed to be almost the same for both types of feedstocks. A study similar to that reported here for catalyst B was conducted on catalyst C using AR and DMDS-AR. The initial activity of the catalyst toward various hydrotreating reactions at 380 °C at different LHSVs was determined. Figures 3 and 4 show the conversions for different reactions as a function of the LHSV for AR and DMDS-AR, respectively. A comparison of the two figures shows that the catalyst’s activity for different reactions for the AR feedstock is in following order:

HDS > HDV > HDAsph > HDCCR ≈

whereas, for DM-AR, the following order is observed:

HDS > HDAsph > HDV > HDNi >

HDCCR > HDN

The results reveal that the catalyst has a strong HDS function for both feeds. Also, the HDM conversion for the DM-AR feedstock is lower, compared to the AR feedstock. This is attributed to the fact that DM-AR is already significantly demetallized, with a total metal content of ∼30% of the original metal content of AR. Furthermore, the remaining metal-containing species are probably more difficult to demetallize. The conversions for both HDN and HDAsph are significantly higher for DM-AR, compared to that of AR. A possible reason for this trend is that nitrogen compounds and asphaltenes were prehydrogenated in the demetallization stage (DM), which paved the way for higher conversion in the desulfurization stage (DS). The demetallization treatment apparently subjects the asphaltenic components to changes that permit their

HDNi > HDN

whereas its activity for various reactions using the DMDS-AR feedstock at low LHSV is in following order:

HDV ) HDS ) HDAsph > HDNi >

HDN ≈ HDCCR

The conversion percentages for all reactions, with the exception of HDAsph, were lower for DMDS-AR than those for AR. This trend can be attributed to the fact that DMDS-AR is already substantially hydrotreated with both catalysts A and B. Table 2 shows that DMDS-AR has very low sulfur, metals, nitrogen, CCR, and asphaltenes contents. Furthermore, it is expected that the remaining heteroatom species are more refractory and more difficult to hydrotreat than the original species existing in AR. The conversion percentage for (16) Absi Halabi, M.; Stanislaus, A.; Owaysi, F.; Khan, Z. H.; Diab, S. Hydroprocessing of Heavy Residues: Relation between Operating Temperature, Asphaltenes Conversion, and Coke Formation. Stud. Surf. Sci. Catal. 1989, 53, 201-212.

Performance on Industrial HDM, HDS, and HDS/HDN

Energy & Fuels, Vol. 17, No. 5, 2003 1195

Figure 5. Second-order kinetics for the HDS reaction in the hydrotreatment of AR and DM-AR feeds over industrial catalyst B.

Figure 4. Comparison of catalyst C activity for different reactions in the hydrotreatment of DMDS-AR feed at different LHSVs. Table 4. Reaction Order for the Hydrotreatment Reactions for Catalysts B and C Catalyst B

Catalyst C

reaction

AR feed

DM-AR feed

AR feed

DMDS-AR

HDS HDV HDNi HDAsph HDCCR HDN

2 2 2 2 2 1

2 2 2 2 2 1

2 2 2 2 2 1.5

1.5 2 2 2 2 1

HDAsph was almost the same for both feedstocks. This can be attributed to the fact that the asphaltenic species in DMDS-AR are probably partially hydrogenated, which makes them more reactive toward further hydrocracking. 3.2. Kinetic Parameters of Hydrotreating Reactions of Catalysts B and C. The kineticssnamely, the reaction order and rate constantssfor the different hydrotreatment reactions over catalysts B and C were investigated. The reaction order and rate constant were calculated in this study using an approach that was similar to that given in an earlier paper.15 Table 4 presents the results of the observed reaction order for both catalysts. For catalyst B, second-order kinetics using both types of feedstocks were observed for all reactions except HDN, which was found to be first order. Figures 5 and 6 show typical second-order plots obtained for catalyst B for the HDS and HDAsph reactions. The data for catalyst C show that the orders of the reactions for both feedstocks are 2, except those for the HDS reaction using DMDS-AR, which was found to be 1.5, and those for the HDN reactions for both AR and DMDS-AR, which were observed to be 1.5 and 1, respectively. The available information in the literature indicates that the apparent reaction orders for hydrotreating reactions (e.g., HDS, HDN, HDM, HDAsph) can vary between 1 and 2, depending on the properties and composition of feedstocks. Thus, co-workers reported

Figure 6. Second-order kinetics for the HDAsph reaction in the hydrotreatment of AR and DM-AR feeds over industrial catalyst B.

first-order kinetics for HDS of residues in trickle bed reactors,17,18 whereas several others found second-order kinetics.19-22 The reaction orders for sulfur removal in asphaltenic and nonasphaltenic fractions of residual oil were found to be 3 and 2, respectively.23 Recent studies confirmed second-order rate behavior for HDS of Kuwait atmospheric residue.24 For vanadium and nickel removal, conflicting results have been reported in the literature. Studies showed first-order kinetics for both vanadium and nickel removal during the hydrotreatment of atmospheric residues.25-28 On the other hand, investigations reported kinetic orders of 1.0 and 1.5 for vanadium removal, depending on reactor configuration.29 Several others found second-order behavior for both vanadium and nickel removal from residues.19,24,30-32 In a recent study with Maya residue, first-order kinetics for vanadium removal and second-order kinetics for nickel removal were found.33 (17) Arey, W. F.; Blackwell, N. E., III; Reichle, A. D. Proceedings of the 7th World Petroleum Congress; 1967; Vol. 4, pp 167-176. (18) Paraskos, J. A.; Frayer, J. A.; Shah, Y. T. Ind. Eng. Chem. Process Des. Dev. 1975, 4, 315. (19) Beuther, H.; Schmid, B. K. Reaction Mechanisms and Rates in Residue Hydrodesulfurization. Proceedings of the 6th World Petroleum Congress; 1963; Vol. 3, pp 297-310. (20) De Bruijn, A.; Naka, I.; Sonnemans, J. W. M. Effect of Noncylindrical Shape Extrudates on the Hydrodesulfurization of Oil Fractions. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 40-46. (21) Scamangas, A.; Papayannakos, N.; Marangozis, J. Catalytic Hydrodesulfurization of Petroleum Residue. Chem. Eng. Sci. 1982, 37, 1810-1818. (22) Papayannakos, N.; Marongozis, J. Kinetics of Catalytic Hydroprocessing of Petroleum Residue in a Batch Recycle Trickle Bed Reactor. Chem. Eng. Sci. 1984, 39, 1051-1061. (23) Philippopoulos, C.; Papayannakos, N. Intra Particle Diffusional Effects and Kinetics of Desulfurization Reactions and Asphaltenes Cracking during Catalytic Hydrotreatment of a Residue. Ind. Eng. Chem. Res. 1988, 27, 415-420. (24) Chen, Y. W.; Hsu; W. C.; Lin, L. S.; Kang, B. C.; Wu, S. T.; Leu, L. J.; Wu, J. C. Hydrodesulfurization Reactions of Residual Oils over Co-Mo/Alumina-Aluminium Phosphate Catalysts in a TrickleBed Reactor. Ind. Eng. Chem. Res. 1990, 29, 1830-1840.

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For asphaltenes conversion, information on the reaction kinetics is very limited. A study on the kinetics of asphaltenes cracking in a trickle bed reactor using two types of Co-Mo/alumina catalysts found second-order kinetics for both catalysts.23 A similar second-order dependence for the decomposition of the overall asphaltenes was also reported.28,34 For hydrodenitrogenation, the apparent reaction order was reported to be 1, with respect to total nitrogen content.34-36 However, in a recent study on the hydroprocessing of Maya residue, a 0.5 order was found for the HDN reaction, with respect to total nitrogen content and asphaltenic nitrogen content.37,38 Such variations in reaction orders were explained on the basis of kinetic aggregation theory.39,40 In accordance with this theory, the mixture of components in an oil feed is treated as a continuum with a distribution function to characterize rates. Despite the fact that the hydrotreatment reactions of individual components are first order, reaction orders up to 2 would arise if fractions of the feed are essentially unreactive, or if the concentration-reaction-rate distribution followed an exponential distribution. Mathematically, it was shown that the rate law for the hydrotreatment reactions will follow the following power law:

dC ) R(C) ) -kCn f dt

(1)

where n ) 1 + (1/γ). A value of γ ) 1 signifies a feed (25) Riley, K. L. The Effect of Catalyst Properties on Heavy Feed Hydroprocessing. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1978, 23, 1104. (26) Dautzenberg, F. M.; Klinken, J. V.; Pronk, K. M. A.; Sie, S. T.; Wijffels, J. B. Catalyst Deactivation through Pore Mouth Plugging. ACS Symp. Ser. 1978, 65, 254-265. (27) Pazos, J. M.; Aquino, L.; Pachano, J. Upgrading of High Metals Venezuelan Residues. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1981, 26, 456. (28) Shimura, M.; Shiroto, Y.; Takeuchi, C. Effect of Catalyst Pore Structure on Hydrotreating of Heavy Oils. Ind. Eng. Chem. Fundam. 1986, 25, 330-337. (29) Van Dongen, R. H.; Bode, D.; Van der Eijk, H.; Van Klinken, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 630-635. (30) Mosby, J. F.; Hoekstra, G. B.; Kleinherz, T. A.; Sroka, J. M. Pilot Plant Proves Resid Process. Hydrocarbon Process. 1973, 52 (5), 93-97. (31) Oleck, S. M.; Sherry, H. S. Freshwater Manganese Nodules as Catalyst for Demetalation and Desulfurization of Petroleum Residua. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 525-528. (32) Stanislaus, A.; Fukase, S.; Koide, R.; Al-Barood, A.; Marafi, A.; Jasem, F.; Absi-Halabi, M. Pilot Plant Study on the Performance of an Industrial HDM (MoO3/Al2O2) Catalyst in Hydrotreating of Kuwaiti Atmospheric Residue. Presented at The 218th Fall Meeting of the American Chemical Society, August 1999, New Orleans, LA. (33) Martinez, M. T.; Callejas, M. A.; Carbo, E.; Harnandez, A. In Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Froment, G. F., Waugh, K. C., Eds.; Elsevier: Amsterdam, 1997; pp 565-570. (34) Marafi, A.; Stanislaus, A.; Fukase, S.; Koide, R.; Al-Baroud, A.; Al-Bazaz, H.; Absi-Halabi, M. An Experimental Pilot Plant Study on the Performance Comparison of Industrial Mo/Al2O2 and Ni-Mo/Al2O2 Catalysts on Hydrotreating Kuwaiti Atmospheric Residue. Presented at The American Institute of Chemical Engineers (AIChE) Symposium on Advances in Hydroprocessing, March 2000, Atlanta, GA. (35) Ho, T. C. Hydrodenitrogenation Catalysis. Catal. Rev. Sci. Eng. 1988, 30, 117-160. (36) Christensen, H.; Cooper, B. H. The Influence of Catalyst and Feedstock Properties in FCC Feed Pretreatment. Presented at the American Institute of Chemical Engineers (AIChE) Spring National Meeting, 1990, Paper 44b. (37) Callejas, M. A.; Martinez, M. T. Hydroprocessing of a Maya Residue. Intrinsic Kinetics of Sulfur, Nitrogen, Nickel, and Vanadium Removal Reactions. Energy Fuels 1999, 13, 629-636. (38) Callejas, M. A.; Martinez, M. T. Hydroprocessing of a Maya Residue. Intrinsic Kinetics of the Asphaltenic Heteroatom and Metal Removal Reactions. Energy Fuels 2000, 14, 1309-1313.

that comprises a distribution with relatively unreactive species, i.e., a tough feed. The smaller the γ value, the more refractory the feed. Thus, the orders observed in this study are probably caused by the differences in the concentration and reactivity of the sulfur, nitrogen, organometallic species, and asphaltenic materials of the feeds. For Kuwait atmospheric residue hydrotreatment, earlier studies have shown second-order kinetics for HDS and HDM reactions.24 The results obtained in this study are in agreement with this observation. In addition, the present study also shows that a second-order power-rate-law equation could still be applied to describe the kinetics of contaminant-removal reactions from the residue, even after partial hydrotreatment. Despite the differences between the properties of AR, DM-AR, and DMDS-AR, the same kinetic orders for most of the hydrotreatment reactions are observed for the three types of feed. It seems that, despite the lower values for sulfur, vanadium, nickel, asphaltenes, nitrogen, and CCR content in DM-AR (in comparison with AR), the range of related species and their reactivities is still very wide, which leads to apparent orders of 2 for most reactions. The reduction of the apparent reaction order of HDS from 2 for AR to 1.5 for DMDSAR for catalyst C reflects the relatively narrower range of sulfur compounds remaining in DMDS-AR. A similar argument can also be presented for the change in the apparent reaction order for the HDN reaction. The observed rate constants and the corresponding ratios of the rate constants for the hydrotreating reactions are presented in Table 5. For catalyst B, the results show that the rate constants for HDS, HDV, HDAsph, and to some extent, HDN, using DM-AR as feedstock, are generally higher than those using AR. The ratios (kDM-AR/kAR) for various reactions have the following order:

HDAsph > HDS ≈ HDV > HDCCR ≈

HDN > HDNi

This order reflects the high hydrogenation function of catalyst B, which became evident when DM-AR was used as feedstock. The differences observed regarding the reactivity and the kinetics of both feeds over catalyst B are inherently related to the differences of the properties of the feedstocks (Table 2) and the changes in molecular composition that took place when AR was treated over catalyst A, with the removal of vanadium being the most prominent change. However, the residue has undergone partial conversion, as manifested by the decrease in density, the increase in viscosity, and the results of distillation. The residue also underwent some hydrogenation, as seen from the increase in the H/C ratio. The total sulfur, asphaltenes, and CCR contents have also decreased. The rate constants of various reactions, in regard to hydrotreatment of the DMDS-AR and AR feedstocks over catalyst C, and the ratios (kDMDS-AR/kAR) are also presented in Table 5. The ratios for HDS and HDN could not be calculated, because of changes in the reaction (39) Ho, T. C.; Aris, R. On Apparent Second-Order Kinetics. AICHE J. 1987, 33, 1050-1051. (40) Ho, T. C. Hydroprocessing Kinetics for Oil Fractions. Stud. Surf. Sci. Catal. 1999, 127, 179-186.

Performance on Industrial HDM, HDS, and HDS/HDN

Energy & Fuels, Vol. 17, No. 5, 2003 1197

Table 5. Rate Constants of Different Reactions on Hydrotreating Different Feedstocks over Catalysts B and C, and the Corresponding Rate Constants and Ratiosa feedstock

HDS (1/(wt %‚h)-1)

HDV (1/(ppm‚h)-1)

0.711 1.420 2.00

0.031 0.063 2.03

1.076 2.239d (1.420)e 2.08

AR DM-AR kDM-AR/kAR AR DMDS-AR kDMDS-AR/kAR h-1

HDNi (1/(ppm‚h)-1)

HDAsph (1/(wt %‚h)-1)

HDCCR (1/(wt %‚h)-1)

HDN (1/h)

Catalyst B 0.077 0.076 0.99

0.155 0.763 4.92

0.063 0.079 1.25

0.210 0.290 1.38

0.027

Catalyst C 0.050

0.291

0.095

0.048

0.057

1.250

0.075

0.787b (0.018)c 0.290

1.15

4.32

0.87

1.8

0.37

For LHSV ) 1.0 and a temperature of 380 °C. Assumed first-order rate constant calculated for comparison. Rate constant for 1.5 order (best fit). d Assumed second-order rate constant calculated for comparison. e Rate constant for 1.5 order (best fit). a

b

order. Hence, the rate constants were also estimated by forcing the HDS and HDN data to fit second-order and first-order kinetics, respectively. These rate constants are also presented in Table 5. The results show that the rate constants in the case of the DMDS-AR, compared to those for AR, were higher for HDS, HDV, and HDAsph, whereas they were practically unchanged for HDNi and HDCCR and significantly lower for HDN. The ratios were observed to have the following order:

HDAsph > HDS ≈ HDV > HDCCR ≈

HDNi > HDN

The differences between the kinetics of the hydrotreatment reactions for the two types of feedstocks over catalyst C are also attributed to the differences in the properties of the feedstocks (Table 2). DMDS-AR is a residual oil that has already been processed over catalysts A and B; hence, its sulfur, metals, asphaltenes, CCR, and nitrogen contents are significantly reduced. The remaining components that are further hydrotreated using catalyst C are probably more refractory. This makes their further reaction with the third-stage catalyst more difficult. Furthermore, the density and viscosity of DMDS-AR are lower, and its H/C ratio is higher, than that for AR, which indicates that significant hydrogenation and hydrocracking have taken place. The kinetic order for the reactions HDV, HDNi, HDAsph, and HDCCR are not different for both feeds. Although most of the heteroatom-containing species have been removed in the treated feedstock, it may still contain a wide distribution of unconverted least-reactive species and result in an overall reaction order of 2. Differences in the nature and reactivity of various types of sulfur, metal, and asphaltenic species in the two feeds may result in different rate-limiting steps and adsorption energies, which, in turn, may influence the temperature effect on the reaction rate and, consequently, the activation energies. This aspect is currently under investigation.

c

4. Conclusions The feedstock used in the evaluation of the performance of residue hydrotreating catalysts that are used as components of a graded catalyst system strongly impact the results of such an evaluation. The residualoil feedstock entering a graded system undergoes gradual changes in composition as it passes from the front-end to tail-end catalysts. In studies related to fixed-bed residual-oil hydrotreating catalyst evaluation, straightrun residual oil is commonly used as feedstock, irrespective of the order in which the catalyst is loaded within a graded catalyst system. The results of such studies are utilized in screening and selecting catalysts, and in developing models for predicting the performance of catalyst systems in industrial units. The results of the present study demonstrate that using straight-run residual oil in such evaluation studies for catalysts that are designed to be loaded toward the tail-end of a graded catalyst system will actually provide erroneous data. The errors include (1) the reactivity pattern of tail-end catalysts toward different hydrotreatment reactions and (2) the kinetic parameters (namely, reactions orders and rate constants). These errors may significantly mask the actual effectiveness of a catalyst toward some important reactions, such as the removal of asphaltenes. Therefore, in developing new catalysts and evaluating their performance, we recommend that feedstocks that are similar to the actual feedstock encountered by a catalyst in an ARDS unit be used. This will lead to a more systematic and accurate approach in defining the actual activity of the catalyst, and, eventually, to a catalyst combination that is better designed for a specific application. Acknowledgment. This work was performed under a joint research project between the Japan Cooperation Center, Petroleum (JCCP), which is funded by the Ministry of Economy Trade and Industry (METI), Japan, and the Kuwait Institute for Scientific Research, Kuwait (KISR). EF020282J