Hydroprocessing of a Maya Residue. 1. Intrinsic Kinetics of

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Energy & Fuels 2000, 14, 1304-1308

Hydroprocessing of a Maya Residue. 1. Intrinsic Kinetics of Asphaltene Removal Reactions Marı´a A. Callejas and Marı´a T. Martı´nez* Instituto de Carboquı´mica CSIC, Apartado 589, 50080 Zaragoza. Spain Received June 12, 2000. Revised Manuscript Received September 18, 2000

The intrinsic kinetics of hydrocatalytic conversion of the asphaltenic fraction from a Maya residue is reported. The kinetic study was carried out at high temperatures (375-415 °C) and hydrogen pressure, 12.5 MPa, in a perfectly mixed reactor in continuos operation. Asphaltenic fraction concentration data fit half-order removal kinetics, the activation energy being 41.5 kcal/ mol. The relationships existing between the deasphalting and important reactions which occur in the hydroprocessing (fraction over 343 °C conversion, desulfuration, denitrogenation, and demetalation) were studied.

Introduction The tensions created by the 1973 and 1979 crude oil crisis and the Gulf War in 1991, which deeply marked the international economic scene and the structure of energy markets, together with the warnings of a foreseen shortage of petroleum resources are provoking the drop of crude oil in the world energy consumption to the benefit of all other energy sources but mainly natural gas, nuclear power, and to a lesser extent coal and hydroelectric power. Nevertheless, hydrocarbons will go on to play an important role in energy consumption by mankind because of their attractive physicochemical properties, their ease of use, and the large capitalistic investments already granted for their valorization. It is certain that, in the history of the petroleum industry, refineries have been endeavoring to produce light and heavy oil products according to their demands from the available barrels of various crude oils, but the next century will be marked by the necessity of producing much less or even no heavy fuel oils. The U.S. Department of Energy1 has assessed that, for the world crude oil supply from 1900 to 2100, extra heavy crude and bitumen will be used as significant portion of the total consumption (40 Mbbl/day) near the year 2060. In that prospect, refining will have to be fully equipped with deep heavy fractions conversion capacity. Heavy residual oils are hard feedstock according to their high content of very complex compounds so-called resins and asphaltenes and to their chemical structure characterized by an high C/H atomic ratio. It is well-known that asphaltenes are, by definition, a solubility class of material that precipitates from the oil to form a solid when the oil is treated with a selected solvent. Asphaltenes are thought to be a complex mixture of high-boiling, polar, aromatic, high-molecular* Author for correspondence. Address: Instituto de Carboquı´mica, P.O. Box 589, Zaragoza, Spain. Fax: 34976733318. E-mail: [email protected]. (1) Yen, T. F. Prepr.-Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 76-79.

weight compounds that, when precipitated from the oil, contain a disproportionate amount of impurities, including the heteroatoms and nickel and vanadium compounds. Asphaltenes are held in petroleum in a delicate balance and this balance can be easily upset. As a result the asphaltenes might precipitate and cause fouling and coking, the process is initiated by a hydroperoxide induced oxidation followed by acid-catalyzed condensation reactions involving specific organo-nitrogen and organo-sulfur compounds.2 During the hydroprocessing, the asphaltenic fraction will limit the efficiency of the refining process3-5 because asphaltenes adversely affect the overall rate of hydrodesulfurization,6,7 act as coke precursors which in turn lead to catalyst deactivation,8 may limit the maximum level of conversion achievable in hydrocracking process due to sludge formation,9 and are also responsible for the high viscosity of residue.10 Since the asphaltenes are the most difficult component in petroleum to process and make catalytic hydrotreating very difficult, studies of petroleum asphaltenes for a better understanding of their properties and changes during processing have rapidly increased during the past years. Although a significant amount of research has been carried out to elucidate the structural characteristics (2) Mushrush, G. W.; Speight, J. G.; Beal, E. J.; Hardy; D. R. Prepr.Am. Chem. Soc., Div. Pet. Chem. 1999, 44, 175-177. (3) Sheu, E. Y.; Detar, M. M.; Storm, D. A.; De Canio, S. J. Fuel 1992, 71, 299-302. (4) Absi-Halabi, M.; Stanislaus, A.; Trimm, D. L. Appl. Catal. 1991, 72, 193-215. (5) Phillips, C. R.; Haidar, N. I.; Poon, Y. C. Fuel 1985, 64, 678-691. (6) Speight, J. G. Fuel Science and Technology Handbook; Marcel Dekker: New York, 1990; p 143. (7) Berti, V.; Iannibello, A. Idrodesolforazione di residui di petrolio; Stazione Sperimentale per i Combustibili: San Donato Milanese, Italy, 1975; pp 42-62. (8) Bartholomew, C. H. In Catalytic Hydroprocessing of Petroleum and Distillates; Oballa, M. C., Shih, S. S., Eds.; Marcel Dekker: New York, 1994; pp 1-32. (9) Miyauchi, Y.; de Wind, M. Proc. Akzo Nobel Catal. Symp. 1994; Hydroprocessing 1994, 123-140. (10) Speight, J. G. The Desulfurization of Heavy Oils and Residue; Marcel Dekker: New York, 1981; pp 145-170.

10.1021/ef000126h CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000

Hydroprocessing of a Maya Residue. 1

of asphaltenes,11-13 there has been less investigation of the reaction kinetics related to their conversion. Because of their complex nature, asphaltenes undergo a multitude of reactions under the conditions used in upgrading processes. The actual chemistry of these reactions is largely unknown. Only for pyrolysis of asphaltene model compounds (alkyl-substituted aromatic and naphtenic hydrocarbons) have the exact reaction pathways and kinetic data of these freeradical reactions been determined.14-22 The reactivity of asphaltenes isolated from petroleum and tar-sand bitumen has so far mainly been investigated for thermal cracking (coking) under inert gases at normal pressure or elevated pressure in autoclaves.23-30 Papers on the reactivity of asphaltenes in cracking under hydrogen (with or without catalysts) have been published for asphaltenes from coal residues28-30 but for petroleum asphaltenes only in basic approaches.31,32 The need for a deeper knowledge of the behavior of this class of substances in a catalytic process has led us to carry out a kinetic study on the asphaltene conversion in this paper. Additionally, the relationships existing among the deasphalting and the heteroatom and metal removal and the hydrocracking have been reported. Experimental Section A Maya heavy residue with a content of asphaltenes of 8.6 wt %, 11.22 wt % of Conradson carbon residue, high content of metals (287 ppm of Ni + V), 3.45 wt % of sulfur, and 0.28 wt % of nitrogen has been catalytically hydroprocessed. The reaction unit, consisting of a continuos stirred tank reactor (CSTR), has a nominal capacity of one liter and it possesses a static basket with a catalyst volume of 185 cm3. The diagram of the pilot plant unit used for experiments has been described in a previous paper.33 (11) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225-230. (12) Trauth, D. M.; Stark, S. M.; Petti, T. F.; Neurock, M.; Yasar, M.; Klein, M. T. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 1993, 38, 434-439. (13) Storm, D. A.; Sheu, E. Y.; Detar, M. M.; Barresi, R. J. Energy Fuels 1994, 8, 567-569. (14) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987, 26, 374376. (15) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987, 26, 488494. (16) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1988, 27, 1348-1356. (17) Savage, P. E.; Klein, M. T. Chem. Eng. Sci. 1989, 44, 393-404. (18) Savage, P. E. Chem. Eng. Sci. 1990, 45, 859-873. (19) Freund H.; Olmstead, W. N. Int. J. Chem. Kinet. 1989, 21, 561574. (20) Smith, C. M.; Savage, P. E. AIChE J. 1991, 37, 1613-1624. (21) Savage, P. E.; Klein, M. T.; Kukes, S. G. Prepr.-Am. Chem. Soc., Div. Fuel Chem. 1985, 30, 408-419. (22) Savage, P. E.; Klein, M. T.; Kukes, S. G. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1169-1174. (23) Speight, J. G. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 1987, 32, 413-418. (24) Schucker, R. C.; Keweshan, C. F. Am. Chem. Soc. Div. Fuel Chem. Preprints 1980, 25, 155-165. (25) Nomura, M.; Terao, K.; Kikkawa, S. Fuel 1981, 60, 699-711. (26) Soodhoo, K.; Phillips, C. R. Fuel 1988, 67, 361-374. (27) Soodhoo, K.; Phillips, C. R. Fuel 1988, 67, 521-529. (28) Martı´nez, M. T.; Benito, A. M.; Callejas, M. A. Fuel 1997, 76, 871-877. (29) Martı´nez, M. T.; Benito, A. M.; Callejas, M. A. Fuel 1997, 76, 899-905. (30) Benito, A. M.; Callejas, M. A.; Martı´nez, M. T. Fuel 1997, 76, 907-911. (31) Sebor, G.; Weisser, O.; Ha´jek, M. Chem. Technol. 1981, 33, 362365. (32) Le Page, J. F.; Morel, F.; Tra´ssard, A. M.; Bousquet, J. Prepr.Am. Chem. Soc., Div. Pet. Chem. 1987, 32, 470-476.

Energy & Fuels, Vol. 14, No. 6, 2000 1305 Table 1. Asphaltenes Content of the Feedstock and of the Liquid Products from the Kinetic Experiments T/WHSV °C/(L/h gcat)

asphaltenes (wt %)

T/WHSV °C/(L/h gcat)

asphaltenes (wt %)

feed 375/2.9 375/4.8 375/6.2 400/2.3

8.6 3.4 4.7 5.2 0.8

400/4.2 400/6.2 415/3.3 415/3.7 415/7.1

1.8 3.5 0.4 0.4 1.1

Experiments have been carried out at 12.5 MPa of hydrogen pressure and the investigated ranges of temperature and weight hourly space velocity, WHSV, have been as follows: 375 to 415 °C and 2.3 to 7.1 L h-1 g-1cat, respectively. The commercial catalyst used, Topsoe TK-711, is of Ni/Mo type, alumina supported. Other catalyst properties have been reported elsewhere.33 After two diagnostic tests,34 3500 rpm stirring speed and 10 000 std ft3/bbl gas/liquid ratio have been used for working in absence of interphase gradients and for minimizing the intraparticle gradients, the catalyst has been crushed at a range of particle size between 53 and 530 µm. After 180 h of running the hydroprocessing unit, steadystate conditions for the catalyst have been reached,34 and at this point, kinetic runs have been started. The feedstock and the product oils were extracted with toluene, subsequently, the toluene-solubles were extracted with n-hexane and separated into oils (soluble) and asphaltenes (insoluble).

Results and Discussion The petroleum residuum hydroprocessed in these experiments was of a heavy nature, having high kinematic viscosity (111.5 cSt at 50 °C) and high percentage of products boiling >343 °C (81.5 vol %). The solvent separation has allowed us to obtain the content of the asphaltenes in the feedstock and in the oil products obtained from the kinetic runs. These values are shown in the Table 1. As observed in the table, a severe decomposition of the asphaltene structure has occurred at the range of temperatures used, being most pronounced at the highest temperatures, 400 and 415 °C. The asphaltene conversion has also increased as the space velocity has decreased. Therefore, the highest asphaltene conversion, 97.7%, has been obtained at 415 °C and 3.3 L h-1 gcat-1 of weight hourly space velocity, it is to say, at the most severe working conditions. The mechanisms of the asphaltene conversion are complex because different physical and chemical processes contribute to the removal of those elements. Initial theories of the thermal decomposition of asphaltenes invoked the concept of sequential reactions but more recent thought postulated that the thermal decomposition of asphaltenes involves several simultaneous reactions.35 The most important reactions which are produced in the thermal hydrocracking of the asphaltenes are (a) breakage of C-C and C-H bonds, (b) breakage of bonds with metals and heteroatoms, (c) aromatization, (d) (33) Trasobares, S.; Callejas, M. A.; Benito, A. M.; Martı´nez, M. T.; Severı´n, D.; Brouwer, L. Ind. Eng. Chem. Res. 1998, 37, 11-17. (34) Martı´nez, M. T.; Callejas, M. A.; Carbo´, E.; Herna´ndez, A. Stud. Surf. Sci. Catal. 1997, 109, 565-570. (35) Speight, J. G. In Asphaltenes and Asphalts, 1; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, 1994; p 7.

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alkylation, (e) condensation, and (f) hydrogenationdehydrogenation. Recently, studies on asphaltene hydrocracking of various petroleum residues have indicated that during hydrocracking the hydrogenation and dealkylation of condensed polyaromatic rings are almost entirely suppressed.36 The main reactions are the destruction of asphaltene micelles caused by the removal of vanadium and the depolymerization of asphaltene molecules by removal of heteroatoms such as sulfur.37-39 The latter is a partial rupture of weakly linked bonds between the polyaromatic rings. Contribution of these two reactions to the asphaltene hydrocracking depends on the kind of feedstocks. For example, the destruction of asphaltene micelles is considered to be particularly pronounced in the asphaltene having a high vanadium content, whereas depolymerization is the principal cause of the changes in the asphaltenes produced from a feed which has a low vanadium content. Although each process is governed by its own kinetics, the total process can be represented by the proposed simple model in which asphaltenes are cracked to form oils, gases, coke, etc.:

Figure 1. Half-order kinetic plot for asphaltene conversion reactions.

asphaltenes + H2 f products (oils, gases, coke, etc.) By applying the rate equation corresponding to a system in continuos operation and assuming an heterogeneous perfectly mixed system in which steady-state conditions have been reached, eq 1, a half-order kinetics has provided an excellent fit for the asphaltene removal from the oil products obtained at 375, 400, and 415 °C.

Co - C )

KCn WHSV

(1)

Co being the initial concentration, C the outlet concentration, WHSV the weight hourly space velocity, n the kinetic order with respect to the asphaltene concentration, and K an intrinsic rate constant (our experiments have been carried out in absence of all type of gradients). By plotting (Co - C)/C0.5 versus 1/WHSV, the rate constants at 375, 400, and 415 °C can be calculated. Figure 1 illustrates half-order kinetic plot at the three temperatures studied. The activation energy was calculated from a semilogarithmic plot of ln K versus 1/T, Figure 2, using the Arrhenius equation

K ) Ko e(-Ea/RT)

(2)

Ko being the preexponential factor, Ea the activation energy, R the molar gas constant, and T the absolute temperature (K). The values of the rate constants and activation energy obtained are detailed in Table 2. High correlation coefficients were found for all temperatures. (36) Zou, R.; Liu, L. In Asphaltenes and Asphalts, 1; Yen, T. F., Chilingarian, G. V., Eds., Elsevier Science: Amsterdam, 1994; p 349. (37) Takeuchi, C.; Fukui, Y.; Nakamura, M.; Shiroto, Y. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 236-242. (38) Asaoka, S.; Nakata, S.; Shiroto, Y.; Takeuchi, C. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 242-248. (39) Shiroto, Y.; Nakata, S.; Fukui, Y.; Takeuchi, C. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 248-257.

Figure 2. Arrhenius plot for the half-order rate constants of deasphalting reactions.

Magaril and Aksenova40 proposed that the asphaltene thermal decomposition is a chain reaction of 1.5 order. Schueker41 used a simple first-order kinetic model to fit the data of asphaltenes nevertheless, Nigan42 reported that the overall asphaltene decomposition followed an approximately second-order dependence at low to moderately high conversion (