Energy & Fuels 2000, 14, 1309-1313
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Hydroprocessing of a Maya Residue. II. Intrinsic Kinetics of the Asphaltenic Heteroatom and Metal 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 asphaltenic metal and heteroatom removal reactions are reported. The asphaltenic sulfur, nitrogen, nickel, and vanadium were determined in isolated asphaltenes from raw material and products. The kinetic study was carried out at high temperatures (375415 °C) and hydrogen pressure, 12.5 MPa, in a perfectly mixed reactor in continuous operation. Concentration data fit half-order kinetics with respect to asphaltenic sulfur, nitrogen, and metal (Ni and V) concentration, the activation energies being 35.6, 39.3, 47.7, and 45.3 kcal/mol, respectively.
Introduction Crude oils and extracts from source rocks are an extremely complex mixture and exhibit wide variations in composition and property. Presently the source of supply of crude oil has become less stable and economics has dictated new thinking to process “the bottom of the barrel”. As a result, the analysis of heavy oils and residues has become increasingly important as crude oil stock sources are becoming heavier and large quantities of high boiling materials are to be processed to derive clean fuels, low boiling products, feedstocks for petrochemicals and downstream industries. Optimum conversion of low-value petroleum heavy residues to high-value products requires adequate compositional information to understand the chemistry of conversion reactions involved in the treatment of heavy oils. Asphaltenes, the heptane-insoluble heavy fraction of crude oils, constitute one of the primary components of crude oil and, as such, are of considerable interest.1-3 Asphaltenes provide very low cracking yields and are of low economic value, they are relatively high in undesired heteroatoms (S, O, and N),4,5 and they contain heavy metals such as Ni and V,6 which can poison catalysts. They have been the subject of wide-ranging studies to elucidate their chemical structure but these studies have been only partially successful, primarily because the asphaltenes exhibit significant complexity and some variability. * Author to whom correspondence should be addressed at Instituto de Carboquı´mica, P.O. Box 589, Zaragoza, Spain. Fax: 34976733318. E-mail:
[email protected]. (1) Chilingarian, G. V.; Yen, T. F. Bitumens, Asphalts and Tar Sands; Elsevier Scientific Publishing Co.: New York, 1978. (2) The Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, DC, 1981. (3) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker Inc.: New York, 1980. (4) Strauz, O. P.; Mojelski, T. W.; Lown, E. M. Fuel 1992, 71, 1355. (5) Waldo, G. S.; Carlson, R. M. K.; Moldowan, J. M.; Peters, K. E.; Penner-Hahn, J. E. Geochim. Cosmochim. Acta 1991, 55, 801. (6) Fish, R. H.; Komlenic, J. J.; Wines, B. K. Anal. Chem. 1984, 56, 2452.
Asphaltenes are a dark brown to black friable, infusible solid component of crude oil. They are characterized1-3 by a hydrogen/carbon ratio of ∼ 1.1. The hydrogen atoms are contained in saturated groups whereas ∼40% of the carbon is contained in aromatic structures.1-3 Generally, asphaltenes contain a greater concentration of heteroatoms1-3 and metals7 (nickel and vanadium) than the original oil. Nickel and vanadium in petroleum exist as soluble organometallic complexes that fall into two categories: metal porphyrins and nonporphyrin metal complexes. Both the porphyrins and the nonporphyrins may be distributed over a wide boiling range (350-650 °C+), reflecting significant variations in molecular weight, structure, and polarity. Metal porphyrins and nonporphyrin metal complexes tend to precipitate as part of the asphaltene material to an extent that varies with the source of the crude oil. Porphyrin and nonporphyrin metals associated with asphaltenes have not been easy to identify in terms of molecular structure. This is partly due to the fact that the characteristics (i.e., spectra) of all possible model nonporphyrin compounds have not been studied. Porphyrins with increased aromaticity and systems with low aromaticity due to discontinued ring conjugation are both characterized as nonporphyrin species. These compounds do not have the characteristic visible absorption spectra and hence are not readily identified. It is also possible that some of the porphyrin in the residuum may not be extracted and identified due to intermolecular association with the asphaltene-generating molecules. Heteroatoms, which occur in polar and even charged groups, have an impact on solubility greater than merely their mole fraction. Because asphaltenes are defined by their solubility characteristics, it is important to understand the heteroatom chemistry. X-ray absorption near-edge structure (XANES) spectroscopic methods, which are nondestructive and direct, (7) Reynolds, J. G. Liquid Fuels Technol. 1985, 3, 73.
10.1021/ef000127+ CCC: $19.00 © 2000 American Chemical Society Published on Web 10/24/2000
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have been very productive in analyzing the heteroatom structures of complex fossil-fuel samples. XANES spectroscopy has been successfully employed to probe the chemical nature of sulfur in different fossil-fuel components8 such as asphaltenes,8-10 crude oils,5,8-12 and coal.8,13,14 In asphaltenes, sulfur occurs as mostly thiophenic (aromatic) and sulfidic (saturated) forms. The oxidized sulfur component in asphaltenes is sulfoxide and appears to have resulted from oxidation of sulfides present in crude oils.8 Generally, the asphaltene sulfoxide fractions are low, 10% or less. In some asphaltenes, thiophenic sulfur dominates, while in others, thiophenic and sulfidic forms are comparable. The thiophene fraction of crude oils and asphaltenes increases for more thermally mature crude oils. Nitrogen has also been explored in XANES studies8,15 on asphaltenes and other carbonaceous materials.8 These studies have shown that asphaltene nitrogen occurs almost entirely in aromatic forms, as predominantly pyrrolic and also pyridinic structures. In this study, the asphaltenic fractions separated from Maya Residue and its hydrotreated products were subjected to elemental and vanadium and nickel analyses and the removal kinetics of these elements were carried out. Experimental Section A Maya residue was hydrotreated by a laboratory-scale plant in continuous operation with an Autoclave-Engineers stirred tank reactor with one liter of capacity using a 6 wt % MoO3/2 wt % NiO on alumina (mean pore diameter: 150 Å) commercial catalyst, TK-711. The residue used had a high concentration of metals (45.17 ppm of nickel and 242.12 ppm of vanadium) and of heteroatoms (3.45 wt % of sulfur and 0.28 wt % of nitrogen). Other important properties of the residue used as feed and of the catalyst were shown previously.16 All experiments were carried out at 12.5 MPa of hydrogen pressure and hydrogen/oil ratio: 10000 std.cu.ft./bbl. The ranges of temperature and weight hourly space velocities (WHSV) were the following: 375-415 °C and between 1.4 and 7.1 L h-1 gcat-1, respectively. The experiments were done in conditions with the absence of interphase gradients from choosing the adequate stirring speeds and gas-liquid ratios and with the absence of intraparticle gradients due to the crushing of the used catalyst. The procedure for the experiments reported here has been described previously and the scheme and the details of the setup shown.16 The feed and product oils were extracted with several solvents (toluene and n-hexane) for obtaining their asphaltenic fractions.17 In the present study, these asphaltenic fractions have been subjected to elemental and Ni and V analysis. The carbon, hydrogen, and oxygen contents in the asphaltenic fraction were measured using an elemental analyzer CARLO ERBA EA1108, the measurements for oxygen contents (8) Mullins, O. C. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; pp 53-96. (9) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182. (10) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53. (11) Strausz, O. P. The Fine Particle. Soc. Meeting, Chicago, IL 1995; Also see Payzant, J. D.; Mojelsky, T. W.; Strausz, O. P. Energy Fuels 1989, 3, 449. (12) Mitra-Kirtley, S.; Mullins, O. C.; Ralston, C. Y.; Pareis, C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 763. (13) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991, 5, 574.
Callejas and Martı´nez Table 1. Elemental Analysis (wt %), H/C Atomic Relation, and Concentration of Nickel and Vanadium (ppm) of the Asphaltenic Fraction of the Feedstock and of the Hydrotreated Products T (°C)/WHSV (L h-1 gcat-1)
C
H
N
S
H/C
Ni
V
feedstock
81
7.4
1.42
6.97
1.1
385
1682
375/2.9 375/4.8 375/6.2 400/2.3 400/4.2 400/6.2 415/3.3 415/3.7 415/7.1
83.05 91.95 82.07 85.12 85.05 84.19
6.41 6.66 6.76 6.59 6.23 6.39 6.0 5.71
4.24 5.08 5.26 3.47 3.83 4.37 5 4.0 5.31
0.93 0.98 0.99 0.93 0.88 0.91
85.28 85.95
1.63 1.58 1.57 1.82 1.8 1.72 2.22 1.88 1.82
462 430 433 502 477 393 294 247 232
1163 1340 983 804 974 1170 629 476 551
0.84 0.80
being much less accurate than those for carbon and hydrogen. Asphaltenic sulfur and nitrogen analyses were carried out in an analyzer ANTEK 7000 ELEMENTAL, and the metals were determined by inductively coupled plasma optic emission spectrometry. The spectrometer used was a Perkin-Elmer model P-400.
Results and Discussion The results of the elemental analysis and metals determination together with the atomic ratio H/C of the asphaltenic fractions from the feedstock and from the oil products are shown in Table 1. Analytical data indicate that in the remaining asphaltenes, the H/C ratio decreases from about 1.1 for feedstock to 0.8 for the asphaltene fraction obtained at the most severe experimental conditions. It is observed that the elemental hydrogen analyses of the hydrocracked asphaltenes are lower than that of the feedstock asphaltene. To the contrary, the carbon content increased compared with the initial asphaltenes. This fact indicates that the remaining, more condensate and stable structures are enriched with aromatic/heterocyclic groups. In addition, regarding the rest of the elements, it can be observed that in the products the concentration of sulfur and vanadium in asphaltenes is lower but the nitrogen is higher and also the content of nickel at hydroprocessing temperatures of 375 and 400 °C. These results confirm the existence of refractory nitrogen difficult to remove. The tendency for nitrogen to remain in the nonvolatile residue that remained after thermal decomposition supports the concept that part of these elements in the asphaltenes are stable because of their location in heterocyclic ring systems. Seki and Kumata18 have reported that the asphaltenes tended to polycondense during the HDS treatment if the temperature was over 400 °C, and this makes the remaining asphaltenic sulfur and nitrogen more difficult to remove. (14) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 945. (15) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252. (16) 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. (17) Callejas, M. A.; Martı´nez, M. T. Hydroprocessing of a Maya Residue. I. Intrinsic Kinetics of Asphaltene Removal Reactions. Energy Fuels 2000, 14, 1304. (18) Seki, H.; Kumata, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 786-789.
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Studies on the disposition of nitrogen in petroleum asphaltenes indicated the existence of various heterocyclic types.19-21 Schmitter et al.22 have brought to light the occurrence of four-ring aromatic nitrogen species in petroleum. These findings are of particular interest because they correspond to the ring systems that have been tentatively identified by application of highperformance liquid chromatography (HPLC) to the basic nitrogen fraction of asphaltenes. The more conventional types of nitrogen (from the organic chemist’s viewpoint), i.e., primary, secondary, and tertiary aromatic amines, have not been established as being present in petroleum asphaltenes. There is evidence for the occurrence of carbazolic nitrogen in asphaltenes.23 Concerning the metals, conversion of the asphaltene generally leads to simultaneous removal of vanadium, whereas nickel removal is more difficult. Reverse-phase HPLC studies by Fish et al.3 have demonstrated that nickel nonporphyrins are significantly more polar than the vanadyl nonporphyrins and are therefore expected to be more strongly associated. By applying the rate equation corresponding to a system in continuous operation and assuming an heterogeneous perfectly mixed system in which steadystate conditions have been reached, eq 1, a half-order kinetics has provided an excellent fit for the asphaltenic sulfur, nitrogen, and metal removal reactions from the oil products:
Co - C )
KCn WHSV
(1)
Co being the initial concentration, C the outlet concentration, WHSV the weight hourly space velocity, n the kinetic order, and K an intrinsic rate constant (our experiments have been carried out in absence of all type of gradients).16 By plotting (Co - C)/C0.5 versus 1/WHSV, the rate constants at 375, 400, and 415 °C can be calculated. Figures 1-4 illustrate half-order kinetic plots at the three temperatures studied. The activation energies were calculated from a semilogarithmic plot of ln K versus 1/T, Figures 5 and 6, using the Arrhenius equation (-Ea/RT)
K ) Koe
Figure 1. Half-order kinetic plot for asphaltenic sulfur removal reactions.
(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 energies obtained are detailed in Tables 2 and 3. High correlation coefficients were found for all temperatures. It can be observed in Figures 1-4 that the experimental data at the three temperatures studied greatly agree with the proposed model. (19) Jacobson, J. M.; Gray, M. R. Fuel 1987, 66, 749. (20) Moschopedis, S. E.; Hawkins, R. W.; Speight, J. G. Fuel 1981, 60, 397. (21) Sinninghe Damste, J. S.; Eglinton, T. I.; de Loeuw, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 710. (22) Schmitter, J. M.; Garrigues, P.; Ignatiadis, I.; De Vazelhes, R.; Perin, F.; Ewald, M.; Arpino, P. Org. Geochem. 1984, 6, 579. (23) Moschopedis, S. E.; Speight, J. G. Prepr. PapsAm. Chem. Soc., Div. Pet. Chem. 1979, 24, 1007.
Figure 2. Half-order kinetic plot for asphaltenic nitrogen removal reactions.
As in the case of the removal of asphaltenes,17 halforder kinetics has provided an excellent fit for the asphaltenic heteroatom and metal removal. The values of the activation energies for the reactions of removal of the asphaltenic sulfur and nitrogen are very similar, indicating the same behavior of those reactions with respect to the temperature and lower than those of the metals. Regarding the kinetics of heteroatom and metal removal from the whole residue,24 nitrogen and vanadium showed the same kinetic order but nickel and sulfur showed higher order, 1 and 2, respectively, and the activation energy was higher in all cases. Previous studies,25 using model compounds have shown fractional order kinetics for total metal removal centered around 0.5 for the demetalation of Ni- and (24) Callejas, M. A.; Martı´nez, M. T. Energy Fuels 1999, 13, 629636. (25) Hung, C. W.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 250-257.
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Figure 3. Half-order kinetic plot for asphaltenic nickel removal reactions.
Callejas and Martı´nez
Figure 6. Arrhenius plot for the half-order rate constants of asphaltenic nickel and vanadium removal reactions. Table 2. Rate Constants [(ppm0.5 L) h-1 gcat-1] and Activation Energies Obtained for Asphaltenic Sulfur and Nitrogen Removal Reactionsa 375 °C
400 °C
Sulfur K 3.9 (5e-17) 8.7 (1.1e-16) correl. coeft. 0.99 0.99 std. err. of estimat. 2.8e-16 6.1e-16 ln Ko - Ea/R correl. coeft. std. err. of estimat. Ea (kcal/mol)
415 °C 20.1 (4.e-17) 1.00 2.2e-16
28.9 (4.5) -17867 (3030) 0.99 0.19 35.6
Nitrogen K 0.97 (2.3e-18) 2.25 (4.9e-18) 6 (1.4e-17) correl. coeft. 0.99 0.99 1.00 std. err. of estimat. 1.2e-17 2.7e-17 8e-17
Figure 4. Half-order kinetic plot for asphaltenic vanadium removal reactions.
ln Ko -Ea/R correl. coeft. std. err. of estimat. Ea (kcal/mol)
30.3 (6) -19726 (4016) 0.98 0.26 39.3
a In parentheses, the standard errors of the estimated coefficients.
Figure 5. Arrhenius plot for the half-order rate constants of asphaltenic sulfur and nitrogen removal reactions.
VO-etioporphyrin and Ni-tetraphenylporphyrin on oxide CoMo/Al2O3 at industrial hydroprocessing conditions. Agrawal and Wei26 confirmed that the apparent fractional-order kinetics resulted from a sequential
mechanism, an initial reversible hydrogenation followed by a terminal hydrogenolysis step. Intrinsic reaction kinetics studies have been summarized by Quann et al.27 and values of reaction order ranging from 0.1 to 1.1 are reported. These discrepancies in reaction order with respect to the total metal concentration reflect the difficulty of representing a complex reaction network by global “pseudo” kinetic expressions. In addition, hydrodemetalation reactions require the diffusion of multiringed aromatic molecules into the pore structure of the catalyst prior to initiation of the sequential conversion mechanism and it is necessary to get an understanding of the molecular diffusion process in porous material to interpret the diffusion-disguised kinetics observed with full-size commercial catalyst. (26) Agrawal, R.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 505-515. (27) Quann, R. J.; Ware, R. A.; Hung, C. W.; Wei, J. Adv. Chem. Eng. 1988, 14, 95-259.
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Table 3. Rate Constants [(ppm0.5 L) h-1 gat-1] and Activation Energies Obtained for Asphaltenic Nickel and Vanadium Removal Reactionsa 375 °C
400 °C
Nickel K 15.4 (1.7e-16) 37.7 (1e-16) correl. coeft. 0.98 0.99 std. err. of estimat. 9.5e-16 5.7e-16 ln Ko -Ea/R correl. coeft. std. err. of estimat. Ea (kcal/mol)
415 °C 144.8 (1.8e-15) 0.98 9.7e-15
39.5 (10.1) -23901 (6740) 0.96 0.43 47.7
Vanadium K 54 (4.2e-16) 135 (2.8e-16) 447 (6e-15) correl. coeft. 0.99 0.99 0.99 std. err. of estimat. 2.3e-15 1.5e-15 3.4e-14 ln Ko -Ea/R correl. coeft. std. err. of estimat. Ea (kcal/mol)
38.9 (8.1) -22691 (5434) 0.97 0.35 45.3
a In parentheses, the standard errors of the estimated coefficients.
Kim and Curtis28 and Girgis and Gates29 reported that the reaction pathways observed for the heteroatomic species containing oxygen, sulfur, and nitrogen followed two paths: one pathway for heteroatom removal prior to saturation of the aromatic ring, producing alkyl aromatics, and one for heteroatom removal following saturation of the aromatic ring, producing alkyl alicyclics. The hydrodesulfurization process is a complex sequence of reactions. Despite a large body of existing information for model compounds, the data available on hydrodesulfurization are often fragmented and catalystdependent. Nevertheless, kinetic studies using individual sulfur compounds have usually indicated that simple first-order kinetics with respect to sulfur is the predominant mechanism by which sulfur is removed from the organic material as hydrogen sulfide. Determination of sulfur forms present in petroleum asphaltenes, by means of XANES and XPS analysis,8,30 indicates that the most prominent sulfur functionalities are of thiophenic (aromatic) (100-51%) and sulfidic (saturated) (0-50%) type while sulfoxide and sulfone make up only a few percent. The removal of sulfur from model compounds such as benzothiophene and its benzologues, which are key compounds in the HDS of heavier feedstocks, has been reported to be achieved by two major pathways.31 In the first, benzothiophene underwent the hydrogenolysis of the carbon-sulfur bond followed by sulfur removal, (28) Kim, H.; Curtis, C. W. Energy Fuels 1990, 4, 206. (29) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021. (30) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939-944.
producing ethylbenzene as the primary product; and in the second and minor pathway, the six-membered rings were hydrogenated first followed by desulfurization.28,29,31-34 The most accepted reaction network for HDS reaction involves series and parallel reactions, and according to the analysis reported by Gray et al.,33 when irreversible series and parallel reactions are lumped together, the first-order reactions can give any apparent reaction order from - ∞ to + ∞. Historically, HDS has been studied much more extensively than HDN because of the relatively low quantities of nitrogen compounds present in traditional petroleum feedstocks compared with the amount of sulfur compounds.35 Hydrodenitrogenation is harder to achieve under processing conditions than hydrodesulfuration.36 Industrial feedstocks have overall nitrogen removal kinetics between one and two of reaction order which are very dependent on feedstock boiling range. The model compound kinetics are generally parallel reactions with very complex kinetics and the likelihood of completely deconvoluting HDN reactions of industrial feeds is remote. Conclusions The results of the elemental analysis and of the metal determination of the asphaltenic fraction have indicated an increase of concentration of nitrogen and nickel, except for the highest temperature used in the remaining asphaltenes. The removal of sulfur, nitrogen, nickel, and vanadium from the asphaltenic fraction has been correctly described by using half-order kinetic equations after experimental data had been tested against the model. The values of the activation energies for the asphaltenic nitrogen and sulfur removal have been similar to the obtained one in the deasphalting, indicating the same behavior of these reactions with respect to the temperature. Acknowledgment. This work was sponsored by the UE Contract No. JOU2-CT92-0206 and the Spanish DGICYT project AMB93-1137-CE. The authors are grateful to Prof. D. Severin of University of Clausthal (Germany) for their assistance in the elemental and Ni and V analysis measurements of the asphaltenic fraction. EF000127+ (31) Van Parijs, I. A.; Froment, G. F. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 431. (32) Curtis, W.; Chem, J. H.; Tang, Y. Energy Fuels 1995, 9 (2), 195. (33) Gray, M. R.; Ayasse, A. R.; Chan, E. W.; Veljkovic, M. Energy Fuels 1995, 9, 500. (34) Vanrysselberghe, V.; Froment, G. F. Ind. Eng. Chem. Res. 1996, 35, 3311. (35) Ho, T. C. Catal. Rev.sSci. Eng. 1988, 30, 117-160. (36) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979.
