Hydroprocessing of a Maya Residue. Intrinsic Kinetics of Sulfur

Energy & Fuels 1999, 13, 629-636

629

Hydroprocessing of a Maya Residue. Intrinsic Kinetics of Sulfur-, Nitrogen-, Nickel-, and Vanadium-Removal Reactions Marıá A. Callejas and Marıá T. Martıńez* Instituto de Carboquı´mica CSIC, Apartado 589, 50080 Zaragoza, Spain Received August 5, 1998

A residue from a Maya crude was processed in a hydrotreating unit with a continuous stirred tank reactor at high temperatures (375-415 °C) and hydrogen pressures (10-15 MPa). A commercial guard-bed demetalation catalyst Ni-Mo supported on γ-Al2O3 was used. In this paper, the intrinsic kinetics of the sulfur-, nitrogen-, nickel-, and vanadium-removal reactions are reported. The sulfur-removal reactions fit second-order kinetics, the nitrogen-removal reactions half-order kinetics, the nickel-removal reactions first-order kinetics, and the vanadium-removal reactions half-order kinetics. The nickel-removal reactions showed the highest value for the activation energy. The sulfur-removal reactions were the only ones that had a dependence on the hydrogen pressure in every range of hydrogen pressures studied, 10-15 MPa, and a kinetic order of 0.4 has been observed. For nitrogen-, nickel-, and vanadium-removal reactions, an increase of the pseudokinetic constants was only observed between 10 and 12.5 MPa. The percentages of sulfur-removal range from 22% to 79.3%, nitrogen conversion from 3.7% to 50.9%, nickel conversion from 32.1% to 98.8%, and vanadium conversion from 40.1% to 99.7% at 375 °C, 10 MPa, and 7.1 L/h gcat and 415 °C, 12.5 MPa, and 3.3 L/h gcat. The relationships between the percentages of sulfur-removal and metal-removal were studied.

Introduction The demand for the light products and middle distillates is increasing1 but the world supply of light, easyto-process crude oils is depleting.2 This evolution of supply and demand implies that heavy crude oils with much higher concentrations of nitrogen, sulfur, and metals must be incorporated into refinery feed slates. Heavy crudes with a low hydrogen/carbon ratio constitute a vast source of untapped energy,2 and refiners have to cope with the upgrading of these heavier crudes. Increased emphasis on environmental problems, especially air pollution, will inevitably lead to more stringent regulations on emissions. Although the product specifications will not change in the same way everywhere, the objectives for both gasoline and diesel oil will be roughly the same: a reduction of their content in sulfur and aromatic compounds. For instance, the European refiners are being asked to produce diesel fuels with reduced sulfur content (50 ppm according to European specifications foreseen for year 2005). A salient feature of residual feedstocks is their large content of coke precursors and contaminants which are generally referred to as heteroatoms, viz. compounds containing sulfur, nitrogen, and metals. During the catalytic hydroprocessing, a collection of reactions takes place, viz. hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallization (HDM), hydrogenation of unsaturated hydrocarbons, and hydrocracking reactions. (1) Decroocq, D. Revue de Inst. Fr. Pe´ t. 1997, 52, 469. (2) Campbell, C. J.; Laherre`re, J. H. Invest. Ciencia 1998, 66-71.

HDS, HDN, and HDM are extremely important in modern refining, so much so that HDS may be the most widely performed chemical reaction currently known. Most of the kinetics of the model compounds indicate first-order reactions in sulfur concentration. However, industrial feeds under similar conditions indicate kinetics ranging from first to second order, depending upon the feed boiling point range.3,4 Kim and Curtis5 and Girgis and Gates6 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 development of generalized kinetic data for the heteroatom removal is complicated. A large amount of polycyclic aromatic derivatives exists in the heavy distillates,7-9 and the reaction environment is much more complex with aromatic species coexisting with various types compounds competing for the active sites in the catalyst surface. Furthermore, the presence of inhibitors (aromatics and nitrogen compounds) present (3) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (4) Cecil, R. R.; Mayer, F. Z.; Cart, E. N. 61st Annual Meeting of AIChE, Los Angeles, CA, 1961. (5) Kim, H. G.; Curtis, C. W. Energy Fuels 1990, 4, 206-214. (6) Girgis, M. J.; Gates B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058. (7) Browarzik, D.; Kehlen, H. Chem. Eng. Sci. 1994, 49, 923-926. (8) Laxminarasimhan, C. S.; Verma, R. P.; Ramachandran, P. A. AICHE J. 1996, 42, 2645-2653. (9) Froment, G. F.; Depauw, G. A.; Vanrysselberghe, V. Ind. Eng. Chem. Res. 1994, 33, 2975-2988.

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630 Energy & Fuels, Vol. 13, No. 3, 1999

in heavy residues and produced by desulfurization (hydrogen sulfide) may modify the reaction pathway. Several workers3,10-15 have shown large inhibiting effects of H2S on HDS. To account for the above, the rate of individual components of compound classes is shown to follow Langmuir-Hinshelwood (L-H) kinetics. 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.16 Hydrodenitrogenation is harder to achieve under processing conditions than hydrodesulfuration.3 Hydrogenation of the nitrogen-containing aromatic ring is particularly difficult because of sterics and poor thermodynamic limitations. In addition, this hydrogenation step can yield condensed products which are even harder to denitrify.17 Industrial feedstocks have overall nitrogen removal kinetics between first and second order which are very dependent on the 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. In the past few decades, much research effort has been devoted to HDS and HDN. The presence of metal compounds in crude oil and their impact on refining processes has received only sporadic interest since the HDS studies have been performed with light fractions. With the increasing amounts of residual feedstocks to be processed, metal compounds are especially problematic. Unlike HDS and HDN, HDM of organometallics (mainly vanadium and nickel compounds) generates deposits on catalysts resulting in decreasing selectivities and activities and, eventually, in a complete and irreversible deactivation. In the improvement of existing processes, as well as in the development of future processes, kinetic data concerning HDM of metallic petroleum constituents are of paramount importance. Reaction studies with model compounds representative of the metal species indicate a sequential hydrogenation-hydrogenolysis global mechanism.18 Bonne´ et al.19 have applied a two-site L-H model with small inhibition by H2 and negligible inhibition by H2S for HDM of Ni-tetraphenylporphyrin. In this model, hydrogenation occurs on one type of site which is surmised to consist of sulfur vacancies associated with molybdenum. Hydrogenolysis occurs on another type of site. (10) Vrinat, M. L. Appl. Catal. 1983, 6, 137-158. (11) van Parijs, I. A.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 431-436. (12) van Parijs, I. A.; Hosten, L. H.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 437-443. (13) Stephan, R.; Emig, G.; Hofmann, H. Chem. Ing. Tech. 1985, 57, 480-481. (14) Papayannakos N.; Marangozis, J. Chem. Eng. Sci. 1984, 39, 1051-1061. (15) Korsten, H.; Hoffmann, U. AIChE J. 1996, 42, 1350-1360. (16) Ho, T. C. Catal. Rev.-Sci. Eng. 1988, 30, 117-160. (17) Trytten, L. C.; Gray, M. R.; Sanford, E. C. Ind. Eng. Chem. Res. 1990, 29, 725-730. (18) Quann, R. J.; Ware, R. A.; Hung, C. W.; Wei, J. Adv. Chem. Eng. 1988, 14, 95. (19) Bonne´, R. L. C.; van Steenderen, P.; van Langeveld, A. D.; Moulijn, J. A. Ind. Eng. Chem. Res. 1995, 34, 3801-3807.

Callejas and Martıńez

L-H models are difficult to apply for petroleum fractions. For such a complex system, it may be more practical to use single-lump models. The performance of single-lump models are reportedly good for specific feedstocks, but they are not able to account for effects of feed variant.20 Studies undertaken with petroleum feedstocks to elucidate an understanding of hydrodemetalation reactions have yielded ambiguous and in some cases conflicting results. Comparison of kinetic phenomena from one study to the next is often complicated. Formulation of a generalized kinetic and mechanistic theory of residuum demetalation requires consideration of competitive rate processes which may be unique to a particular feedstock. Catalyst activity is affected by catalyst size, shape, and pore size distribution and the intrinsic activity of the catalytic metals. Feedstock reactivity reflects the composition of the crude source and the molecular size distribution of the metal-bearing species. Although currently there is much activity in HDS, HDN, and HDM research, the future holds even more challenges. The need to process more heavy, poorer quality crude oils is inevitable. This will stimulate new catalysts and new process designs which will be even more dependent on fundamental knowledge of HDS, HDN, and HDM. The objective of the current investigation was to perform an intrinsic kinetic study of sulfur-, nitrogen-, and metal-removal reactions from an oil residue in the hydrogenation process. Experimental Section The feedstock, a residue from a Maya crude, has been hydrogenated in a continuos hydroprocessing unit. The Maya residue has a high metal content (45.17 ppm of Ni and 242.12 ppm of vanadium), 3.45 wt % of sulfur, and 0.28 wt % of nitrogen. Other feedstock properties have been reported elsewhere.21 In this process, a commercial catalyst, Topsoe TK-711 (6 wt % MoO3 and 2 wt % NiO), supported on γ-Al2O3 was used. This catalyst was specially developed for pretreatment of residual oils for reduction of metals. The catalyst was previously presulfided for 10 h at 350 °C at atmospheric pressure, under a hydrogen flow containing 10 vol % of H2S. The study was performed in a hydroprocessing unit provided with a continuos stirred tank reactor (CSTR). The utilization of a CSTR in the kinetic study provides a tool for working in the absence of intrareactor gradients, but other types of gradients, those at the boundary between different phases (gas/ liquid/solid) in the reaction medium and gradients within the catalyst particle, can also be present. In our kinetic study, working conditions in the absence of interphase gradients were investigated. The stirring speed was varied from 2000 to 3500 rpm and the gas/liquid ratio from 6000 to 10 000 std cu ft./bbl. The content of the sulfur and metals in the samples obtained in that range of experimental conditions indicated that the influence of external masstransfer resistance on the reaction was negligible at 10 000 std cu ft/bbl gas/liquid ratio and 3500 rpm stirring speed, and therefore, the kinetic study was carried out at the experimental conditions mentioned. (20) Sau, M.; Narasimhan, C. S. L.; Verma, R. P. Stud. Surf. Sci. Catal. 1997, 106, 421-435. (21) Trasobares, S.; Callejas, M. A.; Benito, A. M.; Martıńez, M. T.; Severıń, D.; Brouwer, L. Ind. Eng. Chem. Res. 1998, 37, 11-17.

Hydroprocessing of a Maya Residue

Energy & Fuels, Vol. 13, No. 3, 1999 631

Table 1. Operating Conditions for Kinetic Experiments run

temp (°C)

pressure (MPa)

LHSV (L/h gcat)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

375 375 375 375 375 375 375 375 375 400 400 400 415 415 415

10.0 10.0 10.0 12.5 12.5 12.5 15.0 15.0 15.0 12.5 12.5 12.5 12.5 12.5 12.5

2.4 4.8 7.1 2.9 4.8 6.2 1.4 4.3 5.9 2.3 4.2 6.2 3.3 3.7 7.1

Table 2. Sulfur, Nitrogen, Nickel, and Vanadium Analysis of the Feedstock and Products from the Kinetic Experiments run

S (wt %)

N (ppm)

Ni (ppm)

V (ppm)

feed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

3.45 2.24 2.53 2.74 2.31 2.53 2.74 1.86 2.47 2.73 1.47 1.67 2.07 0.88 0.89 1.12

2800 2600 2702 2759 2600 2680 2710 2410 2711 2690 2070 2490 2680 1700 1758 2205

45.17 20.58 26.68 31.39 20.54 25.43 29.63 13.95 23.64 28.83 2.38 4.2 6.5 0.64 0.7 1.42

242.12 62.38 103.54 148.27 56.57 87.18 119.36 19.3 79.13 109.97 7.99 20.17 63.99 0.82 1.12 3.92

To avoid intraparticle gradients, the kinetic runs were carried out with the catalyst crushed at a particle size range between 53 and 530 µm. Twenty grams of presulfided catalyst was located inside bags of metallic mesh of 35 µm of sieve aperture in order to avoid misleading results caused by losses of catalyst by attrition and plugs in the reactor output. After running the hydroprocessing unit at 375 °C and 10 MPa H2 pressure for 180 h, spot checks of replicate samples during this period showed that product composition was constant within the variability of the analyses, indicating that the activity of the catalyst became stable and the runs indicated in Table 1 were started. For each kinetic experiment, the flow of liquid feed and hydrogen gas was started and the liquid product was then discarded until a further residence time had elapsed. Steady-state operation was assumed after this interval, and collection of product was begun. The details of the reactor and the experimental setup have been described previously.21 Sulfur and nitrogen analysis 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 from sulfur, nitrogen, and metals determination are indicated in Table 2. As shown, the outlet concentrations of the elements decrease as the space velocity decreases and the reaction temperature increases. Due to our experiments that have been carried out in the absence of all type of gradients, it is thought that

Figure 1. Second-order kinetic plot for sulfur-removal reactions at 12.5 MPa hydrogen pressure.

our reactions are kinetically controlled, and therefore, the data of Table 2 can be fit to the simplified eq 1, which corresponds to a heterogeneous perfectly mixed system working continuously and after steady-state conditions were reached

Co - C )

m K*CnPH 2

LHSV

(1)

Co being the initial concentration, C the outlet concentration, LHSV the liquid hourly space velocity, K* the intrinsic rate constant, n the kinetic order with respect to total metal or heteroatom concentration, m the order dependence on H2 pressure, and PH2 the hydrogen pressure. Metal or heteroatom removal rates, -r, have generally been interpreted using a simple mass action kinetic expression of the form -r ) K* PH2m Cn To obtain the kinetic order n, the process was carried out at constant pressure, 12.5 MPa, and a pseudokinetic rate constant K was defined.

K ) K*PH2m

(2)

Then eq 1 can be rewritten as

Co - C )

KCn LHSV

(3)

The best fits obtained for the data of sulfur, nitrogen, nickel, and vanadium concentration (Table 2) have been for second-, half-, first-, and half-order kinetics, respectively. Figures 1, 2, 3, and 4 illustrate a second-order kinetic plot for S-removal reactions, half-order kinetic plot for N-removal reactions, first-order kinetic plot for Niremoval reactions (HDNi), and half-order kinetic plot for V-removal reactions (HDV), respectively. The hydrodesulfurization process is a complex sequence of reactions. Despite a large body of existing information for model compounds, the data available on


Figure 2. Half-order kinetic plot for nitrogen-removal reactions at 12.5 MPa hydrogen pressure.

Figure 3. First-order kinetic plot for nickel-removal reactions at 12.5 MPa hydrogen pressure.

hydrodesulfurization is often fragmented and catalyst dependent. 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. However, in the hydrodesulfurization of petroleum feedstocks, a mixture of sulfur-containing compounds with various reactivities exists. Each compound reacts at a different rate because of structural differences as well as differences in molecular weight and concentration, and an overall apparent reaction order dependence higher than first order can be expected.22-25 (22) De Bruijn, A. Proc. 6th Int. Congr. Catal. 1976, 2, 951. (23) Ho, T. C.; Aris, R. AIChE J. 1987, 33, 1050-1051. (24) Gray, M. R.; Ayasse, A. R. Energy Fuels 1995, 9, 500-506. (25) Martıńez, M. T.; Benito, A. M.; Callejas, M. A.; Trasobares, S. Energy Fuels 1998, 12, 365-370.


Figure 4. Half-order kinetic plot for vanadium-removal reactions at 12.5 MPa hydrogen pressure.

The most accepted reaction network for HDS reaction involves series and parallel reactions, and according to the analysis reported by Gray et al.,24 when irreversible series and parallel reactions are lumped together, the first-order reactions can give any apparent reaction order from -∞ to +∞. Numerous investigators have examined the kinetics of metal-removal from petroleum fractions, and a discrepancy in reaction order n with respect to total metal (Ni or V) concentration has been observed.18 Hydrodemetalation kinetic studies of petroleum fractions have been described in the literature by using halforder,26 first-order,27 second-order,28 or between firstand second-order29 rate laws. A spectrum of metal compound reactivities in petroleum could arise for several reasons. Nickel and vanadium exist in a diversity of chemical environments that can be categorized into porphyrinic and nonporphyrinic species, vanadyl and nonvanadyl, or associated with large asphaltenic groups and small, isolated metalcontaining molecules. Each species can be characterized by a unique intrinsic reactivity. Reaction inhibition which occurs between the asphaltenes and the nonasphaltenes, as well as between Ni and V species, can also contribute to reactivity distributions. The interpretation of the observed reaction order discrepancy is therefore compatible with the multicomponent nature of petroleum. The influence of the temperature has been assumed to follow the Arrhenius equation

K ) Ko e(-Ea/RT)

(4)

Ko being the preexponential factor, Ea the activation (26) Hung, C. W.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 250. (27) Riley, K. L. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1978, 23, 1104. (28) Oleck, S. M.; Sherry, H. S. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 525. (29) van Dongen, R. H.; Bode, D.; van der Eijk, H.; van Klinken, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 630.



Figure 5. Arrhenius plot for the second-order rate constants of HDS reactions at 12.5 MPa hydrogen pressure.

Figure 7. Arrhenius plot for the first-order rate constants of HDNi reactions at 12.5 MPa hydrogen pressure.

Figure 6. Arrhenius plot for the half-order rate constants of HDN reactions at 12.5 MPa hydrogen pressure.

Figure 8. Arrhenius plot for the half-order rate constants of HDV reactions at 12.5 MPa hydrogen pressure.

energy, R the molar gas constant, and T the absolute temperature (K). By plotting ln K versus 1/T, the activation energy values were obtained. Figures 5, 6, 7, and 8 show the Arrhenius plot for the sulfur-, nitrogen-, nickel-, and vanadium-removal reactions, respectively. For calculating m from eq 2, a series of experiments at 375 °C and different pressures and space velocities (Table 1) were carried out. A logarithmic-logarithmic plot of K versus hydrogen pressure would have a slope m and ln K* as the intersection with the ordinate axis. The values of the pseudokinetic rate constants at 375, 400, and 415 °C, the intrinsic rate constant at 375 °C, and the activation energy for the S-removal reactions are indicated in Table 3. The values of the pseudokinetic rate constants at 375, 400, and 415 °C and the activation energies for the N, Ni, and V removal reactions are indicated in Tables 4, 5, and 6, respectively.

For the nitrogen-, nickel-, and vanadium-removal reactions, no pressure dependence on the rate constants has been observed in the range of hydrogen pressures studied (10-15 MPa), but nevertheless, an increase of the pseudokinetic constants between 10 and 12.5 MPa has been observed (Tables 4-6). Therefore, it would be necessary to perform a more complete study on the influence of the pressure at values below 12.5 MPa. The sulfur removal reactions have shown to be pressure dependent, Figure 9, in the range of work pressures defined by eq 2, obtaining a kinetic order of 0.4. There are few references in the literature which study the hydrogen pressure dependence on the kinetics, and the obtained results are contradictory because both feedstock and catalyst may influence the magnitude of the hydrogen response. Audibert and Duhaut30 observed first-order dependence on hydrogen when demetalating



Table 3. Pseudokinetic Rate Constants, K [L/ppm h gcat], Intrinsic Rate Constant, K* [L/ppm h gcat (MPa)0.4], and Activation Energy Obtained for Sulfur-Removal Reactions and Standard Errors of the Estimated Coefficients (in Parentheses) 375 °C K10MPa corr coeff. std error of estimation K12.5MPa corr coeff. std error of estimation K15MPa corr coeff. std error of estimation ln K* m corr coeff. std error of estimation K* ln Ko -Ea/R corr coeff. std error of estimation Ea (kcal/mol)

400 °C

415 °C

10-5

6.9 × (5 × 10-6) 0.98 2.5 × 10-6 7.3 × 10-5 (3 × 10-6) 0.99 1.3 × 10-6

2.8 × 10-4 (2 × 10-5) 0.98 1.2 × 10-5

1.8 × 10-3 (2 × 10-5) 1.00 9 × 10-6

8.1 × 10-5 (5 × 10-6) 0.99 3.4 × 10-6

68.6

Table 4. Rate Constants [(ppm0.5 L)/h gcat] and Activation Energy Obtained for Nitrogen-Removal Reactions and Standard Errors of the Estimated Coefficients (in Parentheses) 375 °C

3.2 (0.17) 0.99 8.5 × 10-2 3.8 (0.14) 0.99 6.1 × 10-2 3.6 (0.25) 0.99 0.19 74.6 (5.22) -47511 (3492) 1.00 0.22 95.1

375 °C

2.7 × 10-5 (0.98) 43.2 (12.8) -34271 (8550) 0.97 0.55

14.2 (1.02) 0.98 0.5 17.6 (0.88) 0.99 0.38 16.9 (0.88) 0.99 0.66 36.2 (5.46) -21649 (3654) 0.99 0.23 43.3

375 °C K10MPa corr coeff. std error of estimation K12.5MPa corr coeff. std error of estimation K15MPa corr coeff. std error of estimation ln Ko -Ea/R corr coeff. std error of estimation Ea (kcal/mol)

400 °C

415 °C

44.8 (1.75) 0.99 0.92

281 (10.8) 0.99 4.6

Table 6. Rate Constants [(ppm0.5 L)/h gcat] and Activation Energy Obtained for Vanadium-Removal Reactions and the Standard Errors of the Estimated Coefficients (in Parentheses)

-11.4 (0.4) 0.4 (0.08) 0.98 0.02

K10MPa corr coeff. std error of estimation K12.5MPa corr coeff. std error of estimation K15MPa corr coeff. std error of estimation ln Ko -Ea/R corr coeff. std error of estimation Ea (kcal/mol)

Table 5. Rate Constants [L/h gcat] and Activation Energy Obtained for Nickel-Removal Reactions and Standard Errors of the Estimated Coefficients (in Parentheses)

400 °C

415 °C

47 (3.62) 0.99 1.9

129.3 (4) 0.99 1.72

Middle Eastern Oil, whereas Chang and Silvestry31 and Oleck and Sherry28 observed higher-order dependence with Kuwait residuum on CoMo/Al2O3 catalysts. Hydroprocessing Kuwait residuum on nonconventional catalysts exhibits a hydrogen dependence less than one.28 Galiasso et al.32 studied the kinetic behavior of resins obtained from Jobo Crude Oil, and orders with respect (30) Audibert, F.; Duhaut, P. 35th Midyear Meeting of the American Petroleum Institute Division of Refining, Houston, 1970. (31) Chang, C. D.; Silvestri, A. J. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 161. (32) Galiasso, R.; Garcıá, J.; Caprioli, J.; Souto, A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1985, 30, 50.

K10MPa corr coeff. std error of estimation K12.5MPa corr coeff. std error of estimation K15MPa corr coeff. std error of estimation ln Ko -Ea/R corr coeff. std error of estimation Ea (kcal/mol)

400 °C

415 °C

58.4 (3.67) 0.99 1.79 75.9 (2.65) 198.4 (17.2) 956 (28.7) 0.99 0.98 0.99 1.14 9 12.24 76.6 (2.09) 1.00 1.57 45.7 (12.3) -26885 (8235) 0.96 0.53 53.6

Figure 9. Pressure dependence on the rate constants for sulfur-removal reactions at 375 °C.

to the hydrogen pressure between 0.5 and 1.5 for the demetalation reactions and of 0.8 for the desulfurization reactions were obtained. The order of dependence is likely to be a function of the rate-limiting metal-removal step. It can be observed in Tables 3-6 that intrinsic activation energies for those removal reactions have



Table 7. Rates (ppm L/h gCat) for the Sulfur-, Nitrogen-, Nickel-, and Vanadium-Removal Reactions run

-rN

-rS

-rNi

-rV

(-rV)/(-rNi)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

724.1 738.1 745.9 897.4 911.1 916.2 829.7 879.9 879.9 2138.4 2345.3 2433.1 5331.2 5421.4 6071.6

34621.4 44166.2 51802.4 38953.5 46726.6 54805.5 28022.8 49417.3 60368.5 60505.2 78089.2 119977.2 141044.2 142578 225993.6

65.9 85.4 100.4 78.1 96.6 112.6 50.2 85.1 103.8 99 188.2 291.2 179.8 196.7 399

461.2 594.2 711.1 570.9 708.7 829.2 336.5 681.4 803.3 560.8 891 1587.1 865.7 1011.7 1892.8

7 6.96 7.08 7.31 7.34 7.36 6.7 8.01 7.74 5.66 4.73 5.45 4.81 5.14 4.74

been higher than the values found in the literature32-36 due to the fact that most of these values are apparent and, therefore, lead to lower values.37 The Ni-removal reactions showed the highest value (95.1 kcal/mol), which indicates a higher-temperature dependence for this reaction. The sulfur-, nitrogen-, nickel-, and vanadium-removal rates are shown in Table 7. It was observed that an increase of the temperature favorably affects the heteroatom- and metal-removal reactions and that desulfuration reactions show the highest reaction rates, as observed by Galiasso et al.32, 38 The removal rates decreased in the order HDS > HDN > HDV > HDNi. Most investigations27,28,30,31,39-47 have revealed that vanadium-removal rates exceed those of nickel in petroleum residua. Cases of V/Ni activity ratios of less than 1 have been reported less frequently.32,48 These were generally due to a temperature phenomenon but may also result from unique feedstock or catalyst properties. Inoguchi et al.48 reported that the vanadium-removal rate exceeded that of nickel only at temperatures in (33) Shingal, G. H.; Espino, R. L.; Sobel, J. E.; Huff, G. A. J. Catal. 1981, 67, 457-468. (34) Chakraborty, P.; Kar, A. K. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 252. (35) Reyes, L.; Zerpa, C.; Krasuk, J. H. Stud. Surf. Sci. Catal. 1994, 88, 85. (36) Speight, J. G. The Desulfurization of Heavy Oils and Residua; Dekker: New York, 1981. (37) Rajadhyaksha, R. A.; Doraiswamy, L. K. Catal. Rev. Sci. Eng. 1976, 13, 209. (38) Galiasso, R.; Blanco, R.; Gonzalez, C.; Quinteros, N. Fuel 1983, 62, 817-822. (39) Beuther, H.; Schmid, B. K. 6th Proc. World Petrol. Congr. 1963, Sect. III, 20. (40) Arey, W. F., Jr.; Blackwell, N. E., III; Reichle, A. D. 7th World Petrol. Congr. 1967, 4, 167. (41) Larson, O. A.; Beuther, H. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1966, 11, B95. (42) Ojima, Y.; Shimizu, Y.; Kondo, T.; Ukegawa, K.; Matsumura, A.; Sakabe, T.; Yagi, T.; Yamada, T.; Hamada, S. J. Jpn. Pet. Inst. 1978, 21, 372. (43) Oxenreiter, M. F.; Frye, C. G.; Hoekstra, G. B.; Sroka, J. M. J. Jpn. Pet. Inst. 1972, 30. (44) Shah, Y. T.; Paraskos, J. A. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 368. (45) Dodet, C.; Noville, F.; Crine, M.; Marchot, P.; Pirard, J. P. Appl. Catal. 1984, 11, 251-258. (46) Iannibello, A.; Marengo, S., Girelli, A. Appl. Catal. 1982, 3, 261-272. (47) Sasaki, Y.; Ojima, Y.; Kondo, T.; Ukegawa, K.; Matsumura, A.; Sakabe, T. J. Jpn. Pet. Inst. 1982, 25, 27.

Figure 10. Relationship between the percentages of HDS, HDNi, and HDV.

excess of 400 °C. Experiments with Khafji and Kuwait atmospheric residua revealed higher nickel-removal rates below 400 °C. Model compound porphyrin studies by Hung and Wei26 and Rankel49 also demonstrated a temperature dependence on the relative V/Ni removal rate. A higher apparent activation energy for vanadium removal observed by Hung and Wei26 could explain the reactivity ratio shift with increasing temperature. In our case, in Table 7 it can be observed that the vanadium-removal rates exceed those of the nickel in the totality of the conditions but from 400 °C the difference between them decreases due to the higher activation energy for nickel-removal. Vanadyl and nickel reactivity differences result from the chemistry of the oxygen ligand on vanadium, which provides an enhanced site for adsorption and reaction on the catalyst owing to the electron density associated with the oxygen. Enhanced V reactivity could also arise from molecular size constraints. Beuther and co-workers39,41 speculated that nickel concentrates in the interior of asphaltene micelles while vanadium concentrates on the exterior. Thus, a combination of stronger adsorption due to the oxygen ligand and inhibition of Ni reaction, coupled with the exposed position at the periphery of the asphaltene, may all contribute to the enhanced vanadium reactivity relative to nickel. From the data of sulfur, nitrogen, and metal concentration, indicated in Table 2, the values of conversion were calculated. Sulfur conversion ranged from 22% to 79.3% and nitrogen conversion from 3.7% to 50.9%. The range of nickel conversion was 32.1-98.8%, and the range of vanadium conversion was 40.1-99.7%. The conversions for the metal-removal reactions were the highest ones as they correspond to a specially developed demetalation catalyst. The relationships between sulfur-removal and metalremoval were studied, Figure 10. Due to the observation of a change of slope between the kinetic points obtained at 375 °C and the kinetic points obtained at 400 and 415 °C, different fits were performed. (48) Inoguchi, M.; Kagaya, H.; Daigo, K.; Sakurada, S.; Satomi, Y.; Inaba, K.; Tate, K.; Nushiyama, R.; Onishi, S.; Nagal, T. Bull. Jpn. Pet. Inst. 1971, 13, 153. (49) Rankel, L. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1981, 26, 689.


The linear equations resulting for the relationship between the sulfur-removal and the nickel-removal were (a) 375 °C, HDNi ) 5.46 + 1.35 HDS, r ) 0.99; (b) 400 and 415 °C, HDNi ) 72.68 + 0.34 HDS, r ) 0.98. For the relationship between the sulfur removal and the vanadium removal, the equations were (a) 375 °C, HDV ) 14.04 + 1.64 HDS, r ) 0.95; (b) 400 and 415 °C, HDV ) 54.8 + 0.60 HDS, r ) 0.88. For both relationships, HDS vs HDNi and HDS vs HDV, the slope of the linear regression presented by the kinetic points obtained at 375 °C shows higher values than those presented by the kinetic points at 400 and 415 °C. Besides, the linear relationships between nickel and sulfur removal have smaller slopes than those for the relationship between vanadium and sulfur removal, a fact which was observed by Massagutov et al.50 For asphaltene-containing stocks, as in our feed (8.6 wt %), this phenomenon is interpreted on the basis of heteroatom distribution within the asphaltene micelles.39 Sulfur and vanadium are concentrated on the exterior, whereas nickel is concentrated in the interior. Conversion of the asphaltene generally leads to simultaneous removal of sulfur and vanadium, whereas nickel removal is more difficult. Conclusions The data of S, N, Ni, and V concentration fit second-, half-, first-, and half-order kinetics. The sulfur removal reactions were the only ones whose rate constants showed pressure dependence in the range of hydrogen (50) Massagutov, R. M.; Berg, G. A.; Kulinich, G. M.; Kicillov, T. S. Proc. 7th World Petrol. Congr. 1967, 177.


pressures studied, 10-15 MPa, as defined by the equation K ) K* PH2m obtaining a value of 0.4 for m, at 375 °C. For the nitrogen-, nickel-, and vanadiumremoval reactions, an increase in the values of the kinetic constants was only observed between 10 and 12.5 MPa. The nickel removal reactions showed the highest value for the activation energy, 95.1 kcal/mol, which indicates that these reactions are the most sensitive to temperature. The removal rates decreased in the order HDS > HDN > HDM. In the totality of the conditions, vanadium-removal rates have exceeded those of nickel, decreasing the differences between their values at 400 and 415 °C. The removal degrees of sulfur, nitrogen, nickel, and vanadium from examined feedstock during hydroprocessing were from 22% to 79.3%, from 3.7% to 50.9%, from 32.1% to 98.8%, and from 40.1% to 99.7%, respectively. The conversions for the metal-removal reactions were the highest ones as they correspond to a specially developed demetalation catalyst. The relationships existing between the desulfurization and demetalation have been studied, showing linearity with a greater slope for the kinetic points obtained at 375 °C than those obtained at 400 and 415 °C. The slopes presented in the relationship HDS vs HDV were greater than those presented in the plot corresponding to HDS vs HDNi. Acknowledgment. This work was sponsored by UE Contract No JOU2-CT92-0206 and Spanish DGICYT Project No. AMB93-1137-CE. EF980166+

Hydroprocessing of a Maya Residue. Intrinsic Kinetics of Sulfur

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