Hydrocracking of a Maya Residue. Kinetics and Product Yield

Ind. Eng. Chem. Res. 1999, 38, 3285-3289

3285

Hydrocracking of a Maya Residue. Kinetics and Product Yield Distributions Marı´a A. Callejas and Marı´a T. Martı´nez* Instituto de Carboquı´mica CSIC, P.O. Box 589, 50080 Zaragoza, Spain

A Maya residuum was reacted in a perfectly mixed reactor in continuous operation under hydrogen in the presence of a hydrotreating commercial catalyst supported on γ-Al2O3 at 12.5 MPa of hydrogen pressure, at different temperatures (375, 400, and 415 °C), and at different mass space velocities (between 1.4 and 7.1 L/h‚gcat). The reaction products were separated into fractions with different boiling points and a kinetic scheme involving lumped species consisting of the fraction over the 343 °C boiling point (atmospheric residuum), that below the 343 °C boiling point (light oils), and gases has been proposed. Experimental data at 375 and 400 °C have found a good fit to the model but not at 415 °C. The percent conversion of sulfur, nitrogen, vanadium, and nickel and the average molecular weight reduction have been analyzed as a function of atmospheric residuum conversion. The number of moles of carbon-carbon bonds broken by reaction has also been estimated. Introduction The increasing scarcity of light petroleum feedstocks provides an incentive for the upgrading of high-molecular-weight residual oils and heavy unconventional feedstocks, but because of high concentrations of heavy material and contaminants in those petroleum feedstocks, chemical and material manufacturing companies have been forced, by environmental concerns and economic incentives, to place stringent constraints on acceptable product quality and process operations. It is well-known that these heavy feedstocks can be converted to lighter products by thermal and/or catalytic1-3 processing in the absence4-6 or presence4,7,8 of hydrogen pressure. Examples of such processes include visbreaking, coking, fluid catalytic cracking, hydrovisbreaking, and catalytic hydrocracking. All of these processes have been proposed for residue conversion.9 The preferred process will depend on the properties of the particular residue, on the desired product, and ultimately on the overall economics. Unfortunately, it is difficult to directly compare the activity or product selectivity for these processes since they are rarely compared on equivalent feeds at similar conditions. Hydrogen addition processes have long been used in upgrading heavy crudes and bitumens. Hydroconversion processes can be thermal or catalytic and consist of several stages. These processes are very flexible and can be applied to a wide range of feedstocks from light naphthas to vacuum residues.10 Catalytic hydrocracking is used extensively in petroleum refining processes to produce high-quality gasoline, diesel, and jet fuels.11 The need for process models to predict accurate product yields is of great interest to the petroleum industry. Historical limitations in the analytical chemistry of complex feedstocks often necessitated the modeling of their reactions at a global lumped level. Lumping methodology12-14 reduces complexity by grouping the entire set of molecules into a small, manageable number of lumps which can be correlated with * To whom correspondence should be addressed. Fax: 34976733318. E-mail: [email protected].

process yield data. The two most commonly used globally lumped models are based on the boiling point or solubility characteristics. These global reaction models represent the interconversion of aggregates of many molecules with common attributes. The present paper reports the effects of temperature and space velocity on the extent of residue conversion of a petroleum residue catalytically hydrotreated. The convention used in this paper is that residue conversion means conversion to components whose boiling points are below 343 °C. A kinetic scheme involving lumped species consisting of the fraction over a 343 °C boiling point (atmospheric residuum, AR), the fraction below a 343 °C boiling point (light oils), and gases has been carried out. Furthermore, the number of moles of carbon-carbon bonds broken by reaction (following the method proposed by Trytten and Gray15) has been estimated and the relationships among AR fraction conversion and other important reactions (hydrodesulfurization, hydrodemetallation, etc.) have been studied. Experimental Section A Maya residuum, supplied by Repsol Petro´leo, S.A., was thermally cracked in a hydrogenation process. The residue used had a high concentration of metals (45.17 ppm nickel and 242.12 ppm vanadium) and of heteroatoms (3.45 wt % sulfur and 0.28 wt % nitrogen), with a heavy nature as indicated by the high kinematic viscosity (111.5 cSt at 50 °C). The distillation of the feed going on the D-2887 ASTM procedure16 gave a composition of 81.5% at 343 °C+ with 18.5% light gas oil boiling in the 195-343 °C range. Other important properties of the residue used as feed were shown previously.17 A commercial catalyst, Topsoe TK-711 (6 wt % MoO3 and 2 wt % NiO), supported on γ-Al2O3 was used. It is a catalyst specially developed for the pretreatment of residual oils. The characteristics of the catalyst and the way in which it has been sulfided have been previously described.17,18 Experiments were conducted continuously in an Autoclave-Engineers stirred tank reactor, with a capacity of 1 L and heated with an oven, whose control was

10.1021/ie9900768 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/03/1999

3286 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Table 1. Fractions (vol %) of the Liquid Products (Obtained by Simulated Distillation) and of the Gases from the Kinetic Experiments T(°C)/WHSV (L/h‚gcat) gases T < 195 °C 195 < T < 343 °C T > 343 °C 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

18 9.5 7.3 42.2 27.1 24.7 49.2 37.5 34.5

2.2 1.5 1.2 2.4 2.8 1.2 6.2 4.4 1.8

22 22 20.6 22.4 22.9 19.9 28.1 23.1 21.8

57.8 67 70.9 33 47.2 54.2 16.5 35 41.9

separated in three zones and measured by two thermocouples. The procedure for the experiments reported here has been described previously18 and the scheme and the details of the setup shown.17 All experiments were carried out at 12.5 MPa of hydrogen pressure and at different temperatures (375, 400, and 415 °C) and mass space velocities, WHSV, (between 1.4 and 7.1 L‚h-1‚gcat-1). The experiments were done in conditions with the absence of interphase gradients due to the choice of adequate stirring speeds and gas-liquid ratio and with the absence of intraparticle gradients due to the crushing of the used catalyst.18 A sample of gas from the experiments was removed for analyzing by a chromatograph Hewlett-Packard 5890 series II, using the columns Porapak N, 80/10 (10 ft); molecular sieve 134, 45/60 (3 ft); and xilene polimethil SE 30 (6 ft). The total liquid products from each experiment were analyzed for simulated distillation, which was used to estimate the boiling distribution of the oil samples, by the D-2887 ASTM procedure.16 From the structural analyses of feed and products previously reported,17,18 the number of carbon-carbon bonds per mole of oil sample, using the method proposed by Trytten and Gray,15 was estimated. Results and Discussion The distribution according to boiling points of the obtained liquid products into naphtha (C5-195 °C), middle distillate (195-343 °C), and atmospheric residuum (>343 °C) together with the gases are indicated in Table 1. It can be observed in this table that the lightest liquid fractions and gases increase by increasing the thermal cracking rates at 400 and 415 °C, confirming that the hydrocracking reactions are favored by the temperature, as has been stated by other authors,19,20 and by decreasing the space velocity of the feed mixture, a fact also indicated by Gary and Handwerk.21 Therefore, the maximum amounts of the distillates with boiling points below 343 °C and gases are obtained at the most severe conditions, maximum temperature and minimum WHSV. Reaction Model. Hydrocracking of petroleum feedstocks proceeds through a network of complex reactions involving a large number of components, ranging from molecules that remain in the residue to methane gas, which makes the kinetic study of these reactions extremely difficult. For simplification, both feed and products are divided into several boiling range fractions and each of these lumps or groups is considered to be a single chemical species with a single cracking rate constant.

Ayasse et al.22 demonstrated that the residue yields lighter liquids and light ends, or gases, as primary products upon cracking. Consequently, the simplest model that could capture this chemistry was a threelump model, consisting of one atmospheric residuum lump, light oils (the sum of the middle distillate fraction and the naphtha fraction), and gases. The components in this model are defined as atmospheric residuum (C1), light oils (C2), and gases (C3) where C is the concentration. The reactions light oils f atmospheric residuum and gases f atmospheric residuum were eliminated because the cracking reactions are considered to be irreversible.19 Furthermore, light oils have been only considered to be formed from higher-boiling lumps and therefore the reactions gases f light oils have also been eliminated. The reactions from the light oils lump were not considered because products in the naphtha and middle distillate fractions can be considered essentially unreactive under the conditions used and because the hydrogen is active in suppressing secondary cracking of the initial reaction products in addition to suppressing coke formation.23 The final form of the reaction model was therefore chosen to be

Since the hydrocracking reactions of bitumen and its fractions have been considered as first-order with respect to hydrocarbons19,24 and we work in a heterogeneous perfectly mixed continuously reaction system, equations describing the model are

C10-C1 k1 + k2 ) C1 WHSV

(

)(

)

(1)

k1 C10-C1 C2 - C20 ) C10 C1 WHSV

(2)

k2 (C10-C1)C3 ) C10C1 WHSV

(3)

C10 and C20 being the initial concentrations for the atmospheric residuum and light oils lumps, respectively, and k1 and k2 the intrinsic rate constants since we have worked in the absence of interphase and intraparticle gradients and therefore our reactions are kinetically controlled. The rate constants are listed in Table 2. It can be observed that the values of the kinetic constants increase with the temperature as could be expected. The experimental data at 375 and 400 °C are in agreement with the proposed model; nevertheless, at 415 °C, low correlation coefficients are obtained. For 375 and 400 °C, the proposed model consisting of the breaking of labile carbon to carbon bonds to form mainly distillates with hydrogen transfer to the fragments is likely appropriate25-27 and condensation of radical intermediates to eventually form reactor solids is not a significant reaction at these temperatures.

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Figure 1. . Average number of carbon-carbon bonds per molecule reduction percentages (HC), sulfur conversion, nitrogen conversion, nickel conversion, vanadium conversion, and reduction of the average molecular weights in the products plotted as a function of atmospheric residuum conversion. Table 2. Rate Constants [l/h.gcat] for the Hydrocracking Reactions and in Parentheses, the Standard Errors of the Estimated Coefficients k (AR conversion) correl. coeff. std. err. of estimation k (light oils formation) correl. coeff. std. err. of estimation k (gases formation) correl. coeff. std. err. of estimation

375 °C

400 °C

415 °C

1.13 (0.1) 0.97 0.44 0.073 (0.017) 0.90 0.007 0.21 (0.08) 0.82 0.03

3.26 (0.13) 0.99 0.07 0.25 (0.046) 0.93 0.02 1.5 (0.34) 0.92 0.18

9.2 (3.9) 0.69 1.52 (1) 0.60 5.12 (2.8) 0.64

The lack of fit of the data obtained at 415 °C could be related to the formation of coke due to polymerization reactions and to the increase of the aromaticity which provoke a modification of the distribution of the yields according to their boiling points. At 415 °C, the increase of the aromaticity and of the condensation degree seem to indicate the existence of condensation and coking reactions, which are favored at high temperatures. Nevertheless, the aromatic hydrogen increase and molecular weight decrease jointly with the decrease in the number of alkyl and hydroaromatic groups per molecule seem to suggest that the cracked paraffinic side chains are being cyclizated and, at the same time, dehydrogenated.17 Because of bad fits of the kinetic points obtained at 415 °C, Table 2, it has not been possible to calculate the activation energies and frequency factors using the integrated Arrhenius equation.

The conversion of atmospheric residuum does not allow for the cracking of very heavy material above the boiling point threshold, or for reactions which reduce the boiling point without any breakage of C-C bonds, such as the hydrogenation of aromatic rings and the removal of sulfur, nitrogen, and metals. Trytten and Gray15 presented a method for calculating the number of carbon-carbon bonds per mole of oil sample using a simple additivity method with data from the average molecular weight (AMW), the weight fractions of carbon, hydrogen, oxygen, nitrogen, and sulfur, and the fraction of aromatic carbon. Going on the method proposed by Trytten and Gray, the moles of carbon-carbon bonds broken by reaction without interference from other reactions such as catalytic hydrogenation and heteroatom removal were estimated for our conditions. In Figure 1, the values for the hydrocracking (HC) produced (considered as the number of carbon-carbon bonds per mole of oil sample reduction percentages) are plotted as a function of atmospheric residuum conversion. The values presented for HC have not been corrected for the carbon-carbon bonds lost to gases, but this amount was negligible, and never exceeded 0.03% of the total carbon-carbon bonds present. It can be observed in the figure that the highest value for HC has been 60%, which is lower than the maximum obtained value in the 343 °C+ fraction conversion. In all range of conversions, the values for HC have been lower than AR conversions, indicating that in our

3288 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

conditions the reactions of the removal of heteroatoms and metals and the hydrogenation of aromatic rings have contributed, to a great extent, to the reduction of the boiling point. In a residue hydrotreating unit, two important chemical processes go on, the removal of feed contaminants and the thermal cracking of feed to lighter products. In our experimental conditions, we have studied the way in which changes in conversion of residuum to materials boiling below 343 °C affect the removal of contaminants. The variation of the removal of the sulfur, nitrogen, nickel, and vanadium as a function of the atmospheric residuum conversion has also been plotted. In the figure, it can be observed that most of the sulfur conversion achieved occurs during the first 50% of AR conversion. During the period between 50 and 80% AR conversion, very little additional sulfur is removed from the reactor liquids. Until 50% residuum conversion is obtained, it is believed that side chains are being broken and whole molecules are largely being converted to distillate,28 and in the period between 50 and 80%, residua molecules which are being converted are thought to be highly condensed naphtenic and aromatic compounds.29 Sulfur removal during hydrocracking is generally considered to consist of both thermal and catalytic reactions.30,31 Thermal removal of sulfur most likely takes place from aliphatic sulfides and disulfides, whereas catalytic reactions would be required for the removal of sulfur from substituted thiophenes.29 Catalytic desulfurization takes place mainly from the distillable liquids rather than the unreacted residuum.14,28,32 In the figure, it can also be observed that overall nitrogen conversion increased as AR conversion increased up to approximately 55% AR conversion. As in the case of sulfur, very little additional nitrogen is removed at higher AR conversion. It is observed that the values of denitrogenation are lower than those of desulfurization. It is known33 that nitrogen removal is considered to be harder to achieve than sulfur removal, mainly because most, if not all, of the nitrogen is contained in heteroaromatic rings which require hydrogenation prior to nitrogen removal. Sanford and Chung34 indicated that nitrogen is removed through catalytic reaction as well as through carbonaceous solids being formed on the catalyst. Nevertheless, Sanford29 confirmed that all of the nitrogen which is removed from the liquid product is accounted for by the selective incorporation of nitrogen into the solids in the reactor and on the catalyst. With respect to the vanadium and nickel conversion plotted as a function of AR conversion, the behavior shown is similar. It can be seen that metal removal is progressive up to approximately 50% AR conversion. After this value, the denickelation and the devanadization appear to level off. As expected, most of the metals removed from the feed are deposited on the catalyst in the form of metal sulfides.35 Because of the existing relationship between the increase of the rate of cracking of hydrocarbons with the decrease in molecular weight,36,37 the values of the average molecular weights have also been plotted versus the 343 °C+ fraction conversion. It can be observed that there is a continuous decrease in molecular size as the extent of conversion increases. Average molecular weights, measured by vapor pressure osmometry in toluene, gave a feed molecular weight of 402 compared

to molecular weights between 207 and 348 for the products from the runs.17 Conclusions The process model describing the performance of the hydrotreatment unit and developed using lumped firstorder kinetic equations has been tested against experimental data and found to give a good representation of the measurements at 375 and 400 °C, but at 415 °C, the fits were bad. The number of carbon-carbon bonds per mole of oil sample produced in our kinetic experiments has been evaluated by a method reported by Trytten and Gray in 1990. The number of carbon-carbon bonds reduction percentages obtained by this method are lower than the conversion of atmospheric residuum in all ranges of conversions, indicating that the reactions of the removal of heteroatoms and metals and the hydrogenation of aromatic rings have contributed, to a great extent, to the reduction of the boiling point. The relationships among the desulfurization, the denitrogenation, the demetalation, and the decrease of the average molecular weights versus the conversion of atmospheric residuum have been studied, observing that most of the heteroatom and metal removal takes place during the first 50% of atmospheric residuum conversion. With respect to the relationship with the molecular weight, a continuous decrease has been observed as the extent of the atmospheric residuum conversion increases. Acknowledgment This work was sponsored by the UE Contract No. JOU2-CT92-0206 and the Spanish CICYT Project AMB93-1137-CE. Literature Cited (1) Wiehe, I. A. A Phase-Separation Kinetic Model for Coke Formation. Ind. Eng. Chem. Res. 1993, 32, 2447. (2) Kanda, N.; Itoh, H.; Yokoyama, S.; Ouchi, K. Mechanism of Hydrogenation of Coal-Derived Asphaltene. Fuel 1978, 57, 676. (3) Benito, A. M.; Martı´nez, M. T. Catalytic Hydrocracking of an Asphaltenic Coal Residue. Energy Fuels 1996, 10, 1235. (4) Savage, P. E.; Klein, M. T. Asphaltene Reaction Pathways. 3. Effect of Reaction Environment. Energy Fuels 1988, 2, 619. (5) Savage, P. E.; Klein, M. T.; Kukes, S. G. Asphaltene Reaction Pathways. 1. Thermolysis. Ind. Eng Chem. Process Des. Dev. 1985, 24, 1169. (6) Benito, A. M.; Martı´nez, M. T.; Ferna´ndez, I.; Miranda, J. L. Visbreaking of an Asphaltenic Coal Residue. Fuel 1995, 74, 922. (7) Schucker, R. C.; Keweshan, C. F. The Reactivity of Cold Lake Asphaltenes. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1980, 25, 155. (8) Benito, A. M.; Martı´nez, M. T.; Ferna´ndez, I.; Miranda, J. L. Upgrading of an Asphaltenic Coal Residue: Thermal Hydroprocessing. Energy Fuels 1996, 10, 401. (9) Gulf Publ. Co. Refining Handbook. Hydrocarb. Proc. 1988, 67, 61. (10) Mavity, V. T.; Ward, J. W., Jr.; Whitebread, K. E. Unicracking for Petrochemicals. Hydrocarbon. Process. 1978, 57, 157. (11) Steijns, M.; Froment, G.; Jacobs, P.; Uytterhoeven, J.; Weitkamp, J. Hydroisomerization and Hydrocracking. 2. Product Distributions from Normal-Decane and Normal-Dodecane. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 654. (12) Quann, R. J.; Jaffe, S. B. Structure-Oriented Lumping. Describing the Chemistry of Complex Hydrocarbon Mixtures. Ind. Eng. Chem. Res. 1992, 31, 2483.

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Received for review February 1, 1999 Revised manuscript received June 8, 1999 Accepted June 16, 1999 IE9900768