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Energy & Fuels 1996, 10, 1235-1240
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Catalytic Hydrocracking of an Asphaltenic Coal Residue A. M. Benito* and M. T. Martı´nez Instituto de Carboquı´mica, CSIC, P.O. Box 589, 50080 Zaragoza, Spain Received March 20, 1996X
Catalytic hydrocracking of a residue from deasphalting a syncrude obtained by direct coal liquefaction of a subbituminous Spanish coal was carried out at different temperatures and reaction times. Kinetic study of the cracking reaction has been performed. The viscosity, coke content, boiling point distribution, elemental analysis, and aromaticity of the reaction products have been determined. The experimental data fit with the first-order kinetic model proposed. The main effects observed with the thermal hydrotreatment have been a large decrease of the viscosity of products that varies from 4608 cSt in the feedstock to 79 cSt. The conversion of the heavy fraction (bp > 350 °C and soluble in toluene) increased with the temperature and residence time, and the formation of coke was inhibited even at the softest reaction conditions used.
Introduction Among other future challenges, refineries must cope with heavy feedstocks of lower quality, i.e., with higher viscosity, Conradson carbon, and heteroatom and metal contents. The poor quality of the crudes is generally due to their higher asphaltene content since a big percentages of heteroatoms and coke precursors reside in the asphaltene fraction.1 Consequently, this asphaltenic fraction will limit the efficiency of the refining process, leading to a quicker catalyst deactivation and higher coke yields production. Then, the chemical character of the asphaltenes has significant implication in their behavior during thermal and catalytic cracking, and it is not surprising that there have been numerous attempts to upgrade the technology for refining heavy oils containing asphaltenes and to understand the changes in asphaltenes during processing.2-4 Hydrogen-addition processes have long been used to upgrade heavy crudes and bitumens. Hydroconversion processes can be thermal or catalytic and consist of several thermal and/or catalytic stages. The term hydrocracking is applied to the catalytic hydrogenation of liquid hydrocarbons under the conditions in which the thermal cracking is produced. This process is very flexible and can be applied to a wide range of feedstocks from light naphthas to vacuum residua.5 Catalytic hydrocracking is used extensively in petroleum-refining processes to produce high-quality petroleum, diesel, and jet fuels.6 This process is carried out under hydrogen pressure in the presence of a catalyst or an additive to suppress coke formation. The operation conditions and catalysts utilized vary a great deal depending on the Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Quann, R.; Ware, R. A.; Hung, C. W.; Wei, J. Adv. Chem. Eng. 1988, 4, 95. (2) Calysalemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225. (3) Tranth, D. M. Z.; Start, S. M.; Petti, T. F.; Neurock, M.; Yasar, M.; Klein, M. T. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1993, 33 (2), 434. (4) Storm, D. A.; Sheu, E. Y.; Detar, M. M.; Barresi, R. J. Energy Fuels 1994, 8, 8567. (5) Mavity, V. T.; Ward, J. W.; Whitebread, K. E. Hydrocarbon Process. 1978, 57 (11), 157. (6) Steijus, M.; Froment, G. F.; Jacobi, P.; Vytherhowren, J. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20 (4), 654. X
desirable products. Conversion of aromatics in these feeds yields lower molecular weight species with increased H/C ratios. This requires the catalyst to be dual-functional, having a metal component for hydrogenation and an acid component for cracking.7 It has been assumed that during residuum hydrocracking at least some of the residue-forming aromatics are hydrogenated to the corresponding naphthenes,8 by analogy with hydrotreating, since the catalyst is chemically the same (pore volume, surface area, and metal levels may differ). However, there are many conditions present in residuum hydrocracking that would normally limit aromatic hydrogenation reactions;9 for example, the high temperature required for cracking reactions (400-450 °C) together with the aromatic levels in the residuum and the coke-forming tendency of the residuum itself would all tend to deactivate the catalyst very quickly.10 On the other hand, the high temperatures are thermodynamically unfavorable for aromatic hydrogenation11 and would be therefore expected to produce some dehydrogenation. The way the hydrogen is incorporated during the hydroconversion process has been intensively studied as well.12,13 It has been shown that when residue is hydrocracked in the absence of a catalyst, there is no incorporation of gaseous hydrogen into the residue fraction and only a small amount of hydrogen is incorporated in the distillate fractions. With an active hydrotreating catalyst, gaseous hydrogen is incorporated into all distillate fractions and the unconverted residuum. The incorporation of the hydrogen then is likely due mainly to the hydrogenation of aromatics and heteroaromatics to the corresponding hydroaromatics or naphthenes. (7) Vansina, H.; Baltanos, M. A.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 526. (8) Beaton, W. I.; Bertolacini, R. J. Catal. Rev.-Sci. Eng. 1991, 33, 281. (9) Sanford, E. C. Energy Fuels 1995, 9, 549. (10) Absi-Halabi, M.; Stanislaus, A.; Trinm, D. L. Appl. Catal. 1991, 72, 193. (11) Yui, S. M.; Sanford, E. C. Can J. Chem. Eng. 1991, 69, 1087. (12) Weitkamp, J. Akzo Catalysis Symposium; Scheveningen, The Netherlands, May-June 1988. (13) Russel, C. L.; Klein, M. T. Energy Fuels 1994, 8, 1394-1400.
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1236 Energy & Fuels, Vol. 10, No. 6, 1996
Benito and Martı´nez
Table 1. Raw Syncrude Propertiesa kinematic viscosity (cSt, 65 °C) elemental analysis of the fraction soluble in toluene C (%) H (%) N (%) S (%) O (%) H/C atomic ratio N/C × 103 atomic ratio S/C × 103 atomic ratio boiling point distribution (%) L (bp < 350 °C) H (bp > 350 °C) C distribution according to solubility (%) oil asphaltene coke a
4608 81.78 6.38 1.55 4.65 5.64 0.94 16.25 21.32 23.63 61.42 14.95 19.58 65.47 14.95
L, light fraction; H, heavy fraction; C, coke fraction.
Progress in the hydrocracking processes has been always influenced by the development of more active and selective catalysts. The most used ones have been the supported catalysts such as nickel-tungsten in alumina or cobalt-molybdenum in alumina that contain the hydrogenate function in the metal and the cracking function in the acid component used as support. These acid catalysts are very sensitive to the presence of nitrogen compounds in the feedstock. In this work, a residue from the deasphalting of a synthetic crude obtained by direct coal liquefaction of a subbituminous Spanish coal14 has been processed by catalytic hydrocracking. The raw syncrude was processed through two upgrading routes, the oils were hydrogenated in two steps,15 and the residue has been upgraded by thermal cracking,16 hydrothermal cracking, and catalytic hydrocracking17,18 to get an integral use of natural resources. The aim of this paper is to study the effect of the temperature and residence time during the hydrocracking experiments by measuring the change produced in the kinetic parameters, viscosity, coke content, elemental analysis, aromaticity, and boiling point distribution as well as to compare these results with those obtained in the hydrothermal process of the same residue previously studied in our laboratory.18 Experimental Section The feedstock used in this experiment was an asphaltenic residue from coal liquids. Operating conditions of the liquefaction and deasphalting processes have already been described in detail.14,19 Some of the feedstock properties are shown in Table 1. Experiments were carried out batchwise in a stainless steel tubular reactor; the equipment and experimental procedures were similar to those described by Benito et al.16-18 Experiments were carried out in duplicate, with 25 g of the sample in each reactor. In this process, a supported catalyst commercially available, Harshaw HT-500 E (15% wt MoO3 and (14) Martinez, M. T.; Fernandez, I.; Benito, A. M.; Cebolla, V.; Miranda, J. L.; Oelert, H. H. Fuel Process Technol. 1993, 33, 159. (15) Fernandez, I.; Martinez, M. T.; Benito, A.; Miranda, J. L. Fuel 1995, 74 (1), 32. (16) Benito, A. M.; Martinez, M. T.; Fernandez, I.; Miranda, J. L. Fuel 1995, 74 (6), 922. (17) Benito, A. M.; Martinez, M. T.; Miranda, J. L. Coal Science and Technology 24. In Coal Science; Pajares, J. A., Tascon, J. M. D., Eds.; Elsevier: New York, 1995; pp 1467-1470. (18) Benito, A. M.; Martinez, M. T.; Fernandez, I.; Miranda, J. L. Energy Fuels 1996, 10 (2), 401-408. (19) Benito, A. M.; Brouwer, L.; Martı´nez, M. T.; Severin, D.; Ferna´ndez, I. Fuel Science Technol. Int. 1994, 12, 1509.
3.5% wt NiO), in extrudates of 1/16 in. was used. The catalyst was previously presulfured during 4 h at 400 °C and atmospheric pressure, under a hydrogen flow of 10% in volume of H2S. The catalyst was mixed intimately with the feedstock in the relation 1:5 (catalyst:sample), the same in all of the experiments, and steel balls were added to this mixture. A determined hydrogen pressure different in each experiment was used in such a way that the hydrogen pressure at the operating temperature was the same in all of the processes (15 MPa). Thus, the pressure and catalyst concentration remained constant in all of the experiments, and we studied only the temperature and reaction time effects. This operation is valid since we are working with a large excess of hydrogen. Kinetic experiments were carried out at different temperatures (425, 450, and 475 °C) and reaction times (5, 10, 20, 30, and 40 min). After the reaction, the microreactor was cooled and the gas was recovered in a gas sampling bag for chromatographic analysis. The nonvolatile products of reaction were slurried in toluene and filtered after ultrasonic disgregation. The solvent was then removed from the filtrate on a rotatory evaporator, and the liquid was weighed. The solid was dried and weighed as well. The hydrocracked products obtained in each experiment were analyzed for viscosity (at 65 °C), coke content (insoluble material in toluene), boiling point distribution (determined by gas chromatography), elemental analysis (C, H, S, N), and aromaticity (analyzed by 1H NMR) following the analytical methods in ref 18.
Results In the hydrocracking process coexist three phases: solid (coke and catalyst), liquid (starting syncrude and obtained liquid products), and gas (unreacted hydrogen and gases produced during the process, H2S, CO2, C1C4). We have considered here that the hydrogen concentration in the liquid phase is constant and there are no problems of gas-liquid diffusion as we have done in previous thermal hydroprocessing experiments performed with the same syncrude.18 Coke is not involved in diffusion problems since it is a final product, but there are diffusion limitations for the catalyst mainly due to its porous support. Gollakota20 showed that to minimize the resistance to the mass transference liquid/solid and intraparticle, both affecting the catalyst, it is necessary to minimize the particle size and maximize the frequency and amplitude of agitation. Additionally, using a vertical configuration of the reactor and adding systems that help the mixing inside the reactors, like steel balls, help to avoid the diffusion problems. For our calculations, we assume the hydrocracking process to be kinetically better than a diffusionally controlled one since we used a high speed of agitation (300 cycles/ min) and steel balls together with the feedstock that ensured the perfect mixture of the reactants. The ratio diameter of the reactor to diameter equivalent of the catalyst particle is above the inferior limit of 4 according to the criteria followed by some authors.21-23 The feedstock had a heavy nature (Table 1) as can be seen from the high viscosity and content of the fraction insoluble in n-hexane (80.42%, asphaltene + coke). The high heteroatom content of the syncrude shows as well its polar nature. (20) Gollakota, S. V. Ph.D. Dissertation, Auburn University, 1984. (21) Perry, M. B.; Pukanic, G. W.; Ruether, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34 (4), 1206. (22) Doraiswamy, L. K. Catal. Rev. Sci. Eng. 1974, 10 (2), 177. (23) Chen, Y. W. Ind. Eng. Chem. Res. 1990, 29, 1830.
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Energy & Fuels, Vol. 10, No. 6, 1996 1237
Figure 2. Representation of the logarithm of the heavy fraction vs time.
Figure 3. Representation of the Arrhenius equation. Table 2. Kinetic Parameters Obtained in the Hydroprocessing Experiments temp (K)
Figure 1. Evolution of the different fractions obtained by hydrocracking.
In the hydrocracking experiments, the heavy fraction (H, bp > 350 °C) decreased with time, while the concentration of the light fraction (L, bp < 350 °C) and gas (G) increased with time (Figure 1). This behavior was observed as well in the hydrothermal process (without catalyst) of the same syncrude,18 but in that case the coke content (C) increased with time, while in the hydrocracking experiment the coke content hardly varied with time. This behavior is not surprising since the presence of catalyst in the reaction makes hydrogen more effective in the stabilization of the formed radicals. Coke suppression by hydrogen is normally thought of as capping of free radicals which were generated by homolytic cleavage of carbon-to-carbon bonds. In many cases, such a reaction would result in a distillate molecule and a residuum molecule of lower molecular weight than the original feed molecule. The catalyst would help in this case to stabilize these fragments before any further cracking. The cracking reaction to form distillates followed a first-order kinetic law for all experimental temperatures. The observed inhibition with time of the condensation reaction to produce coke for all of the temperatures essayed made us propose a simple model in which H is cracked to produce distillates (L + G). Taking into account that there is an excess of hydrogen in the reaction, the pressure is the same in all of the experiments, and it is valid to consider constant the concentration of hydrogen in the liquid, we have proposed a pseudo-first-order kinetic with respect to the
425 450 475 Ea (kJ/mol)
thermal process K1 (s-1)
catalytic process K1 (s-1)
3.10 × 10-5 7.50 × 10-5 9.59 × 10-5
4.98 × 10-5 6.16 × 10-5 12.60 × 10-5
97
80
heavy fraction: K1
H 98 L + G The integrated equation that describes the process is
-ln(H/Ho) ) K1t The kinetic constants were obtained from the representation ln H vs time (Figure 2), and the activation energy was calculated from the semilogarithmic representation of ln K vs 1/RT (Figure 3) by applying the Arrhenius equation
k ) Ae-Ea/RT where A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature (K). The kinetic parameters obtained for the catalytic and thermal hydroprocesses for the cracking reaction are detailed in Table 2. Acceptable correlations have been found for all of the experimental temperatures, and the obtained Ea (80 kJ/mol) for the catalytic process is lower than the Ea found in the hydrothermal experiments18 (97 kJ/mol). Then, for the same temperature and time conditions, the conversion to L + G was higher when a catalyst was used. This is in agreement with the role of the catalysts that decrease the energetic requirements needed to produce the reac-
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1238 Energy & Fuels, Vol. 10, No. 6, 1996
Benito and Martı´nez Table 3. Elemental Analysis of the Fraction Soluble in Toluene of the Liquid Fraction Obtained in the Hydrocracking Experiments residence time 5 min 10 min 20 min 30 min 40 min Experiments at 425 °C C (%) 82.45 83.24 83.03 H (%) 6.33 6.49 6.40 N (%) 1.58 1.66 1.68 S (%) 4.42 4.10 3.80 O (%) 5.22 4.51 5.09 H/C atomic ratio 0.92 0.94 0.92 N/C × 103 atomic ratio 16.43 17.09 17.34 S/C × 103 atomic ratio 20.10 18.47 17.16
83.87 6.47 1.71 3.50 4.45 0.93 17.48 15.65
84.50 6.26 1.79 3.18 4.27 0.89 18.16 14.11
Experiments at 450 °C C (%) 82.90 83.58 84.14 H (%) 6.35 6.48 6.23 N (%) 1.64 1.65 1.66 S (%) 4.35 3.86 3.25 O (%) 4.76 4.43 4.72 H/C atomic ratio 0.92 0.93 0.89 N/C × 103 atomic ratio 16.96 16.92 16.91 S/C × 103 atomic ratio 19.68 17.32 14.48
84.70 6.15 1.72 3.06 4.37 0.87 17.41 13.55
84.83 6.13 1.71 2.88 4.45 0.87 17.28 12.81
Experiments at 475 °C C (%) 83.08 83.72 86.88 H (%) 6.33 6.04 5.89 N (%) 1.64 1.68 1.69 S (%) 4.12 3.65 2.92 O (%) 4.83 5.00 2.62 H/C atomic ratio 0.91 0.87 0.81 N/C × 103 atomic ratio 16.92 17.20 16.67 3 S/C × 10 atomic ratio 18.60 16.35 12.60
86.02 5.81 1.68 2.67 3.82 0.81 16.74 11.64
86.43 5.92 1.70 2.58 3.37 0.82 16.86 11.19
Table 4. Hydrogen Distribution and Structural Parameters Obtained by 1H NMR of the Fractions Soluble in Toluene (A) Raw Syncrude NMR data Har fa σ Har/Car
58.45 0.84 0.18 0.78 (B) Experiments residence time
Figure 4. Comparison between the experimental results and the kinetic curves calculated for every pseudocomponent.
tion and consequently increase the rate of the reaction. The kinetic constants for the cracking reaction are higher in the catalytic than in the hydrothermal process. The maximum conversion to L (L - Lo)/Ho × 100) was 28% and to G (G/Ho × 100) was 6.7% at 475 °C and 40 min. When a catalyst was not used, the maximum conversion to L was 22% and to G 8% at the same conditions. Then, the observed effects of using a catalyst are a higher extension of cracking to produce liquid products and lower to produce gases and an inhibition of coke formation. Figure 4 shows a comparison between the experimental results and the kinetic curves calculated for every pseudocomponent. The concordance was good, and it can be said that the cracking reaction fitted with the proposed scheme. The chemical changes produced in this asphaltenic syncrude were also studied in this work. Elemental analysis, Table 3, of the fraction soluble in toluene (that is after removing the coke) gave us information about the removal of heteroatoms and therefore the changes in the polarity of the syncrude. This analysis combined
5 min
30 min
40 min
Har fa σ Har/Car Ho/HR
Experiments at 425 °C 58.71 58.94 57.66 0.84 0.83 0.83 0.18 0.18 0.19 0.79 0.80 0.79 1.17 1.18 1.19
10 min
20 min
59.54 0.84 0.17 0.79 1.14
60.72 0.85 0.17 0.76 1.08
Har fa σ Har/Car Ho/HR
57.65 0.83 0.19 0.79 1.07
Experiments at 450 °C 57.65 60.62 0.84 0.85 0.18 0.16 0.79 0.75 1.03 1.31
60.87 0.86 0.16 0.73 1.30
62.54 0.86 0.16 0.74 1.21
Har fa σ Har/Car Ho/HR
59.41 0.84 0.18 0.79 1.07
Experiments at 475 °C 62.42 65.92 0.86 0.88 0.17 0.13 0.76 0.70 1.09 1.06
67.97 0.89 0.14 0.72 0.91
67.56 0.89 0.14 0.72 0.89
with the data obtained by 1H NMR (Table 4) gave us information about the hydrogen incorporation in the syncrude and the changes of aromaticity produced during the course of the reaction. In the hydrocracking process a decrease of the H/C and S/C atomic ratios with time and temperature was produced while the N/C atomic ratio was hardly modified. It can be said that in our conditions during the
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Energy & Fuels, Vol. 10, No. 6, 1996 1239
Table 5. Composition of the Gaseous Fraction Obtained in the Hydrocracking Experiments (Feedstock Basis, wt %) residence time gas
5 min
10 min
20 min
30 min
40 min
0.150 0 1.090 0.110 0.160 0.190 0.300
0.250 0.009 0.970 0.220 0.230 0.350 0.280
CH4 CO CO2 C2H6 SH2 C3H8 C4H10
0.001 0 0.010 0.001 0.002 0.001 0.005
Experiments at 425 °C 0.003 0.100 0 0 0.020 0.540 0.002 0.080 0.006 0.110 0.003 0.170 0.010 0.160
CH4 CO CO2 C2H6 SH2 C3H8 C4H10
0.004 0 0.040 0.005 0.009 0.006 0.006
Experiments at 450 °C 0.006 0.160 0 0.110 0.080 0.650 0.005 0.140 0.020 0.170 0.010 0.170 0.008 0.230
0.250 0.010 0.810 0.210 0.220 0.260 0.240
0.280 0.010 0.930 0.240 0.290 0.340 0.250
CH4 CO CO2 C2H6 SH2 C3H8 C4H10
0.02 0 0.09 0.01 0.02 0.01 0.02
Experiments at 475 °C 0.12 0.43 0 0.03 0.45 0.93 0.09 0.31 0.13 0.29 0.09 0.29 0.20 0.18
0.71 0.03 1.40 0.53 0.48 0.54 0.25
0.82 0.03 1.41 0.57 0.49 0.54 0.28
hydrocracking process the hydrogenation reaction was not produced. At the same time, the nitrogen content was basically constant, which is logical since it is necessary to hydrogenate the nitrogen heterocyclic before nitrogen removal can occur,24 and H/C decreased. Then, the H/C and N/C tendencies indicated that hydrogenation and denitrogenation reactions were not produced in our conditions. A higher decrease of the S/C atomic ratio and a lower decrease of the H/C atomic ratio were produced in the catalytic hydrocracking compared to the hydrothermal process. Therefore, the main change produced in this process was a removal of the S and O in the structures of the syncrude. These heteroaromatic changes were reflected in the composition and variation with time and temperature of the gaseous fraction obtained (Table 5): H2S and CO2 are produced in high yield, and this increased with time and temperature. This results fitted with the lower S/C atomic ratio and O content of the liquid fraction observed when time and temperature increased. At 425 °C, there is a little cracking production as can be seen for the low percentage of the gaseous fraction C1-C4, and this increased as the time and temperature went up. When a catalyst is used, the production of the gaseous fraction C3-C4 was higher than that of C1-C2, compared to the process without catalyst. Hydrogen has been more effective in stabilizing the alkyl chains before they cracked again. For that the gaseous fragments are bigger. When the alkyl chains are cracked to form gas, radicals CnH2n+1• (n ) 1-4) are formed. After stabilization with hydrogen, the result is that the feed molecule has experienced the loss of two hydrogen atoms for every carbon atom lost. This was the main factor that made the H/C ratio decrease in our process. The main part of the hydrogen in the liquid fraction is aromatic hydrogen (Har) (Table 4), and this increased slightly with time and temperature. This gave a (24) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021.
Figure 5. Boiling point distribution at the three temperatures essayed in the hydrocracking experiments.
relatively high aromaticity parameter (fa) that became higher with time and temperature. These results together with the decrease of the H/C atomic ratio indicated that there has been no apparent incorporation of hydrogen in the liquids in these conditions and the main reaction has been the cracking reaction. The hydrogen mainly contributed to the production of hydrogen-rich volatile compounds and to the stabilization of the free radicals formed during the cracking reaction, decreasing the extension of condensation reactions to produce coke. It has been seen that there is a decrease in the substitution degree of the aromatic rings (σ) and in the length of the aliphatic chains (Ho/HR) greater than that observed in hydrothermal experiments, and the aromaticity (fa) increased only slightly and to a lower degree than in the process without catalyst. The Ho/HR and σ decreases indicated that the cracking reaction has been produced in the aliphatic chains and mainly in those attached to aromatic rings, and the decrease of this parameters with time and temperature fitted with a higher extension of the cracking reaction. The observed aromaticity increase of the liquid fraction was due then
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1240 Energy & Fuels, Vol. 10, No. 6, 1996
to the alkyl chain removal from the aromatic rings as well as the breaking of the alkyl chains bonded to aromatic rings to give shorter ones. The gases formed in these ruptures produced less dehydrogenation than in the hydrothermal process, which can explain the higher effectiveness of the hydrogen in the stabilization of the formed radicals. This was due to the presence of the catalyst in the process. Assuming the role of the catalytic additives25 that can produce atomic hydrogen in their interphase, it is easy to understand that the catalyst acted as a solid hydrogen donor, and then the stabilization of the radicals and hydrogenolysis of the carbon-heteroatom bonds before heteroatom removal were more easily accomplished than in the process without catalyst. Har/Car, the parameter giving information about the aromatic hydrogen to aromatic carbon ratio in the hypothetical case in which the rings were not substituted, had values ranging from 0.7 to 0.8. Therefore, the main aromatic structures had two to three condensed aromatic rings and approached to three when time and temperature increased. So, the liquids had higher aromaticity, shorter alkyl chains, and lower substitution degree when the reaction conditions became harder. From these results it can be inferred that the extension of the condensation reactions has been quite insignificant, which resulted in no manifest increase of coke in the process. The boiling point distribution obtained by GC is shown in Figure 5. An increase of the fraction with bp < 450 °C is observed. The highest increase was produced in the fraction with bp < 350 °C (31% at the hardest conditions). The fractions with boiling point in the interval 350-400 °C increased to a lower degree (12% and 14%, respectively, at the hardest conditions), and a big decrease of the fraction with bp > 450 °C was observed. Then, the cracking reaction was mainly produced in this fraction to give compounds of lower boiling point. The viscosity decreased very quickly with time and (25) Gray, M. R.; Khorasheh, F.; Wanke, S. E. Energy Fuels 1992, 6, 478.
Benito and Martı´nez Table 6. Kinematic Viscosity of the Liquids Obtained in the Hydroprocessing Experiments (cSt, 65 °C) residence time 5 min 10 min 20 min 30 min 40 min experiments at 425 °C experiments at 450 °C experiments at 475 °C
358 293 175
203 166 120
165 112 112
147 85 84
79 82 79
temperature (Table 6). Ninety-eight percent of the viscosity reduction was obtained at the most severe conditions, changing from 4608 cSt in the raw material to 79 cSt. Conclusions The hydrocracking process has been effective in reaching the two main objectives in the treatment of residua: conversion to distillates and viscosity reduction. The presence of hydrogen and catalyst inhibited the condensation reactions and therefore coke formation. The experimental results fitted with the proposed first-order kinetic, and a decrease of the Ea was obtained compared to the hydrothermal process. Increasing the severity of the reaction conditions produced more formation of distillates and gases but no apparent increase of the coke content. Heteroatom removal is another important aim in the conversion of residua. S and O were the heteroatoms removed during the process. However, the removal of N was hardly produced, which indicated that N is integrated in the condensed aromatic structures that are difficult to crack. Aromaticity of the liquid fraction increased in the process when the reaction conditions became harder. This could be due to alkyl chain removal or alkyl chain breakage to give shorter ones. When a catalyst is used, a higher yield of C3-C4 gases is produced, and a lower yield of C1-C2 gases was produced when a catalyst together with hydrogen was present as a result of a more effective stabilization of the radicals formed. EF9600467
