Energy & Fuels 2001, 15, 751-755
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Molar Kinetics and Selectivity in Cracking of Athabasca Asphaltenes Yingxian Zhao† and Murray R. Gray* Department of Chemical and Materials Engineering,University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Keng H. Chung Syncrude Canada Ltd., 9421-17 Avenue, Edmonton, Alberta, Canada T6N 1H4 Received December 22, 2000. Revised Manuscript Received February 15, 2001
The thermal decomposition of Athabasca bitumen asphaltenes was investigated in the temperature range 350 to 430 °C. The cracking kinetics of the asphaltenes and their intermediates were analyzed on a total a molar basis, to avoid the assumptions inherent in lumped kinetic models. The apparent first-order activation energy was 176 kJ/mol over the temperature range. Reaction selectivity changed from evolution of hydrogen sulfide to evolution of hydrocarbon gases and liquids over the same range of temperature. This change was consistent with a shift of the controlling reaction mechanism from the cleavage of C-S bonds to the cleavage of C-C bonds as temperature increased past 400 °C. The formation of hydrocarbon gases was the dominant reaction on a molar basis at temperatures over 400 °C, therefore, these reactions require more attention in mechanistic models for cracking of heavy petroleum fractions.
Introduction The heavy, asphaltenic fractions of heavy oils and bitumens present challenges for both processing and analysis. The toluene-soluble, pentane-insoluble fraction, or asphaltenes, is responsible for much of the cokeforming tendency of the oil.1 Previous work on narrow molecular weight fractions of Athabasca bitumen pitch prepared by super-critical fluid extraction (SCFE) by n-pentane showed that all the asphaltene-free SCFE front-cuts had similar reactivity, except the asphaltenerich SCFE residue.2,3 The SCFE residue had a lower reactivity and much higher coke formation propensity. These findings raise an important question: What physicochemical characteristics cause the dramatic difference in behavior between the SCFE residue and the rest of the 524+ °C fraction of bitumen? Understanding the reactions of asphaltenes also presents a considerable analytical challenge. The starting material can be viewed as a random oligomer of aromatic cores with linking groups and side chains,4 making separation and quantitation of molecular weight distributions by chromatographic means very difficult. The traditional approach has been to define kinetics on the basis of masses of solubility fractions, such as resins, aromatics, and saturates, but this approach is funda* Author to whom correspondence should be addressed. Phone: 780492-7965. Fax: 780-492-2881. E-mail:
[email protected]. † Present address: Value Creation Group, Suite 705, 777-Eighth Avenue SW, Calgary, Alberta, Canada. (1) Wiehe, I. A. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1993, 38, 428-433. (2) Chung, K. H.; Xu, C. M.; Gray, M. R.; Zhao, Y. X.; Kotlyar, L.; Sparks, B. Rev. Process Chem. Eng. 1998, 1, 41-79. (3) Gray, M. R.; Zhao, Y.; McKnight, C. M.; Komar, D. A.; Carruthers, J. D. Energy Fuels 1999, 13, 1037-1045. (4) Wiehe, I. A. Energy Fuels 1994, 8, 536-544.
mentally unsatisfactory. Such mass-based kinetics are not based on changes in the number of moles of species due to cracking, nor are the chemical distinctions between the various classes simple functions of molecular weight.5 Some progress has been made in applying the kinetics from polymer decomposition to the cracking of asphaltenes,6 but the decomposition of asphaltenes to give products from methane through to coke still presents difficulties in determining the characteristics of the overall molecular weight distribution. In addition, the bonds connecting the constituent units of asphaltenes are heterogeneous in their strength, ranging from aliphatic thioethers through to biphenyl linkages,7 therefore, the assumption of uniform cracking kinetics that works so well for polymer decomposition must be examined carefully for the case of asphaltenes. This paper investigates the thermal decomposition of Athabasca asphaltenes, and presents a simple kinetic analysis that minimizes assumptions and analytical complexity. Supercritical fluid extraction (SCFE) was used to prepare the asphaltenic fraction of bitumen vacuum bottoms in sufficient quantity for reaction studies.8 Experiments were performed by reacting asphaltenes in a microbatch reactor at 350 to 430 °C, under hydrogen pressure to minimize formation of toluene insolubles (i.e., coke). The decomposition of asphaltenes was characterized by determining molecular weight and sulfur content of the feed material before and after reaction. This analysis provided a basis (5) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530-536. (6) Kodera, Y.; Kondo, T.; Saito, I.; Sato, Y.; Ukegawa, K. Energy Fuels 2000, 14, 291-296. (7) Strausz, O. P.; Mojelsky, T. W.; Faraji, F.; Lown, E. M. Energy Fuels 1999, 13, 207-227. (8) Chung, K. H.; Xu, C. M.; Hu, Y. X.; Wang, R. N. Oil Gas J. 1997, January, 66.
10.1021/ef000286t CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001
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for analysis of the bond breakage reactions that are important in thermal decomposition of asphaltenes. Theory The network of reactions for asphaltene decomposition can be expressed as a series of reactions as follows:
∑Bi f ∑Pi f ∑Pj f ∑Pk f
(1)
where Bi is the ith component of the SCFE asphaltenes and Pi, Pj, and Pk are the ith, jth and kth intermediate products, respectively. The overall reaction of asphaltenes under hydrogen pressure can be expressed in terms of lumped gas, liquid, and solid products as
∑aB,iBi + aHH2 f ∑aL,iPL,i + ∑aG,iPG,i + ∑aS,i PS,i + a′HH2
(2)
where PL, PG, and PS are the liquid, gas, and solid products, and al,i is a stoichiometric coefficient. Assuming that the hydrogen consumption is negligible (aH ≈ a′H), then at low-severity reaction conditions where the solid yield is negligible (aS,i ≈ 0), eq 2 can be simplified as
B f aLPL + aGPG
(3)
where B is the SCFE asphaltene feed, PL is the liquid product, and PG is the gas product. The material balance on a molar basis can be expressed as
ntot ) (1 - aL - aG)nB + (aL + aG)nB0 + nL0 + nG0 (4) where ntot is the total moles of the feed and products, nB, nG, and nL are the moles of the feed, gas products, and liquid products at time t, respectively, and nB0, nG0 and nL0 are the moles of the feed, gas products, and liquid products at time zero, respectively. Hence, the rate of cracking of asphaltenes is defined using eq 3:
r)-
dnB 1 dnL 1 dnG ) ) ) dt aL dt aG dt dntot 1 1 dntot-G ) (5) aL + aG - 1 dt aL - 1 dt
Assuming a power law rate expression of the form:
-
dnB ) knm B dt
(6)
where k is the apparent rate constant and m is the apparent reaction order, then the kinetic relations can be solved for a given reaction order. For first-order kinetics, m ) 1 and eqs 4, 5,and 6 can be solved to give:
(nL0 + nG0) ntot (aL + aG - 1)k ) t+1+ n B0 nB0 n B0
(7)
For zero-order kinetics, m ) 0 and
(nL0 + nG0) ntot (aL + aG - 1)k ) t+1+ n B0 nB0 n B0
Table 1. Composition of Athabasca Asphaltenes from Supercritical Fluid Extraction yield on Athabasca vacuum bottoms, wt % molecular weight sulfur, wt % nitrogen, ppm aromatic C, mol % Ni, ppm V, ppm micro carbon residue, wt % asphaltene (n-heptane), wt %
40.4 3870 6.51 10500 49 339 877 48.9 88.0
Using this total molar approach, kinetic analysis requires only the masses of the products and their average molecular weights, to calculate ni(t). Unlike the conventional lumped-kinetic analysis, this analysis does not ignore important cracking reactions within lumped species such as asphaltenes. Experimental Section The asphaltenes were obtained as a residual fraction from supercritical fluid extraction (SCFE) of Athabasca vacuum bottoms from Syncrude Canada Limited.8 This material contained 88% heptane insolubles and 5% toluene insolubles (Table 1). Consequently, the fraction provided an asphaltenerich material for kinetic experiments. All reaction experiments were carried out in a 15 mL microbatch reactor at 4.2 MPa initial hydrogen pressure (at room temperature), giving an initial pressure at reaction conditions of 9-10 MPa. The reaction temperature and time were varied from 350 to 430 °C, and from 2.5 to 60 min, respectively. In a typical experiment, 5 g of the asphaltene feed was loaded into the reactor. The reactor was then pressurized with hydrogen, and immersed in a preheated fluidized sand bath at a set temperature and time. After the reaction was completed, the reactor was cooled in water. Gas product was collected and analyzed in selected cases, and the mass of volatiles was determined by weighing the reactor before and after venting the off-gases. The remaining products, solids and liquids, were washed out of the reactor with toluene. The liquid and solid products were separated by filtration using a 0.22 µm Millipore filter. The toluene in the filtered liquid was evaporated using a rotary vacuum evaporator in a hot water bath, and then dried in a vacuum oven at 110 °C for 2 h. The “dried” liquid product was collected for analysis. The solid (toluene-insoluble) on the filter paper was also vacuum-dried, and analyzed. Yields of liquids and solids were calculated on the basis of the initial amount of SCFE asphaltene on a solid-free basis. The yield of gases was calculated with correction for the initial hydrogen, and assuming negligible consumption of hydrogen under these noncatalyzed conditions. The average molecular weight and sulfur content of the solid-free asphaltenes and liquid products were determined in the Micro Analytic Laboratory at University of Alberta. The average molecular weight was measured on a molar or number-average basis by vapor pressure osmometry (VPO; Corona Wescan molecular weight analyzer, Model 1232A). The method was similar to ASTM Method D2503 except that the solvent was o-dichlorobenzene at 120 °C in order to fully disperse the asphaltenes.5 The hydrocarbons in selected gas samples were separated by gas chromatography and detected by flame ionization (Hewlett-Packard 5840A). The yield of hydrogen sulfide was calculated from the sulfur content of the liquid products before and after reaction, for the experiments with negligible coke yield. The yield of hydrocarbons was the total yield of gas less the yield of hydrogen sulfide.
Results and Discussion
(8)
Figure 1 shows representative product yields from asphaltenes over a period of 30 min at 430 °C. The
Cracking of Athabasca Asphaltenes
Figure 1. Product yields in asphaltene decomposition at 430 °C and 4.2 MPa H2 (room temperature) as a function of reaction time.
Figure 2. Average molecular weight of liquid products from asphaltene decomposition as a function of time.
average material balance was 99.3 ( 0.2% (95% confidence interval). Coke yields and gas yields both gave a standard deviation of ( 0.5 wt % on replicate experiments. At 5 min reaction time, the gas and coke yields were insignificant, and liquid yield accounted for more than 98% of the feed. At longer reaction times of 30 and 60 min, the gas and coke yield were higher, as expected. The coke yield showed a 10 min induction time and then increased sharply. Such an induction period for coke formation has been reported by many workers.1,9-12 At lower reaction temperatures, below 400 °C, the formation of coke was much less significant. For example, at 370 °C the liquid yield was 98% after 1 h of reaction. The data of Figure 2 show that the average molecular weight of liquid product decreased rapidly with increasing reaction severity. The molecular weight measurements were repeatable to ( 8% on replicate kinetic experiments. This mean molecular weight included both unreacted starting material as well as products. Some low boiling material (molecular weight less than about 100) was lost upon evaporation of the toluene solvent prior to determining molecular weight, but this would not represent a significant error. At a given reaction (9) Levinter, M. E.; Medvedeva, M. I.; Panchenkov, G. M.; Agapov, G. I.; Galiakbarov, M. F.; Galikeev, R. K. Khim. Tekhol. Topl. Masel. 1967, 4, 20. (10) Magaril, R. Z.; Aksenova, E. I. Khim. Tekhol. Topl. Masel. 1970, 7, 22. (11) Valyavin, G. G.; Fryazinov, V. V.; Gimaev, R. H.; Syunyaev, Z. I.; Mulyyukov, S. F. Khim. Tekhol. Topl. Masel. 1979, 8, 8. (12) Takatsuka, T.; Kajiyama, R.; Hashimoto, H.; Matsuo, I.; Miwa, S. A. J. Chem. Eng. Jpn. 1989, 22, 304.
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Figure 3. Sulfur content of liquid products from asphaltene decomposition at 350 to 430 °C as a function of time.
temperature, the molecular weight of liquid product decreased sharply in the first 5 min of reaction time, and then leveled off. The effect of reaction temperature on the change in molecular weight of liquid products was significant. After 20 min of reaction, the molecular weight dropped by a factor of 1.8 at 370 °C, by a factor of 2.5 at 400 °C, and by a factor of 8 at 430 °C. A comparison of the data at 430 °C in Figures 1 and 2 shows that coke formation was only significant after the average molecular weight had been reduced by a factor of 5. Consequently, the intermediate products of asphaltenes, rather than the feed itself, were the coke precursors. In other words, the asphaltene was not the “monomer” to coke, if one considers coke formation as a polymerization process. A high initial molecular weight may not bring about more coke because such large structures may be very short-lived at reaction conditions; rather it is the chemical structure of the intermediates that defines the coke formation propensity. These data are consistent with the idea that coke precursors separate from the liquid phase due to incompatibility driven by reaction,1 rather than the proposal that coke forms by immediate polymerization of precursors when subjected to thermal cracking conditions.13,14 The sulfur content of liquid product (toluene-soluble) from experiments at 350 to 430 °C as a function of reaction time is shown in Figure 3. The repeatability of the measurement of sulfur content was (0.07 wt % on replicate kinetic experiments. Similar to the changes in molecular weight (Figure 2), the sulfur content of the liquid decreased with increasing reaction time and temperature, but to a much smaller extent. At 430 °C and 1 h, the sulfur content of the liquid product was reduced by 22%, whereas molecular weight was reduced by a factor of 9. The relatively low extent of sulfur reduction was not surprising, because thermal cracking gives a limited contribution to the sulfur conversion of bitumen residue. A comparison of the data in Figures 2 and 3 shows that a small reduction in sulfur content coincides with a significant reduction in molecular weight. This result can be attributed to a macromolecu(13) Rao, B. M. L.; Serrano, J. E. Fuel Sci. Technol. Int. 1986, 4, 483. (14) Speight, J. G. Korean J. Chem. Eng. 1998, 15, 1-8.
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Figure 4. Moles of liquid plus gas products/moles of feed vs reaction time for asphaltene decomposition at 370-430 °C and 4.2 MPa H2 (room temperature).
lar network of residua with sulfur cross-linking among individual molecules7,15,16. Breakage of weak sulfurcarbon bonds would then give a dramatic decrease in molecular weight. Analysis of Reaction Kinetics. The molar amounts of liquid products were determined from the mass yields and average molecular weights. The gas products, on a molar basis, were obtained from the mass and composition of gas products. The hydrocarbon components in the gases had an average molecular weight of 24, with a negligible dependence on reaction temperature and time. Methane was always the dominant light hydrocarbon on a molar basis. The data of Figure 4 show the increase in the number of moles in the reactor due to cracking reactions. In the absence of significant coke formation (i.e., all data except at 430 °C after >15 min reaction) the increase in the number of moles was approximately linear with time at 350, 370, 400, and 430 °C, consistent with eq 8 for zero-order kinetics. Extrapolation of the data to shorter times showed that the rate of reaction became significant only after approximately 3-4 min, consistent with the time to heat the tubing-bomb reactors to the operating temperatures. Use of first-order kinetics ( eq 7) to fit the data gave a sum of squared residuals of 10.0 on 24 measurements of ntot/nB0 at seven different temperatures compared to a sum of squared residuals of 13.1 using zero-order kinetics ( eq 8), therefore, the fit to first-order kinetics was slightly better. Equation 8 for first-order kinetics required an additional adjustable parameter for the ultimate yield of products, (aL + aG). A value of 25 mol/mol was selected from Figure 4 as an estimate of the limiting number of moles. Cracking reactions normally follow first-order kinetics overall,17 therefore, the first-order analysis was used to determine the apparent activation energy of the cracking reactions. An Arrhenius plot of the apparent rate constants for cracking is given in Figure 5. The line shows the best-fit log-linear regression from minimization of the chi-square value, giving an apparent activation energy of 176 kJ/mol. Although the residuals were (15) Le Page, J. F.; Xhatila, S. G.; Davidson, M. Resid and Heavy Oil Processing; Editions Technip: Paris, 1990; pp 41-44. (16) Adam, P.; Mycke, B.; Schmid, J. C.; Connan, J.; Albrecht, P. Energy Fuels 1992, 6, 553-559. (17) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994.
Zhao et al.
Figure 5. Arrhenius plot for SCFE asphaltene decomposition in temperature range 350-430 °C. Error bars show the 95% confidence interval for rate constants at 350, 370, 400, and 430 °C.
not evenly distributed, suggesting that the apparent activation energy may shift with temperature, the deviations from linearity were not statistically significant. The apparent activation energy was below the normal range for breakage of aliphatic carbon-carbon bonds in model compounds (typically 200-300 kJ/ mol).18,19 This result suggests that weaker bonds, such as carbon-sulfur, were important in the thermolysis reactions at this level of conversion, as suggested by previous studies of asphaltene pyrolysis.20,21 The bonding energy of a paraffinic C-C bond is ca. 345 kJ/mol and that of an alkyl C-S bond is 270 kJ/mol,22 therefore, a significant contribution from cracking of C-S bonds would reduce the apparent activation energy to below 200 kJ/mol. Reaction Selectivity. Reaction selectivity is a measurement of the relative rates of different reactions in a complex mixture. The conversion of asphaltene was not determined directly in this study, due to the multiple factors that affect solubility fractions; therefore, the product formation probability (PFP) was defined as the ratio of the rate of formation of one product to the total rate of formation of all products:
PFPi )
fi
)
ri
∑fi ∑ri
(9)
where fi is the molar selectivity of the ith product and ri is the molar formation rate of the ith product. To exclude the effect coke formation, the initial PFP values were calculated from the data of Figure 4, along with the corresponding curves for gas formation, at short reaction times as the reaction time approached zero. The initial PFP values of the gas components and the total liquid and total gas products as a function of temperature are illustrated in Figure 6. The upper panel shows that hydrocarbon gases were insignificant at low temperature, where hydrogen sulfide evolution was (18) Poutsma, M. L. Energy Fuels 1990, 4, 113-131. (19) Khorasheh, F.; Gray, M. R. Ind. Eng. Chem. Res. 1993, 32, 1853-1863. (20) Savage, P. E.; Klein, M. T.; Kukes, S. G. Ind. Eng. Chem. Des. Dev. 1985, 24, 1169-1174. (21) Savage, P. E.; Klein, M. T.; Kukes, S. G. Energy Fuels 1988, 2, 619-628. (22) Benson, S. W. Thermochemical Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976.
Cracking of Athabasca Asphaltenes
Figure 6. Initial probability of formation of products (PFPs) of gas and liquid products vs reaction temperature for the decomposition of asphaltene decomposition at 350-430 °C.
dominant. At temperatures over 400 °C, the product formation probability was significantly higher for hydrocarbon gases. The corresponding PFP curve for liquid products shows a maximum at ca. 380 °C, where the evolution of hydrocarbon gases was not yet significant. Although the overall rate of decomposition rate was higher at higher temperatures, this analysis emphasizes the importance of gas products in determining the molar reaction rates and the nature of the cracking reactions. The data from the PFP analysis suggests a shift in reaction mechanism as a function of temperature. The dominance of the scission of C-S bonds at 350-385 °C was indicated by the importance of hydrogen sulfide relative to other products. The decomposition reactions of the asphaltenes were dominated by the scission of C-C bonds above 400-410 °C. This temperature range is exactly where thermal cracking reactions of hydrocarbons become significant at reaction times of order 1 h, i.e., the residence time of commercial reactors. This switch in mechanism from sulfur-carbon bond breakage to carbon-carbon bond breakage is not a new concept, because the thermal reactivity of the sulfur bonds has
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been pointed out by several investigators,7,15 and previous studies of asphaltene pyrolysis identified the evolution of hydrogen sulfide as a significant reaction at low temperature.20,21 Despite the significant change in PFP with temperature, the apparent activation energy did not demonstrate a statistically significant increase with increasing temperature (Figure 5). The present study demonstrates an alternate approach to analyzing the decomposition of complex asphaltenic mixtures using total moles. Whereas lumping on the basis of solubility fractions tends to emphasize molecular weight and aromaticity of liquid products,5 this approach gives equal weight to gases and highmolecular weight components. The analysis of reaction selectivity as a function of temperature indicated that although asphaltenes can be considered as an oligomeric material, the linking bonds that are important during upgrading are heterogeneous. Consequently, the decomposition kinetics of asphaltenes are considerably more complex than the decomposition of uniform polymers. Application of depolymerization kinetics to asphaltenes, as suggested by Kodera et al.,6 should be extended to incorporate sulfur linkages. The selectivity data indicate that reactions that form light hydrocarbons are very important on a molar basis at temperatures over 400 °C, therefore, these reactions should be included in oligomer-degradation type models. Conclusions The decomposition of asphaltenes gave an apparent activation energies of 176 kJ/mol over the temperature range from 350 to 430 °C. The selectivity data for H2S and hydrocarbon gas formation suggested that the cleavage of C-S bonds dominated the overall decomposition kinetics at low temperatures, but the cleavage of C-C bonds became dominant at temperatures over 400 °C. At these temperatures, formation of light products (H2S and hydrocarbon gas) dominated over liquid products on a molar basis. Acknowledgment. The authors thank Alberta Energy and Syncrude Canada for financial support of this work. EF000286T
