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 frac- tion, or asphaltenes, is responsible for much of the coke- forming 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 asphaltene- rich 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 dif- ference 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 start- ing 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- 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 molec- ular 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 asphalt- enes 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 as- phaltenes 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 molec- ular weight and sulfur content of the feed material before and after reaction. This analysis provided a basis * Author to whom correspondence should be addressed. Phone: 780- 492-7965. Fax: 780-492-2881. E-mail: murray.gray@ualberta.ca. † 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.; Carru- thers, J. D. Energy Fuels 1999, 13, 1037-1045. (4) Wiehe, I. A. Energy Fuels 1994, 8, 536-544. (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. 751 Energy & Fuels 2001, 15, 751-755 10.1021/ef000286t CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001