The dependence of coking kinetics on the chemical structure of asphaltenes was examined by reacting five different asphaltenes in 1-methylnaphthalene and tetralin at 430 °C and ca. 9.8 MPa. The selected heptane-insoluble asphaltenes were Athabasca asphaltenes from Canada, Arabian Light and Arabian Heavy from Saudi Arabia, Maya from Mexico, and Gudao from China. The 13C NMR aromaticity of the asphaltenes ranged from 0.40 to 0.61, and the sulfur contents ranged from 4.44 wt % to 7.47 wt %. The cracking kinetics of the asphaltenes were consistent with a modified kinetic model for coke formation, incorporating phase separation and hydrogen transfer to the asphaltenes. The rate of cracking of asphaltenes in 1-methylnaphthalene correlated with the content of aliphatic sulfur, and the yield coefficient for coke correlated with the aromaticity. These correlations allowed prediction of coking kinetics for Iranian Light and Khafji asphaltenes on the basis of average structural properties of asphaltenes. Hydrogen transfer to the different asphaltenes did not correlate with any single average structural property.

The formation of coke from asphaltenes during thermal cracking is significantly affected by both reactions with the liquid components and the solvent properties of the liquid medium. The roles of both solubility phenomena and chemical reactions were studied by reacting Athabasca asphaltenes in a closed batch reactor at 430 °C. Reactions of asphaltene in maltene at different concentrations and reactions in a series of aromatic solvents (1-methyl naphthalene, naphthalene, and tetralin) were used to investigate the role of solvent properties and hydrogen donation reactions. The most important characteristics of the liquid phase were hydrogen donating ability and the ability to initiate cracking reactions. The latter mechanism was confirmed by adding n-dodecyl sulfide as an initiator compound. A modified kinetic model for coke formation, incorporating phase separation and hydrogen transfer to the asphaltenes, was consistent with the experimental results over a range of asphaltene concentrations and solvent conditions.

Asphaltenic material from Athabasca bitumen, with and without fine solids, was reacted at 430 °C under a nitrogen environment in the liquid phase, mixed with either 1-methyl naphthalene or maltene fractions from Athabasca. Phase behavior during coke formation was investigated by examining the coke produced from selected reactions by scanning electron microscopy (SEM). Both fine solids and solvents were found to assist the dispersion of liquid coke spheres in an oil medium. Phase inversion, to an oil-in-coke structure, was observed in some sections of coke produced from pure asphaltenes. The thermodynamic feasibility of phase inversion was confirmed by calculating the entropy difference for a set of representative conditions. Qualitatively, as the volume fraction of coke increases, the probability of a phase inversion is increased. This result agrees with the behavior of polymer blends, where phase inversion occurs at high concentration.

Asphaltenes from Iranian Light, Khafji and Maya were cracked in batch reactors at 350, 370, 390, 410, and 430 • C under hydrogen to produce liquid products for kinetic analysis. The cracking kinetics of the asphaltenes and their intermediates were analyzed on a total molar basis, to avoid the assumptions inherent in lumped kinetics. The overall reactivity of the three asphaltenes was similar for reaction times from 1 to 37 min. The behavior of Khafji was distinct in its initial high reactivity of sulfur species, while the high yield of hydrocarbon gases from Iranian Light was likely due to the poly-alkyl side chains of the aromatic rings. The apparent first order activation energies were in between 170 and 255 KJ/mol. The activation energies were in the sequence Iranian Light > Maya > Khafji.

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