367
Structure of Petroleum
processing of feedstocks containing asphaltenes is the nature of the primary thermal decomposition
products, particularly those designated as carbenes and carboids (Speight, 1992, 1994).
One of the postulates of coke formation involves the production of coke by a sequence of polymerization and condensation steps from the lightest (lower molecular weight) to the heaviest (higher
molecular weight) fractions. In addition, there is an induction period during which no coke is formed,
and many kinetic models that contain sequences of direct chemical reactions to coke fail to predict
the induction period. This induction period has been observed experimentally by many previous
investigators (Levinter et al., 1966, 1967; Magaril and Akensova, 1968; Magaril and Aksenova,
1970; Valyavin et al., 1979) and makes visbreaking and the Eureka processes possible. The postulation that coke formation is triggered by the phase separation of asphaltenes (Magaril et al., 1971) led
to the use of linear variations of the concentration of each fraction with reaction time, resulting in
the assumption of zero-order kinetics rather than first-order kinetics.
During the coke induction period, the reactant asphaltenes form only lower molecular weight
products and, as long as the asphaltenes remain dissolved, the heptane-soluble materials can provide
sufficient abstractable hydrogen to terminate asphaltene-free radicals, making asphaltene radical–
asphaltene radical recombination infrequent. In support of this assumption, it is known (Langer
et al., 1961) that partially hydrogenated refinery process streams provide abstractable hydrogen and,
as a result, inhibit coke formation during residuum thermal conversion. Thus, the heptane-soluble
fraction of a residuum, which contains naturally occurring, partially hydrogenated aromatics, can
provide abstractable hydrogen during thermal reactions.
The asphaltene concentration varies little in the coke induction period but decreases once coke
begins to form. Observing this, it might be concluded that asphaltenes are unreactive, but it is
the high reactivity of the asphaltenes down to the asphaltene core that offsets the generation of
asphaltene cores from the heptane-soluble materials to keep the overall asphaltene concentration
nearly constant.
Thus, the available data suggest that coke formation is a complex process involving both chemical reactions and thermodynamic behavior. Reactions that contribute to this process are cracking of
side chains from aromatic groups, dehydrogenation of naphthenes to form aromatics, condensation of
aliphatic structures to form aromatics, condensation of aromatics to form higher fused-ring aromatics,
and dimerization or oligomerization reactions. Loss of side chains always accompanies thermal cracking, and dehydrogenation and condensation reactions are favored by hydrogen-deficient conditions.
There is a need to investigate fully the degree of association of asphaltenes in thermal systems
and to determine whether it has any influence on the coke-forming tendencies of the asphaltenes.
REFERENCES
Acevedo, S., Mends, B., Rajas, A., Lairs, I., and Rives, H. 1985. Asphaltenes and resins from the Orinoco basin.
Fuel 64: 1741–1747.
Ali, L.H., Al-Ghannan, A., and Speight, J.G. 1985. Investigations of asphaltenes in heavy crude oils: Part II.
Determination of resins occlusion through swelling by organic solvents. Iraqi Journal of Science 26: 41.
Andersen, S.I. and Birdi, K.S. 1991a. Proceedings of the International Symposium on the Chemistry of
Bitumens. Western Research Institute, Laramie, Wyoming, p. 236.
Andersen, S.I. and Birdi, K.S. 1991b. Aggregation of asphaltenes as determined by calorimetry. Journal of
Colloid and Interface Science 142(2): 497–502.
Andersen, S.I. and Speight, J.G. 1992. Asphaltene precipitation and incipient flocculation in mixed solvents.
Preprints. Division of Fuel Chemistry American Chemical Society 37(3): 1335.
Andersen, S.I. and Speight, J.G. 1993. Observations on the critical Micelle concentration of asphaltenes.
Fuel 72: 1343.
Andersen, S.I. and Speight, J.G. 2001. Petroleum reins: Separation, character, and role in petroleum. Petroleum
Science and Technology 19: 1.
Bardon, C., Barre, L., Espinat, D., Guille, V., Li, M.H., Lambard, J., Ravey, J.C., Rosenberg, E., and Zemb, T.
1996. The colloidal structure of crude oils and suspensions of asphaltenes and resins. Fuel Science and
Technology International 14: 203–242.
Structure of Petroleum
processing of feedstocks containing asphaltenes is the nature of the primary thermal decomposition
products, particularly those designated as carbenes and carboids (Speight, 1992, 1994).
One of the postulates of coke formation involves the production of coke by a sequence of polymerization and condensation steps from the lightest (lower molecular weight) to the heaviest (higher
molecular weight) fractions. In addition, there is an induction period during which no coke is formed,
and many kinetic models that contain sequences of direct chemical reactions to coke fail to predict
the induction period. This induction period has been observed experimentally by many previous
investigators (Levinter et al., 1966, 1967; Magaril and Akensova, 1968; Magaril and Aksenova,
1970; Valyavin et al., 1979) and makes visbreaking and the Eureka processes possible. The postulation that coke formation is triggered by the phase separation of asphaltenes (Magaril et al., 1971) led
to the use of linear variations of the concentration of each fraction with reaction time, resulting in
the assumption of zero-order kinetics rather than first-order kinetics.
During the coke induction period, the reactant asphaltenes form only lower molecular weight
products and, as long as the asphaltenes remain dissolved, the heptane-soluble materials can provide
sufficient abstractable hydrogen to terminate asphaltene-free radicals, making asphaltene radical–
asphaltene radical recombination infrequent. In support of this assumption, it is known (Langer
et al., 1961) that partially hydrogenated refinery process streams provide abstractable hydrogen and,
as a result, inhibit coke formation during residuum thermal conversion. Thus, the heptane-soluble
fraction of a residuum, which contains naturally occurring, partially hydrogenated aromatics, can
provide abstractable hydrogen during thermal reactions.
The asphaltene concentration varies little in the coke induction period but decreases once coke
begins to form. Observing this, it might be concluded that asphaltenes are unreactive, but it is
the high reactivity of the asphaltenes down to the asphaltene core that offsets the generation of
asphaltene cores from the heptane-soluble materials to keep the overall asphaltene concentration
nearly constant.
Thus, the available data suggest that coke formation is a complex process involving both chemical reactions and thermodynamic behavior. Reactions that contribute to this process are cracking of
side chains from aromatic groups, dehydrogenation of naphthenes to form aromatics, condensation of
aliphatic structures to form aromatics, condensation of aromatics to form higher fused-ring aromatics,
and dimerization or oligomerization reactions. Loss of side chains always accompanies thermal cracking, and dehydrogenation and condensation reactions are favored by hydrogen-deficient conditions.
There is a need to investigate fully the degree of association of asphaltenes in thermal systems
and to determine whether it has any influence on the coke-forming tendencies of the asphaltenes.
REFERENCES
Acevedo, S., Mends, B., Rajas, A., Lairs, I., and Rives, H. 1985. Asphaltenes and resins from the Orinoco basin.
Fuel 64: 1741–1747.
Ali, L.H., Al-Ghannan, A., and Speight, J.G. 1985. Investigations of asphaltenes in heavy crude oils: Part II.
Determination of resins occlusion through swelling by organic solvents. Iraqi Journal of Science 26: 41.
Andersen, S.I. and Birdi, K.S. 1991a. Proceedings of the International Symposium on the Chemistry of
Bitumens. Western Research Institute, Laramie, Wyoming, p. 236.
Andersen, S.I. and Birdi, K.S. 1991b. Aggregation of asphaltenes as determined by calorimetry. Journal of
Colloid and Interface Science 142(2): 497–502.
Andersen, S.I. and Speight, J.G. 1992. Asphaltene precipitation and incipient flocculation in mixed solvents.
Preprints. Division of Fuel Chemistry American Chemical Society 37(3): 1335.
Andersen, S.I. and Speight, J.G. 1993. Observations on the critical Micelle concentration of asphaltenes.
Fuel 72: 1343.
Andersen, S.I. and Speight, J.G. 2001. Petroleum reins: Separation, character, and role in petroleum. Petroleum
Science and Technology 19: 1.
Bardon, C., Barre, L., Espinat, D., Guille, V., Li, M.H., Lambard, J., Ravey, J.C., Rosenberg, E., and Zemb, T.
1996. The colloidal structure of crude oils and suspensions of asphaltenes and resins. Fuel Science and
Technology International 14: 203–242.