456
The Chemistry and Technology of Petroleum
reaction progresses, these derived products increase in molecular weight and eventually become
insoluble in the reaction medium, deposit on the catalyst, and form coke.
As predicted from the chemistry of the thermal reactions of the asphaltene constituents, there
is a steady increase in aromaticity (reflected as a decrease in the hydrogen/carbon atomic ratio)
with on-stream time. This is due to (1) aromatization of naphthene ring system that are present
in asphaltene constituents, (2) cyclodehydrogenation of alkyl chains to form other naphthene ring
systems, (3) dehydrogenation of the new naphthene ring systems to form more aromatic rings, and
(4) dealkylation of aromatic ring systems.
As the reaction progresses, the aromatic carbon atoms in the asphaltene constituents show a general increase and the degree of substitution of the aromatic rings decreases. Again, this is in keeping
with the formation of products from the original asphaltene constituents (carbenes, carboids, and
eventually coke) that have an increased aromaticity and decreased number of alkyl chains as well
as a decrease in the alkyl chain length. Thus, as the reaction progresses with increased on-stream
time, new asphaltene constituents are formed that, relative to the original asphaltene constituents,
the new species have increased aromaticity coupled with a lesser number of alkyl chains that are
shorter than the original alky chains.
It may be that the chemistry of hydrocracking has to be given serious reconsideration insofar as
the data show that the initial reactions of the asphaltene constituents appear to be the same as the
reactions under thermal conditions where hydrogen is not present. Rethinking of the process conditions and the potential destruction of the catalyst by the deposition of carbenes and carboids require
further investigation of the chemistry of asphaltene hydrocracking.
If these effects are prevalent during hydrocracking high-asphaltene feedstocks, the option may
be to hydrotreat the feedstock first and then to hydrocrack the hydrotreated feedstock. There are
indications that such hydrotreatment can (at some obvious cost) act beneficially in the overall conversion of the feedstocks to liquid products.
REFERENCES
Allan, D.E., Martinez, C.H., Eng, C.C., and Barton, W.J. 1983. Visbreaking gains renewed interest. Chemical
Engineering Progress 79(1): 85–89.
Ancheyta, J., Centeno, G., Trejo, F., Betancourt, G., and Speight, J.G. 2005. Asphaltene characterization as a
function of time on-stream during hydroprocessing of maya crude. Catalysis Today 109: 162.
Ancheyta, J. and Speight, J.G. 2007. Feedstock evaluation and composition. In Hydroprocessing of Heavy
Oils and Residua, J. Ancheyta and J.G. Speight (Eds.). CRC Press, Taylor & Francis Group, Boca Raton,
FL, Chapter 2.
Bjorseth, A. 1983. Handbook of Polycyclic Aromatic Hydrocarbons. Marcel Dekker, Inc., New York.
Boduszynski, M.M. 1987. Composition of heavy petroleum. 1. Molecular weight, hydrogen deficiency, and
heteroatom concentration as a function of atmospheric equivalent boiling point up to 1400°F (760°C).
Energy & Fuels 1: 2–11.
Boduszynski, M.M. 1988. Composition of heavy petroleum. 2. Molecular characterization. Energy & Fuels 2:
597–613.
Cooper, T.A. and Ballard, W.P. 1962. In Advances in Petroleum Chemistry and Refining, Vol. 6, K.A. Kobe and
J.J. McKetta (Eds.). Interscience, New York, Chapter 4.
Cumming, K.A. and Wojciechowski, B.W. 1996. Catalysis Reviews—Science and Engineering 38: 101–157.
Dias, J.R. 1987. Handbook of Polycyclic Hydrocarbons. Part A. Benzenoid Hydrocarbons. Elsevier, New York.
Dias, J.R. 1988. Handbook of Polycyclic Hydrocarbons. Part B. Polycyclic Isomers and Heteroatom Analogs of
Benzenoid Hydrocarbons. Elsevier, New York.
Dickie, J.P. and Yen, T.F. 1967. Macrostructures of the asphaltic fractions by various instrumental methods.
Analytical Chemistry 39: 1847–1852.
Dolbear, G.E. 1998. Hydrocracking: Reactions, catalysts, and processes. In Petroleum Chemistry and Refining,
J.G. Speight (Ed.). Taylor & Francis Publishers, Washington, DC, Chapter 7.
Dolbear, G.E., Tang, A., and Moorehead, E.L. 1987. Upgrading studies with California, Mexican, and Middle
Eastern heavy oils. In Metal Complexes in Fossil Fuels, R.H. Filby and J.F. Branthaver (Eds.). American
Chemical Society, Washington, DC, pp. 220–232.
The Chemistry and Technology of Petroleum
reaction progresses, these derived products increase in molecular weight and eventually become
insoluble in the reaction medium, deposit on the catalyst, and form coke.
As predicted from the chemistry of the thermal reactions of the asphaltene constituents, there
is a steady increase in aromaticity (reflected as a decrease in the hydrogen/carbon atomic ratio)
with on-stream time. This is due to (1) aromatization of naphthene ring system that are present
in asphaltene constituents, (2) cyclodehydrogenation of alkyl chains to form other naphthene ring
systems, (3) dehydrogenation of the new naphthene ring systems to form more aromatic rings, and
(4) dealkylation of aromatic ring systems.
As the reaction progresses, the aromatic carbon atoms in the asphaltene constituents show a general increase and the degree of substitution of the aromatic rings decreases. Again, this is in keeping
with the formation of products from the original asphaltene constituents (carbenes, carboids, and
eventually coke) that have an increased aromaticity and decreased number of alkyl chains as well
as a decrease in the alkyl chain length. Thus, as the reaction progresses with increased on-stream
time, new asphaltene constituents are formed that, relative to the original asphaltene constituents,
the new species have increased aromaticity coupled with a lesser number of alkyl chains that are
shorter than the original alky chains.
It may be that the chemistry of hydrocracking has to be given serious reconsideration insofar as
the data show that the initial reactions of the asphaltene constituents appear to be the same as the
reactions under thermal conditions where hydrogen is not present. Rethinking of the process conditions and the potential destruction of the catalyst by the deposition of carbenes and carboids require
further investigation of the chemistry of asphaltene hydrocracking.
If these effects are prevalent during hydrocracking high-asphaltene feedstocks, the option may
be to hydrotreat the feedstock first and then to hydrocrack the hydrotreated feedstock. There are
indications that such hydrotreatment can (at some obvious cost) act beneficially in the overall conversion of the feedstocks to liquid products.
REFERENCES
Allan, D.E., Martinez, C.H., Eng, C.C., and Barton, W.J. 1983. Visbreaking gains renewed interest. Chemical
Engineering Progress 79(1): 85–89.
Ancheyta, J., Centeno, G., Trejo, F., Betancourt, G., and Speight, J.G. 2005. Asphaltene characterization as a
function of time on-stream during hydroprocessing of maya crude. Catalysis Today 109: 162.
Ancheyta, J. and Speight, J.G. 2007. Feedstock evaluation and composition. In Hydroprocessing of Heavy
Oils and Residua, J. Ancheyta and J.G. Speight (Eds.). CRC Press, Taylor & Francis Group, Boca Raton,
FL, Chapter 2.
Bjorseth, A. 1983. Handbook of Polycyclic Aromatic Hydrocarbons. Marcel Dekker, Inc., New York.
Boduszynski, M.M. 1987. Composition of heavy petroleum. 1. Molecular weight, hydrogen deficiency, and
heteroatom concentration as a function of atmospheric equivalent boiling point up to 1400°F (760°C).
Energy & Fuels 1: 2–11.
Boduszynski, M.M. 1988. Composition of heavy petroleum. 2. Molecular characterization. Energy & Fuels 2:
597–613.
Cooper, T.A. and Ballard, W.P. 1962. In Advances in Petroleum Chemistry and Refining, Vol. 6, K.A. Kobe and
J.J. McKetta (Eds.). Interscience, New York, Chapter 4.
Cumming, K.A. and Wojciechowski, B.W. 1996. Catalysis Reviews—Science and Engineering 38: 101–157.
Dias, J.R. 1987. Handbook of Polycyclic Hydrocarbons. Part A. Benzenoid Hydrocarbons. Elsevier, New York.
Dias, J.R. 1988. Handbook of Polycyclic Hydrocarbons. Part B. Polycyclic Isomers and Heteroatom Analogs of
Benzenoid Hydrocarbons. Elsevier, New York.
Dickie, J.P. and Yen, T.F. 1967. Macrostructures of the asphaltic fractions by various instrumental methods.
Analytical Chemistry 39: 1847–1852.
Dolbear, G.E. 1998. Hydrocracking: Reactions, catalysts, and processes. In Petroleum Chemistry and Refining,
J.G. Speight (Ed.). Taylor & Francis Publishers, Washington, DC, Chapter 7.
Dolbear, G.E., Tang, A., and Moorehead, E.L. 1987. Upgrading studies with California, Mexican, and Middle
Eastern heavy oils. In Metal Complexes in Fossil Fuels, R.H. Filby and J.F. Branthaver (Eds.). American
Chemical Society, Washington, DC, pp. 220–232.