478
The Chemistry and Technology of Petroleum
In general, none of the fractions or combinations of fractions separated from crude petroleum
is suitable for immediate use as petroleum products. Each fraction must be separately refined by
processes that vary with the impurities in the fraction and the properties required in the finished
product (Chapters 21 and 24). The simplest treatment is the washing of a fraction with a lye solution to remove sulfur compounds. The most complex is the series of treatments—solvent treating,
dewaxing, clay treating or hydrorefining, and blending—required to produce lubricating oils. On
rare occasions, no treatment of any kind is required. Some crude oils yield a light gas oil fraction
that is suitable as furnace fuel oil or as a diesel fuel.
17.6 OPTIONS FOR HEAVY FEEDSTOCKS
In order to further distill the residuum or topped crude from the atmospheric tower at higher temperatures, reduced pressure is required to prevent thermal cracking and the process takes place in
one or more vacuum distillation towers. The principles of vacuum distillation resemble those of
fractional distillation and except that larger-diameter columns are used to maintain comparable
vapor velocities at the reduced pressures, the equipment is also similar. The internal designs of some
vacuum towers are different from atmospheric towers in that random packing and demister pads are
used instead of trays. A typical first-phase vacuum tower may produce gas oil, lubricating-oil base
stock, and a heavy residuum for propane deasphalting. A second-phase tower operating at lower
vacuum may distill surplus residuum from the atmospheric tower, which is not used for lube-stock
processing, and surplus residuum from the first vacuum tower not used for deasphalting.
Vacuum towers are typically used to separate catalytic cracking feedstock from surplus residuum
and heavy oil and tar sand bitumen have fewer components distilling at atmospheric pressure and
under vacuum than conventional petroleum. Nevertheless, some heavy oil still pass through the
distillation stage of a refinery before further processing is undertaken. In addition, a vacuum tower
has recently been installed at the Syncrude Canada plant to offer an additional process option for
upgrading tar sand bitumen (Speight, 2005 and references cited therein). The installation of such a
tower as a means of refining heavy feedstocks (with the possible exception of the residua that are
usually produced through a vacuum tower) is a question of economics and the ultimate goal of the
refinery in terms of product slate. After distillation, the residuum from the heavy oil might pass to
a cracking unit such as visbreaking or coking to produce salable products. Catalytic cracking of
the residuum or the whole heavy oil is also an option but is very dependent on the constituents of the
feedstock and their interaction with the catalyst.
The development of the catalytic or reactive distillation that unites in the same equipment catalyst and distillation devices finds its main applications for reversible reactions, such as methyl tetrabutyl ether (MTBE) ethyl tributyl ether (ETBE) synthesis, so as to shift an unfavorable equilibrium
by continuous reaction product withdrawal (DeCroocq, 1997). But catalytic distillation can provide also several advantages in selective hydrogenation of C 3 , C 4 , and C 5 cuts for petrochemistry.
Inserting the catalyst in the fractionation column improves mercaptans removal, catalyst fouling
resistance, and selective hydrogenation performances by modifying the reaction mixture composition along the column.
Thus, there is the potential for applying a related concept to the deep distillation of heavy oil.
REFERENCES
Bland, W.F. and Davidson, R.L. 1967. Petroleum Processing Handbook. McGraw-Hill, New York.
Burris, D.R. 1992. In Petroleum Processing Handbook, J.J. McKetta (Ed.). Marcel Dekker Inc., New York,
p. 666.
DeCroocq, D. 1997. Major scientific and technical challenges about development of new processes in refining
and petrochemistry. Revue Institut Français de Pétrole 52(5): 469–489.
Gary, J.H., Handwerk, G.E., and Kaiser, M.J. 2007. Petroleum Refining: Technology and Economics, 5th edn.
CRC Press, Taylor & Francis Group, Boca Raton, FL.
The Chemistry and Technology of Petroleum
In general, none of the fractions or combinations of fractions separated from crude petroleum
is suitable for immediate use as petroleum products. Each fraction must be separately refined by
processes that vary with the impurities in the fraction and the properties required in the finished
product (Chapters 21 and 24). The simplest treatment is the washing of a fraction with a lye solution to remove sulfur compounds. The most complex is the series of treatments—solvent treating,
dewaxing, clay treating or hydrorefining, and blending—required to produce lubricating oils. On
rare occasions, no treatment of any kind is required. Some crude oils yield a light gas oil fraction
that is suitable as furnace fuel oil or as a diesel fuel.
17.6 OPTIONS FOR HEAVY FEEDSTOCKS
In order to further distill the residuum or topped crude from the atmospheric tower at higher temperatures, reduced pressure is required to prevent thermal cracking and the process takes place in
one or more vacuum distillation towers. The principles of vacuum distillation resemble those of
fractional distillation and except that larger-diameter columns are used to maintain comparable
vapor velocities at the reduced pressures, the equipment is also similar. The internal designs of some
vacuum towers are different from atmospheric towers in that random packing and demister pads are
used instead of trays. A typical first-phase vacuum tower may produce gas oil, lubricating-oil base
stock, and a heavy residuum for propane deasphalting. A second-phase tower operating at lower
vacuum may distill surplus residuum from the atmospheric tower, which is not used for lube-stock
processing, and surplus residuum from the first vacuum tower not used for deasphalting.
Vacuum towers are typically used to separate catalytic cracking feedstock from surplus residuum
and heavy oil and tar sand bitumen have fewer components distilling at atmospheric pressure and
under vacuum than conventional petroleum. Nevertheless, some heavy oil still pass through the
distillation stage of a refinery before further processing is undertaken. In addition, a vacuum tower
has recently been installed at the Syncrude Canada plant to offer an additional process option for
upgrading tar sand bitumen (Speight, 2005 and references cited therein). The installation of such a
tower as a means of refining heavy feedstocks (with the possible exception of the residua that are
usually produced through a vacuum tower) is a question of economics and the ultimate goal of the
refinery in terms of product slate. After distillation, the residuum from the heavy oil might pass to
a cracking unit such as visbreaking or coking to produce salable products. Catalytic cracking of
the residuum or the whole heavy oil is also an option but is very dependent on the constituents of the
feedstock and their interaction with the catalyst.
The development of the catalytic or reactive distillation that unites in the same equipment catalyst and distillation devices finds its main applications for reversible reactions, such as methyl tetrabutyl ether (MTBE) ethyl tributyl ether (ETBE) synthesis, so as to shift an unfavorable equilibrium
by continuous reaction product withdrawal (DeCroocq, 1997). But catalytic distillation can provide also several advantages in selective hydrogenation of C 3 , C 4 , and C 5 cuts for petrochemistry.
Inserting the catalyst in the fractionation column improves mercaptans removal, catalyst fouling
resistance, and selective hydrogenation performances by modifying the reaction mixture composition along the column.
Thus, there is the potential for applying a related concept to the deep distillation of heavy oil.
REFERENCES
Bland, W.F. and Davidson, R.L. 1967. Petroleum Processing Handbook. McGraw-Hill, New York.
Burris, D.R. 1992. In Petroleum Processing Handbook, J.J. McKetta (Ed.). Marcel Dekker Inc., New York,
p. 666.
DeCroocq, D. 1997. Major scientific and technical challenges about development of new processes in refining
and petrochemistry. Revue Institut Français de Pétrole 52(5): 469–489.
Gary, J.H., Handwerk, G.E., and Kaiser, M.J. 2007. Petroleum Refining: Technology and Economics, 5th edn.
CRC Press, Taylor & Francis Group, Boca Raton, FL.