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The Chemistry and Technology of Petroleum
had become dark in color. At this point, distillation was discontinued and the heavy oils were held in
the hot zone, during which time some of the high-molecular-weight components were decomposed
to produce lower molecular weight products. After a suitable time, distillation was continued to
yield light oil (kerosene) instead of the heavy oil that would otherwise have been produced.
The yields of kerosene products were usually markedly increased by means of cracking distillation, but the technique was not suitable for gasoline production. As the need for gasoline arose in
the early 1900s, the necessity of prolonging the cracking process became apparent and a process
known as pressure cracking evolved.
Pressure cracking was a batch operation in which, as an example, gas oil (200 bbl) was heated
to about 425°C (800°F) in stills that had been reinforced to operate at pressures as high as 95 psi
(6.4 atm). The gas oil was held under maximum pressure for 24 h, while applied heat maintained the
temperature. Distillation was then started and during the next 48 h a low-boiling distillate (100 bbl)
was produced that contained the gasoline components. This distillate was treated with sulfuric acid
to remove unstable gum-forming components and then redistilled to produce a cracked gasoline
(boiling range).
The large-scale production of cracked gasoline was first developed by Burton in 1912. The process employed batch distillation in horizontal shell stills and operated at about 400°C (ca. 750°F) and
75–95 psi. It was the first successful method of converting heavier oils into gasoline. Nevertheless,
heating a bulk volume of oil was soon considered cumbersome, and during the years 1914–1922,
a number of successful continuous cracking processes were developed. By these processes, gas oil
was continuously pumped through a unit that heated the gas oil to the required temperature, held
it for a time under pressure, and then discharged the cracked material into distillation equipment
where it was separated into gases, gasoline, gas oil, and tar.
The tube-and-tank cracking process is not only typical of the early (post-1900) cracking units but
also is one of the first units on record in which the concept of reactors (soakers) being on-stream/
off-stream is realized. Such a concept departs from the true batch concept and allowed a greater
degree of continuity. In fact, the tube-and-tank cracking unit may be looked upon as a forerunner
of the delayed coking operation.
In the tube-and-tank process, a feedstock (at that time a gas oil) was preheated by exchange with
the hot products from the unit pumped into the cracking coil, which consisted of several hundred feet
of very strong pipe that lined the inner walls of a furnace where oil or gas burners raised the temperature of the gas oil to 425°C (800°F). The hot gas oil passed from the cracking coil into a large reaction chamber (soaker) where the gas oil was held under the temperature and pressure conditions long
enough for the cracking reactions to be completed. The cracking reactions formed coke which, in the
course of several days, filled the soaker. The gas oil stream was then switched to a second soaker, and
the first soaker was cleaned out by drilling operations similar to those used in drilling an oil well.
The cracked material (other than coke) left the on-stream soaker to enter an evaporator (tar separator) maintained under a much lower pressure than the soaker where, because of the lower pressure,
all of the cracked material, except the tar, became vaporized. The vapor left the top of the separator
where it was distilled into separate fractions—gases, gasoline, and gas oil. The tar that was deposited in the separator was pumped out for use as asphalt or as a heavy fuel oil.
Early in the development of tube-and-tank thermal cracking, it was found that adequate yields
of gasoline could not be obtained by one passage of the stock through the heating coil; attempts to
increase the conversion in one pass brought about undesirable high yields of gas and coke. It was
better to crack to a limited extent, remove the products, and recycle the rest of the oil (or a distilled
fraction free of tar) for repeated partial conversion. The high-boiling constituents once exposed to
cracking were so changed in composition as to be more refractory than the original feedstock.
With the onset of the development of the automobile, the most important part of any refinery
became the gasoline-manufacturing facilities. Among the processes that have evolved for gasoline
production are thermal cracking, catalytic cracking, thermal reforming, catalytic reforming, polymerization, alkylation, coking, and distillation of fractions directly from crude petroleum.
The Chemistry and Technology of Petroleum
had become dark in color. At this point, distillation was discontinued and the heavy oils were held in
the hot zone, during which time some of the high-molecular-weight components were decomposed
to produce lower molecular weight products. After a suitable time, distillation was continued to
yield light oil (kerosene) instead of the heavy oil that would otherwise have been produced.
The yields of kerosene products were usually markedly increased by means of cracking distillation, but the technique was not suitable for gasoline production. As the need for gasoline arose in
the early 1900s, the necessity of prolonging the cracking process became apparent and a process
known as pressure cracking evolved.
Pressure cracking was a batch operation in which, as an example, gas oil (200 bbl) was heated
to about 425°C (800°F) in stills that had been reinforced to operate at pressures as high as 95 psi
(6.4 atm). The gas oil was held under maximum pressure for 24 h, while applied heat maintained the
temperature. Distillation was then started and during the next 48 h a low-boiling distillate (100 bbl)
was produced that contained the gasoline components. This distillate was treated with sulfuric acid
to remove unstable gum-forming components and then redistilled to produce a cracked gasoline
(boiling range).
The large-scale production of cracked gasoline was first developed by Burton in 1912. The process employed batch distillation in horizontal shell stills and operated at about 400°C (ca. 750°F) and
75–95 psi. It was the first successful method of converting heavier oils into gasoline. Nevertheless,
heating a bulk volume of oil was soon considered cumbersome, and during the years 1914–1922,
a number of successful continuous cracking processes were developed. By these processes, gas oil
was continuously pumped through a unit that heated the gas oil to the required temperature, held
it for a time under pressure, and then discharged the cracked material into distillation equipment
where it was separated into gases, gasoline, gas oil, and tar.
The tube-and-tank cracking process is not only typical of the early (post-1900) cracking units but
also is one of the first units on record in which the concept of reactors (soakers) being on-stream/
off-stream is realized. Such a concept departs from the true batch concept and allowed a greater
degree of continuity. In fact, the tube-and-tank cracking unit may be looked upon as a forerunner
of the delayed coking operation.
In the tube-and-tank process, a feedstock (at that time a gas oil) was preheated by exchange with
the hot products from the unit pumped into the cracking coil, which consisted of several hundred feet
of very strong pipe that lined the inner walls of a furnace where oil or gas burners raised the temperature of the gas oil to 425°C (800°F). The hot gas oil passed from the cracking coil into a large reaction chamber (soaker) where the gas oil was held under the temperature and pressure conditions long
enough for the cracking reactions to be completed. The cracking reactions formed coke which, in the
course of several days, filled the soaker. The gas oil stream was then switched to a second soaker, and
the first soaker was cleaned out by drilling operations similar to those used in drilling an oil well.
The cracked material (other than coke) left the on-stream soaker to enter an evaporator (tar separator) maintained under a much lower pressure than the soaker where, because of the lower pressure,
all of the cracked material, except the tar, became vaporized. The vapor left the top of the separator
where it was distilled into separate fractions—gases, gasoline, and gas oil. The tar that was deposited in the separator was pumped out for use as asphalt or as a heavy fuel oil.
Early in the development of tube-and-tank thermal cracking, it was found that adequate yields
of gasoline could not be obtained by one passage of the stock through the heating coil; attempts to
increase the conversion in one pass brought about undesirable high yields of gas and coke. It was
better to crack to a limited extent, remove the products, and recycle the rest of the oil (or a distilled
fraction free of tar) for repeated partial conversion. The high-boiling constituents once exposed to
cracking were so changed in composition as to be more refractory than the original feedstock.
With the onset of the development of the automobile, the most important part of any refinery
became the gasoline-manufacturing facilities. Among the processes that have evolved for gasoline
production are thermal cracking, catalytic cracking, thermal reforming, catalytic reforming, polymerization, alkylation, coking, and distillation of fractions directly from crude petroleum.