486
The Chemistry and Technology of Petroleum
The Burton cracking process for the large-scale production of cracked gasoline was first used in
1912. The process employed batch distillation in horizontal shell stills and operated at about 400°C
(∼750°F) and 75–95 psi. It was, in fact, 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.
In 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 typical of the early continuous cracking processes. Gas oil,
preheated by exchange with the hot products of cracking, was 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 these
temperatures and pressure conditions long enough for the cracking reactions to be completed. The
cracking reactions formed coke, which over the course of several days filled the soaker. The gas oil
stream was then switched to a second soaker, and drilling operations similar to those used in drilling an oil well cleaned out the first soaker.
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 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.
As refining technology evolved throughout the twentieth century, the feedstocks for cracking
processes became the residuum or gas oil from a distillation unit. In addition, the residual oil produced as the end products of distillation processes, and even some of the higher boiling crude oil,
often contains substantial amounts of asphaltic materials, which preclude use of the residuum as
fuel oils or lubricating stocks (Speight, 2000). However, subjecting these residua directly to thermal
processes has become economically advantageous, since on the one hand the end result is the production of lower boiling products. On the other hand, the asphaltene and resin constituents that are
concentrated in residua are regarded as the precursors to coke.
As the thermal processes evolved and catalysts were employed with more frequency, poisoning
of the catalyst with a concurrent reduction in the lifetime of the catalyst became a major issue for
refiners. To avoid catalyst poisoning, it became essential that as much of the nitrogen and metals
(such as vanadium and nickel) as possible should be removed from the feedstock. The majority of
the heteroatoms (nitrogen, oxygen, and sulfur) and the metals are contained in, or associated with,
the residuum. It became necessary that this fraction be removed from cracking feedstocks and a
number of thermal processes, such as flash distillation, vacuum flashing, visbreaking, and coking,
also came into wider usage by refiners and were directed at upgrading feedstocks by removal of the
nonvolatile fraction (asphaltene plus resin fractions).
18.3 COMMERCIAL PROCESSES
As crude oil slates are expected to continue to grow heavier with higher sulfur contents, while environmental restrictions are expected to significantly reduce the demand for high-sulfur residual fuel oil.
Light sweet crude oils will continue to be available and in even greater demand than they are today.
The Chemistry and Technology of Petroleum
The Burton cracking process for the large-scale production of cracked gasoline was first used in
1912. The process employed batch distillation in horizontal shell stills and operated at about 400°C
(∼750°F) and 75–95 psi. It was, in fact, 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.
In 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 typical of the early continuous cracking processes. Gas oil,
preheated by exchange with the hot products of cracking, was 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 these
temperatures and pressure conditions long enough for the cracking reactions to be completed. The
cracking reactions formed coke, which over the course of several days filled the soaker. The gas oil
stream was then switched to a second soaker, and drilling operations similar to those used in drilling an oil well cleaned out the first soaker.
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 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.
As refining technology evolved throughout the twentieth century, the feedstocks for cracking
processes became the residuum or gas oil from a distillation unit. In addition, the residual oil produced as the end products of distillation processes, and even some of the higher boiling crude oil,
often contains substantial amounts of asphaltic materials, which preclude use of the residuum as
fuel oils or lubricating stocks (Speight, 2000). However, subjecting these residua directly to thermal
processes has become economically advantageous, since on the one hand the end result is the production of lower boiling products. On the other hand, the asphaltene and resin constituents that are
concentrated in residua are regarded as the precursors to coke.
As the thermal processes evolved and catalysts were employed with more frequency, poisoning
of the catalyst with a concurrent reduction in the lifetime of the catalyst became a major issue for
refiners. To avoid catalyst poisoning, it became essential that as much of the nitrogen and metals
(such as vanadium and nickel) as possible should be removed from the feedstock. The majority of
the heteroatoms (nitrogen, oxygen, and sulfur) and the metals are contained in, or associated with,
the residuum. It became necessary that this fraction be removed from cracking feedstocks and a
number of thermal processes, such as flash distillation, vacuum flashing, visbreaking, and coking,
also came into wider usage by refiners and were directed at upgrading feedstocks by removal of the
nonvolatile fraction (asphaltene plus resin fractions).
18.3 COMMERCIAL PROCESSES
As crude oil slates are expected to continue to grow heavier with higher sulfur contents, while environmental restrictions are expected to significantly reduce the demand for high-sulfur residual fuel oil.
Light sweet crude oils will continue to be available and in even greater demand than they are today.