25 April 2014
Thermal cracking process
Process that increased the yield of refined gasoline
extracted from raw petroleum by using heat to convert complex
hydrocarbons into simpler gasoline hydrocarbons, thereby making
possible the development of the modern petroleum industry.
The people behind the invention:
William M. Burton (1865-1954), an American chemist
Robert E. Humphreys (1942- ), an American chemist
Gasoline, Motor Vehicles, and Thermal Cracking
Gasoline is a liquid mixture of hydrocarbons (chemicals made up
of only hydrogen and carbon) that is used primarily as a fuel for internal
combustion engines. It is produced by petroleum refineries
that obtain it by processing petroleum (crude oil), a naturally occurring
mixture of thousands of hydrocarbons, the molecules of which
can contain from one to sixty carbon atoms.
Gasoline production begins with the “fractional distillation” of
crude oil in a fractionation tower, where it is heated to about 400 degrees
Celsius at the tower’s base. This heating vaporizes most of the
hydrocarbons that are present, and the vapor rises in the tower,
cooling as it does so. At various levels of the tower, various portions
(fractions) of the vapor containing simple hydrocarbon mixtures become
liquid again, are collected, and are piped out as “petroleum
fractions.” Gasoline, the petroleum fraction that boils between 30
and 190 degrees Celsius, is mostly a mixture of hydrocarbons that
contain five to twelve carbon atoms.
Only about 25 percent of petroleum will become gasoline via
fractional distillation. This amount of “straight run” gasoline is not
sufficient to meet the world’s needs. Therefore, numerous methods
have been developed to produce the needed amounts of gasoline.
The first such method, “thermal cracking,” was developed in 1913
by William M. Burton of Standard Oil of Indiana. Burton’s cracking
process used heat to convert complex hydrocarbons (whose molecules
contain many carbon atoms) into simpler gasoline hydrocarbons
(whose molecules contain fewer carbon atoms), thereby increasing
the yield of gasoline from petroleum. Later advances in
petroleum technology, including both an improved Burton method
and other methods, increased the gasoline yield still further.
Starting in about 1900, gasoline became important as a fuel for
the internal combustion engines of the new vehicles called automobiles.
By 1910, half a million automobiles traveled American roads.
Soon, the great demand for gasoline—which was destined to grow
and grow—required both the discovery of new crude oil fields
around the world and improved methods for refining the petroleum
mined from these new sources. Efforts were made to increase
the yield of gasoline—at that time, about 15 percent—from petroleum.
The Burton method was the first such method.
At the time that the cracking process was developed, Burton was
the general superintendent of the Whiting refinery, owned by Standard
Oil of Indiana. The Burton process was developed in collaboration
with Robert E. Humphreys and F. M. Rogers. This three-person
research group began work knowing that heating petroleum
fractions that contained hydrocarbons more complex than those
present in gasoline—a process called “coking”—produced kerosene,
coke (a form of carbon), and a small amount of gasoline. The
process needed to be improved substantially, however, before it
could be used commercially.
Initially, Burton and his coworkers used the “heavy fuel” fraction
of petroleum (the 66 percent of petroleum that boils at a temperature
higher than the boiling temperature of kerosene). Soon, they
found that it was better to use only the part of the material that contained
its smaller hydrocarbons (those containing fewer carbon atoms),
all of which were still much larger than those present in gasoline.
The cracking procedure attempted first involved passing the
starting material through a hot tube. This hot-tube treatment vaporized
the material and broke down 20 to 30 percent of the larger hydrocarbons
into the hydrocarbons found in gasoline. Various tarry
products were also produced, however, that reduced the quality of
the gasoline that was obtained in this way.
Next, the investigators attempted to work at a higher temperature
by bubbling the starting material through molten lead. More
gasoline was made in this way, but it was so contaminated with
gummy material that it could not be used. Continued investigation
showed, however, that moderate temperatures (between those used
in the hot-tube experiments and that of molten lead) produced the
best yield of useful gasoline.
The Burton group then had the idea of using high pressure to
“keep starting materials still.” Although the theoretical basis for the
use of high pressure was later shown to be incorrect, the new
method worked quite well. In 1913, the Burton method was patented
and put into use. The first cracked gasoline, called Motor
Spirit, was not very popular, because it was yellowish and had a
somewhat unpleasant odor. The addition of some minor refining
procedures, however, soon made cracked gasoline indistinguishable
from straight run gasoline. Standard Oil of Indiana made huge
profits from cracked gasoline over the next ten years. Ultimately,
thermal cracking subjected the petroleum fractions that were
utilized to temperatures between 550 and 750 degrees Celsius, under
pressures between 250 and 750 pounds per square inch.
In addition to using thermal cracking to make gasoline for sale,
Standard Oil of Indiana also profited by licensing the process for use
by other gasoline producers. Soon, the method was used throughout
the oil industry. By 1920, it had been perfected as much as it
could be, and the gasoline yield from petroleum had been significantly
increased. The disadvantages of thermal cracking include a
relatively low yield of gasoline (compared to those of other methods),
the waste of hydrocarbons in fractions converted to tar and
coke, and the relatively high cost of the process.
A partial solution to these problems was found in “catalytic
cracking”—the next logical step from the Burton method—in which
petroleum fractions to be cracked are mixed with a catalyst (a substance
that causes a chemical reaction to proceed more quickly,
without reacting itself). The most common catalysts used in such
cracking were minerals called “zeolites.” The wide use of catalytic
cracking soon enabled gasoline producers to work at lower temperatures
(450 to 550 degrees Celsius) and pressures (10 to 50 pounds
per square inch). This use decreased manufacturing costs because
catalytic cracking required relatively little energy, produced only
small quantities of undesirable side products, and produced high quality
Various other methods of producing gasoline have been developed—
among them catalytic reforming, hydrocracking, alkylation,
and catalytic isomerization—and now about 60 percent of the petroleum
starting material can be turned into gasoline. These methods,
and others still to come, are expected to ensure that the world’s
needs for gasoline will continue to be satisfied—as long as petroleum
See also: Fuel cell; Gas-electric car; Geothermal power; Internal
combustion engine; Oil-well drill bit; Solar thermal engine ; Thermal cracking process