Thermal cracking process

The invention: 



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.





More Gasoline!



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.



Impact



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

gasoline.

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

remains available.



See also: Fuel cell; Gas-electric car; Geothermal power; Internal

combustion engine; Oil-well drill bit; Solar thermal engine.