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The separation processes described above are based on differences in physical properties of the components of crude oil. By chemically changing their molecular structure, it is possible to convert less valuable hydrocarbon compounds into those in demand.

The first of these conversion processes is cracking, or thermal decomposition of long-chain hydrocarbon molecules into shorter molecules with lower boiling points. For example, a paraffin molecule such as dodecane (C₁₂H₂₆) has such poor «antiknock» properties that it cannot be used in a modern automobile engine, but under intense heat it breaks down into shorter molecules such as paraffins (C₆H,₄; C₇H,₆) or olefins (C₆H_I2; C₅H,₀), which are suitable for motor fuels. The chemical reactions that take place in a cracking operation are complex.

The product derived from a cracking process is, in effect, a synthetic crude oil. Desired gasoline fractions must be separated by distillation.

Thermal cracking and reforming. The earliest cracking techniques are typified by processes in which kerosene or gas oil materials were converted by heating to temperatures between 450° and 540 °C (850° and 1,000 °F) at pressures of 18 to 35 kilograms per square centimetre (250 to 500 pounds per square inch). This process produced gasolines of about 70 octane number (low by modern standards).

Visbreaking, another thermal cracking process, reduces viscosity of heavy crude oil residues to make them more suitable for inclusion in fuel oils.

The steam cracking process, by which ethylene and other olefins are made from naphtha, differs from thermal cracking in that it is carried out at low pressures and higher temperatures.

Thermal reforming, a modification of the thermal cracking process, reforms or alters the properties of low-grade components such as naphthas by converting the molecules into those of higher octane number. Pressures used are somewhat higher than in cracking. At temperatures from 510° to 566 °C (950° to 1,050 °F), it is possible to obtain gasolines with octane numbers of 70 – 80 from components of less than 40.

Catalytic cracking. The development of catalytic cracking was one of the major achievements of the petroleum industry. Enormous catalytic cracking plants dominate the skylines of modern oil refineries and operate continuously for years on end. The catalytic cracking of gas oil is one of the most important processes for manufacture of gasoline.

Use of a catalyst (a material that assists a chemical reaction but does not take part in it) in the cracking reaction increases the yield of improved quality products under much less severe operating conditions than in thermal cracking. Typical temperatures used are from 454 °С to 510 °C (850° to 950 °F) at much lower pressures of 0.7 to 1.4 kilograms per square centimetre (10 to 20 pounds per square inch). At first natural or synthetic clays were used as catalysts. A typical synthetic clay contains 12.5 percent alumina and 87.5 percent silica. In the last few years zeolitic or molecular sieve-base catalysts have been introduced. These give greater gasoline yields while reducing the formation of gas and coke.

Cracking may be accomplished either with a «fixed bed» of catalyst, with a «moving bed» of granular catalyst, or with a «fluid» catalyst, which is a finely divided solid having properties analogous to a liquid when agitated by air or oil vapours.

In this arrangement a reactor and a regenerator vessel are located side by side. The oil is vaporized when it meets the hot catalyst at point A. The vapours force the catalyst up into the reactor, which (because its cross section is larger in area) allows the catalyst to settle into a bed whose depth can be varied to regulate the reaction time. The oil vapours maintain the bed of catalyst in a turbulent condition, thus effecting the close contact between vapours and catalyst necessary for cracking.

As the cracking reactions proceed, carbon is deposited on the catalyst particles.

Inorganic chemicals. Various inorganic chemicals are also made from petroleum, some, particularly ammonia and sulfur, in large quantities. Ammonia production requires hydrogen derived from a hydrocarbon source. Traditionally, this hydrogen was produced from a coke and steam reaction, but today most ammonia is synthesized from liquid petroleum fractions, natural gas, or refinery gases. It has been pointed out that sulfur is an unwanted impurity in many petroleum products. In the process of eliminating it from oil products, it is ultimately recoverable either as elemental sulfur or as sulfuric acid. Though various inorganic chemicals manufactured from petroleum represent a substantial tonnage, the total is still only a modest proportion of the world production of such chemicals.

Aromatic compounds. The aromatic compounds, prominent in the refinery process of catalytic reforming, are major sources of petroleum chemical products as well as the lower olefins mentioned above. In the traditional chemical industry, aromatics such as benzene, toluene, and the xylenes were made from coal during the course of coal carbonization in making coke or town gas. Today a much larger volume of these compounds is made in petroleum refineries by catalytic reforming. A further source of supply now increasingly used in Europe is the aromatic-rich liquid fraction produced in the cracking of naphtha to make ethylene and other olefins.

Polymers. A highly significant proportion of products made from these basic chemicals are the plastics, synthetic rubbers, and synthetic fibres known as high polymers because their molecules are the high-molecular-weight compounds made up of the repeated structural units that have combined chemically, or polymerized. In plastics the four major products are polyethylene, polyvinyl chloride, and polystyrene, all made from the simple structural unit, or monomer, ethylene and polypropylene, made from the monomer propylene. Major raw materials for synthetic rubbers include butadiene, ethylene, benzene, and propylene.

Among synthetic fibres the polyesters, which are a combination of ethylene glycol (made from ethylene) and terephthalic acid (made from one of the xylenes), are the most widely used; they account for about one-half of all synthetic fibres. The second major synthetic fibre is nylon, its most important raw material being benzene. The acrylics, in which the major raw material is the propylene derivative acrylonitrile, make up most of the remainder of synthetic fibres.

By-products. Petroleum chemicals have advanced from the original concept of by-product usage and represent a large investment of money and technology. An additional group of products, however, that represent a direct byproduct of oil-refining operations, the naphthenic acids, can be abstracted for use in paint dryers and other derivatives. Naphthasulfonates recovered in sulfuric acid refining of oil products serve as emulsifying agents. Many cresylic acid products can be recovered as by-products.