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the treatment, involving the decomposition of heavy hydrocarbons, of petroleum and its fractions to obtain motor fuels and chemical raw materials. In addition to decomposition, cracking involves isomerization and the formation of new molecules (for example, as a result of cyclization, polymerization, and condensation).
Cracking is one of the principal methods of obtaining motor fuels (especially of gasolines). It may be carried out as a purely thermal process (known as thermal cracking) or in the presence of catalysts (catalytic cracking). The cleavage reactions involved in thermal cracking are usually considered to be free-radical chain reactions. The products of thermal cracking, which is usually carried out at 470°-540°C and at pressures of 4–6 meganewtons (MN) per sq m (40–60 atmospheres), contain many unstable unsaturated hydrocarbons. Gasolines from these products are not responsive to tetraethyl lead and require further treatment by reforming. Thermal cracking is used for the lowgrade varieties of residual heavy crude.
Low-pressure thermal cracking, carried out at 500°-600°C and at pressures of a few tenths of a MN per sq m (several atmospheres), is also called coking. It is used to convert heavy products, such as tars, into lighter products that can be treated further to produce motor fuels (yield, 60–70 percent). As much as 20 percent coke is also obtained in the process. Coke has a variety of uses, such as in the preparation of electrodes for arc furnaces and galvanic cells.
High-temperature (650°-750°C), low-pressure (near atmospheric) cracking is known as pyrolysis. It is used to convert residual heavy crude to gas containing as much as 50 percent unsaturated hydrocarbons (for example, ethylene and propylene) and aromatic compounds. The products obtained serve mainly as chemical raw materials. Thermal cracking is usually carried out in tube furnaces or in reactors with a heavy circulating heat-transfer agent (the coke formed may serve this purpose).
The main product of catalytic cracking, which is carried out in the presence of artificial or natural aluminosilicates (activated clays, such as montmorillonite), is a high-quality motor gasoline (octane rating, to 85) used for automotive transport and aviation. In addition, kerosine-gas-oil fractions are obtained that are suitable as diesel or jet fuel. The process is carried out at 450°–520°C under pressures of 0.2–.03 MN per sq m (2–3 atmospheres) in reaction towers with an immobile or continuously circulating catalyst. In both cases, the catalyst requires regeneration, since carbonaceous deposits (coke) form on the catalyst and deactivate it. The coke is removed by roasting.
Decomposition proceeds much more rapidly in catalytic cracking than in thermal cracking. Furthermore, isomerization (with the formation of saturated hydrocarbons) takes place in catalytic cracking, yielding more light products than the thermal method. The gasoline obtained in catalytic cracking contains many isoparaffins and few unsaturated hydrocarbons (accounting for its high quality). The raw material for catalytic cracking is usually gas oil, which yields 30–40 percent gasoline (isoparaffm content, to 50 percent), 45–55 percent catalytic gas oil, 10–20 percent gas (including a 6–9 percent butane-butylene fraction, a chemical raw material), and 3–6 percent coke.
Catalytic cracking in the presence of hydrogen, known as hydrocracking, is used widely to treat medium and heavy petroleum distillates with high sulfur and tar components (which are unsuitable for purely catalytic treatment). Hydrocracking is carried out at 350°-450°C under hydrogen pressures of 3–14 MN per sq m (30–140 atmospheres). Between 170 and 350 cu m of hydrogen are consumed per cu m of raw material. Among the catalysts used are molybdenum and nickel oxides and sulfides and cobalt molybdate on cracking carriers (for example, aluminosilicates). The use of hydrogen ensures effective hydrogenation of the macromolecular and sulfur compounds on the catalyst and subsequent decomposition on the cracking component. As a result, the yield of light-colored products is increased as much as 70 percent (relative to the crude oil), and the content of sulfur and unsaturated hydrocarbons in the products is decreased sharply. The motor fuels obtained, such as gasoline and jet and diesel fuels, are of high quality.
Steam cracking has been very useful in the production of unsaturated hydrocarbons as chemical raw materials. Various types of crude oils, from refining gases to the residues of crude-oil distillation, serve as starting materials. The process is carried out at 650°-800°C in the presence of catalysts (for example, nickel oxide) on refractory. The advantages of this method include the low level of coke formation and the high olefin yield.
There are other types of cracking known that find some application in industry. One of these is oxidative cracking (cracking in the presence of oxygen). Electrocracking is used in the production of acetylene (methane being passed through an electric arc).
REFERENCESmidovich, E. V. Destruktivnaia pererabotka nefti i gaza, 2nd ed. Moscow, 1968. (Tekhnologiia pererabotki nefti i gaza, part 2).
V. V. SHCHEKIN