Pyrolysis

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pyrolysis

[pə′räl·ə·səs]
(chemistry)
The breaking apart of complex molecules into simpler units by the use of heat, as in the pyrolysis of heavy oil to make gasoline. Also known as thermolysis.

Pyrolysis

 

the process by which organic compounds are decomposed by heat. The term “pyrolysis” is usually used in a narrower sense to refer to the high-temperature, extensive thermal conversion of crude oil and natural gas through decomposition, isomerization, and other transformations of the molecules of the raw materials; the term “cracking” is equivalent to “pyrolysis” in this narrower sense.

Pyrolysis is one of the most important industrial methods for producing starting materials for petrochemical synthesis. Its desired product is a gas that is rich in the unsaturated hydrocarbons ethylene, propylene, and butadiene, which are used for making polymers for the production of plastics, synthetic fibers, and synthetic rubbers.

The first pyrolysis plants were constructed in Russia in the 1870’s, in Kiev and Kazan. Primarily kerosine was subjected to pyrolysis, with the aim of producing illuminating gas. Subsequently it was proved feasible to separate aromatic hydrocarbons from the tar that is produced by pyrolysis. Pyrolytic methods were extensively developed during World War I, when a large demand arose for toluene, the starting material for the production of trotyl, or trinitrotoluene (TNT).

A wide variety of raw materials can be subjected to pyrolysis, including crude petroleum, heavy distillates, and such gaseous hydrocarbons as ethane and propane. However, pyrolysis is mostly used to treat gaseous hydrocarbons and gasolines, with which the greatest yields of the desired products are obtained with a minimal formation of coke. In industry, the most common pyrolysis units are of the tube type. The stock, for example, gasoline, is passed through a steam preheater and then mixed with superheated steam before entering a furnace, where it is subjected to further heating and pyrolytic decomposition. The final temperature of the reaction at the furnace exit varies from 750° to 850°C. The high temperatures, the short time that the stock spends in the reaction zone, and the dilution of the stock by water vapor promote decomposition to yield significant amounts of gas. A liquid tar is a by-product of pyrolysis. On the average, stocks of gasoline contain 20 percent tar by weight, while gaseous stock contains 5 percent tar.

To stop the pyrolysis reaction, the steam-gas mixture that exits from the furnace is rapidly cooled in a quencher by direct contact with a water condensate, which evaporates in the process. The mixture undergoes further cooling in a waste-heat boiler, where steam is produced under high pressures. The partially cooled steam-gas mixture exits from the waste-heat boiler and is subject to oil scrubbing to remove particles of soot and coke; the heavy tar fraction separates from the mixture. The scrubbed steam-gas mixture is further cooled with subsequent separation of the water and light-hydrocarbon condensates from the pyrolysis gases, which are transferred to a fractional-distillation unit for the separation of ethylene and propylene.

Pyrolysis tar characteristically contains a high concentration of the aromatic hydrocarbons benzene, toluene, and naphthalene. It also contains unsaturated hydrocarbons, including cy-clopentadiene, which is a starting material in the synthesis of many organic products. The components of the tar are used to manufacture high-octane gasoline, aromatic hydrocarbons, such binders as cumarone-indene resins, and petroleum-coke for electrodes. Table 1 shows the approximate yields of the most valuable gas and tar components from various raw materials.

Table 1. Percent yields for the major products of pyrolysis
 Raw material
 EthanePropaneGasoline (light)Gas oil (light)
Ethylene ............77.742.033.526.0
Propylene ...........2.816.815.516.1
Butadiene, butylene.....2.74.38.89.4
Benzene ...........0.92.56.76.0
Toluene............0.20.53.32.9

Pyrolysis of petroleum and the other heavier stocks is accompanied by considerable deposition of coke and requires specially designed apparatus. Circulating heat carriers are used to accelerate pyrolysis. When solid heat carriers are used, for example, quartz sand or petroleum coke, the reaction is accelerated by burning the coke that is formed within the system. When a gaseous heat carrier is used, for example, steam, acceleration results from minimal deposition of coke. Catalytic processes have been developed to reduce the cost of pyrolysis. The optimal yield in the pyrolysis of gaseous stock is obtained at about 1200°C, in which case the major product is acetylene, a starting material in the production of chloroprene rubber and acetalde-hyde.

REFERENCES

Smidovich, E. V. Destruktivnaia pererabotka nefti i gaza, 2nd ed. Moscow, 1968. (Tekhnologiia pererabotki nefti i gaza, part 2.)
Paushkin, la. M., S. V. Adel’son, and T. P. Vishniakova. Tekhnologiia neftekhimicheskogo sinteza, part 1. Moscow, 1973.

E. V. SMIDOVICH