Steam cracking

Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes, including ethylene and propylene.

In steam cracking, a gaseous or liquid hydrcarbon feed is diluted with steam and then briefly heated in a furnace. Typically, the reaction temperature is very hot—over 900°C—but the reaction is only allowed to proceed for a few tenths of a second before being quenched by contact with a colder fluid. The products produced in the reaction depend on the composition of the feed. Light hydrocarbon feeds give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon feeds give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil.

The process also results in the slow deposition of coke, a form of carbon, on the reactor walls. This degrades the effectiveness of the reactor, so reaction conditions are designed to minimize this. Nonetheless, a steam cracking furnace can usually only run for a few months at a time between de-cokings.

Chemistry

A large number of chemical reactions take place during steam cracking, most of them based on free radicals. Computer simulations aimed at modeling what takes place during steam cracking have included hundreds or even thousands of reactions in their models. The major sorts of reactions that take place, with examples, include:

Initiation reactions, where a single molecule breaks apart into two free radicals. Only a small fraction of the feed molecules actually undergo initiation, but these reactions are necessary to produce the free radicals that drive the rest of the reactions. In steam cracking, initiation usually involves breaking a chemical bond between two carbon atoms, rather than the bond between a carbon and a hydrogen atom.

CH₃CH₃ → 2 CH₃•

Hydrogen abstraction, where a free radical removes a hydrogen atom from another molecule, turning the second molecule into a free radical.

CH₃• + CH₃CH₃ → CH₄ + CH₃CH₂•

Radical decomposition, where a free radial breaks apart into two molecules, one an alkene, the other a free radical. This is the process that results in the alkene products of steam cracking.

CH₃CH₂• → CH₂=CH₂ + H•

Radical addition, the reverse of radical decomposition, in which a radical reacts with an alkene to form a single, larger free radical. These processes are involved in forming the aromatic products that result when heavier feedstocks are used.

CH₃CH₂• + CH₂=CH₂ → CH₃CH₂CH₂CH₂•

Termination reactions, which happen when two free radicals react with each other to produce products that are not free radicals. Two common forms of termination are recombination, where the two radicals combine to form one larger molecule, and disproportionation, where one radical transfers a hydrogen atom to the other, giving an alkene and an alkane.

CH₃• + CH₃CH₂• → CH₃CH₂CH₃

CH₃CH₂• + CH₃CH₂• → CH₂=CH₂ + CH₃CH₃