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.
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.