Steam cracking
Heat hydrocarbons until they break, then freeze the wreckage before it reassembles. The single reaction the plastics industry rests on.
Steam cracking heats a hydrocarbon feed — ethane, or naphtha — with steam to above 800 °C for less than a second, then quenches it within milliseconds. The heat breaks large saturated molecules into small unsaturated ones; the quench is what stops them recombining into tar.
Steam is there as a diluent, not a reactant. It lowers the partial pressure of the hydrocarbons, which favours the products wanted, and it suppresses coke laying down on the tube walls. The furnaces still coke up and still have to be shut down and burned clean on a cycle of weeks.
Uses
It makes ethylene and propylene, and with a naphtha feed also butadiene and aromatics. Essentially every commodity polymer starts here.
The feed decides the output. Cracking ethane gives ethylene and very little else; cracking naphtha gives a spread, which is a liability when only one product is wanted and an asset when the co-products have their own markets.
History
Thermal cracking for fuel dates from 1913. Cracking specifically for olefins as chemical feedstock is a 1940s development, and it is what made the polymer industry possible at scale — before it, monomers came from coal chemistry and acetylene in quantities that could not have supplied a mass market.
Economic significance
A world-scale cracker is a multi-billion-pound investment with a forty-year life, and its feedstock choice is fixed in steel at the design stage. That is why the shale gas boom reshaped global petrochemicals so sharply: North American operators with ethane crackers gained a durable cost advantage in ethylene that naphtha crackers in Europe and Asia could not answer, and European crackers have been closing since.
It is also the largest single energy consumer in the chemical industry, which makes it the hardest part of that industry to decarbonise: electrified and hydrogen-fired cracker furnaces are being demonstrated, and none is yet ordinary.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
takes as input
- Naphtha — material · the European and Asian cracker feed; North American crackers mostly run on ethane from natural gas instead, and make a different product slate as a result
produces
- Ethylene — compound · the principal product, and the largest-tonnage organic chemical made anywhere
- Propylene — compound · a co-product from naphtha and barely a product at all from ethane, which is why dedicated propylene plants exist
- Naphtha — material · consumed rather than produced: it is the feed the cracker is designed around, and the choice of feedstock fixes the plant's whole product slate
- Styrene — compound · by way of ethylbenzene, which is ethylene and benzene joined and then dehydrogenated
is used in
- Chemical manufacture — industry · the reaction the whole petrochemical industry is arranged around, and its largest single energy consumer
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing