Butylene is an unsaturated four carbon olefin, basically butane with one double bond.

In a refinery, butylene is important as a feed stock to the alky unit to make C4 Alkylate, the highest-value alkylate. Butylene is also used to make MTBE and IsoOctene, which are both high-quality gasoline blend stocks.

Refinery butylene primarily comes from the FCC unit. Small amounts of butylene are also produced by other cracking units, such as the coker, but these less likely to be separated for use in the alky. Butylene can also be sourced externally from steam crackers, which produce a large amount of butylene when processing light naphtha as a feed stock.

In the absence of an alky unit, the typical disposition of butylene is in LPG blending or fuel gas, both very low-value options.