Propylene is a three carbon olefin, basically propane with one double bond.

In a refinery, propane is important as a feed stock to the alky unit to make C3 Alkylate for blending into gasoline. Propylene is also often separated and sold as a feed stock to the petrochemicals industry to make polypropylene plastics.

Refinery propylene primarily comes from the FCC unit. Small amounts of propylene are also produced other cracking units, such as the coker, but these are less likely to be separated for use in the alky. Propylene can also be produced on purpose by dehydrogenating propane, but this is not typically done in a refinery.

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