Olefins are unsaturated hydrocarbons containing a double bond between two carbon atoms. Examples are butylene and propylene.

Olefins in finished petroleum products

In gasoline, olefins contribute to octane, which is beneficial. However, because they tend to form harmful deposits inside of engines, there are typically limits set on total olefin content.

Refinery streams containing high percentages of olefins such as coker naphtha, can have a tendency to polymerize and form gums that can cause problems in storage and transportation systems so these streams usually are hydrotreated to saturate the olefins

Some refineries produce olefins such as propylene as a product, typically as a feed stock to petrochemicals.

Olefins in refinery feed stocks

In a refinery, olefins are an important feed stock into the alkylation unit to produce alkylate (a high-value gasoline blend stock). The olefins most commonly used in the alkylation unit are butylene and propylene, which primarily come from the FCC unit.