Toluene is a hydrocarbon with 7 carbon atoms that contains an unsaturated aromatic ring structure, plus one additional carbon atom. Essentially benzene plus an extra carbon.
In a refinery, toluene is valued as a source of octane for gasoline blending. However, in gasoline, toluene content is also constrained by a limit on total aromatics content, because aromatics contribute to smog formation when lost to the atmosphere through evaporation.
Toluene is also an important feedstock to the petrochemicals industry, so many refiners produce and extract toluene to sell as a product.
The primary source of toluene in a refinery is from the reformate produced in the reformer.