Also known as: i-Butane, iC4
Isobutane is an branched-chain isomer of normal butane.
Isobutane is particularly valuable in a refinery as a feedstock to the alkyaltion unit, where it is combined with either propylene or butylene to make alkylate (a high-quality gasoline blend stock).
Isobutane is produced in a variety of conversion units in a refinery. However, when a refinery has an alky there is usually a need to make additional isobutane "on purpose" to keep the alky full. This is done by isomerizing normal butane in a C4 Isomerization unit.