The purpose of the C4 isomerization unit is to convert normal butane (n-C4) into isobutane to feed the alkylation unit.
Isobutane is produced in a variety of conversion units in a refinery. However, when a refinery has an alkylation unit, 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.
How it works
A normal butane feed stream is mixed with a small amount of hydrogen, raised to 200-400F (93-204C), and injected into a reactor vessel containing a platinum catalyst. The catalyst and heat convert the C4 molecules into the isomer isobutane. Typically, unreacted C4s are recycled to increase yield.