An alternate method to squeeze much more out of crude oil after preliminary distillation is the use of Alkylation (Alky) Units to combine olefins (propylene and butylene) from the Fluid catalytic cracker (FCC) with isobutane and a sulfuric acid catalyst. It blended vigorously earlier than the sulfuric acid is again eliminated. What left is pumped to distillation towers, where it separated into liquefied petroleum gasoline (LPG), blended butanes and alkylate. Alkylate is a excessive-octane mixing element used in lead-free premium gasoline.

The top tank has inlet and outlet piping plus a smaller diameter pipe that connects to the lower tank. The lower tank additionally has an inlet pipe. The inlet pipes are normally linked to an upstream cracking unit and a storage tank. The outlet piping would go to another distillation tower or to a mixing unit.