The Alkylation unit, Unit 16, reacts C4 olefins with isobutane
in the presence of a sulphuric acid catalyst to form alkylate, a high octane
gasoline blending component. The butane
/ butylenes mix feed stock is produced in both the #1 and #2 Fluid Catalytic
Cracking units with additional isobutane being recovered from the straight run
gas plant and / or imported. The
alkylation unit has a processing capacity rated at 4,000 BPSD of Alkylate production. The unit consists of coalescers, contactors,
acid settling tanks, a flash drum, caustic wash drum, a deisobutanizing column,
and a depropanizing column.
After passing through a heat exchanger where it is cooled by refrigerated effluent from the contactors, the isobutane and olefin feed mixture is passed through coalescers where any entrained moisture is removed. The feed mixture then enters the contactor (stirred reactor) where it is intimately mixed with the catalyst, sulphuric acid. The alkylation reaction of olefins with isobutane is exothermic, and the process is required to take place at relatively low temperatures to suppress and avoid undesirable polymerization reactions. After sufficient residence time in the contactor, the mixture of unreacted isobutane, propane, n-butane, alkylate and acid flows into the Acid Settler where the hydrocarbon and acid phases are allowed to separate. The acid is recycled back to the contactor, while the hydrocarbon mixture flows through a heat exchanger to cool the contactor and then to the Refrigeration Flash Suction Drum where the hydrocarbon stream is flashed to provide the necessary refrigeration.
The vapor from the flash drum is compressed and condensed before being fed to the Depropanizer column to remove any C3 components. The colder liquid from the refrigeration drum passes through a heat exchanger to cool the hydrocarbons entering the coalescer/contactor and also to heat this reactor effluent prior to neutralization. In the neutralization section, traces of entrained acid and acid esters are removed by contacting with a caustic soda solution. Following neutralization, the hydrocarbon mixture is fractionated in the Deisobutanizer column to separate the unreacted isobutane which is recycled back to the feed. A side stream of n-butane is also pulled from this column to allow control of the Alkylate product RVP to meet desired gasoline blending specifications. The Deisobutanizer and Depropanizer columns are reboiled by a common fired heater.