Storage and wash vessels in the alkylation unit include those for fresh and depleted acid and water, an acid analyzer settling pot, and a number of wash tanks. Caustic washes neutralize free acid carried over from the reaction zone and neutralize alkyl sulfates in the net effluent. Water washes decompose remaining esters and remove caustic and salt that might have carried over from the caustic wash.
Interface level control in caustic and water washes prevents corrosion and fouling of the deisobutanizer and other downstream units. Levels rising above high set points can cause carryover of caustic or water, while too low levels can cause hydrocarbon carryunder.