TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: The mild regeneration mode involves contacting the solid acid alkylation catalyst with a portion of the alkylation mode alkylate-containing effluent and hydrogen in the first reactor of the zone, and performing the operation at least once more in a downstream reactor in the same zone employing, instead of the portion of the alkylation mode alkylate-containing effluent, the effluent of the preceding upstream reactor, to produce a regeneration mode effluent.The alkylate is recovered from the regeneration mode effluent and the alkylation mode alkylate-containing effluent.Alternatively, the mild regeneration mode involves contacting the alkylation catalyst with entire alkylation mode alkylate-containing effluent, and the alkylate is recovered only from the regeneration mode effluent.Alternatively, the mild regeneration mode involves contacting alkylation catalyst with the alkylatable compound and hydrogen in the first reactor of the zone, and performing the operation at least once more in a downstream reactor in the same zone employing, instead of alkylatable compound, the effluent of the preceding upstream reactor, to produce a regeneration mode effluent.The alkylate is recovered from the regeneration mode effluent and the alkylation mode alkylate-containing effluent.Alternately, the mild regeneration mode involves contacting the alkylation catalyst with hydrogen in each of at least two reactors.Reactors contain alkylate formed in at least two reactors during their previous cycle of operation in the alkylation mode.The alkylate is recovered from the alkylation mode alkylate-containing effluent.Preferred Apparatus: The alkylation apparatus comprises a separation unit for separating alkylatable compound from the alkylation mode alkylate-containing effluent and from the regeneration mode effluent, a separation unit for separating alkylatable compound from the regeneration mode effluent, and a separation unit for separating alkylatable compound from the alkylation mode alkylate-containing effluent, after which a stream comprising at least a portion of the alkylatable compound is recycled to the zone operating in the alkylation mode or zone operating in the regeneration mode.The apparatus comprises preferably two reactors in zone (A) and two reactors in zone (B), and a separation unit between the two reactors operating in the alkylation mode in which a portion of the alkylate can be removed from the alkylate-containing product stream.The apparatus comprises preferably three reactors in zone (A) and three reactors in zone (B), and separation unit between the first and the second as well as between the second and the third reactor in the zone operating in the alkylation mode, in which a portion of the alkylate can be removed from the alkylate-containing product stream.At least two reactors in zone (A) and at least two reactors in zone (B) each comprises more than one catalyst bed with separate addition of alkylation agent. The catalyst bed at the most downstream end of the zone operating in the alkylation mode is deprived of separate addition of alkylation agent.The frequency with which zone (A) reactors and the zone (B) reactors cycle back and forth between the alkylation mode and the mild regeneration mode is once per 10 hours to 10 times per hour.The apparatus comprises additional solid acid alkylation catalyst-containing reactor(s) which can replace one or more of the reactors in zone (A) or zone (B) to allow the catalyst in the replaced reactor or reactors to be subjected to high temperature regeneration.The high temperature regeneration is performed at 423-873 K.