NOVELTY - The acid alkylation system (100) comprises a cooler-reactor (220), and a settler (280) having a height and a width, where the height exceeds the width and the cooler-reactor receives a feed of one of a stream comprising olefin (114) and a stream comprising isoparaffin (134), which contains isobutane. A portion of one of the streams is bypassed around the cooler-reactor to the settler to control the temperature within the settler, where 10-40 vol.% of the olefin stream is bypassed around the cooler-reactor and 5-30 vol.% of the isobutane stream is bypassed around the cooler-reactor.    USE - The acid alkylation system is useful to produce high octane gasoline from lower boiling feeds.    ADVANTAGE - The acid alkylation system ensures effective and simple production of the high octane gasoline from the lower boiling feeds with high quality and high reliability.    DETAILED DESCRIPTION - The acid alkylation system (100) comprises a cooler-reactor (220), and a settler (280) having a height and a width, where the height exceeds the width and the cooler-reactor receives a feed of one of a stream comprising olefin (114) and a stream comprising isoparaffin (134), which contains isobutane. A portion of one of the streams is bypassed around the cooler-reactor to the settler to control the temperature within the settler, where 10-40 vol.% of the olefin stream is bypassed around the cooler-reactor and 5-30 vol.% of the isobutane stream is bypassed around the cooler-reactor. The cooler-reactor receives an alkylation catalyst in a volume ratio of alkylation catalyst feed of 1:1-3.0:1. The acid alkylation system further comprises a riser-reactor receiving a portion of one of the streams bypassed around the cooler-reactor before entering the settler. The portions of the olefin stream and the isobutane stream are combined before entering the cooler-reactor, where the isobutane:olefin mole ratio is 1:1-20:1. The combined feed is split before entering the cooler-reactor. The cooler-reactor has a height and a width, where the height exceeds the width and one end of the cooler-reactor is adapted to receive a cooling water stream. The cooler-reactor comprises cooling coils adapted to receive the cooling water stream. The olefin comprises C3 or C4 olefin. An INDEPENDENT CLAIM is included for a process for controlling the temperature in a settler.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of an acid alkylation unit or system.    Acid alkylation unit or system (100)    Olefin (114)    Isoparaffin (134)    Cooler-reactor (220)    Settler. (280)