11 12 and 13 are in open fluid communication with coolers 20 21 22 and 23 respectively hydrocarbon feed is provided via conduit 24 to coolers 20 21 22 and 23 along with additional fresh acid where cooled recycled or rerun acid catalyst is picked up to form a hydrocarbon and acid catalyst mixture the hydrocarbon and acid catalyst mixture is dispersed upwardly with high velocity through the coolers 20 21 22 and 23 and into the corresponding inlets of riser reactors 10 11 12 and 13 respectively at the bottom of common settler vessel 14 outlet conduits 26 27 28 and 29 which extend downwardly from settler vessel 14 are provided for the withdrawal of liquid acid catalyst for recycle conduits 26 27 28 and 29 are connected at the lower ends thereof with coolers 20 21 22 and 23 respectively via corresponding conduits 30 31 32 and 33 at an intermediate point along the length of the settler vessel 14 an outlet conduit 34 is provided for the removal of the separated liquid hydrocarbon product in operation a liquid hydrocarbon feed mixture comprising a mixture of an alkylating agent such as a low boiling olefin e g butylene and an alkylatable hydrocarbon such as a low boiling isoparaffin e g isobutane is introduced through conduit 24 as well as fresh makeup acid catalyst the feed mixture is dispersed at high velocity in the shells of coolers 20 21 22 and 23 which contain cooled liquid acid catalyst thus inducing acid catalyst circulation into the hydrocarbon feed mixture by density difference between the settled acid 38 from the settler 14 and the fresh makeup acid catalyst dispersed with the hydrocarbon feed in this manner acid.