A reactor for the autorefrigerant alkylation process has a reactor vessel with a lower end inlet for the refrigerant reactant and the sulfuric acid and a series of inlets for the olefin reactant at vertically spaced intervals. A flow path for the reactants is provided by co-acting baffles which define sequential reaction zones. The baffles interact with a rotary mixer with multiple impellers to provide agitation. Outlets for the vaporized refrigerant and the reaction effluent are provided at the upper end of the vessel. In the alkylation process, the liquid isoparaffin hydrocarbon reactant/refrigerant with a sulfuric acid alkylation catalyst is introduced into the lower end and passed along the extended reactant flow path with the olefin reactant introduced at intervals along the path. The reaction mixture flows in the sequence of serial reaction zones within the reactor to promote mixing of the isoparaffin reactant with the acid catalyst.