An apparatus and method for manipulating the temperature and pressure of the hydrocarbon phase in an acid settler vessel (5) associated with a catalytic alkylation process and apparatus (4), wherein the hydrocarbon phase in the acid settler vessel is regulated by controlling the amount of rerun catalyst stream (19) entering the hydrocarbon phase of the acid settler vessel (5). In response to a detected temperature or pressure of the hydrocarbon phase above a set point, the flow of rerun catalyst stream (19) to the hydrocarbon phase of the acid settler vessel (5) is decreased. This decrease in flow to the hydrocarbon phase may be accompanied by an increase in flow of rerun catalyst vapor (20) to the catalyst phase of the acid settler vessel (5).