A reactor for performing a reaction between two immiscible fluids of different density, comprising an interior formed by a cylindrical, vertically oriented elongate shell, a bottom and a cap, wherein the interior is divided by internals into a backmixed zone, a zone of limited backmixing arranged below the backmixed zone and a plug-flow zone which are at least consecutively traversable by one of the fluids, wherein the backmixed zone comprises at least one inlet and the plug-flow zone comprises an outlet and the backmixed zone comprises at least one mixing apparatus selected from a stirrer, a jet nozzle and means for injecting the fluid of lower density, a first cylindrical internal element which in the interior extends in the longitudinal direction of the reactor, which delimits the zone of limited backmixing from the plug-flow zone and which comprises a first passage to the backmixed zone and a second passage to the plug-flow zone, wherein the first internal element is arranged concentrically to the shell so that the plug-flow zone has an annular horizontal cross section, and wherein the lower edge of the first internal element is arranged at a distance from the bottom of the reactor, a second internal element which delimits the backmixed zone from the plug-flow zone such that there is no direct fluid connection between the backmixed zone and the plug-flow zone, wherein the second internal element extends from the upper edge of the first internal element to the shell, and wherein the second internal element is a horizontally oriented annular plate, and backmixing-preventing third internal elements in the form of random packings, structured packings or liquid-permeable trays arranged in the zone of limited backmixing. wherein the interior is divided by internals into a backmixed zone, a zone of limited backmixing arranged below the backmixed zone and a plug-flow zone which are at least consecutively traversable by one of the fluids, wherein the backmixed zone comprises at least one inlet and the plug-flow zone comprises an outlet and the backmixed zone comprises at least one mixing apparatus selected from a stirrer, a jet nozzle and means for injecting the fluid of lower density, a first cylindrical internal element which in the interior extends in the longitudinal direction of the reactor, which delimits the zone of limited backmixing from the plug-flow zone and which comprises a first passage to the backmixed zone and a second passage to the plug-flow zone, wherein the first internal element is arranged concentrically to the shell so that the plug-flow zone has an annular horizontal cross section, and wherein the lower edge of the first internal element is arranged at a distance from the bottom of the reactor, a second internal element which delimits the backmixed zone from the plug-flow zone such that there is no direct fluid connection between the backmixed zone and the plug-flow zone, wherein the second internal element extends from the upper edge of the first internal element to the shell, and wherein the second internal element is a horizontally oriented annular plate, and backmixing-preventing third internal elements in the form of random packings, structured packings or liquid-permeable trays arranged in the zone of limited backmixing.