Abstract: A seal cartridge for the upper end-adjacent portion of a rotatable agitator shaft which projects upwardly through the drive nozzle of a chemical reactor vessel includes an outer housing which is adapted to be fixedly anchored relative to the drive nozzle, said housing containing sealing means which are removably secured to and carried by the agitator shaft whereby to rotate with the shaft interiorly of the housing and wherein the sealing means are removable from said housing for purposes of inspection and/or replacement after the housing has been removed from the shaft.