an apparatus is provided for introducing a mixture of dry catalyst and a hydrocarbon liquid into a polymerization reactor vessel . the apparatus comprises a enclosed mixing tank with rotating mixing blades and a rotatable shot valve spaced below the mixing tank . the shot valve can be rotated from a first position in communication with an opening in the lower portion of the mixing tank to a second position in communication with a hydrocarbon feed conduit that provides hydrocarbon liquid to the polymerization reactor vessel . a bypass conduit is provided for introducing hydrocarbon liquid from the hydrocarbon feed conduit into the mixing tank wherein the interior of the mixing tank is maintained at the same pressure as the hydrocarbon feed conduit .