Abstract: This invention makes possible substantially continuous flow of uniformly distributed hydrogen and hydrocarbon liquid across a densely packed catalyst bed to fill substantially the entire volume of a reactor vessel by introducing the fluids as alternate annular rings of gas and liquid (i.e. a mixture of liquid hydrocarbon and a hydrogen-containing gas) at a rate insufficient to levitate or ebullate the catalyst bed. Catalyst are selected by density, shape and size at a design feed rate of liquids and gas to prevent ebullation of the packed bed at the design feed rates. The liquid and gas components of the hydrocarbon feed flow into the catalyst bed from concentric annular rings that are coaxial with the catalyst bed. At the desired hydrocarbon flow rate, such catalyst bed continually flows in a plug-like manner downwardly through the reactor vessel. Catalyst is removed on a periodic or semicontinuous basis by laminarly flowing catalyst particles in a liquid stream out of the bottom of the catalyst bed.