A device and a method for operating it for application in a vertical tube reactor with downward flow (downer), has the aim of ensuring the intimate mixing of the solid particulate catalyst with a reagent fluid. Homogeneous distribution of catalyst is due to the use of a plate having perforations. Said perforated plate normally allows the passage of a portion of the stream of catalyst. The other portion overflows the edge of the plate, flowing in the form of an annular curtain, near the inner surface of the surrounding tube of the device. After passing beyond the perforated plate, the curtain-flow undergoes a deflection produced by an annular screen, to be mixed with the stream originating from the orifices in the perforated plate. A hydrocarbon charge is injected below the perforated plate, forming a certain angle with respect to the direction of the downward flow of catalyst, by means of inlets distributed uniformly about cross sections of the surrounding tube. After the mixing region, the diameter of the device, when smaller than the diameter of the associated tube reactor, expands slightly.