A quench zone mixing apparatus that occupies a low vertical height and has an improved mixing efficiency and fluid distribution across the catalyst surface includes a swirl chamber, a rough distribution network, and a distribution apparatus. In the swirl chamber, reactant fluid from a catalyst bed above is thoroughly mixed with a quench fluid by a swirling action. The mixed fluids exit the swirl chamber through an aperture to the rough distribution system where the fluids are radially distributed outward across the vessel to the distribution apparatus. The distribution apparatus includes a plate with a number of bubble caps and associated drip trays that multiply the liquid drip stream from the bubble caps to further symmetrically distribute the fluids across the catalyst surface. Alternatively, deflector baffles may be associated with the bubble caps to provide a wider and more uniform liquid distribution below the plate. The distribution apparatus can be used in the reaction vessel without the swirl chamber and rough distribution system, e.g., at the top of a vessel.