The invention relates to Quench box assembly comprising quench pipe and quench box, to mix quench gas and vapor-liquid effluent from previous catalyst bed to achieve equilibrium temperature before entering the next bed. The quench pipe is in the form of ring having aperture while quench box consists of swirling section and a mixing chamber. The swirling section consists of inclined baffles to provide swirling action to incoming stream and the turbulence created by the swirling action increases the heat transfer rate thus requiring the smaller reactor volume to attain equilibrium temperature. The perforated plate being open from all the sides allowing the liquid to flow uniformly from all directions thus providing uniform distribution on the distributor tray. Hence, eliminates the requirement of rough liquid distributor before the distribution tray.