Catalyst particles are contacted with a liquid and a gas in a vertically extending cylindrical vessel. The particles and liquid are placed in the vessel into which the gas and liquid are continuously fed from the bottom thereof so that an upwardly flowing mixture comprising the particles, liquid and gas is formed. The mixture is introduced into a gas separating zone disposed adjacent to an upper portion of the vessel to separate the mixture by gravity into a gas phase, a supernatant liquid phase and a phase rich in the catalyst particles. The gas and supernatant phases are continuously withdrawn from the separating zone while the catalyst particles-rich phase is continuously recycled to the bottom of the vessel by gravity.