Abstract: Reactions in heterogeneous systems containing reactants in gaseous, liquid and solid phases are carried out in a tower composed of a series of superimposed reaction compartments mutually separated by horizontal shelves, through which the gaseous phase moves countercurrent to the liquid and solid phases and at the same time effects turbulent mixing with the liquid phase. The liquid and solid phases are introduced into the top compartment. The shelves are suitably perforated for upflow of the gas through bodies of mixed liquid and solids which are held on the shelves at levels determined by the heights of overflow pipes which conduct excess reaction mixture from each reaction compartment into the mixture in the compartment next below. Reaction products are taken off from the bottom compartment.