An apparatus for the continuous reaction of a liquid with a gas which comprises: two externally cooled, substantially concentric, stationary circular reaction surfaces, a rotatable circular rotor having a lid as well as a skirt extending from the upper end of the space between said reaction surfaces partway to the bottom of said space, a distribution ring for supplying recycled reaction product for quenching the reaction mixture which ring is equipped with a bedplate devised to form in combination with the lower end of a cooling jacket provided on the outer reaction surface a downwardly opening inlet for the recycled reaction mixture; means for turning the rotor and means for passing the reactant liquid and the reactant gas in parallel streams from the upper part to the space between said reaction surfaces and said rotor; and a circular wall beneath the outer reaction surface which is located concentrically with said reaction surface.