Abstract: Improvements in a reaction vessel which includes a lower cylindrical section conically reduced in diameter toward the outlet end and an upper cylindrical section conically increased in diameter from the inlet end, the two sections being joined endways, and wherein a gas stream is tangentially introduced into the lower cylindrical section and is caused to react, inside the upper section, with a reactant liquid injected into the latter. A horizontal partition is provided within the lower cylindrical section conically reduced in diameter to divide the space therein into an upper and a lower spaces, whereby the gas stream is divided and separately passed, in a spiral flow from the upper space and in a piston flow from the lower space, into the upper cylindrical section.