Abstract: Fractionating apparatus especially designed for use in non-adiabatic distillation comprising two series of corrugated horizontally spaced sheets, each of the two series of sheets extending vertically, one series of such sheets being offset vertically from the other series, and forming a plurality of vertically disposed side channels on each side of the adjacent corrugated metal sheets. The side channels provide a tortuous vertical path for upward flow of vapor, and liquid descending from above is split at each offset horizontal fin of the corrugated sheets and mixed with the neighboring stream, thereby providing efficient mixing of the liquid phase as well as the gas phase, such horizontal fins also providing additional liquid holdup, as the descending liquid tends to puddle on the topside of the fins.