develops the liquid flowing through the reaction zone weeps through the walls 30 of the downcomers and descends along the outer surface of walls 30 onto the upper surface of distillation trays 12 where mixing and fractionation of the liquid occurs some mass transfer also occurs on the surface of wall 30 after leaving the tray 12 the liquid progressively descends through the reaction and distillation zones with at least some of the liquid passing the reaction zones of downcomers 18 and eventually reaching the bottom of the tower where it is drawn off through line 29 or directed to a distillation zone of the column typically a reboiler not shown is utilized to vaporize a portion of the liquid bottoms which is returned to the reactor as the liquid phase is descending through the downcomers 18 under the influence of gravity the vapor phase which is typically generated by distillation of the liquid phase but could be from another source ascends through the smaller apertures 34 in the trays and intermixes with the liquid phase which passes out of the porous downcomers the primary mass transfer between the liquid and vapor phases occurs on the trays but some also occurs within the catalytic reaction zone and in particular on the.