A mass transfer column comprising: a shell (12); an open internal region (14) defined by said shell; and a mass transfer assembly (16) positioned in the open internal region (14), the mass transfer assembly (16) comprising: a dividing wall (18) forming first and second sub-regions; one or more zones of mass transfer structures positioned in the first and second sub-regions (22 and 24); and a liquid flow divider (48) positioned above the dividing wall (18) for delivering a volumetric split of liquid to the first and second sub-regions. The liquid flow divider (48) may comprise a moveable weir (68) or a valve (180) in order the change the ratio of liquid flow between the two sub-regions.