Abstract: A circular section column with helical liquid flow is divided by a vertical wall 10 into two gas and vapor tight sectors A and B, each sector housing a series of trays.The trays disposed are designated respectively 1a, 2a and 3a for sector A and 1b, 2b and 3b for sector B.The distance D separating each two successive half trays of a given series is constant.On the other hand, the half-trays of one series are each staggered in the axial direction of the column with respect to the half-trays of the other series by a spacing d which is less than half the distance D.This arrangement allows a greater number of half-trays to be housed in a column of a given height, or the height of a column for a given number of trays to be decreased.