A process of claim 11 wherein additional b zones are connected in the same way as the zones of the F section described in claim 11 to form a B section for column balancing and are sequentially numbered from zone B(b) to zone B(1) where b is an integer, the B section being positioned upstream of the R section of claim 11 with zone B(1) adjacent to zone R(r), the branched opening of the inlet tube o