Surface active agents have been shown to significantly affect the efficiency of packed distillation columns. Francis and Berg (1967) found that increases in efficiency of as great as 100% could be achieved by the addition of small amounts of such materials. Such improvements were observed only for surface tension negative systems as defined by Zuiderweg and Harmens ( 1958). In these systems, the reflux liquid decreases in surface tension down the column causing channeling due to incomplete wetting of the packing. On the other hand, positive systems increase surface tension down the column and exhibit better wetting characteristics, hence greater interfacial area and higher efficiency. Neutral systems, that is, those for which no significant surface tension gradients are developed, show wetting and efficiency characteristics similar to positive systems in supported area equipment. In the cases examined by Francis and Berg, surface active agents were found to stabilize the liquid films in the negative systems against breakup and channeling in both organic and aqueous test mixtures resulting in the restoration of efficiencies to values comparable to those of positive and neutral systems.
The distillation studies of Francis and Berg were carried out in supported area equipment, that is, a packed column. Zuiderweg and Harmens show, however, that in the * At high vapor rates, however, negative systems may develop a stable spraying action increasing area under such conditions (Fane and Sawistowski 1968).