The wetting behavior of azeotropic systems at atmospheric and reduced pressures

Abstract

Contact angles are reported for the azeotropic systems n-propanol-water and benzene-cyclohexane measured under distillation conditions at total reflux over a pressure range 50 to 760 torr on both copper and polytetrafluoroethylene surfaces. The influences of the solid surfaces, the surface tension positive and negative nature of the liquids, together with the changes accompanying the azeotropic shift with pressure on the wettability, have been established and shown to conform with wetting theory. ADRIAN P. BOYES and ANTHONY B. PONTER Deportment of Chemicol EngineeringUniversity of New BrunswickFredericton, Canada SCOPEThe wettability of binary liquids which form azeotropes and whose surface tensions are strongly dependent on concentration have been determined for a range of pressures in a cell in which the contact angles can be meawred at conditions which simulate total reflux in a distillation column. The object of the study was to establish if the wetting changes actually occurring were responsible for the large changes in efficiency of separation at concentrations near the azeotrope and at near-terminal compositions. No methods are available for predicting performance of columns in these regions and to date no interfacial properties have been reported for these conditions upon which the interfacial areas for transfer and so column efficiencies are dependent. CONCLUSIONS A N D SIGNIFICANCEWhen aqueous mixtures of n-propanol are distilled below the azeotropic composition, representing a surface tension positive system (that is, where the lighter component has the lower surface tension on copper which is a surface with a high surface free energy), a reduction in pressure has been shown to cause a decrease in wettability. Conversely, for n-propanol concentrations above the azeotrope, which is a surface tension negative system, an increase in wetting resulted when the pressure was reduced. With the low energy surface polytetrafluoroethylene for any propanol composition, the wetting was found to be independent of pressure and temnerature. For the benzene-cyclohexane system, however, the wetting behavior was not found to be analogous to that described when using n-nrouanol-water since an additional effect which was attributed to the cyclohexane being absorbed on the polytetrafluoroethylene surface was encountered.The azeotropic shift caused by varying the pressure was shown to result in binary mixtures which have compositions close to the azeotrope, changing from surface tension positive to negative in nature with accompanying differences in wetting behavior. The dependency, wetting on concentration, in particular near the azeotrope and at near-terminal values predicted by considering the relative contributions played by the vapor-liquid, liquid-solid, and vapor-solid interfaces, was in accord with the experimental findings.The data point conclusively to the controlling role played by wetting on column performance using these systems. They also illustrate that design of distillation columns for separation of azeotrop...

Effect of Surfactant Addition upon Packed Distillation Column Performance

“…No improvement was evident when surface active agents were added to positive systems. However, when surface tension positive systems were distilled in film contacting devices, it was observed by a number of workers (Jackson and Geaglske, 1950;Vanwijk and Thijssen, 1954; Liang and Smith, 1962; Norman et al, 1963;Pritchard, 1963; Sawistowski and Smith, 1959; Boyes and Ponter, 1969) that at low compositions significant changes occurred in the values of the Height of a Transfer Unit and Ponter and co-workers (Ponter et al, 1973; Boyes and Ponter, 1972) by mea-suring the contact angles under total reflux conditions, demonstrated the large wetting changes which resulted at near-terminal compositions and were able to explain the phenomenon satisfactorily with simple wetting theory. It was established that the anomalous wetting behavior would only exist on high energy surfaces such as glass or copper and that low energy surfaces such as polytetrafluoroethylene could not produce this effect.…”