Selective evaporation of a binary mixture into dry or humidified air

Abstract

Bench-scale experiments of the selective evaporation of the binary mixture 2-propanol—water into dry or humidified air were carried out in a cylindrical vessel. The stirred and thermostatted liquid was evaporated from its free surface into a preheated and either dry or humidified air stream. The air was preloaded with either alcohol or water. Experimental results show that the selectivity of evaporation can be effected in any direction by preloading the air stream with one of the mixture's components. For example: If the mixture 2-propanol—water at a liquid temperature of 30 °C and a composition of 60 mol% 2-propanol is evaporated into dry air, water evaporates preferentially and, at the end of the experiment, pure alcohol remains in the vessel. If, however, the air is preloaded with water vapor, the liquid at the end of the experiment consists of either pure water at sufficiently high air humidities or, at lower air humidities, of a mixture enriched with alcohol. Furthermore, in cases where evaporation takes place into humidified air, the selectivity may be inverted by increasing or decreasing the liquid temperature, respectively. Theory which involves phase equilibrium and mass transfer in the gas and the liquid phases can explain these effects.