IPA/water and TBA/water systems which show an azeotrope were dehydrated in a wetted-wall column by the diffusion distillation. The wetted-wall column consists of two concentric tubes and a gas layer. The diffusion distillation is the evaporation and diffusion process below the boiling temperature of the mixture in a gap filled with a gas. The essence of the process is that the lighter component condenses at a faster rate through a gas then the heavier. Hence the relative volatility and the difference of diffusivity affect the separation.
Several experiments were carried out with various evaporation side temperatures, and various kinds of gas such as $CO_2$, air and helium. The diffusion length was adjusted by interchange of inner tubes of different diameter.
The degree of separation was presented by selectivity which defined as the difference of composition between evaporation and condensation side. IPA/water and TBA/water systems were found to show maximum selectivity at 40℃ and 35℃ respectively. The separation with $CO_2$ yielded higher selectivities when compared with the cases of air and helium at rather low flux. With decreasing diffusion length, selectivity and flux increased.
To describe mass transfer in our process, we made use of vapor-liquid equilibrium and Maxwell-Stefan equation. The condition at the interface was obtained by the differential energy balance along the column. The experimental data were compared with the results of simulation.
