Steady states in membrane distillation: influence of membrane wetting

Abstract

Non-isothermal transport of water through porous hydrophobic membranes is interpreted on the basis of a modified membrane distillation model. In previous papers, it was suggested that two opposing fluxes can take place through the partially wet membrane. The first is a vapour flux, from hot to cold through dry pores, and the second a liquid flux, from cold to hot through liquid-filled pores. We now present some results that confirm the above hypothesis. Temperature- and/or pressure-driven water transport through two PTFE membranes has been studied. A model is developed which characterizes the temperature- and pressure-driven steady states in accordance with the membrane distillation theory, explains the decrease of the fluxes and the steady-state pressure differences by progressive membrane wetting, and defines and evaluates a new parameter, the percentage of pores filled with liquid water, which describes that membrane wetting.