Membrane desalination performance governed by molecular reflection at the liquid-vapor interface

Abstract

Membrane-based separation processes that utilize liquid-vapor phase changes, such as membrane distillation (MD) and osmotic distillation (OD), form the basis of emerging desalination technologies. Although thin, hydrophobic membranes are required for optimal separation performance, current modelling approaches often neglect the transport resistances associated with phase-changes at liquid-vapor interfaces. We develop a new theoretical framework to analyze water transport across porous hydrophobic membranes by accounting for the transport resistances associated with both diffusion through the membrane and phase-changes at the liquid-vapor interfaces. Applying the developed framework to MD and OD reveals that for thick membranes (≫1μm) water flux is governed by a combination of molecular and Knudsen diffusion, showing an inverse proportionality to membrane thickness. However, the resistances associated with phase-changes, which arise from molecular reflection at liquid-vapor interfaces, become transport-limiting factors as membrane thickness decreases (<∼1μm). Our analysis identifies an optimal membrane thickness of between 1μm and 10μm to maximize the water flux in MD, while the water flux in OD is shown to increase monotonically as the membrane thickness decreases. The framework presented provides key design principles for membranes that utilize vapor transport for desalination, water treatment, and chemical concentration.