Sorption and hydration effects on liquid carbon dioxide transport through mesoporous γ-alumina and titania membranes

Dehydrated and partially hydrated liquid carbon dioxide (>70 bar, 298 K) transport was examined in mesoporous ceramic membranes with γ-alumina (5 nm) and titania (1 and 5 kDa) separation layers. Liquid carbon dioxide, a green solvent alternative, is an apolar solvent with pressure-dependent physicochemical properties. Viscosity-corrected volumetric flux was used to incorporate changes in density and viscosity with increasing feed pressure. It was observed, for both dehydrated and partially hydrated liquid carbon dioxide (∼20 ppm water in feed), that solvent-membrane interactions played a significant role in transport behavior. For dehydrated feeds, consecutive permeation cycles indicated appreciable carbon dioxide sorption, reducing the effective cross-sectional pore area and permeability. When liquid carbon dioxide was partially hydrated, water preferentially adsorbed on the pore surface and led to a significant reduction in permeability relative to dehydrated conditions. Flux of hydrated liquid carbon dioxide was non-linear in all three membranes due to water stripping from the pore surface with increasing pressure (i.e. carbon dioxide density and solvent strength). The permeability of liquid carbon dioxide was compared with reported values for "conventional" solvents. Based on our findings, we discuss potential factors that will influence condensed phase carbon dioxide transport in ceramic membranes.