Porous hydrophobic/hydrophilic composite membranes: Application in desalination using direct contact membrane distillation

A new type of composite hydrophobic/hydrophilic porous membrane is proposed for application in membrane distillation. The membranes were prepared by the phase inversion technique in a single casting step from polymer solutions containing surface-modifying macromolecules. Polyetherimide was used as a hydrophilic base polymer. The effects of polyetherimide concentration in the casting solution on the permeate flux and on the membrane characteristics were investigated. The liquid entry pressure of water, mean pore size, effective porosity and mean roughness of the membranes were determined. Direct contact membrane distillation experiments were performed for pure water and aqueous solutions of sodium chloride as feed. The effect of the mean temperature, stirring rate and salt concentration were studied. Similar experiments were carried out for two commercial polytetrafluoroethylene membranes of different pore sizes, often used for membrane distillation purposes. The water vapor permeability of the proposed membranes was determined and compared with that of the commercial membranes. The new membranes seem to be promising in the field of membrane distillation as they combine a low resistance to mass flux achieved by the diminution of the water vapor transport path length through the hydrophobic thin top-layer of the membrane and a low conductive heat loss through the membrane obtained by using thicker hydrophilic sub-layer of the membrane.