Application of PVDF membranes in desalination and comparison of the VMD and DCMD processes

Abstract

Membrane distillation (MD) is a promising desalination technology that may help to resolve the fresh water shortage. Hydrophobic flat-sheet polyvinylidene fluoride (PVDF) membranes were used for desalination by direct contact MD (DCMD) and vacuum MD (VMD) processes. The effect of the operating conditions, such as the feed temperature, flow velocity on both the hot and cold sides and the concentration of the feed solution were investigated. During the DCMD processing of a 35g/L aqueous NaCl solution, with a hot side temperature of 73°C and a cold distillate water temperature of 25°C, the permeate flux was as high as 18.9kg/m2h with a NaCl rejection of 99.8%. A permeate flux of 22.4kg/m2h and a NaCl rejection of 99.9% were attained during the VMD process at the same feed temperature and a downstream pressure of 31.5kPa. Furthermore, there was hardly any wetting during 6h desalination test. A dusty gas model was used to successfully predict the permeating flux in both the DCMD and VMD processes, and the mass transfer mechanisms through the membrane were determined. Comparison of the flux and the thermal efficiency in the DCMD and VMD processes under the same conditions demonstrated that this type of thin flat-sheet membrane, which had a high surface hydrophobicity and a sponge-like cross-section, was more suitable for use in the VMD process than in the DCMD process.