Desalination by membrane distillation adopting a hydrophilic membrane

Direct contact membrane distillation (MD) by means of composite membranes with a PVA/PEG hydrophilic layer on a hydrophobic PVDF substrate has been developed for desalination. The effects of brine temperature, salt concentration, running time and the addition of ethanol on the flux of composite membranes have been investigated. Results showed that the flux of the composite membrane did not deteriorate by adopting an additional hydrophilic membrane although durability was obviously improved. More than 99% of the separation coefficient in one run was achieved with the conductivity of the produced fresh water in the range of 6–10 µs/cm. The flux of the composite membrane retained 91% flux of substrate at 70°C, being 23.7 kg/h@m2. When the brine temperature rose to 70°C, the composite membrane showed a declined concentration polarisation, with a smaller Cmb /Cb (3.89) than that of the substrate (5.79). Although the flux decreased with the increase of brine concentration, it retained 64% flux of pure water at brine solution containing 20% NaCl and was kept almost constant until 25% NaCl. In the continuous running experiments, there was no obvious drop of flux, even after adding 25% ethanol to the brine and running overnight. It is expected that adopting a hydrophilic layer can prohibit the wetting problem that faced traditional MD with hydrophobic membranes.

Direct contact MD by means of a composite membrane with a hydrophilic layer was developed for desalination where the durability was greatly improved while keeping most of the flux of the substrate. With the conductivity of produced fresh water in one run in the range of 6– 10 µs/cm, the flux of a prepared composite membrane retained 91% flux of the hydrophobic substrate at 70°C, which was 23.7 kg/h@m2. A composite membrane with a hydrophilic layer indicated a declined concentration polarisation, with smaller Cmb/Cb (3.89) than that of the substrate (5.79) at a brine temperature of 70°C, although the membrane coefficient was reduced by 47% due to the concentration polarisation. Although flux decreased with the increase of brine concentration, it remained almost un- changed at above 20% of NaCl, being 64% relative to the pure water. There was no obvious drop of flux in the continuous running experiments, even after adding 25% ethanol to the brine. It is expected that the wetting problem which faced traditional MD with a hydrophobic membrane can be avoided by adopting a hydrophilic layer. The method also has the potential to achieve a practical high flux by adopting a substrate with a pore size larger than 0.2 µm and applying a hydrophilic layer, thus reducing the membrane price by selecting the low-cost substrate.