Direct Contact Membrane Distillation Flux Research Articles

Porous hydrophobic/hydrophilic composite membranes preparation and application in DCMD desalination at higher temperatures

In our previous studies [1,2], porous hydrophobic/hydrophilic membranes have been proposed for desalination by direct contact membrane distillation (DCMD). The membranes were prepared by the simple phase inversion technique from polymer solutions containing a hydrophilic polymer, a solvent, a non-solvent additive and a surface modifying macromolecule (SMM). Membranes of different pore sizes have been prepared using polyetherimide (PEI) as a host polymer. The membranes were characterized by various techniques including atomic force microscopy (AFM), gas permeation test, liquid entry pressure of water (LEPw) measurements, X-ray photoelectron microscopy and contact angle measurements. DCMD experiments were carried out using water and NaCl salt aqueous solutions at different feed and permeate stirring rates (0–740 rpm), feed temperature (25– 50°C), permeate temperature (15–40°C) and feed concentration (0–2 M). The temperature polarization coefficients and the permeability of the membranes were evaluated. The DCMD performance of the proposed hydrophobic/ hydrophilic membranes were similar to those of commercial polytetrafluoroethylene (PTFE) membranes (TF200 and TF450, from Gelman) and one of the membranes, prepared with 12 wt% PEI in the casting solution, showed DCMD fluxes higher than those of PTFE membranes. All tested membranes exhibited very high separation factors, >99.8%, when aqueous salt solutions were used as feed. It was concluded that the porous hydrophobic/hydrophilic composite membranes are promising in the field of membrane distillation because they combine a low resistance to mass transfer achieved by the diminution of the water vapor transport length through the hydrophobic thin-layer of the *Corresponding author.

Membrane distillation. II. Direct contact MD

Pure water direct contact membrane distillation (DCMD) experiments were used to measure the permeability parameter associated with the molecular diffusion in membrane distillation (MD). The fluxes given by a recently reported MD model, which is based on the dusty-gas model of gas transport through porous media, showed good agreement with the experimental results over the entire range of feed temperatures studied. The model was also capable of predicting flux as a function of the difference between bulk feed and permeate temperatures for the limiting case in which only molecular diffusion contributes to flow. The DCMD experiments were performed in this work with a new laboratory-scale module that does not require a support for flat-sheet membranes. The resulting DCMD fluxes were two to three times higher than those reported in the literature for either DCMD or reverse osmosis. The MD model was also used to predict the performance of DCMD desalination, and the results were compared to those of reverse osmosis, in terms of both water production rates and NaCl rejection.