(152d) Application of Direct Contact Membrane Distillation for Treating High Salinity Solutions: Impact of Membrane Structure and Chemistry

Direct contact membrane distillation (DCMD) is a thermally driven membrane process in which water vapor transport occurs through a non-wetted porous hydrophobic membrane. In DCMD, a hot feed solution is flowing on one side of the membrane while a cold distillate solution is flowing on the other side creating a driving force of vapor pressure difference across the membrane. Highly concentrated solutions are required for the forward osmosis as a draw solution for the process. In this work, DCMD is investigated as a separation method for the draw solution of the forward osmosis process. Three different types of membranes with the same pore size of 0.45 µm (i.e. PVDF, PP, and ECTFE) were used in this research: polyvinylidene fluoride (PVDF) from Millipore, poly propylene (PP), and ethylene chloro trifluoro ethylene (ECTFE) from 3M. Also PP-3M membranes with different pore sizes of 0.45, 0.2, and 0.1 µm were used to study the effect of the pore size of the membrane on the process efficiency. Three highly concentrated solutions were used (i.e. MgCl₂, KCl and NaCl) as feed solutions for the MD process. The feed solution and the distillate were kept at 50 ⁰C and 20 ⁰C respectively. The results showed that the ECTFE membrane gives the highest water flux, likely because it is thinner (about half than the thickness of the PVDF and the PP membranes) and therefore exhibits less mass flux resistance. The PVDF membrane showed the lowest flux as it has the lowest porosity (66%) compared to 85% and 81% for the PP and the ECTFE membranes respectively. Membranes with larger pore sizes exhibited slightly higher flux. Feed chemistry also impacted the flux, with KCl solutions having higher flux than either NaCl or MgCl₂. These differences were largely driven by solution vapour pressure differences. Excitingly, all salt rejection measurements were in excess of 99.5%.