(713b) Chemical Vapor Modified Membranes with Large Pore Size for Efficient Desalination

In membrane distillation (MD) process, water vapor is thermally transported through hydrophobic membrane pores that are not wetted by saline water. Though membranes with pore size larger than 1 mm are much desired to increase the MD permeate flux, the vulnerability of large-pore-size membranes to pore wetting results in the penetration of saline water and the consequent failure of MD operation. We used a novel vapor-based modification method to process ultrathin superhydrophobic coatings into the porous structure of MD membranes with minimal altering of the microstructure. The permeate flux and liquid entry pressure of large-pore-size MD membranes were simultaneously improved after the modification. Salt rejection higher than 99.99% was achieved in all the modified MD membranes. As the nominal pore size increased from 0.2 mm to 2 µm, the permeate flux of modified membranes increased by 48.0-73.0%. The percentage increase was 20% higher at 50 °C than they were at 65 °C, indicating that increasing pore size has a more significant effect on the Knudsen diffusion at lower temperature. The modified membrane maintained the wetting resistance and the permeate flux increase over that of pristine membranes at high saline concentration and extended operation time. This study demonstrates the potential to use modified membranes with large pore size to improve MD desalination efficiency by lowering the ratio of energy consumption versus permeate flux.