(539c) Effects of Additives On the Stability of Dual-Layer Hydrophobic-Hydrophilic PVDF Hollow Fiber Membranes for Continuous Membrane Distillation Operation

Water crisis has been regarded as one of the major threat for human life. The problem of water shortage is predicted to become more severe in the future as the world population increases exponentially while the clean water resources continually decrease. Although our earth is mainly composed of water, 97% of it is consisted of saline water creating a necessity of development of a cost effective desalination process. In this regard, membrane distillation (MD) has received attentions due to its ability to be operated under a lower temperature and pressure compared to conventional desalination technologies. Moreover, low-grade or waste energy resources such as solar and geothermal energy can also be utilized in MD process increasing its competitiveness. Despite of MD potencies, most of the desalination plants still rely on reverse osmosis membrane. The reasons may due to the occurrence of membrane wetting in MD. This phenomenon is crucial in MD operation since it may cause permeation flux decay and separation performance deterioration.

 In this study, the advantages of the implementation of dual-layer hydrophobic-hydrophilic PVDF hollow fiber membranes for MD have been highlighted. The effects of incorporating methanol as a non-solvent additive and self-synthesized fluorinated silica (FSi) particles as a hydrophobic modifier on the resultant membrane morphology and MD performance were investigated. Employing 3.5 wt% sodium chloride solution at 80 °C, the highest direct contact membrane distillation (DCMD) flux of 83.40 ± 3.66 kg/(m².hr) and separation factor higher than 99.99% were attained for the membrane spun with methanol additive. Moreover, the stability of the dual-layer hydrophobic-hydrophilic hollow fiber membrane has been demonstrated through continuous DCMD experiments for 5 days. The separation factor was maintained higher than 99.99% for the membrane spun with methanol additive, verifying the suitability of the dual-layer hydrophobic-hydrophilic hollow fiber membrane configuration for desalination processes. The morphological transformation of the outer membrane surface from a porous agglomerated globule structure into a denser interconnected globule structure may be accounted for the stability improvement of the membrane spun with methanol additive. On the other hand, it was found that an enhanced hydrophobicity of the membrane spun with FSi particles did not result in an improvement of the membrane stability. This opposing result may due to the existence of the hydrophilic hydroxyl group on the FSi particle surface that favors the occurrence of membrane wetting.