A novel braided structure with enhanced feed flow state for hollow fiber air-gap membrane distillation performance improvement

The feed flow state affects temperature polarization (TP), concentration polarization (CP), and membrane fouling during the membrane distillation process. A novel braided structure for AGMD was proposed in this paper, in which hollow fiber membranes and heat exchange tubes are braided together. The braided structure can alter feed flow direction and reduces the membrane's contact area. Theoretical calculations show that the braided structure reduces the critical Reynolds number for the feed's unstable flow, and improves the feed flow state by forming Dean vortices. Comparative experiments of the braided module and parallel module were carried out. The results show that the braided module performs better than the parallel module. The braided module has a higher flux (J) and gained output ratio (GOR) when treating the tap water with significant TP; a lower decline rate in J and GOR besides a smaller concentration polarization coefficient (CPC) when treating 3.5 wt% NaCl aqueous solution; a slower formation of membrane fouling and a lower reduction in J and GOR when treating a humic acid/calcium carbonate mixed solution. It is proved that the braided structure can improve the perfor-mance of AGMD by enhancing the feed flow state to mitigate the TP, CP, and membrane fouling.

Automated summary

The feed flow state has a significant impact on temperature polarization, concentration polarization, and membrane fouling in membrane distillation. In this study, a novel braided structure for AGMD was proposed, which effectively improves the feed flow state. Comparative experiments showed that the braided module outperformed the parallel module in terms of flux, gained output ratio, concentration polarization coefficient, and membrane fouling.