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Air Gap Membrane Distillation Process Enhancement for Hypersaline Feed Solution by Application of Vacuum
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This study investigated how the process performance of air gap membrane distillation be enhanced by the application of vacuum to treat hypersaline feed solutions. A vacuum-enhanced air-gap membrane distillation (V-AGMD) configuration was set up and sub-ambient pressure was applied in the air gap to remove non-condensable gases and reduce conductive heat transfer for enhancing the flux and thermal efficiency. The process was evaluated in comparison to the AGMD process while varying operational parameters such as the mean temperature (Tm), difference of temperature in heating and cooling streams (ΔT), flow velocity, and absolute air gap pressure. Initially, tap water was used as feed to set a baseline for the analysis of vapor pressure reduction incurred in hypersaline feed. Two commercial flat sheet PTFE membranes (pore size 0.1 μm and 0.2 μm) were tested. In the hypersaline feed experiments, two air gap widths (0.80 mm and 1.15 mm) were assessed in both ambient and vacuum conditions. Within the scope of this work, the operational conditions which yielded a reasonable trade-off between flux and thermal efficiency were considered optimal. These conditions were Tm 60 °C, ΔT 16 °C, flow velocity 0.1 m/s and absolute air gap pressure of 500 mbar for tap water experiments. For hypersaline feed experiments, the optimal conditions were Tm 60 °C, ΔT 16 °C, flow velocity 0.07 m/s and absolute air gap pressure of 350 mbar. A quantitative comparison between AGMD and V-AGMD setup showed that, for the determined optimal conditions using tap water, the flux improved by 37.6 %, and thermal efficiency improved by 17.7 %. For the hypersaline feed, the V-AGMD setup showed an enhancement of the flux by 279.5 % and thermal efficiency by 149.9 %. The results of this work proved that improving the performance of AGMD process is possible by deaeration of the airgap using vacuum and that V-AGMD is a viable option for treatment of hypersaline feeds.
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Vacuum-enhanced air-gap membrane distillation (V-AGMD) flat sheet membranes thermal efficiency hypersaline feed treatment