Improvement of porous polyvinylidene fluoride-co-hexafluropropylene hollow fiber membranes for sweeping gas membrane distillation of ethylene glycol solution

摘要

Porous polyvinylidene fluoride-co-hexafluropropylene(PVDF-HFP)hollow fiber membranes were fabricated through a wet spinning process.In order to improve the membrane structure,composition of the polymer solution was adjusted by studying ternary phase diagrams of polymer/solvent/non-solvent.The prepared membranes were used for sweeping gas membrane distillation(SGMD)of 20 wt% ethylene glycol(EG)aqueous solution.The membranes were characterized by different tests such as N2 permeation,overall porosity,critical water entry pressure(CEPw),water contact angle and collapsing pressure.From FESEM examination,addition of 3 wt% glycerol in the PVDF-HFP solution,produced membranes with smaller finger-likes cavities,higher surface porosity and smaller pore sizes.Increasing the polymer concentration up to 21 wt% resulted in a dense spongy structure which could significantly reduce the N2 permeance.The membrane prepared by 3 wt% glycerol and 17 wt% polymer demonstrated an improved structure with mean pore size of 18 nm and a high surface porosity of 872 m^−1.CEPw of 350 kPa and overall porosity of 84% were also obtained for the improved membrane.Collapsing pressure of the membranes relatively improved by increasing the polymer concentration.From the SGMDtest,the developed membrane represented a maximumpermeate flux of 28 kg·m^−2·h^−1 which is almost 19% higher than the flux of plain membrane.During 120 h of a long-termSGMD operation,a gradual flux reduction of 30% was noticed.In addition,EG rejection reduced from 100% to around 99.5% during 120 h of the operation.