Removal of 1,1,1-trichloroethane from water using a polyvinylidene fluoride hollow fiber membrane module: Vacuum membrane distillation operation

Polyvinylidene fluoride (PVDF) hollow fiber membranes with an asymmetric structure have been prepared by the phase inversion method using dimethylacetamide (DMAc) as solvent and LiCl–H2O as additives and applied for volatile organic compounds (VOCs) removal from water. The membrane was characterized using scanning electron microscopy (SEM) for observing its microstructures and a gas permeation method for measuring its porosity, pore size and pore size distribution. The results have provided the conditions for tailor-made hollow fiber membranes with the desired morphology suitable for vacuum membrane distillation (VMD). Moreover, the structural parameters such as mean pore size and effective porosity determined are being used as parameters in the mathematical modeling of the VMD process. A hollow fiber membrane module was then fabricated using a selected PVDF hollow fiber membranes and applied for removal of 1,1,1-trichloroethane (TCA) from water. The effects of various operating parameters including downstream vacuum level, feed temperature, feed flow rate and TCA feed concentration on the performance of the module were investigated both theoretically and experimentally. A high feed temperature and/or a low downstream pressure favored the enhancement of TCA removal, but also yielded dramatically a high water permeation flux, resulting in an excessive dilution of the permeated VOC. Under optimal operating conditions, particularly the downstream vacuum level, feed temperature and feed flow rate, TCA removal efficiency up to 97% was achievable. A mathematical model, which takes into consideration of mass transfer resistances in both liquid and membrane phases, predicts well the TCA removal data in comparison with the experimentally observed results under the favorable operation ranges employed in vacuum membrane distillation.