Effect of stretching ratio and heating temperature on structure and performance of PTFE hollow fiber membrane in VMD for RO brine

Membrane distillation (MD) using hollow fiber membrane for desalination is becoming an attractive technology due to high water recovery and salt rejection. Polytetrafluoroethylene (PTFE) hydrophobic hollow fiber membranes were prepared through a cold pressing method including paste extrusion, stretching and heating. The influences of stretching ratio and heating temperature on the morphology, structure, porosity, shrinkage ratio, tensile strength and permeability of the PTFE hollow fiber membranes were investigated. Field emission scanning electron microscopy (FESEM) images showed that the PTFE hollow fiber membranes had the microstructure of nodes interconnected by fibrils. The increase of stretching ratio can improve the pore size and porosity of the membrane. The heating temperature higher than the melting point of PTFE was necessary in the heating process because melting treatment can restrain the shrinkage and improve the tensile strength of the membrane. Finally, the PTFE hollow fiber membranes were tested for their performances in desalination through vacuum membrane distillation (VMD) for the treatment of seawater reverse osmosis (RO) brines. The increase in stretching ratio and heating temperature significantly improved the permeation flux. The salt rejections achieved 99.9%, indicating the prepared PTFE hollow fiber membranes exhibited satisfying salt rejection performances. The hydrophobic PTFE hollow fiber membranes developed in this study are promising for practical application in the treatment of seawater RO brine.