Morphological changes and creep recovery behavior of expanded polytetrafluoroethylene (ePTFE) membranes used for membrane distillation

Abstract

Existing studies of membrane distillation (MD) have shown changes in membrane surface microstructure along with decreased system performance during long-term MD system operation; however, changes in feed-side morphology during long-term operation have not been demonstrated in the absence of foulants. Also, the net effect of morphological changes alone on performance during long-term operation has not previously been quantified. Three 30-day direct contact MD experiments were operated at different feed temperatures and salinities with a flat-sheet expanded polytetrafluoroethylene (ePTFE) membrane. Results showed that changes in membrane morphology (surface morphology, thickness, and bulk porosity) in the absence of foulants can result in decreasing water flux over time. Higher temperature (65 °C) experiments showed greater membrane morphology changes compared to lower temperature (45 °C) experiments. Creep recovery tests performed at different temperatures demonstrated membrane deformation over time under low and constant stress with the potential for permanent deformation after stress removal; these tests also demonstrated that higher temperatures result in greater deformation. Correlation between short-term creep recovery test results and long-term MD experimental results indicates that creep recovery tests may provide a useful shorter-term method for assessing and comparing propensity of different membranes to deform over time and at different temperatures.