Asymmetric membranes for membrane distillation and thermo-osmotic energy conversion

Asymmetric membranes with a thin, small pore size upper layer have the potential to facilitate a high vapor flux while maintaining high liquid entry pressure, which is critical for membrane distillation (MD) and thermo-osmotic energy conversion (TOEC) processes. Here, asymmetric mixed cellulose ultrafiltration membranes were modified with perfluorodecyltrichlorosilane to produce 50 nm and 25 nm pore size membranes with highly hydrophobic surfaces (contact angle >115°) and unprecedented liquid entry pressures >24 bar. The 50 nm membrane performance was evaluated in a series of MD and TOEC mode experiments, where the membrane dense layer faces the hot feed stream or the cold permeate stream, respectively. Our results demonstrate that the membrane water vapor permeability coefficient is significantly higher when operating in MD mode (1.7 × 10⁻⁷ kg m⁻² s⁻¹ Pa⁻¹) compared to TOEC mode (0.9 × 10⁻⁷ kg m⁻² s⁻¹ Pa⁻¹) at a similar temperature difference of 39 °C, suggesting an additional resistance to vapor flux in TOEC mode. We developed a model for mass and heat transfer through the membrane to explain the change in performance due to reversing the asymmetric membrane orientation. As expected, the model and the experimental results show a linear increase in the water vapor permeability coefficient with respect to temperature difference across the membrane for the MD orientation. However, for the TOEC orientation, the water vapor permeability coefficient was relatively constant across all the temperature differences investigated. Our results predict that the additional mass transport resistances in TOEC mode decrease as the transmembrane temperature gradient decreases. We conclude with a discussion on the implications of using asymmetric membranes for MD and TOEC processes.