Low Energy Desalination via Direct Contact Membrane Distillation: Looking into the Concentration Polarization

Membrane fouling due to concentration and temperature polarization is at the top list of restrictions for membrane distillation (MD) development. Yet, concentration polarization (CP) phenomenon has not been taken seriously in the evaluation of MD mass flux throughout the history of this field due to the complexity in the experimental setup. This work gives more insight into the observation and evaluation of CP in direct contact MD numerically. Computational fluid dynamics models of non-isothermal steady multi-species flow are conducted and coupled together to simulate the localized effect of CP at the feed-membrane interface. Additionally, a modified model with integrated spacers is also developed to investigate the practicality of spacers in reducing the CP effect at membrane surface and hence enhancing the performance. The model successfully captures the CP distribution in the feed channel where a concentration polarization coefficient (CPC) of 2.5 is recorded for seawater desalination running under typical operational conditions. Taking this value into consideration, a drop of 6% in mass flux evaluation is observed. On the other hand, spaced feed channel found to reduce CPC down to 1.83 proving the role of spacer in mixing the flow and mitigate the development of concentration polarization.