Insights into the enhanced flux of graphene oxide composite membrane in direct contact membrane distillation: The different role at evaporation and condensation interfaces

Graphene oxide (GO) coating has recently been reported as a novel approach to increase membrane flux of membrane distillation (MD), yet the phenomena underlying the process are still not fully understood. In this study, a mathematical model based on capillary-film assumption was developed and validated with the results (R2>0.99) from a series of MD experiments. According to the model, when GO layer was placed at the evaporation interface, the temperature difference across the membrane surface increases significantly (44.2%-92.0%) and the temperature polarization coefficient is increased greatly from 0.29-0.38 to around 0.55. This leads to a big increase of driving force for higher heat flow and subsequently mass flux (17.8-45.5%). However, the vapor pressure on membrane surface was decreased due to Kelvin effect of GO capillary pores, which has a negative influence on the driving force, accounting for about 26.9% to 52.6% drop in the achieved flux. In comparison, when GO layer was placed at the condensation interface, the temperature difference across the membrane surface decreases slightly (7.2-12.2%), but the reduced vapor pressure on GO capillary pores due to Kelvin effect become the dominant factor affecting membrane flux, resulting in an increase mass flux of 12.4-16.4%. The model developed in this study provides a theoretical foundation for understanding the role of GO coating on flux improvement, and can be used for further development of high flux membranes.

Automated summary

A mathematical model based on capillary-film assumption was developed to understand the effects of graphene oxide (GO) coating on membrane distillation (MD). The results showed that placing GO layer at the evaporation interface significantly increased the temperature difference across the membrane surface, leading to higher heat and mass flux. However, the vapor pressure reduction due to Kelvin effect of GO capillary pores had a negative influence on the driving force. Placing GO layer at the condensation interface reduced the temperature difference but increased the mass flux, primarily due to the reduced vapor pressure on GO capillary pores.