Numerical simulations of conductive heating vacuum membrane distillation: Quantitative analysis of shunted heat flows and preventive strategy of salt crystallization

Abstract

Membrane distillation, a thermal-membrane coupled technology, shows potential in desalination despite issues with temperature polarization. The conductive heating vacuum membrane distillation (CHVMD) process addresses temperature polarization by incorporating a thermal conducting layer on the feed side to transfer external heat to the membrane/feed interface. However, the internal mechanism is difficult to analyze through experiments. Hence, a three-dimensional computational fluid dynamics model was developed to simulate the process of heat and mass transfer in CHVMD, considering the effects of the thermophysical properties of the substances. The numerical model was utilized to investigate the influence of feed velocity, liquid-film layer thickness, input heat and heat input method on the system performance. Results showed that the temperature polarization inside the system had been effectively alleviated because the thermal conducting layer can centrally transfer external heat to the feed near the membrane. As the feed velocity and liquid-film layer thickness decreased, system flux increased. With only 25 W input heat, system flux can reach 9.32 kg/m²·h, leading to a larger area of concentrated salt distribution on membrane surface and increasing the risk of salt crystal deposition in membrane pores. Furthermore, we proposed the heat input methods with a variable heat flux, which can effectively solve the salt crystallization while further increasing system flux.