Effect of microwave irradiation on vacuum membrane distillation

In this paper, a vacuum membrane distillation (VMD) system equipped with a microwave source was designed. And the polyvinylidene fluoride (PVDF) hollow fiber hydrophobic membranes were selected to carry out microwave vacuum membrane distillation (MWVMD). The effects of feed temperature, vacuum degree and feed velocity on microwave strengthening mass transfer process were preliminarily investigated. The results showed that microwave irradiation could effectively induce uniform heating in the radial direction of the membrane module, and significantly improve the mass transfer process of vacuum membrane distillation. The maximum membrane distillation mass transfer coefficient increasing rate in this study was obtained as about 27.7% in the conditions of the temperature of 60 °C, the feed velocity of 0.14 m/s and the vacuum degree of −96 kPa. And the increment was largened with the decrease of the feed temperature and feed velocity as well as the increase of the vacuum degree. The influences of microwave irradiation on the membrane properties were also investigated. The results indicated that microwave irradiation had no significant effect on the mechanical properties and hydrophobicity of membrane materials which were situated in microwave field for more than 120 h. The influence of microwave irradiation on the membrane fouling was also analyzed. For the solution which contains 30 mg/L Ca2+, microwave irradiation aggravated the deposition of calcium carbonate to some extent.