Effect of microwave irradiation on typical inorganic salts crystallization in membrane distillation process

This study is a continuation of our previous work on microwave-assisted membrane distillation. The crystal morphology of sodium chloride and calcium carbonate in the process of membrane distillation and microwave membrane distillation was investigated in this work. The effect of salt concentration on the ability of absorbing microwave of solution and the mass transfer of microwave membrane distillation were also analyzed. The results show that the ionic conduction is the main between two factors (ionic conduction and dipole rotation) which have effect on absorbing microwave. When the mass concentration of sodium chloride solution is up to 5%, the microwave energy absorbed by solution is almost five times as much as pure water. Moreover, the increase in salt concentration has no significant extra influence on the mass transfer of microwave membrane distillation. The crystallization experiments confirm that microwave irradiation do not affect membrane fouling or worsen it. The different effect of microwave irradiation on crystallization of two typical inorganic salts is observed in this study. For sodium chloride, the microwave heating leads to the decline of the total particle number, while the total deposition keeps constant and the size distribution of crystal particles becomes more uniform, compared with conventional heating. As for calcium carbonate, microwave irradiation not only increases the total deposition and total particle number, but also induces the crystals growth. Especially, it is found that microwave irradiation induces the crystals to grow toward aragonite. The ratios of the aragonite number to the total particle number increase from 28.0% and 56.5% to 81.1% and 85.8% at the two higher feed mass concentrations, respectively.