Submerged vacuum membrane distillation crystallization (S-VMDC) with turbulent intensification for the concentration of NaCl solution

The feasibility of simultaneous pure water production and salt crystal recovery from highly concentration NaCl solution was evaluated by utilizing a submerged vacuum membrane distillation crystallization (S-VMDC) system with different turbulent intensification modes. The effect of intensification mode including stirring and aeration on membrane distillation performance and crystallization behavior was investigated systematically. An iterative method was used to evaluate the heat transfer enhancement effect. The results indicated that both stirring and aeration had positive influence on permeate flux due to the improvement of heat transfer and weakening of polarization phenomenon in boundary layer. The dominated crystallization behavior converted from surface crystallization to bulk crystallization with the help of turbulent intensification, which in turn increased the limiting concentration of the feed solution. The size and distribution of crystallized product could be regulated by selecting an optimal intensification mode. With the combined effect of stirring rate at 1350 rpm and aeration rate at 0.20 m3/h, 31.85% NaCl crystals with relative desirable size distribution could be recovered from bulk solution. This study provided an alternative perspective to understand the enhancement effect of turbulent intensification strategy in S-VMDC process for concentrating saturation solution in both qualitative and quantitative ways.