Influence of salt concentration on DCMD performance for treatment of highly concentrated NaCl, KCl, MgCl2 and MgSO4 solutions

Highly concentrated NaCl, KCl, MgCl2 and MgSO4 solutions were treated using DCMD. The effects of salt concentration (1.0 -4.0 mol/L) and circulation velocity (0.1-0.5 m/s), as well as thermodynamic and physical properties of the salt solutions on permeate flux were investigated. Results showed that the permeate fluxes decrease with increasing concentration for the four salts solutions studied, which follows the order of KCl > NaCl > MgSO4 > MgCl2 at the salt concentrations higher than 1.0 mol/L. The different vapor pressure depression caused by reduction of water activity was identified as the main reason behind this. However, the drastic increase of viscosity of MgSO4 and MgCl2 solutions at higher salt concentrations would also have a notable adverse impact on permeate flux. Under these circumstances, change of hydrodynamics, i.e. increase of circulation velocity would be a great help to improve the heat transfer and then the flux. To prevent salt from crystallizing on membrane surface in saturated conditions, the feed inlet temperature should be controlled within a certain range, and it was 40 to 50 °C, 40 to 45 °C and 25 to 35 °C for NaCl, KCl, and MgSO4, respectively in this study.