In fl uence of salt concentration on DCMD performance for treatment of highly concentrated NaCl, KCl, MgCl 2 and MgSO 4 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 N NaCl N MgSO4 N 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. © 2014 Elsevier B.V. All rights reserved.

The influence of different salts concentrations of KCl, NaCl, MgSO4 and MgCl2 solutions on DCMD performance was investigated in this study. The permeate flux decreases with increasing salt concentration, which follows the order of KCl N NaCl N MgSO4 N MgCl2 at salt concentration higher than 1.0 mol/L. The discrepancies of the permeate fluxes are mainly associated with the water activities in the salt solutions, which are dependent on the interactions of ion-ion, ion-solvent in solutions. Water activities of NaCl, KCl, MgCl2 and MgSO4 solutions could be accurately predicted with MSE model embedded in the OLI platform. A reversed flux occurs in the DCMD of highly concentrated MgCl2 solution (N2.5 mol/L) at the tested conditions, which is due to the combined effect of lower water activity and higher viscosity of the fluid. Moreover, 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, e.g. increase of circulation velocity would be a great help to improve the heat transfer and then the flux. DCMD is able to treat highly concentrated brines even in nearly saturated conditions. However, the temperature difference across the membrane must be controlled carefully. Otherwise, a dilution of the brine solution due to reversed transfer of vapor from cold side to hot side and/or a deposition of salt on membrane surface will appear, leading to failure of producing purified water or contamination of the distillate and partially loss of membrane hydrophobicity. Acknowledgment The authors would like to acknowledge the support of the International Science & Technology Cooperation Program of China (No. 2012DFA91500), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20121401120013), the International Science & Technology Cooperation Program of Qinhai (No. 2012-H-803) for this study.