Thermodynamic and kinetic analysis of gas hydrates for desalination of saturated salinity water

Abstract

The shortage of fresh water is among the most serious issues in the world. A representative technology to overcome the problem is desalination, but most conventional methods (RO membrane or thermal distillation) have been focused on the treatment of relatively low salinity water, such as seawater or brackish water. To strengthen water security, in this study, we introduce a possibly economic technology for desalination of high salinity water (over-saturated concentration, in this study, a 30 wt% NaCl system) via gas hydrate formation by coupling LNG waste cold energy. First, the thermodynamic effects of NaCl on CH4 (methane), SF6 (sulfur hexafluoride), and HFC-134a hydrates were investigated. Based on the phase equilibrium of each hydrate, experimental pressures for kinetic experiments were selected under vapor pressure boundaries as follows: 4.5 MPa for CH4, 0.75 MPa for SF6, and 0.16 MPa for HFC-134a at 258.15 K (assuming the use of LNG waste cold energy). The results of the formation kinetics on the basis of gas moles consumed for hydrates showed the order CH4 > HFC-134a > SF6; however, after considering the hydration numbers and structures for each hydrate, surprisingly, the conversion rate of water to gas hydrates showed the order HFC-134a > CH4 > SF6, even though the experimental pressure condition for HFC-134a was very mild (0.16 MPa) compared to CH4 (4.5 MPa). For this interesting phenomenon, we suggest a possible mechanism through visual observations during hydrate formation. We believe these thermodynamic, kinetic, and morphological results show potential as an alternative desalination technology, especially for saturated salinity water, with lower energy consumption.