Study on the desalination efficiency of hydrate phase by a pressure-driven filtration method

Abstract

The mechanism of hydrate-based desalination is that water molecules would transfer to the hydrate phase during gas hydrate formation process, while the salt ions would be conversely concentrated in the unreacted saltwater. However, the salt concentration of hydrate decomposed water and the desalination degree of hydrate phase are still unclear. The biggest challenge is how to effectively separate the hydrate phase and the remaining unreacted salt water, and then decompose the hydrate phase to measure the salt concentration of hydrate melt water. This work developed an apparatus and pressure-driven filtration method to efficiently separate the hydrate phase and the remaining unreacted saltwater. On this basis, the single hydrate phase was obtained, then it was dissociated and the salt concentration of hydrate melt water was measured. The experimental results demonstrate that when the initial salt mass concentration is 0.3% to 8.0%, the salt removal efficiency for NaCl solution is 15.9% to 29.8% by forming CO₂ hydrate, while for CaCl₂ solution is 28.9% to 45.5%. The solute CaCl₂ is easier to be removed than solute NaCl. In addition, the salt removal efficiency for forming CO₂ hydrate is higher than that for forming methane hydrate. The multi-stage desalination can continuously decrease the salt concentration of hydrate dissociated water, and the salt removal efficiency per stage is around 20%.