Effects of Sodium Chloride Dihydrate on the Dissociation Behavior of HFC-125a Hydrates

Abstract

Gas hydrate-based desalination is an energy-efficient method in this regard, particularly when combined with simultaneous salt hydrate precipitation. However, the dissociation behaviors of gas hydrates in the presence of salt hydrates remain poorly understood. This study was conducted to investigate the influence of sodium chloride dihydrate (NaCl<middle dot>2H2O) distribution on HFC-125a hydrate dissociation behavior using temperature-dependent powder X-ray diffraction (PXRD) analysis. Two sample preparation methods were employed: (i) the coformation of gas hydrates and NaCl<middle dot>2H2O from high-salinity solutions and (ii) the physical mixing of preformed components. The results revealed that NaCl<middle dot>2H2O in the coformed sample exhibited dual functional behavior, simultaneously accelerating gas hydrate dissociation through thermodynamic inhibition and inducing anomalous preservation effects that delayed the complete dissociation of the gas hydrates. In contrast, the physically mixed sample exhibited an accelerated one-step dissociation at even lower temperatures without any preservation effects. The PXRD analysis demonstrated that the presence of NaCl<middle dot>2H2O induced lattice expansion in the gas hydrate crystals and influenced local salinity during dissociation. Thus, the findings indicate that NaCl<middle dot>2H2O exhibits a complex dual functionality, depending on the formation method and distribution, and provide crucial insights for optimizing the operating conditions of gas hydrate-based zero liquid discharge desalination processes.