Thermodynamic and Spectroscopic Studies of Phase Equilibria of HFC-134a Hydrate in NaCl Aqueous Solutions

Abstract

Gas hydrates can be applied to desalination because they exclude salts or ions from cage lattices during crystallization. However, gas hydrates normally require high-pressure and low-temperature conditions. For gas hydrate-based desalination to be competitive, guest gases which require low pressures for gas hydrate formation should be sought. In this study, HFC-134a, which is widely used as a refrigerant and forms gas hydrates at mild conditions, was used as a guest gas for desalination. Three-phase (hydrate (H)-liquid water(LW)-vapor (V)) equilibria of the HFC-134a + water mixture in NaCl solutions of 3.5 and 8.0wt% were measured to determine the inhibition effect of NaCl on gas hydrate stability conditions using both a conventional isochoric method and a step-wise differential scanning calorimeter (DSC) method. Heat of dissociation of HFC-134a hydrate was measured to be 147.3kJ/g-hydrate using a high pressure DSC. Furthermore, powder X-Ray diffraction (PXRD) revealed that HFC-134a forms a sII hydrate. The experimental results obtained in this study can be utilized as fundamental data for the hydrate-based desalination process.