Phase Equilibrium Relations and Structural Transition in the 1,1,1,2-Tetrafluoroethane Hydrate System

The three-phase (hydrate, aqueous, and HFC-134a-rich liquid phases) equilibrium relations of the 1,1,1,2-tetrafluoroethane (HFC-134a) + water binary system were measured in a pressure range up to 293 MPa. The HFC-134a hydrate has a structural phase transition point from structure-II to structure-I at (65 +/- 1) MPa and (282.2 +/- 0.1) K, where the slope of the three-phase equilibrium curve is drastically changed. The Raman spectra of the HFC-134a molecule in the structure-I HFC-134a hydrate phase indicate that HFC-134a molecule occupies only large cage of structure-I at a pressure up to 293 MPa.