Effects Of Salinity On Hydrate Phase Equilibrium And Kinetics Of Sf6, Hfc134a, And Their Mixture

In this study, the effects of salinity on the equilibrium and kinetics of sulfur hexafluoride (SF6), 1,1,1,2-tetrafluoroethane (HFC134a), and their mixture were evaluated to analyze their potential applications to hydrate-based desalination. The equilibrium pressure of the mixture gas (SF6/HFC134a) was lower than that of pure SF6 gas but higher than that of pure HFC134a gas. The thermodynamic effects of various concentrations (0, 3.5, 5, and 8 wt %) of NaCl on the gas hydrates were evaluated. Experiments were performed on the formation kinetics in the presence of NaCl solution at the same experimental temperature and pressure (274.15 K and 0.70 MPa). The hydrate growth rates decreased with increasing NaCl concentration. The rates also declined with time, associated with the inhibition of mass transfer between the gas and the liquid. During hydrate formation, the vapor compositions of the mixture gas were analyzed by in situ Raman spectroscopy and the results were consistent with those obtained by gas chromatography. Furthermore, an in situ Raman spectroscopic analysis demonstrated that the SF6 and HFC134a molecules occupy only the large cage (5(12)6(4)) of the structure II (sII) hydrate during SF6/HFC134a formation and the addition of salt did not change the structure of the SF6 and HFC134a hydrates. The hydrate phase behavior and kinetic results of this study can serve as foundational data for hydrate-based desalination technology and fluorinated gases separation and recovery processes.