Effects of Salinity on Hydrate Phase Equilibrium and Kinetics of SF₆, HFC134a, and Their Mixture

Abstract

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 (51264) 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.

Dynamic Analysis of Growth of Ice and Hydrate Crystals by In Situ Raman and Their Significance in Freezing Desalination

“…As the salt was added, the hydrate phase equilibrium boundary shifted a lower temperature and higher pressures (Figure ). Salt ions (Na + and Cl – ) can disturb the hydrogen-bonded frameworks of the hydrate structure, reducing the thermodynamic stability of gas hydrates. , Pure HFC134a formed hydrates under much lower pressure conditions than CO 2 , which is consistent with the results obtained by Lee et al Furthermore, CO 2 forms a structure I hydrate occupying small (5 12 ) and large (5 12 6 2 ) cavities of structure I, whereas pure HFC134a only formed large cages of the structure II hydrate (5 12 6 4 ) …”

“…Figure compares the thermodynamic stability of CO 2 and HFC134a hydrates from pure water and 3.5 wt % NaCl solution. Our previous study referred to the hydrate phase as the equilibrium of HFC134a + NaCl solution (0 and 3.5 wt %) . The HLS correlation was used to estimate the hydrate equilibrium of HFC134a over a wider range of pressure and temperature (shown as green dotted lines in Figure ).…”

“…In this study, kinetic experiments of CO 2 and HFC134a hydrates were conducted at a constant temperature and pressure (Table ). Our previous study contained a more detailed description of the experimental apparatus and procedure. ,,, During the hydrate formation, the temperature and pressure data were recorded using a data acquisition at 10 s intervals. In this study, CO 2 forms a structure I hydrate (assumed hydration number, ∼6), whereas HFC134a forms a structure II hydrate (assumed hydration number, ∼17) due to its molecular size in which each hydrate number is widely accepted and used in the literature. ,, Eq was used for calculation of the hydrate conversion. …”

“…Raman spectroscopy has recently become a more prevalent analytical tool for determining the fundamental properties of gas hydrates. ,, For example, Raman spectroscopy has been used to study the kinetic and phase behavior of hydrate formation and dissociation and the composition and structure of the cage occupancy. ,, Furthermore, Raman measurements of the water phase could also be used to distinguish between hydrate and ice phases, which can give an accurate characteristic spectroscopic signal for O–H bands. , Most of the studies in the literature on gas hydrates have focused on the vibrations of guest molecules. However, literature data and fundamental research were insufficient because they mainly focused on the shape and position of the Raman signals of the host molecule .…”

“…Carbon dioxide (CO 2 ) and 1,1,1,2-tetrafluoroethane (HFC134a) are selected as gas hydrate formers. HFC134a gas was chosen because it forms hydrate structure II and has mild operating conditions, , and CO 2 was used to investigate the hydrate structure I. Based on the volume change of a gas phase, a novel approach for calculating water to ice conversion in a closed system was proposed.…”