Effect of HFC-134a as a Promoter of CO2 Hydrate: Phase Equilibrium, Dissociation Enthalpy and Kinetics

HFC-134a gas was investigated as a potential guest molecule to improve the thermodynamic conditions and formation rate for CO2 hydrate. In the phase equilibrium study, the equilibrium pressure of CO2 + HFC-134a was lower than that of pure CO2 gas, and the equilibrium pressure decreased gradually with increasing HFC-134a concentration. The dissociation enthalpy (Delta H-d) was calculated using the Clausius-Clapeyron equation, and the Delta H-d value also changed with increasing HFC-134a concentration. In particular, the Delta H-d of 8 mol % HFC-134a-added CO2 hydrate was 143.2 kJ/mol, which was similar to that of pure HFC-134a (structure-II). In the kinetic study, the reactor was initially filled with CO2 + HFC-134a gas only and pure CO2 gas was then supplied as a source when the hydrate reaction proceeded. As a result, the formation rate of the HFC-134a mixture in the initial 2 min was faster than that of pure CO2. This was consistent with the gas chromatography results, which showed that HFC-134a occupies the cage at the beginning of hydrate formation. These results suggest that the addition of HFC-134a influences the CO2 hydrate thermodynamic equilibrium and kinetic characteristics.