Phase Equilibrium Conditions of Tetrabutyl Ammonium Nitrate + CO2, N2, or CH4 Semiclathrate Hydrate Systems

Phase equilibrium conditions for semiclathrate hydrates formed from tetrabutyl ammonium nitrate (TBANO(3)) + CO2, N-2, or CH4 + water system were investigated under high-pressure in a "full view" sapphire cell. An isochoric equilibrium step-heating pressure-search method was used for the determination of the hydrate phase transition points through plotting the points of intersection of hydrate-liquid vapor (H-L-V) to liquid vapor (L-V) experimentally. All the experimental equilibrium data were obtained in the presence of TBANO3 solution at ammonium salt mass fraction of 0.3941 corresponding to a stoichiometric composition for TBANO(3)center dot 26H(2)O. Measurements were performed for TBANO(3) + CO2 + water, TBANO3 + N-2 + water, and TBANO(3) + CH4 + water system in the temperature range of (281.5 to 290.9) K and in the pressure range of (1.19 to 32.89) MPa. The three-phase equilibrium lines of TBANO(3) + N-2 and TBANO(3) + CO2 semiclathrate hydrate shifted the hydrate stability region to lower pressure and higher temperature than that of the pure N-2 and CO2 hydrate, respectively. However, the addition of TBANO(3) made TBANO(3) + CH4 semidathrate hydrate phase equilibrium curve at higher pressure than the pure CH4 hydrate.