Phase Equilibria for Gas Hydrates Formed with Methane or Ethane + Tetra- n-Butylphosphonium Bromide + Water

This paper presents the three-phase (liquid-hydrate-vapor) equilibrium conditions for the binary and ternary systems constituted by methane (CH4) or ethane (C2H6) + tetra-n-butylphosphonium bromide (TBPB, C16H36BrP) + water (H2O). The experiments were carried using the isochoric method within the temperature and pressure ranges of (277.0-293.5) K and (0.65-9.22) MPa, respectively. Phase equilibria for binary systems, CH4 + H2O and C2H6 + H2O, were in good agreement when they were compared with previously reported data. Different mass fractions of aqueous TBPB solutions (wTBPB) in the range of (0.0099-0.20) were studied. For the ternary CH4 + C16H36BrP + H2O system, it was observed that the ionic liquid has promotional effects for the gas hydrate formation in the range of wTBPB = 0.05-0.20 since the three-phase equilibrium curves moved toward low-pressure and high-temperature conditions, which agrees with previous results reported in the literature. From comparison, deviations are within the experimental errors. On the other hand, encouraging results were found only at wTBPB = 0.05 for the C2H6 + C16H36BrP + H2O system, since the ionic liquid exhibited promotional effects; meanwhile, this effect decreased for wTBPB = 0.10 and 0.20, since the liquid-hydrate-vapor equilibrium curves were closer to that of ethane hydrate, and under some conditions (wTBPB = 0.20), the ionic liquid had suppressing capabilities where the equilibrium curve moved to temperatures lower than the corresponding ethane hydrate. Standard uncertainties of temperature and pressure in the three-phase equilibrium measurements were estimated to be u(T) = 0.13 K and u(p) = 0.07 MPa, respectively.