Stability Conditions for Semiclathrate Hydrates Formed with Tetrabutylammonium Chloride plus Tetrabutylphosphonium Chloride + CH4

Semiclathrate hydrate is a promising method to ease the thermodynamic conditions of gas hydrate formation for hydrate-based gas storage and transportation on the industrial scale. The phase equilibrium conditions of mixed semiclathrate hydrate formed with tetrabutylammonium chloride (TBAC) + tetrabutylphosphonium chloride (TBPC) + CH4 were measured systematically by employing the isochoric pressure-search method. The data revealed that mixed TBAC + TBPC exerted relatively higher stabilization effect on CH4 hydrate than pure TBAC or TBPC, and the strongest stabilization effect showed up in system with 0.0300 total salt mole fraction (x(salt)) and 0.75 TBAC salt ratio (x(TBAC)/x(salt)). In addition, the dissociation enthalpies of mixed (TBAC + TBPC + CH4) hydrate were calculated with the obtained experimental data by using the simplified Clausius-Clapeyron equation and the Peng-Robinson equation of state. It was found that the mean dissociation enthalpies for mixed (TBAC + TBPC + CH4) hydrate with different salt concentration relied on a (153.04 to 203.92 kJ/molCH(4)) scale, which was significantly higher than that of the pure CH4 hydrate. The data generated in this work showed that mixed TBAC + TBPC could enhance CH4 hydrate stability and the stored energy effectively.