Stability conditions and guest distribution of the methane + ethane + propane hydrates or semiclathrates in the presence of tetrahydrofuran or quaternary ammonium salts

Highlights: • We examined the stability conditions and guest distributions of natural gas hydrates. • THF, TBAB, and TBAF could remarkably stabilize the hydrate dissociation conditions. • Preferential occupation of CH4 molecules was observed at the stoichiometric concentrations. -- Abstract: In this study, the stability conditions and guest distributions of methane (CH4) + ethane (C2H6) + propane (C3H8) hydrates or semiclathrates are examined in the presence of tetrahydrofuran (THF) and quaternary ammonium salts such as tetra-n-butylammonium bromide (TBAB) and tetra-n-butylammonium fluoride (TBAF) through thermodynamic and spectroscopic analyses. The three-phase equilibria of hydrate or semiclathrate (H), liquid water (LW), and vapor (V) for the quinary CH4 + C2H6 + C3H8 + THF + water, CH4 + C2H6 + C3H8 + TBAB + water, and CH4 + C2H6 + C3H8 + TBAF + water mixtures with various concentrations were experimentally measured in order to determine the hydrate or semiclathrate stability conditions. The experimental results demonstrated that all thermodynamic promoters such as THF, TBAB, and TBAF used in this study could remarkably stabilize the hydrate dissociation conditions even though the degree of stabilization was dependent on the type of promoters and on the concentrations of each promoter used. From the 13C NMR analysis, preferential occupation of CH4 molecules in the hydrate or semiclathrate lattices was observed at the stoichiometric concentrations of each hydrate or semiclathrate structure. However, at the THF mole fraction lower than 0.056, the large cages of structure II were shared by CH4, C2H6, C3H8, and THF molecules, whereas the small cages were occupied by only CH4 molecules. The results given in this study are very useful in understanding the thermodynamic stability, structural characteristics, and guest distribution of the hydrates or semiclathrates with multi-components in the presence of thermodynamic promoters for further applications in gas storage and separation processes