Hydrate-Phase Equilibria and 13C NMR Studies of Binary (CH4 + C2H4) and (C2H6 + C2H4) Hydrates

Three-phase equilibria of hydrate + liquid water + vapor phases were investigated at various gas compositions for binary gas mixtures of (CH4 + C2H4) and (C2H6 + C2H4). Hydrate-phase equilibria of the binary (CH4 + C2H4) hydrate show significant changes with changing composition, whereas those of binary (C2H6 + C2H4) hydrates show little difference because of the similar physical properties of the two guest species. In addition to macroscopic equilibrium measurements, solid-state C-13 NMR spectra of hydrate samples were collected to identify both cavity occupancies of guest components and hydrate structures. For the binary (CH2 + C2H4) hydrate, the large-cavity occupancy of C2H4 molecules increased nonlinearly with increasing C2H4 concentration, which supports nonlinear shifts of the equilibrium curves. Meanwhile, the large-cavity occupancy of C2H4 molecules from the (C2H6 + C2H4) gas mixtures increased linearly with increasing C2H4 concentration, which is attributed to linear changes in the distribution or density.