Experimental investigation of the exact role of large-molecule guest substances (LMGSs) in determining phase equilibria and structures of natural gas hydrates

Highlights: • The exact role of LMGSs in phase equilibria and structural coexistence of natural gas hydrates was examined. • Phase equilibria of CH₄ + C₃H₈ + LMGS (NH and MCP) hydrates were measured. • The presence of LMGSs had no effect on the equilibrium shift of CH₄ + C₃H₈ hydrates. • A coexistence of sII and sH hydrates in CH₄ + C₃H₈ + LMGS + water mixtures was revealed by PXRD. • Guest distributions of CH₄ + C₃H₈ + LMGS (NH and MCP) hydrates were analyzed through ¹³C NMR. This study examined the complex phase behavior and structures of natural gas hydrates containing large-molecule guest substances (LMGSs). Methylcyclopentane (MCP) and 2,2-dimethylbutane (neohexane, NH) were selected as LMGSs for CH₄ (90%) + C₃H₈ (10%) hydrates. To verify the exact role of LMGSs, the four-phase (hydrate (H) – liquid water (L_W) – liquid LMGS (L_LMGS) – vapor (V)) equilibria of the CH₄ + C₃H₈ + LMGS + water mixtures were measured. The hydrate phase equilibria demonstrated that the presence of LMGSs did not contribute to shifts in the equilibrium curves of the CH₄ + C₃H₈ hydrates. The structures and guest distributions of the gas hydrates containing LMGSs were analyzed using nuclear magnetic resonance (NMR) and powder X-ray diffraction (PXRD). ¹³C NMR spectra and PXRD patterns revealed the coexistence of structure II (sII) and structure H (sH) hydrates in the CH₄ + C₃H₈ + LMGS + water mixtures. CH₄ and LMGSs were enclathrated in sH, while CH₄ and C₃H₈ were encapsulated in sII. The experimental results covering complex phase equilibria and structural characteristics of natural gas hydrates containing LMGSs will contribute to a better understanding of the exact role of LMGSs in natural gas hydrates.