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

摘要

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 CH_4 (90％) + C_3H_8 (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 CH_4 + C_3H_8 + 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 CH_4 + C_3H_8 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). ~(13)C NMR spectra and PXRD patterns revealed the coexistence of structure Ⅱ (sⅡ) and structure H (sH) hydrates in the CH_4 + C_3H_8 + LMGS + water mixtures. CH_4 and LMGSs were endathrated in sH, while CH_4 and C_3H_8 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.