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

Abstract

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). C-13 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. (c) 2020 Elsevier Ltd. All rights reserved.