Enclathration of CO2 as a Co-guest in sH Hydrates Formed from the CO2 + N2 + Methylcyclopentane Mixtures

Abstract

Among the well-known three types of gas hydrates, structure H (sH) hydrates have several noteworthy features compared to structure I (sI) and structure II (sII) hydrates. sH hydrates can capture large-sized liquid hydrocarbon guests in the presence of help gases such as CH4 and N2. sH hydrates have a higher gas storage potential in their smaller cages compared to sI and sII hydrates, but the function of CO2 as a co-guest molecule in sH hydrate has been still controversial. In order to investigate the enclathration of CO2 as a co-guest in sH hydrates, the four-phase equilibria of the CO2 + N2 + methylcyclopentane (MCP) + water mixtures were measured and compared with three-phase equilibria of the corresponding CO2 + N2 + water mixtures. Furthermore, the structural transition according to the compositions of the gas mixtures (10, 20, 40, 60, and 80% CO2) was verified using Raman spectroscopy and powder X-ray diffraction (PXRD). At lower CO2 concentrations, CO2 was found to function as a co-guest of sH hydrates and due to the enclathration of MCP, four-phase equilibria showed a slight thermodynamic promotion. The structural transition from sH to sI was observed at CO2 concentrations higher than 20%.