Thermodynamic and spectroscopic analysis for swapping carbon dioxide for natural gas in the sH hydrate system

Abstract

$^{13}C$ NMR spectra were obtained for sH isopentane + $CH_4$ hydrate and MCH + $CH_4$ hydrate and compared with liquid isopentane and MCH spectra in order to analyze the conformation of acyclic large guest molecules. For isopentane + $CH_4$ hydrate, the resonance line shifting was detected between isopentane in hydrate and liquid isopentane, in the other hand, any shifting was not detected between MCH in hydrate and liquid MCH. The differences in the chemical shifts between sH hydrate and liquid isopentane was checked through the use of a hydrogen-hydrogen steric perturbation model. In addition $^{13}C$ NMR spectra were also obtained for the replacement processes of $CH_4$ sH hydrates using pure $CO_2$ and $N_2$ + $CO_2$ mixture. Structure transitions were observed while the replacement processes proceeded, and the effect of $N_2$ for attacking sI-S was also found. Additionally, GC data and Raman spectroscopy for $N_2$ + $CO_2$ replacement processes were obtained, and no Raman peaks for $N_2$ and $CO_2$ in sH-M were detected. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities, and that the overall replacement of sH hydrate proceeds slowly, but that the lattice structure of the replaced hydrate changes into sI very rapidly.