Spectroscopic analysis of clathrate hydrates with various guest mole-cules and simulating the methane production from methane hydrate by replacement method with 1D 1m-scale reactor

Abstract

Clathrate hydrates are the types of inclusion compounds that are composed by host - wa-ter and small gaseous/liquid molecules such as methane, ethane, hydrogen and oxygen. Due to the advantages of hydrate application such as energy gas storage (methane, ethane and hydro-gen), gas separation of targeted component and carbon dioxide sequestration (CCS), clathrate hydrates have been applied to many industrial fields. In this thesis, spectroscopic and microscopic identification of host ？ guest inclusion phe-nome-non occurring in clathrate hydrate systems is intensively investigated, especially focused on microscopic behavior of water-host networks and small gaseous guest molecules. Further-more, we simulating the methane production from methane hydrate by replacement method with 1D 1m-scale reactor. First, we investigate that hydrates of two isomers of $C_2H_7N$, dimethylamine (DMA) and ethylamine (EA) which are known to be clathrate hydrate formers by themselves. Here, we in-troduced methane gas as a secondary guest into both dimethylamine and ethylamine clathrate hydrates and identified their structural transitions using powdered X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (NMR). We observed the structural transitions of amine clathrate hydrates from expanded structure I (cubic Pm3n) to structure II (Cubic Fd3m). In addi-tion, from experimental results obtained through neutron powder diffraction (NPD) and XRD, we found that neither temperature nor pressure affected the hydrate structural transition. Raman spectroscopy was used to identify structural transition occurring in these amine clathrate hydrate systems. In addition, we measured the hydrate equilibrium conditions for amine-water-methane hydrates. The DMA and EA act as hydrate inhibitors in DMA/EA + $H_2O$ + $CH_4$ hydrate systems compared with pure methane hydrate in our experimental pressure and temperature range. Second, we investigated the crystal structures and phase equilibria of butanols + $CH_4$ + $H_2O$ systems to reveal the hydroxyl group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers were identified with powder X-ray dif-fraction (PXRD). In addition, Raman spectroscopy was used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hy-drates without any hydrogen bonding interaction between the hydroxyl group of the guests and the water-host framework. However, Raman spectra of the binary $(1-butanol + CH_4)$ hydrate did not show the free OH signal, indicating that there could be possible hydrogen bonding inter-actions between the guests and hosts. We also measured the four-phase equilibrium conditions of the butanols + $CH_4$ + $H_2O$ systems. Third, we studied hydrate with trimethylamine (TMA) which is known to form the semi-clathrate hydrate, and it has been reported that the structural transition of the TMA semi-clathrate hydrate may not occur in the presence of hydrogen gas as a secondary guest molecule. In this study, we investigate the structural transition of TMA hydrate by $CH_4$ gas. Powder X-ray diffraction was used to analyze the crystal structure of a binary (TMA + $CH_4$) clathrate hydrate, and the results showed that the structural transition from semi- to cubic structure II hydrate can occur in the presence of methane gas. $^{13}C$ solid-state NMR and Raman spectroscopy were used to confirm the distribution of guest molecules. Finally, we measured the phase equilibrium con-ditions of a binary $(TMA + CH_4)$ clathrate hydrate. The guest-induced structural transformation in amine semi-clathrate hydrates can provide useful insight into the complex nature of the host？guest inclusion system, and these results can be applied to research on potential gas storage and transportation areas. Finally, the recovery of methane gas from methane hydrate bearing sediments was inves-tigated by using a continuous flow stream of $CO_2$ and $N_2$ gas mixture. Long cylindrical high-pressure reactor was designed to demonstrate the recovery of methane from methane hydrate bearing sediments, and the injection rate of gas mixture was controlled to monitor the recovered methane amount from methane hydrate bearing sediments. The recovery efficiency of methane gas from methane hydrate bearing sediments is in inverse proportion to the flow rate of $CO_2$ and $N_2$ gas mixture. Methane hydrates were synthesized by using two different sediments, hav-ing the particle size distributions of 75 to 150 $\mu$ m and 45 to 90 $\mu$ m with same porosity. The re-covery efficiency of methane from methane hydrates was monitored and we confirmed that there is no significant difference in the replacement characteristics by using two different sedi-ments. Horizontal and vertical flow of $CO_2$ and $N_2$ gas mixture were applied to monitor the ef-fect of flow direction and we also confirmed that similar amount of methane recovery was achieved in horizontal and vertical flow of $CO_2$ and $N_2$ gas mixture at same flow rate. The above results obtained from the spectroscopic and microscopic identification of bi-nary clathrate hydrate systems including host - guest complex dynamics can provide the better under-standing of un-revealed host-guest inclusion complexes as well as can be applied to the applications of hydrate-based industry. Also, our study could help in establishing the process variables for recovering of methane gas from methane hydrate bearing sediments in offshore conditions.