Thermodynamic and spectroscopic investigations of the distinct roles of trimethylene oxide on the formation of CH4 hydrates for potential application to natural gas storage

Abstract

It is expected that natural gas occupies a huge portion of world energy consumption in the near future due to low carbon emission and the need for alternative energy to petroleum. As the natural gas industry has been boosted, various methods have been extensively studied for the storage and transportation of natural gas. The natural gas storage and transportation method via solid gas hydrate is considered as a promising option due to storage capacity for CH4 and favorable storage condition comparing to the conventional liquefied natural gas process. However, to make that gas hydrate become a more viable option, formation pressure and temperature of natural gas hydrate need to be further mitigated to ambient conditions. The formation condition can be thermodynamically enhanced by adding specific chemical as a thermodynamic promoter to CH4 hydrate system meanwhile the CH4 storage capacity of binary CH4 hydrate is inevitably reduced. Although a trade-off between total CH4 storage capacity and achieving the milder formation condition is inherent in the gas hydrate system, it can be balanced by tuning effect occurring in binary gas hydrate system. It was reported that some liquid molecules such as tetrahydrofuran, tert-butylamine, tert-butyl alcohol, pyrrolidine, and piperidine, could provide enhanced CH4 storage capacity while keeping favorable thermodynamic stability in CH4 hydrate system via tuning effect. The tuning phenomenon of binary CH4 hydrate was investigated by some researches, however, more studies need to be conducted to reveal the most efficient liquid guest molecule making tuning phenomenon for application to gas hydrate-based natural gas storage and transportation. In this study, we focused on trimethylene oxide (TMO) + CH4 hydrate and investigated its hidden guest occupation behaviors for potential application to gas hydrate-based natural gas storage and transportation. The occurrence of tuning effect in the TMO + CH4 hydrate was firstly mentioned and tuning factors indicated as the CH4 occupancy ratio between the large and small cages of sII hydrate with corresponding critical guest concentrations, which were reported in previous studies, were summarized. Interestingly, the critical guest concentration (CGC) appeared at 0.8 mol% TMO presenting maximum tuning factor (TF). The tuning factor of TMO + CH4 hydrate exhibited 0.73 at the critical guest concentration (CGC), which is the largest value reported thus far. The three phase (H-L-V) pressure (P) – temperature (T) equilibria at the TMO + CH4 + H2O system were provided with varying TMO concentrations (1.0, 3.0, and 5.56 mol%). According to the phase equilibria result, the thermodynamic stability of CH4 hydrate containing TMO molecule was significantly shifted to the more stable region than pure CH4 hydrate. Synchrotron high resolution powder diffraction (HRPD), dispersive Raman and 13C solid-state high-power decoupling (HPDEC) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrometers were used to investigate experimentally the enclathration behaviors containing the structure properties and cage occupation pattern. The spectroscopic results experimentally revealed the distinct cage occupation behaviors of guest molecules (CH4, TMO) in TMO + CH4 hydrates throughout the occurrence of the tuning effect. The results of this study could broaden fundamental understandings on the tuning phenomenon and give beneficial insights for design and development to natural gas storage and transportation process based on solid gas hydrate.