Investigation of tuning behavior of trimethylene oxide hydrate with guest methane molecule and its critical guest concentration

Abstract

Due to the promising storage capacity for CH4, the method of natural gas storage and transportation using solid gas hydrate is recognized as an alternative to conventional liquefied natural gas process. To make this alternative more viable, however, it is necessary to further shift the formation temperature and pressure of natural gas hydrate closer to ambient conditions. Although there is a trade-off between storing more CH4 and mitigating the formation conditions, it is known that adding specific liquid molecules such as tetrahydrofuran, as a secondary guest to CH4 hydrate could lead to favorable thermodynamic stability while still affording promising CH4 storage capacity via a mechanism known as the tuning phenomenon. Here, this study focused on trimethylene oxide (TMO) + CH4 hydrate to reveal its unique guest occupation patterns for potential application to natural gas storage and transportation. The occurrence of the tuning behavior on the TMO + CH4 hydrate was observed for the first time. Most importantly, the tuning factor referred to as the CH4 occupation ratio between the large and small cages of sII hydrate, exhibited a value of 0.73 at the critical guest concentration, which is the largest value reported to date. The phase equilibria of the TMO + CH4 hydrate were provided for TMO concentrations ranging from 1.0 to 5.56 mol%. The spectroscopic data obtained from synchrotron HRPD, dispersive Raman, and 13C solid-state HPDEC MAS NMR spectrometry provided experimental evidence of the distinct enclathration characteristics of CH4 and TMO molecules during the tuning phenomenon. These findings provide fundamental knowledge on the tuning effect and lend useful insights for designing and developing gas hydrate-based natural gas storage and transportation process.