Investigation of Tetrahydrofuran-CH4 Hydrate Formation in Unstirred Conditions from a Different Perspective: Application to Solidified Natural Gas Storage

In this work, tetrahydrofuran (THF)-CH4 hydrate formation in unstirred conditions was investigated using kinetic measurement, morphology observation, high pressure in situ Raman spectroscopy, and high-pressure DSC. The experiments were conducted at four THF concentrations (1.39, 2.78, 4.17, and 5.56 mol %) and a pressure of 6.0 MPa with the temperature ranging from 277.15 to 283.15 K. The results indicate that THF concentration is a key factor affecting THF-CH4 hydrate formation. When increasing the THF concentration above 2.78 mol %, the emergence of channels on hydrates improved mass transfer for THF-CH4 hydrate growth. The in situ Raman spectroscopy measurement shows that CH4 and THF molecules fill the small and large cavities of the sII hydrate, respectively. With the increase of the THF concentration, more CH4 molecules occupy the small cavities of the sII hydrate. In 2.78 and 4.17 mol % THF solutions, the coexistence of THF-CH4 and CH4 hydrates was determined while the temperature decreased to 277.15 K, and the gas consumption was 2.2 times larger than that obtained at 5.56 mol % THF. This finding suggests that forming two types of hydrates at a low THF concentration will be a viable means to increase the gas storage capacity.