Experimental investigation on the methane storage by forming sH hydrate

Abstract

Methane storage via clathrate hydrates is a promising method, the structure H (sH) hydrate can be formed at more moderate conditions compared to structure I (sI) hydrate and has a larger storage capacity than structure II (sII) hydrate. In this work, CH4 storage by forming sH hydrate in the tert-butyl methyl ether (TBME) solution was explored at an initial pressure of 5.0 MPa and within the temperature range of 274.15– 278.15 K. The CH4/TBME hydrate phase equilibrium condition was first determined with the isochoric pressure searching method, and the results revealed that the measured equilibrium data of CH4/TBME hydrate at different TBME concentrations were almost consistent. The effect of TBME concentration (0– 4.29 mol%), promoter type and sub-cooling on the hydrate formation kinetics and gas uptake was systematically investigated. The results showed that the hydrate formation rate increased with the increase of TBME concentration. The CH4 uptake achieved 117.7 ± 3.6 v/vw (volume ratio of gas to water at standard condition) in the 4.29 mol% TBME system, which was significantly increased than that of pure water (44.1 ± 5.0 v/vw). The cocamidopropyl dimethylamine (CDA) further improved the CH4/TBME hydrate formation kinetics and gas uptake, the CH4 storage capacity attained 145.6 ± 1.7 v/vw, which was slightly higher than that of the sodium dodecyl sulfate (SDS) system (143.6 ± 3.0 v/vw). As the sub-cooling increased, the formation kinetic profiles of CH4/TBME hydrate changed from a two-step to a one-step drop, indicating that the shift of mixed sH and sI hydrates to single sH hydrate. Additionally, the time taken for hydrate growth in the presence of TBME was significantly decreased than the pure water system, and the adding of CDA and SDS further shorten it. The CH4 recovery in each test was above 95.0 % and no foam generated during hydrate dissociation for the CDA-contained system. The stability test of CH4/TBME hydrate indicated that around 86 % CH4 was still enclosed in the hydrate phase after one week. This study would contribute to the application of the hydrate-based methane storage technology in future.