Investigating two synthetic routes for gas hydrate formation to control the trapping of methane from natural gas

Abstract

Gas hydrate is considered as an emerging technique for natural gas storage and CH4 separation from natural gas, because assorted nano-sized cages of gas hydrate allow selective trapping of specific gaseous guests such as CH4, C2H6, and C3H8. When a mixture of CH4, C2H6, and C3H8 forms gas hydrates with a liquid promoter at 5.6 mol%, sII large cages (sII-L) can be fully occupied by a promoter, while only CH4 remains in the hydrate phase by occupying sII small cages (sII-S). Based on this, can CH4 be selectively stored or separated from natural gas? Here, we aimed to elucidate the cage-filling behavior of gaseous and liquid guests in CH4 + C2H6 + C3H8 hydrates formed with the addition of tetrahydrofuran (THF), trimethylene oxide (TMO), or 1,3-dioxolane (DIOX). We examined feasible strategies for synthesizing gas hydrate focusing on ice-borne or water-borne route. Our results suggested that ice-borne gas hydrates were a promising option for CH4 separation, because only CH4 was trapped in ice-borne gas hydrates. In contrast, C2H6 and C3H8 shared sII-L with promoters in water-borne gas hydrates. Although partial occupation of sII-L by C2H6 and C3H8 reduced CH4 purity in the hydrate phase, it resulted in enhancing the CH4 uptake in THF hydrate formed via water-borne route, unlike TMO and DIOX. The present findings will broaden the understanding of the cage occupation behavior in ternary gas hydrates formed with liquid promoters, and provide feasible strategies for synthesizing gas hydrates for storing natural gas or CH4 separation from natural gas.