Morphology Study of Mixed Methane-Tetrahydrofuran Hydrates with and without the Presence of Salt

Storing natural gas (NG) in the form of clathrate hydrates (solidified natural gas, SNG) is a promising technology for its long-term, large-scale storage because it is safe and economically feasible compared to liquefied natural gas and compressed natural gas. Recently, we reported rapid mixed methane hydrate formation kinetics in the presence of saline water and seawater for the SNG technology. In this study, we investigate in detail the morphology of mixed CH4/tetrahydrofuran hydrate formation and dissociation with and without the presence of salt (3 wt % NaCl). Knowledge about hydrate crystal morphology is a key aspect of SNG technology development. This study reports the morphology difference of mixed hydrates in the presence of saline and nonsaline water to shed light on the observed kinetic performance. Morphology changes observed during the nucleation, hydrate growth, and dissociation of mixed hydrates at two different formation pressures of 5 and 3 MPa at 283.15 K are presented. Subtle differences were observed with significant upward growth (above the gas-liquid interface) during the initial hydrate formation (until 5 min) for the saline system in comparison to significant downward growth (below the gas-liquid interface) in the bulk solution for the nonsaline system despite the similar operating conditions. Further, from morphology observations, it could be seen that the hydrate dissociation of mixed hydrates formed using saline water took less time (about 20 min) than that observed in nonsaline water (about 30 min). This can be attributed to the spread of hydrates along crystallizer walls in the case of the saline water system, resulting in better heat transfer during dissociation.