Thermotactic habit of gas hydrate growth enables a fast transformation of melting ice

Abstract

Promoting hydrate formation kinetics is considered a major challenge hindering the application of hydrate-based techniques. Surfactants are thus widely used for a better gas solution and water conversion. Yet significant issues involving an uncontrolled morphology and foaming are encountered. In this work, a unique anisotropic behavior of gas hydrate formation was identified through in-situ X-ray technique with the rate of vertical growth 5 times higher than the lateral. This was ascribed to the varying driving force of hydrate nucleation at the wall surface where the reaction heat could be more easily dissipated. Consequently, a thermotactic habit of gas hydrate growth was proposed: the nucleation and growth would preferentially proceed towards a low temperature region to release the reaction heat. Hence through regulating the temperature and thereby the heat transfer scheme one can design the behavior and morphology of hydrate formation. Specifically, a surprisingly fast hydrate formation was observed when freezing the surfactant-containing solution prior to warming-up and pressurization. The melting ice consuming large amount of heat enabled a sequential transformation of water into hydrate tracing the thawing front. This thermotactic behavior was also consolidated in a more macroscopic process. The findings could provide new insights into the kinetics of gas hydrate formation coupling the heat-associated phase transition; the results could also be of help in the up-scale of the hydrate-based techniques where a concentrated transformation is highly required.