High resolution MRI studies of CO2 hydrate formation and dissociation near the gas-water interface.

[Display omitted] • CO 2 hydrates formed above and inside a water phase differ in morphology and stability. • The dissociation of hydrates inside the water phase is affected by both heat and mass transfer. • A 0.2 mm thick low porosity hydrate film at the original water-CO 2 interface was detected by MRI. • CO 2 hydrate film are supported by hydrate dendrites and sink when these dissociate. Gas hydrates are widely considered as promising candidates for gas storage, energy transportation and seawater desalination. Critical to such applications is a detailed 3D understanding of hydrate formation. To this end, we employ magnetic resonance imaging (MRI) to non-invasively image the gradual formation of opaque hydrate from CO 2 and water in a cylindrical vessel at 1 °C and as a function of pressure between 2.0 and 3.5 MPa. A 200 µm thick dense hydrate layer is consistently observed to form at the gas-water interface accompanied by a ~1.4 mm thick porous hydrate layer above it and frequently complex dendritic hydrate formation in the water phase below it. Dissociation is observed to occur preferentially via the thick hydrate layer with the initial hydrate film retained largely intact for an extended period of time. The sequential images of hydrate dissociation inside the water phase are most consistent with a vertical heat and mass transfer controlled hydrate dissociation process. The observed difference between the hydrates formed above and inside the water phase is of mechanistic value in understanding these complex interfacial phase transitions. [ABSTRACT FROM AUTHOR]