CO<sub>2</sub> Hydrate Thermodynamic and Crystallographic Characterization below the Freezing Point under Low Subcooling

In this study, CO2 hydrate formation-decomposition experiments were carried out below the freezing point in a lab-scale reactor and the in-situ high-pressure optical cell. It was found that, in the low subcooling temperature range below 0 °C, ice formation competed with hydrates. Initially, ice occupied most of the space and hydrates had limited space to grow. On the other side, it would reduce the contact between gas and liquid water, thus hindering the process. Finally, the ice dissolved and the hydrate growth dominated in the system. Otherwise, hydrate crystals grew in supercooled water in the micron-sized tube. Here we provided a comparison between macro and micro-scale properties, which revealed the competition relationship between ice and hydrates, and consequently, it was difficult to define equilibrium curves with similar methods at temperatures below the ice-point.