Crystal Growth of Clathrate Hydrate at the Interface between Hydrocarbon Gas Mixture and Liquid Water

This paper reports the visual observations of the formation and growth of clathrate hydrate crystals on the surface of a water droplet exposed to a methane + ethane + propane gas mixture. The compositions of the methane + ethane + propane gas mixtures are (i) 99.47:0.51:0.02, (ii) 94.1:5.8:0.1, and (iii) 90:7:3 molar ratio. The nucleation of the hydrate first occurred at a random point on the water droplet and then the hydrate grew to form a polycrystalline layer covering the droplet. We visually observed the individual crystals that constitute the polycrystalline hydrate layer and classified the morphology of the hydrate crystals depending on the system subcooling Δ T sub and system pressure. It was found that the size of the individual mixed-gas hydrate crystals decreased with the increasing Δ T sub as observed for the simple hydrates each formed with methane, ethane, or propane. The size of the individual crystals decreased with the increasing molar ratio of methane in the gas mixtures. The mixed-gas hydrate crystals exhibited a morphology different from that of the simple methane hydrate. We also measured the lateral growth rates of the hydrate-film propagation. In any of the systems, the rates increased with increasing Δ T sub as observed in other hydrate-forming systems.