Dynamics of methane-propane clathrate hydrate crystal growth from liquid water with or without the presence of n-heptane

Methane-propane clathrate hydrate crystal growth within an enclosure partially filled with liquid water was examined under different undercooling conditions both with and without the presence of n-heptane. Water saturated with guest gas was rapidly undercooled and maintained at constant temperature. The growth of the hydrate phase always started with the formation of a film at the upper surface of the liquid water pool. The visual observations using a microscope revealed detailed features of crystal nucleation, migration, and growth occurring within the water pool. In all experiments, hydrate crystal growth was initiated by the formation of hydrate film at the interface between liquid water and the adjacent gas or n-heptane layer. Hydrate crystals were then observed to grow downward from the film. Undercooling was found to be a key parameter to control the morphology of hydrates growing underneath the hydrate film as it influences the growth rate and configurations of crystals. Moreover, a number of small crystals were seen in the water ascending toward the hydrate film. The evolution of the shape of these crystals was monitored. These crystals were octahedral or triangular or hexagonal platelets. Finally, the role of constitutional undercooling and the memory of the water to the observed crystal growth are discussed.