Ultrarapid Formation and Direct Observation of Methane Hydrate in the Presence of l-Tryptophan

Environmentally friendly amino acids are promising candidates for the application of hydrate-based gas solidification technology and others. The formation kinetics and macroscopic morphology of methane hydrate in the presence of l-tryptophan were experimentally studied by adopting an isothermal and isochoric method. The experimental results showed that l-tryptophan could not increase the nucleation rates of methane hydrates, but it had better kinetic promotion effects on the growth rates of methane hydrate than sodium dodecyl sulfate, and the minimum concentration required to be effective was 0.1 wt %. The promotion effects almost remained unchanged with the increase of the concentration of l-tryptophan, even at a stirring rate of 200 rpm. For a 0.1 wt % l-tryptophan solution, the ultimate gas consumption was increased by 46.52% and t90 was decreased by 33.67% when the initial experimental pressure was elevated from 7 to 11 MPa. Both ultimate gas consumption and t90 were increased when the temperature was increased from 273.65 to 277.15 K. From direct top view observation, methane hydrate tended to first nucleate at the gas–liquid–solid triple phase lines, then stretched like a thin film along the surface of the solution. The hydrate continued to form at the edge of the film and even climbed on to the inner wall of the container. The solution under the hydrate film was continuously sucked, and the hydrate film could even be cracked by interfacial forces at higher pressure. When the concentration of l-tryptophan was greater than 0.5 wt %, hydrates cropped out from several sites of the hydrate film and continued to grow upward. The formed hydrates were quite loose and porous which provided capillary forces and channels for the upward transportation of the solution.