Effect of Tryptone on Clathrate Hydrate Crystal Growth in the HFC-134a + Tryptone Aqueous Solution System

Clathrate hydrate crystal formation proceeds through hydrate film formation at the gas–liquid interface, followed by crystal growth into the aqueous phase. The growth sequence and morphology of HFC-134a hydrate crystals in aqueous tryptone solutions were visually observed. These visual observations were examined at varying subcooling temperatures. The subcooling temperature (ΔTsub) is defined as the difference between the system temperature and the phase equilibrium temperature at a given pressure. ΔTsub was set from 1.0 to 8.0 K. In the 0.1 mass % solution, polygonal plate or polyhedral crystals were observed at ΔTsub of 2.6 and 4.0 K. At 2.6 K, a plate crystal grew from about 1 mm to about 2 mm in diagonal length within around 40 min after detaching from the liquid and gas interface. The interface was not covered by a hydrate film until 10 min after the first observable detachment crystal. At 7.9 K, a hydrate film formed within several seconds, and columnar or dendritic crystals grew from the film into the bulk liquid. In the 0.4 mass % solution, film formation was delayed; at 7.4 K, a hydrate film formation took 44 min after the first observable crystal, and polygonal crystals with sizes from 0.5 mm to 1 mm were observed in the liquid bulk phase. Polygonal morphologies persisted from 1.9 to 7.4 K, whereas columnar or dendritic morphologies were observed at 8.0 K. These observations of crystal morphologies and growth sequences revealed that tryptone could change the hydrate crystal growth and morphology and delay the hydrate film formation at the gas-liquid interface. The tryptone as an eco-friendly additive may change the growth sequence and the morphology of the hydrate.