Reception of Alcane-Series Hydrocarbon Hydrates during Crystallization of Amorphous Ice Saturated with Gas
Formation of gas hydrates in gas-saturated low-temperature layers of amorphous ice, received by means of settling of molecular beams of expanded steam and gas on a surface chilled with liquid nitrogen was studied for binary systems «water - methane», «water - ethane» and «water - propane». Vitrification (softening) and further spontaneous crystallization are observed during heating of non-equilibrium condensates. Condensate vitrification and crystallization temperatures at different gas concentration values were defined on the basis of changes in dielectric properties during heating. Measurements were executed in the continuous heating mode with a constant rate of 0,03 K/s in the temperature range of 70-200 K. Crystallization of amorphous condensates in the conditions of strong metastability leads to formation of crystallohydrates. Avalanche-type occurrence of crystallization centers captures gas molecules, therefore, they are not displaced by the movement of the crystallization front. Increase of the gas content in amorphous ice layers leads to shifting of the crystallization signal towards high temperatures. No significant impact of the gas concentration change on the vitrification temperature was noticed. During layer vitrification a clear peak of gas emission is observed, caused by creeping of the specimen during transfer from solid amorphous condition to liquid viscous yielding condition. A low-temperature peak of gas emission, caused by gas sublimation from the surface and from the porous specimen structure is observed in the vitrification area for amorphous ice condensates saturated with ethane and propane, in addition to gas emission. Preservation of gas hydrates was observed at atmospheric pressure in the liquid pentane medium up to the temperatures close to 273 K. Self-conservation ensured preservation of hydrates in a metastable condition at temperatures significantly higher than their equilibrium dissociation temperatures. The received specimens of gas hydrates contained methane up to 15, ethane - 12 and propane - 13 % wt. The results of studies show the success of use of the molecular beam non-equilibrium condensation method for gas hydrate reception.