Hydrate Formation in Gas-Saturated Layers of Amorphous Ice with Crystalline Nuclei

Abstract

The formation of propane hydrate in the gas-saturated condensates of amorphous ice in the presence of seed crystals has been studied. The gas-saturated amorphous layers were obtained by condensation of the supersonic flows of rarefied vapor and gas onto a substrate cooled with liquid nitrogen. The samples were prepared using both parallel flows orientated normally to the substrate and counter-flows arranged at an angle to it. The formation of ice nanocrystals during the adiabatic expansion of the vapor flow at the exit of the supersonic nozzle ensured their presence in the condensates. The changes in the specific surface density (porosity) of amorphous gas-saturated condensates caused by the changes in the flow orientation relative to the substrate, as well as the presence of seed crystals in the nonequilibrium condensates, affect their stability and crystallization kinetics. Under conditions of deep metastability, spontaneous crystallization occurs, involving the capture of adsorbed gas molecules and formation of the gas hydrate. The crystallized condensates contained a high gas concentration, exceeding its value for the hydrate in an equilibrium state. The excess gas content indicates the presence of gas in the intercrystalline space and the porous medium of the sample.