Statistical Analysis of Supercooling in Fuel Gas Hydrate Systems

The recently developed high pressure automated lag time apparatus (HP-ALTA) was applied to the study of the formation and growth of interfacial gas hydrate films for three gases; methane (C1), 90% methane/10% propane gas mix (C1/C3), and synthetic gas mix (SGM). The effects of gas pressure and cooling rate were studied for each gas. Some degree of supercooling was observed in all cases. The probability distributions of formation temperature (T-f) were often found to be bimodal, due to the formation of either gas hydrate or ice in sequential experimental runs. The width of the Tf distribution of ice was about 3-4 K. In contrast, the width of the Tf distribution of gas hydrates was about 20 K which reflects the importance of mass transfer (gas diffusion) processes in nucleation. Differences in hydrate and ice nucleation probability distributions were observed for different gases, reflecting differences in both thermodynamic equilibrium phase behavior and hydrate formation mechanisms. For all gases studied, Tf generally increased with increasing gas pressure. A minimum threshold pressure for hydrate formation was observed, with magnitude decreasing in the order C1 > SGM > C1/C3. The effect of cooling rate on gas hydrate nucleation probability was also studied. The median of the distribution of T-f (T-f50) was found to decrease with an increased cooling rate, consistent with the increases in effective induction time as samples were cooled more slowly. Our results clearly highlight the value in collecting large data sets which can be used to assemble probability distributions when studying intrinsically stochastic processes such as gas hydrate nucleation and growth.