Gas hydrate formation probability distributions: Induction times, rates of nucleation and growth

An improved, high pressure, stirred automated lag time apparatus (HPS-ALTA) was used to determine gas hydrate nucleation and growth rates from induction time measurements at fixed subcooling. The improved HPS-ALTA uses multiple thermoelectric elements to allow rapid, radially symmetric heating and cooling, which together with the automatic detection of hydrate formation, makes feasible the measurement of statistically significant induction time distributions. Four induction time probability distributions, with between 83 and 312 points, were measured at subcoolings ranging from (6 to 9.7) K for water with a synthetic natural gas mixture at pressures around 12 MPa. The induction time data measured at each subcooling were exponentially distributed and were fit using a model derived from Classical Nucleation Theory based on the mononuclear nucleation mechanism. The nucleation rates obtained in this work were consistent within a factor of three or better with recent literature measurements made using a completely different apparatus. Hydrate growth rates measured at fixed subcooling using the improved HPS-ALTA were also found to be stochastic but consistent with previous measurements of the kinetic growth rate in a binary gas mixture. The results obtained here establish the need to improve theoretical models of hydrate formation to reconcile predicted nucleation rates with the much slower ones observed experimentally.