Effects of Stirring and Cooling on Methane Hydrate Formation in a High-Pressure Isochoric Cell

Abstract

Higher mixing rates applied to hydrate-forming liquids in laboratory cell systems are commonly assumed to cause a more thorough dispersion of newly formed hydrate nuclei from gas-liquid interface into bulk liquid. There is, however, no simple and direct correlation between stirring rate and nucleation rate. Most experimental studies on hydrate kinetics in literature have been carried out at isobaric conditions. Studies in cooled systems at isochoric conditions (temperaturedependent pressures) are in scarcity. This paper presents an experimental study on effects of varying stirring rates (220 – 660 rpm) and varying cooling rates (0.5 – 10 o C /h) on the point of spontaneous nucleation (PSN) of structure-I methane hydrate. The experiments were conducted in an isochoric system with temperature and pressure both decreasing along with the continuous cooling process. Initial growth rates after nucleation were also monitored. The results suggest that higher stirring rates lead to reduced hydrate induction time and increased hydrate growth rate. The nucleation rate appeared to be slightly reduced at the lowest stirring rate (220 rpm). The degree of sub-cooling at PSN seemed to be of a more stochastic character at cooling rates lower than at 2 o C /h. Increased cooling rates trend to reduce induction time while the sub-cooling at PSN appeared to first oscillate at lower cooling rates then approach some constant level at higher cooling rates. For higher cooling rates, the nucleation rate decreased faster. Hydrate growth behavior in cooling rate experiments (0.5 – 10 o C/h) suggests that the total gas intake and the gas consumption rate increased with increasing cooling rate towards a constant level at 6 – 10 °C/h. A careful selection of stirring rate and cooling rate is essential to hydrate study.