Multicomponent Gas Hydrate Nucleation: The Effect of the Cooling Rate and Composition

The effect of the cooling rate and gas composition on the kinetics of hydrate formation and the stochastic nature of nucleation has been examined by forming structure II hydrates from two different synthetic natural gases: one with two components (SNG2) and the other with seven components (SNG7). The hydrate equilibrium properties of SNG2 and SNG7 were comparable, and all experiments were initiated at the same temperature and pressure conditions in an autoclave cell. The initial degree of subcooling and other parameters that could affect the kinetics were, thus, approximately the same during all nucleation experiments. From the experimental results, SNG2 showed an increasing nucleation rate, while SNG7 showed a decreasing effect as the cooling rate increases in steps of 2 degrees C/h. In addition, it is observed that the rate of nucleation is dependent upon gas compositions, and different gas compositions respond differently on the same cooling rate. In an attempt to understand the experimental results, the classical nucleation theory of multicomponent systems, the probability distribution function, and the principle of irreversible thermodynamics have been employed. The observed effects are, thus, related to the dependence of the critical size upon the temperature and gas composition, chemical oscillations as a result of the change in solubility of the individual gas components, and coupled mass and heat fluxes during cooling. Such findings on system responses are of paramount importance for a reliable evaluation of the effect of additives on hydrate formation processes.