Experimental study and modeling of mixed gas hydrates during crystallization and final state
Résumé
Gas hydrates also known as clathrates, are ice-like crystalline materials where a solid water framework accommodates gas molecules in a cage-like structure under the certain thermodynamic conditions. Gas hydrates formation is a crucial issue in oil and gas industries as they are formed and block the pipelines. Nevertheless, gas hydrates attract the attention of many researchers and industries for several applications like gas separation, energy storage and transportation etc. Moreover, metastable gas hydrates are a promising energy source in the shallow geosphere. Therefore, fundamental understanding of gas hydrates thermodynamics as well as formation and decomposition mechanisms is essential.
During the last decade, our GasHyDyn team has measured equilibrium data for many gas mixtures, and this document adds new data and provides information about the guest distribution in hydrate phase and volume of the mixed hydrates at equilibrium and non-equilibrium conditions. Two different rates of crystallization were performed to evaluate the kinetic effects on final equilibrium state. Furthermore a thermodynamic model based on classical van der Waals and Platteuw approach by Kihara potentials was used to investigate the impact of kinetics. The experimental results show that the final equilibrium pressure and hydrate composition is different according to rate of crystallization. Furthermore, the hydrate volume at slow crystallization was lower than at quick crystallization. Moreover, at a slow crystallization rate, the hydrate crystals appear to be more homogeneous. The modeling results show that the hydrate crystallization of a gas mixture is closer to the thermodynamic equilibrium at slow crystallization rates where the impact of kinetics is slight. These observations could be taken into account to re-design the calculations in which the thermodynamic equilibrium is considered. For instance, amount of kinetic hydrate inhibitors (KHI) to prevent gas hydrate formation in oil and gas industries could be well re-estimate to reduce the operation costs.