A review on hydrate composition and capability of thermodynamic modeling to predict hydrate pressure and composition

Previous experimental data shows that guest distribution in hydrate phase depends noticeably on the guest composition in vapor phase. In addition, composition of larger molecules, such as propane or butane, in the hydrate phase, is notably higher than in vapor phase. Our simulation results demonstrated that the hydrate composition data from direct measurement (microscopic tools) have been well evaluated by the thermodynamic model. Nevertheless, when structural transition can occur in a system, the thermodynamic model is no longer accurate. In the case of indirect measurements, the thermodynamic model usually predicts well the hydrate composition. Nonetheless, its capability does vary with differing hydrate composition and equilibrium pressure, to the extent that in some cases, it completely fails to predict hydrate composition. This could be due to kinetic effects on the enclathration of guest molecules during the crystallization, errors in experimental techniques to measure the hydrate composition or the model parameters like Kihara potential are not properly applied. Finally, these observations show that more reliable experimental database is needed to study the evolution of guest distribution in hydrate phase and some enhancements are required for the standard thermodynamic model.