Influence of water-insoluble organic components on the gas hydrate equilibrium conditions of methane

In this study, certain organic additives to systems of water+methane are investigated for their effect on the hydrate equilibrium pressure. The additives studied are cyclic organic components, i.e., tetrahydropyran (THP), cyclobutanone (CB), both forming structure II (sII), and methylcyclohexane (MCH), forming structure H (sH) and fluoroalkanes, i.e., fluoroform (CHF3) and tetrafluoromethane (CF4), both forming sI or sII depending on the concentration. Hydrate equilibrium data for single hydrates of the fluoroalkanes were determined, of which the ones of CHF3 were compared to previous data from literature. The hydrate equilibrium pressures were experimentally determined using the Cailletet apparatus in a range of pressures from 2 to 14 MPa and temperatures up to approximately 303 K. A reduction of the methane hydrate equilibrium pressure was observed with all additives except for CF4. The experimental data were modelled to retrieve the Kihara fit parameters for the various additives as present in the van der Waals–Platteeuw model. By using these parameters to correlate the hydrate equilibrium pressures, deviations within approximately 3% from the experimental hydrate equilibrium line were obtained. The modelling results showed that reduction of hydrate equilibrium pressures is accompanied by a significant reduction of storage capacity for methane, which is not recommendable for practical applications.