Equilibrium Hydrate Formation Conditions for the Mixtures of Methane plus Ionic Liquids plus Water

The equilibrium hydrate formation conditions for methane in the presence of the aqueous solutions of the five ionic liquids with the mass fraction of 0.1 have been investigated. The data were measured using an isochoric method in the pressure range of (3 to 17) MPa and the temperature range of (276.15 to 289.15) K. It is found that the additions of the ionic liquids shift the methane hydrate equilibrium phase boundary to the temperature and pressure conditions that are unfavorable for the hydrate formation. The dialkylimidazolium-based ionic liquids with the hydroxylated cations exhibit an enhanced effectiveness in inhibiting hydrate formation. For the tetraalkylammonium-based ionic liquids, ones with the shorter alkyl substituents of the cations perform better thermodynamic inhibition effects than ones with the hydroxylated longer alkyl substituents of the cations. Among all of the ionic liquids studied, tetramethyl-ammonium chloride is the most effective one, which is comparable with ethylene glycol.