Phase equilibria of methane and carbon dioxide clathrate hydrates in the presence of aqueous solutions of tributylmethylphosphonium methylsulfate ionic liquid

Abstract : The effect of a tributylmethylphosphonium methylsulfate ionic liquid (IL) aqueous solution on the equilibrium conditions of carbon dioxide and methane clathrate hydrates was studied. An isochoric pressure-search method was used to measure the hydrate dissociation conditions for the carbon dioxide + tributylmethylphosphonium methylsulfate + water and methane + tributylmethylphosphonium methylsulfate + water systems in the temperature ranges of (273.5 to 282.2) K and (273.3 to 288.5) K, and pressures up to (4.35 and 14.77) MPa, respectively. The concentrations of tributylmethylphosphonium methylsulfate in the aqueous solutions were 0, 0.2611, and 0.5007 mass fractions. The good agreement between our experimental hydrate dissociation data in the absence of tributylmethylphosphonium methylsulfate with selected literature experimental data demonstrates the reliability of the experimental method used in this work. The comparison between the hydrate dissociation conditions in the presence and absence of tributylmethylphosphonium methylsulfate shows that the IL has an inhibition effect on carbon dioxide and methane clathrate hydrate formation. Furthermore, a thermodynamic model, developed based on van der Waals-Platteeuw solid solution theory accompanied with the Peng-Robinson equation of state (PR-EoS) and the nonrandom two-liquid (NRTL) activity model, was successfully applied to represent/predict the obtained experimental data.