A thermodynamic model to predict the aqueous solubility of hydrocarbon mixtures at two-phase hydrate-liquid water equilibrium

Understanding the fate and transport of hydrocarbons and hydrocarbon mixtures in the deep sea and underlying sediments requires accurate determination of the two-phase hydrate (H)-liquid water (Lw) thermodynamic equilibrium in the absence of a free gas phase. In addition to controlling hydrate formation directly from the aqueous phase, the H-Lw equilibrium provides the aqueous solubility of dissolving hydrate. The aqueous solubility of hydrocarbon mixture at H-Lw equilibrium was calculated based on the van der Waals and Platteeuw statistical thermodynamic model and the Holder model. Langmuir constants were calculated from cell potential parameters obtained from ab initio intermolecular potentials and thus the model contains no fitting parameters. The model accurately predicts pure methane, ethane, and propane experimental H-Lw equilibrium solubility data, including new data provided here for methane. We present hydrocarbon solubilities in water and seawater at H-Lw equilibrium at deep sea conditions for both methane and a thermogenic mixture based on the methane–ethane–propane ratio from the 2010 Macondo oil spill. We also present model predictions of the proportion of occupied large and small cages at H-Lw equilibrium for both systems, as well as hydrocarbon ratios in the hydrate phase for the thermogenic mixture.

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