Solubility measurements of methane and ethane in water at and near hydrate conditions

The isobaric solubility of pure methane and ethane gases in liquid water was measured under different conditions, including hydrate formation points, using a ramping method in which measurements are made through the entire and continuous hydrate formation/decomposition cycle. The work was conducted for methane at 3.45 MPa (500 psia) and temperatures ranging from 17.0 to 0.0°C (290.2 to 273.2 K), and for ethane at 0.66 MPa (95 psia) and a temperature range of 17.0 to 0.0°C (290.2 to 273.2 K). The isobaric solubilities obtained show a significant divergence from (normal) Henryʹs law solubility as the temperature is lowered. The increase in the solubility is presumably the result of the onset of a sorption process that traps the gas molecules in the water structure. Several sub-processes were observed as the hydrate formation was taking place: (1) the dissolution of the gas in the liquid water, or interstitial solubility, (2) the onset of the sorption sub-process and build-up of the hydrate precursors, (3) the catastrophic formation and the existence of the solid phase with its slushy characteristics, and (4) the solidification of the hydrates, therefore causing the total plugging of the experimental cell.