Measurement of methane solubility in pure water in equilibrium with hydrate by using high–pressure optical capillary cell

Abstract

The solubility of methane in aqueous solution in equilibrium with hydrate is an important control on the crystallization and dissolution of methane hydrate. Accurate calibration of Raman spectroscopic measurements of dissolved CH4 with the newly developed Unsaturated Homogenized Solution Method makes possible more accurate measurement of methane solubility across a wide range of pressure conditions. In this study, methane solubilities in pure water in equilibrium with methane hydrate in the absence and presence of a vapor phase were experimentally determined by Raman spectroscopy at temperatures from 273.15 to 293.15 K and pressures from ~2.6 to 120 MPa. The results show that the solubility of CH4 under H–Lw equilibrium increases with temperature at constant pressure but decreases slightly with increasing pressure at constant temperature. The pressure effect on CH4 solubility in water at H–Lw equilibrium becomes more significant with the increase of temperature. Both the isobaric relationship between ln(mCH4) and 1/T, and the isothermal relationship between ln(mCH4) and P are linear, indicating that the enthalpy change can be considered constant at fixed pressure over the temperature range of 273.15–293.15 K, and the volume change at a given temperature is constant for the methane hydrate dissolution reaction. The values of ΔV range from 9.3 to 12.0 cm3 per mole of hydrate at temperatures between 273.15 and 293.15 K. The Gibbs free energy change is 3.99 to 7.81 kJ∙mol−1 at temperature range from 273.15 K to 293.15 K and pressure range from 10 to 120 MPa. The enthalpy and entropy changes of the methane hydrate dissolution reaction over the studied T–P range are 41.52 to 48.12 kJ∙mol−1 and 0.1282 to 0.1476 kJ∙K−1∙mol−1, respectively. The values increase slightly with increasing pressure.