Phase Equilibrium in Two-Phase, Water-Rich-Liquid, Hydrate Systems:  Experiment and Theory

Abstract

Two-phase equilibrium between CO 2 hydrate (H) and a water-rich liquid (L) are experimentally measured and theoretically described between 273 and 281 K, at pressures below 30 MPa, and at aqueous CO 2 concentrations between 0.0163 and 0.0242 mole fraction. These data represent the conditions where hydrates form from a single-phase aqueous solution of fixed composition. Both theoretical and experimental results indicate that the equilibrium pressure is very sensitive to concentration at all temperatures. The concentrations reported represent the solubility of CO 2 in a water phase in equilibrium with hydrate at the given temperature and pressure. When a constant aqueous composition LH curve is extrapolated to the three-phase VLH curve, the composition characterizing the LH curve also represents the solubility of CO 2 in water at the VLH conditions. Since the solubility of CO 2 in water at hydrate-forming conditions is difficult to obtain, this method provides an excellent way of indirectly measuring this three-phase solubility. The effect of salinity on hydrate formation from water-rich-liquid systems was also studied. A modified model was introduced to describe the experimental results and produced good agreement between calculated and experimental pressures. A simplified version of the model can provide quick and reasonable estimations of the equilibrium conditions of hydrates at low concentrations and medium to low pressures. Interestingly, the increase of salt increases the maximum temperature at which hydrates are stable for a constant pressure and constant composition system. This is because the salt increases the chemical potential of the dissolved gases, which more than offsets the reduction in the chemical potential of the liquid water. The model can also be used for prediction of LH equilibrium for other gas hydrates. An example is given for methane hydrate at three different concentrations of methane in water.

Solubility of CO2 in water and NaCl solution in equilibrium with hydrate. Part II: Model calculation

“…The value of A ij for the CO 2 /H 2 O system is 0.04532 according to the reference results. k ij is fitted again in this work from the experimental data of gas‐liquid‐hydrate three‐phase equilibria of the CO 2 /H 2 O system, because the reference data were obtained at a high temperature without CO 2 hydrate existing. Figure and Equation show the relationship between k ij and the temperature.…”

Biochemical reaction and diffusion in seafloor gas hydrate capillaries: Implications for gas hydrate stability

“…Therefore, gas hydrate is somewhat of a "precipitate" form of gas that resides between dissolved gas saturation and free gas bubbles (Chen et al, 2006). Other research illustrating various relationships of Lw-H systems is available (Kim et al, 2008;Englezos, 2001, 2002;Zhang et al, 2008).…”

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

“…The temperature and pressure dependencies of the solubility of hydrate former in water are different at two-phase liquid water-vapor (L w -V) equilibrium and two-phase hydrate-liquid water (H-L w ) equilibrium [9][10]. The solubility of hydrate-former in water at L w -V equilibrium decreases with an increase in temperature at a given pressure, while at H-L w equilibrium the solubility in water increases with an increase in temperature [7,[10][11][12]. Two-phase H-L w equilibrium conditions can occur at the same high pressure and cold temperature conditions as the three-phase hydrate-liquid water-vapor (H-L w -V) equilibrium, which are common to Arctic regions, the deep sea, and underlying sediments.…”

“…Most of the available hydrate thermodynamic equilibrium experimental data are three-phase H-L w -V equilibria [1]. Experimental two-phase H-L w equilibrium data are comparatively scarce and limited to pure component methane, carbon dioxide, ethane and propane [1,7,[10][11][12]17]. To our knowledge there are no reliable experimental data on the solubility of hydrocarbon mixtures containing methane-ethane and propane in the aqueous phase at H-L w equilibrium.…”