The CO2-H2O system: III. A new experimental method for determining liquid-vapor equilibria at high subcritical temperatures

A highly precise and accurate vibrating U-tube technique was developed to d etermine the upper baric stabilities of liquid-vapor assemblages in the CO2 -H2O system at high subcritical temperatures (similar to 275-360 degreesC). The first step is to create an isobaric-isothermal, physically isolated an d chemically homogeneous sample of "high-pressure" CO2-H2O fluid of known c omposition. Fluid pressure (P) is then lowered slowly at constant temperatu re. Pressure readings and matching values for tau (the period of vibration of the U-tube) are recorded at 0.1 or 0.2 MPa intervals. When the fluid beg ins to separate into two phases (liquid + vapor), a distinct inflection is observed in the trend of P vs. tau. Performing such experiments for fluid c ompositions at 0.05 mole fraction CO2 (X-CO2) intervals in the range 0.05 l ess than or equal to X-CO2 less than or equal to 0.40 at 300 degreesC produ ced a complete high-P liquid-vapor boundary curve for the CO2-H2O system at that temperature. Agreement with corresponding curves determined in previo us studies ranges from poor to excellent.