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The CO₂–H₂O system: IV. Empirical, isothermal equations for representing vapor-liquid equilibria at 110–350 °C, P 150 MPa

Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Building 4500-S, Oak Ridge, Tennessee 37831-6110, U.S.A.

Correspondence: ^* E-mail: blencoejg{at}ornl.gov

Empirical formulae are presented for calculating vapor-liquid equilibria (VLE) in the CO₂-H₂O system at 10 temperatures between 110 and 350 °C. At each temperature, separate functions are used to represent the bubble- and dew-point boundary curves that: originate at the saturation vapor pressure of water (P_sat^H₂O) at X_CO2 = 0; diverge with increasing pressure up to ~P (X_CO2^max) where P/X_CO2 = + along the dew-point curve; then converge with increasing pressure above P(X_CO2^max). At temperatures below 265 °C and pressures > P(X_CO2^max), the compositions of coexisting liquid and vapor [ X_CO2^L(V) and X_CO2^V(L)] do not converge completely with increasing pressure due to the absence of critical behavior. Thus, relatively simple functions suffice to accurately represent VLE at those temperatures. In contrast, at T > 265 °C, X_CO2^L(V) and X_CO2^V(L) converge rapidly as P approaches P_c (the critical pressure in the CO₂-H₂O system at a given temperature between 265 and 374 °C and P 215 MPa). For those temperatures, therefore, more complex VLE formulae are required to achieve close representation of phase relations. For dew-point equations, this includes adding an exponential "correction term" to ensure that P/X_CO2 = 0 at the critical points indicated by corresponding bubble-point functions.

Stable liquid-vapor coexistence in mixed-volatile systems requires ^L_i = ^V_I (isofugacity conditions) for all "i" (volatile components) in the two fluid phases. Thus, the equations presented in this paper specify numerous P-T-X conditions where _H2O^L = _H2O^V and _CO2^L = _CO2^V in the CO₂-H₂O system. These results have important applications in the ongoing effort to develop a more rigorous thermodynamic model for CO₂-H₂O fluids at geologically relevant temperatures and pressures.