Abstract

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 (PsatH2O) at XCO2 = 0; diverge with increasing pressure up to ~P (XCO2max) where ∂P/∂XCO2 = +∞ along the dew-point curve; then converge with increasing pressure above P(XCO2max). At temperatures below 265 °C and pressures > P(XCO2max), the compositions of coexisting liquid and vapor [ XCO2L(V) and XCO2V(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, XCO2L(V) and XCO2V(L) converge rapidly as P approaches Pc (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/∂XCO2 = 0 at the critical points indicated by corresponding bubble-point functions.

Stable liquid-vapor coexistence in mixed-volatile systems requires ƒLi = ƒVI (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 ƒH2OL = ƒH2OV and ƒCO2L = ƒCO2V 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.

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