LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

- 5 -whe~e P*a and P*b a~e the vapor pressures of pure components A and Brespectively at the given temperature. and Xa and ~ are the respectivemole fractions of A and B in the mixture.When boiling is first initiated the ratio of CO to water in the gas2phase tends to be relatively large because most of the carbon dioxideinitially diss olved in the liquid exsolves quickly into the gas phase whileonly a small amount of water changes to steam. with continued boiling, themole fraction of CO in the gas phase steadily decreases because little2additional CO is available to partition into the gas phase while the2fraction of water that is converted to steam increases at a relativelyconstant rate. As the temperature of the ascending gas- water mixturedecreases, the volume of the gas phase increases due to the decrease inhydrostatic load. The net effect is a drastic decrease in the partialpressure of CO as a boiling fluid ascends toward the earth's surface.2For relatively di lute systems and low initial dissolved gas concentrations.a distribution coefficient. B. is defined as the concentration ofgas in the vapor divided by the concentration of gas in the liquid. Thetemperature dependence of B for carbon dioxide in dilute aqueous solutionsis given bylog B = 4.7593 - 0.01092t • (11)where t is temperature in degrees Celsius. Equation (11) is valid from100 0 to 340°C. The following equation can be used to calculate the concentrationof CO remaining in the liquid phase (Cl) for the situation in2which all the evolved gas remains in contact with a boiling fluid duringadiabatic decompression (single-step steam separation).(12)where C oB is theis the initial concentration of dissolved CO 2before boiling.distribution coefficient, and y is the fraction of separatedsteam. The corresponding equation that gives the concentration of CO 2in the coexisting steam (C ) isvCv = -------(l/B)(l - y) + y(13)