Calculating Fugacities from Gas Equations of State

Redlich-Kwong Equation (original, unmodified) lri7fc = ln/fc/X. F 

Modified Redlich-Kwong Equation of Kerrick and Jacobs (a) Pure Gas 

However, CH4 significantly reduces the activity of CO2 in these mixtures. The CO2 and CH4 molecules are similar and non polar, and both are quite different from H2O. Jacobs and Kerrick (1981) attribute the predicted activities to simple dilution of CO2 by CH4 (lowering its effective activity) the H2O molecule presumably interacts in somewhat similar fashion with both CO2 and CIJ4 so that a change in the CO2/CH4 ratio should have little effect on water activity. The addition of CH4 to CO2-H2O fluids has geological implications evident in this figure. First, the predieted CO2-H2O 

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