The equilibrium constants for methane and for water, and for ethane andwater have been calculated from experimental data for the two binary systems.These constants are for the two-phase systems for the temperature range of 100?to 340'F and the pressure range of 200 to 10,000 psia.
The constants for ethane are greater than those for methane. Equilibriumconstants for a natural gas from a natural gas-water system are about the sameas those for methane. Equilibrium constants for water in all three systems arevery nearly the same over the greater part of the range of temperatures andpressures studied.
In ideal solutions, the vapor-liquid equilibrium constants for a componentshould be the same in all such solutions at a given temperature and pressure.Few solutions are ideal, and consequently the use of generalized constantsoften leads to error in situations where they do not actually apply. Indecidedly non-ideal systems, it is desirable to have equilibrium constant datafor the specific system.
Vaporization equilibrium constants may be calculated either by thermodynamicmethods or from experimental vapor-liquid equilibrium data.
Equilibrium constants calculated from Equation (1) have been found to haveonly limited agreement with experimental values, even though the solutions forwhich they were calculated approached ideality. If the more volatile componentis at a temperature above its critical, the accuracy of the equilibriumconstant for the component is made more doubtful by the use of the vaporpressure term under such conditions. In order to make the thermodynamiccalculation on a component above its critical temperature, one must resort toone of the empirical methods of extrapolation of the vapor pressure toconditions above its critical point. Application of the thermodynamicequilibrium constant then to the methane-water or the ethane-water systemswould be risky since the systems are non-ideal, and the methane and ethane areabove their critical in most petroleum operations.