For vapor-liquid equilibrium calculations, the K-value for a component is defined as the mole fraction of the component in the vapor phase divided by the mole fraction of the component in the liquid phase :
where: 
= K-value for component i
= mole fraction of component i in the vapor phase
= mole fraction of component i in the liquid phase
Equilibrium K-values are usually calculated from fundamental thermodynamic relationships which relate the liquid and vapor fugacities for each component in the mixture. For equation of state methods, the fugacity components are computed directly with the equation of state.
Available methods for calculating VLE K-values are listed below. See Section 1.2.1 of the PRO/II Reference Manual for a more complete discussion.