Calculation of the Thermodynamics of Mixing of Fluids Using Simple Equations of State

The Redlich–Kwong and van der Waals equations of state were used to calculate the enthalpy, entropy, and Gibbs free energy of mixing for the CO2/C2H6 binary system, as a function of composition. Comparison with experimental enthalpies of mixing shows that both of these equations perform well qualitatively, including a representation of phase changes from vapor to liquid at high pressures. The Redlich–Kwong equation provides a better quantitative estimate of the enthalpy of mixing for this system. It is observed that the ideal fluid estimate of the free energy of mixing is fairly accurate at high pressures, largely due to a cancellation of enthalpy and entropy terms in the real gas free energy.