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The Prediction of the Speed of Sound in Hydrocarbon Liquids and Gases: The Peng-Robinson Equation of State Versus SAFT-BACK

Two equations from the two classes of thermodynamic functions, in terms of their simplicity and complexity of functional forms, are compared, applying them in prediction of second-order derivative properties such as sound velocity and isobaric and isochoric properties of hydrocarbon fluids. Peng-Robinson (PR) equation of state, from the simple class, and SAFT-BACK equation from the complex family are selected for this purpose and their results are compared in both super and subcritical regions. The results reveal the superiority of the SAFT-BACK equation over the PR equation of state, especially at the liquid region or the high-density conditions. At low pressures (low densities) similar results are observed through both approaches. These results imply the inaccuracy of the cubic equations of state in prediction of second derivative properties, and moreover, suggest the prediction of sound velocity as a consistency test for any cubic equation of state.