Development of a new alpha function for the Peng-Robinson equation of state: Comparative study of alpha function models for pure gases (natural gas components) and water-gas systems

Abstract

Numerous modifications have been suggested for the temperature dependence of the attractive term of the Peng-Robinson equation of state (PR-EOS), through the alpha function. In this work, a new alpha function combining both exponential and polynomial forms is proposed. Pure-compound vapor pressures for different molecular species were fitted and compared using different alpha functions including the Mathias-Copeman and Trebble-Bishnoi alpha functions. The new alpha function allows significant improvements of pure compound vapor pressure predictions (about 1.2% absolute average percent deviations) for all the systems considered, starting from a reduced temperature of 0.4. In addition, a generalization of the classical Mathias-Copeman alpha function was proposed as a function of the acentric factor. These alpha functions were used for VLE calculations on water + various gases including gaseous hydrocarbons. A general procedure is presented to fit experimental VLE data. The corresponding thermodynamic approach is based on the Peng-Robinson equation of state with the above cited alpha functions. It includes the classical mixing rules for the vapor phase and a Henry's law approach to treat the aqueous phase.