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

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 func- tions. The new alpha function allows significant improvements of pure com- pound vapor pressure predictions (about 1.2 % absolute average percent devia- tions) 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 hydro- carbons. A general procedure is presented to fit experimental VLE data. The corresponding thermodynamic approach is based on the Peng--Robinson equa- tion 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.