An evolved cubic equation of state with a new attractive term

Abstract

A large variety of modifications have been presented for the temperature dependent function (α) existing in the attractive term of cubic equations of state (CEOS). Most of α-functions attempted to modify the vapor pressure prediction of polar components while other modifications have focused on both polar and non-polar compounds and other relations have considered an expansion of polynomials in the acentric factor (ω) and reduced temperature (Tr) to predict vapor pressure more accurately. In most cases such as Soave and Peng–Robinson equations of state, the suggested α-functions do not show a limiting behavior when temperature increases infinitely. In addition, the incompetency of many α-functions in describing the supercritical behavior of fluids have been modified by defining specific parameters for some different components or switching α-functions. Such approaches can create difficulties for the calculation of properties in which, there are equations of state derivatives. In this work, a new α-function has been put forward to improve the attractive term of Peng–Robinson EOS. The proposed model is capable of predicting the saturation vapor pressure of light gases, polar, nonpolar and heavy hydrocarbon compounds. The normal boiling point (Tb) is used as an additional parameter along with acentric factor (ω) and reduced temperature (Tr) to enhance the ability of the proposed α-function in the prediction of fluid phase behavior especially for petroleum fractions and heavy cuts existing in reservoir fluids samples. Moreover, two discriminating indexes (λ1, λ2) are introduced, to handle all pure substances and classify them under distinct groups for the allocation of special coefficients to each group. Experimental vapor pressure of 31 different pure components, 11 binary mixtures and 12 reservoir oil samples were collected, in order to evaluate the performance of the proposed model compared to the original Peng–Robinson. A comparison between the proposed model and other powerful equations of state such as modified PR, PSRK and PC-SAFT model was also performed, and the results of which indicate that the new proposed model has better outcomes over the mentioned methods, especially for complex mixtures and reservoir fluids. Furthermore, the proposed α-function shows a reasonable limiting behavior as the temperature approaches infinity and is capable of describing the fluid phase behavior at supercritical region.