Based on molecular theory of hard-core model. The parameters of the ne In this study, an equation of state is derived w cubic equation of state are temperature and acentric factor dependent. To calculate the saturate vapor pressure and volume of vapor and liquid of pure compound, this equation of state requires the critical temperature, the critical pressure and acentric factor of the compound. The proposed equation of state is applied for the calculation of PVT and VLE of various pure fluids and fluid mixtures. The average of absolute deviations of the predicted vapor pressure, vapor volume and saturated liquid density of 30 pure compounds are 0.814, 2.614 and 5.814%, respectively. The results are compared with those obtained by commonly used cubic equations of state. In addition comparisons with the mentioned cubic equations of state to predict some of the thermodynamic properties, including second virial coefficients and thermal properties, are given. The results indicate that the new equation of state yields better results. Moreover, the capability of the equation of state to predict the molar heat capacity of gases at constant pressure and volume, the sound velocity in gases, and the zero line fpr gases are also illustrated.

Equation of state is an essential tool for prediction of the phase equilibria in refinery, petroleum, oil and gas reservoirs and many other chemical industries. In addition, properties of compounds such as vapor pressure, density of vapor and liquid, thermal properties, and solubility of substances in supercritical fluids are calculated by using equations of state [1–5]. From the many equations of state, cubic equations of state are currently the equations of state considered most applicable for such calculations. Cubic equations of state can be classified into two categories. (i)The cubic equations with two parameters for repulsive and attractive terms such as van der Waals [6], Redlich-Kwong [7], Soave-Redlich-Kwong (SRK) [8] and Peng-Robinson (PR) [9], and etc. (ii)The cubic equations with three or more parameters such as Hyene [10], Patel-Teja [11], Trebble-Bishnoi [12], and etc. Although increasing the number of parameters makes a cubic equation of state more accurate for calculating pure component properties, the extension of these type of equations of state to mixtures require more mixing rules and therefore become more difficult,.

The PR and SRK equation of state are the best two parameters cubic equation of state among the others. Many attempts have been made to improve the prediction ability of these two cubic equations of state. Often, in these modified equations of state the attractive parameter generally is considered a function of the reduced temperature while the acentric factor and repulsive parameter are kept constant. Of course, there are equations of state in which the repulsive parameter is considered a function of the temperature, those proposed by Riazi and Mansoori [13], Feyzi et al. [14]. Almost in all of the general cubic equation of state, the vdW form of hard sphere (repulsive) contribution term is used. It is well known that vdW repulsive term is over simplified.