Analysis of the equations of state of the hard convex body chain and hard spheres has been done for predicting the vapor liquid equilibrium (VLE) of simple fluids of n-alkanes. The repulsive part of the Boublik equation of state for the hard convex body chain has been found as an equivalent alternative either for the well known Carnahan-Starling repulsive term or the established van der Waals repulsive part of hard spheres equations of state. The attractive parts of these equations of state have the similar form as that of the van der Waals and are obeying the power-law temperature dependency. Add-on separation method of compressibility factor has been used for these equations of state. The simulated data for VLE densities from these equations of state are found to agree well with the available experimental data for n-alkane fluids.