The conformal solution method based on the hard sphere expansion (HSE) theory predicts thermodynamic properties of mixtures by separating the mixture properties into a contribution from repulsion and other contributions from various types of intermolecular attraction. The original HSE, however, has a tendency to show increasing relative error in the prediction of thermodynamics properties as the difference of molecular sizes in mixtures increases since the radial distribution functions of a mixture are represented by that of a pure reference in the mean density approximation (MDA). When the hard convex body (HCB) equation of state was substituted for the hard sphere equation of state in the repulsion term to overcome the shortcoming of the original HSE, better results on the K-value in binary hydrocarbon mixtures were obtained.