In the petroleum industry, knowledge of phase behavior is essential to solve many problems, such as the design of the catalytic cracking of heavy oil. The catalyst tends to lose activity as a result of the formation of the solid phase, so the calculation of multiphase behavior of systems containing solid phases will serve to solve the problem. The Soave-Redlich-Kwong equation of state is widely employed to evaluate multiphase behavior, but cubic equations of state are incapable of calculating the properties of solid phases and thus cannot evaluate the multiphase behavior of systems containing solid phases. In this research, a new technique for describing the solid phase has been developed. The multiphase behavior of propane binaries with ployaromatic hydrocarbons was then explored. To evaluate the critical end point, which is difficult to calculate, an algorithm combining the method of Heidemann and Khalil (AIChE J. 1980, 26, 769-780) to compute the critical point and the tangent-plane criterion was developed previously (Yang et al. Ind. Eng. Chem. Res. 2009, 48, 6877-6881). Setting the initial guesses with the values at the obtained critical end points, the three-phase loci were then computed successfully. The calculation results show that the three-phase loci terminate at a lower critical end point (LCEP), which is wrong according to the experimental data. To correct this mistake, the newly developed technique was employed, and the quadruple points where the three-phase loci really end were evaluated successfully.