Stability limit determination for pure compounds

The Helmholtz criteria for equilibrium and stability were used with the Peng-Robinson (P-R) equation of state to determine the limits of stability for n-hexane, cyclohexane, and benzene, as representative paraffinic, naphthenic, and aromatic compounds, respectively. The cubic equation that results from the criteria of stability was solved to obtain the three roots of volume at different pressures. The intersection of the stability curve and the temperature isotherm determines the limit of stability for a compound. The stability limits tend to increase with increase in temperature alone for each substance.