Development of an equation of state for the square-well chain molecules ofvariable well width based on a modified coordination number model

An equation of state (EOS) is developed according to the statistical associ ating fluid theory (SAFT) model. The dispersion term in the new EOS is deri ved by a modified coordination number equation. This modified coordination number expression gives correct results at low density and close-packed reg ions. It also agrees well with the computer simulation results for square-w ell (SW) fluids. A modified expression of the chain term compressibility fa ctor for the SW fluid with variable well width is proposed in this study. C omparison of the calculated compressibility factors with molecular simulati on data for tangent hard sphere chains and SW chains indicates that the new EOS yields satisfactory results and relatively smaller deviations than tho se from other equations. Finally, the new EOS is employed on real fluid sys tems of n-alkanes. For each pure fluid, four EOS parameters are regressed. These pure fluid parameters yield reasonable correlation with the molecular weight for n-alkanes up to n-eicosane. The pure fluid properties are well calculated by the new EOS. Prediction of the saturated vapor pressures of h igher carbon number alkanes also shows satisfactory results. The new EOS de veloped in this study is consistent with the molecular simulation data for SW fluids with variable well width. It is simple in mathematical form, and is applicable from small molecular weight compounds to long chain molecules . (C) 2001 Elsevier Science B.V. All rights reserved.