Saturated vapor pressure through a modified Lennard-Jones equation of State

J.M.V. MACHADO; M.S. ZABALOY; E.A. MACEDO

FLUID PHASE EQUILIBRIA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2001 vol. 182 p. 75 - 96

0378-3812

The study of a Lennard-Jones (LJ)-based model for viscosities led us to concentrate on a recent high-quality pressure?density?temperature LJ equation of state (EOS). The application of LJ-EOSs to real fluids typically requires introducing a temperature dependence on the LJ intermolecular potential parameters. Though this goal has been the matter of previousworks, we have found the following problems: (a) relatively lowquality of the unmodified LJ-EOS (U-LJ-EOS) used as the basis for real-fluid computations; (b) uncontrolled use of the U-LJ-EOS outside the range of conditions of the supporting molecular simulation data; (c) poor reproduction of the pure-compound critical temperature and/or critical pressure and (d) need of iterative calculations either to compute the vapor pressure at a given temperature or to regress the values of the LJ parameters from experimental data at saturation. In the present work, we address all these problems, and provide useful equations to compute properties at saturation through non-iterative procedures. These procedures conserve the quality of the output numerical result, in comparison to the corresponding iterative algorithms. We examine a number of options for the LJ temperature dependence. Some of them make possible to represent accurately the vapor pressure, in a wide range of temperature, for non-polar, polar and heavy compounds.We also scrutiny the potential (unwanted) appearance of crossing pressure versus volume isotherms, which can be produced by the introduction of a temperature dependence, and propose a simple test to systematically assess the range of applicability of a chosen temperature dependence.We also illustrate here the effect of a good vapor?pressure reproduction on the prediction of viscosities.