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

Abstract

A study was carried out to address the need to compute Lennard-Jones (LJ) densities as a function of temperature and pressure, in wide ranges of temperature and pressure, for further use in LJ-based viscosity computations. A high-quality LJ-EOS was chosen. Some of the compounds used include n-undecane, n-decane, ethane, methane, sulfur dioxide, propylene, m-xylene, ethyl acetate, isopropanol, and chloroform. An extrapolation scheme that makes possible calculate LJ densities or pressures at lower temperatures was proposed. The original LJ-EOS coupled to the extrapolation schemes was called EXT-LJ-EOS. It was possible to obtain a very good description of the pure compound vapor pressure curve for substances of diverse nature utilizing the EXT-LJ-EOS. However, all the options studied produced violations to the requirement which states that different pressure versus density isotherms should not intersect each other. Violations occurred only at relatively high reduced pressures. The constraint studied was a type of restriction (restriction (32)). It should be inspected in a wide enough temperature-density range whenever a temperature dependence is imposed on an EOS, regardless the nature of the EOS. Compliance with restriction (32) for pure compounds did not guarantee compliance for mixtures when using temperature dependent interaction parameters or temperature-dependent mixture covolume parameters. Restriction (32) could be embedded into constrained optimization computer programs used to fit pure compound or mixture parameters from experimental data. With such programs, restriction (32) should be evaluated at the conditions f the experimental data and within a wide-range temperature-density grid. Any proposed EOS temperature dependence could potentially violate constraint (32). A better representation of vapor pressures had a good compact on the LJ based prediction of viscosities.