(190z) A Systematic Approach to Combination Rules: Unlike Lennard-Jones Parameters for Vapor-Liquid Equilibria

Molecular models show a remarkable predictive power regarding thermo-physical properties. In many fields of process engineering particularly the knowledge on vapor-liquid equilibria (VLE) of mixtures is of crucial importance. Thus, it is worthwhile to systematically investigate how successful pure substance models can be combined to predict mixture VLE.

In molecular simulation of a binary mixture A+B with pairwise additive potentials, three different interactions occur: the two like interactions between molecules of the same type A-A and B-B, which are fully defined by the pure component models, and the unlike interaction between molecules of different type A-B. In mixtures consisting of polar molecules, the electrostatic part of the unlike interaction is fully determined by the laws of electrostatics. However, there is no rigorous physical framework that yields reliable unlike repulsion and dispersion parameters, e.g. the unlike Lennard-Jones (LJ) parameters. To determine unlike LJ parameters, combining rules were developed in the past based on physical, mathematical or empirical considerations.

In the present systematic approach [1], firstly, the sensitivity of VLE predictions on the unlike LJ size and energy parameters is investigated in detail using the mixtures CO+C2H6 and N2+C3H6 as examples. It is found that mixture vapor pressure strongly depends on the unlike size and energy parameters whereas the bubble density depends solely on the size parameter and the dew point composition is rather insensitive to both parameters.

Secondly, for a large set of binary mixtures, eleven combination rules from the literature are assessed. It is shown that none is generally significantly superior to the simple Lorentz-Berthelot rule. A straightforward adjustment of one state-independent binary parameter for the unlike energy parameter to a single binary vapor pressure or Henry's law constant data point ensures an excellent agreement with experiment. This approach was applied to a set of around 300 binaries containing all combinations of non-polar, polar and hydrogen bonding components [2,3].

References

[1] T. Schnabel, J. Vrabec, and H. Hasse, J. Mol. Liq., 2007, 135, 170-178.

[2] T. Schnabel, J. Vrabec, and H. Hasse, Fluid Phase Equilibr., 2008, 263, 144-159.

[3] J. Vrabec, Y.-L. Huang, and H. Hasse, Fluid Phase Equlibr., 2008, submitted.