Thermophysical properties of fluids: towards a single molecular model valid for n-alkanes ?


G. Galliero1 and C. Boned1

1Laboratoire des Fluides Complexes et leurs Réservoirs (UMR 5150 CNRS/TOTAL), Université de Pau et des Pays de l’Adour,PAU Cedex, France

Keywords: molecular based model, Lennard-Jones chain, molecular dynamics
property: transport properties
material: normal alkane

At present time there is no molecular model, with the corresponding theory, which is able to provide both equilibrium and transport properties with the same molecular parameters in the fluid regimes (gas, liquid and supercritical) apart from the low density conditions. This statement is even more pronounced when dealing with non mono-atomic fluids. This lack is largely due to the fact that comprehensive theory is still not available for evaluating the transport properties in dense fluids in terms of a realistic molecular model (interaction potentials + description of the molecules).

However, from the equilibrium properties side, the combination of the Lennard-Jones Chain (LJC) molecular model with the Statistical Associating Fluid Theory (SAFT), a molecular based Equation of State (EoS), is able to represent very well the thermodynamic properties of a great variety of fluids [1]. Furthermore, combined with Density Functional or Density Gradient Theory, these approaches are capable to yield very good results on interfacial properties for the same systems [2]. Additionally, based on Molecular Dynamics simulations results, has been proposed recently semi-empirical accurate correlation to describe viscosity [3] and thermal conductivity [4] of the LJC fluid over a wide range of thermodynamic states for short chains (i.e. shorter than the hexadecamer).

So, in this work, we have applied the LJC molecular model combined with the previously mentioned theory/correlation on normal alkanes (methane, n-butane, n-heptane and n-decane) along the vapour/liquid coexistence line. The idea was to test if, with a single set of molecular parameters for each n-alkane, this model was able to provide a reasonable estimate of both thermodynamic and transport properties.

Good results have been obtained for methane (monomer) and n-butane (dimer) except for thermal conductivity in the gas state. For longer alkanes it has been found that both viscosity and thermal conductivity were underestimated by this approach. It will be shown that these trends are fully related to the bad modelling of the internal degrees of freedom when using the LJC model. Furthermore using molecular dynamics simulation it will be shown that, for viscosity, the addition of a only one rigidity parameter allows to strongly improve all the results.

References
  1. S. P. Tan, H. Adidharma, M. Radosz, Ind. Eng. Chem. Res., 47, 8063 (2008)

  2. P. Paricaud, A. Galindo, G. Jackson, Fluid Phase Equilib 194-197, 87 (2002)

  3. G. Galliero, C. Boned, Phys. Rev. E 80, 061202 (2009)

  4. G. Galliero, C. Boned, Phys. Rev. E 79, 021201 (2009)

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