Molecular Dynamics Simulation of Acid Gas Mixtures:  A Comparison between Several Approximations

Molecular dynamics simulations of CH 4, CO 2, H 2 S, and their mixtures, chosen as representative of acid gas mixtures, have been performed. Static properties (pressure or density) and dynamic properties (shear viscosity) have been computed. A comparison has been performed between results from three different models: a simple Lennard-Jones (LJ) model, an isotropic multipolar (IMP) one, and the usual LJ plus point charge (LJP). For pure fluids, a reasonable agreement is found between all three models and experiments except for CO 2, for which the LJ model suffers from its lack of electrostatic contributions. Concerning CH 4 −H 2 S mixtures, results obtained using the three molecular models are consistent with each other for static and dynamic properties. In addition, no significant differences between the results obtained using both Lorentz−Berthelot and Kong combining rules were noticed. For the CH 4 −H 2 S−CO 2 mixture, the situation is different: a strong dependence of the pressure on the molecular models as well as on the combining rules, in the case of the LJP model, has been noted. Shear viscosity was found to be less dependent on the choice of models and combining rules. Thanks to simulations on H 2 S−CO 2 mixtures, it has been found that the way cross interactions are treated between these two compounds explains the discrepancies for CH 4 −H 2 S−CO 2 mixtures. For the systems studied, the IMP approximation seems to be the best option for engineering calculations of physical properties because it is quick to compute, ensures accuracy, and is weakly dependent on the combining rules employed. In addition, it is shown that a van der Waals one fluid model combined with an accurate LJ equation of state together with a correlation on viscosity is able to provide results consistent with simulations (on both LJ and IMP models). Such a result makes this correlative scheme a good alternative for industrial applications.