Molecular simulation of phase equilibria and transport properties
Abstract
Molecular simulations are used to test models and theories through comparison with experimental results and theoretical predictions. Three areas are covered: transport properties, free energy measurement, and phase diagram calculation. Transport properties of isotropic fluids composed of hard ellipsoids of revolution are studied using molecular dynamics simulation. The self diffusion coefficient, the shear viscosity, and the thermal conductivity are evaluated for a range of densities and elongations and are compared to the predictions of an Enskog kinetic theory for nonspherical bodies. The simulation and the kinetic theory values for the shear viscosity and the thermal conductivity show the same qualitative behavior, that is, increasing with density and with particle nonsphericity. Quantitatively, there is good agreement at low densities (up to 30% of closest packing); at higher densities (60% of closest packing) deviations from Enskog theory are larger than, and in the opposite direction to those seen for hard spheres. The Stokes-Einstein and Debye relations are tested, and indicate a transition from a kinetic theory region towards the hydrodynamic limit as density increases. A new Monte Carlo method of free energy calculation, which does not rely on particle insertion is analyzed. The justification for the use of the method in the NVT ensemble is shown. The method is extended into the isothermal-isobaric ensemble where it is successfully applied to a variety of hard molecules. Analysis shows that the single particle sampling portion of the method should not be extended to the NPT ensemble. Isothermal phase diagrams for a variety of mixtures are determined by a combination of semi-grand ensemble simulation and orthogonal collocation integration. The semi-grand ensemble avoids problems of particle insertion that are encountered in some other widely used techniques. Orthogonal collocation allows the simulations to be run in parallel as opposed to stepwise integration techniques. The orthogonal collocation method is first studied numerically on an ideal system to demonstrate its superiority in suppressing error propagation. Next, Lennard-Jones mixtures, including some which form azeotropes, are examined and compared with prior works. Finally, three systems of real molecules, Krypton-Argon Methane-Ethane, and Ethane-Carbon Dioxide are simulated and compared with experimental results.
Degree
Ph.D.
Advisors
Talbot, Purdue University.
Subject Area
Chemical engineering|Molecules|Chemistry
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