Phase Equilibria Of Fluids By Gibbs Ensemble Monte Carlo Method

Phase equilibria of fluids and their mixtures are of central importance to chemical engineering. However, phase equilibrium from molecular simulation is originally not a simple matter, which can be attributed partly to the difficulty in obtaining free energy or chemical potential data required for phase coexistence. Until the1980s, methodological development rendered the determination of phase equilibrium by molecular simulation much easier than before. Molecular simulation is based on the force fields describing intermolecular forces in a system. Many force fields are available, but most existing force fields have not been tested for whether to reproduce phase coexistence properties of pure fluids and mixtures, which severely limits their reliability and predictive ability. In this thesis phase coexistence properties of some systems of interest for chemical industries were investigated using the NPT-Gibbs ensemble Monte Carlo (MC) simulations. The currently available force fields for systems studied were tested for vapor-liquid equilibria. The NPT-Gibbs ensemble Monte Carlo simulation algorithm was coded in visual C++, an object-oriented language for the lack of availability of portable and high quality codes. Particle identity exchange algorithm to increase the percentage of successful particle insertions was added in this program. Ensemble averages of the quantities of direct interest can be calculated using this program. In addition, some thermodynamic quantities are outputted periodically to monitor convergence profiles. MC simulations in Gibbs ensemble (GE) were carried out to determine the vapor-liquid coexistence curve for the binary mixtures H2+N2, H2+Ar and N2+Ar, and the ternary mixture H2+N2+Ar using the Lennard-Jones models available. Simulation results deviated from experimental data for the pressure-composition diagrams of the binary systems. For the ternary system results were in qualitative agreement with experimental values. These indicated it was difficult to accurately computer the phase behavior of the systems studied using simple models. Using two center Lennard-Jones models for N2 and O2, one center Lennard-Jones model for Ar, equilibrium positions and density of the N2, O2, Ar mixtures by GE MC method. The simulation results were found to be closer to experiments at 2 and 10 absolute pressure, which demonstrated the selected models allowed accurate reproduce the low and high pressure phase equilibria for this ternary system. The OPLS united-atom force field was used to describe the interactions of C2H5OH, EPM2 model was used for CO2 and TIP4P model for H2O, the phase coexistence properties of the binary system CO2+C2H5OH and the ternary system supercritical-CO2+C2H5OH+H2O by MC simulation with Gibbs ensemble method. Good agreement was found between simulation results and the experimental data for the CO2+C2H5OH system, but deviations in the calculated equilibrium compositions from the experimental values for the ternary system were noticeable. The phase equilibria of the ternary mixture, supercritical- C3H8+C2H5OH+H2O were caculated using the OPLS models for C3H8 and C2H5OH, and TIP4P model for H2O, the agreement of data for the compositions of the coexisting phases with experimental results was relatively good. The selectivity, solubility, distribution coefficient and extracted concentration of ethanol under different conditions were investigated. Results indicated that the solubility of ethanol increased with pressure, temperature and the ethanol concentration in the liquid phase under the supercritical conditions. The similar tendencies were observed for the selectivity of ethanol. Concentration of extracted ethanol and distribution coefficient of ethanol increased with pressure and concentration of ethanol in the liquid phase, but decreased with temperature.