Gibbs Ensemble Simulates Vapor-Liquid Equilibria Of Fluids

The Gibbs Ensemble Monte Carlo simulation (GEMC) is a direct computer simulation method for determining phase equilibria, which can be obtained from a single computer experiment. The method is based on performing a simulation simultaneously in two distinct physical regions of generally different densities and compositions. Three types of perturbation are performed, a random displacement of molecules, a change in the volume of two regions and random transfers of molecules between two regions. Since there is no interface between the two regions that represent the coexisting phases and no need to simulate chemical potential, the simulations are faster than indirect methods. While the GEMC does not necessarily provide data of the highest possible accuracy and is not applicable to many important classes of mixtures, it is now commonly used for obtaining the phase behavior of fluids and mixtures. The main reason for this widespread use is not only the simplicity of the method but also the minimal demands on its user in terms of information on the approximate location of the phase diagram or other aspects of the behavior of a new system under study.Three GEMC programs designed by author are used to calculate the coexistence properties of pure L-J fluid, binary mixtures of L-J molecules and mixtures of CO2/CH3CH2CH3 respectively. In the simulation calculations L-J fluid potential models and the isotropic site-site models are applied.The constant-volume GEMC method is applied to simulate the pure L-J fluid. The liquid- vapor phase coexistence curve that is plotted according to the simulation results indicates the simulation calculations are correct and more complete than other simulations. The constant-pressure GEMC method is applied to simulate the mixtures of L-J molecules of which the molecular parameters are similar in order to testify the efficiency of this method. The phase diagram for ideal mixtures at T*=0.928 is completely plotted.Still, the constant-pressure GEMC method is applied to simulate the real mixtures of CO2/CH3CH2CH3. The simulation data of vapor-liquid phase equilibria at T=244.26K and 266.48K are good agreement in general with those from experiments. The isotropic site-site models in which molecular models of 2CLJ is adapted for CO2 and 3CLJ for CH3CH2CH3 are used to calculate the potentials of interactions between sites in the molecules. Moreover, the study testifies the 2CLJ model for CO2 and 3CLJ model forCH3CH2CH3 are sufficiently accurate for GEMC simulation.Although the principle and method of GEMC are concise, it is not easy to get the full phase coexistence curve from the simulations especially in the vicinity of critical points. This topic is discussed in detail in the study as well as the other important specifics. In order to get the full data, it is necessary to adjust the ratios of three kinds of move step in the Monte Carlo cycles and the accepted probability of the volume change.