Molecular Dynamics Simulation Of Equilibrium Properties At Liquid-vapor Interface Of Simple Fluid

In chemical engineering industry, the unit operations such as rectification, absorption and drying concern with mass transfer and heat transfer at liquid-vapor surface. However, the complexity of the processes results in theoretical and experimental study progress slowly. In recent years, with the rapid development of computer science and technology, molecular dynamics (MD) simulations are increasingly used in the study of various chemical processes in the thermodynamics and diffusion properties.In this thesis, thermodynamics properties and condensation process of argon and binary mixture containing argon and methane at liquid-vapor equilibrium condition were studied by molecular dynamics simualtion. The simulations were carried out in the canonical ensemble (NVT) and the interactions between different molecules were modeled by the shifted Lennard-Jones12-6potential.The equilibrium properties of argon were carried out by molecular dynamics under different temperatures (104.3K、113.3K、123.2K and131.5K), and the corresponding thickness and tension of liquid-vapor interface, pressure of vapor were obtained when the liquid phase and vapor phase were in equilibrium. The simulation results are in accord with literature data well. The new method of calculation for the fugacity of vapor was proposed. The results have a quite good correspondence with which obtained from Redlich-Kwang equation of state. Besides, the condensation coefficients were calculated and compared with literatures data, it was evaluated from0.796to0.439and decayed with increasing the temperature, and the tendency is similar with the results in the literatures. Activation energies of condensation were obtained based on the molecular collision theory, the results indicate that the activation energy of condensation connects with latent heat of vaporization and temperature, and it is an increasing function of temperature. The relationship between condensation coefficients and activation energies can be described by Arrhenius law.Besides, the equilibrium properties of binary mixture containing argon and methane over the entire range of compositions were carried out by molecular dynamics under temperature T=104.3K. The relationship of compositions between liquid and vapor, thickness and tension of liquid-vapor interface, Gibbs absorption, pressure and fugacity of vapor were calculated, respectively. The simulation results are in good agreement with literature data. Besides, the activities and activity coefficients of component i were derived from adjusted Raoult’s law fi=fi*γixi. The relationship among tension of interface, activity of component i and Gibbs absorption agree very well with Gibbs equation (?).Finally, the equilibrium properties of binary mixture containing argon and methane at different temperatures were simulated by molecular dynamics under initial mole fraction of argon equal to0.3. The thickness of interface, interface tension and Gibbs absorption of argon were calculated. The results demonstrate that the tendency of curve for thickness of interface, interface tension of binary mixture versus temperature is similar with argon fluid. The Gibbs adsorption of argon is increasing with temperature increase. Besides, the activity coefficients of component argon in liquid mixture and the mole fraction of argon in liquid and vapor were decreasing with temperature increase, respectively, and the deviation between fugacity of vapor and corresponding pressure of mixture and component argon increasing as the corresponding saturated pressure increase, respectively.