Study Of Liquid-vapor Phase Transition Characteristics Of Binary Mixture

Recently, with the rapid development of Chinese economy, the demandsof gases such as Oxygen and Nitrogen in steel, chemical, defense, medical and otherindustries are increasing. The Oxygen and Nitrogen are mostly obtained from airusing deep cooling method. Liquid-vapor phase transitions exist in air liquefactionprocess in turbo-expander and the Nitrogen-Oxygen separation process in thedistillation column in air separation systems. Liquid-vapor phase transitionphenomenon is widespread in different engineering fields. This problem has beeninvestigated by many researchers in the past several years. But few investigationsfocused on the microscopic features and detailed interface characteristics. In presentwork, the molecular dynamics simulation method was adopted to study thethermophysical properties of binary mixtures, liquid-vapor phase transition process,and liquid-vapor interface characteristics from the microscopic view.Firstly, the liquid-vapor thermophysical properties of pure argon were studied.Considering the balance of computation time and accuracy, the cutoff radius wasdetermined to rc=4.5. The present method was first verified with the experimentaldata of the liquid-vapor densities of pure Argon, the variation of saturation pressurewith temperature and the critical temperature. Excellent agreements between themwere achieved. The characteristics of Argon-Krypton binary mixture were thenstudied and the thermophysical properties of this binary mixture under differentinitial concentrations and temperatures were obtained. This method provides a newapproach to predict the thermal properties of binary or multiple mixtures. Themixture properties under extreme conditions as well as under critical conditions,which are difficult to obtain in the experiment, can be obtained and to enrich thethermal property database.Secondly, the detailed characteristics of Argon-Krypton binary mixtures werethen investigated, such as the tangential and normal components of pressure tensor,the surface tension, the interface thickness, etc. The temperature has great influence on interface. The molecular velocity is larger at higher temperature, which meansthat the molecule on the surface is more likely to break the bondage of the interfaceand moves into the vapor region. So the surface tension is smaller. The surfacetension becomes zero at critical temperature. The thickness of the liquid-vaporinterface increases with the increase of temperature. The liquid-vapor interface doesno longer exist at the critical temperature. The critical temperature of mixture liesbetween those of the pure components.Finally, the liquid-vapor phase transition characteristics of Nitrogen-Oxygenbinary mixture in the distillation process in the air separation system wereinvestigated. The diatomic model for Nitrogen and Oxygen was used, instead of themonatomic model in previous studies. The liquid/vapor density, the criticaltemperature, and the variation of saturation pressure with temperature of Nitrogenand Oxygen were studied. These results agree well with experiment data. Thedetailed interfacial properties of Nitrogen-Oxygen mixture under differentconcentrations and temperatures were calculated.