Microscopic Research On The Diffusion Coefficient

Diffusion occurs in every field of chemical engineering and the research on the diffusion coefficient is the most important thing for almost each researcher. Although many researchers have made an exhaustive study on diffusion coefficient, its mechanism is still an insoluble mystery. New models for self-diffusion and mutual diffusion were proposed based on the microscopic mechanism and used to predict the diffusion of different substances. Through this research, we have obtained:(1) New isothermal-isobaric molecular dynamics algorithm was proposed and the temperature and pressure relaxation times were optimized. The optimum value for both relaxation times is 2×10-15 second. Using this arithmetic, we calculated the self-diffusion coefficients of carbon dioxide and argon and the results agree with the experimental values.(2) New model for self-diffusion coefficient was proposed involving both free volume and diffusion potential. The free volume was calculated by using the Generated Van der Waals equation of state and the local critical diffusion potential was computed by proposing a new model on the base of the structure of crystal. We predicted the self-diffusion coefficient of argon by using this self-diffusion model and obtained acceptable results.(3) The monoatomic molecules are bounded by a potential trap and they will have to escape from this trap if a diffusion course happens. However there is always a little region within which molecules are able to fluctuate freely. By calculating the free volumes of different systems under different conditions, we obtained that the free volume is influenced most by the density and little by the temperature.(4) The space structures of ethane, ethane, benzene and ethanol were analyzed by using both the radial distribution functions and space distribution functions. We found that atoms arrange regularly. We also calculated the local critical diffusion energy by using the space distribution functions, obtaining the fact that the potential energy of basic molecule distributes regularly. However, for the associated ethanol systems, it is not the case. Using this method, we calculated the diffusion coefficients of ethane, ethane, benzene and ethanol, the results agree with the experimental values well.(5) A new mutual diffusion coefficient model was proposed based on the relation between self-diffusion coefficient and friction coefficient. The new model was divided into three parts: composition part, self-diffusion coefficient part and the relation between chemical potential and composition. The mutual diffusion coefficients of carbon dioxide in the mixtures was simulated under different compositions at 1132~1810℃, by using the self-diffusion coefficients obtained from literature. The relation between chemical potential and composition was calculated using different equation of state, namely Peng-Robinson, Redlich-Kwong, Van der Waals and Virial equations. The average deviation is less than 5%.(6) A new model of mutual diffusion coefficient was proposed for systems having an only associated component. The one associated system involved benzene and ethanol was analyzed, finding the average association number of ethanol is 1.8.