Study On Diffusivity And Microstructures Of Supercritical Carbon Dioxide System By Molecular Dynamics Simulation

Supercritical fluid technology has got wide attention as a potential green technology, but the lack of the basic data had seriously blocked the research improvement and application. For the complexity of the interaction of supercritical fluid molecules, it's still hard to predict properties correctly in theory, while it's also very hard in experimentation because of the rigorous experimental condition. Owing the power to get over the upper problems, molecular simulation method has made its name in the research of supercritical fluid these years. In this paper, molecular dynamics simulation method was applied to investigate the diffusivity and microstructures of supercritical CO₂ at molecular level.Firstly, a new method was reported in this paper to deal with the simulated data as to get accurate values of diffusion coefficients on the foundation of differential-variation idea of mathematics. Then, the accuracy of COMPASS force field was investigated before the major work, because the force field was the most important factor in molecular dynamics simulation.Based on the former work, the diffusivity of pure CO₂, co-solvent in supercritical CO₂, and the microstructures as well were calculated by molecular dynamics simulation. Focus was concentrated on the interrelationship between the diffusivity and microstructures, and valuable rules were found in this paper. A new equation to predict the self-diffusion coefficients of CO₂ were established, which agreed well with simulation results and experimental data in literature. It was found that the convergence of CO₂ around the solute decreased the diffusion ability of the solute, and that the diffusion coefficients decreased with the increase of the solute's concentration.A proving trial based on Taylor dispersion theory was carried out to validate the simulation results via supercritical fluid chromatogram. The results indicated that the simulation showed good agreement with experimental data with the absolute average deviation of 5.31%.