Self-diffusion And Structure Of Pure Alkanes And Infinitely Dilute Solutions Of Alkanes In Supercritical Carbon Dioxide: A Molecular Dynamics Simulation Study

Molecular self-diffusion is the random translational motion of molecules driven by internal kinetic energy. As one of the most basic molecular movement forms, it can be measured with self-diffusion coefficient. For study on fluid dynamics and mass transfer and process design, molecular diffusion plays an important role in the applications of supercritical fluid technology. The rate of mass transfer in supercritical fluid which contains the rate of mass transfer and phase reaction in supercritical fluid, depends on the dynamics of supercritical fluid system, and the critical section is to obtain accurate diffusion coefficients. By far, many scholars have done different levels of researchs focusing on diffusion coefficients of various systems in supercritical fluid.Generally, there are three methods to obtain diffusion coefficients: experimental determination, theoretical study and molecular dynamics simulation. It is very difficult and expensive to determine diffusion data by experiment because of rigorous experimental condition required. Further more, several correlation parameters used in empirical or theoretical study are difficult to obtain. As a result, both experimental and theoretical investigations are seeking a development progress, and neither of them has formed a maturely systematical theory. Owing the power to get over the upper problems, molecular simulation method has made its name in the research of supercritical fluid these years. With the rapid development of computational techniques, computer simulation which has attracted rather more attention and applied in much wider fields, has already become one of the most important methods for obtaining diffusion coefficients of fluids.In this thesis, molecular dynamics simulation method was applied to investigate the diffusivity and structural properties of n-alkanes, supercritical CO₂ and n-alkanes/ CO₂ binary mixtures over a wide temperature range at high pressure.1. In this work, simulations were performed with the TINKER v5.0 molecular modeling software using the OPLS (Optimized Potentials for Liquid Simulation) force field. First, molecular dynamics simulations were employed to calculate the self-diffusion coefficients of methane, ethane and propane, over a temperature range of 112 K to 454 K and a pressure range of 11 MPa to 207 MPa, whose experimental data can get from reported literature data. Fortunately, the results of MD simulation behaved good accordance with literature data. It was shown that MD simulation could predict the self-diffusion coefficients of n-alkane precisely. So we can make further efforts to simulate the self-diffusion coefficients of methane, ethane and propane at higher temperatures and pressures, and other n-alkanes from n-butane to Dodecane over a wide temperature and pressure range. Second, the microstructural properties such as radial distribution function (RDF) and coordination numbers have been calculated to represent the internal structure of fluid. We analyzed the variation trends of RDFs, coordination numbers and self-diffusion coefficients with the change of temperature and pressure, intending to clarify the relation between self-diffusion coefficients and structural properties.2. Based on the former work, the diffusivity of pure CO₂ and the microstructures as well were calculated by molecular dynamics simulation using the EPM2 force field. By the comparison of experiment and MD simulation data, it was denoted that the simulation values were highly consistent with literature data with the average relative deviation of 4.70%.3. The infinite dilute diffusion coefficients of solutes in supercritical CO₂ are quite essential to applications in chemical industry. In this work, the infinite dilute diffusion coefficients of n-alkanes in supercritical CO₂ were investigated. Unfortunately, the results indicated that the simulation showed poor agreement with the literature data.Therefore, molecular dynamics method could calculate the self-diffusion coefficients of n-alkanes and supercritical CO₂ and accurately. However, the infinite dilute diffusion coefficients of n-alkanes in supercritical CO₂ is far from satisfactory, we need further efforts to seek more appropriate simulation conditions. All the same, MD simulation is a powerful tool to help experiment devise and engineering application to obtain the self-diffusion coefficients and structural properties.