Studies On The Molecular Simulation Of Diffusion Process In Membrane And Water Solution

This paper used molecular Dynamic Simulation method to detailedly study the diffusion process and diffusion coefficient of small molecules in nanofiltration membrane and water solution, and got some inaccessible results with traditional experimental method.The mainly aspects discussed in the paper include:The simulation of diffusion coefficients of oxygen and other 8 kinds of gas molecules in water solution. The simulation of diffusion coefficients of methanol and other kinds of organic substance in water solution. The diffusion process and diffusion coefficients of H₂O and NaCl in membrane.This paper simulated diffusion process of gas solutions at different temperature such as 298.15K,293K,303K and 313K. In the Dynamic Simulation, the models of gas solution were constructed by adding different molecules into the cell. Comparing the simulation values with the experimental results, the sample number of molecules had some effect on the simulation of diffusion coefficients.Results of the investigations showed that the diffusion coefficients of the same solution differed a lot when they were calculated with different molecule-number samples, and the diffusion coefficients at first increased with the increasing of the gas, but then decreased. Comparing calculated diffusion coefficients of four kinds of gas in water with experimental value, the calculated results are reasonable. This paper is aimed to show that the diffusion coefficients for industry use can be calculated with MD simulation method which would be more helpful to the development of transfer theory.Using molecular dynamics (MD) simulation, the diffusion of organic substance such as methanol were simulated at 293.15K and 0.1MPa.The diffusion coefficients were analyzed by differentiation-interval variation method. Results of the investigations showed that along with the same concentration of the number of elements in the sample,simulation results increased gradually closer to the experimental values. By the same method,the diffusion of organic substance such as benzene in salt-water solution has also be calculated. In this way, the diffusion of three kinds of organic substance had been calculated.In this paper,two kinds of nanofiltration membrane and eight kinds of reverse osmosis compound membrane were used as the main base case, in which the diffusion process of the main components(water, sodium chloride) in the seawater within the membrane were simulated and the diffusion coefficient of every particle in the membrane is calculated. Analysed the diffusion coefficient of the NaCl, the result showed that movement of molecule of water was not affected by the kinds of salt molecules in the same membrane.In eight kinds of reverse osmosis membrane, the diffusion coefficient of water molecule in the membranes of HT/MMPD was the biggest. It will be founded that the diffusion coefficient of water molecule in the membrane of TMC-HT/MMPD was higher somewhat than TMC/MMPD which was added TH; Analysis of the water molecule diffusion coefficient in the membrane of TMC/MPD and TMC/MMPD displayed that the water molecule diffusion coefficient in the membrane of TMC/MPD was much bigger than in TMC/MMPD. The same conclusion was obtained by comparing the diffusion coefficient of the molecules of water in the membrane CFIC/MMPD and CFIC/MPD. The diffusion coefficient of TMC/MPD membrane in the system was simulated with the result of 0.0115A²/ps, which was almost inosculated with the existing simulated result 0.015A²/ps for this membrane.The diffusion coefficient of NaCl in nanofiltration membrane was 0.161A²/ps,and the simulation value was closed of it. In a word, the foundation of the model, the method of simulation and the analytic method for the data were effective in the simulation process for this paper.