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

This paper used molecular Dynamic Simulation method to detailedly study the diffusion process and diffusion coefficient of small molecules in reverse osmosis membrane and water solution, and got some inaccessible results with traditional experimental method. There are mainly four aspects discussed in the paper: (1) Research of molecular Dynamic Simulation method for molecular diffusion coefficients. (2) The simulation of diffusion coefficients of Glycine and other 11 kinds of amino acids in water solution. (3) The simulation of diffusion coefficients of oxygen and other 8 kinds of gas molecules in water solution. (4) The diffusion process and diffusion coefficients of H2O, NaCl, MgCl2, CaSO4, K2SO4 in 8 kinds of reverse osmosis membrane, and relationships of the membranous structure monomer and the diffusion coefficients.1) This paper firstly researched the factors which affect diffusion coefficients in the system from the molecular number sample and simulation time step of the model, and advanced a new analytic method with the Dynamic Simulation data.①This paper simulated diffusion process of Glycine solutions of 0.7mol/L and 0.9mol/L. In the Dynamic Simulation, the models of 0.7mol/L Glycine solution were constructed by adding 3, 5, 7, 10, 12 Glycine molecules into the cell, and 4,7,9,13,15 Glycine molecules were added into the cell of 0.9mol/L Glycine solution models. Comparing the simulation values with the experimental results, the sample number of molecules had some effect on the simulation of diffusion coefficients, which was closer to the experimental results with more molecules added into the cell, and approached a steady value when the molecule number exceeds a line.This paper also simulated the diffusion process of oxygen in water of different temperature, such as 303K and 313K. The models were separately constructed by adding 1,2,3 oxygen molecules into the cell . Simulation values were also found approaching experimental results with more molecules in the cell.②Through comparing the use of traditional analysis method in MSD curves with differentiation interval variation method, this paper advanced a new data analysis method-clustering method, which overcome shortcomings of traditional method used in some MSD curves with bad linear direction and modifies differentiation interval variation method. Using clustering method and differentiation interval variation method separately in the analysis of diffusion coefficient of oxygen in the water, the results of them and the experimental value were separately 2.00×10-9m2s-1,1.80×10-9m2s-1,2.10×10-9m2s-1, which obviously proved clustering method effective.③This paper increased simulation time of 0.1fs time step to 2~3 times longer than before in the simulation of diffusion process in water of methane and ammonia, the results showed the increasing time of simulation did not have obvious effect. Then several different time step between 0.1fs to 0.5 fs were used in the simulation of diffusion process in water of methane, ammonia and carbon dioxide, results showed time step was the effective factor to the diffusion coefficient, which rose to a maximum with the increasing of time step. The diffusion coefficients corresponding to the two time steps close to the maximum were most close to the experimental value. This two time steps were 70 and 150 percent of the time step corresponding to the maximum diffusion coefficient value. Considering the effect of time step in the simulation, this paper advised several time steps between 0.1fs to 0.5 fs should be taken in the simulation of a special system, and took 70 percent of the time step corresponding to maximum diffusion coefficient value as the reasonable value.2) The paper dealt with for the first time the diffusion process of amino acid in the water solution by the way of molecular simulation, at the same time, the diffusion coefficient of amino acid in water was calculated by the method of differentiation interval variation. At present, the method of gaining the diffusion coefficient of amino acid in the water mainly based on the experiments, on the contrary, the way of simulation for the diffusion coefficient of amino acid in the water wasn't published preciously. In this thesis, the diffusion coefficient of amino acid, such as Glycine and other 11 kinds of amino acid, was calculated using the method of molecular dynamic simulation. The error between the simulated results and the experimental results was less than seven percent, and therefore the way of calculating diffusion coefficient by molecular dynamic simulation was proved to be effective.3) In this paper, the diffusion process of oxygen and other 8 kinds of gases in the water solution was simulated by the way of molecular simulation, meanwhile, the diffusion coefficient of gas in the water was calculated by the method of clustering method. The work present in this thesis about the simulation of getting diffusion coefficient of the oxygen, carbon dioxide, methanol and ammonia in the water solution was enough to prove the reliability of the method of molecular simulation, in which the error between the simulated data in this paper and the experimental results was less than 30 percent, however, the biggest error between simulated data in the literature and experimental result was more than 50 percent.4) In this paper, 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, magnesium chloride, calcium sulphate and kalium sulphate ) in the seawater within the membrane were simulated and the diffusion coefficient of every particle in the membrane is calculated.①The 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 0.015 A2/ps for this membrane. 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.②The diffusion state of water molecules in the TMC/MPD indicates that there were obvious interaction between water molecules and Functional groups of the polymer. When the Water molecules were in the field nearing benzene fragments there was a significant repulsion action. On the other way, when water molecules near the amido-group there was an attracting tend movement.③The diffusion coefficient of water molecule in the membranes of HT/MMPD was the biggest in all of the membranes we studied. 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. For the monomer of the membrane MMPD had a methyl substitution in the benzene ring, the rigidity of the membrane MMPD was stronger than MPD when the membranes consist of the same account and scale of the monomer. As a result, this did not benefit the molecules of water transitting through the membrane MMPD. The same conclusion was obtained by comparing the diffusion coefficient of the molecules of water in the membrane CFIC/MMPD and CFIC/MPD.④NaCl molecule had the biggest diffusion coefficient in the membrane of HT/MMPD. Then it took turns to the membrane of ICIC-IPC/MPD, TMC-HT/MMPD, CFIC/MPD , TMC/MMPD, and the least in the membrane of ICIC-HT/MMPD. Only ICIC-IPC/MPD had the bigger difference between simulation result and the experiment result.⑤In eight kinds of membranes we simulated, the diffusion coefficient value of Na+ and Cl- had much difference. The movement of NaCl molecule was restricted by the ions which moves slowly. The kind of the ions which constrained the molecule NaCl was variety in different membranes: in the membranes of TMC/MPD and CFIC/MMPD, the constrain ion was Cl-, while was Na+ in other six membranes.⑥For membrane ICIC-HT/MPD ,the diffusion coefficients of water in the systems containing MgCl2,CaSO4 and K2SO4 were consistent with that in the NaCl system, this result demonstrated that movement of molecule of water was not affected by the kinds of salt molecules in the same membrane. ⑦The diffusion coefficients of anions (Cl-,SO42-) in the reverse osmosis membrane ICIC-HT/MPD were much bigger than cations (Mg2+,Ca2+,K+). This result was consistent with the diffusion coefficient of the anions Cl- and cations Na+ in the NaCl-H2O system, which indicated that diffusion process of salt molecule in the membrane was only restricted by the membrane species,without relationship with salt molecules itself.