Analyses On Diffusion Process Of H₂O And Salts In Molecular Sieve Membranes By Molecular Simulations

With the rapid development of quantum mechanics, molecular mechanics andcomputer technology, the molecular simulation technique had been widely applied inmany modern fields of scientific research, and played a more and more important role.Molecular sieve membrane had a widely application perspective in the field ofdesalination, owing to its unique physical, chemical properties and porous structures.The molecular simulations were adopted to investigate the diffusion process of waterand salts in molecular sieve membranes, which can simulate the molecular structuresand behavior at molecular level, and understood the diffusion mechanism of smallmolecules in molecular sieve membrane from microcosmic point of view.In this paper, the diffusion process of H2O and salts in molecular sievemembranes and its’ composite membranes was investigated by GCMC and MDsimulation method. Four kinds of typical molecular sieve membrane (MFI, LTA, SOD,C168) and5kinds of MFI molecular sieve composite membrane (MFI/PASF) withcompound ratio (r=13,9,5,4,3), NaCl, KCl, MgCl2, CaCl2and water moleculeswere studied.1、The diffusion process of H2O and salts in conventional molecular sievemembranesBy caculation the adsorption isotherm of H2O in the MFI, LTA, SOD, C168fourkinds of molecular sieve membrane with GCMC simulation method, the saturatedadsorption quantity of H2O in membrane was obtained.Then, the diffusion process of H2O and salts in MFI, LTA, SOD, C168werestudied by MD simulation method. The results showed that the diffusion coefficientsof H2O and salts decreases with increasing of the concentration of salt solution withinmembrane, which showed that the simulative results were in accordance with those ofthe experimental results. For the same salt concentration, the rate of the diffusioncoefficients of H2O and salts increased with increasing temperature. The diffusion process of H2O in molecular sieve membrane was relatively stable, its MSD curvewas linear related to time. For the same concentration under different salt solution, thediffusion coefficient of H2O and Cl-were not influenced by cation, and cationicmovement was greatly influenced by its radius and valence.2、The diffusion phenomenon of H2O and salts in molecular sieve compositemembrane (MFI/PASF)By caculation the adsorption isotherm of H2O in five different composite ratio (r=13,9,5,4,3) molecular sieve composite membrane (MFI/PASF) with GCMCsimulation method, the saturated adsorption quantity of H2O in membrane wasobtained. And then, by analysis adsorption density distribution and energies of H2O incomposite membrane found that the adsorption of H2O in the system decreases withdecrease of the compound ratio, and tend to be absorbed into the part of MFI.Next, the diffusion phenomenon of H2O、Na+and Cl-in five different compositeratio were studied by MD simulation method. The results showed that the diffusioncoefficients of H2O and salts decreases with increasing of composite ratio. Thediffusion displacement of H2O、Na+and Cl-in the composite membrane were obtained.The results showed that the diffusion of molecules in molecular sieve compositemembrane was jumping.