Molecular Simulation Study On Micro-process And Separation Mechanism Of Membrane Separation

Because of the advantages of high separation precision,low energy consumption,mild condition and easy operation,Membrane separation technology has become one of the most important means of separation.The selection and development of membrane materials is the key to membrane separation.Deep understanding of micro-mass transfer process and mechanism of membrane separation is of great significance to the development of new high-performance membrane materials.In this paper,based on the experimental study of the research group,the molecular simulation of the three membrane separation processes was carried out to explore the membrane material and membrane separation process at the molecular level.Molecular dynamic simulation was used to investigate the process of gasoline desulfurization by pervaporation.The micro diffusion mechanism of diffusion molecule inside polymer membrane was investigated by analyzing the interaction energy between diffusion molecule and polymer membrane,the size of diffusion molecule,fractional free volume?FFV?of polymer membrane,and the mobility of polymer chains.It was concluded that the four factors work together to control the diffusion of molecules in polymer membranes.Due to the interaction energy and the molecular geometrical structure,the presence of olefins and aromatics in the gasoline reduces the desulfurization selectivity,but at the same time facilitates the increase in flux.The results of DPD simulation of membrane swelling show that alkanes,naphthenes,olefins and aromatic hydrocarbons in gasoline can promote the swelling of PEG membrane,among which the aroma is the most significant and the alkane has the least effect.From molecular dynamic insights,we find that PG membranes with appropriate pore size and shape can be applied to separate propylene and propane.100%selectivity of C₃H₆over C₃H₈ can be obtained using pore-11 and pore-12 membranes,but relatively low flux.Surprisingly,N-H modified membranes exhibit extremely superior selectivity for propylene?s=54?while maintaining high permeability.The separation mechanism was investigated from two aspects:molecular adsorption and permeation process.It was found that the PG membranes demonstrate stronger adsorption to C₃H₆ molecules due to the combined effect among VDW and electrostatic interaction,which facilitate their approach to the pores.On the other hand,the N-H modified pore-13 membranes demonstrate an attractive potential energy well toward molecules,but an energy barrier to propane molecules during the pore penetrating process.And this will surely make it much easier for C₃H₆ to enter and pass through the pore.Intrinsically,it is the difference of electron overlap between gas molecules and the membranes that cause the energy difference.The cross-linked polyamide structure was successfully constructed by molecular dynamics method.Based on this,the micro-mechanism of reverse osmosis was studied.The results show that the salt ions are present in the form of hydrated ions in the aqueous solution due to the strong interaction force.The radius of the salt ions is larger than that of the polyamide.Therefore,under the pressure difference,the water molecules can pass through the polyamide membrane and the salt ions are trapped.The increase in salt concentration not only increases osmotic pressure,but also reduces the rate of diffusion of water molecules.Due to the interaction with the polyamide film,the water molecules spontaneously diffuse from the solution to the membrane surface,and the increase of the carboxyl group and the carbonyl group in the membrane material is beneficial to improve the hydrophilicity of the membrane.