A Molecular Dynamics Simulation And Performance Manipulation Of Heat And Mass Transfer Processes In Porous Membranes For Desalination

Permeable membrane has been widely applied in various fields of nature and industrial production.Because of its capability in efficient separation,it is often used in the fields of fossil raw material mining,building material insulation,agricultural product drying,catalytic reaction and water treatment.Therefore,the study on the mechanism of heat and mass transfer of the membrane material is conducive to promoting the development of industry and agriculture,saving resources and benefiting mankind.Currently,the research on membrane materials is mostly focused on the modification of composite materials using experimental methods.Nevertheless the molecular level heat and mass transfer mechanisms inside the membrane are still not clear,which limits the development of the membrane materials.In this study,taking the membranes used in humidification-dehumidification as an example?PVDF/Si O₂ and GO-PVP/PVDF membrane?,the heat and mass transfer inside the membranes are studied by MDS?Molecular Dynamics Simulation?.The multi-scale resistance models of PVDF/Si O₂ and GO-PVP/PVDF are used to link the transport properties in the solid part with the parameters in the macroscopic porous part to obtain the performance of the whole bulk membrane,which can be directly evaluated experimentally.The atoms and functional groups in the bulk and interfaces are adjusted to manipulate the performance of membrane.The main works are summarized as followed:?1?For PVDF/Si O₂ composite membrane,the molecular models of Si O₂ are constructed based on crosslinking algorithm using orthosilicic acid monomers.At the same time,the PVDF/Si O₂ interface models with different functional groups?hydroxyl and methyl?and different proportions?40%,60%,80%,100%hydroxyl?are constructed.Then the reverse non-equilibrium molecular dynamics?RNEMD?simulation and mean square displacements?MSD?are applied to calculate the thermal conductivity and analyze the moisture diffusivity of material,respectively.To provide more physical insights,the vibrational density of states?VDOS?at the material interfaces are calculated.Minimun matching coefficient at the interface when the content of hydroxyl group is 58%,which means that the thermal conductivity is the smallest.?2?For GO-PVP/PVDF membrane,four different interfaces?GO-PVP,GO-PVDF,PVDF-PVP,GO-GO?are constructed and their effects on the heat and mass transfer performance of the whole composite membrane are studied.Combining MDS with a macro-scale resistance-in-series/parallel model for the bulk performance,the effects of four kinds of interfaces,different GO ratios?17%,50%,67%,80%?,different functional groups?-O-,-OH,ODA?on the GO surface and the degree of GO oxidation on the heat and mass transfer performance of the whole composite membrane are evaluated.Based on the density functional theory?DFT?,the energy barriers of water molecules through different GO's six-membered ring are calculated.The values of the energy barriers for different functional groups follow the order of graphene?41.6e V?>-O-?37.2e V?>-OH?30.8e V?>ODA?27.6e V?.A novel GO membrane where water molecules can penetrate directly through the pore of graphene oxide is fabricated.Moisture diffusivityis increased by 38%.?3?In order to build a surface with real roughness and increase the computational domain and time scale,a method of"template"based on dissipative molecular dynamics?DPD?to built simulation models is proposed.The Fast Fourier Transformation and Johnson transform system are used to obtain the coordinates of each bead on the rough surface.The rough parameters obtained from real surfaces,including the Skewness?Sk?,Kurtosis?K?and the Standard deviation?R_q?.?4?The coarse-grained model of GO-PVP/PVDF is constructed,and the force field of each bead is established according to the relationship between the conservative force parameters and the parameters of Flory-Huggins.Based on the molecular simulation theory at the mesoscopic scale,the dispersion characteristics of GO in PVP,the effects of rough interface on the thermal conductivity and moisture diffusivity of composite membrane,as well as the the flow characteristics of water molecules in nano channel of bulk materials are all studied.The heat and mass transfer properties of the PVDF membrane under deformation are also analyzed.In summary,the heat and mass transfer mechanism of membrane materials used in seawater desalination are investigated through the combination of experiments and MDS on macroscopic,mesoscopic and microscopic scales.The research methods and results of this paper provide theoretical guidance and suggestions for the preparation of new permeable membranes with low thermal conductivity and high moisture permeability.