Study On Fluid Transport Characteristics In Nanochannel Of Active Layer Of Forward Osmosis Membrane

Forward osmosis,which is a new concentration-driven membrane separation technology,is widely used in water treatment,power generation,food processing,medical and other fields as it has advantages of low energy consumption,low membrane fouling and high water recovery,compared with other traditional membrane separation technology.The decline of water flux and water production quality caused by membrane fouling,reverse solute diffusion and concentration polarization becomes a bottleneck for the application of forward osmosis.Membrane fouling,reverse solute diffusion and concentration polarization are closely linked with the complex transport of water molecules and ions in forward osmosis membrane.Therefore,the study on the transport mechanism of water molecules and ions in forward osmosis membrane nanochannels contributes to providing a theoretical guidance for the further development of forward osmosis.Due to the confinement effect,the fluid mass transfer behavior in the forward osmosis membrane nanochannels is very different from that in the macroscopic condition,and it is difficult to experimentally observe and study the structure and dynamic behavior of fluid in the nanochannels.The study of fluid transport characteristics in forward osmosis membrane nanochannels requires the use of molecular dynamics simulation method combined with statistical thermodynamics and software was used to explore its physical mechanism.In this thesis,carbon nanotube was used to construct forward osmosis membrane nanochannel.Molecular dynamics simulation was used to study the transport of fluid in forward osmosis membrane nanochannels.The structure and dynamic characteristics of water molecules in the nanochannels was analyzed.The selective transport mechanism of forward osmosis membrane ions was revealed.In this thesis,molecular dynamics simulation was used to simulate fluid transport in different sizes of forward osmosis membrane nanochannels which are carbon nanotubes,and the effect of pore size on fluid transport was analyzed.It is found that water molecules exhibit different structural characteristics in the nanochannels due to the confinement effect,and the arrangement of water molecules in the nanochannels changes with the increasing of pore size.Different structures lead to different dynamic behaviors of water molecules in the nanochannel.By calculating the diffusion coefficient and potential of mean force of water molecules in the nanochannel with different pore sizes,it is found that the water molecules in the channel at 10.85（?） diffuse the fastest,and that the potential of mean force of water molecules entering the channels decreases with the increasing of pore sizes.The ion rejection ratio becomes 100% when the pore size is less than 10.85 （?）.By analyzing the potential of mean force and hydration radius of ions entering the nanochannels with different sizes,it is revealed that the physical mechanism of ion interception by forward osmosis membrane is the size selectivity of ion hydration diameter.Through calculating water flux and ion rejection ratio,10.85（?） is determined to effectively separate water and ions and maintain high water flux.Due to the different preparation conditions and methods of forward osmosis membrane,the properties of the membranes differ greatly.The characteristics of fluid transport across membranes are related to the size of nanochannels,and the hydrophilicity of forward osmosis membranes.In this thesis,the transport behavior of fluid through nanochannels of the forward osmosis membrane under different hydrophilic conditions was simulated.The influence of hydrophilicity and hydrophobicity of forward osmosis membrane on the transport performance of fluid was studied.It is found that increasing the hydrophilicity of forward osmosis membrane is not likely to the diffusion of water molecules and the interception of ions.The structure characteristics,dynamics and resistance of water molecules in nanochannels are affected by membrane hydrophilic,when membrane hydrophilic,wall adsorption of water molecules and the density of water in the channel increase.Water molecules transform to a highly ordered structure similar to ice through hydrogen bond.This structure intensifies the friction between water molecules and the pore wall and reduces the water flux.Due to the increasing hydrophilicity of the membrane,the adsorption effect of the forward osmosis membrane on water molecules is enhanced,and the constraint of water molecules on ions near the membrane pores is reduced.Thus,ions pass through the channel more freely,the mass transfer rate of the ions is improved,and the ability of the membrane to reject ions is reduced.In this thesis,the study of fluid transport characteristics in active layer nanochannels of forward osmosis membrane is helpful to further understand the fluid transport process in forward osmosis membranes,and is of great importance in theoretically guiding the preparation of high efficient forward osmosis membranes.