| At present,reverse osmosis seawater desalination technology is developing rapidly,and the installed capacity and processing capacity continue to increase.However,the cost problem has always restricted the development of seawater desalination industry.Reverse osmosis membrane is the core component of seawater desalination system.Most of the research on membrane permeability characteristics focuses on the fields of membrane preparation process and membrane modification.How to reduce the concentration polarization caused by transmembrane reverse osmosis and improve water production is still an important issue.In addition,the flow of water in the reverse osmosis membrane is affected by the nanoscale effect,and the permeability of the membrane cannot be fully explained by its microstructure.Therefore,it is of great significance for the development of industrial seawater desalination to establish the calculation and analysis method of multi-scale transmembrane reverse osmosis characteristics,study the dynamic characteristics of reverse osmosis(RO)membrane,and master the correlation between water production,desalination rate and driving pressure difference.As a mesoscopic numerical method based on statistical physics,lattice Boltzmann method(LBM)is widely used in the simulation of confined water flow because of its natural parallelism and easy handling of boundary conditions.Therefore,this paper systematically studies the permeability characteristics and influencing factors of reverse osmosis membranes through experiments and numerical simulations.The main contents are as follows:Firstly,the permeability of reverse osmosis membrane was tested by transmembrane seepage experiment platform.The transmembrane reverse osmosis characteristics of pure water and brine under different driving pressures,the influence of ionic strength on reverse osmosis water flux and desalination,and the difference of salt ion type transmembrane reverse osmosis were studied.The results show that:(1)The permeability of the reverse osmosis membrane does not change with pressure,but varies with the concentration and type of salt solution.This is mainly because the resistance of the convection diffusion process generated by the concentration polarization layer is different.When the salinity rises from 0 to 35,the permeability loss is as high as 86%;(2)Different from the water flux,the rejection rate is almost not affected by the concentration polarization,which is basically maintained at 97%.This is mainly related to the dense separation layer of the reverse osmosis membrane and the peak-valley structure of the membrane surface characterized by scanning electron microscopy.(3)Increasing magnetic stirring can weaken the concentration polarization layer,thereby increasing the water flux of transmembrane reverse osmosis while maintaining a high rejection rate.When the salinity is 35,the water flux is increased by about 7.2%.Secondly,the physical model of single-layer reverse osmosis membrane was constructed,and the numerical method of channel flow was verified.Based on the lattice Boltzmann method,the influence of nano-scale effect on the permeability of reverse osmosis membrane was numerically studied,and the flow difference of water in reverse osmosis membrane with different hydrophilic and hydrophobic materials was compared and analyzed.The results show that:(1)The fluid-solid contact angle is an important factor affecting the permeability of the reverse osmosis membrane.The membrane structure with porosity of 0.436 and 0.594,the contact angle 120° is 3.57 times and 2.75 times higher than the average water flow velocity of 30°,respectively.(2)The membrane permeability increases exponentially with the increase of contact angle.In the case of extreme hydrophobicity,the water flux is increased by 5 times compared with that without considering the nano-scale effect,and the contribution of nano-scale effect to permeability is as high as 96%.(3)The nano-scale effect decreases with the increase of pore size,and the relative flow capacity of large pores and small pores in the membrane is closer when hydrophobic,which is why the traditional Hagen-Poiseuille equation cannot be used to describe the flow law of reverse osmosis membrane.Finally,the physical model of the gradient reverse osmosis membrane was constructed based on the real gradient membrane structure,and the correlation between the permeability of the reverse osmosis membrane and its microstructure was analyzed by combining the nanopore scale flow characteristics of the previous chapter.The results show that:(1)The gradient membrane can improve the water flux of the membrane under certain conditions.Structurally,it is because the porous sublayer has a larger porosity or a smaller specific surface.The specific surface of the porous sublayer decreases from 0.24nm-1 to 0.11nm-1,the permeability of the hydrophobic membrane increases by 2.22 m D,the permeability of the hydrophilic membrane increases by1.79 m D,the porosity of the porous sublayer increases from 0.425 to 0.612,the permeability of the hydrophilic membrane increases by 0.7m D,and the permeability of the hydrophobic membrane increases by 1.12 m D;(2)The thickness of the membrane can be increased by increasing the porosity of the porous sublayer without sacrificing the water flux,which will make the thick film have good permeability and show better mechanical strength;(3)The structure of the membrane can be characterized by a characteristic factor γ.The smaller the γ,the greater the permeability of the membrane.When γ decreases from 2.14 to 1.51,the permeability of the hydrophilic membrane and the hydrophobic membrane increases by 22% and 31%,respectively.The permeability of the membrane is more sensitive to the hydrophobicity of the membrane skeleton.In this paper,the external factors affecting the flow of transmembrane reverse osmosis and the internal factors causing the difference of hydraulic characteristics of reverse osmosis are revealed from experiments and simulations,respectively.The method of improving the water flux of transmembrane reverse osmosis is proposed,which provides a new structure-permeability insight for the development of high-performance reverse osmosis membranes,and helps to reduce the cost of industrial desalination water. |
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