Study On Concentration Polarization Characteristics Of Reverse Osmosis For Desalination

With the rapid development of our society and national economy,the demand for freshwater resources for industrial production in coastal areas has been significantly increased.Seawater desalination technology is an effective way to solve this problem.Among various existing seawater desalination technologies,reverse osmosis technology dominates.In the reverse osmosis process,water pass through the reverse osmosis membrane,and impurities such as salt are trapped by the membrane,gradually forming a concentration layer on the membrane surface,which is called concentration polarization.Concentration polarization is a major factor in reducing membrane performance and increasing the energy consumption.Fully understanding and accurately predicting concentration polarization is crucial for membrane design,performance prediction,and membrane pollution prediction,which has strong theoretical research value and engineering guidance significance.The Lattice Boltzmann Method has gradually become an effective tool for studying reverse osmosis processes due to its advantages of simplicity and ease of implementation in dealing with complex systems.However,when dealing with solute mass transfer,the small diffusion coefficient will lead to computational instability.Because of such problems,this paper first established a numerical model that is more suitable for reverse osmosis processes,then simulated the reverse osmosis solution flow and solute mass transfer processes,explored the formation mechanism and characteristics of concentration polarization,and finally conducted the transmembrane reverse osmosis experiment,discussed the actual factors affecting membrane performance.The main research contents are as follows:(1)Based on the Lattice Boltzmann Method,a numerical model capable of simultaneously solving reverse osmosis solution flow and solute mass transfer was constructed,and a new method was proposed to avoid computational instability caused by the small diffusion coefficient.The boundary conditions that can achieve membrane permeability characteristics was constructed.(2)The empty channel was simulated to analyze the formation process of concentration polarization and its influencing factors.The results show that concentration polarization is gradually formed along the membrane,and its thickness is about one tenth of the height of the membrane flow path.Concentration polarization is affected by velocity,transmembrane pressure difference,and solution concentration.(3)The spacers were added into the empty channel to analyze the impact of spacer and its type on the characteristics of solution flow characteristics and solute mass transfer.The results showed that the spacer can increase the local flow rate and promote solute mass transfer.The impact of spacer layout,size,and geometry on reverse osmosis varies.The stronger the inhibition effect of spacer on concentration polarization,the more membrane pollution and energy consumption loss.(4)Based on the transmembrane reverse osmosis experimental platform,the actual effects of different operating conditions and concentration polarization on membrane performance were analyzed.Based on experimental data,a prediction model for the removal of NaCl molecules by reverse osmosis membranes was established and its accuracy was verified.The results show that operating pressure and solution concentration can affect the effective transmembrane pressure difference on both sides of the membrane,thereby affecting membrane performance.Eliminating or weakening concentration polarization can significantly improve membrane performance.