Simulation Of Solution Structure And Mechanism Of Process Control In Membrane Distillation Crystallization

The global water shortage was one of the key problems that restrict human development.Among them,the treatment and reuse of high concentration salt containing wastewater has been highly concerned.In the research and development of high concentration brine treatment technology,membrane distillation crystallization technology has received extensive attention because of its low energy consumption and high equipment integration.In recent years,in the study of membrane distillation crystallization,the single component salt solution was usually used to simulate wastewater to reduce the variables in the experiment.However,the solution structure and ionic interactions of multi-ion systems were complicated,which led to the lack of clarity on the mechanism of membrane and crystallization process.At the same time,the composition of high concentration brine produced in the actual production and life was often more complex,so it was very important to explore the solution structure and revealed the ion interaction in the solution for further promoting the study of membrane distillation crystallization process.This thesis studies the solution structure of Na⁺/Mg²⁺-Cl^-and Na⁺/Ca²⁺-Cl^-which was a typical high concentration brine system.The membrane distillation crystallization method was used to treat the corresponding multi-element high concentration brine.The mechanism of the solution structure and ion interaction in membrane flux,membrane fouling and membrane crystallization was also studied.Firstly,the Na⁺/Mg²⁺-Cl^-and Na⁺/Ca²⁺-Cl^-solution models were constructed by molecular dynamics simulation.The interaction and hydration structure of different ions in the solution were studied by molecular dynamics simulation.The results shown that the coordination number of Mg²⁺-Cl^-was significantly higher than that of Na⁺-Cl^-in Na⁺/Mg²⁺-Cl^-solution system,while in Na⁺/Ca²⁺-Cl^-solution,although Ca²⁺and Mg²⁺were double electron ions,the coordination number of Na⁺-Cl^-was higher than that of Ca²⁺-Cl^-due to the larger ion radius of Ca²⁺.In terms of hydration structure,Ca²⁺and Mg²⁺have a much more stable hydration layer structure than Na⁺,so the introduction of Ca²⁺/Mg²⁺would lead to the reduction of free water molecules and the decrease of water activity in the corresponding solution system.At the same time,because of the high charge density and complex coordination structure of Mg²⁺,the excess Mg²⁺would lead to the increase of the space potential resistance,thus reducing the diffusion coefficient of Na⁺in the solution.These simulation results were of great significance in the study of the distillation performance and crystallization process of the membrane.Furthermore,the effects of solution composition and temperature on membrane distillation flux and crystal products were studied by experimental research.The results of vacuum membrane distillation crystallization treatment of high concentration Na⁺/Mg²⁺-Cl^-solution system shown that with the increase of Mg²⁺,the flux of the membrane decreased slightly,the flux decay slows down,and the surface defects of the crystal increase.This was because the addition of Mg²⁺hindered the formation of initial nucleus of Na Cl,reduced the diffusion ability of Na⁺,which led to the deviation of growth diffusion equilibrium in the formation of Na Cl crystal.The increase of temperature would lead to the increase of membrane flux,increase of crystal growth rate,and increase of particle size and surface defects of crystal products.As comparison,the experimental method of membrane distillation crystallization was also used to treat the highly concentrated Na⁺/Ca²⁺-Cl^-solution system.The mechanism of the process regulation of membrane distillation crystallization was analyzed by the change of the structure and operating conditions of the solution through the membrane flux and the properties of the crystallization products.The results shown that the flux of the membrane would also decrease slightly with the increase of Ca²⁺,because the introduction of Ca²⁺/Mg²⁺would lead to the decrease of water activity and the reduction of the driving force of mass transfer in the membrane distillation process.However,the introduction of Ca²⁺would not lead to defects on the crystal surface,but make the inner cavity appear in Na Cl crystal products.Because the coordination number of Ca²⁺-Cl^-was lower than that of Na⁺-Cl^-,and the addition of Ca²⁺did not prevent the formation of initial nucleus of Na Cl.The increase of Cl^-in solution might even lead to the increase of Na Cl nucleation,this result shown that the Ostwald ripening effect appears in the process of crystal formation,and finally forms hollow crystal,which may cause the crystal purity obtained by the process to decrease.In conclusion,the study explored the solution structure of Na⁺/Mg²⁺-Cl^-and Na⁺/Ca²⁺-Cl^-systems by molecular dynamics simulation,and revealed the regulation mechanism of solution composition in the process of membrane distillation crystallization,which has important guiding significance for the design and optimization of the membrane distillation crystallization process of complex solution.