| Nowadays,water resources problems such as lack of fresh water resources and water pollution have become increasingly prominent,which has increasingly restricted the sustainable development of human society.The desalination of seawater or brackish water has become an inevitable choice for solving the water crisis and opening up new safe and environmentally friendly water sources.At present,the desalination technologies commonly used in the world have multi-stage flash(MSF),multi-effect distillation(MED),reverse osmosis(RO),electrodialysis(ED),etc.,but these technologies have high energy consumption or high cost or secondary pollution and other issues.Therefore,the development of low-energy,simple operation,and environmentally friendly desalination technology has become the focus of attention.Compared to other traditional desalination technologies,Capacitive Deionization(CDI)technology is energy efficient(operating at low voltages,eliminating the need for high pressure pumps,heat pumps,heaters or high DC voltages),environmentally friendly(no chemicals,no Secondary pollution)and other advantages.In recent years,researchers have built inorganic salt,metal oxides,organic small molecules and polymer-functionalized carbon-based composite electrode materials,and supplemented with acid-base activation,nitrogen doping,heat treatment and other processes to prepare CDI electrode materials,making The gradual improvement of CDI performance demonstrates its technical superiority.In this paper,for the seawater desalination process of(membrane)capacitive deionization,the multi-physics coupling problem analysis in the unit electrode is taken as the object.Using numerical simulation method and COMSOL Multiphysics software,combining the mass,momentum,porous Darcy seepage,charge conservation equation and coupling adsorption kinetic equation,a three-dimensional transient analysis model of the seawater desalination process of(membrane)capacitive deionization was established.On the basis of verifying the reliability of the analytical model,the performance influencing factors and influence rules of the multi-physics of the flow field,ion concentration field and electric field in the unit electrode during the seawater desalination process of membrane capacitance method were analyzed by simulating the complete"adsorption-desorption"process in the unit,then the effects of different plate voltages,inlet concentrations,inlet flow rates,plate thicknesses,flow path spacings and film thicknesses on the adsorption and desorption processes,and compared with the capacitive deionization.After the mechanism analysis of membrane/capacitance desalination unit is completed,the middle inflow type membrane capacitance desalting module is further explored.The main research work is as follows:(1)According to the relevant knowledge of MCDI/CDI,the geometric model,physical model and mathematical model of desalting unit in MCDI/CDI module were established.Linear regression was performed for the results of MCDI simulation under different voltages by using the quasi-first order and quasi-second order adsorption kinetic equations.Thermodynamic fitting was performed on the results of MCDI simulation under different concentrations by using the Langmuir and Freundlich equations of adsorption thermodynamics.The results show that the multi-physics coupled numerical analysis model established in this paper can more accurate description of the MCDI/CDI seawater desalination process,suitable for numerical simulation of the unit electrode desalination process in MCDI/CDI devices.(2)Using COMSOL software to simulate the complete"adsorption-desorption"process in a desalination unit of MCDI/CDI module,study and analyze the variation of flow field,ion concentration field and electric field with time and the distribution of each domain in the unit.The results show that:In the MCDI unit,the impedance at the boundary layer of the membrane diffusion is reduced by the increase of the ion concentration near the membrane,so that the thickness of the diffusion boundary layer formed at this point is correspondingly reduced,resulting in an increase in the fluid flow velocity near the membrane boundary.During the adsorption process,the MCDI unit relies on the membrane that restrain the common-ion effect,its internal diffusion adsorption takes the adsorption maximum time as the boundary,and presents the characteristics of first slow then fast,and it can adsorb more ions than CDI.During desorption process,the polar plate surface ion concentration of MCDI unit is higher than CDI,and the CDI unit is in the disadvantage that the large amount of ions in it that diffused into the solution is attracted by the electrode on the opposite side,resulting in the amount of ion desorption of the MCDI unit polar plate is greater than CDI.The direction of the electric field force and the direction of the surface potential of cation exchange membrane and the move direction of the sodium ions are in the same direction,which makes the effect of potential superimposition,lead to the potential distribution of MCDI(t=118 s)in the flow channel region(except the flow channel region is 0.04 mm away from the negative polar plate)is generally higher than that of CDI(t=92 s),and the difference is gradually obvious.The influence of MCDI/CDI desalination unit on the desalination performance under different parameters was also analyzed and compared,and the internal physics coupling mechanism and the influencing factors affecting seawater desalination during MCDI/CDI desalination were revealed.Finally,through parameter optimization analysis,put forward a MCDI/CDI desalting unit the best operation parameter optimization scheme:considering the choice of1.5 V,500 mol/m~3,0.10 m/s,0.08 mm,0.8 mm and 0.02 mm as the optimum plate voltage,inlet concentration,inlet flow rate,plate thickness,flow path spacing,and film thickness.(3)Based on the above work,a three-dimensional analysis model of the middle inflow type-MCDI desalting module was established for the capacitor desalination device component,the influences of various parameters(plate voltage,inlet aperture,inlet concentration,inlet flow rate,plate thickness,and flow path spacing)on the desalination process of the desalting module under two different desorption methods(short connection and reverse connection)were analyzed and compared in detail,and the change rules of the concentration,absorption efficiency and desorption efficiency with time.The results show that:The outlet minimum concentration of MCDI desalting assembly decreases with the increase of plate voltage,and increases with the increase of inlet aperture,inlet flow rate and flow path spacing;The higher the inlet concentration is,the greater the concentration difference between MCDI desalting assembly and the lowest outlet concentration is.The thicker the plate thickness is,the slower the rising rate of the outlet concentration of MCDI desalting assembly is;The adsorption efficiency of the desalting assembly increases with the increase of plate voltage,and decreases with the increase of the inlet aperture,inlet flow rate and flow path spacing,regardless of the inlet concentration and plate thickness;In the desorption phase,the maximum outlet concentration increases with the increase of plate voltage,inlet concentration and plate thickness,and decreases with the increase of inlet aperture,inlet flow rate and flow path spacing;The desorption time increases with the increase of plate voltage,inlet aperture and inlet concentration,and decreases with the increase of inlet flow rate,and has nothing to do with plate thickness and flow path spacing;The desorption efficiency of the desalting assembly increases with the increase of plate voltage,inlet aperture and plate thickness,and decreases with the increase of inlet flow rate and flow path spacing,regardless of inlet concentration.And under the effect of additional reverse voltage,the reverse desorption method under the different condition parameters to achieve the highest outlet concentration were higher than the short method,the next cycle of adsorption in reverse method reached the lowest concentration were linked to the value of the previous cycle were similar,the peak of desorption efficiency in reverse method and the speed and time which droped after reaching the peak was superior to that in short. |
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