Mechanism And Experimental Study On Solution Regeneration By Electrodialysis

Solution regeneration is an important energy consuming process in liquid desiccant air dehumidification and heating tower heat pump systems.The traditional thermal regeneration methods have the disadvantages of high energy consumption,liquid carry-over and high environmental dependence.Electrodialysis is a solution regeneration technology with energysaving potential.Compared with the traditional thermal regeneration method,electrodialysis has many advantages,such as operating at room temperature,low dependence on the environment,and avoiding liquid carry-over in air.In recent years,electrodialysis has attracted the attention of many researchers.However,there is a lack of understanding of the theoretical mass transfer limit and energy consumption limit of electrodialysis solution regeneration,and the actual performance and applicability of electrodialysis solution regeneration are still unclear.In view of the above problems,this paper studies and evaluates the theoretical mass transfer limit,energy consumption limit,experimental performance and applicability of electrodialysis solution regeneration by combining theoretical analysis,experimental test and numerical simulation,which provides theoretical and experimental guidance for electrodialysis solution regeneration technology.Secondly,from the perspective of thermodynamics,the theoretical minimum energy consumption of electrodialysis solution regeneration is analyzed and calculated.It is found that the theoretical minimum energy consumption of electrodialysis solution regeneration is only related to the initial and end states of the solution.The higher the initial concentration of the solution and the larger the regeneration ratio,the greater the theoretical minimum energy consumption of the regeneration process.The principle of minimizing energy consumption in the regeneration process of electrodialysis solution is described.The loss of irreversible work in the regeneration process can be reduced by reducing the driving force and using multi-stage system.Based on the concept of theoretical maximum heat utilization ratio for thermal regeneration process,a thermodynamic method for comparing the actual energy efficiency of electrodialysis and thermal regeneration process is proposed.These studies reveal the energy consumption limit of electrodialysis solution regeneration from a theoretical point of view.Thirdly,from the experimental point of view,the effects of different factors on the mass transfer performance of electrodialysis are studied.It is found that the initial concentration has a significant effect on the salt transport number and the salt hydration number,but the current density,volume ratio,flow rate and flow mode have no significant effect on them.On this basis,the theoretical and system concentration characteristic curves are proposed.The results show that the concentration performance of electrodialysis under experimental conditions is significantly lower than the theoretical performance.In high concentration,multi-stage electrodialysis process is required to achieve the target concentration.The thermodynamic efficiency of electrodialysis and thermally driven solution regeneration process is compared.On the premise of separating liquid water,the actual thermodynamic efficiency of multi-stage electrodialysis system is lower than that of thermal regeneration system at higher concentration.It was also found that the salt transport number and the free water content decreased significantly at high concentration,while the resistance of ion exchange membrane increased significantly due to membrane shrinkage.According to the salt transport number,free water content and membrane resistance in the experiment,the suitable concentration range for electrodialysis solution regeneration is analyzed.The results show that the performance of electrodialysis in treating salt solution in the lower concentration range of heating tower heat pump system is acceptable,so it has a certain application potential in solution regeneration in heating tower heat pump system,However,the performance decreases significantly when dealing with liquid desiccants with high concentration,indicating that it has little application potential in solution regeneration in liquid desiccant air dehumidification system.These studies reveal the applicable concentration range of electrodialysis solution regeneration from an experimental point of view.Finally,from the perspective of numerical simulation,different simplified theoretical models are used to calculate the concentration distribution in the electrodialysis channel,and the concentration polarization degree of the membrane surface under different conditions is compared.The results showed that the higher the current density and the higher the solution concentration,the lower the concentration polarization on the membrane surface.For the design of multi-stage process system,the simplified model can be used for calculation to make the solution concentration meet the design requirements at the outlet of membrane stack.The results also show that the central solution concentration along the electrodialysis channel hardly changes with the channel position,while the solution concentration on the membrane surface gradually increases with the channel direction.In addition,it is also found that the transverse convective mass transfer flux at the cross section of the channel can not be ignored compared with the transverse diffusion mass transfer flux,otherwise the calculated concentration polarization degree of the membrane surface will be higher.These studies are helpful to understand the concentration polarization phenomenon in solution regeneration by electrodialysis.