| Sodium sulfate is a chemical with abundant resources and huge output in China,which is also one of the main products for the resource treatment of industrial wastewater.However,it’s the economy of the producing process is poor due to its low prices.Therefore,the conversion of sodium sulfate to produce potassium sulfate has high economic value and environmental significance.Electrodialysis metathesis is a promising technology in this field,due to its high production purity,convenient operation and ease of control.However,the industrial application data is lacking,and there is no mass transfer model and process simulation for EDM.A pilot experiments on the conversion of sodium sulfate to potassium sulfate by electrodialysis metathesis was carried out in this paper.A mathematical model of this process was established based on EDM.The economic analysis of the whole process was carried out to lay the foundation for its industrial application.A pilot device for continuous electrodialysis metathesis was constructed in this paper,and the effects of current density,raw water concentration and raw water flow rate on the conversion of sodium sulfate to potassium sulfate were explored in detail.The results showed that the increase of current density,raw water concentration and raw water flow rate could effectively increase the ion flux and product concentration.The increase of the raw water concentration and the raw water flow rate will reduce the water flux in the membrane process,while the increase of the current density will increase the water flux.The production corresponding purity of the product potassium sulfate solution was 97%,the maximum concentration was 107.9 g/L,and the energy consumption was 183.7-269.9 k Wh/ton K2SO4.Based on the Nernst-Planck equation,the equations of ion flux and water flux were obtained from the experiment data.A semi-empirical mathematical model of continuous EDM was established and verified.According to the actual needs of the application process,a process of the metathesis electrodialysis-reverse osmosis-evaporation was established,and the influence of the current density and raw water concentration on the economy of the integrated process was explored using the model.The results showed that in the process of constant potassium sulfate raw water concentration,the total membrane area and capital cost of EDM was gradually decreased with the increase of current density,while the total membrane area and energy consumption of RO gradually increased.When the current density was 200 A/m2,the specific cost of the coupling process was the lowest,which was 1059￥/ton K2SO4.With the increase of raw water concentration,the total membrane area of EDM and RO gradually increased,while energy consumption,capital cost,and specific cost were gradually decreased.In the process of constant sodium sulfate concentration and raw solution concentration ratio,the changes of current density and raw water concentration to the total membrane area,energy consumption,asset cost,and unit product cost of EDM in the process were consistent with the constant potassium sulfate process.The exergy analysis of the process with the lowest specific cost showed that the total exergy efficiency of the coupling process was 1.5%.Since the evaporation process was a phase change process,the exergy loss in the evaporation was the largest,accounting for 80.4%of the total exergy loss in the coupling process,followed by the reverse osmosis process,and again the preheater process.Above research results provided effective industrial application data,and was beneficial to realize the optimal design of the metathesis electrodialysis process,which was significance for enterprises to reduce energy consumption and increase profits. |
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