Carbon Fixation By Bipolar Membrane Electridialysis And High-value Utilization Of Sodium Resources

As a new type of mass transfer process,bipolar membrane electrodialysis process（BMED）, =with the advantages of simple operation,high energy utilization rate,less pollution discharge,and simple equipment,is widely used to provide important technical support and guarantee for solving production problems in chemical,environmental,marine,biological and other fields.The massive emission of carbon dioxide（CO₂）leads to the greenhouse effect and causes various environmental hazards.In this paper,BMED was used for carbon sequestration to prepare sodium carbonate and sodium bicarbonate.BMED was also used for carbon absorption and utilization of sodium resources,with the view to prepare concentrated solutions of Na₂CO₃ and Na HCO₃ to accomplish carbon reduction while increasing the utilization value of sodium resources.Firstly,the process parameters of BMED for the preparation of concentrated Na HCO₃solution were optimized.The investigation process took the factors of product concentration,carbon fixation rate,carbon fixation energy consumption and product energy consumption as the investigation indexes.The experimental results showed that when initial concentration of the salt solution increased,the concentration of Na⁺difference on both sides of the membrane increased,so the Na⁺diffusion flux increased and the product concentration of alkali cell increased.The pH value of the alkali cell was the key factor affecting the main ion species and ion ratio in the solution.Changing the pH value can change the content and ratio of HCO₃^-and CO₃^2-in the solution.The current density and CO₂ aeration rate were important conditions to control the pH value of the alkali cell solution.The former regulated OH^-production rate of the bipolar membrane,while the latter controlled the amount of CO₂dissolved.The research results found that the current density increased,the CO₃^2-content in the solution increased,and the energy consumption increased.Reducing the CO₂ aeration rate could increase the pH of the solution and increase the carbon fixation rate.The smaller gas-liquid flow ratio was helpful for ion transmembrane migration,reducing carbon sequestration energy consumption and product energy consumption.Under the optimal operating conditions,the acid-base titration method was used to determine the concentration of HCO₃^-in the alkali cell up to 1.186 mol/L,the carbon fixation rate was 57.65%,the carbon fixation energy consumption was 3.1 k W·h/m³ CO₂,and the solution preparation energy consumption was 716.9 k W·h/t Na HCO₃.The solution production cost was obtained through economic analysis.On the basis of the explanation of the effect of solution pH on the dissolution and presence of CO₂ in the solution and the preparation of a concentrated solution of Na HCO₃,solution of Na₂CO₃ was obtained.By changing the concentration of Na⁺,current density,CO₂ content and gas-liquid flow ratio,the effect of pH change on the concentration of the experimental product and the purity of Na₂CO₃ was studied.The experimental results showed that the difference of Na⁺concentration affected the pH of the alkali cell by changing the migration amount of Na⁺.The higher the current density,the higher the pH of the alkali cell solution,and the higher the concentration and purity of Na₂CO₃.The CO₂ aeration rate directionally regulated the pH range of the solution and affected the ion ratio in the solution.The gas-liquid flow ratio was reduced,the gas-liquid contact was more sufficient,the concentration and purity of CO₃^2-was increased,and the pH of the solution was also increased.The phenomenon of water migration from the salt cell to the alkali cell during BMED reduces the product concentration.Therefore,the preliminary study on the phenomenon of water migration was carried out.The study found that the CO₂ aeration rate and the gas-liquid flow ratio had little effect on the water migration.At the same time,it was proved that the anions in the solution have little effect on the water migration.However,the change of the current density and the initial concentration of the salt solution made the cation migration rate change significantly,resulting in a large change in the water migration.It showed that the water migration phenomenon caused by cations in the solution is dominant.