| Desulfurization wastewater from thermal power plant was the high salinity wastewater,which had the characteristic of complicated composition and instability of water quantity and quality.Bipolar membrane electrodialysis(BMED)had been considered as one of the most effective next-generation membrane separation technologies.It could be used to generate the acid and base by means of the corresponding saline ions.Therefore,BMED hold a great promise in the practical application of environmental protection and resource reclamation.In this paper,we primarily investigated the feasibility of BMED for treating the pretreated desulfurization wastewater.Evolution of acid and base concentration under feeding-bleeding(FB)mode,mechanism of base concentration reduction,acids transfer through anion exchange membrane(AEM),stabilization of base concentration and optimization of acid concentration and base energy consumption were emphasically proposed and discussed.The primary research contents and conclusions were summarized as follows:(1)Variation trend of acid and base concentration under the FB mode was simulated by establishing a mathematical model.Simulation results indicated that acid and base concentration could ultimately reach the stable state with its concentrations positively and negatively related to current density and feeding-bleeding rate,respectively.Besides,either initial volume or initial concentration could not influence the final equilibrium value except for equilibrium time.In order to investigate the reliability of this model,Na2SO4 and NaCl solutions were selected for real verification in BMED operation.Experimental results showed that acid concentration change was highly consistent with the simulation results.However,the evolution of base concentration increased first and then declined after reaching the maximum.Through the investigation of conductivity,pH,Na/H concentration ratio and base current efficiency,it could demonstrate the acid leakage from acid compartment was the main reason resulting in the decrease of base concentration.Therefore,controlling the suitable Na/H concentration ratio could efficiently eliminate the effect of acid leakage from acid compartment on base concentration.In the case of NaCl and Na2SO4 solutions,the critical Na/H concentration ratio for holding the base concentration stabilization was 14 and 6,respectively.(2)A mathematical model was established to describe the acid leakage process on AEM through determination of HCl and H2SO4 concentration and diffusion coefficient on AEM and combining Nernst-Planck equation and electric neutrality principle.The adsorption results suggested the acid adsorption on AEM were much higher than that calculated from Donnan mode,the Gibbs free energy of-1.10 kJ·mol-1 for HCl and-0.49 kJ·mol-1 for H2SO4 indicated acids adsorption on the AEM were spontaneous process and both the acids and AEM showed high affinity.It could be found the volume proportion of gel phase was far outweigh interstitial phase,which accounted for 96.6%of AEM,implying the acid in AEM was mostly concentrated in the gel phase.According to analysis of the diffusion coefficient,it could be found the diffusion coefficients of both Cl-and SO42-were lower than H+and the ratio of Cl-(SO42-)and H+ were 0.22(0.46).Through the mathematical model calculation,the results indicated the model value was in a good agreement with experimental value,and with the increase of acid concentration the differences of which would decrease gradually.(3)Two systems for elimination of acid leakage on the influence of base concentration were proposed through controlling the Na/H concentration ratio,i.e.pH adjustment in the three compartment configuration(System 1)and addition of extra AEM forming four compartment configuration(System 2).For System 1,the experimental results showed when the pH in the salt compartment was around to 2.0,the base concentration could keep stability.With regard to System 2,the objective could be easily achieved with the salt 2 concentration set to the initial value without any concentration.Compared to apparent current efficiency(CEapp)from both Systems,it could be found there were no obvious differences in CEapp for acid and base production.Nevertheless,as for energy consumption(EC),the apparent energy consumption(ECapp)for acid and base production from System 2 was always higher than from that of System 1.Whereas,System 2 presented a certain superiority for absolute energy consumption(ECabs)of base production,the required ECabs value was about 88%of the System 1.On the other hand,in comparison to System 1,System 2 could further shorten the operation time,increasing production efficiency.(4)The response surface methodology(RSM)using Box-Behnken design was employed to establish the quadratic polynomial model to optimize acid concentration(AC)and base EC.According to analysis applying Design Expert software,the optimized operating parameters were obtained which values were initial salt concentration ×3,current density 40.3 mA·cm-2 and feeding-bleeding rate 10.8 ML·min-1.Verification test under this condition showed experimental value of 0.99 mol·L-1 for AC and 2.24 kWh·kg-1 NaOH for base EC had nearly no significant difference with the predicted value of 1.0 mol·L-1 for AC and 2.22 kWh kg-1 NaOH for base EC,proving the quadratic polynomial model possessed high accuracy for prediction of AC and base EC.By evaluating the economic of BMED process,it was concluded the total energy cost and total fixed cost were 0.236 $ kg-1 NaOH and 0.866 $·kg-1 NaOH,respectively.With the characterization of ion membranes by means of SEM and FTIR techniques,it could be confirmed there was no membrane fouling discovered throughout the BMED running. |
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