Calculation of limiting current density by microscopic numerical simulation including porous spacer in an electrodialysis
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The limiting current density is of an important parameter in an electrodialysis system, which is the maximum available current density in electro-dialysis procedures. Therefore, it is certainly beneficial to estimate the value of the limiting current density. In this study, the numerical method for estimating the limiting current density is proposed in an electrodialysis with a spacer, which has an important role to mix the ionic solution in term of mass transfer. Moreover, a set of exhaustive experiments for measuring the limiting current density were conducted in order to examine validation of the proposed method. It was found that the present numerical method agrees well with experimental data.Keywords:
Limiting current
Limiting
Current limiting
The limiting current density is of an important parameter in an electrodialysis system, which is the maximum available current density in electro-dialysis procedures. Therefore, it is certainly beneficial to estimate the value of the limiting current density. In this study, the numerical method for estimating the limiting current density is proposed in an electrodialysis with a spacer, which has an important role to mix the ionic solution in term of mass transfer. Moreover, a set of exhaustive experiments for measuring the limiting current density were conducted in order to examine validation of the proposed method. It was found that the present numerical method agrees well with experimental data.
Limiting current
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Current limiting
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The features of the electrochemical behavior of experimental heterogeneous ion-exchange membranes with different mass fractions of sulfonated cation-exchange resin (from 45 to 65 wt%) have been studied by voltammetry during electrodialysis. Electromembrane systems with 0.01 M NaCl solution and with a mixed 0.01 M NaCl + 0.05 M phenylalanine (Phe) solution have been investigated. A significant influence of the ion-exchanger content on the parameters of current-voltage curves (CVCs) was established for the first time. Electrodialysis of the sodium chloride solution revealed a decrease in the length of the limiting current plateau and in the resistances of the second and third sections of the CVCs with an increase in the resin content in the membrane. The fact of the specific shape of the CVCs of all studied cation-exchange membrane samples in mixed solutions of the mineral salt and the amino acid was established. A specific feature of current-voltage curves is the presence of two plateaus of the limiting current and two values of the limiting current, respectively. This phenomenon in electromembrane systems with neutral amino acids has not been found before. The value of the first limiting current is determined by cations of the mineral salt, which are the main current carriers in the system. The presence of the second plateau and the corresponding second limiting current is due to the appearance of additional carriers due to the ability of phenylalanine as an organic ampholyte to participate in protolytic reactions. In the cation-exchange electromembrane system with the phenylalanine containing solution, two mechanisms of H+/OH− ion generation through water splitting and acid dissociation are shown. The possibility of the generation of H+/OH− ions at the enriched solution/cation-exchange membrane interface during electrodialysis of amino acid containing solutions is shown for the first time. The results of this study can be used to improve the process of electromembrane demineralization of neutral amino acid solutions by both targeted selection or the creation of new membranes and the selection of effective current operating modes.
Ion Exchange Membranes
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Concentration polarization
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The use of currents below and above the limiting one for the treatment by electrodialysis of a solution prepared as a surrogate of a rinse water of the acid zinc electroplating processes was studied. The ionic transport behavior was evaluated by chronopotentiometry. The limiting current values were obtained through current–voltage curves. The trials performed with different current densities showed that the electrodialysis removed at least 98% of the contaminants without insoluble species precipitation on membrane surfaces. The water reuse is possible after electrodialysis treatment with current densities above the limiting one and considering a demineralization rate of 60%.
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Electrodialysis is a promising technology to remove low concentrations of target ions from multi-ionic mixtures. While the synthesis of selective membranes is a prominent topic in research, few studies have been presented on selectivity-enhancing process design. This work investigates the limiting current density as a selectivity promoter in removing dilute target ions from a concentrated solution. Ambiguities and challenges in the prevailing definitions of the limiting current density are identified, and a new approach based on the Nernst equation is proposed, the boundary-layer method. Chloride and fluoride with starting concentrations of 10 mM were removed from 1 M sodium sulfate base electrolyte with varying current density levels around the limiting value. Removal rates, separation efficiencies, and energy consumption were compared. The separation efficiencies between chloride and sulfate and fluoride and sulfate had their highest values at 0.93 and 0.81, respectively, when operating at 130 A/m2. We demonstrate that increasing the ion selectivity through the ion-specific limiting current density is possible and only requires standard current-voltage data. The experimental results suggest that process optimization is an essential supplement to membrane development to enhance the selective removal of target ions by electrodialysis.
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Membrane Technology
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Electromigration
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Reversed electrodialysis
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The main objective of this research was to investigate the performance of electrodialysis using various types of ion exchange membranes and the behavior of the mass transfer phenomena occurring in electrodialysis process for the recovery of citric acid and sodium citrate. An understanding of the membranes properties is a crucial step in their selection for a practical application. Thus, the properties such as thickness, ion exchange capacity, water content, and permselectivity were evaluated. The membranes behavior was also studied in term of current - voltage responses at different current densities for the limiting current density determination. It was found that the limiting current density increases linearly with the increases of feed concentration. At the feed sodium citrate concentration of 15.0 g dm-3, the AFN anion exchange membrane and CMX.SB cation exchange membrane exhibited the highest limiting current density with the value of 7.20 A dm-2 and 8.2 A dm-2, respectively. For performance of electrodialysis using ion exchange membranes in the citric acid and sodium citrate recovery process, the effect of operating parameters such as current density, feed concentration, and water transport was studied. From the experiment results indicated that the AFN anion exchange membrane and the C66-10F cation exchange membrane performed the best for the recovery of sodium citrate with the flux of 4.346 kg m-2 hr-1 and 3.55 kg m-2 hr-1 at the current density of 5.0 A dm-2. The combination of type C66.10F-BP.I-AFN was the best ion exchange membranes for the production of citric acid from sodium citrate. At the current density of 10.0 A dm-2, the highest citric acid concentration was obtained, which was 97.8 g dm-3 with the energy consumption of 12.02 W hr m-2 kg-1. It was also found that the water transport due to the electroosmosis in the electrodialysis process would limit the achievable of the maximum concentration of citric acid and sodium citrate. The highest water transport rate of ion exchange membranes was the membrane type AMI 701-CMI 7000 with the values of 0.168 ml min-1. In the laboratory-scale electrodialysis, the study also demonstrated that over 90% of citric acid and sodium citrate could be recovered and concentrated using electrodialysis approach. The productivity rate achieved for citric acid was from 0.525 kg m2 hr-1 to 0.707 kg m-2 hr-1 for current densities of 2.16 A dm-2 to 4.32 A dm-2. However, the productivity rate of sodium citrate was found to be higher than citric acid, with the value ranges from 1.438 kg m-2 hr-l to 1.587 kg m-2 hr -1 under the range of current densities from 1.72 A dm-2 to 5.17 A dm-2. A limiting current density model was developed in this study and the calculated results were shown in good agreement with the experimental results. After that, a complete ion transport model, based on the description of the different mass transfer phenomena involved in the electrodialysis system, was also developed. The model was able to elucidate the electrodialysis process and determine the operating parameters, which controls the performance of electrodialysis system. The optimum experimental conditions in the electrodialysis system were also evaluated, which was based on the developed ion transport model. Finally, a preliminary pilot scale plant design for the recovery of sodium citrate and citric acid was carried out.
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The electrodialysis with ion-exchange membranes is a very useful method for separating an objective amino acid from mixed amino acids. The values of the limiting current density in the electrodialysis for various kinds of amino acids and pH were measured in this work. It was found that the values of limiting current density decreased with increasing the molecular weight of amino acids, viz., glycine (Gly), alanine (Ala), and arginine (Arg). It was evident that the relationship between the concentration of total ions in the feed solution and the limiting current density showed a straight line which passed through the origin. The value of the limiting current density of amino acids is smaller than that of inorganic ions.
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Alanine
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