Electrodialysis cells

An electrodialysis cell has the following dimensions (110 cm X 60 cm x 0.04 cm (thickness), and is used to treat water with a throughput velocity of 10 cm/sec. The product concentration is 0.0092 eq/Liter. The cell current efficiency is 0.892. Resistance across the cell is 0.205 ohm. The influent concentration is 125 mg/Liter of NaCl. Calculate the following (a) cell current, (b) cell power output, (c) the cell voltage, and (d) the energy consumption per equivalent of product transferred. 

Figure 14.3 Schematic drawing of an electrodialysis cell. A, Anion-exchange membranes C, cation-exchange membranes.

Let us elaborate upon some of the physical objects mentioned so far as well as on the ionic concentrations and the electric potential field that form in each spatial element of an electrodialysis cell. 

Let us begin to write down the essentials of the above verbal image in terms of a very classical phenomenological model. Consider a mixture of M ionic species i = 1,2, , M fed into an electrodialysis cell from a reservoir where the ionic concentrations C are maintained at the values Cjo (mol/cm3) and the electric potential

cartesian coordinates x (unity vector i) and z (unity vector k) directed, respectively, across the cell and along the cell in the direction of the mean flow, with the origin placed at the symmetry plane at the entrance to the channel. ... 

To illustrate some of the notions introduced so far, let us consider the Ci and p fields in an electrodialysis cell at equilibrium. For simplicity, let us limit our consideration to a 1,1 valent electrolyte at bulk (feed) concentration Co- Assume a constant fixed charge density N(—N) for the an- (cat-) ion membrane. 

Ibanez, J.P., Aracena, A., Ipinza, J., and Cifuentes, L. 2004. Modeling for copper transport within the boundary layer in an electrodialysis cell. Revista de Metalurgia (Madrid, Spain) 40, 83-88. 

Transport of ions in an electrodialysis cell, in which the salt solutions in the chambers formed between the ion exchange membranes are very well stirred, is shown in Figure 10.7. In this example, chloride ions migrating to the left easily... 

Figure 10.7 Schematic of the concentration and potential gradients in a well-stirred electrodialysis cell...

The membranes in an electrodialysis cell are separated by spacer gaskets as indicated in Figure 5.3, which shows schematically the design of a so-called sheet flow electrodialysis stack. The spacer gasket consists of a screen that supports the membranes and controls the flow distribution in the cell and a gasket that seals the cell to the outside and also contains the manifolds to distribute the process fluids in... 

The simultaneous separation and recovery of acidic and basic bioactive peptides by employing electrodialysis with ultrafiltration membranes has also been investigated recently [30]. This work aims at demonstrate the feasibility of separating peptides from a beta-lactoglobulin hydrolysate, using an ultrafiltration membrane stacked in an electrodialysis cell, and a study of the effect of pH on the migration of basic/ cationic and acid/anionic peptides in the electrodialysis configuration. 

Josefsson [5] determined soluble carbohydrates in seawater by partition chromatography after desalting by ion exchange membranes. The electrodialysis cell used had a sample volume of 430mL and an effective membrane surface area of 52cm2. Perinaplex A-20 and C-20 ion exchange membranes were used. The water-cooled carbon electrodes were... 

Electrodialysis — In electrodialysis electrically charged - membranes and an electrical potential difference are used to separate ionic species from an aqueous solution and uncharged components. It refers to an industrial-scale process of electrolyte concentration/depletion due to separation on anion- and cation-exchange membranes under the influence of an electric field. The electrodialysis cell is constructed like a bipolar filter-press electrolyzer, with anion-exchange membranes sandwiched alternately with cation-exchange membranes, see following Figure. 

Electrodialysis is a well-proven technology with a multitude of systems operating worldwide. In Europe and Japan, electrodialysis dominates as a desalting process with total plant capacity exceeding that of reverse osmosis and distillation [3]. Electrodialysis with monopolar membranes is applied to different food systems, to demineralization of whey [5-8], organic acids [9], and sugar [10,11], separation of amino acids [12] and blood treatments [13], wine stabilization [14—16], fruit juice deacidification [17-19], and separation of proteins [20-22]. These applications use the sole property of dilution-concentration of monopolar lEMs in a stack of as many as 300 in an electrodialysis cell. 

As in any electrochemical process, the products to be treated by EDBM must possess a relatively high mineral content to allow a good electrical conductivity to decrease the global resistance of the electrodialysis cell. Moreover, Bazinet et al. [108] demonstrated that the electrical efficiency of skim milk electroacidification is decreased, due to a lack of sufficiently mobile ions such as potassium (Figure 21.17). Consequently, H ions have to migrate across the CEM to ensure the electroneutrality. 

FIGURE 21.20 Electrodialysis cells for inhibiting enzymatic browning in cloudy apple juice, (a) Electroacidification step and (b) electro-alkalinization step. CEM, cation-exchange membrane BPM, bipolar membrane AEM, anion-exchange membrane. 

The basic principle of electrodialysis for desalination is to drive the cations and anions from saline water feeds under the influence of an electric potential gradient through cation- and anion-selective membranes. The electric potential prevents diffusion of oppositely charged ions in the other direction. A schematic of the process is shown in Figure 29.7. In a typical electrodialysis cell to deionize a salt solution, anion- and cation-exchange membranes are arranged alternatively in a... 

The energy consumption of an electrodialysis cell is determined effectively by the cell voltage and the current efficiency. The current efficiency is established by the membrane properties. In practice, the cathodic reaction almost always results in hydrogen evolution while the anodic reaction leads to the evolution of oxygen. The cathodic reaction increases the pH of the solution in this compartment, while the anodic reaction decreases the pH. 

In the endothermic HI reaction, hydriodic acid Hix from Bunsen reaction is concentrated in a number of steps and the resulting hydrogen iodide concentrate is decomposed into reagent iodine and product hydrogen gas. The HI reaction steps appear to have the largest room for process improvement, for which several innovative process techniques have been incorporated in the present flowsheet. The HI concentration steps combine electro-electrodialysis cell and carbonized osmosis membrane to reduce excess iodine and water prior to final distillation. An iodine absorber is integrated into the HI decomposer to improve decomposition ratio in a newly proposed Co- regenerated process ... 

Concentration of HI over Hix solution by polymer electrolyte membrane electrodialysis was investigated using galvanodynamic and galvanostatic polarisation method. For this purpose, Hix solution with sub-azeotrope composition (HI L HjO = 1.0 0.5 5.8) was prepared. It was noticed that the electrical energy demand for electrodialysis of Hix solution decreases with increasing temperature. From the experimental results, it is concluded that the system resistance crucially affects the electrodialysis cell overpotential and hence the optimisation of cell assembly as well as the selection of low resistance materials should be carried out in order to obtain high performance electrodialysis cell. 

Figure 2. displays cell potential measured from the electrodialysis cell as a function of operation temperature. As operation temperature increased from 35 to 90°C, the cell potential decreased from 0.68 to 0.41 V at the applied current of 2 A. This is because high temperatures reduce the amount of electrical energy required to concentrate HI molarity from a thermodynamic standpoint and hence electrical energy demand AG for electrodialysis of Hix solution decreases with increasing temperature... 

From the experimental results, we concluded that electrical energy demand AG for electrodialysis of Hix solution decreases with increasing temperature. Moreover, the electrodialysis cell overpotential is primarily influenced by the system resistance and hence the thermal efficiency of the cell could be improved by reducing the constituent resistance and by optimizing the cell assembly. 

Figure 1. Schematic diagram of (a) electrodialysis cell and (b) experimental arrangements... 

Figure 2. Plots of cell potential against operation temperature measured on the electrodialysis cell at the applied current of 2A. The apparent area of activated carbon electrode was 25 cm ...

Figure 3. Galvanodynamic polarization curves measured on the electrodialysis cell... 

Figure 4. Plots of (a) cell potential and (b) HI molarity against elapsed time measured on the electrodialysis cell at the operation temperature of 90°C. The applied current on the cell remained constant with 2 A throughout the measurement.

Haddad, Laksana and Simons [3] described a device for off-line neutralization of strongly alkaline samples. The method uses an electrodialysis cell comprising three compartments separated from each other by cation-exchange membranes (Fig. 9.2). 

The current density to normality ratio describes the capacity of an electrodialysis cell to pass an electric current. This ratio may vary from 400 to 700 (when the current density is expressed as mA/cm ). If the ratio is too high, regions of low ionic concentration will form near the membranes, which results in polarization. This causes high electrical resistance and a higher electrical consumption. 

D.A. Cowan and J.H. Brown, Effect of turbulence on limiting current density in electrodialysis cell, Ind. Eng. Chem., 1959, 51, 1445-1448. 

V. Perez-Herranz, J.L. Guiiion and J. Garacia-Anton, A new technique for visualization of the concentration boundary layer in an electrodialysis cell, J. Appl. Electrochem., 2000, 30, 809-816. 

In an electrodialysis cell, the solutions flow past the membranes and a continuous velocity gradient extends from the membrane face to the center of the channel. It is customary, however, to describe the situation by a simplified model. It is assumed that there is a completely stationary layer close to the surfaces and... 

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