Electrodialysis cell Electrolyte

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. 

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. 

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. 

The previous sections dealt with processes of charge transport in electrically neutral electrolytes as applied to electrolytic and electrodialysis cells. Most substances, when at contact with a surrounding water (polar) medium, acquire a surface electric charge due to ionization, adsorption of ions, and dissociation [21, 22]. If the charged surface is placed into an electrolyte solution, then ions of the opposite sign (counter-ions) contained in the solution will be attracted to the surface, while ions of the same sign (co-ions) will be repelled from the surface (Fig. 7.7). 

Voltage and amperage of the electrodialysis cell and calculated electrical resistance of the electrolyte and current density in the electrolyte cell compartment... 

Electrodialysis and electrolytic treatments are providing an important contribution to the recycling and safe disposal of chromium containing wastes. A wide range of process strategies and cell designs have been considered at the laboratory and pilot-plant level. In this section, we will consider a number of industrial-scale processes and devices to illustrate some of the possibilities. 

Aquatech Systems, a business unit of Allied-Signal, Inc., has patented the SOXAL process, which is a process for regenerating the spent scrubbing solution of an alkaline sodium salt scrubber using electrodialysis cell stacks (electrolytic cells with ion-selective membranes) (Byszewski and Hurwitz, 1991). 

Consequently, by placing a neutral solution of the protein hydrolyzate in the central compartment of a multicompartment cell, it can be expected that during electrolysis the monoamino acids will remain, in the central compartment owingto the fact that they exist in the form of dipolar ions dicarboxylic amino acids will migrate into the anode compartment and diamino acids into the cathode compartment. Electrodialysis or electrolytic fractionation of amino acids is based on this principle. 

FIG. 22-56 Schematic diagram of electrodialysis. Solution containing electrolyte is alternately depleted or concentrated in response to the electrical field. Feed rates to the concentrate and dduate cells need not be equal. In practice, there would he many cells between electrodes. 

Electrodialysis can be performed with two main cell types multi-membrane cells for dilution-concentration and water dissociation applications, and electrolysis (or electro-electrodialysis [EED]) cells for oxidoreduction reactions. In multimembrane cells, only the membrane transport phenomena intervene, while electrochemical reactions occurring at the electrodes do not interact with the separation process the electrodes are simple electrical terminals immersed in electrolytes allowing the current transfer. The electrolysis cell operates with only one membrane that separates two solutions circulating in each electrode compartment. This application is based on electrode redox reactions, which are electrolysis specific properties. The anode induces oxidations, and reductions occur at the cathode [4]. 

Membranes for electrodialysis and polymer electrolyte membrane fuel cell (PEMFC) have electric charges. Most of the nanofiltration membranes also carry negative charges. The content of electric charge in a polymer is given by ion-exchange capacity (meq (milliequivalent)/g of dry polymer). 

Electrodialysis involves the use of a selectively permeable membrane, but the driving force is an electrical potential across the membrane. Electrodialysis is useful for separating inorganic electrolytes from a solution, and can therefore be used to produce freshwater from brackish water or seawater. Electrodialysis typically consists of many cells arranged side by side, in a stack. Figure 9.12 illustrates a two-cell stack. 

Example 4. The transport number during electrodialysis (dynamic state transport number) may be measured using a similar two-compartment cell with reversible electrodes such as silver-silver chloride electrodes (Figure 4.6). The transport number is calculated from the transported ions and the amount of electricity passed through the membrane, which is measured with a coulometer. To eliminate the effect on the transport number of electrolytes diffusing through the membrane, it is desirable that a solution of the same concentration be used on both sides of the membrane. When there is a concentration difference between the two sides, the transport number is affected by diffusion of electrolytes through the membrane. 

Ion exchange membranes have been used in various industrial fields, and have great potential for use in new fields due to their adaptable polymer membrane. As mentioned in the Introduction, membranes are characterized mainly by ion conductivity, hydrophilicity and the existence of carriers, which originate from the ion exchange groups of the membrane. Table 6.1 shows reported examples of applications of ion exchange membranes and the membrane species used in various fields. Various driving forces are usable for separation electrochemical potential, chemical potential, hydraulic pressure such as piezodialysis and pervaporation, temperature difference (thermo-osmosis), etc. Of these, the main applications of the membrane are to electrodialysis, diffusion dialysis, as a separator for electrolysis and a solid polymer electrolyte such as in fuel cells. 

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