Electrodialysis solution resistivity

Ion-exchange membranes also show some promise in the solution of waste problems, inter alia the treatment of spent pickle liquors by electrodialysis is discussed (80). A very high degree of deionization can be achieved with ion-selective membranes. P. Cohen investigated the electrodialysis of simulated pressurized water reactor coolant (34). The specific resistance of the treated water was as low as 0.5 to 3,0 Mi cm. 

Exceeding the limiting current density in practical applications of electrodialysis can affect the efficiency of the process severely by increasing the electrical resistance of the solution and causing water dissociation, which leads to changes of the pH values ofthe solution causing precipitation of metal hydroxide on the membrane surface. 

Another approach to achieve purification of rinses and recovery in one step, electrodialysis has been suggested for chromic acid recovery and removal of metallic impurities [108]. As the authors point out there are two main process limitations first, the poor stability of most anion-exchange membranes against the oxidative chromic acid solution and secondly the increase in membrane resistance due to the formation of polychromates in the membrane. 

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. 

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. 

The data in Figure 8.16 show the results of an experiment to determine the values of i jm. Those data were obtained from electrodialysis of a solution of NaCl in a stack containing ten cell pairs.21 The applied voltage, current, and pH of the depleted product water were monitored. A control experiment with one cell pair in the stack was used to determine the voltage drop attributable to the electrodes and rinse streams. The cell-pair resistance, Rcp = (Vapp - V ec) x Acp/I, was observed to increase slightly for moderate increases in the polarization parameter, i/N. [The polarization parameter is defined as the current density, i = l/Acp, divided by the log-mean concentration of the depleting stream, N = (C n - Cout)/ln(Cjn/Cout).]... 

Figure 3.17 Organic fouling of an anion exchange membrane. Electrodialysis was carried out using 0.05 N sodium chloride solution containing sodium dodecylbenzene sulfonate (100ppm) at a current density of 3.5 mA cm 2 at 25.0 °C. (Membrane strongly basic anion exchange electrical resistance measured in 0.05 N sodium chloride solution 3.5 Q cm2).

Figure 3.18 Change in electrical resistance of an anion exchange membrane (strongly basic anion exchange) with and without anionic polyelectrolyte layers in the presence of sodium tetradecyl sulfate (STS). 1. without the layers and with STS 2. with the layers (immersion time 4 h) and with STS 3. with the layers (immersion time 24 h) and with STS 4. with the layers and without STS 1 left vertical axis 2,3 and 4 right vertical axis. After an anion exchange membrane had been immersed in 100ppm anionic polyelectrolyte (polycondensation product of sodium naphthalene sulfonate and formaldehyde MW ca. 1000) solution for the respective time at room temperature, electrodialysis was carried out at a current density of 2.5 mAcmr2 using 0.10 N sodium chloride solution containing 2.16 X 10 3 mol dm3 of STS.

Table 3.3 Current efficiency in electrodialysis of hydrochloric acid solution and electrical resistance of anion exchange membranes and composite membranesa...

Figure 5.5 Transport properties of a cation exchange membrane having a cationic polyelectrolyte layer formed by electrodeposition. (A) PNaCa ( ) current efficiency (%) ( ) electrical resistance of the membrane during electrodialysis for 1 h. After solutions containing 0.0416N sodium chloride and poly(3-methylene-N, N-dimethylcyclohexylammonium chloride) of various concentrations had been electrodialyzed, for 60 min at a current density of 10 mA cm 2, as anolyte to electrodeposit the polyelectrolyte on the membrane surface (catholyte was 0.0416N sodium chloride), a 1 1 mixed solution of 0.208N calcium chloride and 0.208 N sodium chloride was electrodialyzed at a current density of 10 mA cmr1 for 60 min (cation exchange membrane NEOSEPTA CH-45T).

Figure 5.39 Change in transport numbers of bromide, nitrate and sulfate ions relative to chloride ions in anion exchange membranes reacted with various amines. ( ) trimethylamine (H) ethylenediamine and then trimethylamine until electrical resistance of the membrane attained was ca. 10 Qcm2 (2 h) (3) tetraethyle-nepentamine and then trimethylamine until the resistance attained was ca. 10 Qcm2 (32 h) ( ) polyethyleneimine and then trimethylamine until the resistance attained was ca. 10 Qcm2 (64 h). The membranes were immersed in 1.0 N hydrochloric acid solution for 2 h before electrodialysis and PaA was measured by electrodialysis of 1 1 mixed salt solutions (concentration of sodium ions 0.04 N) at 1.0 mA cm 2 at 25.0 °Cfor 60 min.

A combination of equations (6) and (7) gives the energy consumption in electrodialysis as a function of the current applied in the process, the electrical resistance of the stack, i.e., the resistance of the membrane and the electrolyte solution in the cells, the current utilization, and the amount of ions removed from the feed solution ... 

The gaskets not only separate the membranes but also contain manifolds to distribute the process fluids in the different compartments. The supply ducts for the diluate and the brine are formed by matching holes in the gaskets, the membranes, and the electrode cells. The distance between the membrane sheets, i.e. the cell thickness, should be as small as possible to minimize the electrical resistance. In industrial size electrodialysis stacks membrane distances are typically between 0.5 to 2 mm. A spacer is introduced between the individual membrane sheets both to support the membrane and to help control the feed solution flow distribution. The most serious design problem for an electrodialysis stack is that of assuring uniform flow distribution in the various compartments. In a practical electrodialysis system, 200 to 1000 cation- and anion-exchange membranes are installed in parallel to form an electrodialysis stack with 100 to 500 cell pairs. 

Electrodialysis (ED) is used to remove ionized substance from hquids through selective ion-permeable membranes. ED is the most widely commercialized electromembrane technology. Desalination of brackish water is the area of electrodialysis application with the largest number of installations. This chemical-free technology competes with reverse osmosis. Electrodialysis shows better resistance to fouling and scaling. It also has an economical advantage in desalination of low-salinity solutions [13]. Also, it should be kept in mind that because of small material consumption ED is the most environmental friendly process for solution desalination [14]. 

There are several common elements in the design of any electrodialysis stack end blocks, end frames with electrodes, membranes, spacers between membranes, and manifolds for inlet and outlet of fluids. The electrodes used in an electrodialysis stack must withstand the electrochemical reactions and the solutions that circulate within the electrode chambers, as well as to the electrolytes, which are carried there due to electrodialysis and form as a result of electrolysis. Residue must not be formed inside the electrode chambers, while the chamber itself must be thin enough to not create too much resistance to the electric current... 

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