Functionalized membranes electrodialysis

Electrodialysis equipment and process design The performance of electrodialysis in practical applications is not only a function of membrane properties but is also determined by the equipment and overall process design. As far as the stack design is concerned there are two major concepts used on a large scale. One is the sheet-flow concept, which is illustrated in Figure 5.3 and the other is the so-called tortuous path concept, which is illustrated in Figure 5.5. 

The key element in electrodialysis with bipolar membranes is the bipolar membrane. Its function is illustrated in Figure 5.11(a), which shows a bipolar membrane consisting of an anion- and a cation-exchange layer arranged in parallel between two electrodes. 

As shown in Eqs. (3.9) and (3.10), the concentration of migrating cations is a squared function of the adsorbate concentration in the outer volume of the solution. The Donnan distribution is a source of serious error when determining M of ionic polysaccharides by membrane osmometry. Polyanions may be freed of cations and excess HsO+ by electrodialysis. 

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 ... 

When the removal of alkali metal ions is carried out by electrodialysis in any step of the process, it is essential that the sol-electrolyte contain an alkali metal salt of a nonsilic-eous anion to function as a supplementary elctrolyte, as is disclosed in U.S. Pat. No. 3,668,088 referred to herein above. The purpose of the supplementary electrolyte is to lower the electrical resistance of the sol-electrolyte by providing anions other than silicate and hydroxyl ions to carry the current toward the anode. In the absence of supplementary electrolyte, the current is carried by hydroxyl ions and silicate ions which are present only at very low concentrations in the pH range 8-9.5. Furthermore, when the current is carried by silicate ions, these ions migrate towards the anode and deposit sihca upon the membrane, thus further increasing the resistance and even-tuaUy terminating the process. There is an upper limit to the amount of supplementary electrolyte that can be present. If more than about 0.1 normal sodium salt such as sodium sulfate is present, the colloidal silica particles in the mixture tend to aggregate, even at the preferred pH of 9. 

Membranes have applications in electrochemical separations in the areas of effluent treatment and recycling. Electrodialysis, ED, is a process in which electrolyte solutions are either concentrated or diluted (or deionised). The process has over the years been the dominant technique for the desalination of brackish water. Electrodialysis has many potential applications for the removal or recovery of ionic species and generally the process can be used to perform a number of functions such as ... 

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