Process Design and Modes of Operation

The efficiency of electrodialysis in a given application depends greatly on the process, design and mode of operation. Two different operating modes are currently used: the first is referred to as unidirectional electrodialysis and the second as elec-trodialysis reversal.

A flow diagram of a typical unidirectionally operated electrodialysis plant is shown in Figure 5. Feed solution pumped into the stack is converted to a diluate and a concentrate which are collected in storage tanks when the desired degree of concentration or depletion is achieved. To prevent the formation of free chlorine by anodic oxidation, the electrode cells are rinsed with a separate solution that

Concentrate recycle Figure 5 Flow scheme of the unidirectional electrodialysis operating mode.

Figure 6 Flow scheme of the electrodialysis reversal operating mode.

does not contain chloride ions. Unidirectionally oper- given by: ated electrodialysis plants are rather sensitive to membrane fouling and scaling and often require careful feed solution pretreatment and stack-cleaning procedures.

Membrane fouling and scaling can be greatly reduced by operating in the electrodialysis reversal mode. In this operating mode, the polarity of the current is changed periodically every few minutes to a few hours. Simultaneously, the hydraulic flow streams are reversed, as shown in Figure 6. The advantage of the electrodialysis reversal operating mode is that precipitates that are formed the concentrate cells are redissolved when the flow is reversed and these cells become the diluate cells. In the elec-trodialysis reversal operating mode there is a brief period when the concentration of the desalted product does not meet the product quality specification. Thus, a certain amount of the product will be lost to the waste stream.

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The required membrane area for a given capacity electrodialysis plant is proportional to the amount of ions removed from a given feed solution and inversely proportional to the applied current density.

As indicated earlier, the applied current density should not exceed a certain limiting value. According to eqn [23] this value is proportional to the diluate concentration and the mass transfer in the boundary layers at the membrane surfaces. The mass transfer depends on the boundary layer thickness, which is a function of flow velocity. For given stack and feed solution properties, the limiting current density is given by:

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