Electrodialysis, defined

Potable Water RO and NF both play a major role in providing potable water, defined either by the WHO criterion of <1000 ppm total dissolved solids (TDS) or the U.S. EPA limit of 500 ppm TDS. RO is most prominent in the Middle East and on islands where potable-water demand has outstripped natural supply. A plant awaiting startup at Al Jubail, Saudi Arabia produces over 1 mVs of fresh water (see Table 22-17). Small units are found on ships and boats. Seawater RO competes with multistage flash distillation (MSF) and multieffect distillation (MED) (see Sec. 13 Distillation ). It is too expensive to compete with conventional civil supply (canals, pipelines, w ls) in most locations. Low-pressure RO and NF compete with electrodialysis for the desalination of brackish water. The processes overlap economically, but they are sufficiently different so that the requirements of the application often favor one over the others. 

The membranes in electrodialysis stacks are kept apart by spacers which define the flow channels for the process feed. There are two basic types(3), (a) tortuous path, causing the solution to flow in long narrow channels making several 180° bends between entrance and exit, and typically operating with a channel length-to-width ratio of 100 1 with a cross-flow velocity of 0.3-1.0 m/s (b) sheet flow, with a straight path from entrance to exit ports and a cross-flow velocity of 0.05-0.15 m/s. In both cases the spacer screens are... 

In a hydrodynamically free system the flow of solution may be induced by the boundary conditions, as for example when a solution is fed forcibly into an electrodialysis (ED) cell. This type of flow is known as forced convection. The flow may also result from the action of the volume force entering the right-hand side of (1.6a). This is the so-called natural convection, either gravitational, if it results from the component defined by (1.6c), or electroconvection, if it results from the action of the electric force defined by (1.6d). In most practical situations the dimensionless Peclet number Pe, defined by (1.11b), is large. Accordingly, we distinguish between the bulk of the fluid where the solute transport is entirely dominated by convection, and the boundary diffusion layer, where the transport is electro-diffusion-dominated. Sometimes, as a crude qualitative model, the diffusion layer is replaced by a motionless unstirred layer (the Nemst film) with electrodiffusion assumed to be the only transport mechanism in it. The thickness of the unstirred layer is evaluated as the Peclet number-dependent thickness of the diffusion boundary layer. 

An isoionic solution is defined as a solution which contains only protein ions of average charge Z, ions, and OH ions. No other ions may be present. An isoionic solution can be prepared by means of a mixed-bed ion-exchange column (Dintzis, 1952), or by electrodialysis, and only for proteins which are soluble in salt-free water. (Refer to Section 111, B). 

Gering and Scamehorn [86] studied the removal of CdCl2 and CdS04 from water by electrodialysis using a platinum-coated columbium anode and a Hastelloy cathode. Ionics (Watertown, MA) cation and anion exchange membranes were used in the electrodialysis stack. The effective cell pair area, defined as half the area of... 

The state-of-the-art in membrane techniques is shown in the figure ( via[8])where the advances in individual membrane operations are defined in terms of availability, service reliability, and price warranty. Research on the first group of methods primarily covers process optimization problems. From the data included in the figure, it is obvious that by far the most advanced methods are the long established membrane processes such as dialysis, microfiltration, ultrafiltration, hyperfiltration, reverse osmosis and electrodialysis. It is also evident that the dynamics of sale shows a tendency to decrease. 

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).]... 

In the electrodialysis of a mixed salt solution, preferential permeation of specific ions is interesting in academic studies and important in practical application. The preferential permeation of ions is defined as the permselectivity between two ions or the transport number of A ions relative to B ions. Details are given in Chapter 5. 

Montonnet et al. (1994) did a great deal of work on defining the conditions for using electrodialysis to stabilize wine, identifying suitable membranes and process control so that each wine is treated according to its specific level of instability. This has made electrodialysis technology reliable and effective, while avoiding excessive alterations in the wine s chemical composition. 

Electrodialysis. Electrodialysis - is effective to remove formate ions, sulfate ions, and carbonate ions which are by-products of reactions defined in Eqs. (31.1), (31.2),... 

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