Concentration polarization electrodialysis

Electrodialysis can be applied to the continuous-flow type of operation needed in industry. Multi-membrane stacks can be built by alternately spacing anionic- and cationic-selective membranes. Among the technical problems associated with the electrodialysis process, concentration polarization is perhaps the most serious (discussed later). Other problems in practical applications include membrane scaling by inorganics in feed solutions as well as membrane fouling by organics. 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

Figure 10.11 Comparison of the theoretical energy consumption and the actual energy consumption of electrodialysis desalination systems. Most of the difference results from concentration polarization effects [24]...

Most inefficiencies in electrodialysis systems are related to the difficulty in controlling concentration polarization. The second cause is current utilization losses, arising from the following factors [10] ... 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

The efficiency of electrodialysis is determined to a large extent by the properties of the membranes. But it is also affected by the process and system design that determine the limiting current density, the current utilization, the concentration polarization and the overall efficiency and costs [20, 21]. 

The limiting current density is determined by concentration-polarization effects at the membrane surface in the diluate containing compartment that in turn is determined by the diluate concentration, the compartment design, and the feed-flow velocity. Concentration polarization in electrodialysis is also the result of differences in the transport number of ions in the solution and in the membrane. The transport number of a counterion in an ion-exchange membrane is generally close to 1 and that of the co ion close to 0, while in the solution the transport numbers of anion and cations are not very different. 

The concentration polarization occurring in electrodialysis, that is, the concentration profiles at the membrane surface can be calculated by a mass balance taking into account all fluxes in the boundary layer and the hydrodynamic conditions in the flow channel between the membranes. To a first approximation the salt concentration at the membrane surface can be calculated and related to the current density by applying the so-called Nernst film model, which assumes that the bulk solution between the laminar boundary layers has a uniform concentration, whereas the concentration in the boundary layers changes over the thickness of the boundary layer. However, the concentration at the membrane surface and the boundary layer thickness are constant along the flow channel from the cell entrance to the exit. In a practical electrodialysis stack there will be entrance and exit effects and concentration... 

Investment costs in electrodialysis with bipolar membranes Investment costs include nondepreciable items such as land and depreciable items such as the electrodialysis stacks, pumps, electrical equipment, and monitoring and control devices. The investment costs are determined mainly by the required membrane area for a certain plant capacity. The required membrane area for a given capacity plant can be calculated from the current density in a stack that is in electrodialysis with a bipolar membrane not limited by concentration-polarization effects. The required membrane area for a given plant capacity is given by ... 

Electrodialysis as a method of demineralization of natural and wastewater is ecologically expedient and economically profitable. However, the use of ion-selective membranes is complicated by the phenomenon of concentration polarization, which... 

The detailed nature of ion transfer through a system of solutions and ion-exchange membranes warrants thorough discussiou because it is not only the source of the depletion and concentration that occurs in electrodialysis bat also the eource of excessive concentration polarization, which is responsible for most of the difficulties encountered in elect rod ialytic processing. 

Because of these detrimental conseqoences of excessive concentration polarization, the minimization of polarization is an important factor in the design of electrodialysis stacks. 

The phenomenon of concentration polarization is important in electrodialysis, because it ultimately limits the rate at which ion transport can occur. Concentration polarization will be discussed first in a descriptive manner. Then, a simplified mathematical model will be used to show the quantitative aspects of concentrations polarization and explain the important concept of "limiting current density". 

R. Yamane, T. Sata, Y. Mizutani and Y. Onoue, Concentration polarization phenomena in ion-exchange membrane electrodialysis. II. The effect of the condition of the diffusion-boundary layer on the limiting current density and on the relative transport numbers of ions, Bull. Chem. Soc. Jpn., 1969, 42, 2741. 

Figure 6.8 Current-voltage relation during electrodialysis of a sodium chloride solution using an ion exchange membrane (concentration polarization). Ion exchange membrane cation exchange membrane (NEOSEPTA CL-25T) measured under natural convection of a 0.104 N NaCl solution at 25.0 °C.

In connection with these studies, the thickness of the diffusion boundary layer can be directly observed by optical methods such as the Schlieren-diagonal method,7 linear laser interferometry8 and by the change in color of an indicator such as methyl red at the membrane-solution interface.9 Further, the concentration polarization at membrane-solution interfaces in electrodialysis has been experimentally and theoretically analyzed in detail.10... 

B.A Cooke, Concentration polarization in electrodialysis-I. The electrometric measurement of interfacial concentration, Electrochim. Acta, 1961, 3, 307-317. 

V.A. Shaposhnik, VI. Vasileva and E.V Reshetnikova, Concentration polarization of ion-exchange membranes in electrodialysis An interferometric study, Russian J. Electrochem., 2000, 36, 773-777. 

Y. Tanaka, Concentration polarization of ion exchange membrane, J. Membr. Sci., 1991, 57 Water dissociation in ion-exchange membrane electrodialysis, J. Membr. Sci., 2002, 203, 227-244. 

M.E. Green and M. Yafuso, A study of the noise generated during ion transport across membranes, J. Phys. Chem., 1968, 72, 4072-4078 I. Rubinstein. Mechanism for an electrodiffusional instability in concentration polarization, J. Chem. Soc., Faraday Trans. 2. 1981, 77, 1595-1609 F. Maletzki, H.-W. Rosier and E. Staude, Ion transport across electrodialysis membranes in the overlimiting current density range Stationary voltage current characteristics and current noise power spectra under different conditions of free convection, J. Membr. Sci., 1992, 71, 105-115. 

This analysis of individual costs making up the total cost of demineralization by electrodialysis helps to pinpoint the areas needing the greatest research effort. The costs are based on two published cost estimates, revised on a comparable basis for a 2,000,000-gallon-per-day plant. These studies indicate that development of methods to reduce concentration polarization within the compartments offers the best means of reducing the cost of demineralization by electrodialysis, not only because the limiting current density might be increased, but also because of the reduction in resistance that occurs when concentration polarization is eliminated. 

The solutions were 0.01N KC1 or 0.01N NaCl. The membrane was supplied by T.N.O., The Hague, Netherlands. Its exposed area was about 1 sq. cm. and the current density in the different experiments was varied up to 25 ma. per sq. cm. The experiments were carried out at room temperature without stirring. Under these conditions concentration polarization and water splitting may be expected to occur to a larger extent and hence be more readily observed than in a flow compartment of an electrodialysis stack. 

To illustrate the concentration polarization phenomenon, we consider an infinitely long electrodialysis cell pair having parallel channels in which the flow is fully developed and laminar. The qualitative behavior of the development of the salt concentration and potential distributions along the channels of a dialysate and concentrate cell pair are shown schematically in Fig. 6.2.3 for the case where the inlet salt concentrations are the same in both channels (Probstein 1972). 

As any membrane separation process elcctrodialysis is effected by concentration polarization and membrane fouling. The magnitude of concentration polarization is largely determined by the electrical current density, by the cell and particularly spacer design, and by the flow velocities of the diluate and brine solutions [39]. Concentration polarization effects electrodialysis lead to a depletion in the laminar boundary layer at the membrane... 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

Concentration polarisation is not generally severe in dialysis and diffusion dialysis because of the low fluxes involved (lower than in reverse osmosis) and also because the mass transfer coefficient of the low molecular solutes encountered is of the same order of magnitude as in reverse osmosis. In carrier mediated processes and in membrane contactors the effect of concentration polarization may become moderate mainly due to the flux through the membrane. Finally, the effect of concentration polarisation may become ver severe in electrodialysis. In the following sections concentration polarization will be described more in detail. In some module configurations such as plate-and-frame and spiral wound spacer materials are used in the feed compartment (see chapter VIII). These spacers effect the mass transfer coefficient and can be considered as turbulence promoters. 

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