Electrodialysis limiting current density

The value of /jim is determined by the discontinuity in the dependence of cell current on applied cell voltage which occurs when the interfacial concentration approaches zero. The polarisation parameter is convenient in the design and scale-up of electrodialysis equipment. It can be easily measured in small-scale stacks at a given value of bulk concentration and then used to predict limiting current densities in larger stacks at other concentrations. Most stacks use operating values of the polarisation parameter that are 50-70 per cent of the limiting values. 

This limiting current, /liin, is the maximum current that can be employed in an electrodialysis process. If the potential required to produce this current is exceeded, the extra current will be carried by other processes, first by transport of anions through the cationic membrane and, at higher potentials, by hydrogen and hydroxyl ions formed by dissociation of water. Both of these undesirable processes consume power without producing any separation. This decreases the current efficiency of the process, that is, the separation achieved per unit of power consumed. A more detailed discussion of the effect of the limiting current density on electrodialysis performance is given by Krol et al. [20],... 

The limiting current density for an electrodialysis system operated at the same feed solution flow rate is a function of the feed solution salt concentration, as shown in Equation (10.17). As the salt concentration in the solution increases, more ions are available to transport current in the boundary layer, so... 

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. 

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. 

Since the thickness ofthe laminar boundary in an electrodialysis stack is difficult to determine in an independent measurement, the limiting current density in practical application is generally not calculated by Equation 5.18 but by an experimentally determined relation which describes the limiting current density as a function to the feed-flow velocity in the electrodialysis stack [4]. The limiting current density is expressed by ... 

Because of the relatively high concentrations of the acid and base as well as the salt solution the limiting current density is in general no problem and a bipolar membrane stack can generally be operated at very high current densities compared to an electrodialysis stack operated in desalination. However, membrane scaling due to precipitation of multivalent ions such as calcium or heavy-metal ions is a severe problem in the base-containing flow stream and must be removed from the feed stream prior to the electrodialysis process with a bipolar membrane. 

Enoch et al. [90] used electrodialysis reversal (EDR) to prepare boiler makeup water for Dutch power stations from several types of surface waters. EDR uses automatic reversal of electrode polarity at regular time intervals to minimize membrane scaling. The EDR unit contained 200 anion and cation exchange membrane pairs, each with a surface area of 0.47 m. Polarity reversal occurred every 15, 20, or 25 min. Samples of surface water were desalted by 96% at an energy consumption of 1 kWh/m of product water and at a current density (8.3 A/cm ) that was 80% of the limiting current density (current density when the surface water cation concentration at the membrane surface drops to zero). 

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. 

D.A. Cowan and J.H. Brown, Effect of turbulence on limiting current density in electrodialysis cell, Ind. Eng. Chem., 1959, 51, 1445-1448. 

T.-C. Huang and L.-Y. Yu, Correlation of ionic transfer rate in electrodialysis under limiting current density, J. Membr. Sci., 1988, 35, 193-206 L.J. Mas, P.M. Pierrarad and PA. Prax, Behavior of an electrodialysis unit cell, Desalination, 1970, 7,285-296. 

H. Miyoshi and K. Kataoka, Estimation of the limiting current density in electrodialysis with both spacer and space, Sep. Sci. Technol., 1989, 24, 507-513. 

H. Miyoshi, Flow distribution, concentration distribution, and limiting current density in electrodialysis with ion-exchange membranes, Rec. Res. Develop. Chem. Eng., 1998, 2, 57-65. 

The cost data of Katz and Volckman were used as a basis for illustrating the approximate effects of membrane polarization, limiting current density, cost, operating life, and resistance of membranes, and temperature on the over-all costs of demineralization of water with electrodialysis. The major objective was to pinpoint the problem areas needing greatest research effort and to show the probable effect of solving these problems upon the cost of demineralization. 

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. 

Electrodialysis as a unit operation is determined by several process and equipment design parameters, such as feed flow velocities, cell and spacer construction, stack design etc. These parameters effect the costs of the process directly and also indirectly by means of the limiting current density and the current utilization [33]. 

The limiting current density determines the minimum membrane area required to achieve a certain desalting effect. Another very important parameter for the overall performance of the electrodialysis process is the current utilization. The current utilization determines the portion of the total current that passes through an electrodialysis stack that is actually used to transfer ions from a feed solution. The current utilization which is always less than 100 % is affected by three factors [38] (1) The membrane selectivity, (2) osmotic and ion-bound water transport, and (3) current passing through the stack manifold. 

In electrodialysis, there are three categories of costs those that increase with current density (the cost of energy) those that decrease with current density (the cost of membrane replacement and amortization of capital investment) and those that are essentially invariant with current density (cost of chemicals, maintenance, and labor). Because two of these categories of costs vary in opposite directions with current density, there is an economically optimum current density. However, for most applications of electrodialysis, the limiting current density is lower than the ofXimum current density. Therefore, determination of the limiting current density in by the previously described methods is usually the first step in design. 

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