Electrodialysis unit-cell

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. 

The process operates at current densities of about 1 kA/m2 and unit cell voltage of 1.5 V. The specific energy consumption is about 2 kWh/kg NaOH. Under the influence of the electric gradient the H + and OH ions emerge on opposite faces of the membrane. Bipolar membrane electrodialysis is being developed by several companies, e.g. WSI Technologies Inc. [270] and Aquatech Systems [129,275,276], Typical product specification ranges for the ICI electrodialysis process is summarized in Table 19. 

Determine the area and power required to demineralize 4000 m3/d of treated wastewater to be used for industrial cooling water using an electrodialysis unit composed of 240 cells. Assume the following data apply ... 

Because one equivalent of a substance is equal to one equivalent of electricity, in electrodialysis calculations, concentrations are conveniently expressed in terms of equivalents per unit volume. Let the flow to the electrodialysis unit be Q . The flow per deionizing compartment or cell is then equal to QJ ral2). If the influent ion concentration (positive or negative) is [C ] equivalents per unit volume, the total rate of inflow of ions is [C ]2o/(m/2) equivalents per unit time per cell. One equivalent is also equal to one Faraday. Because a Faraday or equivalent is equal to 96,494 coulombs, assuming a coulomb efficiency of r, the amount of electricity needed to remove the ions in one cell is equal to 96,494[C ](2ot7/(ni/2) coulombs per unit time. Coulomb efficiency is the fraction of the input number of equivalents of an ionized substance that is actually acted upon by an input of electricity. 

If time is expressed in seconds, coulomb per second is amperes. Therefore, for time in seconds, 96A C ]Q r KmJ2) amperes of current must be impressed upon the membranes of the cell to effect the removal of the ions. The cells are connected in series, so the same current must pass through all of the cells in the electrodialysis unit, and the same 96,494[C ](2ot7 (ni/2) amperes of current would be responsible for removing the ions in the whole unit. To repeat, not only is the amperage impressed in one cell but in all of the cells in the unit. 

A recent innovation by a U.S. manufacturer of ED systems has significantly minimized the need for pretreatment. This innovation, known as electrodialysis reversal (EDR), operates on the same basic principle as the standard electrodialysis unit, except that both the product and the brine cells are identical in construction. At a frequency of 3-4 times/hr, the polarity of the electrodes is reversed and the flows are simultaneously switched by automatic... 

In the examples following in which the invention is demonstrated in whole or in part by electrodialysis, the electrodialysis unit is a cell pair comprised of a single coated titanium base anode common to two cells in the following sequence ... 

The AMJ Electro Cell System is an electrodialysis unit with a tubular design claimed to be free of leaks and more efficient than the conventional rectangular stack design. This is a two-compartment system which according to AMJ can be adapted to make the three-compartment cell required by our sodium silicate electrodialysis process to make colloidal silica. 

AMJ offered to build a 15 square foot unit guaranteed to perform as they claim which we could test for 30 to 60 days with their help. The cost of the electrodialysis (ED) cell as such would be around 18,000. Fifty percent would be paid on delivery and the balance would be paid only if the cell performs as guaranteed. 

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

Stack design in bipolar membrane electrodialysis The key component is the stack which in general has a sheet-flow spacer arrangement. The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. As indicated in the schematic diagram in Figure 5.10 a repeating cell unit in a stack with bipolar membranes is composed of a bipolar membrane and a cation- and an anion-exchange membrane and three flow streams in between, that is, a salt... 

The second typical technology applied for d. of water is -> electrodialysis. After appropriate pretreatment (as above), the feed solution is pumped through the unit of one or more stacks in series or parallel. The concentrated and depleted process streams leaving the last stack are recycled, or finally collected in storage tanks. The plants operate unidirectionally, as explained, or in reverse polarity mode, i.e., the current polarity is changed at specific time intervals (minutes to hours), and the hydraulic flow streams are reversed simultaneously, thus preventing the precipitation in the brine cells. 

But the conventional membrane setup described is only one possibility of a sodium sulfate electrolysis. Other proposals include the use of an ODC or even an HDA (Hydrogen Depolarized Anode) for cell voltage reduction or the use of anion exchange membranes instead of the cation variety [7]. Last but not least electrodialysis may be used in a three compartment unit the salt is converted into free acid and the free base [40]. 

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]. 

Concentration of Sodium Sulfate Solutions. A concentration unit is always necessary for both two- and three-compartment cells when the feed consists of dilute sodium sulfate solutions, because the cell voltage increases rapidly when the sodium sulfate concentration is <10%. The methods available are reverse osmosis, electrodialysis, and evaporation. With the present average European prices of steam and electric energy, evaporation seems to be the best method for concentrating up to 20% diluted feed solutions having a salt content around 3-5%. 

The electrodialysis cell is described in the following section and the rest of the components of the unit in Appendix C. 

---