Electrodialysis efficiency

The ion-permeable membrane properties vital for the electrodialysis efficiency are discussed below. 

Efficient separation or pretreatment in the influent streams can include activated carbon absorption to reduce or prevent such problems. Principal applications of electrodialysis include ... 

An electrodialysis cell has the following dimensions (110 cm X 60 cm x 0.04 cm (thickness), and is used to treat water with a throughput velocity of 10 cm/sec. The product concentration is 0.0092 eq/Liter. The cell current efficiency is 0.892. Resistance across the cell is 0.205 ohm. The influent concentration is 125 mg/Liter of NaCl. Calculate the following (a) cell current, (b) cell power output, (c) the cell voltage, and (d) the energy consumption per equivalent of product transferred. 

Lantagne and Velin [267] have reviewed the application of dialysis, electrodialysis and membrane cell electrolysis for the recovery of waste acids. Because of the new trends governed by environmental pressures, conventional treatment methods based on neutralization and disposal are being questioned. Membrane and electromembrane technologies are considered to be potential energy-efficient substitutes for conventional approaches. Paper mills will focus on the application of ion-exchange membranes namely dialysis, electrodialysis and membrane cell electrolysis for recovery of waste acids. 

In AAC technologies, water is exposed to an AAC material, and metals in the water are adsorbed by the material. AAC systems can be designed and built as stand-alone units or integrated to work efficiently in concert with complementary water treatment systems designed for hydrocarbon removal, pH control, particulate removal, or electrodialysis. AAC systems can tolerate hard water (calcium and magnesium) and high temperatures (up to 200°F) without a decrease in performance. 

In the study described in Christensen et al. (2004, 230), the arsenic content of the wood before electrodialysis was 837 114 mg kg-1 (95 % confidence level) based on 95 samples. The electrodialytic process was more effective if the wood was first broken down into <2 cm chips and soaked in phosphoric acid followed by oxalic acid (Christensen et al., 2004, 236). The soaking probably leaches a significant amount of the arsenic and metals from the wood, which allows the electrodialytic process to remove most of the remaining arsenic, copper, and chromium (Christensen et al., 2004, 235-236). The most efficient results for all three contaminants, which included >95 % removal of arsenic, used 100 kg of wood chips with a 60-cm spacing between the electrodes. The electrodialysis lasted for 21 days (Christensen et al., 2004, 231). 

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],... 

A key factor determining the overall efficiency of an electrodialysis process is the energy consumed to perform the separation. Energy consumption E in kilowatts, is linked to the current I through the stack and the resistance R of the stack by the expression... 

The addition of small amounts of amines and ammonium salts makes it possible to increase selectivities and current efficiencies to about 90 % over a prolonged period, even at Pb cathodes516 517). Recent Japanese applications claim special process modifications, such as the reactivation of the cathodes by pole reversal in alkaline electrolytes 518), special temperature programs during electrolysis519), and working up the discharged electrolysis solution by electrodialysis methods 520). The use of porous cathodes 521) was also proposed. 

Water transport in electrodialysis from the diluate to the concentrate process stream can affect the process efficiency significantly. If a convective flux as a result of pressure differences between flow streams can be excluded there are still two sources for the transport of water from the diluate to the concentrate solution. The first one is the result of osmotic-pressure differences between the two solutions, and the second is due to electro-osmosis that results from the coupling of water to the ions being transported through the membrane due to the driving force of an electrical potential. 

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]. 

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. 

The efficiency of producing hydrogen through a sulphur-iodine process is very sensitive to the HI decomposition process. In Korea, an electrodialysis method was selected to concentrate HI in the solution. The current status of HI concentration and decomposition characteristics with electrodialysis, distillation and HI decomposition catalyst is presented. Another sensitive item of iodine content is also briefly described. A future plan on how to demonstrate a lab-scale SI closed loop is also presented with key issues. 

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. 

Example 8.1 A brackish water of 378.51 m /day containing 4,000 mg/L of ions expressed as Nad is to be de-ionized using an electrodialysis unit. There are 400 membranes in the unit each measuring 45.72 cm by 50.8 cm. Resistance across the unit is 6 ohms and the current efficiency is 90%. COIN to avoid polarization is 700. Estimate the impressed current and voltage, the coulomb efficiency, and the power requirement. 

A brackish water containing 4,000 mg/L of ions expressed as NaCl is to be deionized using an electrodialysis unit. The input power required to run the unit is 93.3 kW. The inflow to the unit is 379 mVday, the coulomb efficiency is 0.78, and 400 membranes are in the unit each measuring 51 cm X 46 cm. The current efficiency is 90%. What is the electric resistance across the unit ... 

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