Electrodialysis current efficiency

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. 

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 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. 

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 ... 

Xue et al. [94] used electrodialysis to purify and recover spent alkaline process streams containing potassium hydroxide, lithium hydroxide and potassium carbonate using nickel-plated steel cathodes and a stainless steel anode. A potential of 3 - 5 V was required at operating temperatures of 80-105 °F (26.7-40.6 °C) to obtain current densities between 170 and 200mA/cm with current efficiencies of 75-60%. 

The energy consumption of an electrodialysis cell is determined effectively by the cell voltage and the current efficiency. The current efficiency is established by the membrane properties. In practice, the cathodic reaction almost always results in hydrogen evolution while the anodic reaction leads to the evolution of oxygen. The cathodic reaction increases the pH of the solution in this compartment, while the anodic reaction decreases the pH. 

Figure 3.15 Transport number of hydrogen ions relative to sodium ions (PNaH) and current efficiency of cations to the composition of an amphoteric ion exchange membrane. (O, ) quarternized membranes (N-methyl pyridinium and sulfonic acid groups) (A, A) tertiary amino groups membrane (pyridinium hydrochloride and sulfonic acid groups) (, A) PNa 11 (O, A) current efficiency (%). Electrodialysis was carried out at a current density of 20 mA cm 2 using a mixed solution of 0.25 N sodium chloride and 0.25 N hydrochloric acid for 60 min at 25.0 °C.

Table 3.3 Current efficiency in electrodialysis of hydrochloric acid solution and electrical resistance of anion exchange membranes and composite membranesa...

Because there is a concentration difference across the ion exchange membrane in practical electrodialysis, the transport number (current efficiency) in the presence of the concentration difference is required. When the current efficiency (dynamic state transport number) is measured in the presence of a concentration difference, the current efficiency in the absence of the concentration difference may be basically obtained by subtracting the amount of electrolyte diffusing through membrane from the measured current efficiency. In all cases, when the transport number is measured, the solutions in both compartments should be vigorously agitated to eliminate the effect of diffusion boundary layers on the transport number. 

When an ion exchange membrane is used in electrodialysis, one side of the membrane contacts a dilute solution and the other a concentrated solution. There is a diffusion flux of electrolyte and non-electrolyte of low molecular weight through the membrane. Because the diffused amount directly affects current efficiency and the purity of products, the diffusion coefficient of electrolytes through membranes is important in practical applications. The following simplified method may be used to determine the coefficient after one side of the membrane has contacted a concentrated solution and the other side a dilute solution (in some cases, pure water) for a given period, the amount of electrolytes diffused through the membrane into the dilute solution is determined, and the diffusion coefficient is calculated by the Fick equation. 

Figure 5.5 Transport properties of a cation exchange membrane having a cationic polyelectrolyte layer formed by electrodeposition. (A) PNaCa ( ) current efficiency (%) ( ) electrical resistance of the membrane during electrodialysis for 1 h. After solutions containing 0.0416N sodium chloride and poly(3-methylene-N, N-dimethylcyclohexylammonium chloride) of various concentrations had been electrodialyzed, for 60 min at a current density of 10 mA cm 2, as anolyte to electrodeposit the polyelectrolyte on the membrane surface (catholyte was 0.0416N sodium chloride), a 1 1 mixed solution of 0.208N calcium chloride and 0.208 N sodium chloride was electrodialyzed at a current density of 10 mA cmr1 for 60 min (cation exchange membrane NEOSEPTA CH-45T).

Figure 5.18 Change in transport number of potassium ions relative to sodium ions in the presence of diethylene glycol. (A) PNaK (O) current efficiency. After the cation exchange membrane (NEOSEPTA CM-1 Na+ form) had been immersed in diethylene glycol until equilibrium (192 h at 60 °C), electrodialysis of a 1 1 mixed salt solution of 0.250 N potassium chloride and 0.250 N sodium chloride was carried out at 10 mA cm 2 for 60 min in the presence of diethylene glycol in the desalting side solution.

When the transport number of a cation exchange membrane is t+ and that of an anion exchange membrane t, the current efficiency in electrodialysis is expressed by... 

Figure 6.17 Transport number of protons relative to sodium ions (PNaH) and the current efficiency of protons in cation exchange membranes with cationic charged layers. (O) PnaH ( ) current efficiency. Electrodialysis was carried out at 20mAcmr2 using a mixed solution of 0.208 N NaCl and 0.208 N HCl after the cation exchange membrane (Na+ form) had been immersed in polyethyle-neimine solutions of various concentrations.

Electrochemical regeneration of the mixed-bed ion-exchange resin can take place in both desalination and concentration chambers of electrodialysis apparatus. Withdrawal of ions from desalination chambers would be responsible for the resin regeneration in this chamber. Ion desorption on the C A contact points and water splitting on the A C contact points would carry out the regeneration in the concentration compartment. In all cases the current efficiency will increase as the current density grows. 

Division of a cell by means of a separator is often practised, despite the additional costs, the need for additional seals and possible maintenance problems. A separator may (1) allow a more independent choice of anode/anolyte or cathode/catholyte (2) enable current efficiency to be maintained due to the exclusion of redox shuttles (3) help to isolate electrode products (4) enable electrodialysis to be practised or (5) prevent the formation of explosive or toxic mixtures, e.g. H2/O2 or H2/CI2. 

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... 

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