What Eletcrodialysis Is

The principle behind electrodialysis is that electrical potential gradients will make charged molecules diffuse in a given medium at rates far greater than attainable by chemical potentials between two liquids as in conventional dialysis. When a DC electric current is transmitted through a saline solution, the cations migrate toward the negative terminal, or cathode, and the anions toward the positive terminal, the anode. By adjusting the potential between the terminals or plates, the electric current and, therefore, the flow of ions transported between the plates can be varied.

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

As noted, the principle of ED is that I. electrical potential gradients will make charged molecules diffuse in a given medium at rates far greater than obtained by chemical potentials between two liquids, as in conventional dialysis. When a DC electric current is transmitted through a saline solution, most salts and minerals are dissolved in water as positively charged particles (anions, for example, Na*) and negatively charged particles (anions, for example, CI). The cations migrate toward the negative terminal, or cathode, and the anions toward the positive terminal, the anode. By adjusting the potential between the terminals or plates and the electric current, the flow of ions transported between the plates can be varied.

(1) Recovery of materials from liquid effluents, such as processes related to conservation, cleanup, concentration, and separation of desirable fractions from undesirable ones; (2) Purification of water sources; (3) Effluent water renovation for reuse or to meet point source disposal standards required to maintain suitable water quality in the receptor streams.

Cconcentration-polarization is a problem which also exists in reverse osmosis systems, and is due to of a build-up in the concentration of ions on one side of the membrane and a decrease in concentration on the opposite side. This adversely affects the operation of membranes and can even damage or destroy them. Polarization occurs when the movement of ions through the membrane is greater than the convective and diffusional movements of ions in the bulk solutions toward and away from the membrane. Along with a deleterious pH shift occurring at the membrane surface, polarization may cause solution contamination and sharply decrease energy efficiency. Commercial electrodialyzer designs incorporate various baffles or turbulence promoters and limit current densities to avoid these effects. Increased feed flow also assists mixing but requires additional power for pumping.

Treatment of brackish waters in the production of potable supplies has been the largest application of electrodialysis. Costs associated with electrodialysis processes depend on such factors as the total dissolved solids (TDS) in the feed, the level of removal of TDS (percent rejection), and the size of the plant. In brackish water treatment, operating costs for very large ED installations (on the order of millions of gallons a day) have been between 40 cents to 50 cents per 1,000 gallons for brackish feed waters, which compares favorably with RO costs.

A rough rule of thumb for the energy requirements for dernineralizing 1,000 gallons of salt water by ED in large capacity plants (4 mgd) is 5 to 7 kWh per 1,000 ppm of dissolved solids removed. Since the efficiency of electrodialytic demineralization decreases rapidly with increasing feed concentrations, this process is best utilized for treatment of weakly saline (brackish) waters containing less than 5,000 ppm of total dissolved solids. In fact, for waters at the low-concentration end of the brackish scale, ED may be the most cost-effective process of all. Electrodialysis is widely used in the United States in the dairy industry, namely in the desalting of cheese whey. Electrical requirements may vary from 5 to 14 kWh per pound of product solids. Another application of ED is the sweetening of prepared citrus juices. Other less extensive uses of electrodialysis in commercial operations in the United States include tertiary or advanced treatment of municipal sewage water and treatment of industrial wastewaters such as metal-plating baths, metal-finishing rinse waters, wood pulp wash water, and glass-etching solutions. Potential applications of ED are many. A particular advantage of the electrodialysis process is its ability to produce solutions of high concentrations of soluble salts. A combination of electrodialysis with conventional evaporation, for example, may be substantially cheaper than evaporation alone for the production of dry salt from saline solutions. Competing technologies include reverse osmosis and crystallization.

Added control of the movement of the ions can be obtained by placing sheet-type membranes of cation- or anion-exchange material between the outer plates, as shown diagrammatically in Figure 1. These sheets of cationselective resins and anion-selective resins permit the passage of the respective ions in the solution. Under an applied DC field, the cations and anions will collect on one side of each membrane through which they are transported and vacate the other side. Thus, if a NaCl solution is supplied to the central zone of the cell shown in Figure 1, the Na+ ions will migrate through Membrane A, depleting the central zone (termed the diluting or product feed stream) of the salt ions. The two outer zones where the ions collect are commonly known as the concentrating or brine streams.

Electrodialysis Cell

NOTE:

C - Cation Selective Membrane A - Anion Selective Membrane

Figure 1. Electrodialysis cell diagram,

NOTE:

C - Cation Selective Membrane A - Anion Selective Membrane

Figure 1. Electrodialysis cell diagram,

Multi-membrane stacks can be built from alternately spacing anionic- and cationic-selective membranes. Flow of solutions through specific compartments and appropriate recombination of transported ions permit desired enrichment of one stream and depletion of another. A schematic view of a typical stack based on the concept of alternating these concentrating and diluting compartments is shown in Figure 2. The feed stream enters each compartment along the top of the figure and flows downward toward the lower exit ports and manifolds. However, as the ionized streams move tangentially along the membranes, cations are transported, or attempt movement, toward the left and anions to the right, causing an alternate build-up and a depletion of ions in adjoining compartments. Thus, one resultant output stream is a diluted product water and the other is a concentrated stream of dissolved salt.

The process flow stream through a commercial demineralizer, incorporating two stacks in series dernineralized water, is shown in Figure 3. Several of the refinements required for continuous-flow operational systems are shown on this diagram, representing a two-stage demineralizer.

Although ED is more complex than other membrane separation processes, the characteristic performance of a cell is, in principle, possible to calculate from a knowledge of ED cell geometry and the electrochemical properties of the membranes and the electrolyte solution.

Another kind of electrodialysis cell configuration, shown in Figure 4, is a multiple electrodialysis system consisting of ten-unit cells, in series rather than manifolded in parallel. The feed solution is introduced at four points: It enters at both upper end points to sweep directly through both electrode chambers and is introduced into the working chambers near either end. The feed solution into the left side traverses depleted chambers and exits as depleted effluent at the right. The feed solution into the rightmost enriching cell flows in the other direction and exits as enriched effluent at the left side.

Feed Water

Cathode Plaie

Cathode Wane ~

"7

Was this article helpful?

0 0
Trash Cash Machine

Trash Cash Machine

How recyclable trash can save the world and bank us huge profits! Get All The Support And Guidance You Need To Be A Success At Recycling! This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Make Profits With Trash!

Get My Free Ebook


Post a comment