Electrodialysis membrane stack

The current utilization refers to the fraction of the total current that passes through an electrodialysis membrane stack that actually is used to transfer anions or cations from a feed solution. The current utilization is always less than 100% due to (1) co-ion intrusion into the ion-exchange membrane (i.e., no ion-exchange membrane will completely exclude co-ions), (2) osmotic and ion-bound water transport (water will flow into the concentrate compartment due to osmosis and the electro-osmotic drag of water molecules with the transporting ions), and (3) shunt currents that skirt around the membranes and pass through the stack manifold. 

Since electrodialysis membranes are subject to fouling, it is sometimes necessaiy to disassemble a stack for cleaning. Ease of reassembly is a feature of ED. 

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. 

Figure 10.17 Flow schematic of electrodialysis systems used to exchange target ions in the feed solution, (a) An all-cation exchange membrane stack to exchange sodium ions for calcium ions in water softening, (b) An all-anion exchange membrane stack to exchange hydroxyl ions for citrate ions in deacidification of fruit juice...

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... 

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. 

The simultaneous separation and recovery of acidic and basic bioactive peptides by employing electrodialysis with ultrafiltration membranes has also been investigated recently [30]. This work aims at demonstrate the feasibility of separating peptides from a beta-lactoglobulin hydrolysate, using an ultrafiltration membrane stacked in an electrodialysis cell, and a study of the effect of pH on the migration of basic/ cationic and acid/anionic peptides in the electrodialysis configuration. 

Fig. 5 Schematic of electrodialysis showing the electrodes and membrane stack. (View this art in color at www.dekker.com.)...

The energy necessary to remove salts from a solution by electrodialysis is directly proportional to the total current flowing through the membrane stack and the total voltage drop between the two electrodes in the stack. The energy consumption is described mathematically by [132]... 

Electrodialysis uses stacks of pairs of anion- and cation-exchange membranes in deionizing water and in recovery of formic, acetic, lactic, gluconic, citric, succinic, and glutamic acids from their sodium and potassium salts in fermentation broths.114 This may have an advantage over processes that involve purification through calcium salts. Electrodialytic bipolar membranes have been used to recover concentrated mineral acids from dilute solution.115 They can be used to convert sodium chloride to hydrogen chloride and sodium hydroxide in a process that avoids the use of chlorine.116 Soy protein has been precipitated by... 

The use of electrodialytic water demineralizers with daily capacity of several hundred thousand to several million gallons has been stimulated by the availability of a field-tested basic membrane stack of suitable size to form a logical building block for such plants. This stack, the Mack III stack, was first described two years ago (5). Since that time, field tests at Oxnard, Calif., have been completed, improvements have been made in stack production and characteristics, and commercial plants are now being designed. In this paper, we describe briefly the Oxnard field tests, present and discuss the revised characteristics of the Mark III stack, and describe certain aspects of the design and economics of the Buckeye unit as a typical example of a municipal electrodialysis plant. 

The basic building block of an electrodialysis unit is the array of alternating anion and cation membranes (with interposed water-carrying spacers) called the membrane stack. The characteristics of an Ionics Mark III stack are described in Table IV. 

Electrodialysis ED stack, spacers, end plates, electrodes and stack hardware. FOB cost = 2 800 000 for a membrane area = 3000 m with n = 0.7 for the range 1500-7000 m. Factor FOB, X 1.00 TM unit including stacks, rectifiers, auxiliary equipment, treatment building and pumping stations, X 2.7. 

An electrodialysis (ED) stack is composed of several flow chambers separated by imi exchange membranes and superimposed by an electric field. In most cases, an alternating arrangement... 

Aquatech Systems, a business unit of Allied-Signal, Inc., has patented the SOXAL process, which is a process for regenerating the spent scrubbing solution of an alkaline sodium salt scrubber using electrodialysis cell stacks (electrolytic cells with ion-selective membranes) (Byszewski and Hurwitz, 1991). 

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

FIG. 22-61 Electrodialysis water dissociation (water splitting) membrane inserted into an ED stack. Starting with a salt, the device generates the corresponding acid and base by supplying and OH" from the dissociation of water in a bipolar membrane. CouHesy Elsevier.)... 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 20-84. The bipolar membrane is inserted in the ED stack as shown. Salt is fed into the center compartment, and base and acid are produced in the adjacent compartments. The bipolar membrane is placed so that the cations are paired with OH" ions and the anions are paired with H. Neither salt ion penetrates the bipolar membrane. As is true with conventional electrodialysis, many cells may be stacked between the anode and the cathode. 

The membranes in electrodialysis stacks are kept apart by spacers which define the flow channels for the process feed. There are two basic types(3), (a) tortuous path, causing the solution to flow in long narrow channels making several 180° bends between entrance and exit, and typically operating with a channel length-to-width ratio of 100 1 with a cross-flow velocity of 0.3-1.0 m/s (b) sheet flow, with a straight path from entrance to exit ports and a cross-flow velocity of 0.05-0.15 m/s. In both cases the spacer screens are... 

Fig. 11. Study of the mobility of valinomycin within a stack of membranes before 0 = 0), immediately after (t = 3 hr) electrodialysis as well as after a five-day period of restacking the membranes 0 = 5 days). c initial ligand concentration Ac, change of total ligand concentration. The size of the circles denotes 95% confidence limits.11 72...

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