Electrodialysis reversal plant

In the practical application of electrodialysis there are two main process operation modes. The first one is referred to as the unidirectional electrodialysis and the second as electrodialysis reversal [22]. In a unidirectional operated electrodialysis system the electric field is permanently applied in one direction and the diluate and concentrate cells are also permanently fixed over the period of operation. Unidirectional operated electrodialysis plants are rather sensitive to membrane fouling and scaling and often require a substantial feed-solution pretreatment and stack-cleaning procedures in the form of periodical rinsing of the stack with acid or detergent solutions. The unidirectional operating concept is mainly used today for applications in the... 

Electrodialysis/electrodialysis reversal (ED/EDR) represents 3% of aU the desalination capacity in the world [48]. It is, however, used mainly for desalinating brackish water. It can achieve 95% water recovery with minimal chemical feed. However, it can only reject ionised matter. Substances such as colloids, silica and boron at pH <8.0 are not removed. A triple-membrane system using UF-EDR-RO has been very effective in producing purified water for power plants [50]. In this integrated system, UF is used for removing suspended solids and macromolecules ... 

A pilot plant was huilt hy Montana et al. (2013) to assess the effectiveness of ultrafiltration (UF), reverse osmosis (RO), and electrodialysis reversal (EDR) in improving the quahty of the... 

Ionics introduces the reverse polarity process, a breakthrough in electrodialysis plant reliability -1970... 

However, reversing the polarity of a stack has to be accompanied with a reversal of the flow streams. This always leads to some loss of product and requires a more sophisticated flow control. The flow scheme of an electrodialysis plant operated with reversed polarity is shown in Figure 5.7. In the reverse-polarity operating mode, the hydraulic flow streams are reversed simultaneously, that is, the diluate cell will become the brine cell and vice versa. In this operating mode, the polarity of the current is changed at specific time intervals ranging from a few minutes to several hours. 

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. 

Like many other specialities, electrodialysis plants are purchased as complete packages from a few available suppliers. Membrane replacement is about 10% per year. Even with prefiltering the feed, cleaning of membranes may be required at intervals of a few months. The comparative economics of electrodialysis for desalting brackish waters is discussed by Belfort (1984) for lower salinities, elecfrodialysis and reverse osmosis are competitive, but for higher ones elecfrodialysis is inferior. Elecfrodialysis has a number of important unique applications, for removal of high contents of minerals from foods and pharmaceuticals, for recovery of radioactive and other substances from dilute solutions, in electro-oxidation reduction processes and others. 

Electrodialysis is a well-proven technology with a multitude of systems operating worldwide. In Europe and Japan, electrodialysis dominates as a desalting process with total plant capacity exceeding that of reverse osmosis and distillation [3]. Electrodialysis with monopolar membranes is applied to different food systems, to demineralization of whey [5-8], organic acids [9], and sugar [10,11], separation of amino acids [12] and blood treatments [13], wine stabilization [14—16], fruit juice deacidification [17-19], and separation of proteins [20-22]. These applications use the sole property of dilution-concentration of monopolar lEMs in a stack of as many as 300 in an electrodialysis cell. 

The ionic concentration to be treated is an overriding consideration governing the cost of plant of a given design and therefore for very high ion concentrations it is foreseeable that membrane pretreatments such as reverse osmosis and electrodialysis will continue to fulfil an important role as might a more widespread revival of continuous countercurrent ion exchange. 

In rain starved regions (southern Mediterranean, northern desert belt) potable water is produced on an industrial scale from sea- and brackish water using distillation plants (older technology), reverse osmosis (newer technology) and to a small extent electrodialysis plants (brackish water). 

R. Rautenbach, W. Kopp, G. van Opbergen and R. Hellekes, Nitrate reduction of well water by reverse osmosis and electrodialysis - studies on plant performance and costs, Desalination, 1988, 65, 241. 

In addition to the actual stack and the power supply unit, an electrodialysis plant consists of several components essential for proper operation, such as pumps, process monitoring and control devices, feed solution pretreatment systems, etc.. There are two operating modes for the electrodialytic process described in the literature [44]. The first is referred to as the unidirectionally operated electrodialysis plant and the second is a reversed polarity operated electrodialysis plant [7]. A flow diagram of a typical unidirectional operated electrodialysis plant is shown in Figure 10. 

A further application of electrodialysis is the concentration of reverse osmosis brines. Because of limiting membrane selectivity and the osmotic pressure of concentrated salt solutions, the concentration of brine in reverse osmosis desalination plants can not exceed... 

A new application area for membranes is energy production. Reverse electrodialysis and pressure retarded osmosis could provide significant quantities of energy from the mixing of fresh water with seawater or mixing the concentrated brine effluent of desalination plants with seawater. However, these applications will require significant reduction in membrane and module costs. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Electrodialysis has the ability to concentrate salts to high levels with much less energy consumption than evaporation would require. This capability has been utilized in Japan to make edible salt by recovering NaCl from seawater and concentrating it to 20% before evaporation. The plants there are huge some have greater than 100 000 square meters of membrane. Salt recovered by electrodialysis in Kuwait is the raw material for a chlor-alkali plant there. Electrodialysis has also been used to concentrate salts in reverse osmosis brines [32]. 

The need for high purity in a separations process is common in many industries semiconductor manufacture, pharmaceuticals processing, and the foods industry, as weil as in many cases of more-conventional chemical processing. It is also very important in separation processes that are oriented to cleaning gas, liquid, and solid streams for environmental purposes. The low concentrations required of many environmentally significant compounds prior to discharge from a chemical plant have created a need for a new class of separation methods and have focused attention on many techniques that often have been ignored. Adsorption, ultraflitration, electrostatic precipitation, reverse osmosis, and electrodialysis are just a few examples of separation processes in which there has been an increased level of interest partly because of their potential in environmental applications. 

This is a second interesting technology proposed to exploit salinity gradient for direct electric power generation. As for the PRO technology, the reverse electrodialysis (RED) concept has been proved by experimental laboratory-scale plants to be capable of generating a net power output. A detailed description of this technology is presented in this section. 

Veerman, J., Saakes, M., Metz, S.J. and Harmsen, G.J. (2010) Electrical power from sea and river water by reverse electrodialysis a first step from the laboratory to a real power plant. Environmental Science and Technology, 44 (23), 9207-9212. 

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