Electrodialysis control systems

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. 

Nomura, Y., Iwahara, M., and Hongo, M. 1988. Acetic acid production by an electrodialysis fermentation method with a computerized control system. Appl. Environ. Microb. 54, 137-142. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

The model of electric field-controlled artificial muscles has been described in 1972 [5], Fragala et al. fabricated an electrically activated artificial muscle system which uses a weakly acidic contractile polymer gel sensitive to pH changes. The pH changes are produced through electrodialysis of a solution. The response of the muscle as a function of pH, solution concentration, compartment size, certain cations, and gel fabrication has been studied. The relative change in length was about 10%, and the tensile force was 1 g/0.0025 cm2 under an applied electric field of 1.8 V and 10 mA/cm2. It took 10 min for the gel to shrink. 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

Most inefficiencies in electrodialysis systems are related to the difficulty in controlling concentration polarization. The second cause is current utilization losses, arising from the following factors [10] ... 

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

Three different membrane processes, ultrafiltration, reverse osmosis, and electrodialysis are receiving increased interest in pollution-control applications as end-of-pipe treatment and for inplant recovery systems. There is no sharp distinction between ultrafiltration and reverse osmosis. In the former, the separation is based primarily on the size of the solute molecule which, depending upon the particular membrane porosity, can range from about 2 to 10,000 millimicrons. In the reverse-osmosis process, the size of the solute molecule is not the sole basis for the degree of removal, since other characteristics of the... 

The gaskets not only separate the membranes but also contain manifolds to distribute the process fluids in the different compartments. The supply ducts for the diluate and the brine are formed by matching holes in the gaskets, the membranes, and the electrode cells. The distance between the membrane sheets, i.e. the cell thickness, should be as small as possible to minimize the electrical resistance. In industrial size electrodialysis stacks membrane distances are typically between 0.5 to 2 mm. A spacer is introduced between the individual membrane sheets both to support the membrane and to help control the feed solution flow distribution. The most serious design problem for an electrodialysis stack is that of assuring uniform flow distribution in the various compartments. In a practical electrodialysis system, 200 to 1000 cation- and anion-exchange membranes are installed in parallel to form an electrodialysis stack with 100 to 500 cell pairs. 

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 volume of wine is pumped into the treatment vat and then into the diluate circuit of the electrodialysis cells. When conductivity reaches the set point, determined by an instability test, the wine is automatically pumped into a reception vat using a system controlled by solenoid valves. A new volume of wine is then pumped into the system and stabilized under the same conditions. Treatment time and, consequently, the performance of the system depend on the wine s degree of instability. Treatment flow rates vary from 50 to 150 1/h/m, depending on this parameter. The concentrate circuit consists of a saline solution that collects the ions extracted from the wine. The ion load is adjusted by adding water to avoid the precipitation of bitartrate crystals inside the small, easily blocked cells. This function is also automatically controlled by conductivity measurements. 

J.W. Post, H.V.M. Hamelers, C.J.N. Buisman, Energy recovery from controlled mixing salt and fresh water with a reverse electrodialysis system. Environmental Science Technology 2008, 42, 5785-5790. 

Individual dissolved monosaccharides have been determined after desalting of the sample by ion-exchange membrane electrodialysis followed by evaporative concentration and ion-exchange LC using colorimetric detection with Cu -aspartic acid-disodium-2,2 bic-inchoninate. More recently a number of papers have described the use of anion-based LC with pulsed am-perometric detection for individual monosaccharide analysis though it has been noted that temperature control of the system is particularly important. 

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