Electrodialysis Process Costs

The total costs in electrodialysis are the sum of fixed charges associated with the amortization of the plant capital costs and the plant operating costs. Both the capital costs as well as the plant operating costs per unit product are proportional to the number of ions removed from a feed solution, that is, the concentration difference 

Capital-related costs The capital costs are determined mainly by the required membrane area for a certain plant capacity and feed and required product concentration. Other items such as pumps and process control equipment are considered as a fraction ofthe required membrane area. This fraction depends on the plant capacity. The same is true for the required land that also depends on the location of the plant. 

The required membrane area for a given capacity plant can be calculated from the current density in a stack that again depends on feed and product solution concentration. It can be calculated for a solution containing a single monovalent salt such as NaCl from the total current passing through the stack which is given by  

I and i are the electric current and the current density passing through a cell pair, A is the cell area, f) is the volume flow, C is the concentration expressed in equivalent per volume, F is the Faraday constant, and E, the current utilization. The subscripts cell refers to the diluate cell, and the superscripts d and f refer to diluate and feed solution, respectively. 

The voltage drop across a cell pair is constant over the entire length of a cell pair from the feed entrance to the product exit while the resistance ofthe cell pair is changing from the feed inlet to the product exit due to a decrease of the resistance of the diluate concentration. Therefore, the current density is also decreasing along the length of a cell pair. 

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Fig. 14 Schematic diagram of the electrodialysis process costs as a function of the...

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. 

Electrodialysis with Bipolar Membrane Process Costs... 

Strathmann, H. (2004) Assessment of electrodialysis water desalination process costs. Proceedings of the International... 

Electrodialysis might also be an option, but it needs to remove large amounts of salt, which will increase the process cost [15],... 

The process design and economics are closely related in electrodialysis. The total process costs are the sum of flxed charges associated with amortization of the plant capital costs and operating costs, such as energy and labour costs. Membrane replacement costs are sometimes regarded as a separate item because of their relatively short life of 5 to 7 years. 

Cost estimate to compare the electrodialysis process with the ion exchange resin process were made and showed that an electrodialysis plant would cost less than a completely new ion exchange resin plant. 

The electrodialysis process to make silica aquasols commercial electrodialysis units is technically feasible. The new variable temperature method allows to make very small particle size sols (4-6 nm) and concentrated small particle sols (7 nm) at higher, more efficient temperatures. Electrodialysis remains an attractive technical option for new plant an electrodialysis plant would cost less than a completely new ion exchange resin plant. Electrodialysis is an energy intensive process but it is essentially pollution-free and recovers substantial NaOH values for credit. Location of a plant in a low-cost power area could be attractive. Electrodialysis would also be a technical option in case environmental pressures require further reduction of sodium sulfate level in the silica sol waste of the ion exchange resin process. 

Industrial cells consist of one or two hundred modules with membrane surfaces that can reach up to one m. These facilities are able to soften brackish water with flow rates spanning from a few hundred up to one thousand m per day, and all with an energy cost of about 1 kWh per m . The precise nature of the electrode reactions taking place in the compartments at both ends of the cell plays no direct role in the electrodialysis process. The intermembrane space has a thickness lower than 1 mm, in order to decrease the ohmic drop. However if the solution rec uires a stronger demineralising effect, then the ohmic drop can be very large because this solution will become poorly conducting. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

Equipment and Economics A veiy large electrodialysis plant would produce 500 /s of desalted water. A rather typical plant was built in 1993 to process 4700 mVday (54.4 /s). Capital costs for this plant, running on low-salinity brackish feed were 1,210,000 for all the process equipment, including pumps, membranes, instrumentation, and so on. Building and site preparation cost an additional 600,000. The building footprint is 300 itt. For plants above a threshold level of about 40 m Vday, process-equipment costs usually scale at around the 0.7 power, not too different from other process eqiiip-ment. On this basis, process equipment (excluding the ouilding) for a 2000 mVday plant would have a 1993 predicted cost of 665,000. 

Electrodialysis is a particularly economic process for low-salinity waters when compared to RO because, although the initial capital cost may be 10 to 15% higher, it generally requires no pretreatment, it produces a higher recovery rate (around 80-85%), it has a lower operating and maintenance cost, and the membranes last twice as long (up to 10 years). 

There are several companies and groups that are developing bio-based succinic acid production for commercial use. The Showa group possesses a unique technology for purification of succinic acid from fermentation broth. This is the fractional crystallization method starting from sodium succinate. The yield by this method is around 70%, but we can recycle the residual solution so that we can minimize the loss of the product. We also compared the cost-effectiveness of this method with the bipolar electrodialysis method. The cost of our purification process seemed to be about half (our internal data). 

Table 1 shows treatment costs for the technology (based on a processing rate of 20 gpm) in comparison to other groundwater treatment technologies (i.e., chemical reduction and precipitation, chemical precipitation with sedimentation or filtration, activated carbon adsorption, ion exchange, reverse osmosis, and electrodialysis) (D168869, Table 13). 

The AQUATECH technology, used in conjunction with conventional separation techniques such as filtration and electrodialysis is able to overcome the disposal problem in a cost effective manner while recycling valuable resources back to the processing plant. The net result is a cleaner environment and the avoidance of long term liabilities to the processor. 

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... 

The efficiency of electrodialysis is determined to a large extent by the properties of the membranes. But it is also affected by the process and system design that determine the limiting current density, the current utilization, the concentration polarization and the overall efficiency and costs [20, 21]. 

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