Lenntech [email protected] www.lenntech.com Tel. +31-15-261.09.00 Fax. +31-15-261.62.89 Application of Electrodeionization in Ultrapure Water Production: Performance and Theory Authors: Brian P. Hernon, R. Hilda Zanapalidou, Li ion-exchange resin. When EDI is used for the pro- Zhang, Linda R. Siwak, and Erik J. Shoepke, Ionics duction of high purity water, the ion-exchange resin beads enhance mass transfer, facilitate water Presented at the 55th Annual Meeting International splitting, and reduce stack resistance. The ion- Water Conference Pittsburgh, PA, Oct. exchange resin exchanges ions with the incoming 30-Nov. 2, 1994 feed stream. The DC electrical field splits water into Note: GE Water & Process Technologies purchased hydrogen and hydroxyl ions, which, in turn, con- Ionics in 2005. tinuously regenerate the ion-exchange resins. The exchanged ions are transferred through the mem- Introduction branes to the brine stream and flushed from the system.1,2 Compared to conventional ion exchange, Ultrapure water is critical in a number of industrial EDI has the advantage of being a continuous proc- applications, such as power generation, semicon- ess with constant stable product quality, which is ductor manufacturing, and pharmaceutical formu- able to produce ultra high purity water without the lation. Traditionally, ion exchange has been used to need for acid or caustic regeneration. provide ultrapure water in these industries but membrane processes are becoming increasingly This paper presents data from five commercial popular as pretreatment steps or as replacements scale EDI installations now in operation. Four of for ion-exchange systems altogether. Membrane these EDI installations are located at electric gen- processes can provide very high levels of deminer- eration plants. The fifth EDI system is part of a sili- alization and offer the advantage of continuous con wafer manufacturing facility located in the operation. Moreover, membrane processes are not Midwest (Si FAB). The electric power plants are as mechanically complex as ion-exchange sys- Grand Gulf Nuclear Station (GGNS), Arkansas tems, and they require no acid and caustic regen- Nuclear One (ANO), Florida Power & Light Turkey eration, nor waste neutralization. Point Power Station (TPPS), and the Riverbend Nuclear Station (RBNS). In the power plants, the In semiconductor fabrication, the use of reverse ultrapure water is used for makeup to high- osmosis (RO) is considered essential to achieve the pressure boilers and for plant usage. In the semi- purity levels necessary for wafer production. In conductor plant, ultrapure water is used for wafer rinsing. power plants, increased recognition of the effect of ultrapure water quality on system component life In each of the five installations discussed in this has resulted in demand for membrane systems paper, the EDI units are integral parts of multi-step which can render an even higher purity of water purification systems that supply high purity water. than mixed-bed deionization systems alone can The EDI process pretreatment includes filtration achieve (e.g. lower TOC, particles, etc.). Such high and partial demineralization by reverse osmosis levels of demineralization can be obtained using (RO). RO removes many of the contaminants which electrodeionization (EDI). EDI combines ion- could harm an EDI system, such as organics, which exchange resins, ion-exchange membranes, and a can foul anion and cation ion-exchange resins, and direct (DC) electric field. Basically, EDI is an elec- particulate matter, which would be difficult to trodialysis (ED) process modified by the addition of remove from an EDI unit. Scaling ions, such as TP1079EN.doc Mar-10 magnesium and calcium which, at high enough ity where regeneration of resins can be tightly concentrations, could necessitate the use of con- monitored and controlled. tinuous acid addition to the EDI, are removed at least 95 percent by an RO system. The combination Conductivity Removal of reverse osmosis and EDI allows the EDI to per- form at its optimum level. In a process train in The performance of EDI can be determined in a which RO is followed by EDI, water can be pro- number of different ways. One simple method is to duced that is comparable to mixed-bed ion- measure the change in conductivity, which is exchange treated water.3 directly related to total dissolved solid (TDS), across the EDI unit. Figures 1-4 show time averaged con- Performance ductivity removal accomplished by the EDI units at GGNS, ANO, TPPS, and RBNS. Conductivity data In the installations reported in this paper, EDI is was obtained using on-line Thornton Dot Two part of water treatment processes that deal with series and 770 series conductivity instruments. In quite different feedwater conditions. The processes the figures, 100% removal corresponds to reaching vary in feedwater sources, composition, and pre- the lowest possible conductivity, which stems from treatment equipment upstream of the RO unit. In water dissociation. all cases, the water treatment systems also have to EDI has been operating at GGNS since September handle feed water quality variations, such as sea- 1991 and was the subject of previous papers.3,4,5,6 sonal changes in the ionic load, temperature, or- The data shown in Figure 1 covers the operation of ganics and pH. Such changes, as well as the an upgraded EDI stack in operation since July process conditions, affect the performance of the 1993. This stack has shown conductivity removal in EDI demineralizers. Table 1 lists the unit processes the range of 99.5%+ since its installation. that are used in each installation and summarizes key variables describing the feedwater. Table 1: Unit Processes and Feedwater Variables for Each EDI Installation Feed Temp Flowrate Feed TDS Location F° gpm System Flowsheet Source gpm (C°) (m3/h) (m3h) GGNS Surface 400 55-82 50 MMF/ACF/UF/EDR/ (1.5) (13-28) (0.2) RO/EDI ANO Surface 100 50-86 200 MMF/ACF/UF/EDR/ (0.4) (10-30) (0.8) RO/EDI Turkey Point Well 475 77-82 200 MMF/DEGAS/UF/ Figure 1: Electrodeionization Conductivity Removal, (1.8) (25-28) (0.8) RO/EDI GGNS Riverbend Well 230 82-88 50 CF/RO/EDI (0.9) (28-31) (0.2) Figure 2 shows conductivity removal for one 100 3 Silicon Fab Well 900 66-72 100 MMF/DEGAS/UF/ gpm unit of a 200 gpm (0.8 m /h) EDI system (3.4) (19-22) (0.4) RO/EDI located at ANO. This unit was also discussed in 4 (MMF= multimedia filtration, ACF=activated carbon filtration, UF=ultrafiltration, detail previously. This unit has been in operation EDR=electrodialysis reversal, DEGAS=forced draft deaerator, CF=cartridge since April 1993 (i.e. over 5000 operating hours). filtration) Since its operation was optimized (after around In all of the above cases the EDI product water is 1200 hours of operation), the conductivity removal further polished using portable ion exchange beds. has been above 99 percent. Portable ion-exchange beds are economical to use Figure 3 shows the conductivity removal over time in these cases since the EDI product water has a for one 100 gpm (0.2 m3/h) unit of a 200 gpm very low ionic load which translates into long bed (0.4 m3/h) EDI system located at Turkey Point life, usually several months in length. Portable ion- Power Station (Fossil). The unit has been in opera- exchange beds also offer higher water quality than tion since September 1993. The system at Turkey is normally achieved in conventional in-situ ion- Point is unique in that it is the first installation by exchange systems, since resin regenerations are Ionics (now GE) of EDI without the use of EDR up- conducted in small volumes at one dedicated facil- stream of the RO unit. The unit has performed very Page 2 TP1079EN well over the first year of operation with conductiv- Specific Ion Removal ity removal in the 98% to 99% range. A more informative measure of EDI performance is the specific ion removal across the unit. Table 2 summarizes results of EDI feed and product water analyses of samples from the five EDI sites. These samples were analyzed by capillary electrophore- sis for their sodium, calcium, magnesium, chloride, and sulfate contents. Table 3 presents the corre- sponding specific ion in terms of percentages. The percent rejection of specifications is consistent from site to site. At each site the EDI reduces speci- fications to low ppb levels with most ions removed Figure 2: Electrodeionization conductivity down to the detection level of the analytical Removal, ANO method. The specific ion levels produced by the EDI units at GGNS, ANO and RBNS are comparable to the level achieved by using mixed-bed ion- exchange resin alone. Silica and Carbon Dioxide Removal Silica removal is very important in power plants since silica deposition on the turbines can result in low efficiencies of power generation and eventu- ally in downtime necessary to remove silica depos- its. In the semiconductor industry, low silica Figure 3: Electrodeionization Conductivity Removal, content in the water is also critical because silica TPPS deposits on the wafers, which can cause costly defects. Figure 4 shows conductivity removal over time Removal of CO2 is another important aspect in ion- for a relatively new installation at Riverbend exchange demineralization. Often, CO2 is the pre- Nuclear Station. The feedwater to this plant is a low dominant anion present in the feedstream to the TDS, low organic, well water which allows this plant ion-exchange bed, especially when RO is used upstream. Since CO2 competes with silica ions for exchange sites, the amount of CO2 in a feedstream affects bed life and silica removal efficiency in the ion-exchange bed. CO2 is weakly ionized, which makes it difficult to remove from water.