effective deoxygenation by a hybrid process combining gas transfer membranes with catalytic oxygen reduction

Transcription

1 Water Technologies & Solutions technical paper effective deoxygenation by a hybrid process combining gas transfer membranes with catalytic oxygen reduction Authors: S. B. Gorry, SUEZ, W. E. Haas, SUEZ and J. W. Mahaffee, Baltimore Gas & Electric IWC Summary: This paper focuses on the deoxygenation of RO permeate by a hybrid process combining GTM technology with catalytic oxygen reduction. The paper discusses the roles that each technology plays in deoxygenation, describes the benefits allowed by combining them, and presents a case history where the hybrid process has successfully deoxygenated water as part of a complete water treatment system since Data from this site shows that the hybrid system consistently maintains oxygen levels of <5 ppb. In addition, the paper examines what role the hybrid process plays in the overall TOC removal by the system. introduction The presence of dissolved oxygen in water poses various problems for different industries. In the pharmaceutical industry, the presence of oxygen can favor the growth of organisms and can alter the stability of some drug formulations. These concerns, however, seldom require specific efforts at oxygen removal within the high-purity water treatment systems. In the semiconductor industry, the presence of dissolved oxygen in silicon wafer wash water can lead to the formation of silicon dioxide films. Some chip manufacturers do address this concern by employing treatment strategies such as nitrogen sparging 1. In boiler systems, like those found in refineries, paper mills, and particularly power plants, dissolved oxygen can cause significant problems by accelerating corrosion rates. The power industry pays particular attention to boiler chemistry, due to the critical nature of these highpressure systems. Power plants use mechanical and chemical processes in an effort to virtually eliminate dissolved oxygen from the boiler system. Mechanical devices such as vacuum degasifiers and deaerators can effectively reduce oxygen levels. These devices are typically located just upstream of the boiler. In addition, power plants usually employ one of several chemical strategies for corrosion control within the boiler system, including phosphate treatment, all volatile treatment, and oxygenated treatment. These strategies often include the use of oxygen scavengers and reducing agents as well as ph control chemicals to minimize the potential for corrosion. Some power plants choose to include a process for deoxygenation in their make-up water systems. By maintaining a large volume of deoxygenated water in a blanketed storage tank, a plant avoids having to rely on the success of a mechanical device as water enters the boiler. Total Organic Carbon (TOC) can also pose problems for a boiler system. When heated, organic compounds break apart into smaller compounds, and individual ionic components. Since these remaining constituents can also increase boiler corrosion rates, most plants set limits for TOC in make-up water. A majority of organic compounds that occur naturally in water are large enough to be removed by a physical process like ultrafiltration, or they can be slightly ionized, allowing removal by ion exchange. However, some compounds exhibit neither tendency, and require additional efforts at removal or destruction. As stated previously, the objective for this paper is to document an effective hybrid process which removes dissolved oxygen during the production of make-up water. The process combines two proven technologies for deoxygenation. Other authors have documented GTM technology and its ability to remove dissolved gases 2, and activated carbon s Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. TP1065EN.docx Apr 10

2 ability to catalyze the reaction between hydrazine and oxygen 3. Incorporating both of these technologies into a two-step, hybrid system takes advantage of their respective benefits while lessening any potential problems that could exist as a result of using them separately. The authors also investigated whether or not the hybrid system contributes to the TOC reduction of the make-up water system as a whole. While the system does contain reverse osmosis, many low molecular weight, nonionic organic molecules are not rejected. Some such compounds belong to a group called Volatile Organic Carbon compounds (VOCs). VOCs by definition are organic compounds that evaporate readily at normal temperatures and pressures 4. The principles that govern gas removal by GTMs would also apply to VOCs. Air stripping has proven effective at removing VOCs 5, so it stands to reason that if VOCs are present in the feed water, we could expect some reduction. GTM principles It is important for this paper to review the principals behind the success of GTM technology. Gas Transfer Membranes work by taking advantage of the natural, physical action of gases. Whether dissolved in water or not, gases act independently of one another. The partial pressure of a gas is the pressure each gas would exert if it alone occupied the whole volume. Dalton s Law states that each gas in a mixture exerts a partial pressure proportional to its molecular concentration, and that the total pressure of the mixture is equal to the sum of the partial pressures 6. In addition, Henry s Law states that the solubility of a gas in water is directly proportional to the partial pressure of that gas above the water 7. By manipulating the partial pressure of a gas at the air-water interface, we can create a driving force for the mass transfer of a gas in or out of solution. The patented Liqui-Cel (registered trademark of Celgard, LLC) GTM unit contains bundled, microporous, polypropylene, hollow fibers, and an internal baffle to promote turbulent flow 8,9. Individual GTM components are shown in Figure 1. Since the membrane material is hydrophobic, only gases can move through the membrane. Figure 1: Components of a GTM Unit There are three ways to alter the partial pressure of a gas in a GTM system. The first way is to apply a vacuum to the inside of the hollow fiber, which would lower the partial pressure of all gases. The second is to apply a pure sweep gas, typically nitrogen, which would effectively lower the partial pressure of other gases like oxygen or carbon dioxide. The third way is to use both a vacuum and a sweep gas. Lowering the partial pressure of oxygen causes a mass transfer of the oxygen out of solution. The degree of success for this technology depends on the water flow rate through the system, the temperature of the water, the level of vacuum applied, and the volume and purity of the sweep gas. catalytic deoxygenation principles The reaction between oxygen and hydrazine is well known and is expressed as follows: N 2 H 4 + O 2 N H 2 O The reaction is stoichiometric, requiring equal amounts of hydrazine, and oxygen to reach completion. At ambient temperatures, this reaction proceeds very slowly. However, in the presence of a catalyst such as activated carbon, the reaction occurs rapidly, producing an effluent containing <10 ppb dissolved oxygen 3. The system presented in the following case history uses a patented process which includes this carbon catalyzed hydrazine oxygen reaction, followed by a suitable ion exchange bed to remove excess hydrazine, and carbon leachables 10. Page 2 TP1065EN.docx

3 case history The hybrid process is part of a complete water treatment system that provides make up water for steam generators at a North American nuclear power plant. The system demineralizes and deoxygenates well water using reverse osmosis, hybrid deoxygenation, and three step ion exchange. The outsourced system started producing make up water for the plant in September of 1996 and has continued to exceed the strict quality specifications for dissolved oxygen, TOC, and ionic constituents demanded by the nuclear industry. See Figure 2 for a Process Flow Diagram. The system provides up to 300 gpm (68 m 3 /h) to handle the maximum plant requirement and operates twenty-four hours per day, seven days per week. The feed water comes from a well and has a Total Dissolved Solids (TDS) content of <200 ppm. A typical feed water analysis appears in Table 1. Figure 2: System Process Flow Diagram Table 1: Typical Raw Water Analysis Cations (mg/l as CaCO 3) Anions (mg/l as CaCO 3) Ca HCO Mg Cl Na SO 4-2 K +1 9 NO 3-1 TDS (mg/l as Ions) SiO 2 (mg/l as SiO 2) Conductivity (µmhos) 10 Oxygen (mg/l as O 2) ph TOC (mg/l as C) < level of approximately 10 ppm TDS. Pretreatment for the RO unit includes cartridge filtration for SDI control and acid feed to prevent scaling. The well water is not chlorinated, so reduction of oxidants is not necessary. The RO system contains two separate arrays and operates in single or two pass mode depending on the make up requirement. Because of the acid feed, the RO permeate contains a significant quantity of gaseous CO 2, which must be removed prior to the downstream ion exchange polishing. Since the principles discussed above for the removal of dissolved oxygen also apply to CO 2 removal, the gas transfer membranes accomplish the necessary decarbonation. The GTM system consists of three skid-mounted GTM contactors in parallel, a vacuum pump, valving to allow air sweep, and instrumentation to monitor vacuum level and sweep volume. As the permeate enters the gas transfer membranes and a vacuum with some air sweep is applied, carbon dioxide and oxygen levels drop significantly. Hydrazine injection occurs at the GTM effluent, and is accomplished with a small diaphragm type pump. Any remaining dissolved oxygen reacts with the hydrazine as the RO permeate water passes through two carbon vessels in parallel. This carbon has been in service for three years. Thus far there has been no decline in its ability to catalyze the reaction. The deoxygenated permeate enters the three step ion exchange polishing system. This system is a mobile, trailer-mounted unit consisting of hydrogen form, strong acid cation resin SAC(H + ), hydroxide form, strong base anion resin SBA(OH - ), and mixed bed resin (H + /OH - ). The SAC(H + ) removes any residual hydrazine. The outsourcing supplier regenerates the polishing system at a regional service center, eliminating on-site regenerant waste disposal issues. Also of note is the polishing mixed bed, which the supplier regenerates using a proprietary process to achieve a very high-purity effluent. System effluent values are listed in Table 2. The well water is low in dissolved oxygen, but becomes almost saturated while sitting in a large storage tank. Reverse osmosis, using polyamid membranes, provides primary demineralization to a TP1065EN.docx Page 3

4 Table 2: System Effluent Values Parameter Unit Values Conductivity µmho < 0.06 Oxygen ppb < 5 Silica ppb < 5 Sodium ppb < 0.2 Chloride ppb < 0.5 Sulfate ppb < 0.5 TOC ppb < 10 GTM operating specifics The GTM system serves two functions at this site, and operating strategies differ slightly for removal of carbon dioxide versus oxygen. For this system the goal is to optimize the CO 2 removal in an effort to maximize runlengths on the polishing demineralizer. The GTMs operate in the combo mode meaning with vacuum and air sweep. While operating in this fashion causes slightly higher dissolved oxygen concentrations than would vacuum only mode, oxygen removal is still very good. catalytic deoxygenation operating specifics This is a straight forward application. Successful operation requires only a reliable hydrazine injection pump at the proper setting. Although the reaction of hydrazine and oxygen is 1:1, a slight overfeed of hydrazine is necessary to accommodate fluctuations in GTM effluent oxygen and flow rate. A hydrazine injection rate of roughly 1 ppm is appropriate for this site. Hydrazine is typically available as a 35% solution. Because 1 ppm of 35% solution is such a small volume, a 20% solution is used for this site. The 20% hydrazine is contained in a tote with a nitrogen blanket to minimize natural decay. hybrid system effluent The hybrid system has effectively reduced oxygen levels since start up in September of Figure 3 shows dissolved oxygen data for the last two years (June 1997 to June 1999). Included in this graph are values for Inlet Oxygen, GTM Effluent Oxygen and Final Effluent Oxygen. Effluent from the gas transfer membranes consistently measures 0.5 to 1 ppm of dissolved oxygen. The operator on site collects the GTM Effluent data manually with colorimetric tests using glass ampoules manufactured by Chemetrics. The effluent from the carbon beds consistently measures <5 ppb. An in-line dissolved oxygen analyzer collects this data. The instrument, an Orbisphere 3600 Analyzer, constantly monitors the oxygen content of the mixed bed effluent. The unit is cross checked weekly and calibrated several times a year. The membrane in the probe is cleaned when needed. The analyzer values are confirmed by colorimetric testing using another Chemetrics product. The sudden drop in final effluent dissolved oxygen from 2.4 to 0.1 ppb, shown in Figure 3, coincided with a membrane cleaning. The deoxygenated, demineralized water is stored in a tank with a nitrogen blanket. The tank effluent is tested daily and consistently measures <5 ppb dissolved oxygen. A Total Organic Carbon (TOC) specification is required of the system as a whole. However, analyzing for TOC content within the hybrid system is not common practice. We collected 29 data points between May and July of 1999 in effort to support our hypothesis that the GTM system aids in the removal of TOC. The data is inconsistent at best. Some samples show removal while others suggest an increase in TOC. See Figure 4 for results. The samples were refrigerated on site and shipped in groups to the outsourcing supplier s lab. The lab analyzed the samples with an O.I. Corporation, Model 700 TOC Analyzer. This instrument uses sodium persulfate oxidation, and is calibrated quarterly. While every effort was made to ensure the integrity of the samples, the fact that they were not analyzed immediately could have contributed to the inconsistency. Perhaps, analyses using in-line instrumentation would provide more reliable data. Also, one set of samples went to an independent laboratory in an effort to determine the type of organic molecules present. The lab checked for the presence of over 30 volatile organic compounds. None of these compounds was detected at a level above the lower detection limit of 1 ppb. The results show that, at least for this sample, VOCs are not present. Page 4 TP1065EN.docx

5 Figure 3: Oxygen Results Figure 4: TOC Results TP1065EN.docx Page 5

6 benefits of hybrid system Combining these two technologies in one system allows the following benefits over using either technology separately. Advantages of the Hybrid System over Carbon Catalyzed Deoxygenation: Reduces hydrazine consumption Reduces hydrazine handling Reduces impact of hydrazine overfeed on downstream demineralization Provides system redundancy Does not increase the manpower requirement Advantages of the Hybrid System over Deoxygenation by GTM: Does not require nitrogen sweep, which would include providing liquid nitrogen storage and delivery systems Requires one third the amount of GTM contactors per unit of flow Produces effluent with lower dissolved oxygen levels Provides more consistent results Requires less operator attention Some important benefits to this system center around hydrazine. Since the hybrid system requires significantly less hydrazine than a system using only the catalytic process, there is an immediate financial benefit when comparing monthly operating costs. This savings is offset some by the electrical cost to operate the vacuum pump. Based on the current cost of hydrazine, the average monthly system production at this site, and the average values for oxygen presented in Figure 3, the hybrid system saves roughly $1200 per month in hydrazine when compared to an identically sized system using only catalytic deoxygenation. Using $0.06 per kwh and actual electrical data from the site, the power to operate the vacuum pump costs about $300 per month. This represents a net monthly savings of $900. Of course, the additional capital cost of the GTM system and other factors must be included to complete an accurate comparison. A detailed analysis of this type is not within the scope of this paper. Also, providing system redundancy with the two step process reduces the potential for a complete system failure that could result in an effluent with saturated Page 6 levels of dissolved oxygen. In the hybrid system if the GTM portion fails, an operator can maintain a low dissolved oxygen level in the system effluent by increasing the hydrazine injection. Similarly, in the unlikely event that both hydrazine pumps fail, the operator can manipulate the GTM system to optimize dissolved oxygen removal and can achieve effluent oxygen levels of 500 ppb (or lower with the addition of nitrogen sweep). conclusions Combining Gas Transfer membranes with a catalytic oxygen reduction as part of a high purity water treatment system is an effective way to maintain <5 ppb dissolved oxygen in the system effluent. The hybrid system provides a cost benefit over a system using only the catalytic deoxygenation process. Additionally, with a two stage process, the potential for a complete catastrophic failure is reduced. So far, data from this site could not prove conclusively that GTM technology reduces TOC levels. One reason for this is that the RO permeate may not contain a significant amount of volatile organic compounds. The authors still believe that GTM technology will remove volatile organic compounds. Interestingly, initial data from another site is providing significantly different results. This data will be the subject for a following paper. references 1. T.H. Meltzer, High Purity Water Preparation for the Semiconductor, Pharmaceutical, and Power Industries, (Littleton, CO: Tall Oaks Publishing, Inc., 1993), pp S.H. Macklin, W.E. Haas, W.S. Miller, Carbon Dioxide and Dissolved Oxygen Removal from Makeup Water by Gas Transfer Membranes, 56th International Water Conference, Pittsburgh, PA, October, W.S. Miller, Oxygen Removal by Catalyzed Carbon Beds, EPRI Condensate Polishing Workshop, Richmond, VA, October, The American Heritage Dictionary, 2nd Edition, (Boston, MA, Houghton Mifflin Company, 1991), p B.A. Okoniewski, Remove VOCs from Wastewater by Air Stripping, Chemical Engineering Progress, February, TP1065EN.docx

7 6. D.C. Giancoli, General Physics, (Englewood Cliffs, NJ, Prentice-Hall, Inc., 1984), pp T.L. Brown, H.E. LeMay, Chemistry: The Central Science, 2nd Edition, (Englewood Cliffs, NJ, Prentice-Hall, Inc., 1981), pp and R. Prasad, C. Runkle, H. Shuey, U.S. Patent 5,264,171, Method of Making Spiral-Wound Hollow Fiber Membrane Fabric Cartridges and Modules Having Flow-Directing Baffles November, R. Prasad, C. Runkle, H. Shuey, U.S. Patent 5,352,361, Spiral-Wound Hollow Fiber Membrane Fabric Cartridges and Modules Having Flow- Directing Baffles, February 16, R.C. Dickerson, W.S. Miller, U.S. Patent 4,556,492, Deoxygenation Process, December 3, TP1065EN.docx Page 7