Water Technologies & Solutions technical paper maintaining storage tank dissolved oxygen levels utilizing gas transfer membranes Authors: W.E. Haas, SUEZ, J. Helmrich, Florida Power and Light and J.E. Stanton, SUEZ introduction In many high purity water applications dissolved oxygen is removed and controlled as a product contaminant or for corrosion control. Many make-up systems incorporate a dissolved oxygen removal step. Once the dissolved oxygen is removed, the concern focuses on how to maintain low dissolved oxygen levels in the storage tank. A nitrogen blanket or floating roof is typically employed to prevent atmospheric oxygen ingress into the tank. Low level dissolved oxygen levels can be produced and maintained utilizing gas transfer membranes (GTM*). This paper discusses continuous dissolved oxygen removal by GTM on a side stream recirculation loop of a demineralized water storage tank. The data presented establishes GTM technology as an effective means to produce and control a storage tank s dissolved oxygen levels. GTM technology utilizes a hollow fiber configuration with an internal baffle patented by Celgard 3, 4, trade named Liqui-Cel (Liqui-Cel is a registered trademark of Celgard, LLC). As illustrated in Figure 1, the internal baffle promotes turbulent flow in and around the hollow fibers while the tube interior operates under vacuum, nitrogen sweep gas or both in combination. The hollow fiber membrane is hydrophobic polypropylene thus allowing only gases to pass through it. Each threadlike fiber has a 300- micron outer diameter with 0.05-micron gas permeable pores. There are three ways to alter the partial pressure of a gas in a GTM system. The first is to apply a vacuum to the inside of the hollow fiber, which lowers the partial pressure of all gases. The second is to apply a pure sweep gas, typically nitrogen, which effectively lowers 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 any gas causes a mass transfer of the oxygen out of solution. GTM principles Whether dissolved in water or not, gases act independently of one another. According to Dalton s Law, the total pressure in a contained gas mixture is the sum of the individual pressures contributed by each gas 1. Furthermore, the pressure each gas exerts is proportional to its molecular concentration. 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. 2 Therefore, by manipulating the partial pressure of a gas at the airwater interface, we can create a driving force for the mass transfer of a gas in or out of a solution. figure 1: GTM Liqui-Cel contactor Find a contact near you by visiting www.suezwatertechnologies.com and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries. 2017 SUEZ. All rights reserved. TP1067EN.docx Feb-10
application The information presented in this paper is based on a full flow pilot test operated at a pressurized water reactor (PWR) nuclear power plant in the southeastern United States. PWR power plants maintain a demineralized water storage tank typically known as a primary water storage tank (PWST). The water in this tank is used primarily for make-up to the nuclear reactor. The dissolved oxygen in this tank is controlled for corrosion control purposes. The tank is filled from the Water Treatment Plant (WTP) with demineralized water that has been deoxygenated to less than 10 ppb. The tank has a bladder to help minimize atmospheric ingress however, without continuous removal, the dissolved oxygen level in this tank eventually rises to 1 to 2 ppm. The PWST storage capacity is 160,000 gallons (606 m 3 ) gallons. The level in this tank is maintained between 75,000 and 125,000 gallons (473 m 3 ). As illustrated in Figure 2, the GTM was installed in a 100 gpm (0.4 m 3 /h) recirculating loop on the PWST. The GTM recirculation on the PWST is constant except when it is being filled. During filling, the water from the water treatment plant is directed entirely through the GTM system to prevent increasing the dissolved oxygen level in the tank. The average make-up to the PWST is 1000 gpd (3.8 m 3 /day). Previous to the GTM installation, dissolved oxygen levels were controlled by a vacuum deaerator operating on a kidney loop. The deaerator was aged and unable to maintain less than 100 ppb dissolved oxygen in the tank. In order to avoid the documentation and engineering evaluation required by the nuclear power industry the power company desired to replace the vacuum deaerator with modern technology that required little pre-engineering to install. The GTM has been in operation since November 1, 1999. The testing and validation were completed on November 30, 1999. The goal to maintain less than 100 ppb dissolved oxygen (per the Electric Power Research Institute Nuclear Chemistry guidelines) is being achieved. Six months later the dissolved oxygen level in the PWST is still below 100 ppb with levels less than 50 ppb being recorded. figure 2: GTM/tank flow path GTM Design. The GTM system is a skid mounted, single module design. Including the vacuum pump, the system measures 7 ft. wide by 7.3 ft. long by 7 ft. high (2.1 m wide x 2.2 m long x 2.1 m high). All piping is 304 stainless steel. The vacuum pump is rated at 3 Hp and has a local disconnect mounted on the skid. A basic piping diagram of the unit is shown in figure 3. figure 3: basic piping diagram The GTM system operates on vacuum, treating 100 gpm (0.4 m 3 /h) from the PWST continuously. It was delivered to the site pre-constructed requiring only inlet, outlet, and electrical connections. The GTM module is designed to operate under pressure therefore, re-pumping is not required in order to maintain flow rate through the system. Design dissolved oxygen removal was targeted at 75% to 80% removal at 28 inches (69 cm) of vacuum with a raw feed level of 1 ppm. The system sizing is not straight forward. The PWST s dissolved oxygen levels drop when there is zero out flow from the tank, while the dissolved oxygen level increases slightly when filling the tank. Page 2 TP1067EN.docx
system sizing Theoretical Calculations. The GTM design and the recirculating flow path can have a dramatic effect on the dissolved oxygen level in the tank and the water subsequently sent to process. For instance, according to the equation for purification by dilution5, it takes 7 to 9 times longer to remove the oxygen from the tank than it does to remove it in a single step while filling it, refer to Equation 1. dissolved oxygen at various starting concentrations. Graphs 1 and 2 clearly illustrate that much more effort is required to remove the dissolved oxygen by recirculation than it is to remove it when filling the tank. equation 1: purification by dilution graph 1: days dissolved oxygen Equation 1 is illustrated in Graph 1 for saturated oxygen levels at 3.0 ppm and 10 ppm using 160,000 gallons (606 m 3 /h) for the tank volume and 100 gpm (0.4 m 3 /h) for the recirculating flow rate. Accordingly, if the tank starts at 3 ppm, the dissolved oxygen in the tank will fall below 0.1 ppm in approximately 15 days. This information is tabulated in Table 1 for different initial dissolved oxygen (D.O.) levels using the same conditions previously stated. table 1: tank turnovers to 0.1 ppm dissolved oxygen graph 2: initial tank DO vs. turnovers required to reach 0.1 ppm DO Tank turnover rate is equal to the amount of time required to process a tank volume and is calculated by multiplying the recirculation flow rate in gpm by the time in minutes divided by the tank volume in gallons, Equation 2. equation 2: turnover rate When the product k equals the volume in the tank one turnover has been achieved. Using a logarithmic scale to produce a straight line, Graph 2 illustrates the number of tank turnovers required to reach 0.1 ppm A more successful design removes as much dissolved oxygen as possible prior to entering the tank with the emphasis on maintaining low levels of dissolved oxygen by constant recirculation through the GTM. In order to maintain decreasing dissolved oxygen levels in the tank the recirculation flow rate must be greater than the flow rate leaving the tank. Ideally, the GTM is designed for the greatest possible dissolved oxygen removal for a recirculation flow rate greater than the flow rate going to process. Graph 3, based on mass balance, gives the maximum size in gpm for the GTM system required to maintain 0.1 ppm dissolved oxygen in the storage tank at varying make-up dissolved oxygen concentration. TP1067EN.docx Page 3
graph 4: storage tank dissolved oxygen graph 3: GTM sizing vs. make-up at varying DO When sizing the GTM system, a balance must be made between the recirculation rate and the daily volume required for make-up. The storage tank volume represents a major buffering capacity against variations in feed water dissolved oxygen levels. At minimum, the GTM recirculation system should be sized to handle the maximum make-up flow rate to the tank. It is important to remove most of the dissolved oxygen before it enters the tank since it requires more effort to remove it by recirculation. Most of the dissolved oxygen can be removed in a single pass. However, multiple passes are required to reach the same level in a recirculating method. System Data. The GTM recirculation system s performance, with the theoretical dissolved oxygen reduction, is illustrated in Graph 4. The system began operation on November 1, 1999 and operated continuously at 100 gpm (0.4 m 3 /h) for 27 days. During this period, an average 1000 gallons per day were made up to the tank and as previously stated the tank capacity is 160,000 gallons (606 m 3 ). A dramatic dissolved oxygen reduction from 2.5 ppm to 0.2 ppm was realized after 4 days of operation. The 100-ppb oxygen level was accomplished after 15 days. As Graph 4 illustrates, the data compares favorably with the theoretical oxygen removal based on Equation 1. The difference is explained by the equation s assumptions. First, the equation assumes absolute contaminant removal. Second, it assumes zero contaminant ingress. Thirdly, the equation makes no correction for GTM s removal efficiency. In this case, the third assumption has only a minor impact since the amount of make-up contains less than 3 ppm dissolved oxygen and is less than 1% of the tank s volume. Considering these assumptions, a closer correlation can be achieved by designing the GTM system to remove dissolved oxygen to less than 10 ppb. Graph 5 documents the GTM system s percent dissolved oxygen removal capabilities at different feed water levels. This does not represent the GTM s best performance capability since percent removal is a function of the system s overall design. This system was designed to reduce dissolved oxygen to a 100-ppb limit as evident by the rapid drop off in percent removal at this level. Graph 6 illustrates the tank and the GTM outlet dissolved oxygen levels on a logarithmic scale for better readability. Even though the GTM system was designed with a 100-ppb dissolved oxygen limit, 20 ppb effluent was recorded with tank levels as low as 50 ppb. As of June 2000, tank dissolved oxygen levels in the 20 to 50 ppb range are routinely observed. Page 4 TP1067EN.docx
conclusion graph 5: percent removal vs. influent DO graph 6: D.O. levels & GTM outlet The data demonstrates that a recirculating GTM system is a viable alternative to installing a nitrogen blanket, floating roof or conventional vacuum deaerator, for maintaining low level dissolved oxygen in a storage tank. Installation is simple and operator attention is minimal making GTM a strong candidate for a tank retrofit if new water specifications call for low level dissolved oxygen make-up. Operating in this mode the system should be sized to handle the maximum make-up flow rate to the tank. Although the GTM can remove dissolved oxygen in a recirculating mode, at least partial deoxygenation should be considered in the make-up system. This paper demonstrates that removing a contaminant in the make-up by a single step is more efficient than removing the contaminant by recirculation or dilution. If make-up water specifications demand the GTM to protect the treated water from oxygen ingress and to act as the primary method for removing dissolved oxygen, the system should be sized at the maximum make-up flow rate while taking into account the turnover rate and the maximum dissolved oxygen level that can be tolerated. However, the absence of full or partial deoxygenated make-up is far from being insurmountable. Some make-up water treatment designs incorporate dissolved carbon dioxide removal that may result in some level of deoxygenation as a side benefit. If the make-up system does not incorporate deoxygenation in any form, a GTM system can easily be incorporated. This paper was presented at IWC as IWC-00-20. TP1067EN.docx Page 5
references 1. D.C. Giancoli, General Physics, (Englewood Cliffs, NJ, Prentice-Hall, Inc., 1984), pp. 337-343. 2. T.L. Brown, H.E. LeMay, Chemistry: The Central Science, 2nd Edition, (Englewood Cliffs, NJ, Prentice-Hall, Inc., 1981), pp. 245-270 and 345-375. 3. 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, 1993. 4. 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, 1993. 5. The Permutit Company, Inc, Water and Waste Treatment Data Book, (15th printing 1986), Section 70, p. 108. Page 6 TP1067EN.docx