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New technology for oxygen removal in steam boilers used for thermal recovery: the water film method. Liu Meicheng1, Jiang Hailong1, Zhai Zhenshan1, Kong Ying2, Shi Deqing2 (1. Anqiu Wenrui Membrane Technology Development Engineering Company, Anqiu 262100)； 2. China University of Petroleum, Dongying 257061) Abstract: This paper briefly describes the traditional principles and methods for deoxidizing boiler feedwater, with a focus on the application of MTY membrane deoxidizers in the feedwater of steam boilers used in oilfield thermal recovery processes. Through the analysis of the operating conditions of membrane deaerators, their advantages and disadvantages were summarized, and the optimal deaeration process suitable for the characteristics of the Yellow River’s surface water, such as poor water quality and high suspended solids levels, was determined. Keywords: boiler feed water ； Deoxygenation methods ； Membrane deaerator. Chinese classification number: TQ028.8 Document code: A. Deoxygenating boiler feed water is crucial for ensuring the safe operation of boilers and extending their lifespan. If the boiler feedwater is not deoxygenated, not only is safe operation not guaranteed, but its lifespan also decreases by 66%-75%; therefore, deoxygenation is a very critical step (1). The use of a wet steam generator for heating heavy oil involves a type of direct-fired steam boiler. Its operation requires strict control over the oxygen content in the water; excessive oxygen levels can cause oxygen corrosion of the boiler tubes. This oxygen corrosion directly affects the rate at which the thickness of these tubes decreases, thereby impacting their service life. The control of oxygen content in the water used for thermal recovery boilers plays a key role in ensuring the safe and economical operation of such boilers. Therefore, it is highly necessary to apply, develop, and promote new, efficient, and economical membrane-based deoxidation technologies. 1. Current status of deoxygenation technologies for boiler water 1.1 Chemical deoxygenation Chemical deoxygenation is achieved by consuming oxygen through the reaction between oxygen and a reducing agent. Chemical deoxygenation involves adding dissolved or diluted chemical agents directly into the boiler itself, the feedwater main, or the hot water piping system of hot water boilers. Commonly used chemical agents include sodium sulfite, hydrazine, and tannin-based substances such as tannic acid. Chemical deoxygenation has the advantages of simple equipment, easy operation, and good deoxygenation effects, but it also has the drawback of high reagent costs. Therefore, this method is rarely used alone for deoxygenation; on small and medium-sized boilers, it is often combined with deoxygenation using steel scrap ； In the high-pressure boilers of power plants, it is used as a supplementary measure for thermal deoxidation; recently, there has been little development in methods that rely solely on chemical agents for deoxidation. 1.2 Deoxygenation using deoxidizers: Deoxygenating water with deoxidizers involves removing oxygen from the water through physical methods. Based on their principles, these methods can be classified as follows: 1.2.1 Thermal deoxygenation: The principle of thermal deoxygenation is based on the law of gas dissolution (Henry’s law), which states that at a constant temperature, the solubility of any gas in water is related to the partial pressure of that gas at the water-gas interface as well as the water temperature. The solubility of oxygen in water decreases as the water temperature rises; when the water reaches boiling point, its solubility is zero. In boilers with a capacity of 2–6.5 t/h and in other situations where low-temperature deoxygenation is required, thermal deoxygenation has significant limitations. Based on the results of field applications, these deoxidizers, after being optimized, achieve a deoxidation effect of 0.018 mg/L. Since thermal deaeration requires the outlet temperature of the deaerator to be reduced to the range permitted by the feed water pump of the wet steam generator, this affects the deaeration efficiency, leaving a certain gap between the deaeration results and the quality requirements for the water used in the wet steam generator. 1.2.2 Vacuum Deoxidation The basic principle of vacuum deoxidation is the same as that of thermal deoxidation; it takes advantage of the fact that the boiling point of water decreases as pressure drops. This process is carried out at a pressure lower than atmospheric pressure. First, the water is heated to a temperature 0.5°C–1°C above its saturation temperature at the corresponding deoxidation pressure, after which it is sent to the deoxidizer. If the water has excess heat, part of it vaporizes while the remaining water remains in a boiling state. The gases dissolved in the water are then desorbed and drawn out of the deoxidizer using a ejector. Vacuum deoxidation is divided into two methods: high-level deoxidation and low-level deoxidation. The disadvantage of these methods is the high initial investment and large floor space required. 1.2.3 Desorption Deoxygenation The basic principle of desorption deoxygenation is also based on Henry’s law: the solubility of a gas in water is proportional to the partial pressure of that gas in the gas in contact with the water, and the partial pressure of a gas in a gas mixture is proportional to its concentration in that mixture. According to this law, as long as deoxygenated water is strongly mixed with already deoxygenated gas, the oxygen dissolved in the water will diffuse into the gas in large quantities, thereby achieving the removal of oxygen from the water. The disadvantage of this deoxygenation method is that its deoxygenation efficiency varies depending on the oxygen content in the water quality; its application range is limited. Moreover, it requires continuous addition of activated carbon and catalysts, which increases the workload and also leads to the shutdown of the wet steam generator. Based on the current performance in actual use, analytical deoxidization requires electric heating, which results in high energy consumption and costs. Moreover, due to design limitations, its deoxidization efficiency is poor and the equipment fails frequently, affecting the operation of the wet steam generator. High-vacuum deoxidization involves high initial investment and takes up a lot of space. Thermal deoxidization technology is relatively mature, but it also has high energy consumption and is not suitable for low-temperature deoxidization, as its deoxidization efficiency is low. Therefore, thermal recovery boilers for heavy oil require new, efficient deoxidization technologies. 2. Characteristics of membrane-based degassing and deoxygenation technology 2.1 Principle and process Membrane-based degassing and deoxygenation technology is a new type of deoxygenation method that utilizes the principles of membrane separation; it is implemented through membrane contactors. Membrane electrolyzers come in three types: hollow fiber, spiral-wound, and flat-plate (3). The membrane used is a hydrophobic microporous membrane, which can be made from fluoroplastics or polyolefins. The principle of degassing and deoxygenation using this membrane method is shown in Figure 1. The membrane contactor is separated into two phase regions by a hydrophobic microporous membrane: one is the aqueous phase region and the other is the gas phase region. The gas phase zone is connected to the vacuum system. After the oxygenated raw water enters the contactor, since the membrane is a hydrophobic microporous membrane, water cannot pass through the membrane pores into the gas phase region, whereas gases can pass through these pores and diffuse into the gas phase region. Under the effect of vacuum negative pressure, a partial pressure difference exists for oxygen between the aqueous phase and the gas phase on either side of the membrane; as a result, oxygen diffuses through the membrane pores into the gas phase and is evacuated, thereby achieving deoxygenation of the raw water. The deoxygenated water is then discharged from the membrane module. 2.2 Hydrophobic hollow fiber microporous membrane module: The membrane module of the MTY10 membrane deaerator is of the combined type produced by Membran’s U.S. subsidiary. The polypropylene hollow fiber membranes used possess strong hydrophobicity, high gas permeability, high tensile strength and rupture strength, as well as certain resistance to acid and alkali corrosion and heat, thereby meeting the requirements of membrane-based deoxygenation technology regarding membrane performance. A schematic diagram is shown in Figure 2. 2.3 Process flow: Considering characteristics such as high levels of suspended solids in the water quality of this region, a special process flow diagram for membrane-based degassing and deoxygenation has been developed, as shown in Figure 3. 2.4 Process flow diagram: The filter system employs two stages of filtration – pre-filtration and final filtration – to address the high levels of sediment and suspended solids present in the water from the Yellow River. The equipment undergoes automatic backwashing at regular intervals for pre-treatment, while the deoxygenation components are cleaned chemically on a periodic basis; as a result, the residual oxygen content after deoxygenation is no more than 0.2 mg/L. 3. Results and Analysis 3.1 Relationship between Deoxidation Pressure and Oxygen Removal Rate Figure 5 shows the oxygen removal rate at different deoxidation pressures (vacuum levels). It can be seen that the oxygen removal rate η increases gradually as the vacuum level rises; this is because when the vacuum pump creates a pressure gradient, gas is forced to pass from the liquid phase through the vacuum pump. According to Dalton’s law: the total pressure of a gas mixture is equal to the sum of the partial pressures of its constituent gases, that is, P_total = P1 + P2 + P3 + …… + Pn. According to Henry’s law as well, the solubility of a gas in water is directly proportional to its partial pressure in water; that is, P1 = H*X, where P1 represents the partial pressure of the gas, H is the Henry’s constant, and X represents the concentration of the gas dissolved in water. By reducing the partial pressure of the gas, some of the dissolved gases escape from the water, and this reduction in partial pressure can be achieved by lowering the total pressure of the gas or by reducing the concentration of the individual gases in the gas phase. As the vacuum is increased, the total pressure P_total of the gas decreases. Since the Henry’s law constant remains constant at a given temperature, the concentration X of the gas dissolved in water decreases, which means that the degassing efficiency η increases. 3.2 Relationship between liquid flow rate and oxygen removal rate Figure 6 shows the O2 removal efficiency at different liquid flow rates; it can be seen that the O2 removal rate η decreases as the liquid flow rate increases. This is because the residence time of the gas within the component decreases, resulting in a reduced proportion of O2 undergoing absorption reactions. The increase in flow rate enhances the turbulence of the gas (2), accelerating its diffusion from the gas/liquid interface layer into the liquid itself. As a result, the liquid consumed at the gas/liquid interface is replenished promptly, allowing a high liquid concentration to be maintained at the absorption interface. Meanwhile, the mass transfer boundary thickness decreases, increasing the liquid mass transfer coefficient and thereby raising the liquid’s ability to absorb O2; consequently, the efficiency of O2 removal decreases. 3.3 Relationship between liquid temperature and oxygen removal rate Figure 7 shows the relationship between liquid temperature (4) and the oxygen removal rate; the higher the liquid temperature, the lower the oxygen content in the liquid. According to Henry’s law, for a given gas and a specific solvent, the Henry coefficient changes with temperature. Generally, as the temperature rises, the H value (Henry coefficient) increases. This reflects the trend that gas solubility decreases as temperature rises. According to Henry’s law: P1=H*X. At the same pressure, an increase in temperature leads to an increase in the value of H, which results in a decrease in the concentration X of the gas dissolved in the liquid. Under the same conditions, the solubility of gases in water decreases, as does the deoxygenation efficiency η of the liquid. 4. Conclusions 1) As the vacuum degree increases, the O2 removal rate η gradually increases. 2) As the flow rate of the liquid increases, the O2 removal rate η gradually decreases. 3) The higher the liquid temperature, the lower the O2 removal rate η gradually becomes. 5. Conclusion Since its introduction in a certain oil production plant, this membrane-based degassing and deoxygenation device has operated well and features advanced technology. As a new technique that has emerged in recent years, it offers the following advantages over traditional deoxygenation methods such as thermal deoxygenation, vacuum deoxygenation, and desorption deoxygenation: 1) It can remove oxygen at low temperatures without the need to heat the water, resulting in low energy consumption. 2) High deoxygenation efficiency, with a deoxygenation rate of over 99%. 3) The equipment is small in size and light in weight; with the same processing capacity, a membrane deaerator is an order of magnitude smaller in size than a vacuum degassing tower, making it highly suitable for onboard systems. 4) Easy to operate and maintain, with low operating costs. 5) It can be designed in a modular manner, allowing for expansion and modification as the processing volume changes. References: (1) Shao Gang, Editor. Membrane-Based Water Treatment Technologies and Engineering Examples. ISBN 7-5025-3675-2/X.131. (2) Chen Wei, Zhu Baoku, Wang Jianli et al. Study on the separation of CO2/N2 mixtures using hollow fiber membrane separators. Membrane Science and Technology, 2001, 27(11): 48-50. (3) Li Xuxiang, Editor, Preparation and Applications of Separation Membranes, ISBN 7-5025-5101-8/TQ.1987. (4) Shi Jun, Yuan Quan, Gao Cong (eds.), Handbook of Membrane Technology, ISBN 7-5025-2760-5/TQ.1216.