Electrochemical-vacuum trinity deaerator

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I. Overview of Vacuum Electrochemical Deoxidizers and Two-Pole Vacuum Deoxidizers In the boiler feedwater treatment process, deoxidation is a very critical step. Oxygen is the main corrosive agent in water supply systems and boilers; oxygen present in the water must be removed promptly, otherwise it will corrode the water supply system and components of the boiler. The corrosion product, iron oxide, enters the boiler and deposits or adheres to the boiler tube walls and heating surfaces, forming insoluble iron scale that hinders heat transfer. Moreover, corrosion causes pitting on the inner walls of the pipes, resulting in an increased resistance coefficient. When pipe corrosion is severe, pipe explosions can even occur. Over the years, many professionals working in boiler water treatment have been seeking efficient and economical methods for deoxygenation. An deaerator that combines two-stage vacuum, electrochemical, and chemical deoxygenation methods is suitable for deoxygenating water at normal temperatures; it possesses advantages that no other deoxygenation method can match, making it highly versatile. II. Structure of the vacuum electrochemical deaerator: It is mainly composed of a housing, a new type of spinning film device, a water spraying grid, a heat storage filler and vapor network, a water tank, and a device for preventing water from entering the exhaust gas, etc.: 1. Housing: It is formed by welding a cylinder body with a stamped circular end cap together. Medium and low-pressure deaerators are equipped with a pair of flanges connecting the upper and lower sections for assembly and maintenance purposes, while high-pressure deaerators have manholes for maintenance. 2. New type film spinner: It consists of a water chamber, a steam chamber, a film spinning tube, a condensate water connection pipe, a make-up water connection pipe, and a primary steam inlet pipe. The film-forming tube of the new type of film former is equipped with a water film guiding device, which enables strong film formation even at low load conditions and maintains a good water film skirt. The condensate water and chemical make-up water are sprayed in a spiral pattern through membrane-forming nozzles at a certain angle, thereby creating a water film skirt. This film skirt undergoes heat exchange with the heating steam introduced via the primary heating steam pipes, resulting in primary deoxidation. The feed water comes into contact with the secondary heating steam rising through the water grates, and is heated to a temperature close to the saturation temperature at the operating pressure of the deoxidizer – that is, 2-3°C below the saturation temperature – thereby achieving rough deoxidation. Generally, about 90-96% of the oxygen content in the feed water can be removed through this rotating film section. 3. Water distribution grating: It is composed of several layers of angular steel components arranged in an interlaced pattern. The feed water, which has been preliminarily deoxygenated in the spinning film section, is distributed here a second time, falling in a uniform mist-like form onto the liquid-vapor mesh located beneath it. 4. Heat storage packed liquid-vapor network: It is a SW-type corrugated packed material composed of many units of identical shape and size, forming a cylindrical structure. This structured packing retains the advantages of both wire mesh corrugated packing and perforated plate corrugated packing, while also featuring high flow capacity, low pressure drop, great operational flexibility, high separation efficiency, low energy consumption, and the property of never falling off. The heat storage filler itself acts as a heat storage device for secondary steam; thorough heat exchange between the feedwater and this heat storage device enables effective deoxygenation, with the low-pressure atmospheric deoxygenator achieving levels below 10 ug/L and the high-pressure deoxygenator achieving levels below 5 ug/L. 5. Water tank: The deoxygenated feed water is collected in the container located below the deoxygenation head, namely the water tank. This deoxygenation water tank is equipped with a scientifically designed high-efficiency heat exchange and reboiling device. Such a device enables efficient heat exchange, rapid increase in water temperature, more thorough deoxygenation, reduction of vibrations in the water tank, and lowering of noise levels; all these advantages contribute to extending the equipment’s service life and ensuring its safe and reliable operation. III. Principles of vacuum electrochemical deoxidizers and two-stage vacuum deoxidizers: Step 1: Two-stage vacuum deoxidation. The two-stage vacuum deoxidizer features a fully sealed structure and remains in a vacuum state throughout its operation. The water supply first enters the high-speed centrifugal membrane device through the electrolytic cell. The working principle of two-stage vacuum deoxygenation relies on Henry’s law and Dalton’s law. According to Henry’s law, in a closed container, when any gas is present above the water surface, its solubility is proportional to its partial pressure, and this solubility depends solely on that gas’s partial pressure. At a certain pressure, as the water temperature rises, the partial pressure of water vapor increases, while the partial pressures of air and oxygen decrease. At 100°C, the partial pressure of oxygen drops to zero, and the dissolved oxygen in water also drops to zero. When the pressure on the water surface is lower than atmospheric pressure, the solubility of oxygen can also reach zero at lower water temperatures. In this way, the oxygen molecules in the water surface space are separated or converted into other gases, resulting in a zero partial pressure of oxygen; as a result, the oxygen in the water continuously escapes, and 90% of the dissolved oxygen is removed under negative pressure. Two-stage vacuum deoxidation is used to lower the boiling point under vacuum in order to achieve deoxidation. Step 2: Electrochemical deoxygenation. The entire system is equipped with an electrolytic separation tank, which consists of an anode and a cathode that form the water decomposition channel, thus constituting the electrolytic separation assembly; water flows between these two electrodes. Under the action of an external DC power supply, at the anode: the iron plate loses electrons through electrolysis, resulting in the formation of ferrous ions – Fe0 → Fe+2 + 2e-. At the cathode: oxygen gains electrons and forms hydroxide ions – O2 + 2e- + 2H2O → 4OH-. In water, the electrochemical products are Fe+2 + 2OH- → Fe(OH)2. The third step is chemical deoxygenation. When it comes to chemical deoxygenation, people usually think of adding chemicals; however, the third step in this device does not require the use of any chemicals. Instead, it makes use of Fe(OH)2, which is produced as a result of the electrochemical deoxygenation in the first step, to react with the dissolved oxygen in water, thereby achieving chemical deoxygenation. The chemical reaction equation is: 4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3. It is clear that this deoxygenation device enables rapid and thorough deoxygenation – it not only removes the dissolved oxygen from water but also eliminates any residual dissolved oxygen in the system. Two-stage deoxygenation was carried out in the same closed container, enhancing the reliability of electrochemical and chemical deoxygenation. IV. Composition of vacuum electrochemical deaerators and two-pole vacuum deaerators: Vacuum electrochemical deaerators and two-pole vacuum deaerators are mainly composed of components such as the deaerator (deaeration head), high-efficiency rotary film device, continuous reaction electrolysis tank, booster pump, water-jet vacuum pump unit, water intake pump unit, and PLC control box. Vacuum electrochemical deoxidizers and two-pole vacuum deoxidizers are equipped with electrolysis cells that contain high-efficiency swirl film devices; these devices transform the natural falling film effect into a strong swirl film effect, increasing the frequency of film renewal and causing the liquid film to rotate vigorously along the tube walls, thereby enhancing dispersion and mass transfer capabilities ； Change the corresponding bubbling to suspended bubbling. Overcome splashing when the water vapor flow velocity in the layer is high, and maintain the gas channel ； The three separate modes of heat and mass transfer are combined into one, carried out within the components of a single unit. When a vacuum electrochemical deaerator or a bipolar vacuum deaerator is in operation, the deaerator is under negative pressure. When installed at a high position, the distance between the outlet of the electrolysis cell and the inlet of the boiler feed water pump should be large; it is advisable that this distance be no less than 10 meters ； When installed at a low position, the outlet of the electrolysis cell is connected to the feedwater pump unit, so that the under-pressure water is pressurized before entering the boiler feedwater pump. Long-nozzle water ejectors are used in water-jet vacuum pump systems; these ejectors feature a compact design and extremely low power consumption. It also features high suction efficiency, with an air extraction volume that is twice that of older models under the same conditions. Low noise, no vibration, and simpler installation. The function of the feedwater pump unit is to raise the pressure of the water under negative pressure, using a feedwater pump, when the deaerator is installed at a low level, and then supply this water to the boiler feedwater pump. The vacuum electrochemical deaerator and the two-pole vacuum deaerator are put into operation. As a first step, the water level in the water injection tank of the deaerator is raised to the overflow level; the overflow valve is set to an open position, and water is also supplied to the water injection tank as usual to maintain the water temperature in that tank at 30°C. After that, the relevant valves are closed, and the water level in the deaerator is adjusted to the standard operating level. Start the vacuum water ejector pump and open the gate valve on the suction pipeline. When the vacuum level in the deaerator reaches -0.075 Mpa, open the water inlet valve and start the boiler feed water pump. The deoxygenated water enters the rotary film device, where a high-efficiency water film is formed under a water pressure of 0.2 Mpa. Operating under high vacuum raises the water temperature by approximately 30°C to 60°C, allowing oxygen to be effectively removed and exhausted into the water ejector tank. After the water is deoxygenated through electrolysis in the electrolytic tank, it enters a secondary electrolytic tank for further chemical deoxygenation. The deoxygenated water is then sent to a tertiary electrolytic tank where it undergoes additional chemical deoxygenation and pressure enhancement, after which the boiler feed pump delivers this deoxygenated water to the boiler. When the deaerator is taken out of service, it is necessary to first close the gate valve on the suction pipeline before stopping the feed water pump. If the feed water pump is stopped first without closing the gate valve on the suction pipeline, the deaerator will be in a vacuum state, which will cause the water in the feed water tank to be drawn back into the deaerator, thereby affecting its deoxygenation efficiency. V. Automatic control systems for vacuum electrochemical deoxidizers and two-pole vacuum deoxidizers 1. The normal operation of these system devices is achieved through fully automatic control without the need for human intervention; the entire operation is managed by an PLC, which automatically monitors and adjusts all operating parameters and control points. All valves and vacuum pump sets are automatically controlled to open (they can also be opened manually). 2. The vacuum level during the online operation of the equipment should be maintained at above -0.05 to -0.06 Mpa. Ensure that the deionized water enters the deaerator and rotates vigorously within the high-efficiency spinning film device to form a water film skirt, allowing oxygen to be dissolved out. Once the vacuum level becomes unstable, the equipment emits an automatic alarm, at which point personnel need to make on-site adjustments. 3. The water level is controlled by an automatic level controller; if the water level falls below the designed level, the device will issue an automatic alarm. 4. The PLC performs automatic tracking control of the electric feedwater control valve and the vacuum level, ensuring that the oxygen content in the feedwater remains at ≤0.05~0.1 mg/L at all times.