What is air separation? Air separation units and systems

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Everyone is familiar with various types of compressors and turbines, but do you truly understand their role in the air separation process? Do you know what an air separation plant in a factory looks like? Air separation, simply put, is a set of industrial equipment used to separate the various gases present in air in order to produce oxygen, nitrogen, and argon. There are also the rare gases helium, neon, argon, krypton, xenon, radon, etc. Air separation equipment is a type of device that uses air as raw material; it converts air into a liquid state through compression and cryogenic processes, and then uses distillation to gradually separate inert gases such as oxygen, nitrogen, and argon from the liquid air. This equipment is widely used in traditional metallurgy, advanced coal chemical processing, large-scale nitrogen fertilizer production, and specialized gas supply industries. In simple terms, the system processes in an air separation unit include: a compression system, a pre-cooling system, a purification system, a heat exchange system, a product delivery system, an expansion refrigeration system, a distillation tower system, a liquid pump system, and a product compression system. We will introduce each of these components one by one based on the system processes of an air separation unit: The compression system includes self-cleaning air filters, turbines, air compressors, boosters, and gauge compressors. (1) Self-cleaning filters generally have an increasing number of filter cartridges as the air flow rate increases, with a higher number of layers as well; typically, there are two layers for models of 25,000 units or above, and three layers for models of 60,000 units or above ； Generally, a separate filter is required for each compressor, and it should be installed at the upper air intake. (2) A turbine is a type of device in which high-pressure steam expands to do work, driving a coaxial impeller to rotate, thereby performing work on the working fluid. There are generally three common types of steam turbines: fully condensing, fully back-pressure, and extraction-condensing; the extraction-condensing type is the most commonly used. (4) In large air separation plants, single-axis isothermal centrifugal compressors are generally used for air compression; those imported have a power consumption about 2% lower than domestic ones, but their cost is 80% higher ； The air compressor features an outlet vent and no return pipeline; it generally has a minimum suction flow requirement to prevent surging, and inlet guide vanes are used for flow regulation. Domestic models of such compressors employ four stages of compression with three stages of cooling (the last stage is without cooling). The main air compressor is equipped with a water washing system to flush the deposits on the surfaces of the impellers and volutes at various stages. The system comes as a set with the host. (5) In large-scale air separation units, two types of compressors are generally used for investment purposes: single-axis isothermal centrifugal compressors and gear-type centrifugal compressors. The gear-type compressors have a significant advantage in terms of energy consumption, especially under conditions involving high pressure. (6) Instrument air compressors generally come in three types: oil-free screw compressors, piston compressors, and centrifugal compressors. Since piston and centrifugal types are naturally oil-free, no oil removal device is required; only a drying unit (for water removal) and a precision filter (for solid particles) are needed ； Screw compressors are generally available in oil-based and oil-free versions, as well as in types with oil removal functionality. Screw compressors that use oil require the installation of oil removal devices, along with high-precision oil removal filters to meet the requirements of the manufacturing process. The advantage of this type of compressor is its relatively low cost ； Oil-free screw compressors use a dry rotor or water lubrication; the advantage of this type is that it contains absolutely no oil, while the disadvantage is its higher cost. For gas volumes of 500 Nm³/h or less, piston-type compressors are suitable ； For gas flow rates of 2000 Nm³/h or less, screw compressors or piston compressors are suitable choices ； For gas flow rates of 2000 Nm³/h or higher, any of the three models can be chosen. At higher gas flow rates, centrifugal compressors have advantages: they have fewer vulnerable components, are easier to maintain, and offer a good cost-performance ratio. The gauge compressor is used during startup, and after normal operation, the gas is drawn from behind the molecular sieve purifier. Pre-cooling system: There are two types of air-cooled towers in the pre-cooling system: closed-loop (the air-cooled tower is divided into upper and lower sections, with chilled water circulating between the upper section of the tower and the water-cooled tower) and open-loop (with a circulating water system). The closed-loop system is mainly used in chemical plants where the water quality is poor, as it requires the addition of fresh water and chemicals ； Open-loop circulation is widely used, but circulating water systems also require regular addition of fresh water, and pre-cooling systems need to take into account operating conditions in summer. Air-cooled towers are generally designed with stainless steel Buerle rings with a diameter of 76 mm and a length of 1 meter at the bottom (heat-resistant), enhanced polypropylene Buerle rings with a diameter of 76 mm and a length of 3 meters (high flow rate), and enhanced polypropylene Buerle rings with a diameter of 50 mm and a length of 4 meters. There are also two types of water cooling towers: two-stage type (when no external cooling source is available, the cooling capacity for drying the nitrogen gas is sufficient, ensuring a reliable pre-cooling system; however, the resistance is double, using 7 meters + 7 meters of φ50 polypropylene Bower rings), and single-stage type (when an external cooling source is available, using 8 meters of φ50 polypropylene Bower rings). Furthermore, filter devices are generally installed at all water inlets of the pre-cooling system (usually 6 in total: 4 for water pumps, the inlet to the water cooling tower, and the inlet on the evaporation side of the chiller), to prevent impurities from entering the system. The effectiveness test of the pre-cooling system showed that the outlet temperature at the 4-meter fill section at the lower part was 1 degree lower than the inlet temperature℃ ； At the outlet of the 8-meter filler section in the upper section, the temperature is 1°C higher than that of water; generally, a thermometer is installed in the middle of the air-cooling tower (extending into its interior). Purification system: The adsorbers used in the purification system include vertical axial flow, horizontal double-bed, and vertical radial flow types. Vertical axial flow is mainly used in conjunction with air separation units of 10,000-class capacity or lower (with diameters up to 4.6 m). The bed thickness ranges from 1550 to 2300 mm, and both double-layer and single-layer configurations are possible; vertical axial flow adsorbers offer the best airflow distribution. Horizontal twin beds are mainly used in conjunction with large and medium-sized air separation units, with a bed thickness of 1150 mm (molecular sieve) + 350 mm (aluminum adhesive). Vertical radial-flow adsorbers can make effective use of the internal space of the container, increasing the area of the adsorption layer with the same diameter by about 1.5 times. This allows for a significant reduction in the height of the tower, while the vertical configuration also results in less floor space being required. Due to the uniform airflow distribution, unlike horizontal adsorbers where the airflow is uneven, the amount of molecular sieve used is reduced by 20%, and the energy required for regeneration is also saved by 20%. However, the disadvantage of vertical radial flow is the concentration of airflow at the center (in a fan-shaped area), which results in a faster penetration time compared to horizontal flow (requiring CO2 < 0.5 ppm). The bed thickness is 1000 mm + 200 mm, and the vertical radial flow configuration can meet the requirements of air separation units of 20,000 class and above. There are two methods for regenerative heating: electric heaters and steam heaters. Steam heaters come in horizontal types (for capacities below 40,000 units) and vertical types (for capacities above 40,000 units). There are also high-efficiency vertical steam heaters (which offer a high steam utilization rate and save 20% in energy consumption). The installation options include using a single steam heater (with H2O leakage detection points) ； Electric heaters (two in use with one as backup, or one in use with one as backup) connected in parallel (interlock shutdown at high temperature and low flow rate to prevent damage; the heating tube material is 1Cr18Ni9Ti) ； An electric heater (for activation and regeneration, 250∽300℃) is connected in parallel with a steam heater ； An electric heater and a steam heater are connected in series (when the steam temperature is low, this results in greater regeneration resistance). A throttling regeneration pipeline also needs to be installed in the purification system to meet the requirements for startup. In addition, safety valves are installed on the regasified gas side and on the steam heater side to prevent leakage or overpressure on the high-pressure side of the equipment or valves, as well as throttling-induced overpressure. Manual butterfly valves are installed in the regeneration flow path to adjust the resistance, thereby ensuring stable operation of the main tower (or they may not be used, and timing control is achieved through control valves installed in the main pipeline). Heat exchange system: Strictly speaking, a heat exchange system involves designing multiple flow streams of mixed media within the same heat exchanger, thereby allowing for automatic balance in heat transfer between these media and resulting in minimal energy consumption. However, this approach means that in cases of internal compression processes, all heat exchangers will have to be high-pressure types, which increases the overall investment cost. Therefore, for internal compression heat exchangers of 20,000 class or higher, it is more economical to use separate high-pressure and low-pressure heat exchangers; whereas for those below 20,000 class, high-pressure heat exchangers are used exclusively. The product delivered is a low-pressure oxygen-nitrogen product; a product control valve and a venting circuit are provided, with the vent going to a silencer (the nitrogen-related components are made of carbon steel, while those related to oxygen are made of stainless steel). The contaminated nitrogen gas is directed to the water-cooled tower exhaust system; this serves to discharge the contaminated nitrogen gas, to prepare the regenerated gas, and to adjust the pressure in the tower. It is necessary that the diameter of the water-cooled tower be sufficient to meet these discharge requirements. Especially in situations where nitrogen gas is also introduced, it is important to prevent the pressure in the tower from rising too high. The resistance in the water-cooled tower is 6 kPa (for a filler height of 8 meters); the resistance in the pipelines and valves is 4 kPa, while the pressure difference required for discharge to the atmosphere is 2 kPa. In total, this amounts to 12 kPa. High-pressure oxygen products are vented using a two-stage throttling process: first, the high-pressure gas is throttled to 10 barG, passes through an eccentric reducer with a Monel noise-reducing plate installed in between, and then the diameter of the pipeline is increased again via another eccentric reducer. The flow velocity of the oxygen stream is kept below 10 m/s, after which it enters a silencing tower for further throttling and venting; the noise-reducing elements in this tower are made of stainless steel ； High-pressure nitrogen products: The nitrogen is first throttled to 10 bar, passed through a stainless steel noise reduction plate, and then sent to a silencing tower where it is throttled and vented; the noise reduction elements are made of carbon steel ； Oxygen valves must not be operated by hand (control valves must not have handwheels, and manual valves should be placed inside explosion-proof walls). The silencing tower can also be integrated with the compressor system’s venting mechanism; the noise reduction is achieved by using the compressor or booster (based on the capacity of the compressor), with the air being directed into the silencing tower. Additionally, it handles the pressure release from the purification system, with the booster facilitating the return flow and thus allowing for the discharge of some of that air. Expansion refrigeration systems: There are generally three types of expanders, namely low-pressure expanders, medium-pressure expanders, and liquid expanders. For a certain type of gas expander, the higher the volumetric flow rate of the working fluid, the higher the efficiency. For general flow rates of 8000 Nm³ or more, the efficiency of low-pressure expanders is 85∽88%; when the flow rate is less than 3000∽8000 Nm³, the efficiency drops to 70∽80%. For medium-pressure expanders, it is common to use one imported unit and one domestically produced unit (as a backup). For inlet expanders with a capacity of 8,000 Nm³/h or more, the efficiency is 82–91% (4 percentage points lower at the booster stage) ； The efficiency of domestic expanders is 78–87% (5 percentage points lower at the pressurization end). Before starting the expander, it is necessary to purge it first (to remove impurities from the piping system and those inside the expander’s volute), then introduce sealing gas (which is normally supplied from the pressurization side). After that, an external circulation and an internal circulation of the oil system are carried out. Interlock tests must be completed before the expander can be started. Once the cold test is successful, a cold tightening is performed ； Cold start requires the fuel tank heater to be activated, but it is not needed during normal operation, as the temperature of the bearings is then balanced. A liquid expander essentially utilizes the pressure head of high-pressure liquid to perform hydraulic work (while the enthalpy of the liquid decreases, but to a much lesser extent compared to gases). Generally, in air separation units of 40,000 series and above, liquid expanders can be used as a substitute for high-pressure liquid air throttle valves. Its advantage lies in using the liquid expansion mechanism for cooling and generating electricity from the expansion work, thereby achieving energy savings; generally, around 2% energy savings can be achieved, although the investment required is in the tens of millions of yuan. In distillation tower systems, sieve tray towers are more commonly used for columns of 15,000 to 50,000 capacity units. Circulating tray towers have an advantage in columns with a smaller diameter below 15,000 capacity units (the liquid flow path is longer compared to convection-type towers, but their construction is more complex). Convection-type towers are more widely used below 30,000 capacity units, while they hold an advantage at capacities above 15,000 units. Four-way overflow trays are more suitable for large columns above 30,000 capacity units. Packing towers require less energy, but the height of the column needs to be increased by about 5 meters. Air separation systems at the 50,000-class level and above have an advantage, especially when the upper and lower towers are arranged in parallel. The upper tower, the crude argon tower, and the refined argon tower all use packed towers; the manufacturers are usually Sulzer or TianDa Beiyang. For the crude argon tower, the cooling source is typically oxygen-enriched liquid air, and the waste gas can be discharged into the waste nitrogen pipeline. The energy consumption is low when the argon system is not in operation ； The heat source for the refined argon column is oxygen-enriched liquid air or nitrogen from the lower column; the cold source can be oxygen-depleted liquid air or liquid nitrogen. There are two types of feed inputs: liquid and gaseous. It should be noted that the sealing performance of the plate fins in the crude argon column condenser is required to be high; otherwise, the argon product will not meet the quality standards. Primary coolers come in single-layer, vertical double-layer, horizontal row double-layer, vertical triple-layer types, as well as falling-film primary coolers (where liquid oxygen and gaseous oxygen flow downward in the same direction as nitrogen). There are 6 ways to arrange distillation tower systems: (1) The upper and lower towers are arranged vertically, which is the conventional arrangement; it results in a lower height, and there is no issue where liquid from the lower tower cannot enter the upper tower or the crude argon tower condenser (the back pressure resulting from the liquid-phase flow in the pipes is sufficient, and in such cases the pipe diameter cannot be too small) ； (2) The lower and upper towers are arranged vertically, which is the conventional layout; their height is moderate. It is difficult for the liquid in the lower tower to reach the upper tower. Alternatively, a stripping pipeline can be used to convey liquid from the crude argon tower to the upper tower (it is required that the outlet of this pipeline satisfy ρυ²＞3000, where ρ is density and υ is flow velocity). The inlet should be located at a height where the vaporization rate in the pipeline is 1%, at which point the pipe diameter needs to be reduced appropriately, and the degree of subcooling of the liquid should not be too high ； (3) The upper column is arranged at the bottom of the argon distillation section and connected by two circulating oxygen pumps; reducing the height of the upper column helps to prevent the liquid from the lower column from entering the upper column or the crude argon column condenser ； (4) The upper column is arranged below the argon distillation section and connected using circulation pumps; the top section of the crude argon column is located above the upper part of the upper column, which allows the space in the cold box to be reduced ； (5) The autonomous cooling system is arranged at the bottom of the upper tower, connected by circulation pumps; the main cooler is located at the top of the lower tower. The advantage is that the main cooler can be made quite large ； (6) The autonomous cooling system is arranged at the top of the tower, connected by circulation pumps; the uppermost section of the crude argon tower is located above the main tower. The advantage of this approach is that the main cooler can be made quite large, which in turn allows the size of the cooling chamber to be reduced. In a liquid pump system with horizontal-mounted pumps (where the inlet pipe is below the outlet pipe), it is necessary to provide heating gas (placed behind the pump or in front of the filter to prevent impurities from entering), sealing gas, outlet and exhaust valves (for discharging liquid at lower levels and exhaust gas at higher levels), as well as a return line for the liquid and gas. The rotation speed of horizontal pumps should not be too high; generally, the discharge pressure should be below 30 barG. Due to their horizontal installation, horizontal pumps can withstand axial forces better during cold contraction, but high rotation speeds make it difficult to maintain proper rotor dynamic balance. Vertical pumps feature a bearing-suspended design (with the inlet pipe positioned above the outlet pipe), allowing them to withstand significant downward forces; the center of gravity of the rotor coincides with that of the shaft, enabling very high rotation speeds ； Generally, at pressures above 30 bar, the following need to be installed: backflow air before the pump (note that this is not required for horizontal pumps), heating air (placed before the pump filter, with air intake at a higher level), sealing air, liquid and air discharge valves (liquid discharged at a lower level and air discharged at a higher level; check during pre-cooling to ensure proper cooling), and a return line for the liquid and air. Vertical pumps are generally multi-stage, and the return gas pipeline must not point downward (it should be horizontal or inclined upward); otherwise, gas cannot be discharged, which can easily lead to cavitation in the pump. Additionally, a blower duct needs to be installed for the cryopump motor to prevent overheating in summer and frosting in winter. The liquid oxygen pump and liquid nitrogen pump are in online cold standby mode; the seal gas pressure of the liquid nitrogen pump is above 7 barG ； Oxygen pump seal gas pressure: 4 barG (nitrogen pressure at the lower column is sufficient) ； Circulating liquid argon pumps: one in use and one as a standby. The sealing gas is generally provided by vaporizing liquid argon; it is required that the flow rate has a 20% margin. Typically, a liquid argon pump is equipped with dual-loop control: pressure-bypass control for its own reflux valve, and flow-level control for its outlet valve. Product compression systems: Ordinary compressed air is sufficient for nitrogen permeation; for nitrogen turbine compressors, which operate at higher pressures, gear-driven types are more energy-efficient. Depending on the discharge pressure, oxygen compressors can be single-stage (for low pressures) or two-stage (consisting of a high-pressure stage and a low-pressure stage), with compression levels ranging from 8 stages up to 30 bar. For pressures below 30 barG, it is necessary to supply sealing gas at 5 barG; pressurized nitrogen can be used for this purpose. Additionally, due to the risks of fire associated with oxygen under high pressure and temperature conditions, all components that come into contact with the oxygen must be made of copper alloy. A safety nitrogen system must also be installed; this is typically determined by the engineering design institute ； Imported oxygen permeators are relatively expensive, costing about twice as much as domestic ones, and thus are not generally used. Currently, Hangyang oxygen permeators are commonly employed; a discharge pressure of 3 to 30 barG and a flow rate of over 8000 Nm³/h are sufficient to meet the requirements. However, the flow rate is low, resulting in a lower oxygen permeation efficiency, generally ranging from 8000 Nm³/h (55%) to 80000 Nm³/h (68%). Oxygen turbines are generally used in external compression processes; they are available for pressures ranging from 3 to 30 barG. However, it is necessary to compare their energy consumption with that of internal compression processes equipped with boosters (which typically have an efficiency exceeding 70%). Internal compression processes also have flow rate limitations; nevertheless, their efficiency is at least 10 percentage points higher than that of oxygen turbines. This difference can even offset the advantage of external compression in terms of reduced additional energy consumption due to reheating. Nevertheless, the use of internal compression for pressure relief in steel plants needs to be increased, so as to prevent fluctuations in the heat exchange system. Ultimately, the optimal solution is determined based on such comparisons.