Principle of pressure swing adsorption air separation oxygen production process

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★ Principle of oxygen production by pressure swing adsorption air separation: The main components of air are nitrogen and oxygen. By using adsorbents that exhibit different adsorption selectivities for nitrogen and oxygen, and by designing an appropriate process, nitrogen and oxygen can be separated to produce oxygen. Both nitrogen and oxygen possess quadrupole moments, but nitrogen’s quadrupole moment (0.31 Å) is much larger than that of oxygen (0.10 Å); therefore, nitrogen has a stronger adsorption capacity on zeolite molecular sieves than oxygen (the force between nitrogen and the ions on the surface of the molecular sieve is stronger, as shown in Figure 1). Therefore, when air passes through an adsorption bed containing zeolite molecular sieve adsorbent under pressure, nitrogen is adsorbed by the molecular sieve, while oxygen, being less adsorbed, becomes enriched in the gas phase and flows out of the adsorption bed, thereby enabling the separation of oxygen from nitrogen to obtain oxygen. When the molecular sieve has adsorbed nitrogen to near saturation, the supply of air is stopped and the pressure in the adsorption bed is reduced; the nitrogen adsorbed by the molecular sieve can then be desorbed, allowing the molecular sieve to be regenerated and reused. By alternating between two or more adsorption beds, oxygen can be produced continuously. Figure 1: Schematic diagram of the basic principle of pressure swing adsorption gas separation. Argon and oxygen have similar boiling points, making it difficult to separate them; they end up being enriched together in the gas phase. Therefore, pressure swing adsorption oxygen production units can typically produce oxygen with a concentration of 90% to 95% (the maximum concentration of oxygen is 95.6%, with the remainder being argon); compared to the oxygen with a concentration of over 99.5% produced by cryogenic air separation units, this type of oxygen is also referred to as enriched oxygen. ★ Brief description of the process for oxygen production using pressure swing adsorption air separation units. As can be seen from the above principles, the adsorption beds in such units must undergo at least two operating steps: adsorption and desorption. Therefore, when there is only one adsorption bed, the production of product oxygen is intermittent. In order to continuously obtain product gas, two or more adsorption beds are usually installed in oxygen production units, and additional necessary auxiliary steps are taken to achieve energy savings, reduce consumption, and ensure stable operation. Each adsorption bed generally goes through steps such as adsorption, forward pressure release, evacuation or reduced-pressure regeneration, flushing and replacement, and pressure equalization and increase, with these operations being repeated periodically. At the same time, each adsorption bed is in a different operating stage, and they are switched at regular intervals under computer control, allowing the multiple adsorption beds to operate in coordination while being staggered in time, so that the pressure swing adsorption unit can run smoothly and continuously produce product gas. Depending on the desorption method, pressure swing adsorption for oxygen production is divided into two processes (see Table 1): 1. PSA process: pressure adsorption (0.2–0.6 MPa) and desorption at atmospheric pressure. It requires low investment and simple equipment, but has high energy consumption; it is suitable for small-scale oxygen production. 2. VPSA process: Adsorption is carried out at atmospheric pressure or slightly above it (0–50 KPa), with desorption achieved by vacuum pumping. The equipment is relatively complex, but it offers high efficiency and low energy consumption, making it suitable for large-scale oxygen production. Table 1: Comparison of key parameters for PSA and VPSA oxygen production systems. Process flow, appropriate scale in m3/h, adsorption pressure in KPa, desorption pressure in KPa, oxygen purity in %, electricity consumption per m3 of oxygen produced in kWh/m3, and oxygen yield in %. PSA: ≤200; 200–600 atm; adsorption pressure: 80–93 KPa; electricity consumption: 0.7–2 kWh/m3; oxygen yield: 30–45%. VPSA: 100–10,000; adsorption pressure: 0–50 KPa; desorption pressure: –45––80 KPa; oxygen purity: 80–95%; electricity consumption: 0.3–0.5 kWh/m3; oxygen yield: 46–68%. For actual separation processes, other trace components present in air must also be taken into consideration. Carbon dioxide and water generally have a much higher adsorption capacity on common adsorbents than nitrogen and oxygen; therefore, appropriate adsorbents can be added to the adsorption bed (or the oxygen-production adsorbent itself can be used) to adsorb and remove them. The number of adsorption towers required for an oxygen production unit depends on the scale of oxygen production, the performance of the adsorbent, and the design principles of the process. Operating with multiple towers generally results in better stability, but it requires higher capital investment. The current trend is to use efficient oxygen production adsorbents, minimize the number of adsorption towers, and adopt short operation cycles in order to improve the efficiency of the equipment and save costs as much as possible. Related expansion: Comparison between cryogenic air separation oxygen production process and pressure swing adsorption oxygen production process. Category, Item: Cryogenic air separation oxygen production plant; Pressure swing adsorption oxygen production plant. Separation principle: Air is liquefied, and separation is achieved based on the different boiling points of oxygen and nitrogen. Pressure adsorption followed by vacuum desorption is used, with separation accomplished due to the differing adsorption capacities of oxygen and nitrogen. Main characteristics of the plants: The process flow is complex, and there are many pieces of equipment, including an air compression system, an air pre-cooling system, an air purification system, an expansion unit, a heat exchange system, and distillation columns. The process flow is simple, with few pieces of equipment, including blowers, vacuum pumps, and adsorption towers. Process characteristics: operation at low temperatures of -160 to -190°C, as well as operation at normal temperatures. Operational features: long startup time, typically between 15 and 40 hours; it must operate continuously without interruption, and any short shutdown results in a long time required to resume normal operation. It has a short startup time, generally ≤30 minutes, and can operate continuously or intermittently. Maintenance characteristics: The equipment has a complex structure, requires high precision in processing, presents significant challenges in terms of maintenance techniques, and incurs high maintenance costs. The equipment has a simple structure, low technical complexity for maintenance, and low maintenance costs. Civil engineering and installation characteristics: large land area required, high demands on factory buildings and foundations, and high project costs. The installation process is lengthy, technically challenging, and costly. It requires little space, has no special requirements for the factory building, and has a low cost. The installation period is short, and the installation cost is low. Oxygen production cost: Small and medium-sized oxygen production units have high power consumption, around 0.5–1.0 KW/Nm3, while those with lower power consumption require about 0.32–0.35 KW/Nm3. Safety: It is subject to strict regulations regarding pressure vessels. It can cause localized accumulation of hydrocarbons, posing a risk of explosion. The operating pressure is low, it is not subject to the regulations governing pressure vessels, and it does not lead to localized accumulation of hydrocarbons