Principles of pressure swing adsorption technology

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Pressure Swing Adsorption (PSA for short) is a new type of gas adsorption and separation technology, which has the following advantages: (1) High product purity. ⑵It can generally operate at room temperature and under low pressure; no heating is required during bed regeneration, making it energy-efficient and cost-effective. ⑶The equipment is simple, with easy operation and maintenance. ⑷Continuous cyclic operation can achieve full automation. Therefore, when this new technology emerged, it attracted the attention of industrial sectors around the world, which competed to develop and research it; as a result, it advanced rapidly and became increasingly mature. In 1960, Skarstrom filed a patent for PSA; using 5A zeolite molecular sieves as the adsorbent and a two-bed PSA unit, he separated oxygen-enriched air from the air. This process was improved and put into industrial use in the 1960s. In the 1970s, significant progress was made in the industrial application of pressure swing adsorption technology, which was mainly used for oxygen-nitrogen separation, air drying and purification, as well as hydrogen purification. Among these, progress in oxygen-nitrogen separation technology involves combining new adsorbents such as carbon molecular sieves with pressure swing adsorption to separate O2 and N2 from air, thereby obtaining nitrogen. As the performance and quality of molecular sieves improve, along with continuous advancements in pressure swing adsorption technology, the purity and recovery rate of the products increase, which in turn facilitates the economic viability and industrialization of pressure swing adsorption. Principle: For any type of adsorption, regarding the same gas to be adsorbed (adsorbate), at adsorption equilibrium, the lower the temperature and the higher the pressure, the greater the amount of adsorption. Conversely, the higher the temperature and the lower the pressure, the smaller the adsorption amount. Therefore, gas adsorption separation methods typically employ two cyclic processes: temperature swing adsorption or pressure swing adsorption, both of which are shown in Figure 1. If the pressure remains constant and adsorption occurs at normal or low temperatures, with desorption carried out at high temperatures, this is known as temperature-swapped adsorption (abbreviated as TSA). Obviously, temperature-dependent adsorption involves the regulation of adsorption and desorption by changing temperature. Thermosorption operations are carried out along the perpendicular line between the adsorption isotherm at low temperatures (room temperature) and that at high temperatures (see Figure 1). Due to the high specific heat capacity of the adsorbent and its low thermal conductivity, it takes a long time to heat or cool it, which makes the process cumbersome. Therefore, thermosorption is mainly used for the purification of gases containing low amounts of adsorbate. If the temperature remains constant and adsorption occurs under pressure, with desorption achieved by reducing pressure (vacuuming) or at atmospheric pressure, this is known as pressure swing adsorption. It can be seen that pressure swing adsorption involves adsorption and desorption by changing pressure. In pressure swing adsorption operations, due to the low thermal conductivity of the adsorbent, the temperature changes in the adsorbent bed caused by the adsorption heat and desorption heat are minimal; therefore, it can be considered an isothermal process. Its operating conditions follow approximately the isotherm for adsorption at normal temperatures (see Figure 1), with adsorption occurring at higher pressures (P2) and desorption at lower pressures (P1). Since pressure swing adsorption proceeds along an adsorption isotherm, from the perspective of static adsorption equilibrium, the slope of this isotherm has a significant impact on it. At a constant temperature, the relationship between pressure and the amount of adsorption is shown in the graph; here, PH represents the adsorption pressure, while PL represents the desorption pressure (after pressure reduction). The difference between the amounts of adsorption corresponding to PH and PL is essentially the effective amount of adsorption, denoted as Ve. Clearly, the effective adsorption capacity of the linear adsorption isotherm is greater than that of the curved (Langmuir-type) isotherm. Adsorption is often carried out under pressure conditions, and pressure swing adsorption introduces a method that combines pressurization and depressurization; it typically consists of an adsorption-desorption system that involves pressurized adsorption followed by depressurization. Under isothermal conditions, a adsorption operation cycle is formed by combining pressure-induced adsorption and pressure-reduced desorption. The amount of adsorbate absorbed by the adsorbent increases as pressure rises and decreases as pressure falls. Meanwhile, during depressurization (to atmospheric pressure or under vacuum), the adsorbed gas is released, regenerating the adsorbent; no external heat supply is required for this regeneration process. Therefore, pressure swing adsorption is also known as isothermal adsorption as well as heat-free regeneration adsorption. In pressure swing adsorption, the compressed air from the air compressor first passes through a cold dryer to have its moisture removed, and then enters a PSA nitrogen production unit consisting of two adsorption towers. The specialized carbon molecular sieve adsorbents installed in these towers selectively adsorb impurity gases such as O2 and CO2, while the N2 gas, as the product, is discharged from the top of the towers with a purity of 99%. During pressure reduction, the oxygen adsorbed by the adsorbent is desorbed and discharged through the bottom of the tower; after washing, the adsorbent is regenerated. After regeneration, the adsorbent can be put back into adsorption after pressure equalization and product pressure increase. The two towers are used alternately to achieve continuous air separation for nitrogen production. Nitrogen production using carbon molecular sieves is based on the difference in diffusion rates of oxygen and nitrogen within these sieves. At pressures of 0.7–1.0 Mpa, oxygen diffuses more rapidly across the surface of the carbon molecular sieves than nitrogen does; as a result, the carbon molecular sieves preferentially adsorb oxygen, while most of the nitrogen remains in the non-adsorbed phase. Carbon molecular sieves inherently have the property that their oxygen adsorption capacity increases under pressure and decreases under reduced pressure. Taking advantage of this property, pressure swing adsorption is used for the separation of oxygen and nitrogen. Thereby obtaining 99.99% nitrogen.