The main components in the air are nitrogen and oxygen. By selecting the adsorbents with different adsorption selectivity for nitrogen and oxygen, the appropriate technological process is designed to separate nitrogen and oxygen to produce oxygen.
Both nitrogen and oxygen have four-pole moment, but the four-pole moment (0.311) of nitrogen is much larger than that of oxygen (0.10, therefore, the adsorption capacity of nitrogen on the zeolized molecular sieve is stronger than that of oxygen (nitrogen has strong interaction with ions on the surface of the molecular sieve, as shown in Figure 1 ). Therefore, when the air passes through the adsorption bed equipped with zeolized molecular sieve adsorbents under pressure, nitrogen is adsorbed by the molecular sieve, and oxygen is enriched in the gas phase and flows out of the adsorption bed due to less adsorption, separate oxygen and nitrogen to obtain oxygen. When the molecular sieve adsorbs nitrogen until it is nearly saturated, stop air ventilation and reduce the pressure of the adsorption bed, then the nitrogen adsorbed by the molecular sieve can be desorbed, and the molecular sieve can be regenerated and reused. More than two adsorption beds are switched to work in turn, thus oxygen can be produced continuously.
Figure 1. Schematic diagram of basic principle of pressure-changing adsorption gas separation
The boiling point of argon and oxygen is close, and it is difficult to separate them, which can be enriched together in the gas phase. Therefore, the pressure-changing adsorption oxygen generation device can only obtain the concentration of 90% ~ 95% oxygen (the limit concentration of oxygen is 95.6%, and the rest is argon), which is also called oxygen-enriched compared with oxygen whose concentration is above 99.5% in the deep cooling and air separation unit.
★Brief description of the process of pressure conversion adsorption air separation oxygen generation device
According to the above principles, the adsorption bed of the pressure-changing adsorption air separation oxygen generation unit must contain at least two operation steps: adsorption and deabsorption. Therefore, when there is only one adsorption bed, the acquisition of product oxygen is intermittent. In order to obtain the product gas continuously, generally more than two adsorption beds are set in the oxygen generation unit, and some necessary auxiliary steps are set from the perspective of energy saving, consumption reduction and stable operation.
Generally, every adsorption bed needs to go through steps such as adsorption, forward pressure release, emptying or decompression regeneration, washing replacement and average pressure and boosting, and repeat the operation periodically. At the same time, each adsorption bed is in different operation steps respectively, switching regularly under the control of the computer, making several adsorption beds operate cooperatively, and staggered in the pace of time, make the pressure conversion absorption device run smoothly and obtain product gas continuously.
According to different desorption methods, the oxygen generation by pressure swing adsorption can be divided into two processes (see table 1 ):
1. PSA process: pressurized adsorption (0.2 ~ 0.6MPa), normal pressure desorption. Small investment, simple equipment, but high energy consumption, suitable for small-scale oxygen production occasions.
2. VPSA process: normal pressure or slightly higher than normal pressure (0 ~ 50KPa) adsorption, vacuum desorption. The equipment is relatively complex, but it has high efficiency and low energy consumption, which is suitable for the occasion of large scale of oxygen generation.
For the actual separation process, other micro components in the air must also be considered. Generally, the adsorption capacity of carbon dioxide and water on common adsorbents is much larger than that of nitrogen and oxygen, so appropriate adsorbents can be filled in the adsorption bed (or using oxygen-making adsorbents themselves) make it be adsorbed and cleared. The number of absorption towers required by the oxygen generation unit depends on the scale of oxygen generation, the performance of the absorbents and the design idea of the process. The operation stability of multi-tower operation is relatively better, but the equipment investment is relatively high. The current trend is: use high-efficiency oxygen-generating adsorbents, reduce the number of absorption towers as much as possible and adopt short operation cycle to improve the efficiency of the unit and save investment as much as possible.
