The traditional industrial air separation method is cryogenic separation, that is, air is cooled to below-150℃ and then separated by cryogenic rectification. This method can obtain both nitrogen and oxygen, as well as liquid nitrogen and liquid oxygen. However, the low-temperature distillation method has the disadvantages of high energy consumption, long process, long start-up process and high equipment maintenance requirements, so it has been severely challenged by new separation methods such as pressure swing adsorption and membrane separation in recent ten years.
There are two mechanisms to separate air by pressure swing adsorption. One is to use the selective adsorption characteristics of 5A zeolite molecular sieve, that is, the equilibrium adsorption capacity of 5A zeolite molecular sieve for nitrogen is greater than that for oxygen, so that when air passes through zeolite bed, nitrogen is adsorbed and oxygen flows out as a product. When the nitrogen adsorbed by zeolite is saturated, the air is stopped, and the bed is vacuumized, and the pumped nitrogen is used as the product. The other is to use the transport and adsorption characteristics of carbon molecular sieve, that is, the equilibrium adsorption capacity of carbon molecular sieve for oxygen and nitrogen is similar, but the molecular size of oxygen (2.8×3.9) is smaller than that of nitrogen (3.0×4. 1), so the diffusion speed of oxygen in carbon molecular sieve is fast and the adsorption capacity is large, so the diffusion speed of oxygen in carbon molecular sieve is fast and the adsorption capacity is also large. After a period of time, the air is stopped and the bed is vacuumized to regenerate the carbon molecular sieve. This method is usually carried out under the conditions that the adsorption stage is 0. 1 ~ 0.5× 106 Pa and the desorption stage is normal pressure or vacuum room temperature, which is easy to realize in industry.
Oxygen-enriched air and 99.9% pure nitrogen can be obtained by pressure swing adsorption separation of air, and the power consumption is less than 1.0kwh/m3. Japan is the most mature country in the world to screen 5A zeolite to produce oxygen, with the oxygen concentration as high as 96% and power consumption of only 0.4kwh/m3.
In a word, PSA air separation has the advantages of low energy consumption, short process, short start-stop time, automatic control and adjustable product concentration, which is expected to be greatly developed.
Membrane separation air membrane separation uses the permeation principle, that is, the diffusion rates of oxygen and nitrogen in non-porous polymer membranes are different. When oxygen and nitrogen are adsorbed on the surface of polymer membrane, due to the concentration gradient on both sides of the membrane, the gas diffuses through the polymer membrane and then desorbs on the other side of the membrane. Since the volume of oxygen molecules is smaller than that of nitrogen molecules, the diffusion rate of oxygen in polymer membranes is higher than that of nitrogen, so that when air enters one side of the membrane, oxygen-enriched air can be obtained on the other side and nitrogen can be obtained on the same side.
Nitrogen and oxygen-enriched air can be continuously obtained by separating air by membrane method. At present, the selectivity coefficient of polymer membrane for oxygen-nitrogen separation is only about 3.5, and the permeability coefficient is also small. The nitrogen concentration of the separated product is 95-99%, and the oxygen concentration is only 30-40%. Membrane air separation is generally carried out at normal temperature and pressure of 0. 1 ~ 0.5× 106 Pa.
Due to the rise of pressure swing adsorption method and membrane method, small and medium-sized cryogenic air separation equipment began to give up some markets. At present, the main disadvantages of pressure swing adsorption method and membrane method are that the product concentration is not high enough and the recovery rate is low, which should be overcome by improving adsorbent and polymer membrane