Molecular sieve is the key device for purification of air separation before air enters the cold box. Molecular sieve is a kind of gas solid adsorbent. After heating, molecular sieve loses crystal water, and many holes are formed in the crystal. The pore size is similar to the molecular diameter of the adsorbed gas, and it is very uniform. It can suck molecules smaller than pore size into pores and keep molecules larger than pore size outside the pores. Therefore, it can separate various components according to the molecular size.
Because the diameter of oxygen molecule is smaller than nitrogen molecule and the diffusion speed is hundreds times faster than nitrogen, carbon molecular sieve can simultaneously absorb oxygen and nitrogen in air in air separation unit, and its adsorption capacity increases with the increase of pressure, and there is no obvious difference in the equilibrium adsorption capacity of oxygen and nitrogen under the same pressure. Therefore, if the adsorption speed is considered further, the adsorption characteristics of oxygen and nitrogen can be effectively separated. Therefore, the adsorption rate of oxygen on carbon molecular sieve is also very fast, and the adsorption rate reaches more than 90% in about 1 minute; At this time, the adsorption capacity of nitrogen is only about 5%, so the adsorption is mostly oxygen, and the rest is mostly nitrogen. In this way, if the adsorption time is controlled within 1 minute, oxygen and nitrogen can be separated initially.
Therefore, the adsorption and desorption are realized by pressure difference. The difference between oxygen and nitrogen is based on the difference of adsorption speed. It is realized by controlling the adsorption time. The time is controlled very short, oxygen has been fully absorbed, and the adsorption process is stopped before nitrogen can be absorbed. Therefore, pressure change is needed to change the pressure of nitrogen production by pressure adsorption and the time should be controlled within 1 minute.
