The main components in the air are oxygen and nitrogen, which respectively exist in molecular state. Molecule is the smallest particle that maintains its original property, and the order of magnitude of diameter is 10-8Cm, and the number of molecules is very large, and they are constantly moving irregularly. Therefore, the molecules such as oxygen and nitrogen in the air are evenly mixed with each other, it is difficult to separate them. Currently, there are mainly three separation methods.
(1) low temperature method
Air is cooled down through compression and expansion until air liquefaction, then the gasification temperature (boiling point) of using oxygen and nitrogen is different (under atmospheric pressure, the boiling point of oxygen is 90 K, the boiling point of nitrogen is 77 K). Compared with oxygen, nitrogen with low boiling point is easier to Gasification. In the distillation column, steam with higher temperature keeps in touch with liquid with lower temperature, the nitrogen in the liquid evaporates more and the oxygen in the gas condenses more, which makes the nitrogen content in the rising steam increase continuously and the oxygen content in the downstream liquid increase continuously, so as to realize the separation of air. To liquefied the air, it is necessary to cool the air to a temperature below 100 k, which is called deep freezing; while the process of separating the liquid-air by using boiling point difference is called rectification process.
(2) absorption method
Let the air pass through the absorption tower filled with some kind of porous substance-molecular sieve, take advantage of the characteristic that molecular sieve has selective adsorption on different molecules, and have relatively strong adsorption performance on nitrogen molecules through molecular sieve, let the oxygen molecules pass through, thus the oxygen with higher purity can be obtained. Due to the limited adsorption capacity of the adsorbents, when the adsorption of a certain molecule reaches saturation, there is no ability to continue the adsorption, and the adsorbated substance needs to be driven away to recover the adsorption capacity. This process is called "regeneration ". Therefore, in order to ensure continuous gas supply, more than two absorption towers need to be used alternately. The process of this method is simple, the operation is convenient, and the operation cost is low, but it is difficult to get high-purity products, and the oxygen purity of the products is about 93%.
(3) membrane separation method
Taking advantage of the permeability selectivity of some organic polymer membrane, when the air passes through the membrane (0.1mm) or hollow fiber membrane, the speed of oxygen penetrating through the membrane is about 4 ~ of nitrogen ~ 5 times, so as to realize the separation of oxygen and nitrogen. This method is easy to install, easy to operate, quick to start, less investment, but the oxygen concentration is generally suitable for 28% ~ 35%, the scale is only suitable for medium and small size, so it is only applicable to oxygen-enriched combustion and medical care, etc. Currently, it has been applied in the glass kiln.
