In the air separation plant, the principle that the return sewage nitrogen takes away the frozen water and carbon dioxide is the result of the synergistic action between the physical characteristics and the equipment mechanism.
Seen from the physical characteristics, the waste nitrogen discharged from the upper column of the rectifying column is an unsaturated gas with high nitrogen content and basically no water and carbon dioxide, and has the ability to absorb impurities. At the same time, its pressure is much lower than the positive flow air, and its volume is 3-4 times larger than the positive flow air. Although the saturation content of impurities in unit volume of contaminated nitrogen is low, the total volume is large, and the actual capacity of containing impurities is strong. In addition, the low temperature characteristics of contaminated nitrogen combined with volume advantages can make frozen impurities easier to sublimate or evaporate into the nitrogen.
In terms of the operating mechanism of the equipment, when the air passes through the cooler or the switching heat exchanger, the temperature drops, the water is frozen into ice, and the carbon dioxide is frozen into dry ice, which is deposited on the surface of the heat exchanger. If not cleaned, the passage will be blocked and the safety will be affected. At this time, return sewage nitrogen plays a key role. As an unsaturated gas, it provides evaporation or sublimation environment for frozen materials. Dry ice sublimes into gaseous carbon dioxide, and ice evaporates into water vapor, which are carried out of the equipment by sewage nitrogen. In addition, the large volume flow of contaminated nitrogen can ensure the complete removal of frozen materials and realize "self-removal". Regular switching of reflux gas path can ensure continuous smoothness of heat exchanger channel, create conditions for subsequent air treatment and ensure stable operation of air separation equipment.
