In the plate heat exchanger of the air separation unit, the positive air is cooled continuously, and the water and carbon dioxide in the air will be frozen into solid ice crystal and dry ice successively with the temperature decreasing, and attached to the wall of the heat exchange channel. If the impurities accumulate for a long time, the flow passage will be blocked, the heat exchange resistance will be increased, and the normal operation of the equipment will be affected, while the return sewage nitrogen can effectively remove such frozen impurities.
The removal process mainly relies on sublimation and partial pressure purging principle. Reflux waste nitrogen comes from the upper column of the rectifying column, with the temperature higher than the sublimation temperature of frozen impurities. Compared with the low-temperature positive air, it belongs to the relatively high-temperature gas flow. When the return sewage nitrogen flows through the channel attached with solid impurities, the solid water and carbon dioxide do not need to be melted and directly sublime into the gas state. At the same time, the partial pressure of water vapor and carbon dioxide in the return sewage nitrogen is very low, forming obvious concentration difference with solid impurities, providing power for sublimation.
In addition, the flow velocity of reflux sewage nitrogen is fast, which can continuously carry away the sublimed gaseous impurities and discharge them from the heat exchanger. This self-cleaning method requires no additional heating and disassembly equipment. Relying on the difference in air flow temperature and partial pressure, it can continuously clean frozen impurities, ensure smooth heat exchange channel and maintain stable and efficient operation of air separation equipment.
