The air positive flow expansion process is generally applicable to occasions where the pressure level of the nitrogen product is not very high (such as lower than 0.2MPa), and its process is shown in the figure. The raw air is divided into two streams after dedusting, compression, pre-cooling and purification: one stream enters the main heat exchanger, is cooled to a certain temperature by the returned dirty nitrogen, and then is pumped out from the middle of the main heat exchanger for expansion. The bottom of the nitrogen tower participates in rectification; another stream of air is cooled to the saturation temperature in the main heat exchanger, and is directly throttled into the nitrogen tower to participate in rectification.

In this way, the product nitrogen is obtained at the top of the nitrogen tower, and the oxygen-rich liquid air at the bottom of the nitrogen tower is throttled and then enters the evaporation side of the condensing evaporator to condense the gas nitrogen at the top of the nitrogen tower. The oxygen-enriched air extracted from the top of the condensing evaporator directly enters the cold end of the main heat exchanger, exchanges heat with the positive air, and is discharged out of the cold box after reheating to normal temperature. Part of it is used for the regeneration of the molecular sieve adsorber, and the rest is emptied. The nitrogen gas exiting from the top of the nitrogen tower is reheated by the main heat exchanger to normal temperature and exits the cold box for supply to users.

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