The purified air enters the fractionator and exchanges heat with the returned polluted nitrogen, oxygen and nitrogen through the main heat exchanger. After cooling, a small part of it is liquefied, and the gas-liquid mixed air is throttled to the pressure of 0.55 MPa. After distillation, 99.999% pure nitrogen is obtained at the top of the lower column, and then it enters the main cooling column, and is condensed into liquid nitrogen by evaporation of liquid oxygen in the upper column. A part of liquid nitrogen is returned to the lower column as reflux liquid, and the remaining liquid nitrogen is throttled after cooling. Enter the sprayer at the top of the upper column. The bottom residue is liquid air containing 38% oxygen. After passing through the liquid air subcooler, it enters the middle of the upper column to participate in distillation. At the same time, the polluted liquid nitrogen flow is drawn out from the middle of the lower column and then enters the upper column as reflux liquid.
A part of the air extracted from the middle of the main heat exchanger enters the turbo expander for adiabatic expansion, thus producing most of the cooling capacity required for the operation of the unit. The expanded air is partially superheated through a thermosyphon heat exchanger, and then enters the middle of the upper column to participate in distillation.
The four kinds of fluids in different states enter the upper column and separate again, and the high purity nitrogen is at the top of the upper column. After the liquid nitrogen subcooler and the main heat exchanger are reheated from the fractionator to about 9 ℃, the liquid oxygen at the bottom of the upper column absorbs the heat evaporation generated by nitrogen condensation in the lower column during the main cooling process. The main heat exchanger reheats 99.6% oxygen to about 9 ℃. From the fractionator, the rest is used as rising steam to participate in distillation. There is still polluted nitrogen in the upper part of the upper column, which is extracted by liquid air subcooler, and then heated by the main heat exchanger and discharged from the fractionator. After regeneration of the molecular sieve adsorber, exhaust the purification system.
The liquid oxygen is extracted from the bottom of the main cooler to the oxygen liquid measuring cylinder, the steam and liquid are separated, and the oxygen cryogenic liquid storage tank is quickly filled after the cylinder is filled. When preparing liquid nitrogen, the liquid nitrogen in the liquid nitrogen subcooler is injected into the nitrogen liquid measuring cylinder, and then the measuring cylinder is quickly injected into the nitrogen cryogenic liquid storage tank. After evaporation, it provides gaseous nitrogen products for users. After the qualified oxygen leaves the fractionator, it is pressurized by the compressed oxygen system for use.