Liquid air separation plant steel structure cold box

In general, the liquefaction temperature of nitrogen is higher than that of oxygen, but why can the condensation evaporator between the upper and lower columns of the air separation column use gas nitrogen condensation to release heat to vaporize liquid oxygen? This is because we usually think that the liquefaction temperature of nitrogen is lower than that of oxygen under the same pressure. However, the pressure at the bottom of the upper column of the air separation unit is generally 0.149 MPa, and the pressure at the top of the lower column is about 0.58 MPa. The reason for the different pressure causes the inversion of the saturation temperature of oxygen and nitrogen. It is precisely by using this principle that the condensing evaporator can continuously provide liquid nitrogen for the lower column At the same time, it provides gas oxygen for the upper column as the rising gas to participate in fractionation.

What is the reason why gas nitrogen and liquid nitrogen are in coexistence at the top of an air separation tower?