After the air of the raw material entering the lower tower is rectified, the nitrogen with higher purity is obtained on the top. Nitrogen enters the main cold to release heat to liquid oxygen, and finally condenses into liquid nitrogen. If the temperature of liquid oxygen is constant, the heat transfer area of main cooling is constant, and the condensation of gas and nitrogen is more, the more heat needed to be released (the load of main cooling and heat is larger ), this requires that the temperature difference of main cooling is larger, that is, the temperature of gas and nitrogen is higher.
The gas in the tower is saturated vapor, and there is a fixed relationship between its temperature, pressure and purity:
1) when the pressure is constant, the higher the nitrogen content of the gas is, the lower the liquefaction temperature is. For example, when 0.55MPa and the gas nitrogen content is 100%, the corresponding liquefaction temperature is-177.8 ℃; when the gas nitrogen content is 96%, the liquefaction temperature is-176.9 ℃.
2) when the temperature of the top of the tower remains unchanged (it is equivalent to the pressure of the upper tower, the purity of liquid oxygen and the liquid oxygen level remain unchanged, and the condensation of gas and nitrogen remain unchanged), the higher the nitrogen content is, the higher the corresponding liquefaction pressure is. For example, when the temperature of the top of the tower is-177.1 ℃ and the gas nitrogen content is 99.99%, the corresponding liquefaction pressure is 0.58MPa; when the nitrogen content is 98%, the pressure is 0.57MPa; when the nitrogen content is 94%, then the pressure is 0.55MPa.
In the large air separation plant, the cost of producing pure oxygen and pure nitrogen at the same time is higher than that of producing pure oxygen alone. The main reason is that when producing pure nitrogen, the purity of liquid nitrogen provided by the lower tower is required to be improved, and the pressure of the lower tower is correspondingly increased, so that the power consumption will be larger and the cost of oxygen will be increased.
3) when the purity of gas and nitrogen is constant, the higher the temperature is, the higher the pressure required for liquefaction is. For example, when the nitrogen content at the top of the tower is 96% and the temperature is-178.1 ℃, the liquefaction pressure is 0.5MPa; when the temperature is-174.7 ℃, the liquefaction pressure is 0.65MPa.
When the air volume in the Tower increases, the main cold heat transfer surface appears insufficient or the main Cold Heat transfer effect decreases due to the dirty heat transfer surface, in order to ensure a certain heat transfer (a certain heat load and liquid oxygen evaporation ), only the heat transfer temperature difference can be enlarged. This requires that when the temperature of gas and nitrogen on the top of the tower rises, the pressure of the tower must rise.
