Although the chemical properties of N2O are inactive, neither corrosion nor explosion will occur, its physical properties are harmful to air separation. Its critical temperature is 309.7k, critical pressure is 7.27mpa, and its three-phase point is 182.3k and 0.088mpa. Under the condition of pressure and temperature of air separation device, it has sublimation property. At atmospheric pressure, its boiling point is 185K, which is higher than that of N2, O2 and ar. therefore, it will be concentrated in liquid oxygen during the separation of oxygen and nitrogen.The expanded air remains gaseous at a very low temperature due to its low pressure. For example, when the air absolute pressure is 0.105 MPa, the temperature drops to - 190 ℃ and it is still gaseous. It is lower than the liquefying temperature of normal high pressure air. For small medium and high pressure oxygen generators, in the late start-up stage, part of the liquid will be produced at the lower part of the main heat exchanger, which plays the role of liquefier; for low-pressure air separation units, another liquefier is set, which uses the expanded low-temperature and low-pressure air to cool the positive flow high-pressure (about 0.6MPa) low-temperature air, so as to partially liquefy it.
In the start-up stage of air separation plant, the cooling capacity is first used to cool the unit, reduce the temperature, produce liquid air, and accumulate the liquid needed for distillation in the tower. When the internal temperature, liquid level and other working conditions reach normal, the required cooling capacity is greatly reduced than that in the start-up stage, mainly to maintain the normal working conditions in the tower. At this time, when the equipment is in a low temperature state, there must be continuous heat transfer from the outside, and there must be heat transfer temperature difference at the hot end of the heat exchanger.It can be seen that in the air separation plant, throttle valve and expander have their own advantages and disadvantages, and they are used together to meet the requirements of refrigeration capacity. The refrigeration capacity is adjusted by adjusting the refrigeration capacity of the expander, and the lowest temperature (- 193 ℃) in the air separation tower is achieved by liquid throttling.
Because the temperature of bypass air is the saturation temperature under the tower inlet air pressure, which is basically unchanged, when the expansion volume increases, the temperature in front of the expander decreases, and the superheat after expansion decreases. If the expansion is within the allowable range of the upper column distillation potential, the oxygen extraction rate will be reduced. On the contrary, the temperature in front of the expander increases with the decrease of expansion.The throttling temperature drop plays an important role in the refrigeration of small high and medium pressure oxygen generators. In the start-up stage, in order to speed up the cooling rate, the method of increasing the pressure is often used to increase the cooling capacity of throttling effect. After the normal operation of the equipment, the required cooling capacity is reduced, and then the working pressure is reduced.If the dew point temperature is measured, the water vapor content can be found from the table of water vapor saturation content. Because the water vapor content does not change in the process of temperature reduction, the determination of dew point is actually the determination of absolute humidity in the air. The lower the dew point, the less moisture in the air.
If the liquid oxygen surface is full to the bottom tray of the upper tower, the distillation effect of the tray will be lost, and the liquid will be carried in the rising steam. Therefore, the liquid oxygen level should not be too low or too high, generally controlled at 80% - 90% of the pipe height. For example, for 150m3 / h air separation plant, the pressure of liquid oxygen column is generally controlled at 48 × 102 ~ 58 × 102 PA.In the switching heat exchanger of the air separation unit, the air temperature decreases continuously. Although the relative humidity of the air is always 100%, the absolute humidity decreases continuously. Finally, the moisture in the air can be separated out completely, almost without moisture.DEAR Liquid air separation unit.In the stage of liquid accumulation, the temperature of the switching heat exchanger tends to be normal, and the refrigeration capacity of the expander should be transferred to the tower to accumulate liquid except for making up for the cold loss. Its cold recovery mainly depends on the liquefier.
