In order to realize the separation of oxygen and nitrogen in the air separation unit, the first thing is to make the air liquefaction, which must try to reduce the air temperature to the liquefaction temperature. The absolute pressure of the lower tower of the air separation tower is about 0.6MPa, and the temperature at which the air begins to liquefaction under this pressure is about-172 ℃. Therefore, in order to make the air liquefaction, there must be a cold fluid with lower temperature to cool the air.
As we know, the air separation plant is cooled by the inflated low temperature air. If the Air wants to expand, it needs to be compressed first, and the compression needs to consume energy.
Air expansion can be through throttling expansion or expansion machine expansion. However, the temperature drop of this kind of expansion is limited. For high pressure air of 20MPa and 30℃, the temperature drop is only 32℃ when throttling to 0.1MPa. The maximum temperature drop of the air from 0.55MPa to 0.135MPa in the turbine expander is only 50℃, which is far from reaching the temperature required by air liquefaction.
The main heat exchanger and condensing evaporator in the air separation plant play a key role in the generation of liquid. The main heat exchanger uses the low-temperature and low-pressure gas after expansion as the return gas of the heat exchanger to cool the high-pressure positive-flow air, making its temperature gradually lower before expansion. At the same time, the temperature after expansion gradually decreases to a lower level, until the temperature required for liquefaction can be reached in the end, making the positive air partial liquefaction. The cooling process of the air separation plant in the start-up stage is such a gradual cooling process.
Because of the low pressure of the air after expansion, it still keeps the gas state at very low temperature. For example, when the absolute air pressure is 0.105MPa, it will still be pneumatic when the temperature drops to-190℃. It is lower than the liquefaction temperature of positive flow high pressure air. For small medium-pressure and high-pressure oxygen generator, in the late stage of start-up, at the bottom of the main heat exchanger, some liquid will be produced, acting as a liquid purifier; for low-pressure air separation plant, another liquid purifier is set, use the expanded low-temperature and low-pressure air to cool the positive-current and high-pressure (about 0.6MPa) low-temperature air to make it partial liquefaction. At the same time, the condensing evaporator also plays the role of liquid purifier in the late stage of start-up. The low-temperature air entering the upper tower after expansion cools down the low-temperature pressure gas from the lower tower in the condensing evaporator, and some of them generate condensation and then flow to the upper tower to further lower the temperature, become a part of low-temperature and low-pressure return gas, and gradually increase the amount of accumulated liquid.
