To achieve oxygen nitrogen separation in an air separation plant, the first step is to liquefy the air, which requires efforts to reduce the air temperature to the liquefaction temperature. The absolute pressure of the lower tower of the air separation tower is around 0.6MPa, and the temperature at which air begins to liquefy is about -172 ℃. Therefore, in order for air to liquefy, it is necessary to have a cold fluid at a lower temperature to cool the air.
In the air separation equipment, the expansion of low-temperature air is used to cool the positive flow pressure air. To expand air, the first step is to compress it, which consumes energy.
Air expansion can be achieved through throttling or expander expansion. However, the temperature drop caused by this expansion is limited. For high-pressure air at 20MPa and 30 ℃, the temperature drop when throttling to 0.1MPa is only 32 ℃. The maximum temperature drop for air to expand from 0.55MPa to 0.135MPa in a turbine expander is only 50 ℃, which is far from reaching the temperature required for air liquefaction.
The main heat exchanger and condenser evaporator in the air separation equipment play a crucial role in the generation of liquid. The main heat exchanger uses the expanded low-temperature and low-pressure gas as the reflux gas of the heat exchanger to cool the high-pressure positive flow air, gradually reducing its temperature before expansion. At the same time, the temperature after expansion gradually decreases to a lower level until it finally reaches the temperature required for liquefaction, causing partial liquefaction of the positive flow air. The cooling process of the air separation equipment during the start-up phase is such a gradual cooling process.
