Air liquefaction is a core process in air separation plants and is achieved through a number of physical processes including compression, cooling, expansion and heat exchange.
Air is first drawn in by the compressor and compressed to a high pressure. During compression, the distance between air molecules decreases and internal energy increases, resulting in a significant increase in air temperature.
The high-temperature, high-pressure air then enters the cooling system, where it exchanges heat with a cooling medium (e.g. water or air) via a heat exchanger to reduce the temperature of the air. This step is intended to reduce the temperature rise during the subsequent expansion process and to approach the liquefaction point of the air.
The cooled air enters the expander or throttle for expansion. During the expansion process, the pressure of the air is reduced and its volume is increased, while the internal energy is reduced and the temperature is further lowered. The temperature drop of throttling expansion and expander expansion is limited and cannot directly liquefy the air to the desired temperature.
Expanded low-temperature air through the heat exchanger and high-pressure positive flow air for heat exchange, the expanded low-temperature air absorbs the heat of high-pressure positive flow air, so that its temperature is further reduced. At the same time, the temperature of the high-pressure positive flow air is gradually reduced to below its liquefaction point, thus achieving liquefaction.
