The water cooling tower is the core equipment of the air precooling system in an air separation unit. It relies on the countercurrent contact between low-temperature dry nitrogen and circulating water, achieving temperature reduction of high-temperature circulating water through the dual effects of sensible heat exchange and evaporation heat absorption. This ensures the reuse of cooling water in the air cooling tower and achieves energy-saving operation of the unit.
During the process operation, the high-temperature circulating water, after heat exchange in the air cooling tower, is sprayed down from the top of the water cooling tower and dispersed into tiny droplets within the packing layer of the tower, thereby increasing the gas-liquid contact area. Meanwhile, the low-temperature dry and contaminated nitrogen gas from the upper distillation tower rises from the bottom to the top in a countercurrent manner. The two streams fully contact each other to complete heat and mass exchange.
The cooling process is divided into two stages. The first stage involves sensible heat exchange due to temperature difference. The initial temperature of the contaminated nitrogen is lower than that of the circulating water, and heat is directly transferred from the hot water to the low-temperature contaminated nitrogen gas, achieving initial cooling of the water body. The second stage is the core evaporation and endothermic cooling. The contaminated nitrogen gas has extremely low humidity and strong water absorption capacity. After contacting water droplets, some liquid water quickly evaporates into water vapor. The vaporization of water absorbs a large amount of latent heat of vaporization, taking away most of the heat from the water body. This is also the main source of refrigeration for the water cooling tower.
The contaminated nitrogen gas, after absorbing heat and being humidified, is directly vented from the top of the tower. The cooled water collects at the bottom of the tower and is then re-sent to the air cooling tower to participate in the air cooling cycle. This entire process eliminates the need for additional refrigeration units, fully recovers the cold energy from the waste contaminated nitrogen of the air separation unit, and effectively reduces the energy consumption of the unit. It is a key process link for low-cost precooling of the air separation system.
