Nitrogen-water precooler is generally installed in all low-pressure air separation plant. It mainly uses the unsaturation of the water in the nitrogen to make part of the water evaporate. When the water evaporates, it absorbs the latent heat of vaporization to lower the cooling water temperature, and then uses it to cool the processed air to lower the air temperature entering the tower. Therefore, it includes air cooling tower and water cooling tower.
The fundamental purpose of setting nitrogen-water precooler is to reduce the temperature of air entering the air separation tower and avoid the sharp fluctuation of the temperature entering the tower. Because the air temperature in the tower directly affects the working condition of switching heat exchanger and distillation column and the economy of the whole air separation plant. When designing, the temperature of air entering the tower is generally set at 30℃. When the inlet temperature of the tower is higher than the design index during operation, the throttling refrigerating capacity of the compressed air will be reduced, and the temperature difference at the hot end of the switching heat exchanger and the heat load will be increased, which will lead to the increase of cold loss and energy consumption.
For example, if sewage nitrogen reheat is insufficient, if it increases by 1℃, the energy consumption of the air separation plant will increase by about 2%. Reducing the air temperature into the tower can not only improve the economy of the air separation plant, but also reduce the saturated moisture content in the air. For example, when the absolute pressure of the air is 0.6MPa and the air temperature drops from 40℃ to 30℃, the saturated moisture content per kg of air will decrease by about 40%. This greatly reduces the burden of self-cleaning of water in switching heat exchanger, which is beneficial to self-cleaning. Therefore, the nitrogen water precooler should be well managed and used to reduce the temperature of air entering the tower as much as possible.
Secondly, in the air cooling tower (Spray Cooling Tower), air and water are in direct contact, which can not only exchange heat but also be washed, and can remove the dust in the air and dissolve some corrosive impurity gas, for example, the material of the aluminum alloy of the Plate-fin switching heat exchanger can be prevented from being corroded and the service life can be prolonged. Because the air cooling tower has a large volume, it can buffer the processed air, and the air compressor is not easy to overpress when switching.
For the adsorption purification process of molecular sieve, the adsorption capacity of molecular sieve is related to temperature. The lower the temperature is, the larger the adsorption capacity is. The working cycle of the adsorber of a certain size can be long, or for a certain adsorption cycle, the volume of the designed adsorber can be small, so it is also required to cool the air to the lowest possible temperature in advance. Fully recycle and use the cooling capacity of nitrogen in the nitrogen water precooler to cool the air.
