Cryogenic nitrogen plant is a complex system for producing liquid nitrogen or gaseous nitrogen. Its working principle involves several key physical processes and components.
1. Pretreatment of raw air
1. Filtration
- First, the raw air needs to be filtered. The air contains particles such as dust and impurities. If not filtered, these impurities will block the pipeline or affect the normal operation of the equipment in the subsequent cryogenic process. This process is like setting a sieve at the entrance of a complex mechanical system, allowing only pure air to enter the next processing flow.
2. Compression
- The filtered air will be compressed by the compressor. The purpose of compressing air is to increase the pressure of the air, so that it is easier to achieve the transformation from gas to liquid in the subsequent cooling process. According to the ideal gas state equation \(PV = nRT\) (where \(P\) is pressure, \(V\) is volume, \(n\) is the amount of substance, \(R\) is the gas constant, and \(T\) is temperature), when the pressure \(P\) increases, the gas is more likely to liquefy when the temperature \(T\) decreases. Generally speaking, the compressor can increase the pressure of the air to several atmospheres or even higher.
2. Cooling and liquefaction of air
1. Precooling
- The compressed air will enter the precooler for precooling. The precooler usually uses a refrigerant medium (such as water or other cryogenic fluids) to reduce the temperature of the air. This step of precooling can initially reduce the temperature of the air and reduce the load of the subsequent deep cooling process. For example, the compressed air at room temperature is reduced from about \(20 - 30^{\circ}C\) to about \( - 50^{\circ}C\).
2. Deep cooling and liquefaction
- The precooled air then enters the main heat exchanger for deep cooling. In the main heat exchanger, air exchanges heat with cryogenic liquid nitrogen or other cryogenic media. Since the temperature of the cryogenic medium is very low (for example, the boiling point of liquid nitrogen is -196^{\circ}C\), the air will be cooled to a temperature close to its liquefaction temperature in this process. As the temperature continues to drop, the air gradually liquefies to form liquid air (liquid air).
III. Production of liquid nitrogen and separation of nitrogen
1. Distillation
- Liquid air will enter the distillation tower for distillation separation. The distillation tower is one of the core components of cryogenic nitrogen equipment. In the distillation tower, the components such as liquid nitrogen and liquid oxygen are separated by different boiling points. The boiling point of liquid nitrogen is lower than that of liquid oxygen. At different heights in the distillation tower, the temperature and pressure conditions are different, so that liquid nitrogen is separated at the top of the tower. This process is like a fine screening according to the different characteristics of the substance, extracting liquid nitrogen from the mixed liquid air.
2. Nitrogen acquisition
- The liquid nitrogen obtained from the top of the distillation tower can be stored as a product. If gaseous nitrogen is needed, liquid nitrogen can be vaporized through an evaporator or other heating equipment to obtain high-purity nitrogen. The purity of nitrogen can reach \(99.999\%\) or even higher according to specific production requirements and equipment performance.
IV. Auxiliary systems of equipment
1. Refrigeration system
- The entire cryogenic nitrogen equipment requires a refrigeration system to provide a low-temperature environment. The refrigeration system usually adopts the compression-expansion cycle principle. For example, the refrigerant is compressed by a compressor, and then the refrigerant is expanded and depressurized through an expansion valve. In this process, the refrigerant absorbs heat, thereby providing low-temperature conditions for cooling and liquefaction of the air. Common refrigerants include liquid nitrogen itself (in some self-circulating systems) or other low-temperature refrigerants such as Freon (in some specific refrigeration units).
2. Control system
- In order to ensure the stable operation of the equipment, the cryogenic nitrogen equipment is equipped with a control system. The control system can monitor parameters such as temperature, pressure, flow, etc. of each key part of the equipment. For example, when the air pressure or temperature deviates from the set value, the control system will automatically adjust the compressor speed, valve opening, etc. to ensure the stability of the production process and product quality.
Through the above series of complex processes, the cryogenic nitrogen equipment converts the raw air into liquid nitrogen or high-purity gaseous nitrogen to meet the needs of various industries, medical treatment, scientific research and other fields.
