What is the function of the air precooling system in air separation equipment?
Air separation equipment is the core device for producing key industrial gases such as oxygen and nitrogen. At the very front end of this equipment, the air pre-cooling system plays a crucial role – it not only cools the air but also undertakes the core tasks of pre-treatment, system protection, and energy efficiency optimization, laying the foundation for the stable and efficient operation of the entire air separation process.
I. Core Function: Creating Conditions for Subsequent Separation
The essence of the air separation process is a physical process carried out at extremely low temperatures. However, after being processed by the compressor, the air temperature usually exceeds 100°C. If this high-temperature and high-humidity air is directly introduced into the subsequent system, it will cause two major problems:
1. Energy consumption surges: Deep cryogenic separation requires the air to be cooled to around room temperature before entering the main heat exchanger. If high-temperature air is directly introduced, the workload of the subsequent refrigeration equipment will increase, leading to a sharp rise in power consumption.
2. Safety and failure risks: Molecular sieves are key components for removing moisture and carbon dioxide from the air. However, high temperatures significantly reduce their adsorption capacity, allowing these impurities to penetrate and enter the subsequent low-temperature system. Once in the low-temperature zone, the moisture will freeze and the carbon dioxide will turn into dry ice, thereby blocking pipelines and equipment, ultimately causing severe accidents such as equipment failure and shutdown.
Therefore, the most critical function of the air pre-cooling system is to cool the air above 100°C to around 10-15°C using water cooling or other methods. This step of cooling creates the necessary conditions for the subsequent processes.
II. Key Benefits of Cooling
The cooling in the pre-cooling system mainly brings two key benefits:
1. Enhancing volumetric efficiency and reducing energy consumption: According to gas laws, a decrease in temperature causes the volume of gas to contract and its density to increase. As a result, the same amount of air requires less work from the subsequent equipment, making the overall process more energy-efficient.
2. Deep dehumidification and prevention of freezing: The saturation water content of air drops sharply as the temperature decreases. The pre-cooling process causes most of the water vapor in the air to condense into liquid water and be removed. This eliminates potential hazards and prevents the water from freezing into ice in the subsequent sub-zero environments, which could block pipelines and equipment.
It can be said that the air pre-cooling system is the first critical step in the entire air separation equipment. By cooling and dehumidifying the air first, it lays a stable, clean, and efficient foundation for all subsequent low-temperature separation steps.
III. Technical Principle: Direct Contact Cooling and Heat and Mass Transfer
Modern large-scale air separation systems employ an efficient cooling method, with the core equipment being the air cooling tower and water cooling tower.
The working principle is as follows:
High-temperature compressed air enters from the bottom of the air cooling tower and comes into direct contact with the low-temperature cooling water sprayed from the top in a counter-flow manner. This is an efficient heat exchange and dehumidification process: the air transfers heat to the water, reducing its own temperature; at the same time, since the water temperature is lower than the dew point of the air, the water vapor in the air rapidly condenses into water droplets and is carried away by the water flow, thus making the air dry. The heated water is sent to the water cooling tower, where it is cooled through natural evaporation or by an external cold source, and then pumped back to the top of the air cooling tower for circulation, forming a closed and energy-efficient water circulation system.
The key to the system’s energy efficiency lies in the cooling method of the circulating cooling water. There are mainly two methods: the traditional mechanical refrigeration (electric refrigeration); and a more efficient process that utilizes the cold energy from the waste nitrogen gas produced by the air separation equipment itself for cooling. The latter achieves internal energy recovery and significantly reduces operational energy consumption.
In summary, the air pre-cooling system ensures the safe, stable, and long-term operation of subsequent molecular sieve adsorbers and low-temperature heat exchangers by cooling, dehumidifying, and preliminarily purifying the air. It can reduce the overall energy consumption of the entire air separation equipment by 5% to 10%. As the first critical link in the system, its performance directly affects the economic efficiency and reliability of air separation production. Under the trend of energy conservation and consumption reduction in industry, optimizing pre-cooling systems (such as adopting waste heat recovery, high-efficiency packing, and zero-emission design, etc.) has always been a key focus of technological development.
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