22/06/2026
Basic Knowledge of Air Separation
What are the causes of molecular sieve water inflow accident?
1.When the liquid level of the air cooling tower exceeds 3500 mm, the water level surpasses the air inlet pipe, and a large amount of water enters the molecular sieve adsorber along with air.
2.When the resistance of the air cooling tower rises to 7 kPa, liquid suspension will form inside the tower, leading to fluctuating liquid level at the bottom and fluctuating tower resistance, and water will be carried into the molecular sieve adsorber by air flow.
3.Disorder of molecular sieve switching procedure causes sudden air venting through the adsorber, sharply accelerating air flow velocity in the air cooling tower and bringing water into the molecular sieve adsorber.
4.Decreased instrument air pressure leads to out-of-control self-regulation of pneumatic valves and rising liquid level.
5.Excessive flow of cooling water and chilled water causes liquid suspension in the air cooling tower.
2γWhat is reflux ratio? What impact does it have on rectification? Reflux ratio refers to the ratio of the downward liquid flow rate to the upward v***r flow rate in the rectification column. Given a fixed number of trays, the purity of rectified products depends on the reflux ratio. A high reflux ratio results in high nitrogen purity in gas phase and low oxygen purity in liquid phase; a low reflux ratio leads to low nitrogen purity in gas phase and high oxygen purity in liquid phase.
3γ What are the preventive measures for expander accidents?
1.The alarm temperature of the front bearing of expander is 70Β°C, and the interlock trip temperature is 75Β°C.
2.The reflux valve opens gradually when expander rotating speed exceeds the alarm value; the expander shuts down once the speed exceeds the interlock trip value.
3.The expander reflux valve fully opens when the booster flow rate drops below the minimum alarm value.
4.The expander inlet valve closes when inlet temperature is lower than -180Β°C.
4γWhat are the operation precautions for booster expander?
1.It is forbidden to remove interlocks to start the expander under any circumstances.
2.Keep the expander inlet temperature no lower than -118Β°C.
3.Do not start the expander when the booster bypass valve FCV401 is closed. Pre-cooling is required before first commissioning or hot startup of the expander.
5γWhat impurities are contained in air? Besides oxygen and nitrogen, air contains trace impurities including water v***r, carbon dioxide, acetylene, other hydrocarbons, as well as tiny solid impurities such as dust.
6γWhy must impurities be removed during air separation process? As air is cooled, frozen water and carbon dioxide will deposit on low-temperature heat exchangers, turbo expanders and rectification columns, blocking flow channels, pipelines and valves. Accumulated acetylene in liquid oxygen poses explosion risks, and dust will abrade rotating machinery. Special purification equipment must be equipped to remove these impurities to guarantee long-term stable and reliable operation of air separation units.
7γHow to judge whether the heating regeneration of molecular sieve is complete? First, the gas pressure and flow rate for molecular sieve heating shall meet process requirements. The heating regeneration process can be judged via regeneration curve. The "cold blow peak temperature" is the indicator of complete regeneration of the whole molecular sieve bed.
8γWhat is oxygen venting rate? It refers to the proportion of oxygen products (gaseous and liquid) produced by oxygen generators that are vented unused. Oxygen venting rate is a key indicator reflecting the matching capacity of equipment and production management level. Higher oxygen venting rate means more energy waste and poorer overall economic benefits. Therefore, multiple measures shall be adopted to reduce oxygen venting rate.
9.What is dew point and air-to-nitrogen ratio? Dew point refers to the temperature at which water v***r in gas reaches saturation and condenses into liquid water. The higher the dew point, the more water v***r the gas contains. The dew point must be strictly controlled in air separation to avoid ice blockage of cryogenic equipment.
10.Air-to-nitrogen ratio means the ratio of total feed air volume to produced nitrogen volume. It reflects the raw material consumption efficiency of nitrogen generation unit. Lower ratio stands for higher air utilization rate and lower energy consumption.
