Oxygen extraction rate (oxygen recovery rate) is the core economic index of air separation unit, which measures the efficiency of oxygen in the air being separated into qualified product oxygen.
Calculation formula: ψ=(product oxygen output × product oxygen purity) ÷ (processing air volume × oxygen content in air 20.95%) × 100%. For example, the processing air is 18100 nm ³/h, the oxygen output is 3200 nm ³/h, the purity is 99.6%, and the extraction rate is about 84.2%, that is, 84.2% of the oxygen in the air is recovered, and 15.8% is lost with the venting of waste nitrogen.
Material balance logic: there are only two directions for oxygen in the air entering the tower - to become product oxygen (effective) or to be discharged with polluted nitrogen and pure nitrogen (loss). The lower the oxygen content of waste nitrogen, the less oxygen taken away by venting, and the higher the extraction rate, which is the core logic of on-site regulation.
Conventional interval: 82% -86% for small and medium-sized external compression air separation units; 86% ～ 90% for air separation with large structured packing and argon; The operating condition of large amount of liquid oxygen production will drop to 75% -82%.
Key influencing factors: ① oxygen content in waste nitrogen (the core, normally controlled at 0.5% -1.5%); ② Reflux ratio and distillation conditions; ③ Expansion air distribution; ④ Output ratio of liquid products; ⑤ Temperature difference between cooling loss and main heat exchanger; ⑥ Variable load operation fluctuation.
Industry significance: for every 1% increase in extraction rate, the energy consumption per unit of oxygen decreases by about 0.8% -1.2%, which directly determines the operation cost and output income of the unit, and is also an important basis for judging whether the distillation condition is normal.
