Excessive oxygen withdrawal rate. From the material balance perspective, when the processing air volume and nitrogen purity are fixed, only a certain amount of oxygen can be withdrawn to maintain the desired oxygen purity. If the withdrawal rate is too high, the purity will inevitably fail to meet requirements. From the distillation perspective, excessive oxygen withdrawal reduces the rising vapor volume in the stripping section of the upper column, increases the reflux ratio, and results in insufficient evaporation of nitrogen (argon) from the liquid, causing oxygen purity to decrease. In this case, the oxygen delivery valve should be partially closed to reduce oxygen flow, while the nitrogen delivery valve should be opened wider to maintain upper column pressure.
Low oxygen purity in liquid air. Low oxygen purity in liquid air indicates excessive liquid air volume. This increases the separation load in the stripping section of the upper column and provides too much reflux liquid for nitrogen (argon) components to evaporate fully, resulting in reduced oxygen purity. The lower column distillation conditions should be adjusted to appropriately increase the oxygen content in liquid air.
Excessive expanded air flow to the upper column. The greater the expanded air flow entering the upper column, the lower the nitrogen exhaust purity. To maintain oxygen delivery volume, oxygen purity must decrease. When expanded air flow is excessive, it disrupts normal distillation conditions in the upper column, causing significant drops in oxygen purity. If there is excess cooling capacity in the column, the expander flow should be reduced. If the liquid oxygen level is normal, part of the expanded air should be bypassed to reduce flow to the upper column.
High liquid oxygen level in the condenser-evaporator. When the main condenser liquid oxygen level rises, it indicates that the descending liquid volume exceeds the evaporation rate, increasing the reflux ratio in the stripping section and reducing oxygen purity. The expander flow should be reduced in this case. If the liquid oxygen level is very high and oxygen purity is poor and difficult to adjust, some liquid oxygen should be discharged to fully utilize the heat exchange area of the condenser-evaporator, then re-adjusted. A rising main condenser liquid oxygen level may also be caused by large amounts of solid carbon dioxide entrained in the liquid oxygen, deteriorating heat transfer and preventing liquid oxygen evaporation, forcing reduced oxygen withdrawal. If necessary, the unit must be shut down for warming.
Decreased tray efficiency. If distillation trays become deformed, the column body tilts, or sieve holes are blocked by solid impurities, the gas-liquid mass transfer efficiency on each tray will be affected, causing purity to drop. If the operating cycle has been long and tray resistance has increased, the unit should be shut down for warming.
Abnormal distillation conditions. When flooding or weeping occurs in the distillation column, normal distillation conditions are disrupted, causing purity to decrease. Specific measures should be taken based on the situation to eliminate abnormal operating conditions.
Main condenser leakage. Poor brazing quality in the condenser-evaporator, worn evaporator tubes, local explosions, or minor leaks can all cause higher-pressure gaseous nitrogen to leak into the lower-pressure oxygen side, reducing oxygen purity. When the purity drop is minor, the difference between liquid oxygen and gaseous oxygen purity can be analyzed to determine leakage. When this difference exceeds the normal vapor-liquid equilibrium concentration difference, leakage is usually the cause, and shutdown for maintenance is required.
