Once an air separation unit (ASU) enters continuous operation, the real challenge is no longer startup, but maintaining long-term stability. Most failures are not sudden-they begin with subtle shifts in temperature, pressure, flow rate, or product purity. Detecting these early signals is often far more valuable than reacting after a shutdown.
Based on field experience, common issues in ASU operation typically fall into six categories: cold box, purification system, main heat exchanger, distillation column, rotating equipment, and instrumentation. These systems are closely interconnected, and a minor deviation in one area can gradually propagate across the plant.
1. Cold Box Internal Leakage
Leakage inside the cold box is one of the most concealed and time-consuming problems to handle. In early stages, it may only appear as localized frosting on the outer shell or abnormal temperature drops at the foundation, without immediately affecting production.
Because the cold box is insulated with perlite, small leaks are difficult to locate. By the time visible deformation or perlite discharge occurs, the issue has often persisted for an extended period.
The root cause is usually thermal stress. Frequent startups, shutdowns, or load fluctuations lead to repeated expansion and contraction of piping, gradually creating micro-cracks at weld joints. In operation, large load swings should be avoided, and temperature distribution on the cold box surface should be monitored closely. Once leakage is confirmed, excavation of insulation (perlite removal) is typically required, resulting in high maintenance time costs.
2. Molecular Sieve Breakthrough in the Purification System
Molecular sieve breakthrough allows moisture and carbon dioxide to enter the cryogenic section, where they freeze in the heat exchanger or distillation column. This leads to increased pressure drop, larger temperature differences, and in severe cases, forced shutdown for defrosting.
Common causes include aging adsorbents, insufficient regeneration temperature, or deteriorated inlet air conditions-especially during high-humidity summer periods.
Judgment should not rely solely on experience. Continuous monitoring of outlet CO₂ concentration and regeneration temperature profiles is essential. If readings show a sustained upward trend, the regeneration system should be checked immediately or the molecular sieve replaced to prevent downstream impact.
3. Main Heat Exchanger Blockage or Internal Leakage
Abnormalities in the main heat exchanger are typically reflected by rising air-side pressure drop, increasing warm-end temperature difference, and declining production capacity. Blockages are usually caused by ice, CO₂, or particulates, often originating from upstream purification issues.
Another risk is internal leakage. High-pressure air leaking into product channels can cause fluctuations in product purity. Diagnosis requires a combined analysis of pressure, flow, and temperature trends.
Minor blockage may be mitigated through operational adjustments and controlled defrosting. However, confirmed internal leakage generally requires shutdown and repair.
4. Distillation Column Instability
The distillation column directly determines product purity. Two common operational issues are flooding and weeping.
Flooding occurs when vapor or reflux rates are too high, leading to increased column pressure drop and reduced purity. The typical response is to reduce load and gradually restore normal operation.
Weeping happens when vapor velocity is insufficient to support liquid on trays, causing liquid to fall through and reducing mass transfer efficiency. In this case, increasing gas flow or adjusting reflux is necessary.
Over long-term operation, dust accumulation inside the column should also be monitored. If necessary, shutdown cleaning should be performed to maintain separation efficiency.
5. Compressor and Expander Issues
Problems in rotating equipment are usually indicated by increased vibration, rising bearing temperatures, or seal leakage.
Vibration is often related to rotor imbalance or bearing wear, while high bearing temperature is typically linked to lubrication system issues such as oil degradation or insufficient cooling.
For expanders, excessive seal clearance can result in cold energy loss, reducing liquefaction efficiency. Routine monitoring of vibration, maintenance of the oil system, and regular seal inspection are essential to ensure stable operation.
6. Instrumentation and Control Errors
Abnormal readings from oxygen analyzers, temperature sensors, and flow meters are relatively common in ASU operation. Issues such as zero drift, loose wiring, or blocked impulse lines can compromise data accuracy and mislead operational decisions.
While these problems are not complex, they require disciplined management. Regular calibration of key instruments, inspection of impulse lines and wiring, and maintaining historical records are recommended to distinguish between random errors and equipment aging.
The core of stable ASU operation lies in continuous monitoring of temperature, pressure, flow rate, and purity. The cold box should be observed for frosting and temperature distribution, the purification system for CO₂ levels, the heat exchanger for pressure drop, and the distillation column for pressure and temperature profiles.
Most failures provide early warning signs. The key is timely identification and response.
From a long-term perspective, stable operating conditions and standardized maintenance practices are far more critical than handling individual failures. With a strong operational foundation, both the reliability and service life of the air separation unit can be significantly improved.
