Energy-Saving Optimization Operations For A 45,000 M³/h Air Separation Unit

The air separation unit (ASU) adopts molecular sieve ambient-temperature pretreatment, air boost turbine expansion cooling, double-column distillation, and dual-pump internal compression for liquid oxygen and liquid nitrogen. The designed oxygen production capacity is 45,000 m³/h, with the air compressor and booster driven by a steam turbine in a "one-to-two" configuration. Under the premise of ensuring stable oxygen and nitrogen supply to downstream gasification and synthesis units, this study explores energy-saving and consumption-reduction strategies through energy consumption analysis and optimized operational measures. The goal is to improve overall system efficiency and reduce operating costs.

 

 

 

Analysis of Current Energy Consumption

 

The ASU is designed for an oxygen production capacity of 45,000 m³/h, with a supporting steam turbine high-pressure steam design flow of 168 t/h, including an extraction flow of approximately 28 t/h. The air compressor has a shaft power of 21,000 kW, and the booster has a shaft power of 18,000 kW. Energy efficiency calculations indicate that the air compressor unit accounts for about 95% of the total energy consumption.

In practice, the ASU usually operates at around 36,000 m³/h, approximately 80% of the design load, while the steam turbine consumption remains at roughly 160 t/h, close to the design level. This mismatch between load and energy consumption highlights that the key to energy saving lies in optimizing the operation of the air compressor unit.

 

Optimization Measures

 

●Turbine Expander Operation Optimization

The original anti-surge curve of the expander was set too high, and the reflux valve remained open at 15%, resulting in low cooling efficiency. By adjusting the anti-surge curve, closing the reflux valve, and increasing the guide vane opening to raise the expander speed, the unit's cooling capacity is ensured, booster outlet pressure decreases, and high-pressure steam consumption by the turbine is reduced.

●Heat Exchanger Improvement

Poor circulating water quality reduces heat exchanger efficiency. By installing bypass valves and implementing regular online backwashing, the booster-end heat exchanger temperature decreased by 4–5 K, significantly improving expander cooling performance. Additionally, monitoring the temperature difference at the thermal end of low-pressure plate heat exchangers prevents cooling loss.

●Coordination of Air Compressor and Booster

Appropriately reduce air compressor load and lower speed while maintaining stable low-column pressure; reduce inlet guide vane angle.

Ensure clean air filters to lower inlet resistance and improve compression efficiency.

Adjust booster anti-surge valve opening to 5% to maintain stable second- and third-stage pressures.

Optimize air compressor discharge pressure to better match ASU output with downstream demand.

●Distillation Column Adjustment

By adjusting the reflux ratio, the oxygen content in nitrogen waste is reduced to 2–3%, ensuring liquid nitrogen purity in the lower column and improving oxygen recovery, which reduces air compressor load.

●Molecular Sieve Adsorber Operation Optimization

Extend molecular sieve pressurization time to 25 minutes to reduce airflow fluctuations and minimize the switching impact on the distillation system. Maintain cold purge temperature above 125 ℃ and extend switching cycle from 4 h to 6 h to save steam consumption and reduce operating costs.

 

Optimization Effects

After optimization, the ASU operates stably, and overall energy consumption decreases significantly. A comparison of operating indicators before and after optimization is shown in Table 1.

Table 1. Comparison of ASU Operating Indicators Before and After Optimization

 

Operating Parameter	Before Optimization	After Optimization
Steam turbine high-pressure steam consumption, t/h	135	125
Air compressor discharge pressure, MPa	0.498	0.490
Booster second-stage outlet pressure, MPa	2.70	2.55
Booster third-stage outlet pressure, MPa	6.6	6.3
Expander reflux valve opening, %	15	0
Steam turbine speed, r/min	4450	4250

 

Calculations indicate that approximately 70,000 t of high-pressure steam can be saved annually, demonstrating significant energy-saving and economic benefits.

 

Conclusion

By optimizing the expander, heat exchangers, air compressor and booster coordination, distillation columns, and molecular sieve adsorbers, the ASU achieves significantly improved energy efficiency and reduced operating costs. The energy-saving measures proposed in this study are applicable to turbine-driven internal-compression ASUs and have broad promotion value for large-scale ammonia synthesis and coal chemical enterprises, effectively reducing the comprehensive energy consumption per ton of ammonia.