1. Process introduction
The air enters from the air suction tower, is filtered, pressurized by an air compressor, enters the air pre-cooling tower, and is pre-cooled with cooling water. The cooled air is sent to the molecular sieve purification system (MS system), and the air is purified by the molecular sieve adsorber Finally, the moisture, CO2 and hydrocarbons in the air are removed. The purified air is divided into two parts. One part enters the air separation tower after passing through the expander system and main heat exchanger. The other part enters the lower tower of the fractionation tower after exchanging heat with product oxygen and nitrogen. In the fractionating tower system, the air that has been pressurized, purified, and pre-cooled in the previous section will be separated, and oxygen and nitrogen will eventually be obtained. Oxygen and nitrogen are compressed by the compressor system and then used in other sections.
2. Control strategy
Most of the air separation units adopt conventional control. The difficulties and key points are the chain protection and anti-surge control of the air compressor unit and the timing control of the molecular sieve system.
1. Air compressor system
The condition parameters cited in the air compressor alarm interlock protection include shaft vibration, shaft displacement, oil pressure before the filter, oil pressure after the filter, main oil pressure, oil temperature, main motor current, third-stage exhaust pressure, etc.
2. Molecular sieve (purifier) timing control system
The entire process is automatically sequence controlled and fault alarms are provided to reduce work intensity and ensure safe production.
3. Air compression and expansion system
The main controls of the expander system include: startup judgment and startup process sequence control, normal shutdown sequence control, severe fault condition judgment and sequence control shutdown, nitrogen injection shutdown judgment and sequence control, oil heater and oil pump start interlock, expander overspeed alarm interlock stop wait.
4. Air separation system
Since fully automatic control involves the adjustment and execution of multiple adjustments, we adopt complete variable operating conditions control to maximize the use of compressor air and reduce energy consumption. However, due to the strong targeting of large-scale air separation units and the large range of cooling capacity adjustment, the operating conditions of the oxygen generating unit will fluctuate abnormally, which will have a great impact on its stability. Therefore, strategies such as multiple adjustments, fine adjustments, and buffer transitions are used in the control. In order to avoid fluctuations and achieve control purposes.
3. Summary
The air separation unit is a general device used in the petrochemical, metallurgical and coalification fields. Air separation is an industrial system with strong coupling, non-linearity, ultra-high purity and large energy consumption. There are two major difficulties in controlling air separation units: difficulty in energy-saving optimization and difficulty in high-purity control. In the long-term application practice in the air separation industry, our company has established a nonlinear fluctuation model of the process, which not only achieves energy saving and consumption reduction, but also controls the purity to 99.999%.
