Air separation nitrogen production

When the demand for nitrogen downstream of the air separation nitrogen production system increases, the system raises the nitrogen production rate, which may lead to a decrease in the efficiency of separating oxygen from nitrogen, resulting in a relatively lower oxygen content in the nitrogen. .

1. System operation mechanism and characteristics of contaminated nitrogen gas In air separation nitrogen production systems (taking deep cryogenic air separation and PSA nitrogen production as examples): Sources of contaminated nitrogen gas: Oxygen-enriched residual gases that remain after separation in the main distillation tower or adsorption tower and are not fully utilized. Typical oxygen content: The oxygen content in contaminated nitrogen from cryogenic air separation is about 0.5%-2%, while in PSA systems it can reach 5%-15%.7814 2. The direct impact of increased nitrogen usage on the system: (1) Adjustment of the distillation column reflux ratio. Phenomenon: Increased demand for nitrogen downstream → Increased amount of vapor rising from the main column → Relative decrease in the amount of reflux liquid. Mechanism: The reduction in reflux liquid leads to more thorough separation of the oxygen component in the gas phase of the upper distillation section. More oxygen molecules enter the liquid oxygen product, reducing the oxygen residue in the contaminated nitrogen gas. Formula support: Oxygen extraction rate η = (Amount of O₂ in the product) / (Total amount of O₂ in the raw air)↑; accordingly, the oxygen content in the contaminated nitrogen is Y = 1 – η×914. (2) Changes in PSA adsorption cycle dynamics: An increase in nitrogen volume leads to a faster switching frequency of the adsorption tower (e.g., from 300 seconds per cycle to 200 seconds per cycle). The adsorption of oxygen by carbon molecular sieves is at a more efficient stage, with the penetration curve shifting forward. Data example: Reducing the adsorption time by 20% can lower the content of nitrogen and oxygen in contaminants from 12% to 8%512. (3) System pressure balance reconstruction: Pressure gradient change: Increased nitrogen extraction volume → Decrease in the pressure in the upper column (e.g., from 0.55 MPa to 0.48 MPa). A low-pressure environment facilitates an increase in the relative volatility of oxygen to nitrogen (α=1.5→1.6), improving separation efficiency by 1415%. 3. Dynamic compensation mechanism of the control system (1) Automatic adjustment of loop response: When the oxygen analyzer detects a decrease in the nitrogen purity of the product, the PID controller makes adjustments by increasing the amount of expanded air (in cryogenic systems) or raising the adsorption pressure (in PSA systems). Typical adjustment range: The compressor speed increases by 5%-10%, and the levels of nitrogen and oxygen in the waste gas decrease by 1-3 percentage points1314. 2) Material balance redistribution: Nitrogen production Q_N2 ↑ → Amount of waste nitrogen Q_waste ↓ (total processed air volume Q_air remains constant). According to mass conservation: Q_air×21% = Q_N2×(1-P_N2) + Q_waste×P_waste. When Q_N2↑ and P_N2 remains constant, P_waste must decrease by 914. 4. Exception handling: It is necessary to be aware of the following situations that may disrupt the aforementioned pattern: Equipment overload: When the amount of nitrogen used exceeds 115% of the designed value, there may be an abnormally high level of nitrogen oxides. Adsorbent failure: When the adsorption capacity of the molecular sieve in the PSA system is below 80%, the level of contaminants such as nitrogen and oxygen may experience abnormal fluctuations 513. Instrument deviation: Malfunctions of the oxygen analyzer can result in false data, so it is necessary to perform regular calibrations (it is recommended to calibrate it once per quarter)14. Operational recommendation: When the content of nitrogen and oxygen in the waste gas remains below 0.8%, it is necessary to check for any abnormalities in the argon fraction to prevent disruptions to the operation of the argon system68].