Working Principle:
The fundamental principle of air separation involves utilising cryogenic distillation to condense air into liquid form, subsequently separating its constituents based on their differing evaporation temperatures. Pure nitrogen and pure oxygen are simultaneously obtained at the top and bottom of the upper column in a two-stage distillation tower; alternatively, liquid oxygen and liquid nitrogen can be extracted from the evaporation side and condensation side of the main condenser respectively. Air separation within the distillation tower occurs in two stages: the air undergoes an initial separation in the lower column, yielding liquid nitrogen whilst producing oxygen-enriched liquid air; The oxygen-enriched liquid air is then fed into the upper tower for further distillation, yielding pure oxygen and pure nitrogen. The upper tower is subdivided into two sections: - The upper section, demarcated by the liquid air feed inlet, serves as the distillation section. It distils the rising vapour to recover oxygen components and enhance nitrogen purity. - The lower section functions as the stripping section, separating nitrogen components from the liquid to increase the oxygen purity of the liquid product.
Process Flow:
1. Air Compression: Air is compressed by compressors to 0.5–0.7 MPa;
2. Pre-cooling: Air is pre-cooled to 5–10°C in pre-cooling units and moisture is separated;
3. Purification: Residual moisture, carbon dioxide, and hydrocarbons are removed in molecular sieve purifiers;
4. Air Expansion: Air undergoes expansion cooling in expansion units, providing required cooling capacity for the plant;
5. Heat Exchange: Air exchanges heat with refluxing oxygen, nitrogen, and contaminated nitrogen in the fractionation tower heat exchanger, cooling to near liquefaction temperature while repeatedly heating the reflux gases to ambient temperature;
6. Subcooling: Nitrogen subcools liquid air and liquid nitrogen prior to throttling within the subcooler;
7. Distillation: Air undergoes distillation separation within the distillation column, yielding product nitrogen at the column top and product oxygen at the column bottom.