Air separation and its Distillation Process

Air separation plants separate atmospheric air into its main components, usually nitrogen and oxygen, and sometimes argon and other rare and inert gases.The most common method of air separation is fractional distillation.Cryogenic Air Separation Units (ASUs) are used to supply nitrogen or oxygen and often produce argon at the same time.Other methods such as membranes, pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) are used commercially to separate single components from ordinary air. High-purity oxygen, nitrogen, and argon gases used in semiconductor device fabrication require cryogenic distillation.Likewise, the only viable source of the noble gases neon, krypton, and xenon is the distillation of air using at least two distillation columns.Helium is also recovered in advanced air separation processes.

Cryogenic Distillation Process

Pure gases can be separated from air by first cooling until liquefied, then selectively distilling the components at different boiling temperatures.The process can produce high-purity gases, but is energy-intensive.The process was pioneered by Carl von Linde in the early 20th century and is still used today to produce high-purity gases.He developed it in 1895; the process remained purely academic for seven years before it was first used in industrial applications (1902).The cryogenic separation process requires the heat exchanger and separation column to be very tightly coupled for good efficiency, and all energy used for refrigeration is provided by air compression at the inlet of the unit.

1.To achieve cryogenic distillation, the ASU requires a refrigeration cycle that operates using the Joule-Thomson effect, and the cold equipment must be kept in an insulated enclosure (often called a "cold box").The cooling of the gas requires a lot of energy to make this refrigeration cycle work and is provided by the air compressor.Modern ASUs use an expansion turbine for cooling; the output of the expander helps drive an air compressor for greater efficiency.The process consists of the following main steps: Pre-filtering of dust in the air prior to compression.

2.The air is compressed, and the final delivery pressure is determined by the recovery rate of the product and the state of the fluid (gas or liquid).Typical pressure ranges are between 5 and 10 bar gauge.Airflow can also be compressed to different pressures to increase the efficiency of the ASU. During compression, water condenses out in the interstage cooler.

3.Process air is typically passed through a bed of molecular sieves to remove any residual water vapor as well as carbon dioxide, which can freeze and plug cryogenic equipment.Molecular sieves are generally designed to remove any gaseous hydrocarbons from the air, as these could cause explosions during subsequent air distillation.Molecular sieve beds must be regenerated.This is achieved by installing multiple units operating in alternating mode and using dry co-generated exhaust air to desorb water.

4.The process air passes through an integrated heat exchanger (usually a plate-fin heat exchanger) and is cooled in the product (and waste) low temperature stream. Part of the air liquefies to form an oxygen-rich liquid.The remaining gas, enriched in nitrogen, is distilled to nearly pure nitrogen (typically < 1 ppm) in a high pressure (HP) distillation column.The condenser of this column requires refrigeration,which is obtained by further expanding the more oxygen-enriched stream through a valve or through an expander (reverse compressor).

5.Alternatively, when the ASU is producing pure oxygen, the condenser can be cooled by heat exchange with the reboiler in the low pressure (LP) distillation column (operating at 1.2-1.3 bar abs).To minimize compression costs, the combined condenser/reboiler of the HP/LP column must operate with a temperature difference of only 1-2 K, requiring plate-fin aluminum heat exchangers.Typical oxygen purities are between 97.5% and 99.5%, affecting maximum oxygen recovery.The refrigeration required to produce the liquid product is obtained using the Joule-Thomson effect in the expander, which feeds the compressed air directly into the low pressure column. Therefore, a certain portion of the air is not separated and must leave from the upper part of the low pressure column as an exhaust stream.

6.Since the boiling point of argon (87.3 K under standard conditions) is between that of oxygen (90.2 K) and that of nitrogen (77.4 K), argon accumulates in the lower part of the low pressure column.When argon is produced, side draw steam is drawn from the low pressure column with the highest concentration of argon.It is sent to another column where the argon is rectified to the required purity, from which the liquid is returned to the same location in the LP column.Argon impurity levels 1 ppm can be achieved using modern structured packing with very low pressure drop.Although the argon content is less than 1% in the feed, the air argon column requires a lot of energy due to the high reflux ratio (approximately 30) required by the argon column. Cooling of the argon column can be provided by cold expansion rich liquid or liquid nitrogen.

7.Finally, the product produced in gas form is heated to ambient temperature relative to the incoming air.This requires carefully designed thermal integration, which must take into account robustness against perturbations (due to switching of molecular sieve beds.It may also require additional external cooling during startup.