An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen, and sometimes argon and other rare inert gases.
Three air separation methods are commonly used for separating constituent gases from the air.
In this separation process, equipment pumps air into the membrane module, and the targeted gases like oxygen and nitrogen are separated based on differences in diffusivity and solubility.
For example, oxygen can be separated from the ambient air and collected at the upstream side, and nitrogen at the downstream side.
Pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) are commercially used to separate a single component from ordinary air.
The principle of Pressure Swing Adsorption (PSA) is the amount of adsorbate deposited on the adsorbent increases with increasing pressure. Adsorption increase with increasing pressure, and desorption occurs at low pressure. The technique is applied for the adsorptive recovery of O2 and N2 from the air.
The most common method for air separation is fractional distillation. Cryogenic air separation units (ASUs) are built to provide nitrogen or oxygen and often co-produce argon.
Pre-filtered air is compressed into a compressor called Feed Air Compressor. The air stream may also be compressed to different pressures to enhance the efficiency of the ASU. During compression, water is condensed out in inter-stage coolers.
The processed air is then passed through a molecular sieve bed, which removes any remaining water vapor and carbon dioxide, which would freeze and plug the cryogenic equipment.
Molecular sieves are designed to remove gaseous hydrocarbons from the air, which can be a problem in the subsequent air distillation and lead to explosions. This is done by installing multiple units operating alternating modes and using the dry co-produced waste gas to desorb the water. Purified air is then compressed in another compressor called Recycle or booster compressor. Processed air then passes through an integrated plate-fin heat exchanger, where air cools to near its liquefaction temperature.
A cold production unit is required to maintain the proper operating temperatures in the process and offset the heat leak into the cold box. Normally for a low–pressure gas plant, an expansion turbine is provided as a cold production unit. In the more recently designed plants, the turbine operates on a small, clean airflow that has been boosted in pressure by its compressor brake (expander–booster).
An air separation unit requires a refrigeration cycle that operates the Joule–Thomson effect to achieve low distillation temperatures. The cold equipment must be kept in an insulated enclosure called a “cold box”. The cooling of the gases requires a lot of energy to make this refrigeration cycle work and is delivered by an air compressor. Modern ASUs use expansion turbines for cooling; the expander’s output helps drive the air compressor for improved efficiency.
A double column is used for the cryogenic distillation of air. In air separation plants is, a combination of two columns used. This 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 before it was used in industrial applications for the first time (1902).
The lower part is a “half” single column with a condenser at the top, and an air feed at the bottom, whereas the upper part, is a single column without a condenser but with a reboiler.
The condenser in the lower column acts as the reboiler for the upper column; those two are thermodynamically attached. Air, normally at a temperature just above the dew point, is fed to the bottom of the lower column.
Vapor raises the column to the condenser and forms reflux. “Rich liquid” contains approx. 35-40 % oxygen is taken out as the bottom product, and nitrogen is the top product. The bottom product, the rich liquid, is then fed to the center part of the upper column for further separation. In this upper column, there is no condenser, but reflux is taken from the pure top product of the lower column.
A reboiler at the bottom of the upper column is heated from the condensing of nitrogen in the lower column. The upper column can have pure oxygen at the bottom, as oxygen has a higher boiling point than nitrogen.
An argon-rich stream is withdrawn from the middle of the LP column and refined to a pure product in other 03 distillation columns, which are called crude Argon column, semi-pure Argon column and Nitrogen removal column
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