Air separation is the most common process used to extract one or all of the main constituents of atmospheric air. The initial measurements of the constituents of air, with the exception of argon, were improved and summarized in the early 1800s by John Dalton. Earth's atmosphere is 78.09 % nitrogen, 20.94 % oxygen, 0.93 % argon, and 0.04 % carbon dioxide with very small percentages of other elements and normally not recovered (neon, helium, krypton, methane, hydrogen, xenon, and radon). In very large air separation units (ASU) Neon, Xenon and Krypton are recovered in small amounts.
Cryogenic air separation utilizes the differing condensing/boiling points of the components of air to enable separation by distillation at cryogenic temperatures. At atmospheric pressure the main components of the air have the following condensing/boiling points:
- Nitrogen -196° C
- Oxygen -183° C
- Argon -186° C
BASIC ASU COMPONENTS
Main Air Compressor (MAC)
The MAC compresses atmospheric air, generally to 4-7 BARG, and delivers it to the system. These compressors are normally driven by electric motors. Interstage coolers are provided to remove the heat of compression between each stage of the compressor, of which there are normally 2-3.
Front End Clean Up
Modern ASUs utilize a Prepurifier Unit (PPU), which removes moisture, CO2, and most hydrocarbons from the air. Moisture and CO2 must be removed to prevent ice and dry ice from forming later in the process. A PPU is typically made up of a chiller to cool the air to 4-13 C, a condensate separator to remove free water, and 2 vessels filled with desiccant and mole sieve material, which adsorbs the contaminants while allowing the air to pass through. One bed is always online to the process, while the other bed is regenerated with heated waste Nitrogen to remove accumulated contaminants. Beds automatically switch every 5-8 hours. The air from the PPU is very close to moisture and CO2-free.
The ColdBox contains the cryogenic heat exchangers, distillation columns, and associated valves and piping. Because parts of this system are very cold, all components are mounted inside the ColdBox and then encased in insulation. Cold boxes can be rectangular or cylindrical and are usually tall, some over 200′ depending on capacity and type of Argon system. Modern cold boxes are filled with perlite insulation, which is light and easy to install and remove, when necessary. Older cold boxes may be tightly packed with cryogenic Rockwool, which is very time-consuming to install and remove.
All ASUs except some very small units have expanders. Expanders provide the required refrigeration to produce liquids in the distillation column system. Air, Nitrogen, or Waste Nitrogen is fed to the expander, causing the wheel to turn and transfer energy to a compressor, generator, or oil brake. This transfer of energy causes the gas to cool. As the process continues, the outlet temperature of the expander eventually reaches the design temperature while cooling the column system.