The components of air (Table 1), especially nitrogen (N2) and oxygen (O2), are critical for many modern industrial processes. Primary metals production, chemical manufacturing, gasification processes, clay, glass and concrete production, welding and other processes depend on O2 from air, while the chemical, petroleum-refining and electronics industries utilize N2 for its inert properties. In addition, liquid N2 is used for cryogenic grinding, freeze-drying, cryogenic storage of biological materials, food freezing and other applications. Argon (Ar) is used as an inert material in welding, steelmaking, heat-treating and electronics manufacturing. This one-page reference discusses the main methods by which these common industrial gases can be derived from air.
Air separation approaches
Separation of air into its constituent parts for industrial use can be divided into two main categories: cryogenic air separation and non-cryogenic processes. Cryogenic air separation processes produce N2, O2 and Ar as either gases or liquids by employing low-temperature distillation to separate the fluids. Cryogenic separation processes are most commonly used when high-purity products and high production rates are required. Non-cryogenic processes for air separation include the use of selective adsorption to separate air components, and differential permeation through membranes. These techniques use differences in molecular structure, size and mass to separate air components. Non-cryogenic processes are carried out at near-ambient temperatures, and are most often used in cases when high-purity gases and liquid products are not needed and when production volumes are relatively small.
Non-cryogenic air separation
The following describes two of the main types of non-cryogenic air separation methods.
Adsorption. Zeolites (highly porous aluminosilicate materials) have non-uniform electric fields in their void spaces. These differences can be capitalized upon to preferentially adsorb N2, because N2 molecules are more strongly adsorbed than O2 molecules or Ar atoms. The zeolite adsorbents are paired with a pressure-swing system, whereby pressurized air enters a vessel containing the adsorbent. N2 is adsorbed preferentially and an O2-rich effluent stream is produced until the bed has been saturated with N2. When saturation is reached, the feed air is routed to another vessel and the regeneration of the first bed begins. Regeneration is achieved by reducing the pressure in the bed, which reduces the equilibrium N 2 -holding capacity of the adsorbent and N2 is released.
Membrane systems. Gas separation processes using polymeric or ceramic membranes are based on differences in the rates at which O2 and N2 diffuse through a membrane that separates high- and low-pressure process streams. Membrane air-separation systems are less energy-intensive than cryogenic separation, but are generally more limited in performance. Due to O2’s smaller molecular size, most membrane materials are more permeable to O2 than to N2. Membrane systems are usually limited to the production of O2-enriched air (25–50% O2, rather than 21 vol.% in regular air).
Cryogenic air separation is currently the most efficient and cost-effective technology for producing large quantities of O2, N2 and Ar as either gaseous or liquid products. By taking advantage of boiling-point differences of the species at very low temperatures (see Table), a distillation column can be used to separate them.
There are five major unit operations required to cryogenically separate air into useful products. Key steps of the cryogenic air distillation process include air compression, air-cooling and purification, heat exchange, refrigeration, internal product compression and rectification (countercurrent distillation, where the separation of components occurs).
An air-separation unit (ASU) using a conventional, multi-column cryogenic distillation process can produce O2 from compressed air at high recoveries and purities. The most common design is a double-column system with an adjacent Ar unit.
To start the separation process, a large quantity of incoming air needs to be liquefied. This is accomplished by cooling the air by decreasing its temperature and manipulating the pressure until condensation begins.
The first air-separation process was developed by Carl von Linde. Started up in 1902, it separated O2 from air, and was eventually developed into the double-column mechanism in 1910 to allow the production of both O2 and N2 simultaneously. The ability to extract Ar was developed in 1913.
The energy needed for the very low temperatures accounts for most of the cost of production, so the efficiency of the compression and heat exchange are of great interest. While ASUs require high initial capital costs, they can realize relatively high yields and can obtain large volumes of high-purity gases or liquids.
Vinson, D.R., Air Separation Control Technology, Computers and Chemical Engineering, vol. 30, pp. 1,436–1,446, 2006.
Smith, A.R. and Klosek, J., A review of air separation technologies and their integration with energy conversion processes, Fuel Processing Technology, vol. 70, pp. 115–134, 2001.
Alekseev, A., Basics of Low-Temperature Refrigeration, Linde AG, https://cds.cern.ch/record/1974048/files/arXiv:1501.07392.
Easterbrook, N., Boland T. and Farese, D., Extremely Low-Temperature Systems, Chem. Eng., August 2015, pp. 38–44.
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