Liquid air is a cryogenic liquid substance created by cooling atmospheric air until it transitions from a gas to a fluid state. This transformation occurs at extremely low temperatures, resulting in a mobile, pale blue liquid. It is essentially a condensed form of the air we breathe, composed predominantly of liquid nitrogen and liquid oxygen. Liquid air must be stored in specialized containers to maintain its temperature, as it rapidly absorbs heat at room temperature and reverts to gas.
Composition and Liquefaction Process
Gaseous atmospheric air is composed of approximately 78.1% nitrogen, 21.0% oxygen, and 0.9% argon. To convert this mixture into its liquid form, the air must be cooled to its approximate boiling point of -195°C (-319°F). Achieving this deep cooling requires a complex industrial process based on compression and expansion.
The process begins by filtering the air to remove impurities like carbon dioxide and water vapor, which would solidify and clog the machinery. The cleaned air is then highly compressed, which raises its temperature significantly. This hot, compressed air is then pre-cooled and allowed to expand rapidly, utilizing the Joule-Thomson effect, causing a dramatic drop in temperature.
The cycle of compression, cooling, and expansion is repeated multiple times, progressively lowering the air’s temperature until it liquefies. Industrial facilities often use advanced refrigeration cycles, such as the Linde or Claude processes, to achieve the necessary cryogenic temperatures. Once liquefied, the mixture can be further processed using fractional distillation, which separates the components based on their individual boiling points.
Extreme Cryogenic Properties
As a cryogenic substance, liquid air is characterized by its intensely cold temperature, posing a significant hazard upon contact. Exposure to the liquid or its extremely cold vapor can cause severe cold burns, or frostbite, by instantly freezing human tissue. Additionally, the extreme cold can cause materials like carbon steel, plastics, and rubber to become brittle and fracture.
The handling of liquid air is governed by its enormous expansion ratio. When liquid air warms and returns to a gaseous state, its volume increases dramatically by approximately 700 to 860 times. This massive volume change means that even a small amount of liquid air vaporizing in a sealed container can generate immense pressure. Safe storage and transport require specialized, heavily insulated containers, known as Dewar flasks, equipped with pressure relief devices to prevent rupture.
Current Industrial Uses
Liquid air’s unique properties make it valuable for several industrial applications. Its most significant emerging use is in Liquid Air Energy Storage (LAES) systems. These systems store excess electricity, often from intermittent renewable sources like wind or solar, by using the energy to power the air liquefaction process.
The stored liquid air is held in large, insulated tanks at low pressure. When power is needed, the liquid air is pumped to a higher pressure and then heated, causing it to vaporize and expand forcefully. This rapid expansion drives a turbine to generate electricity, feeding power back into the grid. This technology offers a solution for long-duration energy storage and grid stabilization.
Liquid air is also commercially used as the initial product in the air separation process, where fractional distillation yields high-purity industrial gases. These gases—including nitrogen, oxygen, and argon—are used for manufacturing, medical, and welding applications. It also provides a method for high-capacity cryogenic cooling in various industrial processes.