Molecular oxygen ($\text{O}_2$) is an odorless and colorless gas that sustains nearly all complex life on Earth, driving the metabolic processes that generate energy. Beyond its biological necessity, oxygen is a powerful industrial agent used across manufacturing, aerospace, and medical fields. Oxygen is generated through vast, planet-scale natural cycles and highly controlled, human-engineered processes designed for specific purity and volume requirements.
Earth’s Primary Oxygen Factory
The vast majority of the free oxygen present in Earth’s atmosphere is generated through photosynthesis, a biochemical process performed by plants, algae, and certain bacteria. This reaction uses sunlight to convert water ($\text{H}_2\text{O}$) and carbon dioxide ($\text{CO}_2$) into glucose (a sugar) and molecular oxygen. The oxygen released is a byproduct of splitting water molecules during the light-dependent reactions of photosynthesis.
While terrestrial forests are often recognized as oxygen producers, the single largest contribution comes from marine organisms, specifically phytoplankton and various forms of marine algae. These microscopic organisms inhabit the sunlit layer of the ocean and collectively account for an estimated 50% to 80% of the total oxygen added to the atmosphere annually.
Extracting Oxygen from the Atmosphere
Because oxygen makes up approximately 20.95% of the Earth’s atmosphere, the most common industrial methods for obtaining it involve separating it from the other atmospheric gases, primarily nitrogen. The two dominant commercial technologies used globally for this purpose are cryogenic distillation and pressure swing adsorption, each suited for different scales and purity needs.
Cryogenic Distillation
Cryogenic distillation is the method of choice for producing very large volumes of high-purity oxygen, nitrogen, and argon. This process begins by compressing and chilling ambient air until it reaches temperatures below -180 degrees Celsius, causing the air to liquefy. The liquid air mixture is then slowly warmed inside a distillation column where the various components separate based on their distinct boiling points. Since nitrogen boils at a lower temperature (approximately -196°C) than oxygen (approximately -183°C), it rises higher in the column, allowing high-purity liquid oxygen to be drawn off at a lower point.
Pressure Swing Adsorption (PSA)
For smaller to medium-scale production or applications where extremely high purity is not required, Pressure Swing Adsorption (PSA) is often utilized. PSA operates at ambient temperatures and employs specialized materials called molecular sieves, typically zeolites, to filter the air. These sieves have a stronger affinity for nitrogen molecules than oxygen molecules.
Compressed air is cycled through a vessel containing the sieve material, which physically adsorbs the nitrogen and other trace gases onto its surface. This leaves a concentrated stream of oxygen gas to exit the vessel. When the sieve material becomes saturated with nitrogen, the pressure is rapidly reduced (the “swing”), allowing the adsorbed nitrogen to desorb from the sieve material so the process can begin again. PSA technology is effective for generating oxygen on-site for facilities like hospitals or small industrial operations.
Generating Oxygen Through Chemical Reactions
When highly specific purity, portability, or localized generation is required, producers use methods that involve breaking down chemical compounds.
Electrolysis
Electrolysis of water is one such method, relying on electrical energy to split the $\text{H}_2\text{O}$ molecule. When a direct electric current is passed through water, the water decomposes, releasing hydrogen gas ($\text{H}_2$) at the cathode and oxygen gas ($\text{O}_2$) at the anode. The oxygen produced via electrolysis is extremely pure, often exceeding 99.99% concentration, making it valuable for specialized industrial processes or laboratory use. However, the process is highly energy-intensive and is significantly more expensive per unit of gas compared to cryogenic distillation, restricting its use to niche applications.
Chemical Oxygen Generators (COGs)
Another specialized method involves Chemical Oxygen Generators (COGs), designed to produce oxygen rapidly and reliably in emergency or confined situations, such as on commercial aircraft or in submarines. These generators utilize solid chemical compounds, most commonly sodium chlorate ($\text{NaClO}_3$) or potassium perchlorate ($\text{KClO}_4$), stored in a sealed canister. When the generator is activated, a primer ignites the chemical “candle,” causing the compound to undergo a decomposition reaction that releases oxygen gas. These generators are compact, have a long shelf life, and do not require external power, making them reliable for immediate use. They are, however, single-use devices that generate heat and are not suitable for continuous, large-volume supply.
Practical Applications of Manufactured Oxygen
Manufactured oxygen is used across numerous sectors, driven by the gas’s powerful oxidizing properties and its necessity for human life. In the medical field, manufactured oxygen is routinely used to support patients with respiratory deficiencies, utilizing concentrators based on PSA technology or liquid oxygen stored in portable tanks. The precise concentration and flow of oxygen are managed through specialized delivery systems to treat conditions ranging from chronic obstructive pulmonary disease (COPD) to acute trauma.
Aerospace and high-altitude flight rely heavily on manufactured oxygen to ensure the safety and survival of personnel and passengers. In pressurized aircraft cabins, oxygen is deployed as a supplementary supply during rapid decompression events, often sourced from the chemical generators described previously. For space travel, particularly in closed-loop life support systems, stored or electrolytically generated oxygen is a necessary component of the breathable atmosphere.
Industrial applications consume the largest volumes of manufactured oxygen, utilizing its ability to enhance combustion and chemical reactions. In steel production, large quantities of high-purity oxygen are injected into basic oxygen furnaces to rapidly oxidize impurities like carbon and silicon, significantly speeding up the conversion of iron into steel. Furthermore, the metalworking industry relies on oxygen for oxy-fuel welding and cutting, where combining oxygen with fuel gases like acetylene creates a flame hot enough to melt and manipulate metals effectively.