Nitrogen ($N_2$) is a colorless, odorless diatomic gas that constitutes approximately 78% of Earth’s atmosphere, making it the most abundant gas in the air. Despite this abundance, the element is chemically unavailable for most life forms and industrial processes in its atmospheric form. The two nitrogen atoms are held together by an extremely strong triple covalent bond, requiring significant energy to break. This inert nature means the dinitrogen molecule must be “fixed” or isolated—converted into a more reactive compound or separated from other gases—before it can be used. Production processes range from microscopic bacterial action in the soil to massive, energy-intensive industrial plants.
Nature’s Production Line: Nitrogen Fixation
The natural conversion of atmospheric nitrogen into usable compounds is biological nitrogen fixation (BNF). This is performed by prokaryotic microorganisms, including soil bacteria and cyanobacteria, which possess the unique ability to break the dinitrogen triple bond. These microbes convert the inert gas directly into ammonia ($NH_3$), a form that plants can readily absorb and use to build proteins and nucleic acids.
The enzyme responsible for this transformation is nitrogenase, a protein complex containing iron, often with molybdenum or vanadium. The reaction is highly energy-demanding, requiring substantial energy, which the bacteria obtain from the host plant during a symbiotic relationship. A well-known example involves Rhizobia bacteria that live within root nodules of leguminous plants like clover and soybeans. A secondary natural process, atmospheric fixation, occurs when lightning strikes cause nitrogen and oxygen in the air to react, forming nitrogen oxides that are then washed into the soil by rain.
Large-Scale Isolation of Nitrogen Gas
For industrial applications requiring bulk quantities of high-purity nitrogen gas, the primary method is Cryogenic Air Separation (CAS). This process physically isolates nitrogen from other gases based on differences in their boiling points. Production begins by drawing in ambient air, compressing it to high pressure, and then purifying it to remove contaminants like moisture, carbon dioxide, and hydrocarbons.
The purified, compressed air is then cooled to extremely low, cryogenic, temperatures, which causes the air to liquefy. This liquid air is fed into a distillation column. Nitrogen has a lower boiling point (-196°C) compared to oxygen (-183°C).
Within the distillation column, the liquid mixture is separated through fractional distillation. As the liquid air warms slightly and rises, the nitrogen vaporizes first, collecting at the top as a high-purity gas. This method is highly effective, capable of producing nitrogen with a purity exceeding 99.999%, necessary for sensitive industrial uses like electronics manufacturing and bulk liquid nitrogen production for cooling.
Manufacturing Nitrogen Compounds for Fertilizers
The production of nitrogen compounds, particularly ammonia ($NH_3$), is dominated by the industrial-scale Haber-Bosch process. This chemical synthesis converts atmospheric nitrogen into the backbone chemical for nearly all nitrogen-based fertilizers. The process combines nitrogen gas from the air with hydrogen gas, typically derived from natural gas or methane, in a 1:3 volume ratio.
This reaction is conducted under harsh, controlled conditions to overcome the dinitrogen molecule’s stability. The gases are subjected to high pressures (150 to 250 atmospheres) and high temperatures (400°C and 450°C). A finely divided iron-based catalyst accelerates the reaction rate, allowing the nitrogen triple bond to be broken and the new ammonia molecule to form.
The resulting ammonia is cooled until it liquefies, allowing continuous removal from the reaction chamber. Removing the product helps shift the chemical equilibrium toward greater ammonia production, maximizing the yield. While only about 15% of the reactants convert to ammonia in a single pass, the unreacted nitrogen and hydrogen gases are recycled back into the system, achieving an overall conversion rate of around 98%.
Localized and On-Demand Nitrogen Generation
Not all applications require the ultra-high purity or massive volumes supplied by cryogenic distillation, leading to the use of localized, on-demand nitrogen generators. These systems use two primary non-cryogenic technologies to separate nitrogen from compressed air. One method is Pressure Swing Adsorption (PSA), which relies on specialized materials called Carbon Molecular Sieves (CMS).
In a PSA unit, compressed air is pushed through a vessel containing the CMS material. Under pressure, oxygen molecules are selectively adsorbed onto the porous surface of the sieves, while nitrogen molecules pass through and are collected. These systems typically use two towers that alternate between adsorption and regeneration (venting the oxygen) to ensure a continuous flow of nitrogen, often achieving purities up to 99.999%.
The second common method is Membrane Separation, which uses a bundle of hollow polymer fibers. Compressed air is passed through these fibers, and separation occurs based on the different permeation rates of the gases. “Fast” gases like oxygen, water vapor, and carbon dioxide permeate through the fiber walls more quickly, leaving the “slower” nitrogen molecules to travel down the length of the fiber and exit as the product gas. Membrane systems are reliable and simple, generating nitrogen with purities commonly ranging from 95% to 99.5% for applications like tire inflation and fire prevention.