Atmospheric air is a vast reservoir of nitrogen, constituting approximately 78% of its volume. This non-reactive gas is sought after by industries for its inert properties, making it ideal for various processes. Nitrogen is widely used to prevent combustion and oxidation in chemical manufacturing, protect sensitive electronics, and play a significant role in food preservation by displacing oxygen in packaging. Extracting this element requires specialized industrial processes to achieve the necessary purity levels.
Cryogenic Distillation: The High-Purity Standard
Cryogenic distillation is the oldest and most energy-intensive method for nitrogen production, but it is unmatched in its ability to yield ultra-high purity gas, often exceeding 99.999%. This large-scale process separates air components by cooling them to extremely low temperatures, taking advantage of the difference in their boiling points. Nitrogen boils at approximately -195.8°C, which is colder than oxygen, which boils at -183°C.
The process begins with atmospheric air being filtered to remove dust, then compressed to a high pressure. This compressed air is then pre-treated by passing it through molecular sieves to scrub out impurities like water vapor, carbon dioxide, and hydrocarbons, which would otherwise freeze and clog the equipment at cryogenic temperatures. The purified air is then gradually cooled down in heat exchangers, using the cold products returning from the process to maximize energy efficiency.
As the air cools, it eventually liquefies and is fed into a distillation column where fractional separation occurs. The liquid air is separated in a high-pressure column into a nitrogen-rich vapor and an oxygen-rich liquid, based on their boiling points. The nitrogen-rich vapor moves into a separate, low-pressure column where it is further refined through continuous vaporization and condensation. The high-purity nitrogen product is collected as a vapor from the top of the low-pressure column. This method is primarily used for bulk production requiring high volume and the highest purity levels, such as in the electronics and specialized chemical industries.
Separating Nitrogen Using Pressure Swing Adsorption
Pressure Swing Adsorption (PSA) offers a non-cryogenic alternative for generating nitrogen, typically for medium-volume, on-site production where ultra-high purity is not a necessity. This technology operates by exploiting the physical phenomenon of adsorption, where gas molecules adhere to the surface of a solid material under pressure. The core of a PSA system consists of twin adsorption vessels filled with Carbon Molecular Sieve (CMS).
Compressed air is fed into one of the vessels at high pressure, forcing the separation to occur. The CMS material is engineered to selectively adsorb oxygen, carbon dioxide, and other trace contaminants onto its porous structure. Nitrogen molecules, which have a lower affinity for the CMS and are physically larger, are allowed to bypass the sieve material and pass through the vessel as the desired product gas. This process yields nitrogen purity levels typically ranging from 95% up to 99.999%.
The system relies on a continuous “swing” cycle between two beds to ensure uninterrupted nitrogen delivery. While the first vessel is under high pressure and actively producing nitrogen, the second vessel is depressurized. This depressurization causes the adsorbed oxygen and other contaminants to detach from the CMS surface (desorption or regeneration), allowing them to be vented safely to the atmosphere. The beds automatically switch roles, cycling between adsorption at high pressure and regeneration at low pressure, making the process highly efficient for industrial uses, including fire prevention and food packaging.
Filtration Using Membrane Technology
Membrane technology provides the simplest and most compact method for nitrogen generation, operating on the principle of selective permeation. This method is characterized by its simplicity and low maintenance, as it involves very few moving parts. It is best suited for lower-flow applications or where purity requirements are modest, typically between 95% and 99.5%.
The separation occurs when pre-treated compressed air is passed through bundles of thousands of hollow, non-porous polymer fibers. Each gas component has a different characteristic speed at which it can dissolve into and pass through the membrane wall. Oxygen, water vapor, and carbon dioxide are considered “fast” gases because they permeate, or pass through the fiber walls, much faster than nitrogen.
As the compressed air travels through the hollow fibers, the faster-permeating gases are vented out of the membrane housing. The slower-moving nitrogen molecules remain inside the hollow fibers, creating a concentrated stream of nitrogen gas collected at the outlet. This pressure-driven selective permeation results in a very dry nitrogen product. The compact, modular design makes these systems easy to install directly on-site, offering a cost-effective solution for continuous, non-ultra-high-purity nitrogen streams.