Nitrogen is highly abundant, accounting for around 78% of the earth’s atmosphere meaning we breathe in more nitrogen than any other element every day. The main methods for nitrogen generation includes cryogenic air separation, pressure swing adsorption (PSA) and membrane nitrogen generators.
Cryogenic Air Separation
Based on the different boiling points of oxygen and nitrogen, the preparation of oxygen from air is called air separation. Firstly, the air is pre-cooled and purified (remove a small amount of water, carbon dioxide, acetylene, hydrocarbons and other gases and impurities such as dust), and then compressed and cooled to make it into liquid air. Since the boiling points of oxygen and nitrogen are different, the liquid air is evaporated and condensed many times in the distillation tower to obtain pure oxygen (up to 99.6% purity) and pure nitrogen (up to 99.9% purity).
Membrane nitrogen generators
The principle of operation for a membrane nitrogen generator is where atmospheric air is compressed by an air compressor and passed through a hollow fibre membrane, filtering out parts of air (primarily oxygen) leaving behind a high purity nitrogen gas. More advanced nitrogen generation systems can include multiple stages of filtration before reaching the membrane, this removes more of the other particles in the gas such as dust and moisture. Fine mesh filters are commonly used in the nitrogen generation process, these are for removing particles in air whilst air dryers are used to remove moisture. Beyond ensuring higher purity of nitrogen, these extra filtration stages will also protect the membrane.
Pressure Swing Adsorption (PSA)
A PSA nitrogen generator works in a similar way to a membrane nitrogen generator in terms of compressing atmospheric air with an air compressor. Instead of passing through a membrane, a PSA nitrogen generator will pass air through a column of tightly packed material at pressure made of carbon called a Carbon Molecular Sieve (CMS). The CMS only allows the smaller nitrogen molecules to pass through whilst the larger molecules of oxygen and other molecules are adsorbed by the CMS leaving high purity nitrogen. Two CMS columns are required for a PSA nitrogen generator, as one column is deprussirized the other pushes the air through the sieve at pressure hence the name Pressure Swing Adsorption. Similar again to the membrane nitrogen generator, an advanced PSA nitrogen generator will also contain prefiltration stages to remove dust particles and moisture from the air before entering the BECMS.