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Did you know the air around us is mostly nitrogen? Everyone needs oxygen to survive. However, the air we breathe is made up of 78% nitrogen, only a mere 21% oxygen and tiny amounts of other gases. The human body does not use this nitrogen, it is however very helpful in various industrial applications.
Let’s start with the basics. Nitrogen is an inert gas that is odorless, colorless, and does not sustain life. However, it is important for plant growth and is a key additive in fertilizers. Its usage ranges far beyond your garden. Nitrogen usually appears in either liquid or gas form (although it is possible to attain solid nitrogen as well). Liquid nitrogen is used as a refrigerant, which is able to rapidly freeze foods and subjects in medical research, as well as reproductive technology. For the purpose of this explanation, we will stick with nitrogen gas.
Nitrogen is widely used, mainly, due to the fact that it does not react when exposed to other gas, unlike oxygen, which is very reactive. Due to its chemical composition, nitrogen atoms need more energy to be broken and react with other substances. Oxygen molecules on the other hand are easier to break apart, therefore, making the gas much more reactive. Nitrogen gas is the opposite, providing unreactive environments where needed.
The lack of reactivity of nitrogen is its biggest quality and as a result, the gas is used to prevent slow and fast oxidation. The electronics industry presents a perfect example of this use, as, during the production of circuit boards and other small components, slow oxidation can occur in the form of corrosion.
Slow oxidation is also no stranger to the food and beverage industry, wherein this case, nitrogen is used to displace or replace the air in order to better preserve the end product. Explosions and fires are a good example of fast oxidation since they need to be fueled by oxygen. Removing the oxygen from a vessel with the help of nitrogen reduces the likelihood of these accidents from occurring.
When producing your own nitrogen, it is important to know and understand the purity level you want to achieve. Some applications require low purity levels (between 90 and 99%), such as tire inflation and fire prevention, while others, such as applications in the food and beverage industry or plastic molding, require high levels (from 97 to 99.999%). In these cases PSA technology is the ideal and easiest way to go. In essence, a nitrogen generator works by separating nitrogen molecules from the oxygen molecules within the compressed air. Pressure Swing Adsorption does this by trapping oxygen from the compressed air stream using adsorption.
Adsorption takes place when molecules bind themselves to an adsorbent, in this case the oxygen molecules attach to a carbon molecular sieve (CMS). This happens in two separate pressure vessels, each filled with a CMS, that switch between the separation process and the regeneration process. For the time being, let us call them tower A and tower B. For starters, clean and dry compressed air enters tower A and since oxygen molecules are smaller than nitrogen molecules, they will enter the pores of the carbon sieve. Nitrogen molecules on the other hand cannot fit into the pores so they will bypass the carbon molecular sieve. As a result, you end up with nitrogen of desired purity.
This phase is called the adsorption or separation phase. It does not stop there however. Most of the nitrogen produced in tower A exits the system (ready for direct use or storage), while a small portion of the generated nitrogen is flown into tower B in the opposite direction (from top to bottom). This flow is required to push out the oxygen that was captured in the previous adsorption phase of tower B. By releasing the pressure in tower B, the carbon molecular sieves lose their ability to hold the oxygen molecules. They will detach from the sieves and get carried away through the exhaust by the small nitrogen flow coming from tower A.
By doing that the system makes room for new oxygen molecules to attach to the sieves in a next adsorption phase. We call this process of ‘cleaning’ an oxygen saturated tower regeneration.