One of the many advantages of producing your own nitrogen is that you can set the purity level to what your application requires. That is a major difference to purchased nitrogen, which is always of the highest purity – even though most jobs don't need it. So what affects nitrogen purity? And why does it matter? The second question is easy to answer: High-purity nitrogen costs more because it consumes more energy. Therefore, you want to make sure that you get the purity just right for your application. This is especially true because increasing it beyond what is necessary yields no additional benefits. In other words, using nitrogen with a purity of 99.99% for a job that calls for a purity of 99.5% wastes energy and money. To answer the first question, we first have to look at how nitrogen gas is generated. 

Two different technologies are used for the on-site production of nitrogen: pressure swing adsorption (PSA) and membrane. The key ingredient in each case is compressed air, which contains 78% nitrogen. In the generator, that nitrogen is separated from the rest of the air.

In a PSA generator, this happens through adsorption. The generator consists of two separate pressure vessels. One of them is used to filter out the nitrogen while the other is regenerating.

As the name indicates, a membrane generator contains a membrane filled with small, hollow polymer fibers. Not all gases, vapors and contaminants can pass through the membrane fiber wall with the same ease. This allows the nitrogen gas to be separated from the compressed air. This is called “permeation.”

A PSA generator allows you to produce a nitrogen with a purity of up to 99.999%. It also requires less compressed air. However, the intake air for the compressor has to be very clean and dry. The purity a membrane generator can achieve is a bit lower. However, as noted above, most applications do not require the highest purity. One of the benefits of this technology is that the intake air can contain water vapor. 

There are range of factors that could affect purity, some of which depend on the type of generator you use. The amount of air that flows through the generator is the most important factor. If that intake air volume is too high, it will impact the nitrogen purity because the pressure in the generator rises too fast. That means that the separating technology of the generator can no longer keep up. As a rule of thumb, you want to keep you adsorption pressure between 0.6-0.8 Mpa. An easy solution is to adjust the amount of air that comes from the compressor.

Another factor could be that the intake air contains too many contaminants. This could clog up the molecular sieve, which would have to be replaced. To prevent this from happening, you can install air treatment equipment such as filters. However, at some point, the performance of the molecular sieve will diminish, so you should always follow a maintenance schedule.

PSA generators feature a piece of equipment called a solenoid valve. Its failure is most frequently responsible for a decline in nitrogen purity. When you notice that this purity drops, this is one of the first things you should check. 

The purity of your nitrogen is especially important when the nitrogen gas comes into contact with an end product. It matters less if you are using it for fire suppression or inflating tires. On the other hand, the gas has to be very pure in the food industry or the pharmaceutical sector. For these and other industrial applications, you need high purity nitrogen. Many applications are subject to standards that will define the required purity. For example, there are specific requirements for food grade nitrogen.