Pressure Swing Adsorption

Nitrogen makes up 78% of the atmosphere, it is also one for the most widely used of the Noble gasses in the world. The reason it is so useful is it’s abundance, however like all the noble gasses the benefit can only be realised in levels of high purity. As this does not occur naturally it has to be created artificially, there are many ways to achieve the same goal.


Pressure swing adsorption (PSA) is one of the most effective methods of generating a flow of high purity nitrogen. For everything but the largest volumes, this is the system that will be what is employed to provide the nitrogen.


How does it work?


PSA works exactly the way it describes, the pressure is swung between high and low pressure where a “catalyst” is used to “separate” the oxygen from the nitrogen.


More technically a cylinder with a molecular sieve is pressurised with air to around 6-10 bar depending on the system, size of the cylinders and cycle time. During this phase the oxygen is forced into the pores of the sieve where it is held in place, this allows the nitrogen to flow through untampered with. This cylinder leads to a holding or reservoir tank where the nitrogen is stored before it is used.


This reservoir tank also provides a key function to the performance of the sieve in the first cylinder; backpressure. Without backpressure, the first cylinder would not reach the desired pressure range of 6-10 bar and the nitrogen quality would be reduced.


After the cycle time has elapsed the pressure is released where the cylinder is vented to the atmosphere, the sieve releases the oxygen it was holding onto and is “re-charged”, able to start another cycle after a period of time has passed.


On Smaller systems this is the full process, however, for most applications, there are two cylinders when one is active the other is re-charging after which they swap.


This process can explain many of the maintenance requirements and performance characteristics of this type of generator;


  • Purity, the sieve relies on the surface area in one cylinder the surface area used to hold onto the oxygen can be in the region of football pitches. This gives unprecedented levels of purity in the order of 99.9995% pure

  • Compressor usage, due to the cyclical nature of the PSA system the input (volume of compressed gas) is drastically higher than the output (volume of nitrogen). This means there is greater wear and tear on the part of the system and is why regular maintenance is vital to long term functioning of the whole system.


Sources:

Apex Gasgen (www.apexgasgenerators.com/nevis-range)

Air Products (http://www.airproducts.com/products/gases/supply-options/onsite-gas-generation/~/media/ED77CF85C5EF40119FD964535E80E4D2.pdf)

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