Membrane nitrogen generators are based on a simple working principle. The main part of a membrane generator is the membrane module (+- 10 cm in diameter), filled with small, hollow polymer fibers.
The dry and clean compressed air enters and due to the structure of these fibers, different components of the air flow to the outside of the fiber. This process is called permeation. During this process, water and oxygen exit through the membrane sides of the fibers. In the end, only nitrogen remains. This is possible since different molecules permeate at different speeds. H2O will permeate very quickly; oxygen takes a little longer. Because of the permeation through the fiber wall, an overpressure would occur inside the membrane housing. The fibers would clog, and the permeation efficiency would be significantly lowered. To prevent that from happening there is an opening in the housing, the permeate vent, where these 'exhaust' gases can escape.
It is very important for the intake air to be clean and dry before it enters the membrane. If this is not the case, the shallow fibers will quickly clog. To prevent this from happening, correct air treatment of the feed air must be installed. In some cases, the needed filters and dryers will already be built into the generator itself. This would mean that, at times, no additional filters should be installed between the compressor and generator. The fibers of the membrane can handle water vapor without many problems. It is however very important that the air be devoid of liquid water, since this will have a detrimental impact on the membrane. Therefore, it is required for a good water separation solution to be in place upstream the generator, a refrigerant dryer for example. Taking care of the intake air of the generator will protect the membrane and ensure a long lifetime.
Adsorption is the process in which atoms, ions or molecules from a substance (compressed air in this case) adhere to a surface of an adsorbent.
A PSA nitrogen generator isolates nitrogen, and the other gases in the compressed air stream (oxygen, CO2 and water vapor) are adsorbed, leaving behind essentially pure nitrogen. PSA nitrogen generator technology is a simple, reliable and cost-effective approach to nitrogen generation that enables continuous, high-capacity nitrogen flow at the desired level of purity. PSA traps oxygen from the compressed air stream when molecules bind
themselves to a carbon molecular sieve. This happens in two separate pressure vessels (tower A and tower B), each filled with a carbon molecular sieve, that switch between a separation process and a regeneration process. Clean and dry compressed air enters tower A.
Since oxygen molecules are smaller than nitrogen molecules, they pass through the pores of the sieve. Nitrogen molecules cannot fit through the pores, so they bypass the sieve resulting in nitrogen of desired purity. This phase is called the adsorption or separation phase. Most of the nitrogen produced in tower A exits the system, ready for direct use or storage. Next, a small portion of the generated nitrogen is flowed into tower B in the opposite direction.
This flow pushes 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, which detach from the sieves and get carried away by the small nitrogen flow coming from tower A. This ‘cleaning’ process makes room for new oxygen molecules to attach to the sieves in a next adsorption phase. The two tower PSA system switches between separation and regeneration to provide continuous nitrogen production at a desired purity level. The Atlas Copco NGP+ nitrogen gas generator provides all the advantages of PSA technology in a reliable and efficient plug-and-play solution.
It is important to understand the level of purity that is needed for each application to purposefully generate on-site nitrogen. Nonetheless, there are some general requirements regarding the intake air. The compressed air must be clean and dry before entering the nitrogen generator, as this positively affects the nitrogen quality and also prevents the CMS from being damaged by moisture.
To treat the air properly, there should be a dryer between the compressor and the generator. If the intake air is generated by an oil lubricated compressor installing an oil coalescing and carbon filter will get rid of any impurities prior to the compressed air reaching the nitrogen generator. Pressure, temperature and pressure dew point sensors can be installed in the generators as a fail-safe, preventing contaminated air from entering the PSA system and therefore ensuring a long lifetime.
ACHIEVABLE PURITY
EFFICIENTLY UP TO 99.999%
EFFICIENTLY UP TO 99.5%
EFFICIENCY
HIGHER
HIGH
PERFORMANCE VS. TEMP.
LOWER AT HIGH TEMP.
HIGHER AT HIGH TEMP.
SYSTEM COMPLEXITY
MEDIUM
LOW
SERVICE INTENSITY
VERY LOW
PRESSURE STABILITY
STABLE WITH EXTRA TANKS
STABLE WITHOUT EXTRA TANK
FLOW STABILITY
START-UP SPEED AFTER SHUTDOWN
FAST
WATER (VAPOR) SENSITIVITY
LOW (NO LIQUID WATER)
OIL SENSITIVITY
NOT ALLOWED (<0,01mg/m3)
NOISE LEVEL
WEIGHT
