Generating Nitrogen with Pressure Swing Adsorption (PSA) Technology

Industrial Gases Compressed Air Wiki Nitrogen

Having the ability to create your own nitrogen means being in full control of your N2 supply. This could be beneficial to a lot of companies that need nitrogen on a daily basis. What does that mean for your company? When nitrogen is generated in-house, you do not have to rely on third parties for the supply, therefore eliminating the need for processing, refills and delivery costs. One way of generating nitrogen is through Pressure Swing Adsorption.

How does Pressure Swing Adsorption work?

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.

Graphic showing the nitrogen generating process. First, tank A is in the adsorption phase while tank B regenerates. In the second stage both vessels equalize pressure after which tank A starts regenerating while tank B generates nitrogen.

First, tank A is in the adsorption phase while tank B regenerates. In the second stage both vessels equalize pressure to prepare for the switch. After the switch, tank A starts regenerating while tank B generates nitrogen.

At this point, the pressure in both towers will equalize and they will change phases from adsorbing to regenerating and vice versa. The CMS in tower A will get saturated, while tower B, due to the depressurization, will be able to restart the adsorption process. This process is also referred to as ‘swing of pressure’ , which means that it allows for certain gases to be captured at higher pressure and released at lower pressure. The two tower PSA system allows for continuous nitrogen production at a desired purity level.

Nitrogen purity and requirements for the intake air

It is important to understand the level of purity that is needed for each application in order to purposefully generate your own nitrogen. Nonetheless, there are some general requirements regarding the intake air. The compressed air has to 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. Furthermore, the inlet temperature and pressure should be controlled between 10 and 25 degrees C, while keeping the pressure between 4 and 13 bar. 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, you should also install an oil coalescing and carbon filter to get rid of any impurities prior to the compressed air reaching the nitrogen generator. There are pressure, temperature and pressure dew point sensors installed in most generators as a fail-safe, preventing contaminated air from entering the PSA system and damaging its components.


A typical installation: Air compressor, dryer, filters, air receiver, nitrogen generator, nitrogen receiver. The nitrogen can be consumed directly from the generator or through an additional buffer tank.

A typical installation: Air compressor, dryer, filters, air receiver, nitrogen generator, nitrogen receiver. The nitrogen can be consumed directly from the generator or through an additional buffer tank (not shown).

Another important aspect in PSA nitrogen generation is the air factor. It is one of the most important parameters in a nitrogen generator system, as it defines the compressed air required to obtain a certain nitrogen flow. The air factor thus indicates a generator’s efficiency, meaning a lower air factor indicates a higher efficiency and of course lower overall running costs.

Choosing between a PSA and Membrane generator

 

PSA

MEMBRANE

ACHIEVABLE PURITY

EFFICIENTLY UP TO 99.999%

EFFICIENTLY UP TO 99.9%

EFFICIENCY

HIGHER

HIGH

PERFORMANCE VS. TEMP.

LOWER AT HIGH TEMP.

HIGHER AT HIGH TEMP.

SYSTEM COMPLEXITY

MEDIUM

LOW

SERVICE INTENSITY

LOW

VERY LOW

PRESSURE STABILITY

FLUCTUATING IN/OUTLET

STABLE

FLOW STABILITY

FLUCTUATING IN/OUTLET

STABLE

START-UP SPEED

MINUTES/HOURS

SECONDS

WATER (VAPOUR) SENSITIVITY

PDP MAX 8°C

NO LIQUID WATER

OIL SENSITIVITY

NOT ALLOWED (< 0,01mg/m³)

NOT ALLOWED (< 0,01mg/m³)

NOISE LEVEL

HIGH (blow-off peaks)

VERY LOW

WEIGHT

MEDIUM

LOW

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