Pressure Swing Absorption vs Membrane: Which Nitrogen Generator should I use?

Oxygen permeation while nitrogen is retained

Managing safety on a ship involves considering many different factors, however this article will be focusing on Nitrogen Generators. When ships the size of buildings are travelling several thousand miles with precious and hazardous materials, it is crucial that these safety measures are in place. While the presence of heat and fuel are beyond our control, we can eliminate oxygen from the fire triangle through Nitrogen Purging. 

The Fire Triangle (Credit: City Fire Protection)

Nitrogen makes up 78% of our atmosphere, and is an inert gas. Nitrogen’s unreactive properties make it an excellent choice to protect flammable cargo. 

There are two main types of nitrogen generators: (1) Pressure Swing Adsorption, and (2) Membrane Nitrogen Generators.

  1. Pressure Swing Adsorption (PSA) Nitrogen Generators
Annotated Diagram of a PSA System N2 Generator 

Principle of Pressure Swing Adsorption

PSA generators use the Pressure Swing Adsorption principle where gases under high pressure tend to be attracted to solid surfaces. Different gases will be more or less strongly attracted to specific surfaces.

Mechanism of the PSA Nitrogen Generator

Nitrogen generators include two towers filled with carbon molecular sieve (CMS) which adsorb oxygen molecules from the air. The pre-treated compressed air goes through an inlet at the bottom of the tower and passes through the carbon sieves. Under pressure, the oxygen particles will get trapped by the pores of the CMS. Meanwhile, nitrogen gas will not be trapped. The nitrogen gas will pass through the CMS, and escape through an outlet as the product gas. 

While one tower is in operation, the other tower regenerates by venting out the oxygen-enriched air. These two towers work in tandem to produce a continuous gas stream of nitrogen in the generator. 

When should I use the PSA Nitrogen Generator?

PSA nitrogen generators produce nitrogen gas that can be up to 99.999% pure, which makes it ideal for vessels that require high levels of purity. 

  1. Membrane Nitrogen Generators
Annotated Diagram of a Membrane System N2 Generator 

Principle of Selective Permeation

Different gases have different permeation rates which is their ability to diffuse through a membrane material. The separation principle takes advantage of this trait, separating fast gases that more easily permeate through the membrane from the slower ones. 

In the case of the nitrogen generator, nitrogen is characterized as a slower gas than oxygen, which creates the difference needed to separate the two within the compressed air. This process is driven by the pressure difference between the inlet feed of high-pressure compressed air and the low-pressure side of the membrane. 

Mechanism of the Membrane Nitrogen Generator

Compressed air is passed through hollow membrane fibres in a cylindrical shell. The oxygen and water vapour permeate rapidly through the fibre walls while the nitrogen molecules continue to pass along the tubes. The nitrogen is then pushed through an outlet under pressure. 

Different variables affect the nitrogen production. Membrane nitrogen generators give an option to control the nitrogen purity by adjusting the outlet, pressure, and airflow. The nitrogen purity can range between 90-99.9%.

When should I use the Membrane Nitrogen Generator?

Any application on board a vessel that requires nitrogen gas with purity less than 99.9% will do best with a membrane system.

In Summary…

It is important to understand the underlying principles of what is onboard your vessel. The best type of nitrogen generator will depend on your vessel’s individual needs. 

This article is brought to you by Giantech Engineering Pte Ltd. Find out how Giantech may help meet your vessel’s N2 system needs:

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