Pressure Swing Adsorption (PSA) and production of nitrogen?

Pressure Swing Adsorption

Nitrogen gas has a very high demand in the chemical industry because of its inertness. Since it does not react normally, we can use it for blanketing and purging purposes to protect important substances from contamination.

It can also help to protect explosives and their safe storage. Pressure Swing Adsorption is an efficient method of onsite production of nitrogen with high purity these days.

Pressure Swing Adsorption

How does Pressure Swing Adsorption work?

The pressure swing adsorption system separates nitrogen from the compressed air by trapping Oxygen, using the method of adsorption. Adsorption is actually the adhesion of the molecules of a substance to a surface (adsorbent).

In this case, the adsorbent is a carbon molecular sieve which binds oxygen at higher pressure usually between 5-10 bar.

The PSA cycle works on two mechanisms:

  • Pressurization (Adsorption)
  • Depressurization (Desorption)

The PSA plant has two towers, when one undergoes adsorption the other goes on desorption.

Air coming from the compressor at high pressure enters the first tower, where oxygen, carbon dioxide, and moisture adsorb on the surface of the carbon molecular sieve.

As soon as the operating pressure reaches, nitrogen comes out as a product and flows toward the nitrogen receiver before it could enter the processing pipeline.

At the same time, the second tower gets depressurized into the atmosphere.

Picture credits: Linde Group

When the first tower gets saturated with oxygen, CO2 and moisture an equalization step occurs.

Some of the gas that remains in the first tower flows into the second to bring it to an intermediate pressure. During this process, zero amount of nitrogen is generated.

Therefore, the nitrogen receiving vessel helps to avoid fluctuations of nitrogen in the downstream processing units.

The first tower then depressurizes into the atmosphere to vent the waste gas.

And tower B starts its adsorption cycle and produces nitrogen.

Controlling the Pressure Swing Adsorption (PSA) Plant

A programmable logic controller (PLC) helps in the safe and smooth operation of the PSA plant.

It serves the following purposes:

  • Automatic operation of the PSA plant.
  • Monitoring the process variables.
  • Generating safety alarms.

An oxygen analyzer helps in monitoring the purity of nitrogen. This tells us about the ppm of oxygen carried over with Nitrogen.

Advantages of PSA Plant

  • It is quite feasible for the production of nitrogen anytime on the facility. Its size ranges as per demand of the nitrogen flowrate.
  • It has a comparatively low operating cost.
  • Provides exact purity depending upon the requirements of the process.
  • No environmental concerns.
  • Portable and has the capability of inhouse placement and production.

The operating cost of a Pressure Swing Adsorption Plant depends on the purity of nitrogen gas. The higher the purity the higher will be the unit operating cost.

For example, the quality of some food items remains intact by blanketing them with nitrogen of 99.5% purity. However, the quality of nitrogen for blanketing some of the flammable substances depends upon that substance’s limiting oxygen concentration (LOC).

We can find the LOC values of the flammable substances in chemical engineering or chemistry books as wells as in the National Fire Protection Association’s NFPA 69: Standard on Explosion Prevention Systems.

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