Thursday, July 29, 2021

The science behind converting a PSA nitrogen unit into a PSA oxygen unit

Amid the surge of Covid-19 cases in India, the need for liquid medical oxygen has grown exponentially. The nation is doing its best to narrow down the gap between supply vs demand that has been created by the second wave of coronavirus. IIT Bombay has come up with an innovative idea that can help Covid patients across the nation in very little time.

The innovative idea by IIT Bombay

The institute came up with the idea of converting the traditional PSA (Pressure Swing Adsorption) nitrogen unit into a PSA oxygen unit by making small changes in the existing nitrogen PSA nitrogen unit. One major change is replacing Carbon Molecular Sieve (CMS) with Zeolite Molecular Sieve (ZMS). The changes can be done in a few days and the plant can be transferred to any hospital for a continuous supply of oxygen.

To understand this innovative idea we need to have a better understanding of the processes involved in this mechanism.

What is Pressure Swing Adsorption technology?

It is a simple technology used to separate gases like Hydrogen, Nitrogen, and Oxygen from ambient air. The technology uses the fact that certain gases under high pressure tend to get attracted to solid surfaces or absorbent materials like zeolites, activated carbon, and molecular sieves.

Why Zeolite Molecular Sieve (ZMS) will replace Carbon Molecular Sieve (CMS)?

To put things into perspective, consider the absorbent material like magnets which attract certain gases according to their characteristics. This is the principle for Pressure Swing Adsorption technology. For example, in Nitrogen PSA plants, Carbon Molecular Sieve acts as a magnet for oxygen and other gases in the chamber. This allows only Nitrogen to pass through the chamber, and the remaining gases are vented out after the pressure drop in the chamber.

This is the very reason why the Carbon Molecular Sieve (CMS) will be replaced by Zeolite Molecular Sieve (ZMS) in the PSA oxygen unit. Zeolite is an absorbent material that has a greater tendency to attract Nitrogen molecules. So when the mixture of gas will enter a chamber with Zeolite Molecular Sieve (ZMS), Nitrogen and other gases will stick to the surface of the sieve, allowing only oxygen molecules to pass through which will result in pure oxygen.

How will the PSA oxygen unit work?

The PSA oxygen unit will be almost the same as the PSA nitrogen unit but with some changes. The Plant has three components: a compressor, Air vessel, and a PSA unit.

– Compression state

The process will start from the compressor. It will extract air from the surroundings and will compress it to almost 8 to 9 bar. The atmospheric pressure that we all exert on earth is 1 bar. The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure.

The compression is necessary to attain the special characteristics of gases like their affinity for an adsorbent material.

– Air vessel part

The air from the compressor then moves to the air vessels which we can see as a storage vessel for the compressed air. In the storage vessel large impurities, carbon molecules, and water vapors get separated giving us only oxygen and nitrogen.

– PSA Unit

From there the gases will move to the PSA unit which will have chambers with Zeolite Molecular Sieve. Here, the sieve will act as a filter and a magnet for nitrogen gas. Nitrogen molecules are large in size compared to oxygen molecules. The size of the oxygen molecule is 292 picometers and the size of the nitrogen molecule is 300 picometers. One picometer is equal to one trillionth of a meter (1 m / 1,000,000,000,000).

The Zeolite sieve will trap the nitrogen molecule and will allow only oxygen molecules to pass through. The oxygen then will be compressed further in the liquid state and can be stored in the cylinder, providing a continuous supply of oxygen.

PSA Oxygen plants have attained 95% purity

A pilot project by IIT Bombay has shown that the oxygen that has been generated has attained an oxygen purity of 95%. This purity of oxygen can be used for mild and moderate covid patients. Work is going on to increase the purity of the oxygen gas so as to provide oxygen for severe covid patients. it is a milestone that has been achieved in no time.

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