Pressure Swing Adsorption (PSA)

The Pressure Swing Adsorption (PSA) technology is in the news because of the shortage of medical oxygen that India is facing in the wake of the COVID-19 pandemic. How does pressure swing adsorption PSA work? Read on to find out more. This is an important topic for the UPSC 2021 Prelims exam, in the science and technology segment.

What is Pressure Swing Adsorption (PSA)?

Pressure swing adsorption or PSA is a process that separates single gases from a gas mixture. PSA is a non-cryogenic air separation process that is commonly used in commercial practice. 

• It is an economic and reliable method for separating many gases and achieving a very high purity level for them.
• PSA is mainly employed in the chemical and petrochemical processes.
• It is used to recover hydrogen from coking or conversion gases, or to separate oxygen and nitrogen from air.
• In the process, gases are separated under pressure based on the species’ molecular characteristics and affinity for an adsorbent material.
• It operates at near-ambient temperatures which is in contrast to the cryogenic distillation techniques of gas separation, which takes place at very low temperatures.
• Specific adsorbent materials (e.g., zeolites, molecular sieves, activated carbon, etc.) are used as a trap, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed material.
• The adsorption process is based on gas molecules binding to an absorbent material. 
  • The adsorbent bed is specially selected depending on the gas to be absorbed. 
  • Ideally, only the gas to be separated is adsorbed, while all other gases in the mixture pass through the adsorbent bed. 

Pressure Swing Adsorption Working Principle

First, air from the ambient atmosphere is compressed into high-pressure air. This gas is then transferred into a vessel or column which is filled with the adsorbent material (activated carbon, zeolite, etc.). The selection of the adsorbent depends on the gas to be extracted. This system is then pressurised and depressurised cyclically, wherein the low sorbing gas will gradually leave the column first, followed by the other gases.

There are four main phases of the pressure swing adsorption process:

1. Adsorption: The adsorber starts off pressurized with pure gas. The impure gas is fed into the column which contains the adsorber. Adsorption takes place and the pure gas is released from the top of the column. This takes place until the adsorber has reached its adsorption capacity.
2. Depressurization: The adsorber is depressurized over in several small steps to recover additional pure gas still in the adsorber. Once all pure gas has been recovered, the desorbed impurities are dumped into the PSA off-gas line.
3. Regeneration: The adsorbent is purged with high-purity gas at constant off-gas pressure to further regenerate the adsorbent bed.
4. Repressurization: The adsorber is repressurized with pure gas and is now ready to receive more feed gas to start the process over. 

PSA Application

The PSA process is used in several applications across several industries. It is used in hydrogen recovery and purification, oxygen production, carbon dioxide removal and purification, helium recovery, nitrogen generation, methane recovery, etc. It is used in the chemical industry, the petrochemical industry, iron and steel industries and refining.

PSA in Oxygen Generation

The process is being used to generate medical oxygen to supply to hospitals. It is used as a substitute for bulk cryogenic or compressed-cylinder storage, which is the primary oxygen source for any hospital. Hospitals and medical applications require oxygen purity to be between 95 – 99%, and the PSA process can produce oxygen at this purity level. 

In the wake of the severe oxygen shortage in the country in the ongoing COVID-19 pandemic, the government has approved the allocation of funds for setting up 551 Pressure Swing Adsorption medical oxygen generation plants at public health facilities across the country.

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