PSA Oxygen plants

Context:

Prime Minister of India announces 551 PSA oxygen plants from PM CARES fund, one for each district

What is Oxygen PSA PLANT?

A PSA oxygen plant employs a technology that absorbs nitrogen from ambient air to concentrate oxygen for supply to hospitals. They operate at near-ambient temperatures and use specific adsorbent materials (that trap a substance on their surface) like zeolites, activated carbon, molecular sieves etc to trap oxygen at high pressure.

How Oxygen PSA plant works:

Air contains 21% Oxygen, 78% Nitrogen, 0.9% Argon and 0.1% other trace gases. Oxygen generation systems separate this oxygen from Compressed Air through a unique process called Pressure Swing Adsorption (PSA)
The Pressure Swing Adsorption process for the generation of enriched oxygen gas from ambient air utilizes the ability of a synthetic Zeolite Molecular Sieve to absorb mainly nitrogen. While nitrogen concentrates in the pore system of the Zeolite, Oxygen Gas is produced as a product.

This process is based on the fact that different gases have the propensity to be attracted to different solid surfaces more or less strongly. This happens with the nitrogen, which is attracted to the zeolites. As the air is compressed, the nitrogen is forced into the crystalline cages of the zeolite, and the oxygen and nitrogen is less adsorbed and conveyed to the end of the zeolite bed and ultimately recovered in the oxygen buffer tank. Two zeolite beds are used together: One filters air under pressure until it gets saturated with nitrogen while oxygen passes through. The second filter begins to do the same while the first one is regenerated as nitrogen is expulsed (desorbed) by releasing the pressure. The process begins again, storing the oxygen and argon in a tank. The argon could be separated afterwards increasing the amount of oxygen up to 99%. Using a carbon molecular sieve (CMS) based adsorbent, which absorbs the oxygen, allowing the impurities to be scrubbed. The maximum purity achievable in such systems is 99.3%. Typically the system is operated at a design point of 99.1% to optimize the output. In such a system there is about a 35% loss in the 94% feed product gas. This loss of product is sensitive to the purity level that is lower purity, less product loss. The whole process is intelligently controlled with help of automated valves.

Molecular sieve:

A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecules migrate through the stationary bed of porous, semi-solid substance referred to as a sieve (or matrix), the components of highest molecular weight (which are unable to pass into the molecular pores) leave the bed first, followed by successively smaller molecules. Some molecular sieves are used in chromatography, a separation technique that sorts molecules based on their size. Other molecular sieves are used as desiccants (some examples include activated charcoal and silica gel).

What are Zeolites?

Zeolites are microporous, three dimensional crystalline solid of aluminium silicate. Zeolites have small openings of fixed size in them which allow small molecules to pass through them easily but larger molecules cannot pass through them; that is why they are sometimes called molecular sieve.

About Oxygen:

Oxygen is one of the basic chemical elements. In its most common form, oxygen is a colorless gas found in air. It is one of the life-sustaining elements on Earth and is needed by all animals. Oxygen is also used in many industrial, commercial, medical, and scientific applications. It is used in blast furnaces to make steel, and is an important component in the production of many synthetic chemicals, including ammonia, alcohols, and various plastics. Oxygen and acetylene are combusted together to provide the very high temperatures needed for welding and metal cutting. When oxygen is cooled below -297° F (-183° C), it becomes a pale blue liquid that is used as a rocket fuel.

Oxygen is one of the most abundant chemical elements on Earth. About one-half of the earth’s crust is made up of chemical compounds containing oxygen, and a fifth of our atmosphere is oxygen gas. The human body is about two-thirds oxygen. Although oxygen has been present since the beginning of scientific investigation, it wasn’t discovered and recognized as a separate element until 1774 when Joseph Priestley of England isolated it by heating mercuric oxide in an inverted test tube with the focused rays of the sun. Priestley described his discovery to the French scientist Antoine Lavoisier, who experimented further and determined that it was one of the two main components of air. Lavoisier named the new gas oxygen using the Greek words oxys, meaning sour or acid, and genes, meaning producing or forming, because he believed it was an essential part of all acids.

In 1895, Karl Paul Gottfried von Linde of Germany and William Hampson of England independently developed a process for lowering the temperature of air until it liquefied. By carefully distillation of the liquid air, the various component gases could be boiled off one at a time and captured. This process quickly became the principal source of high quality oxygen, nitrogen, and argon.

In 1901, compressed oxygen gas was burned with acetylene gas in the first demonstration of oxy-acetylene welding. This technique became a common industrial method of welding and cutting metals.

The first use of liquid rocket propellants came in 1923 when Robert Goddard of the United States developed a rocket engine using gasoline as the fuel and liquid oxygen as the oxidizer. In 1926, he successfully flew a small liquid-fueled rocket a distance of 184 ft (56 m) at a speed of about 60 mph (97 kph).

After World War II, new technologies brought significant improvements to the air separation process used to produce oxygen. Production volumes and purity levels increased while costs decreased. In 1991, over 470 billion cubic feet (13.4 billion cubic meters) of oxygen were produced in the United States, making it the second-largest-volume industrial gas in use.

Worldwide the five largest oxygen-producing areas are Western Europe, Russia (formerly the USSR), the United States, Eastern Europe, and Japan.