Cryogenic Distillation Process (How Oxygen is Produced ?)

Oxygen Production Plant, Image: Linde engineering

After a year of suffering from Covid-19 in 2020, the world has already lost millions of lives. And, at the beginning of 2021, the 2nd wave hit the Indian subcontinent making the situation even worst. As the case rises, many people experienced severe illnesses like respiratory failure, causing the oxygen demand to skyrocket and resulting in a shortage of hospital beds and medical oxygen supplies. But, what caused the shortage of medical oxygen production? To get the answer to this question, we have to understand the process involved in separating oxygen from ambient air.

Source: Method of indirect measurement of oxygen concentration in the air (by Artur Zaporozhets, Oleksandr Redko, Vitaliy Babak, Valentin Mokiychuk), ResearchGate

   The atmospheric air is composed of 78.08% of nitrogen, 20.95% of Oxygen, 0.93% of Argon, 0.03140% of Carbon dioxide. Among them, Oxygen and Nitrogen are the most usable gas in different industries. These gases are separated according to the need of the industries like the Cutting and fabrication industries, Healthcare industries, Food industries, and Chemical industries. Ambient air may also have up to about 5% (by volume) of water content and may contain a number of other gases (usually in trace amounts) that are removed at one or more points in the air separation process. 

Over the past centuries, different processes have been developed for producing pure oxygen. Some process cycles minimize capital cost, some minimize energy usage, some maximize product recovery, and some allow maximum operating flexibility. Generally, there are two commonly used processes for the separation of oxygen from ambient air. They are,

• Cryogenic Distillation Process

• Pressure Swing Adsorption Process

Cryogenic Distillation Process for Oxygen Production:  

 The cryogenic distillation process includes the separation of pure gases from the atmospheric air by first cooling it until it liquefies and then selectively distilling the various gases at their specific boiling point. This process was developed by German scientist  Carl von Linde in the early 20th century and is still in use for the industrial production of pure oxygen. Compared to other technology, this process can produce high-quality oxygen (with around 99.5% pure O2) and is also suitable for large production. In contrast, it requires more operational energy as compared to other technologies. 

Working of Cryogenic Distillation Process:

Cryogenic Distillation Setup

The steps involved in separating pure oxygen from atmospheric air are,

1. At First, the air is sucked from the inlet with the help of a compressor. Before the compressor, there is an air filter at the inlet which removes the dust particle and water vapor allowing only air to pass through it.
2.  Inside Compressor, the air is compressed to somewhere between 5 and 8 bar (about 75 to 115 PSI), depending upon the desired product pressure. After compression, the temperature of air increases (As the air molecules are compressed in a smaller volume of space, the energy applied for compression is converted to heat energy which is given by Charle’s Law)
3.  The compressed warm air is now passed through a discharge pipe of the compressor to the heat exchanger. The heat exchanger consists of a coiled tube covering the discharge pipe from which the warm air flows. A cooling fluid is passed through that coiled tube (it can be cold water or liquid nitrogen or any other deep freezing fluid), which cools down the compressed and warm air to negative temperature (below 0 °C).
4.  The cold compressed air continues to flow through the pipe to the Separator Unit, where the compressed air is partially expanded. With the expansion, the temperature of the liquid drops down (which is given by the Joule–Thomson effect ) to -80 °C. As the temperature drops, the carbon dioxide in the air solidifies (since the freezing temperature of CO2 is -79 °C). And, the expanded air is then passed through a molecular sieve, which separates solidified carbon dioxide, as well as any remaining water vapor (if present) from the expanded air.
5.  After CO2 purification, the air ( after CO2 separation only Nitrogen, Oxygen, and Argon remain as the main components ) is passed to the expansion unit where it is further expanded through a nozzle, forming a jet. While expanding, the air temperature drops from -80°C to -200°C. At this temperature liquefaction of air occurs, which means all the air condenses to form a liquid ( Since Nitrogen, Oxygen, and Argon liquifies at -196°C, -183°C, -186°C respectively).
6.  At last, the Liquid air now passed to the Air Distillation Column. Inside Air Distillation Column, the liquified air is slowly warmed up to separate different gases. While slowly warming, Nitrogen gets separated first as it has the lowest boiling point (-196°C) after nitrogen Argon gets separated as its boiling point is -186°C and at the end, only the Oxygen which has the highest boiling point (-183°C) is left behind. And is collected at the bottom of the distillation column.
7. Finally, the pure Oxygen is collected at the reservoir by a pipe connecting the reservoir with the bottom of the distillation column. Now it is ready for medical supply and other purposes.

This pure form of liquid oxygen is now pressurized according to the need and supplied for various purposes like for filling the medical oxygen cylinder or other industries. The Oxygen obtained from Cryogenic Distillation Process is more than 99%. This process is suitable for high-volume production. The steps involved in supplying the pure oxygen from the production plant to the ready to use oxygen cylinder is shown below: