How Oxygen is Separated from Gas Mixtures

Oxygen, one of the most essential components of Earth's atmosphere, can be separated from gas mixtures using various techniques. This article explores the most common method, fractional distillation, as well as other advanced technologies. Understanding the process is crucial for industries such as semiconductor fabrication, medical, and gas production.

Understanding Fractional Distillation

Each gas in a mixture begins to separate based on its boiling point. In the case of air, nitrogen has a boiling point of -196 °C, while oxygen has a slightly lower boiling point of -183 °C. The process of fractionating air to separate oxygen and other gases involves several steps. Initially, air is compressed and cooled to a liquid state, a process called liquefaction.

Liquid Air and Fractional Distillation

1. Liquefaction: Air is cooled and compressed to the point where it turns into a liquid. At this state, the various components of air can be separated based on their boiling points.

2. Fractional Distillation: The liquid air is then passed into a fractionating column, where it is gradually heated from the bottom. As the liquid ascends the column, it undergoes partial vaporization and condensation. This process allows different gases to separate based on their boiling points. The lighter gases, such as oxygen, rise to the top and are collected at the column's top, while the heavier gas, nitrogen, collects at the bottom.

3. Collection: The oxygen is collected as a liquid, which can then be further purified or used directly as an oxygen stream.

Commercial Methods and Applications

Besides fractional distillation, there are other advanced methods used to separate oxygen from gas mixtures, including:

Membrane gas separation: This method uses specialized polymer membranes that allow certain gases to pass through more easily than others. It is particularly useful for applications requiring high purity oxygen and nitrogen. Pressure Swing Adsorption (PSA) and Vacuum Pressure Swing Adsorption (VPSA): These techniques use adsorbent materials to separate gases based on their adsorption properties. They are popular for applications in gas purification and manufacturing.

Cryogenic Distillation: This is the most common method for air separation and involves bringing air to its boiling point and then using fractional distillation to separate the various components. Cryogenic distillation is particularly used in industries that require high purity oxygen, nitrogen, and argon, such as semiconductor fabrication.

Overview of Air Separation Plants

An air separation plant operates by separating atmospheric air into its primary components: nitrogen, oxygen, and sometimes argon and other rare inert gases. The most common method employed is fractional distillation. Cryogenic distillation units (ASUs) are built to provide nitrogen and oxygen and often co-produce argon. Membrane PSA and VPSA are commercially used to separate a single component from ordinary air.

High purity oxygen, nitrogen, and argon are essential in semiconductor device fabrication, making cryogenic distillation the only viable source for these rare gases. For smaller stand-alone units, air separation is often used to separate oxygen and nitrogen, the major components of air.

Conclusion

Separating oxygen from gas mixtures is a critical process with a wide range of applications across various industries. Fractional distillation, along with other advanced methods, ensures the production of high-purity gases for applications requiring precise control over their quality and composition.