Air separation

We can pull air apart.

Big machines take the air. They make the air very cold. This helps us get the parts we need.

First, the machines clean the air. They take out dust and water. This keeps the machines safe.

Next, the air gets very cold. It turns into a liquid. This liquid is easy to separate.

We can get parts like nitrogen and oxygen. We even get rare gases. These parts help doctors help people.

It is amazing how we use air!

We can pull the air apart to find its parts.

Most air is made of nitrogen and oxygen. We can also find rare gases like argon. One common way to do this is called fractional distillation. This is a set of steps that uses cold to separate gases.

First, machines clean the air. They use a molecular sieve to remove dust and water. This sieve acts like a tiny sponge. It also removes carbon dioxide so the machines do not clog up.

Next, the air is squeezed by a compressor. This makes the air very cold. The air turns into a liquid. This happens in a cold box to keep the heat out.

Then, the liquid is heated just a little. Different gases boil at different temperatures. Nitrogen boils first, so it rises to the top. Oxygen stays lower. Argon sits in the middle. This lets us collect each gas separately.

We use these gases for many things. Doctors use oxygen to help patients. Steel makers use oxygen to make metal. Even rocket companies use these gases.

Air separation plants are special places that pull the air apart.

Most of the air around us is made of nitrogen and oxygen. These plants can also find rare gases like argon. Some advanced plants even find helium. This is very important for many different jobs. We need these pure gases for medicine and making steel. We even use them to help rockets fly into space.

One common way to separate air is called cryogenic distillation. This process starts by cleaning the air very carefully.

Machines use a molecular sieve to soak up water and carbon dioxide. This is important because those things could freeze and clog the machines. Next, a large compressor squeezes the air. This squeezing helps create the cold needed for the next step. The air is then cooled until it turns into a liquid.

This method of using cold to separate gases was started by Carl von Linde. He developed his ideas in the year 1895. For seven years, his work was mostly just for study. In 1902, people began using it for real industrial work. Today, we still use his ideas to make high purity gases. It is a way of working that has lasted over a hundred years.

Inside the plant, the liquid air goes into tall columns.

These columns use different temperatures to sort the gases. Nitrogen boils at a very low temperature of 77.4 K. Oxygen boils at a slightly higher temperature of 90.2 K. Argon sits in the middle at 87.3 K. Because they boil at different times, they can be collected separately. This allows us to get oxygen that is 97.5% to 99.5% pure.

There are other ways to separate air without using extreme cold. Some machines use a process called pressure swing adsorption.

This uses a special material that acts like a tiny sponge. Another way uses membranes, which are thin layers that let some gases through.

These smaller methods are used in portable oxygen machines for doctors. They are often much smaller than the huge cryogenic plants.

Air separation is the industrial process of splitting atmospheric air into its primary components. The most common gases produced are nitrogen and oxygen. Specialized plants can also isolate argon and other rare inert gases. Some advanced processes even recover helium. This separation is vital for modern life. We use these pure gases for medical treatments, making steel, and fueling rockets.

The most widespread method is called cryogenic distillation. This process relies on the fact that different gases turn into liquids at different temperatures. To begin, the air must be extremely clean. First, the air is pre-filtered to remove dust. Next, it passes through a molecular sieve bed. A molecular sieve is a material that acts like a tiny sponge to soak up impurities. It removes water vapor and carbon dioxide. This step is critical because those substances would freeze and clog the cryogenic equipment. The sieve also removes gaseous hydrocarbons to prevent potential explosions.

Once cleaned, the air enters a large compressor. This machine squeezes the air to a pressure between 5 and 10 bar gauge. Compression is a key part of the energy cycle. During this stage, inter-stage coolers condense water out of the stream. The compressed air then enters an integrated heat exchanger. Here, the air is cooled by exchanging heat with already cold product streams. This cooling eventually causes part of the air to liquefy. Modern plants often use expansion turbines to improve efficiency. The energy from these turbines can help drive the main air compressor.

Inside the plant, the liquid air moves into distillation columns. These columns separate the components based on their boiling points. Nitrogen has a very low boiling point of 77.4 K. Oxygen boils at a slightly higher 90.2 K. Argon sits in the middle at 87.3 K. In a high-pressure (HP) column, the air is distilled to almost pure nitrogen. This nitrogen can reach purity levels of less than 1 part per million. The remaining liquid is richer in oxygen and moves to a low-pressure (LP) column. This column operates at 1.2 to 1.3 bar abs.

Argon presents a unique challenge in the distillation process. Because its boiling point is between nitrogen and oxygen, it builds up in the lower section of the LP column. To collect it, a vapor side draw is taken from the area where argon concentration is highest. This stream is sent to a separate argon column to reach the desired purity. This process requires a high reflux ratio of about 30. Even though argon makes up less than 1% of the air, this step uses significant energy. Modern structured packings help keep impurities in the argon below 1 ppm.

Not all air separation requires extreme cold. Some methods work at ambient temperatures. One method is pressure swing adsorption (PSA). In PSA, a zeolite material is exposed to high-pressure air. The zeolite adsorbs a specific gas, much like a sponge soaks up water. When the pressure is released, the gas is released from the zeolite. A similar method is vacuum pressure swing adsorption (VPSA), which uses sub-atmospheric pressure.

These methods allow for much smaller compressors. They are used in portable oxygen concentrators for medical purposes.

Another approach involves membrane technology. Membranes are thin layers that can separate gases. Polymeric membranes can produce oxygen-enriched air containing 25% to 50% oxygen. For much higher purity, ceramic membranes can be used. These ceramic types, such as ion transport membranes (ITM), can provide oxygen purity of 90% or more. However, they require very high operating temperatures between 800 and 900 degrees Celsius.

Membrane separation is also used on jet liners to fill fuel tanks with nitrogen-rich air. This reduces the risk of accidental fires. It can also provide oxygen-enriched air to pilots flying in unpressurized cabins.

The scale of these operations is massive. In steelmaking, the basic oxygen steelmaking process requires nearly two tons of oxygen for every ton of steel produced. Some coal gasification projects use cryogenic plants that produce 3,000 tons of oxygen per day. Once separated, gases are often sent through pipelines to nearby industrial users. For long distances, companies ship liquid products or use gas cylinders and dewar flasks for smaller amounts.