Understanding Air Separation Plants: How They Divide Atmospheric Air Into Valuable Gases

An air separation plant is an industrial facility that separates naturally occurring air, or atmospheric air, into its component gases. The gases harvested in the process include oxygen, nitrogen, and argon, as well as some inert gases such as neon, xenon, and krypton. Since nitrogen and oxygen are the two most common gases in the earth's atmosphere, these two elements make up the bulk of the gases harvested in this process.

The process employed in an air separation plant is called cryogenic distillation. By cooling the atmospheric air to extremely low temperatures, the component gases can be separated and harvested. Liquid forms of nitrogen or oxygen may also be harvested at some plants.

The cryogenic distillation process is a four step procedure. In the first step, atmospheric air is cleaned and compressed to a pressure range of 5 to 10 bars (4-9 atmospheres). The cleaning process removes dust and contaminants that could interfere with the cooling process. The compression process heats the air to remove traces of moisture before passing the compressed air to a heat exchanger for cooling.

When the compressed air has been cooled, the next step is purification. This process involves passing the air through a molecular sieve, which filters the air by a process called adsorption. In this process, gases or molecules are attracted to an adsorbent surface and form a layer on top of the adsorbent material. The molecular sieves are designed to remove carbon dioxide, water, and hydrocarbon gases.

After the air is purified, it can safely be passed to the cooling and distillation chamber. The cooling chamber consists of a heat exchanger and cryogenic streams. When the air is passed through this chamber, it reaches sufficiently low temperatures to enable distillation of the component gases.

The distillation of the air occurs in the distillation column. Depending upon the gases to be harvested, the air may pass through more than one distillation column. This component uses refrigerant and an expander or valve to generate a Joule-Thomson effect, which causes the component gases of the air to cool and reach a liquid state. The liquefied gases are then collected and warmed to ambient temperature for storage or use.

The gases produced by an air separation plant are used in many industrial processes. In some cases, these gaseous forms of these elements are transported to industrial users via direct pipelines. In other cases, the harvested elements may be stored in the liquid or gaseous forms for later transportation and use. Larger quantities of the distilled gases are usually transported in their liquid forms by tanker trucks. Smaller quantities may be transported in pressurized cylinders as a gas.

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