Liquid nitrogen is collected in some air separation plants.
An air separation plant is an industrial facility that separates natural air, or atmospheric air, into its component gases. Gases collected in the process include oxygen, nitrogen, and argon, as well as some inert gases such as neon, xenon, and krypton. As nitrogen and oxygen are the two most common gases in the earth’s atmosphere, these two elements make up the majority of the gases collected in this process.
The process used in an air separation plant is called cryogenic distillation. By cooling atmospheric air to extremely low temperatures, the component gases can be separated and collected. Liquid forms of nitrogen or oxygen can also be collected from 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 bar (4 to 9 atmospheres). The cleaning process removes dust and contaminants that can 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.
Once the compressed air has cooled down, the next step is purification. This process involves passing air through a molecular sieve, which filters the air through a process called adsorption. In this process, gases or molecules are attracted to an adsorbent surface and form a layer on the adsorbent material. Molecular sieves are designed to remove carbon dioxide, water, and hydrocarbon gases.
Once the air is purified, it can be safely passed to the cooling and distillation chamber. The cooling chamber consists of a heat exchanger and cryogenic flows. When air passes through this chamber, it reaches temperatures low enough to allow distillation of the component gases.
Air distillation takes place in the distillation column. Depending on the gases to be collected, the air can pass through more than one distillation column. This component uses refrigerant and an expander or valve to generate a Joule-Thomson effect, which causes component gases in the air to cool to a liquid state. The liquefied gases are then collected and heated to room 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 through direct pipelines. In other cases, the collected items may be stored in liquid or gaseous form for later transportation and use. Large quantities of distilled gases are normally transported in their liquid form in tanker trucks. Smaller amounts can be transported in pressurized cylinders as a gas.