Industrial separation of air
The mixture of air is found in the atmosphere of the earth and when dry, it is relatively uniform in composition, consisting of only four gases, whereas air that is more ambient contains a number of other gases. The process used to separate air in industries is known as cryogenic distillation, or fractional distillation. This process is can be very simple (depending on the number of products being separated) and involves separating each element one at a time using their boiling points. Once the air is separated into its components, many elements are produced. These include nitrogen, oxygen, argon, krypton, hydrogen, helium, carbon dioxide, xenon and neon.

Process: Cryogenic distillation consists of two phases (gas and liquid) and is usually used in medium to large scale plants to produce elements such as nitrogen, oxygen and argon as gases and/or liquid products. This process is the preferred method for generating high purity oxygen and nitrogen and is common amongst many plants. The process begins by compressing the air to somewhere between 5 to 8 bars. The compressed air is then cooled and the majority of the water vapour in the incoming air is condensed and removed, as the air passes through several coolers, including an aftercooler. Next, the components of air must be removed. First, the carbon dioxide and water are removed through the use of a molecular sieve. The molecules of these elements are absorbed onto the surface of the sieve and then contained in vessels.

The next step involves heat transfer against product and waste gas streams to bring the air to cryogenic temperatures, which is approximately -185ºC. The cooling of the air is done in aluminium heat exchangers, which allows exchange of heat between the incoming cool air and the waste gas streams exiting the separation process. This minimises the amount of refrigeration that is required by the plant, thus saving energy. Next, the air undergoes fractional distillation in order to separate it into the desired components. Oxygen (bp: -183ºC) and nitrogen (bp: -195.8ºC) are removed using two distillation columns in series. The varying pressures inside these columns ensure that nitrogen leaves the top of each column and oxygen leaves the bottom. However, because argon (bp: 185.8ºC) has a similar boiling point to oxygen, it will most likely stay with it, and therefore needs to be removed. Its removal occurs in the low pressure distillation column where the argon concentration level is at its highest. The removed argon is then processed through a third column made especially for it. The argon is then vented, processed further or collected as a liquid. As indicated via the process, the separation of air is based primarily on temperature difference and different boiling points of each of the elements. This is a key factor in the separation