All of the component gases in the Earth’s atmosphere have unique and useful properties and this explains why separation processes have been developed.
Cryogenic air separation is an entirely physical process that is most often used to produce nitrogen, oxygen and argon. A cryogenic air separation unit (ASU) exploits the fact that air can be cooled sufficiently for it to become a mixture of liquids and the difference in their boiling temperatures allows the component gases to be separated by distillation.
Air enters the process through filters that remove dust, soot and other suspended contaminants. Cooling the air stream down is achieved by alternate stages of compression and heat exchange. The first compression stage increases the pressure to approx. 6 bar and this raises the temperature of the process stream to around 185°C.
Heat is then expelled through either air-to-water or air-to-air heat exchangers that are chilled by cold gaseous streams routed back from the separation columns, to reduce the temperature back to ambient or below.
Carbon dioxide and water vapour are two components that must be removed prior to reducing the temperature to cryogenic levels, because both would form solid contaminants that interfere in the distillation process and may damage the process equipment. Most of the water is condensed and removed between successive compression stages. The final traces, together with remaining carbon dioxide and hydrocarbons, are removed using molecular sieve pre-purification units in newer plants, or reversing heat exchangers in older installations.
Gas temperatures can be lowered by reducing pressure in exactly the opposite way to the temperature increase that occurs under compression, and this is the basis of the refrigeration process employed to reach the cryogenic temperatures needed for air separation. Depending on the specific plant design, a high pressure stream of nitrogen, waste gas, feed gas, or product gas, is fed through an expander, which is a form of turbine.
This absorbs mechanical energy from the stream as it expands to lower pressure, causing the gas temperature to lower rapidly.
Energy recovered by the expander is typically used to drive a process compressor, an electrical generator, or other energy-consuming device such as an oil pump or air blower. Exiting gas streams are warmed to near ambient temperature by recovering refrigeration from the gaseous product streams and waste stream, to minimise the amount of refrigeration that must be produced by the plant. Energy is the major cost of operating an ASU and plants are designed to optimise efficiency.
At the core of the air distillation process is a series of columns housed inside an insulated chamber called the cold box; here the super-cooled air is condensed into liquid and the component gases separated. Partially condensed air flows into the lower end of the cold box and as it rises, continues to condense on the cold surfaces and trickle down the column, while cold gas bubbles up through liquid air that has collected in the perforated trays attached to the columns.
This interaction accumulates liquid oxygen at the bottom of the distillation column, as its higher boiling point (-183°C) allows it to condense more readily. While nitrogen with its lower boiling point (-196°C) concentrates in the gas phase at the top of the column, nitrogen exits from the top of each distillation column and oxygen exits at the bottom. Argon has a boiling point similar to that of oxygen (-186°C) and additional processing is required to extract and purify both it and the rare gas components.
Various configurations of high and low pressure columns enable plants to produce product streams compatible with the expected demand. The energy efficiency of an ASU is maximised by designing it to deliver the final product steam at the minimum required pressure. If a substantial proportion of liquefied product is required then supplemental refrigeration capacity is provided either as a stand-alone liquefier or integrated with the ASU in more modern systems.