Oxygen is the third most widely used chemical in the world, with an annual worldwide market of over $9bn. Uses for pure oxygen range from relatively small scale breathing oxygen systems for patients with pulmonary disorders and medical oxygen systems for hospitals, to large industrial applications such as chemical synthesis, enriched air for refining systems, syngas generation for GTL (Gas to Liquids) plants, or commercial coal or coke gasification systems.
Oxygen is produced commercially as a gas or as a liquid by several methods.
These include cryogenic air separation, a process that compresses and cools atmospheric air, then – relying on different boiling points – separates the resulting liquid into its components in a distillation column. Another method is that of vacuum pressure swing adsorption (VPSA), a non-cryogenic technology that produces oxygen from air by using an adsorbent in a pressure swing process to remove nitrogen.
Colorless, odorless and tasteless as mentioned, oxygen has poor solubility in water and a specific gravity of 1.105, which makes it slightly heavier than air. When cooled to its boiling point of -183°C, oxygen becomes a transparent, pale blue liquid that is slightly heavier than water.
This gas of respiration reacts with all elements except inert gases, to form compounds called oxides. The rate of reaction – known as oxidation – varies, with magnesium for example, oxidizing very rapidly and igniting spontaneously in air. In contrast however, noble metals such as gold and platinum, oxidize only at very high temperatures.
Although oxygen itself is nonflammable, it enhances combustion and enables all materials that are flammable in air to burn much more vigorously. These combustion-supporting properties account for its wide use in many industrial applications.
The principal uses of oxygen stem from its strong oxidizing and life-sustaining properties, used in medicine for therapeutic purposes and in the metals industry for steelmaking and metal-cutting applications. In the chemical and petroleum industries, oxygen is used in the production of a wide variety of fuels and chemicals, while it also finds utility in the pulp and paper industry for a variety of applications including pulp bleaching, black liquor oxidation, and lime kiln enrichment. In the glass industry, oxygen/fuel combustion is used to reduce particulate and NOx emissions in melting operations. O2 is also used for gasification applications for the production of synthesis gas to make chemicals, fuels, electricity, hydrogen or steam.
Smelting of iron ore into steel consumes 55% of commercially produced oxygen, injected through a high pressure lance into molten iron, which removes sulphar impurities and excess carbon as the respective oxides, SO2 and CO2. The reactions are exothermic so the temperature increases to 1700°C.
Another 25% of commercially-produced oxygen is used by the chemical industry, where ethylene is reacted with O2 to create ethylene oxide which, in turn, is converted into ethylene glycol – used to manufacture a host of products including antifreeze and polyester polymers, the precursors of many plastics and fabrics.
Most of the remaining 20% of commercially-produced oxygen is used in medical applications; metal cutting and welding, as an oxidizer in rocket fuel, and in water treatment, O2 is used in oxyacetylene welding burning acetylene with O2 to produce a very hot flame. In this process, metal up to 60cm thick is first heated with a small oxy-acetylene flame and then quickly cut by a large stream of O2. Rocket propulsion requires a fuel and an oxidizer and larger rockets use liquid oxygen as their oxidizer, which is mixed and ignited with the fuel for propulsion.
Two Mega Plants
Oxygen plants are mushrooming in all the parts of the world, Air Liquide has recently signed two contracts to design and build very large units, each of them with a production capacity of over 3,000 tpd: with Posco in South Korea and with Shenhua Ningxia Coal in China. These are the largest production capacities ever installed for a steelmaker and the oxygen produced will allow the Gwangyang and Pohang sites to meet the Posco Group’s increase in global steel production in South Korea. In China, it has completed contract with the Shenhua Ningxia Coal Industry Group for two air separation units, each with a production capacity of 3,000 tones of oxygen per day.
These two units will supply the oxygen needed for the synthesis of methanol and propylene from coal gasification.