Inert gas

Inert gases and noble gases are not exactly synonyms although some of the objects they describe overlap. "Noble gas" refers to inert elemental gases only whereas "inert gas" refers to molecular as well as elemental gases which are inert. Also "inert gas" is an archaic term for noble gas.

An inert gas is any gas that is not reactive under normal circumstances. Unlike the noble gases an inert gas is not necessarily elemental and are often molecular gases. Like the noble gases the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases. This is a tendency, not a rule, as noble gases and other "inert" gases can react to form compounds.

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Although the term "rare gases" is sometimes used as a synonym for the elemental inert gases, i.e. noble gases—they are only rare relative to other gases found in Earth's atmosphere (i.e. air) with the exception of argon which makes up a significant portion of air, around %0.934; hardly rare at all. Because of their unreactivity, and perhaps their relative scarcity, the inert gases were not discovered until helium was discovered to exist in the Sun, where it is abundant, before it was discovered to exist in Earth's atmosphere. This is possible through the analysis of spectral lines.

Helium and neon are the only true elemental inert gases, because they do not form any (known) true chemical compounds, unlike the heavier noble gases (argon, krypton, xenon and radon).

In marine applications, inert gas refers to gases with a low content of oxygen that are used to fill void spaces in and around tanks for explosion protection. There are two types of inert gas which are either based on nitrogen or on flue gas.

Production

The elemental inert gases are usually obtained by evaporating them off from condensed air at their respective vapor pressures.

Nitrogen based inert gas is produced on board of chemical tankers and product carrieres (smaller vessels) with compressors and a Nitrogen specific membrane.

Inert gas is produced on board crude oil carriers (above 20000 tonnes) by using either a Flue Gas system or by burning kerosene in a dedicated Inert Gas Generator. The flue gas system uses the boiler exhaust as it's source, so it's important that the fuel/air ratio in the boiler burners is properly regulated to ensure that high quality IG is produced. Too much air would result in an oxygen content exceeding 5%, too much fuel oil would result in carryover of dangerous hydrocarbon gas. The flue gas is cleaned and cooled by the scrubber tower. Various safety devices prevent overpressure, return of hydrocarbon gas to the engine room or supply of IG with too high oxygen content. Gas tankers and product carriers cannot rely on flue gas systems (because they require IG with O2 content of 1% or less) and so use IGGs instead. The Inert Gas Generator consists of a combustion chamber and scrubber unit supplied by fans and a refrigeration unit which cools the gas. A drier in series with the system removes moisture from the gas before it is supplied to the deck. Regular calibration and testing to equipment is required to ensure that it works correctly.

Applications

Because of the non-reactive properties of inert gases they are often useful to prevent undesirable chemical reactions from taking place. For example molecular nitrogen, a molecular inert gas, is often used in food packaging to ensure that food does not spoil in transit since no bacteria or fungi can flourish without the reactive gases oxygen or carbon dioxide, which the molecular nitrogen displaces, since most extant cells on Earth require the reactions which these gases are involved in to function. Most importantly since molecular nitrogen is inert it will not cause any reactions to take place in the food, possibly changing the intrinsic taste or smell, nor will it cause any chemical reactions in the human body. Thus the inert gas is used as a passive preservative, preventing biological decay, while being undetectable to the consumer since taste and olfactory senses require a chemical reaction to take place in order to send a signal to the brain. This is in contrast to active preservatives which react with the biological material of bacteria, fungi, and possibly the food itself changing the food's intrinsic taste or smell, or may even act directly on the consumer's taste and olfactory mechanisms.

As chemists sometimes need to perform experiments on air-sensitive compounds, air-free techniques have been developed to handle them under inert gas.

In gas metal arc welding inert gases are used to shield the heated metal from the reactive gases in air which can cause undesirable reactions in the metal. Some gases which are not usually considered inert but which behave like inert gases in all the circumstances likely to be encountered in some use can often be used as a substitute for an inert gas. This is useful when an appropriate pseudo-inert gas can be found which is inexpensive and common. For example carbon dioxide is sometimes used in gas mixtures for gas metal arc welding because it is not reactive in the circumstances encountered in arc welding even though it is often reactive in other circumstances. Carbon dioxide is usually used as a 25% mix with argon because pure carbon dioxide is too reactive.

On oil tankers IG is used to prevent the atmosphere in cargo tanks from coming into the explosive range. IG keeps the oxygen content of the tank atmosphere below 8% (on crude carriers, less for product carriers and gas tankers), thus making any air/hydrocarbon gas mixture in the tank too lean to ignite. IG is most important during discharging and during the ballast voyage when more hydrocarbon vapour is likely to be present in the tank atmosphere. IG can also be used to purge the tank of the volatile atmosphere in preparation for gas freeing - replacing the atmosphere with breathable air - or visa versa. Cargo tanks on Gas Carriers are not inerted, but the Hold space around them is. This arrangement allows the tanks to be kept cool using a small heel of cargo while the vessel is in ballast while retaining the explosion protection provided by the inert gas.

See also

• inerting system