Tony Wheatley explores the role of gases in the environment, both naturally and from an industrial perspective.
The industrial gas industry has enjoyed continuous growth worldwide over many decades and the most plausible reason, is that the products delivered enable or enhance many industrial processes. Gas applications aim to match the chemical and physical properties of gases, with process difficulties that various industries are faced with.
All of the components of the Earth’s atmosphere
are exploited as raw materials for use in a wide range of applications.
Oxygen in the environment
Nearly all living things need oxygen to stay alive: it combines with other chemicals in plant and animal cells to produce the energy needed for life processes. Oxygen is also needed to make most fuels burn, by combining with the fuels in a chemical reaction and releasing heat. It is non-flammable, odourless, colourless in the gas phase but condenses to a pale blue liquid when cooled sufficiently under pressure.
Used for its properties as a strong oxidiser, oxygen is the most abundant element on Earth. Reactive with virtually all other elements, except the noble gases, oxygen is therefore usually bound in other compounds like silicates, oxides and water. Molecular oxygen occurs almost entirely in the atmosphere, but is also dissolved in rivers, lakes, and oceans.
The only natural process that restores gaseous oxygen to the environment is photosynthesis: by which plants use solar energy to convert water and carbon dioxide into sugar, while releasing oxygen as a by-product.
It is valued for its reactivity and used, with or instead of air, to increase the oxygen available for combustion or biological activity. This increases reaction rates and boosts plant efficiency. It finds many uses in steelmaking, metal refining, fabrication processes, in chemicals, pharmaceuticals, petroleum processing, glass and ceramic manufacture, and pulp and paper manufacture. It aids environmental protection through treatment of municipal and industrial effluents and has numerous uses in healthcare.
Some applications, such as effluent treatment and pulp and paper bleaching, require its even more reactive form called ozone, to ensure full reaction with undesired compounds. Ozone forms naturally in the stratosphere layer of the atmosphere where the sun’s ultraviolet radiation coverts O2 molecules into O3.
Oxygen’s reactivity ensures that only negligible quantities of pure oxygen will be returned to the atmosphere after use in most applications.
Nitrogen in the environment
Nitrogen is a common normally colourless, odourless, tasteless and mostly diatomic, non-metal gas.
In the gas phase, pairs of nitrogen atoms bond together to form very stable molecules called diamers. Nitrogen gas will only react with other elements under special conditions produced by temperature, catalysis, high voltage or pressure.
Nitrogen has been extracted commercially from the atmosphere since 1903, with the greatest demand being for the manufacture of ammonia. Nitric acid salts include some important compounds such as potassium nitrate, nitric acid, and ammonium nitrate. Nitrated organic compounds, such as nitro-glycerine and trinitrotoluene, are often explosives.
As a cryogenic liquid, nitrogen is a useful refrigerant in food processing and transport, industrial cooling, the preservation and storage of bodies, reproductive cells and biological samples. Nitrogen is used as a carrier gas, for pressure testing, blanketing and purging where a dry, inexpensive
and relatively inert gas is required.
Nitrogen is an essential element for life, because it is a part of vital organic compounds in micro-organisms like amino acids, proteins and DNA. Industrial processes that emit vast amounts of nitrogen thereby increasing the nitrate and nitrite supplies in soil and water, have altered the proportions of nitrate and nitrite in the environment radically. Excessive use of nitrogenous fertiliser and the application of nitrogen containing pesticides affect the environment negatively and accumulate in groundwater and drinking water supplies.
Nitrogen molecules occur mainly in air, do not react with water but are soluble in it. In water and soils nitrogen can be found as nitrates and nitrites, all these substances being interconnected in the natural ‘Nitrogen cycle’.
Although a dietary requirement for all organisms in the form of proteins and nucleic acids, elementary nitrogen cannot be taken up directly. It must first be converted to nitrate or nitrite by a process called ‘nitrification’ that is performed by bacteria. This releases energy and establishes a nitrate stock in the soil where is it available to plants.
Ammonium and nitrate are absorbed by plants and these are then converted into organic molecules like amino acids and DNA. Animals obtain their vital supplies of nitrates by eating either plant material or other animals that eat plants. After nitrogen nutrients have served their purpose in plants and animals, other bacteria start a process called ammonification that converts them back into ammonia and water soluble ammonium salts. Anaerobic bacteria then complete the cycle by releasing nitrogen gas, through a process named dentrification.
Argon in the environment
Argon is continuously formed during the radioactive decay of K-40 (an isotope of Potassium) in subsurface nuclear activity. The density and low chemical reactivity of argon ensures that its molecules find their way back to the Earth’s atmosphere unchanged after use.
Argon is also very stable and popular as an inert gas with an extensive range of applications, due to its availability and low cost as a by-product of nitrogen and oxygen by cryogenic air separation.
It also accumulates during the production of ammonia by reforming natural gas, although in both cases the atmosphere is the source of argon.
A monatomic, colourless, odourless, tasteless and nontoxic gas, argon is non corrosive and non-flammable, with similar solubility in water to oxygen and low thermal conductivity and ionisation potential.
The largest volume of argon demand is for making and refining steel and stainless steel. Welding and processing of many metals especially titanium, aluminium and magnesium alloys would be impossible without inert gas. ICP spectroscopy and Scanning Electron Microscopy use argon. It provides a protective atmosphere for growing semiconductor crystals and for heat treating metal components and in graphite electric furnaces. It prevents oxidation and preserves intravenous drug preparations, wine, old materials, documents.
Argon can also be used for insulating buildings when used to fill the void between the glass panels of themo-pane windows. It is integral to the manufacture of incandescent and fluorescent light bulbs and even finds use in cryosurgery. In the weapons industry, argon cools the seeker head of missiles.
Hydrogen in the environment
Hydrogen, the first element in the periodic table, forms only 0.15% of the mass of the Earth’s crust and 0.5 ppm of its atmosphere. The small molecular size and mass of hydrogen imply that it cannot be retained by Earth’s gravitational field and also allows it to diffuse through many substances, making leak tight systems difficult to engineer.
In normal conditions it is a colourless, odourless and an insipid gas formed of diatomic molecules. The most flammable of all known substances but not very reactive at ambient temperature, it reacts explosively with oxygen only if accelerated by a catalyst like platinum or an electric spark.
Hydrogen does occur naturally in underground mines and oil wells, but is most often produced by steam reforming of natural gas. This functional gas product can be produced by the electrolysis of water but this is only deemed viable where high purity is required and electric power is inexpensive.
Hydrogen’s established uses include: synthesis of ammonia, ethanol, hydrogen chloride and hydrogen bromide; the hydrogenation of vegetable oils; hydro-cracking, hydro-forming and hydro refining of petroleum; atomic-hydrogen welding; instrument-carrying balloons; fuelling rockets; and cryogenic research. After use any un-reacted hydrogen will escape into space, so none will be returned to the Earth’s environment.
The dream of a pollution free hydrogen economy has gained popularity because of environmental concerns about climate change, the political vulnerability of oil supplies and rising cost of crude oil and natural gas, yet two serious obstacles delay its realisation:
Hydrogen will only be a dream fuel if it can be produced in massive volumes using renewable energy.
The physical properties of hydrogen dictate that it is the most difficult fuel to store and distribute cost effectively.
Helium in the environment
Helium often occurs in underground gas deposits and is separated during the production of natural gas. The first member of the ‘noble’ gas group, it is the least reactive element and doesn’t essentially form chemical compounds.
Helium has many unique properties: low boiling point, low density, low solubility, high thermal conductivity and inertness, and can be liquefied – but its condensation temperature is the lowest among all the known substances.
Applications include filling balloons and dirigibles, autogenous welding, cooling of superconducting magnets in medical scanners and for research, pressurizing gas in liquid propellants for rockets, breathing gases for divers, as working fluid in nuclear reactors and as gas carrier in chemical analysis by gas chromatography. After use helium atoms enter the atmosphere but most eventually escape into space.
Carbon dioxide is produced naturally by respiration of plants and animals in balance with it’s consumption during daylight hours by plants in photosynthesis. Centuries of fossil fuel combustion have produced vast a amount of surplus in the atmosphere that has been partially absorbed by the ocean waters.
Carbon dioxide produced for industrial applications is normally separated from waste gases flowing from existing combustion activities and therefore, may partially offset the accumulation of atmospheric carbon dioxide.
At current levels of output, the total volume of industrial gases extracted globally from the atmosphere, is a very tiny fraction of the available mass of gas molecules.