The life blood of existence on earth oxygen supports a number of applications but requires effective handling practices.
Oxygen, though discovered almost 250 years ago, still remains an invaluable contributor to the Industrial World. Its discovery is hotly disputed by historians, between Joseph Priestley, a British clergyman who first published his findings in 1774; a Swedish pharmacist Carl Scheele who claimed to have produced it in 1772 but did not publish until 1777, and a French chemist Antoine Lavoisier whose experimentation in 1774 was not published also until 1777.
The intrigue of this saga was compounded when a copy of a letter from Scheele to Lavoisier describing a ‘new substance’ was found in the former’s belongings when he died, which the latter had claimed he had not received!
But it is Lavoisier who first named the ‘new substance’ oxygen, (oxygene to be more precise in French) from oxys, which in Greek is ‘acid’, and genes, which means ‘producer’. Lavoisier wrongly assumed that oxygen was a constituent of all acids.
Many English scientists of the day refused to accept this name ‘oxygen’ due to its incorrect description but more because it was not the chosen name by Priestley, who they considered to be the legitimate discoverer. But by the 1790’s ‘OXYGEN’ was widely used in the English language.
Oxygen, the promoter of life on Earth, is also the latter’s biggest constituent by weight, comprising almost half of the Earth’s mass, at 90% of the seas and some 23% of the mass of our atmosphere.
This high level of oxygen in our air is the result of the oxygen cycle based on photosynthesis. Humans require around 120g of oxygen per hour to survive - the equivalent of around one tonne per annum.
Just as it supports human life it also acts as a powerful supporter of combustion. But at concentrations above about 50% oxygen can be toxic. When other elements are burned in oxygen almost all form oxides. The most familiar is of course water. Oxygen’s oxidising nature has opened up many industrial applications (see later).
A gas, a liquid, a solid
Oxygen is a gas at normal pressure and temperature, but upon cooling it turns into a cryogenic, blue coloured liquid (LOX) at -183˚C (90K) and finally into a solid substance at -219ºC (54K). Cryogenic oxygen is magnetic and when solid oxygen is pressurised at low temperatures, it behaves like a metallic acquiring superconducting properties.
The first recorded quantity of LOX (albeit only as a few drops) was by a Swiss scientist, Raoul Pictet in late 1877. But it was not until 1891 that Dewar had produced enough LOX to enable it to be studied. Within a few years engineers Hampson and Linde from Britain and Germany respectively had produced liquid air in commercial quantities. By carefully boiling this liquid air, large quantities of pure gaseous oxygen (and nitrogen) were obtained.
This fractional distillation of liquid air is still the most widely used technique for obtaining pure gaseous oxygen. Though if a purity of only 90% oxygen is required, which for some applications is quite acceptable, then a technique known as ‘pressure swing absorption’ (PSA) is employed.
Here clean dry air under pressure is forced through a pair of zeolite (a naturally occurring mineral with a high volume of porosity) molecular sieves, which absorb most of the nitrogen thus releasing an almost pure stream of oxygen.
By reducing to a low-pressure mode in one tube, the sieve is forced to desorb the first gas (nitrogen) while the second tube is used to once again absorb nitrogen. This enables a continuous process to be maintained. PSA is gradually replacing some cryogenic uses, where ‘nearly pure’ oxygen
Since the cryogenic technique is heavily dependant on the use of large quantities of electricity, with industrial gas companies being one of the largest consumers of electricity in their local country, its popularity is bound to decrease as energy costs continue to escalate.
Gaseous oxygen (GOX)
The storage of GOX has as its roots as a small, brazed copper sphere with a water capacity of just 2 litres and working at a pressure of 20 bar. That was over 150 years ago.
Progress has been rapid in developing the storage capacity such that the seamless steel (carbon steel only) cylinders in the early 20th century, which were capable of transporting 0.03m³ of oxygen per kilogram of cylinder weight, have now been transformed into carbon-wrapped composite vessels capable of carrying almost twenty times that amount. But enjoying an efficiency of 0.60m³ / kg of cylinder weight such composites have a much higher cost associated with them.
Such is the usefulness of this abundant element oxygen, especially in the medical gases arena, that users are prepared to pay the high costs associated with high efficiency such as a light transportable package.
In applications such as domiciliary use and emergency response a cylinder carrying over 400 litres of oxygen and weighing just 2kgs is ‘worth its weight in gold’!
But the real benefits of GOX are enjoyed by the industrial market place, where oxygen’s combustibility is exploited. It increases the productivity of the iron and steel making processes (where over 50% of commercially produced oxygen is used) by reducing the power consumption in a plant, while the non-ferrous industry sees higher recovery ratios during the mineral extraction processes.
One of its earliest applications was in the cutting industry where combined with a fuel gas such as acetylene (see gasworld, December 2007), it can produce excellent surface finish on slabs up to 2m thick. Combined with ‘assist gases’ such as carbon dioxide its use in lasers for precision-cutting metals up to 30mm is also widespread.
Its capability of producing high temperatures is exploited also by welders worldwide in conjunction with acetylene for joining a wide range of ferrous and non-ferrous alloys.
Finally, oxygen’s role in the waste treatment industry is unique. A prime example is the ‘Thames Bubbler’ which by pumping oxygen into the River Thames has brought back over a hundred different types of fish to an otherwise ‘dead’ river in the past 20 years.
All that glitters is not gold
Or should that be ‘Too much of a good thing can be bad’? Without oxygen life on earth would be impossible. But an excess can lead to disastrous results. If administered in high doses it is toxic to man.
Oxygen enrichment can be highly dangerous and is very hard to defeat. Oxygen is easily absorbed into clothing, and operators who fill oxygen cylinders have been known to ‘catch fire’. Hence now such workers have special PPE, which are resistant to spontaneous ignition.
Oxygen’s reaction with hydrocarbons such as oil, is often violent and promotes combustion with an explosive force. There have been numerous serious accidents, which have been attributed to this ferocity in the gases industry.
Traces of oil have resulted in ‘burn outs’ in oxygen filling lines. Here we can see that the cylinder’s steel, which forms an endothermic reaction, has extinguished the fire. But when in contact with materials which result in an exothermic reaction just as with aluminium for example, oxygen’s reaction is indeed very violent. Witness the aluminium alloy warships, which perished in recent conflicts.
Due to oxygen’s very reactive nature, its use in contact with other materials is strictly regulated. A major source of information for its safe use is via an International Standard ISO 11114-3 (Transportable gas cylinders, Autogenous ignition test in oxygen atmosphere).
There have been many conferences on the subject of ‘oxygen compatibility’ and for interested readers the proceedings of ASTM G4 committee are a useful starting source. Here you will find that the cleanliness from hydrocarbons apart, pressure within the component, velocity of the oxygen through a component, the overall component design and a host of other criteria govern the seriousness of a potential accident.
It is however accepted that some hydrocarbon deposits will always find their way into a gas cylinder, either during its production or during its lifetime use. Most organisations have their own specifications of the maximum permissible CxHy per unit surface area of the cylinder. Similar hydrocarbon considerations are also needed for the PTFE tape use as a sealing agent on oxidising gas cylinder valve steams.
Then again we have its oxidising properties to control. Gas cylinder users can protect against the ingress of moisture into an oxygen steel cylinder by fitting a suitable valving system. But if oxygen cylinders do permit moisture ingress, the results can be equally disastrous. Wet oxygen will corrode the internals of steel cylinders very rapidly eventually leading to a failure.
Oxygen is indeed a remarkable gas. We are the only planet in our solar system to have it in just the correct concentration to promote life, as we know it. It is without doubt a major contributor to our way of life and in many cases our saviour.
But if used outside the strictly controlled limits, it acts as our foe. Use of standards and best operating practices is the way ahead in the industrial gases arena.