Splitting air opens up new opportunities

Tom Shelley reports on the raft of new opportunities for products and processes that stem from the ability to separate nitrogen and oxygen from air on a small scale New nano scale membranes render economic the separation of air into nitrogen up to 98 or even 99.5 pure plus oxygen up to 40 per cent. On the one hand, it becomes economic to work with nitrogen as opposed to air to blanket fuel tanks in aircraft, prevent food going off in domestic refrigerators, inflate tyres so as to leak less, and undertake various operations more safely in the oil and gas industries. On the other hand, enriched oxygen allows lower grade methane biogas to be used in engines to generate power small furnaces to be heated more economically. The ability to use membranes to separate nitrogen and oxygen from air has been around for some time, but it is only recently that it has become cheap enough to be considered for general use, and for users to start considering ways to employ it. Parker Filtration and Separation at Etten Leur in The Netherlands makes its 0.5mm diameter membrane fibres out of polyphenylene oxide (PPO) with a separating top skin layer only 40nm thick. Work is currently under way to reduce this figure to 20nm to increase process production rates still further. Despite their thin skins, the fibres are nonetheless strong enough to withstand up to 60 bar pressure. The details of the manufacturing process are proprietary but we can say that the porosity of the fibres is achieved by spinning them in the presence of methyl alcohol which is subsequently evaporated out leaving the pores. Nitrogen and argon diffuse only slowly through the fibre walls whereas carbon dioxide and oxygen molecules diffuse faster. Water and hydrogen molecules and helium atoms diffuse faster still, but it is the separation of oxygen and nitrogen that is of most economic interest. The oxygen, being the faster diffusing species comes through the sides of the fibres, while the nitrogen tends to continue down the length of the fibres. The maximum nitrogen concentration that can be achieved is 98 per cent, or in extreme cases, 99.5 per cent, although reducing oxygen to 14 per cent is sufficient to protect against fire into most cases, while allowing humans to continue to breathe without ill effects. The hazards to business of fires should not be underestimated. It is said that 43 per cent of firms close immediately after a major fire, and 28 per cent more after three years. The effect of water sprinklers and the efforts of fire brigades often cause as much damage as the fire itself. Nitrogen is becoming more and more popular to inflate tyres with, because its low diffusivity means that they then leak less. It has long been used as the energy absorbing gas in hydraulic accumulators, because highly compressed air simply is not safe, and is widely used for blanketing and purging tanks in the oil and gas industries, for flare head purging, and for purging barges prior to loading. It is also beginning to be injected into old oil wells at 700 to 800 bar pressures to restore production, where it causes much less trouble than water. Among other problems, injecting high pressure sea water has been found to introduce bacteria which feed on the oil. It is also alleged that high pressure water injection can trigger small earthquakes, and possibly large ones as well. The practice of injecting high pressure air is also a less than a good idea, because oil and air combustion is almost inevitable. For the same reason, there is also development work under way to flood underground coal cutting heads with nitrogen, particularly when mining sub bituminous coals that contain much methane and volatiles which are liable to ignite spontaneously. In this instance, the miners work the cutting heads by remote control. A recent application that will improve the safety of certain aircraft significantly is the requirement that all civil aircraft are to be equipped with On Board Inert Gas GeneratorS (OBIGS) so they can have nitrogen blanketed fuel tanks from 2006/7. On visiting Etten Leur, we were shown prototype fibre separators for use in the A380, which would perhaps require 8 or 16 units each aircraft. We were also told that in 2006, Turkish made domestic refrigerators will be equipped with nitrogen flooded cool drawers to extend food life - a world first. Nitrogen is already quite widely used to fill plastic bags of salad, to extend life, to blanket wine, and to pressurise beer delivery in some bars. Having on site generation of nitrogen is somewhat cheaper and more convenient than deliveries of bottled gas, although there is usually a nitrogen gas bottle around in such installations in case of emergencies. Tonnage amounts of oxygen for steel works are still most cheaply made by cryogenic plants, but on a small scale, the membranes have no competitors, apart from deliveries of liquid oxygen or oxygen in cylinders, if a producing plant happens to be near enough. Engines fuelled by methane biogas or gases from old coal mines normally cease to function if the methane concentration drops below 40 per cent. If the air is enriched to 27 per cent oxygen, it is possible to make the engines continue to function down to 20 per cent methane, extending the obtaining of useful fuel gas from typical land fill sites by six to eight years, Parker is currently engaged in developing the technology in conjunction with the Umsicht section of the Fraunhofer Instiute in Germany, with trials under way on a landfill site in that country. In trials conducted in conjunction with Thyssen, it has been found that enriching combustion air to 26 per cent reduces fuel consumed to pre-heat steel crucibles by one third. Thyssen has access to even cheaper lower cost cryogenic produced oxygen in its plants, but many smaller furnace users do not. The maximum oxygen concentration that the membranes can stand is about 40 per cent, which is also, incidentally, the maximum amount that humans can breathe without ill effects at ground level pressure. Parker Parker Filtration and Separation Economatics Eureka says: Separating air into oxygen reduced and oxygen enhanced fractions offers many benefits. Pointers * The oxygen content of air only has to be reduced to 14 per cent to prevent most things burning * Nitrogen inflated tyres leak less than those filled with air, and nitrogen has many safety related uses in the oil and gas industries, as well as preventing the spoiling of foodstuffs * Oxygen enriched air greatly assists combustion, especially of low grade biogas and reduces fuel consumption in gas and oil fired heaters and furnaces