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|The Molecular Sieves for Gas Separating Membranes|
Dr. Roman Denissov, Dipl. eng. Sergei Yakushev.
Abstract: In article there are presented a photo and properties of Zhdanov zeolite transparent polycrystalline plates and being it derivatives the numbers of non-stoichiometrical cathode-chromic sodalites. It is offered to extend created plates to get technology on other zeolites to manufacture the small-size and productive membranes required to solve the ecological and energy problems. Informing there are three selecting mechanisms in membane air separating in zeolites. All of them act for the oxygen using as permeate - a membrane getting through gas, where is expected for air separating high degree. The membrane makes small-size and productive devices to get and apply accumulator free oxygen directly on a place and to have the fuel complete combustion in automobile engine cylinders too.
Last years by growth of ecological and power problems there was appeared the complex of technologies - «zero emission fuel combustion» where the oxygen atmosphere combustion (complete combustion) is the most prospective. There are already some projects of the thermal power plants focused on oxygen coal combustion, diluted by CO2.
In oxygen concentrating a burning process rate is increased, so even kinds of bad quality fuel well burn in pure oxygen. It not allows to get traditional fuel complete combustion only, and also opens different new kinds of biofuel and garbage using possibilities to do no essential preparing transformations with. But while the share of renewed energy sources, excepting hydroelectric power stations, does not reach in 2 %. There is a criteria of price - 40 $ oil barrel being a "rubicon" which starts an economic reason to make motor biofuel and to develop new complex technologies resource savings.
By some estimations producing by widely used cryogenic way oxygen (freezing up to -150°C) makes 30% cost rising of the electric power. Besides this method implies to have oxygen producing plant, that can be expedient only for scale manufactures and is unacceptable for a small one.
There are looking for a way in development of air separating membrane technologies. Known technologies consist in oxygen molecules or ions selective passage through a dense material film (membrane).But dense materials have the large flow resistance. Real polymeric membrane is a complex expensive product having hundreds square meters of finely thin film fixed by collection of its position elements is assembled in the hermetic module . In single cycle there is capable to increase oxygen concentration from 21 % (air) up to 32 %.
Small molecule membrane separation development has been caused by independent living condition requirements to make, for example,in submarine or space vehicles. The basic results have been achieved in 60th years. For next 50 years of membrane technologiesdevelopment, because of the dense materials applying, it has not been achieved essential productivity increasing or dimension decreasing.
Progress is seen in creation the membrane as a molecular sieve having the calibrated apertures which the selection is being. The sieve has incomparably smaller resistance, than dense film. As a material the zeolites are considered to be strong alumino-silicate very complex structure porous crystals, which single element contains many tens of atoms. Regular geometric shaping pores form through channels incorporating each other by "windows". The "windows" have the selecting molecules sizes. The free space volume formed by pores depends on zeolite type and can make half of crystal volume.
Preliminary estimations perform the membrane surface will be comparable to air line section. I. e. the membrane can be made as an airline small insert.
Natural zeolites have imperfect structure and, as a rule, are polluted a much by other minerals, and having qualitative structure synthetic ones are as microcrystals for the sizes ~1µm.
Therefore, main efforts of researchers are directed on studying of a polycrystalline zeolitic film growth conditions on a porous substrate (together membrane is). Until films are got out rather looseness and nondense.
In the 80th we had been developed technology to compact polycrystalline plates using a qualitative Zhdanov zeolite powder and being it derivatives numbers of non-stoichiometrical cathode-chromic sodalites (is patented [3-8]). Thus initial microcrystals physical properties are kept the same.
The plate shown on a photo is optical transparent, its density corresponds to crystal one (>>99%). On mechanical properties it is close to glass, vibration-proof, permits mechanical clearing and washing.
In technological view, Zhdanov zeolite optical transparent polycrystalline monoliths and sodalites to get the fact shows, that zeolite alumino-silicate structure is capable to be reconstructed with structure conserving being participate in recrystallizating processes, where is separate grains growing or reducing with free space filling and strong couplings forming. Recrystallizating processes of too complex systems revealing and it real time activating possibility is experimental mineralogy the event. It means also on principle possibility other of zeolites dense polycrystalline forms to get.
To make the membrane it is necessary to expand recrystallizating compaction technology on other zeolites, having air separating suitable channels. Researches and developments will take 10 years about. Such membrane will be favourable differ by absence of a substrate and connected it bonding problems. It can act in all of temperature range, including high, up to a zeolite stability limit. The membrane will not be expensive as it can be organized their lot or mass production as polycrystalline zeolitic disks.
We had been developed zeolitic membranes air separating physical model. It is shown, that process is defined by acting simultaneously three selection mechanisms caused by parities of the molecules and channels sizes, on inlet “windows” , zeolite inside moving and nitrogen molecules quadrupole interaction with zeolite lattice cations also. All of them act for the oxygen using as permeate - a membrane getting through gas that is supposed a high level of mix separating. This fact with high throughput ability makes the zeolitic membranes the most prospect one for air separating, to use gas directly on a place, accumulator free.
We assume such membranes consisting simple oxygen-producing devices will not be suitable in large and small power engineering only. But small-sized and operational-proof ones they can be used for various fuels ecology compatible combustion in an oxygen atmosphere the automobile engine cylinders to be. Increasing of environment oxygen content stimulates vital activity (programmes of agriculture, health care, refuses deactivation). A parallel product of air separating is nitrogen being sufficiently neutral gas which atmosphere food stuffs are preserved for a longer and ignition is prevented.
Zeolites form the big class of minerals and differ by ‘windows’ and pores sizes. Using different zeolites it is possible to get membranes to separate other gas mixes, i.e. it is wide application spectrum technology. For a long term, it can tell to create by use of zeolites new polycrystalline materials large series.
Patents, physical mechanisms of zeolitic membranes O2 and N2 molecules selection and other information are presented on a personal site http://www.hot.ee/romulus1 (RUS ENG) the same name article. It contains 18 pages, 10 figures and photos.
Authors express thanks to A.Borshchevski for useful discussions and helping in graphic design.
31.08.2006 Tallinn Estonia
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