Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Reexamination Certificate
1999-08-11
2001-02-20
Wong, Edna (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C204S158200, C204S157300, C423S210000
Reexamination Certificate
active
06190510
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for purifying a gas, in particular a plasmagenic rare gas, intended for example for the purification of krypton and xenon containing tetrafluoromethane and methane, and a purification apparatus for implementing such a process.
2. Description of the Related Art
Rare gases are generally extracted from air by distillation. After distillation, they contain impurities that have to be extracted before they are used.
Thus, in the case of krypton and xenon, the impurities mainly consist of tetrafluoromethane (CF
4
) and methane (CH
4
) at levels which may be as high as several hundreds of parts per million by volume (ppmv). The commonest applications of these rare gases require high-purity products and the presence of these foreign molecules is an unacceptable drawback for these applications.
For example, krypton is widely used for the partial-vacuum filling of filament lamps. In this application, the very high temperature to which the tungsten filament in these lamps is heated is sufficient to cause dissociation of the tetrafluoromethane into highly corrosive fluorinated radicals, which causes rapid degradation of the filament by etching of the metal.
Furthermore, methane is also dissociated under these temperature conditions and generates unstable gaseous precursors which form carbon-containing solid deposits of a brownish appearance on the internal surface of the bulb.
As is known and for technical reasons, the purification of rare gases cannot be accomplished by distillation.
The purification techniques known hitherto are essentially based on thermal phenomena.
Such techniques allow CH
4
to decompose and/or oxidize easily, but they are unsuitable for eliminating CF
4
, which is an extremely stable and barely reactive molecule.
Moreover, since the CF
4
molecule does not react significantly with any solid or liquid medium at room temperature, the chemical conversion of CF
4
requires very high reaction temperatures to be reached and a considerable enthalpy to be transferred to the gaseous medium.
If a combustion technique is used, very high flow rates of combustible gas, generally hydrogen, are required, which raises treatment-cost problems, safety problems and plant-constraint problems because of the heat released. Moreover, such a technique is not suitable for reducing already low impurity concentrations to a negligible level.
Another known purification technique is based on thermochemical decomposition by reaction over a suitable metal, for example zirconium. This technique is relatively effective and does not require a very large supply of combustible gas. However, it has many disadvantages, in particular because of the fact that it is not possible to purify gases with a high flow rate and that its implementation requires an apparatus which takes up a lot of room.
Furthermore, this technique requires quite a high operating temperature, for example about 900 to 1000° C., and the beds of metals used are very expensive and have a limited lifetime.
For the reasons explained above, the purification of rare gases currently constitutes the main cause of limited efficiency of lines for producing these gases.
SUMMARY OF THE INVENTION
The object of the invention is to provide a gas purification process and a purification apparatus for the implementation of such a process helping to overcome the abovementioned drawbacks.
The subject of the invention is therefore a process for purifying a gas, characterized in that it includes the steps consisting in:
making the gas to be purified flow through a hollow dielectric tube;
creating an electric field in the gas by means of a travelling electromagnetic wave which travels in the dielectric tube and is suitable for creating, in the gas, an atmospheric-pressure plasma which is not in local thermodynamic equilibrium for the purpose of dissociating the impurities in the gas in order to form reactive compounds; and
making the reactive compounds formed react with a corresponding reactive element for the purpose of eliminating them from the gas to be purified.
The process according to the invention may furthermore include one or more of the following characteristics:
the electromagnetic wave is a surface wave produced by means of a surface-wave exciter of the surfatron-guide type;
the electric field being created in a region of the dielectric tube, it furthermore includes a step of adjustment of the longitudinal dimension of the region in order to adjust the length of the plasma;
oxygen is added to the gas to be purified, prior to the step of making the gas flow through the dielectric tube;
the step consisting in making the reactive compounds react consists in making the reactive compounds react with an alkaline element, in particular soda lime or an alkaline aqueous solution;
the process furthermore includes a step of dehydration of the gas after the step of reacting the reactive compounds with a corresponding reactive element;
the rare gas to be purified is composed of krypton or of xenon;
the impurities in the gas are selected from methane and perfluorinated gases, in particular tetrafluoromethane.
The subject of the invention is also an apparatus for purifying a gas, for the implementation of a purification process, characterized in that it includes at least one high-frequency travelling-wave exciter combined with a waveguide suitable for guiding the travelling waves produced by the at least one wave exciter to at least one hollow dielectric tube in which the gas is intended to flow, in order to create therein an atmospheric-pressure plasma for ionizing and exciting the molecules of the gas to be purified for the purpose of dissociating the impurities in the gas in order to form reactive compounds, in particular fluorinated compounds, and at least one unit for treating the reactive compounds being arranged on the exit side of a corresponding hollow dielectric tube.
This apparatus may furthermore include one or more of the following characteristics:
each exciter consists of a surface-wave exciter and a cylindrical sleeve made of conductive material, inside which is mounted a corresponding dielectric tube, and includes a region for concentrating the incident waves in a space lying between the internal wall of the exciter and a free end of the sleeve for the purpose of creating therein an electric field coaxial with the tube;
the sleeve includes an inner cylinder, in which the dielectric tube is mounted, and an outer cylinder which, with the inner cylinder, delimits a cooling chamber supplied with a liquid coolant;
the sleeve and the dielectric tube are separated by a cylindrical space and the apparatus includes means for supplying the space with a gaseous coolant;
the dielectric tube consists of silica;
each exciter is provided with impedance-adjustment means which include a plunger forming a quarter-wave trap which is coaxial with the sleeve and mounted so as to be able to move axially in the exciter;
the apparatus furthermore includes means for adjusting the longitudinal dimension of the plasma created in the hollow dielectric tube;
the wall of each exciter is thinned down close to the incident-wave concentration region;
the thinned-down part of the wall of each exciter consists of an added piece welded to the rest of the wall;
the at least one unit for treating the reactive compounds each consist of a soda-lime cartridge;
the apparatus includes a humidifier arranged upstream of the soda-lime cartridge;
the apparatus furthermore includes a unit for dehydrating the gas to be purified, arranged downstream of the unit for treating the reactive compounds;
the surface-wave exciters each consist of an exciter of the surfatron-guide type.
REFERENCES:
patent: 3909206 (1975-09-01), Katz
patent: 5137701 (1992-08-01), Mundt
patent: 5360485 (1994-11-01), Bou et al.
patent: 5453125 (1995-09-01), Krogh
patent: 5750823 (1998-05-01), Wofford et al.
patent: 43 19 118 (1994-12-01), None
patent: 0781599 (1997-07-01), None
patent: 0820801 (1998-01-01), None
Bryselbout Francis
Moisan Michel
Parent Jean-Claude
Rostaing Jean-Christophe
Burns Doane Swecker & Mathis L.L.P.
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Wong Edna
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