Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – With means applying electromagnetic wave energy or...
Reexamination Certificate
1998-04-27
2001-05-01
Gorgos, Kathryn (Department: 1741)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
With means applying electromagnetic wave energy or...
C118S72300R
Reexamination Certificate
active
06224836
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 97-05,147 filed in France on Apr. 25, 1997; the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a device for exciting a gas, of the surfaguide type, in which the gas is excited by a surface wave plasma, in particular an atmospheric-pressure surface wave plasma.
The invention also relates to an apparatus for treating a gas incorporating such an excitation device.
Another effective exciting device for this application is known by the name “surfatron-guide”.
(ii) Description of Related Art
One particularly advantageous application example of these types of devices is the plasma treatment of a chemically non-reactive gas containing impurities consisting of perfluorinated greenhouse-effect gaseous compounds or of volatile organic compounds.
To do this, the gas to be treated and the impurities which it contains are placed in an electric field which is intense enough to produce an electrical discharge by ionizing the gas molecules, this discharge being caused by stripping off electrons from the initially neutral gas molecules.
Under the action of the discharge, the molecules of the gas are dissociated in order to form radicals of smaller sizes than the initial molecules and, consequently, when appropriate, individual atoms, these atoms or fragments of molecules thus excited not appreciably giving rise to any chemical reaction.
Thus, after passing through the discharge, the gas atoms or molecules become de-excited and recombine respectively, before becoming intact again on leaving the discharge.
In contrast, the impurities undergo, by excitation, irreversible dissociation and irreversible transformation by forming new molecular fragments having chemical properties different from those of the initial molecules, which are consequently capable of being extracted from the gas by an appropriate subsequent treatment.
A surfatron-guide has a hollow structure made of an electrically conductive material, having a first end closed off by a moveable waveguide plunger forming a short-circuit and a second part which extends perpendicularly to the first part and in which is coaxially mounted a tube made of a dielectric material, through which tube the gas to be treated flows.
The second part is provided with a tuning plunger which can move axially in order to adapt the impedance of the device.
This type of electromagnetic field applicator is satisfactory for creating a surface wave plasma at atmospheric pressure.
However, it has a certain number of drawbacks, in particular due to its cost, because of the greater complexity of its construction.
However, another type of gas-exciting device is known, this being called a “surfaguide”.
This type of excitation device has a hollow structure forming a waveguide, made of electrically conductive material, which is intended to be connected to a microwave generator provided with a passage through which a hollow discharge tube made of a dielectric material is intended to pass, the gas to be excited flowing through the tube, and with a wave-concentrating region designed to concentrate the microwave radiation produced by the generator onto the tube, during operation of the device, for the purpose of producing a surface wave plasma in the gas.
The surfaguide has no tuning plunger and is therefore less expensive than the surfatron-guide. Furthermore, the length of the plasma created by the surfaguide is, for the same power, slightly longer than that of the plasma created by the surfatron-guide.
However, the density of the plasma column produced by the surfatron-guide is locally higher than for the surfaguide.
In addition, under certain operating conditions, the surfaguide is less effective than the surfatron-guide when discharge tubes having a diameter greater than 20 mm are used at a frequency of 2.45 Ghz.
Moreover, for high operating powers, radiation losses occur in the environment of the surfaguide, these being highly prejudicial to the energy balance of the device and also causing reliability and safety problems.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the invention is to help to overcome the drawbacks of the devices of the state of the art and to provide a device for exciting a gas which is less expensive than the surfatron-guide and is capable also of working at atmospheric pressure.
The subject of the invention is therefore a device for exciting a gas, of the surfaguide type, comprising a hollow structure forming a waveguide, made of an electrically conductive material, this hollow structure being intended to be connected to a microwave generator and provided with a passage through which a hollow dielectric tube is intended to pass, the gas to be excited flowing through the tube, and with a wave-concentrating region designed to concentrate the microwave radiation produced by the generator onto the tube, during operation of the device, for the purpose of producing a surface wave plasma in the gas, characterized in that it furthermore includes at least one electromagnetic screening sleeve, made of a conductive material, fastened to the structure and extending along the extension of the passage so as to surround the hollow tube.
The exciting device according to the invention may furthermore include one or more of the following characteristics:
the hollow structure forming a wave-guide has a longitudinal general shape and includes a first open end intended to be connected to the microwave generator, a second open end intended to be provided with means forming a guide short-circuit, and a region of narrowed cross-section which extends between the first end and the second end and delimits the wave-concentrating region;
the region of narrowed cross-section includes a central part of constant cross-section equipped with the passage and extending between two parts of cross-sections which increase linearly towards the ends;
the at least one sleeve has a length at least equal to the length of the plasma created in the gas;
the free end of each sleeve has a flange provided with a hole for passage of the dielectric tube;
the at least one sleeve has a length equal to the sum of the length of the plasma and of the wavelength of the microwave radiation in vacuum;
the wall of the at least one sleeve is provided with at least one orifice for viewing the plasma, the dimensions of which are designed to prevent penetration of the radiation;
the at least one sleeve has a cylindrical general shape of cross-section at least equal to twice the cross-section of the hollow tube;
it includes two sleeves which extend along the extension of one with respect to the other, on each side of the central part;
each sleeve includes an end mounting plate, each mounting plate extending laterally beyond the central part for the purpose of fixing the sleeves to the structure, by bolting the mounting plates together; and
the diameter of the passage is greater than the external diameter of the hollow tube.
The subject of the invention is also an apparatus for treating a gas, comprising a device for exciting the gas which is connected to a microwave generator and through which a hollow dielectric tube passes, the gas to be excited flowing through the tube, the device comprising means for concentrating the microwave radiation produced by the generator onto the dielectric tube so as to produce, in the gas, an atmospheric plasma for ionizing and exciting the molecules of the gas to be treated for the purpose of forming reactive gaseous compounds, the apparatus furthermore including at least one unit for treating the reactive compounds, these units being placed on the downstream side of the hollow dielectric tube, characterized in that the device for exciting the gas consists of an excitation device as defined above.
Other features and advantages will emerge from the following description, given solely by way of example and with reference to the appended drawings.
REFERENCES:
patent: 4698822 (1987-10-01), Leprince et al.
pa
Etemadi Roxane
Moisan Michel
Rostaing Jean-Christophe
Burns Doane Swecker & Mathis L.L.P.
Gorgos Kathryn
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Tran Thao
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