Electric heating – Metal heating – By arc
Reexamination Certificate
1998-12-28
2001-05-22
Walberg, Teresa (Department: 3742)
Electric heating
Metal heating
By arc
C356S316000, C219S121590
Reexamination Certificate
active
06236012
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a plasma torch intended for the excitation of a gas for the purpose of analyzing it.
The invention also relates to a gas analyzer using such a plasma torch.
(ii) Description of Related Art
Currently, gas analysis techniques are indirect techniques, such as filtration, hydrolysis or sparging, in which the impurities, the concentration of which is to be determined, are extracted from the gas before analysis.
Thus, for example, the filtration analysis technique uses a membrane for filtering the gas to be analyzed for the purpose of retaining the impurities that it contains. Next, these impurities are dissolved in an acid solution and then analyzed, for example by spectroscopy, for the purpose of determining the nature and concentration thereof.
These conventional analytical techniques have a number of drawbacks.
First of all, because of their nature and particularly because there is a step of extracting the particles to be analyzed, these techniques are not suitable for the quality of a gas to be analyzed continuously.
Furthermore, they give relatively inaccurate results since these techniques only allow an average concentration value corresponding to the total amount of the sample to be obtained. They therefore do not allow instantaneous variations in the concentrations to be detected.
In addition, some impurity particles are likely to be in the form of volatile compounds which cannot be extracted from the gas using such techniques. The result obtained is thus likely to be underestimated.
Finally, these techniques entail a not insignificant risk of contaminating the gas and require relatively complex equipment.
It has been attempted to remedy these drawbacks by using a direct gas analysis technique.
According to this technique, a gas sample to be analyzed is introduced into a heat source, such as a plasma, capable of dissociating the chemical species present in the sample into free atoms and then of exciting and optionally ionizing the atoms obtained. Next, these excited atoms are detected by measuring the various wavelengths that they emit or, if they are ionized, by measuring their mass.
Although this technique allows a gas to be analyzed continuously it also has a number of drawbacks, especially because of the gas recirculation movements produced by the action of Lorentz forces near the inductor used for generating the plasma.
These recirculation movements will force the gas to the periphery of the plasma and cause the decomposition products to be deposited on the torch and therefore cause undesirable contamination of the latter, impeding optical detection, as well as modifying the energy transfer between the induction coil and the plasma.
Moreover, the gas flowing in this peripheral region undergoes less excitation, thereby helping to reduce the accuracy of the measurement.
The studies completed by the Applicant on this subject have moreover demonstrated that, depending on the nature of the gas to be analyzed (for example, depending on whether or not it is a diatomic gas), there is a major risk of the plasma being blown out when introducing the gas to be analyzed into this plasma.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the invention is to overcome the aforementioned drawbacks.
The subject of the invention is therefore particularly a plasma torch for the excitation of a gas for the purpose of analyzing it, comprising an injector, configured in the form of a main tube intended to be connected to a source for supplying a gas to be analyzed, and an external cylindrical sleeve which is double walled, coaxial with the injector and defining, between its consecutive internal and external walls, a cylindrical annular channel for supplying a plasma gas, intended to be connected to a corresponding supply source for the purpose of generating a plasma at the outlet of the sleeve, being characterized in that the diameter of the injector can vary.
The plasma torch according to the invention may furthermore include one or more of the following characteristics:
the diameter of the injector can vary, taking at least two values, by adopting the following configuration: the injector is formed from at least two coaxial tubes, one internal and the other external, the internal tube being capable of sliding vertically inside the external tube;
according to one of the embodiments of the invention, the diameter of the injection tube lies in the range going from 0.8 to 3 mm;
according to one of the embodiments of the invention, the diameter of the injection tube lies in the range going from 1.3 to 2 mm;
the injector includes an additional outer tube coaxial with the main tube and defining two coaxial channels, an internal and an external coaxial channel, one of these being intended for supplying the torch with a gas to be analyzed and the other being intended to supply the torch with a gas for guiding the said gas to be analyzed in the plasma, respectively;
the plasma gas and/or the guiding gas comprise argon or helium or any other gas capable of creating a plasma, or a mixture of such gases;
the external wall of the sleeve forms the external wall of the torch;
the torch includes a coil placed near the end face of the external wall of the torch and connected to a high-frequency current source for the purpose of creating, in the path of the plasma gas, an electromagnetic field and of creating the plasma in the gas;
the torch furthermore includes an intermediate cylindrical tube coaxial with the sleeve and lying inside the sleeve, between its internal wall and its external wall, the intermediate cylindrical tube and the external wall of the sleeve defining a channel for supplying a gas for shielding the internal surface of the external wall of the torch from solid deposits;
the channel for supplying a shielding gas forms a channel for supplying a gas containing a chemical compound suitable for reacting with the solid deposits liable to be formed on the external wall of the torch in order to form a volatile compound.
The subject of the invention is also a gas analyzer, characterized in that it comprises a plasma torch as defined above, connected to a source for supplying a gas to be analyzed to a source for supplying a plasma gas and, advantageously, also to a source of gas for guiding the gas to be analyzed in the plasma generated at the outlet of the torch in the plasma gas, and optical detection means which are capable of measuring the light intensity emitted by the impurities present in the plasma and are connected to a processing unit which includes means for calculating the concentration of impurities from the measured value of the light intensity and from at least one predetermined reference value which is stored in a memory associated with the processing unit and is obtained by prior calibration.
According to one particular characteristic, the analyzer includes a unit for the production of standard specimens, comprising:
a source of solutions of dissolved salts of one or more elements;
a nebulizing unit;
a solvent-stripping unit;
one outlet of the production unit being connected to the channel for supplying the torch with a gas to be analyzed.
Further features and advantages will emerge from the following description, given by way of example, and with reference to the appended drawings:
REFERENCES:
patent: 4482246 (1984-11-01), Meyer et al.
patent: 4766287 (1988-08-01), Morrisroe et al.
patent: 5051557 (1991-09-01), Satzger
patent: 5223711 (1993-06-01), Sanderson et al.
patent: 5233156 (1993-08-01), Chan et al.
patent: 5908566 (1999-06-01), Seltzer
patent: 0 263 031 (1988-04-01), None
patent: 0 296 921 (1988-12-01), None
patent: 0 358 212 (1990-03-01), None
patent: 0 397 468 (1990-11-01), None
Patent Abstracts of Japan, vol. 13, No. 1 (P-808), Jan. 6, 1989 & JP 63 210754 A (Shimadzu Corp) Sep. 1, 1988.
Trassy et al., “Dosage des éléments métalliques dans les gaz. Etude d'une méthode directe” Journal of High Temperature Chemical Processes, No. 2, Dec. 1993, pp. 439-447, XP-002076765.
Patent
Carre Martine
Coffre Eric
Trassy Christian
Burns Doane Swecker & Mathis L.L.P.
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Van Quang
Walberg Teresa
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