JP2007057295A - Catalytic combustion type gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalytic combustion type gas sensor for sensing an alcohol or DME with high sensitivity from a low to high concentration. <P>SOLUTION: The catalytic combustion type gas sensor has a heating control device having a bridge circuit, which has a bridge circuit formed of a detection element wherein a conductive coil is covered with a combustion catalyst, a compensation element obtained by coating the conductive coil with the combustion catalyst and a bridge resistor and holding the detection element and the compensation element to a predetermined temperature within a range of 250-350°C and the adsorbent arranged to the flow channel for introducing a specimen gas into the compensation element to adsorb alcohol and/or dimethyl either. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a catalytic combustion type gas sensor that senses alcohol and dimethyl ether with high sensitivity.

  In recent years, with the decrease in oil reserves, the supply has become unstable and the price of crude oil has risen. Moreover, petroleum-based fuels not only generate CO, which causes global warming, but also emit various harmful gases. Also, petroleum-based fuels cause environmental pollution by simply flowing out even if they do not burn. Therefore, various attempts have been made to put to practical use a new fuel that can prevent global warming and environmental pollution and can be stably supplied.

  In particular, alcohol (for example, methyl alcohol, ethyl alcohol, etc.) and dimethyl ether (hereinafter referred to as DME) are attracting attention as next-generation fuels. Alcohol and DME can be produced from raw materials other than petroleum and are highly volatile liquids. Therefore, when such liquid leaks from a pipe or tank in an alcohol or DME production plant, it instantly evaporates. Therefore, in an alcohol or DME manufacturing plant, in order to detect such leakage, it is necessary to detect alcohol and DME in the air using a sensor to prevent ignition and explosion.

Sensors that detect specific components in the air (hereinafter referred to as gas sensors) are roughly classified into semiconductor type gas sensors and catalytic combustion type gas sensors.
The semiconductor gas sensor can detect low concentration gas components with high sensitivity, but the selectivity of the gas components to be detected is inferior. There are drawbacks such as difficulty in removing and regenerating.

  On the other hand, the catalytic combustion type gas sensor is excellent in the selectivity of the gas component to be detected, and it is easy to detect a high concentration gas component and to remove and regenerate the adsorbed component. Suitable as However, the sensitivity is poor when sensing low concentration gas components. For this reason, various techniques for increasing the sensitivity of the catalytic combustion type gas sensor for low concentration gas components have been studied.

For example, Patent Document 1 discloses a catalytic combustion type gas sensor that senses CO. The catalytic combustion type gas sensor disclosed in Patent Document 1 increases the sensitivity to CO by adding Ag and Pt to the combustion catalyst of the detection element to reduce the sensitivity to H ₂ and C ₂ H ₅ OH. It is a thing. However, in this technique, Ag and Pt are added to the combustion catalyst of the sensing element, but Ag and Pt are not added to the combustion catalyst of the compensation element, so that a combustion catalyst having different components is used in the sensing element and the compensation element. Become. Therefore, when the CO concentration is low, the CO sensing reproducibility is not always sufficient. That is, the catalytic combustion type gas sensor disclosed in Patent Document 1 has room for improvement from the viewpoint of increasing sensitivity to low concentrations of CO.

Applying the technique described in Patent Document 1 to use as a catalytic combustion type gas sensor that senses alcohol or DME in the air, the sensitivity of the detection element to CO or H ₂ is lowered in the combustion catalyst. By adding an element, the sensitivity to alcohol and DME is increased. That is, since combustion catalysts having different components are used for the detection element and the compensation element, sufficient reproducibility cannot be obtained when sensing low-concentration alcohol or DME.

As described above, considering that the use of a catalytic combustion type gas sensor that senses alcohol and DME is detection of leakage in a manufacturing plant, a small amount of alcohol vaporized in the air (ie, low concentration) and DME Must be detected with high sensitivity. However, even if the technique described in Patent Document 1 is applied to the detection of alcohol or DME, it cannot be expected to detect low-concentration alcohol or DME with high sensitivity.
JP 2002-71614 A

  An object of the present invention is to provide a catalytic combustion type gas sensor that solves the above problems and senses alcohol and DME with high sensitivity from a low concentration to a high concentration.

  The present invention provides a bridge circuit formed using a sensing element formed by coating a conductive coil with a combustion catalyst, a compensation element formed by coating the conductive coil with the combustion catalyst described above, and a bridge resistance. And a heating control device that maintains the sensing element and the compensation element at a predetermined temperature within a range of 250 to 350 ° C., and is disposed in a flow path for introducing the analyte gas into the compensation element. It is a catalytic combustion type gas sensor having an adsorbent that adsorbs dimethyl ether. Here, the conductive coil of the detection element and the compensation element uses the same metal wire. Moreover, the combustion catalyst which coat | covers those electroconductive coils uses the catalyst of the same material.

In the catalytic combustion type gas sensor of the present invention, it is preferable that the conductive coil of the sensing element and the conductive coil of the compensation element have the same shape. Here, the shape of the conductive coil refers to the diameter and length of the metal wire formed into the coil, and the diameter, pitch, and number of turns of the formed coil.
Moreover, it is preferable to use a nickel wire or a nickel alloy wire as the conductive coil.

Furthermore, it is preferable to use cobalt oxide as a combustion catalyst. Copper oxide may be mixed with cobalt oxide.
As the adsorbent, activated carbon may be used, or a molecular sieve (for example, synthetic zeolite) called molecular sieve (trade name) may be used. Activated carbon and molecular sieve (for example, synthetic zeolite) may be used alone or in combination.

  According to the present invention, alcohol and DME can be sensed with high sensitivity from a low concentration to a high concentration. Therefore, the catalytic combustion type gas sensor of the present invention can be used without any trouble in order to detect the leakage of alcohol or DME at a production plant of alcohol or DME.

  FIG. 1 is a circuit diagram showing an example of a bridge circuit of a catalytic combustion type gas sensor of the present invention. In the catalytic combustion type gas sensor of the present invention, the detection element 2, the compensation element 3, and the bridge resistors 5a, 5b, and 5c form a bridge circuit 1 as shown in FIG. In addition, although the example which arrange | positions the detection element 2 and the compensation element 3 in series was shown in FIG. 1, you may arrange | position the detection element 2 and the compensation element 3 in parallel.

In FIG. 1, the detection element 2 and the compensation element 3 are indicated by symbols used in the circuit diagram. FIG. 2 is a perspective view schematically showing an example of the detection element 2 and the compensation element 3. In the catalytic combustion type gas sensor of the present invention, since the detection element 2 and the compensation element 3 have the same configuration, the detection element 2 will be described first with reference to FIG.
As shown in FIG. 2, the detection element 2 is obtained by covering a conductive coil 7 with a combustion catalyst 8. The metal wire 9 that forms the conductive coil 7 is a metal wire 9 that is conductive and generates heat when energized. In order to detect low-concentration alcohol or DME, it is preferable to use a nickel wire or a nickel alloy wire as the conductive coil 7.

  The combustion catalyst 8 that covers the conductive coil 7 uses an active material for alcohol and DME. The alcohol and DME are oxidized (that is, burned) on the surface of the combustion catalyst 8 and the temperature of the detection element 2 rises. Sense the presence of alcohol and DME. The principle will be described later. In order to sense low concentrations of alcohol and DME, it is preferable to use cobalt oxide as the combustion catalyst 8. Copper oxide may be mixed with cobalt oxide.

  The combustion catalyst 8 is electrodeposited on the conductive coil 7 by an electrodeposition coating method. At this time, the combustion catalyst 8 may be formed in a columnar shape (that is, a hollow portion is not provided inside), but the combustion catalyst 8 is formed in a cylindrical shape as shown in FIG. 8 is preferably embedded. The reason is that by forming the combustion catalyst 8 in a cylindrical shape as shown in FIG. 2, the contact area between the alcohol and DME and the combustion catalyst 8 is increased, and the sensitivity to low concentrations of alcohol and DME is increased. is there.

Next, the compensation element 3 used in the catalytic combustion type gas sensor of the present invention will be described with reference to FIG.
The conductive coil 7 of the compensation element 3 uses a metal wire made of the same material as the conductive coil 7 of the detection element 2. Further, it is preferable that the conductive coil 7 of the compensation element 3 and the detection element 2 have the same shape. The shape of the conductive coil 7 refers to the diameter and length of the metal wire 9 to be formed into the coil, and the diameter, pitch, and number of turns of the formed conductive coil 7.

  The combustion catalyst 8 of the compensation element 3 is also made of the same material as the combustion catalyst 8 of the detection element 2. Further, it is preferable that both the compensation element 3 and the combustion catalyst 8 of the detection element 2 have the same shape. The shape of the combustion catalyst 8 indicates an outer diameter (and inner diameter in the case of being formed in a cylindrical shape) and a length formed in a cylindrical shape or a columnar shape. By unifying the conditions for electrodeposition coating, the combustion catalyst 8 can be formed in the same shape.

In this way, the compensation element 3 having the same electrical characteristics as the sensing element 2 can be manufactured.
Next, the structure of the gas detection part of the contact combustion type gas sensor of the present invention will be described. FIG. 3 is a cross-sectional view schematically showing an example of the gas sensing unit. As shown in FIG. 3, the gas sensing unit 15 is configured by providing an inlet and an outlet for the specimen gas 14 in a sealed space.

  The gas sensing unit 15 is installed in a state where the sensing element 2 and the compensating element 3 are electrically insulated. The detection element 2 and the compensation element 3 are both protected by a protective tube 11 in order to prevent damage due to foreign matter in the sample gas 14. A mesh-like plate material 12 (hereinafter referred to as a mesh material) is attached to the opening at the upper end of the protective tube 11. This mesh material 12 prevents foreign matter in the sample gas 14 from entering the protective tube 11 and introduces only the sample gas 14 into the protective tube 11.

An adsorbent 13 is disposed in the flow path for introducing the sample gas 14 into the compensation element 3. As the adsorbent 13, a substance that adsorbs alcohol and DME is used. For example, activated carbon or molecular sieve (for example, synthetic zeolite) can be used. Activated carbon and molecular sieve (for example, synthetic zeolite) may be used alone or in combination.
The sensing element 2 and the compensation element 3 installed in the gas sensing unit 15 generate heat when energized to the metal wire 9 (that is, the conductive coil 7), but each of them is 250 to 350 ° C. by a heating control device (not shown). Is maintained at a predetermined temperature within the range of. When the temperature of the detection element 2 and the compensation element 3 is less than 250 ° C., the combustion of the specimen gas 14 described later becomes insufficient, and the sensitivity of the catalytic combustion type gas sensor is lowered. On the other hand, when the temperature of the sensing element 2 and the compensating element 3 exceeds 350 ° C., not only the life of the conductive coil 7 is shortened, but also the reactivity to unsaturated hydrocarbons (for example, propylene) occurs. Sensitivity to alcohol and DME decreases.

  In this way, the detection element 2 and the compensation element 3 installed in the gas detection unit 15 are heated to a predetermined temperature in the range of 250 to 350 ° C. and come into contact with the sample gas 14. However, the specimen gas 14 that comes into contact with the combustion catalyst 8 of the detection element 2 contains alcohol or DME, but the specimen gas 14 that comes into contact with the combustion catalyst 8 of the compensation element 3 does not contain alcohol or DME. Therefore, alcohol and DME are combusted on the surface of the combustion catalyst 8 of the detection element 2, and the temperature of the detection element 2 rises above the temperature maintained by the heating control device. On the other hand, the temperature of the compensation element 3 is kept constant by the heating control device. That is, the temperature difference between the sensing element 2 and the compensation element 3 is caused by the combustion of alcohol or DME.

If the temperature of the sensing element 2 changes, its resistance value also changes. Therefore, alcohol or DME can be sensed by detecting a change in the resistance value caused by the temperature rise of the sensing element 2 by the bridge circuit 1.
When the sample gas 14 includes not only alcohol and DME but also other combustible gases (for example, H ₂ , CO, etc.), the combustible gas contacts the detection element 2 and the compensation element 3 and burns. That is, the temperature of the detection element 2 and the compensation element 3 both rises equally by the combustion of combustible gas. Therefore, even when the specimen gas 14 contains other combustible gas, the temperature difference between the detection element 2 and the compensation element 3 is caused by the combustion of alcohol or DME. By detecting the change by the bridge circuit 1, alcohol or DME can be sensed.

As described above, the catalytic combustion type gas sensor of the present invention can detect alcohol and DME with high sensitivity from a low concentration to a high concentration.
The adsorbent 13 gradually reduces its adsorption effect by adsorbing alcohol or DME in the sample gas 14. If the adsorbent 13 whose adsorption effect is reduced is continuously used, the sensitivity of the catalytic combustion type gas sensor is lowered. Therefore, it is necessary to periodically check the adsorption effect of the adsorbent 13 using a check gas. The inspection gas is a gas with a certain amount of alcohol or DME added. If the contact combustion gas sensor cannot detect alcohol or DME in the inspection gas, it means that the adsorbing effect of the adsorbent 13 has disappeared. . In that case, the adsorbent 13 is replaced with a new one.

  The adsorbent 13 disposed in the flow path for introducing the specimen gas 14 to the compensation element 3 can be easily attached and detached (for example, a cassette method), so that the adsorbent 13 can be replaced in a short time. It becomes.

  The sensing element 2 and the compensating element 3 shown in FIG. 2 were manufactured and installed in the gas sensing unit 15 shown in FIG. 3 to form the bridge circuit 1 shown in FIG. The conductive coil 7 of the detection element 2 and the compensation element 3 has a coil diameter of 0.8 mm, a pitch of 0.1 mm, and a number of turns of 22. The metal wires 9 formed on the conductive coil 7 were all nickel wires having a diameter of 0.018 mm and a length of 55 mm. The combustion catalyst 8 was a mixture of cobalt oxide and copper oxide, and was formed in a cylindrical shape (outer diameter 1.2 mm, inner diameter 0.2 mm, length 2.5 mm) as shown in FIG.

As the adsorbent 13, activated carbon was used. The protective tube 11 used a stainless steel cap, and the mesh material 12 used 40 mesh.
The temperature of the sensing element 2 and the compensation element 3 installed in the gas sensing unit 15 was maintained at 280 ° C., and a test sample gas was supplied to the gas sensing unit 15 to perform a sensing test. As the test sample gas, a gas in which alcohol (that is, C ₂ H ₅ OH) is added to air at various concentrations (that is, 50 volume ppm, 100 volume ppm, 1000 volume ppm, 10000 volume ppm) is used. A sensing test was performed twice for the test sample gas. In addition, a gas in which DME is added to air at various concentrations (that is, 50 volume ppm, 100 volume ppm, 1000 volume ppm, and 10000 volume ppm) is used as a test sample gas. Sensitive tests were performed one by one. The results are shown in Table 1.

As is clear from Table 1, C ₂ H ₅ OH and DME were detected for all the test sample gases. Therefore, it was confirmed that by using the catalytic combustion type gas sensor of the present invention, alcohol and DME can be sensed with high sensitivity from a low concentration to a high concentration. Moreover, the reproducibility of alcohol and DME sensing was recognized.

It is a circuit diagram which shows the example of the bridge circuit of the contact combustion type gas sensor of this invention. It is a perspective view which shows typically the example of the detection element and compensation element which are used with the contact combustion type gas sensor of this invention. It is sectional drawing which shows typically the example of the gas detection part of the contact combustion type gas sensor of this invention.

Explanation of symbols

1 Bridge circuit 2 Sensing element 3 Compensating element 4 Gas sensitivity meter
5a Bridge resistance
5b Bridge resistance
5c Bridge resistance 6 Power supply 7 Conductive coil 8 Combustion catalyst 9 Metal wire
10 Insulation material
11 Protection tube
12 Mesh material
13 Adsorbent
14 Sample gas
15 Gas detector

Claims (5)

  A sensing element comprising a conductive coil covered with a combustion catalyst; a compensation element comprising the conductive coil covered with the combustion catalyst; and a bridge circuit formed using a bridge resistor, the sensing element And a heating control device that maintains the compensation element at a predetermined temperature within a range of 250 to 350 ° C., and is disposed in a flow path for introducing a sample gas into the compensation element to adsorb alcohol and / or dimethyl ether. A catalytic combustion type gas sensor characterized by having an adsorbing agent.   The catalytic combustion type gas sensor according to claim 1, wherein the conductive coil of the sensing element and the conductive coil of the compensation element have the same shape.   The catalytic combustion type gas sensor according to claim 1 or 2, wherein a nickel wire or a nickel alloy wire is used as the conductive coil.   The catalytic combustion type gas sensor according to claim 1, 2 or 3, wherein cobalt oxide is used as the combustion catalyst. 5. The catalytic combustion type gas sensor according to claim 1, wherein activated carbon and / or molecular sieve are used as the adsorbent.

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