Method for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit and device for carrying out the method

Abstract

Method for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit (18) with a conveying channel (10) through which the sample gas stream flows from the exhaust gas analysis unit and a first catalyst (30) in the conveying channel (10) with a catalytically active surface made of one or more metals and / or metal oxides, characterized in that the first catalyst (30) is operated at a temperature of 180°C to 250°C and downstream of the first catalyst (30) in the conveying channel (10) there is arranged a second catalyst (34) with a catalytically active surface made of one or more metals and / or metal oxides, which is operated at a temperature of 10°C to 80°C.

Description

[0001] The invention relates to a method for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit and a device for carrying out the method with a conveying channel through which the sample gas stream from the exhaust gas analysis unit flows and a first catalyst in the conveying channel with a catalytically active surface made of one or more metals and / or metal oxides.

[0002] Exhaust gas analysis units analyze exhaust gases, for example, from internal combustion engines. Due to legislation, particular attention is paid to pollutants produced in engines and emitted into the air, for example, by vehicles. These pollutants include, in particular, nitrogen monoxide, the concentration of which in exhaust gases is usually determined using chemiluminescence analysis.

[0003] This analysis is performed in an exhaust gas measurement system by adding ozone to the exhaust gas, triggering a spontaneous reaction in which nitrogen monoxide and ozone are converted into oxygen and nitrogen dioxide. A portion of the resulting nitrogen dioxide is in an excited electron state. The molecules spontaneously release this excess energy in the form of optically measurable radiation, which is proportional to the nitrogen monoxide concentration in the sample gas.

[0004] Since the described reaction only applies to nitrogen monoxide molecules, the nitrogen dioxide components in the exhaust gas are reduced to nitrogen monoxide before entering the analysis chamber. This occurs in a thermal or thermal / catalytic converter at temperatures above 200°C. Furthermore, ozone is generated from oxygen in an ozone generator immediately upstream of the chemiluminescence detector for the reaction. To achieve a light output proportional to the nitrogen oxide concentration, the volume flow of the exhaust gas is kept constant. The radiation is converted into an electrical signal using a cooled photodetector. This signal is fed to the evaluation electronics as a measure of the nitrogen oxide emission.

[0005] However, the problem is that the ozone is not completely converted in the chemiluminescence detector, but is pumped from the analysis chamber into the downstream lines. However, because ozone is a strong oxidizing agent, as well as toxic and reactive, and can cause respiratory irritation, it is not permitted to remove the remaining ozone from the system via the ambient air. For this reason, catalysts are installed downstream of the chemiluminescence detectors in exhaust gas measurement systems, where the ozone is catalytically converted to oxygen.

[0006] Catalysts include pure metal catalysts, mixed metal oxide catalysts or activated carbon catalysts.

[0007] US2007 / 0154375 A1 discloses a device for air purification in aircraft in which two catalysts are connected in series. The first catalyst consists of a metallic foam that serves to absorb inorganic materials from the aircraft, such as oily substances. A second catalyst for ozone purification is connected downstream of this.

[0008] Furthermore, WO00 / 13772 A1 discloses a two-stage ozone trap, wherein the first catalytically active coating is either a manganese oxide or a palladium-containing material, and the second catalyst is platinum, rhodium-containing materials, or silver oxides. The first catalyst may additionally contain copper oxides. The reaction takes place at low temperatures of approximately 38°C.

[0009] For example, DE 39 20 428 A1 discloses a supported catalyst for the decomposition of ozone at very low ozone concentrations. Hopcalite, a catalytically active material composed of manganese oxide and copper oxide, is applied to a foamed, open-pore organic polymer as a support. The polymer can be made, for example, from polyurethane, with the catalytically active component present in particles with a size of 10 to 500 µm. This catalyst can be used to decompose ozone at temperatures below 50°C. The prior art also includes the publications DE 30 42 455 A1 and US 2010 / 0226845 A1.

[0010] Furthermore, DE 44 31 139 A1 discloses a process in which ozone-containing gases or liquids are first freed of ozone by catalysis and then by contact with an ozone-binding polymer. The catalysis can be carried out using catalysts based on precious metals or inorganic oxides, while polyarylene thioethers, polyarylene ethers, or other macroporous copolymers are used as the ozone-binding polymer.

[0011] Furthermore, DE 690 21 267 T2 discloses a process for adsorption combustion / decomposition in which a catalyst, an adsorbent and a catalyst are arranged one behind the other upstream in the flow direction of the gas.

[0012] However, the problem with ozone decomposition in an exhaust gas measuring system downstream of a chemiluminescence detector is that the relatively high ozone content leads to damage to seals and lines in the system, so the ozone should be decomposed as directly downstream of the detector as possible. However, this has the disadvantage that the catalyst is exposed to the hot exhaust gas stream, whereby it cannot be guaranteed that the catalytically active layer will remain active in this environment over a long period of time, or that defects could form that lead to deactivation of the catalytic surface. Furthermore, repercussions on the chemiluminescence detector must be minimized, as it has been shown that the use of some catalysts can result in falsified measured values due to increased flow resistance, as the flow through the analysis chamber changes.In addition, cooling or heating systems must often be used to operate the catalyst at its optimal temperature.

[0013] The task therefore arises of creating a method for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit, as well as a device for carrying out the method, which, on the one hand, ensures the most complete decomposition of the ozone possible over a long period of time and, on the other hand, avoids repercussions on the exhaust gas analysis unit. Furthermore, seals and other plastic parts of the exhaust gas measuring system should be protected as effectively as possible from the aggressive ozone. Despite the aggressive exhaust gas stream, sufficient decomposition of the ozone should be ensured even with partial deactivation of the catalyst. Energy consumption for cooling or heating should be avoided wherever possible.

[0014] This object is achieved by a method for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit having the features of main claim 1 and a device for carrying out the method according to claim 2.

[0015] By operating the first catalyst (30) at a temperature of 180°C to 250°C and arranging a second catalyst with a catalytically active surface made of one or more metals and / or metal oxides downstream of the first catalyst in the conveying channel, said second catalyst being operated at a temperature of 10°C to 80°C, it is ensured that the majority of ozone is initially decomposed in the first catalyst and any residual ozone present is removed from the sample gas. By arranging the first catalyst in the immediate vicinity of the exhaust gas analysis unit, the elevated exhaust gas and analysis temperature present there can be utilized in the downstream catalyst without the need for cooling or heating.Accordingly, the second catalyst also does not require thermal treatment, as the temperature of the exhaust gas being tested decreases with distance from the analysis unit, allowing the optimal conversion temperature to be automatically achieved here as well, thereby reducing energy consumption. Because the majority of the ozone is decomposed immediately downstream of the analysis chamber, the downstream components of the exhaust gas measurement system, and in particular the downstream plastic seals, are also protected from the aggressive ozone. This results in a significantly increased service life of the exhaust gas measurement system, as wear is reduced and the required emission values of the system are still achieved even if the first catalyst is subject to deactivation or other aging processes, such as the formation of deposits on the catalytic surface, due to the aggressive exhaust gas.

[0016] This process is carried out using a device in which a second catalyst with a catalytically active surface made of one or more metals and / or metal oxides is arranged downstream of the first catalyst in the conveying channel. The first catalyst has a catalytic surface made of copper oxide and is a bulk catalyst with electrolyte copper spheres with a diameter of 1-2 mm, and the second catalyst has a catalytic surface made of manganese oxide and copper oxide. The catalytic surface of the first catalyst is created by the rapid oxidation of the copper spheres on their surface, which then acts as a catalyst. With such a catalyst, high conversion rates for ozone are achieved. These catalysts have a conversion rate of almost 100% at high gas temperatures and corresponding temperatures in the catalyst of around 200°C.These catalysts operate reliably over long periods, even at very high ozone concentrations. The first catalyst is a bulk catalyst with electrolytic copper spheres with a diameter of 1-2 mm. Electrolytic copper is defined as copper with a purity of over 99.5%. A catalyst constructed in this way exhibits low flow resistance, reliably preventing any interference with the flow in the analysis chamber caused by the catalyst located directly behind the analysis chamber. The second catalyst has a catalytic surface made of manganese oxide and copper oxide. These catalysts are particularly suitable for achieving a conversion rate of up to 100%, even at low temperatures, particularly at ambient temperature. These catalysts are also suitable for converting very small amounts of ozone in the sample gas.

[0017] Advantageously, the first catalyst has a porosity of 0.3 to 0.45. Such a porosity provides a sufficiently low flow resistance while simultaneously providing a large effective catalytic surface area for the conversion.

[0018] The specific surface area of the second catalyst is advantageously 100 to 250 m 2 / g. Such a large specific surface area enables very good efficiencies even at low concentrations, since a large catalytically active surface is provided in a relatively small space.

[0019] Preferably, the second catalyst is a hopcalite catalyst, which is particularly suitable for use in exhaust gases at low conversion temperatures. Hopcalite can be used, for example, as a catalyst known as Carulite. ® offered material can be used, which is present as granules in the catalyst.

[0020] In a further advantageous embodiment, the second catalyst comprises a support material with a catalytically active coating. Although this usually has a higher flow resistance, it also increases the residence time with a large surface area. Efficiency can thus be ensured over a long period of time. The increased flow resistance does not have any adverse effects on the analysis chamber, since the catalyst can be positioned at a sufficient distance from the chamber.

[0021] This device is designed to decompose ozone contained in a sample gas stream from an exhaust gas analysis unit, ensuring complete conversion of the ozone to oxygen over a long period of time. At the same time, highly accurate measurement values are achieved throughout the entire service life, as feedback effects on the exhaust gas analysis unit are avoided. Energy consumption is low, as the existing temperatures of the exhaust gas and the ambient temperature are utilized in the respective catalyst stages. Damage to pumps, seals, and lines of the exhaust gas measurement system due to aggressive ozone is also avoided.

[0022] An embodiment of a device according to the invention for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit is shown in the figures and is described below.

[0023] Fig.1 schematically shows an exhaust gas measuring system with a device according to the invention for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit.

[0024] The exhaust gas measuring system consists of a feed channel 10 into which a sample gas stream, in particular an exhaust gas stream from an internal combustion engine, is drawn by means of a pump 12. The exhaust gas stream can be present either in pure form or mixed with air in a defined ratio. This exhaust gas stream contains, among other substances, nitrogen monoxide and nitrogen dioxide. In a thermal or thermal / catalytic converter 14 arranged in the feed channel 10, the nitrogen dioxide is reduced to nitrogen monoxide at temperatures above 200°C, since only the nitrogen monoxide is converted into oxygen and nitrogen dioxide in a spontaneous reaction with ozone, with a portion of the nitrogen dioxide formed being in an excited electronic state.The molecules spontaneously release this excess energy in the form of optically measurable fluorescence radiation, which is proportional to the nitrogen monoxide concentration in the sample gas, so that the radiation can serve as a measure of the nitrogen oxides present. This radiation is detected by a photomultiplier 16 of a chemilumiscence detector, which serves as the exhaust gas analysis unit 18, and converted into an electrical signal. For this purpose, the sample gas stream is fed into an analysis chamber 20 of the chemilumiscence detector 18 and mixed there with a gas stream containing ozone, which is fed to the analysis chamber 20 via a connecting line 22. This gas stream is generated in an ozone generator 24, which is supplied with oxygen or air via a supply line 26 and in which an ozone content of, for example, approximately 5% is produced in the gas stream by silent electrical discharge.

[0025] The ozone entering the analysis chamber 20 is not completely converted into oxygen or nitrogen dioxide during the reaction, so that a significant portion of the ozone leaves the analysis chamber 20 again and enters the following section 28 of the conveying channel 10.

[0026] However, ozone is an extremely reactive and aggressive gas that causes health problems in humans, particularly in the respiratory tract, which is why it must not be discharged into the room where the exhaust gas measuring system is located or into the surrounding area.

[0027] For this reason, a first catalyst 30 is often arranged directly downstream of the analysis chamber 20, within the housing of the chemiluminescence detector 18. This catalyst 30 consists of a bed of solid spheres 32 made of electrolytic copper with a diameter of approximately 2 mm, resulting in a porosity of approximately 0.4. The catalyst 30 achieves its best conversion efficiency at a temperature above 200°C, at which the converter 14 also operates. In this catalyst 30, the residual ozone from the analysis chamber 20 is ideally converted into oxygen by almost 100%, without any adverse effects on the analysis chamber 20, because this catalyst 30 has a very low flow resistance, which prevents backflow or vortexes from occurring in the analysis chamber 20. As a result, the subsequent section 28 of the conveying channel 10 and the seals no longer come into contact with the aggressive ozone.

[0028] However, due to the exhaust gas components present in the sample gas stream, it can happen that zones of the catalyst 30 are deactivated and no complete conversion takes place. To ensure that no ozone leaves the conveying channel 10, a second catalyst 34 is arranged in section 28 of the conveying channel 10, spaced apart from the first catalyst 30. This second catalyst 34 is operated at room temperature and has a material known as hopcalite, such as carulite, as its catalytic surface. ® This material is a fine granulate mixture of manganese oxide and copper oxide, optionally with aluminum oxide components, and exhibits excellent catalytic activity for the decomposition of ozone at low temperatures. Since the hopcalite has an open-pore structure, this catalyst 34 has a specific surface area of approximately 200 m2. 2 / g, providing a very large surface area for conversion. The large surface area and longer residence time ensure that even the smallest ozone residues in the sample gas stream are almost completely converted. The sample gas stream can then be removed from the exhaust gas measurement system.

[0029] This ozone trap, with two catalysts connected in series, which achieve optimal results at different temperatures, operates very reliably over a long period of time. Unacceptable emissions are largely eliminated and the components of the exhaust gas measurement system are protected from damage. The different physical conditions in the two catalysts ensure almost complete conversion of the ozone into harmless oxygen.

[0030] It should be clear that the scope of protection of the present main claim is not limited to the described embodiment. Other differently operating catalysts with modified catalytically active coatings can also be used if necessary. Furthermore, the catalyst arrangement according to the invention can also be successfully used in other analysis units that operate with ozone.

Claims

[1] Method for decomposing ozone contained in a sample gas stream from an exhaust gas analysis unit (18) with a conveying channel (10) through which the sample gas stream flows from the exhaust gas analysis unit and a first catalyst (30) in the conveying channel (10) with a catalytically active surface made of one or more metals and / or metal oxides, characterized by , that the first catalyst (30) is operated at a temperature of 180°C to 250°C and downstream of the first catalyst (30) in the conveying channel (10) there is arranged a second catalyst (34) having a catalytically active surface made of one or more metals and / or metal oxides, which is operated at a temperature of 10°C to 80°C. [2] Device for carrying out the method according to claim 1, with a conveying channel (10) through which the sample gas stream flows from the exhaust gas analysis unit and a first catalyst (30) in the conveying channel (10) with a catalytically active surface made of one or more metals and / or metal oxides, characterized by , that downstream of the first catalyst (30) in the conveying channel (10) there is arranged a second catalyst (34) having a catalytically active surface made of one or more metals and / or metal oxides, wherein the first catalyst (30) has a catalytic surface made of copper oxide and is a bulk catalyst with electrolyte copper spheres (32) having a diameter of 1-2 mm and the second catalyst (34) has a catalytic surface made of manganese oxide and copper oxide. [3] Device according to claim 2, characterized by that the first catalyst (30) has a porosity of 0.3 to 0.

45. [4] Device according to claim 2, characterized by that the specific surface area of the second catalyst is 100 to 250 m 2 / g. [5] Device according to one of claims 2 to 4, characterized by that the second catalyst (34) is a hopcalite catalyst. [6] Device according to one of claims 2 to 5, characterized by that the second catalyst (34) has a carrier material with a catalytically active surface.

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