JP4817294B2 - Method for selective reduction of nitrogen oxides - Google Patents

Description

  The present invention relates to a selective reduction method of nitrogen oxide (NOx) using a reducing agent in an oxygen residual atmosphere.

  A selective reduction method using ammonia, hydrocarbons, carbon monoxide, hydrogen, oxygen-containing compounds, and nitrogen-containing compounds as reducing agents has been proposed as a method for removing nitrogen oxides in various exhaust gases in which oxygen remains, and is related. A number of patents and papers have been reported.

  In the conventionally known nitrogen oxide removal method using a reducing agent, the temperature of the catalyst bed is assumed to be uniform. In the treatment operation seen in automobile exhaust gas, etc., it may be inevitably used under conditions that are outside this operation range, and the treatment reaction temperature may deviate from the active temperature range where the catalyst works effectively. In addition to being able to obtain a high NOx removal efficiency, there may be a problem that the reaction cannot proceed efficiently. In this case, the catalyst does not work effectively, and the solution to deal with such a case has not been sufficiently solved.

In the field of industrial production, the reaction is carried out as steady as possible, and the temperature distribution in the reactor is such that the reactor inlet is lowered and a positive temperature gradient is provided with respect to the reactor outlet to improve the efficiency of hydrocarbon cracking reaction. The reaction is carried out under as constant conditions as possible. A specific reaction is “hydrocarbon cracking with a positive temperature gradient” (Patent Document 1), and “method and apparatus for producing hydrogen by thermal catalytic cracking of hydrocarbons” (Patent Document 2).
These are all for performing exothermic reactions that proceed constantly in industrial production, and the inlet temperature of the reaction gas is kept constant, and against sudden changes seen in exhaust gas treatment etc. It is not for responding, but exclusively for carrying out the exothermic reaction continuously under stable conditions.

When the reducing agent and NOx are treated in the presence of the catalyst, it is required that the treatment be performed under certain conditions even when the temperature of the catalyst bed is out of the uniform condition. Processing operations found in automobile exhaust gas, etc. are inevitably used under conditions that deviate from this operating range, and the development of a processing method that can perform certain processing under such conditions is desired. .
Special Table 2002-504169 JP 2000-281304 A

  The problem to be solved by the present invention is that when the reducing agent and NOx are treated in the presence of a catalyst, even when the temperature of the catalyst bed is out of a uniform condition, even under such a condition, the temperature is kept under a certain condition. It is possible to provide a processing method that can be processed and can efficiently proceed with the reaction.

  As a result of research conducted to solve the above problems, a positive temperature is set so that the inlet temperature of the catalytic reactor is low and the outlet temperature is high with respect to the flow of reaction components in the catalytic reaction layer. Specifically, the reactor inlet temperature of the processing gas is 150 to 300 ° C, and the reactor outlet temperature is maintained at 350 to 600 ° C. In the selective reduction and removal reaction of NOx using a reducing agent, it was found that the utilization efficiency of the reducing agent can be improved in response to a sudden change in reaction conditions. In this case, it is effective that the catalytic reactor has a length at least 5 times the inner diameter of the reaction tube with respect to the positive direction of the reaction gas flow.

According to the present invention, the following inventions are provided.
At least one component selected from hydrogen, carbon monoxide, saturated and unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, oxygen-containing organic compounds, and nitrogen-containing organic compounds , for automobile exhaust gas containing nitrogen oxides (NOx) fed into the catalytic reactor with a reducing agent containing, the catalytic reactor is a catalytic reactor having at least 5 times the length of the reaction tube inner diameter with respect to the positive direction of the reaction gas flow, Pt metal component 0 .01 to 2.0% by weight of a catalyst loaded with ZSM-5 metal oxide is packed, and the catalyst reactor inlet temperature is maintained at 150 to 300 ° C and the catalyst reactor outlet temperature is maintained at 350 to 600 ° C. A method for reducing nitrogen oxide (NOx) contained in exhaust gas, wherein the exhaust gas for automobiles containing nitrogen oxide (NOx) is extracted as nitrogen gas.

According to the method of the present invention, a stable NOx removal rate can be obtained which is higher than that of a catalyst bed having a uniform temperature using the same catalyst.
The method of the present invention is effective in exhaust gas treatment with a wide range of temperature changes such as automobile exhaust gas, increases NOx removal rate, and can be used as an exhaust gas purification converter for both lean burn gasoline vehicles and diesel vehicles.

In the present invention, a catalytic reactor comprising a catalytic reaction layer having a nitrogen oxide removing catalyst using a reducing agent is used.
The exhaust gas containing NOx gas is supplied to the catalytic reactor as a reaction gas together with the reducing agent gas, and NOx contained in the exhaust gas is reduced to N ₂ gas in the catalytic reactor. For this treatment, a certain length is required with respect to the positive direction of the reaction gas flow. It is necessary to have a length at least 5 times the inner diameter of the reaction tube with respect to the positive direction of the reaction gas flow. If it is less than 5 times, the reaction does not proceed sufficiently and satisfactory results cannot be obtained. If this is exceeded, the reaction can complete the reaction. It is necessary to surpass this in anticipation of safety so that the reaction can be completed steadily, but it need not be unnecessarily long. If it is unnecessarily long, the reaction may not be completed and may not have an economic meaning. For this reason, there is no meaning for the case of exceeding 10 times.
This can be determined experimentally.

The reaction is an exothermic reaction, and the exhaust gas emitted from the engine is supplied to the catalytic reactor. The exhaust gas guided to the catalytic reactor may be supplied after being cooled in the middle or may be supplied without being cooled by a special cooling means. The temperature gradient is set so that the inlet temperature of the catalyst reactor is low and the outlet temperature is high. By installing the tail end of the catalyst layer (catalyst layer outlet) at a position where the temperature of the electric furnace becomes the highest, it is possible to have a positive temperature gradient from the catalyst layer inlet to the outlet.
The catalyst layer temperature does not depend on the change in the inlet reaction gas temperature, and the outlet temperature is controlled by paying attention to the outlet catalyst layer temperature. In the demonstration test of the present invention, the electric furnace temperature is controlled so that the outlet temperature is 350 to 600 ° C. This may be the temperature at which the process gas is supplied to the catalyst layer. In the case of mounting as a catalyst layer for exhaust gas treatment for automobiles, the outlet temperature range can be similarly maintained by applying heating means of an electric system. In this case, the reaction gas rises in this temperature range toward the exit temperature as the reaction proceeds. In the case of 100 ° C. or lower, heating is appropriately performed so that the outlet temperature is within the above range and the catalyst layer inlet temperature is 150 to 300 ° C.

The reaction gas contains nitrogen oxide (NOx). The nitrogen oxide content is in the range of 500 ppm to 2000 ppm.
The reducing agent to be coexisted contains at least one component selected from hydrogen, carbon monoxide, saturated and unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, oxygen-containing organic compounds and nitrogen-containing organic compounds.
The saturated and unsaturated aliphatic hydrocarbons have 1 to 15 carbon atoms. These may be linear or may have a branched chain. The unsaturated aliphatic hydrocarbon is an aliphatic hydrocarbon having a double bond, and a plurality of double bonds may be present.
The aromatic hydrocarbon is a hydrocarbon having an aromatic hydrocarbon as a basic skeleton, and may have an alkyl group as a substituent. Specifically, benzene, toluene, xylene and the like can be mentioned.
The oxygen-containing organic compound is an alcohol in which an oxygen-containing aliphatic hydrocarbon is substituted with an OH group, or a compound having an ether or aldehyde group bonded through oxygen. Specifically, methanol, ethanol, acetaldehyde, dimethyl ether, diethyl ether and the like can be mentioned.
The nitrogen-containing organic compound is an aliphatic hydrocarbon substituted with a cyano group, and specific examples include acetonitrile and acrylonitrile.
The content of these reducing agents is 500 ppm to 5000 ppm.
The reaction gas contains up to 10% O ₂ .

It is necessary that the catalytic reactor has a length of at least 5 times the inner diameter of the reaction tube with respect to the positive direction of the reaction gas flow.
The catalyst reactor is filled with the following catalyst.
A component selected from Pt, Pd, Rh, Ir, Cu, Co, and Ag as the metal component in the catalyst, and silica gel, zeolite (ZSM-5, ferrierite, mordenite), or Al ₂ O ₃ as the metal oxide component , TiO ₂ , or ZrO ₂ .

When the metal component as a catalyst is supported on the carrier, the ratio of the metal component is (0.01 to 2.0) wt%. A metal component such as platinum is supported on a zeolite catalyst (Pt / H-ZSM-5) (Pt support rate = 1.0 wt%).
When the catalyst is filled in the catalyst reactor, the reaction layer is filled. An example is as follows.
This sample was mixed with granular silica gel or granular zeolite as a diluent at a ratio of 1: 9 (Pt catalyst = 0.5 g, silica gel or zeolite = 4.5 g), and about the direction of the reaction component flow direction in the tubular reactor having an inner diameter of 10 mm. Packed over a range of 6.5 cm (silica gel mixed sample) or 8.5 cm (zeolite mixed sample).
The left side of FIG. 1 shows a catalytic reactor (Example 1 described later) in which a positive temperature gradient is provided in the catalyst layer of the present invention.
Further, the right side of FIG. 1 shows a conventional catalytic reactor ( Comparative Example 1 described later) having no ordinary temperature gradient.

  The outlet temperature is controlled at 350 to 600 ° C. by changing the installation position in the electric furnace.

The rate of change of each component according to the temperature distribution in the reactor is as shown in FIG.
FIG. 2 shows the results when the silica gel diluted sample is used, and FIG. 3 shows the results when the zeolite diluted sample is used.
Improved N ₂ selectivity and NO _x removal rate can give better results towards the reactor outlet.

Examples of the present invention will be described below.
Particulate zeolite (H-ZSM-5) was impregnated with H ₂ PtCl ₆ aqueous solution and calcined at 500 ° C to obtain a zeolite catalyst carrying Pt (Pt / H-ZSM-5) (Pt support rate = 1.0 wt%). This sample was mixed with granular silica gel or granular zeolite as a diluent at a ratio of 1: 9 (Pt catalyst = 0.5 g, silica gel or zeolite = 4.5 g), and about the direction of the reaction component flow path in the tubular reactor having an inner diameter of 10 mm. Packed over a range of 6.5 cm (silica gel mixed sample) or 8.5 cm (zeolite mixed sample) (inner diameter: catalyst reactor length = 1 to 6.5, or 1 to 8.5).
Thermocouples were installed at the reaction gas inlet, center, and outlet of the catalyst bed, respectively. The installed state of the catalyst is shown on the left side of FIG. The reaction temperature of the catalyst bed was the lowest at the inlet, increased monotonically toward the outlet, and was highest at the outlet. The NOx selective reduction reaction test was conducted under flow reaction conditions, and the NOx conversion rate in the steady state at each temperature of the catalyst heating furnace set by the temperature controller was investigated. 1000mLNOx + 400ppm n-octane + 10% O ₂ (He dilution) flows as the reaction gas at 500ml / min, and the outlet gas is FT-IR (NO, NO ₂ with micro gas chromatograph (measures CO ₂ and N ₂ ) and gas cell. Analysis). The activity was evaluated based on the NOx (= NO + NO ₂ ) conversion rate and N ₂ selectivity shown in the following equation.
FIG. 2 shows the result of treatment with a NO-nC ₈ H ₁₀ —O ₂ mixed gas using a Pt / H-ZSM-5 catalyst (catalyst supported on SiO ₂ ) with the temperature gradient of the present invention. Show.
FIG. 3 shows the result of treatment with a NO-nC ₈ H ₁₀ -O ₂ mixed gas using the Pt / H-ZSM-5 catalyst (catalyst supported on HZSM-5) with the temperature gradient of the present invention. Indicates.
The results were organized according to the following formula.
NOx conversion rate = [NOx outlet concentration (ppm) / NOx inlet concentration (ppm)] x 100 (%),
N ₂ selectivity = [(N ₂ outlet concentration x 2 (ppm)) / total N concentration in product (ppm)] x 100 (%),

Comparative Example 1

  In Example 1, the NOx selective reduction reaction test was conducted in the same manner except that the Pt catalyst was not diluted with silica gel or zeolite, the catalyst bed length was shortened to about 10 mm, and no temperature gradient was provided in the catalyst reaction layer. It was. The result is shown in FIG.

Comparing FIGS. 2 and 3 with FIG. 4, in FIGS. 2 and 3 of the present invention, by providing a positive temperature gradient in the catalyst reaction layer as compared with the case where the catalyst bed temperature is uniform in the conventional method, It can be seen that the NOx conversion is a high value in a wide maximum temperature range. Furthermore, the N ₂ selectivity was improved. These results indicate that the active temperature region of the Pt catalyst having the highest activity for NOx removal but a narrow temperature range can be expanded.

The figure on the left shows a reactor with a positive temperature gradient in the catalyst layer of the present invention, and the figure on the right shows a conventional catalyst reactor without a temperature gradient. The Pt / silica gel supported catalyst of the present invention, showing the results of processing the NO-nC ₈ H ₁₀ -O ₂ gas mixture. Shows the Pt / H-ZSM-5 supported catalysts of the present invention, the results of processing the NO-nC ₈ H ₁₀ -O ₂ reaction gas mixture. In no conventional temperature gradient condition, the Pt / H-ZSM-5 catalyst, shows the result of processing NO-nC ₈ H ₁₀ -O ₂ antiferromagnetic mixed gas FIG.

Claims (1)

At least one component selected from hydrogen, carbon monoxide, saturated and unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, oxygen-containing organic compounds, and nitrogen-containing organic compounds , for automobile exhaust gas containing nitrogen oxides (NOx) fed into the catalytic reactor with a reducing agent containing, the catalytic reactor is a catalytic reactor having at least 5 times the length of the reaction tube inner diameter with respect to the positive direction of the reaction gas flow, Pt metal component 0 .01 to 2.0% by weight of a catalyst loaded with ZSM-5 metal oxide is packed, and the catalyst reactor inlet temperature is maintained at 150 to 300 ° C and the catalyst reactor outlet temperature is maintained at 350 to 600 ° C. A method for reducing nitrogen oxide (NOx) contained in exhaust gas, wherein the exhaust gas for automobiles containing nitrogen oxide (NOx) is extracted as nitrogen gas.

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