JPH119964A - Method for removing ammonia-containing exhaust gas - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To decompose ammonia in exhaust gas containing no oxygen into a safe and harmless substance by a reaction at a relatively low temperature. SOLUTION: An oxygen-containing gas is brought into contact with a nickel catalyst to oxidize the nickel catalyst, and then an ammonia-containing exhaust gas is brought into contact with the oxidized nickel catalyst to decompose ammonia in the exhaust gas into nitrogen and water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing ammonia-containing exhaust gas, and more particularly, to decomposing and removing ammonia-containing gas containing no oxygen gas, for example, ammonia contained in ammonia-containing exhaust gas discharged from a CVD apparatus. On how to do it.

[0002]

2. Description of the Related Art For example, when a nitrogen compound semiconductor thin film such as gallium-nitrogen (GaN) is manufactured by a CVD method (chemical vapor deposition), an ammonia gas as a nitrogen source is supplied to a CVD apparatus by a hydrogen gas. It is supplied together with a carrier gas such as nitrogen or nitrogen gas. Therefore, exhaust gas containing unreacted ammonia is discharged from the CVD apparatus. Ammonia (NH ₃ ) is not only a flammable gas that has the danger of explosion, but also a toxic gas that is harmful to the human body. It is necessary to remove (remove) ammonia.

As a method for removing ammonia contained in exhaust gas, a wet method in which ammonia is dissolved in a solution by a scrubber and a dry adsorption method in which ammonia is adsorbed by an adsorbent have been known. In recent years, various catalyst decomposition methods for decomposing and removing ammonia molecules into harmless molecules using a catalyst have been proposed as methods more advantageous than these methods.

For example, Japanese Patent Application Laid-Open No. 2-198638 describes a method in which ammonia molecules are decomposed into hydrogen molecules and nitrogen molecules at 300 to 700 ° C. on a nickel-based catalyst. The nickel-based catalyst used here is
Nickel is the main component of the reaction.
It is a material to which an iron group metal or metal compound is added.
Nickel-based catalysts to which such transition metals are added are commercially available. For example, sulfur-resistant nickel catalysts N111 and N112 manufactured by Nikki Chemical Co., Ltd. correspond to these, and are used for hydrogenation of organic compounds such as benzene and phenol. Besides being used, it is also used for removing oxygen contained in hydrogen, argon or nitrogen, and for purifying carbon monoxide and carbon dioxide in hydrogen by methanation.

[0005] Japanese Patent Application Laid-Open No. 8-961 discloses a method in which ammonia is oxidized and decomposed by a copper oxide catalyst comprising a metal oxide containing copper oxide. In this method, an exhaust gas in which ammonia and oxygen gas coexist is brought into contact with the copper oxide catalyst at 20 to 500 ° C., and ammonia in the gas is decomposed into water molecules and nitrogen molecules by oxygen in the gas.

[0006]

However, when the nickel-based catalyst is used, if the operating temperature is lower than 500 ° C., the decomposition rate of ammonia is low, so that the operating temperature must be 500 ° C. or higher. Further, in this method, since highly ignitable hydrogen is generated by decomposition of ammonia, special consideration is required for the material and structure of the processing apparatus in order to prevent the ignition of hydrogen.

Further, when hydrogen gas is used as a carrier gas in a CVD apparatus, hydrogen gas is present in the exhaust gas. Therefore, it is preferable that the ammonia removal treatment be performed at a temperature as low as possible. By
There is an advantage that various considerations for the apparatus can be reduced and the energy required for heating can be reduced.

On the other hand, when the copper oxide-based catalyst is used, the detoxification treatment can be performed in a low temperature range of 350 ° C. or less, so that the above-mentioned consideration for hydrogen is not necessary. When oxygen-free gas such as the exhaust gas from the CVD apparatus is treated, it is necessary to add oxygen gas to the exhaust gas. In addition, this method has a problem that nitrogen oxides (NOX) are easily generated, and particularly when the temperature exceeds 300 ° C., the amount of generated NOX increases.

As described above, a method conventionally proposed as a method for decomposing ammonia molecules into harmless molecules by a catalytic reaction is a method for decomposing ammonia molecules in an oxygen-free exhaust gas such as an exhaust gas discharged from a CVD apparatus. There are many unsuitable methods for removing harmful substances.

Accordingly, the present invention provides an ammonia-containing exhaust gas which can decompose ammonia in an oxygen-free exhaust gas into harmless molecules by a reaction at a relatively low temperature and does not generate harmful or ignitable substances. The purpose is to provide a method for eliminating harmful substances.

[0011]

In order to achieve the above object, the method for removing ammonia-containing exhaust gas according to the present invention is directed to an ammonia-containing exhaust gas which decomposes and removes ammonia in an oxygen-free exhaust gas from a nickel catalyst. The method of removing, wherein after contacting the nickel catalyst with an oxygen-containing gas to oxidize the nickel catalyst, the ammonia-containing exhaust gas is brought into contact with the oxidized nickel catalyst to convert ammonia in the exhaust gas into nitrogen and water. It is characterized by decomposition.

Further, the present invention is characterized in that the nickel oxidizing step of oxidizing the nickel catalyst and the ammonia decomposing step of decomposing the ammonia are alternately and repeatedly performed, and the nickel catalyst comprises The component nickel contains at least one metal component selected from iron, cobalt, manganese, chromium, copper and zinc.
The process is performed in a temperature range of 370 ° C.

The nickel catalyst used in the present invention is obtained by a known production method, for example, calcining a nitrate of nickel, or reducing an oxide obtained by precipitation from an aqueous solution of a nickel salt with hydrogen. And various nickel catalysts (including nickel-based catalysts) commercially available as nickel catalysts can be used. Since the nickel catalyst may be oxidized by oxygen in the air in a normal use mode and its catalytic activity may be reduced, attention is paid to prevent oxidation during storage. Also, to prevent oxidation,
In some cases, the surface is stored in an oxidized state, and hydrogen is reduced before use. Further, in order to impart resistance to catalyst poisoning, various multi-component nickel-based catalysts in which a transition metal is added to nickel, which is a main component of the reaction, are also known.In the present invention, these nickel-based catalysts can also be used. is there.

It is widely known that the nickel catalyst decomposes ammonia into nitrogen and hydrogen at a high temperature as shown in the following reaction formula. In this catalytic reaction, hydrogen is generated as described above. There is an inconvenience. 2NH ₃ → N ₂ + 3H ₂

On the other hand, according to the present invention, after the nickel catalyst is oxidized by bringing the nickel catalyst into contact with an oxygen-containing gas in advance, and then the oxidized nickel catalyst is brought into contact with ammonia, as shown in the following reaction formula, Nickel and ammonia react with each other to reduce nickel oxide to produce metallic nickel, and ammonia is decomposed into nitrogen and water. 3NiO + 2NH ₃ → N ₂ + 3H ₂ O +
3Ni

The decomposition reaction of ammonia proceeds at a low temperature of about 150 ° C., but is preferably performed at a temperature in the range of 330 to 370 ° C. That is, the reactivity is low at a temperature lower than 330 ° C., and the high reactivity is obtained at a temperature of 330 ° C. or higher. On the other hand, when the temperature exceeds 370 ° C., the reduced metallic nickel acts as an ammonia decomposition catalyst, and ammonia is decomposed to generate hydrogen as described above.

The metal nickel reduced by the above reaction formula and having lost the oxidizing ability of ammonia is brought into contact with an oxygen-containing gas again to become nickel oxide as shown in the following reaction formula. It can be recovered and used repeatedly. The temperature at this time is not particularly limited, and the reaction can be performed at room temperature. 2Ni + O ₂ → 2NiO

As described above, after performing the nickel oxidation step of oxidizing the nickel catalyst by bringing the oxygen-containing gas into contact with the nickel catalyst, the nickel catalyst (nickel oxide) oxidized in the nickel oxidation step contains no oxygen. By performing the ammonia decomposition step of bringing the ammonia-containing exhaust gas into contact, the ammonia in the exhaust gas can be decomposed into safe nitrogen and water. In addition, since the treatment can be performed at a relatively low temperature, the heat energy for heating can be reduced, and the treatment apparatus when hydrogen is contained in the ammonia-containing exhaust gas requires little consideration.

Further, the nickel oxidation step and the ammonia decomposition step are alternately and repeatedly performed, whereby the nickel catalyst can be used semi-permanently while being filled in the filling cylinder. Further, the nickel oxidation step and the ammonia decomposition step may be performed independently in separate places, but usually, the oxygen-containing gas and the ammonia-containing exhaust gas are alternately flowed into a filling cylinder filled with a nickel catalyst. do it. In particular, by installing a plurality of filled cylinders filled with a nickel catalyst and sequentially switching between the nickel oxidation step and the ammonia decomposition step for each filled cylinder, it is possible to continuously perform the abatement treatment of the ammonia-containing exhaust gas. .

[0020]

【Example】

Example 1 Nickel nitrate _{(Ni (NO 3) 2 ·} 6H 2 O) 600g
Was melted, 150 g of pumice powder was added thereto, mixed, and evaporated to dryness. This was formed into a pellet having a diameter of 3 mm and a length of 3 mm, calcined at 400 ° C. for 4 hours, and then calcined at 650 ° C. for 4 hours.

166 g of the obtained fired product was packed in a stainless steel column having an inner diameter of 43 mm (filling height 100 m).
m). An activation treatment was performed at 180 ° C. for 10 hours while flowing hydrogen gas through the column to obtain a nickel catalyst, and then purged with nitrogen gas for 8 hours to reach room temperature. Continued
Nitrogen gas containing 5% oxygen gas as oxygen-containing gas,
Nickel was oxidized by flowing at a hollow cylinder speed of 1 cm / sec for 16 hours, and then purged with nitrogen gas for 8 hours to obtain a reactant.

As the ammonia-containing gas, a nitrogen gas containing 1 mol% of ammonia in a nitrogen gas was used. The nitrogen gas was passed through the column at a cylinder speed of 1 cm / sec, and the concentrations of ammonia, moisture and hydrogen at the column outlet were measured. . As a measuring instrument, ammonia is used with an ammonia monitor TG2400 manufactured by Bionics Instruments Co., Ltd. used.

The ammonia removal experiment was carried out by setting the heating temperature of the column variously. From the time until the ammonia concentration at the column outlet reached 25 ppm, the ammonia treatment amount (liter) per kg of the reactant at each temperature was calculated. Calculated. The result is shown in FIG.

From FIG. 1, it can be seen that the throughput is small up to a column temperature of 300 ° C., and that the throughput rapidly increases at 330 ° C. or higher. However, when the column temperature was 380 ° C., hydrogen was detected at the column outlet. This is presumably because the metallic nickel reduced by reacting with ammonia acts as an original ammonia decomposition catalyst, and decomposes ammonia to generate nitrogen and hydrogen as described above.

Example 2 As a nickel-based catalyst, a sulfur-resistant nickel catalyst N112 manufactured by Nikki Chemical Co., Ltd. was used. The composition of this catalyst is 45-47% nickel, 2-3% chromium, 2-3% copper, 27-29% diatomaceous earth, and 4-5% graphite. 166 g of this catalyst was packed in a stainless steel column having an inner diameter of 43 mm (filling height 100 mm). At the column outlet, the same measuring device as in Example 1 was provided.

The following operations (1) to (3) were repeated for the above column. Purge with nitrogen gas at room temperature for 15 hours. A nitrogen gas containing 5% oxygen gas is flowed at room temperature for 15 hours (nickel oxidation step). The column is preheated while purging with nitrogen gas for 15 hours. The column is heated to 350 ° C., and a nitrogen gas containing 1 mol% of ammonia is flowed at a cylinder speed of 1 cm per second (ammonia decomposition step). The time when the ammonia concentration at the column outlet reaches 25 ppm is set as a breakthrough point, and the operation for removing ammonia is terminated, and the operation returns to the operation of (1).

The above operations (1) to (6) are repeated six times, and the operation time of each operation and 1k of the reactant calculated from the operation time
The ammonia treatment amount per g is shown below. Time [min] Ammonia treatment amount [liter] 1st 587 30.6 2nd 562 29.3 3rd 559 29.1 4th 571 29.8 5th 565 29.5 6th 558 29.1

[0028]

As described above, according to the method for removing ammonia-containing exhaust gas of the present invention, the ammonia contained in the oxygen-free ammonia-containing exhaust gas is separated from water and nitrogen safely under relatively low temperature conditions. Can be decomposed into Also,
Even when the ammonia resolution is lost, the ammonia resolution can be restored only by contacting the oxygen-containing gas.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a column temperature and an ammonia treatment amount.

Continuing on the front page (72) Inventor Shuichi Koseki 1-16-7 Nishi-Shimbashi, Minato-ku, Tokyo Inside Nippon Sanso Corporation

Claims (4)

[Claims] 1. A method for abating an ammonia-containing exhaust gas, which comprises decomposing and removing ammonia in an oxygen-free ammonia-containing exhaust gas with a nickel catalyst, comprising contacting the nickel catalyst with an oxygen-containing gas to oxidize the nickel catalyst. A method for removing ammonia-containing exhaust gas, which comprises contacting the ammonia-containing exhaust gas with an oxidized nickel catalyst to decompose ammonia in the exhaust gas into nitrogen and water. 2. The method of claim 1, wherein the nickel oxidation step of oxidizing the nickel catalyst and the ammonia decomposition step of decomposing the ammonia are alternately repeated. 3. The nickel catalyst according to claim 1, wherein nickel as a main component of the reaction contains at least one metal component selected from iron, cobalt, manganese, chromium, copper and zinc. The method for removing ammonia-containing exhaust gas according to claim 1. 4. The method according to claim 3, wherein the step of decomposing ammonia comprises the steps of
The method for removing ammonia-containing exhaust gas according to claim 1, wherein the method is performed in a temperature range of 70 ° C.