JPH0824572A - Treatment of methyl bromide-containing waste gas - Google Patents

Abstract

PURPOSE:To provide a treating method of a methyl bromide-containing waste gas capable of removing, decomposing and making gaseous methyl bromide harmless at low cost efficiently. CONSTITUTION:The methyl bromide-containing waste gas is supplied from a fumigating storehouse 10 to activated carbon towers 12a-12c and methyl bromide in the waste gas is adsorbed on the activated carbon. The methyl bromide-adsorbed activated carbon 12a is heated and the methyl bromide adsorbed on the activated carbon is desorbed from the activated carbon. The heat of the combustion waste gas of a catalyst tower 16 is used as a heat source at this time. The desorbed methyl bromide is heated by the heat of the combustion waste gas of the catalyst tower 16 with a heat exchanger, then supplied to the catalyst tower 16 and subjected to combustion decomposition there to produce hydrogen bromide. This decomposed gas containing the hydrogen bromide is absorbed in a sodium hydroxide soln. at an absorption tower 20 to produce harmless sodium bromide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating exhaust gas containing methyl bromide, and more particularly to a method for removing, decomposing and detoxifying methyl bromide from exhaust gas containing methyl bromide from equipment such as fumigation warehouses.

[0002]

2. Description of the Related Art In a fumigation warehouse, compressed methyl bromide is vaporized from a methyl bromide cylinder 60 by a vaporizer 62 and then supplied to a fumigation warehouse 64 as shown in FIG. Fumigation of various kinds. Conventionally, when the gas in the fumigation warehouse 64 is discharged, after diluting the air 66 forcibly into the fumigation warehouse 64 to dilute the methyl bromide gas in the warehouse and lower the concentration of methyl bromide in the exhaust gas 68. , Through the fan 70 into the atmosphere.

However, this method does not mean that the total amount of methyl bromide released into the atmosphere is reduced, and it cannot be said to be a sufficient method in terms of preventing pollution. Methyl bromide is also a cause of ozone-depleting substances like freon, and development of a technique for reducing the amount released to the atmosphere is strongly desired. As a method for removing methyl bromide in the exhaust gas, an activated carbon adsorption method or a combustion oxidative decomposition method can be considered. However, in the activated carbon adsorption method, it is possible to remove methyl bromide in the exhaust gas by adsorbing methyl bromide on the activated carbon and reduce the exhaust gas concentration, but the methyl bromide itself is not harmed. It does not translate into a final solution.

On the other hand, in the combustion oxidative decomposition method, the exhaust gas containing methyl bromide is heated to 600 ° C. to 900 ° C. for complete oxidative decomposition, as disclosed in JP-B-55-50458, and it is 99% or more. It shows a decomposition rate and is considered to be a sufficient method to reduce the amount released to the atmosphere. However, this combustion oxidative decomposition method has a problem in the economical efficiency of operation.

That is, as shown in FIG. 7, the concentration of methyl bromide-containing exhaust gas from the fumigation warehouse is about 10,000 ppm during the fumigation operation at the beginning of evacuation, but just before the end of the diluted evacuation, the entry into the warehouse is It is as low as 15ppm, which means that the air itself is almost heated, and a large amount of fuel such as kerosene and propane gas is used, so it is hard to say that it is an economical method from the viewpoint of running cost. .

Further, in order to achieve a high efficiency of decomposing methyl bromide, combustion decomposition at a high temperature is required, which may cause secondary pollution due to thermal NO _x, which is a cause of decomposition technology at a low temperature. Development is desired. Therefore, a method of combusting and decomposing exhaust gas containing methyl bromide on a catalyst at a low temperature of about 300 ° C. is proposed in Japanese Patent Publication No. 54-22792 and Japanese Patent Laid-Open No. 23598/1993.

These can reduce the amount of fuel used by reducing thermal NO _x and combustion decomposition at low temperatures, but since they are diluted and exhausted as described above, auxiliary combustion is required to maintain the reaction temperature, and maintenance is also complicated. . Further, since the diluted exhaust amount needs to be 10 times or more the warehouse volume, the equipment scale is large and the initial cost is high.

[0008]

DISCLOSURE OF THE INVENTION The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to treat methyl bromide-containing exhaust gas capable of detoxifying and detoxifying methyl bromide gas at low cost and with high efficiency. To provide.

[0009]

Means for Solving the Problems In the present invention, after methyl bromide is adsorbed and removed on activated carbon, methyl bromide is desorbed from the activated carbon, and then the desorbed methyl bromide is burned and decomposed by a combustion catalyst. The gas is absorbed by an alkaline solution.

[0010]

[Function] Methyl bromide-containing exhaust gas from equipment such as a fumigation warehouse is adsorbed on the activated carbon in the activated carbon layer. Activated carbon having adsorbed methyl bromide is heated, and methyl bromide is desorbed from the activated carbon. The desorbed methyl bromide is combusted and decomposed by the combustion catalyst to be decomposed gas containing hydrogen bromide. When this decomposed gas is absorbed in an alkaline solution such as sodium hydroxide, it becomes a harmless component such as sodium bromide.

[0011]

EXAMPLE FIG. 1 is a system diagram showing an example of a method for treating exhaust gas containing methyl bromide according to the present invention. In this system, roughly, in the adsorption step, the methyl bromide of the methyl bromide-containing exhaust gas in the fumigation warehouse 10 is adsorbed by the activated carbon tower 12, and the methyl bromide adsorbed by the activated carbon is desorbed from the activated carbon in the desorption / decomposition step. After that, it is decomposed by combustion in the catalyst tower 16, and then hydrogen bromide generated by the combustion decomposition is absorbed in the alkaline solution in the absorption tower 20.

The exhaust gas in the fumigation warehouse 10 containing methyl bromide, while being diluted with the dilution air 26, is ventilated through the activated carbon tower 12 provided in multiple stages to capture the methyl bromide on the activated carbon. At this time, the concentration of methyl bromide entering the activated carbon tower 12 linearly decreases in a logarithmic manner, as shown in FIG. 12a is large, and the latter is less in the activated carbon tower 12c.

As shown in FIG. 2, the concentration of methyl bromide is 100.
It is about 30% (0.3
(g-methyl bromide / g-activated carbon) is adsorbed and captured, and when the concentration of methyl bromide is 1000 ppm, it is about 1 in the middle activated carbon tower 12b.
It is adsorbed and captured by 5%, and when the concentration of methyl bromide is 100 ppm, it is adsorbed by about 7% by the activated carbon tower 12c in the subsequent stage. Also, after the activated carbon tower 12a in the preceding stage reaches a saturated adsorption amount at a high concentration, there is no methyl bromide trapping effect when the concentration of methyl bromide decreases, and rather, methyl bromide is desorbed from the activated carbon of the activated carbon tower 12a. However, there is a possibility that it will flow out to the activated carbon towers 12b and 12c in the subsequent stage. From this, it is desirable that the multistage activated carbon tower is ventilated sequentially to the activated carbon tower 12b and the activated carbon tower 12c in the subsequent stage to efficiently use the activated carbon.

Next, the activated carbon tower 12a which has captured a large amount of methyl bromide in the fumigation warehouse 10 is sent to a desorption / decomposition process to decompose and detoxify the methyl bromide. The activated carbon tower is supplemented with the desorbed activated carbon tower 12d. In the desorption / decomposition step, the activated carbon tower 12a is heated to desorb the adsorbed and trapped methyl bromide, and the methyl bromide is discharged in a high concentration state. At this time, the higher the temperature, the higher the concentration of desorbed methyl bromide. Specifically, as shown in FIG. 3, it desorbs rapidly at a high concentration at 200 ° C. and slowly desorbs at a concentration of ½ or less at 100 ° C. Therefore, in the desorption process, the heating temperature is
About 60 to 259 ° C is desirable.

Next, this high-concentration methyl bromide is preheated by exchanging heat with the combustion heat in the catalyst tower 16 in the heat exchanger 14, and then sent to the catalyst tower 16 to be combusted and decomposed on the catalyst.
In this case, if the concentration of methyl bromide is high, specifically, if it is 8000 ppm to 10000 ppm, auxiliary fuel such as kerosene is not used, and the heat generated by combustion of methyl bromide also causes thermal deterioration of the catalyst. Never. From this, in the heating and desorption of the activated carbon tower 12a, it is possible to make the composition in the gas decomposed by combustion uniform concentration by controlling the heating temperature by the air volume of the combustion exhaust gas 18 and the like.

FIG. 4 shows the changes over time in the desorbed methyl bromide concentration, the heating temperature and the residual ratio of methyl bromide in the activated carbon. Desorption of methyl bromide gas, catalyst tower 1
Can self-burn at 6. In the catalyst tower 16, it is desirable to combust and decompose with an oxidation catalyst that has a resistance to poisoning of halides, and in the case of these catalysts, exhaust gas of low temperature and large air volume can be treated, and further the life of the catalyst can be increased. it can.

As the catalyst, those supporting alumina, titania, zirconia, manganese, copper, nickel, cobalt, platinum or palladium can be used. FIG. 5 shows the results when methyl bromide was decomposed using one of these. Two
The decomposition rate was 60% at 50 ° C and almost 100% at 300 ° C. Therefore, it is desirable to operate the catalyst tower 16 at 250 ° C. or higher.

Here, in the catalyst tower 16, methyl bromide is almost completely decomposed into hydrogen bromide, carbon dioxide gas and water vapor. That is, 2CH ₃ Br + 30 ₂ → 2HBr + 2CO ₂ + 2H ₂ O This combustion exhaust gas 18 is heat-exchanged with the methyl bromide gas in the heat exchanger 14. The combustion decomposition gas (including hydrogen bromide) from the catalyst tower 16 is cooled in a cooler (not shown in FIG. 1), cooled to 50 ° C. or lower, and then absorbed in the sodium hydroxide solution 22 in the absorption tower 20, It becomes harmless sodium bromide (NaBr) and is taken out through the pipe 24.

In this case, if the temperature of the decomposition gas is too high, the absorption efficiency is lowered and the amount of the absorbing liquid evaporated is increased, which may cause clogging of the pipe. The gas after being absorbed is released to the atmosphere by a fan or the like, but hydrogen bromide in this case can be absorbed almost 100%. The alkaline solution after the absorption operation can be neutralized with hydrochloric acid (HCl) or the like and discharged. In addition, in the above-mentioned embodiment, all the methyl bromide exhaust gas in the fumigation warehouse was ventilated to the activated carbon tower, but at first, 10,000 p
The concentration is as high as pm, and catalytic combustion may be performed as it is. This has the effect of saving labor and energy in the desorption process of activated carbon.

Further, in the above-mentioned embodiment, the method for treating the methyl bromide-containing exhaust gas for fumigation has been described, but the present invention is not limited to these exhaust gases.
It can also be applied to methyl bromide-containing exhaust gas in which methyl bromide in exhaust gas changes from high concentration to low concentration in equipment using methyl bromide.

[0021]

EFFECTS OF THE INVENTION According to the present invention, exhaust gas containing methyl bromide from equipment such as fumigation warehouses having a high concentration to a low concentration can be removed, decomposed, and rendered harmless with high efficiency.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of a method for treating an exhaust gas containing methyl bromide according to the present invention.

FIG. 2 is a relationship diagram between the concentration of methyl bromide and the amount of adsorption by activated carbon.

FIG. 3 is a relationship diagram between the elapsed time in the desorption process and the methyl bromide concentration at each heating temperature.

FIG. 4 is a diagram showing the relationship between the heating temperature and the desorption concentration of methyl bromide.

FIG. 5 is a diagram showing the relationship between the decomposition rate of methyl bromide by a catalyst and the reaction temperature.

FIG. 6 is a system diagram showing a conventional method for treating an exhaust gas containing methyl bromide.

FIG. 7 is a diagram showing the relationship between the change over time in the concentration of methyl bromide in the warehouse and the concentration of methyl bromide.

[Explanation of symbols]

 10 Fumigation Warehouse 12a-12d Activated Carbon Tower 14 Heat Exchanger 16 Catalyst Tower 18 Combustion Exhaust Gas 20 Absorption Tower 22 Sodium Hydroxide Solution

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location B01D 53/68 53/77 53/86 B01D 53/34 134 D 53/36 G (72) Inventor Mitsuo Watanabe 1-1-14, Kanda, Chiyoda-ku, Tokyo Within Hitachi Plant Construction Co., Ltd. (72) Inventor Hiroshi Kawagoe 7-1-1, Omika-cho, Hitachi-shi, Ibaraki Hitachi, Ltd. Hitachi Research Laboratory

Claims (7)

[Claims] 1. An adsorption step of introducing an exhaust gas containing methyl bromide into an activated carbon layer to adsorb and remove the methyl bromide in the exhaust gas to the activated carbon, and an odor adsorbed by heating the activated carbon adsorbing the methyl bromide. A desorption step of desorbing methyl bromide, a combustion decomposition step of contacting the methyl bromide desorbed from the activated carbon in the desorption step with a combustion catalyst to decompose by combustion, and a hydrogen bromide generated by the combustion decomposition of methyl bromide in an alkaline solution. A method for treating exhaust gas containing methyl bromide, which comprises: 2. The method for treating a methyl bromide-containing exhaust gas according to claim 1, wherein the exhaust gas is a fumigation-containing methyl bromide-containing exhaust gas. 3. The method for treating an exhaust gas containing methyl bromide according to claim 1, wherein combustion heat generated in the combustion decomposition step is utilized as a heat source necessary for the desorption step. 4. The method for treating exhaust gas containing methyl bromide according to claim 1, wherein methyl bromide supplied to the combustion decomposition step is preheated by combustion heat generated in the combustion decomposition step. 5. The activated carbon layer in the adsorption-removal step comprises a multi-stage layer, and the activated carbon layer at the exhaust gas inlet portion where the saturated adsorption amount is first reached is taken out, and the activated carbon layer can be sequentially added to the exhaust gas outlet portion. The method for treating a methyl bromide-containing exhaust gas according to claim 1, wherein 6. The methyl bromide-containing product according to claim 1, wherein in the desorption step, the activated carbon that has reached the saturated adsorption amount is heated in a range of 60 to 250 ° C., and is sequentially shifted to a high temperature. Exhaust gas treatment method. 7. The combustion decomposition step is characterized in that the desorbed methyl bromide is contacted with a combustion oxidation catalyst which is resistant to poisoning of halides at a temperature of 250 ° C. or higher.
Method for treating exhaust gas containing methyl bromide of.