JPH0910553A - Treatment of discharged gas containing volatile organic halide - Google Patents

Abstract

PURPOSE: To decompose a volatile organic halide efficiently and economically without producing a reaction by-product by adjusting a desorption temperature and preheating a mixed gas from a gas adjustment step within a specified range. CONSTITUTION: A gas 12 containing a volatile organic halide is sucked into adsorption devices 2, 3 filled with adsorbents 13, 14. An air 17 from a blower 16 is heated using a heater 18 and is guided into the adsorption devices 2, 3 as a recycled gas to desorb the volatile organic halide from adsorbents 13, 14. After that, a vapor 21 is added to the recycled gas 20 containing the volatile organic halide, then the gas 201 is heated at 130-300 deg.C using a preheater 4, and this preheated gas is guided into a catalytic reaction device 5. Thus the desorption temperature, and the preheating temperature are specified, then the degradation of the function or performance of a catalyst by an excess reaction heat is prevented from occurring, and the production of dioxins is also prevented from being entailed. The catalyst to be used should preferably be composed of one or more kinds of noble metals selected from among Pt, Pd, Rh and Au, borne by one or more kinds of oxides selected from among titania, zirconia, boria and phosphorus.

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 a volatile organic halogen compound.
More specifically, the present invention relates to a treatment method for efficiently decomposing and detoxifying volatile organic halogen compounds such as trichlorethylene and tetrachloroethylene contained in the atmosphere, groundwater and the like.

[0002]

Fluorocarbons, methylene chloride, organic halogen compounds such as trichloroethylene and tetrachloroethylene are widely used as refrigerants, degreasing agents for electronic parts and metal products, and solvents for dry cleaning.
This is because under normal circumstances these compounds are chemically very stable. However, these compounds are substances that cause ozone depletion and are also suspected to have carcinogenicity. As a result, pollution of soil and groundwater due to volatile organic halogen compounds generated in the past has become extremely problematic.

As a method for treating a volatile organic halogen compound contained in groundwater or the like, a method in which the volatile organic halogen compound is separated by a vacuum extraction method and the like is adsorbed on an adsorbent such as activated carbon and recovered is generally used. However, there remains a fundamental problem of how to detoxify volatile organic halogen compounds adsorbed on activated carbon in terms of complete detoxification.

As a method of detoxifying the volatile organic halogen compound itself, a thermal decomposition method, a photodecomposition method, a catalytic decomposition method and the like have been proposed. The thermal decomposition method burns a volatile organic halogen compound at high temperature and high pressure, but has problems such as a large-scale apparatus and high processing cost.
The photolysis method irradiates ultraviolet rays in the presence of ozone or a photocatalyst to decompose volatile organic halogen compounds, but when the concentration of the compound in the processing gas is high, it is inappropriate or the processing efficiency is high. Has the drawback of being low. On the other hand, the catalytic cracking method using a catalyst is a simple method,
The processable concentration range is also a technology that has received widespread attention.

Japanese Patent Application Laid-Open No. 6-63357 discloses a method for treating organic halogen compounds in soil and water using a catalytic decomposition method.
There is an apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) Publication. This equipment consists of an adsorption device for concentrating a gas containing a volatile organic halogen compound into an adsorbent and then desorbing it with a regeneration gas, a catalytic decomposition device filled with an organic halogen compound decomposition catalyst, and an organic halogen compound decomposition device. It is composed of a washing tower for removing the generated hydrogen halide gas, and by constructing the apparatus in this way, the organic halogen compound can be efficiently treated at a low temperature. As the catalyst, Si, Al,
One or more oxides selected from Ti, Zr, etc., specifically, those having a strong acid point such as zeolite are used.

However, in this apparatus, when treating the organohalogen compound, the organohalogen compound is exposed to a high temperature of 400 ° C. or higher before the decomposition treatment. Is disassembled,
There is a high possibility that a highly toxic substance such as dioxins will be generated due to the polymerization reaction.

Further, in Japanese Patent Laid-Open No. 6-134241, the volatile organic compounds in the waste water are stripped to concentrate the exhausted volatile organic compounds to an adsorbent, which is then desorbed by a regeneration gas, and the gas is removed. A treatment method of contacting a catalyst and decomposing volatile organic substances is disclosed.

However, since the regenerated gas having a constant temperature is used in the step of desorbing the volatile organic substance from the adsorbent with the regenerated gas, as shown in the comparative example described later, the volatile organic substance in the regenerated gas after desorption is shown. The concentration (hereinafter referred to as "desorption concentration") is estimated to draw one parabola with respect to the time axis. That is, it can be said that the desorption concentration cannot be controlled within a certain range. If the desorption concentration cannot be controlled within a certain range, the catalyst will deteriorate due to sintering, or the total processing efficiency will decrease.

That is, if the desorption concentration is too high, the calorific value of the catalyst increases, and the catalyst temperature rises too much to cause sintering. Also, if the low desorption concentration region is long, the processing time becomes long and the total processing is performed. It leads to a decrease in efficiency.

[0010]

SUMMARY OF THE INVENTION In consideration of the above, the present invention efficiently and economically produces volatile organic halogen compounds contained in wastewater or the atmosphere without producing a side reaction product. It is an object to provide a method of decomposing and detoxifying.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have reached the present invention.

That is, according to the present invention, the volatile organic halogen compound contained in the waste water is stripped and exhausted, or the volatile organic halogen compound contained in the gas is adsorbed to the adsorbent. A step, a desorption step of desorbing an adsorbed component adsorbed from the adsorbent with a regeneration gas, and transferring the gas into the gas to regenerate the adsorbent; and adding steam to the gas containing the desorbed volatile organic halogen compound, A gas adjusting step of mixing, a preheating step of heating the mixed gas obtained in the gas adjusting step, a catalytic cracking step of bringing the preheated mixed gas into contact with a cracking catalyst to decompose, and a hydrogen halide produced by the cracking. In the method for treating volatile organic halides, which consists of a detoxification process of absorbing the Adjust the desorption temperature, and 130 to a temperature to preheat the gas mixture obtained from the gas adjustment step
It is in the range of 300 ° C.

[0013]

The volatile organic halogen compound to be treated in the present invention includes carbon tetrachloride, chloroform, chloroethylene, 1,1,1-trichloroethane, trichloroethylene, tetrachloroethylene, trifluoromethane, dichlorodifluoromethane and the like.

The method of the present invention will be described below with reference to the drawings.

FIG. 1 is a conceptual diagram showing an example of equipment to which the processing method of the present invention is applied.

The equipment of FIG. 1 mainly comprises a stripping device 1, adsorption devices 2 and 3, a preheating device 4, a catalytic reaction device 5 and a cleaning device 6. When the volatile organic halogen compound contained in the wastewater such as groundwater is treated using the equipment of FIG. 1, the volatile organic halogen compound-containing wastewater 7 is supplied to the upper part of the stripping device 1 by the pump 8. Air 9 is blown into the stripping device 1 from below. In the stripping device 1, the volatile organic halogen compound-containing wastewater 7
And air 9 come into countercurrent contact with each other, the volatile organic halogen compound moves to the air 9 side, and the volatile organic halogen compound-containing gas 1
The drainage 7 is discharged as treated water 11 outside the system.

Atmosphere 12 Containing Volatile Organic Halogen Compounds
In the case of processing the above, the atmosphere 12 containing the volatile organic halogen compound is directly sucked into the adsorption devices 2 and 3 by the pump 8.

Gas containing volatile organic halogen compound 12
Is sucked into the adsorbing devices 2 and 3 filled with the adsorbents 13 and 14, and the volatile organic halogen compound in the gas is adsorbent 1
The air 15 adsorbed by 3, 14 and from which the volatile organic halogen compound has been removed is released into the atmosphere. Usually, as the adsorbents 13 and 14, granular activated carbon, fibrous activated carbon, zeolite or the like is used.

In FIG. 1, two towers are used as the adsorption devices 2 and 3. This is because while performing the adsorption treatment on the one hand, the desorption regeneration treatment on the other hand and by repeating this alternately, the treatment for removing the volatile organic halogen compound is continuously performed, and the treatment efficiency is improved. Depending on the object to be treated and the treatment conditions, there is no problem even if only one adsorption device is used. Further, the adsorption device system may be a fixed bed system, a fluidized bed system or the like, but is not particularly limited.

The air 17 sent from the blower 16 is heated by a heater 18 to become a regeneration gas 19 which is introduced into the adsorbers 2 and 3 to desorb the volatile organic halogen compound from the adsorbent to regenerate the adsorbent, The volatile organic halogen compound is transferred into the regeneration gas to obtain the volatile organic halogen compound-containing regeneration gas 20.

Next, water vapor 21 is added to the volatile organic halogen compound-containing regeneration gas 20. This is because, in the decomposition of the volatile organic halogen compound, the coexistence of water vapor as a hydrogen source facilitates the decomposition of the volatile organic halogen compound and suppresses the generation of highly toxic halogen gas or phosgene.

As a method of adding this steam, a method of adding steam generated by a boiler or about 200.degree.
A method in which a volatile organic halogen compound-containing regenerated gas and water are introduced into a vaporizer heated to the above to vaporize the water can be considered, but any method can be used as long as it can introduce water vapor into the reaction system. For example, in the case of using a vaporizer, if the vaporizer is filled with beads or the like formed by foaming alumina as a filler, mixing and diffusion of gas and water is improved, and vaporization of water is facilitated.

The regenerated gas 20 containing the volatile organic halogen compound, to which steam has been added, is heated by the preheating device 4 to 130
After being heated to 300 ° C., introduced into the catalytic reaction device 5,
It is brought into contact with the catalyst layer 22 and decomposed. The reason why the preheating device 4 heats at 130 ° C. or higher is to prevent water vapor in the volatile organic halogen compound-containing regeneration gas from condensing. On the other hand, if the temperature exceeds 300 ° C., dioxins may be by-produced as described above.

It is said that dioxins are easily formed as a by-product due to thermal combustion, and that a large amount of dioxins is produced particularly in the vicinity of 300 to 500 ° C.

By the way, when the volatile organic halogen compound to be decomposed is carbon tetrachloride or the like having a catalytic reaction temperature of about 200 ° C., heating by the preheating device 4 is sufficient. However, if trichlorethylene, tetrachloroethylene, or the like is to be decomposed by a catalytic reaction, the required catalytic reaction temperature will be 400 ° C. or higher. Therefore, a sufficient catalytic reaction cannot be expected by heating with the preliminary or heating device 4. If a method of further heating the volatile organic halogen compound-containing regenerated gas 20 to 300 to 500 ° C. before introducing it into the catalytic reactor is used, a part of the volatile organic halogen compound is thermally decomposed and the polymerization reaction proceeds, resulting in severe poisoning. Dioxins may form as by-products.

Therefore, for this purpose, there are a method of heating the entire catalytic reaction apparatus 5 and a method of heating the catalyst layer 22. Which one is adopted depends on the size of the device itself and design problems. For example, when a large amount of catalyst is used, heating the catalyst layer 22 is effective,
When the amount of catalyst used is small, it is preferable to heat the catalytic reaction device 5 itself from the viewpoint of device assembly.

The catalyst for decomposing the volatile organic halogen compound used in the present invention is a catalyst having acid resistance and increased solid acidity, and is one or two selected from titania, zirconia, boria, phosphorus and the like. Pt, Pd,
It carries one or more precious metals selected from Rh, Au and the like. An example of such a catalyst is the platinum-supported zirconia-phosphoric acid catalyst disclosed in JP-A-5-317645. The catalyst may have a cylindrical shape, a spherical shape, a honeycomb shape, or the like, and a desired shape suitable for actual reaction conditions may be selected.

The processing gas 23 that has been decomposed by the catalytic decomposition reaction is introduced into the cleaning device 6, and the hydrogen halide in the processing gas is absorbed and excluded by the alkaline cleaning liquid 24.

The cleaning device 6 comprises a pre-cleaning tower 25, a main cleaning tower 26 and an absorbent tank 27. The processing gas 23 is introduced from the upper part of the pre-cleaning tower 25 and passes through the space above the absorption tank 27. It is discharged into the atmosphere from the upper part of the main washing tower 26. In both the pre-cleaning tower 25 and the main cleaning tower 26, the alkaline cleaning liquid 24 is dropped from the upper part thereof, comes into contact with the processing gas 23, and absorbs and removes hydrogen halide. The alkaline cleaning liquid 24 is circulated and used by a circulation pump 28, but alkali may be replenished as the pH decreases,
A new alkali cleaning liquid may be replenished while discharging a part of the alkali cleaning liquid. It should be noted that the absorption device basically does not cause any problem if it can absorb and remove hydrogen halide in the processing gas 23, and the type of the absorption tower and the number of stages thereof may be selected according to this purpose.

In order to control the desorption concentration of the organic halogen compound using the apparatus of FIG. 1, for example, the temperature of the regeneration gas as shown in FIG. 2 is changed up and down at regular time intervals. In this case, it is desirable to increase the temperature of the regeneration gas sequentially as the residual concentration of the organic halogen compound in the adsorbent decreases.
As another example, as shown in FIG. 3, there is a method in which the regeneration gas temperature is changed up and down for one cycle only as in the method shown in FIG. 2, and then the temperature is gradually and linearly increased. In any case, as long as the desorption concentration can be controlled to be constant, it may be either as shown in FIG. 2 or as shown in FIG.

When the desorption treatment is carried out at a constant regeneration gas temperature, the desorption concentration continues to increase with time, eventually reaches the maximum value, and thereafter the desorption concentration gradually continues to decrease. Along with this, a rapid heat generation occurs in the catalytic reaction device, and the reaction temperature also rises rapidly. Although it depends on the structure of the apparatus and the like, when trichloroethylene is taken as an example, the reaction temperature rises by about 100 ° C. at combustion of 5000 ppm and about 200 ° C. at combustion of 10,000 ppm. Since it is quite difficult to control the reaction temperature thus rapidly rising within a certain range by an external heat source, the reaction temperature may exceed 600 ° C. Under such a condition, the specific surface area and the metal surface area of the catalyst are remarkably reduced due to sintering, resulting in accelerated catalyst deterioration.

Further, the decomposition performance also depends on the concentration of volatile organic halides in the processing gas, and for example, the concentration is 5000
In case of ppm and 15000ppm, the concentration is 5000pp
m has higher decomposition performance. Therefore, it is not always preferable to raise the regeneration temperature to raise the desorption concentration too much. Naturally, it is not preferable that the desorption concentration is extremely different. On the other hand, if the temperature of the regenerated gas is lowered and the maximum desorption concentration is optimized, the desorption time will be long and the low desorption concentration state will continue for a long time, which will increase the processing time and eventually reduce the processing efficiency. In addition, since the heat generated by combustion is small, the load on the external heat source increases in order to obtain the reaction temperature required for catalytic decomposition, and the running cost increases.

[0033]

EXAMPLES In the following examples, the analysis value of chlorobenzene is shown as a substitute value for the analysis value of dioxins. Chlorobenzene is a substance that becomes a precursor for the formation of dioxins, and it has already been confirmed that the higher the analysis value of chlorobenzene, the higher the analysis value of dioxins.

The gas analysis was carried out by using GC-MS, and the specific surface area was determined by the nitrogen adsorption BET method.

(Embodiment 1) A vacuum pump was used to supply 11
Air containing 150 ppm of trichloroethylene obtained by aerating the stock solution of trichlorethylene by suctioning at 0 Nm ³ / h was introduced into an adsorption device filled with 20 liters of granular activated carbon made of coconut shell to adsorb trichloroethylene to the activated carbon. The gas concentration after the adsorption treatment was analyzed, and the adsorption treatment was stopped when the trichlorethylene concentration reached 0.1 ppm. During this time, it took about 38 hours.

Then, as shown in FIG.
The regeneration gas obtained by changing the temperature between 1 ° C. and 120 ° C. for 1 hour was charged into the adsorption device at a rate of ³ Nm ³ / h to desorb trichlorethylene and regenerate the activated carbon. The regeneration gas temperature was increased by 10 ° C. at the upper and lower limits every 4 hours. As a result, the concentration of trichlorethylene in the regeneration gas at the outlet of the adsorption device was almost constant as shown by the solid line A in FIG.

This regeneration gas containing trichlorethylene was mixed with 2
Introduced into a vaporizer heated to 00 ° C, steam 10 vol%
Was added and mixed in such a proportion that it was maintained at 200 ° C by a preheating device.

This trichlorethylene-containing regeneration gas is
The catalyst was introduced into a catalytic reaction apparatus filled with 800 ml of a zirconia-phosphoric acid catalyst in which Pt was carried at a rate of 0.5 w%, and catalytically reacted at a reaction temperature of 470 to 510 ° C. The treated gas was introduced into a washing tower and washed with a 5N sodium hydroxide solution. When the concentration of trichlorethylene in the exhaust gas after cleaning was analyzed, it was about 1 ppm, and hydrogen chloride, carbon monoxide, chlorobenzene, etc. were not detected. The specific surface area of the catalyst after use was measured and found to be almost the same value as when it was not used.

(Example 2) As shown in FIG. 3, the temperature of the regeneration gas was first set to 150 ° C. and then lowered to 120 ° C. over 1 hour, after which the temperature of the regeneration gas was raised to 6.7 ° C. /
A treatment test of trichlorethylene was carried out in the same manner as in Example 1 except that the temperature was raised by h.

The same results as in Example 1 were obtained except that the concentration of desorbed trichloroethylene from the activated carbon showed the change with time as shown by the chain line B in FIG.

(Comparative Example 1) The regeneration gas temperature of activated carbon was set to 15
A treatment test of trichlorethylene was conducted in the same manner as in Example 1 except that the temperature was kept constant at 0 ° C.

The concentration of desorption of trichlorethylene from activated carbon shows the change with time as shown by the broken line C in FIG.
It reached 7,000 ppm. At that time, the catalyst reaction temperature is about 6
It was 50 ° C.

When the concentration of trichlorethylene in the exhaust gas after cleaning was analyzed, it was about 1 ppm, and hydrogen chloride, carbon monoxide, chlorobenzene, etc. were not detected. However, when the specific surface area of the catalyst after use was measured, it was reduced by 30% as compared with that when it was not used.

(Comparative Example 2) The same procedure as in Example 1 was carried out except that the temperature of the trichlorethylene-containing regeneration gas was kept constant at 400 ° C by the preheating device.

The concentration of trichlorethylene in the exhaust gas after cleaning was about 1 ppm, and hydrogen chloride, carbon monoxide, etc. were not detected, but about 25 μg / Nm ^{3 of} chlorobenzene was detected. This can be said to indicate that the exhaust gas after cleaning contains harmful dioxins at a high concentration.

When the specific surface area of the catalyst after use was measured, it was about the same as when it was not used, and there was no change in the catalyst itself.

[0047]

According to the method for treating exhaust gas containing a volatile organic halogen compound of the present invention, it is possible to keep the desorption concentration constant, so that it is possible to prevent catalyst deterioration due to generation of excessive reaction heat. Therefore, efficient processing is possible,
Moreover, since the gas before catalytic decomposition is not exposed to high temperature, dioxins and the like are not generated. Therefore, the volatile organic halogen compound can be economically decomposed without the problem of air pollution.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an example of equipment to which a processing method of the present invention is applied.

FIG. 2 is a diagram showing a change with time of a temperature of a regeneration gas adopted in Example 1.

FIG. 3 is a diagram showing a change with time of a temperature of a regeneration gas adopted in Example 2.

FIG. 4 is a diagram showing changes with time in desorption concentration in Examples 1 and 2 and Comparative Example 1.

[Explanation of symbols]

1 ---- stripping device 2,3--adsorption device 4 ---- preheating device 5 ---- catalytic reaction device 6 ---- cleaning device 7 ---- wastewater containing volatile organic halogen compounds 8 ------ Pump 9 --- Air 10 --- Gas containing volatile organic halogen compound 11 --- Processed water 12 --- Air containing volatile organic halogen compound 13,14-Adsorbent 15 ------ Air from which volatile organic halogen compounds have been removed 16 --- Blower 16 ---
Air 17 --- Heater 18 ---
Regeneration gas 19 --- Regeneration gas containing volatile organic halogen compound 20 --- Steam 21 ---
Catalyst layer 22 --- Processing gas 23 ---
Alkaline cleaning liquid 24 --- Preliminary cleaning tower 25 ---
Main washing tower 26 ---- Absorbing liquid tank 27 ---
Circulation pump

Claims (3)

[Claims] 1. An adsorption step of adsorbing a volatile organic halogen compound contained in a waste gas by stripping a volatile organic halogen compound contained in a waste gas, or adsorbing a volatile organic halogen compound contained in a gas to an adsorbent, A desorption step of desorbing an adsorbed component adsorbed from the adsorbent with a regeneration gas, transferring the gas to the gas to regenerate the adsorbent, and a gas for adding and mixing water vapor to the gas containing the desorbed volatile organic halogen compound. Adjustment step, a preheating step of heating the mixed gas obtained in the gas adjustment step, a catalytic decomposition step of decomposing the preheated mixed gas with a decomposition catalyst, and an alkali solution of hydrogen halide produced by the decomposition In the treatment method for volatile organic halides, which consists of a detoxification process of absorption by desorption, the desorption temperature is adjusted so that the desorption concentration becomes almost constant. And a method for treating an exhaust gas containing a volatile organic halogen compound, wherein the temperature for preheating the mixed gas obtained in the gas adjusting step is in the range of 130 to 300 ° C. 2. As a catalyst for decomposing a volatile organic halogen compound, Pt, Pd and R are added to one or more oxides selected from titania, zirconia, boria, phosphorus and the like.
The method according to claim 1, wherein a material carrying one or more kinds of noble metals selected from h, Au and the like is used. 3. The method according to claim 1, wherein the catalytic reaction temperature is 300 to 600 ° C.