JP2005095786A - Purifying method of ammonia-containing exhaust gas and ammonia-containing waste water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a purifying method of ammonia-containing exhaust gas efficiently purifying ammonia-containing exhaust gas by remarkably reducing a discharge amount of nitrous oxide, and a purifying method of ammonia-containing waste water efficiently treating the ammonia-containing exhaust gas generated in the case of purifying the ammonium-containing waste water by stripping ammonia from the ammonia-containing waste water, by remarkably reducing a discharge amount of nitrous oxide. <P>SOLUTION: Ammonia is decomposed by bringing the ammonia-containing exhaust gas into contact with an ammonia decomposing catalyst, then bringing the treated gas into contact with the nitrous oxide decomposing catalyst to decompose nitrous oxide contained in the treated gas. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for purifying ammonia-containing exhaust gas and a method for purifying ammonia-containing waste water.

When the ammonia-containing exhaust gas is contacted with an ammonia decomposition catalyst to decompose and purify ammonia in the exhaust gas into nitrogen gas and water, the treated gas in addition to undecomposed ammonia is a by-product of nitric oxide. And nitrogen dioxide (hereinafter collectively referred to as “NOx”) and nitrous oxide (N ₂ O). However, these ammonia, NOx, and nitrous oxide are harmful substances to the environment, and it is not environmentally preferable to discharge the processing gas containing them into the atmosphere as they are.

  In order to solve the above problem, it has been proposed that NOx is brought into contact with a reduction catalyst together with residual ammonia to be decomposed into nitrogen gas and water (see Patent Document 1). However, nitrous oxide is not removed here, and nitrous oxide remains in the treated gas. This nitrous oxide is a harmful substance that affects global warming and the destruction of the ozone layer. The negative impact on the environment becomes a problem.

  For the purpose of preventing the generation of nitrous oxide, ammonia-containing exhaust gas is introduced into a two-stage catalyst layer comprising an ammonia oxidation catalyst layer and a nitrogen oxidation-reduction catalyst layer, A method has been proposed in which the nitrogen oxide concentration of the gas is measured, a part of the processing gas is divided so that the nitrogen oxide concentration becomes a predetermined concentration, and injected into the subsequent nitrogen redox catalyst layer (Patent Document) 2).

  Similarly to the above method, using a two-stage catalyst layer comprising an ammonia oxidation catalyst layer and a nitrogen oxidation-reduction catalyst layer, the nitrogen oxide concentration is always 50 ppm or less relative to the unreacted ammonia concentration in the ammonia oxidation catalyst. A method of controlling the temperature of the ammonia oxidation catalyst layer in the previous stage has been proposed (Patent Document 3).

  In the techniques described in Patent Documents 2 and 3, ammonia is decomposed so that nitrous oxide is not generated as much as possible in the preceding catalyst layer, and NOx produced as a by-product in the preceding catalyst layer is dividedly injected from the processing gas in advance in the latter catalyst layer. By using the remaining ammonia or unreacted ammonia remaining by controlling the temperature of the previous stage ammonia oxidation catalyst layer, it is decomposed in a well-balanced manner so as not to generate nitrous oxide, and the entire process is attempted to reduce nitrous oxide. It is.

  When purifying ammonia-containing wastewater, it is generally performed that ammonia-containing wastewater is subjected to stripping treatment, and the ammonia-containing exhaust gas thus obtained is brought into contact with a catalyst to decompose ammonia. The problem of nitrous oxide generation occurs.

  Therefore, a method for adjusting the gas flow rate in the catalyst layer or adjusting the contact time of the gas in the catalyst layer is proposed so that the concentration of nitrous oxide generated when decomposing ammonia is within a predetermined range. (Patent Document 4).

  Further, a method has been proposed in which the oxygen concentration in the gas introduced into the catalyst layer is adjusted so that the generated nitrous oxide concentration falls within a predetermined range when decomposing ammonia (Patent Document 5).

  The techniques described in Patent Documents 4 and 5 use a single-stage catalyst layer composed of a catalyst having a specific composition, adjust the gas flow rate in the catalyst layer, or adjust the oxygen concentration in the gas, thereby reducing nitrous oxide. This is to decompose ammonia into nitrogen and water while suppressing by-products.

Japanese Patent Laid-Open No. 54-1978857 JP-A-10-309437 Japanese Patent Laid-Open No. 10-314548 JP 2002-52381 A JP 2002-66538 A

  In any of the above prior arts, in order to reduce the nitrous oxide concentration, it is necessary to strictly define the operating conditions, and a complicated control system is necessary to realize this. For this reason, when the concentration of ammonia or NOx in the exhaust gas to be treated fluctuates, there is a possibility that the response is delayed and nitrous oxide is discharged.

  Therefore, the present invention is intended to provide a method for purifying ammonia-containing exhaust gas that efficiently purifies ammonia-containing exhaust gas by significantly reducing nitrous oxide emissions.

  In addition, the present invention is an ammonia-containing wastewater that efficiently treats ammonia-containing exhaust gas that is generated by stripping ammonia from wastewater containing ammonia, significantly reducing emission of nitrous oxide. It is intended to provide a purification method

According to the studies by the present inventors, it has been found that the object can be achieved by the following invention.
(1) A method for purifying ammonia-containing exhaust gas, which purifies exhaust gas containing ammonia according to a process including the following steps.
(A) A process of decomposing ammonia by bringing ammonia-containing exhaust gas into contact with an ammonia decomposition catalyst.
(B) A step of bringing the processing gas from step (a) into contact with a nitrous oxide decomposition catalyst to decompose nitrous oxide in the processing gas.
(2) A method for purifying ammonia-containing wastewater, comprising purifying ammonia-containing wastewater according to a process including the following steps.
(C) A step of transferring ammonia into the gas phase by subjecting the ammonia-containing waste water to a stripping treatment.
(D) A step of decomposing ammonia by bringing the ammonia-containing gas from step (c) into contact with an ammonia decomposition catalyst.
(E) A step of decomposing nitrous oxide in the processing gas by bringing the processing gas from step (d) into contact with a nitrous oxide decomposition catalyst.

  According to the present invention, it is possible to efficiently purify ammonia-containing exhaust gas and ammonia-containing wastewater without maintaining strict operating conditions or installing a complicated control system for realizing this.

  According to the present invention, it is possible to reduce the concentrations of residual ammonia, NOx and nitrous oxide contained in the exhaust gas after the purification treatment to such an extent that there is no environmental problem.

  Further, according to the present invention, even if the ammonia concentration in the exhaust gas to be treated fluctuates, the concentration of residual ammonia, NOx and nitrous oxide in the exhaust gas after the purification treatment is promptly changed in response to this. It can be reduced to the extent that there is no problem.

  First, the method for purifying ammonia-containing exhaust gas of the present invention will be described.

  The method of the present invention basically comprises (a) a step of bringing ammonia-containing exhaust gas into contact with an ammonia decomposition catalyst, and (b) a step of bringing the treatment gas from step (a) into contact with a nitrous oxide decomposition catalyst. And the step of decomposing nitrous oxide in the processing gas.

  In the step (a), ammonia in the exhaust gas is decomposed while suppressing by-product generation of NOx to the extent that there is no environmental problem. Therefore, the catalyst used, the processing conditions, etc. when carrying out the step (a) are such that, as described above, ammonia in the exhaust gas can be decomposed while suppressing by-product generation of NOx to the extent that there is no environmental problem. What is necessary is just to select suitably.

  It should be noted that the concentration of the present invention that does not cause environmental problems means that ammonia is 5 ppm or less, NOx is 50 ppm or less, and nitrous oxide is 50 ppm or less. Therefore, in the step (a), ammonia is oxidatively decomposed so that the ammonia concentration and the NOx concentration in the processing gas become 5 ppm or less and 50 ppm or less, respectively.

Examples of the ammonia decomposition catalyst are as follows.
(1) Oxide catalyst comprising Co, Ce and Mn (2) Cu as the first active component, Fe, Ni, V, Mo, W, Na, K, Li, Mg, Ba and Sn as the second active component Catalyst having a structure in which one or more selected oxides are supported on an oxide support (3) One or more selected from oxide support and Ag, or Ag and Fe, Mn, Zn, Mo, W and W (4) (A) a composite oxide of Ti-Si, Ti-Zr or Ti-Si-Zr, (B) at least one oxide selected from V, W and Mo, C) Catalyst containing at least one precious metal selected from Pt, Pd, Rh, Ru and Ir or a compound thereof (5) Catalyst in which Cu ions are exchanged and Pt is supported on ZSM-5 (6) γ-Al ₂ O ₃ carrier Ni, it was supported Mn oxide Perovskite oxide catalyst represented by medium (7) A′xA1-xBO3-y (A ′: Ca, Sr, A * La, B: Mn, Fe, Co) (8) Cu ion exchange Y-type zeolite, Or Cu supported Al ₂ O ₃ catalyst (9) MoO ₃ supported SiO ₂ catalyst (10) Composite oxide catalyst represented by BaMnAl ₁₁ O ₁₉ (11) At least one selected from Pt, Pd, Rh, Ru and Ir (12) Catalyst containing Cu-Mn composite oxide as an active component The ammonia-containing exhaust gas to be purified in the present invention usually has an ammonia concentration of about 100 to 30,000 ppm. belongs to. In the step (a), in order to oxidatively decompose such ammonia, a necessary amount, specifically, an equivalent amount or more of oxygen is supplied as air or an oxygen-containing gas according to a conventional method.

  Since the processing gas from step (a) contains nitrous oxide, the processing gas from step (a) is introduced into step (b), where the processing gas contacts the nitrous oxide decomposition catalyst. To decompose nitrous oxide to an extent that there is no environmental problem. Therefore, the catalyst used and the treatment conditions for carrying out the step (b) are appropriately selected so that the nitrous oxide in the treatment gas can be decomposed to an extent that there is no environmental problem, specifically to 50 ppm or less. do it.

Examples of the nitrous oxide decomposition catalyst are as follows.
(13) A catalyst in which Ag is supported on an alumina carrier having a BET surface area of 5 to 25 m ² / g (14) A catalyst in which Cu is supported on a zeolite composed of silicon and oxygen represented by the formula (SiO ₂ ) ₅₅ (15) Catalyst comprising at least one noble metal selected from Rh, Pd and Ru on SiO ₂ or SiO ₂ —Al ₂ O ₃ (16) Fe ion exchange β-type zeolite (17) Rh ₂ O ₃ or Co ₂ O ₃ and Catalyst composed of these mixtures (18) A composite in which only zinc oxide obtained by firing a hydrotalcite-type compound containing Zn (II) ions, Rh (II) ions and Al (III) ions exists in a crystalline state Oxide catalyst The above-mentioned ammonia decomposition catalyst and nitrous oxide decomposition catalyst can be used as appropriate. Alternatively, when (4) is used and (15) or (17) is used as a nitrous oxide decomposition catalyst, NOx is hardly produced as a by-product, and ammonia-containing exhaust gas is reduced while greatly reducing nitrous oxide. This is preferable because it can be efficiently purified.

In this case, the gas temperature in the ammonia decomposition catalyst (1) or (4) is 250 to 450 ° C., preferably 300 to 400 ° C., and the space velocity (SV) is 500 to 100,000 hr ⁻¹ , preferably 1000 to 50000 hr ^{−. 1} . Further, the gas temperature in the nitrous oxide decomposition catalyst (15) or (17) is 300 to 600 ° C., preferably 350 to 550 ° C., and the space velocity (SV) is 1000 to 100,000 hr ⁻¹ , preferably 3000 to 50000 hr ^{−. 1} .

  The ammonia decomposition catalyst and the nitrous oxide decomposition catalyst may be charged in two different reaction tubes, or the ammonia decomposition catalyst and the nitrous oxide decomposition catalyst may be charged in two stages in one reaction tube.

  Since the concentrations of ammonia, NOx and nitrous oxide contained in the processing gas obtained through the above steps (a) and (b) are such that there is no environmental problem, the processing gas is released into the atmosphere as it is. be able to.

  Next, a method for purifying ammonia-containing wastewater will be described.

  The method of the present invention basically includes (c) a step of subjecting ammonia-containing wastewater to stripping treatment to transfer ammonia into the gas phase, and (d) ammonia-containing gas from step (c) as an ammonia decomposition catalyst. It comprises a step of decomposing ammonia by contacting, and (e) a step of decomposing nitrous oxide in the processing gas by contacting the processing gas from step (d) with a nitrous oxide decomposition catalyst.

  In the step (c), ammonia in the ammonia-containing waste water is stripped and transferred to the gas phase. This stripping process can be performed according to a generally known method. As the carrier gas, water vapor or heated air can be used.

  Step (d) is for decomposing ammonia in the ammonia-containing exhaust gas obtained in step (c), and is the same as step (a). Therefore, step (d) may be performed in the same manner as described in step (a) above.

  Step (e) decomposes nitrous oxide contained in the processing gas from step (d), and is the same as step (b). Therefore, step (e) may be performed in the same manner as described in step (b) above.

  Since the concentrations of ammonia, NOx and nitrous oxide contained in the processing gas obtained through the above steps (d) and (e) are such that there is no environmental problem, the processing gas is released into the atmosphere as it is. be able to.

  According to the present invention, nitrogen-containing compounds other than ammonia, for example, amines such as methylamine, dimethylamine, and trimethylamine, imines such as ethyleneimine, nitriles such as acetonitrile, and gas containing hydrocyanic acid can be similarly purified. it can. In the present invention, the term “exhaust gas containing ammonia” includes gases containing nitrogen-containing compounds as described above in addition to ammonia-containing exhaust gases. Similarly, the term “wastewater containing ammonia” in the present invention means methylamine, dimethylamine, and trimethylamine that can be transferred from the wastewater to the gas phase by stripping treatment in the same manner as ammonia in addition to ammonia-containing wastewater. Such waste water containing nitrogen-containing compounds is also included.

  The invention is further illustrated by the following examples, which illustrate advantageous embodiments of the invention.

<Ammonia decomposition catalyst 1>
873 g of cobalt (II) nitrate hexahydrate, 1303 g of cerium (II) nitrate hexahydrate and 1292 g of manganese (II) nitrate hexahydrate were uniformly dissolved in pure water. The aqueous solution was heated to evaporate water and obtain a mixed powder containing Co, Ce, and Mn. Next, this mixed powder was fired at 550 ° C. for 5 hours to prepare an oxide powder composed of Co, Ce, and Mn. After adding 400 g of γ-alumina powder to 800 g of this oxide powder, mixing to form a spherical shape (average particle diameter 5 mm), drying at 120 ° C. for 5 hours, and then firing at 550 ° C. for 3 hours to obtain Co A -Ce-Mn based ammonia decomposition catalyst was obtained. The catalyst composition was Co ₃ O ₄ : CeO ₂ : MnO ₂ : Al ₂ O ₃ = 1: 2.1: 1.6: 2.4 (mass ratio).
<Ammonia decomposition catalyst 2>
Titanium oxide powder (trade name DT-51, manufactured by Millennium Co., Ltd.) was formed into a spherical shape (average particle size 5 mm), dried at 120 ° C. for 5 hours, and then fired at 500 ° C. for 3 hours. This spherical titanium oxide was impregnated with an aqueous silver nitrate solution. After impregnation, it was dried at 120 ° C. for 5 hours and then calcined at 500 ° C. for 3 hours. Next, it was impregnated with an aqueous ferrous sulfate solution, dried at 120 ° C. for 5 hours, and then calcined at 500 ° C. for 3 hours to obtain an ammonia decomposition catalyst 2. The composition of this catalyst was TiO ₂ : Ag: Fe = 80: 17: 3 (mass ratio).
<Nitrous oxide decomposition catalyst>
Γ-alumina pellets (manufactured by Sumitomo Chemical Co., Ltd., average particle size 5 mm) are immersed in an aqueous rhodium nitrate solution adjusted to a predetermined concentration for 5 minutes, then dried at 120 ° C. for 5 hours, and then at 600 ° C. for 5 hours. Rh-supported γ-Al ₂ O ₃ catalyst was prepared by calcining for a period of time. The catalyst composition was Rh: Al ₂ O ₃ = 5: 95 (mass ratio).
Example 1
A gas inlet side of a stainless steel reaction tube having an inner diameter of 45 mm and a length of 950 mm was filled with 1140 ml of the ammonia decomposition catalyst, and the gas outlet side was filled with 140 ml of the nitrous oxide decomposition catalyst. Then, a simulated ammonia-containing gas composed of an ammonia concentration of 4000 ppm, a water vapor concentration of 20%, and the remaining air is heated and adjusted to a predetermined temperature (360 ° C., 380 ° C. and 400 ° C.), and then ammonia is supplied at a supply rate of 12 L / min (STP). It flowed to the cracking catalyst layer and then to the nitrous oxide cracking catalyst layer. When the temperature of each catalyst layer becomes almost the same as the temperature of the simulated ammonia-containing gas, measure the ammonia concentration, NOx concentration and nitrous oxide concentration in the treated gas from the nitrous oxide decomposition catalyst layer according to the following method The ammonia conversion rate was determined according to the following formula. The results are shown in Table 1.
Ammonia concentration:
A certain amount of gas is passed through an absorption bottle containing 0.5 mass% boric acid aqueous solution to absorb the ammonia contained in the gas into the boric acid aqueous solution, and the boric acid aqueous solution containing ammonia after the absorption operation is purified. After adjusting to a predetermined amount with water, the absorbed ammonia was calculated by quantifying NH ₄ ⁺ in the liquid with a cation chromatograph manufactured by Hitachi, Ltd., and the ammonia concentration in the gas was calculated.
NOx concentration:
Measurement was performed by a chemiluminescence method using a NOx meter MODEL42C manufactured by Nippon Thermo Electron.
Nitrous oxide concentration:
It was measured with a Shimadzu gas chromatograph (TCD).
Ammonia conversion (%):
(4000 ppm-ammonia concentration in treated gas (ppm)) / (4000 ppm) (x100)
Example 2
In Example 1, the treatment of the simulated ammonia-containing gas was performed in the same manner as in Example 1 except that the ammonia decomposition catalyst 1 was changed to the ammonia decomposition catalyst 2. The results are shown in Table 1.
Comparative Example 1
In Example 1, the simulated ammonia-containing gas was treated in the same manner as in Example 1 except that the latter nitrous oxide decomposition catalyst layer was not provided. The results are shown in Table 1.

  In Table 1, the concentrations of NOx and nitrous oxide are volume concentrations based on wet gas.

From Examples 1 and 2 and Comparative Example 1, the concentration of nitrous oxide in the treatment gas can be significantly reduced by providing a treatment step (step (b)) using a nitrous oxide decomposition catalyst.

Claims (2)

A method for purifying ammonia-containing exhaust gas, comprising purifying exhaust gas containing ammonia according to a process including the following steps.
(A) A process of decomposing ammonia by bringing ammonia-containing exhaust gas into contact with an ammonia decomposition catalyst.
(B) A step of bringing the processing gas from step (a) into contact with a nitrous oxide decomposition catalyst to decompose nitrous oxide in the processing gas. A method for purifying ammonia-containing wastewater, comprising purifying wastewater containing ammonia according to a process including the following steps.
(C) A step of transferring ammonia into the gas phase by subjecting the ammonia-containing waste water to a stripping treatment.
(D) A step of decomposing ammonia by bringing the ammonia-containing gas from step (c) into contact with an ammonia decomposition catalyst.
(E) A step of decomposing nitrous oxide in the processing gas by bringing the processing gas from step (d) into contact with a nitrous oxide decomposition catalyst.