Acid gas treatment agent and acid gas treatment method

Abstract

Provided is an acid gas treatment agent and an acid gas treatment method which, in order to reduce the frequency of regeneration treatment of the treatment agent, employ an Mg-Al-based layered double hydroxide that has high adsorption capacity with respect to an acid substance and a long breakthrough time. An acid gas treatment agent according to the present invention comprises an Mg-Al-based layered double hydroxide that has an average pore diameter of not less than 30 nm as measured by a nitrogen gas adsorption method and that has a sulfate ion content of not more than 1 mass%.

Description

Acid gas treatment agent and acid gas treatment method

[0001] The present invention relates to an acid gas treatment agent and an acid gas treatment method for removing acidic substances such as hydrogen chloride, sulfur oxides, and nitrogen oxides from combustion exhaust gas.

[0002] The combustion exhaust gases generated in thermal power plants and waste incineration contain hydrogen chloride, sulfur oxides (SO ｘ ) and nitrogen oxides (NO ｘ Therefore, various methods are being used to treat acid gases containing such acidic substances to remove the acidic substances.

[0003] With regard to a treatment method for removing such acidic substances, the present applicant has proposed a treatment method and treatment agent for acidic exhaust gases that utilizes an Mg—Al-based layered double hydroxide (hereinafter, layered double hydroxide may be abbreviated as LDH (Layered Double Hydroxide)) as an efficient treatment technology capable of simultaneously treating multiple types of acidic substances (see Patent Document 1, Non-Patent Documents 1 and 2). For example, Non-Patent Document 1 describes that a carbonate-type Mg—Al-based LDH can efficiently remove hydrogen chloride gas, and that a carbonate-type Mg—Al-based LDH slurry can remove not only hydrogen chloride gas but also sulfur oxide gas. Non-Patent Document 2 describes that the removal rate of hydrogen chloride gas increases by increasing the amount of carbonate-type Mg—Al-based LDH.

[0004] Furthermore, the present applicant has proposed a system for regenerating and repeatedly using a used treating agent of an Mg-Al-based LDH (see Patent Documents 2 and 3). Patent Document 2 proposes an on-site regeneration method in which treated exhaust gas is used for regeneration, and Patent Document 3 proposes a method for treating acidic exhaust gas using a hydroxide-type Mg-Al-based LDH and for regenerating a used treating agent.

[0005] Patent No. 6954569 Patent No. 6898627 Patent No. 7130197

[0006] Journal of the Society of Inorganic Materials, 2014, Vol.21, p.197-203Atmospheric Pollution Research, 2020, Vol.11, Issue 2, p.290-295

[0007] Conventional Mg-Al-based LDHs used in the treatment of acidic exhaust gases do not necessarily have a high adsorption capacity for acidic substances such as nitrogen oxides and sulfur oxides, and they quickly reach saturation, requiring frequent regeneration treatment, which results in a large burden of labor and cost for the regeneration treatment process. Therefore, there is a demand for Mg-Al-based LDHs with a long breakthrough time in order to increase the adsorption capacity for the acidic substances in acidic exhaust gases and reduce the frequency of regeneration treatment.

[0008] The present invention has been made to solve these problems, and aims to provide an acidic gas treatment agent and an acidic gas treatment method using an Mg-Al-based LDH that has a high adsorption capacity for acidic substances and a long breakthrough time, thereby reducing the frequency of regeneration treatment of the treatment agent.

[0009] The present invention is based on the finding that an acid gas treatment agent having a high adsorption capacity for acidic substances and a long breakthrough time can be obtained by setting the average pore size and sulfate ion content of the Mg—Al-based LDH within a predetermined range.

[0010] The present invention provides the following means: [1] An agent for treating acidic gas, comprising an Mg—Al-based layered double hydroxide having an average pore size of 30 nm or more as measured by nitrogen gas adsorption and a sulfate ion content of 1 mass % or less. [2] The agent for treating acidic gas of [1], wherein the Mg—Al-based layered double hydroxide is in a carbonate or hydroxide form. [3] The agent for treating acidic gas of [1] or [2], wherein the Mg—Al-based layered double hydroxide has an elemental molar ratio of Mg to Al of 2 to 3. [4] The agent for treating acidic gas of any of [1] to [3], wherein the Mg—Al-based layered double hydroxide has a cumulative volume 50% particle diameter D50 of 13.0 to 16.0 μm and a ratio of the cumulative volume 90% particle diameter D90 to the cumulative volume 10% particle diameter D10 (D90 / D10) of 55 to 75. [5] A molded product comprising the agent for treating acidic gas of any of [1] to [4]. [6] A method for treating an acidic gas, comprising contacting the agent for treating an acidic gas according to any one of [1] to [4] or the molded article according to [5] with an acidic gas containing an acidic substance to remove the acidic substance.

[0011] The present invention provides an agent for treating acidic gas and a method for treating acidic gas using an Mg-Al-based LDH that has high adsorption capacity for acidic substances and a long breakthrough time. Consequently, the present invention can reduce the frequency of regeneration of the agent for treating acidic gas and improve the efficiency of removing acidic substances from acidic gas.

[0012] 1 is a graph showing a breakthrough curve in an acid gas adsorption test of an example.

[0013] In this specification, when a numerical range is expressed using "to", the numerical values ​​before and after "to" indicate the lower and upper limits. A preferred numerical range can be formed by arbitrarily combining the preferred lower and upper limits.

[0014] The agent for treating acidic gas of the present invention comprises an Mg—Al-based LDH having an average pore diameter of 30 nm or more as measured by nitrogen gas adsorption method and a sulfate ion content of 1 mass % or less.

[0015] The Mg-Al-based LDHs have a hydroxide base layer ([Mg ２＋ １－ｘ Al ３＋ ｘ (OH)２ ]) and an interlayer anion (A ｎ－ ) and an intermediate layer consisting of interlayer water ([(A ｎ－ ) ｘ／ｎ ・yH ２ These nanoparticles have a structure in which hydroxide base layers have a positive charge equivalent to x, and negatively charged anions that compensate for this are present between the layers. Mg-Al-based LDHs are also found in naturally occurring clay minerals, but are generally synthesized by known methods.

[0016] When an acidic gas is contacted with an Mg-Al-based LDH, it can capture acidic substances such as hydrogen chloride, sulfur oxides, and nitrogen oxides between the layers while maintaining the hydroxide base layer. The interlayer anions are ion-exchanged with anions derived from the acidic substances, thereby exhibiting adsorption capacity for acidic substances. This adsorption capacity for acidic substances decreases as the interlayer anions decrease with the progress of ion exchange, and the agent for treating acidic gases using the Mg-Al-based LDH breaks through. The Mg-Al-based LDH of the agent for treating acidic gases that has broken through in this way can be regenerated and reused by anion exchange with interlayer anions using a known method.

[0017] From the viewpoints of good adsorption ability for acidic substances and ease of repeated use through regeneration, the Mg—Al-based LDH is preferably a carbonate- or hydroxide-type Mg—Al-based LDH in which the interlayer anion is a carbonate ion or a hydroxide ion.

[0018] Furthermore, in the Mg-Al-based LDH, the element molar ratio of Mg to Al (Mg / Al molar ratio) is preferably 2 to 3, more preferably 2 to 2.8, and even more preferably 2 to 2.5, from the viewpoints of the ion (anion) exchange capacity of interlayer anions and the solid solubility limit of aluminum ions, which are trivalent metal ions.

[0019] The agent for treating acidic gas of the present invention has a high adsorption capacity for acidic substances and a long breakthrough time because the Mg—Al-based LDH has an average pore size and a sulfate ion content within a predetermined range.

[0020] Specifically, the average pore diameter measured by nitrogen gas adsorption is 30 nm or more, preferably 31 nm or more, and more preferably 32 nm or more. The average pore diameter measured by nitrogen gas adsorption is calculated as 4V / A from the specific surface area A of the Mg-Al-based LDH determined by the BET single-point method from the adsorption isotherm and the single-point total pore volume V. Specifically, the average pore diameter is determined by measurement using a specific surface area / pore size distribution measurement device.

[0021] There is a correlation between the average pore size of the Mg—Al-based LDH and its adsorption capacity for acidic substances, and an average pore size of 30 nm or more facilitates the intrusion of acidic substances into the adsorption sites between the layers of the Mg—Al-based LDH, thereby enhancing the adsorption capacity for acidic substances. From a practical standpoint, the upper limit of the average pore size is preferably 50 nm or less, more preferably 48 nm or less, and even more preferably 45 nm or less.

[0022] The method for controlling the average pore diameter of the Mg-Al-based LDH is not particularly limited. The average pore diameter can be enlarged to 30 nm or more by, for example, a mechanochemical method, specifically, a method in which the Mg-Al-based LDH is brought into contact with water, dried, and then pulverized and mixed by a mechanochemical method. When mechanical stress is applied to the Mg-Al-based LDH, some of the pores are deformed and enlarged, thereby enlarging the average pore diameter. Examples of pulverization methods include methods using a cone crusher, a roll crusher, a cutter mill, an autogenous pulverizer, a stamp mill, a stone mill, or the like. When the amount of Mg-Al-based LDH to be pulverized is small, a household coffee mill can also be used.

[0023] The sulfate ion content of the Mg—Al-based LDH in the acidic gas treatment agent of the present invention is 1 mass % or less, preferably 0.9 mass % or less, and more preferably 0.8 mass % or less. The sulfate ion content is determined by conversion from the elemental analysis value of sulfur element obtained by X-ray fluorescence spectrometry. Note that the sulfate ions in the Mg—Al-based LDH may be contained, for example, as those derived from the raw materials for producing the Mg—Al-based LDH.

[0024] There is a correlation between the sulfate ion content of the Mg-Al-based LDH and its adsorption capacity for acidic substances, and sulfate ions present between the layers of the Mg-Al-based LDH inhibit the adsorption of acidic substances. When the sulfate ion content is 1 mass% or less, anion exchange between the interlayer anions and anions derived from acidic substances is facilitated, resulting in a significant improvement in the adsorption capacity for acidic substances and a longer breakthrough time for the acid gas treatment agent.

[0025] The method for controlling the sulfate ion content of the Mg—Al-based LDH is not particularly limited. For example, a method for reducing the sulfate ion content to 1% by mass or less includes contacting the Mg—Al-based LDH with an aqueous sodium carbonate solution to replace the sulfate ions contained in the Mg—Al-based LDH with carbonate ions. Contact with an aqueous sodium carbonate solution is preferred because it can increase the pore size of the Mg—Al-based LDH through the effects of hydration swelling and the like.

[0026] When Mg—Al-based LDH is brought into contact with an aqueous sodium carbonate solution, it is preferable to control the average pore size by pulverizing and mixing using a mechanochemical method or the like after contact and drying. Because Mg—Al-based LDH tends to form clumps in a wet state, it is more rational to pulverize it only once after sufficiently drying it as a process for controlling the average pore size and sulfate ion content.

[0027] Furthermore, from the viewpoint of the flow efficiency of acidic gases, the Mg—Al-based LDH in the acidic gas treatment agent of the present invention preferably has a cumulative 50% volume particle diameter D50 of 13.0 to 16.0 μm and a ratio of the cumulative 90% volume particle diameter D90 to the cumulative 10% volume particle diameter D10 (D90 / D10) of 55 to 75. It is more preferable that the Mg—Al-based LDH has a D50 of 13.5 to 15.5 μm and a D90 / D10 ratio of 60 to 70. Note that D50, D10, and D90 can be determined from a volume-based particle size distribution measured using a laser diffraction / scattering particle size analyzer. Specifically, they can be measured by the method described in the examples.

[0028] The form of the acidic gas treatment agent of the present invention is not particularly limited as long as the average pore size and sulfate ion content of the Mg—Al-based LDH are within the ranges specified in the present invention. The acidic gas treatment agent of the present invention may be processed as appropriate from the viewpoint of its usage mode and ease of handling, and may be in the form of granules granulated by a known method such as extrusion granulation. It can also be used as a molded product formed into pellets, spherical particles, or the like using a binder or the like. That is, the molded product of the present invention contains the acidic gas treatment agent.

[0029] From the viewpoint of ease of handling, the particle size of the molded body is preferably 1 to 5 mm, more preferably 2 to 4 mm, and even more preferably 2.5 to 3.5 mm. Here, the particle size of the molded body corresponds to the diameter if the molded body is spherical, and in the case of a pellet, it means the length of the part where the gap between the plates is greatest when the pellet is sandwiched between two parallel plates (the maximum gap between the two plates). A molded body with a particle size of 1 to 5 mm is a granular material that does not pass through a sieve with 1 mm openings but passes through a sieve with 5 mm openings when sieved.

[0030] The method for treating an acidic gas of the present invention involves contacting the above-described agent or molded article for treating an acidic gas of the present invention with an acidic gas containing the acidic substance to remove the acidic substance. The acidic substance is not particularly limited, but as described above, for example, hydrogen chloride, sulfur oxides, and nitrogen oxides can be suitably adsorbed and removed by the agent or molded article for treating an acidic gas of the present invention.

[0031] As described above, the acidic gas treatment agent of the present invention exhibits the excellent effects of the present invention by virtue of the Mg-Al-based LDH having an average pore size and a sulfate ion content within a predetermined range. However, the agent can also be used in combination with, for example, an LDH composed of another metal element that has the ability to adsorb acidic substances.

[0032] The present invention will be described in detail below with reference to examples. The present invention is not limited to the following examples, and various modifications are possible within the scope of the present invention.

[0033] [Sample Analysis] The average pore diameter and sulfate ion content of the samples in the following Examples and Comparative Examples were measured as follows. (Average Pore Diameter) The average pore diameter was measured by nitrogen gas adsorption using a specific surface area / pore distribution measuring device ("BELSORP (registered trademark)-mini", manufactured by Microtrac-Bell Corporation). (Sulfate Ion Content) Elemental analysis was performed using an X-ray fluorescence analyzer, and the sulfur element content was converted into the sulfate ion content.

[0034] (Particle diameter at 50% cumulative volume D50, particle diameter at 90% cumulative volume D90, and particle diameter at 10% cumulative volume D10) Using a laser diffraction / scattering particle size distribution analyzer (MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.), a sample powder was added to a sample circulator in which a 0.025% by mass aqueous solution of sodium hexametaphosphate was circulated (transmittance 80 to 95%), and the volume-based particle size distribution was measured to determine D50 and D90 / D10.

[0035] [Sample Preparation] (Example 1) 8.6 g of sodium carbonate was dissolved in 1 L of ion-exchanged water and heated to 70°C to prepare an aqueous sodium carbonate solution. 100 g of Mg—Al-based LDH (1) (manufactured by Sakai Chemical Industry Co., Ltd.; Mg / Al molar ratio 2) was added to this aqueous sodium carbonate solution and stirred at 70°C for 1 hour. The resulting mixture was filtered, and the solids on the filter paper were washed with ion-exchanged water until the conductivity of the wash liquid was 100 μS / cm or less. The washed solids were dried at 105°C and then ground in a household coffee mill for 30 seconds to obtain a sample of Mg—Al-based LDH (2) (average pore diameter: 32.0 nm, sulfate ion content: 0.55% by mass, D50: 14.5 μm, D90 / D10: 65.8).

[0036] Comparative Example 1 The Mg—Al-based LDH (1) used in Example 1 was used as a sample without being treated (average pore diameter: 28.5 nm, sulfate ion content: 2.76 mass %, D50: 30.2 μm, D90 / D10: 81.0).

[0037] (Comparative Example 2) The Mg-Al-based LDH (1) used in Example 1 was ground for 30 seconds with a household coffee mill to obtain a sample of Mg-Al-based LDH (3) (average pore diameter: 29.5 nm, sulfate ion content: 2.75 mass%, D50: 12.0 μm, D90 / D10: 48.7).

[0038] [Acid Gas Adsorption Test] 0.15 g of sample was placed in a reaction tube (inner diameter 17 mm) packed with glass wool, and the glass wool was placed on top to prepare a reaction tube with the sample placed therein so as to have breathability. The reaction tube was heated in a tubular electric furnace set at a temperature of 150°C. Hydrogen chloride gas was passed through the reaction tube at a downward flow (linear velocity 1.0 m / min) while controlling the supply rate with a mass flow controller to a concentration of 300 ppm by mass (balance gas: nitrogen). The concentration of hydrogen chloride gas at the reaction tube outlet was measured using an infrared spectrometer for gas analysis.

[0039] In FIG. 1, the horizontal axis represents the flow time of hydrogen chloride gas, and the vertical axis represents the concentration ratio of hydrogen chloride gas at the inlet and outlet of the reaction tube (outlet concentration C ｏｕｔ / inlet concentration C ｉｎ 1 shows the breakthrough curves of the samples when the temperature was 100°C or higher. As can be seen from FIG. 1, the samples of Comparative Examples 1 and 2 had breakthrough times of less than 60 minutes, whereas the sample of Example 1 had a long breakthrough time of 110 minutes or more. Furthermore, the sample of Example 1 had a smaller concentration ratio of hydrogen chloride gas at the outlet of the reaction tube than the samples of Comparative Examples 1 and 2, confirming that the sample had a high adsorption capacity for hydrogen chloride gas.

Claims

1. An acid gas treatment agent comprising an Mg-Al-based layered double hydroxide having an average pore diameter of 30 nm or more as measured by nitrogen gas adsorption method and a sulfate ion content of 1 mass% or less.

2. The agent for treating acidic gas according to claim 1, wherein the Mg-Al-based layered double hydroxide is a carbonate type or a hydroxide type.

3. The agent for treating acidic gas according to claim 1, wherein the elemental molar ratio of Mg to Al in the Mg-Al based layered double hydroxide is 2-3.

4. The acidic gas treatment agent according to claim 1, wherein the Mg-Al-based layered double hydroxide has a cumulative 50% volume particle diameter D50 of 13.0 to 16.0 μm and a ratio of the cumulative 90% volume particle diameter D90 to the cumulative 10% volume particle diameter D10 (D90 / D10) of 55 to 75.

5. A molded article comprising the acid gas treatment agent according to claim 1.

6. A method for treating an acidic gas, comprising contacting the agent for treating an acidic gas according to claim 1 or the molded article according to claim 5 with an acidic gas containing an acidic substance, thereby removing the acidic substance.

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