A NOx removal catalyst containing an inorganic fiber

Abstract: NOx removal catalyst can be obtained by kneading various kinds of inorganic fibers and water containing a surfactant or its similars to make a pulp-like substance, adding a catalystic component thereto and then further kneading the resultant mixture to prepare a catalyst paste. The thus obtained catalyst paste is shaped into a honeycomb-like, plate-like or column-like form according to conventional various molding methods, and then dried and calcinated for use. Instead of the above catalystic components, catalystic carriers such as titanium, zeolite, zirconia and the like may be shaped and calcinated in the same manner and used, carrying a catalystic component. A practically high performance and strength can be obtained to bear a good result when a composition produced by calcinating molybdenum (Mo), vanadium (V) or tungsten (W) compound born on titanium oxide at 400 to 700°C is used.

Description: The present invention relates to a NOx removal catalyst containing an inorganic fiber and a process for producing the same, more particularly to a NOx removal catalyst containing an inorganic fiber with a high strength to be used as a NOx removal catalyst for ammonia catalystic reduction. PRIOR ART NOx contained in flue gases from power stations, various factories and automobiles etc. is a substance to cause photochemical smog and acid rain. As an effective NOx removal method, a method of removing NOx in flue gases by selective catalystic reduction using ammonia (NH 3 ) as a reducing agent is widely used, mainly in thermal power stations. As a catalyst, there is used a titanium oxide (TiO 2 )-based catalyst containing vanadium (V), molybdenum (Mo) or tungsten (W) as an active component. In particular, a titanium oxide-based catalyst containing vanadium as one of active components is a mainstream NOx removal catalyst nowadays, since not only it has high activity but also it is less deteriorated by impurities contained in an exhaust gas and can be used even at a low temperature (Japanese Laid-Open Patent Publication No. 128681/1975, etc.). The above catalysts are applied to the above-desired exhaust gas treatment by being formed into a particulate, plate-like or honeycomb-like shape to fill a catalystic reactor with. Nowadays, a method of removing NOx by catalystic reduction using ammonia is used mainly for purifying exhaust gases from boilers for electricity generation, large-capacity boilers and gas turbines. The amount of a catalyst to be used is several hundreds to one thousand m 3 per plant and usually a catalyst is not changed for 2 to 10 and several years. It is necessary that a catalyst to be employed for such use is in a large form and has a high strength so that the wear and impairment of the catalyst do not occur for a long term. Accordingly, many inventions have been made including a method of reinforcing a catalyst by adding an inorganic fiber into the catalyst in order to improve the strength of the catalyst (Japanese Laid-Open Patent Publication No. 65191/1977), a method of obtaining a dense catalystic material by adding an organic or inorganic binder at the time of forming a catalyst (Japanese Laid-Open Patent Publication No. 122293/1977), and a method of obtaining a forming material with a high strength by sintering catalystic particles by calcination at a high temperature. Among the above, a method of adding an inorganic fiber is generally employed widely since it can improve the fragility of a sintering material and obtain a large catalyst with a high strength. In the above prior arts, a method has been employed, which comprises kneading, as required, a binder component and water with a catalyst or a catalystic carrier component powder in advance to make a paste-like or clay-like substance and adding an inorganic fiber thereinto. Generally speaking, inorganic fibers such as a silica-alumina-based fiber and a non-alkali glass fiber are originally low in affinity with water. In addition to it, since they are spun by using various mineral oils, the surface of fibers are coated with a very small amount of oil and their water repellency further increases. When such an inorganic fiber 1 is made to mix with a catalyst paste 2 as above, the inorganic fiber 1 repels the paste 2 and the inorganic fiber 1 alone becomes a large lump as shown in Fig. 4 (A) and is hard to mix with the paste 2. When an attempt of mixing it with the paste 2 by further kneading is made, the lump of the inorganic fiber 1 is pressed to be dense. At the same time it is cut and, as shown in Fig. 4 (B), it becomes dispersed in the paste 2 in a form of a small, hard lump. It is remarkably difficult to make the inorganic fiber 1 uniformly disperse in the paste 2 once the inorganic fiber 1 falls into such a state. Therefore, the prior art has had no choice but to use a catalyst paste in a state of Fig. 4 (B) in one degree or another. The catalyst paste 2 containing a small lump of an inorganic fiber 1 shown in Fig. 4 (B) is poor in paste properties and not suitable for a molding method such as extrusion molding and rolling molding usually employed for molding a catalyst. In addition, since the added inorganic fiber 1 is omnipresent, an improving effect of strength cannot be obtained against expectation. Thus, said catalyst paste 2 has some problems. In Fig. 5 (A), (B), (C) is shown a plate-like product having a gather portion 3 of a predetermined pitch, which is produced by putting a catalyst paste 2 made of a silica-alumina-based fiber mixed with a catalystic component according to the prior art between two inorganic fiber 1 woven cloths (ceramic screens) 4, 5, passing said paste 2 between a pair of pressurizing-coating rollers 6, 7 at the upper and lower sides to coat the ceramic screens 4, 5 by pressurizing with said paste 2 and shaping it into a form. In the gather portion 3 occurs peeling between the upper and the lower screens 4, 5, and the rigidity of a catalystic material reduces remarkably. Because of this, deflection of the catalystic material occurs, and the gather portion 3 collapses and extends. The reason of it seems to be that there exists a lump of an inorganic fiber 1 containing water (trade name: Kaowool (phonetic)) in the catalyst paste 2 and that water separates at the time of pressurizing and coating. FR-A-2 435 967 relates to a process for the production of a catalyst and a support for said catalyst, which comprises treating inorganic heat-resistant fibers with water to form an aqueous slurry, mixing said slurry with a catalytically active compound to form a paste and shaping, drying and calcinating said paste. SUMMARY OF THE INVENTION The object of the present invention is to provide a NOx catalyst containing an inorganic fiber and a process for producing the same characterized by that, in a process for producing a catalyst to which an inorganic fiber is added, the difficulty of an inorganic fiber to mix is improved to obtain a catalyst paste suitable for various molding methods and that simultaneously an improving effect of strength due to the added inorganic fiber can be fully exhibited by removing the omnipresence of the inorganic fiber. In order to accomplish the above-mentioned object, a first invention of the present application relates to a NOx removal catalyst containing an inorganic fiber obtained by kneading an inorganic fiber and water containing a surfactant to disperse the fiber in a pulp-like admixture, adding a catalytic component to said pulp-like admixture, and further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture and shaping the paste-like mixture into a predetermined form and then drying and calcining the resultant product. A second invention of the present application relates to a process for producing a NOx removal catalyst containing an inorganic fiber, which comprises kneading the hydrophobic inorganic fiber and water containing a surfactant until the hydrophobic nature of the inorganic fiber is changed to hydrophilic, to disperse the fiber in a pulp-like admixture; and then adding a catalytic component to said pulp-like admixture; further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture; and shaping the paste-like mixture into a predetermined form and then drying and calcining it to obtain a shaped catalytic material. A third invention of the present application relates to a NOx removal catalyst containing an inorganic fiber obtained by kneading a hydrophobic inorganic fiber and water containing a surfactant until the hydrophobic nature of the inorganic fiber is changed to hydrophilic, to disperse the fiber in a pulp-like admixture and then adding a catalytic carrier substance to said pulp-like admixture; further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture; then shaping the paste-like mixture to obtain a shaped catalytic material; then adding a catalytically active component to the mixture, shaping the mixture into a predetermined form and then drying and calcining it to obtain a shaped, supported catalyst. A fourth invention of the present application relates to a process for producing a NOx removal catalyst containing an inorganic fiber characterized by kneading a hydrophobic inorganic fiber and water containing a surfactant until the hydrophobic nature of the inorganic fiber is changed to hydrophilic, to disperse the fiber in a pulp-like admixture; and then adding a catalytic carrier substance to said pulp-like admixture; further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture; then shaping the paste-like mixture to obtain a shaped catalytic material; then adding a catalytically active component to the mixture, shaping the mixture into a predetermined form and then drying and calcining it to obtain a shaped, supported catalyst. In each invention of the present application, it is preferable that as an inorganic fiber is used a silica-alumina-based fiber or a non-alkali glass fiber, and that as a surfactant is used methyl cellulose, alkyl sodium sulfonate or polyvinyl alcohol. However, both an inorganic fiber and a surfactant to be used in the present invention are not restricted to the above-mentioned substances. In Fig. 2 is shown a process for producing 2 kinds of catalysts of the present invention. When water containing a surfactant and an inorganic fiber are kneaded by a kneader in advance according to the present invention, the inorganic fiber becomes wet with water due to the effect of the surfactant and part of it is cut and dispersed in water to obtain a pulp-like water-inorganic fiber aggregate. When an oxide of a catalytic component is added thereto, catalytic component particles suspend into water containing an inorganic fiber as shown in Fig. 1 (A), differing from a conventional method of Fig. 4 and then kneaded. Because of this, as shown in Fig. 1 (B) showing an intermediate state before reaching the final paste, it is in a state in which the gap of aggregation 11 of catalytic component particles is filled with a coagulation 12 of inorganic fiber 1 containing catalyst paste 2 to be easily kneaded. Accordingly, such a phenomenon as seen in the prior art does not occur that an inorganic fiber 1 containing air is compressed to become a hard lump and that the inorganic fiber 1 remains ununiform after kneading. When the kneading time is lengthened, the inorganic fiber 1 is dispersed easily and uniformly in the catalyst paste 2 as shown in Fig. 1 (C), and an improving effect of strength, which is the aim of the addition of an inorganic fiber 1, can be fully exhibited. In addition to the above-mentioned effect, the paste 2 with an inorganic fiber 1 dispersed uniformly therein is excellent in a rolling property into a honeycomb-like form. Molding with few defects and distortion can be obtained from said paste 2 and it is also easy to obtain moldings with a high strength from said paste. Though the slight omnipresence of an inorganic fiber 1 added to the paste 2 leads to defects such as the bend and cutting of moldings, particularly in honeycomb molding of small pitch cells of 3 to 5 mm, said defects can be greatly reduced by using the paste 2 with s fiber 1 dispersed uniformly therein of the present invention. According to the present invention, there can be provided a catalyst, which is characterized by that at the production of a catalyst by adding an inorganic fiber 1, the mixing of the inorganic fiber 1 can be easily and uniformly carried out and that said catalyst can exhibit improving effects of strength due to the added inorganic fiber 1 and that of Young's modulus of elasticity. By this can be obtained a catalyst with a high strength free from wear and impairment for a long term, particularly a catalyst with a high strength to be used in a large NOx removal apparatus. Moreover, since the mixing of an inorganic fiber 1 becomes easy, the kneading time shortens. Thus, it is also effective in the production cost. Furthermore, a catalyst paste 2 with an inorganic fiber 1 dispersed uniformly therein can be obtained easily. By employing the present process in honeycomb extrusion molding, wherein the omnipresence of an inorganic fiber 1 tends to cause the bend and crack of moldings, can be obtained moldings with few defects and a high strength, needless to say. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 (A), (B) and (C) are schematic views showing the behavior of an inorganic fiber in the process of producing a catalyst to show the characteristics of the catalyst of the present invention. Fig. 2 shows a process for producing 2 kinds of catalysts of the present invention. Fig. 3 shows the piston extrusion pressure and the displacement of paste obtained in Example 1-3 and Comparative Example 1 and 2 of the present invention. Fig. 4 shows the process of kneading an inorganic fiber in the case of a conventional process for producing a catalyst. Fig. 5 shows a plate-like catalyst containing an inorganic fiber obtained according to the prior arts. EXAMPLES (i) Outline of the Constitution The present invention is a catalyst and a process for producing the same, said catalyst being characterized, as mentioned above, by kneading various kinds of inorganic fibers and water containing a surfactant or its similars (hereinafter referred to as a surfactant) by a kneader to make a pulp-like substance, adding a catalytic component thereto and then further kneading the resultant mixture to prepare a catalyst paste. The thus obtained catalyst paste is shaped into a honeycomb-like, plate-like or column-like form according to conventional various molding methods, and then dried and calcinated for use. Instead of the above catalytic components, catalytic carriers such as titanium, zeolite, zirconia and the like may be shaped and calcined in the same manner and used, carrying a catalytic component. (ii) Mutual Effects and Effects of Constitutional Parts The surfactant to be used here to make an inorganic fiber hydrophilic are as mentioned above and the amount to be added is not particularly restricted. Usually, in order to impart hydrophilic nature to a fiber, it is used in an amount of exceeding 0 ( not including 0 ) to about 5 weight %. When the amount is too small, an effect of removing the hydrophobic nature of an inorganic fiber is insufficient. When the amount is too large, the binding of catalystic particles is prevented and catalystic strength reduces to the contrary. For mixing an inorganic fiber and a surfactant, mixing and kneading machines such as a kneader and stirrer with a propeller are used. Since an inorganic fiber is easily ground, such a kneader with high grindability as a ball mill etc. is unsuitable. Besides, when an inorganic fiber of a long fiber is used, it gives a good effect when it is cut to a certain length. On the other hand, whatever catalytic components they may be, the effect of the present invention is exhibited. In the case of a NOx removal catalyst, a practically high performance and strength can be obtained to bear a good result when a composition produced by calcining molybdenum (Mo), vanadium (V) or tungsten (W) compound supported on titanium oxide at 400 to 700°C is used. Concretely, a catalytic component to be used is produced as follows: mixing a slurry or a power of titanium oxide, orthotitanate or metatitanate with water added, an oxide of molybdenum, vanadium or tungsten, and a compound forming an oxide by thermal decomposition such as oxo acid salt etc., evaporating water according to an ordinary method of producing a catalyst such as heating and kneading, evaporating to dryness, etc. for carrying, drying the obtained paste and further calcining it at 400 to 700°C to use as a catalytic component. In order to add the above to a pulp-like substance of an inorganic fiber and knead, the above-mentioned kneader can be used and kneading is carried out till the water content of the paste becomes 20 to 40 %. In the case of the above NOx removal catalyst, the obtained catalyst paste is shaped into a honeycomb-like, column-like or cylindrical form by means of an extruder, or a plate-like form by coating a metal base such as a metal lath etc., a ceramic or a glass net woven cloth by means of a roller. The shaped product is then cut into a required shape, formed, dried and calcined a temperature in the range of 400 to 700°C. Hereinafter, the present invention will be explained minutely according to concrete examples. Example 1 To 67 kg of a slurry of metatitanate (TiO 2 content: 30 weight %, SO 4 content: 8 weight %) was added 7.50 kg of ammonium paratungstate ((NH 4 )10H 10 ·W 12 O 41 ·6H 2 O). The mixture was kneaded by a heat-kneader while evaporating water to obtain a paste containing about 36 % of the water content. The resultant paste was extruded into a column-like shape of 3, to make particles. The obtained particles were dried by a fluid-bed dryer and calcined at 550°C for 2 hours in an atmosphere. The obtained granules were pulverized by means of a hammer mill so that the granules with a diameter of 1 µm or less are 60 % or more and a catalytic powder was obtained. The composition of W/Ti was 1/9 (atomic ratio). On the other hand, to 5.2 kg of a Al 2 O 3 ·SiO 2 -based inorganic fiber (trade name: Kaowool (phonetic), SiO 2 /Al 2 O 3 weight ratio: about 1) was added 30 g of a methyl cellulose-based surfactant (trade name: Metholose (phonetic)) dissolved in 9 kg of water. The mixture was kneaded by means of a kneader for 5 minutes to be pulp-like. To the resultant product was added 20 kg of the above catalystic powder and the mixture was further kneaded for 30 minutes to obtain a catalyst paste containing 27 % of water. Examples 2 and 3 The procedure as in Example 1 was repeated except that 6 g of sodium dodecyl sulfate and 10 g of polyvinyl alcohol were used instead of the methyl cellulose-based surfactant of Example 1 to obtain a catalyst paste. Example 4 To 67 g of a slurry of metatitanate (TiO 2 content: 30 weight %, SO 4 content: 8 weight %) were added 2.5 kg of ammonium paramolybdate ((NH 4 )6Mo 7 O 24 ·4H 2 O) and 4.2 kg of ammonium metavanadate (NH 4 VO 3 ). The mixture was kneaded by means of a heat-kneader while evaporating water to obtain a paste containing about 36 % of water. The resultant paste was extruded into a column-like shape of 3 to make particles. The obtained particles were dried by a fluid-bed dryer and calcinated at 550°C for 2 hours in an atmosphere. The obtained granules were pulverized by means of a hammer mill so that the granules with a diameter of 1 µm or less are 60 % or more and a catalytic powder was obtained. The composition of V/Mo/Ti was 4.5/5/90.5 (atomic ratio). A catalyst paste was obtained using this catalytic powder in the same manner as in Example 1. Example 5 The same procedure as in Example 1 was repeated except that 5.2 kg of a filament wool of a non-alkali glass fiber (fiber diameter: 6 µm) was used instead of the inorganic fiber of Example 1 to prepare a catalyst paste. Comparative Example 1 The same procedure as in Example 1 was repeated except that the surfactant of Example 1 was not used to prepare a catalyst paste. Comparative Example 2 Instead of the kneading method of Example 1, 9 kg of water containing a surfactant was added to 20 kg of a catalystic powder. To the mixture was added 5.2 kg of a SiO 2 ·Al 2 O 3 -based inorganic fiber of Example 1 and kneaded by a kneader for 30 minutes to obtain a paste-like substance. Comparative Example 3 The same procedure as in Example 4 was repeated except that the kneading method of Example 4 was replaced by that of Comparative Example 2 to prepare a catalyst paste. Test Example 1 In order to evaluate the properties of the paste obtained in Examples 1-3 and Comparative Examples 1-2, an extrusion test was carried out using a piston extrusion molder with a cylinder diameter of 20 mm and a mouthpiece diameter of 3 mm, and extrusion pressure and the displacement of the piston were measured. The results are shown in Fig. 3. The results show that the catalyst paste of the present invention can be extruded under a low and specific pressure. It is shown in the pastes of Comparative Examples 1 and 2 that pressure increases according to small displacement and the pastes solidify at the portion of the mouthpiece or a lump of an inorganic fiber contained in the paste blocks the mouthpiece gradually. Thus, the process for producing the catalyst of the present invention is a suitable method for obtaining a paste suited for extrusion molding. Test Example 2 A net-like substance base imparted with rigidity to be used as a base for the coating of a catalyst paste was obtained by impregnating an inorganic fiber woven cloth, which is produced by plainly weaving a yarn composed of 1,400 filaments with a diameter of 9 µm of an E glass at a rate of 10 yarn per inch, with a slurry with a weight ratio of SiO 2 /polyvinyl alcohol/TiO 2 of 10/2/50 and drying it at 150°C. Between two bases were put the catalyst pastes of Examples 1-5 and Comparative Examples 1-3, and a plate-like catalyst with a thickness of about 1 mm was obtained by contact bonding of a catalyst at the intervals and the surface of the meshes of the bases by passing said pastes between a pair of pressurizing rollers at the upper and lower sides. The obtained catalyst was dried at 180°C for 2 hours and calcined at 550°C for 2 hours in an atmosphere. The obtained plate-like catalyst was cut into a test sample with a length of 50 mm and a width of 20 mm, and strength and Young's modulus of elasticity were measured by means of an all-purpose bending tester. The results are shown in Table 1 Catalyst paste used Strength (kg/cm 2 ) Young's modulus of elasticity (kg/mm 2 ) Example 1 240 151 Example 2 226 122 Example 3 208 98 Example 4 256 135 Example 5 230 110 Example 6 210 185 Comp.Example 1 152 32 Comp.Example 2 163 45 Comp.Example 3 145 56 The above reveals that the catalysts using the pastes of Examples have a high strength, compared with those using the pastes of Comparative Examples. In addition, Young's modulus of elasticity of Examples is almost three times that of Comparative Examples. Thus, according to the process of the present invention, it is possible to obtain a catalyst with a high strength and remarkably excellent elastic properties. The addition of an inorganic fiber aimes at improving the fragility of a metallic oxide molded product and the present invention exhibits a remarkable effect in this point. Example 6 The same procedure as in Example 1 was repeated except that a slurry of metatitanate was used instead of ammonium paratungstate to obtain a titanium-based catalytic carrier paste. A plate-like product with a thickness of about 1 mm was obtained by contact bonding of the paste on the bases in the same manner as in Test Example 2. The resultant product was dried, calcined impregnated with a solution containing ammonium metavanadate and calcined at 450°C. In the present Example, too, a catalyst with a high strength and high Young's modulus of elasticity was obtained. The result is shown in Table 1.

Claims: A NOx removal catalyst containing an inorganic fiber obtained by kneading an inorganic fiber and water containing a surfactant to disperse the fiber in a pulp-like admixture, adding a catalytic component to said pulp-like admixture, and further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture and shaping the paste-like mixture into a predetermined form and then drying and calcining the resultant product. The NOx removal catalyst containing an inorganic fiber as set forth in claim 1, wherein said inorganic fiber is a silica-alumina-based fiber or a non-alkali glass fiber, and said surfactant is methyl cellulose, alkyl sodium sulfonate or polyvinyl alcohol. A process for producing a NOx removal catalyst containing an inorganic fiber obtained by kneading the hydrophobic inorganic fiber and water containing a surfactant until the hydrophobic nature of the inorganic fiber is changed to hydrophilic, to disperse the fiber in a pulp-like admixture; and then adding a catalytic component to said pulp-like admixture; further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture; and shaping the paste-like mixture into a predetermined form and then drying and calcining it to obtain a shaped catalytic material. A process for producing the NOx removal catalyst containing an inorganic fiber as set forth in claim 3, in which said inorganic fiber is a silica-alumina-based fiber or a non-alkali glass fiber, and said surfactant is methyl cellulose, alkyl sodium sulfonate or polyvinyl alcohol. A NOx removal catalyst containing an inorganic fiber obtained by kneading a hydrophobic inorganic fiber and water containing a surfactant until the hydrophobic nature of the inorganic fiber is changed to hydrophilic, to disperse the fiber in a pulp-like admixture and then adding a catalytic carrier substance to said pulp-like admixture; further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture; then shaping the paste-like mixture to obtain a Shaped catalytic material; then adding a catalytically active component to the mixture, shaping the mixture into a predetermined form and then drying and calcining it to obtain a shaped, supported catalyst. A NOx removal catalyst containing an inorganic fiber as set forth in claim 5, in which said inorganic fiber is a silica-alumina-based fiber or a non-alkali glass fiber, and said surfactant is methyl cellulose, alkyl sodium sulfonate or polyvinyl alcohol. A process for producing a NOx removal catalyst containing an inorganic fiber obtained by kneading a hydrophobic inorganic fiber and water containing a surfactant until the hydrophobic nature of the inorganic fiber is changed to hydrophilic, to disperse the fiber in a pulp-like admixture; and then adding a catalytic carrier substance to said pulp-like admixture; further kneading the resultant mixture to make a uniform dispersion of said fiber in a paste-like mixture; then shaping the paste-like mixture to obtain a shaped catalytic material; then adding a catalytically active component to the mixture, shaping the mixture into a predetermined form and then drying and calcining it to obtain a shaped, supported catalyst. A process for producing the NOx removal catalyst containing an inorganic fiber as set forth in claim 7, in which said inorganic fiber is a silica-alumina-based fiber or a non-alkali glass fiber, and said surfactant is methyl cellulose, alkyl sodium sulfonate or polyvinyl alcohol.

Inventors: KATO YASUYOSHI, KONISHI KUNIHIKO, KATO, YASUYOSHI, KONISHI, KUNIHIKO
Applicants: BABCOCK HITACHI KK, BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1993
Main classification: B01D53
IPC codes: B01J23, B01D53, B01J35, B01J37, B01J21