Use of porous membrane in reaction process

Abstract: A porous membrane (11) adapted for use in a reaction process, the porous membrane or a porous layer therein being formed with interconnected micropores having an average pore size less than 100 nm and containing uniformly distributed therein a catalytic component such as molyb­ denum sulfide, platinum-alumina or palladium-silica-­ alumina.

Description: The present invention relates to a chemical reaction process employing a porous membrane. In the field of gas-separation in which a gas diffusion method is adapted to separate a specific gas from a mixture of gases, there has been utilized a porous membrane which is formed with interconnected micropores having an average pore size of between several nanometres and several ten nanometres (several ten Å and several hundred Å), smaller than the mean free patch of gas molecules. JP-A-59-59223 discloses such a porous membrane adapted to separate hydrogen gas from a mixture of hydrogen and nitrogen or carbon monoxide the molecular weight ratio of which is relatively large. The separation of hydrogen gas from the mixture is carried out based on the difference in the rates of permeability of the gases in Knudsen flow. In recent years, it has been found that the characteristic of the porous membrane in gas separation is useful for a chemical reaction process. Where the reaction rate of a catalytic reaction process (A ⇄ B + P) is noticeably decreased due to the pressure of product gas P or the reaction is restricted by equilibrium, the porous membrane is effective to exhaust the product gas P from the reaction system therethrough to enhance the percent conversion of the product gas and the efficiency of separation and concentration of the product gas P. On Pages 58-61 of Chemical Engineering issued on February, 1984, such a reaction process is disclosed as a decomposition reaction process of hydrosulfide (H₂S = H₂ + S), wherein as shown in Fig. 5, the porous membrane is in the form of a porous glass pipe 1 which is filled with catalytic particles 2 of molybdenum sulfide to effect the decomposition reaction of raw material gases supplied therein and to concentrate the product gas, dependent upon a pressure difference between the interior and exterior of pipe 1. In the reaction process described above, however, insufficient separation of the product gas sometimes occurs due to insufficient membrane area per unit volume of the porous membrane and insufficient contact of the product gas with the porous membrane. This results in low percent conversion of the product gas. Additionally, the presence of catalytic particles (2) results in pressure loss of the raw material gases passing through the glass pipe 1, and the pores of glass pipe 1 are clogged with powder of the catalytic particles. It is, therefore, a primary object of the present invention to provide a process employing a porous membrane, which enhances the percent conversion of the product gas in the chemical reaction and the efficiency of separation and concentration of the product gas without causing any of the problems as described above. According to the present invention, there is provided a process for effecting a chemical reaction employing a porous membrane which is formed with numerous interconnected micropores and is superior in selective permeability and catalytic activity. The process comprises applying the reaction fluid to a porous membrane comprising a porous support (13a, 14a or 15a) formed with interconnected micropores, a porous thin layer (13b, 14c or 15c) of thickness less than 500 µm formed with interconnected micropores having an average pore size less than 100 nm (1000 Å), and an optional further porous layer (14b or 15b) formed with interconnected micropores which is present either between the support (14a) and said porous thin layer (14c) or on the side of the support (15a) remote from said porous thin layer (15c), all layers present being integrally formed, either the porous support or the optional further porous layer being formed of, or containing uniformly distributed therein, a catalyst for catalyzing the chemical reaction process in reaction fluid applied thereto, and the porous support (13a, 14a or 15a) and the optional further porous layer (14b or 15b) having a pore size greater than the pore size of said porous thin layer (13b, 14c or 15c). The fluid is applied to the side of the porous membrane remote from the porous thin layer, so that reaction occurs in the catalyst-containing layer, and the product gas is separated from the fluid through the porous thin layer by Knudsen diffusion. Examples of the catalytic component are molybdenum sulfide, platinum-alumina and palladium-silica-alumina essentially in a uniform condition. For a better understanding of the present invention, and to show how it may be used to effect a chemical reaction, reference will be made, by way of example, to the accompanying drawings, in which: Figs. 1 to 3 schematically illustrate respective enlarged cross-sections of porous membranes used in the present invention; Fig. 4 (a) is a sectional view of a reaction apparatus in which the porous membranes each are utilized as a module of the apparatus; Fig. 4 (b) is a front view of the reaction apparatus depicted along arrows in Fig. 4 (a); and Fig. 5 is a sectional view of a conventional reaction apparatus. Fig. 1 illustrates a double layered porous membrane 13 which is composed of a porous support 13a containing therein a catalytic component essentially in a uniform condition, and a porous thin layer 13b integrally formed on the porous support 13a. In the double layered porous membrane 13, it is preferable that the porous support 13a is a made of an inorganic raw material such as alumina, silica-alumina, mullite, cordierite, zirconia or carbon. The porous support 13a is formed in a desired configuration under the same conditions as the conventional conditions for forming a ceramic porous support and subjected to a firing or heating treatment. During the forming process of the porous support 13a, the catalytic component raw material may be uniformly mixed with the inorganic raw material, for example by co-precipitation or kneading, and the mixture used to form the porous support 13a. Alternatively, the inorganic raw material is formed into the desired configuration and the catalytic compound raw material is added to the inorganic raw material by means of impregnation, adsorption or ion-exchange. The catalytic raw material may be a precious metal catalyst such as platinum-alumina or palladium-silica-alumina as appropriate for the chemical reaction process. When the porous membrane is used to effect decomposition of hydrogen sulphide, molybdenum sulphide is used as the catalytic component raw material. The porous support 13a is formed with numerous interconnected micropores having an average pore size of about 0.5 µm to 30 µm, preferably about 0.5 µm to 5 µm. In this instance, the average pore size of the micropores is chosen so as to reliably effect gas diffusion and to eliminate the occurrence of cracks or pin-holes in the porous support. From the point of view of strength and processability, it is preferable that the thickness of the porous support be about 1 mm. The porous thin layer is made of an inorganic raw material such as alumina, silica-alumina or zirconia and is integrally formed on one surface of the porous support 13a by means of a sol-gel method, a high pressure adherence method of fine powder, an adherence method of porous glass or the like. The porous thin layer 13b is formed with numerous interconnected micropores having an average pore size less than 100 nm (1000 Å). The thickness of the porous thin layer 13b is less than 500 µm, desirably 1 nm (10 Å) to 100 µm. This is preferable to reliably effect gas-diffusion and to eliminate the occurrence of cracks or pin-holes in the porous thin layer. In this respect, it has been also found that the average pore size of the micropores in porous thin layer 13b and the thickness of the same is closely related to the selective permeability of the double layered porous membrane 13. In the double layered porous membrane 13, it is advantageous that the porous support 13a contains therein a sufficient amount of catalytic component so as to enhance the catalytic activity of the porous membrane 13. In this sense, the porous support 13a itself may be made of the catalytic component. In Fig. 2 there is schematically illustrated a triple layered porous membrane 14 which is composed of a porous support 14a, a first porous thin layer 14b integrally formed on the porous support 14a, and a second porous thin layer 14c integrally formed on the first porous thin layer 14b. In the porous membrane 14, the first porous thin layer 14b contains therein the same catalytic component as that of porous support 13a, and the second porous thin layer 14c is formed to provide selective permeability. In Fig. 3 there is schematically illustrated another triple layered porous membrane 15 which is composed of a porous support 15a, and first and second porous thin layers 15b and 15c integrally formed on the opposite surfaces of the porous support 15a. In the porous membrane 15, the first porous thin layer 15b contains therein the catalytic component, and the second porous thin layer 15c is formed to provide selective permeability. The composition of the respective porous supports 14a and 15a is substantially the same as that of the inorganic component of porous support 13a. The catalytic component of the respective porous thin layers 14b and 15b is substantially the same as that of the porous support 13a. The composition of the respective porous thin layers 14c and 15c is substantially the same as that of the porous thin layer 13b. In addition, the average pore size of respective micropores in the first porous thin layers 14b, 15b and the thickness of the same are greater than those of the porous thin layer 13b, and the second porous thin layers 14c, 15c may be modified to contain therein the catalytic component. In a practical embodiment of the present invention, the pore size in the foregoing porous membranes 13-15 can be adjusted by the composition of the raw material, mechanical treatment, heat treatment or firing. For instance, the adjustment of the pore size to less than 100 nm (1000 Å) can be carried out by the following method. In a sol-gel process for gelidifying fine sol-primary particles obtained by hydrolysis of metal-alcoxide, the sol-primary particles are merely gelidified to provide a porous body formed with numerous micropores having an average pore size less than 20 nm (200 Å). To obtain a porous body with numerous micropores having an average pore size of about 20 nm (200 Å) to about 100 nm (1000 Å), the sol-primary particles are temporarily fired at a temperature below 900°C to obtain fine powder thereof. Subsequently, an appropriate amount of the fine powder is added to the sol-particles or pulverized in a dry condition to be used as a carrier slurry. In a method for manufacturing porous glass, the average pore size in the porous membrane can be adjusted by an appropriate selection of the composition of phase-separable glass particles, the temperature of phase-separation treatment, and the melting treatment condition. In an adherence method of fine powder under a high pressure, an isostatic press method is applied to powder of particle diameter less than 1 µm to obtain a porous body formed with micropores having an average pore size less than 100 nm (1000 Å). In a practical embodiment of the present invention, each of the porous membranes 13-15 is used as a desired module in the form of, for example, a plate member, a pipe element or a honeycomb structure. Assuming that in such a module an amount of reaction fluid flows along one side of the porous membrane, a chemical reaction of the fluid is caused by catalytic activity of the porous membrane such that a specific product fluid is separated from the reaction fluid and concentrated by the selective permeability of the porous membrane. In such a use of the porous membrane, it is not necessary to contain catalytic particles within the porous membrane pipe as in the conventional reaction process. For this reason, the membrane area in a volume unit of the porous membrane is increased to ensure sufficient contact of the reaction fluid with the porous membrane, the pressure loss of the reaction fluid is decreased, and clogging of the pores in the porous membrane is avoided. It is, therefore, possible to enhance percent conversion of the product fluid from the reaction fluid. Owing to these facts, the reaction apparatus can be constructed in a small space. Although the porous membrane can be utilized in various reaction processes for a dehydrogenation reaction, a hydrogenation reaction, a hydrogenolysis reaction and the like, it is desirable that the porous membrane be utilized in a reaction process for gas-separation, for instance, in a process for collecting hydrogen from purge gases in a decomposition reaction of hydrosulfide, ammonia synthesis or methanol synthesis. Hereinafter, the respective manufacturing methods of the porous membranes 13-15 for use in a decomposition reaction process of hydrosulfide will be described in detail. Example 1 For manufacturing a porous membrane 13 in the form of a pipe element, molybdenum sulfide having an average particle diameter of 1 µm was added to an organic binder and formed by extrusion into a pipe-shaped element. The pipe-shaped element was dried at a temperature of 100°C for two hours, fired at a temperature of 400°C for three hours in atmospheric air, and further heat treated at a temperature of 800°C for three hours in a flow of nitrogen to obtain a pipe-shaped porous support (13a) of molybdenum sulfide. Subsequently, boehmite-sol obtained by hydrolysis of aluminum isopropoxide was applied to cover the external surface of the pipe-shaped porous support (13a) and fired at a temperature of 400°C for three hours in atmospheric air to form a porous thin layer (13b) on the external surface of the pipe-shaped porous support (13a). The composition of the resultant porous pipe element is listed in the Table. Example 2 For manufacturing a porous membrane 14 in the form of a pipe element, slurry A in the form of molybdenum sulfide having an average particle diameter of 1 µm was applied to cover the internal surface of a pipe-shaped porous support (14a) of cordierite, dried at a temperature of 100°C for two hours, thereafter fired at a temperature of 400°C for three hours in atmospheric air, and further heat treated at a temperature of 800°C for three hours in a flow of nitrogen to form a first porous thin layer (14b) on the internal surface of the pipe-shaped porous support (14a). Subsequently, slurry B in the form of boehmite-sol obtained by hydrolysis of aluminum isopropoxide was applied to cover the internal surface of the first porous thin layer (14b), dried at a temperature of 100°C for two hours, thereafter fired at a temperature of 400°C for three hours in the atmospheric air to form a second porous thin layer (14c) on the first porous thin layer (14b). The composition of the resultant porous pipe element is listed in the Table. Example 3 For manufacturing the porous membrane 15 in the form of a pipe element, the pipe-shaped porous support of Example 2 was used as a pipe-shaped porous support (15a), and the slurry A of Example 2 was applied to cover the internal surface of the pipe-shaped porous support (15a), fired and heat treated in the same manner as in Example 2 to form a first porous thin layer (15b) on the internal surface of the pipe-shaped porous support (15a). Additionally, the slurry B of Example 2 was applied to cover the external surface of the pipe-shaped porous support (15a) and fired in the same manner as in Example 2 to form a second porous thin layer (15c) on the external surface of the pipe-shaped porous support (15a). The composition of the resultant porous pipe element is listed in a column of Example 3 on the table. Comparative Example A first comparative example 1 was prepared in the form of a porous pipe element (16) made of cordierite and filled with beads of molybdenum sulfide having an average particle diameter of 1mm. A second comparative example 2 was prepared in the form of a porous pipe element (17) made of cordierite, the external surface of which was covered by the slurry B of Example 2, fired and heat treated to provide a porous thin layer (14c), and filled with beads of molybdenum sulfide. The respective compositions of the porous pipe elements (16) and (17, 14c) are listed in each column of Comparative Examples 1 and 2 on the table. Test The respective porous pipe elements in the foregoing Examples 1-3 and Comparative Examples 1 and 2 were tested for the decomposition reaction of hydrosulfide in a reaction apparatus such as is illustrated in Figs. 4(a) and 4(b). The reaction apparatus comprises a plurality of parallel pipe elements 10 mounted within a cylindrical casing 20. In this test, the respective porous pipe elements in the foregoing Examples were used as the parallel pipe elements 10 to measure the decomposition activity of them in reaction of hydrosulfide. The cylindrical casing 20 is formed at its opposite ends with an inlet port 21 and a first outlet port 22 and formed at its intermediate portion with a second outlet port 23. In using the reaction apparatus, an amount of hydrosulfide gas was supplied into the parallel pipe elements 10 through the inlet port 21 of casing 20, a mixture of sulfur gas and non-reacted hydrosulfide gas was exhaused from the first outlet port 22 of casing 20, and the hydrogen gas produced was exhausted from the second outlet port 23 of casing 20. The test results given in the table were obtained by measurement of concentration of the separated hydrogen gas using a gas chromatographic adsorption under the following condition. SV *1 : 500hr⁻¹ Pressure of supplied gas : 3.8 atm Pressure at the second outlet port : 0.5 atm Reaction temperature 800 °C where SV *1 represents an amount of supplied gas per a volume unit of catalyst. From the table, it will be understood that in Examples 1-3, the concentration of hydrogen gas exhaused from the second outlet port 23 was a high. This means that the porous pipe elements in Examples 1-3 were effective to enhance percent conversion of hydrogen gas from hydrosulfide and the efficiency of separating and concentrating the hydrogen gas. In contrast, that in the Comparative Examples 1 and 2, the concentration of hydrogen gas was low.

Claims: A process for effecting a chemical reaction in a reaction fluid to yield at least one product gas, which process comprises applying the fluid to a membrane comprising a porous support (13a, 14a or 15a) formed with interconnected micropores, a porous thin layer (13b, 14c or 15c) of thickness less than 500 µm made of alumina, silica-alumina or zirconia and having an average pore size less than 100 nm (1000 A), and an optional further porous layer (14b or 15b) formed with interconnected micropores which is present either between the support (14a) and said porous thin layer (14c) or on the side of the support (15a) remote from said porous thin layer (15c), all layers present being integrally formed, either the porous support or the optional further porous layer being formed of, or containing uniformly distributed therein, a catalyst for catalysing the chemical reaction process in the reaction fluid applied thereto, and the porous support (13a, 14a or 15a) and the optional further porous layer (14b or 15b) having a pore size greater than the pore size of said porous thin layer (13b, 14c or 15c), wherein the fluid is applied to the side of the porous membrane remote from the porous thin layer (13b, 14c or 15c), the reaction occurs in the catalytic layer, and the product gas is separated from the fluid through the porous thin layer (13b, 14c or 15c) by Knudsen diffusion. A process according to claim 1 wherein the catalyst is molybdenum sulfide. A process according to claim 1 wherein the catalyst is a precious metal catalyst on a support. A process according to claim 3 wherein the supported catalyst is platinum-alumina or palladium-silica-alumina. A process according to any one of the preceding claims wherein the porous support (13a, 14a or 15a) is made of an inorganic raw material. A process according to claim 5 wherein the porous support (13a, 14a or 15a) is made of alumina, silica-alumina, mullite, cordierite, zirconia or carbon. A process according to any one of the preceding claims wherein said porous thin layer (13b, 14c or 15c) is integrally formed on a porous support (13a, 14a or 15a) having an average pore size of 0.5 to 30 µm. A process according to any one of claims 1 to 6 wherein the porous membrane consists of, in order, a porous support (14a), a further porous layer (14b) as the catalytic layer, and a porous thin layer (14c). A process according to any one of claims 1 to 6 wherein the porous membrane consists of, in order, a further porous layer as the catalytic layer (15b), a porous support (15a), and a porous thin layer (15c).

Inventors: ABE FUMIO, ABE, FUMIO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1986
Main classification: B01J35
IPC codes: B01J19, B01J35, B01D71, B01D53, B01J15, B01D69, B01J8, B01J4