Exhaust purifier

Abstract

To provide an exhaust gas purification device that can improve the exhaust gas purification performance. [Solution] An exhaust gas purification device 1 is provided with a cylindrical casing 2, in which a plurality of gas passages 6 extending in the axial direction of the casing 2 are arranged radially within the casing 2, and a catalyst for purifying exhaust gas G flowing through the gas passages 6 is supported on the inner surface of the gas passages 6, wherein the flow velocity u of the exhaust gas G differs depending on the position inside the casing, and the flow length Lc of the gas passages 6 at positions with a high flow velocity u is longer than the flow length Lo of the gas passages 6 at positions with a low flow velocity u.

Description

Technical Field

[0001] The present invention relates to an apparatus for purifying exhaust gas by removing pollutants from the exhaust gas generated by combustion, and particularly to an apparatus for purifying exhaust gas by oxidizing or reducing pollutants using a catalyst.

Background Art

[0002] A honeycomb structure as an example of a substrate for supporting a catalyst for purifying exhaust gas (exhaust gas) generated by combustion in an internal combustion engine is described in Patent Document 1. The honeycomb structure is configured by forming a large number of cells serving as exhaust gas flow paths in a plane direction perpendicular to the flow path. In the honeycomb structure described in Patent Document 1, the density and shape of these cells are made different between the central portion and the outer peripheral portion. That is, the honeycomb structure of Patent Document 1 aims to improve the mechanical strength in a honeycomb structure in which two or more cell structures are separated by a boundary wall, and to avoid or suppress catalyst clogging when the catalyst is supported. The cell structure is made different between the outside and the inside of the boundary wall provided at a position of a predetermined radius, and the cell thickness at the intersection of the partition walls is thickened in a predetermined range on both sides sandwiching the boundary wall. Further, in the drawings described in Patent Document 1, the orientation of the square cells on the inner peripheral side and the outer peripheral side of the boundary wall is approximately 45 degrees different, and the opening area is smaller in the cells on the inner peripheral side.

[0003] On the other hand, a honeycomb structure aimed at improving catalyst performance is described in Patent Document 2. The honeycomb structure described in Patent Document 2 is a structure that focuses on the boundary film that inevitably occurs in the flow path, has a plurality of parallel gas flow paths, and a large number of cut portions with an interval of 300 mm or less are provided so as to cross the gas flow paths.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

【Patent Document Japanese Patent Application Publication No. 11-290699 [Overview of the project] [Problems that the invention aims to solve]

[0005] It is known that when a fluid such as exhaust gas flows inside a closed pipe, such as a cylindrical pipe, the flow velocity in the center of the pipe is faster than the flow velocity on the outer periphery. In the honeycomb structure described in Patent Document 1, the flow velocity in the cells on the central side is faster than the flow velocity in the cells on the outer periphery. However, in the structure described in Patent Document 1, a boundary wall is provided at a location radially outward from the center of the flow path, and the wall thickness at the intersection of the partition wall is increased in a predetermined range on the inner and outer circumferential sides of the boundary wall. As a result, the opening area of ​​the cells, i.e., the flow path, is reduced, making it difficult for the fluid to flow. In particular, the opening area of ​​the cells on the inner circumferential side of the boundary wall is smaller than the opening area of ​​the cells on the outer circumferential side, which hinders or suppresses the flow of fluid in the central side where the flow velocity is high. If this is used as a catalyst, contact between the exhaust gas and the catalyst may be hindered, potentially reducing the exhaust gas purification performance.

[0006] Furthermore, when examining the structure of Patent Document 1 in terms of boundary film diffusion resistance as described in Patent Document 2, the structure of Patent Document 1 has a uniform cell length, i.e., flow path length. Therefore, if the length of the cells on the central side where the flow velocity is high is made to match the flow velocity, the length of the cells on the outer side where the flow velocity is low becomes excessively long, and the purification performance decreases due to the boundary film contact resistance. In other words, the flow path length does not match the flow velocity on either the inner or outer side, and the overall exhaust gas purification performance of the catalyst may not be sufficiently high.

[0007] On the other hand, in the structure described in Patent Document 2, the boundary film diffusion resistance can be reduced by shortening the catalyst length to such an extent that the velocity boundary layer caused by the gas flow does not develop sufficiently, but the channel length at which the velocity boundary layer develops is unknown. Therefore, in the honeycomb catalyst described in Patent Document 2, the relationship between the honeycomb length and the mass transfer coefficient is determined by experiment, and based on this relationship, the spacing of the notches is set to 300 mm or less. However, the boundary film begins to develop immediately after the exhaust gas enters the gas channel, and in a gas channel of about 300 mm, the majority of it is filled with the boundary film. The honeycomb catalyst described in Patent Document 2 is a technology targeting honeycomb denitrification catalysts, and is a large catalyst targeting exhaust gases such as gases emitted from various factories, etc., where the pollutants to be removed, the composition of the exhaust gas, and the amount of emissions fall within a specific range. With this type of catalyst, it may be possible to improve the diffusion of exhaust gas to the catalyst, but this effect is not attributed to a reduction in the influence of the boundary film, and at least it does not reduce the boundary film diffusion resistance. In other words, the technology described in Patent Document 2 cannot be generalized, and when dealing with exhaust gases that differ from factory exhaust gases in terms of combustion form, quantity, composition, and pollutants to be removed, such as those from vehicle engines, the purification efficiency is not necessarily good, and there is still room for improvement.

[0008] This invention was made in view of the above-mentioned technical problems, and aims to provide an exhaust gas purification device that can improve the exhaust gas purification performance. [Means for solving the problem]

[0009] To achieve the above objective, this invention provides an exhaust gas purification device in which a plurality of gas passages extending in the axial direction of the casing are arranged radially within a cylindrical casing, and a catalyst for purifying exhaust gas flowing through the gas passages is supported on the inner surface of the gas passages, characterized in that the flow velocity of the exhaust gas differs depending on the position inside the casing, and the length of the gas passages at positions with high flow velocity is longer than the length of the gas passages at positions with low flow velocity.

[0010] In this invention, the flow velocity at the central part of the casing is faster than the flow velocity at the outer periphery of the casing, and the flow path length of the gas flow path at the central part of the casing is longer than the flow path length of the gas flow path at the outer periphery of the casing.

[0011] In this invention, the position where the exhaust gas flow velocity increases is the position where the temperature of the exhaust gas becomes higher than the temperature of other positions due to the purification reaction of the exhaust gas by the catalyst, and the flow path length of the gas flow path at the position where the temperature is higher may be longer than the flow path length of the gas flow path at other positions where the temperature is lower than the position where the temperature is higher.

[0012] In this invention, the exhaust gas purification reaction by the catalyst is an exothermic reaction, the position where the temperature is highest is downstream in the direction of exhaust gas flow, and the flow path length of the gas flow path downstream in the direction of exhaust gas flow may be longer than the flow path length of the gas flow path upstream in the direction of exhaust gas flow.

[0013] In this invention, the base plate has a plurality of gas passages formed thereon, and the plurality of base plates may be stacked in the axial direction of the casing with the gas passages facing in the axial direction of the casing. [Effects of the Invention]

[0014] According to this invention, the length of the gas flow path supporting the catalyst on its surface is set to a length corresponding to the exhaust gas flow velocity. That is, the length of the gas flow path at high flow velocities is longer than the length of the gas flow path at low flow velocities. As a result, it is possible to effectively utilize areas where the diffusion resistance due to the boundary film is small to bring the exhaust gas into contact with the catalyst and improve the purification performance. In other words, by minimizing the length of the portion within the gas flow path where the boundary film occupies the interior of the flow path and obstructs contact of the exhaust gas with the catalyst, the length of the portion where the exhaust gas simply flows without purification can be shortened. As a result, the purification efficiency can be improved, and an increase in pressure loss can be avoided or suppressed. [Brief explanation of the drawing]

[0015] [Figure 1] This is a partially broken perspective view schematically showing an example of an exhaust gas purification system. [Figure 2] This is a schematic diagram showing the stacking state of the catalyst, as well as the flow velocity distribution and temperature distribution. [Figure 3] (a) is a perspective view showing a part of a substrate in which through-holes, which serve as gas passages, are formed in a flat plate material, and (b) is a perspective view showing a part of the mesh material that serves as the substrate. [Figure 4] This is an enlarged cross-sectional view showing a portion of a substrate on which a catalyst is attached to the surface. [Figure 5] This is a model diagram illustrating the gas flow path used to indicate the run-up distance. [Figure 6] This diagram shows the relationship between the ratio of contact strength to pressure loss and the distance from the inlet, based on measurements. [Figure 7] This is a diagram showing the relationship between flow rate and run-up distance. [Figure 8] This is a schematic cross-sectional view showing an example where the channel length in the center of the substrate or catalyst differs from the channel length in the peripheral area. [Figure 9] This is a schematic cross-sectional view showing an example where the channel length on the inlet side and the channel length on the outlet side of the substrate or catalyst are different. [Modes for carrying out the invention]

[0016] Next, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the embodiments described below are merely examples of implementing the present invention and do not limit the present invention.

[0017] The exhaust gas purification device according to the present invention can be used as a device for treating various exhaust gases generated during combustion. An example thereof is a device for purifying exhaust gas generated from an internal combustion engine (engine) of a vehicle. The purification of exhaust gas is to render harmless carbon monoxide, hydrocarbons, or nitrogen oxides contained in the exhaust gas by oxidizing or reducing them using a catalyst. The exhaust gas purification device includes a catalyst therefor.

[0018] An example of the exhaust gas purification device 1 is schematically shown in FIG. 1. A catalyst body 3 is housed inside a casing 2. The casing 2 is a cylindrical body having an appropriate cross-sectional shape such as circular, elliptical, or rectangular, and an inlet 4 is provided on one end side in the axial direction, and an outlet 5 is provided on the other end side. By passing the exhaust gas G introduced from the inlet 4 through the catalyst body 3, contaminants such as carbon monoxide contained in the exhaust gas G are oxidized or reduced and rendered harmless.

[0019] The catalyst body 3 has a number of passages through which the exhaust gas G passes. As shown in FIG. 2 for an example thereof, the catalyst body 3 has a thin flat plate shape, and a number of sheets are stacked and housed inside the casing 2. Note that the catalyst bodies 3 may be in close contact with each other, or a predetermined gap may be provided between the catalyst bodies 3. The catalyst body 3 is provided with a number of flow paths, that is, gas flow paths 6 through which the exhaust gas G passes. The gas flow path 6 may be a through hole penetrating the catalyst body 3 in its thickness direction, and it is preferable that the number and the ratio of the area to the surface area of the catalyst body 3 are as large as possible.

[0020] An example of the substrate 7 constituting the catalyst 3 is shown in Figure 3. Figure 3(a) shows the basic or fundamental structure of the substrate 7, in which numerous through holes are formed in a heat-resistant plate material (substrate plate), such as a metal plate, and these through holes serve as gas flow paths 6. A more specific example is shown in Figure 3(b). In this example, a metal plate is sheared at regular intervals, and the sheared portions are stretched in the thickness direction of the metal plate. The portions between the sheared sections, i.e., the unsheared portions, become so-called connecting portions 3a, and the sheared portions open up vertically, resulting in a configuration in which numerous rectangular or diamond-shaped openings are arranged vertically and horizontally. This mesh-like substrate 7 can be processed using conventionally known apparatus and methods.

[0021] In Figure 3, the symbol "L" indicates the flow path length of the gas flow path 6 for each substrate 7. In the example shown in Figure 3(a), the thickness of the substrate 7 corresponds to the flow path length L, and in the example shown in Figure 3(b), the width across which the metal plate is sheared is the flow path length L. Therefore, the gas flow path 6 described here is a hollow section where the hydraulic diameter is constant, and the flow path length can be said to be the length in the axial direction of that hollow section where the hydraulic diameter is constant.

[0022] The catalyst 8 is supported on the entire surface of the substrate 7, particularly on the inner surface of the gas flow path 6. This state is schematically shown in Figure 4. In the exhaust gas purification device 1 for vehicles, the catalyst 8 includes known catalytic materials such as platinum (Pt) and rhodium (Rh). Multiple substrates 7 or catalysts 3, configured in this way, are stacked with their gas flow paths 6 oriented in the axial direction of the casing 2.

[0023] In this embodiment of the invention, the length (flow path length) L of the gas flow path 6 is set to the length described below, taking into consideration the diffusion resistance due to the boundary film. Figure 5 is a model diagram for explaining the flow of exhaust gas G inside a gas flow path Cm with a rectangular cross-section. When exhaust gas G flows into the gas flow path Cm, the flow state of the exhaust gas G changes because the region in which it flows is restricted to the gas flow path Cm. That is, the region in which the exhaust gas G flows is narrowed to the gas flow path Cm. Therefore, turbulence occurs near the inlet of the gas flow path Cm due to the change in the flow state of the exhaust gas G. This turbulent state gradually subsides as the exhaust gas G flows through the gas flow path Cm, and becomes a laminar flow state. The section from the inlet, or starting point, of the gas flow path Cm to the point where the laminar flow state begins is the so-called abrupt section L1, and in this abrupt section L1, the exhaust gas G repeatedly comes into contact with and separates from the catalyst forming the inner surface of the gas flow path Cm. That is, diffusion of exhaust gas G to the catalyst occurs actively.

[0024] After passing the abruptly contracted section L1, the exhaust gas flow G stabilizes, becoming laminar, and the boundary film Fb gradually grows. The thickness δ of the boundary film Fb gradually increases, and eventually the inside of the gas channel Cm is filled with the boundary film Fb. The section from when the boundary film Fb begins to form until the inside of the gas channel Cm is filled with the boundary film Fb is the so-called boundary film growth section L2. After passing this boundary film growth section L2, the diffusion of exhaust gas G or pollutants to the catalyst 8 forming the inner surface of the gas channel Cm hardly occurs or becomes very slow, and the purification of exhaust gas G becomes difficult.

[0025] The thickness δ of the boundary layer Fb can be estimated by equation (1).

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[0026] The state in which the boundary film Fb fills the interior of the gas channel Cm is when the thickness δ of the boundary film Fb is half of the hydraulic diameter dH of the gas channel Cm (dH / 2). Therefore, we set the thickness δ in equation (1) to (dH / 2). Furthermore, if we let the run-up distance L0 be the sum of the above-mentioned abrupt section L1 and boundary film growth section L2, then equation (1) can be rewritten as equation (2).

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[0027] In equation (2), the hydraulic diameter dH can be determined geometrically based on the shape of the gas flow path 6 in the catalyst 3. For example, from the cross-sectional area Ac of the flow path and the total circumference of the flow path, i.e., the circumference Lr... dH = 4 × Ac × Lr It can be calculated as follows. Therefore, by measuring the gas density ρ, gas flow velocity u, and gas viscosity μ, the run-up distance L0 can be calculated. When the inside of the gas flow path 6 is filled with the boundary film Fb, an improvement in the purification rate of exhaust gas G by the catalyst 8 cannot be expected. Therefore, in this embodiment of the invention, it is preferable to set the run-up distance L0 calculated by equation (2) as the upper limit of the flow path length L. In other words, the flow path length L is determined based on the run-up distance L0.

[0028] The aforementioned abrupt section L1 occurs at the inlet side of the gas flow path 6. In this section, the diffusion of exhaust gas G is promoted, but on the other hand, pressure loss is likely to occur. The flow path length L, which is determined based on the run-up distance L0, includes the abrupt section L1, so it is preferable to set the flow path length to take into account the pressure loss and the diffusivity of exhaust gas G or the contact of exhaust gas G with the inner surface of the gas flow path 6. The length of the abrupt section L1 can generally be expressed by equation (3).

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[0029] In this embodiment of the invention, the flow path length L is the length including the abrupt section L1, and the abrupt section L1 is determined by characteristics such as the structure of the gas flow path 6 and the composition of the exhaust gas G. Therefore, if the flow path length L is shortened to a range less than or equal to the above-mentioned run-up distance L0, the boundary film growth section L2 is shortened. That is, the contactability of the exhaust gas G with the catalyst 8 decreases. Here, "contactability" refers to the number of times (frequency) that pollutants contained in the exhaust gas G come into contact with the surface of the catalyst 8 per unit time. The flow path length L that maximizes the purification efficiency while simultaneously reducing pressure loss and improving contactability exists in the range from the abrupt section L1 to the upper limit of the run-up distance L0. Figure 6 shows the results of measuring the relationship between the ratio of contactability to pressure loss and the distance from the inlet of the gas flow path 6. If the ratio of contactability to pressure loss is large, even if pressure loss, i.e., energy loss occurs, the diffusion of exhaust gas G to the catalyst 8 is promoted and the purification efficiency is improved. As shown in the measurement results in Figure 6, the above ratio (i.e., efficiency) increases sharply when the distance from the inlet of the gas flow path 6 is short, reaches a maximum point, then drops sharply to a predetermined value, and then gradually decreases as the distance from the inlet of the gas flow path 6 increases. In short, active contact of the exhaust gas G or pollutants contained therein with the catalyst 8 occurs in both the rapid contraction section L1 and the boundary film growth section L2, and in the boundary film growth section L2, the contactability gradually decreases on the forward side in the flow direction of the exhaust gas G, so it is preferable to set the flow path length L with the length of the rapid contraction section L1 as the lower limit.

[0030] In the exhaust gas purification device 1 according to the present invention, the flow path length L of the gas flow path 6 is set based on the run-up distance L0. The run-up distance L0 is obtained by the above-described formula (2). As is clear from formula (2), the higher the gas flow velocity u, the longer the run-up distance L0, and thus the longer the flow path length L based thereon. On the other hand, when a fluid flows through a cylindrical pipe, the flow velocity u is the fastest at the center and slower toward the peripheral part side, as shown together with the velocity lines in FIG. 2. FIG. 7 shows the relationship between the flow rate and the run-up distance L0. Since the flow rate is proportional to the gas flow velocity u, the higher the gas flow velocity u, the longer the run-up distance L0. Therefore, the run-up distance L0 at the peripheral part side is shorter than that at the center part of the base material 7. For this reason, the flow path length L of the gas flow path 6 based on the run-up distance L0 is longer at the center of the flow of the exhaust gas G, that is, at the center of each base material 7, and shorter at the peripheral part side than at the center part. This state is shown in a schematic cross-sectional view in FIG. 8. If the flow path length at the center part is “Lc”, the flow path length Lo at the peripheral part is “Lo < Lc”. This can be configured, for example, by processing the thickness of the base material 7 so as to gradually become thinner from the center part toward the outer peripheral part side.

[0031] On the other hand, if the purification reaction by the catalyst 8 is an exothermic reaction, the temperature inside the casing 2 becomes higher on the outlet 5 side, as shown together in FIG. 2. Therefore, the exhaust gas G thermally expands on the outlet 5 side (downstream side in the flow direction), and its flow velocity becomes faster than the flow velocity on the inlet 4 side (upstream side in the flow direction). That is, the run-up distance L0 is short in the gas flow path 6 on the inlet 4 side and long in the gas flow path 6 on the outlet 5 side. Based on such a difference in the run-up distance L0, the flow path length Ld of the gas flow path 6 on the outlet 5 side is configured to be longer (Lu < Ld) than the flow path length Lu of the gas flow path 6 on the inlet 4 side. This state is shown in a schematic cross-sectional view in FIG. 9. The flow path length Ld of the gas flow path 6 on the outlet 5 side is longer (Lu < Ld) than the flow path length Lu of the gas flow path 6 on the inlet 4 side. This can be configured, for example, by gradually increasing the thickness of the laminated base materials 7 from the inlet 4 side toward the outlet 5 side.

[0032] In the exhaust gas purification device 1 according to the present invention described above, similar to conventional devices, it is installed in the middle of a predetermined exhaust system. Exhaust gas G generated by an engine or the like is introduced from the inlet 4, and the exhaust gas G passes through the gas flow path 6 of the stacked base material 7 or catalyst 3, where a purification reaction occurs, and the purified exhaust gas G is discharged from the outlet 5. Inside each gas flow path 6, for example, as schematically shown in Figure 5, the turbulent flow state in the abruptly contracted section L1 actively contacts or diffuses with the catalyst 8, promoting the purification reaction. In the subsequent section where the boundary film Fb gradually grows, the diffusion of exhaust gas G or pollutants contained therein to the catalyst 8 and the accompanying purification reaction gradually decrease. When the exhaust gas G passes the boundary film growth section L2 or the run-up distance L0, the diffusion of exhaust gas G or pollutants to the catalyst 8 almost ceases, and the gas flow path 6 beyond that point hardly functions as a flow path for purifying exhaust gas G. In the exhaust gas purification device 1 described above, the flow path length L of the gas flow path 6 is set to a length that allows exhaust gas G or its pollutants to diffuse with respect to the catalyst 8. The exhaust gas G that has passed through the designated gas flow path 6 flows into another gas flow path 6 located downstream, and, as described above, is purified in the abrupt section L1 and the boundary film growth section L2 before flowing downstream.

[0033] As described above, the flow path length L of the gas flow path 6 in the exhaust gas purification device 1 according to this invention is set based on the run-up distance L0 corresponding to the flow velocity of the exhaust gas G, and is configured to be shorter where the flow velocity u is slow and longer where the flow velocity u is fast. Therefore, exhaust gas G or pollutants can be efficiently diffused into the catalyst 8 within the gas flow path 6 to generate a purification reaction and improve purification performance. Furthermore, since the gas flow path 6 is not longer than the length required for such a purification reaction to occur, the flow path length L of the gas flow path 6 is not unnecessarily long, and as a result, the catalyst 8 can function effectively to purify the exhaust gas G efficiently. In other words, according to this invention, an exhaust gas purification device with excellent exhaust gas purification performance can be obtained.

[0034] Although one embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and for example, the shape of the catalyst or substrate described above may be any shape as needed. Furthermore, in this invention, the flow path length of all gas flow paths does not need to be the length based on the aforementioned run-up distance L0, and the flow path length of gas flow paths in areas with high flow rates may be the length based on the run-up distance. In short, the present invention is sufficient if the flow path length of the gas flow paths is a length corresponding to the exhaust gas flow velocity, and if the flow velocity is different in some areas, the flow path length should be different according to the difference in flow velocity. [Explanation of symbols]

[0035] 1. Exhaust purifying device 2 Casing 3 Catalyst 3a Connection part 4 Inlet 5 Outlet 6 Gas flow path 7 Base material 8 Catalyst δ thickness Cm gas flow path Fb membrane G Exhaust L flow path length L0 Approach distance L1 Rapidly shortened section L2 boundary film growth zone Lc (central) flow path length Ld (Outlet side) flow path length Lo (outer circumference) flow path length Lu (inlet side) flow path length dH hydraulic diameter u Gas flow velocity (flow rate)

Claims

1. An exhaust gas purification device comprising a cylindrical casing, wherein a plurality of gas passages extending in the axial direction of the casing are arranged radially within the casing, and a catalyst for purifying exhaust gas flowing through the gas passages is supported on the inner surface of the gas passages, The exhaust flow velocity differs depending on the position inside the casing. The length of the gas flow path at the location with the high flow velocity is configured to be longer than the length of the gas flow path at the location with the low flow velocity. An exhaust gas purification device characterized by the following features.

2. An exhaust gas purification device according to claim 1, The flow velocity at the central side of the casing is faster than the flow velocity at the outer periphery side of the casing. The flow path length of the gas flow path on the central side of the casing is longer than the flow path length of the gas flow path on the outer peripheral side of the casing. An exhaust gas purification device characterized by the following features.

3. An exhaust gas purification device according to claim 1 or 2, The position where the exhaust flow velocity increases is the position where the temperature becomes higher than the temperature at other positions due to the exhaust purification reaction by the catalyst. The length of the gas flow path at the location where the temperature is higher is longer than the length of the gas flow path at the other location where the temperature is lower than the location where the temperature is higher. An exhaust gas purification device characterized by the following features.

4. An exhaust gas purification device according to claim 3, The exhaust gas purification reaction by the catalyst is an exothermic reaction. The position where the temperature is highest is a position downstream in the direction of exhaust flow. The flow path length of the gas passage on the downstream side in the direction of exhaust flow is longer than the flow path length of the gas passage on the upstream side in the direction of exhaust flow. An exhaust gas purification device characterized by the following features.

5. An exhaust gas purification device according to claim 1 or 2, The base plate has the aforementioned plurality of gas flow channels formed therein, Multiple base plates are stacked in the axial direction of the casing, with the gas flow path facing the axial direction of the casing. An exhaust gas purification device characterized by the following features.

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