JP5208458B2 - Honeycomb structure - Google Patents

Description

  The present invention relates to a honeycomb structure, and more particularly to a honeycomb structure suitable for removing particulate matter from exhaust gas from a diesel engine.

  As a filter that collects and removes particulate matter (particulate matter, hereinafter referred to as “PM”) contained in the gas discharged from a diesel engine, it is conventionally made of porous ceramics. A honeycomb structure including a plurality of cells, which is partitioned by a plurality of partition walls extending in the direction of, is used. In such a honeycomb structure, in general, cells sealed at one end and cells sealed at the other end are alternately arranged, and gas flows in from a cell opened in one direction, and is porous. After passing through the partition wall, it flows out from the cell opened in the other direction. When the gas passes through the partition wall, PM in the gas is collected on the surface of the porous partition wall and in the pores and removed.

  Therefore, when the pore diameter is too large, PM that passes through the partition without being collected increases, and the collection efficiency decreases. On the other hand, if the pore diameter is too small, the pressure loss increases due to the resistance to the passage of gas, and the load on the engine increases. In view of this, a filter has been proposed in which the pore size and pore size distribution of the porous ceramics constituting the partition walls are controlled to achieve harmony between the trapping efficiency and the pressure loss, which are in a contradictory relationship (for example, Patent Documents). 1, Patent Document 2).

  These filters are intended to increase the relative number of pores suitable for PM collection by setting the pore diameter of the partition wall to be within a limited narrow range. The average value of the pore diameter measured by the above (Patent Document 1) is 1 μm to 15 μm, the latter (Patent Document 2) is 20 μm to 60 μm, and the standard deviation in the pore diameter distribution when the pore diameter is expressed in common logarithm Are both 0.20 or less.

Japanese Patent No. 3272746 JP 2002-242655 A

  However, in order to collect PM in the honeycomb structure, the gas must pass through the partition walls. Therefore, the magnitude of the pressure loss depends on the thickness of the partition walls. Even if the pore size distribution is controlled, there is a problem that the honeycomb structure has a high collection efficiency and a small pressure loss.

  Accordingly, in view of the above circumstances, the present invention has a high efficiency of collecting particulate matter (PM) in exhaust gas, a small initial pressure loss, and a honeycomb in which an increase in pressure loss accompanying PM collection is suppressed. It is an object to provide a structure.

  In order to solve the above-mentioned problem, a honeycomb structure according to the present invention is a honeycomb structure including a plurality of cells defined by a plurality of partition walls made of porous ceramics and extending in a single direction. The average value of the pore diameter of the partition wall measured by mercury porosimetry is 1 μm to 20 μm, and the standard deviation of the pore diameter distribution when the pore diameter is expressed in a common logarithm is 0.20 or less. And the thickness of the partition wall is 0.05 mm to 0.7 mm ”.

  “Ceramics” is not particularly limited, and silicon carbide, silicon nitride, cordierite, alumina, mullite, and the like can be used.

  In the “mercury intrusion method”, mercury is introduced into the open pores by applying pressure, and the pore diameter assumed to be cylindrical is calculated from the Washburn equation using the pressure value and the volume of mercury infiltrated at that time. This is a method, and JIS R1655 prescribed for ceramic molded bodies can be applied mutatis mutandis. The “average pore diameter measured by mercury porosimetry” refers to the diameter (median diameter) when the cumulative pore volume is 50% of the total pore volume.

  When the pore diameter and the pore size distribution are controlled within a range suitable for PM collection, in the initial stage after collection, PM is collected and a gas flow path is secured, but PM deposition proceeds. As a result, the resistance to the passage of gas increases and the pressure loss increases. And the resistance with respect to passage of gas becomes so large that a partition is thick. On the other hand, if the partition walls are made thin, the resistance to the passage of gas decreases, and the degree to which the pressure loss increases with the deposition of PM is considered to be reduced. On the other hand, there is no gap between the partition walls. There is a risk that the mechanical strength of the honeycomb structure having the lowering may decrease. If the honeycomb structure has insufficient mechanical strength and cracks are generated in the partition walls, gas passes through the cracks, and PM is not collected in the partition walls. It is meaningless to control to suit the collection.

  As a result of the study, the inventor found that the average pore diameter of the partition walls measured by the mercury intrusion method is 1 μm to 20 μm and the standard deviation of the pore diameter distribution is 0.20 when the pore diameter is expressed in common logarithm. In the honeycomb structure described below, it was found that by increasing the partition wall thickness to 0.05 mm to 0.7 mm, an increase in pressure loss due to PM deposition can be suppressed while maintaining the mechanical strength. When the thickness of the partition wall exceeds 0.7 mm, the initial pressure loss increases, the degree of increase in the gas passage resistance with PM deposition increases, and the increase in pressure loss becomes significant. On the other hand, when the partition wall thickness is less than 0.05 mm, the mechanical strength decreases, and the risk of cracks occurring in the partition wall increases. The partition wall thickness is more preferably 0.1 mm to 0.7 mm, and even more preferably 0.2 mm to 0.5 mm.

  Therefore, according to the present invention having the above-described configuration, the pore diameter and the pore size distribution of the partition walls are controlled to a range suitable for PM collection, and the risk of causing a decrease in mechanical strength can be reduced. By reducing the thickness, it is possible to collect PM with high collection efficiency while maintaining a small initial pressure loss.

  The honeycomb structure according to the present invention may be “the porous ceramic is silicon carbide”.

  According to the present invention having the above configuration, the use of silicon carbide having high strength and excellent heat resistance as the porous ceramic makes the structure having many voids and more suitable as a honeycomb structure used in a high temperature environment. It will be. Further, by having excellent heat resistance, even when the honeycomb structure is heated in order to burn the accumulated PM, deformation or melting damage due to heating is unlikely to occur.

  As described above, as an effect of the present invention, there is provided a honeycomb structure having high PM collection efficiency in exhaust gas, low initial pressure loss, and suppressed increase in pressure loss due to PM collection. Can do.

  Hereinafter, a honeycomb structure which is the best embodiment of the present invention will be described with reference to FIGS. 1 to 4. Here, FIG. 1 is a (a) side cross-sectional view and (b) cross-sectional view schematically showing the configuration of the honeycomb structure of the present embodiment, and FIG. 2 shows the relationship between the average pore diameter and the initial pressure loss. 3 is a graph showing the relationship between average pore diameter and compressive strength, FIG. 4 is a graph showing the relationship between partition wall thickness and pressure loss, and FIG. 5 is a honeycomb according to the present embodiment. It is a graph which shows the change of the pressure loss accompanying the increase in PM deposition amount about a structure with a comparative example. In the present embodiment, a case where the honeycomb structure of the present invention is applied as a diesel particulate filter (hereinafter referred to as “DPF”) that collects PM from exhaust gas of a diesel engine is exemplified.

  As shown in FIGS. 1 (a) and 1 (b), the honeycomb structure 10 of the present embodiment includes a plurality of partition walls 2 that are made of porous ceramics and that are partitioned and extended in a single direction. The average value of the pore diameter of the partition wall 2 measured by the mercury intrusion method is 1 μm to 20 μm, and the standard deviation of the pore diameter distribution when the pore diameter is expressed in common logarithm is 0.20 or less. In addition, the partition wall thickness d is 0.05 mm to 0.7 mm. 1A is a side sectional view parallel to the direction in which the gas flows in and out, and FIG. 1B is a transverse sectional view orthogonal to the direction.

  More specifically, the porous ceramic of the present embodiment is silicon carbide, and in the plurality of cells 3 arranged in a row, the cells 3a opened in one direction and the cells 3b opened in the other direction are alternated. In addition, one end of each cell 3 is sealed by a sealing portion 6.

  In the honeycomb structure 10 having such a configuration, as shown by a one-dot chain line in FIG. 1A, when diesel exhaust containing PM is caused to flow from the open end of the cell 3a, the gas passes through the porous partition wall 2 and then flows. Then, it flows out from the open end of the cell 3b opened in the other direction. When the gas passes through the partition wall 2, PM is collected on the surface of the partition wall and in the pores.

Next, the result of examining the relationship between pore diameter and initial pressure loss, and pore diameter and compressive strength is shown. For the study, a honeycomb structure having the above-described configuration except for the average pore diameter, a porosity of about 57%, and a cell density of 169 cells / square inch (2.62 cells / 10 ⁻³ m ² ) was used. A plurality of products having different diameters were produced and used. The initial pressure loss is such that a cylindrical honeycomb structure having a diameter of about 100 mm and a length of about 140 mm is installed in a gas flow path in a state where no PM is collected, and an air flow rate of 5 Nm ³ / min. And the pressure difference between the inflow side and the outflow side was measured. The average pore diameter was determined as the median diameter from the pore diameter distribution measured by the mercury intrusion method using a pore sizer 9310 manufactured by Shimadzu Corporation. The porosity was determined by the Archimedes method.

  As a result, as shown in FIG. 2, in the range where the average pore diameter is about 25 μm or more, the initial pressure loss is almost constant and small as less than 6 kPa, but when the average pore diameter is smaller than a dozen μm, the initial pressure loss is suddenly reduced. Increased.

  On the other hand, the compressive strength in the gas flow direction (A axis) and the direction orthogonal to the gas flow direction (B axis) is set to 1 mm / crosshead speed based on the Japan Society of Automotive Engineers standard JASO M505-87. Measurement was performed in min, and the relationship between the pore diameter and the compressive strength was examined. The result is shown in FIG. In general, the mechanical strength and the pore diameter are in a contradictory relationship, but the tendency is also remarkable in FIG. 3, and when the pore diameter is larger than about 20 μm, the compressive strength is greatly reduced. .

  From the above results, the larger the pore diameter, the smaller the initial pressure loss, but it was considered that the limit of increasing the pore diameter while maintaining the mechanical strength to some extent was about 20 μm. Therefore, as described above, according to the honeycomb structure of the present embodiment in which the average value of the pore diameters is set to 1 μm to 20 μm, the initial pressure loss can be suppressed while maintaining the mechanical strength to some extent.

Next, the result of examining the relationship between the thickness of the partition wall and the pressure loss is shown. The honeycomb structure used for the examination was manufactured by the following method. First, 75% by weight of SiC powder having an average particle diameter (diameter) of 12 μm, 20% by weight of Si ₃ N ₄ powder having an average particle diameter (diameter) of 10 μm, and 5% by weight of C powder having an average particle diameter (diameter) of 15 μm The powder was mixed and kneaded with an organic binder (methylcellulose), water, and a surfactant, and then the kneaded product was formed into a honeycomb by extrusion. At this time, a plurality of types having different partition wall thicknesses were prepared depending on the extrusion mold setting. After that, each molded body was fired at 2300 ° C. for 10 minutes in a non-oxidizing atmosphere to produce a honeycomb structure. Each honeycomb structure had a cell density of 200 cells / square inch (3.10 cells / 10 ⁻³ m ² ), a cylindrical shape having a diameter of about 140 mm and a length of about 150 mm.

  With respect to the obtained honeycomb structure, the pore size distribution was measured by the mercury intrusion method using the above apparatus, the average pore diameter (median diameter) was obtained from the cumulative pore size distribution, and further measured by the mercury intrusion method. When the standard deviation of the pore diameter distribution when the pore diameter was expressed in the common logarithm was determined, the average pore diameter was 8 μm and the standard deviation was 0.16.

The above honeycomb structures having different partition wall thicknesses are respectively installed in the gas flow paths, and a gas containing PM is circulated at a flow rate of 5 Nm ³ / min. PM is 2 g per liter of the honeycomb structure volume (honeycomb structure) at the time of the 1 m ³ per PM2kg) deposition was measured differential pressure of the inlet side and the outlet side. The result is shown in FIG.

  As is apparent from FIG. 4, the pressure loss increases as the partition wall thickness increases, and the degree of increase in the pressure loss increases as the partition wall thickness increases. And when the thickness of the partition exceeded 0.7 mm, the pressure loss was almost 10 kPa. Here, when the pressure loss exceeds 10 kPa, the load on the engine increases. Therefore, 10 kPa is a value that serves as a standard for regenerating DPF by burning normal PM. Thereby, it was considered that the thickness of the partition wall was appropriately 0.7 mm or less.

  Hereinafter, specific examples of the present embodiment will be described in comparison with comparative examples. The honeycomb structures of Examples and Comparative Examples 1 and 2 were produced in the same manner as described above except for the thickness of the partition walls. As shown in Table 1, the partition wall thickness was 0.3 mm in the example, 1.2 mm in the comparative example 1, and 0.03 mm in the comparative example 3. That is, Comparative Example 1 is an example in which the thickness of the partition wall exceeds the range of 0.05 mm to 0.7 mm, and Comparative Example 2 is an example of less than the range of 0.05 mm to 0.7 mm.

Next, pressure loss was evaluated for the honeycomb structures of Examples and Comparative Examples 1 and 2. The evaluation was performed by passing a gas containing PM through the honeycomb structure installed in the gas flow path at a flow rate of 5 Nm ³ / min and measuring a change in pressure loss accompanying an increase in the amount of PM deposited. The result is shown in FIG.

  As is apparent from FIG. 5, in the example, the initial pressure loss when PM is not deposited is very small, and the pressure loss gradually increases as PM is deposited, but the value of small pressure loss is maintained. It was. On the other hand, in Comparative Example 1 where the partition walls are thick, the initial pressure loss is as large as about four times that of the example, and further, the pressure loss greatly increases as PM is deposited. It spread so that it accumulated. From this, it was considered that when the partition walls are thick, the effect of the deposited PM on the gas passage resistance is extremely large.

  In Comparative Example 2, cracks occurred in the partition walls due to the gas flow, and the gas circulated through the cracks, so that PM could not be collected. Moreover, the pressure loss could not be correctly evaluated.

  As described above, the two requirements of “the average value of the pore diameter measured by the mercury intrusion method is 1 μm to 20 μm” and “the standard deviation of the pore diameter distribution when the pore diameter is expressed in a common logarithm is 0.20 or less” are satisfied. It is considered that only satisfying is insufficient as a honeycomb structure used as a practical DPF, and it is necessary to appropriately set the partition wall thickness. And by setting the thickness of the partition wall to 0.05 mm to 0.7 mm, the partition wall can be thinned to the extent that the possibility of reducing the mechanical strength such as the occurrence of cracks can be reduced, and the initial pressure loss is reduced. In addition, it was considered that PM could be collected with high collection efficiency while maintaining a small initial pressure loss.

  The present invention has been described with reference to the preferred embodiments. However, the present invention is not limited to the above-described embodiments, and various improvements can be made without departing from the scope of the present invention as described below. And design changes are possible.

  For example, in the above embodiment, the case where silicon carbide is used as the porous ceramic is exemplified, but the present invention is not limited to this, and silicon nitride, cordierite, alumina, mullite, or the like can be used. In addition, aluminum titanate having a small coefficient of thermal expansion and excellent thermal shock resistance can be used.

  In addition, the case where the honeycomb structure of the present invention is applied as a DPF has been exemplified, but the present invention is not limited to this. For example, the honeycomb structure can be widely applied as a filter for purifying gas discharged from an internal combustion engine such as a gasoline engine or a boiler. Can do.

FIG. 2A is a side cross-sectional view schematically showing a configuration of a honeycomb structure of the present embodiment, and FIG. It is a graph which shows the relationship between an average pore diameter and an initial pressure loss. It is a graph which shows the relationship between an average pore diameter and compressive strength. It is a graph which shows the relationship between the thickness of a partition, and a pressure loss. It is a graph which shows the change of the pressure loss accompanying the increase in PM deposition amount about the honeycomb structure of this embodiment as contrasted with a comparative example.

Explanation of symbols

2 Partition wall 3, 3a, 3b Cell 6 Sealing portion 10 Honeycomb structure

Claims (1)

A plurality of cells made of porous ceramics and partitioned by a plurality of partition walls extending in a single direction, and a plurality of cells opened in one direction and a cell opened in the other direction A honeycomb structure provided with a sealing portion that seals one end of each of the cells so as to alternate, wherein exhaust gas is allowed to flow from the cells opened in the one direction, and is passed through the partition walls and the other It is used as a diesel particulate filter that flows out of a cell opened in the direction,
The porous ceramic is a silicon carbide sintered body obtained from silicon carbide, silicon nitride, and carbon,
The average value of the pore diameter of the partition wall measured by mercury porosimetry is 8 μm;
The standard deviation of the pore diameter distribution when the pore diameter is expressed as a common logarithm is 0.16,
A honeycomb structure, wherein the partition wall functioning as a filter for collecting particulate matter has a thickness of 0.3 mm.

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