Apparatus for coating catalyst slurry

A coating apparatus includes: a supply frame for supplying slurry into channels from one end of a honeycomb substrate; and a blower for evacuating a wind box. An annular resistive member is attached to the circumference of the opening of the wind box, and the honeycomb substrate is arranged, with a spacer placed on the resistive member. When the blower is operated and the slurry is supplied, the coat width of the slurry coated on the inner surfaces of the channels in an outer circumferential area is less than the coat width of the slurry coated on the inner surfaces of the channels in a center area.

FIELD

The present invention relates to an apparatus for coating slurry containing a catalyst material on a monolithic catalyst honeycomb substrate used for purifying the exhaust gas of an automobile.

BACKGROUND

A purification apparatus using a monolithic catalyst is conventionally known as an apparatus for purifying the exhaust gas of an automobile. The monolithic catalyst includes a substantially cylindrical honeycomb substrate having a large number of parallel channels for permitting a gas to flow in one direction, and slurry containing a catalyst material is coated on the inner surfaces of the channels of the honeycomb substrate. When the exhaust gas flows through the channels of the monolithic catalyst in the axial direction of the honeycomb substrate, chemical reaction takes place between the exhaust gas and the catalyst material, and the exhaust gas is purified thereby.

When the exhaust gas flows through the cylindrical monolithic catalyst, the exhaust gas flowing through the circumferential channels does not pass as smoothly as the exhaust gas flowing through the central channels as viewed in the radial direction of the monolithic catalyst. For this reason, the exhaust gas purification effect is lower in the circumferential portions of the monolithic catalyst than in the central portion thereof.

In an effort to solve this problem, various measures are taken to improve the exhaust gas purification effect at the time of manufacturing the catalyst, such as coating a larger amount of slurry in the central portion of the honeycomb substrate than in the circumferential portions thereof. For example, in the coating apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2009-136833, a resistive member such as a net member or a plate member is arranged on the slurry supply side of the honeycomb substrate in such a manner that the slurry flow is decelerated in the circumferential portions and consequently the slurry is provided more in the central portion than in the circumferential portions.

SUMMARY

However, where a resistive member is arranged on the slurry supply side of the honeycomb substrate, as in the coating apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2009-136833, the slurry inevitably attaches to the resistive member and remains on it, resulting in an increase in the amount of slurry consumed. Since the catalyst material included in the slurry contains a noble metal such as platinum or palladium, the manufacturing cost of the monolithic catalyst increases in accordance with an increase of the amount of slurry consumed. In addition, if the slurry attaches to the resistive member ununiformly, the amount of slurry coated on the honeycomb substrate may also become ununiform, with the result that monolithic catalysts may vary in quality.

If the slurry attaching to the resistive member dries and hardens, agglomerates of such slurry may be mixed in the liquid slurry to be coated. If this happens, the agglomerates may enter and clog the channels. As a result, the monolithic catalyst may be degraded in performance and quality.

Where a resistive member is arranged on the slurry supply side, an inorganic oxide included in the slurry acts as an abrasive, and the resistive member is abraded thereby. That is, the resistive member has to be replaced with a new one after a certain period of time. This also increases the manufacturing cost of the monolithic catalyst.

Accordingly, an object of the present invention is to provide a catalyst slurry coating apparatus which enables a monolithic catalyst improved in quality and performance to be manufactured at low cost.

A coating apparatus for coating catalyst slurry according to the present invention is coating slurry containing a catalyst material on inner surfaces of a plurality of channels which extend through a substrate in a first direction and are adjacent to one another. The coating apparatus comprising slurry supply means for supplying the slurry into the channels from one end of the substrate, as defined in the first direction, air stream generation means for generating air streams flowing through the channels from the one end of the substrate to another end, as defined in the first direction, such that the slurry supplied from the one end of the substrate by the slurry supply means flows from the one end of the substrate through the channels toward said another end and reaches an intermediate point, as defined in an overall length of the substrate, and air stream suppression means, arranged away from said another end of the substrate and facing downstream-side ends of first channels, as viewed in the first direction, for suppressing the air streams in the first channels such that the air streams in the first channels are slower than the air streams in second channels.

DESCRIPTION

FIG. 1is a schematic view showing an example of a coating apparatus10configured to coat slurry on the inner surfaces of channels of a honeycomb substrate1. The slurry contains, for example, a catalyst material for purifying the exhaust gas of an automobile. The catalyst material includes a noble metal such as platinum or palladium. The coating apparatus10of the embodiment is an apparatus for manufacturing a monolithic catalyst used for purify the exhaust gas of an automobile.

The exhaust gas of an automobile provides different flow rate distributions, depending upon the type of automobile, when it is made to flow through the monolithic catalyst. In addition, the exhaust gas flows in different ways, depending upon the canning shape of the catalyst and the bent state of a pipe. In order to improve the exhaust gas purification effect, it is preferred that the coat shape of the slurry for the honeycomb substrate1be changed in accordance with the type of automobile or the installation position of the catalyst. The coating apparatus10of the present embodiment enables control of the slurry coat shape for the honeycomb substrate1.

As shown, for example, inFIG. 2, the honeycomb substrate1has a substantially cylindrical outer shape, and a plurality of channels2extending in the axial direction are defined inside the honeycomb substrate1. InFIG. 1, the channels2are indicated by solid lines for the sake of easy understanding though they cannot be viewed in actuality. The channels2extend in the axial direction and are arranged in parallel to one another. In the present embodiment, all the channels2have the same cross sectional area. The cross sectional shape of each channel2may be any desirable shape, including circular shape and hexagonal shape. The honeycomb substrate1may be fabricated using such a ceramic material as cordierite, or stainless steel.

The coating apparatus10is provided with a support base11for supporting the axially lower end1bof the honeycomb substrate1. The support base11is hollow and functions as a wind box12. The top plate11aof the support base11has a circular opening11bcommunicating with the hollow section of the wind box12. A resistive member5(air stream suppression means), such as the annular net (net member)3shown inFIG. 3or the annular plate (plate member)4shown inFIG. 4, is attached to the circumference of the opening11b. An annular spacer6(adjusting means) is arranged between the resistive member5and the lower end1bof the honeycomb substrate1in such a manner that the honeycomb substrate1is located at a position above the resistive member5, as viewed inFIG. 1. That is, the distance between the resistive member5and the lower end1bof the honeycomb substrate1can be changed in accordance with the thickness of the spacer6.

The coating apparatus10is provided with a blower7(air stream generation means) which evacuates the hollow section of the wind box12. Although the blower7is employed as the air stream generation means in the present embodiment, compressed air may be supplied to the channels2from the upper end1aof the honeycomb substrate1to cause air streams flowing through the channels2.

When the blower7is driven in the state where the honeycomb substrate1is set on the support base11, with the spacer6interposed, the hollow section of the wind box12is evacuated, causing a negative pressure in the opening11b. As a result, air flows through the channels2from the upper end1aof the honeycomb substrate1to the lower end1bthereof. Then, slurry is supplied from a funnel-shaped supply frame8(supply means) attached to the upper end1aof the honeycomb substrate1. The slurry is sucked into the channels2from the upper end1aof the honeycomb substrate1, and the inner surfaces of the channels2are coated with the slurry.

The amount of slurry to be supplied is determined in such a manner that when all of the slurry supplied at one time is made to flow from the supply frame8into the channels2, the slurry reaches an intermediate point of the overall length of the channels2. In other words, according to the coating apparatus10of the embodiment, the slurry does not flow out of the lower end1bof the honeycomb substrate1, and the resistive member5does not get wet with the slurry. Since the slurry is hardly wasted, the manufacturing cost can be lowered, accordingly.

The spacer6has a circular opening6ahaving substantially the same diameter as the outer diameter of the honeycomb substrate1. Likewise, the support base11has a circular opening11bhaving substantially the same diameter as the outer diameter of the honeycomb substrate1. Let us assume that the blower7is driven in the state where the resistive member5is not attached to the circumference of the opening11bof the support base11. In this case, the air in every channel2of the honeycomb substrate1flows at the same speed, and the slurry coat width is substantially the same for all channels2. The “slurry coat width” is intended to refer to the distance between the upper end1aof the honeycomb substrate1to a position which the slurry reaches.

According to the present embodiment, the blower7is driven and the slurry is supplied in the state (the state shown inFIG. 1) where the annular resistive member5is attached to the circumference of the opening11bof the support base11. In this case, the suction force with which the slurry is sucked into the channels2(first channels) located in the outer circumferential area2bradially outward of the honeycomb substrate1is weaker than the suction force with which the slurry is sucked into the channels2(second channels) located in the center area2aclose to the radial center of the honeycomb substrate1. As a result, the slurry coat width differs between the channels2located in the center area2aof the honeycomb substrate1and the channels2located in the outer circumferential area2b.

The resistive member5has an opening smaller than the outer diameter of the honeycomb substrate1and faces the channels2(first channels) located in the radially outward of the honeycomb member1, namely, the outer circumferential area2b. The opening5aof the resistive member5oppose to the channels2(second channels) located in the center area2a. With this structure, the speed of the air flow is lower in the channels2located in the outer circumferential area2bwhich the resistive member5faces than in the channels2in the center area2a. As a result, the slurry coat width is shorter in the outer circumferential area2bthan in the center area2a.

In order to control the coat width difference (i.e., the difference between the slurry coat width in the outer circumferential area2band the slurry coat width in the center area2a) to be a desirable value, the inventors measured the coat width difference, using different types of resistive members5(the net member3and the plate member4) and spacers6having different thicknesses (the thickness of a spacer6is equal to the distance between the honeycomb substrate1and the resistive member5), and examined how the types of resistive member5and the thickness of a spacer6had an effect on the coat width difference. An example of the combination between the types of resistive member5and the thicknesses of the spacers6is shown inFIG. 5. The measurements of the coat width difference are shown inFIG. 6.

The other measurement conditions were determined as follows:

the outer diameter of the honeycomb substrate1: 103 mm

the outer diameter of the resistive member5: 103 mm, the inner diameter of the opening5a:60 mm

the mesh of the net member3: 250

the suction time by the blower7: 5 sec, wind speed: 40 m/s

The wind speed of the air flow caused by the blower7was measured before the slurry was supplied.

Under the above conditions, the honeycomb substrate1was coated with the slurry. After the slurry dried, the center of the honeycomb substrate1was cut in the longitudinal direction (first direction), and the coat width difference of the slurry was measured in practice.

The results are shown inFIG. 6. As can be seen, it was found that in both the case where the plate4was employed as the resistive member5and the case where the net member3was employed as the resistive member5, the coat width difference tended to increase when the distance (clearance) between the lower end1bof the honeycomb substrate1and the resistive member5(namely, the thickness of the spacer6) was short, and tended to decrease when the distance was long. The results are attributable to the fact that the resistive member5provided close to the honeycomb substrate1slows the speed of the air streams at the exit of the channels2located in the outer circumferential area2bwhich the resistive member5faces.

In other words, it was found that the coat width difference could be controlled by changing the distance between the lower end1bof the honeycomb substrate1and the resistive member5, namely, the thickness of the spacer6. That is, with respect to the channels2in the center area2awhich faces to the opening5aand is not influenced by the resistive member5, the slurry coat width does not vary in accordance with the thickness of the spacer6. The coat width varies in accordance with the thickness of the spacer6only in the channels2in the outer circumferential area2b. It is also found that if the resistive member5is away from the honeycomb substrate1more than 30 mm, the slurry coat width remains substantially the same as the case where the resistive member5is not provided.

Where the net member3is used as the resistive member5, it can be arranged in contact with the honeycomb member1, without using the spacer6. Where the plate4is used, it must be away from the lower end1bof the honeycomb substrate1. Even where the net member3is employed, it should not be arranged in contact with the lower end of the honeycomb substrate1.

If the net member3is in contact with the lower end1bof the honeycomb substrate1, it may happen that the honeycomb substrate1will break or crack. To prevent the honeycomb substrate1from breaking, the honeycomb substrate1may be brought into contact with the net member3at a low speed (slowly) when the honeycomb substrate1is set. If this is done, however, the takt time is inevitably long, and the productivity lowers. It may be thought to make the net member3, using a material softer than the material of the honeycomb substrate1. If this is done, however, the net member3is not rigid and is easy to deform. Such a soft net member may not satisfactorily function as the resistive member5.

In the present embodiment, the resistive member5(even where the net member3is used) is arranged at a position away from the lower end1bof the honeycomb substrate1. As described above, in order to provide the coat width difference, the distance between the lower end1bof the honeycomb substrate1and the resistive member5should be more than 0 mm and less than 30 mm. In practice, if the distance exceeds 15 mm, the coat width difference becomes 10 mm or less. Under the circumstances, the resistive member5should be away from the honeycomb substrate1, and the distance between them should desirably be 20 mm or less.

As shown inFIG. 6(which illustrates the case where the net member3is employed as the resistive member5and the case where the plate4is employed as the resistive member5), the coat width difference is larger in the case where the plate4is employed than in the case where the net member3is employed, provided that the thickness of the spacer6is the same. This is attributable to the fact that the air can flow through the net member3, whereas the air cannot flow through the plate4. That is, the use of the plate member4as the resistive member5is effective in providing a comparatively large coat width difference. As can be seen inFIG. 6, where the plate4is arranged at a position 1.8 mm away from the lower end1bof the honeycomb substrate1, the coat width difference is maximal, and the air stream can be suppressed most effectively. How the slurry is actually coated on the honeycomb substrate1at the time is shown inFIG. 7.

Where a comparatively-large-mesh net member3having a mesh of 200 is arranged in contact with the lower end1bof the honeycomb substrate1, the coat width difference is less than 5 mm, as shown inFIG. 6. In this case, the net member3hardly functions as the resistive member5. That is, the net member3hardly functions as the resistive member5unless it has a mesh smaller than 200 mesh.

As should be clear from the above, in order to control the coat width difference, the type of resistive member5(including the mesh of a net member) should be properly selected, and the thickness of the spacer6has to be properly determined. Needless to say, the slurry coat shape on the honeycomb substrate1can be changed in accordance with the shape of the resistive member5and the position at which the resistive member5is arranged in the plane orthogonal to the axial direction of the honeycomb substrate1.

In the following, other conditions having an effect on the coat width difference will be considered. Parameters having an effect on the coat width difference include the clearance between the resistive member5and the honeycomb substrate1, the type of resistive member5, the opening ratio of the resistive member5(plate4′), the diameter of the channels2of the honeycomb substrate1, the suction wind speed provided by the blower7, the suction time, the viscosity of the slurry, the amount of slurry coated, etc.

As basic conditions, a honeycomb substrate having a diameter of 103 mm and an axial length of 83 mm was set on the coating apparatus10, a plate4′ (not shown) having a large number of openings (0.8 mmϕ) was arranged at a position 3 mm away from the lower end1bof the honeycomb substrate1, slurry having a viscosity of 130 mPas was supplied in an amount of 180 g, and the blower7was operated such that the suction wind speed was 40 m/s and the suction time was 3 sec. A simulation was performed based on these basic conditions. The opening ratio of the plate4′ (the ratio of the total area of the openings to the area of the plate4′ without the opening5a) was set at 30%. The simulation result of the coat width difference obtained when the slurry was coated under the basic conditions was 13.6 mm.

How the coat width difference varied was examined by changing the parameters one by one. The results of examination are shown inFIGS. 8 to 14.

As shown inFIG. 8, the less the opening ratio of the plate4′ is (i.e., the smaller the number of openings is), the smaller amount of air flowing through the openings of the plate4′. As a result, the coat width difference increases. Conversely, the more the opening ratio of the plate4′ is, the larger amount of air flowing through the openings of the plate4′. As a result, the coat width difference decreases. If the opening ratio of plate4′ is decreased and the amount of air following through the openings is decreased, the slurry coating portion on the channels2in the center area2aof the honeycomb member1and the slurry coating portion on the channels2in the outer circumferential area2bare known to form a steep step shape. If the opening ratio of plate4′ is increased and the amount of air following through the openings is increased, the slurry coating portions are known to form a gentle step shape.

As shown inFIG. 9, even if the openings of the plate4′ are changed in diameter (with the opening ratio kept constant), the coat width difference remains substantially the same.

As shown inFIG. 10, if the clearance between the lower end1bof the honeycomb substrate1and plate4′ is decreased, the air does not flow smoothly through the channels2facing the plate4′. As a result, the coat width difference increases. Conversely, if the clearance between the lower end1bof the honeycomb substrate land plate4′ is increased, the air flows smoothly in the channels2facing the plate4′. As a result, the coat width difference decreases. If the clearance of the plate4′ relative to the honeycomb substrate1is decreased, the slurry coating portion on the channels2in the center area2aof the honeycomb member1and the slurry coating portion on the channels2in the outer circumferential area2bare known to form a steep step shape, and that if the clearance of the plate4′ relative to the honeycomb substrate1is increased, the slurry coating portions are known to form a gentle step shape.

As shown inFIG. 11, if the suction wind speed by the blower7is decreased, the air flows slowly in the channels2facing the opening5aas well, resulting in a small coat width difference. Conversely, if the suction wind speed by the blower7is increased, the air flows rapidly in the channels2facing the opening5aas well, resulting in a large coat width difference. If the suction wind speed by the blower7is decreased, the slurry coating portion on the channels2in the center area2aof the honeycomb member1and the slurry coating portion on the channels2in the outer circumferential area2bare known to form a gentle step shape, and that if that suction wind speed is increased, the slurry coating portions are known to form a steep step shape.

As shown inFIG. 12, if the suction time of the blower7is short, the amount of air stream (the amount of flowing air) is small, resulting in a small coat width difference. Conversely, if the suction time of the blower7is long, the amount of air stream (the amount of flowing air) is large, resulting in a large coat width difference.

As shown inFIG. 13, if the viscosity of the slurry is decreased, the air flows easily in the channels2facing the opening5a, resulting in a large coat width difference. Conversely, if the viscosity of the slurry is increased, the air flows slowly in the channels2facing the opening5a, resulting in a small coat width difference.

As shown inFIG. 14, even if the amount of slurry supplied is changed, the coat width difference remains substantially the same.

Provided that the above-mentioned parameters of the coating apparatus10are set at appropriate values, the present embodiment enables the coat width difference between the center area2aof the honeycomb substrate1and the outer circumferential area2bof the honeycomb substrate1to be controlled at a desired value by simply adjusting the thickness of the spacer6for example. Accordingly, a monolithic catalyst improved in both quality and performance can be manufactured at lost cost and with a simple structure.

In the above-mentioned embodiment, the slurry coat width is controlled to be shorter in the outer circumferential area2bthan in the center area2a, because the air tends to flow at a lower speed through the channels2of the outer circumferential area2bof the honeycomb substrate1than through the channels2of the center area2a. However, it is preferred that the air flow speed through the channels2of the outer circumferential area2bbe equal to the air flow speed through the channels2of the center area2a. For this purpose, it is thought to increase the diameter of the channels2of the outer circumferential area2bto be larger than the diameter of the channels2of the center area2a.

In recent years, monolithic catalysts are being developed wherein the diameter of the channels2of the outer circumferential area2bof the honeycomb substrate1is larger than the diameter of the channels2of the center area2a.

FIG. 15shows an example of a honeycomb substrate21wherein the diameter of channels22of the center area2ais comparatively small and the diameter of channels24of the circumferential area2bsurrounding channels22is comparatively large. This honeycomb substrate21has the same shape and dimensions as the above-described honeycomb substrate1, except that channels22of the center area2aand channels24of the outer circumferential area2bare different in diameter. To be more specific, in the center area2a,600 channels22are provided in a square of (1 inch×1 inch), and in the outer circumferential area2b,400 channels24are provided in a square of (1 inch×1 inch).

Where the diameter of the channels24in the outer circumferential area2bis larger than the diameter of the channels22in the center area2a, the air can easily flow through the channels24in the outer circumferential area2b. As a result, the speed at which the air flows through the channels22in the center area2aand the speed at which the air flows through the channels24in the outer circumferential area2bcan be made substantially the same.

However, where this honeycomb substrate21is set in the coating apparatus10without the resistive member5, and slurry is supplied, with an air stream generated in all channels22and24, the slurry flows more smoothly in the channels24of the outer circumferential area2bthan in the channels22of the center area2a. As a result, the slurry coat width in the channels24of the outer circumferential area2bis greater than the slurry coat width in the channels22of the center area2a, as shown inFIG. 16.

In contrast, where the honeycomb substrate21depicted inFIG. 15is set in the coating apparatus10together with the resistive member5, and slurry is supplied by operating the blower7, the slurry coat width in the channels22of the center area2aand the slurry coat width in the channels24of the outer circumferential area2bare substantially equal, as shown inFIG. 17. In other words, as long as the above-mentioned parameters of the coating apparatus10are set at respective appropriate values, the slurry coat width in the center area2aand the slurry coat width in the outer circumferential area2bcan be made equal to each other by simply adjusting the thickness of the spacer6. In the present embodiment, the slurry coat widths of all channels22and24are made equal to one another by designing the cross sectional area of the channels24of the outer circumferential area2bto be 1.5 times as wide as the cross sectional area of the channels22of the center area2a. It should be noted, however, the optimal cross sectional area ratio can be changed in accordance with the opening ratio of the resistive member5or the thickness of the spacer6.

As another type of monolithic catalyst that comes to be put to practical use, different kinds of slurry (or the same kind of slurry) are coated from the respective axial ends of the honeycomb substrate. The coating apparatus10of the present embodiment can be advantageously applied to an apparatus for manufacturing this type of monolithic catalyst. Where this type of monolithic catalyst is manufactured, the coat width of the slurry applied from one end of the honeycomb substrate and the coating width of the slurry applied from the other end of the honeycomb substrate are related to each other, so that it is useful to control the coat widths.

The embodiment described above does not limit the present invention and is presented by way of example. The scope of the invention is in no way restricted by the above-described embodiment. The above embodiment may be modified in various manners without departing from the gist of the invention.

For example, in connection with the above embodiment, reference was made to the case where a resistive member, such as a net member or a plate member, is provided at a position away from the downstream-side end of the honeycomb substrate with respect to the slurry supply direction, and the air flow speed through the channels is made different, depending upon where the channels are located. However, this is not restrictive. Any means may be employed as a resistive member, as long as it can suppress the air flow through the channels in a state where it does not contact the slurry. In the above embodiment, the resistive member opposed to the honeycomb substrate was described as being annular, but the resistive member may have any shape as long as it can be opposed to channels where the slurry coat width should be decreased.