Ceramic honeycomb filter

Details Ceramic honeycomb filter Ceramic honeycomb filter

2002-09-12

2004-12-07

Smith, Duane (Department: 1724)

Gas separation

Specific media material

Ceramic or sintered

C055S282300, C055S385300, C055SDIG001, C055SDIG003, C060S311000

Reexamination Certificate

active

06827754

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a ceramic honeycomb filter for removing particulates from an exhaust gas from diesel engines.
BACKGROUND OF THE OPTION
To remove particulates from an exhaust gas emitted from diesel engines, investigation has been made to use ceramic honeycomb filters having porous partition walls through which the exhaust gas containing particulates is caused to flow. Such filters are called particulates-capturing filters (diesel particulate filters). FIG.
2
(
a
) is a front view showing a ceramic honeycomb filter
11
as a particulates-capturing filter, FIG.
2
(
b
) is a partially cross-sectional side view showing the ceramic honeycomb filter
11
of FIG.
2
(
a
), and
FIG. 3
is a schematic cross-sectional view showing the ceramic honeycomb filter
11
of FIG.
2
. As shown in
FIGS. 2 and 3
, a substantially cylindrical ceramic honeycomb filter
11
comprises an outer peripheral wall
11
a
, and porous partition walls
11
b
disposed inside the outer peripheral wall
11
a
, with flow paths
11
c
surrounded by the outer peripheral wall
11
a
and the porous partition walls
11
b
or by the adjacent porous partition walls
11
b
sealed alternately by sealers
12
a
,
12
b
at inlet-side ends
11
d
and outlet-side ends
11
e
. The outer peripheral wall
11
a
of the ceramic honeycomb filter
11
supported by holding members
13
a
,
13
b
are received in a metal housing
14
.
An exhaust gas containing particulates flows into the flow paths
11
c
of the ceramic honeycomb filter
11
through the inlet-side ends
11
d
(shown by
10
a
), passes through the porous partition walls
11
b
, and goes out of the adjacent flow paths
11
c
through the outlet-side ends
11
e
(shown by
10
b
). In this process, particulates contained in the exhaust gas are captured by pores in the porous partition walls
11
b
. When the captured particulates are excessively accumulated in the ceramic honeycomb filter
11
, the pressure loss of the filter
11
increases, likely resulting in decrease in an engine output. Accordingly, the captured particulates are periodically burned by an external igniting means such as an electric heater and a burner to regenerate the ceramic honeycomb filter
11
. Generally employed, when such a ceramic honeycomb filter
11
is used as a particulates-capturing filter, are (a) an alternate regeneration method using a pair of ceramic honeycomb filters, in which one filter is used while the other filter is regenerated, (b) a-continuous regeneration method, in which particulates are burned by the action of a catalyst while capturing particulates, to regenerate the filter, etc.
Important to such a particulates-capturing filter are a pressure loss, particulates-capturing efficiency, and particulates-capturable time, a time period from the start of capturing particulates to a point at which a pressure loss reaches a predetermined level. The capturing efficiency and the pressure loss are in a contradictory relation; the higher capturing efficiency results in increase in the pressure loss, while decrease in the pressure loss results in the reduction of the capturing efficiency. To satisfy these contradictory properties of the filter, it was conventionally investigated to control the porosity and average pore diameter of the porous partition walls of the ceramic honeycomb filter. It is further necessary that particulates captured by the ceramic honeycomb filter can be burned at high efficiency, and that the ceramic honeycomb filter is not broken by thermal stress generated by the burning of particulates. Thus, investigation has been conducted to meet these requirements.
Under such circumstances, JP 3-10365 B discloses a filter for cleaning an exhaust gas with little pressure loss, the filter having partition walls whose pores are composed of small pores having a pore diameter of 5-40 &mgr;m and large pores having a pore diameter of 40-100 &mgr;m, the number of the small pores being 5-40 times that of the large pores, whereby high capturing efficiency can be maintained from the start. In this filter, pores in the partition walls preferably have an average pore diameter of more than 15 &mgr;m, and the cumulative pore volume of the pores is preferably within a range of 0.3-0.7 cm
3
/g. Though JP 3-10365 B does not describe the porosity P (volume %) of the partition walls, assuming that cordierite used in Examples has a density p of 2.5 g/cm
3
, the porosity P of the partition walls can be calculated from the cumulative pore volume V (cm
3
/g) by the following formula:
P
(%)=100
V
&rgr;/(11
+V
&rgr;).
According to the above formula, a preferred range (0.3-0.7 cm
3
/g) of the cumulative pore volume of pores in the partition walls is converted to a porosity of 42.8-63.6% by volume.
JP 61-54750 B discloses that by adjusting an open porosity and an average pore diameter, it is possible to design a filter from a high-capturing rate to a low-capturing rate. JP 61-54750 B provides a preferred specific example of the open porosity and the average pore diameter in a region defined by points
1
,
5
,
6
and
4
in FIG.
8
. The open porosity and the average pore diameter of each point are as shown below.
Open Porosity
Average Pore
Point
(volume %)
Diameter (&mgr;m)
1
58.5
1
5
39.5
15
6
62.0
15
4
90.0
1
JP 9-77573 A discloses a honeycomb structure having a high capturing rate, a low pressure loss and a low thermal expansion ratio, which has a porosity of 55-80% and an average pore diameter of 2540 &mgr;m, pores in its partition walls being composed of small pores having pore diameters of 5-40 &mgr;m and large pores having pore diameters of 40-100 &mgr;m and the number of small pores being 5-40 times that of large pores.
Though a good balance between the pressure loss and the particulates-capturing efficiency of a filter can be achieved to some extent by optimizing the porosity and the average pore diameter of a ceramic honeycomb structure and the pore diameters of its partition walls, however, increase in the porosity and the average pore diameter inevitably results in decrease in the strength of the porous partition walls of the filter. The reason therefor is that the strength of the porous partition walls is in a contradictory relation with the porosity and the average pore diameter of the porous partition walls. Particularly when the porosity is increased to 60% or more, or the average pore diameter is increased to 15 &mgr;m or more to provide a filter with a low pressure loss, there is remarkable decrease in the strength of the porous partition walls. Accordingly, it has been impossible to obtain ceramic honeycomb filters having low pressure loss and high capturing efficiency as well as high durability, which are not broken by thermal stress and shock generated when used as particulates-capturing filters for diesel engines, or by mechanical stress generated by fastening in assembling and vibration, etc.
At the time of regenerating a conventional honeycomb filter in which particulates are captured, because an exhaust gas passes through flow paths adjacent to an outer peripheral wall
11
a
as shown in
FIG. 3
, heat generated by the combustion of the captured particulates dissipates to a metal housing via the outer peripheral wall
11
a
and holding members
13
a
,
13
b
. Accordingly, there is a large temperature gradient between a center portion of the filter
11
and an outer periphery portion, resulting in the problems that the filter is cracked by thermal stress, and that particulates are insufficiently burned because there is no enough temperature increase in the outer periphery portion.
Further, because sealers on the exhaust-gas-entering side have flat outer end surfaces in conventional ceramic honeycomb filters as shown in
FIGS. 2 and 3
, particulates are accumulated on the outer end surfaces of sealers on the exhaust-gas-entering side. In addition, because particulates extremely strongly tend to be aggregated, the accumulation of particulates gradually grows. When there is a large accumulation of particulates, flow paths

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