U.S. patent application number 09/943258 was filed with the patent office on 2003-03-06 for honeycomb with varying channel size.
Invention is credited to Beall, Douglas M., Marcher, Johnny.
Application Number | 20030041730 09/943258 |
Document ID | / |
Family ID | 25479330 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030041730 |
Kind Code |
A1 |
Beall, Douglas M. ; et
al. |
March 6, 2003 |
Honeycomb with varying channel size
Abstract
A honeycomb structure which includes an inlet end and an outlet
end opposing each other and a plurality of cell channels extending
along an axis from the inlet end to the outlet end, the cell
channels having different hydraulic diameters and being arranged in
a checkerboard pattern between large-diameter and small-diameter
cell channels, and an extrusion die for making the same.
Inventors: |
Beall, Douglas M.; (Painted
Post, NY) ; Marcher, Johnny; (Helsinge, DK) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
25479330 |
Appl. No.: |
09/943258 |
Filed: |
August 30, 2001 |
Current U.S.
Class: |
95/273 ; 428/117;
55/523 |
Current CPC
Class: |
Y10S 55/30 20130101;
B01D 46/0001 20130101; B23H 9/00 20130101; Y10T 428/131 20150115;
Y10T 428/24744 20150115; B01D 46/2451 20130101; B01D 46/249
20210801; B01D 46/2498 20210801; B01D 46/2425 20130101; B01D
46/2466 20130101; F01N 3/022 20130101; B23H 2200/30 20130101; Y10T
428/24157 20150115; B01D 39/2068 20130101; B01D 46/247 20130101;
F01N 2330/06 20130101; F01N 2330/30 20130101; Y02T 10/12 20130101;
B01D 46/2429 20130101; Y10T 428/24149 20150115 |
Class at
Publication: |
95/273 ; 428/117;
55/523 |
International
Class: |
B32B 003/12; B01D
046/00 |
Claims
What is claimed is:
1. A honeycomb structure comprising: an inlet end and an outlet end
opposing each other; a plurality of cell channels extending along
an axis from the inlet end to the outlet end, wherein the cell
channels have different hydraulic diameters and are arranged in a
checkerboard pattern between large-diameter and small-diameter cell
channels.
2. The honeycomb in accordance with claim 1 wherein the large
channels have a hydraulic diameter 1.1-2 times larger than the
small channels.
3. The honeycomb in accordance with claim 2 wherein the large
channels have a hydraulic diameter 1.3 times larger than the small
channels.
4. A ceramic filter for trapping and combusting diesel exhaust
particulates, the filter comprising a honeycomb body, the honeycomb
body comprising: an inlet end; an outlet end opposite the inlet
end; a plurality of parallel cell channels extending in an axial
direction between the inlet and outlet end, the plurality of cell
channels having thin porous walls, the plurality of cell channels
comprising: a group of outlet cell channels each end-plugged at the
inlet end and open and the outlet end; a group of inlet cell
channels each open at the inlet end and end-plugged at the outlet
end, the inlet channels having a larger hydraulic diameter than the
outlet channels; wherein each inlet channel adjoins an outlet
channel in a vertical direction and in a horizontal direction, such
that inlet channels and outlet channel alternate across the
honeycomb body.
5. A ceramic filter in accordance with claim 4 wherein the inlet
channels have a hydraulic diameter 1.1-2 times larger than the
outlet channels.
6. A ceramic filter in accordance with claim 5 wherein the inlet
channels have a hydraulic diameter 1.3 times larger than the outlet
channels.
7. A ceramic filter in accordance with claim 5 which is made of
cordierite.
8. A ceramic filter in accordance with claim 5 which is made of
silicon carbide.
9. A method of removing particulates from an exhaust gas, the
method comprising passing the exhaust gas through the open inlet
cell channels at the inlet end of filter of claim 4, through the
cell walls, and out of the honeycomb through the open outlet cell
channels at the outlet end, whereby the particulates are removed
from the exhaust gas and retained on the honeycomb filter.
10. A honeycomb extrusion die comprising a die body, the die body
comprising: an inlet face, a discharge face opposite the inlet
face, a plurality of feedholes extending from the inlet face into
the body, an intersecting array of discharge slots extending into
the body from the discharge face to connect with the feed holes at
feed hole/slot intersections within the die, the intersecting array
of discharge slots being formed by side surfaces of a plurality of
pins of two different cross sectional areas forming a checkerboard
matrix of pins alternating in size.
11. The honeycomb extrusion die according to claim 10 wherein pins
have a square cross section.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a multicellular structure,
such as a honeycomb, particularly for trapping and combusting
diesel exhaust particulates.
[0002] Wall-flow filters are used in the purification of diesel
exhaust. Typically such diesel particulate filters are made of
cordierite or silicon carbide and include a honeycomb body having
thin interconnecting porous walls which form parallel cell channels
of equal hydraulic diameter, longitudinally extending between the
end faces of the structure. Alternating cells on one end face of
the honeycomb are plugged with a ceramic filler material to form a
"checkerboard" pattern. The pattern is reversed on the opposite
side, so that the ends of each cell are blocked at only one end of
the structure. When diesel exhaust enters the filter through one
end face (i.e., inlet end), it is forced to pass through the porous
walls in order to exit through the opposite end face (i.e., outlet
end).
[0003] For diesel particulate filtration, honeycomb structures
having cellular densities between about 10 and 300 cells/in.sup.2
(about 1.5 to 46.5 cells/cm.sup.2), more typically between about
100 and 200 cells/in.sup.2 (about 15.5 to 31 cells/cm.sup.2), are
considered useful to provide sufficient thin wall surface area in a
compact structure. Wall thickness can vary upwards from the minimum
dimension providing structural integrity of about 0.002 in. (about
0.05 mm.), but are generally less than about 0.060 in. (1.5 mm.) to
minimize filter volume. A range of between about 0.010 and 0.030
inches (about 0.25 and 0.76 mm.) e.g., 0.019 inches, is most often
selected for these materials at the preferred cellular
densities.
[0004] Interconnected open porosity of the thin walls may vary, but
is most generally greater than about 25% of thin wall volume and
usually greater than about 35% to allow fluid flow through the thin
wall. Diesel filter integrity becomes questionable above about 70%
open pore volume; volumes of about 50% are therefore typical. For
diesel particulate filtration it is believed that the open porosity
may be provided by pores in the channel walls having mean diameters
in the range of about 1 to 60 microns, with a preferred range
between about 10 and 50 microns.
[0005] Filtration efficiencies up to and in excess of 90% of the
diesel exhaust particulates (by weight) can be achieved with the
described cordierite materials. The filtration of a lesser but
still significant portion (i.e. less than 50%) of the particulates
may be desirable for other filtering applications including exhaust
filtering of smaller diesel engines. Efficiencies, of course, will
vary with the range and distribution of the size of the
particulates carried within the exhaust stream. Volumetric porosity
and mean pore size are typically specified as determined by
conventional mercury-intrusion porosimetry.
[0006] U.S. Pat. No. 4,420,316 to Frost et al. discusses cordierite
wall-flow diesel particulate filter designs. U.S. Pat. No.
5,914,187 discusses silicon carbide wall-flow diesel particulate
filters.
[0007] There are significant problems associated with conventional
filters of the type described herein. Specifically, as the exhaust
passes through the filter, particulate matter (i.e., carbon soot)
accumulates on the wall of the cell channels or in the pores of the
wall and forms a soot layer. This soot layer decreases the
hydraulic diameter of the cell channels causing a pressure drop
across the length of the filter and a gradual rise in the back
pressure of the filter against the engine, triggering the engine to
work harder, and affective engine operating efficiency.
[0008] Eventually, the pressure drop becomes unacceptable which can
be remedied by regeneration of the filter. In conventional systems,
the regeneration process involves heating the filter to initiate
combustion of the carbon soot layer. Normally, during regeneration
the temperature in the filter rises from about 400-600.degree. C.
to a maximum of about 800-1000.degree. C. Under certain
circumstances, a so-called "uncontrolled regeneration" can occur
when the onset of combustion coincides with, or is immediately
followed by, high oxygen content and low flow rates in the exhaust
gas (such as engine idling conditions). During an uncontrolled
regeneration, the combustion of the soot may produce temperature
spikes within the filter which can thermally shock and crack, or
even melt, the filter. The highest temperatures during regeneration
tend to occur near the exit end of the filter due to the cumulative
effects of the wave of soot combustion that progresses from the
entrance face to the exit face of the filter as the exhaust flow
carries the combustion heat down the filter.
[0009] In addition to capturing the carbon soot, the filter also
traps metal oxide "ash" particles that are carried by the exhaust
gas. These particles are not combustible and, therefore, are not
removed during regeneration. However, if temperatures during
uncontrolled regenerations are sufficiently high, the ash may
eventually sinter to the filter or even react with the filter
resulting in partial melting.
[0010] It would be considered an advancement in the art to obtain a
diesel particulate filter which not only survives the numerous
controlled regenerations over its lifetime, but also the much less
frequent but more severe uncontrolled regenerations, while at the
same time combining good fuel economy. This survival includes not
only that the diesel particulate filter remains intact and
continues to filter, but that the back pressure against the engine
remains low.
SUMMARY OF THE INVENTION
[0011] The present invention provides porous ceramic honeycomb
structures suitable for use in diesel particulate filters, the
structures offering improved configurations that are significantly
more resistant to thermal cracking and melting damage under typical
diesel exhaust conditions than current designs. At the same time,
the inventive structures offer significantly lower pressure drops
across the filter and hence superior resistance to soot-induced
engine back pressure build-up.
[0012] Temperature spikes which occur during regeneration, and
especially during uncontrolled regeneration, are reduced in the
inventive structures. At the same time, the inventive design
provides structures with low initial filter pressure drop and a
reduction in the system pressure drop during use.
[0013] In particular the invention relates to a honeycomb structure
which includes an inlet end and an outlet end opposing each other
and a plurality of cell channels extending along an axis from the
inlet end to the outlet end, the cell channels having different
hydraulic diameters and being arranged in a checkerboard pattern
between large-diameter and small-diameter cell channels.
[0014] For a diesel particulate filter the large-diameter channels
are inlet channels which are open at the inlet end and plugged at
the outlet end. The small-diameter channels are outlet channels
which are plugged at the inlet end and open at the outlet end. Each
inlet channel adjoins an outlet channel in a vertical direction and
in a horizontal direction, such that inlet channels and outlet
channel alternate across the inlet and outlet ends. Preferably, the
inlet cell channels have a hydraulic diameter of about 1.1-2.0
times, preferably 1.3 times larger than the outlet hydraulic
diameter. For purposes of the present invention hydraulic diameter
of the cell channels simply refers to the effective diameter of a
cell channel.
[0015] The honeycomb structures may be formed of cordierite,
silicon carbide or of other similarly porous but thermally durable
ceramic material. Although the cell density is not critical in the
present invention, it is preferred that the honeycomb structures
have a cell channel density of about 100-300 cells/in.sup.2
(15.5-46.5 cells/cm.sup.2), and more preferably about 200
cells/in.sup.2 (15.5-31 cells/cm.sup.2) and a wall thickness about
0.01 to 0.25 inches (0.25-0.64 mm).
[0016] The invention also relates to an extrusion die for
fabricating the inventive honeycomb structures. The novel die
includes a die body which incorporates an inlet face, a discharge
face opposite the inlet face, a plurality of feedholes extending
from the inlet face into the body, and an intersecting array of
discharge slots extending into the body from the discharge face to
connect with the feed holes at feed hole/slot intersections within
the die, the intersecting array of discharge slots being formed by
side surfaces of a plurality of pins of two different cross
sectional areas, alternating in size such as to form a checkerboard
matrix.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 illustrates a prior art end-plugged honeycomb
structure with inlet and outlet cell channels of equal
diameter;
[0018] FIG. 2 illustrates an embodiment of an end-plugged honeycomb
structure according to the present invention with inlet cell
channels of larger diameter than outlet cell channels; and,
[0019] FIG. 3 illustrates the effect of the modified cell geometry
according to the present invention on the soot loaded pressure
drop.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Referring to FIG. 1 therein illustrated is a top view of a
prior art end-plugged honeycomb structure with cell channels of
equal hydraulic diameter. A top view of an embodiment of the
present invention is illustrated in FIG. 2. Honeycomb 10 has a
front or inlet end 12. Although not shown, the outlet end is
opposite the inlet end 12. A plurality of cell channels which are
divided into inlet cell channels 14 and outlet cell channels 16
extend between the inlet and outlet ends. The cell channels have
porous walls 18 and run substantially longitudinal and mutually
parallel between the inlet and outlet ends of the structure.
[0021] The cell channels are arranged to alternate between inlet
cell channels 14 and outlet cell channels 16, resulting in a
pattern of alternating cell channel with small and large hydraulic
diameters. Therefore, each inlet cell channel 14 is bordered on all
sides by outlet cell channels 16 and vice versa. The hydraulic
diameter of inlet cell channels 14 is about 1.1-2.0 times,
preferably about 1.3 times the hydraulic diameter of the outlet
cell channels 16. Although not critical, preferably the structures
have a cell density of about 100-300 cells/in.sup.2 (15.5-46.5
cells/cm.sup.2), and more preferably about 200 cells/in.sup.2
(15.5-31 cells/cm.sup.2), and preferably a wall thickness about
0.001 to 0.025 inches (0.25-0.64 mm), and more preferably about
0.019 inch (0.486 mm).
[0022] Both inlet cell channels 14 and outlet cell channels 16 are
plugged along a portion of their lengths, either at the inlet end
or the outlet end. In FIG. 2 since the inlet end is shown, outlet
cell channels 16 are plugged. The plugs thickness is preferably
about 2 mm to 5 mm in depth. Therefore, inlet cell channels 14 are
open at the inlet end and plugged at the outlet end. Conversely,
outlet cell channels 14 are plugged at the inlet end and open at
the outlet end. This plugging checkerboard configuration allows
more intimate contact between the fluid stream and the porous walls
of the structure. Fluid stream flows into the honeycomb structure
through inlet cell channels, then through the porous cell walls,
and out of the structure through the outlet cell channels.
[0023] The inventive structures are especially suited as diesel
particulate filters, especially in applications where regeneration
of the filter by burning of the carbon soot can result in locally
high temperatures within the filter, thus necessitating excellent
thermal shock resistance and high melting point of the filter
material. The inventive honeycomb structures may be formed of
cordierite, silicon carbide or of other similarly porous but
thermally durable ceramic material. The honeycomb structures may be
either circular or square in cross-section. However, the filter of
course need not have these shapes, but may be oval, rectangular, or
any other cross-sectional shape that may be dictated by the
particular exhaust system design selected for use.
[0024] An advantage of the present inventive filters is a low
pressure drop across the length of the filter and therefore lower
back pressure against the engine, resulting in better fuel
efficiency during use. As it is known the pressure drop across an
end-plugged honeycomb structure depends on the resistance to
laminar flow of gas down the cell channels and, as a second order
effect, the extend of gas contraction and expansion occurring as
the gas traverses the cellular structure. Specifically, the
pressure drop is directly related to the hydraulic diameter of the
cells. In a conventional size cell channel as soot accumulates or
builds up on the cell walls, the effective hydraulic diameter of a
cell decreases resulting in an increase in the pressure drop.
[0025] However, in the inventive filters the inlet cell channels
have a larger hydraulic diameter to start with and as such the
effective hydraulic diameter after the soot accumulates is larger
translating into a decrease in the rate of increase in the pressure
drop across the length of the filter. This effect is presented in
FIG. 3. 2" diameter by 6" long round filters were mounted in an
airstream into which was added dispersed carbon black powder
(Printex U brand). The filter was held in the airstream for a
period of time for artificial soot build-up and then transferred to
a pressure drop apparatus where the pressure drop was measured for
a range of flowrates using air. After recording the pressure drop,
the part was transferred back to the soot loading rig and
additional soot was loaded into the filter. The part was again
transferred to the pressure drop rig to measure pressure drop as a
function of flowrate. This procedure was repeated several times and
the data is plotted in FIG. 3 for a flowrate of 26.25 cfm. Pressure
drop behavior is given for a conventional diesel particulate filter
having a cell density of 200 cells/in.sup.2 and a channel wall
thickness of 0.019 inches (a so-called 200/19 honeycomb) and equal
diameter cell channels, as well as for a diesel particulate filter
made according to the present invention. The data confirm the
benefit in soot loaded pressure drop for the structures of the
present invention. It shows an approximate 25% lower pressure drop
at higher soot loadings as compared with the conventional 200/19
geometry.
[0026] Another advantage of the present invention is expected to
reside in a reduction in peak filter temperatures encountered
during regeneration. For purposes of the present invention the term
"peak filter temperature" refers to the maximum temperatures
reached within the filter during regeneration. Specifically, the
peak filter temperature is maintained at or below about
1050.degree. C., in the inventive structures. Hence, the inventive
filters are significantly more resistant to thermal cracking and
melting than conventional cordierite filters under conditions
encountered in diesel exhaust systems. This advantage is secured by
an increase in the inlet open frontal area due to the larger
diameter of the unplugged inlet cell channels at the inlet end. At
the same time the open frontal area on the outlet end is decrease
since the larger inlet cell channels are plugged and open are the
smaller outlet cell channels. This has the effect of increasing the
thermal mass of the filter at the outlet end, and securing
resistance of the filter to temperature increase from regeneration.
Therefore, the inventive filters have a large inlet open frontal
area which maximizes the surface area for low pressure drop and
increased thermal mass from the small open outlet cell channels at
the outlet end.
[0027] The invention also relates to an extrusion die for
fabricating the inventive honeycomb structures. Honeycomb extrusion
dies suitable for the manufacture of honeycomb bodies with
alternating channel diameters, as described in the present
invention, will have pin arrays comprising pins of alternating
size. It is not critical to the present invention how such dies are
fabricated and as such could be provided by any one of a number of
known methods, including the assembly of arrays of plates as
disclosed in U.S. Pat. No. 4,468,365 or by bonding pin arrays to a
suitable die base plate as described in U.S. Pat. No. 5,761,787. A
preferred method however would be to use a plunge EDM process with
an EDM electrode configured to have multiple rows of
parallel-aligned tabs of two different cross-sectional areas
alternatively arranged.
[0028] Therefore, the novel die includes a die body which
incorporates an inlet face, a discharge face opposite the inlet
face, a plurality of feedholes extending from the inlet face into
the body, and an intersecting array of discharge slots extending
into the body from the discharge face to connect with the feed
holes at feed hole/slot intersections within the die, the
intersecting array of discharge slots being formed by side surfaces
of a plurality of pins of two different cross sectional areas,
alternating in size such as to form a checkerboard matrix.
[0029] A suitable method for fabricating the inventive structures
is by forming a plasticized mixture of powdered raw materials which
is then extruded through into a honeycomb body with alternating
cell channel diameters, optionally dried and then fired to form the
product structure. The fired honeycomb filter is typically mounted
by positioning the filter snugly within a cylindrical steel filter
enclosure with a refractory resilient mat disposed between the
filter sidewall and the wall of the enclosure. The ends of the
enclosure may then be provided with inlet and outlet cones for
channeling exhaust gas into and through the alternately plugged
channels and porous wall of the structure.
[0030] A still further advantage of the inventive filter design is
that the plugging process is made easier due to the differences in
size of the inlet and outlet cell channels. Typically the plugging
process is carried out manually and involves the employment of a
flexible mask in one of the filter to cover every other cell
channel in a checkerboard pattern. The exposed channels are then
filled with a ceramic paste (of a material similar the honeycomb
structure) that can be fired to result in a ceramic plug. The
pattern is reverse on the opposite end of the structure to plug
each cell channel only at one end. A disadvantage of this procedure
is that after plugging the first side, it is difficult to determine
which holes to insert mask into on the second end. In a preferred
embodiment, separate masks are fitted to the front or inlet end and
to the back or outlet end to overcome the aforementioned
disadvantage. The honeycomb structures may be plugged either before
or after firing with a paste having the same or similar composition
to that of the green body, using appropriate amounts of a liquid
phase to impart a workable viscosity, optionally with the addition
of binders and plasticizers, as described in U.S. Pat. No.
4,329,162.
[0031] In addition to the embodiments presented herewith, persons
skilled in the art can see that numerous modifications and changes
may be made to the above invention without departing from the
intended spirit and scope thereof.
* * * * *