Gas separation – Specific media material – Ceramic or sintered
Reexamination Certificate
2000-05-31
2001-11-27
Hopkins, Robert A. (Department: 1724)
Gas separation
Specific media material
Ceramic or sintered
C055SDIG003, C060S311000
Reexamination Certificate
active
06322605
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to filters for use in exhaust streams for capturing particulate material. In particular the present invention relates to porous ceramic diesel exhaust filters which can be regenerated by microwave energy.
Recently much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material (DPM) present in the diesel exhaust stream. In 1998 DPM was declared a toxic air contaminant by the California Air Resources Board. As mentioned herein above legislation has been passed that regulates the concentration and particle size of DPM pollution originating from both mobile and stationary sources.
DPM which is mainly carbon particulates, is also known as soot. One way of removing diesel soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter (DPF) which is used to capture the soot. The DPF is designed to provide for nearly complete filtration of the soot without hindering the exhaust flow. However, as diesel soot accumulates, exhaust flow becomes increasingly difficult and the DPF must either be replaced or the accumulated diesel soot must be cleaned out. Cleaning the accumulated diesel soot from the DPF is achieved via burning-off or oxidation to CO
2
and is known in the art as regeneration. Regeneration is considered to be a superior approach over DPF replacement since no interruption for service is necessary.
The regeneration process can be either passive or active. In a passive system, regeneration occurs when the DPF becomes so filled with carbon particulates that heat accumulated in the exhaust system due to excessive back pressure raises the temperature of the carbon to a point where it ignites. This design can result in thermal shock or melt down of the filter, high fuel penalty and poor filtering action. Active regeneration is considered to be a superior approach over passive regeneration. In an active system, heat required to initiate combustion of the soot is generated by an outside source. Electrical power, fuel burners and microwave energy have all been studied as heat sources. Microwave energy is considered to be a superior approach over electrical power and fuel burners because it is highly efficient cost- and energy-wise.
Microwave regeneration has been addressed, for example in U.S. Pat. No. 5,087,272 (Nixdorf) which discloses a microwave regenerated filter made of single crystal silicon carbide whiskers which are consolidated into a preform of cylindrical configuration or into a thin layer such as a paper, which is then folded into a multicellular form. A problem associated with the proposed filter is that it is labor intensive and time consuming to manufacture, and hence not adaptable to high efficiency production methods.
Standard commercially available filters are made of cordierite (2MgO-2Al
2
O
3
-5SiO
2
). However, cordierite is transparent to microwaves and is not regenerable upon exposure to microwave energy.
Therefore a need exists for a filter for trapping and combusting particulates from a diesel exhaust stream which can undergo regeneration by microwave energy and which can be manufactured according to high efficiency production methods (i.e., extrusion), while at the same time exhibiting a high filtration efficiency.
It is the purpose of the present invention to provide such a filter.
SUMMARY OF THE INVENTION
The present invention provides a filter for trapping and combusting diesel exhaust particulates comprising a microwave-absorbing filter body formed from a refractory oxide ceramic material having a loss tangent which decreases with increasing temperature, such that upon exposure to a microwave source the temperature of the filter as a function of time reaches an equilibrium at about 1100° C., and preferably around 900-1000° C.
In particular the refractory oxide ceramic material is selected from the group consisting of A
1−x
M
x
B
1−y
M′
y
O
3−&agr;
, where A and M are selected from the group consisting of Na, K, Rb, Ag, Ca, Sr, Ba, Pb, La, Pr, Nd, Bi, Ce, Th and combinations thereof; where B and M′ are selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Rh, Ru, Pt, Zn, Nb, Ta, Mo, W and combinations thereof; wherein, the chemical formula is electrostatically balanced;(A′
a
R
r
M″
m
)(Z)
4
(X)
6
O
24
, where A′ is from Group IA metals; where R is selected from Group IIA metals; where M″ is selected from the group consisting of Mn, Co, Cu, Zn, Y, lanthanides and combinations thereof; where Z is selected from the group consisting of Zr, Hf, Ti, Nb, Ta, Y, lanthanides, Sn, Fe, Co, Al, Mn, Zn, Ni, and combinations thereof; where X is selected from the group consisting of P, Si, As, Ge, B, Al, and combinations thereof; wherein, the chemical formula is electrostatically balanced.
In particular the filter body of the present invention is a honeycomb substrate having an inlet and outlet end and a multiplicity of cells extending from said inlet end to said outlet end, said cells having porous walls, wherein part of the total number of cells at said inlet end are plugged along a portion of their lengths, and the remaining part of cells that are open at said inlet end are plugged at said outlet end along a portion of their lengths, so that a gaseous stream passing through the cells of said honeycomb from said inlet end to said outlet end flows into said open cells, through said cell walls and out of said honeycomb substrate through said open cells at said outlet end, and having a cell density in the range of about 100-300 cells/in
2
, and a cell wall thickness in the range of about 0.008-0.030 inches.
In particular the filters have an open porosity of at least 25% by volume, a pore diameter in the range of 10 to 50 microns, and a filtration efficiency of at least 90%.
REFERENCES:
patent: 4329162 (1982-05-01), Pitcher, Jr.
patent: 4477771 (1984-10-01), Nagy et al.
patent: 4781831 (1988-11-01), Goldsmith
patent: 5009781 (1991-04-01), Goldsmith
patent: 5087272 (1992-02-01), Nixdorf
patent: 5108601 (1992-04-01), Goldsmith
patent: 5622680 (1997-04-01), Monceaux et al.
patent: 0 420 513 (1995-01-01), None
patent: 6-241022 (1994-08-01), None
“Preparation of Bulk and Supported Perovskites”, Twu and Gallagher, Chapter 1, pp. 1-9, Properties an Applications of Perovskite-Type Oxides.
“Development of a Microwave Assisted Regeneration System for a Ceramic Diesel Particulate System” Gautam et al., SAE Technical Paper Series 1999-01-3565, pp. 1-16.
Application 60/157,895 filed Oct. 5, 1999.
He Lin
Merkel Gregory A.
Tanner Cameron W.
Wexell Dale R.
Corning Incorporated
Gheorghiu Anca C.
Hopkins Robert A.
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