Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1999-05-19
2001-08-14
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C060S297000, C060S300000, C060S301000, C423S239100, C423S244020
Reexamination Certificate
active
06274106
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to absorbing arrangements and methods for temporarily storing oxides of nitrogen and sulfur and removing such oxides from the exhaust gas from internal combustion engines.
As used herein, the term “absorb” includes the chemical process for storing gases such as, for example, by conversion of barium oxide to barium nitrate for storage of nitrogen oxide.
U.S. Pat. No. 4,755,499 discloses an arrangement for the reversible storage of oxides of nitrogen and sulfur, for example from motor vehicle exhaust gases, in which the absorber is regenerated by heating in a reducing atmosphere. In this arrangement, a reduction of the nitrogen oxides takes place at the same time.
A storage catalyst of that type for use in motor vehicles is described in more detail in U.S. Pat. No. 5,402,641, in which high temperatures above 500° C. are necessary to regenerate the absorber. Consequently, use of the storage catalyst is possible only for motor vehicles having a high exhaust-gas temperature, in particular for motor vehicles with an Otto engine.
In this case, however, the possibility of use is limited since, under certain operating conditions of internal combustion engines, such as occur for example in city traffic, the acceleration phases cause a large emission of nitrogen oxide, but no long-lasting high temperature condition such as is required to regenerate the absorber, especially with respect to oxides of sulfur, is attained.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and arrangement for releasably absorbing oxides of sulfur and nitrogen which overcomes disadvantages of the prior art.
Another object of the invention is to provide an absorber for nitrogen oxides and a corresponding method, suitable especially for use with fuel consumption-optimized engines such as Diesel and/or direct injection Otto engines, in which regeneration of the absorber is possible even at low exhaust gas temperatures.
These and other objects of the invention are attained by providing an arrangement for absorbing oxide gases which includes an absorption layer for absorbing oxides of nitrogen and/or sulfur and a support for the absorption layer which has a wall thickness ≦0.16 mm. The support preferably has a wall thickness ≦0.1 mm and desirably ≦0.05 mm. The support may be a metal sheet or foil and may be heatable by application of an electric current. Preferably the support is formed so as to provide flow passages with a structure arranged to make the flow turbulent. With this arrangement, regeneration of the absorber is possible even at very low exhaust gas temperatures such as occur for example in the case of direct injection Diesel engines.
According to the invention, the usual gas absorbing materials may be employed, for example as described in U.S. Pat. No. 4,755,499, and also in U.S. Pat. Nos. 5,402,641 and 5,362,463. A common feature of all these storage materials is that they have an elevated absorption temperature, while a still higher regeneration temperature is required especially for removing the oxides of sulfur. For most storage media of this kind, temperatures in the range from 150° to 700° C., in particular temperatures above 300° C., are required. Such temperatures commonly occur in motor vehicles with Otto engines, but are comparatively rare with Diesel engines and especially in internal combustion engines having direct fuel injection.
The preferred NO
x
storage materials are distinguished in that, under conditions of net oxidation, i.e., a stoichiometric excess of oxidizing agents, such as occurs in the exhaust gas during the operation, they will store nitrogen oxides and, upon a reduction of the excess of oxygen, may reduce them. For this purpose, the NO
x
storage catalysts usually include a precious metal, in particular the usual precious metal coatings for three-way catalysts. The NO
x
-laden storage material is advantageously regenerated in a regenerating phase at &lgr;≦1.
Ordinarily, various reactions take place successively or simultaneously on the NO
x
storage catalyst, the most important reactions being: oxidation of the NO in the exhaust gas to NO
2
, storage of the NO
2
as nitrate, decomposition of the nitrate, and reduction of the re-formed NO
2
to nitrogen and oxygen.
As described above, the course of the reactions depends, among other things, not only on the temperature of the catalyst but also on the concentration of the reagents at the active region of the catalyst and the flow velocity of the gas.
According to the invention, it has now been found that, with various factors capable of being combined with each other, it is possible also, at little cost, to optimize the known exhaust-gas absorbers so that they may be employed for internal combustion engines with direct injection and for Diesel engines. For this purpose, the wall thickness of the supporting member on which the absorption layer is applied preferably should be ≦160 microns, and desirably ≦140 microns and if a metal support is used, a wall thickness ≦50 microns, preferably ≦40 microns, and desirably ≦30 microns, and the absorber should preferably be heated to a temperature above the temperature of the exhaust gas.
According to the invention, it has been found that, with the use of thin-walled ceramic supports for the absorption layer, i.e. supporting members having a wall thickness ≦0.14 mm, not only is a more rapid temperature rise of the absorption layer possible, but also a thicker absorption layer may be used. This accomplishes two objectives: in the first place, even short periods of high-temperature operation can be utilized for regeneration since the storage layer temperature will be increased to the required temperature more quickly, and in the second place, by providing a thicker absorption layer, a higher NO
x
or SO
x
storage capacity can be achieved so that during operation of the internal combustion engine a longer period of time can elapse before the storage layer must be regenerated. Consequently, despite the less frequent occurrence of temperature peaks in the exhaust gas of consumption-optimized internal combustion engines, no failure of the storage layer resulting from exceeding its storage saturation limit will occur.
According to the invention, absorbers having a support member made of metal foil are especially suitable, and the metal foil may advantageously be connectable to an electric power source for resistance heating so that, even at low exhaust gas temperatures, the absorber can be brought to the necessary regenerating temperature by passing an electric current through the metal support. Furthermore, by using a metal support member, the gas passages which are coated with the absorption layer may be variously shaped, so that, for example, a controlled turbulent flow vortex of the exhaust gas in the passages can be established.
With especial advantage, according to the invention, supports with a variety of passage segments may be used for the absorber where, for example, an intermediate segment of the passages is modified to produce a turbulent flow. This can be done, for example, by varying the passage cross-section, or by a twisting or distortion of the passages. In this way, the support may be adapted in a controlled way for especially favorable reaction conditions along the flow passages. Another special feature of the support, beside a possible variation in number of passages in the flow direction and the provision of changes of cross section along the flow direction, is the segmentation of the support where, for example, one segment with an absorption layer is disposed near the engine outlet and another segment with an absorption layer is located somewhat farther away. Thus, even with the most variable operating conditions, good NO
x
purification results can be obtained with fuel consumption-optimized engines.
According to the invention, it has been found that the NO
x
storage arrangement will have especially good absorption and desorption properties if t
Baker & Botts LLP
Griffin Steven P.
Medina Maribel
Volkswagen AG
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