Honeycomb

Details Honeycomb Honeycomb

2000-05-15

2004-09-21

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S177000, C422S211000, C422S222000, C502S439000, C502S527190, C428S593000

Reexamination Certificate

active

06793896

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a honeycomb, particularly as a catalytic converter substrate, with a honeycomb structure comprising a large number of ducts running in the longitudinal direction of the honeycomb, through which a fluid can flow, where the honeycomb possesses foil layers arranged one above the other.
BACKGROUND OF THE INVENTION
Honeycombs of this type are particularly used as exhaust gas catalytic converters in motor vehicles, although they can also be used in other combustion installations or in chemical engineering, if appropriately dimensioned.
Honeycombs of this kind are known which possess an alternating arrangement of plane and corrugated foils soldered to each other, thus producing fairly small ducts with a cross-section that is essentially sinusoidal or encloses an incircle. In order to achieve the most complete possible catalytic conversion in the honeycomb, the honeycomb must have an appropriate volume to ensure that the retention time of the substances to be converted in the honeycomb is sufficiently long. However, the surface area of the duct walls contained in the honeycomb volume is an essential determinant of the costs, as it causes a corresponding increase in the use of structural material and catalytically active coating material, which usually contains precious metals, such as platinum, palladium and/or rhodium. The increase in substrate mass, as well as in binder and carrier material for the catalytically active precious metals, also increases the thermal inertia of the catalytic converter, as a result of which, for example, the pollutant emission of exhaust gas catalytic converters is elevated during the warm-up phase of vehicle engines. Moreover, an increase in the internal surface area also increases the pressure loss in these and thus the power loss of the drive unit.
The object of the invention is to create a honeycomb that permits highly effective catalytic conversion with low thermal inertia, as well as the inexpensive manufacture of catalytic converters.
SUMMARY OF THE INVENTION
According to the invention, the object is solved in that ducts are provided, whose cross-sectional dimension in a first direction is several times the cross-sectional dimension in another direction, particularly a direction perpendicular to the first it has been found that ducts of this kind display a far more effective duct geometry that sinusoidal ducts, for example, in which the gussets or acute-angled inside corners of the ducts have virtually no catalytic effect, despite being coated with catalytically effective material. The efficiency is also improved in comparison with ducts with an approximately circular or isogonal duct cross-section, because the elongated or gap-like duct cross-sections permit better mass transfer transverse to the flow ducts owing to a more favourable ratio of cross-section to wall circumferential area, this being favourable in the case of laminar flow, in particular. Due to the elongated, non-isogonal cross-sections of the ducts, where the duct walls are at very different distances from the centre of the duct, the volume percentage with stagnating or only slow-flowing gas boundary layers, and thus the inhibition of diffusive transport of the pollutants to the catalytically active coatings, is reduced, and the area-specific reactivity, and thus the efficiency of the resultant catalytic converter, is substantially increased. Moreover, savings can be made on substrate and coating material.
The ducts can display a straight, curved or kinked, parallel profile.
The non-isogonal or non-isometric ducts according to the invention preferably account for a volume percentage of the honeycomb that constitutes a non-negligible proportion of the total capacity of the catalytic converter, e.g. more than 5% of the total catalytic converter capacity, particularly preferably almost the total catalytic converter capacity. The honeycomb area designed in accordance with the invention can thus, for example, account for more than 10 percent by volume and preferably more than 25 or 50 percent by volume of the total honeycomb volume.
The areas of the honeycomb designed according to the invention are preferably at a distance from its marginal areas, i.e. its face and/or lateral surfaces. The distance from the marginal areas can be several times the duct height, e.g. more than 5 or 10 times, or a fraction of the honeycomb width, e.g. {fraction (1/20)} to {fraction (1/10)} or more. Advantageously, the entire honeycomb structure is composed of non-isogonal ducts according to the invention, where different areas of the honeycomb can, however, display different duct cross-sections.
In order to improve the efficiency of the catalytic converter, the non-isogonal ducts can extend over a substantial portion of the length of the honeycomb, e.g. more than one-quarter or one-half of the same, preferably over virtually the entire length of the same, where the ducts can be interrupted by areas having different cross-sectional geometries. However, it could be sufficient that the ducts having non-isogonal cross-sections or having a substantially constant heigt over a width of several foil layer distances do extend with their cross-sections as defined above over a length corresponding to several foil layer distances.
The ducts advantageously extend over the entire width of the honeycomb, as a result of which temperature equalisation is not impeded by partition walls and mass transfer is possible over the entire width of the honeycomb, this leading to a more uniform distribution of both velocity and mass over the cross-section of the honeycomb. The retention time of the fluid in the honeycomb, which defines the lower limit of the honeycomb volume, is equalised and increased as a result, thus also increasing the efficiency.
In particular, the ducts can have at least locally or over a portion or preferably over the entire length of the honeycomb a height being at least substantially constant, the ducts extending with this height over a width of the honeycomb of a length corresponding to a multiple (for instance the 2 or 3-to 5-fold or even more) of the average or the greatest height of the ducts or the foil layer distance. Duct walls or duct sections having smaller cross sections can be adjacent these duct sections. Adjacent foil layers therefore extend at least over this width or over a larger width, for instance {fraction (1/10)} or ½ or over the entire width of the honeycomb substantially in parallel to each other. Accordingly substantially no or only small cross sections lowerings, for instance being about 25% or less of the duct height, are present over this width.
The ducts can, in particular, be designed in such a way that they extend with at least an approximately equal height over a width corresponding to a multiple (e.g. 3 to 5 times or more) of the mean of maximum duct height. These areas can be followed by duct walls or duct constructions.
In particular, the foil layers have a profile, the profile height of which is small in comparison with the distance between opposite foil layers. This profile can be provided, for example, in the form of punctiform elevations and depressions, these increasing the stiffness of the foil layers and simultaneously improving the adhesion of a ceramic substrate material on the foil layers.
In addition, or as an alternative, the foils can also be provided with a profile, the profile height of which, i.e. the distance between the upper and lower vertex of the profile, is large compared to the distance between the foil layers. In addition to stabilising the dimensions of the foils, this can also influence the flow characteristics of the honeycomb, e.g. in relation to a mass transfer in the transverse direction of the same.
For special applications, e.g. with partial lateral inflow into the ducts, it may be desirable to provide a profile which is of asymmetrical design in relation to a reference plane running through one vertex of the profile, perpendicular to the foil layer. The flow resistance in opposite directions

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