Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
2000-05-15
2003-07-08
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
Reexamination Certificate
active
06589910
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a catalytic converter substrate with a large number of continuous flow paths for a fluid medium and with carrier elements for a catalyst material extending in the longitudinal direction of the flow paths.
BACKGROUND OF THE INVENTION
A substrate of this type is known from EP 0 430 945 B1, which is constructed of at least partially structured foils, where the sequence of the smooth and corrugated foils positioned against each other in the honeycomb forms ducts that are permeable to a fluid and separated from each other. The foil stacks are wrapped around each other in order to simplify manufacture of the honeycomb.
Catalytic converter substrates of this kind are capable of improvement in relation to various aspects.
Due to the very small flow ducts with sinusoidal cross-section, areas occur in the gussets or narrow areas of the ducts which have difficulty in coming into contact with the fluid medium in the central, hydraulically effective flow cross-sections, as thicker flow boundary layers develop, or dead spaces containing inert, i.e. already reacted, medium that is not transported away, or only after a delay, occur here as a result of the viscous adhesion of the fluid medium to the partial surfaces of the duct circumference. The catalytic efficiency of the partial surfaces covered in this way is low and the overall efficiency of the precious-metal coatings used in the catalytic converter thus capable of improvement. The result of this is that the minimum volume of the catalytic converter, and thus also its weight, are subject to a lower limit. Therefore, the medium or gas, reacted as effectively as possible directly on the coating, without inhibition of reaction and mass transfer, should be transported away as quickly and uniformly as possible with the through-flow, rather than occupying and reducing the efficiency of a major proportion of the coating surface in the form of thicker, inert, exceedingly slow-flowing and occasionally stagnating medium layer.
Moreover, the ready-for-use catalytic converter, and thus also the substrate, must meet the requirement that the pressure loss when the fluid medium flows through the substrate be as low and uniform as possible. Moreover, while offering high efficiency, the overall weight of the catalytic converter should be as low as possible, particularly giving consideration to the additionally applied, porous, catalytically effective coating, so that the inertia of the catalytic converter during the warm-up phase up to reaching operating temperature remains low, as is desirable in the automotive sector, for example. Consequently, there are limits to the increase in catalyst activity that can be achieved by constantly reducing the diameter of the flow ducts and increasing the total internal surface area.
Consequently, the generally harmful proportion of the volume where the substrate has only a supporting and separating function, comprising unused structural walls, coating material and inert, static medium volumes, which can account for up to half the cross-section and more than half the volume, is thus to be minimised.
Furthermore, high demands are to be placed on catalytic converters, particularly also in the automotive sector, in relation to temperature uniformity in the structure, as catalytic converters are exposed to severe temperature fluctuations, particularly during the warm-up and cool-down phases, this resulting in prolonged inhibition of the chemical reaction during cold-start, re-start, part-load and cold-weather operation, especially in the peripheral area of the substrate.
Furthermore, the catalytic converter substrate should be designed in such a way as to achieve the most homogeneous possible pressure and velocity distribution of the fluid medium over the substrate cross-section, thus making the effective flow retention time more uniform and, consequently, longer.
SUMMARY OF THE INVENTION
The object of the invention is therefore to create a catalytic converter substrate which is optimised in relation to the above-mentioned problems and, in particular, leads to a catalytic converter with reduced precious-metal and material input in a reduced or optimised overall volume, while maintaining or improving efficiency.
According to the invention, this object is solved in that carrier elements are provided that display edges around which flow is possible in the longitudinal direction of the flow paths and to which, therefore, only minimal boundary layers can adhere. It was found that the edges introduced according to the invention, which are flowed around by the laminar flow of the fluid medium without significantly increasing any pressure loss and with which, owing to their extending in the longitudinal direction of the flow paths, there is comparatively intensive contact and, on average, a long contact time with a given volume increment of the fluid medium, lead to a significant improvement in catalytic converter efficiency. The edges around which flow is possible project into the flowing medium, increasing the turbulence therein and generally increasing the specific conversion capacity of the coating carrier surface substantially compared to the plane areas of the side walls of narrow, non-round flow ducts, thus constituting catalytically particularly effective areas. In this context, the edges can be either angular or rounded, e.g. in the form of wires or small tubes with a circular cross-section.
Hereafter, the term “carrier elements” is always intended to mean those with edges around which flow is possible, including those permitting full circumferential flow, unless expressly stated otherwise.
The edges around which flow is possible preferably lie parallel or at an acute angle to the longitudinal direction of the catalytic converter substrate.
In this context, the total length of the edges around which flow is possible in relation to the total internal surface of the substrate and/or their arrangement and frequency distribution over the cross-section and the volume of the substrate is designed such that the catalytic activity assignable to the carrier elements according to the invention is significant in relation to the overall efficiency of the catalytic converter, e.g. amounts to more than 10% of the total efficiency. In particular, the catalytic efficiency assigned to the carrier elements can be greater than that of the surface and volume areas not provided with the carrier elements according to the invention. In this context, a carrier element is assigned a cross-sectional area of the substrate in which contact or material conversion of the fluid medium takes place on the edge of the carrier element.
The ratio of the length of the edges around which flow is possible to the substrate volume through which flow is possible, which can be calculated as the total of the hydraulic cross-sections divided by the substrate length, can be 1 cm per 5 to 0.8 cm
3
or less, averaged over the entire substrate volume, this being equivalent to a continuous edge around which flow is possible in a duct with a hydraulic radius of approx. 15 or 5 mm. In this context, the hydraulic cross-section is in each case defined by an inscribed circle touching the duct walls and, in the case of a contiguous gas plenum, by the radius of adjacent circles. In the case of multi-rib webs, a corresponding multiple of the web length must be taken, while at least twice the length must be used for round wires or profiles, as flow from opposite sides is possible here.
The total length of the edges around which flow is possible along a flow path can be a multiple of the distance between adjacent edges. This particularly applies to the continuous length of a single edge around which flow is possible, where the carrier elements or edges can also be axially interrupted or several carrier elements with edges around which flow is possible can be arranged one behind the other a distance apart in the longitudinal direction of a flow path. If the edges around which flow is possible are of arched or helical design, for exa
Browdy and Neimark , P.L.L.C.
Johnson Edward M.
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