Coating processes – Interior of hollow article coating – Removing excess coating material
Reexamination Certificate
1999-08-19
2003-09-30
Beck, Shrive P. (Department: 1762)
Coating processes
Interior of hollow article coating
Removing excess coating material
C427S238000, C427S294000, C427S430100, C427S443200
Reexamination Certificate
active
06627257
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a process for coating the flow channels in a monolithic, cylindrical catalyst carrier with a coating dispersion.
Monolithic, catalyst carriers are used on a large scale for the production of automotive vehicle exhaust gas catalysts used for pollution abatement. They have a cylindrical shape and a large number of flow channels for the exhaust gases from the internal combustion engine passes through them, the channels lying parallel to the axis of the cylinder. These carriers are frequently also called honeycomb carriers.
The cross-sectional shape of the catalyst carrier depends on how and where it is to be physically incorporated into the vehicle. Catalyst carriers with a round cross-section, an elliptical or triangular cross-section are widely used. The flow channels generally have a square cross section and are arranged in a tight grid over the entire cross-section of the catalyst carrier. Depending on the actual application, the channel density, or cell density, of the flow channels is between 10 and 120 cm
−2
. Catalyst carriers with cell densities of up to 250 cm−
2
or more are under development.
For the purification treatment of vehicle exhaust gases, catalyst carriers which have been obtained by the extrusion of ceramic materials are used. As an alternative, catalyst carriers made from corrugated and rolled-up metal foils are available. Currently, catalyst carriers with cell densities of 62 cm
−2
are still mainly used for exhaust gas treatment in private cars. The cross-sectional dimensions of the flow channels in this case are 1.27×1.27 mm
2
. The wall thicknesses in these kinds of catalyst carriers are between 0.1 and 0.2 mm.
In order to convert the harmful substances present in automotive vehicle exhaust gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds very finely divided platinum group metals are generally used, the catalytic effect of which can be modified by compounds of base metals. These catalytically active components have to be deposited onto the catalyst carrier. However, it is difficult to ensure the requisite very fine distribution of.catalytically active components by depositing these components onto the geometric surfaces of the catalyst carrier. This applies equally to both non-porous metallic and porous ceramic catalyst carriers. A sufficiently large surface area for the catalytically active components can only be provided by applying a support layer consisting of finely divided, high surface area materials.
The present invention provides a process for applying this type of support layer to the internal surfaces of the flow channels of honeycomb-shaped catalyst carriers. In the context of this invention, the support layer for the catalytically active components is called a dispersion coating. The dispersion coating consists of finely divided, high surface area materials and is produced using a so-called coating dispersion. The coating dispersion is a slurry of the finely divided materials, generally in water.
Various processes for depositing the coating dispersion on the catalyst carriers are known from the prior art. After the coating procedure, the catalyst carriers are dried and then calcined in order to consolidate the dispersion coating. The catalytically active components are introduced into the dispersion coating by impregnating with, generally, aqueous solutions of precursor compounds of the catalytically active components. As an alternative, the catalytically active components may be added to the coating dispersion itself. Subsequent impregnation of the final dispersion coating with catalytically active components is not required in this case.
GB 1 515 733 describes a coating process for ceramic catalyst carriers. The porous catalyst carriers are inserted upright, that is with the flow channels in a vertical alignment, into a pressure-resistant coating chamber and degassed by applying a reduced pressure of 0.84 bar (25 inches of mercury). Then the coating chamber is filled with coating dispersion to above the upper end face of the catalyst carrier and this is forced into the pores of the catalyst carrier by applying a pressure which is greater than atmospheric. After reducing the pressure back to atmospheric and opening a discharge valve in the base of the coating chamber, excess coating dispersion flows out of the flow channels in the catalyst carrier. Then any flow channels which are blocked with coating dispersion are blown clear from top to bottom using compressed air. The cycle time for this coating process is from less than 1.5 to 2 minutes.
U.S. Pat. No. 4,208,454 also describes a process for coating porous ceramic catalyst carriers. The lower end faces of the catalyst carriers to be coated are placed on the opening of a collection vessel in which the pressure is reduced to 5 to 16 inches of water below atmospheric pressure by means of a large volume fan. This reduced pressure is held constant during the entire coating period. A predetermined volume of coating dispersion is distributed over the upper end face of the catalyst carrier and drawn uniformly through the flow channels into the collection vessel. The suction process is maintained for at least about 30 seconds. After the first 5seconds the entire amount of coating has been drawn through the catalyst carrier. During the remainder of the time the air flowing through the flow channels ensures that any flow channels blocked by coating dispersion are cleared. The amount of coating remaining on the catalyst carrier can be affected by the duration of the total suction time and by the extent to which the pressure is reduced. Axial uniformity of the coating on the catalyst carrier can be improved by turning the catalyst carrier over after about half the suction time and applying the suction in the reverse direction. Using this process, coating dispersions with 30 to 45% solids contents and a viscosity between 60 and 3000 cps can be processed. The preferred solids content is 37 wt % and the preferred viscosity is 400 cps. The reproducibility for the amount of coating applied using this process is given as ±5%.
EP 0 157 651 B1 also describes a process for coating ceramic catalyst carriers with a predetermined amount of a coating dispersion. Here, the pre-weighed amount of coating dispersion is placed in an open, wide, vessel and the lower end face of the catalyst carrier is immersed in the dispersion. Then the dispersion is drawn into the flow channels of the catalyst carrier under suction, by applying a pressure which is slightly below atmospheric to the upper end face. To improve axial uniformity of the coating, it is also recommended here that the coating process be allowed to proceed in two steps.
In the first step, only about 50 to 85% of the total amount of coating is placed in the vessel and drawn into the catalyst carrier under suction. Afterwards, the catalyst carrier is turned over and the remainder of the coating is drawn into the catalyst carrier under suction in the reverse direction. This coating process does not require a separate step for clearing any blocked flow channels. The cycle time for this process is somewhat less than 1 minute. Using this process, coating dispersions which have a solids content between 35 and 52% and viscosities between 15 and 300 cps can be processed.
U.S. Pat. No. 5,182,140 describes a process for coating ceramic and metallic catalyst carriers. In this case, the coating dispersion is pumped from below into the vertically arranged catalyst carrier until the dispersion reaches a height which s well above the upper end face of the catalyst carrier. Then excess coating dispersion is removed from the carrier applying compressed air to the upper end face of the catalyst carrier. This simultaneously blows out any flow channels which are still blocked. In accordance with example 1 in this patent document, the coating dispersion is adjusted to reach an ultimate height of 2 cm above the upper end face of the catalyst carrier
Domesle Rainer
Foerster Martin
Krampe Willi
Piroth Josef
Schlachter Ulrich
Beck Shrive P.
Degussa-Huls Aktiengesellschaft
Jolley Kirsten Crockford
Kalow & Springut LLP
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