Plastic and nonmetallic article shaping or treating: processes – With step of cleaning – polishing – or preconditioning...
Reexamination Certificate
1999-04-19
2001-12-11
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
With step of cleaning, polishing, or preconditioning...
C264S229000, C264S621000, C501S095200
Reexamination Certificate
active
06328915
ABSTRACT:
BACKGROUND—FIELD OF INVENTION
This invention relates to monolithic aerogel catalysts and composite materials, specifically to a method to allow internal molding and facilitate rapid drying for aerogel honeycomb catalyst monoliths.
BACKGROUND—DESCRIPTION OF PRIOR ART
Aerogel catalysts are generally used in the form of fine powders or lumps which are fragile, loose, and difficult to handle in chemical reactors. Severe pressure drops and heat and mass transfer limitations occur in fixed bed reactors where aerogels are used in these types of physical forms. Other alternative forms such as aerogel coatings on rashig rings or aerogels being embedded into alundum boiling stones have been tried with limited success to assist in improving on the above limitations. Fluidized bed reactors have also been piloted using the “lumps” form of aerogels with limited success.
European Patent 0186149 by Stauffer Chemical Company describes the preparation of non-aged, inorganic oxide containing aerogels. The method comprises the steps of dissolving the alkoxide in a solvent, optionally adding a catalytic amount of a base or acid, and hydrolyzing the metal compound to produce a gel. The solvent in the gel is exchanged with an extraction fluid, and the fluid in the gel is supercritical extracted to form an aerogel. The patent describes the preparation of amorphous, granular metal oxide aerogels, rather than monolithic forms.
Transparent metal oxide aerogel monoliths have been successfully formed by Lawrence Livermore National Laboratory, U.S. Pat. No. 5,395,805 to Droege (1995), in samples approximately 1 inch in diameter and 0.25 inches thick. This type of small monolith has extremely limited commercial catalytic applications due to its essentially inaccessible internal surface area. The pressure drop that is required to access the internal surface area is tremendously high. Per the LLNL patent, the fabrication of these small monoliths requires a containment vessel that is sealed in such a way as to be gas permeable.
Conventional honeycomb monolith chemical reaction beds for NO
x
reduction are typically at least 20 feet in depth (a 20 foot superficial gas flow path) and have the disadvantages of relatively high pressure drop, laminar flow in the honeycomb channels, and active catalyst surface limited to the surface washcoating of the catalyst impregnated on a ceramic honeycomb monolith.
Current catalyst pore structures depend on the micropore and macropore structure of the material of the base monolith and the ability to uniformly apply a washcoat of material over the monolith. Washcoat connections with the support via thin branches in small pores are highly vulnerable to thermal stress cracking. Typical internal surface areas for a titania monolith are approximately 50 M
2
per gram of material. The washcoat layer surface area is normally in the range of 100 to 200 M
2
per gram of material. Once a thin washcoat has been poisoned by materials such as alkalies and sulfur oxides, the catalyst will be deactivated.
A conventional composition for a NO
x
reduction catalyst that utilizes ammonia for its reduction agent is in the range of four to eight weight percent vanadium oxide or tungsten oxide coated over a titania monolith. The current commercial catalysts have a formulation tradeoff limitation between more Vanadium which increases the activity toward NO
x
reduction but also increases the activity of the unwanted oxidation reaction of SO
2
to SO
3
. SO
3
combines with the ammonia to form ammonium bi-sulfate or ammonium sulfate which can cause corrosion and plugging of the downstream heat exchange equipment. The vanadium oxide allows activity toward NO
x
in lower operating temperature zones than the tungsten oxide.
Aerogel matrix composites using fibers dispersed within the bulk aerogel have been successfully formed by Battelle Memorial Institute U.S. Pat. No. 5,306,555 to Ramamkurthl (1994). These samples were formed with a high weight percentage of fibers, from 9 to 35, and had relatively low surface areas from 147 to 303 M
2
per gram of material.
Although these related patents discuss the formulation of metal oxide aerogels and methods of fabrication of small aerogel monoliths over long time periods (days), none address the practical application of aerogels as catalysts. Economic fabrication techniques for aerogel catalyst sections where the inherently large internal surface area characteristics can be fully exploited at low pressure drops in gas reacting systems are not addressed. The present invention addresses the need for a catalyst that allows selective catalyst reduction of NO
x
by using large ultra-thin honeycomb aerogel catalyst sections that allow the unique surface area of aerogels to be fully exploited at very low gas pressure drops.
OBJECTS AND ADVANTAGES
The following lists several objects and advantages of the invention:
(a) The aerogel mold design will allow the mold to serve multiple uses: as the frame for the pre-tensioned reinforcing fibers, as the gas porous mold during the polymerization (gelation) process, and as the gas porous mold during the supercritical drying process.
(b) Allowing the gas porous mold to serve as both the polymerization and drying vessel enables the matrix of reinforcing fibers and the aerogel inorganic matrix to achieve its maximum composite strength.
(c) The gas porous mold and its gas porous fingers which penetrate through the monolith, are the internal molds for the gas channels and will allow extremely rapid supercritical drying in minutes rather than hours or days.
(d) The ceramic tensioning caps that are the integral part of the pre-stressing system for reinforcing fibers used in the aerogel catalyst, also serve as a permanent, extremely tough handling surface for the large ultra-thin catalyst sections.
REFERENCES:
patent: 5207814 (1993-05-01), Cogliati
patent: 5242647 (1993-09-01), Poco
patent: 5306555 (1994-04-01), Ramamurthi
patent: 5395805 (1995-03-01), Droege
patent: 5538931 (1996-07-01), Heinrichs
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