Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Producing microporous article
Reexamination Certificate
1999-02-02
2001-01-30
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Producing microporous article
C264S635000, C156S089110, C156S089220, C055S523000
Reexamination Certificate
active
06180054
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a composite ceramic candle filter for removing particulates from a hot gas stream, and a method for making said filter.
DESCRIPTION OF RELATED ART
Ceramic filters have been tested in processes such as coal gasification and coal combustion to remove particulates from hot flue gases to protect downstream equipment from corrosion and erosion and to comply with EPA NSPS (New Source Performance Standards) regulations. Ceramic filters in a tubular (candle) form, with one end closed and the other end open have been shown to remove the particulates efficiently. The hot gas to be filtered typically flows from the outside to the inside of the filter, with particulate-free gas exiting from the open end. The candle geometry is also suited for removal of the filtered cake by backpulsing with compressed gases.
Ceramic hot-gas candle filters must withstand exposure to chemically corrosive gas streams at temperatures in excess of 800° C. In addition, they are subjected to significant thermal stresses during backpulse cleaning which can cause catastrophic failure of the ceramic candle filter element.
Ceramic hot-gas candle filters known in the art are generally fabricated from either porous monolithic materials or porous ceramic fiber-containing composite materials. Monolithic ceramic candle filters are either weak or can fail catastrophically in use. Composite filters are less susceptible to catastrophic failure and generally have improved strength, toughness, and thermal shock resistance versus monolithic ceramic filters.
Candle filters may have relatively uniform porosity throughout the filter or they may comprise a porous support with a thin layer, or membrane, of fine porosity on the outer surface of the support. The membrane layer is typically applied to the filter using a variety of methods such as coating from a dispersion containing finer grains than those used in the support for smaller membrane pore sizes, bonding randomly arranged chopped ceramic fibers to the support using colloidal (or sol) materials, or forming a ceramic matrix by chemical vapor infiltration.
Materials used to fabricate ceramic hot-gas filters generally include oxides such as aluminosilicates, glass, and alumina, and non-oxides such as silicon carbide and silicon nitride. Oxide-based ceramic filters have adequate resistance to flue gas atmospheres and fly-ash for the design life of the filters; however, they generally have low thermal shock resistance. Non-oxide ceramics generally have good thermal shock resistance, however they are susceptible to oxidation in the corrosive environment to which they are subjected which results in a degradation of mechanical properties.
The disadvantages of ceramic candle filters known in the art include failure, often catastrophic, due to thermally induced stresses caused by backpulse cleaning, chemical degradation caused by species present in the hot gases being filtered, delamination of the membrane layer, incomplete removal of the filter cake upon backpulsing, and high cost. They also tend to be heavy, requiring expensive support structures to hold an array of the candles in the filter unit.
SUMMARY OF THE INVENTION
The present invention is directed to a ceramic hot gas filter comprising a porous elongated filter support and a porous membrane layer on at least one surface thereof. Specifically, the porous membrane may be on the outer surface, the inner surface, or both the outer and inner surface of the porous elongated filter support. The membrane layer(s) is firmly adherent to the support and therefore does not suffer from delamination problems. The porosities of the support and membrane are controlled such that the support functions as a bulk filter and the membrane layer functions as a surface filter. The support has an opening at one end into a hollow interior, a closed end opposite the open end, and an external flange integral with the open end. The support is formed of a plurality of layers of oxide ceramic support yarn, each layer being arranged in a crisscrossing relationship with neighboring layers to form a plurality of quadrilateral-shaped openings. The yarn in the support is coated with a first oxide ceramic material which, upon heat treatment, forms a porous refractory oxide support matrix. The membrane layer(s) may be formed of an ordered arrangement of continuous filament oxide ceramic membrane yarn, a uniform coating of ceramic filler material, or some combination of the two. Any yarn present in the membrane layer is (preferably prior to winding) coated with a second oxide ceramic material which, upon heat treatment forms a porous refractory oxide membrane matrix. Preferably, the support yarn and the continuous filament membrane yarn each contain at least 20 weight percent alumina (Al
2
O
3
) and have softening points above about 750° C. The ceramic coating materials are generally particulates of oxides or oxide compounds, or mixtures thereof and may also include oxide precursor materials. The membrane layer(s) has a porosity that is less than that of the support. Preferably the quadrilateral-shaped openings have dimensions of about 100 to about 500 microns after heat treatment so that the support functions as a bulk filter. The membrane layer(s) preferably has pore diameters of about 0.1 to 50 microns and functions as a surface filter. In a preferred embodiment of the invention, the support yarn has generally the same composition as the membrane yarn and the support matrix has generally the same composition as the membrane matrix.
The present invention also provides a method for making a ceramic hot gas filter involving the steps of fabricating an elongated porous filter support by coating a ceramic oxide support yarn with a first coating composition, winding the coated support yarn onto a mandrel to form a plurality of layers of the coated support yarn, each layer being arranged in a crisscrossing relationship with neighboring layers to form a plurality of quadrilateral-shaped openings. The mandrel may be contoured to provide an integral external flange adjacent to one end of the support. Alternatively, a separate collar insert may be slid onto a uniformly cylindrical mandrel to form the flange portion of the support. The resulting support has an open end adjacent to the flange, an outside surface, and a second open end opposite the flanged end.
A membrane layer is then formed on at least one surface of the support. For example, the membrane layer may be formed on the outer surface by coating a continuous filament oxide ceramic membrane yarn with a second coating composition and applying the coated membrane yarn in an ordered arrangement on the outer surface of the support. Methods for forming the ordered arrangement membrane layer(s) include hoop winding a single yarn, multiple yarn winding, fabric wrapping and coating with a particulate slurry or a solution containing ceramic precursor materials. In a preferred embodiment, the ordered arrangement comprises a circular or hoop winding of the continuous filament oxide ceramic so as to define a gap of predetermined width between adjacent windings. The gap is then filled with additional ceramic filler material, preferably an oxide material, which upon subsequent heat treatment, forms a porous refractory membrane matrix. The width and uniformity of the gap between adjacent hoops or windings is not particularly critical; however, uniform filling of the gap with filler material is desirable, both around the circumference and along the length of the filter. In another embodiment, a membrane layer is formed by winding the coated continuous filament such that adjacent hoops or windings are as close to one another as possible and no such filler material is applied. Yet another embodiment features a membrane layer comprising ceramic filler material but without hoop-wound filaments; i.e., an infinitely large gap between hoop windings. In this embodiment, for example, ceramic particulates, preferably of an oxide material and preferably in the form of a slurry, are
Chambers Jeffrey Allen
Connolly Elizabeth Sokolinski
Domanski Daniel Matthew
Forsythe George Daniel
Rajendran Govindasamy Paramasivam
AlliedSignal Composites Inc.
Fiorilla Christopher A.
Palguta Larry J.
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