Abstract:
A filter comprising (i) a filter bag formed of a filter medium comprising glass fibers, the fibers being fixed to one another at crossover points by a ceramic binder coating, and (ii) a transversely adjustable support member to which the filter medium is fixed. This invention also relates to a method of filtering a gas stream using such a filter and support member arrangement

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to elongated bag-like tubular filters (“filter bags”) for filtering particulate material carried in a gas or liquid stream in a filter housing and, more particularly, relates to a ceramic coated filter medium and internal support for use in gas filtration. 
     2. Related Background Art 
     Filter bags are typically mounted in filter housings or in a baghouse with supporting frames or cages positioned therein to hold the filter bags in an open, hollow tubular configuration. The bags are open at an outlet end and closed at the other end. Particulate-laden gas or liquid is directed into the baghouse and through the gas-permeable filter bags. The particulate material is filtered and retained on the exterior surface of the filter bags while the filtered gas passes through the filter bag. Periodically, a reverse purge of gas or fluid is directed into the open outlet end of the filter bags for dislodging any trapped particulate material from the bags and thus, cleaning the filter bags. Hence, filter media must be able to withstand not only the normal conditions of gas filtration, but also the periodic cleaning process to which they will be subjected. 
     The temperatures at which gas filtration is conducted are greatly limited by the properties of the filter media employed. Synthetic filter media typically have insufficient heat resistance beyond approximately 250° C. At temperatures above 250° C., loss of mechanical strength and/or thermal decomposition is observed with such conventional synthetic filter media. For temperatures up to about 300° C., woven or non-woven glass can be used. However, filters of glass fibers are often brittle and do not withstand the mechanical stress associated with typical baghouse operations. For temperatures above 300° C., metallic filter media may be utilized. However, due to their very high price and limited corrosion resistance, the field of application for metallic filter media is very limited. Vacuum formed fiber ceramics or sintered grain ceramics are also available. However, due to the limited filter area per ceramic filter element, substantially more space is required for the total filter, thereby resulting in elevated cost. 
     Hence, there is great industrial demand for inexpensive, compact filtration equipment operational within a high-temperature range of about 300° C. to about 500° C. or even as high as 700° C. or higher. Glass fibers are an attractive filter media in this temperature range due to their desirable physical and chemical properties and low cost. However, as described previously, the use of glass fiber based filter media is limited by the brittleness of the media which results in fiber breakage due to friction and bending during use. 
     To reduce friction between the glass fibers, the art has suggested applying various coatings, such as silicon, starch, graphite and PTFE to glass fibers. However, above 300° C., these coatings degrade, thus resulting in the loss of fiber protection and therefore, rapid destruction of the filter media. 
     Therefore, for economical and process reasons, a need exists for a filter medium operational at temperatures above about 300° C., exhibiting good chemical resistance, long lifetime, a larger filter area per element, and an attractive price. 
     SUMMARY OF THE INVENTION 
     In the filter media of the present invention, individual fibers of woven or non-woven glass or mineral are fixed at the fiber crossover points through coating with organic-inorganic or inorganic binders. Breakage at the fiber crossover points is avoided by substantially eliminating movement of the filter medium through the use of a support. The support is desirably dimensionally adjustable (e.g., transversely adjustable) to provide a close fit between the filter medium and the support. 
     The present invention provides for high-temperature (e.g., above about 300° C. to at least about 500° C. or even as high as about 700° C.) filter media of ceramic coated woven or non-woven glass fibers with optimal filtration properties, such as filter rate and dust-collecting efficiency and without significant pressure drop. This is accomplished by coating the glass fibers with an organic-inorganic or inorganic binder to effect fixation of the fibers at the crossover points. Additionally, through the use of a special support member, breakage of the fixed crossover points, both as a result of the normal conditions of gas filtration and as a result of periodic cleaning processes, is eliminated. Thus, the filter medium of the present invention results in good operation in the temperature range of about 300° C. to at least about 500° C. (or even as high as about 700° C.) and exhibits good chemical resistance, long lifetime, a larger filter area per element (e.g., 1-2 m 2 /element) and an attractive price. 
     Accordingly, in a first aspect, this invention is directed to a filter comprising a filter bag formed of a filter medium comprising glass fibers, the fibers being fixed to one another at crossover points by a ceramic binder coating. This invention is also directed to such a filter medium wherein the filter medium is fixed on a dimensionally adjustable (i.e., transversely adjustable) support member. 
     Preferably, the glass fibers suitable for use in the present invention include materials such as glass, ceramic minerals, and a mixture thereof. 
     In the preferred embodiment of the present invention, the filter medium operates at temperatures up to about 700° C. 
     In yet another preferred embodiment, the filter medium weighs from about 100 g/m 2  to about 2000 g/m 2 . 
     Preferably, the binder coating is an organic-inorganic or inorganic substance. 
     In another preferred embodiment of the present invention, the binder comprises SiO 2 , Al 3 O 3 ZrO 2 , TiO 2 , SiO 2 /TiO 2 , SiO 2 /ZrO 2 , B 2 O 3 /SiO 2 , Na 2 O/B 2 O 3 /SiO 2  or Na 2 O/Al 2 O 3 /B 2 O 3 /SiO 2  composites. 
     In yet another preferred embodiment, the binder composition is applied to the filter medium by dipping, rolling or spraying. 
     In the preferred embodiment of the present invention, the support member is dimensionally adjustable by being transversely adjustable. A suitable support member may comprise a cylindrical metal support comprising metal wires, slotted support rings, spacers, and V- or U-shaped rails. For example, the support member may have a circumference which can be fixed in position by spacers guided in parallel or conically arranged V-shaped or U-shaped guiding rails. 
     This invention is also directed to a method of filtering a gas stream, comprising the steps of providing a support member being transversely adjustable, supporting, by the support member, a filter medium comprising glass fibers, wherein the fibers are fixed to one another at crossover points by a ceramic binder which is applied to the filter medium, and passing a gas stream with particulates therein through the filter medium supported by the support member, wherein the filter medium traps the particulates and removes the particulates from the gas stream. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic, front view of the filter and internal support of the present invention. 
     FIG. 2 is a schematic, top view of the filter and internal support of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The filter media of the present invention are preferably comprised of glass. As used herein, the term “glass” includes glass, ceramic and glass-ceramic formulations including fibers made from minerals, such as rock. Preferably, conventional glass fibers are used, which may be one or more of many formulations well known in the art. The glass fibers may be woven or non-woven and may be prepared by any technique known in the art. 
     Any one of a number of suitable binders, or combinations thereof, may be used for the purposes of the present invention. However, the binder should be organic-inorganic or inorganic, in order to attain the optimal heat resistance. The binder typically is in the form of a sol-gel. A sol-gel form is generally one which converts a colloidal dispersion to a gel. Thus, the binder of the present invention typically comprises water glass, or a colloidal suspension of organic-inorganic or inorganic particles in water or an organic solvent, or a mixture of colloidal suspensions with water glass, or hydrolysis and condensation products, which ultimately form SiO 2 , Al 3 O 3 ZrO 2 , TiO 2 , SiO 2 /TiO 2 , SiO 2 /ZrO 2 , B 2 O 3 /SiO 2 , Na 2 O/B 2 O 3 /SiO 2  or Na 2 O/Al 2 O 3 /B 2 O 3 /SiO 2  composites. 
     In a preferred embodiment of the present invention, the filter medium has a weight of from about 100 g/m 2  to about 2000 g/m 2 . 
     In general, the filter bag of the present invention is used with a support member on which the filter bag is supported facing an opening through a cell plate in a baghouse. The support member includes a generally tubular support frame, which is affixed to the cell plate for supporting the filter bag. It is preferred that the support frame be configured to provide a tight fit with the filter bag, in order to reduce the potentially detrimental flexing that can occur during the filtration and reverse-purge cleaning cycles to which the bag will be subjected. 
     One such support frame is described in a copending, commonly-assigned application, application Ser. No. 09/061,242, filed concurrently herewith by Käppeler et al., entitled “Adjustable Filter Bag Retainer,” and incorporated by reference herein in its entirety. The support frame described therein includes a longitudinal gap extending along its entire length. In order to provide a tight fit between the filter bag and the frame, a mechanism is provided for spreading open the gap in order to expand the frame transversely. 
     FIGS. 1 and 2 schematically illustrate such a support member, which is made up of a matrix of peripheral rings  3  affixed to substantially longitudinal beams  1 . Each of the rings  3  has a disjunction, and the disjunctions of the rings  3  are oriented transversely between an adjacent pair of the beams  1 , creating a longitudinal gap in the frame. In order to provide a close fit with the filter bag (not shown), the frame can be expanded transversely. This is done by a mechanism that selectively spreads the rings  3  open. In order to accomplish this, a pair of rails  4  is affixed to the frame on opposite sides of the gap. A spacer  2  slidably engages each of the rails  4 . The spacer  2  biases the rails apart, and the peripheral rings  3  are held slightly open due to the presence of the spacer  2 . The rails are non-parallel, and the spacer maintains its shape as it slides. Thus, as illustrated schematically in FIGS. 1 and 2, sliding the spacer  2  up and down the rails (i.e., in the directions indicated by arrows) will alternately force the rails  4  apart causing the rings  3  to open so as to expand the frame transversely, and permit the rings  3  to relax inwardly to allow the frame to contract. 
     The frame is preferably formed of heavy wire. The beams  1 , of which there are preferably 10 to 25 disposed around the perimeter of the frame, are typically formed of a 3 mm wire. The supporting rings should be spaced between about 100 and about 400 mm apart along the length of the frame. In the illustrated frame, sixteen longitudinal beams  1  are spot welded around the periphery of eleven peripheral rings  3 , which are spaced approximately 200 mm apart. The rails  4  can be affixed to the inside of the rings  3 , by means of spot welding or the like. As discussed, the rails are not parallel in this frame, but are disposed at a slight angle with respect to each other. This angle should be less than about 5 degrees, preferably in the approximate range of about 3 to about 5 degrees. 
     As can be seen in FIG. 2, the rails are generally concave and face one another. The opposing surfaces of the rails provide a seat in which the spacer  2  can fit. In the illustrated embodiment, the rails are substantially U-shaped. However, the rails can take another suitable shape, such as V- or C-shaped, and can be angled or arcuate. 
     Depending upon the stiffness of the various materials used for the spacer and the peripheral rings  3 , it is preferred that approximately two to ten spacers  2  be disposed along the length of the frame. 
     Suitable alternate designs for the support member are also discussed in the above-referenced copending application. 
     The filter medium is coated with the organic-inorganic or inorganic binder. The binder may be applied by any suitable method, including dipping, rolling or spraying. 
     The resulting filter element is non-self-supporting. Thus, the support member is inserted into the filter element. Preferably, the support member is adjustable in order to provide a close fit between the filter element and the support. Any deformation or movement caused by suction or reverse jet cleaning during the filtration process is thus eliminated, resulting in no breakage of the fixed fiber crossover points. 
     The examples which follow are intended as an illustration of certain preferred embodiments of the invention, and no limitation of the invention is implied. 
     EXAMPLE 1 
     Binder 
     Methyltriethoxysilane is mixed with tetraethoxysilane in molar ratios from 1:1 to 10:1. Then, water is added in molar ratios, water to hydrolyzable groups, from 1:0.1 to 1:4. Hydrolysis and condensation reactions are initiated by adding an acid, e.g., HCl, as a catalyst so that the pH of the solution is adjusted to 1. After reaction, the resulting coating solution can be used as prepared or diluted with an organic solvent such as ethanol or propanol. 
     EXAMPLE 2 
     Binder 
     Highly, dispersed fused silica is suspended in propanol with an ultrasonic disintegrator. The amount of fused silica is from 0.25% and 5% by weight. Another solution is prepared by hydrolysis and condensation of tetraethoxysilane (TEOS) with water, ethanol and HCl whereby the molar ratios of TEOS:water:ethanol:HCl vary from 1:0.5:0:0.01 and 1:8:10:0.1. Then, the suspension of the SiO 2 -particles in propanol is mixed with the tetraethoxysilane solution in a volume ratio from 1:1 to 1:10. 
     EXAMPLE 3 
     Binder 
     Methyltriethoxysilane is mixed with tetraethoxysilane in molar ratios from 1:1 to 10:1. Then, water is added in molar ratios of water to hydrolyzable groups from 1:0.1 to 1:4. The water is introduced by a suspension of colloidal SiO 2 -particles in water with a solids content of 30% by weight. Hydrolysis and condensation reactions are initiated by adding an acid, e.g., HCl, as a catalyst so that the pH of the solution is adjusted to 1. After the reaction occurs, the resulting coating solution can be used as prepared or diluted with an organic solvent such as ethanol or propanol. 
     EXAMPLE 4 
     Binder 
     Highly dispersed fused silica is suspended in distilled water with an ultrasonic disintegrator. The amount of fused silica is from 0.25% and 5% by weight. Then, water glass solution is added in a volume ratio of 1:1 and mixed for an additional 20 minutes. 
     EXAMPLE 5 
     Binder 
     Methyltriethoxysilane is mixed with tetraethoxysilane in molar ratios from 1:1 to 10:1. Then, water is added in molar ratios of water to hydrolyzable groups from 1:0.1 to 1:10. The water is introduced by a suspension of colloidal ZrO 2 -particles in water with a solids content of 30% by weight. Hydrolysis and condensation reactions are initiated by adding acid, e.g., HCl, as a catalyst so that the pH of the solution is adjusted to 1. After the reaction occurs, the resulting coating solution can be used as prepared or diluted with an organic solvent, such as ethanol or propanol. 
     EXAMPLE 6 
     Binder 
     A colloidal suspension of ZrO 2 -particles in water with a solids content of 30% by weight is mixed with water glass solution in volume ratios from 1:1 to 1:10. 
     EXAMPLE 7 
     Binder 
     A colloidal suspension of Al 2 O 3 -particles in water with a solids content of 20% by weight is mixed with water glass solution in volume ratios from 1:1 to 1:10. 
     While a preferred form of the invention has been shown in the drawings and described, since variations in the preferred form will be apparent to those skilled in the art, the invention should not be construed as limited to the specific form shown and described, but instead is as set forth in the following claims.