Patent Publication Number: US-2005115276-A1

Title: Method and system for reducing a foam in a glass melting furnace

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part application of application Ser. No. 10/440,631, filed May 19, 2003, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method for selectively reducing or removing a foam present in a glass melting furnace. The present invention also relates to a system for reducing or removing a foam present in a glass melting furnace.  
      2. Description of the Related Art  
      A glass melting furnace is conventionally used to melt an initial raw material to form a molten material which can be subsequently processed into a glass product. For example, the molten material can be used to form glass fibers. Such glass fibers can be used, for example, in insulation and structural reinforcement applications. In addition to glass fibers, the molten material provided by a glass melting furnace can be used to form, for example, flat glass, glass containers and various specialty glass products.  
      During the melting of the initial raw material introduced into the glass melting furnace, a foam is typically formed above the molten material in the furnace. The foam can be formed from the evolution of gas during the melting of the initial raw material. Generally, the foam contains small bubbles held together by a matrix of molten material, and forms a layer over at least a part of the surface of the molten material. The physical characteristics of such foam layer can depend on the conditions in the furnace. Foam layer thickness, for example, typically can be from about 2 inches (5.1 cm) to about 4 inches (10.2 cm).  
      The presence of foam can impede the transfer of heat from a heat source of the glass melting furnace, to the initial raw material and/or the molten material present underneath the foam. In conventional systems, the heat source typically must therefore provide an additional amount of heat in order to compensate for the insulating effect of the foam. As a result, the presence of the foam can increase the operating costs of the glass melting furnace. Further, the increased temperature in the furnace can shorten the operating life of the furnace and/or increase the production of particular exhaust gases such as, for example, NO x  gases and toxic metal oxide gases.  
      In light of the above, reducing the foam in a glass melting furnace can be advantageous at least because such reduction can result in an increase in energy efficiency. For example, it has been estimated that the cumulative impact of removing about half of the foam in U.S. combustion-heated glass furnaces could result in an annual energy savings of as much as 12 to 14 trillion BTU. Further, abating the foam in a furnace can extend furnace life as well as reduce the production of particular exhaust gases.  
      Reducing or removing the foam present above the molten material in a glass melting furnace can be difficult to achieve. For example, various attempts at foam abatement including adjusting the glass chemistry (e.g., by using chemical additives in the initial raw material) and varying the furnace crown heating profile, have been ineffective and/or unpredictable. In addition, using an oxygen rich combustion heat source in place of an air combustion heat source can actually result in an increase in foam generation.  
      In view of the foregoing, it is apparent that a need exists for a method and system for effectively reducing or removing a foam present in a glass melting furnace.  
     SUMMARY OF THE INVENTION  
      According to one aspect, a method for selectively reducing or removing a foam present in a glass melting furnace is provided, comprising: 
          selecting a portion of a foam present above the surface of a molten material in a glass melting furnace; and     providing an ultrasonic energy emitted from at least one ultrasonic energy source to the selected portion of the foam, wherein the ultrasonic energy is effective to reduce or remove the selected portion of the foam.        

      According to another aspect, a system for reducing or removing a foam present in a glass melting furnace is provided, comprising: 
          at least one ultrasonic energy source for emitting an ultrasonic energy, wherein the at least one ultrasonic energy source is arranged such that a majority of the ultrasonic energy is reflected off of an acoustic reflecting surface prior to being provided to a foam present above a molten material present in the glass melting furnace.       

    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a perspective view of a system which includes a glass melting furnace wherein ultrasonic energy sources are arranged at the sidewalls of the furnace, in accordance with one aspect.  
       FIG. 2  is a perspective view of a system which includes a glass melting furnace wherein ultrasonic energy sources are arranged at the crown of the furnace, in accordance with another aspect.  
       FIG. 3  is a perspective view of a system which includes a glass melting furnace wherein ultrasonic energy sources are arranged at the sidewalls and the crown of the furnace, in accordance with another aspect.  
       FIG. 4A  is a perspective view of a system which includes a glass melting furnace wherein an ultrasonic energy source alternately provides ultrasonic energy to two non-overlapping zones of the foam layer, in accordance with another aspect.  
       FIG. 4B  is a perspective view of a system which includes a glass melting furnace wherein an ultrasonic energy source alternately provides ultrasonic energy to two overlapping zones of the foam layer, in accordance with another aspect.  
       FIG. 5  is a perspective view of a system which includes a glass melting furnace wherein two ultrasonic energy sources provide different ultrasonic energies to two zones of the foam layer, in accordance with another aspect.  
       FIG. 6  is a cross-sectional side view of a system for reducing or removing a foam present in a glass melting furnace, in accordance with a further aspect. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
      Reducing or removing the foam present in a glass melting furnace in accordance with the present invention can provide several advantages. For example, reducing or removing the foam can lead to the reduction of the energy needed to melt the initial raw material introduced into the glass melting furnace and/or the energy required to maintain the molten material at an optimal temperature. In addition, a reduction in energy usage can in turn extend furnace life and/or reduce the production of particular furnace exhaust gases such as, for example, NO x  gases and toxic metal oxide gases.  
      The selective reduction or removal of at least a portion of the foam present in the glass melting furnace can provide additional advantages. For example, the foam present above the molten glass can act as an insulation for maintaining heat in the molten glass. Selectively removing a portion of the foam, for example, from an area of the molten glass that is subjected to heating, can enable the molten glass to be effectively heated while at the same time using the remaining foam as insulation for maintaining the heat in the molten glass.  
      Referring to  FIG. 1 , a furnace such as a glass melting furnace  10  is provided for melting an initial raw material introduced thereto, and providing a flow of molten material  18  therefrom. The molten material can include molten glass as the primary component. The furnace  10  can have any shape suitable for melting the initial raw material and providing the molten material  18 , preferably an elongated shape. In an exemplary embodiment, the furnace  10  includes two side walls  26 ,  27  and a crown  28 , which is the upper surface of the furnace  10 .  
      The initial raw material can include any material suitable for forming the molten material  18  such as, for example, limestone, glass, sand, soda ash, feldspar and mixtures thereof. In one embodiment, a glass composition for producing glass fibers is “E-glass,” which typically includes 52-56% SiO 2 , 12-16% Al 2 O 3 , 0-0.8% Fe 2 O 3 , 16-25% CaO, 0-6% MgO, 0-10% B 2 O 3 , 0-2% na20+K 2 O, 0-1.5% TiO 2  and 0-1% F 2 . The initial raw material can be provided in any form such as, for example, relatively small particles. Upon entry into the furnace  10 , the initial raw material can form a batch blanket  14  at one end of the furnace  10 .  
      The initial raw material can be introduced into the furnace  10  on a batch, semi-continuous or continuous basis. In some embodiments, a port  12  is arranged at an end of the furnace  10  through which the initial raw material is introduced. The port  12  can be positioned above the surface of the molten material  18 . The amount of the initial raw material introduced into the furnace  10  can be a function of, for example, the capacity and operating conditions of the furnace  10  as well as the rate at which the molten material  18  is removed from the furnace  10 .  
      The molten material  18  formed from the initial raw material can be removed from the furnace  10  via a throat  16  located at an end of the furnace  10  that is opposite the end at which the port  12  is positioned. Preferably, the throat  16  is arranged below the surface of the molten material  18 . The molten material  18  can be removed from the furnace  10  on a batch, semi-continuous basis or continuous basis. In an exemplary embodiment, the molten material  18  continuously flows in the furnace  10  from the point at which it is formed to the throat  16  where it is removed. Thereafter, the molten material  18  can be processed by any suitable known technique, for example, a process for forming glass fibers.  
      The glass melting furnace  10  utilizes at least one heat source  16  which provides heat to the initial raw material and/or molten material  18  in the furnace  10 . Preferably, a plurality of heat sources  16  is used. The at least one heat source  16  can be, for example, an air combustion burner, an oxygen combustion burner or a combination of air and oxygen combustion burners. Other types of heat sources known in the art, such as electrical or induction, can be used in conjunction with or in place of the combustion burner.  
      The at least one heat source  16  can be arranged at any position in the furnace  10  which is suitable for heating the initial raw material and/or molten material  18 . In many embodiments, a heat source or plurality of heat sources  16  are arranged at each sidewall  26 ,  27  of the furnace  10 . At least one heat source or plurality of heat sources  16  can also be positioned at the crown  28  of the furnace  10 .  
      In an exemplary embodiment, the at least one heat source  16  can directly or indirectly provide heat to at least one area of the molten glass. For example, in the case where foam has been entirely removed from above an area of the molten glass, the at least one heat source  16  can provide heat directly to the molten glass. Alternatively, in the case where foam has been partially removed from above an area of the molten glass, the at least one heat source  16  can provide heat indirectly to the molten glass through the remaining foam.  
      The at least one heat source  16  may provide an amount of heat which is effective to melt the initial raw material to form the molten material  18 , and to maintain the molten material  18  in its molten state. The optimal temperature for melting the initial raw material and maintaining the molten material  18  in its molten state can depend on, for example, the composition of the initial raw material and the rate at which the molten material  18  is removed from the furnace  10 . For example, the maximum temperature in the furnace  10  can be at least about 1400° C., preferably from about 1400° C. to about 1650° C. The temperature of the molten material  18  can be from about 1050° C. to about 1450° C.; however, the present invention is not limited to operation within the above temperature ranges. The molten material  18  removed from the furnace  10  is typically a substantially homogeneous composition, but is not limited thereto.  
      A foam layer  24  is present above the surface of the molten material  18  in the furnace  10 . As used herein, the term “above the surface of the molten material” includes foam that is in the upper most region of, or on the surface of the molten material  18 .  
      At least a part of the foam layer  24  can be formed during the melting of the initial raw material. The foam layer  24  can at least partially include “primary foam” which refers to foam generated near the batch blanket  14  and which typically includes CO 2 -rich bubbles. The foam layer  24  can also at least partially include “secondary foam” which refers to foam generated from the evolution of gases formed from the chemistry of the fining process including, for example, sulfur dioxide (SO 2 ), oxygen (O 2 ) and carbon dioxide (CO 2 ).  
      Also, the foam layer  24  can be at least partially formed from additional sources of foam in the furnace  10 . For example, the foam layer  24  can include foam produced from injecting or bubbling a gas into the molten material  18  and/or the agitation of the molten material  18 .  
      The foam layer  24  is present above at least a portion of the surface of the molten material  18 , and can also be present above unmelted raw material particles present at the surface of the molten material  18 . For example, the foam layer  24  can be present above the majority of the surface of the molten material  18 , or substantially above the entire surface of the molten material  18 .  
      While not wishing to be bound to any particular theory, the amount and type of foam generated during the melting process is believed to be a function of, for example, the composition of the initial raw material, the presence of contaminants such as organic contaminants in the initial raw material, the redox state of the molten glass, the furnace temperature and atmosphere, and/or the rate of removal of the molten material  18  from the furnace  10 . The foam layer  24  can have any thickness and is not limited to having a uniform thickness. For example, the thickness of the foam layer  24  can be about from about 1 inch (2.5 cm) to 4 inches (10.2 cm), or greater.  
      According to one aspect of the present invention, at least one acoustic energy source  22  is arranged to provide acoustic energy to at least a portion of the foam layer  24  present in the furnace  10 . In some embodiments, the at least one acoustic energy source  22  comprises at least one ultrasonic energy source  22  which provides ultrasonic energy. The ultrasonic energy provided by the at least one ultrasonic energy source  22  is effective to reduce or remove at least a part of the foam layer  24  present in the glass melting furnace  10 .  
      For example, at least a portion of the foam layer  24  can be selected for reduction or removal. In an exemplary embodiment, only the selected portion of the foam layer  24  is reduced or removed, that is, the foam layer  24  preferably is not entirely reduced or removed. The selected portion of the foam layer  24  can include a contiguous part of the foam layer  24  or separate parts of the foam layer  24 . In an exemplary embodiment, the method can include selecting a portion of the foam layer  24  present above an area of the molten glass which is subjected to heating. By removal of a selected portion of the foam layer  24 , heat can be effectively delivered to the molten glass, and the remaining foam layer  24  can be used as insulation for maintaining heat in the molten glass.  
      While not wishing to be bound to any particular theory, it is believed that the ultrasonic energy can reduce or remove the foam by destabilizing the matrix of molten material which traps gas in the foam. For example, the ultrasonic energy can cause the reduction or removal of the foam through a plurality of mechanisms, for example, transient and stable cavitation. In transient cavitation, the binding between molecules in the foam is broken when the foam is subjected to a relatively high level of acoustic energy. As a result, microscopic bubbles can be formed which are typically highly unstable and subsequently implode. In stable cavitation, the violent rupturing of the foam structure generates eddies in the surrounding molten material. The generation of eddies in the molten material, also known as microstreaming, can cause further rupturing of the foam structure. Other mechanisms can occur instead of or in addition to the transient and stable cavitation discussed above, and the present invention is not limited to any particular theory of the removal of the foam above the surface of molten material by acoustic energy.  
      According to the present invention, the at least one ultrasonic energy source  22  can emit ultrasonic energy at a frequency which is effective to reduce or remove at least a part of the portion of the foam layer  24  to which the ultrasonic energy is provided. By use of the at least one ultrasonic energy source  22 , the part of the foam layer  24  to which the ultrasonic energy is provided can be reduced by from about 25% to about 100% of the thickness of the foam, preferably from about 50% to about 100% of the thickness of the foam.  
      The frequency and intensity of the emitted ultrasonic energy can effect the degree of reduction or removal of the foam layer  24 . For example, the ultrasonic energy emission can be at a frequency of from about 25 kHz to about 125 kHz, more preferably from about 25 kHz to about 75 kHz, and most preferably from about 25 kHz to about 50 kHz.  
      The intensity of the ultrasonic energy can be from about 100 to 160 dB, more preferably from about 100 to 140 dB, and most preferably from about 100 to 120 dB. One ultrasonic energy source suitable for use in the present invention is available from the Power Ultrasonic Group of the Instituto de Acustica, located in Madrid, Spain.  
      The at least one ultrasonic energy source  22  can be arranged in any position which enables the at least one ultrasonic energy source  22  to provide ultrasonic energy to the foam layer  24 . In some embodiments, the at least one ultrasonic energy source  22  is arranged to provide ultrasonic energy to the upper surface of the foam layer  24 , such as being arranged above the surface of the molten material  18 . For example, the at least one ultrasonic energy source  22  can be arranged at one or both of the sidewalls  26 ,  27  and/or the crown  28  of the furnace  10 .  
      The ultrasonic energy source  22  preferably introduces ultrasonic energy into the space in the furnace  10  above the surface of the molten material  18 . That is, the ultrasonic energy source  22  preferably is not in direct contact with the molten material  18  itself.  
      In one embodiment, the at least one ultrasonic energy source  22  can be arranged to provide ultrasonic energy to an upstream portion of the foam layer  24 , for example, the foam that is adjacent to the batch blanket  14 . The term “upstream portion of the foam layer” as used herein refers to the half of the foam layer  24  that is closer to the port end of the furnace  10 . Because there can be a current of molten material  18  flowing from the port end to the throat end in the furnace  10 , treating the upstream portion of the foam layer  24  can be particularly effective to reduce the overall amount of foam present in the furnace  10 .  
      The at least one ultrasonic energy source  22  can be effective to provide ultrasonic energy to any amount of the surface area of the foam layer  24 , preferably from about 10% to about 50% of the surface area of the foam layer  24 . In other embodiments, the at least one ultrasonic energy source  22  is effective to provide ultrasonic energy to substantially the entire surface area of the foam layer  24 .  
      Referring to FIGS.  1  to  3  which illustrate exemplary embodiments of the present invention, a plurality of ultrasonic energy sources  22  can be utilized, and various configurations of the ultrasonic energy sources  22  can be used. As shown in  FIG. 1 , a plurality of ultrasonic energy sources  22  can be positioned at each sidewall  26 ,  27  of the furnace  10 . Referring to  FIG. 2 , a plurality of ultrasonic energy sources  22  can be arranged at the crown  28  of the furnace  10 . In  FIG. 3 , a plurality of ultrasonic energy sources  22  can be arranged at each sidewall  26 ,  27  as well as the crown  28  of the furnace  10 .  
      Referring to  FIGS. 4A and 4B , and in accordance with additional aspects of the present invention, the at least one ultrasonic energy source  22  can alternately provide ultrasonic energy to a plurality of zones A, B of the foam layer  24 . The plurality of zones A, B can each encompass a separate part of the foam layer  24  as shown in  FIG. 4A , or can overlap with another zone as shown in  FIG. 4B .  
      This can be implemented by using, for example, an ultrasonic energy source  22  which is moveable and/or rotatable. A motor or other suitable device (not shown) can be used to move and/or rotate the ultrasonic energy source  22 , thereby redirecting the ultrasonic energy between the plurality of zones A, B. In the above exemplary embodiments, a single ultrasonic energy source  22  can be used to provide ultrasonic energy to a plurality of zones A, B of the foam layer  24 , thereby potentially reducing the total number of ultrasonic energy sources  22  needed to provide ultrasonic energy to a large surface area of the foam layer  24 .  
      The foam layer  24  can contain different types of foams at different zones of the furnace  10 . To achieve the reduction or removal of different types of foams present in the furnace  10 , a plurality of ultrasonic energy sources  22  can be used wherein each ultrasonic energy source  22  provides a particular type of ultrasonic energy which is suitable for removing or reducing a particular type of foam.  
      For example, referring to  FIG. 5 , a first type of foam can be present at a first zone X of the foam layer  24 , and a second type of foam (which is different from the first) can be present at a second zone Y of the foam layer  24 . A first ultrasonic energy source  22   a  can be provided which is directed to the first zone X and is effective for reducing the amount of the first foam. A second ultrasonic energy source  22   b  can be provided which is directed to the second zone Y and is effective for reducing the amount of the second foam. Similarly, an nth ultrasonic energy source  22  can be used which is effective for reducing an nth foam present in an nth additional zone. The zones of foam can occupy separate areas or overlap.  
      As shown in  FIG. 4A , the heat from the at least one heat source  16  and the ultrasonic energy from the at least one ultrasonic energy source  22  can be provided to the same area of the foam layer  24 . This embodiment can provide the efficient transfer of heat to the molten material  18  because the heat is directed to a portion of the foam layer  24  which has been reduced or removed.  
      In an exemplary embodiment, the at least one ultrasonic energy source can be located in an environment maintained at a temperature that is lower than the temperature in the glass melting furnace. Maintaining the at least one ultrasonic energy source in a relatively cool environment can, for example, extend the service life of the at least one ultrasonic energy source. For example, the at least one ultrasonic energy source can be maintained at a temperature of from about 300 to about 600° C.  
      The use of an acoustic conduit can enable the at least one ultrasonic energy source to be remotely located from the hot gas present in the glass melting furnace. The acoustic conduit can include any structure suitable for conveying the acoustic energy. For example, referring to  FIG. 6 , the at least one ultrasonic energy source  16  can be arranged in a cavity  30  in a sidewall of the glass melting furnace  10 , wherein the cavity can function as an acoustic conduit. A cooling system  40  can provide at least one flow of cool gas, such as air, to the cavity  30 . The at least one flow of cool gas can, for example, result in a gas flow out of the cavity  30  which is effective to substantially prevent hot gas from the glass melting furnace  10  from entering into the cavity  30 .  
      An acoustic reflecting surface  50  can be arranged such that an amount of the acoustic energy  52  emitted from the acoustic energy source  16  is reflected off of the acoustic reflecting surface  50  prior to being provided to the foam layer  24 . For example, the acoustic reflecting surface  50  can be arranged such that a majority of the acoustic energy  52 , preferably substantially all of the acoustic energy  52 , is reflected off of the acoustic reflecting surface  50 . The acoustic reflecting surface  50  can be formed from a material suitable having a high impedance for efficiently reflecting acoustic energy. Preferably, the material used to form the acoustic reflecting surface can be a castable refractory material. The acoustic reflecting surface  50  can be formed from a material that is the same as a material used to form the sidewall and/or crown of the furnace, or preferably a material that reflects acoustic energy more efficiently than a material used to form the sidewall and/or crown of the furnace  10 . For example, the acoustic reflecting surface  50  can be formed from a ceramic, a metal or a combination thereof.  
      The acoustic reflecting surface  50  can be fixed or moveably attached to the sidewall and/or crown of the furnace  10 . In an exemplary embodiment, an adjustable connection  54  between the acoustic reflecting surface  50  and the sidewall and/or crown can be used to adjust the angle of the acoustic reflecting surface  50  relative to the emitted ultrasonic energy  52 . The position of the adjustable connection  54  can be controlled remotely, for example, by a motor used with a controller (not shown).  
      While the invention has been described with preferred embodiments, it is to be understood that variations and modifications can be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and the scope of the claims appended hereto.