Patent Publication Number: US-2007107818-A1

Title: Deoxygenation of furnaces with hydrogen-containing atmoshperes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This invention claims the benefit of U.S. Provisional Application 60/737,276, filed Nov. 16, 2005 titled, “in Situ Catalytic Deoxygenation of Heat Treating Furnaces Containing H2 Atmosphere. The disclosure of this provisional application is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      100% hydrogen atmospheres and hydrogen containing atmospheres have been routinely used by the heat treating industry both in batch and in continuous furnaces for metal powder reduction, bright annealing of ferrous and non-ferrous metals and other materials requiring thermal processing. During the operation of the furnaces containing 100% hydrogen atmosphere, the ingression of external air into the furnace is a safety concern, especially in the case where the furnace temperature is lower than the auto-ignition temperature of hydrogen, such as in the exit end or cooling zone of the furnace where the furnace temperature is low. In this case the ingressed air will not react with the furnace hydrogen atmosphere to form moisture, and it can consequently accumulate inside the furnace. When the accumulated air or oxygen level inside the furnace reaches an unsafe level, the furnace will have to be shut down and purged with nitrogen (inert) gas. High oxygen levels can also result in the oxidation of materials being treated inside the furnace. This furnace shut-down and purging will decrease productivity and increase production cost.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention is a method of removing oxygen molecules present in a furnace atmosphere comprising the step of: reacting the oxygen molecules from the furnace atmosphere on one or more catalytic reactors.  
      The present invention is a method for using a catalytic deoxygenation reactor having a furnace atmosphere comprising hydrogen gas (up to 100%) in order to avoid oxygen build-up in the furnace. The reactor may be placed inside the furnace and may be located along the pathway of ingressing gas whereby the oxygen in the ingressing gas will be catalytically converted with hydrogen from the furnace&#39;s atmosphere to water vapor (H 2   0 ). The reactor comes into contact with the oxygen molecules in the atmosphere by the movement of the gases in the furnace atmosphere. Alternatively, atmosphere moving equipment, for example, fans, pumps, or vacuums, can be provided to move the gases in the furnace atmosphere to foster the contact between the oxygen and hydrogen molecules and the reactor. Alternatively, fan(s), pump(s) or vacuum(s) could be used to draw the atmosphere into a reactor that is located external to the furnace, and the atmosphere gases with the reduced molecular oxygen content could be reintroduced into the furnace downstream of the reactor. The reactor may be located adjacent to or mounted onto the furnace and the gases having the reduced molecular oxygen content could be immediately returned to the furnace atmosphere after passing through the reactor. The invention can lower oxygen levels in the furnace and may prevent oxidation of materials that are being processed in and by the furnace.  
      Additionally, this invention provides a furnace comprising an atmosphere and at least one catalytic reactor for removing oxygen molecules present in the atmosphere.  
      These inventions may provide one or more of the following benefits: furnaces and methods for safe operation of furnaces (avoiding the accumulation of oxygen to an unsafe level in the furnace) having a hydrogen-containing atmosphere; furnaces and methods that may avoid or reduce the number of shut-downs and/or nitrogen purgings of furnaces due to air ingression; furnaces and methods that eliminate or reduce the amount of unwanted oxidation of the materials processed in furnaces; furnaces and methods that increase the productivity of furnaces; and furnaces and methods that decrease the production costs of the materials treated in furnaces.  
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       FIG. 1  is a schematic longitudinal cross-sectional view of the exit end of a furnace with the installation of a reactor according of the present invention.  
       FIG. 2  is a schematic longitudinal cross-sectional view of the exit end of a furnace with the installation of a reactor according of the present invention.  
       FIG. 3  is a schematic transverse cross-sectional view of a reactor useful in the present invention.  
       FIG. 4  is a schematic longitudinal cross-sectional view of the entrance end of a furnace showing the placement of a reactor in the front end of the furnace.  
       FIG. 5  is a schematic longitudinal cross-sectional view of the entrance end of a furnace showing the placement of a reactor in the supply line to a recovery system from the front end of the furnace. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      This invention is directed to a method of using a catalytic reactor typically inside a furnace, for example at the exit end curtain area of a furnace or kiln. The furnace comprises hydrogen gas in its atmosphere (up to 100%). The reactor catalytically converts to water any oxygen leaking-in from the outside of the furnace or internal sources within the furnace.  
      The term furnace will be used to mean any piece of thermal process equipment with protective or reactive atmosphere capability. Examples of furnaces may include heat-treating furnaces, continuous belt furnaces, rotary furnaces, box furnaces, strand furnaces, strip furnaces, roller-hearth furnaces, float glass furnaces, ceramic kilns, and others that would be known to the art. These furnaces can be used in processing metal, composites or ceramic materials, including sintering, annealing, carburizing, decarburizing, hardening, brazing, nitriding, melting, glass-making and the like.  
      The atmosphere in the furnace is typically a hydrogen-containing atmosphere. The desired hydrogen-containing atmosphere can contain any amount of hydrogen, typically between 5 and 100%. The balance of the hydrogen-containing atmosphere can be made up of inert gases and/or reactive gases depending upon the process requirements. Examples of inert gases can be nitrogen, argon and helium. Examples of reactive gases may include CO, CO 2 , propane, methane, ethane, and other alkanes, natural gas, silane, ammonia, and the like. This invention is used to maintain a desired atmosphere within the furnace. In some embodiments, the reactor and method of this invention may be used in conjunction with separate atmosphere generating equipment that may be located inside or outside the furnace. The atmosphere generating equipment will have one or more sources outside the furnace for the gases to be fed, or to be generated and then fed, into the furnace atmosphere.  
      The term ingressing gas will be used to describe an undesired source of oxygen into the furnace atmosphere. The present invention is a method to prevent undesired ingressing gas from causing a build up of oxygen in a furnace having a hydrogen-containing atmosphere. The ingressing gas can be air that enters the furnace, or it can be a gas that is present as an impurity or impurities in a source for the atmosphere of the furnace (for example, oxygen in a hydrogen gas supply or unreacted oxygen from atmosphere generating equipment). Ingress of air can be from leaking flanges, pipe connections, ports, the entrance and exit for the materials to be processed in the furnace or other openings into the furnace. Ingressing gas, particularly air may result from furnace design, furnace wear, or from changing the size or shape of a material to be processed in a furnace. Further, the source of the ingressing gas may be from the material being processed in the furnace.  
      The catalytic reactor comprises catalytic materials, such as platinum, palladium, nickel, noble-metals, rhodium, ruthenium, or other materials that promote the reaction of hydrogen and/or reactive gases/hydrocarbons (if present in the furnace atmosphere) with oxygen. The reactor may comprise catalyst alone or a support to hold the catalyst or onto which the catalyst may be coated or adhered. Examples of support materials include alumina, or other ceramic or metallic powders, strips, honeycombs, screens, semi-conductive materials, porous silicon, other porous materials and designs or other support forms.  
      The reactor may be located anywhere in the furnace. Typically the reactor is located near a source of the ingressing gas which is often at the entrance or exit of the furnace. The reactor is near the source of the ingressing gas so that a majority of the ingressing gas contacts the reactor before it has dispersed within the atmosphere as a whole. The reactor is considered near the source for the ingressing gas if it is less than ten feet away from the source of the ingressing gas. The reactor is particularly useful in furnaces or in sections of furnaces having a hydrogen-containing atmosphere and/or in sections of the hydrogen-containing atmosphere where the temperature of the atmosphere is lower than the auto-ignition temperature of the gases in the atmosphere, such as in the exit end or cooling zone of a furnace. For a hydrogen-containing atmosphere containing mostly hydrogen, the temperature of the furnace may be below 579° C. at atmospheric pressure (auto-ignition temperature for hydrogen). The auto-ignition temperature for the atmosphere will depend on the composition of gases in the atmosphere and the pressure in the furnace. At temperatures below the auto-ignition temperature for the composition of the atmosphere, if the reactor of this invention were not present, the oxygen in the ingressing gas (for example, air) would not react with the hydrogen (and hydrocarbons) in the furnace atmosphere to form water vapor (and CO or CO 2 ), and would consequently accumulate inside the furnace.  
      The catalytic reactor could also take the form of a furnace curtain material or coating on a curtain material, for example a fiberglass curtain material, or other air ingress blocking means, such as a flap or flexible or inflexible physical barrier being constructed out of catalytically active material. Alternatively, the catalyst could be powders or beads or catalytic coatings on beads or powders that are used to block the furnace exit, which would ensure that oxygen is reacted at the point of entering the furnace or upon initially mixing with the furnace atmosphere with the hydrogen or hydrocarbon to form water vapor or carbon dioxide or carbon monoxide, which is less reactive than oxygen to the materials being processed and safer if present in a hydrogen-containing atmosphere.  
      In one embodiment, as shown in  FIG. 3 , the catalytic reactor comprises the catalytic materials packed inside a support which can be made of stainless steel and can have two screened walls on opposite ends of the reactor that allow gas to readily pass through the reactor. The reactor can be sized, for example having the same width as the width of the interior of the furnace, and otherwise designed for maximum ingressing-gas contact, and located in the furnace that a large portion or a majority of the ingressing gas cannot by-pass the reactor. By optimizing the sizing and location of the reactor a large portion of the ingressing gas should pass through the catalytic reactor where the oxygen of the ingressing gas will catalytically react as previously described.  
      In one embodiment, the operation of the catalytic reactor inside the furnace can be monitored by placing a thermocouple inside the catalytic reactor, and another thermocouple outside the catalytic reactor inside the furnace, for example, facing the furnace exit end as shown in  FIG. 1 . Based on the exothermic characteristics of hydrogen (or hydrocarbon) and oxygen reaction (H 2 + 0   2 →H 2   0 ), heat will be generated inside or on the surface of the reactor and the reactor&#39;s temperature will rise. Therefore, the temperature difference between the two thermocouples (or multiple sets of thermocouples) can be used as an indication of the presence and/or the amount of oxygen inside the furnace.  
      Alternatively, oxygen monitors, other gas analyzers, and/or dew point monitors can be used to measure (directly or indirectly) the oxygen concentration or dew point inside the furnace at the appropriate location(s), which can be used to indicate if deoxygenation reactions due to the presence of the reactor are occurring.  
      The ingression of air is also a safety and process concern when recapturing and recycling the hydrogen containing atmosphere from the furnace. Since recycling systems must draw the hydrogen containing atmosphere from the furnace into the recycling equipment, there may be a tendency for air to be drawn into the furnace and into the recycling system as well. Furnaces that operate at atmospheric pressure—or close to it are especially prone to air ingression. The use of an in situ catalytic deoxygenation reactor inside the supply piping or across or over the opening to the supply piping from the furnace to a recycle system will improve safety of the furnace from a flammability perspective and also eliminate the need to remove oxygen in the subsequent purification process. Additionally, it may be beneficial to use additional reactors near places of ingressing gas near the opening to the supply piping, for example, near the entrance or exit of the furnace depending upon where the supply piping to the recycling system is located. The reactor of this invention can be used for removing oxygen from the gases in the atmosphere that pass through the reactor at low pressures, for example, less than 1 psig or less than 0.5 psig. The pressures in or near the inlet piping to the recycling system from the furnace may be less than 1 psig or less than 0.5 psig.  
     EXAMPLES  
       FIG. 1  shows a schematic representation of the exit end of a continuous belt furnace  10  for processing a material  15  in the furnace, for example, annealing of metallic powders  15 . The furnace includes a hydrogen-containing gas inlet  12  to supply hydrogen-containing gas atmosphere for the furnace, a solid conveyor belt  11  to carry metallic powders  15  through the furnace, an inert gas, for example nitrogen, purge inlet  13 , a physical curtain  14  at the exit end of the furnace to prevent air from leaking into the furnace, and an oxygen content monitoring sample port  16 . The atmosphere  112  is all the gas inside the furnace containing the materials  15  to be processed in the furnace. The material to be processed in the furnace may move in the direction indicated by arrow A, the gas in the atmosphere may move in the direction indicated by arrow B. When the oxygen monitor  16  shows a high oxygen level in the furnace, the furnace will be shut down and purged with the inert gas; however, the reactor  17  will help to reduce the frequency for an inert gas safety purge and shut down. (The shut-down and/or purge steps may be fully automated by a processing means (for example, a computer and electronic controls) that is not shown. Alternatively, the shut down and/or purge steps may be controlled by an operator.) The door  19  and physical curtain  14  (as depicted in  FIG. 1 ) of the current furnace design cannot adequately prevent all the ingressing gas (for example, air) from entering the furnace  10 . As shown, an in situ catalytic deoxygenation reactor  17  in accordance with the present invention and an optional differential thermocouple monitor  18  are provided in the furnace. With the invented method, via the presence of the reactor, oxygen that is within the ingressing gas that passes through the physical curtain  14  will be catalytically converted in the catalytic reactor  17  to water vapor with the furnace&#39;s hydrogen-containing atmosphere. An embodiment of a reactor is shown in greater detail in  FIG. 3 .  
      The operation of the catalytic reactor  17  is monitored by the differential thermocouple  18  based on the exothermic characteristic of the chemical reaction between oxygen and hydrogen.  
      Alternatively, although not shown, the reactor could comprise catalytic material coated on the physical curtain  14  alone or in addition to the reactor  17  as shown. If desired, catalyst could be provided on one or more other fixtures or structures within the furnace and/or on the internal surfaces of the furnace walls if desired.  
       FIG. 2  shows a similar furnace  20  as in  FIG. 1  but as shown the furnace  20  has a powder, bead, or shell barrier or lock  24  instead of a physical curtain  14  as shown in  FIG. 1 . The furnace  20  has an in situ catalytic deoxygenation reactor  27  and an optional differential thermocouple monitor  28 . The catalytic reactor is shown as located within the furnace  20 . With the invented method, any oxygen leaking through the physical curtain  24  will be catalytically converted in or on the catalytic reactor  27  to water vapor by reacting the oxygen molecules with hydrogen present in the furnace&#39;s atmosphere  122 . The additional elements of the furnace  20  shown in  FIG. 2  are the hydrogen inlet  22 , the gas sample port  26  for the atmosphere  122 , and the exit door  29  for the furnace.  
       FIG. 3  shows the front view of the catalytic reactor  70  similar to the one shown in a side view in  FIG. 1  or  2 . The reactor includes a catalytic material  71 , for example, a palladium metal supported on aluminum oxide (Al 2   0   3 ) powders, packed inside a porous container support  72 , for example, a stainless steel container with two screened openings  74  on its front and back walls facing each other to allow gases to pass through the reactor. At the bottom of the stainless steel container  72 , a flexible foil or flap  73 , which may comprise stainless steel or any useful composition, is attached and may contact the material to be processed in a furnace to prevent ingressing gas or oxygen molecules in the atmosphere from passing under the reactor  70 . The reactor  70  may be sized so that the top  76  of the reactor contacts and/or may be mounted on the ceiling of the furnace, the sides  77  and  78  of the reactor contact and/or may be mounted on the side walls of the furnace and the bottom  75  is in contact with or nearly contacting the material to be processed by the furnace. With the invented method, oxygen accumulation in the furnace may be reduced or may not occur, and the furnace can run continuously for a longer campaign without the need for shutting down and purging the furnace with inert gas. Additionally less of the material to be processed by the furnace may be oxidized.  
       FIG. 4  shows the front end of a furnace  40  similar to the furnaces previously shown, but with the installation of an in situ catalytic deoxygenation reactor  47  according to the present invention located in the front end of the furnace. With the invented method, any oxygen leaking through the entrance end  49  will be catalytically converted in the reactor  47  to water vapor with the furnace&#39;s hydrogen atmosphere. The reactor located at the entrance end  49  of the furnace may be used to reduce the amount of oxygen in the furnace  40 . The materials  45  being processed in the furnace may move in the direction indicated by arrow A on the conveyor belt  41 . The gases in the atmosphere  142  mayflow in the direction indicated by arrow B.  
       FIG. 5  shows an embodiment in which a reactor  57  is used in the intake pipe  81  of a recovery or recycling system. In this embodiment, the recycling system  80  of the hydrogen-containing atmosphere from the furnace is used to remove impurities from the hydrogen-containing atmosphere, typically to create a recycled hydrogen gas stream with less than 5%, less than 3% or less than 1% impurities, and then the hydrogen may be reused in the furnace atmosphere. To prevent oxygen and other impurities, which may be components of the ingressing gas, from entering the intake pipe  81  the reactor  57  is placed in the intake pipe  81  as shown. Alternatively, the reactor could be placed across the entrance  82  to the intake pipe or multiple reactors could be used, for example the reactor shown in  FIG. 4  could be used with the reactor shown in  FIG. 5 .  
      One or more reactors can be used in a furnace to remove the oxygen from the atmosphere. If multiple reactors are used they can be located in various locations and have different designs. For example, the reactor shown in  FIG. 1  can be used in conjunction with the reactor shown in  FIG. 5 , or the reactor shown in  FIG. 1  can be used with a reactor that is coated on an exit curtain (not shown) and/or with the reactor shown in  FIG. 4 .  
      The inventions have been described with reference to particular embodiments. The inventions shall not be limited to the particular embodiments, and shall only be limited by the scope of the claims.