Abstract:
A compact and efficient fuel reformer which is operable to produce a hydrogen-enriched process fuel from a raw fuel such as natural gas, or the like includes a compact array of catalyst tubes which are contained in a heat-insulated housing. The catalyst tube array preferably includes a multitude of catalyst tubes that are arranged in a hexagonal array. The housing includes internal hexagonal thermal insulation so as to ensure even heating of the catalyst tubes. The diameter of the tubes is sized so that spacing between adjacent tubes in the array can be minimized for efficient heat transfer. The interior of each of the catalyst tubes includes a hollow dead-ended central tube which serves as a fines trap for collecting catalyst fines that may become entrained in the fuel stream. The catalyst tubes are also provided with an upper frusto-conical portion which serves to extend the catalyst bed and provide a catalyst reserve. The assembly includes a side-fired startup burner which allows for an improved diffusion burner orifice array at the top of the reformer. The catalyst tubes are supported by side walls of the assembly in a manner that stabilizes the tubes in the assembly. In the assembly, the internal transverse manifold plates are tied together by portions of the tube assemblies so as to form a composite beam that supports the weight of the catalyst tube array.

Description:
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. RX-7502-810-IFC awarded by the United States Department of Transportation through Georgetown University. 
    
    
     TECHNICAL FIELD 
     This invention relates to a catalytic reaction system. More particularly, this invention relates to a system having an array of catalyst tubes, each of which has an annular catalyst bed. 
     BACKGROUND OF THE INVENTION 
     Catalytic reaction apparatus have been commonly used in industry to produce industrial gases such as a hydrogen enriched fuel gas and are therefore well known in the art. The most common approach for producing hydrogen is the steam reforming process in which a raw fuel gas is mixed with steam and passed through catalyst beds disposed in a tubular reformer. Heat for this endothermic reaction is provided from a furnace in which the tubes are locoted in a widely spaced apart configuration. 
     As a result of the large size and limited operating flexibility characteristics of these industrial units, steam reforming technology was not successfully integrated for use with power plants that incorporated hydrogen consuming fuel cells until the successful application as disclosed in U.S. Pat. Nos. 4,098,587; 4,098,588 and 4,098,589. The new design represented by these patents consisted of a compact reaction apparatus with a number of important features that made it suitable for use within a fuel cell power plant. 
     Namely, it is a compact reaction apparatus for steam reforming a raw fuel that is mainly characterized by having: a plurality of vertical tubular reformers closely packed (by the then standard of the art) within a furnace and shielded so as to produce an evenly heated tube at any location within the array of tubes; having a burner cavity area and an enhanced heat transfer area; and having annular reformers incorporating regenerative heat transfer capability between the reaction products and the process stream. 
     This design resulted in a steam reformer apparatus that met the size and operating characteristic requirements of a fuel cell power plant while maintaining a high thermal efficiency that is necessary to ensure a competitive overall power plant operating efficiency. 
     While the design disclosed in the aforesaid patents was a milestone achievement for the application of hydrogen generation technology to fuel cell power plants, these early designs were in need of improvements to make it truly more compact, lighter in weight, more uniform in its heat distribution and catalyst bed stability. Chief among these problems is the need to develop an efficient supporting structure that keeps the tube bundle aligned and properly distributes the loading forces resulting from tubes and catalyst and ancillary equipment without undue weight penalty or complex and costly structural fixtures. 
     DISCLOSURE OF THE INVENTION 
     This invention relates to a compact and efficient reformer which is operable to produce a hydrogen-enriched process fuel from a raw fuel such as natural gas, or the like. The reformer of this invention includes a compact array of catalyst tubes which are contained in a heat-insulated housing. The catalyst tube array preferably includes a multitude of tubes that are arranged in a hexagonal array. The housing is preferably circular for manufacturing and structural efficiency, and the interior of the circular housing is fitted with a geometrically matching insulation. For example, when the hexagonal array of reformer tubes is employed, the insulation will provide a hexagonal perimeter which faces the reformer tube array. The outermost tubes in the array are thus equally efficiently insulated against heat loss. The diameter of the tubes is also sized so that spacing between adjacent tubes in the array can be minimized for efficient heat transfer. The stiff tube support structure maintains the critical spacing between tubes under dead weight loading at reformer operating temperatures. 
     The interior of each of the catalyst tubes includes a hollow dead-ended central tube over which processed fuel is passed after leaving the catalyst reaction bed. The dead-ended tube serves as a fines trap for collecting catalyst fines which become entrained in the fuel stream as the latter passes through the catalyst bed. The catalyst tubes are also provided with an uppermost conical cap which serves to extend the catalyst bed so that an excess of catalyst can be loaded into the bed when the tubes are constructed and assembled. The assembled and closed catalyst tubes thus will contain an excess of catalyst so as to maintain a desired height to the catalyst beds even when catalyst slumping and settling occurs. Catalyst settling is also controlled by the respective size of the catalyst pellets and the radial thickness of the catalyst bed. In addition, the conical cap shape prevents the fluidization of the catalyst bed in the upper portion of the catalyst reaction zone by lowering the gas flow velocity as the flow area increases. This is important because fluidization leads to excessive settling and crushing of the catalyst in this region with each thermal cycle. This exacerbates the catalyst layer height loss that is desirable to minimize. 
     This design is also characterized by the use of a side-fired startup burner instead of a central-fired startup burner as has been previously used. The side-fired burner allows for an improved diffusion burner orifice array at the top of the reformer. Thus, a burner orifice array which is not interfered with by the centrally located startup burner is achieved so as to make the heat distribution from the diffusion burners more easily and efficiently achieved. It will be appreciated that the presence of a centrally located startup burner will disrupt the diffusion burner pattern and will create a void in the central portion of the upper,end of the furnace when the startup burner is shut down. This undesirable result does not occur when the side-fired startup burner of this invention is used. 
     The design is also characterized by the use of reformer tube caps having a thickness that is greater than conventionally used so as to provide an added temperature operating range because the operating range is limited mainly by corrosion and strength requirements. Increasing the thickness of the cap, which is disposed in the hottest part of the reformer tube, improves the capability of the reformer to deal with design and structural variations, and increases the safety margin of the design. 
     The catalyst tubes are supported by side walls of the assembly housing in a manner that stabilizes the tubes in the assembly, and allows the assembly to take advantage of assembly components which provide unique structural features affording improved strength and stiffness, and also the resistance to thermal stresses without increasing weight or volume. In the aforesaid U.S. Pat. No. 4,098,587, the weight of the catalyst tubes is supported by the bottom wall of the apparatus, which is also a pressure boundary for the vessel. In the assemblage of this invention, the internal transverse manifold plates are tied together by portions of the tube assemblies so as to form a composite beam that supports the weight of the catalyst tube array. The manifold plates and the tying tube assembly portions interact with each other in a manner which creates the structure and effect of a composite beam that transfers the load from the tubes out to the cylindrical side wall of the assembly. The resultant structure provides Increased load bearing strength in a manner similar to a honeycomb panel. 
     The two internal transverse manifold plates serve as face sheets of the honeycomb-like structure, in which the tube sections between the manifold plates serve as a core for the honeycomb-like structure. By freeing the bottom area of the assembly from the need to provide tube weight and load support, the bottom area can be utilized for other functions such as the additional capture of fines, or integrated heat exchange options. This is a desirable feature which enables the achievement of maximum packaging density in a weight and volume sensitive power plant design. 
     It is therefore an object of this invention to provide a more efficient and compact apparatus for reforming a fuel supply so as to adapt the latter for use in a fuel cell power plant. 
     It is a further object of this invention to provide an apparatus of the character described wherein the apparatus has a structural tube support configuration which efficiently transfers catalyst tube support loads to the existing cylindrical housing in the apparatus. 
     It is an additional object of this invention to provide an apparatus of the character described wherein catalyst bed compaction is remedied. 
     It is another object of this invention to provide an apparatus of the character described wherein improved heat transfer from fuel process burners to the catalyst tubes is provided. 
     It is yet another object of this invention to provide an apparatus of the character described wherein improved catalyst bed support is provided. 
     These and other objects and advantages of the invention will become more readily apparent from the following detailed description thereof when taken in conjunction with the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial cross-sectional view of a reformer assembly formed in accordance with this invention; 
     FIG. 1A is an enlarged and fragmented view of the annular fuel gas inlet passage shown in FIG. 1; 
     FIG. 2 is a fragmented axial cross-sectional view of the upper portion of the reformer assembly of FIG. 1; 
     FIG. 3 is a transverse cross-sectional view of the reformer assembly; 
     FIG. 4 is an axial cross sectional view of one of the catalyst tube assemblies illustrating the manner in which the catalyst tube assemblies are mounted in the reformer assembly; and 
     FIG. 5 is a fragmented axial cross sectional view of a lower portion of one of the catalyst tube assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIG. 1 discloses an embodiment of a reformer apparatus  10  which is formed in accordance with this invention, and which includes a housing  12  that contains a plurality of catalyst tubes  14  in which the processing of a raw hydrocarbon fuel stock takes place. The reformer apparatus  10  includes a raw fuel inlet  16  through which raw hydrocarbon fuel is introduced to the apparatus  10 , and a reformed fuel outlet  18  through which the reformed hydrocarbon fuel is removed from the apparatus  10 . The apparatus  10  further includes a burner fuel inlet  20  through which a burner fuel is introduced into the apparatus  10 , and a burner air inlet  22  through which ambient air, or another source of oxygen, is introduced into the apparatus  10  in order to support combustion of the burner fuel so as to provide heat for the processing of the raw hydrocarbon fuel stock. A burner exhaust outlet  24  is provided to vent the burner fuel exhaust stream from the apparatus  10 . In the drawings, arrows A and B indicate the direction of flow of the burner gases and burner air streams respectively. Arrows C and D indicate the direction of flow of the start burner gas stream and the process fuel being reformed, respectively. 
     As shown in FIGS. 1,  2  and  3 , there are nineteen individual catalyst tube assemblies  14  which are arranged in a closely packed hexagonal array. The catalyst tube assembly array has a central group of seven catalyst tube assemblies and an outer group of twelve catalyst tube assemblies. Each catalyst tube assembly  14  includes an outer reformer tube  30  having an inner surface  32  and an outer surface  34 . Sealed upper ends  36  of the reformer tubes  30  are provided by end caps  38  which define uppermost portions of the reformer tubes  30 . The reformer tubes  30  have lower ends  40  and tube bodies  42  which extend from the lower ends  40  to the end caps  38 . Regenerator tubes  50  are positioned concentrically inside of the reformer tubes  30 . The regenerator tubes  50  have inner and outer surfaces  52  and  54 , respectively and extend from the lower ends  56  of the catalyst tube assemblies  14  to the upper ends  64  thereof. As seen in FIG. 2, the regenerator tubes  50  are formed in three major sections, which are: cylindrical body sections  60  which extend upwardly from the lower ends  40  a height H 1 ; frustoconically inwardly tapered Intermediate sections  62  which extend upwardly from the body sections  60  a height H 2 ; and smaller cylindrical upper sections  64  which extend upwardly from the upper end  82  of intermediate sections  62  a height H 3 . The intermediate sections  62  have an angle of convergence Ø toward the axis  500  of the tube  30 . The heights and angle are selected so that the extra catalyst volume which is located in the space H 2 , will provide a catalyst reserve to compensate for the volume reduction in catalyst that occurs during thermal cycling, and also to limit fluidization of the catalyst bed and minimize pressure drop through the catalyst bed. The upper end of the upper section  64  of the regenerator tube  50  is open so that the gas stream from the catalyst bed may enter the tube  64  and  74  as indicated by the arrows D. A perforated plate  58  is secured to the tube  64  and allows the reformed gas to leave the upper end of the catalyst bed and enter the tube  64 ,  74 . A cross member  66  in the form of a rod is positioned so that its ends may contact the inner surface of the end cap  38  so as to center the regenerator tube assembly  50  in the upper end of the reformer tube  30 . The taper angle Ø in the medial section  62  is selected so as to fit the size, flow and pressure drop requirements for the unit. 
     An annular space  70  is defined between the inner surface of the reformer tube  30  and the outer surface of the regenerator tube  50 . The space  70  has three major sections associated with and adjacent to the three major sections of the regenerator tube  50 . In the exemplary embodiment, a lower section and an intermediate section. The intermediate section has a thickness which matches the lower section at its lower end and which increases in an upward direction. The upper section of the space  70  has a thickness further increasing in an upward direction in accordance with the taper of the upper section  64  of the regenerator tube  50 . In the aforesaid embodiment of the invention, the majority of the annular space  70  contains a catalyst bed. The catalyst bed is formed from cylindrical pellets  72  having an outer surface formed of an appropriate catalytic material such as nickel. The regenerator tube  50  is formed with a plurality of spacers  73  which extend radially outwardly from the outer surface of the tube  50  along the regenerator tube body section  60 . The spacers  73  maintain the regenerator tube  50  centered within the reformer tube  30 . 
     A fines-collecting system comprising upper and lower interior tubes  74  and  76  is provided within the regenerator tube  50 . The upper tube  74  is open at its upper and lower ends. The upper end of the upper tube  74  is welded to the inner surface of the intermediate section  62  of the regenerator tube  50  and is recessed below the upper end of the intermediate section  62 . The upper end of the lower tube  76  is substantially level with the upper end of the regenerator tube body section  60 . The lower end of the upper tube  74  extends slightly into the lower tube  76 . The inner diameter of the lower tube  76  is sufficient to accommodate the outer diameter of the upper tube  74  and define an annular space  78  laterally between the tubes  74  and  76 . The closed lower end of the lower tube protrudes beyond the lower end of the body section  60  of the regenerator tube  50 . An annular space  79  is defined between the outer surface of the lower tube  76  and the inner surface of the regenerator tube  50  to serve as a regeneration chamber. Process fuel from the catalyst bed enters the upper tube  74  through the perforated section  64  and flows downwardly into the lower tube  76  to fill the latter in a relatively quiescent manner. The lower tube  76  is dead ended and thus serves as a trap for collecting catalyst pellet fines which may become entrained in the process fuel stream as the latter passes through the catalyst bed. The process fuel stream spills out over the top of the tube  76  and flows through the regenerator chamber  79  where the thermal energy of the process gas is transferred back into the incoming process flow stream to assist in supplying the necessary heat of reaction. 
     To assemble the reformer, reformer tubes  30  are located within the associated apertures in the plate  84  and then attached by welds  31  to fix their locations. Then the assembled regenerator tubes  50  are inserted into the reformer tubes  30  and the combination inverted. In the inverted condition, the regenerator tubes  50  are supported by contact between the cross member  66  and the end cap  38 . The spacers  73  hold the regenerator tubes  50  centered within the reformer tubes  30  allowing the annular spaces  70  therebetween to be filled with the catalyst pellets  72 . After a proper amount of the catalyst has been introduced into the spaces  70 , the catalyst support assembly is inserted into the spaces  70 . The catalyst support assembly includes an annular perforated plate  81  which is welded to support rods  75 . The rods  75  are supported by an annular plate  80 . The lower boundary of the spaces  70  are sealed by welding the inside edges of the solid discs  77  to the regenerator tubes  50  by means of welds  37 , and by welding the outside edges of the discs  77  to the lower support plate  86  by means of welds  35 . Once so sealed, the reformer can be righted whereupon the catalyst pellets  72  will essentially fill the spaces  70 . 
     The burner cavity  100  consists of lower and upper regions  99  and  101  respectively, with the upper portions of each catalyst tube assembly  14  projecting into the lower region  99  of the burner cavity  100 . The upper region  101 , which is the open region, is sized so that the volume of this region of the burner cavity  100  ensures complete burner gas combustion, and therefore, low emissions. In addition, the burner cavity width above the catalyst tubes in either the upper or lower regions  101  and  99  is sized to promote uniform flow to each of the individual tube assemblies  14  forming the tube array. The lower region  99  which contains the projecting tube array, is sized to maximize heat transfer to the tube assemblies  14 . The perimeter of the lower region  99  of the burner cavity  100  is bounded by a hexagonal insulation wall  102 . The wall  102  is formed of panels of ceramic fiber insulation board. The panels are positioned in close facing relationship to the six sides of the hexagonal array of catalyst tube assemblies  14 . An exemplary spacing between the wall  102  and the perimeter catalyst tube assemblies  14  is approximately the same as the spacing between adjacent catalyst tube assemblies  14  in the array. The hexagonal configuration of the wall  102 , and its close proximity to the catalyst tube assembly array maintains temperature uniformity across the array so that the perimeter catalyst tube assemblies  14  and, more particularly, their outboard sides, will be at substantially the same temperature as the interior catalyst tube assemblies  14 , so as to maximize system efficiency. The upper end of the wall  102  extends above the upper ends of the catalyst tube assemblies  14 , and defines the border between the upper and lower regions  101  and  99  of the burner cavity  100 . An additional element which is not shown in the drawings, but which may be included in the assembly, are ceramic caps for the catalyst tube assemblies  14 , which caps are described in U.S. Pat. No. 4,740,357, which is incorporated herein in its entirety. 
     Immediately below the lower region  99  of the burner cavity  100  is an enhanced heating portion  104  of the furnace  12  which is configured so as to enhance heat transfer from the burner gas to the catalyst tube assemblies  14 . In the enhanced heating portion  104 , each catalyst tube assembly  14  is located within an associated concentric sleeve  106 . The sleeves  106  have inner diameters which form annular spaces  108  between the inner surfaces of the sleeves  106  and the outer surfaces of the reformer catalyst  30 . A support plate  112  supports a ceramic fiber insulation  114  which fills the space between the sleeves  106  and extends upward to the boundary  110 . 
     FIGS. 1,  1 A,  4  and  5  disclose details of the catalyst tube assemblies  14 , and the manner in which they are mounted in the reformer apparatus  10 . Upper and lower plates  84  and  86  span the reformer housing  12  proximate the lower end wall  95  of the housing  12 . Each plate  84  and  86  has a plurality of apertures  85  and  87  respectively. Each catalyst tube assembly  14  extends through associated apertures  85  in the upper plate  84  and associated apertures  87  in the lower plate  86 . The catalyst tubes  30  are welded to the upper plate  84  via welds  31  and also welded to the lower plate  86  via welds  33 . It will be noted that the catalyst tubes  30  combine with the plates  84  and  86  to form a rigid structure analogous to a honeycomb panel, in which the plates  84  and  86  are the face sheets, and the catalyst tubes  30 , which extend between the plates  84  and  86 , act as the core. This welded structure which serves as the process gas inlet manifold is also operable to support the catalyst tube assemblies  14  during normal operation and during transient transportation and seismic loads. The weight of the internal components in the catalyst tube assemblies  14 , which consists of the catalyst beds  72 , the catalyst support plates  81 , the support rods  75 , and the regenerator tubes  50  is supported by annular plates  77 , the internal diameters of which are welded to the regenerator tubes  50  by welds  37 , and the outer diameters of which are welded to the plate  86  by welds  35 . The outer edge portions of the upper and lower plates  84  and  86  are supported by surfaces  92  and  98 , respectively, on the reformer outer shell  94  and  96 . The fact that the plates  84  and  86  are secured to the reformer shell side wall  94  and  96  ensures that the weight of the catalyst tube assemblies  14  is transferred outwardly to the side wall  94 ,  96  by the plates  84  and  86 . The tube  76  is secured to the tube  50  by means of a plurality of spaced-apart clips  7  which allow the reformed gas stream to flow from the annulus  79  into the manifold  144 . 
     The assembly  10  operates as follows. Burner fuel enters the system through the burner fuel inlet  20  which is located at a height approximately even with a lower region  99  of the burner cavity  100 . The fuel enters an annular manifold  120  which leads to an annular passageway  122 . Walls  123  and  125 , which define the passageway  122 , encircle the array of catalyst tubes  14  so as to allow the fuel to evenly descend through the passageway  122 . The fuel flows downwardly through the passageway  122 , acquiring heat as it progresses downwardly to an annular manifold  124  at a lower portion of the enhanced heating portion  104 . 
     A vertical conduit  126  then ducts the preheated burner fuel upward from the manifold  124  to a fuel manifold  128  located in upper region  101  of the burner cavity  100 . The burner fuel passes from the manifold  128  through tubular nozzles  130  which extend downwardly from a lower wall of the manifold  128 . The nozzles  130  pass through an air manifold  132  which is coupled to the inlet  22  and through one or more insulating panels between the upper region  101  of the burner cavity  100  and the air manifold  132 . Apertures in such panels have sufficient clearance around the nozzles  130  to define corresponding annular passageways through which air is drawn from the air manifold  132  to combustor with the gas introduced to the burner cavity  100  through the nozzles  130 . A start burner  140  is provided in a side wall of the burner cavity  100  above the catalyst tube assemblies  14  and a flame sensor  141  is provided on an opposite side of the burner cavity  100 . Arrow C in FIG. 1 indicates the direction of flow of the start burner gas. 
     The hot combustion gases from the burner fuel and air proceed downward through the burner cavity  100 , through inlets to the annular spaces  108  at the upper ends of the associated sleeves  106  thereby transferring heat to the catalyst tube assemblies  14 . Optionally, the spaces  108  are maintained as shown in U.S. Pat. No. 4,847,051, the disclosure of which is incorporated herein in its entirety. When the combustion gases leave the annular space  108  at the lower end of the sleeve  106 , they enter an exhaust plenum  152  between the plate  112  and the plate  84 . The burner gases proceed to the outer perimeter of the plenum  152  and upward therefrom through an annular passageway  146  immediately inboard of the passageway  122 . The burner gases moving upward through the passageway  146  transfer heat to the incoming heating fuel proceeding downward through the passageway  122 . Upon reaching the upper end of the passageway  146 , the burner gases are collected in an annular collection space  148  and therefrom exit via the exhaust outlet  24 . 
     The process fuel enters through the inlet  16  and is directed via appropriate conduits to the process fuel gas inlet plenum  150  between the plates  84  and  86 . From the plenum  150 , the process fuel gas passes through openings in the lower portion of the reformer tube  30  which is located within the plenum  150 . The process fuel proceeds upward through the catalyst bed, receiving heat both from the downward flowing burner gas in the annular space  108  outboard of the reaction chamber, and from the downward flowing processed gas in the regeneration chamber to be described below. The process fuel gas exits the upper surface of the catalyst bed in the processed state and passes through the holes in the perforated upper section  64  of the regenerator tube  50 . The processed gas then passes downward through the upper filter tube  74 . Upon exiting the lower end of the tube  74 , the processed gas must change direction, proceeding upward through the annular space  78  as indicated by arrows D. During this flow direction change, particulate matter (e.g., certain reaction byproducts, catalyst particles, and the like) will fall to the closed lower end of the lower tube  76  and collect there. After passing through the open upper end of the tube  76 , the flow of processed gas again reverses direction and flows downwardly through the regeneration chamber  79 . During this downward flow, the processed gas transfers heat to the incoming process fuel in the reaction chamber immediately outboard thereof. At the lower end of the regenerator tube, the processed gas enters a processed fuel outlet plenum  144  between the plate  86  and a bottom of the reformer housing  12 . From the plenum  144 , the processed gas proceeds through conduits to the processed fuel outlet  18 . 
     Additionally, the structural coupling of the plates which define the process fuel inlet plenum increases the overall rigidity of the system, allowing for use of thinner and lighter material; reducing the possibility of damage during transport; and reducing the possibility of damage during use. A rigid tube support structure is required to minimize the tendency of the upper ends of the catalyst tubes to move toward each other as the tube support structure deflects under dead weight loads at elevated temperatures. Excess deflection can lead to catalyst tube temperature mal-distribution by causing non-uniformity in the various gas flow paths at the upper end of the catalyst tubes. 
     Another of the areas of damage during reformer use involves crushing of the catalyst material. The more rigid mounting of the catalyst tubes can reduce their movement relative to the regenerator tubes. Such movement may be caused by vibration or by thermal cycling as the reformer is used. Such relative movement first allows shifting of the catalyst, followed by crushing of the catalyst as the relative movement reverses and the tubes seek to resume their previous relative position. The frustoconical intermediate and upper portions of the regenerator tube allow for the storage of an reserve amount of catalyst which compensates for catalyst pellet crushing or slump, should such occur, and also stabilizes the upper portion of the catalyst bed against fluidization during operation of the system. 
     The aforesaid design results in a nineteen catalyst tube array reformer assembly employing four inch diameter catalyst tubes. With the four inch diameter tubes, the center void space used as a fines catcher is markedly reduced. Since the center void space is unused volume, it should be made as small as possible. A small center also reduces catalyst crushing effects since the amount of thermal growth in the catalyst cavity annulus is proportional to the tube diameter. Keys to maintaining operating temperature uniformity in the catalyst tubes are the hexagonal shape in the burner cavity, the rigidity of the tube support, and the provision of a multiple burner tube array above the catalyst tubes. 
     Since many changes and variations of the disclosed embodiment of the invention may be made without departing from the inventive concept, it is not intended to limit the invention otherwise than as required by the appended claims.