Abstract:
A chemical reactor is provided that includes:
       a plurality of tubes having a catalytic reactor substrate therein, each of the tubes having an inlet at one end for receiving a fluid flow and an outlet at an opposing end for discharging fluid flow, and a longitudinal axis parallel to the direction of fluid flow through the tube, the longitudinal axis of each tube being parallel to one another; and   a plurality of heat exchange fins disposed on and extending radially from the exterior surface of each tube;   the heat exchange fins on each tube independently having a profile along a cross-section perpendicular to the longitudinal axis of each tube, and the tubes being arranged with respect to one another, such that the heat exchange fin profiles together form a tessellated pattern in a plane that is perpendicular to the longitudinal axes of the tubes.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under contract DE-FC36-04GO14319. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Tubular reactors are often used for carrying out reactions involving fluid components, such as reforming hydrocarbon fuel. Hydrocarbon fuel reformers such as diesel fuel reformers can be used in various applications, including providing a hydrogen and carbon monoxide reformate that can be used as fuel for a solid oxide fuel cell. These reactors typically include a catalyst monolith disposed inside a tube, through which fuel flows. The catalyst catalyzes a reaction of the hydrocarbon fuel to convert it into hydrogen and carbon monoxide. 
     One important requirement for reactors such as hydrocarbon fuel reformers is to provide adequate heat transfer into the catalyst to maintain endothermic reaction conditions. Multiple techniques and concepts may be utilized to provide the required thermal conditions. As an example, reducing the diameter of a catalyst while maintaining its length both reduces the catalyst mass and therefore its heat capacity, and also reduces the heat transfer path. This approach, however, requires the use of multiple catalyst segments to maintain total catalyst volume and the resultant space velocity at a given flow rate. When such a strategy is utilized, the system must therefore be engineered to create uniform space velocities among all of the catalyst segments. Another technique to promote heat transfer is to increase the surface area separating the two fluids across which the heat transfer occurs. In practice, this is typically accomplished by incorporating heat transfer fins that are intimately attached to the catalyst tube(s) that project into the hot fluid flow path. 
     In order to exercise a high degree of control over the respective orientations of the tubes in a multi-tube reactor, a typical manufacturing approach is illustrated in  FIGS. 1-4 . The first step in this approach, as shown in  FIG. 1 , is the provision of a housing  12  having a number of fins or foils  14  disposed therein. Next, as shown in  FIG. 2 , a series of holes  16  are formed in the fins or foils  14  in a configuration to provide the desired distribution of fluid velocity and thermal kinetics. These holes may be formed by any of a number of known techniques, such as electronic discharge machining (EDM), laser machining, or mechanical boring. Then, as shown in  FIG. 3 , tubes  18  having catalyst monoliths  20  disposed therein are inserted into each of the holes  16 , and fixed in place with adhesive or metal brazing. 
     This prior art process, however, is subject to several problems. The forming of the holes  16  through the relatively delicate (typically about 0.04 mm thick) fins or foils  14  using conventional techniques such as EDM can be a time-consuming and expensive process. Additionally, insertion of the tubes  18  into the holes  16  subjects the foil along the periphery of the hole to damage and/or distortion because of the fine gauge of the foil. This can lead to small gaps  22  between the outside edge of the tube  18  and the foil  14 , as shown in  FIG. 4  (gaps may be exaggerated for purposes of illustration), which reduces the effectiveness of the heat transfer between the warmer fluid in the area of the foil matrix  14  and the colder fluid within the volume of the catalyst monolith  20 . 
     Accordingly, there is a need for a more effective multi-tube reactor design and manufacturing method that is less susceptible to the above-described problems. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment of the present invention, a chemical reactor is provided comprising:
         a plurality of tubes comprising a catalytic reactor substrate therein, each of the tubes having an inlet at one end for receiving a fluid flow and an outlet at an opposing end for discharging fluid flow, and a longitudinal axis parallel to the direction of fluid flow through the tube, the longitudinal axis of each tube being parallel to one another; and   a plurality of heat exchange fins disposed on and extending radially from the exterior surface of each tube;   the heat exchange fins on each tube independently having a profile along a cross-section perpendicular to the longitudinal axis of said each tube, and the tubes being arranged with respect to one another, such that the heat exchange fin profiles together form a tessellated pattern in a plane that is perpendicular to the longitudinal axes of the tubes.       

     According to another exemplary embodiment of the invention, a method of manufacturing a chemical reactor, comprising the steps of:
         (a) providing a plurality of tubes, each tube defining a longitudinal axis;   (b) affixing heat exchange fins to each of said plurality of tubes, the fins on each tube independently having a predetermined profile along a cross-section perpendicular to the longitudinal axis of said tube;   (c) disposing a catalytic reactor substrate inside of each tube;   (d) arranging the tubes parallel to one another, such that the heat exchange fin profiles together form a tessellated pattern in a plane that is perpendicular to the longitudinal axes of the tubes; and   (e) securing the tubes to maintain their positions with respect to one another.       

     The multi-tube reactors according to these exemplary embodiments provides good contact between reactor tube and the heat transfer fins or foils and also avoids the need for expensive and time-consuming hole forming techniques such as EDM. These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts an end view of a prior art configuration having heat transfer fins or foil in a housing; 
         FIG. 2  depicts an end view of a prior art configuration having holes formed in the heat transfer fins or foil of  FIG. 1 ; 
         FIG. 3  depicts an end view of a prior art configuration having catalytic reactor tubes disposed in the holes of the structure of  FIG. 2 ; 
         FIG. 4  depicts an end view of a magnified view of one of the catalytic reactor tubes from  FIG. 3  disposed in a surrounding heat transfer fin or foil; 
         FIG. 5  depicts an end view of a hexagonal catalytic reactor tube; 
         FIG. 6  depicts an end view of a heat transfer fin or foil stock disposed around the reactor tube of  FIG. 5 ; 
         FIG. 7  depicts an end view of a group of seven of the structures from  FIG. 6  formed in a tessellated pattern; 
         FIG. 8  depicts the structure of  FIG. 7  where the multiple structures are secured together; 
         FIG. 9  depicts a perspective view of the structure of  FIG. 8 ; 
         FIG. 10  depicts the structure of  FIG. 8  disposed in an external housing; and 
         FIG. 11  depicts a perspective view of the structure of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring further below to the Figures, the invention will be described with reference to specific embodiments, without limiting same. 
     Turning now to  FIG. 5 , an end view of a single hexagonal reactor tube  18  is shown having a catalyst monolith  20  disposed therein. Such catalyst monoliths are well-known in the art, and typically have a porous ceramic substrate or an oxidized metallic substrate that has been coated with a catalyst-containing washcoat. Of course, it is not essential that the reactor tube  18  begin with a catalyst disposed therein, as the catalyst monolith  20  can be inserted into the reactor tube  18  at any later stage of the manufacturing process, such as after placement of the heat transfer fins or foil or even final assembly of the multi-tube reactor device. In some exemplary embodiments, it may be desirable to place the catalyst monolith inside the tube later in the manufacturing process to avoid subjecting it to potential damage during implementation of the other manufacturing steps (e.g., brazing). 
       FIG. 6  shows an end view of a catalyst monolith-containing reactor tube  18  as shown in  FIG. 5 , which has had heat transfer foils  14  wrapped around it to form heat transfer reactor assembly  24 . The heat transfer foils  14  have a substantially uniform thickness (as measured from the outer surface of reactor tube  18 ) so that the hexagonal shape of reactor tube  18  is carried through to the outer surface of heat transfer foils  14 , which therefore also have a hexagonal outer shape as viewed from the end of the reactor tube  18 . The heat transfer foils  14  may be affixed to the reactor tube by heat transfer foil winding techniques that are well-known in the art. In one exemplary embodiment, the foil material (e.g., in a reel or coil cut or shaped to match the shape of the reactor tube is affixed) to the reactor tube (also known in the heat transfer art as a core) and then wound around the reactor tube. The number and configuration of the foils may be based upon cell geometry and allocated process time, as known in the art. If a shaped foil such as a foil having a shape such as a sinusoidal wave pattern is used, a flat may be wound along with and interspersed with the shaped foil to help avoid nesting of adjacent rows of shaped foil that could result in corrupted cell densities, poor dimensional control, delamination between layers, and disruptions to the heat exchange fluid flow path. 
     The foil(s) to be wound may be affixed to the core using any of a number of known methods. For example, the foil may be spot-welded, pinched/constrained by a wire or other mechanical affixing means (which may be later removed), restrained magnetically, and other known methods. Once the foil is initially affixed, it may be wound or wrapped around the core either by rotating the core or by moving the foil circumferentially around the core (holding the core stationary) while gradually adjusting the respective position of the foil along the core&#39;s longitudinal axis to dispose the foil in helically wound rows along the longitudinal axis of the core. Foil tension should be maintained during the wrapping/winding process in order to achieve good contact between the foil and the core without deforming the foil and thereby altering the targeted final cell density. 
     Depending on the expected severity of the conditions on the exterior of the reactor tube  18  (e.g., flow velocity, temperature), the foil may be maintained in position on and in thermal contact with the reactor tube simply by the tension that was maintained on the foil during the winding/wrapping process. However, in some exemplary embodiments, further affixing techniques such as brazing may be used. If brazing is used, the brazing materials (e.g., a brazing paste) may be applied along the edge of the foil that will interface with the core while discharging the foil(s) from the reel before winding onto the core. 
     In one exemplary embodiment, the catalytic substrate  20  is disposed into the reactor tube  18  after the heat transfer foils or fins  14  are affixed to the outside of the reactor tube  18 . In another exemplary embodiment, the ends of the tube  18  are not covered with heat transfer fins, which can allow for the tubes to extend beyond the end of a heat exchanger housing in which the tube  18  will ultimately be disposed. 
     After the heat transfer foils or fins  14  are affixed to the reactor tube  18 , seven of the hexagonal-shaped heat transfer reactor assemblies  24  are shown arranged together in a tessellated fashion to form multi-tube assembly  27 , as shown in  FIG. 7 . By “tessellated fashion”, it is meant that the cross-sectional footprints of the reactor assemblies  24  in a plane perpendicular to the longitudinal axes of the parallel tubes  18  will together fill a substantially contiguous area in that plane with no overlaps or gaps between the footprints of the individual reactor assemblies  24 . Of course, the number of assemblies is not limited to seven, as the tessellated hexagonal (i.e., honeycomb) pattern can be continued indefinitely to form a variety of sizes and/or shapes of different configurations of tessellated reactor assemblies  24 , as desired. After the heat transfer reactor assemblies  24  have been arranged together as exemplified in  FIG. 7 , they may be fixed together by a collar or band  26  around the ends of tubes  18  on each end of assembly  27 , as shown in  FIGS. 8 and 9 . The band or collar  26  may be made of any suitable material (e.g., steel, high temperature silicone rubber) that can withstand the conditions to which the heat exchange reactor will be subjected. In one exemplary embodiment, the band or collar is made of steel that is welded or brazed to the ends of tubes  18 . 
     Turning now to  FIGS. 10 and 11 , the multi-tube assembly  27  ( FIGS. 7-9 ) is disposed in housing  12 , which in this exemplary embodiment is hexagonal shaped in order to accommodate the generally hexagonal shape of multi-tube assembly  27 , thereby forming the heat exchange reactor assembly  29 . Housing  12  is generally enclosed around the multi-tube reactor assembly, and is shown as such in  FIG. 11  with an opening on each end of the housing  12 , through which the ends of reactor tubes  18  protrude.  FIG. 10  is shown in a cut-away view with the end of housing  12  not shown so that the manner in which the multi-tube assembly  27  (FIGS.  7 - 9 ) fits into housing  12  can be illustrated. Heat transfer fluid (e.g., air or any known organic heat transfer fluid) to heat or cool the reactor tubes  18  as necessitated by the desired reaction conditions is directed through the open end of the housing  12  and through the matrix formed by reactor foils  14  ( FIG. 6 ) to transfer heat into or out of the reactor tubes  18 . Reactants may be introduced into and reaction product may be collected from the reactor tubes  18  through manifolds (not shown) on each end of the assembly  29 , which manifold may be integrated with the collar  26 . 
     In one or more exemplary embodiments, reactor tubes can be engineered in a multi-tube reactor configuration to promote highly uniform fluid flow rates among the tubes, and also for uniform fluid flow rates outside the tubes across the heat exchanger fins or foils. If the incoming gas(es) has/have velocity gradients, the local flow impedance can be increased or decreased by one or more of the following techniques: changing the cell shape, changing the cell density, changing the wall thickness of the substrate, changing the foil thickness. This may be advantageous for catalytic chemical reaction such as the reforming of hydrocarbon fuel, as it allows the reaction to proceed in a uniform and controllable manner. 
     The tessellated pattern shown in  FIGS. 7-10  is based on hexagons, like a honeycomb. This pattern, often found in both nature and industrial design, may offer advantages in structural integrity, ease of fabrication, and is readily formed into a regular tessellated pattern using uniform hexagonal shapes. However, other shapes such as triangles, squares, or rectangles, may also be formed into regular tessellated patterns using uniform shapes, and are also available as exemplary embodiments of the invention. Other combinations of regular polygons may also be tessellated, such as octagons with squares (also known as truncated square tiling), squares with triangles (such as snub square tiling or elongated triangular tiling), hexagons with triangles (such as snub hexagonal tiling, snub hexagonal tiling reflection, or trihexagonal tiling), dodecagons with triangles (also known as truncated hexagonal tiling), or dodecagons with squares and hexagons (also known as truncated trihexagonal tiling). Exemplary embodiments of the invention include these, as well as numerous other tessellated patterns known in the art, as described for example by D. Chavey, “Tilings by Regular Polygons-II: A Catalog of Tilings”,  Computers  &amp;  Mathematics with Applications  17: 147-165 (1989), the disclosure of which is incorporated by reference in its entirety. Irregular tessellated patterns and semi-regular tessellated patterns are also well-known, and are included as exemplary embodiments of the invention. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.