Abstract:
A modular fluid processing architecture is provided that consists of a matrix of nested tubes secured between end block manifolds. Multiple chemical reactors may be housed in the annular spaces formed by the nesting of the tubes, and the processes may be integrated through flow splitting, mixing, switching and heat exchange in the manifolds. A flow switching system may provide the ability to switch the flows on or off in individual processors or in banks of such processors. The switching may effect the operation of some or all of the processes. Such switching can facilitate rapid and close following of demand for the processor output while allowing each processor to run within a range of high efficiency, since processors may be turned off or on in response to falling or rising demand for the output.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to the field of micro-reactors and methods for operating such micro-reactors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Significant efforts have been made toward developing meso-scale chemical processing systems for a variety of applications. These applications typically consist of one or more chemical reactors coupled with one or more heat exchangers and associated flow manipulation operations. One application in particular that has received considerable attention is that of fuel processing systems for fuel cells (U.S. Pat. Nos. 5,861,137, 5,938,800 and 6,033,793.) Other applications that have received attention include fuel vaporizers and personal heating and cooling devices.  
         [0003]     Common challenges facing developers of these systems include slow load-following response, poor part-load efficiency, and difficult manufacturing. Poor load-following response is a legacy of the large-scale industrial process designs on which many of the meso-scale designs are based. Packed bed reactors and heat exchangers used in these designs operate with a thermal and chemical inertia that limits the ability of these systems to respond quickly to changes in the processing throughput or load. These designs typically operate well over a relatively narrow and tightly-controlled range of process conditions, with significant efficiency penalties for operation away from the design point. Manufacturability is hindered by difficult scale-up and scale-down challenges encountered when changing process throughput capacity. Process reactors and heat exchangers, for example, must often be redesigned to accommodate changes in material stream flow rates and heat transfer rates.  
         [0004]     Recent advances in the field of micro-chemical processing systems (U.S. Pat. Nos. 6,192,596, 5,961,932, 5,534,328, 5,595,712 and 5,811,062) have begun to address some of the aforementioned challenges. By providing increased heat transfer area from a relatively small thermal mass, high surface-to-volume ratios inherent in some micro-reactor designs (e.g., parallel micro-reactor channels) may decrease thermal inertia effects and may allow more-precise control over reaction temperatures and heat exchange rates. Load-following problems are improved to some extent by high heat fluxes and accelerated apparent reaction rate. Heat exchange surface thicknesses on the order of hundreds of microns are offered by microfabrication techniques, enabling increased heat fluxes due to shortened conduction paths. Apparent reaction rates are accelerated as they approach the intrinsic kinetics of the chemical reactions at hand as heat and mass transfer lengths are decreased through miniaturization. These designs may be scalable to some extent, as reactors typically consist of arrays of parallel micro-channels, and can be scaled simply by adding or subtracting channels. Manufacturing difficulties have been further addressed through the use of laminated sheet assemblies (U.S. Pat. No. 6,192,596).  
         [0005]     Notwithstanding the foregoing, to date, micro-reactor systems have failed to adequately address the issue of part-load efficiency penalties, as they still are optimized to operate over a narrow throughput range.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a fluid processing device of simplified construction and manufacture that may be modular in nature with a unitized architecture that can afford easy scaling and independent control of constituent integrated micro-reactor processors units in which the various constituent sub-processes of the desired process may occur. According to one aspect of the invention, each subsystem unit may be optimized for high efficiency execution of the complete chemical process in a system of nested tubes and connecting manifolds. The tubes may have any of a variety of cross-sectional geometries including circular, elliptical, square, rectangular, polygonal, or irregular shape depending on the desired heat transfer and fluid flow characteristics for the process. The tubes need not be of uniform or regular cross-section along their length. The integrated chemical processing device consists of one or more subsystem units that may communicate with one another via heat exchange, fluid mixing, and/or flow splitting in connecting manifolds. The manifolds may be configured to mechanically secure the tubes in the desired positions relative to one another.  
         [0007]     In accordance with another aspect of the invention, independent control of the subsystem units may be provided by one or more micro-valve arrays appropriately positioned in the endplates to control the flow of material streams into each unit. Individual subsystem units may be switched on or off, or may be throttled in response to changes in process load. Selected material streams may be switched on or off for banks of subsystem units (or individual units) when it is beneficial to do so. Low thermal inertia of the micro-reactor geometry and heat integration between subsystem units may help to provide rapid start-up capability of individual reactors in response to load changes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an isometric view of a four-module fuel processing device.  
         [0009]      FIG. 2  is a sectional view of the nested tubes of one of the processors of  FIG. 1  with portions removed.  
         [0010]      FIG. 3  is an exploded perspective view of two identical four-valve arrays show in opposite orientations.  
         [0011]      FIG. 4  is an exploded view of modular nested-tube reactor assemblies connecting to a manifold end block.  
         [0012]      FIG. 5  is an exploded view of an end block manifold assembly that includes flow channels in various of the laminates for directing fluid flow from a common inlet.  
         [0013]      FIG. 6  is an exploded view of an end block manifold showing a manifold plate in which heat exchangers are formed by cutout patterns.  
         [0014]      FIG. 7  is an exploded view of an end block assembly that manifolds a fluid flow from a common inlet.  
         [0015]      FIG. 8  is an exploded view of an end block assembly in which fluid channels conduct parallel fluid flows to a pattern of eight heat exchangers.  
         [0016]      FIG. 9  is an exploded view of an end block assembly with two sets of counter-flow heat exchangers formed by cutout patterns in adjacent end block plates.  
         [0017]      FIG. 10  is an exploded view of an end block assembly with fluid channels to conduct gas flow to and from a heat exchanger.  
         [0018]      FIG. 11  is an exploded view of an end block assembly with fluid channels that conduct fluid flow to and from a second heat exchanger.  
         [0019]      FIG. 12  is a process flow diagram for a simple steam reforming process.  
         [0020]      FIG. 13  is a block diagram of a control architecture for a four-module fuel processing device.  
         [0021]      FIG. 14  is a flowchart of control logic for a four-module fuel processing device.  
         [0022]      FIG. 15  is an isometric view of a 64 module fuel processing device directly coupled to a fuel cell stack to form an integrated power generation module.  
         [0023]      FIG. 16  is an isometric view of the fuel processing device of  FIG. 15  rotated 180°.  
         [0024]      FIG. 17  is an exploded view of  FIG. 16  with a detail view of a nested tube micro-reactor architecture consisting of six concentric tubes.  
         [0025]      FIG. 18  is a process flow diagram for a fuel processor integrated with a fuel cell stack. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     The invention is described herein with reference to embodiments of fuel processor systems, but is equally applicable to other fields and types of chemical reactions and the like.  
         [0027]      FIG. 1  shows an embodiment of a modular fluid processing system  10  that executes steam reforming, combustion for production of heat required by the system, and water-gas shift reaction in a four-processor apparatus that can serve as part of a fuel processor for small (50-100 W) proton exchange membrane (PEM) fuel cell once coupled to a carbon monoxide (CO) polishing reactor and appropriate ancillary equipment, including filters, compressors, and pumps (not shown). The device consists of four processor modules  11 A-D attached to two end block manifolds  12  and  13 . Fluid streams enter the device through tubes  14 - 18  and pass through valve array assemblies  5 - 9  en route to a number of chemical processor operations located both in the four processor modules  11  and in the end block manifolds  12  and  13 , exiting through tubes  20  and  21  as summarized as Table 1.  
                                 TABLE 1                                   Fluid Stream                                    Inlet Tube           14   Natural Gas combustor fuel       15   Combustion air       16   Auxiliary steam for water-gas shift       17   Primary steam for reformer       18   Natural Gas reformer feedstock       Outlet Tube       20   Hydrogen-rich product stream       21   Combustor exhaust                    
         [0028]     Referring next to  FIG. 2 , in this embodiment, each processor module  11  comprises three concentric stainless steel tubes  22 - 24  of 6 mm, 4 mm and 2 mm outer diameter. While the base module geometry chosen here consists of three concentric tubes  22 - 24  of uniform, circular cross-section, the tubes  22 - 24  may be of any cross-sectional shape including but not limited to, rectangular, elliptical, polygonal, and triangular and may be arranged in any configuration. The tubes and end block manifolds of this embodiment may be made of stainless steel, as this material provides good corrosion resistance and good thermal conductivity, has a high melting point, and is widely available in standard tube sizes from a variety of manufacturers. Alternative tubematerials that may be appropriate for this or other processes include but are not limited to metals and metal alloys, ceramics, polymers and composites.  
         [0029]     Chemical reactors are formed in the annular spaces  25 - 27 . It should be noted that, although the present embodiment discusses the reactors as having chemical reactions conducted therein, the reactor spaces  25 - 27  may also be used for heating of fluids, such as air or natural gas, for cooling, as may be achieved by passing a two-phase water-steam stream through the reactor space, for evaporation of a fluid, as for fuel vaporization or evaporative cooling, and for other processes. The appropriate length, diameter and wall thickness of the tubes  22 - 24  may be determined based on considerations of heat transfer between adjacent reactors and on desired flow properties within each reactor including residence time, pressure drop, and fluid turbulence. For the processor module  11  of the present embodiment, tube lengths, wall thickness and diameters set forth in Table 2 below should be sufficient for the process described below.  
                           TABLE 2                       Tube   Diameter (mm)   Wall Thickness (mm)   Length (mm)                   22   2   0.25   44       23   4   0.50   42       24   6   0.50   40                  
 
         [0030]     Catalyst materials may be applied to one or both of the inner and/or outer surfaces of the tubes  23 ,  24  and to the inner surface of the tube  22  to promote chemical reactions in the spaces  25 - 27  within or between the tubes  22 - 24 . Catalysts may be applied to the surfaces of the tube walls using a number of known techniques, including chemical vapor deposition (CVD), physical vapor deposition (PVD), and sol-gel methods. Catalysts may also be provided in the spaces  25 - 27  on or as packed granule beds, in a porous ceramic monolith, or in a sol-gel-created matrix or by other means known in the art. For the reactions of the present embodiment, space  27  may be packed with granules of alumina-supported platinum combustion catalyst (e.g., Aesar #11797 available from Alfa Aesar, a Johnson Matthey company, of Ward Hill, Mass., USA), space  26  may be packed with granules of alumina-supported nickel steam reforming catalyst (e.g., ICI 57-3, ICI-25-4M available from SYNETIX of Billingham, UK or BASF G1-25S available from BASF Corporation of Houston, Tex.), and space  25  is packed with granules of alumina-supported copper-zinc water-gas shift catalyst (e.g., Süd Chemie G66-B); however, alternative catalysts formulations and supports could be used.  
         [0031]     Valve array assemblies  5 - 9  break inlet fluid flows into four parallel streams for processing in processor modules  11  and allow independent switching of the process streams to control the operation of individual modules  11 . Referring to  FIG. 3 , each valve array may consist of a plenum  63  mounted to a valve substrate  66  with gasket  65  forming a fluid-tight seal. The valve assembly may be secured to the endblock manifolds  12  and  13  using bolts inserted through hole patterns  57 - 59  and fastened into tapped holes in the endblocks. Alternatively, the valve assemblies may be secured to the endblocks using an adhesive. Valve assemblies  5 - 9  are located on the surface of manifold endblocks  12  and  13  such that valve openings  68  communicate with appropriate fluid channels in the endblock. Valves  67  may be fabricated on a silicon substrate  66  using standard microfabrication techniques known to those skilled in the art of micro-electro mechanical systems (MEMS). Actuation for valves  67  may be accomplished using forces generated by one of the following phenomena: shape memory alloy phase transition, thermal expansion of a bimetallic junction, electrostatic force, piezoelectric force, or thermopneumatic force. The present embodiment employs valve arrays based on shape memory alloy technology such as those manufactured by TiNi Alloy Company of San Leandro, Calif.  
         [0032]     End block manifolds  12  and  13  may be constructed of multiple laminates with apertures and channel patterns that are joined together to form gas flow paths to execute flow switching, heat exchange, flow splitting, and gas mixing operations as shown in  FIGS. 4 through 11  and discussed in detail below. In the present embodiment, the laminates may be fabricated by stamping stainless steel sheets ranging in thickness from 50 μm to 2 mm. The laminates should be joined so as to substantially prevent leakage from the channels. This may be accomplished through diffusion bonding of the laminates by aligning the stack of laminates comprising the end blocks  12 ,  13 , and compressing them at high pressures and temperatures in a vacuum, as is known in the art of diffusion bonding. Other laminate thicknesses may be used as appropriate when considering fabrication techniques and/or process requirements. Other laminate materials may include, but are not limited to, other metals and metal alloys, ceramics, polymer, and composites. Alternative laminate fabrication methods may include, but are not limited to, water-jet cutting, powder injection metal forming, chemical etching, laser cutting, casting, plating and conventional machining. Alternative joining methods may include but are not limited to bolt and gasket assemblies, ultrasonic welding, conventional welding, brazing, and adhesive bonding.  
         [0033]     Referring in particular to  FIG. 4 , the tubes  22 - 24  of the processor modules  11  may be connected to end block manifold  13  via successive attachment to individual laminate sheets  30 - 33 . Laminate  30  has four apertures  34  through which the outer tubes  22  of the processor modules  11  are passed. The ends  35 - 37  of the tubes  22 - 24  abut and are sealed to laminate plates  31 - 33 , respectively, with the middle tube  23  passing through the aperture  40  in laminate  31  and with the end  36  of the middle tube  22  being sealed to laminate  32 . The inner tube  24  extends through the aperture  41  in laminate  31  and the end  37  of the inner tube  24  abutting and being sealed to the laminate  33 .  
         [0034]     Still referring to  FIG. 3 , the apertures  40  in the laminate  31  are generally circular in shape, but the laminate  31  is notched at one side of each of the apertures  40  to provide a fluid channel  42  that communicates with the reactor formed in the space  25  between the outer and middle tubes  22 ,  23 . Similarly, the aperture  41  in the laminate  32  includes a fluid channel  44  at one side thereof that communicates with the reactor formed in the space  26  between the middle and inner tubes  23 ,  24 . The reactor formed in the interior space  27  of the tube  24  is in fluid communication with the aperture  45  in the laminate  33 . Fluids may be communicated between the other laminates of the end block  13  and the reactor formed in the space  25  through the fluid channels  42 ,  43  and  46  in laminates  31 - 33 , respectively. Similarly, fluids may be communicated with the reactor remaining laminates of the end block  13  and the reactor formed in the space  26  through fluid channels  44  and  47  in laminates  32  and  33 , respectively.  
         [0035]     The present embodiment may employee a combination of compression fitting and diffusion bonding to secure and seal tubes  22 - 24  to endblock  13  as in the following process. After endblock  13  has been formed e.g., through diffusion bonding, internal surfaces of laminates  30 - 33  that are exposed through apertures  34 ,  40 , and  41  may be plated with a thin film of metal that exhibits a higher thermal expansion coefficient than that of the endblock material. In the present embodiment, the endblock material being stainless steel, an appropriate plating metal may be silver. The endblock is next raised in temperature (e.g., to 400° C.) such that apertures  34 ,  40 , and  41  expand to allow a clearance fit for insertion of tubes  22 - 24 . The room-temperature tubes  22 - 24  are held in alignment by a jig as they are inserted into the apertures  34 ,  40 , and  41  such that they each abut one of laminates  31 - 33  as described above. Endblock  13  is next cooled, yielding a compression interference fit to secure tubes  22 - 24  in place. The above process is repeated to secure the opposite ends of tubes  22 - 24  to endblock  12 . The assembled device is then placed in a vacuum furnace to cure at elevated temperature such that the mismatch in thermal expansion coefficients between the endblock material and the plating metal results in a stress-induced diffusion bond between the endblocks  12  and  13 , the plating metal, and the tubes  22 - 24 . Diffusion bonding is a desirable technique for bonding the tubes to the laminates in this particular embodiment, but any number of bonding techniques including swaging the ends  35 - 37  into annular grooves on the laminates  31 - 33 , ultrasonic welding, adhesive bonding, laser welding, brazing or conventional welding may be employed.  
         [0036]     Cross-sectional dimensions for fluid passages  42 - 47  may range from 250 μm to 2 mm for height and width as determined by pressure drop and heat transfer considerations for the respective fluid flows. In the present embodiment, fluid channels  42 ,  43 ,  44 ,  46 , and  47  are 1 mm wide by 2 mm high, while fluid channel  45  is 0.75 mm wide by 1.5 mm high. These dimensions are characteristic of channel cutouts throughout the assembly.  
         [0037]     Referring next to  FIG. 5 , in particular, plates  50 - 53  of the end block manifold  13  are shown in exploded form. Flow channels  54 - 56  on laminate  50  and fluid channels  60 - 62  on laminate  51  communicate, respectively, with fluid channels  46 ,  45 ,  47  on laminate  33 . The fluid channels  55  in laminate  50  are connected to the fluid inlet  14  through valve array assembly  5 . Fluid from the inlet  14  is thus divided into four streams that are conducted through the fluid channels  55  and  45  and ultimately to the reactor formed in the space  27  in the interior of the tube  24 .  
         [0038]     Referring in particular to  FIG. 5 , in the present embodiment, the flow channels  70  in the laminate  52  are connected to fluid inlet tube  16  through valve array assembly  7  to conduct a fluid stream (referred to herein as the third fluid stream) from inlet tube  16  to the reactor modules.  
         [0039]     Referring to  FIG. 6 , laminates (plates)  71 - 77  cooperate to provide a counter-flow heat exchanger for exchange of heat between two fluid streams (referred to herein as the second and fourth fluid streams). The laminates  71 ,  72  contain fluid channels  80 - 83 , best shown in Detail A of  FIG. 6 , that conduct said fourth fluid stream to and said second fluid stream away from counter-flow heat exchangers  84  located in identical laminates  73 ,  74 . The number and geometry of channels in the heat exchangers  84  may be determined to satisfy heat transfer requirements between said fourth fluid stream and said second fluid stream. Laminate  75  includes header channels  85 , as best shown in Detail B of  FIG. 6 , to conduct said fourth fluid stream from heat exchanger  84  to fluid channels  86  in the laminates  73 ,  74 . Elongated flow channels  87  in laminate  76  conduct the second fluid stream from fluid channels  90  in laminate  77  to the heat exchangers  84  of laminates  73 ,  74 .  
         [0040]     Referring more specifically to  FIG. 7 , the four holes  88  conduct the second fluid stream, having entered the device through inlet tube  15  and having been split into up to four parallel streams by valve array assembly  6 , to fluid channels  89  and  91 , which conduct said fluid stream to fluid channels  90 .  
         [0041]     Referring next to  FIG. 8 , laminates  94 - 97  are analogous to laminates  30 - 33  shown in  FIG. 4 , and serve the function of joining and sealing reactor module tubes  22 - 24  to manifold end block  12  and conducting fluid streams flowing to and from reactor spaces  25 - 27  to fluid channels  100 - 102 . Reactor space  25  connects to channel  100 , reactor space  26  connects to fluid channel  101  and reactor space  27  connects to fluid channel  102 . Fluid channel  106  conducts a fifth fluid stream (product of reactor  27 ) to the counter flow heat exchanger  113  where said fifth fluid stream transfers heat to a sixth fluid stream. Fluid channel  104  conducts a seventh fluid stream (product of reactor  25 ) to counter-flow heat exchanger  112  where the seventh fluid stream transfers heat to an eighth fluid stream. Manifold fluid channel  109  collects the sixth and eighth fluid streams from heat exchangers  113  and  112 , respectively, and conducts the mixed streams to the fluid channel  105  for subsequent introduction to reactor module  26 .  
         [0042]     Referring next to  FIG. 9 , laminate  114  contains counter-flow heat exchangers  112 ,  113 . The number and geometry of heat exchanger channels  112 ,  113  in the laminate  114  may be selected to achieve the desired heat transfer between the seventh and eighth and fifth and sixth fluid streams respectively.  
         [0043]     The fluid channels  115 ,  116 ,  118 , and  119  in laminate  121  conduct the eighth fluid stream, having entered the device through inlet tube  18  and having been split in up to four parallel flows by valve assembly  9 , to the heat exchanger  112 .  
         [0044]     As shown in  FIG. 10 , the fluid channels  122  in laminate  123  conduct the seventh fluid stream from the heat exchanger  112  to the fluid channel  130  in the laminate  124 , where the portions of the seventh fluid stream that were divided for processing in the four reactor modules  11  are combined and conducted to outlet tube  20 .  
         [0045]     Referring to  FIG. 11 , the fluid channels  135 - 138  in laminate  126  conduct the sixth fluid stream, having entered the device through inlet tube  17  and having been split in up to four parallel flows by valve array assembly  8 , to the heat exchanger  113 .  
         [0046]     Fluid channels  128  in the laminate  132  conduct the fifth fluid stream from the heat exchanger  113  to the “U”-shaped fluid channel  139  formed in laminate  133 , where the portions of said fifth fluid stream that were divided for processing in the four base modules are mixed and conducted to outlet tube  21 . Laminate  134  does not contain flow channels, and serves as the end plate of the end block manifold  12 .  
         [0047]      FIG. 12  shows a process flow diagram for a steam reforming process implemented in the four module apparatus described above in accordance with one embodiment of the invention. The system produces nominally 0.06 Nm 3  (normal cubic meters)/hr product gas  156  with a nominal hydrogen content of 67% by volume from 0.016 Nm 3 /hr natural gas used both as combustor fuel  146  and reformer feedstock  140 . Thus each of the four process modules  11  produces up to 0.015 Nm 3 /hr product gas. Part load efficiency of the system is improved because, with appropriate switching of flows in the fluid channels of the end block manifolds  12 ,  13 , only one reactor needs to operates outside its optimal load range while the system supplies processing loads ranging from 0 to 0.06 Nm 3 /hr. The remaining modules operate at either zero or at a desired maximum load.  
         [0048]     Natural gas feedstock stream  140  enters the device through inlet tube  18  and is split in up to four flows  141  controlled by a valve array  9 . Combustion air stream  142  enters through inlet tube  15  and is split in up to four flows  143  by valve array  6 . Reformer steam stream  148  enters through inlet tube  17  and is split in up to four flows  149  by valve array  8 . Combustion fuel stream  146  enters through inlet tube  14  and is split in up to four flows  147  by valve array  5 . Auxiliary steam stream  144  enters through inlet tube  16  where it is split in up to four flows  145  by valve array  7 . The up to four flows of each process inlet stream  141 ,  143 ,  149 ,  147 , and  145  undergo the remainder of the process in parallel but in their respective, separate processor modules  11 . The remainder of the process is described below for one example module.  
         [0049]     The feedstock stream  141 , which is described in the present embodiment as natural gas, flows through heat exchanger  112  to cool the product gas stream  155  to 100° C., an appropriate temperature for introduction of the product gas stream  156  into a CO polishing reactor and subsequently to a proton exchange membrane (PEM) fuel cell stack. Steam stream  149  flows through the heat exchanger  113  where it is heated by 750° C. combustion products  158 . Hot steam stream  151  and hot feedstock stream  150  are mixed to form the steam reformer input stream  152  before entering steam-reforming reactor space  26  in the processor module  11 . The endothermic steam reforming reactions are maintained at 725° C. by heat flux  160  supported by the exothermic combustion reaction in the adjacent reactor space  27  in the processor module  11 . The wall thickness and geometry of the tubes  23 ,  24  may be chosen to provide appropriate thermal resistance between reactor spaces  26 ,  27  while maintaining structural integrity and manufacturability of the reactor module  11 . The molar steam-to-carbon ratio of the steam reformer input stream  152  is maintained at 2.5 in the present embodiment to promote complete conversion of the natural gas feedstock to hydrogen and carbon monoxide and to inhibit carbon deposition on the steam reforming catalyst. Reformate stream  153  then flows to heat exchanger  84  where it is cooled by incoming combustion air  143  to 300° C. for introduction to water-gas shift reactor  25 . Auxiliary steam stream  145  may be mixed with the steam reformate stream to form a stream  154  with increased water content to further promote conversion of carbon monoxide and water to carbon dioxide and hydrogen in the water-gas shift reactor  25 . Material and wall thickness and geometry for the tubes  22 ,  23  may be chosen such that reactor space  25  is thermally insulated from reactor space  26  and maintained below 350° C. Product stream  155  from the water-gas shift reaction in reactor space  25  flows through the heat exchanger  112  to heat incoming feedstock stream  141  before leaving the apparatus through the outlet tube  20 . Incoming combustion fuel  147  (which may, in various embodiments, be, or include, natural gas, fuel cell anode purge stream gas, other hydrocarbon or alcohol fuel) mixes with the air stream  157  heated by the heat exchanger  84  for introduction for combustion into the reactor space  27 . The fuel and air flows may be controlled such that the combustion reaction in the reactor space  27  produces sufficient heat to maintain the gas flow through the reactor space  27  at 725° C. Combustion products  158  exit the reactor space  27  after combustion and flow through heat exchanger  113  to heat the steam flow  149 , as previously discussed, before leaving the apparatus through outlet tube  21 .  
         [0050]     The flow stream switching control system architecture, shown in  FIG. 13  switches the valve arrays  5 - 9  to control the operation of the four processor modules  11  in response to process load changes. The system controller may also control ancillary equipment (not shown, e.g. water pumps, fuel compressors, feedstock and combustor fuel control valves, air compressor) to maintain appropriate process flows in the active portion of the processor modules  11 . For example, air compressor flow rate may be set to 75% of full load if only three modules are active.  
         [0051]     The control system of the present embodiment may operate in accordance with the logic structure shown in  FIG. 14 . The control system may operate within a general or special purpose computer or microcontroller. In the present embodiment, a microcontroller having appropriate inputs and outputs, processor circuitry, program memory, and the like is used. Upon completion of the necessary startup steps, the system proceeds to the next step of sensing fuel cell stack power load, using conventional electrical sensors. Alternatively, or in combination, hydrogen sensors could be used to monitor the partial pressure of hydrogen in the hydrogen-side outlet from the fuel cell. As generation of electricity by a fuel cell results in removal of hydrogen from the gas stream on the hydrogen side of a proton exchange membrane of a PEM fuel cell, a lowering of the partial pressure of hydrogen in the outfeed indicates that the generation of additional hydrogen is needed to sustain power production.  
         [0052]     In the next step  172  system calculates the hydrogen output needed and the desired number of processor modules needed in operation to achieve this output level based on the electrical output of the fuel cell. This may be accomplished in various ways, including the use of a look-up table, an algorithm, a predictive model or a combination of the foregoing. For a predictive model, the calculated demand for hydrogen could be increased or decreased more sharply if demand over a specified number of previous cycles of the control system had calculated successively increasing or decreasing hydrogen demand.  
         [0053]     Once the required output has been determined, the system proceeds to the next step  173  of determining whether the number of operating processor modules  11  is sufficient to supply the desired hydrogen output. If the number of operating processor modules  11  is not sufficient, or if there are more processor modules  11  in operation than are needed to fill the demand, then, in the next step  174 , one or more of the processor modules  11  may be turned on or off by the system by operating the valves  5 - 9  to control the various process gas streams. Of course, the valves  5 - 9  may also be used to operate all of the operating modules at a higher or lower output or to operate all but one of the operating processor modules  11  at the maximum desired capacity, and to operate the remaining module at less than the maximum desired capacity in order to produce the desired hydrogen output level. In addition, in this step, if the control system senses that demand is increasing and an additional processor module  11  may soon be needed, the control system may begin the startup procedure for such processor module  11 , for example, by starting the combustion process in the reactor space  27  so that the heat exchanger  113  can begin to be warmed to operating temperature by the combustion gas stream  158 .  
         [0054]     To fine-tune the reactor selection, the system may then, in the next step  175 , read hydrogen partial pressure information from hydrogen sensors. The system next performs the step of determining if the proper hydrogen concentration is present in the fuel cell outfeed (or, alternatively, infeed). If hydrogen needs to be produced at an increased or decreased rate to maintain proper operating conditions for the fuel cell, the number of processors and their load levels may be adjusted in the step  177  to meet demand, in a manner analogous to that described above in connection with the steps  173 ,  174 .  
         [0055]     In the final step  178 , the system loops back to the step  171  to begin the control process anew. Of course, the ancillary equipment referenced in  FIG. 13  may be controlled in reference to hydrogen demand and/or power load, as well as in response to other feedback mechanisms. For example, if power output of the fuel cell decreases and hydrogen demand is therefor reduced, the demand for air from the compressor may be reduced. Of course, factors such as compressor outlet pressure may also be used in controlling the compressor.  
         [0056]     The unitized design of this embodiment allows each micro-reactor subsystem to operate at high process efficiency over a narrow throughput range while the device as a whole operates at the same high process efficiency over a much wider throughput range determined by the total number of micro-reactor subsystems in the device. Rapid load-following may be achieved by the switching on and off of fluid flow to individual processes in the processor modules  11 , which have low thermal inertia and hence relatively quick startup times and from the process intensification inherent in the micro-reactor design. Embodiments of the invention can provide scalability of the unitized micro-reactor architecture. Designs may be scaled quickly by either changing the size of the base subsystem unit, or alternatively, by adding or subtracting individual subsystem units. Construction may be made in many cases using readily available or easily manufacturable components and processes, such as stainless steel plates for the laminates and stainless steel or other metal tubing. The control of flow in the fluid channels can be achieved with available microvalve arrays, and through the proper choice of fluid channel length and cross-sectional area.  
         [0057]     While the embodiments of the invention have been discussed with concentric tubes disposed between two end blocks, the invention could be embodied in other configurations, for example, between one center block with tubes extending from the opposite surfaces thereof and mounted at their distal ends to endblocks. Further, the process could be carried out with tiers of tubes extending between disposed in either direction away from the center block. Tiers of blocks extending between layers of laminates that valve, join, and split fluid flows and that provide evaporators and condensers for the fluid streams before passing them to the next tier could be provided.  
         [0058]      FIG. 15  shows another embodiment of the invention. This embodiment provides an integrated power generation module  195  consisting of a fuel processing system  196  coupled directly to a 1 kilowatt PEM fuel cell stack  224 . As best shown in  FIG. 17B , the apparatus consists of 64 processor modules  230 , which are similar to the processor modules  11  described above. Each processor module  230  consists of six concentric tubes  232 ,  234 ,  236 ,  238 ,  240  and  242 , with catalyst applied to the inner and/or outer wall surfaces of the tubes as desired. End block manifolds  219  and  220  consist of 36 and 47 laminates respectively that form flow manifolds, valve arrays, and heat exchangers analogous to those described previously with respect to the end blocks  12 ,  13  of the fuel processor  10 , though scaled up to accommodate 64 parallel and process flows. These plates may range in thickness from 250 μm to 5 mm.  
         [0059]     As shown in  FIG. 15 , the fuel cell stack  224  consists of 15 single cell assemblies  223  and four coolant flow fields  217  connected electrically in series. Each single cell assembly  223  consists of a membrane electrode assembly  215  between an anode flow field plate  214  and a cathode flow field plate  216 . Fuel cell stack layers  214 - 217  are held in engagement with one another by eight nuts  222  on threaded rods  221  that are welded to the end block assembly  220 . The fuel cell stack connects to an external load circuit via electrodes  204  and  205 .  
         [0060]     The nested tube reactor modules  230  of the fuel processor  196  are configured as follows. Tube dimensions may be selected such that relative wall thicknesses and areas promote desired levels of heat exchange between adjacent reactor spaces  231 ,  233 ,  235 ,  237 ,  239 ,  241 . Relative tube diameters and lengths may be selected to obtain appropriate reactor volumes for desired residence times. In the present embodiment, the innermost tube  232  may be 60 mm long with 2 mm outer diameter and 200 μm wall thickness. The reactor space  231  inside this tube  232  houses a combustion reactor with a nominal duty of 8 W. The next tube  234  may be 58 mm long with 4 mm outer diameter and 600 μm wall thickness. The reactor space  233  formed between tubes  232  and  234  houses a steam reforming reactor with a nominal processing rate of 0.19 standard liters per minute of natural gas at 750° C. with a steam to carbon ratio of 2.5. Tube  236  may be 56 mm long with 6 mm outer diameter and 700 μm wall thickness. Reactor space  235  formed between tubes  234  and  236  conducts superheated steam stream  279  from end block  219  to end block  220  where it subsequently flows to the inlet of the steam reformer in reactor space  233 . Tube  238  may be 54 mm long with 8 mm outer diameter and 500 mm wall thickness. Reactor space  237  formed between tubes  236  and  238  houses a water gas shift reactor where steam and carbon monoxide (CO) in the process stream are reacted at 300-350° C. on a water-gas shift catalyst. Tube  240  may be 52 mm long with 10 mm outer diameter and 700 mm wall thickness. The reactor space  239  formed between tubes  238  and  240  houses an evaporator that cools water gas shift reactor  237  as a two-phase water/steam stream  278  flows from end block  220  to  219 . Tube  242  may be 50 mm long with 12 mm outer diameter and 500 mm wall thickness. The reactor space  241  formed between tubes  240  and  242  houses a preferential oxidation (PROX) reactor that reacts small amounts of air with the reformate gas over an oxidation catalyst with high CO selectivity to further remove CO from the product reformate to a level below 10 ppmv. As shown in  FIG. 17B , the space  243  outside the processor modules  230  is bounded by a shell  218  that conducts an air stream  262  from the inside face of end block  219  to outlet tube  226  to cool the PROX reactor  241  and maintain it at a temperature below 120° C. to promote high CO selectivity of the PROX catalyst. The inside face of endblock  220  contains an orifice for the PROX reactor  241  of each processor module to draw heated air  264  from the air flow  262  flowing (counter to the flow direction of reactants in PROX reactor  241 ) in space  243  to cool the PROX reactor  241 . Appropriate design of said orifices provides for the metering of the air flow into the PROX reactor  241 .  
         [0061]     Tubes  211  conduct  64  parallel flows of preheated combustion fuel  260  from end block  220  to end block  219  for introduction to combustion reactor  231 . Tubes  210  conduct  8  parallel flows of preheated combustion air  267  from end block  220  to end block  219  for introduction to combustion reactor  231 . In the present embodiment, combustion air flow is controlled in banks of eight reactor modules by an eight valve array to allow rapid startup of combustion reactors  231  and steam reforming reactors  233  in response to process load changes. Alternatively air flow could be individually controlled for each processor module by means of a 64-valve array. This rapid start-up capability is enabled by hot air flow through the combustion reactor  231  even if a particular module is turned off. The hot air flow maintains the combustion reactor  231  and adjacent steam reformer reactor  233  at elevated temperatures sufficient for ignition of the combustion fuel upon its introduction.  
         [0062]     The process flow diagram for the power generation apparatus heretofore described is shown in  FIG. 18 . Reformer feedstock natural gas stream  250  enters end block  220  of the fuel processor  196  from inlet tube  208 , where it is divided into 64 parallel streams, each controlled by valves in an analogous arrangement to that described previously in reference to the four-module embodiment. These steams flow to heat exchangers  285  in the end block  220  where they are heated by the 760° C. combustion exhaust stream  269  from the catalyst-induced combustion process occurring in reactor space  231 .  
         [0063]     Hot feed stream  251  then mixes with superheated steam stream  279  to produce a steam to carbon ratio of 2.5 prior to entering steam reforming reactor  233 . Steam reformer  233  is maintained at 20 psig and 750° C. by heat  280  from adjacent combustion reactor  231 . Hot reformate stream  252  is cooled to 300° C. by steam flow  278  in heat exchanger  286  in end block  219  heating stream  278  to make superheated steam  279 . The reactor space  237  in which the water gas shift reaction takes place is maintained at 300-350° C. by cooling from adjacent stream  278 , flowing through the evaporator in the adjacent reactor space  239  to promote conversion of carbon monoxide in stream  253  into carbon dioxide. The heat exchange from the water gas shift reaction to the steam flow is shown as the heat flow  281 .  
         [0064]     Water gas shift products  254  are cooled in heat exchanger/evaporator  287  located in endblock  220  by a portion  282 A of water stream  282 , heating and evaporating water stream  282 A. Stream  255  then enters PROX reactor  241  where it reacts with heated air stream  264  over an oxidation catalyst with high CO selectivity to further convert CO to CO 2 , lowering the concentration of CO in the product reformate to a level below 10 ppmv. Air stream  264  mixes with process stream  255  at the inlet to PROX reactor  241  after entering the reactor through orifices in the face of endblock  220 . The 64 parallel product streams  256  are mixed back to one stream  257  after being cooled to 85° C. by air stream  261  in heat exchanger  288  located in endblock  219 . The product stream  257  then flows through tube  212  and through endblock  220  to the anode flow fields  214  of fuel cell stack  224 .  
         [0065]     Air stream  261  enters the processor  196  at about 20° C. through inlet tube  225  in end block  219  where it flows to heat exchanger  288  in the end block  219  beating to 40° C., before passing from the end block  219  through fluid channels (not shown) into the space  243  bounded by the shroud  218 , where the airstream  262  helps maintain PROX reactor  241  at the desired operating temperatures near 100° C. Air stream  264  is split from stream  262  to supply PROX reactor  241  by the aforementioned orifices in the inside face of endblock  220 . The remaining air  265  exits the device through tube  226  where it is plumbed to inlet tube  202  for introduction to the cathode flow fields  216  of fuel cell stack  224 .  
         [0066]     The process air streams are not split into separate streams upstream of fuel cell stack  224 . Anode exhaust stream  258  is plumbed from the fuel cell stack anode outlet tube  203  to a mixer (not shown) where it is mixed with inlet fuel stream  259  to provide a fuel mixture for combustion reactor  231 . Inlet tube  206  may provide a connection to re-introduce a portion of the anode effluent to the fuel cell stack  224  if an anode fuel recycling scheme is employed. The combustion fuel mixture enters the processor  196  in two equal flows through inlet tubes  213  and  227  where it is split into 64 parallel streams by two 32-valve arrays in an analogous arrangement to that described previously in reference to the fuel processor  10  before it flows to heat exchanger  290  located in endblock  220  to recover heat from exhaust stream  271 . Fluid channels in multiple laminates that may communicate with one another through overlaying apertures in successive laminates may be used to route and communicate fluids between the valves in each bank, as needed, in order to achieve the appropriate channeling of the fluid. Preheated fuel stream  260  flows to endblock  219  through tubes  211  where it mixes with preheated air stream  267  before entering combustion reactor  231 . Cathode exhaust stream  266  flows from the fuel cell stack to end block  220  where it is split into 8 parallel streams for blocks of 8 modules, each stream controlled by valves as described previously. Air stream  266  next flows to heat exchanger  289  located in endblock  220  where it is heated by combustion exhaust stream  270  before flowing through tubes  210  to endblock  219  for mixing with fuel stream  260  as described above. Combustion reactor  231  is maintained at 760° C. to supply heat  280  consumed by steam reforming reactions in reactor  233 . Combustion exhaust stream  268  exits the combustion reactor  231  and enters endblock  220  where it is subsequently split into streams  269  and  270  to provide two heat transfer streams for use in preheating reformer feedstock  250  in heat exchanger  285 , combustor fuel  259  in heat exchanger  290 , combustion air  266  in heat exchanger  289 , and reformer steam  282 B in heat exchanger  293 . Exhaust streams  273  and  274  may be mixed in end block  220  prior to leaving the device through outlet tube  207 . Stack coolant water stream  276  enters through tube  201  and is heated to 80° C. by fuel cell waste heat. Hot water  291  is taken from stack coolant outlet stream  277  and exits the device through tube  206  for potential use in cogeneration applications. The remaining coolant water  282  is split into parallel flows,  282 A and  282 B, for beating and vaporizing in heat exchangers  287  and  293  respectively. The streams are remixed to stream  278  before flowing to evaporator  239  and heat exchanger  286  to generate superheated steam  279  for use in reformer reactor  233 . The process steam is split into 64 valved streams for individual reactor modules prior to flowing through heat exchangers  287  and  293 .  
         [0067]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.