Patent Application
20070280862
The present invention relates to the field of micro-reactors and methods for operating such micro-reactors.
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.
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.
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).
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.
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.
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.
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.
Referring next to
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.
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.
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
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
Referring in particular to
Still referring to
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.
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.
Referring next to
Referring in particular to
Referring to
Referring more specifically to
Referring next to
Referring next to
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.
As shown in
Referring to
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.
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.
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.
The flow stream switching control system architecture, shown in
The control system of the present embodiment may operate in accordance with the logic structure shown in
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.
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.
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.
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
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.
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.
As shown in
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
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.
The process flow diagram for the power generation apparatus heretofore described is shown in
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.
Water gas shift products 254 are cooled in heat exchanger/evaporator 287 located in endblock 220 by a portion 282A of water stream 282, heating and evaporating water stream 282A. 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 CO2, 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.
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.
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 282B 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, 282A and 282B, 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.
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.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US02/21860 | 6/26/2002 | WO | 6/21/2007 |
| Number | Date | Country | |
|---|---|---|---|
| 60301493 | Jun 2001 | US |
