An apparatus and method for enhancing the yield and purity of hydrogen when reforming hydrocarbons is disclosed in one embodiment of the invention as including receiving a hydrocarbon feedstock fuel (e.g., methane, vaporized methanol, natural gas, vaporized diesel, etc.) and steam at a reaction zone and reacting the hydrocarbon feedstock fuel and steam in the presence of a catalyst to produce hydrogen gas. The hydrogen gas is selectively removed from the reaction zone while the reaction is occurring by selectively diffusing the hydrogen gas through a porous ceramic membrane. The selective removal of hydrogen changes the equilibrium of the reaction and increases the amount of hydrogen that is extracted from the hydrocarbon feedstock fuel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to microreactors and more particularly to microreactors for reforming hydrocarbon fuels and generating hydrogen gas.

2. Description of the Related Art

A microreactor (or microstructured reactor or microchannel reactor) is a device in which chemical reactions are designed to take place in confined spaces having lateral dimensions of less than 1 mm. Currently, there are major technological issues that prevent current technology from meeting the needs of microreactors for generating hydrogen, syngas, or performing specialty chemical synthesis. In general, gas-phase reactions for generating hydrogen and other specialty chemicals require microfabricated components that can perform under harsh operating conditions such as high temperatures, high temperature transients, or corrosive or erosive environments.

Current microfabrication processes (e.g., wet etching, dry etching, lithography, LIGA, etc.) are primarily applicable to silicon, photoresists, and metals. These materials readily corrode when subjected to hot gas streams that contain corrosive ingredients such as oxygen, steam, CO2, sulphur, and trace metals, each of which may be present when generating H2from natural gas or using gas-phase specialty chemical synthesis. Another important factor when generating hydrogen gas for fuel cells is the H2to CO ratio, since a higher ratio reduces the cost of CO removal.

The few processes that are available for microfabricating ceramics are either prohibitively expensive due to the need for very expensive precursor materials, such as pre-ceramic polymers, or are unable to attain the high precision required as a result of shrinkage that occurs during sintering. The shrinkage problem in particular often creates a need for very expensive secondary machining operations. Furthermore, many high temperature gas-phase reactions require passing gas streams over catalysts. The introduction of catalysts into silicon, metal, or sintered-ceramic-based microreactors is typically a complex multi-step process that requires a high surface area (i.e., highly porous) wash-coat to be applied inside the channels of the microreactor prior to catalyst deposition. This wash-coat is necessitated by the very low component surface area of silicon and metals. The wash-coat is typically easily damaged during operation because it poorly bonds with silicon, metal or sintered ceramic. This characteristic undesirably shortens the life of the microreactor.

In view of the foregoing, what are needed are robust ceramic materials for fabricating microreactors that are able to withstand high temperatures, high temperature transients, or corrosive or erosive environments and thus have excellent thermal shock resistance and thermal cycling properties. Ideally, such materials would enable features to be fabricated in net-shape and net-size with very high precision. Further needed are microreactors that increase the H2to CO ratio to reduce the cost of CO removal by secondary operations such as membrane reactors. Further needed are microreactors to increase the amount of hydrogen that can be extracted from hydrocarbon feedstock fuels. Further needed are ceramic materials that enable cost-effective fabrication of microreactors. Yet further needed are porous ceramic materials with intrinsically high surface area that can be infiltrated with catalysts to increase hydrocarbon reformation efficiency.

SUMMARY OF THE INVENTION

Consistent with the foregoing and in accordance with the invention as embodied and broadly described herein, a method for enhancing the yield and purity of hydrogen when reforming hydrocarbons is disclosed in one embodiment of the invention as including receiving a hydrocarbon feedstock fuel (e.g., methane, vaporized methanol, natural gas, vaporized diesel, etc.) and steam at a reaction zone and reacting the hydrocarbon feedstock fuel and steam in the presence of a catalyst to produce hydrogen gas. The hydrogen gas is selectively removed from the reaction zone while the reaction is in process by selectively diffusing the hydrogen gas through a porous ceramic membrane. The selective removal of hydrogen changes the equilibrium of the reaction and increases the amount of hydrogen that can be extracted from the hydrocarbon feedstock fuel.

In selected embodiments, the porous ceramic membrane is fabricated from a mixture of alumina powder and a phosphate-containing reagent to react with the alumina powder. For example, HSA-CERCANAM® provides one such material. This material is able to withstand high temperatures, high temperature transients, and corrosive and erosive environments. Thus, this material has excellent thermal shock resistance and thermal cycling properties. In selected embodiments, the method further includes providing heat to the reaction zone to react the hydrocarbon feedstock fuel and steam. This heat may be generated by combusting one or more residual reactants or reaction products such as hydrocarbons, hydrogen gas, and carbon monoxide that are left over or are byproducts of the hydrocarbon/steam reaction.

In another aspect of the invention, a device for enhancing the yield and purity of hydrogen when reforming hydrocarbons may include an inlet for receiving a hydrocarbon feedstock fuel and steam. A reaction zone may be placed in communication with the inlet to react the hydrocarbon feedstock fuel and steam in the presence of a catalyst to produce hydrogen gas. A porous ceramic membrane is provided to selectively remove hydrogen gas from the reaction zone while the reaction in occurring, thereby increasing the extent of reaction between the hydrocarbon feedstock fuel and the steam and increasing the yield of hydrogen.

In yet another aspect of the invention, a ceramic microchannel device for reforming a hydrocarbon fuel to produce hydrogen gas is disclosed. This ceramic microchannel device is fabricated at least in part from a ceramic made be mixing alumina powder with a phosphate-containing reagent to react with the alumina powder.

In yet another aspect of the invention, a method for enhancing the yield and purity of hydrogen when reforming hydrocarbons includes receiving a hydrocarbon feedstock fuel and steam at a reaction zone. The hydrocarbon feedstock fuel and steam are then reacted at the reaction zone to produce hydrogen gas. The hydrogen gas is then selectively removed from the reaction zone while the reaction is occurring by extracting hydrogen gas or hydrogen ions from the reaction zone. In selected embodiments, the hydrogen gas is removed from the reaction zone by diffusing the hydrogen gas through a porous ceramic membrane. In other embodiments, hydrogen gas is removed from the reaction zone by conducting hydrogen ions through an ionically-conductive ceramic membrane.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, in selected embodiments, a catalytic microchannel reformer100in accordance with the invention may be adapted to receive an input102, comprising a hydrocarbon feedstock fuel and steam, at a reaction zone104. For the purposes of this description, a hydrocarbon includes any molecule containing both hydrogen and carbon. In selected embodiments, the reaction zone104may exist within one or more microchannels of the device100, as will be explained in more detail hereafter. In selected embodiments, an input102comprising methane (CH4) (the primary constituent in natural gas) and steam (H2O) may be used as the inputs to the reaction zone104. For the purposes of this disclosure, methane will be used as the hydrocarbon feedstock fuel. In other embodiments, however, other hydrocarbon feedstock fuels such as propane, methanol, diesel, or the like, may be used in place of methane. Where the feedstock fuel is a liquid, some pre-heating may be performed to vaporize the liquids prior to input to the reformer100.

At the reaction zone104, the reactants102may be heated to approximately 650° C. to 700° C. in the presence of a catalyst, such as a nickel-based catalyst or other catalyst known in the art. The catalyst will cause the steam and methane to react to form carbon monoxide and hydrogen in accordance with the following reaction:
CH4+H2O→CO+3H2

This reaction is commonly referred to as steam methane reforming (SMR), which is one of the most common and least expensive methods for producing bulk hydrogen. In selected embodiments, the reactant stream102may be pre-heated prior to being input to the device100to aid the reformation process and reduce the time and amount of heat that is required to bring the reactants102to the necessary temperature.

In selected embodiments, additional hydrogen may be recovered at the reaction zone104using a lower temperature (550° C. to 600° C.) gas-shift reaction. This reaction may react the carbon monoxide (CO) generated above with steam to produce hydrogen gas and carbon dioxide in accordance with the following equation:
CO+H2O→CO2+H2

This reaction is commonly referred to as a water-gas-shift (WGS) reaction. Both of the above reactions require steam as one of the reactants. In selected embodiments, a high steam-to-methane ratio (e.g., 2:1) may be provided in the input stream102to ensure that coking is minimized during the SMR and WGS reactions.

In selected embodiments, the microchannel reformer100may be designed such that the SMR and WGS reactions take place isothermally. Because the microchannel reformer100may be very small (with dimensions on the order of several inches), the small size may impose substantially isothermal conditions. In general, a higher temperature (generally above 600° C.) favors the forward reaction in the case of SMR and the reverse reaction in the case of WGS. This is one reason why these two reactions are generally not combined in a single reactor. However, in selected embodiments, a microchannel reformer100in accordance with the invention may combine both of these reactions in a single device100. Both of these reactions may be driven to near completion by continuously removing hydrogen gas from the reaction zone104. This may be accomplished using a porous ceramic membrane106(with micro- or nano-sized pores) located adjacent to the reaction zone104. As the SMR and WGS reactions proceed in a forward direction, hydrogen gas may selectively diffuse through the membrane106, creating a deficit of product (hydrogen) in the reaction zone104. This deficit may drive the SMR and WGS reactions nearer to completion. In alternative embodiments, an ionically-conductive membrane106may be used to selectively remove hydrogen gas from the reaction zone104by transporting hydrogen ions through the membrane106.

For the purpose of this description, “selectively” removing hydrogen from the reaction zone104may also include removing other gaseous species from the reaction zone104by diffusion through the membrane106, although this may occur at a significantly lower diffusion rate. Thus, the term “selectively,” as used herein, does not necessarily mean exclusively removing hydrogen, but rather that the membrane106may remove hydrogen gas from the reaction zone104at a faster rate than it removes other gaseous species. The higher diffusion rate of hydrogen is primarily due to hydrogen's lower molecular weight compared to other gaseous species in the reformer100. The diffusion rate is proportional to the square root of the molecular weight of each gas. By removing hydrogen from the reaction zone104faster than other gaseous species, the equilibrium of the SMR and WGS reactions may be shifted, driving the reactions nearer to completion.

As shown inFIG. 1, a product stream108may contain primarily H2, but may also include secondary gaseous species such as CO, CO2, H2O, and CH4. Reactants and reaction products that do not diffuse through the membrane106, which may include primarily CO2and H2O, but may also include residual CO, H2, and CH4, may be conveyed to a combustion zone110. Here, the reactants and reaction products may be combusted in the presence of air114to generate heat and fully oxidize the remaining reactants in accordance with the following equations:
2CO+O2→2CO2+Heat
2H2+O2→2H2O+Heat
CH4+2O2→CO2+2H2O+Heat

After combustion, an exhaust stream112containing primarily CO2and H2O may be output from the microchannel reformer100. Ideally, the microchannel reformer100will be designed such that the amount of combustible gases conveyed to the combustion zone110will produce enough heat to drive the SMR reactions in the reaction zone104, while still maximizing the amount of hydrogen in the product stream108. Table I below shows various calculations with respect to the gas compositions at various locations in the microchannel reformer100. The calculations in Table I assume that the extents of reaction for both the SMR and WGS reactions stay constant through the reaction zone104. More specifically, Table I shows approximate gas compositions inside the reaction zone104and the product zone107assuming that the extent of reaction for both the SMR and WGS reactions is 0.9 and the initial steam-to-methane ratio is 2:1.

As indicated in Table I, the diffusion rate of hydrogen is almost three times greater than the next closest gas. Furthermore, Table I shows that the gas composition in the reaction zone104may include about 73.1 percent H2after the SMR and WGS reactions have progressed to near completion. The H2concentration in the product stream108may increase to about 91.4 percent after the hydrogen and other gases diffuse through the membrane106. This concentration may increase to about 93.8 percent after water is removed (e.g., condensed) from the product stream108. The hydrogen concentration may be increased even further if the CO2is removed from the product stream through a process such as compression and liquefaction.

As mentioned previously, gas-phase reactions for generating hydrogen or syngas typically require microfabricated components that can perform under harsh operating conditions such as high temperatures, high temperature transients, or corrosive or erosive environments. Such materials should have excellent thermal shock resistance and thermal cycling properties. Further needed are materials that enable features to be fabricated in net-shape and net-size with very high precision.

In selected embodiments in accordance with the invention, ceramic materials marketed under the tradename CERCANAM® may be used to fabricate all or part of the catalytic microchannel reformer100. These ceramic materials may be classified as either HAS-CERCANAM®, which has a very high surface area and a continuous nanopore network, or regular CERCANAM® or LSA-CERCANAM®, which is substantially the same material but is designed to have a lower surface area and thus a reduced pore structure. In general, these ceramic materials may be classified as phosphate-bonded ceramic materials. That is, each of these CERCANAM® compositions may be fabricated from ceramic powders (e.g., alumina powder) combined with phosphate-containing reagents (e.g., phosphoric acid). The phosphate-containing reagents may react with the ceramic powders to bond the ceramic powders together.

In selected embodiments, the pore-structure of the HSA-CERCANAM® may be created or enhanced simply by adding a pore former to the LSA-CERCANAM® slip. CERCANAM® and similar materials are disclosed, for example, in U.S. patent application Ser. No. 11/464,476 filed on Aug. 14, 2006 and entitled PROCESS FOR MAKING CERAMIC INSULATION and U.S. patent application Ser. No. 11/781,125 filed on Jul. 20, 2007 and entitled METHOD FOR JOINING CERAMIC COMPONENTS, both of which are incorporated by this reference.

The CERCANAM® material described herein is particularly suitable for fabricating a catalytic microchannel reformer100in accordance with the invention. For example, both HSA-CERCANAM® and LSA-CERCANAM® exhibit thermal stability in oxidizing and reducing environments up to 1000° C. for at least 16 hours. Thus, this material may be used to fabricate a microchannel reformer100with long life and minimal component degradation. Furthermore, both HSA-CERCANAM® and LSA-CERCANAM® exhibit excellent thermal shock resistance when rapidly cycled between room temperature and 800° C. This facilitates rapid heating or cooling of the microchannel reformer100during startup or shutdown. In selected embodiments, fuel in the microchannel reformer100may be spark-ignited for rapid heat-up to 700° C.

Another benefit of both HSA-CERCANAM® and LSA-CERCANAM® is that these ceramic materials may be microfabricated in net-shape and net-size with very high precision. In selected embodiments, these materials may be cast in various shapes and forms to fabricate monolithic components. The unique characteristics of CERCANAM® allow it to be cast on and around features, either sacrificial or permanent. Using sacrificial features, for example, microchannels may be incorporated internally into a monolithic CERCANAM® piece in a simple one-step process. Where structures include both HSA-CERCANAM® and LSA-CERCANAM®, the structure may be fabricated in two casts, one each for HSA-CERCANAM® and LSA-CERCANAM®. The ability to fabricate CERCANAM® components using minimal fabrication steps significantly lowers component costs.

In selected embodiments in accordance with the invention, a microchannel reformer100may be fabricated from both HSA-CERCANAM® and LSA-CERCANAM® compositions. For example, the cross-hatched portions116of the reformer100may be fabricated from LSA-CERCANAM® while the cross-hatched portions118may be fabricated from HSA-CERCANAM®. Thus, in selected embodiments, the membrane106may be fabricated from HSA-CERCANAM® because it provides a material with high surface area and a network of sub-micron and non-sized pores. The pore size and structure may be tailored, as needed, to provide a membrane106with desired characteristics. HSA-CERCANAM® has been found to provide an effective membrane106to selectively remove hydrogen from the reaction zone104and thereby drive the SMR and WGS reactions nearer to completion.

In selected embodiments, a wall120or surface120of the reaction zone104may also be fabricated from HSA-CERCANAM®. This wall120may be infiltrated or embedded with a catalyst material as previously discussed herein. The intrinsically high surface area of HSA-CERCANAM® improves the contact between the hydrocarbon feedstock and the catalyst, thereby improving the yield of hydrogen gas in the product stream108compared to other materials. This improvement in efficiency will be discussed in association withFIG. 9.

Although specific reference has been made herein to CERCANAM®, the reformer100is not limited these materials. Indeed, any reformer100which utilizes a porous or ionically-conductive membrane106to selectively remove hydrogen from the reaction zone104, while the reactions therein are occurring, is intended to fall within the scope of the invention. CERCANAM® or similar materials simply provide one example of materials that may be used to fabricate a reformer100in accordance with the invention.

Referring toFIG. 2, in other embodiments, a microchannel reformer100in accordance with the invention may be designed in a symmetric configuration to improve efficiency and generate additional product. For example, a microchannel reformer100may receive dual inputs streams102a,102b, each containing a hydrocarbon feedstock fuel and steam. These streams102a,102bmay be conveyed to dual reaction zones104a,104b, which may exist within microchannels of the device100. A porous ceramic membrane106a,106bmay be placed adjacent to each reaction zone104a,104b. Each of these ceramic membranes106a,106bmay communicate with a single product zone107, which may output a product stream108. Similarly, each of the reaction zones104a,104bmay communicate with a different combustion zone110a,110b, where residual reactants and reaction products may be combusted to produce heat to drive the SMR and WGS reactions. By using dual reaction zones104a,104b, more catalyst-infiltrated surface area is available to increase the efficiency of the reactor100.

Referring toFIG. 3, in selected embodiments, a catalytic microchannel reformer100in accordance with the invention may be scalable in order to produce hydrogen at a desired rate. For example, in selected embodiments, multiple microchannel reformers100a-d, each working in accordance with the reformer100ofFIG. 2, may be fabricated as interconnectable modules100a-d. These modules100a-dmay be linked together to create a stack300having a desired hydrogen production rate. Furthermore, modules100a-dmay be added or removed from the stack300, as needed, to increase or decrease the hydrogen production rate.

Referring toFIG. 4, in selected embodiments, a microchannel reformer module100may include one or more input ports400to receive an input stream102, namely a hydrocarbon feedstock fuel and steam. A central product port402may output a product stream108containing hydrogen gas as the primary constituent. Exhaust ports404may be used to expel an exhaust stream112as well as supply air (i.e., oxygen) to an internal combustion zone110. As shown, in selected embodiments, the input ports400and product port402may be located on a central interface plate406, allowing the ports400,402to interface with corresponding ports400,402on adjacent modules100a-d.

Referring toFIG. 5, a cutaway perspective view of the microchannel reformer module100ofFIG. 4is illustrated. The module100includes input streams102, a product stream108, and exhaust streams112to show the flow of gases through the module100. These streams102,108,112may be compared to the schematic diagram ofFIG. 2to more fully understand the flow and operation of the microchannel reformer100. As shown, ports are provided on a top and bottom side of the module100to allow the input stream102to flow to other downstream modules100as well as receive a product stream108from downstream modules100. The reactions zones104, combustion zones110, and porous ceramic membranes106a,106bare also shown within the device100.

As explained previously, in selected embodiments, where CERCANAM® is used as the fabrication material, the module100may be fabricated in as few as two processing steps, while understanding that the fabrication process is not limited to any specific number of steps. That is, the entire structure100may be fabricated in two casts, namely, one cast for the HSA-CERCANAM® portions and one cast for the LSA-CERCANAM® portions. Channels in the structure100may be formed by inserting sacrificial organic inserts into the CERCANAM® slip, such as mylar or plastic inserts. When the CERCANAM® structure is fired, these organic materials may burn away to leave the desired channels in the structure100.

Referring toFIG. 6, an alternative cutaway perspective view of the microchannel reformer module100ofFIG. 4is illustrated. Like the cutaway view ofFIG. 5, the module100is illustrated with the input stream102, product stream108, and exhaust stream112to show the flow of gases through the module100. The porous ceramic membranes106a,106band product zones107are also shown.

Referring toFIGS. 7 and 8, as mentioned in relation to Table I, the combination of a higher initial concentration of H2in the reaction zone and the higher diffusivity of H2compared to other gas species will result in a product gas stream108with very high purity (e.g., greater than 91 percent hydrogen on a wet basis). We can carry this simplified calculation a bit further by exploring the variation of the product gas composition108as a function of the extent of reaction for each reaction (i.e., the SMR and WGS reactions), while keeping the extent of reaction for the other reaction constant.FIG. 7shows the product gas composition108as the extent of the SMR reaction goes from zero to full completion.FIG. 8shows the product gas composition108as the extent of the WGS reaction goes from zero to full completion.

As shown inFIG. 7, when the extent of the SMR reaction exceeds 0.6, the product gas composition is over 85 percent H2, even where the extent of the WGS reaction is as low as 0.1. As shown inFIG. 8, where the extent of the SMR reaction is held constant, the H2concentration is only a mild function of the extent of the WGS reaction. Thus, as long as the extent of the SMR reaction is 0.9 or higher, an H2concentration of over 90 percent can be expected in the product gas stream108.

The calculations provided inFIGS. 7 and 8are based on a number of simplifying assumptions and thus should only be used as a guideline. For example, these calculations do not consider the thermodynamics of the SMR and WGS reactions. Furthermore, these calculations do not account for the fact that the reformer gas concentration may vary along the length of the microchannels, resulting in variations in gas concentration percolating across the channels at different locations in the reformer100. Furthermore, the extents of reaction are not independent of each other. For example as the extent of the SMR reaction increases, more CO and H2may be generated in the reactor100. Because H2is removed from the reaction zone104much faster than CO, this would create an excess of reactants for the WGS reaction, thereby driving the equilibrium composition further towards the product side of the reaction. In addition, the selective removal of H2also drives the SMR reaction further towards the product side.

Other simplifying assumptions ignore other possible reactions that may occur in the reactor100such as “coking reactions” as indicated by the following equations:
CO+H2→H2O+C
CH4→2H2+C

The first coking reaction is not favored in the presence of excess steam since steam is on the product side of the reaction, and excess steam favors the reverse reaction. On the other hand, too much steam in the feed gas is also non-ideal since it reduces the overall efficiency of the microreactor due to the energy required to heat the excess steam.

Referring toFIG. 9, the difference in methane reforming capability for catalysts embedded in different CERCANAM® materials is illustrated. The upper curve900represents the extent of the SMR reaction where the catalyst is infiltrated or embedded in a wall120made of HSA-CERCANAM®. The lower curve902represents the extent of the SMR reaction where the catalyst is infiltrated or embedded in a wall120made of LSA-CERCANAM®. The graph shows that the extent of the SMR reaction was significantly better for HSA-CERCANAM® than it was for the LSA-CERCANAM® for all measured flow rates. These results show that the higher H2yields in the product gas stream108are likely due to the increased intimacy of contact between the catalyst and methane in the catalyst-infiltrated HSA-CERCANAM® wall120.