2. Methods
Steam reforming is a process in which hydrogen is stripped from a hydrocarbon fuel by thermal energy provided by a combustor. In alcohol steam reforming, the feed stream contains steam and an alcohol or alcohols. In the present design, methanol, ethanol, and propanol are preferred with methanol being especially preferred. In an integrated reactor, combustion or heat generation should occur in close proximity to the endothermic reaction. Preferably, an exothermic reaction occurs in microchannels that are interleaved with microchannels in which there is an endothermic reaction. Co-flow of endothermic and exothermic reaction streams is preferred; however, cross-flow or countercurrent flow are also options. The heat of an exothermic reaction is conducted from the exothermic reaction to the endothermic reaction catalyst, where it drives the endothermic reaction. Preferably an exothermic reaction channel and endothermic reaction channel in the integrated reactors is a microchannel, that is, a channel having at least one dimension of 2 millimeters or less. The use of channels having a minimum dimension of more than 2 mm may be less effective since heat and mass transfer limitations may be magnified. An integrated combustor can use the high surface area of reactor microchannels to remove heat as it is produced, thus keeping microreactor components from exceeding material temperature constraints while combusting with much less excess air or diluent than would be necessary for an external combustor. The flow rate of reactants will depend on the desired amount of hydrogen to be produced and on the minimum or maximum capacity of the steam reformer. The rate of combustion can be controlled to provide the desired amount of heat to a steam reforming reaction in an adjacent reforming chamber. The steam reforming reaction can be run over a broad pressure range from sub-ambient to very high. The alcohol steam reforming reaction is preferably carried out at temperatures of 200-400 °C, more preferably 220-300 °C, and in some cases 240-270 °C. In some preferred configurations, the combustion temperature is approximately the same as the average reformer temperature, that is, the average temperature of the reforming catalyst.
The design provides a method of conducting an endothermic reaction in an integrated combustion reaction, comprising: passing an endothermic reaction composition into at least one endothermic reaction chamber, passing a fuel and an oxidant into at least one exothermic reaction chamber wherein the fuel and oxidant each have a contact time in the combustion chamber of 50 milliseconds or less, wherein the exothermic reaction chamber comprises at least one exothermic reaction chamber wall that is adjacent at least one endothermic reaction chamber. The endothermic reaction chamber comprises an endothermic reaction catalyst in contact with at least the at least one endothermic reaction chamber wall that is adjacent at least one exothermic reaction chamber, and transferring heat from the at least one exothermic reaction chamber into the at least one endothermic reaction chamber at a rate as based on the internal surface area of the endothermic reaction chamber. The heat flux can be measured based on either a single exothermic reaction chamber or multiple chambers in a multi-chamber device. The reformate stream usually comprises hydrogen, carbon dioxide, and carbon monoxide. Proton exchange membrane fuel cells operate have a very low tolerance for carbon monoxide. They can generally tolerate carbon dioxide and some other gases such as nitrogen, but only up to a certain amount. Clean-up of a reformate stream can be performed, for example by a multi-step process consisting of water gas shift reactors, combined with selective oxidation and carbon monoxide methanation, or by the use of a hydrogen permeable membrane. Direct methanization of a reformate stream is important without first passing the reformate through a hydrogen-selective membrane, preferential oxidation, or water gas shift reactor. This is highly desirable since hydrogen-selective membranes are expensive, and additional process steps can be costly and result in lowered yield. Eliminating the requirement of a preferential oxidation also eliminates the need to add oxygen, including the need to vary oxygen content to account for fluctuations in carbon monoxide concentration. In preferred cases, the process adds heat to the steam reforming step but does not have additional heat exchangers or heat exchange steps for methanation or for other carbon monoxide-reducing steps.
The exothermic reaction chamber has an internal dimension of less than 0.8 mm and a volumetric heat flux, based on reaction chamber volume of greater than 8 Watts per cubic centimeter. Contact times in the exothermic and endothermic reaction chambers are preferably less than 600 milliseconds. Area heat flux for the area of either reaction chamber is preferably 0.8 Watts per square centimeter or more. The design provides a method of steam reforming in an integrated combustion-reforming reactor, comprising: step a) passing steam and hydrocarbon into at least one endothermic reaction chamber wherein the steam to carbon ratio is less than 2:1 with a pressure drop through the endothermic reaction chamber of less than 6000 kPa, step b) passing a fuel and an oxidant into at least one exothermic reaction chamber wherein the fuel and oxidant each have a contact time in the combustion chamber of 600 milliseconds or less, wherein the exothermic reaction chamber comprises at least one exothermic reaction chamber wall that is adjacent at least one endothermic reaction chamber, wherein the endothermic reaction chamber comprises an endothermic reaction catalyst in contact with at least the at least one endothermic reaction chamber wall that is adjacent at least one exothermic reaction chamber, step c) converting the steam and hydrocarbon to form carbon monoxide and hydrogen such that the at least one endothermic reaction chamber has an output demonstrating a conversion of at least 50 percent of the hydrocarbon with a selectivity to carbon monoxide of at least 50 percent. A device is characterized by operation for 200 or 500 hours and then cut open to reveal less than 0.08 gram of coke per each kilogram of the fuel processed.
The endothermic reaction chamber and the combustion chamber are separated by a thermally conductive wall. The endothermic reaction composition endothermically reacts to form products. Where not otherwise specified, the front of the combustion chamber is defined as where the flow of fuel contacts a combustion catalyst and an oxidant, and the back of the combustion chamber is defined as the last part of the reaction chamber that contains a combustion catalyst and is in direct thermal contact with an endothermic reaction chamber. The exhaust section is not in direct thermal contact with the endothermic reaction chamber. The design also provides a method of simultaneously conducting an endothermic and an exothermic reaction in an integrated combustion-reforming reactor, comprising: passing a mixture comprising hydrogen and methanol through a microchannel in an integrated combustion-reforming reactor; reacting the hydrogen and methanol with an oxidant to form water, carbon dioxide and carbon monoxide and produce heat, thus removing hydrogen and methanol from the mixture; wherein a greater percentage of methanol is removed from the mixture than the percentage of hydrogen removed from the mixture, as measured by comparing the levels of hydrogen and methanol in the mixture before passing through the microchannel with the levels of hydrogen and methanol at any point after passing through the microchannel. This is an extremely surprising result. The removing steps are by chemical reactions, not separation techniques.
An integrated combustion-reforming reactor refers to an integrated reactor that includes at least one combustion channel adjacent to at least one endothermic steam reforming reaction channel. During operation, a reactant enters a combustion or reaction chamber in a bulk flow path flowing past and in contact with a porous material or porous catalyst. In these cases, a portion of the reactant molecularly transversely diffuses into the porous catalyst and reacts to form a product or products, and then the products diffuse transversely into the bulk flow path and out of the reactor. The term bulk flow region or bulk flow path refers to open areas or open channels within the reaction chamber. A reaction chamber with a bulk flow path or region will contain a catalyst and there is a gap between the catalyst surface and a reaction chamber wall or a second catalyst surface. A contiguous bulk flow region allows rapid gas flow through the reaction chamber without large pressure drops. In preferred cases, there is laminar flow in the bulk flow region. Equilibrium conversion is defined in the classical manner, where the maximum attainable conversion is a function of the reactor temperature, pressure, and feed composition. For the case of hydrocarbon steam reforming reactions, the equilibrium conversion increases with increasing temperature and decreases with increasing pressure. Fuel flow to combustor can be initiated at this point. Once the fuel has begun reacting, the hydrogen flow is tapered off and the fuel flow is increased. The excess air should not be too much, since the extra air removes heat from the steam reformer. Air and methanol flows are adjusted until the steam reformer is at the desired temperature. The reformer fuel mixture flow is initiated at this point. Combustor flows are adjusted as necessary to maintain desired temperatures. Preferred forms of porous supports are foams and felts and these are preferably made of a thermally stable and conductive material.
The methanol conversion is calculated by using a carbon balance on the system. Reaction chamber volume is the internal volume of a reaction chamber either exothermic or endothermic. This volume includes the volume of the catalyst, the open flow volume if present and metal support ribs or fins if present within the reaction chamber volume. This volume does not include the reaction chamber walls. The reaction chamber volume must contain a catalyst somewhere within its cross-section and must be directly adjacent another reaction chamber for heat transport. This volume is used for calculations of endothermic reaction chamber volumetric heat flux, area heat flux, and endothermic reaction contact time. The reactor core volume is defined as the reaction chamber volume and all combustion chamber volume and the metal webs that separate the two chambers. The combustion chamber volume is defined as the chamber volume in which the exothermic heat generating reaction occurs and is adjacent to the reaction chamber volume. Perimeter metal is not included in reactor core volume. The reactor core volume does not include any preheat exchanger zone volume that may or may not be attached to the reactor core volume. The preheat exchanger zone may be attached to the reactor but does not contain an endothermic reaction catalyst along any plane that bisects the device orthogonal to the direction of flow. The use of electric heating for system start-up is eliminated by following the subsequent procedure. Hydrogen and air are fed to combustor to initiate combustion and heat the vaporizers. Once the vaporizers are heated to approximately 80 °C, methanol is fed to the vaporizer. The hydrogen is slowly tapered off as the methanol feed is increased until only methanol and air are being fed to the combustor and the device is completely self-sustaining. The methanol-air mixture is adjusted until the steam reformer reaches the desired temperatures depending on the conditions being tested. The methanol-water solution feed is then initiated.
Endothermic reaction chamber heat flux is defined as the endothermic reaction heat duty divided by the reaction chamber volume. Reactor core volume heat flux is defined as the endothermic reaction heat duty divided by the reactor core volume. Heat exchanger flux is defined as the total heat transferred to the cold streams divided by the heat exchanger core volume. Heat exchanger core volume is defined as the total heat exchanger volume inclusive of microchannels, ribs between microchannels, and the walls separating microchannels for all fluid streams transferring heat. The heat exchanger volume is inclusive of the heat exchanger zone. The heat exchanger core volume does not include the perimeter metal or manifolds or headers. The heat exchanger core volume does not include the endothermic reaction chamber nor any volume that could be included within any plane that bisects the endothermic reaction chamber orthogonal to the direction of flow. Average area heat flux is defined as the endothermic reaction heat duty divided by the area of the endothermic reaction chamber heat transfer surface. The endothermic heat transfer surface is defined by a planar area, which may be intermittent in the case of ribs or other structures in the endothermic reaction chamber, above which there is area for flow of reactants and below which there is a wall that separates the endothermic reaction chamber and the exothermic reaction chamber. This area is the path for heat transfer from the exothermic reaction chamber to the endothermic reaction chamber. The apparent equilibrium conversion temperature is the apparent temperature based on methane conversion or, more generally, hydrocarbon conversion or the temperature required to produce an equilibrium methane conversion equal to the measured methane conversion at the measured average process pressure. Average process pressure is assumed to be the average of the measured inlet and outlet pressures.