Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
1998-12-04
2001-03-27
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S652000, C423S653000
Reexamination Certificate
active
06207132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and apparatus for producing high purity hydrogen by steam-reforming a carbonaceous fuel. The apparatus comprises a reforming chamber provided with a hydrogen-permeable membrane tube, and an oxidation chamber with an oxidation catalyst bed disposed therein.
2. Description of the Related Art
Steam-reforming of hydrocarbon or carbonaceous fuels, such as methanol, ethanol, gasoline, petroleum and the like, for producing hydrogen for various applications are known in the art. This invention is particularly concerned with steam-reforming of carbonaceous fuels to generate hydrogen for use in fuel cell for electric vehicles and on-site electric power plants.
Steam-reforming of hydrocarbon, such as methanol, is a reversible and endothermic reaction. Thus, heat must be supplied to allow the reaction to proceed and to allow the reaction to reach the equilibrium state. The maximum yield of a reforming reaction can only be reached at the equilibrium state of the reaction. Due to the poor yield at low temperature, the temperature of the steam-reforming reaction often requires the temperature to be raised to 700-900° C. in order to obtain a satisfactory hydrogen yield. Such high reaction temperatures can be reduced upon interrupting the equilibrium state by withdrawing one of the products during the progress of the reaction, while maintaining the same yield.
Traditionally, the purity of hydrogen obtained from the steam-reforming of carbonaceous fuels normally reaches to about 70%, which requires further purification, for example to a level of greater than 95% purity, before application to the fuel cells for technical and economic feasibility.
High purity hydrogen can be obtained by using a thin metal layer of palladium or palladium-alloy membrane as seen in the semiconductor industry. However, such membranes are impractical for application on an industrial scale because of their low hydrogen flux which would result in a demand of greater surface area of the membrane and tremendous expense. Furthermore, the thickness of such thin metal layer is normally too thin to possess sufficient mechanical strength during application, especially under elevated temperatures and pressures.
U.S. Pat. No. 5,451,386 discloses a hydrogen-selective membrane comprising a tubular porous ceramic support having a palladium metal layer deposited on an inside surface of the ceramic support. Using such membrane can provide a hydrogen flux and hydrogen selectivity that is significantly higher than the traditional membranes described above and can possess good mechanical strength for high temperature hydrogen separations, such as when applied in the promotion of ammonia decomposition. While the ceramic supported membrane exhibits a mechanical strength that can be used at high temperatures and pressures, the thickness of the palladium layer of the membrane has to be greater than 10 &mgr;m to avoid any defects from taking place. Such limitation would restrict the promotion of hydrogen flux which requires a reduction in thickness of the palladium layer of the membrane.
According to the aforesaid U.S. patent, in Buxbaum et al. it is disclosed that a 2 &mgr;m thick of palladium film deposited on a niobium disk fails when applied in the extractions of hydrogen at temperatures above about 500° C. because of the diffusion of niobium into the palladium metal under such high temperatures, and eventually becomes impermeable to hydrogen.
U.S. Pat. No. 5,741,474 discloses a system for producing high-purity hydrogen by reforming a hydrocarbon and/or an oxygen atom-containing hydrocarbon to form a reformed gas containing hydrogen and by separating the hydrogen from the reformed gas. Such system includes a reforming chamber provided with a catalyst for steam-reforming and partial oxidation, and a hydrogen-separating membrane, such as a palladium or palladium-silver alloy membrane. The heat generated by the partial oxidation maybe used as heat to continue the reforming by cooperating with the heat generated by the burning of the non-permeated gas. The burning of the non-permeated gas would, however, require additional fuels to burn the gas. Also, the burning often needs to employ an extremely high temperature, which implies a need of special materials for construction and for prevention of a significant amount of heat loss. Furthermore, the burning of the non-permeated gas in this process is unable to convert completely the gas into a non-polluting gas for discharge into environment.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a process and apparatus for producing high-purity hydrogen by reforming a hydrocarbon, that would be capable of overcoming the disadvantages described above.
According to one aspect of the present invention, an apparatus for producing high purity hydrogen by steam-reforming a carbonaceous fuel comprises: at least one reforming chamber containing a reforming catalyst bed for catalyzing the formation of hydrogen from the carbonaceous fuel via a steam-reforming reaction; at least one hydrogen-permeable membrane tube disposed inside the reforming chamber to be surrounded by the reforming catalyst bed and to confine therein a hydrogen compartment, the membrane tube being permeable to hydrogen, thereby permitting the hydrogen formed within the reforming catalyst bed to enter the hydrogen compartment; and at least one oxidation chamber provided adjacent to the reforming catalyst bed for burning the gas not permeable to the membrane tube and for supplying heat to the reforming chamber, the oxidation chamber having an oxidation catalyst bed, and being spaced apart from the membrane tube by at least a portion of the reforming catalyst bed.
According to another aspect of the invention, a process for producing high purity hydrogen by steam-reforming a carbonaceous fuel and by instantaneous separation of hydrogen, comprises the steps: providing a reforming chamber which contains a reforming catalyst bed, and a hydrogen-permeable membrane tube which is disposed inside the reforming chamber so as to be surrounded by the reforming catalyst bed, the membrane tube confining a hydrogen compartment; providing an oxidation chamber adjacent to the reforming chamber, and an oxidation catalyst bed inside the oxidation chamber; feeding a mixture including the carbonaceous fuel into the reforming catalyst bed to undergo a steam-reforming reaction which produces hydrogen and the other reformed gases; withdrawing hydrogen from the hydrogen compartment; and allowing the other reformed gases to enter the oxidation chamber and catalytically oxidating the other reformed gases to form a non-polluting gas.
The hydrogen-permeable membrane tube used in the present invention includes a porous substrate and a thin metal layer which is preferably selected from the group consisting of palladium and a palladium alloy and which is deposited on the surface of the substrate. The porous substrate is selected from the group consisting of a porous stainless steel and a porous ceramic material. The substrate has a thickness that ranges from 0.5 mm to 2 mm. The thin metal layer has a thickness of 1 to 20 &mgr;m.
The reforming catalyst bed is selected from the group consisting of a copper-based catalyst, an iron-based catalyst, and a nickel-based catalyst. The oxidation catalyst bed comprises a catalyst selected from the group consisting of palladium and platinum. The steam-reforming reaction of the carbonaceous fuel may be carried out at a temperature of 250 to 550° C. The reformed hydrogen is withdrawn under a transmembrane pressure difference of 2 to 20 atm. The carbonaceous fuel is selected from the group consisting of methanol, ethanol, gasoline, petroleum, and the mixtures thereof. In an embodiment of the present invention, the mixture fed to the reforming chamber includes methanol and steam. The methanol is fed at a WHSV of from 0.5 to 30 h
−1
, and the mole ratio of steam to methanol ranges from 1 to 2.
Due to
Lin Yu-Ming
Rei Min-Hon
Chinese Petroleum Corporation
Knobbe Martens Olson & Bear LLP
Langel Wayne
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