System for extracting hydrogen from a gas containing methane
A system for extracting hydrogen from a gas containing methane, especially natural gas. Hydrocarbons contained in the gas are catalytically broken down in a reformer by steam in order to form hydrogen, carbon monoxide and carbon dioxide. Catalytic conversion of the obtained carbon monoxide with steam occurs in a downstream conversion step in order to form carbon monoxide and water. Carbon dioxide is removed from the converted gas flow by gas washing, and the washed hydrogen-rich gas flow is subsequently divided in a pressure-swing adsorption system into a product gas flow made of hydrogen and a waste gas flow. The waste gas flow is introduced with hydrogen, which is separated from the gas flow after gas washing, into a reformer which is essentially a carbon-free combustible gas, and is combusted there.
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CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of and Applicants claim priority under 35 U.S.C. §§120 and 121 of parent U.S. patent application Ser. No. 10/566,268 filed Mar. 10, 2006. Applicants claim priority under 35 U.S.C. §119 of German Application No. 103 34590.6 filed Jul. 28, 2003. Applicants also claim priority under 35 U.S.C. §365 of PCT/EP2004/008322 filed Jul. 24, 2004. The international application under PCT article 21(2) was not published in English.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for extracting hydrogen from a gas that contains methane, particularly natural gas.
2. The Prior Art
A conventional hydrogen plant is known from U.S. Pat. No. 5,131,930, which is operated with natural gas as the starting material. In the plant, catalytic splitting, generally operated with steam, of hydrocarbons contained in the natural gas first takes place, in a heated reformer, to produce carbon monoxide and synthesis gas that contains hydrogen. Afterwards, catalytic conversion of the carbon monoxide to hydrogen takes place, and subsequently, pure characterization of the hydrogen takes place, using a pressure swing adsorption system. The waste gases of the adsorption system are passed back to the combustion chamber of the reformer, and there they are burned, together with additional natural gas that is supplied. It is also known to use refinery gas or other combustion gases as the additional fuel. A significant amount of carbon dioxide is produced by means of the steam splitting of the methane, in accordance with the water gas equilibrium
which increases further in the conversion stage, by means of the carbon monoxide conversion, to a concentration of approximately 16 vol.-% (dry), in general. This amount of carbon dioxide gets into the atmosphere by way of the chimney of the combustion chamber, together with the carbon dioxide produced by the firing of additional fuels that contain carbon. The CO2 content in the flue gas generally lies above 20 vol.-% (dry). In a refinery, a hydrogen plant designed in this manner therefore represents one of the major carbon dioxide emitters.
A method for extracting hydrogen is known from U.S. Pat. No. 4,553,981, in which a gas that contains hydrocarbon is reformed with steam and converted. In a scrubber, a CO2 waste gas stream is then separated from the converted gas stream. Subsequently, isolation of hydrogen takes place using a pressure swing adsorption system. The waste gas stream of the adsorption system is compressed and passed back into the reformation and/or conversion. This results in great circulating streams. To avoid an accumulation of inert gases, such as nitrogen, a purge stream must be removed from the waste gas stream of the pressure swing adsorption system. Firing of the reformer takes place in conventional manner. The method is complicated and expensive.
SUMMARY OF THE INVENTION
The invention is based on the task of indicating a simple and inexpensive method for extracting hydrogen from a gas that contains methane, particularly natural gas, in which only small amounts of carbon dioxide are given off into the environment.
The object of the invention and the solution for this task is a method for extracting hydrogen from a gas that contains methane, particularly natural gas. Hydrocarbons contained in the gas are catalytically split into hydrogen, carbon monoxide, and carbon dioxide, in a reformer, by means of steam, and in a subsequent conversion stage, catalytic conversion of the carbon monoxides that have been formed to carbon dioxide and hydrogen takes place, with steam. The carbon dioxide is removed from the converted gas stream by means of gas scrubbing, and the scrubbed, hydrogen-rich gas stream is subsequently separated into a product gas stream that consists of hydrogen, and a waste gas stream, in a pressure swing adsorption system. The waste gas stream is passed to the reformer, together with hydrogen that is branched off from the gas stream behind the gas scrubber, as a fuel gas that is extensively free of carbon, and combusted there.
While almost complete splitting of the hydrocarbons into hydrogen, carbon monoxide, and carbon dioxide takes place in the reformer, the carbon monoxide that has formed is subsequently converted to carbon dioxide in the conversion stage, and the latter is removed in the subsequent gas scrubber. The waste gas of the pressure swing adsorption system therefore contains essentially hydrogen, and only small remaining amounts of carbon. The same holds true for the hydrogen that is branched off from the gas stream behind the gas scrubber. During the joint combustion of these two gas streams in the reformer, a waste gas that consists predominantly of nitrogen and water is therefore formed, while the carbon dioxide content is low. Because of the gas recirculation, additional firing of the reformer with fuels that contain carbon is eliminated, so that the carbon dioxide emissions are clearly reduced. In comparison with conventional methods, the carbon dioxide emissions can be reduced by approximately 75%. The process technology steps that are used within the framework of the teaching according to the invention are, without exception, proven technologies that have already been used successfully in hydrogen production for a long time. The effort and expense required to achieve the carbon dioxide reduction described are comparatively slight. The possibility therefore also exists to retrofit an existing, conventional hydrogen plant, in order to operate the method according to the invention with this system.
Preferably, a conversion reactor that is operated at medium temperature, or a high-temperature conversion reactor with a subsequent low-temperature conversion reactor are used for the conversion stage. In this way, almost complete conversion of the carbon monoxide that has been formed to carbon dioxide is guaranteed, where the latter can subsequently be removed from the gas stream by way of the gas scrubber. When using a subsequent low-temperature conversion reactor, there is the advantage that the high-temperature conversion reactor of an existing hydrogen plant can continue to be used, thereby clearly lowering the retrofit costs for an existing plant.
Preferably, technically pure carbon dioxide is separated in the gas scrubber, which is used for technical applications or processed further to produce a product having a quality that makes it suitable for use in the foods industry. In addition to use as a material for the foods industry, another possible use of the technically pure carbon dioxide is, for example, filling a petroleum bore as a measure for more efficient petroleum recovery. Alternatively, the carbon dioxide can also be used as a raw material for methanol synthesis. In this connection, the carbon dioxide scrubber can be operated with known physical methods, such as Rectisol, Selexol, or Genosorb, or instead, with a chemical or physical/chemical method, e.g. aMDEA (aqueous solution of N-methyl diethanolamine) or sulfinol.
If an existing H2 plant is retrofitted for the purpose of minimizing CO2, it is practical to compress the converted gas stream before it enters into the newly built CO2 scrubber, in order to equalize the pressure loss that results from this. In this way, the effectiveness of the CO2 scrubber is increased.
The object of the invention is also a system for carrying out the method.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.
In the drawings, wherein similar reference characters denote similar elements throughout the several views:
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The method steps described, which are used within the scope of the teaching according to the invention, are all technically mature technologies that have proven themselves both in the production of hydrogen and in the production of ammonia. The reformer 4 merely has to be sized sufficiently large to guarantee the H2 production, including the fuel gas supply after the CO2 scrubbing. The conversion reactor 5 is operated at medium temperature, in order to ensure almost complete conversion of the carbon monoxide that has formed to carbon dioxide. The carbon dioxide 21 that is obtained by means of the purification step 9 in the exemplary embodiment can be processed further in the foods industry. Alternatively to this, however, there is the possibility of utilizing the technically pure carbon dioxide 18 that was extracted in the scrubber 7 directly for technical applications. Possibilities here are filling a petroleum bore as a measure for efficient petroleum transport, or use as a raw material for methanol synthesis.
The effort and expense for carrying out the method described are relatively slight. In particular, there is the possibility of retrofitting an existing, conventional hydrogen plant in such a manner that the method according to the invention can be operated with it.
Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.
1. A system for extracting hydrogen from a gas that contains methane comprising:
- at least one reformer having a combustion chamber for catalytic splitting of gaseous hydrocarbons with steam;
- a conversion stage having at least one conversion reactor for catalytic conversion of carbon monoxide to carbon dioxide and hydrogen, with steam;
- a gas scrubber for separating carbon dioxide from the gas stream that leaves the conversion stage; and
- a subsequent pressure swing absorption system for isolating hydrogen, to which a gas line that is passed back to the combustion chamber is connected, for firing the reformer with a gas stream that exits from the adsorption system,
- wherein an additional device is provided for passing back part of the hydrogen-rich gas stream that leaves the gas scrubber into the combustion chamber of the reformer.
2. The system according to claim 1, wherein the conversion stage comprises a conversion reactor operating at temperatures between 360 and 500° C. and a subsequent conversion reactor operating at temperatures between 210 and 270° C.
3. The system according to claim 1, further comprising a purification stage for concentrating the separated carbon dioxide after the carbon dioxide exits the gas scrubber.
International Classification: B01J 8/00 (20060101);