PATENT
Patent number: 7931888
Issue date: Apr 26, 2011
Application number: 12/234,781
Inventors: Raymond Francis Drnevich, Ramchandra M. Watwe
Original Assignee: Praxair Technology, Inc.
FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
Hydrogen has many current industrial uses and potential 20 future uses involving the supply of energy in fuel cells. For example, there presently exists a continuing need for hydrogen to treat high sulfur content crude oil in the production of fuels. In the future, hydrogen may be used as an automotive fuel or more generally, in the generation of electricity. 25
Hydrogen is currently produced in steam methane reforming installations. In such installation, a hydrocarbon containing feed, typically natural gas, is combined with steam and then introduced into reformer tubes located within a fired furnace of the reformer. The reformer tubes contain a catalyst 30 to catalyze the well known, endothermic steam methane reforming reaction in which methane and steam are reacted to form carbon monoxide and hydrogen. In addition, an exothermic water gas shift reaction occurs in which carbon monoxide and steam are reacted to form carbon dioxide and additional 35 hydrogen. Typically, the hydrocarbon containing feed is natural gas and part of such feed is combined with steam and introduced into the reformer tubes and another part of the feed is fed to burners firing into the furnace section to support the steam methane reforming reaction. In typical steam methane 40 reformers, the steam to carbon molar ratio is set at about 2.8.
The heated product stream of the steam methane reforming reaction is cooled and subjected to a high temperature shift reaction to react the carbon monoxide with residual steam to produce additional hydrogen. The resulting shifted stream is 45 then introduced into a pressure swing adsorption unit in which the hydrogen is separated to form a product stream and a stream of tail gas is produced that can be introduced into the burners to help fire the furnace section of the steam methane reformer. 50
Steam methane reformers also have a convective section connected to the furnace section in which flue gas is routed to heat boiler feed water and to produce steam. The boiler feed water after deaeration and heating to near its boiling temperature is then introduced into a steam drum. Water from the 55 steam drum is partially vaporized in the boiler and returned to the steam drum as low quality steam. Steam from the steam drum is introduced into a superheater in the convective section to form superheated steam. The superheated steam is combined with the hydrocarbon feed to produce the reactant 60 stream for the steam methane reformer and part of the superheated steam can be advantageously exported at a profit. The flue gas is discharged from the convective section through a stack.
Steam methane reformers can utilize a variety of feed 65 stocks, for instance, refinery off-gases, natural gas, butane, light naphtha and naphtha. All of these are hydrocarbon containing feeds. In Broadhurst et al., "Effects of Hydrocarbon Feed Type on Operating Costs and Environmental Impact on a Steam Reforming Based Hydrogen Plant", AICHE (2005), various feeds were tested against one another by simulation at a steam to carbon ratio of 3.0 and a reformer exit temperature of 880° C. The feed rates were allowed to vary so that a target hydrogen output of 100 MMSCFD was produced. From the simulations, it was concluded that the environmental impact increases in terms of higher carbon dioxide emissions and lower process efficiency as the feed stock becomes heavier. This being said, it was surmised in this reference that the selection of the feed stock will be dominated by feed stock/ fuel costs.
Another method of producing hydrogen is gasification in which a carbonaceous material such as coal, petroleum or biomass is converted into a synthesis gas that contains hydrogen and carbon monoxide. The carbonaceous material is reacted at high temperatures with oxygen addition within a gasifier to produce the synthesis gas. For example, in one type of gasifier that is used in the gasification of coal, the coal is pulverized and fed into the gasifier. Other types of gasifiers utilize a coal slurry. Within the gasifier, the coal is heated and volatiles are released creating a char. The volatile product and some of the char is reacted with the oxygen to form carbon dioxide and carbon monoxide. The char also reacts with the carbon dioxide and steam to produce the carbon monoxide and hydrogen. In addition, carbon monoxide and steam also react in water gas shift reactions to produce carbon dioxide and additional hydrogen. The resulting hydrogen and carbon monoxide containing synthesis gas can be processed and hydrogen can be separated from the synthesis gas by pressure swing adsorption.
In Gray et al. "Polygeneration of SNG, Hydrogen, Power, and Carbon Dioxide from Texas Lignite", Mitretek Systems (2004), the gasification of lignite is discussed in connection with the production of electric power, hydrogen, synthetic natural gas and carbon dioxide. In this reference, the purpose of such gasification is to allow the electric power to be sold to the grid, the hydrogen to be sold to a pipeline for use in oil refining operations and the synthetic natural gas to be sold as a natural gas supplement or to replace natural gas in steam methane reforming operations, thereby to provide hydrogen for the refining operations. The carbon dioxide that is generated by the gasification can be sequestered or used for enhanced oil recovery.
In one plant configuration shown in Gray, the lignite is gasified to produce a synthesis gas. The synthesis gas is subjected to water gas shift reactions to increase the hydrogen. After removal of mercury, sulfur and carbon dioxide, the shifted stream is then passed into a sulfur guard bed and then into a methanation unit to produce synthetic natural gas. Sulfur can be extracted in a Claus unit for sulfur recovery. The carbon dioxide can be compressed to 2,000 psi and fed to a pipeline. To protect the methanation catalyst, the purified and shifted syngas is sent to a sulfur polishing reactor to remove the last traces of hydrogen sulphide before being sent to the methanation reactor. In the methanation reactor the carbon dioxide and hydrogen are reacted to produce methane. The resulting synthetic natural gas is compressed for delivery to a natural gas pipeline. Some of the synthesis gas can be sent to a gas turbine where electric power is generated. The hot effluent from the gas turbine can be used in a heat recovery steam generatorto generate highpressure steam that isused in a steam turbine to generate additional electrical power.
In another plant design that is shown in Gray, after carbon dioxide removal, the purified and shifted syngas is sent to a polymer membrane separation system followed by a pressure swing adsorption unit where hydrogen is removed. The remaining synthesis gas is then sent to a gas turbine for electric power generation. The heated effluent from the turbine is then sent to a heat recovery steam generator to generate high pressure steam for use in a steam turbine to generate 5 additional power.
As is apparent from the above description of the prior art, steam methane reformers can utilize a variety of feeds, natural gas, refinery off-gas, synthetic natural gas and mixtures of synthetic natural gas and natural gas. Obviously, the amount 10 of synthetic natural gas utilized as a feed to a steam methane reformer will depend upon such economic factors as the price of natural gas.
As will be discussed, the present invention provides a closer integration between a gasifier and a steam methane 15 reformer then has been contemplated in the prior art. Among other advantages, a method in accordance with the present invention allows the hydrogen to be produced in a steam methane reformer with the use of less natural gas or synthetic natural gas than is possible in the prior art. Thus, the present 20 invention permits use of natural gas or production of synthetic natural gas to be more widely varied in response to external economic conditions than is possible in the prior art. As will be discussed, yet further advantages of the present invention concern the possibility of simplifying the construction of the 25 gasification facility and providing for a more reliable hydrogen supply than is currently possible with the use of a gasification facility alone.
SUMMARY OF THE INVENTION 30
The present invention provides a method of producing a hydrogen product stream. In accordance with the method, a steam stream is combined with a hydrocarbon containing stream to produce a reactant stream. Hydrocarbons are 35 reacted with steam contained in the reactant stream in a steam methane reformer to produce an intermediate product stream. The hydrocarbon stream is alternately formed from a first feed stream and a second feed stream without substantially changing flow rate of the steam stream and while maintaining 40 reformer exit temperatures at both the reactant and flue gas side essentially constant and flow rate of the flue gas essentially constant. The first stream is made up of at least one of natural gas, refinery off-gas, naphtha and synthetic natural gas. The second feed stream is made up of a hydrogen and 45 carbon monoxide containing gas and at least one of natural gas, the refinery off-gas, naphtha and the synthetic natural gas. A part of a synthesis gas stream, formed by a gasifier, is utilized in production of the hydrogen and carbon monoxide containing gas. The intermediate product stream produced by 50 the steam methane reformer is subjected to a water gas shift reaction to produce a shifted stream. The hydrogen is separated from the shifted stream in a pressure swing adsorption unit to produce the hydrogen product stream.
It is appropriate here to point out, that by maintaining the 55 reformer exit temperature at the reactant side, in other words, the temperature of the intermediate product stream upon directly leaving reformer tubes essentially constant, methane slip is controlled to maximize hydrogen production. The maintenance of the flue gas side temperature essentially con- 60 stant, or in other words, the flue gas temperature before reaching the convective section of the steam methane reformer essentially constant, coupled with the maintenance of the flue gas flow rate essentially constant, the amount of steam generated in the convective section of the steam methane 65 reformer can be maintained at an essentially constant flow rate. And be maintaining the steam flow rate essentially constant, the steam to carbon ratio will increase with the use of the hydrogen and carbon monoxide containing stream in the second feed stream to allow the increased carbon monoxide content of the feed to be subjected to the water gas shift reaction to an increased extent within the steam methane reformer and to provide steam to the water gas shift reaction occurring in the shift conversion unit used in the processing of the intermediate product stream. The end result of such control is to allow the same amount of hydrogen to be produced when the feed to the steam methane reformer is switched to the second feed stream.
As is apparent from the above description, the use of part of the synthesis gas to form a hydrogen and carbon monoxide containing stream that can serve as part of the feed to a steam methane reformer has the obvious advantage of not using the same amount of natural gas or synthetic natural gas or refinery off-gas in the production of the hydrogen. As a result, there can be less utilization of these gases to not only produce a cost savings but to allow the synthetic natural gas to be available for sale. A yet further savings is brought about due to the fact that the carbon monoxide content of the feed to the steam methane reformer when synthesis gas is used will allow the exothermic water gas shift reaction to occur at a greater extent. The heat produced by such reaction as well as the reduction in hydrocarbon feed rate to the reformer will reduce the amount of fuel that would otherwise be required to fire burners utilized within the furnace section of the steam methane reformer. Such fuel is provided by part of the feed to be reacted that can contain natural gas refinery off-gas, naphtha or synthetic natural gas and combinations thereof.
Additionally, as mentioned above, the present invention allows for certain economic advantages to be realized in the construction of the gasification facility 2 and for the hydrogen to be supplied on a more reliable basis. If hydrogen production were to be made part of the gasification project, then the gasification facility would require a second shift conversion and a pressure swing adsorption unit as well as associated heat exchangers, piping and etc. Although the use of a hydrogen and carbon monoxide containing stream from a unit designed to produce hydrogen is contemplated by the present invention, the present invention certainly allows for a simplified and less expensive gasification train within the gasification facility. Additionally, a gasifier has between about a 85 percent and about 90 percent on stream factor while a steam methane reformer has about a 98 percent on-stream factor. The steam methane reformer can be switched between the two types of feed streams thereby increasing the reliability of the hydrogen supply. This is important where a steam methane forming facility exists and the gasifiers are being built at a later time. Other advantages will become apparent from the discussion that will follow below.
The part of a synthesis gas stream, formed by the gasifier can also be utilized in production of the synthetic natural gas. However, as will be discussed, the gasifier facility might also use an excess synthesis gas production capacity to generate electrical power.
In a specific embodiment where the gasifier facility also produces synthetic natural gas, the water gas shift reaction that is used in connection with the intermediate product stream produced by the steam methane reformer can be a second water gas shift reaction. The synthesis gas stream can be utilized to produce synthetic natural gas and the hydrogen and carbon monoxide containing gas by subjecting the synthesis gas stream to a first water gas shift reaction to produce the partially shifted gas stream. The partially shifted gas stream can be subjected to acid gas removal to form a purified partially shifted gas stream. The hydrogen and carbon monoxide containing gas can be formed from a portion of the purified partially shifted gas stream and a remaining portion of the purified partially shifted gas stream can be subjected to methanation to produce a synthetic natural gas. In such case, the first feed stream is formed from at least one of a natural gas 5 stream containing the natural gas, a refinery off-gas stream containing the refinery off-gas and a synthetic natural gas stream containing at least part of the synthetic natural gas produced by the methanation. The first feed stream is introduced into a hydrotreater to convert any sulfur species within 10 the first feed stream to hydrogen sulphide and then into an adsorbent bed to adsorb the hydrogen sulphide. The second feed stream is formed by passing the at least one of the natural gas stream, the refinery off-gas stream and the synthetic natu- 15 ral gas stream into the hydro treater and then into an adsorbent bed and combining the portion of the partially shifted gas stream therewith, either prior to or after the adsorbent bed.
In another embodiment, again, the water gas shift reaction that is used in connection with the intermediate product 20 stream produced by the steam methane reformer can be a second water gas shift reaction. The synthesis gas stream is utilized to produce a synthetic natural gas and the hydrogen and the carbon monoxide containing gas is produced by forming the hydrogen and carbon monoxide containing gas stream 25 from part of the synthesis gas stream. A remaining part of the synthesis gas stream is subjected to a first water gas shift reaction to produce a partially shifted gas stream. The partially shifted gas stream is subjected to acid gas removal to form a partially shifted gas stream. The partially shifted gas 30 stream is subjected to methanation to produce a synthetic natural gas. In such case, the first feed stream can be formed from at least one of the natural gas streams containing the natural gas, a refinery off-gas stream containing the refinery 35 off-gas and synthetic natural gas stream containing at least part of the synthetic natural gas produced by the methanation. The first feed stream is introduced into a hydrotreater to convert any sulfur species within the first feed stream to hydrogen sulphide and then into an adsorbent bed to adsorb 40 the hydrogen sulphide. The second feed stream is formed by passing the at least one of the natural gas stream, the refinery off-gas and the synthetic natural gas stream into a hydrotreater and then combining the part of the synthesis gas stream therewith to form a combined stream and then passing 45 the combined stream into the adsorbent bed.
In any embodiment, the hydrogen can be separated from a shifted stream in a pressure swing adsorption unit that also produces a tail gas stream. At least part of the tail gas stream can be used as part of the fuel supplied to burners firing into 50 a furnace section of the steam methane reformer. Another part of the tail gas stream can be used as a fuel for duct burners firing into a convective section of the steam methane reformer.
The flow rate of steam should be sufficient to produce a 55 steam to carbon molar ratio of between about 2.0 and about 4.0 when the first feed stream is used. Preferably, the reformer exit temperature at the flue side is between about 1600° F. and about 2000° F. and at the reactant side is between about 1450° F. and about 1700° F. The partially shifted synthesis gas 60 stream can have a hydrogen to carbon monoxide molar ratio of between about 2.8 and about 3.3.
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