Patent Application
20250115480
This disclosure relates to methods of producing hydrogen from natural gas using a reforming process that recycles heat to decrease the amount of carbon released.
Hydrogen is used in many commercial processes, including fertilizer production and oil refining. Currently, most hydrogen used is produced from fossil fuels, which emit large amounts of carbon into the atmosphere. While green hydrogen can be produced directly from the electrolysis of water, the low efficiency of the process and the limits on renewable energy, limit the amount produced through this technique.
Reforming processes can be used to generate hydrogen for the hydrogen economy. However, the processes are generally provided with the heat used for the endothermic reforming reaction by burning natural gas or other hydrocarbons to generate the heat. This results in significant amounts of carbon emissions.
An embodiment described herein provides a method for producing hydrogen. The method includes desulfurizing a natural gas stream to form a sweet gas stream, converting higher hydrocarbons in the sweet gas stream to methane to form a methane stream, converting a portion of the methane in the methane stream to a syngas stream in a membrane reformer, and separate a portion of hydrogen from the syngas stream as a permeate stream from the membrane reformer. The retentate stream from the membrane reformer is fed to an autothermal reformer to form an oxidized stream. The membrane reformer is heated with the oxidizer stream.
Another embodiment described herein provides a system for producing hydrogen from natural gas while recovering heat energy. The system includes a desulfurizer reactor coupled to a natural gas feed, a pre-reformer coupled to an effluent from the desulfurizer, and a membrane reformer coupled to an effluent from the prereformer, wherein a permeate outlet from the membrane reformer removes a hydrogen stream from the membrane reformer. An autothermal reactor (ATR) is coupled to a retentate outlet from the membrane reformer.
Various embodiments described herein provide integrated autothermal and heat-exchanger reformer systems, and a method of using the systems to make low carbon hydrogen. The systems utilize a membrane-based hydrogen separation in conjunction with both steam methane reforming reactions in a heat-exchanger reformer and autothermal reforming reactions. The waste heat from the exit stream of the autothermal reformer provides the heat input required by the endothermic reaction in the heat-exchanger reformer. Thus, these systems utilize the waste heat generated in an autothermal reactor, lowering the carbon footprint of the processes.
Further, the systems integrate hydrogen selective membranes that include palladium and other metals into the thermo-neutral reforming process. In various embodiments, the hydrogen selective membranes are used for separating H2 and CO2, in a membrane water-gas shift reactor, or as a membrane reformer, or combinations thereof. The use of the hydrogen selective membrane increases the efficiency of the process for low carbon hydrogen production.
The method begins at block 102, when a natural gas stream is desulfurized to form a sweet gas stream. The compressed hydrocarbon feed may be fed to a sulfur-removal unit (hydrodesulfurization unit) to remove sulfur compounds. Sulfur compounds can be poisonous to the catalysts used in the pre-reformer or the reformer. Hydrogen is fed to the sulfur removal unit to hydrogenate the sulfur compounds to remove the sulfur from the hydrocarbon feed. Typically, the sulfur removal unit operates, for example, at temperatures between about 250° C. and about 450° C. and pressures between about 1 bar and about 50 bar or between about 20 bar and about 40 bar. The sulfur-free hydrocarbon feed, for example, less than 1 ppm sulfur, leaves the sulfur removal unit.
The hydrodesulfurization unit may include a catalytic reactor, such as a fixed-bed reactor that is a reactor vessel having a fixed bed of catalyst. In operation, the fixed-bed reactor may convert sulfur compounds in the hydrocarbon feed to H2S for ease of removal. In implementations, the fixed-bed reactor may be characterized as a hydrotreater that performs hydrogenation. In operation for some implementations, the hydrocarbon feed may be pre-heated, for example, in a heat exchanger, and fed to the fixed-bed reactor. Hydrogen is also fed to the fixed-bed reactor for the hydrodesulfurization as a hydrogenation reaction. The source of the hydrogen can be the membrane water gas shift reactor. The catalyst in the fixed bed may be hydrodesulfurization catalyst. For example, the hydrodesulfurization catalyst may be molybdenum disulfide (MoS) or tungsten. The catalyst may be based on MoS supported on γ-alumina. The catalyst may be a cobalt-modified MoS. The hydrodesulfurization catalyst may have an alumina base impregnated with cobalt and molybdenum, generally termed a CoMo catalyst.
The hydrodesulfurization reaction occurs in presence of the catalyst in the fixed-bed reactor at a temperature for example, in the range of about 300° C. to about 400° C. and a pressure, for example, in the range of about 30 bar to about 130 bar. As mentioned, the hydrodesulfurization reaction in the fixed-bed reactor may be a hydrogenation reaction, i.e., giving addition of hydrogen (H). In particular, the type of hydrogenation reaction is hydrogenolysis that cleaves the C—S bond and forms C—H and H—S bonds. The hydrodesulfurization (hydrogenation) reaction with the example of propanethiol (C3H7SH) as a sulfur impurity in the hydrocarbon feed is as follows: C3H7SH+H2→C3H8+H2S.
The fixed-bed reactor may additionally include a bed (e.g., packed bed) of absorbent (e.g., zinc oxide or ZnO) to remove (absorb) the H2S from the hydrocarbon (e.g., naphtha). The H2S removed from the hydrocarbon via capture of the H2S into the absorbent may include the H2S formed in the hydrodesulfurization conversion of sulfur compounds and also the H2S that entered the fixed-bed reactor in the hydrocarbon feed. The fixed bed reactor may discharge the hydrocarbon, for example, having less than less than 1 ppm sulfur. In some implementations, the absorbent is not in the fixed-bed reactor but instead in a second vessel that receives the hydrocarbon having the H2S from the fixed-bed reactor. Thus, in those implementations, the second vessel discharges the hydrocarbon 214 having, for example, less than 1 ppm sulfur. In either configuration, the ZnO bed that captures the H2S may be replaced with a fresh ZnO bed including over the maintenance cycle.
At block 104, higher hydrocarbons in the sweet gas stream are converted to methane in a pre-reforming reactor to form a methane stream. For example, the higher hydrocarbons can include ethane, propane, butane, pentane, hexane, naphtha, liquid petroleum gas (LPG), natural gas (NG), and higher hydrocarbons. Further, the higher hydrocarbons can include any isomers of these compounds, including branched compounds and compounds with double or triple bonds, such as ethylene, acetylene, propylene, butane, and the like. The conversion is performed by steam reforming the sweet gas stream under relatively mild conditions, for example, the inlet stream of the pre-reformer is maintained at 450° C. and 34 bar.
The pre-reformer is typically fed with steam to crack, in the presence of pre-reforming catalyst, the long hydrocarbon molecules into methane. Different catalysts are developed to pre-reform different types of hydrocarbon feeds. The pre-reformer may operate between about 300° C. and about 650° C., or between about 400° C. and about 600° C., and between about 8 bar and about 50 bar, or between about 10 bar and about 40 bar.
The pre-reformer may be a vessel having a pre-reforming catalyst to convert higher molecular-weight hydrocarbons to methane. A feed conduit may flow the feed hydrocarbons to the pre-reformer. A steam conduit may flow steam to the pre-reformer. In implementations, the steam conduit may introduce the steam into the hydrocarbons flowing in the feed conduit to the pre-reformer.
The hydrocarbons fed to the pre-reformer may be liquid hydrocarbons, e.g., with a final boiling point of at least about 630 K. The hydrocarbons may be condensates from natural gas stream (C5-C6 hydrocarbons), liquefied petroleum gas (LPG), naphtha, kerosene, diesel, or other refined petroleum products. The catalyst in the pre-reformer may be a bed (e.g., packed bed) of pre-reforming catalyst. The catalyst in the pre-reformer may be a nickel-based catalyst, noble-metal based catalyst, transition-metal based catalyst, and the like. In operation, the hydrocarbons and steam react in presence of the pre-reforming catalyst to generate methane. The reaction in the pre-reformer may generate reformate including primarily methane. As discussed, the operating temperature in the pre-reformer may be, for example, in the range of about 500° C. to about 600° C. In embodiments, electrical heaters (e.g., resistive heaters) may be dispose in or on the pre-reformer vessel to provide heat for the reaction. On the other hand, the pre-reformer vessel may be insulated (thermal insulation) without electrical heaters. The pre-reforming reaction may operate in adiabatic mode under targeted operating conditions generally not utilized additional heat other than heating the feed to input temperatures and providing sufficient thermal insulation to avoid heat loses. The operating pressure in the pre-reformer may be, for example, in the range of about 10 bar to about 50 bar
At block 106 a portion of the methane in the methane stream is converted to a syngas, e.g., hydrogen and carbon monoxide, in a membrane reformer. This forms a mixed syngas/methane stream.
At block 108, a portion of the hydrogen is separated from the syngas through a hydrogen separation membrane in the membrane reformer and exits as a permeate stream. The heat for the membrane reformer is provided by an autothermal reformer.
At block 110, a retentate stream from the membrane reformer is fed to an autothermal reformer. The retentate stream is reacted with an oxygen stream to form an oxidized stream. At block 112, the oxidized stream is passed through a heat exchanger in the membrane reformer to heat the membrane reformer to operating temperatures.
Further, in addition to providing the heat from the ATR, the oxidized stream from ATR undergoes a water-gas shift conversion in which CO and H2O are converted to CO2 and H2. In this second pass, a substantially complete conversion of CO or carbon to CO2 is achieved, while produced hydrogen is separated. This results in product streams of nearly pure H2, and a CO2 product stream that includes small amounts of unconverted CO, CH4 or unrecovered H2.
Not all of the steps listed are required in every embodiment. Further, additional steps may be included, for example, for hydrogen purification.
After desulfurization, steam 216 is injected into the sweet gas stream 214, and the mixed stream is passed through a heater 218, before being sent to a pre-reformer 220. As discussed herein, the pre-reformer 220 converts higher hydrocarbons, such as propane, butane, pentane, hexane, contained in the sweet gas stream 214 to methane.
The methane stream 222 from the pre-reformer 220 is passed through a heat exchanger 224 where it is heated by a syngas stream from the reforming process 226. The heated stream is passed to a heat-exchanger reformer (HER) 228 where the steam methane reforming reaction occurs. The methane/syngas stream 230 exiting the HER 228 is sent to an autothermal reformer (ATR) 232. An oxidizer stream 234, such as oxygen, is also fed to the ATR 232. The ATR 232 performs two reactions, a further steam methane reforming reaction and an exothermic partial oxidation reaction. In the ATR 232, the unreacted methane in the methane/syngas stream 230 from the HER 228 participates in these reactions to produce hydrogen and generate thermal energy in the partial oxidation reaction.
The syngas stream 236 exiting the ATR 232 is sent through a heat exchanger in the HER 228 to provide the heat for the endothermic steam-methane reforming reaction. In addition, after leaving the HER 228, the hot gaseous mixture is also passed through the heat exchanger 224 to pre-heat the methane stream 222 for the HER 228.
The syngas stream 236 is sent to the water gas shift (WGS) reactor 238 where the carbon monoxide in the syngas stream 236 reacts with water to be converted to hydrogen and carbon dioxide, forming a raw hydrogen stream 240. The raw hydrogen stream 240 is fed to a membrane separator 242 that separates the hydrogen 202 from carbon dioxide 244 and other gases.
The WGS reactor 238 and the membrane separator 242 form a hydrogen formation and separation system 246. In some embodiments, the hydrogen formation and separation system 246 are incorporated into a single reactor, as discussed further with respect to
The membrane used in the membrane-HTWGS 402 has a bore or lumen. The bore is the permeate side of the tubular membrane. The membrane material may be, for example, palladium (Pd) or Pd alloy. In some embodiments, the membrane material, or wall, of the tubular membrane is thin, such as less than about 10 μm, or between about 2 μm and about 4 μm.
The membrane may be formed on a tubular support, such as a porous ceramic, with a hydrogen-selective membrane material disposed on the tubular support. Thus, the wall of the tubular membrane includes the tubular support and the membrane material. The membrane material of the tubular membrane may be, for example, palladium or palladium alloy. In various embodiments, the palladium alloy includes a palladium-platinum (Pd—Pt) alloy, a palladium-gold (Pd—Au) alloy, a palladium-ruthenium (Pd—Ru) alloy, or tertiary alloys of these elements, Pt, Au, or Ru with palladium. In some embodiments, the membrane material has a thickness of greater than about 2 microns or greater than about 3 microns, or in a range of between about 2 and about 20 microns, between about 3 and about 10 microns, or between about 3 and about 6 microns. The thickness of the membrane material may be less than about 30 microns, less than about 20 microns, or less than about 10 microns. As indicated, the membrane material may be disposed (e.g., deposited) on a tubular substrate such as a dense or porous tubular support that is ceramic or metallic with a ceramic interlayer.
Accordingly, this configuration allows simultaneous steam methane reforming reaction and membrane-based hydrogen separation. The unreacted reactants leaving the membrane HER 602 are sent to the ATR 232 where both exothermic partial oxidation and endothermic steam methane reforming take place. The syngas stream 236 from the ATR 232 provides thermal energy to the membrane HER 602. As hydrogen 202 is separated by a hydrogen permeable membrane and the membrane HER 602, a pure stream of hydrogen 202 is obtained from the membrane HER 602 at the permeate side. The retentate side of the reactor primarily includes carbon dioxide 244, mixed with unreacted steam and traces of carbon monoxide.
Lab scale testing was completed using developed membranes and a high temperature water gas shift (WGS) catalyst, as described with respect to configuration 2 described with respect to
The nitrogen leak rate is shown in
As shown by these results, the technical problem of providing thermal energy for endothermic steam-methane reforming without utilizing carbon-based fuels is solved through the integration of the heat-exchanger reformer with an autothermal reformer. The unreacted reactants leaving the HER firstly react in the autothermal reformer producing more hydrogen and the waste heat entailed in the exit stream of the autothermal reformer is utilized for the endothermic gas heated reformer. In addition, the membrane-based configurations developed in this invention also provide different methods for hydrogen separation. These include the in-situ separation of hydrogen in a membrane high temperature water gas shift reactor or in-situ hydrogen separation in a membrane reformer. Both configurations provide higher reaction rates and higher conversions while utilizing the waste heat entailed in the output stream of an integrated autothermal reformer. The input reactant stream of the autothermal reformer comprises the output product stream of the heat-exchanger reformer. Hence, this also allows the unreacted reactants leaving the gas heated reformer to react in the autothermal reformer to produce more hydrogen.
The systems developed in the present invention solve several technical problems associated with conventional steam methane reforming. Conventional HERs utilize natural gas to generate the required thermal energy input which results in significant carbon emissions that are environmentally detrimental. The present invention disclosure develops membrane-based integrated gas heated and autothermal reformers. The waste heat entailed in the output stream of the autothermal reformer is utilized to operate the endothermic heat-exchanger reformer. In addition, the system configurations developed also include membrane-based hydrogen separation. Conventionally, pressure swing adsorption (PSA)-based hydrogen separation techniques are utilized to separate hydrogen from the product gas mixtures. However, system configurations 2 and 3 in the present invention include in-situ hydrogen separation via selective hydrogen permeable membranes. The system configuration 2 includes in-situ hydrogen separation during a water gas shift reaction while the system configuration 3 includes in-situ hydrogen separation in a membrane reformer. These configurations provide a pure stream of hydrogen, higher reaction rates, and higher conversions (based on the Le Chatelier's principle) which aid in eliminating the need of utilizing a PSA-based hydrogen separation system. Thus, leading to a more intensified and efficient process.
An embodiment described herein provides a method for producing hydrogen. The method includes desulfurizing a natural gas stream to form a sweet gas stream, converting higher hydrocarbons in the sweet gas stream to methane to form a methane stream, converting a portion of the methane in the methane stream to a syngas stream in a membrane reformer, and separating a portion of hydrogen from the syngas stream as a permeate stream from the membrane reformer. The retentate stream from the membrane reformer is fed to an autothermal reformer to form an oxidized stream. The membrane reformer is heated with the oxidizer stream.
In an aspect, combinable with any other aspect, desulfurizing the natural gas stream includes passing the natural gas stream through a hydrodesulfurization reactor.
In an aspect, combinable with any other aspect, the higher hydrocarbons include ethane, propane, butane, pentane, hexane, or any isomer thereof, or any combination thereof.
In an aspect, combinable with any other aspect, converting the higher hydrocarbons to methane includes passing the sweet gas stream over a nickel catalyst in a pre-reforming reactor.
In an aspect, combinable with any other aspect, converting a portion of the methane in the methane stream to a syngas stream includes performing a steam reforming reaction in the membrane reformer.
In an aspect, combinable with any other aspect, the retentate stream from the membrane reformer is reacted with oxygen to form the oxidized stream.
In an aspect, combinable with any other aspect, the oxidized stream is converted to syngas in the membrane reformer.
In an aspect, combinable with any other aspect, further hydrogen is separated from the syngas in the permeate stream.
Another embodiment described herein provides a system for producing hydrogen from natural gas while recovering heat energy. The system includes a desulfurizer reactor coupled to a natural gas feed, a pre-reformer coupled to an effluent from the desulfurizer, and a membrane reformer coupled to an effluent from the prereformer, wherein a permeate outlet from the membrane reformer removes a hydrogen stream from the membrane reformer. An autothermal reactor (ATR) is coupled to a retentate outlet from the membrane reformer.
In an aspect, combinable with any other aspect, an oxidized stream from the ATR passes through a heat exchanger in the membrane reformer. In an aspect, the oxidized stream passes through the retentate side of the membrane reformer.
In an aspect, combinable with any other aspect, the desulfurizer includes a hydrogen feed.
In an aspect, combinable with any other aspect, the desulfurizer includes a hydrodesulfurization catalyst.
In an aspect, combinable with any other aspect, the pre-reformer includes a nickel catalyst.
In an aspect, combinable with any other aspect, the membrane reformer is a steam reforming reactor configured to use the ATR as a heat source and wherein the membrane reformer includes a hydrogen selective membrane. In an aspect, the membrane reformer includes a hydrogen selective membrane including palladium.
In an aspect, combinable with any other aspect, the ATR includes an oxygen feed.
In an aspect, combinable with any other aspect, the system includes an outlet stream of substantially pure hydrogen.
In an aspect, combinable with any other aspect, the system includes an outlet stream of carbon dioxide including carbon monoxide, steam, and hydrogen.
Other implementations are also within the scope of the following claims.
