Reducing or removing fouling during thermal hydrocarbon decomposition

Abstract

Operating a hydrocarbon decomposition reactor includes initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, and directly contacting the heating element with the reactant gas in the reactor zone, thereby initiating decomposition of the one or more hydrocarbons in the reactant gas to yield a decomposition product including carbon and a product gas including hydrogen. The reactant gas includes one or more hydrocarbons and particulate matter, and the reactor zone includes a heating element and a reactor wall. The decomposition product deposits on the particles, and the heating element, the reactor wall, or both comprise a ceramic coating that inhibits deposition of the decomposition product.

Description

Attorney Docket No.: 44807-0492WO1 / JHU 18344REDUCING OR REMOVING FOULING DURING THERMAL HYDROCARBON DECOMPOSITIONCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Patent Application No. 63 / 682,023 filed on August 12, 2024, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present invention relates to systems and methods for reducing or removing unwanted carbon deposits, commonly referred to as fouling or coking, on the surface of reactor components in chemical reactors designed to thermally decompose hydrocarbons into their constituent elements of hydrogen and solid carbon.BACKGROUND

[0003] Hydrocarbons (e.g., methane, ethane, propane, ethene, propene, ethylene, propylene, higher alkanes, higher alkenes, and higher alkynes, and polymers such as polyethylene) are molecules that contain only carbon and hydrogen as elemental components. Many fuels that include hydrocarbons create significant quantities of greenhouse gasses (e.g., carbon dioxide) during combustion. Methods and systems such as plasma decomposition, molten bubbler reactors, and thermocatalytic decomposition have been used to split hydrocarbons into solid carbon and hydrogen. The hydrogen can be used as a clean burning fuel, and the solid carbon can be used or sequestered. These methods typically involve heating the hydrocarbon to be decomposed to an elevated temperature (e.g., above 500 °C). As a result, surfaces of some reactor components (e.g., tubes, pipes, reactor chambers, and heat exchangers) are also heated to temperatures of at least 500 °C. Sometimes, these reactor components reach even higher temperatures, as the heat that drives the chemical reaction passes through these components.

[0004] It is generally understood that unwanted carbon can deposit on hot surfaces of hydrocarbon decomposition reactors. This deposition process and the resulting deposit are often referred to as “fouling.” The resulting deposit can also be referred to as a “fouling deposit.” Depending on the size of the reactor and how long the deposit has been allowed to grow, foulingAttorney Docket No.: 44807-0492WO1 / JHU 18344 deposits can be microns to meters in thickness. Removal of fouling deposits can potentially damage reactor components and can also lead to downtime of the reactor.SUMMARY

[0005] This disclosure describes systems and methods designed to reduce fouling or remove fouling deposits from hydrocarbon decomposition reactors. Methods described herein include incorporation of selected components and materials, including selected heating elements and / or insulation materials coated with fouling-resistant ceramics. Examples of suitable heating elements include electric (e.g., resistive) heating elements coated with ceramic materials (e.g., nitrides, oxides, carbides) configured to provide heat directly to the reactor volume without separating the source of heat from the reactor volume through the wall of a reaction tube. In the absence of the reaction tube, the temperature gradient between the heat source and the reactor volume is reduced or minimized, and fouling or fouling deposits can also be reduced or minimized via, for example, a reduced chemical affinity to decompose on the metal nitride surface. For hydrocarbon decomposition reactors described herein, pure hydrocarbons can be run through the reactors in a production mode, or periodic pulses of air or carbon dioxide can be run through the reactors in a self-cleaning mode in which early-stage fouling is burned off. In some examples, small fractions of oxygen, water, carbon dioxide, or any mixture thereof can be included in a hydrocarbon stream to react with carbon deposited on hot surfaces to form volatile species and thus prevent or reduce fouling.

[0006] Although the disclosed inventive concepts include those defined in the attached claims, it should be understood that the inventive concepts can also be defined in accordance with the following embodiments.

[0007] Embodiment 1 is method of operating a hydrocarbon decomposition reactor, the method comprising: initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, wherein the reactant gas comprises one or more hydrocarbons and particulate matter, and the reactor zone comprises a heating element and a reactor wall; and directly contacting the heating element with the reactant gas in the reactor zone, thereby initiating decomposition of the one or more hydrocarbons in the reactant gas to yield a decomposition product comprising carbon and a product gas comprising hydrogen,Attorney Docket No.: 44807-0492WO1 / JHU 18344 wherein the decomposition product deposits on the particulate matter, and the heating element, the reactor wall, or both comprise a ceramic coating that inhibits deposition of the decomposition product.

[0008] Embodiment 2 is the method of embodiment 1, wherein the one or more hydrocarbons comprise methane, ethane, propane, ethene, propene, ethylene, propylene, C4+ alkanes, C4+ alkenes, C4+ alkynes, or a combination thereof.

[0009] Embodiment 3 is the method of any one of embodiments 1-2, wherein the heating element comprises a ferritic iron-chromium-aluminum alloy.

[0010] Embodiment 4 is the method of embodiment 3, wherein the ceramic coating comprises an oxide, a nitride, or a carbide.

[0011] Embodiment 5 is the method of embodiment 4, wherein the ceramic coating comprises an oxide, nitride, or carbide of aluminum, chromium, iron, or silicon.

[0012] Embodiment 6 is the method of embodiment 5, wherein the ceramic coating does not chemically react with air or deteriorate in a reducing environment.

[0013] Embodiment 7 is the method of any one of embodiments 5-6, wherein the ceramic coating is resistant to degradation in atmospheres containing water, carbon dioxide, carbon monoxide, hydrogen, or a combination thereof.

[0014] Embodiment 8 is the method of any one of embodiments 5-7, wherein the ceramic coating is resistant to degradation in the reactant gas.

[0015] Embodiment 9 is the method of any one of embodiments 3-8, wherein the alloy comprises 5 wt% to 6 wt% aluminum, 20 wt% to 24 wt% chromium, and 65 wt% to 75 wt% iron.

[0016] Embodiment 10 is the method of any one of embodiments 3-9, wherein the alloy comprises trace amounts of carbon, silicon, manganese, or a combination thereof.

[0017] Embodiment 11 is the method of any one of embodiments 3-10, wherein the heating element comprises silicon carbide or molybdenum disilicide.

[0018] Embodiment 12 is the method of any one of embodiments 3-11, wherein the heating element is configured as a rod-over-bend, tubular, or porcupine heating element.

[0019] Embodiment 13 is the method of any one of embodiments 3-12, wherein the reactor zone comprises a plurality of heating elements.Attorney Docket No.: 44807-0492WO1 / JHU 18344

[0020] Embodiment 14 is the method of embodiment 13, wherein the plurality of heating elements are distributed throughout the reactor zone to maximize heat transfer to the reactant gas.

[0021] Embodiment 15 is the method of any one of embodiments 1-14, wherein a temperature difference between a surface of the heating element and a reaction temperature required for decomposition of the hydrocarbon in the reactant gas is less than 250 °C or less than 50 °C.

[0022] Embodiment 16 is the method of any one of embodiments 1-15, wherein the heating element is an electric (e.g., a resistive) heating element.

[0023] Embodiment 17 is the method of embodiment 16, further comprising powering the heating element by an external source of electricity.

[0024] Embodiment 18 is the method of embodiment 17, further comprising generating the external source of electricity by providing the hydrogen in the product gas to a generator or fuel cell.

[0025] Embodiment 19 is the method of any one of embodiments 1-18, further comprising terminating the flow of the reactant gas, and initiating a flow of cleaning gas to the reactor zone, wherein the cleaning gas comprises air or carbon dioxide.

[0026] Embodiment 20 is the method of any one of embodiments 1-19, wherein the reactor zone comprises carbon deposit on surfaces of the components or walls of the reactor zone, and the cleaning gas reacts with the carbon deposit to yield an exit stream comprising carbon dioxide, thereby removing at least a portion of the carbon deposit from the surfaces.

[0027] Embodiment 21 is the method of embodiment 20, wherein initiating the flow of cleaning gas occurs when a thickness of the carbon deposit is less than 10 cm.

[0028] Embodiment 22 is the method of any one of embodiments 19-21, further comprising flowing the cleaning gas to the reactor zone until the carbon dioxide content of the exit stream is at least 95 mol% of the carbon dioxide content of the cleaning gas.

[0029] Embodiment 23 is the method of any one of embodiments 19-22, further comprising terminating the flow of cleaning gas, and re-initiating the flow of the reactant gas.

[0030] Embodiment 24 is the method of any one of embodiments 19-23, wherein the heating element provides heat to the reactor zone during the terminating and the initiating.Attorney Docket No.: 44807-0492WO1 / JHU 18344

[0031] Embodiment 25 is the method of any one of embodiments 19-24, further comprising, after terminating the flow of the reactant gas and before initiating the flow of cleaning gas, initiating a flow of inert gas to the reactor zone, thereby flushing the reactant gas from the reactor zone before initiating the flow of cleaning gas.

[0032] Embodiment 26 is the method of embodiment 25, wherein the inert gas comprises argon, neon, xenon, nitrogen, or a combination thereof.

[0033] Embodiment 27 is the method of any one of embodiments 23-26, further comprising, after terminating the flow of cleaning gas and before re-initiating the flow of the reactant gas, initiating a flow of inert gas to the reactor zone, thereby flushing the cleaning gas from the reactor zone before re-initiating the flow of the reactant gas.

[0034] Embodiment 28 is the method of embodiment 27, wherein the inert gas comprises argon, neon, xenon, nitrogen, or a combination thereof.

[0035] Embodiment 29 is the method of any one of embodiments 1-28, wherein the particulate matter comprises aerosolized particles.

[0036] Embodiment 30 is the method of any one of embodiments 1-29, wherein the aerosolized particles comprise an aerosolized hydrocarbon decomposition catalyst.

[0037] Embodiment 31 is a method of reducing fouling in a hydrocarbon decomposition reactor, the method comprising: initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, wherein the reactor zone comprises a heating element and a reactor wall; and directly contacting the heating element and the reactor wall with the reactant gas in the reactor zone, thereby initiating decomposition of the hydrocarbon in the reactant gas to yield carbon and hydrogen, wherein the reactant gas comprises one or more hydrocarbons and an additive comprising water, carbon dioxide, oxygen, air, or any combination thereof.

[0038] Embodiment 32 is the method of embodiment 31, wherein the reactant gas comprises 0.01 vol% to 30 vol% of the additive.

[0039] Embodiment 33 is the method of any one of embodiments 31-32, wherein the heating element comprises a carbon deposit on a surface of the heating element or reactor wall, and the additive reacts with the carbon deposit to yield an exit stream comprising carbon dioxide orAttorney Docket No.: 44807-0492WO1 / JHU 18344 carbon monoxide, thereby removing at least a portion of the carbon deposit from the surface of the heating element.Embodiment 34 is the method of any one of embodiments 31-33, wherein reactant gas comprises 90 vol% to 95 vol% of the one or more hydrocarbons.

[0040] Embodiment 35 is the method of any one of embodiments 31-34, wherein the additive comprises water, the water is in the form of water vapor, and a concentration of the water in the reactant gas is less than 100 mg / cm3.

[0041] Embodiment 36 is the method of any one of embodiments 31-35, wherein the additive comprises carbon dioxide, and a concentration of the carbon dioxide in the reactant gas is 30 vol% or less.

[0042] Embodiment 37 is the method of any one of embodiments 31-36, wherein additive comprises oxygen, and a concentration of the oxygen in the reactor zone is less than 100 ppm by volume.

[0043] Embodiment 38 is the method of any one of embodiments 31-37, wherein the additive comprises air, and a concentration of the air is less than 1000 ppm by volume.

[0044] Embodiment 39 is the method of any one of embodiments 31-38, wherein the heating element is an electric (e.g., a resistive) heating element.

[0045] Embodiment 40 is a hydrocarbon decomposition reactor comprising: a reactor zone; a heating element positioned in the reactor zone; an inlet configured to provide a reactant gas to the reactor zone; and an outlet configured to remove product gas from the reactor zone, wherein a surface of the heating element is coated with a ceramic coating and configured to be in direct contact with the reactant gas in the reactor zone.

[0046] Embodiment 41 is the reactor of embodiment 40, wherein the heating element is an electric (e.g., resistive) heating element.

[0047] Embodiment 42 is the reactor of any one of embodiments 40-41, wherein the heating element comprises a ferritic iron-chromium-aluminum alloy.

[0048] Embodiment 43 is the reactor of any one of embodiments 40-42, wherein the ceramic coating comprises an oxide, a nitride, or a carbide.Attorney Docket No.: 44807-0492WO1 / JHU 18344

[0049] Embodiment 44 is the reactor of any one of embodiments 40-43, wherein the ceramic coating comprises an oxide, a nitride, or a carbide of aluminum, chromium, iron, or silicon.

[0050] Embodiment 45 is the reactor of any one of embodiments 40-44, wherein the heating element comprises silicon carbide or molybdenum disilicide.

[0051] Embodiment 46 is the reactor of any one of embodiments 40-45, wherein the heating element is configured as a rod-over-bend or tubular heating element.

[0052] Embodiment 47 is the reactor of any one of embodiments 40-46, wherein the reactor zone comprises a plurality of heating elements.

[0053] Embodiment 48 is the reactor of embodiment 47, wherein the plurality of heating elements are distributed throughout the reactor zone to maximize heat transfer to the reactant gas.

[0054] The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.BRIEF DESCRIPTION OF DRAWINGS

[0055] FIG. 1 depicts a reactor zone of a hydrocarbon decomposition reactor, with heating elements extending into the reactor zone.

[0056] FIG. 2 depicts heating elements separated from a process gas with a retort or sheath made of or coated with a material resistant to fouling.

[0057] FIG. 3. shows a photograph of a heating element half-coated with aluminum nitride and exposed to natural gas at 1150 °C, showing that carbon has substantially deposited on the uncoated portion of the element.DETAILED DESCRIPTION

[0058] A common reactor configuration in high temperature processing of gaseous hydrocarbons (e.g., thermal cracking, steam reforming of methane) includes reaction tubes compatible with the temperature and chemical environment of the reaction positioned inside a firebox. Heat in the firebox may be supplied by combustion of fuel or by electrical resistive heating elements. During operation, heat generated outside the reaction zone is drawn into theAttorney Docket No.: 44807-0492WO1 / JHU 18344 reaction zone by the creation of a temperature gradient. For the temperature in the interior of the reaction tube to be sufficiently high to run the reaction, the temperature gradient is typically also quite high, sometimes as high as 350 °C. This temperature gradient can result in walls of the reaction vessel that are sometimes hundreds of degrees greater than the temperature of the gas inside the reactor vessel.

[0059] When the reaction of interest is hydrocarbon decomposition (e.g., pyrolysis of methane to solid carbon and gaseous hydrogen), solid carbon is ideally produced in the gas stream and then carried with the gas stream as a fine powder. A number of strategies to promote carbon production in the gas stream are used, including adding catalysts into the gas stream or adding an aerosol of carbon powder to the gas stream, both of these strategies providing “surfaces” on which carbon in the gas stream can deposit and be transported out of the reactor. In practice, however, some fraction of the solid carbon product is deposited on the hot walls of the reaction vessel. Over time, this layer of carbon can become thick and interfere with reactor function, for example, by affecting gas flow (e.g., by reducing the volume of the reactor), by affecting heat transfer into the reactor (e.g., by creating an effectively thicker reactor tube wall with different thermal conductivity), or both. In addition, the carbon deposited on hot surfaces may be of a different kind (morphology and microstructure) than the carbon produced in the gas stream; for instance, a dense graphitic carbon may deposit on (and then flake off) the reactor walls while the desired product may be an amorphous sooty carbon. Mixing of these two types of carbon may lead to production of an inhomogeneous batch of carbon. Such unwanted deposition of carbon on reactor wall components is referred to as “fouling” or “fouling deposit.” When the carbon deposit is thick enough, “fouling” is sometimes referred to as “coking.” “Fouling” and “coking” are often used interchangeably.

[0060] Fouling is not necessarily limited to the primary reactor volume (or reactor zone) of a hydrocarbon decomposition reaction. Fouling may occur on any hot surface within a hydrocarbon decomposition reactor, including the hot surfaces of heat exchangers, piping, and heating elements. For instance, if the hydrocarbon vapor is pre-heated via a recuperative heat exchanger prior to introduction into the hydrocarbon decomposition reactor, then fouling may occur on the hot surfaces of the heat exchanger, reducing its efficiency.

[0061] This disclosure describes systems, materials of construction, and reactor operations designed to reduce, remove, or inhibit fouling from hydrocarbon decomposition reactors. SomeAttorney Docket No.: 44807-0492WO1 / JHU 18344 of these systems and methods include coating or manufacturing components designed for hot surfaces, such as resistive heating elements or reactor walls, with materials that do not promote hydrocarbon decomposition. As described herein, suitable fouling-resistant surfaces include ceramic materials such as inorganic oxides, carbides, and nitrides (e.g., aluminum oxide, aluminum nitride, silicon oxide, silicon nitride, aluminum nitride, silicon carbide, and combinations thereof.

[0062] FIG. 1 depicts an exemplary reactor hot zone 100 of a hydrocarbon decomposition reactor showing different positions of hot surfaces. Heating elements 102 are distributed throughout reactor hot zone 100, including on an interior surface of reactor zone 100 and on heating elements 104 extending into an interior of reactor hot zone 100, such that reactant gases entering reactor hot zone 100 through inlet 101 directly contact heating elements 102 or the insulating walls of the reactor. In one example, heating elements 104 are bayonet heaters. The interior volume of reactor hot zone 100 is referred to as the “reactor volume.” Outlet 105 is configured to remove product gas from the reactor hot zone 100. In some embodiments, as shown in FIG. 2, spacer 108 may be placed between the heating elements 104 and reactor volume. In one example, spacer 108 is a retort placed between the reactor volume and the heating elements 104 in a “rod-over-bend” configuration around the inner circumference of the reactor, or the bayonet heaters may be covered with a sheath.

[0063] Heating elements 102 are resistive heating elements. In some examples, the resistive heating elements are composed of a ferritic iron-chromium-aluminum alloy (FeCrAl alloy) suitable for use at temperatures up to 1425 °C (e.g., KANTHAL APM). The alloy can include 5 wt% to 6 wt% aluminum, 20 wt% to 24 wt% chromium, and 65 wt% to 75 wt% iron, with trace amounts of carbon, silicon, manganese, or a combination thereof. In other examples, the resistive heating elements are composed of silicon carbide, molybdenum disilicide, or graphite.

[0064] Prior to operation, metallic heating elements are often pre-treated by operating them at a sufficiently high temperature in air to form a thin ceramic oxide layer (e.g., aluminum oxide) on the surface of the heating elements. For silicon carbide or molybdenum disilicide elements, the ceramic oxide layer may be composed of silicon oxide. These oxide layers protect the interior metal of the heating element and do not further react with the atmosphere to which they are exposed (e.g., the oxide layer does not react with hydrogen or oxygen-containing gasses or form chemical carbides by reaction with gaseous hydrocarbons). As described herein, nitriding theAttorney Docket No.: 44807-0492WO1 / JHU 18344 surface of a heating element to form a “nitrided” heating element (e.g., by heating them in a substantially pure nitrogen environment or otherwise coating with an inorganic nitride layer) is an effective way to reduce carbon deposition on the heating elements when used in a reactor. FIG. 3 shows a photograph of a piece of Kanthal APM heating wire half-coated with aluminum nitride and then exposed to natural gas at 1150 °C. While essentially no carbon deposit is formed on the coated portion 112 of the wire, a substantial carbon deposit is formed on the uncoated portion of the wire 114.

[0065] In a first general aspect, operating a hydrocarbon decomposition reactor includes initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, and directly contacting the heating element with the reactant gas in the reactor zone, thereby initiating decomposition of the one or more hydrocarbons in the reactant gas to yield a decomposition product and a product gas. The reactant gas includes one or more hydrocarbons and particulate matter. The particulate matter can include aerosolized particles. In one example, the aerosolized particles are aerosolized catalyst particles (e.g., particles that catalyze decomposition of the hydrocarbon). The reactor zone includes a heating element (or a plurality of heating elements) and a reactor wall (or a plurality of reactor walls). The decomposition product deposits on the particulate matter. The heating element, the reactor wall, or both have a ceramic coating that inhibits deposition of the decomposition product. The decomposition product includes, consists essentially of, or consists of carbon (e.g., one or more forms of elemental carbon). The product gas includes, consists essentially of, or consists of hydrogen (e g., hydrogen gas).

[0066] Implementations of the first general aspect may include one or more of the following features.

[0067] The one or more hydrocarbons can include methane, ethane, propane, ethene, propene, ethylene, propylene, C4+ alkanes, C4+ alkenes, C4+ alkynes, or a combination thereof. During operation, a temperature difference between a surface of the heating element and a reaction temperature required for decomposition of the hydrocarbon in the reactant gas is less than 250 °C or less than 50 °C.

[0068] The heating element can be an electric (e.g., a resistive) heating element. In one example, the heating element is composed of a ferritic iron-chromium-aluminum alloy (e.g., 5 wt% to 6 wt% aluminum, 20 wt% to 24 wt% chromium, and 65 wt% to 75 wt% iron). In otherAttorney Docket No.: 44807-0492WO1 / JHU 18344 examples, the heating element is composed of silicon carbide or molybdenum disilicide. The alloy can include trace amounts of carbon, silicon, manganese, or a combination thereof. The heating element(s) can be configured as a rod-over-bend, tubular, or porcupine heating element. Operating the reactor can include powering the resistive heating element by an external source of electricity (e.g., generating the external source of electricity by providing the hydrogen in the product gas to a generator or fuel cell).

[0069] The ceramic coating can include, consist essentially of, or consist of an oxide, a nitride, or a carbide (e g., an oxide, nitride, or carbide of aluminum, chromium, iron, or silicon). Nonlimiting examples include aluminum nitride, silicon carbide, and silicon nitride. A thickness of the ceramic coating is typically in a range of about 0.1 microns to about 1000 microns. The ceramic coating can independently cover all or a portion of the heating element(s), the reactor wall(s), and other components in the reactor zone (e g., insulation). The ceramic coating does not chemically react with air or deteriorate in a reducing environment. The ceramic coating is resistant to degradation in atmospheres containing water, carbon dioxide, carbon monoxide, hydrogen, or a combination thereof. The ceramic coating is resistant to degradation in the reactant gas.

[0070] Operating the hydrocarbon decomposition reactor can include terminating the flow of the reactant gas, and initiating a flow of cleaning gas to the reactor zone, wherein the cleaning gas comprises air or carbon dioxide. The reactor zone includes carbon deposit on surfaces of the components or walls of the reactor zone, and the cleaning gas reacts with the carbon deposit to yield an exit stream comprising carbon dioxide, thereby removing at least a portion of the carbon deposit from the surfaces. Initiating the flow of cleaning gas can occur when a thickness of the carbon deposit is less than 10 cm. Operating the hydrocarbon decomposition reactor can include flowing the cleaning gas to the reactor zone until the carbon dioxide content of the exit stream is at least 95 mol% of the carbon dioxide content of the cleaning gas. The flow of cleaning gas can be terminated, followed by re-initiating the flow of the reactant gas. The electric heater provides heat to the reactor zone during the terminating and the initiating.

[0071] After terminating the flow of the reactant gas and before initiating the flow of cleaning gas, a flow of inert gas to the reactor zone can be initiated, thereby flushing the reactant gas from the reactor zone before initiating the flow of cleaning gas. Examples of suitable inert gases include argon, neon, xenon, nitrogen, or a combination thereof. After terminating the flowAttorney Docket No.: 44807-0492WO1 / JHU 18344 of cleaning gas and before re-initiating the flow of the reactant gas, a flow of inert gas to the reactor zone can be initiated, thereby flushing the cleaning gas from the reactor zone before reinitiating the flow of the reactant gas.

[0072] In a second general aspect, reducing fouling in a hydrocarbon decomposition reactor includes initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, and directly contacting the heating element and the reactor wall with the reactant gas in the reactor zone, thereby initiating decomposition of the hydrocarbon in the reactant gas to yield carbon and hydrogen. The reactor zone includes a heating element (or heating elements) and a reactor wall (or reactor walls). The reactant gas includes one or more hydrocarbons and an additive including water, carbon dioxide, oxygen, air, or any combination thereof.

[0073] Implementations of the second general aspect may include one or more of the features described with respect to the first general aspect as described above, as well as one or more of the following features.

[0074] The reactant gas typically includes 70 vol% to 95 vol% of the one or more hydrocarbons (e.g., 90 vol% to 95 vol%) and 0.01 vol% to 30 vol% of the additive (e.g., 0.01 vol% to 10 vol%). In one example, the additive includes, consists essentially of, or consists of water, the water is in the form of water vapor, and a concentration of the water in the reactant gas is less than 100 mg / cm3. In one example, the additive includes, consists essentially of, or consists of carbon dioxide, and a concentration of the carbon dioxide in the reactant gas is 30 vol% or less. In one example, the additive includes, consists essentially of, or consists of oxygen, and a concentration of the oxygen in the reactor zone is less than 100 ppm by volume. In one example, the additive includes, consists essentially of, or consists of air, and a concentration of the air is less than 1000 ppm by volume.

[0075] The heating element (e.g., an electric heating element) includes a carbon deposit on a surface of the heating element or reactor wall, and the additive reacts with the carbon deposit to yield an exit stream including carbon dioxide or carbon monoxide, thereby removing at least a portion of the carbon deposit from the surface of the heating element.

[0076] In a third general aspect, a hydrocarbon decomposition reactor includes a reactor zone, a heating element positioned in the reactor zone, an inlet configured to provide a reactant gas to the reactor zone, and an outlet configured to remove product gas from the reactor zone. AAttorney Docket No.: 44807-0492WO1 / JHU 18344 surface of the heating element is coated with a ceramic coating and configured to be in direct contact with the reactant gas in the reactor zone.

[0077] Implementations of the third general aspect may include one or more of the features described with respect to the first and second general aspects as described above, as well as one or more of the following features.

[0078] The heating element can be an electric heating element (e.g., resistive) heating element. In one example, the heating element is composed of a ferritic iron-chromium- aluminum alloy (e g., 5 wt% to 6 wt% aluminum, 20 wt% to 24 wt% chromium, and 65 wt% to 75 wt% iron). In other examples, the heating element is composed of silicon carbide or molybdenum disilicide. The alloy can include trace amounts of carbon, silicon, manganese, or a combination thereof. The heating element(s) can be configured as a rod-over-bend, tubular, or porcupine heating element. Operating the reactor can include powering the resistive heating element by an external source of electricity (e.g., generating the external source of electricity by providing the hydrogen in the product gas to a generator or fuel cell).

[0079] The ceramic coating can include, consist essentially of, or consist of an oxide, a nitride, or a carbide (e.g., an oxide, nitride, or carbide of aluminum, chromium, iron, or silicon). Nonlimiting examples include aluminum nitride, silicon carbide, and silicon nitride. A thickness of the ceramic coating is typically in a range of about 0.1 microns to about 1000 microns. The ceramic coating can independently cover all or a portion of the heating element(s), the reactor wall(s), and other components in the reactor zone (e.g., insulation). The ceramic coating does not chemically react with air or deteriorate in a reducing environment. The ceramic coating is resistant to degradation in atmospheres containing water, carbon dioxide, carbon monoxide, hydrogen, or a combination thereof. The ceramic coating is resistant to degradation in the reactant gas.

[0080] In a first exemplary system and method to remove or reduce fouling in hydrocarbon decomposition reactors, nitrided electric heating elements are positioned directly in a reactor zone of the hydrocarbon decomposition reactor. Gaseous species, either alone or containing entrained aerosolized catalyst, are introduced into a hot zone of the reactor and are thus heated directly by the heating elements. That is, no reactor component is positioned between or separates the heating element and the gas. The particular heating element configuration, as wellAttorney Docket No.: 44807-0492WO1 / JHU 18344 as selected parameters (e.g., operating temperatures and pressures, thermal diffusion) are selected for compatibility with the particular hydrocarbon provided to the reactor.

[0081] Configurations of heating elements may include “rod over bend” configurations, in which the heating elements are bent into a serpentine line and attached to an outer wall of the reaction zone, tubular heating elements in which bundles of heating elements are consolidated into a tube hanging into the reaction zone, or porcupine heating elements in which a spiral of heating elements is created by winding the wire around a mandrel. In each of these configurations, the reactive gases pass close by a high volumetric density of heating elements, thereby rapidly heating the reactive gases.

[0082] In a second exemplary system and method to remove or reduce fouling in hydrocarbon decomposition reactors, heating elements (which may be nitrided) are physically separated from the flow of reactant gasses and products by a material resistant to fouling, such material being composed of aluminum nitride, silicon carbide and / or silicon nitride, or a metal coated with a dense coating of aluminum nitride, silicon carbide and / or silicon nitride. The physical barrier may be a retort, or a sheath covering tubular heating elements.

[0083] In a third exemplary system and method, fouling is removed from a hydrocarbon decomposition reactor by alternately providing reactive gasses, air, or carbon dioxide to the reactor through the inlet stream, such that the electric heating elements are alternately run in the presence of the reactive gases and in the presence of air or carbon dioxide. When the electric heating elements are run in the presence of air, there is little or no carbon deposition. Rather, fouling in the reactor is burned off. When the electric heating elements are run in the presence of carbon dioxide, there is also little or no carbon deposition. Rather, fouling in the reactor is gasified according to the reverse Boudouard reaction, which transforms fouling deposits of carbon into carbon monoxide. Alternating the input to the reactor between reactive gases and air or carbon dioxide can be effected without cooling the reactor, thereby reducing downtime compared to a mechanical cleaning process.

[0084] In a fourth exemplary system and method, small quantities of additives (e.g., water, oxygen, carbon dioxide, or any combination thereof) are combined with the reactive gases (e.g., hydrocarbon stream), thereby reducing the level of fouling during hydrocarbon decomposition. Suitable concentrations of these additive include, for example, water up to 100 mg / cm3gases in the inlet stream, carbon dioxide concentrations in the inlet stream up to 30%, and oxygenAttorney Docket No.: 44807-0492WO1 / JHU 18344 concentrations in the inlet stream up to 100 ppm. The quantities (e.g., concentrations) of water, oxygen, and carbon dioxide are selected such that spontaneous reaction of these additives with the reactant hydrocarbon or product hydrogen does not occur (e.g., there is no self-propagating combustion). Instead, the combustion reaction (in the case of oxygen) or reforming reaction (in the case of carbon dioxide or water) occur preferentially, with carbon deposits (“coked carbon”) on hot surfaces in the reactor providing the necessary activation energy for reaction. That is, the carbon in the reactor most likely to react with the water, oxygen, or carbon dioxide in the inlet stream is the carbon in the fouling deposits (the “coked carbon”), due at least in part to the elevated temperature of these deposits. The reaction product of coked carbon with these additives yields, for example, carbon monoxide, a gaseous species that is carried with product hydrogen and can be separated from the reactant stream once cooled.

[0085] Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.

[0086] Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.

Claims

Attorney Docket No.: 44807-0492WO1 / JHU 18344WHAT IS CLAIMED IS:

1. A method of operating a hydrocarbon decomposition reactor, the method comprising: initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, wherein the reactant gas comprises one or more hydrocarbons and particulate matter, and the reactor zone comprises a heating element and a reactor wall; and directly contacting the heating element with the reactant gas in the reactor zone, thereby initiating decomposition of the one or more hydrocarbons in the reactant gas to yield a decomposition product comprising carbon and a product gas comprising hydrogen, wherein the decomposition product deposits on the particulate matter, and the heating element, the reactor wall, or both comprise a ceramic coating that inhibits deposition of the decomposition product.

2. The method of claim 1, wherein the one or more hydrocarbons comprise methane, ethane, propane, ethene, propene, ethylene, propylene, C4+ alkanes, C4+ alkenes, C4+ alkynes, or a combination thereof.

3. The method of claim 1, wherein the heating element comprises a ferritic iron-chromium- aluminum alloy.

4. The method of claim 3, wherein the ceramic coating comprises an oxide, a nitride, or a carbide.

5. The method of claim 4, wherein the ceramic coating comprises an oxide, nitride, or carbide of aluminum, chromium, iron, or silicon.

6. The method of claim 5, wherein the ceramic coating does not chemically react with air or deteriorate in a reducing environment.Attorney Docket No.: 44807-0492WO1 / JHU 183447. The method of claim 5, wherein the ceramic coating is resistant to degradation in atmospheres containing water, carbon dioxide, carbon monoxide, hydrogen, or a combination thereof.

8. The method of claim 5, wherein the ceramic coating is resistant to degradation in the reactant gas.

9. The method of claim 3, wherein the alloy comprises 5 wt% to 6 wt% aluminum, 20 wt% to 24 wt% chromium, and 65 wt% to 75 wt% iron.

10. The method of claim 3, wherein the alloy comprises trace amounts of carbon, silicon, manganese, or a combination thereof.

11. The method of claim 1, wherein the heating element comprises silicon carbide or molybdenum disilicide.

12. The method of claim 1, wherein the heating element is configured as a rod-over-bend, tubular, or porcupine heating element.

13. The method of claim 1, wherein the reactor zone comprises a plurality of heating elements.

14. The method of claim 13, wherein the plurality of heating elements are distributed throughout the reactor zone to maximize heat transfer to the reactant gas.

15. The method of claim 1, wherein a temperature difference between a surface of the heating element and a reaction temperature required for decomposition of the hydrocarbon in the reactant gas is less than 250 °C or less than 50 °C.

16. The method of claim 1, wherein the heating element is a resistive heating element.Attorney Docket No.: 44807-0492WO1 / JHU 1834417. The method of claim 16, further comprising powering the resistive heating element by an external source of electricity.

18. The method of claim 17, further comprising generating the external source of electricity by providing the hydrogen in the product gas to a generator or fuel cell.

19. The method of claim 1, further comprising terminating the flow of the reactant gas, and initiating a flow of cleaning gas to the reactor zone, wherein the cleaning gas comprises air or carbon dioxide.

20. The method of claim 19, wherein the reactor zone comprises carbon deposit on surfaces of the components or walls of the reactor zone, and the cleaning gas reacts with the carbon deposit to yield an exit stream comprising carbon dioxide, thereby removing at least a portion of the carbon deposit from the surfaces.

21. The method of claim 20, wherein initiating the flow of cleaning gas occurs when a thickness of the carbon deposit is less than 10 cm.

22. The method of claim 19, further comprising flowing the cleaning gas to the reactor zone until the carbon dioxide content of the exit stream is at least 95 mol% of the carbon dioxide content of the cleaning gas.

23. The method of claim 19, further comprising terminating the flow of cleaning gas, and reinitiating the flow of the reactant gas.

24. The method of claim 19, wherein the electric heater provides heat to the reactor zone during the terminating and the initiating.Attorney Docket No.: 44807-0492WO1 / JHU 1834425. The method of claim 19, further comprising, after terminating the flow of the reactant gas and before initiating the flow of cleaning gas, initiating a flow of inert gas to the reactor zone, thereby flushing the reactant gas from the reactor zone before initiating the flow of cleaning gas.

26. The method of claim 25, wherein the inert gas comprises argon, neon, xenon, nitrogen, or a combination thereof.

27. The method of claim 23, further comprising, after terminating the flow of cleaning gas and before re-initiating the flow of the reactant gas, initiating a flow of inert gas to the reactor zone, thereby flushing the cleaning gas from the reactor zone before re-initiating the flow of the reactant gas.

28. The method of claim 27, wherein the inert gas comprises argon, neon, xenon, nitrogen, or a combination thereof.

29. The method of claim 1, wherein the particulate matter comprises aerosolized particles.

30. The method of claim 29, wherein the aerosolized particles comprise an aerosolized hydrocarbon decomposition catalyst.

31. A method of reducing fouling in a hydrocarbon decomposition reactor, the method comprising: initiating a flow of reactant gas to a reactor zone of a hydrocarbon decomposition reactor, wherein the reactor zone comprises a heating element and a reactor wall; and directly contacting the heating element and the reactor wall with the reactant gas in the reactor zone, thereby initiating decomposition of the hydrocarbon in the reactant gas to yield carbon and hydrogen, wherein the reactant gas comprises one or more hydrocarbons and an additive comprising water, carbon dioxide, oxygen, air, or any combination thereof.Attorney Docket No.: 44807-0492WO1 / JHU 1834432. The method of claim 31 , wherein the reactant gas comprises 0.01 vol% to 30 vol% of the additive.

33. The method of claim 31, wherein the heating element comprises a carbon deposit on a surface of the heating element or reactor wall, and the additive reacts with the carbon deposit to yield an exit stream comprising carbon dioxide or carbon monoxide, thereby removing at least a portion of the carbon deposit from the surface of the heating element.

34. The method of claim 31, wherein the reactant gas comprises 90 vol% to 95 vol% of the one or more hydrocarbons.

35. The method of claim 31, wherein the additive comprises water, the water is in the form of water vapor, and a concentration of the water in the reactant gas is less than 100 mg / cm3.

36. The method of claim 31, wherein the additive comprises carbon dioxide, and a concentration of the carbon dioxide in the reactant gas is 30 vol% or less.

37. The method of claim 31, wherein additive comprises oxygen, and a concentration of the oxygen in the reactor zone is less than 100 ppm by volume.

38. The method of claim 31, wherein the additive comprises air, and a concentration of the air is less than 1000 ppm by volume.

39. The method of claim 31, wherein the heating element is an electric heating element.

40. A hydrocarbon decomposition reactor comprising: a reactor zone; a heating element positioned in the reactor zone; an inlet configured to provide a reactant gas to the reactor zone; and an outlet configured to remove product gas from the reactor zone,Attorney Docket No.: 44807-0492WO1 / JHU 18344 wherein a surface of the heating element is coated with a ceramic coating and configured to be in direct contact with the reactant gas in the reactor zone.

41. The reactor of claim 40, wherein the heating element is a resistive heating element.

42. The reactor of claim 40, wherein the heating element comprises a ferritic iron-chromium- aluminum alloy.

43. The reactor of claim 40, wherein the ceramic coating comprises an oxide, a nitride, or a carbide.

44. The reactor of claim 43, wherein the ceramic coating comprises an oxide, a nitride, or a carbide of aluminum, chromium, iron, or silicon.

45. The reactor of claim 40, wherein the heating element comprises silicon carbide or molybdenum disilicide.

46. The reactor of claim 40, wherein the heating element is configured as a rod-over-bend or tubular heating element.

47. The reactor of claim 40, wherein the reactor zone comprises a plurality of heating elements.

48. The reactor of claim 47, wherein the plurality of heating elements are distributed throughout the reactor zone to maximize heat transfer to the reactant gas.

Images