Process for producing hydrogen

Abstract

The present invention relates to a process for the production of hydrogen, comprising steps of hydrodesulfurization, reforming in a reforming section comprising a gas-heated reformer and an autothermal reformer, a water-gas shift step, a carbon dioxide separation step, and a hydrogen purification step. The hydropurification step produces a hydrocarbon-containing stream that is split, one portion of which is used as a fuel gas stream supplied to one or more combustion heaters used to heat one or more process streams in the process, and the other portion of which is split into a hydrodesulfurization recycle stream that is compressed and used in a hydrodesulfurization unit, and a process recycle stream that is returned to the process. The present invention also relates to a chemical plant configured to carry out the process.

Description

[Technical Field]

[0001] The present invention relates to a process for converting hydrocarbons to hydrogen while minimizing carbon dioxide production. [Background technology]

[0002] Processes for generating hydrogen are well known and generally involve combustion steam methane reformers combined with water-gas shift and carbon dioxide (CO2) removal. Such processes produce significant volumes of carbon dioxide in the flue gas at pressures that are not suitable for efficient CO2 capture. There is a need for hydrogen production processes that produce lower levels of carbon dioxide emissions and allow for more efficient CO2 capture.

[0003] Some processes use a combustion reformer to produce syngas. In a combustion reformer, fuel is burned in a radiant box of the combustion reformer to provide heat to drive the steam reforming reaction. For example, EP 2103569(A2) discloses a method for generating hydrogen and / or synthesis gas within a production facility, where little or no exiting steam is produced within the facility. Most or all of the steam generated from waste heat from the process is used in a steam-hydrocarbon reformer. The flowsheet shown in Figure 1 of this reference includes a combustion steam reformer (650), an optional water-gas shift reactor (602), and a pressure swing adsorber (330). The pressure swing adsorber produces residue gas (698) that is separated. A portion (630) is used as fuel in the combustion steam reformer. Another portion is compressed, optionally first processed, and then used as feed to the combustion steam reformer for hydrodesulfurization and / or pre-reforming.

[0004] In an alternative process, an autothermal reformer is used instead of a combustion steam reformer. For example, U.S. Patent Application Publication No. 2022 / 194789 (A1) describes in Figure 2 of this reference a flowsheet including, in sequence, a pre-reforming unit (140), an autothermal reformer (110), a high-temperature shift unit (115), a low-temperature shift unit (150), a water wash section (160), a CO removal section (170), and a hydrogen purification unit (125). The hydrogen purification unit generates a high-purity H stream (8) and an off-gas stream (9). The off-gas stream can be fed to the feed side of the pre-reformer unit and / or the feed side of the ATR and / or the feed side of the shift section. The same arrangement is seen in Figure 2 of WO 2022 / 038089 (A1).

[0005] Arrangements using gas-fired reformers and autothermal reformers are also known. In known hydrogen plants that include a gas-fired reformer, an autothermal reformer, a water-gas shift unit, a carbon dioxide separation unit, and a purification unit, a stream containing unreacted hydrocarbons, called off-gas, is often separated from the hydrogen product in the purification unit. The off-gas can be used as a fuel to provide some or all of the heat load of the plant.

[0006] WO 2019 / 162236 A1 describes a method for producing hydrogen, comprising receiving a feed gas containing hydrocarbons and carrying out a reforming process in response to the feed gas to generate hydrogen, the reforming process comprising both a gas-heated reforming process and an autothermal reforming process, heat generated by the autothermal reforming process being supplied to the gas-heated reforming process, and the method being carried out in a hydrogen plant integrated with one or more further processing plants, such as a methanol plant. In one embodiment, synthesis gas generated from the gas-heated reforming and autothermal reforming processes is shifted in a water-gas shift unit and processed to remove carbon dioxide, and then processed to produce a hydrogen stream and a residue gas stream containing CO, CH, CO, and H residues. The remaining gas may alternatively be referred to as tail gas or recycle gas. The remaining gas may be utilized as fuel in a combustion heater to preheat the feed gas or recycled to the gas-heated or autothermal reformer for maximum carbon efficiency.

[0007] WO 2022 / 003312(A1) describes a process for producing hydrogen, comprising the steps of: (i) subjecting a gaseous mixture comprising hydrocarbons and steam and having a steam to carbon ratio of at least 2.6:1 to steam reforming in a gas-heated reformer, followed by autothermal reforming using an oxygen-rich gas in an autothermal reformer to generate a reformed gas mixture; (ii) increasing the hydrogen content of the reformed gas mixture by subjecting the reformed gas mixture to one or more water-gas shift stages in a water-gas shift unit to provide a hydrogen-enriched reformed gas; (ii) cooling the hydrogen-enriched reformed gas and separating condensed water therefrom to provide a dehydrated hydrogen-enriched reformed gas; (iv) passing the dehydrated hydrogen-enriched reformed gas through a carbon dioxide separation unit to provide a carbon dioxide gas stream and a crude hydrogen gas stream; and (v) passing the crude hydrogen gas stream from the carbon dioxide removal unit through a purification unit to provide purified hydrogen gas and a fuel gas, wherein the fuel gas is supplied to one or more fired heaters used to heat one or more process streams in the process.

[0008] While the above process has good efficiency with very high carbon dioxide capture rates, there is a need for a hydrogen production process with an even higher yield of hydrogen per unit of hydrocarbon feed. The present invention addresses this problem. Summary of the Invention

[0009] The present inventors have improved the process described in WO 2022 / 003312 A1 to increase the yield of H per unit of hydrocarbon feed. Thus, the present invention provides a process for producing hydrogen, comprising: (i) passing a feed stream comprising a hydrogen stream and a hydrocarbon stream through a hydrodesulfurization unit to hydrodesulfurize and produce a refined hydrocarbon stream; (ii) adding steam to the refined hydrocarbon stream to produce a gaseous mixture comprising hydrocarbons and steam; (iii) subjecting the gaseous mixture comprising hydrocarbons and steam to steam reforming in a reforming section comprising a gas-heated reformer and an autothermal reformer to generate a reformed gas mixture; (iv) increasing the hydrogen content of the reformed gas mixture by subjecting the reformed gas mixture to one or more water-gas shift stages in a water-gas shift unit to provide a hydrogen-enriched reformed gas; (v) cooling the hydrogen-enriched reformed gas and separating condensed water therefrom to provide a dehydrated hydrogen-enriched reformed gas; (vi) passing the dehydrated hydrogen-enriched reformed gas through a carbon dioxide separation unit to provide a carbon dioxide gas stream and a crude hydrogen gas stream; (vii) passing the crude hydrogen gas stream from the carbon dioxide removal unit through a purification unit to provide a purified hydrogen gas stream and a hydrocarbon-containing off-gas stream; (viii) dividing the off-gas stream into a fuel gas stream, the fuel gas stream being supplied to one or more fired heaters used to heat one or more process streams within the process, and a recycle stream; (ix) compressing the recycle stream; (x) dividing the compressed recycle stream into a desulfurization recycle stream and a process recycle stream; (xi) feeding the hydrodesulfurization recycle stream to a desulfurization unit; (xii) separating the process recycle stream into (xii-a) downstream of the hydrodesulfurization unit and upstream of the gas-heated reformer; (xii-d) downstream of the gas-heated reformer and upstream of the autothermal reformer; (xii-c) downstream of the autothermal reformer and upstream of the water gas shift unit; (xii-d) downstream of a water-gas shift unit and upstream of a carbon dioxide separation unit; or (xii-e) returning the carbon dioxide to one or more locations selected from downstream of the carbon dioxide separation unit and upstream of the purification unit.

[0010] One aspect in which this process differs from the arrangement of WO 2022 / 003312(A1) is that instead of using all of the off-gas from the refinery unit as fuel gas for one or more combustion heaters, the off-gas is split into two streams: a recycle stream and a fuel gas stream. The fuel gas stream is fed to one or more combustion heaters, which are used to heat one or more process streams within the process. The recycle stream, which contains some unreacted hydrocarbons, is compressed and then split into a hydrodesulfurization recycle stream used in the hydrodesulfurization unit and a process recycle stream returned to the process to convert as many hydrocarbons as possible. The use of a portion of the hydrodesulfurization recycle replaces the pure hydrogen feed sent to the hydrodesulfurization unit, thereby improving overall efficiency. The use of process recycle maximizes hydrogen conversion and, if the recycle is returned upstream of the water-gas shift unit, maximizes CO conversion and CO capture. Surprisingly, despite the added complexity of this arrangement, the overall hydrogen efficiency of the process (H produced per unit of hydrocarbon feed) is increased. This is achieved without sacrificing the efficiency of CO2 capture, which can be 98% or greater in the process of the present invention.

[0011] In a second aspect, the present invention provides a chemical plant comprising: (i) a hydrodesulfurization unit (203, 303, 403, 503) configured to receive a hydrogen stream (202, 302, 402, 502) and a hydrocarbon-containing feed stream (201, 301, 401, 501) and to hydrodesulfurize the hydrogen stream to produce a refined hydrocarbon stream; (ii) means for adding steam (204, 304, 404, 504) to the refined hydrocarbon stream to produce a gaseous mixture (205, 305, 405, 505) comprising hydrocarbons and steam; (iii) a reforming section (207, 307, 407, 507) comprising a gas-heated reformer (207a, 307a, 407a, 507a) and an autothermal reformer (207b, 307b, 407b, 507b) arranged to receive a gaseous mixture comprising hydrocarbons and steam and to generate a reformed gas mixture (208, 308, 408, 508); (iv) a water-gas shift (210, 310, 410, 510) unit configured to receive the reformate mixture and generate a hydrogen-enriched reformate (211, 311, 411, 511); (v) means for cooling the hydrogen-enriched reformed gas and separating condensed water therefrom to provide a dehydrated hydrogen-enriched reformed gas; (vi) a carbon dioxide separation unit (212, 312, 412, 512) configured to receive the dehydrated hydrogen-enriched reformate gas and generate a carbon dioxide gas stream (213, 313, 413, 513) and a crude hydrogen gas stream (214, 314, 414, 514); (vii) a purification unit (215, 315, 415, 515) configured to receive the crude hydrogen gas stream and generate a purified hydrogen gas stream (216, 316, 416, 516) and a hydrocarbon-containing off-gas stream (217, 317, 417, 517); (viii) means for dividing the off-gas stream into a fuel gas stream (218, 318, 418, 518) and a recycle stream (219, 319, 419, 519), and means for supplying the fuel gas stream to one or more fired heaters used to heat one or more process streams within the process; (ix) means for compressing the recycle stream; and (x) means for dividing the compressed recycle stream into a hydrodesulfurization recycle stream (220, 320, 420, 520) and a process recycle stream (221, 321, 421, 521); (xi) means for supplying a hydrodesulfurization recycle stream to the hydrodesulfurization unit; (xii) separating the process recycle stream into (xii-a) downstream of the hydrodesulfurization unit and upstream of the gas-heated reformer; (xii-d) downstream of the gas-heated reformer and upstream of the autothermal reformer; (xii-c) downstream of the autothermal reformer and upstream of the water gas shift unit; (xii-d) downstream of a water-gas shift unit and upstream of a carbon dioxide separation unit; or (xii-e) means for returning the carbon dioxide to one or more locations selected from downstream of the carbon dioxide separation unit and upstream of the purification unit.

[0012] A chemical plant may be constructed from scratch (e.g., a "grassroots" chemical plant). Alternatively, an existing chemical plant may be retrofitted with the necessary units and associated piping, etc., to produce a chemical plant according to the present invention.

[0013] The chemical plant is preferably a hydrogen plant, i.e. it produces hydrogen as an end product. [Brief explanation of the drawings]

[0014] [Figure 1]1 is a simplified diagram of the process described in WO 2022 / 003312 A1. A hydrocarbon stream (101) and a hydrogen stream (102) are fed to a desulfurization unit (103). Steam (104) is added to the output from the desulfurization unit to produce a stream (105) having a steam-to-carbon ratio of at least 2.6:1, which is sent to a reforming section (107). The reforming section includes a gas-heated reformer (107a) and an autothermal reformer (107b). An oxygen-rich gas stream (106) is also fed to the autothermal reformer. Steam reforming reactions occur in the gas-heated reformer and the autothermal reformer. Hot gas exiting the autothermal reformer is used to provide heat for the endothermic steam reforming reaction occurring in the gas-heated reformer by passing the hot gas through the shell side of the gas-heated reformer. The reformate stream may optionally be mixed with additional steam (109) (not used in the modeled configuration) and sent to a water-gas shift unit (110) to generate a hydrogen-rich reformate gas (111). The hydrogen-rich reformate gas is fed to a carbon dioxide separation unit (112) where it is separated into a carbon dioxide gas stream (113) and a crude hydrogen gas stream (114). The crude hydrogen gas stream is sent to a purification unit (115) where it is separated into a purified hydrogen gas stream (116) and an off-gas stream (117). The off-gas stream is used as fuel gas. [Figure 2] 1 shows an arrangement according to the present invention based on the arrangement shown in Figure 1. The off-gas stream (217) is split to produce a fuel gas stream (218) and a recycle stream (219). The recycle stream is compressed (not shown) and then split into a desulfurization recycle stream (220) which is fed to a purification unit, and a process recycle stream (221) which is reintroduced into the process downstream of the desulfurization unit (203) and upstream of the gas-heated reformer (207a). [Figure 3]1 shows an arrangement according to the present invention based on the arrangement shown in Figure 1. The off-gas stream (317) is split to produce a fuel gas stream (318) and a recycle stream (319). The recycle stream is compressed (not shown) and then split into a desulfurization recycle stream (320) that is fed to a desulfurization unit, and a process recycle stream (321) that is reintroduced into the process downstream of the autothermal reformer (307b) and upstream of the water-gas shift unit (310). [Figure 4] 1 shows an arrangement according to the present invention based on the arrangement shown in Figure 1. The off-gas stream (417) is split to produce a fuel gas stream (418) and a recycle stream (419). The recycle stream is compressed (not shown) and then split into a desulfurization recycle stream (420) which is fed to the desulfurization unit, and a process recycle stream (421) which is reintroduced into the process downstream of the water gas shift unit (410) and upstream of the carbon dioxide separation unit (412). [Figure 5] 1 shows an arrangement according to the present invention based on the arrangement shown in Figure 1. The off-gas stream (517) is split to produce a fuel gas stream (518) and a recycle stream (519). The recycle stream is compressed (not shown) and then split into a desulfurization recycle stream (520) which is fed to a desulfurization unit, and a process recycle stream (521) which is reintroduced into the process downstream of the carbon dioxide separation unit (512) and upstream of the purification unit (515). DETAILED DESCRIPTION OF THE INVENTION

[0015] Subheadings are included for clarity and are not intended to limit the invention.

[0016] The features relating to the layout of the chemical plant described in relation to the process also apply to the chemical plant according to the second aspect of the invention.

[0017] Treatment before gas-heated reformer The gaseous mixture fed to the gas-fired reformer comprises hydrocarbons and steam. Preferably, the mixture comprises 90% by volume or more methane, such as 95% by volume or more methane, based on the % of hydrocarbons present in the mixture and excluding all steam. The hydrocarbon-containing feed is pre-treated upstream of the gas-fired reformer to remove contaminants, including at least a step of hydrodesulfurization.

[0018] A wide variety of feeds may be used, such as natural gas, associated gas, LPG, petroleum distillates, diesel, naphtha, or mixtures thereof, or hydrocarbon-containing off-gases from chemical processes, such as refinery off-gases or pre-reformed gases.

[0019] Hydrodesulfurization Step (i) comprises passing a feed stream comprising a hydrogen stream and a hydrocarbon through a hydrodesulfurization unit to effect hydrodesulfurization and produce a refined hydrocarbon stream.

[0020] The feed stream may be compressed before or after hydrodesulfurization, preferably before hydrodesulfurization. The feed may be compressed to a pressure in the range of 10 to 100 bar (absolute). The pressure of the feed stream provides useful pressure control throughout the process. The operating pressure is preferably in the range of 15 to 50 bar (absolute), more preferably 25 to 50 bar (absolute), which provides improved performance from the process.

[0021] The hydrodesulfurization step is typically catalytic hydrodesulfurization, which can be achieved using known catalysts such as CoMo or NiMo catalysts. This process generates hydrogen sulfide, which is absorbed using a suitable hydrogen sulfide adsorbent, such as a zinc oxide adsorbent. Ultra-purified adsorbents may be usefully used downstream of the hydrogen sulfide adsorbent to further protect the steam reforming catalyst. Suitable ultra-purified adsorbents may include copper-zinc oxide / alumina materials and copper-nickel-zinc oxide / alumina materials. It is preferable to add hydrogen to the compressed hydrocarbon to facilitate hydrodesulfurization and / or reduce the risk of carbon laydown in the reforming process. The amount of hydrogen in the resulting mixed gas stream can be in the range of 1 to 20% by volume, on a dry gas basis, but is preferably in the range of 1 to 10% by volume, and more preferably in the range of 1 to 5% by volume. In a preferred embodiment, a portion of the desulfurized recycle stream (described below) can be mixed with the compressed hydrocarbon.

[0022] The hydrogen stream to the hydrodesulfurization unit is provided at least in part by the hydrodesulfurization recycle stream separated in step (x) described below. Although additional hydrogen can be added to the hydrodesulfurization stream, to maximize hydrogen yield, the hydrodesulfurization recycle stream is preferably fed to the hydrodesulfurization unit without the addition of additional hydrogen. In the latter case, the hydrogen stream and the hydrodesulfurization recycle stream are one and the same.

[0023] Compared to the arrangement described in WO 2022 / 003312 A1, in which part of the crude hydrogen gas stream (from the carbon dioxide separation unit) or the purified hydrogen gas stream (from the purification unit) can be used to provide hydrogen for desulfurization, this arrangement uses a different, less hydrogen-rich feed, which helps to improve the yield of hydrogen per unit of hydrocarbon feed.

[0024] If the feed contains other contaminants, such as chlorides or heavy metal contaminants, these can be removed either upstream or downstream of hydrodesulfurization using conventional adsorbents prior to upgrading. Adsorbents suitable for chloride removal are well known and include alkalized alumina materials. Similarly, adsorbents for heavy metals such as mercury or arsenic are known and include copper sulfide materials.

[0025] The feed may be preheated in one or more stages, preferably after compression and before desulfurization. The process provides a variety of hot gas sources that can be used for this application. For example, the feed may be heated by heat exchange with a shifted gas stream recovered from a water-gas shift stage, preferably a high-temperature shift stage. The desulfurized feed, also referred to herein as a refined hydrocarbon stream, may be heated, for example, in a combustion heater fueled by fuel gas.

[0026] Steam addition Step (ii) comprises adding steam to the refined hydrocarbon stream to produce a gaseous mixture comprising hydrocarbons and steam. Steam introduction may be carried out by direct injection of steam and / or by saturating the refined hydrocarbon stream by contact with a heated water stream. The steam added in step (ii) is preferably generated by burning a fuel gas stream in one or more combustion heaters. In a preferred embodiment, the gaseous mixture comprising hydrocarbons and steam is formed by directly mixing the refined hydrocarbon stream with steam, preferably steam generated in one or more combustion heaters, and / or by cooling the reformed gas mixture with water.

[0027] The amount of steam introduced is sufficient to provide a steam-to-carbon ratio (defined as the steam to hydrocarbon carbon ratio at the inlet to the gas-fired reformer) of 2.0:1 to 3.5:1, such as 2.6:1 to 3.5:1. For the avoidance of doubt, a feed containing 75 mol% HO and 25 mol% CH would have a steam-to-carbon ratio of 3.0:1, a feed containing 75 mol% HO, 23 mol% and 2 mol% CH would have a steam-to-carbon ratio of 2.8:1, etc. Operating the reforming section at a steam-to-carbon ratio toward the lower end of the 2.0:1 to 3.5:1 range, for example, 2.0:1 to 2.4:1, reduces the heating requirements and oxygen demand for the reforming stage and has the advantage of smaller and lower cost front-end equipment (e.g., combustion heater, pre-reformer, and autothermal reformer), but typically requires the addition of additional steam to the reformed gas mixture upstream of the water-gas shift unit. In one embodiment, the steam-to-carbon ratio of the gaseous mixture comprising hydrocarbons and steam at the inlet to the gas-heated reformer in step (iii) is between 2.0:1 and 2.4:1, and additional steam is added to the reformed gas mixture upstream of the water-gas shift unit. If the steam-to-carbon ratio is in the range of 2.4:1 to 3.5:1, additional steam addition upstream of the water-gas shift unit is not required, which may be useful in situations where adding steam to the reformed gas is impractical.

[0028] The gaseous mixture containing hydrocarbons and steam is then desirably preheated prior to reforming. In a preferred embodiment, the gaseous mixture is heated by passing it through a combustion heater fueled by at least a portion of the fuel gas, particularly the same combustion heater used to preheat the hydrocarbons. Desirably, the mixed stream is heated to between 400°C and 500°C, preferably between 420°C and 460°C.

[0029] Pre-reforming The gaseous mixture fed to the gas-heated reformer preferably contains 90% or more by volume of methane, excluding all steam, based on the percentage of hydrocarbons present in the mixture. While not generally necessary for light gaseous hydrocarbon feedstocks containing higher hydrocarbons, it may be preferable in some cases to include an adiabatic pre-reforming stage upstream of the gas-heated reformer. In these cases, the gaseous mixture containing hydrocarbons and steam is first subjected to adiabatic steam reforming in a pre-reformer vessel. In such a process, the gaseous mixture containing hydrocarbons and steam is adiabatically passed through a bed of a steam reforming catalyst, usually with a high nickel content, e.g., greater than 40% by weight, at an inlet temperature typically in the range of 400°C to 650°C. During such an adiabatic pre-reforming step, any hydrocarbons higher than methane are reacted with steam to produce a mixture of methane, carbon oxides, and hydrogen. The use of such an adiabatic steam reforming step, commonly referred to as pre-reforming, may be desirable to ensure that the feed to the gas-fired reformer does not contain hydrocarbons higher than methane and also contains some hydrogen.

[0030] Modification Section Following purification and, if necessary, pre-reforming, the gaseous mixture containing hydrocarbons and steam is subjected to steam reforming in a reforming section including a gas-heated reformer and an autothermal reformer. The gas-heated reformer and the autothermal reformer are arranged so that the hot gas exiting the autothermal reformer is used to provide heat for the steam reforming reaction occurring in the gas-heated reformer. Any suitable arrangement may be used in the reforming section, such as a series or parallel arrangement. In a series arrangement, all of the gas exiting the gas-heated reformer is supplied to the autothermal reformer, and all of the hot gas exiting the autothermal reformer is supplied to the shell side of the gas-heated reformer to provide heat for the steam reforming reaction. This arrangement is preferred for grassroots plants. In a parallel arrangement, the gas-heated reformer and the autothermal reformer are each supplied with their own hydrocarbon-containing feed, and the gas from the outlet of the gas-heated reformer and the gas from the outlet of the autothermal reformer are combined and supplied to the shell side of the gas-heated reformer to provide heat for the steam reforming reaction. This arrangement may be suitable in situations where an existing chemical plant containing a reforming section having either a gas-heated reformer or an autothermal reformer is being retrofitted to produce the described plant.

[0031] In one type of gas-fired reformer, the catalyst is disposed in tubes extending between a pair of tube sheets through a heat exchange zone. Reactants are fed to the zone above the upper tube sheet and pass through the tubes into the zone below the lower tube sheet. A heating medium passes through the zone between the two tube sheets. This type of gas-fired reformer is described in GB Patent Application Publication No. 1578270 and WO 97 / 05947. Another type of gas-fired reformer that can be used is a double-tube gas-fired reformer, such as that described in U.S. Pat. No. 4,910,228, in which the reformer tubes each comprise an outer tube having a closed end and an inner tube concentrically disposed within the outer tube, the closed end of the outer tube communicating with the annular space between the inner and outer tubes, with a steam reforming catalyst disposed within the annular space. The outer surface of the outer tube is heated by the autothermal reforming gas. A reaction mixture is fed to the end of the outer tube remote from the closed end so that the mixture passes through the annular space, undergoes steam reforming, and then passes through the inner tube.

[0032] A compressed, preheated gaseous mixture containing hydrocarbons and steam passes through catalyst-packed tubes in a gas-heated reformer. During the reforming catalyst passage, the endothermic steam reforming reaction is driven by the heat required for the reaction, supplied by the autothermal reformed gas, which flows through the outer surface of the tubes. The steam reforming catalyst used in the gas-heated reformer may include nickel supported on a particulate refractory support, such as calcium aluminate, magnesium aluminate, alumina, titania, or zirconia rings or porous pellets. Alternatively, a combination of nickel and a noble metal, such as ruthenium or rhodium, may be used. Instead of, or in addition to, a particulate steam reforming catalyst, the steam reforming catalyst may include one or more structured catalyst units, which may be in the form of a metal or ceramic monolith or folded metal structure on which layers of nickel and / or noble metal steam reforming catalyst are deposited. Such structured catalysts are described, for example, in WO 2012 / 103432 A1 and WO 2013151885 A1.

[0033] The temperature of the autothermal reforming gas used to heat the gas-heating reformer is preferably sufficient so that the gas undergoing steam reforming exits the catalyst tubes at a temperature in the range of 600°C to 850°C, preferably 650°C to 750°C, more preferably 680°C to 720°C. In the present invention, the reformed gas containing methane, hydrogen, steam, and carbon oxides is preferably supplied directly to the autothermal reformer without any dilution or heat exchange, where it is subjected to autothermal reforming, also referred to as secondary reforming. Therefore, steam reforming in the gas-heating reformer can be referred to as primary reforming.

[0034] An autothermal reformer may comprise a burner located at the top of the reformer to which the steam reformate gas and oxygen-rich gas are supplied, a combustion zone below the burner through which a flame extends, and a fixed bed of particulate steam reforming catalyst located below the combustion zone. Thus, in autothermal reforming, heat for the endothermic steam reforming reaction is provided by combustion of some of the hydrocarbons in the feed gas. The steam reformate gas is typically supplied to the top of the reformer, and the oxygen-rich gas is supplied to the burner, with mixing and combustion occurring downstream of the burner to generate a heated gas mixture whose composition is equilibrated as it passes through the steam reforming catalyst. The autothermal steam reforming catalyst may comprise nickel supported on a refractory support such as rings or pellets of calcium aluminate, magnesium aluminate, alumina, titania, zirconia, and the like. In a preferred embodiment, the autothermal steam reforming catalyst comprises a layer of catalyst comprising Ni and / or Ru on zirconia over a bed of Ni on alumina catalyst to reduce catalyst support volatilization which can result in reduced performance of the autothermal reformer.

[0035] The oxygen-rich gas may contain at least 50% O by volume, or may be an oxygen-enriched air mixture. However, in the present invention, the oxygen-rich gas preferably comprises at least 90% O by volume, more preferably at least 95% O by volume, and most preferably at least 98% O by volume, or at least 99% O by volume, such as a pure oxygen gas stream obtainable using a vacuum pressure swing adsorption (VPSA) or air separation unit (ASU). The ASU may be electrically powered, preferably using renewable electricity, to further improve process efficiency and minimize CO2 emissions.

[0036] The amount of oxygen-rich gas added is preferably such that 40 to 60 moles of oxygen are added per 100 moles of carbon in the hydrocarbons fed to the process. The amount of oxygen added is preferably such that the reformed gas leaves the catalyst in the autothermal reformer at a temperature in the range of 800°C to 1100°C, more preferably 900°C to 1100°C, and most preferably 970°C to 1070°C. In a preferred embodiment, a small steam purge may be added to the oxygen-rich gas to prevent backflow in the event of a plant trip.

[0037] The reformed gas produced by the autothermal reformer is used to provide the heat required for the primary steam reforming step by flowing the hot gas through the tubes in the gas-fired reformer. During this heat exchange, the reformed gas transfers heat to the gas undergoing steam reforming, cooling it. Preferably, the reformed gas cools by a few hundred degrees Celsius, but exits the gas-fired reformer at a temperature somewhat higher than the temperature at which the gaseous mixture containing the hydrocarbon and steam mixture is fed to the gas-fired reformer. Preferably, the reformed gas exits the gas-fired reformer at a temperature between 450°C and 650°C, more preferably between 450°C and 580°C.

[0038] It will be understood that the flow of gas in step (iii) flows from the tube side of the gas-heated reformer to the autothermal reformer to the shell side of the gas-heated reformer and then to the water gas shift unit. The shell side of the gas-heated reformer and any downstream locations thereafter are "downstream of the autothermal reformer" as that term is used herein.

[0039] After leaving the reforming section (via the shell side of the gas-heated reformer), the reformed gas is typically then further cooled in one or more steps of heat exchange. Heat recovered during this cooling may be employed to preheat reactants and / or heat water used to provide steam employed in the steam reforming step. As described below, the recovered heat may additionally or alternatively be used in the carbon dioxide separation step. In a preferred embodiment, the reformed gas mixture exiting the shell side of the gas-heated reformer is used to heat water supplied to a saturator.

[0040] Water Gas Shift Unit The reformed gas contains hydrogen, carbon monoxide, carbon dioxide, steam, and a small amount of unreacted methane, and may contain small amounts of inert gases such as nitrogen and argon. Preferably, the hydrogen content of the reformed gas is in the range of 30-45% by volume, and the carbon monoxide content is in the range of 5-15% by volume. In the present invention, the hydrogen content of the partially cooled reformed gas mixture is increased by subjecting it to one or more water-gas shift stages, thereby producing a hydrogen-enriched reformed gas and simultaneously converting the carbon monoxide in the reformed gas to carbon dioxide. The reaction can be illustrated as follows: CO+H2O⇔CO2+H2

[0041] Because steam reforming is carried out with excess steam, it is generally not necessary to add steam to the reformed gas mixture recovered from the autothermal reformer to ensure that sufficient steam is available for the water-gas shift reaction, however, additional steam may be added if desired.

[0042] The partially cooled reformate gas may be subjected to one or more water-gas shift stages in a water-gas shift unit to form a hydrogen-enriched reformate gas stream or "shift" gas stream. The one or more water-gas shift stages may include a high-temperature shift, a medium-temperature shift, an isothermal shift, and a low-temperature shift stage.

[0043] The high temperature shift operates adiabatically in a shift vessel having an inlet temperature in the range of 300° C. to 400° C., preferably 320° C. to 360° C., over a bed of reduced iron catalyst such as chromia-promoted magnetite. Alternatively, a promoted zinc aluminate catalyst may be used.

[0044] The medium-temperature and low-temperature shift stages may be carried out using a shift vessel containing a supported copper catalyst, particularly a copper / zinc oxide / alumina composition. In the low-temperature shift, gas containing carbon monoxide (preferably not more than 6% CO by volume on a dry basis) and steam (at a steam-to-total dry gas molar ratio in the range of 0.3 to 1.5) may be passed over the catalyst in an adiabatic fixed bed with an outlet temperature in the range of 200°C to 300°C. The outlet carbon monoxide content may be in the range of 0.1% to 1.5%, particularly less than 0.5% by volume on a dry basis, if additional steam is added. Alternatively, in the medium-temperature shift, gas containing carbon monoxide and steam may be fed to the catalyst at an inlet temperature in the range of 200°C to 240°C, although the inlet temperature may be as high as 280°C. The outlet temperature may be up to 300°C, but may be as high as 360°C.

[0045] While one or more adiabatic water-gas shift stages, such as a high-temperature shift stage, optionally followed by a low-temperature shift stage, may be employed, the partially cooled reformate gas is preferably subjected to an isothermal water-gas shift stage in a cold-shift vessel, optionally followed by one or more adiabatic intermediate-temperature or low-temperature water-gas shift stages in the non-cooled vessels described above. The use of an isothermal shift stage, i.e., heat exchange in a shift converter, such that an exothermic reaction occurs on a catalyst bed in contact with a heat-removing heat exchange surface, offers the possibility of using the reformate gas stream in a highly efficient manner. While the term "isothermal" is used to describe a cold-shift converter, the temperature of the hydrogen-enriched reformate gas stream at the outlet of the isothermal shift converter may be 1°C to 25°C higher than the inlet temperature, due to the possibility of a slight increase in gas temperature between the inlet and outlet. The coolant may conveniently be water under pressure such that partial or complete boiling occurs. The water may be in tubes surrounded by the catalyst, or vice versa. The resulting steam may be used, for example, to drive a turbine for electricity or to provide process steam for the process. In a preferred embodiment, the steam generated by the isothermal shift stage is used to supplement the steam addition to the gaseous mixture comprising hydrocarbons and steam upstream of the gas-heated reformer, improving the efficiency of the process and allowing relatively high steam-to-carbon ratios to be achieved at low cost.

[0046] The addition of an adiabatic medium-temperature or low-temperature shift stage downstream of the isothermal shift stage offers the potential to increase the CO capture efficiency from this process to over 98%; however, the inventors have found that a single isothermal shift converter can provide superior efficiency.

[0047] Following one or more shift stages, the hydrogen-enriched reformed gas is cooled to a temperature below the dew point so that the steam condenses. The liquid condensate can then be separated using one or more gas-liquid separators, which may have one or more additional cooling stages between them. Any cooling agent may be used. Preferably, the hydrogen-enriched reformed gas stream is first cooled by heat exchange with the process condensate. This results in a heated water stream that can be used to provide some or all of the steam required for reforming. Thus, in one embodiment, the condensate recovered from the hydrogen-enriched reformed gas is used to provide at least a portion of the steam in the gas mixture fed to the steam reforming step of the gas-heated reformer. Because the condensate may contain ammonia, methanol, hydrogen cyanide, and CO2, returning the condensate to steam provides a useful process for returning hydrogen and carbon to the process.

[0048] One or more additional cooling stages are desirable. Cooling may be achieved in one or more stages using demineralized water, air, or a combination thereof during heat exchange. In a preferred embodiment, cooling is achieved by heat exchange with one or more liquids in a CO2 separation unit. In a particularly preferred arrangement, the hydrogen-enriched reformed gas stream is cooled by heat exchange with the condensate, followed by cooling with CO2 reboiler liquid. The cooled shift gas may then be fed to a first gas-liquid separator, and the separated gas may be further cooled with water and / or air and fed to a second separator, and then further cooled with water and / or air and fed to a third separator. A second or third stage condensate separation is preferred. Some or all of the condensate may be used to generate steam for steam reforming. Any condensate not used to generate steam may be sent to water treatment as effluent.

[0049] Carbon Dioxide Separation Unit The carbon dioxide separation step may be carried out using a physical or reactive scrubbing system, preferably a reactive scrubbing system, particularly an amine scrubbing system. Carbon dioxide may also be separated by acid gas recovery (AGR) processes. In the AGR process, the dehydrated hydrogen-enriched reformed gas stream (i.e., the dehydrated shifted gas) is contacted with a suitable absorbent stream, such as an amine, particularly a methyl diethanolamine (MDEA) solution, so that the carbon dioxide is absorbed by the liquid, providing a loaded absorbent and a gas stream (having a reduced carbon dioxide content). The loaded absorbent is then regenerated by heating and / or depressurization to desorb the carbon dioxide and obtain a regenerated absorbent, which is then recycled to the carbon dioxide adsorption step. Alternatively, methanol or glycol may be used to capture carbon dioxide in a manner similar to that of amines. In a preferred arrangement, at least a portion of the heating to regenerate the absorbent is performed using steam generated in one or more fired heaters. If the carbon dioxide separation step operates as a single pressure process, i.e., essentially the same pressure is employed in the adsorption and regeneration steps, then little recompression of the recycled carbon dioxide is required.

[0050] For example, recovered carbon dioxide from AGR can be compressed and used to manufacture chemicals, sent to storage or sequestration, used in enhanced oil recovery (EOR) processes, or used to generate other chemicals. Compression may be achieved using an electrically driven compressor powered by renewable electricity. If the CO2 is to be compressed for storage, transportation, or use in an EOR process, the CO2 may be dried to prevent traces of liquid water from condensing. For example, the CO2 may be dried to a dew point of −10° C. or below by passing it through a bed of a suitable desiccant, such as zeolite, or by contacting it with glycol in a glycol drying unit.

[0051] Upon separating the carbon dioxide, the process provides a crude hydrogen gas stream. The crude hydrogen stream may contain 85-99% by volume hydrogen, preferably 90-99% by volume hydrogen, with the remainder comprising methane, carbon monoxide, carbon dioxide, and inert gases. While this hydrogen gas stream is sufficiently pure for many loads, in the present invention the crude hydrogen gas stream is passed through a purification unit to provide purified hydrogen gas and off-gas, resulting in the use of fuel gas in the process as an alternative to an external fuel source to minimize CO2 emissions from the process.

[0052] Refining Unit The role of the purification unit is to receive the crude hydrogen gas stream from the water-gas shift unit and separate it into a purified hydrogen gas stream and an off-gas stream. Any suitable purification unit can be used. Preferred examples include membrane systems, temperature swing adsorption systems, or pressure swing adsorption systems. Such systems are commercially available. The purification unit is preferably a pressure swing adsorption unit or a temperature swing adsorption unit. Such units include a regenerable porous adsorbent material that selectively captures and thereby purifies gases other than hydrogen. The purification unit produces a pure hydrogen stream, preferably greater than 99.5% by volume, more preferably greater than 99.9% by volume, which may be compressed and used in downstream power or heating processes, for example, by using it as fuel in a gas turbine (GT) or by injection into a domestic or industrial network gas piping system. The pure hydrogen may also be used in downstream chemical synthesis processes. Thus, the pure hydrogen stream may be used to produce ammonia by reaction with nitrogen in an ammonia synthesis unit. Alternatively, the pure hydrogen may be used with a carbon dioxide-containing gas to produce methanol in a methanol production unit. Alternatively, pure hydrogen may be used with carbon monoxide-containing gases to synthesize hydrocarbons in a Fischer-Tropsch production unit. Any known ammonia, methanol, or Fischer-Tropsch production technology may be used. Alternatively, the hydrogen may be used to upgrade hydrocarbons, for example, by hydrotreating or hydrocracking hydrocarbons in a hydrocarbon refinery, or in any other process in which pure hydrogen may be used. Compression may again be achieved using an electrically driven compressor powered by renewable electricity.

[0053] In the present invention, the off-gas stream typically has an H content of less than 90 vol.%, typically 70-90 vol.%, such as 75-85 vol.%. The off-gas typically has a hydrocarbon content of typically 2.5-7.5 vol.%, such as 4-6 vol.%. The off-gas has a higher hydrocarbon content and a lower hydrogen content than the off-gas in the arrangement described in WO 2022 / 003312 A1, stream (142) having a hydrogen content of 87.56 mol.% and a methane content of 2.19 mol.%.

[0054] recycling The off-gas stream is split into a fuel gas stream and a recycle stream (step (viii)). The fuel gas stream is fed to one or more fired heaters that are used to heat one or more process streams within the process. The recycle stream is then compressed (step (ix)) and split into a hydrodesulfurization recycle stream and a process recycle stream (step (x)). The hydrodesulfurization recycle stream is fed to a hydrodesulfurization unit (step (xi)) to provide hydrogen for the reaction occurring therein. The process recycle stream is returned to one or more locations (step (xii)). The process recycle stream can be reintroduced into the process at a variety of different locations (configurations (xii-a) to (xii-e)).

[0055] The relative mass flow rates of fuel gas, process recycle stream, and hydrodesulfurization recycle stream depend on several factors, including the combustion heater demand, the H demand of the dehydrodesulfurization unit, and the H content of the hydrogen-containing off-gas stream. The percentage used as process recycle is determined by subtracting the percentage required for the combustion heater duty, then subtracting the percentage required for the HDS duty. The remainder is sent to recycle.

[0056] In preferred embodiments, the recycle stream is reintroduced into the process downstream of the hydrodesulfurization unit and upstream of the gas-heated reformer (arrangement (xii-a)), or downstream of the gas-heated reformer and upstream of the autothermal reformer (arrangement (xii-b)). In relation to arrangement (xii-a), if a pre-reformer is present, the recycle stream is preferably reintroduced downstream of the pre-reformer and upstream of the gas-heated reformer. An advantage of these arrangements is that they provide another opportunity to convert residual hydrocarbons in the off-gas to hydrogen, thus improving the hydrogen yield per unit of hydrocarbon. A further advantage of these arrangements is that they provide another opportunity to convert residual carbon monoxide in the off-gas to carbon dioxide, since the residual carbon monoxide in the off-gas further passes through a water-gas shift section. This reduces carbon monoxide emissions from the process. A still further advantage of these arrangements is that they provide another opportunity to capture residual carbon dioxide in the off-gas, since the residual carbon dioxide in the off-gas further passes through a carbon dioxide separation unit. It will be appreciated that the feed to a gas-heated or autothermal reformer is typically much hotter than the off-gas from the purification unit, and the off-gas may need to be heated before being reintroduced.

[0057] In one embodiment of configuration (xii-a), the relative mass flow rates (e.g., in tonnes / h) of fuel gas:process recycle stream:hydrodesulfurization recycle stream are 66±3:28±3:6±3, and the sum of these values ​​is 100. Preferably, the relative mass flow rates of these streams are 66±2:28±2:6±2, such as 66±1:28±1:6±1.

[0058] In one embodiment, the recycle stream is reintroduced into the process downstream of the autothermal reformer and upstream of the water gas shift unit (arrangement (xii-c)). This arrangement offers the opportunity to reduce the carbon monoxide content of the off-gas.

[0059] In one, the recycle stream is reintroduced into the process at a location downstream of the water-gas shift unit and upstream of the carbon dioxide removal unit (configuration (xii-d)). This configuration provides an additional opportunity to separate hydrogen from unconverted hydrocarbons.

[0060] In one, the recycle stream is reintroduced into the process at a location downstream of the carbon dioxide removal unit and upstream of the purification unit (configuration (xii-e)). This configuration provides an additional opportunity to separate the hydrogen and unconverted hydrocarbons.

[0061] It will be appreciated that more complex arrangements are possible, for example where the recycle stream is introduced at two or more different locations of the options specified. For simplicity of design, it is preferred that the recycle be introduced at only a single location (xii-a) through (xii-e), preferably (xii-a) or (xii-b).

[0062] The combination of steps described herein provides sufficient fuel gas to heat the process streams used in the process without significant additional fuel during normal operation. The volume of supplemental fuel in the process is desirably kept to a minimum to maximize CO2 capture efficiency. The amount of supplemental fuel, e.g., natural gas, supplied to one or more fired heaters along with the fuel gas is preferably less than 5% by volume of the total fuel provided, more preferably less than 3% by volume of the total fuel provided, and most preferably less than 2% by volume of the total fuel provided.

[0063] In some situations, such as during start-up of the process, it may be necessary to temporarily supplement the fuel gas with hydrocarbon fuel, but this should not significantly reduce the efficiency of the process; however, during normal operation, the fuel gas recovered from the refinery unit is the primary source of fuel provided to one or more fired heaters.

[0064] In some embodiments, a single combustion heater fueled at least in part by fuel gas is sufficient to heat hydrocarbons, reformed gas recovered from a pre-reforming stage upstream of the autothermal reforming stage, and water to generate at least a portion of the steam for the process.

[0065] While all process streams requiring heating can be heated within a single fired heater, in a preferred arrangement, one fired heater is used for the hydrocarbon- and / or hydrogen-containing process gas stream and another is used solely to boil water for steam generation. Therefore, the latter could also be described as a boiler. Thus, fuel gas can be split between the first fired heater, used to heat the hydrocarbon- and / or hydrogen-containing stream, and the second fired heater, used to boil water to generate steam. Using two fired heaters in this manner offered several distinct advantages: steam could be raised in the second fired heater and thereby used as part of plant startup; steam could be generated in the second fired heater while the plant was shut down and supplied to the plant during the shutdown process; the first and second fired heaters could be operated independently, facilitating startup because no coils would be heated in a no-flow situation; and the isolation of the first fired heater allowed the second fired heater to either run or warm nitrogen as part of the startup procedure while it was being started. The fuel gas split to the first and second combustion heaters can be in the range of 10-90% by volume to 90-10% by volume, respectively, but is preferably in the range of 60-80% by volume for the first combustion heater and 40-20% by volume for the second combustion heater.

[0066] The steam generated in the second fired heater can be used to heat the CO2 absorption liquid in the carbon dioxide separation unit. The second fired heater can also be used to superheat steam recovered from a steam drum coupled to a waste heat boiler heated by the reformed gas. The waste heat boiler is also preferably used to generate steam used to preheat the oxygen-rich gas and / or to provide process steam added upstream of the water-gas shift unit to maximize the conversion to hydrogen and carbon dioxide. A portion of the steam from the waste heat boiler can also be passed through a steam expander to generate electricity. [Example]

[0067] Example 1 (Comparative Example) This example corresponds to the flowsheet shown in Figure 1 of WO 2022 / 003312 A1. The steam-to-carbon ratio of the feed to the gas-fired reformer is 3.1:1. Selected heat and mass balance calculations from the example modeled in WO 2022 / 003312 A1 are shown in Table 1. A simplified version of the flowsheet is shown in Figure 1.

[0068] Example 2 (according to the present invention) The flowsheet shown in WO 2022 / 003312 A1 was modified as follows: The steam-to-carbon ratio of the feed to the gas-fired reformer was 2.6:1. The off-gas was split into a fuel gas stream and a recycle stream. The recycle stream itself was split into a process recycle stream (219) and a desulfurization recycle stream (220). This example corresponds to the arrangement shown in Figure 2. Heat and material balance calculations for selected streams are shown in Table 2. The relative mass flow rates of fuel gas:process recycle stream:hydrodesulfurization recycle stream are 66:28:6 in this example.

[0069] [Table 1]

[0070] [Table 2]

Claims

1. 1. A process for producing hydrogen, comprising: (i) passing a hydrogen stream (220, 320, 420, 520) and a hydrocarbon-containing feed stream (201, 301, 401, 501) through a hydrodesulfurization unit (203, 303, 403, 503) to hydrodesulfurize and produce a refined hydrocarbon stream; (ii) adding steam (204, 304, 404, 504) to the purified hydrocarbon stream to produce a gaseous mixture (205, 305, 405, 505) comprising hydrocarbons and steam; (iii) subjecting said gaseous mixture comprising hydrocarbons and steam to steam reforming in a reforming section (207, 307, 407, 507) comprising a gas-heated reformer (207a, 307a, 407a, 507a) and an autothermal reformer (207b, 307b, 407b, 507b) to generate a reformed gas mixture (208, 308, 408, 508); (iv) increasing the hydrogen content of the reformed gas mixture by subjecting the reformed gas mixture to one or more water-gas shift stages in a water-gas shift unit (210, 310, 410, 510) to provide a hydrogen-enriched reformed gas (211, 311, 411, 511); (v) cooling the hydrogen-enriched reformed gas and separating condensed water therefrom to provide a dehydrated hydrogen-enriched reformed gas; (vi) passing the dehydrated hydrogen-enriched reformed gas through a carbon dioxide separation unit (212, 312, 412, 512) to provide a carbon dioxide gas stream (213, 313, 413, 513) and a crude hydrogen gas stream (214, 314, 414, 514); (vii) passing the crude hydrogen gas stream from the carbon dioxide removal unit through a purification unit (215, 315, 415, 515) to provide a purified hydrogen gas stream (216, 316, 416, 516) and a hydrocarbon-containing off-gas stream (217, 317, 417, 517); (viii) dividing the off-gas stream into a fuel gas stream (218, 318, 418, 518), which is supplied to one or more fired heaters used to heat one or more process streams within the process, and a recycle stream (219, 319, 419, 519); (ix) compressing the recycle stream; (x) dividing the compressed recycle stream into a hydrodesulfurization recycle stream (220, 320, 420, 520) and a process recycle stream (221, 321, 421, 521); (xi) feeding the hydrodesulfurization recycle stream to the hydrodesulfurization unit; (xii) subjecting said process recycle stream to (xii-a) downstream of the hydrodesulfurization unit and upstream of the gas-heated reformer; (xii-d) downstream of the gas-heated reformer and upstream of the autothermal reformer; (xii-c) downstream of the autothermal reformer and upstream of the water gas shift unit; (xii-d) downstream of the water gas shift unit and upstream of the carbon dioxide separation unit; or (xii-e) returning the carbon dioxide to one or more locations selected from downstream of the carbon dioxide separation unit and upstream of the purification unit.

2. 2. The process of claim 1, wherein the steam to carbon ratio of the gaseous mixture comprising hydrocarbons and steam at the inlet to the gas-heated reformer in step (iii) is from 2.0:1 to 3.5:

1.

3. 2. The process of claim 1, wherein the steam to carbon ratio of the gaseous mixture comprising hydrocarbons and steam at the inlet to the gas-heated reformer in step (iii) is from 2.6:1 to 3.5:

1.

4. 2. The process of claim 1, wherein the steam-to-carbon ratio of the gaseous mixture comprising hydrocarbons and steam at the inlet to the gas-heated reformer in step (iii) is from 2.0:1 to 2.4:1, and additional steam is added to the reformed gas mixture upstream of the water-gas shift unit.

5. The process of any one of claims 1 to 4, wherein the recycle stream is returned to the process at location (xii-a).

6. The process of any one of claims 1 to 5, wherein the process recycle stream is returned to the process at a single location.

7. The process of any one of claims 1 to 6, wherein the gas-heated reformer and the autothermal reformer are arranged in series.

8. 8. The process of any one of claims 1 to 7, wherein the off-gas stream separated in step (vii) comprises 70 to 90% by volume of hydrogen.

9. 9. The process of any one of claims 1 to 8, wherein the off-gas stream separated in step (vii) comprises 2.5 to 7.5 vol% hydrocarbons.

10. 10. The process of any one of claims 1 to 9, wherein the gaseous mixture comprising hydrocarbons and steam is formed by hydrocarbons including steam generated by the one or more combustion heaters and / or by cooling the reformed gas mixture with water.

11. The process of any one of claims 1 to 10, wherein the purification unit is a pressure swing adsorption unit or a temperature swing adsorption unit.

12. 12. The process of any one of claims 1 to 11, wherein the steam added in step (ii) is generated by combusting the fuel gas stream in the one or more fired heaters.

13. A chemical plant, (i) a hydrodesulfurization unit (203, 303, 403, 503) configured to receive a hydrogen stream (202, 302, 402, 502) and a hydrocarbon-containing feed stream (201, 301, 401, 501) and to hydrodesulfurize the hydrogen stream to produce a refined hydrocarbon stream; (ii) means for adding steam (204, 304, 404, 504) to said refined hydrocarbon stream to produce a gaseous mixture (205, 305, 405, 505) comprising hydrocarbons and steam; (iii) a reforming section (207, 307, 407, 507) comprising a gas-heated reformer (207a, 307a, 407a, 507a) and an autothermal reformer (207b, 307b, 407b, 507b) arranged to receive the gaseous mixture comprising hydrocarbons and steam and to generate a reformed gas mixture (208, 308, 408, 508); (iv) a water-gas shift (210, 310, 410, 510) unit arranged to receive the reformed gas mixture and generate a hydrogen-enriched reformed gas (211, 311, 411, 511); (v) means for cooling the hydrogen-enriched reformed gas and separating condensed water therefrom to provide a dehydrated hydrogen-enriched reformed gas; (vi) a carbon dioxide separation unit (212, 312, 412, 512) arranged to receive the dehydrated hydrogen-enriched reformed gas and generate a carbon dioxide gas stream (213, 313, 413, 513) and a crude hydrogen gas stream (214, 314, 414, 514); (vii) a purification unit (215, 315, 415, 515) configured to receive the crude hydrogen gas stream and generate a purified hydrogen gas stream (216, 316, 416, 516) and a hydrocarbon-containing off-gas stream (217, 317, 417, 517); (viii) means for splitting said off-gas stream into a fuel gas stream (218, 318, 418, 518) and a recycle stream (219, 319, 419, 519), and means for supplying said fuel gas stream to one or more fired heaters used to heat one or more process streams within said process; (ix) means for compressing said recycle stream; (x) means for dividing the compressed recycle stream into a hydrodesulfurization recycle stream (220, 320, 420, 520) and a process recycle stream (221, 321, 421, 521); (xi) means for supplying said hydrodesulfurization recycle stream to said hydrodesulfurization unit; (xii) subjecting said process recycle stream to (xii-a) downstream of the hydrodesulfurization unit and upstream of the gas-heated reformer; (xii-d) downstream of the gas-heated reformer and upstream of the autothermal reformer; (xii-c) downstream of the autothermal reformer and upstream of the water gas shift unit; (xii-d) downstream of the water gas shift unit and upstream of the carbon dioxide separation unit; or (xiii-e) means for returning the carbon dioxide to one or more locations selected from downstream of the carbon dioxide separation unit and upstream of the purification unit.

14. 14. The chemical plant of claim 13, wherein the plant comprises a pre-reformer either upstream of the hydrodesulfurization unit or downstream of the hydrodesulfurization unit and upstream of the gas-heated reformer.

15. The chemical plant according to claim 13 or 14, wherein the recycle stream is returned to the process at position (xii-a).

16. The chemical plant of any one of claims 13 to 15, wherein the process recycle stream is returned to the process at a single location.

17. The chemical plant according to any one of claims 13 to 16, wherein the gas-heated reformer and the autothermal reformer are arranged in series.

18. 18. The chemical plant of any one of claims 13 to 17, wherein the steam added in step (ii) is generated by burning a fuel gas stream in the one or more fired heaters.

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