Reforming process with post-treatment of a hydrogen-containing gas

Abstract

A process (1) comprising: (I) generating pure oxygen or oxygen-enriched air (2) by an air separation unit (3); (II) reforming (4) a desulphurized hydrocarbon feedstock (5) in presence of said pure oxygen or oxygen-enriched air (2) and steam (6) to obtain a hydrogen-containing synthesis gas (7); (III) post-treatment of said hydrogen-containing synthesis gas (7) comprising CO shift (8), CO2 removal (9), and hydrogen purification (10); said hydrogen purification (10) being arranged to separate a hydrogen-rich gas (11, 30) having a hydrogen content ≥ 98.5% molar, and a methane-containing tail stream (12); (IV) recycling at least a portion of said methane-containing tail stream (12, 12') as feed to step (II).

Description

[0001] Reforming process with post-treatment of a hydrogen-containing gas

[0002] Field of the invention

[0003] The present invention relates to the field of the production of a synthesis gas by reforming of a hydrocarbon-containing gas, particularly for the production of an ammonia makeup gas. More precisely, the invention relates to recycling purification process streams for reducing a carbon footprint in the production of a hydrogen-rich gas.

[0004] Prior art

[0005] The production of a hydrogen-containing gas, particularly of ammonia makeup gas, is typically based on the conversion of a hydrocarbon source into a synthesis gas in a reformer. The so obtained reformed synthesis gas is further processed in order to increase the hydrogen content thereof and to separate a stream of concentrated carbon dioxide CO2. The processed synthesis gas may be mixed with nitrogen to give a makeup gas for ammonia synthesis.

[0006] In the art there is a growing interest in minimizing the carbon dioxide emission of the hydrogen plants especially in view of the continuously evolving regulations concerning the emission of greenhouse gas.

[0007] Several solutions are available to reduce the carbon dioxide emissions from a hydrogen plant, but the most applied solution is to capture the carbon dioxide from the synthesis gas and to store the sequestrated CO2 underground.

[0008] Unfortunately, the carbon dioxide sequestrated from the synthesis gas only addresses a limited fraction of the total carbon dioxide emissions of the plant. In practice, the CO2 emitted from the plant is the sum of several contributions including the CO2 generated by the chemical conversion of the hydrocarbons into syngas, but also the CO2 generated by the operation of auxiliary systems of the plant, e.g., of the fired heater(s). This latter contribution is not negligible and is seldom taken into consideration in the art. EP 4140945 A1 and WO 2022 / 038089 A1 belong to the prior art.

[0009] Summary of the invention

[0010] The invention concerns a process and plant according to the attached independent claims. Preferred embodiments are described in the dependent claims.

[0011] An object of the present invention is a process comprising the following steps:

[0012] (I) generating pure oxygen or oxygen-enriched air by an air separation unit (ASU);

[0013] (II) reforming a desulphurized hydrocarbon feedstock, e.g., natural gas, in presence of said pure oxygen or oxygen-enriched air and steam to obtain a hydrogen-containing synthesis gas;

[0014] (III) post-treatment of said hydrogen-containing synthesis gas comprising CO shift, CO2 removal, and hydrogen purification; said hydrogen purification being arranged to separate a hydrogen-rich gas having a hydrogen content > 98.5% molar, and a methane-containing tail stream;

[0015] (IV) recycling at least a portion of said methane-containing tail stream as feed to step (II).

[0016] Another object of the present invention is a plant comprising:

[0017] - an air separation unit arranged to generate pure oxygen or oxygen-enriched air, said oxygen-enriched air having an oxygen content > 50% molar, preferably > 70% molar, more preferably > 90% molar;

[0018] - a reforming section arranged to reform a desulphurized hydrocarbon feedstock, e.g., natural gas, in presence of said pure oxygen or oxygen- enriched air and steam to obtain a hydrogen-containing synthesis gas;

[0019] - a CO shift unit, a CO2 removal unit, and hydrogen purification unit for post- treatment of said hydrogen-containing synthesis gas;

[0020] - said hydrogen purification unit being arranged to separate a hydrogen-rich gas having a hydrogen content > 98.5% molar, and a methane-containing tail stream;

[0021] - a line for recycling at least a portion of said methane-containing tail stream as feed to the reforming section.

[0022] In practice, the present invention allows to reduce the duty of the fired heater, the size of the ASU and recycle most of the unconverted carbon back to the process as reforming feed. Overall, process or plant CO2 emissions are reduced and the efficiency of the process or plant is increased. Further also the Capex of the plant are reduced as consequence of the reduction in size of the ASU and the fired heater.

[0023] Thanks to the invention, a hydrogen-rich gas is produced and the carbon intensity of the process can be reduced to a desirable value of 0.1 kg CO2 / kg H2.

[0024] Description of the preferred embodiments of the present invention

[0025] According to an embodiment, said process or plant is for obtaining a hydrogen-rich gas 11 or an ultra-high-purity hydrogen 30.

[0026] According to another embodiment, said process or plant is for obtaining an ammonia- containing stream 34, preferably ammonia or substantially pure ammonia.

[0027] According to still another embodiment, said process or plant is for obtaining a urea- containing stream 58, preferably a urea solution or a urea melt.

[0028] Said oxygen-enriched air preferably has an oxygen content > 50% molar, more preferably > 70% molar, even more preferably > 90% molar.

[0029] According to an embodiment, said pure oxygen or oxygen-enriched air 2 is provided at a pressure of at least 20 bar, preferably comprised from 20 bar to 80 bar, more preferably comprised from 30 bar to 70 bar, even more preferably comprised from 40 bar to 60 bar, still more preferably comprised from 45 bar to 55 bar. According to another embodiment, a nitrogen-rich stream 32 is also generated by said ASU 3 of step (I). Said nitrogen-rich stream 32 is preferably at room temperature and at atmospheric pressure.

[0030] According to different embodiments, said reforming step (II) or reforming section 4 comprises an autothermal reforming (ATR) step or section, or a partial oxidation (POX) step or section, or an ATR + gas-heated reforming (GHR) step or section.

[0031] According to an embodiment, said ATR + GHR step or section comprises: an autothermal reforming (ATR) arranged to receive a first portion of the desulphurized hydrocarbon feedstock and generating a first stream of hydrogen-containing synthesis gas; a gas-heated reforming comprising a first side (e.g., a tube side) and a second side (e.g., a shell side), wherein said gas-heated reforming is arranged to receive a second portion of said desulphurized hydrocarbon feedstock in said first side and to generate a second stream of hydrogen-containing synthesis gas in the same side, and said first stream of hydrogen-containing synthesis gas and said second stream of hydrogencontaining synthesis gas are mixed in said second side to generate said hydrogencontaining synthesis gas.

[0032] Preferably, said hydrogen-rich gas 11 has a hydrogen content comprised from 98.5% molar to 99.5% molar.

[0033] Said methane-containing tail stream 12, 12’ is preferably entirely recycled as feed to said step (II).

[0034] Preferably, said methane-containing tail stream 12, 12’ has a methane content > 90% molar, preferably > 93% molar, more preferably > 95% molar. More preferably, said methane-containing tail stream 12, 12’ has a low nitrogen content (i.e., below 1 % molar) so that no nitrogen is accumulated in the process gas.

[0035] According to other embodiments, said portion of methane-containing tail stream 12, 12’ is recycled in step (IV) in at least one of the following locations: upstream of a desulphurization 41 of a hydrocarbon feedstock to be desulphurized, and / or upstream of a pre-reforming 38 of said desulphurized hydrocarbon feedstock, and / or between a pre-reforming 38 of said desulphurized hydrocarbon feedstock and said reforming 4; preferably upstream of the pre-reforming 38 of said desulphurized hydrocarbon feedstock.

[0036] According to different embodiments, said hydrogen purification 10 comprises a pressure swing absorption (PSA), and / or a membrane separation, and / or a cryogenic purification step, preferably a cryogenic purification step.

[0037] According to a preferred embodiment, said hydrogen purification 10 comprises, in sequence:

[0038] - PSA and cryogenic purification on a PSA effluent; or

[0039] - PSA, methanation 28 and cryogenic purification on a methanation effluent; to separate the hydrogen-rich gas and the methane-containing tail stream.

[0040] According to another embodiment, at least a portion of the hydrogen-rich gas is fed as fuel to a fired heater of the reforming step or section, and at least a portion of said methane-containing tail stream is recycled as feed to the reforming step (II) or section.

[0041] Preferably, a cryogenic purification section 13 is used in said hydrogen purification 10.

[0042] According to an embodiment, said cryogenic purification section 13 comprises an indirect heat exchange 20 downstream of said CO2 removal 9 and of an optional drying to obtain a refrigerated and optionally dried modified syngas 15 at a temperature below -100 °C, said cryogenic purification section 13 further comprising a first liquid-gas separator 14 of the refrigerated and optionally dried modified syngas 15. Preferably, said refrigerated and optionally dried modified syngas 15 is at a temperature below - 130 °C, more preferably below -150 °C, even more preferably below -170 °C, e.g., comprised from -180 °C and -190 °C.

[0043] According to different embodiments, drying may be performed by physical adsorption (e.g., on molecular sieves or other desiccant), or by absorption (i.e. , using a solvent), in a drying unit. Preferably, drying involves removing only water, or removing water and carbon dioxide, depending on the type of drying unit.

[0044] According to another embodiment, said process comprises: heating said methane- containing tail stream 12 in said cryogenic purification section 13 by indirect heat exchange 20, a heated methane-containing tail stream 12’ being recycled in step (IV).

[0045] According to a first embodiment, said process comprises: heating at least part of (e.g., all) the hydrogen-rich gas 11 in said cryogenic purification section 13 by indirect heat exchange 20.

[0046] According to a second embodiment, said process comprises: heating a first portion 16 of the hydrogen-rich gas 11 in said cryogenic purification section 13 by indirect heat exchange 20, a second portion 17 of the hydrogen-rich gas 11 being further treated in a second cryogenic purification section 18 to obtain an ultra-high-purity hydrogen 30 having a hydrogen content > 99.5% molar.

[0047] Preferably, the hydrogen content of the ultra-high-purity hydrogen 30 is > 99.7% molar, more preferably > 99.9% molar, even more preferably > 99.99% molar, e.g., 99.999% molar.

[0048] More preferably, said hydrogen-rich gas 11 and / or ultra-high-purity hydrogen 30 is / are used for producing an ammonia synthesis makeup gas 33, in particular by mixing said hydrogen-rich gas 11 and / or ultra-high-purity hydrogen 30 with said nitrogen-rich stream 32 to provide said ammonia synthesis makeup gas 33 having an appropriate hydrogen to nitrogen (H / N) molar ratio. Preferably, said H / N molar ratio is comprised from 2.8 to 3.2, more preferably from 2.9 to 3.1 .

[0049] According to other embodiments, said hydrogen-rich gas 11 and / or ultra-high-purity hydrogen 30 gas are fed as fuel to a fired heater, e.g., to pre-heat process streams prior to desulphurization 41 , and / or pre-reforming 38, and / or reforming 4.

[0050] Preferably, said second cryogenic purification section 18 is arranged for: (V.A) washing said second portion 17 of the hydrogen-rich gas 11 with a liquid nitrogen solution 23 in a contacting unit 19, e.g., an absorption column, to obtain a washed hydrogen-rich gas 21 and an impurities-loaded solution 22;

[0051] (V.B) treating the washed hydrogen-rich gas 21 of step (V.A) by indirect heat exchange 25 with a cryogenic expander or a cryogenic refrigeration unit 24 comprising a helium or hydrogen refrigerant cycle; to obtain said ultra-high-purity hydrogen 30 and a tail liquid stream 26 comprising liquid nitrogen and impurities; and

[0052] (V.C) recycling the tail liquid stream 26 to the contacting unit of step (V.A).

[0053] According to an embodiment, step (III) comprises - in sequence - CO shift 8, first steam condensation 27, CO2 removal 9, methanation 28 and second steam condensation 29 to obtain a modified syngas 31 ; followed by feeding said modified syngas 31 to said hydrogen purification 10.

[0054] According to possible embodiments, superheated steam 39 is generated by cooling the hydrogen-containing synthesis gas 7 or a shifted gas 47 (i.e. , an effluent of a CO shift unit 8) in a heat exchanger 44, preferably a steam superheater (SSH).

[0055] According to another embodiment, at least part of a CO2-rich stream 35 obtained in the CO2 removal 9 is subjected to compression 36 at a pressure above 100 bar (preferably comprised from 150 bar to 200 bar), and a so obtained compressed carbon dioxide 37 is exported. For instance, the compressed carbon dioxide 37 may be stored under pressure for carbon capture, and / or used for enhanced oil recovery, and / or reacted with an ammonia-containing stream 34 in a urea synthesis 57.

[0056] Preferably, said plant or process does not include neither a gas turbine engine for producing mechanical power, nor an electrical energy generator coupled to said gas turbine engine.

[0057] According to a preferred embodiment, step (II) is performed with a steam to carbon (S / C) ratio of no more than 3.0, preferably of no more than 2.0, with pre-reforming 38 in an adiabatic reactor, without a primary reforming.

[0058] According to another preferred embodiment, superheated steam 39 is generated by cooling the hot hydrogen-containing synthesis gas 7 prior to CO2 removal 9.

[0059] According to an embodiment of said plant, said hydrogen purification unit 10 comprises a cryogenic purification section 13 comprising an indirect heat exchanger 20 downstream of said CO2 removal unit 9 and of an optional drying unit to obtain a refrigerated and optionally dried modified syngas 15 at a temperature below -100 °C (preferably below -130 °C, more preferably below -150 °C, even more preferably below -170 °C, e.g., comprised from -180 °C and -190 °C); said cryogenic purification section 13 further comprising a first liquid-gas separator 14 of the refrigerated and optionally dried modified syngas 15.

[0060] Preferably, said methane-containing tail stream 12 is heated in said indirect heat exchanger 20, said plant further comprising a line for recycling a heated methane- containing tail stream 12’ in the reforming section 4.

[0061] Said plant is preferably characterized in being an ammonia synthesis front-end for producing an ammonia synthesis makeup gas 33 having a hydrogen to nitrogen (H / N) molar ratio for ammonia synthesis 40.

[0062] Advantages of the present invention

[0063] Advantageously, steam superheating is performed using a portion of a waste heat at the outlet of a waste heat boiler of an autothermal reformer used in the reforming process. This feature results in a significant reduction of the amount of steam that is generated by process waste heat recovery, for example in an ammonia production process section.

[0064] Advantageously, the fired heater(s) is / are fired with a fuel gas including a CO2-depleted hydrogen-containing gas generated in the process, optionally mixed with natural gas. This feature reduces the CO2 emissions caused by the fired heater(s).

[0065] Advantageously, the fired heater(s) is / are co-fired with a mixture of natural gas and CCh-depleted gas, depending on the decarbonization level required for the overall plant which is strongly related with the maximum CO2 emission limit.

[0066] Advantageously, the invention in its various embodiments introduces a significant reduction of the overall natural gas consumption and of overall CO2 emissions to atmosphere compared to the prior art.

[0067] An advantage of a low-S / C pure ATR is reducing the amount of steam generated in the process and reducing the combustion of carbon-containing fuel in the reforming process. Reducing the amount of steam is an advantage because steam needs be superheated to produce useful energy and superheating is typically made at the expense of carbon-containing fuel. Higher single-train capacities of the front-end and better economy of scale of the overall plant or process can be obtained, because less steam has to be preheated in the stream feeding the ATR. Also, the process and plant can be operated without any export of steam outside of the process.

[0068] The eventual, consequent drawback of having more slip of methane from the reforming section is compensated - in a synergistic manner - by the optional cryogenic separation of methane. More methane in the feed (i.e. lower S / C) produces more residual methane, and the methane-containing tail stream is more concentrated. The temperature difference between the warm streams and cold streams in the cryogenic heat exchange is consequently increased, so that cryogenic separation of methane is more effective.

[0069] Advantageously, at least some of the carbon dioxide removed from the reformed gas is compressed at a high pressure. The so obtained high-pressure carbon dioxide may be stored under pressure (carbon capture) or further used for process purposes; a preferred use for the so obtained CO2 under pressure is enhanced oil recovery (EOR) and a particularly preferred use is a feed for the synthesis of urea.

[0070] More advantageously, the process of the invention may be used in ammonia-urea combined production wherein the process of the invention is used to produce ammonia makeup gas; the ammonia feeds the urea synthesis 57 together with compressed CO2 37 removed during purification of the synthesis gas. C02compression for use as EOR or storage is typically above 100 bar and preferably in a range comprised from 150 bar to 200 bar. This pressure is close to a typical synthesis pressure of urea from ammonia and carbon dioxide. Hence an advantageous aspect of the invention is the use of compressed CO2in the synthesis of urea.

[0071] Advantageously, more than 95% molar (preferably more than 97% molar) of carbon dioxide can be removed from the reformed gas. This leads to the following advantages:

[0072] - almost complete carbon separation from the process streams containing hydrogen or ammonia;

[0073] - carbon can be recycled as feed to the reforming process;

[0074] - obtaining a better energy efficiency with respect to known alternative processes that, as an example, operate with higher S / C ratios.

[0075] Advantageously, a combination of PSA, optionally methanation, and cryogenic purification brings to a desired reduction in size of the reforming section.

[0076] Advantageously, a concentrated methane-containing stream is obtained already at the second steam condensation, i.e., before the modified syngas enters the hydrogen purification.

[0077] Advantageously, the hydrogen purification can be operated so as to leave certain amounts of nitrogen for the optional and subsequent ammonia synthesis.

[0078] Advantageously, the present process and plant were designed to avoid freezing (e.g., of water and CO2) at cryogenic temperatures.

[0079] Advantageously, reaching temperatures that are lower than the dew point temperature of the dried gas has the advantage that the formed liquid phase contains mainly methane: the condensation temperature of methane is from about -150 °C to -160 °C but the condensation temperature of hydrogen is much higher at a pressure > 20 bar, preferably > 30 bar, more preferably > 40 bar. Advantageously, also some CO may be separated in the condensed phase. Operating under these pressures allows to separate higher amounts of liquid phase at equilibrium. Advantageously, a substantially inert-free make-up gas involves the following advantages:

[0080] - lower syngas compression power;

[0081] - higher process efficiency;

[0082] - lower fuel consumption (e.g., up to 5% of the total energy consumption).

[0083] Advantageously, cryogenic purification results in a higher efficiency conversion to ammonia with respect to methanation because hydrogen is not consumed in the methanation section to reduce residual oxygenated compounds (e.g. CO) up to ppm level, and consequently more ammonia can be obtained with reforming equipment and lines having a same size, which in turns enables a higher single train capacity of the plant or process.

[0084] However, an embodiment may contemplate that cryogenic purification is used downstream of methanation after all the oxygenated compounds (CO, CO2) have been reduced to methane.

[0085] Advantageously, a process or plant with no primary reforming (SMR) can reduce fuel requirements to indirectly provide the reaction duty for the endothermic reforming process in a radiant section of a furnace. In an ATR, heat for the endothermic reforming reactions is directly provided in the same reactor by coupling exothermic partial oxidation reactions. In this way, the CO2 produced in the ATR is more concentrated and at higher pressure than the CO2 that can be produced in the exhaust gas from the fuel side of a primary reformer furnace, and it is more easily separated.

[0086] Advantageously, lower Capex can be obtained for separating CO2 from a pressurized and concentrated gas (downstream reforming and CO shift) than from a flue gas of a primary reformer furnace.

[0087] Advantageously, pre-reforming increases flexibility to operate with different feeds, especially in refineries where it is important to use the most available and economic feed. Advantageously, pre-reforming leads to lower Capex due to a smaller size of a downstream ATR.

[0088] Advantageously, pre-reforming eases operating the reformer at a higher inlet temperature without any preheat coil cracking issues as higher hydrocarbons are already converted to methane and hydrogen.

[0089] Advantageously, pre-reforming enables operation at low steam to carbon ratios.

[0090] Advantageously, adiabatic pre-reforming enables to drop the temperature after such step, and requires to put more heat in the process by heat recovery. This increases the overall efficiency by further reducing any steam export.

[0091] The invention is now further elucidated with reference to preferred embodiments and with the help of the figures.

[0092] Description of figures

[0093] Fig. 1 : block diagram showing a process or plant of the present invention, according to a first possible embodiment;

[0094] Fig. 2: block diagram showing a process or plant of the present invention, according to a second possible embodiment;

[0095] Fig. 3: block diagram showing a process or plant of the present invention, according to a third possible embodiment;

[0096] Fig. 4: block diagram showing a process or plant of the present invention, according to a fourth possible embodiment;

[0097] Fig. 5: block diagram of a hydrogen purification step or unit according to a first preferred embodiment;

[0098] Fig. 6: block diagram of a hydrogen purification step or unit according to a second preferred embodiment. Detailed description of the invention

[0099] Fig. 1 is a simplified scheme of a first preferred embodiment of the present invention.

[0100] A hydrocarbon feedstock 42 is preheated and fed to a desulphurization unit 41 .

[0101] The so desulphurized hydrocarbon feedstock 5 is then fed to a pre-reforming unit 38 together with steam 6 to obtain a pre-reformed effluent 43.

[0102] The pre-reformed effluent 43 is then fed to an autothermal reforming (ATR) section 4 wherein such effluent 43 is converted in a hydrogen-containing synthesis gas 7 containing water vapour (H2O), hydrogen (H2), amounts of carbon monoxide (CO), carbon dioxide (CO2), unreacted desulphurized hydrocarbon feedstock, other impurities and optionally nitrogen. The hot hydrogen-containing synthesis gas 7 is cooled in a downstream heat exchanger 44, preferably a steam superheater (SSH) or a combination of waste heat boiler (WHB) and steam superheater (SSH). A superheated steam 39 can be generated by superheating a stream of water 45 in the same heat exchanger 44.

[0103] Said steam 6 fed upstream of the pre-reforming unit 44 is obtained with a separate steam generator (not shown) or with said heat exchanger 44.

[0104] In an air separation unit (ASU) 3 an air stream 46 is separated into a nitrogen-rich stream 32 and pure oxygen or oxygen-enriched air 2. The pure oxygen or oxygen- enriched air 2 is fed to the ATR section 4. The nitrogen-rich stream 32 may be exported or vented.

[0105] The hydrogen-containing synthesis gas 7 cooled in the heat exchanger 44 is then treated - in sequence - in a CO shift unit 8, a first steam condensation unit 27, a carbon dioxide removal unit 9, a methanation unit 28 and a second steam condensation unit 29 to obtain a modified syngas 31 .

[0106] The CO shift unit 8 converts CO and water vapor to CO2 and H2 thereby producing a shifted gas 47. The CO shift unit 8 may comprise a high-temperature (HT) shift subunit and a low-temperature (LT) shift sub-unit, or one or more medium-temperature (MT) shift sub-unit(s).

[0107] In the first steam condensation unit 27 the shifted gas 47 is divided into a first condensed fraction 48 (e.g., that may be exported or fed into the line of steam 6) and a first gaseous fraction 49 fed to the carbon dioxide removal unit 9.

[0108] In the carbon dioxide removal unit 9 (e.g., comprising an amine-based washing column), the first gaseous fraction 49 is separated into a CCh-rich stream 35 and a CCh-depleted stream 50. The CCh-rich stream 35 is sent to compression 36 to thereby obtain compressed carbon dioxide 37.

[0109] The CCh-depleted stream 50 is fed to the methanation unit 28 wherein residual CO2 and traces of carbon monoxide react with hydrogen to give a methanated stream 51 containing mainly hydrogen, methane, and steam, and impurities in minor amounts.

[0110] In the second steam condensation unit 29, the methanated stream 51 is separated into a second condensed fraction 52 (e.g., that may be exported or fed into the line of steam 6) and a second gaseous fraction making the modified syngas 31 . The modified syngas 31 contains hydrogen and methane as main components, and other impurities.

[0111] The modified syngas 31 may be dried and is then fed to hydrogen purification 10 arranged to separate said (optionally dried) modified syngas 31 into a hydrogen-rich gas 11 , 30 and a methane-containing tail stream 12. Said hydrogen purification 10 may comprise pressure swing absorption (PSA), membrane separation, and / or cryogenic purification.

[0112] At least a portion of said methane-containing tail stream 12 is recycled as feed to step (II).

[0113] Said portion of methane-containing tail stream 12 may be recycled in at least one of the following locations: upstream of the desulphurization unit 41 of the hydrocarbon feedstock 42 to be desulphurized, and / or upstream of the pre-reforming unit 38 of said desulphurized hydrocarbon feedstock 5, and / or between the pre-reforming unit 38 and said reforming 4 (i.e. , by mixing with the pre-reformed effluent 43). Fig. 2 is a simplified scheme of a second preferred embodiment of the present invention.

[0114] With respect to the embodiment of Fig. 1 , the hydrogen-rich gas 11 , 30 may be mixed with the nitrogen-rich stream 32 separated in the ASU 3 to provide an ammonia synthesis makeup gas 33 having an appropriate hydrogen to nitrogen (H / N) molar ratio for ammonia synthesis 40. Said ammonia synthesis may comprise an ammonia synthesis loop comprising an ammonia reactor at ammonia-forming conditions. An ammonia-containing stream 34 is withdrawn from ammonia synthesis 40.

[0115] Fig. 3 is a simplified scheme of a third preferred embodiment of the present invention.

[0116] With respect to the embodiment of Fig. 2, the compressed carbon dioxide 37 and the ammonia-containing stream 34 are fed to urea synthesis 57 and are reacted under urea-forming conditions to obtain a urea-containing stream 58. The urea-containing stream 58 may be a urea solution or a urea melt.

[0117] Fig. 4 is a simplified scheme of a fourth preferred embodiment of the present invention.

[0118] With respect to the embodiments of Fig. 1 , Fig. 2 or Fig. 3, the hydrogen-containing synthesis gas 7 is treated in a CO shift unit 8 (preferably comprising a HT shift subunit and an optional LT shift sub-unit), and the thus resulting shifted gas 47 is cooled in the heat exchanger 44. A cooled shifted gas 59 is then treated - in sequence - in the first steam condensation unit 27, the carbon dioxide removal unit 9, the methanation unit 28 and the second steam condensation unit 29 to obtain the modified syngas 31 .

[0119] The process or plant according to the embodiment of Fig. 4 may be used to produce the hydrogen-rich gas 11 , 30, or the ammonia-containing stream 34 (as in Fig. 2), or the urea-containing stream 58 (as in Fig. 3).

[0120] Fig. 5 is a schematization of a hydrogen purification step or unit according to a first preferred embodiment.

[0121] The modified syngas 31 is refrigerated in an indirect heat exchanger 20 of the cryogenic purification section 13 to obtain a refrigerated modified syngas 15. The refrigerated syngas 15 may have a temperature comprised from -180 °C and -190 °C and a pressure comprised from 30 barg to 35 barg. The necessary refrigeration is provided by a refrigeration loop 53 providing a heat exchange medium indirectly exchanging heat with the modified syngas 31 .

[0122] The refrigerated modified syngas 15 is subsequently fed to a first liquid-gas separator 14 separating a hydrogen-rich gas 11 and the methane-containing tail stream 12.

[0123] The methane-containing tail stream 12 is heated in the indirect heat exchanger 20, and is then recycled as heated methane-containing tail stream 12’ as feed in the reforming section 4, in any of the locations discussed herein before.

[0124] The hydrogen-rich gas 11 is heated in the indirect heat exchanger 20 and may be used to fuel of a fired heater and / or fed to ammonia synthesis 40.

[0125] Fig. 6 is a schematization of a hydrogen purification step or unit according to a second preferred embodiment.

[0126] The embodiment of Fig. 6 differs from the embodiment of Fig. 5 in that the hydrogen- rich gas 11 is split into a first portion 16 and a second portion 17. The first portion 16 is heat exchanged in exchanger 20 as in the embodiment of Fig. 5. The second portion 17 is treated in a second cryogenic purification section 18 to obtain ultra-high-purity hydrogen 30.

[0127] The second portion 17 is washed in a contacting unit 19, preferably an absorption column, with a liquid nitrogen solution 23 to obtain a washed hydrogen-rich gas 21 and an impurities-loaded solution 22. The impurities-loaded solution 22 may be treated (e.g., purified) and then recycled as purified liquid nitrogen 54 in the liquid nitrogen solution 23. The washed hydrogen-rich gas 21 is recovered at the top of the contacting unit 19 and is chilled in a second indirect heat exchanger 25 to obtain a chilled hydrogen-rich gas 55. The chilled hydrogen-rich gas 55 is subsequently fed to a second liquid-gas separator 56.

[0128] The second liquid-gas separator 56 is arranged to separate the chilled hydrogen-rich gas 55 in the ultra-high-purity hydrogen 30 and a tail liquid stream 26 comprising liquid nitrogen and impurities. The tail liquid stream 26 is recycled in the liquid nitrogen solution 23 to feed the contacting unit 19. The ultra-high-purity hydrogen 30 may be sent to compression and storage, to fuel a fired heater, and / or to ammonia synthesis 40.

[0129] A cryogenic expander or a cryogenic refrigeration unit 24 provides net refrigeration to the second cryogenic purification section 18. The cryogenic expander may be fed with a compressed hydrogen refrigerant. The cryogenic refrigeration unit may comprise a helium or a hydrogen refrigerant cycle. An expanded hydrogen refrigerant or the helium or hydrogen refrigerant indirectly exchange heat with the washed hydrogen-rich gas

[0130] 21 in the second indirect heat exchanger 25.

[0131] LIST OF THE REFERENCE SIGNS

[0132] 1 process or plant

[0133] 2 pure oxygen or oxygen-enriched air

[0134] 3 air separation unit (ASU) 4 reforming step or section, preferably ATR step or section, or POX step or section, or ATR + GHR step or section

[0135] 5 desulphurized hydrocarbon feedstock

[0136] 6 steam

[0137] 7 hydrogen-containing synthesis gas 8 CO shift step or unit

[0138] 9 CO2 removal step or unit

[0139] 10 hydrogen purification step or unit

[0140] 11 hydrogen-rich gas

[0141] 12 methane-containing tail stream 12’ heated methane-containing tail stream

[0142] 13 (first) cryogenic purification step or section

[0143] 14 first liquid-gas separator

[0144] 15 refrigerated modified syngas

[0145] 16 first portion of hydrogen-rich gas 17 second portion of hydrogen-rich gas

[0146] 18 second cryogenic purification step or section

[0147] 19 contacting unit, preferably absorption column

[0148] 20 indirect heat exchange step or indirect heat exchanger 21 washed hydrogen-rich gas

[0149] 22 impurities-loaded solution

[0150] 23 liquid nitrogen solution

[0151] 24 cryogenic expander or cryogenic refrigeration unit

[0152] 25 indirect heat exchange step or second indirect heat exchanger 26 tail liquid stream

[0153] 27 first steam condensation

[0154] 28 methanation

[0155] 29 second steam condensation

[0156] 30 ultra-high-purity hydrogen 31 modified syngas

[0157] 32 nitrogen-rich stream

[0158] 33 ammonia synthesis makeup gas

[0159] 34 ammonia-containing stream, preferably ammonia stream or substantially pure ammonia stream 35 CCh-rich stream

[0160] 36 compression step or unit

[0161] 37 compressed carbon dioxide or compressed CO2

[0162] 38 pre-reforming step or unit 39 superheated steam

[0163] 40 ammonia synthesis step or loop

[0164] 41 desulphurisation, preferably hydrodesulphurisation (HDS), step or unit

[0165] 42 hydrocarbon feedstock

[0166] 43 pre-reformed effluent 44 heat exchanger, preferably steam superheater (SSH)

[0167] 45 stream of water

[0168] 46 air stream

[0169] 47 shifted gas

[0170] 48 first condensed fraction 49 first gaseous fraction

[0171] 50 CO2-depleted stream

[0172] 51 methanated stream

[0173] 52 second condensed fraction

[0174] 53 refrigeration loop 54 purified liquid nitrogen

[0175] 55 chilled hydrogen-rich gas

[0176] 56 second liquid-gas separator

[0177] 57 urea synthesis step or unit 58 urea-containing stream

[0178] 59 cooled shifted gas

Claims

CLAIMS1 ) A process (1 ) comprising the following steps:(I) generating pure oxygen or oxygen-enriched air (2) by an air separation unit (3), said oxygen-enriched air having an oxygen content > 50% molar, preferably > 70% molar, more preferably > 90% molar;(II) reforming (4) a desulphurized hydrocarbon feedstock (5), e.g., natural gas, in presence of said pure oxygen or oxygen-enriched air (2) and steam (6) to obtain a hydrogen-containing synthesis gas (7);(III) post-treatment of said hydrogen-containing synthesis gas (7) comprising CO shift (8), CO2 removal (9), and hydrogen purification (10); said hydrogen purification (10) being arranged to separate a hydrogen-rich gas (11 , 30) having a hydrogen content > 98.5% molar, and a methane- containing tail stream (12, 12’);(IV) recycling at least a portion of said methane-containing tail stream (12, 12’) as feed to step (II).2) The process according to claim 1 , wherein said portion of methane-containing tail stream (12, 12’) is recycled in step (IV) in at least one of: upstream of a desulphurization (41 ) of a hydrocarbon feedstock to be desulphurized, and / or upstream of a pre-reforming (38) of said desulphurized hydrocarbon feedstock, and / or between a pre-reforming (38) of said desulphurized hydrocarbon feedstock and said reforming (4).3) The process according to any of claims 1 -2, wherein a cryogenic purification section (13) is used in said hydrogen purification (10), said cryogenic purification section (13) comprising an indirect heat exchange (20) downstream of said CO2 removal (9) and of an optional drying to obtain a refrigerated and optionally dried modified syngas (15) at a temperature below -150 °C, preferably below -170 °C, more preferably comprised from -180 °C and -190 °C, said cryogenic purificationsection (13) further comprising a first liquid-gas separator (14) of the refrigerated and optionally dried modified syngas (15); said process comprising: heating said methane-containing tail stream (12) in said cryogenic purification section (13) by indirect heat exchange (20), a heated methane-containing tail stream (12’) being recycled in step (IV).4) The process according to any of claims 2-3, wherein step (III) comprises - in sequence - CO shift (8), first steam condensation (27), CO2 removal (9), methanation (28) and second steam condensation (29) to obtain a modified syngas (31 ); followed by feeding said modified syngas (31 ) to said hydrogen purification (10).5) The process according to any of the previous claims, wherein said hydrogen purification (10) comprises a pressure swing absorption (PSA) and / or a membrane separation.6) The process according to any of the previous claims, wherein said methane- containing tail stream (12, 12’) has a methane content > 90% molar.7) The process according to any of the previous claims, wherein a nitrogen-rich stream (32) is generated by said ASU (3) of step (I), at room temperature and atmospheric pressure, said nitrogen-rich stream (32) being mixed with said hydrogen-rich gas (11 ) to provide an ammonia synthesis makeup gas (33) having an appropriate hydrogen to nitrogen (H / N) molar ratio for the ammonia synthesis (40); preferably said H / N molar ratio being comprised from 2.9 to 3.1 .8) The process according to any of the previous claims, wherein:- said pure oxygen or oxygen-enriched air (2) is provided at a pressure of at least 20 bar, preferably comprised from 20 bar to 80 bar, more preferably comprised from 30 bar to 70 bar, even more preferably comprised from 40 bar to 60 bar; and / or- at least part of a CCh-rich stream (35) obtained in the CO2 removal (9) is subjected to compression (36) at a pressure above 100 bar, preferablycomprised from 150 bar to 200 bar, and the so obtained compressed carbon dioxide (37) is exported, e.g., stored under pressure for carbon capture, or used for enhanced oil recovery, or reacted with an ammonia-containing stream (34) in a urea synthesis (57).9) The process according to any of the previous claims, wherein:- step (II) is performed with a steam to carbon ratio of no more than 2.0, with pre-reforming (38) in an adiabatic reactor, without a primary reforming.10) The process according to any of the previous claims, wherein:- superheated steam (39) is generated by cooling the hot hydrogen-containing synthesis gas (7) prior to CO2 removal (9).11 ) The process according to any of the previous claims, wherein said process does not include producing neither mechanical power with a gas turbine engine, nor electrical energy with a generator coupled to said gas turbine engine.12) A plant (1 ) comprising:- an air separation unit (3) arranged to generate pure oxygen or oxygen- enriched air (2), said oxygen-enriched air having an oxygen content > 50% molar, preferably > 70% molar, more preferably > 90% molar;- a reforming section (4) arranged to reform a desulphurized hydrocarbon feedstock (5), e.g., natural gas, in presence of said pure oxygen or oxygen- enriched air (2) and steam (6) to obtain a hydrogen-containing synthesis gas (7);- a CO shift unit (8), a CO2 removal unit (9), and hydrogen purification unit (10) for post-treatment of said hydrogen-containing synthesis gas (7);- said hydrogen purification unit (10) being arranged to separate a hydrogen- rich gas (11 , 30) having a hydrogen content > 98.5% molar, and a methane- containing tail stream (12);- a line for recycling at least a portion of said methane-containing tail stream(12) as feed to the reforming section (4).13) The plant according to claim 12, wherein said hydrogen purification unit (10) comprises a pressure swing absorption (PSA) unit and / or a membrane separation unit.14) The plant according to any of claims 12-13, wherein said hydrogen purification unit (10) comprises an indirect heat exchanger (20) downstream of said CO2 removal unit (9) and of an optional drying unit to obtain a refrigerated and optionally dried modified syngas (15) at a temperature below -150 °C, preferably below -170 °C, more preferably comprised from -180 °C and -190 °C, said cryogenic purification section (13) further comprising a first liquid-gas separator (14) of the refrigerated and optionally dried modified syngas (15); said methane- containing tail stream (12) being heated in said indirect heat exchanger (20), and a line for recycling a heated methane-containing tail stream (12’) in the reforming section (4).15) The plant according to any of claims 12-14, wherein said plant does not include neither a gas turbine engine for producing mechanical power, nor an electrical energy generator coupled to said gas turbine engine.16) The plant according to any of claims 12-15, wherein the plant is an ammonia synthesis front-end for producing an ammonia synthesis makeup gas (33) having a hydrogen to nitrogen (H / N) molar ratio for the ammonia synthesis (40).

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