Production of ammonia make-up syngas with cryogenic purification

Abstract

A process and plant (1) for producing high-purity hydrogen (10). Such process comprises steps of: (I) reforming (2) a desulphurized hydrocarbon feedstock (3) in presence of steam (4) to obtain a raw syngas (5); (II) subjecting said raw syngas (5) to CO shift (6), carbon dioxide removal (7), methanation (8) and steam condensation (9, 11) to obtain a modified syngas (12); (III) treating the modified syngas (12) in a cryogenic purification section (13) to obtain said high-purity hydrogen (10) and a methane-containing tail stream (14); wherein net refrigeration of said cryogenic purification section (13) is at least partially provided by an expansion (19) of a compressed nitrogen-rich gas (15) generated by an air separation unit (16).

Description

[0001] Production of ammonia make-up syngas with cryogenic purification

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The invention relates to the production of high-purity hydrogen with cryogenic purification. More in detail, the invention relates to production of a high-purity hydrogen by steam reforming of a hydrocarbon feedstock, such as natural gas, and treatment of a modified syngas by cryogenic purification.

[0005] Prior art

[0006] Conventionally hydrogen is produced from fossil fuel by steam reforming in a process at high temperature (> 1000 °C) and relatively high pressure depending on the process parameters.

[0007] In a so-called reforming front-end, fossil fuel, water and oxygen are reacted to produce a synthesis gas containing mainly hydrogen, carbon monoxide and some carbon dioxide. Such synthesis gas is then treated and purified to increase the hydrogen content in the syngas up to 98% molar, the balance being unreacted fossil fuel, carbon monoxide and non-condensable gases (such as argon, nitrogen, and carbon dioxide).

[0008] The purity of the so obtained purified synthesis gas is influenced by different process parameters, such as temperature of the reforming reactor(s), steam to carbon (S / C) ratio at an inlet to such reactor, and relative oxygen rate.

[0009] A desired purity degree of hydrogen depends on the intended use of such gas.

[0010] Fuel grade hydrogen does not require a particularly high purity degree, such degree depending generally on allowed emissions. In contrast, fuel cell (FC) applications require high-purity hydrogen of grade 4N or 5N (wherein N stands for “nines", a logarithmic notation). Also, synthesis loops of ammonia production plants are very sensitive to hydrogen impurities typically oxygen containing molecules, so that a best possible purification of the synthesis gas must be achieved.

[0011] Commonly used hydrogen purification methods to obtain high-purity hydrogen streams are pressure swing adsorption (PSA), chemical adsorption, membrane separation, and cryogenic partial condensation.

[0012] PSA has the advantage to accept large gas volumes of variable composition. However, a major drawback is that its purification process comprises a regeneration step that uses produced hydrogen as a purge gas, so that about 10% - 15% of the overall hydrogen production goes lost, or in any case needs to be recycled in the reforming front-end to be reprocessed.

[0013] Chemical adsorption and membrane separation have similar drawbacks as PSA, and additionally require very specific inlet specifications for concentration and impurities.

[0014] Cryogenic partial condensation is a well-defined and widely used commercial route to carbon monoxide (CO) production, is sometimes used in hydrogen separation plants in oil and gas industries, and in hydrogen recovery in refinery processes. This condensation takes advantage of different boiling points of two or more gases to be separated from a gaseous mixture.

[0015] While the process simplicity of cryogenic partial condensation makes it attractive, the involved refrigeration requirements are significant. Among other factors cooling agents are expensive, or not always readily available.

[0016] US 3,361 ,534 A, US 2020 / 141637 A1 , WO 2008 / 113494 A2, US 2018 / 298292 A1 , and GB 1 025 104 A are known art for the present invention. Summary of the invention

[0017] The invention aims to overcome the above drawbacks of the prior art.

[0018] In particular, the present invention aims to provide a simple and cost-effective process for producing high-purity hydrogen, e.g., to fulfill carbon emission target or chemical requirements. Such process takes advantage of a traditionally unused gaseous stream, i.e. , a nitrogen-rich gas from an air separation unit, to provide at least part of the cooling power (refrigeration) to a hydrogen cryogenic purification section.

[0019] The problem underlying the invention is to solve the above listed limitations in a cost-effective way. This problem is solved by a process, a plant, and a method for revamping according to the following disclosure.

[0020] A process for producing high-purity hydrogen comprises steps of:

[0021] (I) reforming a desulphurized hydrocarbon feedstock in presence of steam to obtain a raw syngas;

[0022] (II) subjecting said raw syngas to CO shift, carbon dioxide removal, methanation and steam condensation (or water removal) to obtain a modified syngas;

[0023] (III) treating the modified syngas in a cryogenic purification section to obtain said high-purity hydrogen and a methane-containing tail stream, optionally recycling said methane-containing tail stream, preferably after heat exchange, as feed to step (I); wherein net refrigeration of said cryogenic purification section is at least partially provided by an expansion of a compressed nitrogen-rich gas generated by an air separation unit (ASU).

[0024] Another object of the present invention is a plant for producing high-purity hydrogen comprising: a reforming section configured for reforming a desulphurized hydrocarbon feedstock in presence of steam to obtain a raw syngas;

[0025] - a CO shift unit, a carbon dioxide removal unit, a methanation unit and at least a steam condensation (or water removal) unit configured for processing said raw syngas to obtain a modified syngas;

[0026] - a cryogenic purification section of the modified syngas to obtain said high-purity hydrogen and a methane-containing tail stream;

[0027] - optionally a line recycling said methane-containing tail stream, preferably after heat exchange, as feed to the reforming section;

[0028] - an air separation unit (ASU) providing a nitrogen-rich gas, and optionally an oxygen-rich stream;

[0029] - an expansion unit of said nitrogen-rich gas in a compressed state to provide a heat exchange medium to refrigerate said cryogenic purification section.

[0030] In the present description the expression “compressed” means a state or condition of said nitrogen-rich gas above an atmospheric pressure of 101 ,325 Pa.

[0031] Still another object of the present invention is a method for revamping a plant for producing high-purity hydrogen, said plant to be revamped comprising a reforming section for reforming a desulphurized hydrocarbon into a raw syngas, a CO shift unit, a carbon dioxide removal unit, a methanation unit and at least a steam condensation (water removal) unit configured for processing said raw syngas to obtain a modified syngas. Said method comprises at least the steps of:

[0032] - installing an air separation unit (ASU) providing a nitrogen-rich gas, if not already available in the plant to be revamped;

[0033] - providing a cryogenic purification section of the modified syngas, if not already available in the plant to be revamped;

[0034] - optionally installing a compression unit and a cooling unit of said nitrogen- rich gas;

[0035] - optionally installing an expansion unit of the compressed and cooled nitrogen-rich stream;

[0036] - installing a line feeding the nitrogen-rich gas or the optional expanded nitrogen-rich gas to at least one indirect heat exchanger of the cryogenic purification section, to provide a heat exchange medium to refrigerate said cryogenic purification section;

[0037] - optionally installing a line recycling a methane-containing tail stream or a heat-exchanged methane-containing tail stream provided by the cryogenic purification section as feed to the reforming section;

[0038] - optionally installing liquid-gas separator separating a high-purity hydrogen and a methane-containing tail stream.

[0039] Description of the preferred embodiments of the present invention

[0040] Preferably, said high-purity hydrogen 10 has a hydrogen content > 98.5% molar, more preferably comprised from 98.5% molar to 99.999% molar or from 98.5% molar to 99.5% molar.

[0041] More preferably, said hydrocarbon feedstock is natural gas or substitute natural gas (SNG), but any suitable reformable hydrocarbon may be used. Natural gas is most preferred.

[0042] According to an embodiment, said nitrogen-rich gas 15, before expansion 19, is cooled in the cryogenic purification section 13 by heat exchange (preferably: indirect heat exchange, more preferably performed with a plate-fin heat exchanger, even more preferably an aluminum plate-fin heat exchanger) with already expanded nitrogen-rich gas 37. According to another embodiment, said nitrogen-rich gas 15 is generated by the ASU 16 in a gaseous state, preferably at room temperature and at atmospheric pressure and said nitrogen-rich gas 15 is then compressed, or at room temperature and above atmospheric pressure, e.g., at a pressure comprised from 2 bar to 50 bar, preferably from 3 bar to 30 bar, more preferably from 5 bar to 15 bar.

[0043] According to still another embodiment, said modified syngas 12 in the cryogenic purification section 13 has a pressure comprised from 2 bar gauge (barg) to 100 barg, more preferably comprised from 10 barg to 70 barg, even more preferably comprised from 20 barg to 50 barg, still more preferably from 30 barg to 40 barg.

[0044] Preferably, step (I) comprises reforming 2 said hydrocarbon feedstock in further presence of an oxygen-rich stream 22 or oxygen-enriched air as oxidant, whereby said oxygen-rich stream 22 or oxygen-enriched air is provided by treating an air stream 23 in said ASU 16. Said oxygen-rich stream 22 or oxygen-enriched air preferably has an oxygen content > 50% molar, more preferably > 70% molar, even more preferably > 90% molar.

[0045] Said oxygen-rich stream 22 or oxygen-enriched air is preferably 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 45 bar to 60 bar.

[0046] More preferably, said reforming 2 of step (I) comprises a step of partial oxidation (POX) or of autothermal reforming (ATR) 40. Even more preferably, said POX or ATR step is performed with a steam to carbon (S / C) ratio < 5, preferably < 3, more preferably < 2, even more preferably comprised from 1 .4 and 1 .95.

[0047] According to an embodiment, said reforming 2 of step (I) comprises: an autothermal reforming (ATR) arranged to receive a first portion of the desulphurized hydrocarbon feedstock and generating a first stream of raw syngas; 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 raw syngas in the same side, and said first stream of raw syngas and said second stream of raw syngas are mixed in said second side to generate said raw syngas.

[0048] According to another embodiment, the modified syngas 12 is treated in the cryogenic purification section 13 at a temperature below -100 °C (i.e., 100 °C below zero Celsius degrees), preferably below -130 °C, more preferably below - 150 °C, even more preferably below -170 °C, e.g., comprised from -180 °C and - 190 °C.

[0049] According to another embodiment, the process comprises:

[0050] (IV) generating said nitrogen-rich gas 15 with said ASU 16;

[0051] (V) compression 17 and cooling 18 of said nitrogen-rich gas 15 of step (IV);

[0052] (VI) expansion 19 of said nitrogen-rich gas of step (V) to provide said net refrigeration - by indirect heat exchange - to said modified syngas 12 in said cryogenic purification section 13.

[0053] According to still another embodiment, the process further comprises:

[0054] (II. A) drying the modified syngas 12 of step (II) with a molecular sieve drying unit 20 before such modified syngas 12 is treated in step (III);

[0055] (II. B) regenerating molecular sieves of said molecular sieve drying unit

[0056] 20 with at least part of an expanded nitrogen-rich gas after providing said net refrigeration;

[0057] (II. C) optionally venting 21 , 2T said nitrogen-rich gas after step (VI) and / or after step (II. B).

[0058] The process preferably comprises indirect heat exchange of said methane- containing tail stream 14 in said cryogenic purification section 13; followed by recycling a heat-exchanged methane-containing tail stream 24 as feed in step (I). More preferably, the heat-exchanged methane-containing tail stream 24 is mixed to the desulphurized hydrocarbon feedstock 3 upstream of reforming 2, or is fed to reforming separately from said desulphurized hydrocarbon feedstock 3.

[0059] Preferably, said methane-containing tail stream 24 has a methane content > 90% molar, preferably > 93% molar, more preferably > 95% molar. The balance may be argon, and traces of hydrogen and nitrogen.

[0060] According to a preferred embodiment, the process further comprises:

[0061] (VII) treating at least a portion 26 of the high-purity hydrogen 10 obtained in step (III) in a second cryogenic purification section 25 to obtain ultra-high-purity hydrogen 27 having a hydrogen content > 99.5% molar, preferably > 99.7% molar, more preferably > 99.9% molar, even more preferably > 99.99% molar, e.g., 99.999% molar.

[0062] Said step (VII) preferably comprises:

[0063] (VILA) washing said portion 26 of the high-purity hydrogen 10 with a liquid nitrogen solution 38 in a contacting unit 28, e.g., an absorption column, to obtain a washed high-purity hydrogen 29 and an impurities-loaded solution 30;

[0064] (VII. B) treating the washed high-purity hydrogen 29 of step (VILA) in indirect heat exchange in one of:

[0065] (i) a cryogenic expander or a cryogenic turbo expander 31 fed with a compressed ultra-high-purity hydrogen 32; or

[0066] (ii) a cryogenic refrigeration unit 33 comprising a helium or hydrogen refrigerant cycle 34; to obtain said ultra-high-purity hydrogen 27 and a tail liquid stream 35 comprising liquid nitrogen and impurities; and

[0067] (VII. C) recycling the tail liquid stream 35 to the contacting unit 28 of step (VILA).

[0068] According to a preferred embodiment, said liquid nitrogen solution 38 used in step (VILA) for washing said portion 26 of the high-purity hydrogen 10 in the contacting unit 28 is generated by an air separation unit (ASU), preferably by the same ASU providing net refrigeration to the cryogenic purification section of step (III).

[0069] Preferably, said cryogenic purification section is refrigerated by the heat exchange medium through at least one indirect heat exchanger 60.

[0070] More preferably, said plant further comprises:

[0071] - a compression unit 17 and a cooling unit 18 of said nitrogen-rich gas 15 provided by the ASU 16;

[0072] - an expansion unit 19 of the compressed and cooled nitrogen-rich stream 36 to provide a heat exchange medium to refrigerate said cryogenic purification section 13; the cryogenic purification section 13 comprising at least one indirect heat exchanger 60 between said modified syngas 12 and said expanded nitrogen- rich gas 37.

[0073] More preferably, said cooling unit 18 and said indirect heat exchanger 60 are integrated in a same apparatus.

[0074] Even more preferably, said compression unit 17 comprises a water-cooled compressor to obtain a compressed nitrogen-rich stream 62 at room temperature. According to a preferred embodiment, the plant further comprises:

[0075] - a molecular sieve drying unit 20 for drying the modified syngas 12 upstream of the cryogenic purification section 13;

[0076] - a line 39 for feeding at least part of the expanded nitrogen-rich gas 37 - after passing in said indirect heat exchanger 60 - to said molecular sieve drying unit 20 for regeneration of molecular sieves of said molecular sieve drying unit 20.

[0077] According to another embodiment, the plant further comprises a line for feeding said methane-containing tail stream 14 to said indirect heat exchanger 60 of the cryogenic purification section 13, and a line for recycling a heat-exchanged methane-containing tail stream 24 as feed to the reforming section 2.

[0078] According to still another embodiment, the plant further comprises a second cryogenic purification section 25 of at least a portion 26 of the high-purity hydrogen 10, to obtain ultra-high-purity hydrogen 27.

[0079] Said second cryogenic purification section 25 preferably comprises:

[0080] - a contacting unit 28, e.g., an absorption column, for washing said portion 26 of the high-purity hydrogen 10 with a liquid nitrogen solution 38 to obtain a washed high-purity hydrogen 29 and an impurities-loaded solution 30;

[0081] - one from among a cryogenic expander or turbo expander 31 fed with a compressed ultra-high-purity hydrogen 32, or a cryogenic refrigeration unit 33 comprising a helium or a hydrogen refrigerant cycle 34; to obtain said ultra-high-purity hydrogen 27 and a tail liquid stream 35 comprising liquid nitrogen and impurities; and a line for recycling the tail liquid stream 35 to the contacting unit 28. Advantages of the present invention

[0082] Advantageously, the method and plant of the present invention ensure the required purity degree, as well as low energy consumption.

[0083] Advantageously, the method and plant of the present invention require standard construction materials and equipment to ensure robustness and stability.

[0084] Advantageously, the method and plant of the present invention have been designed to work in a wide range of process parameters.

[0085] Advantageously, the method and plant of the present invention make use of unused nitrogen, typically available in significant quantities, capable of obtaining condensation of a desired fix concentration or purity of hydrogen.

[0086] Advantageously, the method and plant of the present invention allow to reduce carbon emission below a certain level for the fuel grade production, for instance 40 gco2 / kgH2or more, such as 100 gco2 / kgn2(this latter being equivalent to 36.4 gcH4 / kgH2or a CH4 concentration in H2 of 4.55 mmolcH4 / molH2).

[0087] Advantageously, the use of a methanation step or unit before cryogenic purification allows to remove (by chemical conversion) traces of CO and CO2 which would cause problems in the cryogenic purification section or step and, eventually, in downstream uses of the high-purity hydrogen.

[0088] In first instance, CO has a much lower boiling point compared to methane, hence it would end up in the product stream.

[0089] Secondly, CO2 has a much higher boiling point and it would freeze at the operation temperatures of the cryogenic purification section.

[0090] Thirdly, cryogenic purification of gas mixtures having more than two gas components (“multi-gas mixtures”) would involve a much more complex separation process compared to a binary mixture. In contrast to that, the present invention allows to make use of a simple single-stage two phase separation separation.

[0091] Advantageously, the nitrogen-rich gas used for regeneration of the molecular sieve do not need to be ultrapure and trace amount of oxygen and argon are acceptable this is extremely important because the use of such nitrogen do not increase the heavy-duty power consumption of oxygen production in the ASU. This because it is not a synthesis nitrogen but is only used for the refrigeration purpose and for the regeneration step as it is readily available and dry.

[0092] Advantageously, the method and plant of the present invention are technically robust and can be applied to separations of different processes, even in presence of variable process parameters.

[0093] Advantageously, a methanator combined with a drying section ensures no oxygen poisoning of the final H2-rich stream, allowing a poisoning-free stream for an optional ammonia synthesis.

[0094] Advantageously, the fact that liquefied methane-containing tail stream may be recycled as feed to the reforming section or step increases the overall conversion of the process.

[0095] Advantageously, the use the nitrogen-rich gas as a refrigeration gas allows to recycle methane at pressure without expansion through a laminated valve to reduce its temperature, this is a benefit compared to PSA process which deliveries low pressure tail gas to the recycle.

[0096] The use of nitrogen-rich stream as a cooling medium for the cryogenic section has been found to be an effective measure to increase the capability of the plant and to substantially maintain the overall LHV efficiency of the process. A first advantage is that the invention makes use of nitrogen-rich stream as a cooling medium to provide the net refrigeration to the cryogenic section, instead of energy-consuming expansion of the raw syngas, as suggested in the prior art. A further advantage is that the nitrogen-rich stream is used in a highly efficient way, i.e., first as a refrigerating medium for the cryogenic separation section, avoiding the feed of a substantial amount of inert nitrogen through the purification equipment downstream of reforming. Hence, a significant advantage is obtained without the drawback of a substantial increase of the volumetric flow rate processed in the reformers, CO shift unit(s) and CO2-removal unit(s).

[0097] The invention, moreover, is particularly efficient in the removal of methane, and other impurities from the modified syngas, thanks to the treatment in the nitrogen- refrigerated cryogenic purification section.

[0098] Integration with an air separation unit is particularly efficient, making also available an oxygen-rich stream which is advantageously injected into the reforming section or step (e.g., ATR or POX), thus boosting the capability of the front-end section in terms of production of raw syngas.

[0099] Advantageously, if the ASU provides the nitrogen-containing stream under pressure, the process or plant may renounce one or more compressor(s).

[0100] The advantages will be more evident with the following detailed description of a preferred embodiment.

[0101] Description of the figures

[0102] Fig. 1 : simplified block scheme of the process or plant according to a first embodiment of the present invention;

[0103] Fig. 1A: simplified block scheme of the process or plant according to a second possible embodiment of the present invention;

[0104] Fig. 2 and Fig. 3: simplified block scheme of the process or plant wherein the second cryogenic purification section, according to different embodiments, is shown in a greater detail;

[0105] Fig. 4: vapor-liquid equilibrium (VLE) diagram of an ammonia and water binary system at atmospheric pressure.

[0106] Detailed description of the invention

[0107] Fig. 1 is a simplified scheme of a preferred embodiment of the present invention. A hydrocarbon feedstock 50 is preheated in a feedstock preheater 41 , is fed to a hydrodesulphurisation (HDS) unit 42 and is then further preheated by indirect heat exchange with hot flue gases of a gas turbine 43.

[0108] The so preheated desulphurized hydrocarbon feedstock 3 is then fed to a prereforming unit 44 together with steam containing stream 4 to obtain a prereformed effluent 45 after indirect heat exchange with said hot flue gases of the gas turbine 43.

[0109] The pre-reformed effluent 45 is then fed to an autothermal reforming (ATR) unit 40 wherein such effluent is converted in a raw syngas 5 containing water vapor (H2O), hydrogen (H2), amounts of carbon monoxide (CO), carbon dioxide (CO2), unreacted desulphurized hydrocarbon feedstock, other impurities and optionally nitrogen. The hot raw syngas 5 is cooled in a downstream heat exchanger 46.

[0110] Said steam 4 fed to the pre-reforming unit 44 is obtained with a steam generator 47 fed with a stream of water 48. Heat is transferred to said steam generator 47 by an indirect exchanger 68 in thermal contact with the hot flue gases of the gas turbine 43 and by the hot effluent raw syngas of the ATR through the downstream heat exchanger 46.

[0111] In an air separation unit (ASU) 16 an air stream 23 is separated into a nitrogen- rich stream 15 and an oxygen-rich stream 22. The oxygen-rich stream 22 is fed to the ATR 40 as such, or is mixed with air to obtain oxygen-enriched air before being fed to the ATR 40. The nitrogen-rich stream 15 may be (depending if it is already compressed or not) fed to a compression unit 17 to obtain a compressed nitrogen-rich stream 62 and then to a cooling unit 18 to give a compressed and cooled nitrogen-rich stream 36. The hot effluent raw syngas 5 of the ATR 40 is cooled in the downstream heat exchanger 46 and in the feedstock preheater 41 , and is then treated - in sequence - in a CO shift unit 6, a first steam condensation unit 9, a carbon dioxide removal unit 7, a methanation unit 8 and a second steam condensation unit 11 to obtain a modified syngas 12.

[0112] The CO shift unit 6 - converting CO and water vapor to CO2 and H2 - comprises a high-temperature shift sub-unit 49 and a low-temperature shift sub-unit 51 producing a shifted gas 52.

[0113] In the first steam condensation unit 9 the shifted gas 52 is divided into a first condensed fraction 53 fed to the steam generator 47 and a first gaseous fraction 54 fed to the carbon dioxide removal unit 7.

[0114] In the carbon dioxide removal unit 7 (e.g., comprising an amine-based washing column), the first gaseous fraction 54 is separated into a CCh-rich stream 55 and a CO2-depleted stream 56. The CCh-rich stream 55 is sent to a compression and storage unit 57 or, as an alternative, to a urea synthesis plant.

[0115] The CO2-depleted stream 56 is fed to the methanation unit 8 wherein residual CO2 and traces of carbon monoxide react with hydrogen to give a methanated effluent 58 containing mainly hydrogen, methane, and steam, and impurities in minor amounts.

[0116] In the second steam condensation unit 11 , the methanated effluent 58 is separated into a second condensed fraction 59 fed to the steam generator 47 and a second gaseous fraction making the modified syngas 12. The modified syngas contains hydrogen and methane as main components, and other impurities.

[0117] The modified syngas 12 may be heat exchanged in a chilling unit 61 (e.g., an ammonia chilling unit), and then passed through a molecular sieve drying unit 20 to obtain a dried modified syngas 12’. The molecular sieve drying unit 20 may comprise at least a first drying sub-unit 63 and a second drying sub-unit 64 in a parallel arrangement. These drying sub-units 63, 64 may be managed so that, when the first drying sub-unit 63 is crossed by the modified syngas 12 for drying, the second drying sub-unit 64 is regenerated, and vice versa, so that the molecular sieve drying unit 20 can operate continuously.

[0118] The dried modified syngas 12’ is refrigerated in an indirect heat exchanger 60 of the cryogenic purification section 13 to obtain a refrigerated syngas 65. The refrigerated syngas 65 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 the compressed nitrogen-rich stream (and, more precisely, by the compressed and cooled nitrogen-rich stream 36) that is cold expanded in an expansion unit 19. The expanded nitrogen-rich gas 37 provides a heat exchange medium to refrigerate said cooling unit 18 and / or said cryogenic purification section 13 wherein it indirectly exchanges heat with the compressed nitrogen-rich stream 62 and / or the modified syngas 12.

[0119] The refrigerated syngas 65 is subsequently fed to a first liquid-gas separator 66 separating a high-purity hydrogen 10 and a methane-containing tail stream 14.

[0120] Such separation is possible taking advantage of the vapor-liquid equilibrium (VLE) diagram - see, as an example, Fig. 4 - of the quas / '-binary gaseous mixture: from a region of gaseous mixture (see point (a)), once the temperature of the mixture is decreased till point (c) is reached, a two-phase mixture is formed. The gaseous phase will have the higher concentration of the more volatile component (point (d)), and the liquid phase will be more concentrated in the less volatile component (point (e)). The compositions of these two phases can be read in the coordinate of point (d) for the gaseous phase and of point (e) for the liquid phase. The mole fraction of the first component at that temperature is indicated in the coordinate corresponding to point (d) or (e), the mole fraction of the second component is 1 minus the mole fraction of the first component.

[0121] The methane-containing tail stream 14 is heated in the indirect heat exchanger 60, and is then recycled as heat-exchanged methane-containing tail stream 24 as feed in the reforming section 2 by injecting said tail stream 24 in the same line of the hydrocarbon feedstock 50 upstream of the feedstock preheater 41 .

[0122] The high-purity hydrogen 10 is heated in the indirect heat exchanger 60 and then divided into a first portion 26 and a second portion 67. The first portion 26 is treated in a second cryogenic purification section 25 to obtain ultra-high-purity hydrogen 27. The second portion 67 may be fed to fuel a combustor of the gas turbine 43.

[0123] To sum up, in the indirect heat exchanger 60 the modified syngas 12 (or dried modified syngas 12’) and the compressed nitrogen-rich stream 62 transfer their heat to another fluid. The methane-containing tail stream 14, the high-purity hydrogen 10 and the expanded nitrogen-rich gas 37 are heated by another fluid.

[0124] A first part 21 of the expanded nitrogen-rich gas 37 bay be vented in position 21 , optionally after a hydrogen recovery from such expanded nitrogen-rich gas 37. At least a second part 39 of such expanded nitrogen-rich gas 37 may be used to regenerate molecular sieves alternatively of the first drying sub-unit 63 and of the second drying sub-unit 64 after such expanded nitrogen-rich gas 37 has been heated in the indirect heat exchanger 60. After regeneration, the effluent of the first drying sub-unit 63 and of the second drying sub-unit 64 may be vented in position 2T.

[0125] The block scheme of Fig. 1 A shows a second embodiment of the present process or plant. Reference signs are the same as in Fig. 1 unless otherwise specified.

[0126] In Fig. 1A high-purity hydrogen 10 separated in the first liquid-gas separator 66 is divided into the first portion 26 and the second portion 67 upstream of the indirect heat exchanger 60. The first portion 26 is treated in the second cryogenic purification section 25 to obtain ultra-high-purity hydrogen 27. Only the second portion 67 is heated in the indirect heat exchanger 60. Two embodiments of the second cryogenic purification section 25 are shown in Fig. 2 and Fig. 3.

[0127] In both these embodiments, the first portion of the high-purity hydrogen 26 is washed in a contacting unit 28, preferably an absorption column, with a liquid nitrogen solution 38 to obtain a washed high-purity hydrogen 29 and an impurities-loaded solution 30. The impurities-loaded solution 30 may be vented, optionally after heating in the indirect heat exchanger 60. The washed high-purity hydrogen 29 is recovered at the top of the contacting unit 28 and is chilled in a second indirect heat exchanger 70 to obtain a chilled high-purity hydrogen 71. The chilled high-purity hydrogen 71 is subsequently fed to a second liquid-gas separator 72.

[0128] In the embodiment of Fig. 2, the second liquid-gas separator 72 separates the chilled high-purity hydrogen 71 in the ultra-high-purity hydrogen 27 and a tail liquid stream 35 comprising liquid nitrogen and impurities. The tail liquid stream 35 is recycled in the liquid nitrogen solution 38 to feed the contacting unit 28. The ultra-high-purity hydrogen 27 may be sent to storage. Such storage may be performed at low temperature (liquid hydrogen transportation) or, after compression, at room temperature and high pressure.

[0129] A cryogenic refrigeration unit 33 provides net refrigeration to the second cryogenic purification section 25 in the embodiment of Fig. 2. Such cryogenic refrigeration unit 33 comprises a helium or a hydrogen refrigerant cycle 34. A liquid refrigerant cycle 34 typically comprises a compressor 73, a condenser 74 (such as a water-cooled condenser), an expander device 75 and an evaporator connected in a closed circuit. In this embodiment, the second indirect heat exchanger 70 makes the evaporator of such liquid refrigerant cycle 34. The liquid refrigerant cycle 34 may also comprise an additional heat exchanger 76 exchanging heat between a stream of evaporated refrigerant 77 and a stream of condensed refrigerant 78.

[0130] In the embodiment of Fig. 3, the second liquid-gas separator 72 separates the chilled high-purity hydrogen 71 in a tail liquid stream 35 (comprising liquid nitrogen and impurities) and a compressed ultra-high-purity hydrogen stream 32. As in the previous embodiment, the tail liquid stream 35 is recycled in the liquid nitrogen solution 38 to feed the contacting unit 28, preferably after heat exchange in the second indirect heat exchanger 70. The compressed ultra-high-purity hydrogen stream 32 is fed as a working fluid to a cryogenic expander or cryogenic turbo expander 31 .

[0131] The cryogenic (turbo) expander 31 comprises an expander device 75 and a third liquid-gas separator 79. The third liquid-gas separator 79 is fed with an expanded ultra-high-purity hydrogen stream 80 obtained downstream of the expander device 75 to separate a third condensed fraction 81 and a third gaseous fraction 82. Both the third condensed fraction 81 and the third gaseous fraction 82 are heated in the second indirect heat exchanger 70 to obtain a heated condensed stream 83 and the ultra-high-purity hydrogen 27. Optionally, the heated condensed stream 83 and the ultra-high-purity hydrogen 27 may be subjected to a further heat recovery in the indirect heat exchanger 60 of the cryogenic purification section 13.

[0132] The heated condensed stream 83 may be used as fuel in the reforming section 2 or as a fuel-grade hydrogen. The ultra-high-purity hydrogen 27 may be sent to compression and storage.

[0133] LIST OF REFERENCE SIGNS

[0134] 1 plant or process for producing high-purity hydrogen

[0135] 2 reforming step or section

[0136] 3 desulphurized hydrocarbon feedstock

[0137] 4 steam

[0138] 5 raw syngas

[0139] 6 CO shift step or unit

[0140] 7 carbon dioxide removal step or unit

[0141] 8 methanation step or unit

[0142] 9 first steam condensation step or unit or first water removal step or unit

[0143] 10 high-purity hydrogen

[0144] 11 second steam condensation step or unit or second water removal step or unit

[0145] 12 modified syngas

[0146] 12’ dried modified syngas

[0147] 13 cryogenic purification section

[0148] 14 methane-containing tail stream

[0149] 15 nitrogen-rich gas

[0150] 16 air separation unit (ASU)

[0151] 17 compression step or unit 18 cooling step or unit

[0152] 19 expansion step or unit

[0153] 20 drying step or molecular sieve drying unit

[0154] 21 venting step or vent

[0155] 2T venting step or vent

[0156] 22 oxygen-rich stream

[0157] 23 air stream

[0158] 24 heat-exchanged methane-containing tail stream

[0159] 25 second (or ultra-low-temperature) cryogenic purification section

[0160] 26 portion of the high-purity hydrogen, preferably first portion

[0161] 27 ultra-high-purity hydrogen

[0162] 28 contacting unit, preferably absorption column

[0163] 29 washed high-purity hydrogen

[0164] 30 impurities-loaded solution

[0165] 31 cryogenic expander or cryogenic turbo expander

[0166] 32 compressed ultra-high-purity hydrogen stream

[0167] 33 cryogenic refrigeration unit

[0168] 34 helium or a hydrogen refrigerant cycle

[0169] 35 tail liquid stream

[0170] 36 compressed and cooled nitrogen-rich stream 37 expanded nitrogen-rich gas

[0171] 38 liquid nitrogen solution

[0172] 39 feeding line of the expanded nitrogen-rich gas to molecular sieve drying unit

[0173] 40 autothermal reforming (ATR) step or unit

[0174] 41 feedstock preheater

[0175] 42 hydrodesulphurisation (HDS) step or unit

[0176] 43 gas turbine

[0177] 44 pre-reforming step or unit

[0178] 45 pre-reformed effluent

[0179] 46 downstream heat exchanger

[0180] 47 steam generator

[0181] 48 stream of water

[0182] 49 high-temperature shift step or sub-unit

[0183] 50 hydrocarbon feedstock

[0184] 51 low-temperature shift step or sub-unit

[0185] 52 shifted gas

[0186] 53 first condensed fraction

[0187] 54 first gaseous fraction

[0188] 55 CO2-rich stream 56 CO2-depleted stream

[0189] 57 compression and storage unit

[0190] 58 methanated effluent

[0191] 59 second condensed fraction

[0192] 60 indirect heat exchanger

[0193] 61 chilling unit, for example ammonia chilling unit

[0194] 62 compressed nitrogen-rich stream

[0195] 63 first drying sub-unit

[0196] 64 second drying sub-unit

[0197] 65 refrigerated syngas

[0198] 66 first liquid-gas separator

[0199] 67 second portion of the high-purity hydrogen

[0200] 68 indirect exchanger

[0201] 69 purified liquid nitrogen

[0202] 70 second indirect heat exchanger

[0203] 71 chilled high-purity hydrogen

[0204] 72 second liquid-gas separator

[0205] 73 compressor

[0206] 74 condenser, preferably water-cooled condenser

[0207] 75 expander device 76 additional heat exchanger

[0208] 77 stream of evaporated refrigerant

[0209] 78 stream of condensed refrigerant

[0210] 79 third liquid-gas separator 80 expanded ultra-high-purity hydrogen stream

[0211] 81 third condensed fraction

[0212] 82 gaseous fraction

[0213] 83 heated condensed stream

Claims

CLAIMS1 . A process (1 ) for producing high-purity hydrogen (10) comprising steps of:(I) reforming (2) a desulphurized hydrocarbon feedstock (3), e.g., natural gas, in presence of steam (4) to obtain a raw syngas (5);(II) subjecting said raw syngas (5) to CO shift (6), carbon dioxide removal (7), methanation (8) and steam condensation (9, 11 ) to obtain a modified syngas (12);(III) treating the modified syngas (12) in a cryogenic purification section (13) to obtain said high-purity hydrogen (10) and a methane-containing tail stream(14); optionally recycling said methane-containing tail stream, preferably after heat exchange, as feed to step (I); wherein net refrigeration of said cryogenic purification section (13) is at least partially provided by an expansion (19) of a compressed nitrogen-rich gas(15) generated by an air separation unit (ASU; 16).

2. The process according to claim 1 , wherein said nitrogen-rich gas (15), before expansion (19), is cooled in the cryogenic purification section (13) by heat exchange with already expanded nitrogen-rich gas (37).

3. The process according to any of the previous claims, wherein said nitrogen- rich gas (15) is generated by the ASU (16) in a gaseous state, preferably at room temperature and at atmospheric pressure and is then compressed, or at room temperature and above atmospheric pressure, e.g., at a pressure comprised from 2 bar to 50 bar, preferably from 3 bar to 30 bar, more preferably from 5 bar to 15 bar.

4. The process according to any of the previous claims, comprising:(IV) generating said nitrogen-rich gas (15) with said ASU (16);(V) compression (17) and cooling (18) of said nitrogen-rich gas (15) of step (IV);(VI) expansion (19) of said nitrogen-rich gas of step (V) to provide said net refrigeration - by indirect heat exchange - to said modified syngas (12) in said cryogenic purification section (13).

5. The process according to any of the previous claims, further comprising:(II. A) drying the modified syngas (12) of step (II) with a molecular sieve drying unit (20) before such modified syngas (12) is treated in step (HI);(II. B) regenerating molecular sieves of said molecular sieve drying unit (20) with at least part of an expanded nitrogen-rich gas after providing said net refrigeration.

6. The process according to any of the previous claims, wherein step (I) comprises reforming (2) said hydrocarbon feedstock in further presence of an oxygen-rich stream (22) or oxygen-enriched air as oxidant, whereby said oxygen-rich stream (22) or oxygen-enriched air is provided by treating an air stream (23) in said ASU (16).

7. The process according to any of the previous claims, comprising: indirect heat exchange of said methane-containing tail stream (14) in said cryogenic purification section (13); followed by recycling a heat-exchanged methane- containing tail stream (24) as feed in step (I); optionally said methane- containing tail stream (24) having a methane content > 90% molar.

8. The process according to any of the previous claims, further comprising:(VII) treating at least a portion (26) of the high-purity hydrogen (10) obtained in step (III) in a second cryogenic purification section (25) to obtain ultra-high- purity hydrogen (27) having a hydrogen content > 99.5% molar, preferably >99.7% molar, more preferably > 99.9% molar, even more preferably > 99.99% molar, e.g., 99.999% molar.

9. The process according to claim 8, wherein said step (VII) comprises:(VILA) washing said portion (26) of the high-purity hydrogen (10) with a liquid nitrogen solution (38) in a contacting unit (28), e.g., an absorption column, to obtain a washed high-purity hydrogen (29) and an impurities-loaded solution (30);(VII. B) treating the washed high-purity hydrogen (29) of step (VILA) in indirect heat exchange in one of:(i) a cryogenic turbo expander (31 ) fed with a compressed ultra- high-purity hydrogen stream (32); or(ii) a cryogenic refrigeration unit (33) comprising a helium or hydrogen refrigerant cycle (34); to obtain said u Itra-h igh-purity hydrogen (27) and a tail liquid stream (35) comprising liquid nitrogen and impurities; and(VII.C) recycling the tail liquid stream (35) to the contacting unit (28) of step (VILA).

10. A plant (1 ) for producing high-purity hydrogen (10) comprising:- a reforming section (2) configured for reforming a desulphurized hydrocarbon feedstock (3) in presence of steam (4) to obtain a raw syngas (5); a CO shift unit (6), a carbon dioxide removal unit (7), a methanation unit (8) and at least a steam condensation unit (9, 11 ) configured for processing said raw syngas (5) to obtain a modified syngas (12);a cryogenic purification section (13) of the modified syngas (12) to obtain said high-purity hydrogen (10) and a methane-containing tail stream (14);- optionally a line recycling said methane-containing tail stream (14), preferably after heat exchange, as feed to the reforming section (2);- an air separation unit (ASU; 16) providing a nitrogen-rich gas (15), and optionally an oxygen-rich stream (22);- an expansion unit (19) of said nitrogen-rich gas (15) in a compressed state to provide a heat exchange medium to refrigerate said cryogenic purification section (13).11 . The plant according to claim 10, further comprising:- a compression unit (17) and a cooling unit (18) of said nitrogen-rich gas (15) provided by the ASU (16);- an expansion unit (19) of the compressed and cooled nitrogen-rich stream (36) to provide a heat exchange medium to refrigerate said cryogenic purification section (13); the cryogenic purification section (13) comprising at least one indirect heat exchanger (60) between said modified syngas (12) and said expanded nitrogen-rich gas (37).

12. The plant according to claim 11 , wherein said cooling unit (18) and said indirect heat exchanger (60) are integrated in a same apparatus.

13. The plant according to any of claims 10-12, further comprising: a molecular sieve drying unit (20) for drying the modified syngas (12) upstream of the cryogenic purification section (13); a line (39) for feeding at least part of the expanded nitrogen-rich gas (37)- after passing in said indirect heat exchanger (60) - to said molecular sieve drying unit (20) for regeneration of molecular sieves thereof.

14. The plant according to any of claims 10-13, further comprising a line for feeding said methane-containing tail stream (14) to said indirect heat exchanger (60) of the cryogenic purification section (13), and a line for recycling a heat-exchanged methane-containing tail stream (24) as feed to the reforming section (2).

15. The plant according to any of claims 10-14, further comprising:- a second cryogenic purification section (25) of at least a portion (26) of the high-purity hydrogen (10), to obtain ultra-high-purity hydrogen (27); said second cryogenic purification section (25) comprising:• a contacting unit (28), e.g., an absorption column, for washing said portion (26) of the high-purity hydrogen (10) with a liquid nitrogen solution (38) to obtain a washed high-purity hydrogen (29) and an impurities-loaded solution (30);• one from among a cryogenic expander or turbo expander (31 ) fed with a compressed ultra-high-purity hydrogen stream (32), or a cryogenic refrigeration unit (33) comprising a helium or hydrogen refrigerant cycle (34); to obtain said ultra-high-purity hydrogen (27) and a tail liquid stream (35) comprising liquid nitrogen and impurities; and• a line for recycling the tail liquid stream (35) to the contacting unit (28).

16. A method for revamping a plant for producing high-purity hydrogen; said plant to be revamped comprising a reforming section for reforming a desulphurized hydrocarbon into a raw syngas, a CO shift unit, a carbondioxide removal unit, a methanation unit and at least a steam condensation unit configured for processing said raw syngas to obtain a modified syngas; the method comprising at least the steps of:- installing an air separation unit (ASU; 16) providing a nitrogen-rich gas (15), if not already available in the plant to be revamped;- providing a cryogenic purification section (13) of the modified syngas (12), if not already available in the plant to be revamped;- optionally installing a compression unit (17) and a cooling unit (18) of said nitrogen-rich gas (15);- optionally installing an expansion unit (19) of the compressed and cooled nitrogen-rich stream (36);- installing a line feeding the nitrogen-rich gas (15) or the optional expanded nitrogen-rich gas (37) to at least one indirect heat exchanger (60) of the cryogenic purification section (13), to provide a heat exchange medium to refrigerate said cryogenic purification section (13);- optionally installing a line recycling a methane-containing tail stream (14) or a heat-exchanged methane-containing tail stream (24) provided by the cryogenic purification section (13) as feed to the reforming section (2);- optionally installing a liquid-gas separator (66) separating a high-purity hydrogen (10) and a methane-containing tail stream (14).

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