Blue ammonia and hydrogen co-production

Abstract

Process and plant for producing hydrogen and ammonia, the process comprising: i) supplying a hydrocarbon feed gas to a reforming unit and withdrawing from the reforming unit a raw synthesis gas stream; ii) supplying at least a portion of the raw synthesis gas stream to a water gas shift (WGS) unit and withdrawing from the WGS unit a shifted synthesis gas stream; iii) supplying at least a portion of the shifted synthesis gas stream to a CO2-removal unit and withdrawing from the CO2-removal unit: a CO2-rich stream and a CO2-depleted synthesis gas stream; iv) separating the CO2-depleted synthesis gas stream into at least a first and second portion; v) supplying the first portion of the CO2-depleted synthesis gas stream to a hydrogen purification unit and withdrawing from the hydrogen purification unit: a hydrogen product stream and a first off-gas stream; vi) combining at least a portion of the first off-gas stream with the second portion of the CO2-depleted synthesis gas stream into a main synthesis gas stream; vii) supplying the main synthesis gas stream to a purification unit and withdrawing from the purification unit: an ammonia synthesis gas stream and a second off-gas stream; viii) supplying the ammonia synthesis gas stream to an ammonia synthesis unit and withdrawing from the ammonia synthesis unit an ammonia product stream; ix) supplying at least a portion of the second off-gas stream to the reforming unit.

Description

[0001] Title: Blue ammonia and hydrogen co-production

[0002] The present invention relates to hydrogen production and ammonia synthesis, specifically focusing on a process and plant for co-producing ammonia and hydrogen.

[0003] Ammonia is a key component in many industrial processes, particularly in the production of fertilizers as well as a hydrogen carrier for use in e.g. transportation. Traditionally, ammonia is produced from a hydrocarbon feed by reforming it in a reforming unit into a raw synthesis gas. This gas is then shifted into a shifted synthesis gas, from which carbon dioxide (CO2) is removed, providing a CCh-depleted synthesis gas. This gas is then split into two portions. One portion is supplied to a pressure swing adsorption (PSA) unit, producing hydrogen and a first off-gas stream. The second portion of the CO2-depleted synthesis gas is purified, resulting in a second off-gas stream and an ammonia synthesis gas which is further converted to ammonia. These off-gas streams are combined into a single off-gas stream, of which a portion is recycled back to the reforming unit, while another portion is used as fuel gas. However, this traditional method has certain limitations and inefficiencies, particularly in terms of the capacity of the reforming, shift and CO2 removal sections, which can be strained by the need to recycle large volumes of gas.

[0004] Applicant’s DK 202100461 A1 discloses a process and plant for co-producing ammonia, optionally methanol, and at least one of: hydrogen and carbon monoxide, from a hydrocarbon feedstock.

[0005] EP 2845837 A1 discloses a process and plant for the production of ammonia, which comprises the production of ammonia make-up gas including the reforming of a hydrocarbon feedstock into a gas product comprising hydrogen, purification of said gas product into substantially pure hydrogen gas, and addition of nitrogen.

[0006] It would be desirable to provide a superior process and plant for producing hydrogen an ammonia which is capable of at least reducing the required capacity of the reforming unit, water gas shift (WGS) unit and CCh-removal unit. It would also be desirable to provide a superior process and plant for producing hydrogen and ammonia, with increased ammonia production and lower specific energy consumption.

[0007] In accordance with embodiments, a process is provided for producing hydrogen and ammonia. The process involves supplying a hydrocarbon feed to a reforming unit and withdrawing a raw synthesis gas. This raw synthesis gas is then supplied to a water gas shift (WGS) unit to produce a shifted synthesis gas. The shifted synthesis gas is then supplied to a CCh-removal unit, which produces a CCh-rich stream and a CCh-de- pleted synthesis gas stream. This CCh-depleted synthesis gas stream is then separated into at least a first and second portion. The first portion is supplied to a hydrogen purification unit, which produces a hydrogen-rich stream and a first off-gas stream. At least a portion of the first off-gas stream is combined with the second portion of the CC>2-depleted synthesis gas stream to form a main synthesis gas stream. This main synthesis gas stream is then supplied to a purification unit which produces an ammonia synthesis gas stream and a second off-gas stream. The ammonia synthesis gas stream is then supplied to an ammonia synthesis unit, which produces an ammonia product stream. At least a portion of the second off-gas stream is then supplied to the reforming unit.

[0008] In accordance with other embodiments, a plant is provided for carrying out the process for producing hydrogen and ammonia. The plant comprises a reforming unit, a water gas shift (WGS) unit, a 002-removal unit, a splitting point for separating the thus CO2- depleted synthesis gas, a hydrogen purification unit, a mixing point to combine off-gas from the hydrogen purification unit with CO2-depleted synthesis gas, a purification unit, an ammonia synthesis unit, and a conduit for supplying at least a portion of the second off-gas stream to the reforming unit.

[0009] Now more specifically, in a first general aspect, the invention is a process for producing hydrogen and ammonia comprising: i) supplying a hydrocarbon feed gas to a reforming unit and withdrawing from the reforming unit a raw synthesis gas stream; ii) supplying at least a portion of the raw synthesis gas stream to a water gas shift (WGS) unit and withdrawing from the WGS unit a shifted synthesis gas stream; iii) supplying at least a portion of the shifted synthesis gas stream to a CCh-removal unit and withdrawing from the CCh-removal unit: a CCh-rich stream and a CCh-depleted synthesis gas stream; iv) separating i.e. splitting the CCh-depleted synthesis gas stream into at least a first and second portion; v) supplying the first portion of the CCh-depleted synthesis gas stream to a hydrogen purification unit and withdrawing from the hydrogen purification unit: a hydrogen product stream and a first off-gas stream; vi) combining at least a portion of the first off-gas stream with the second portion of the CC>2-depleted synthesis gas stream into a main synthesis gas stream; vii) supplying the main synthesis gas stream to a purification unit and withdrawing from the purification unit: an ammonia synthesis gas stream and a second off-gas stream; viii) supplying the ammonia synthesis gas stream to an ammonia synthesis unit and withdrawing from the ammonia synthesis unit an ammonia product stream; ix) supplying at least a portion of the second off-gas stream to the reforming unit.

[0010] Hence, the off-gas from the hydrogen purification unit, such as off-gas from a pressure swing adsorption (PSA) unit, is returned to becoming part of the main syngas stream instead of being combined with the off-gas stream from downstream purification of the synthesis gas (syngas) prior to ammonia synthesis farther downstream.

[0011] Thereby less hydrogen is recycled with the associated benefit of reducing the net recycle flow to the reforming unit and thereby reduce the required capacity of the reforming, shift and CO2 removal sections. There is an associated increase in ammonia production with lower specific energy consumption.

[0012] The hydrogen product stream, containing 99.99 vol.% H2 or more, and thus having quality of 99.99 vol.% H2 or more, is suitably exported to outside battery limits of the process or plant.

[0013] For the purposes of the present application:

[0014] The term “present invention” or simply “invention” may be used interchangeably with the terms “present application” or simply “application”, respectively.

[0015] The term “first aspect of the invention” means the process of the invention. The term “second aspect of the invention” means the plant (process plant) of the invention. The term “and / or” means in connection with a given embodiment any of three options. The term “and / or” may be used interchangeably with the term “at least one of” the three options.

[0016] The term “comprising” includes “comprising only” i.e. “consisting of”.

[0017] The term “suitably” means “optionally”, i.e. an optional embodiment.

[0018] The term “synthesis gas” may be used interchangeably with the term “syngas”. For instance, the same meaning applies to the terms: “raw synthesis gas”, “raw synthesis gas stream”, or “raw syngas”, all defining here the stream withdrawn from the reforming unit.

[0019] The terms “water gas shift (WGS)” and “shift” or “shifting” are used interchangeably. The term “hydrogen purification unit” and “purification unit” refer to different and separate units.

[0020] The term “at least a portion” of a certain item means a portion thereof or the entire portion. For instance, “at least a portion of a given stream” means “a portion of the stream or the entire stream”.

[0021] The use of the article “a” or “an” means at least one.

[0022] The term “unit” may be used interchangeably with the term “section” and means a physical section comprising one or more units.

[0023] For the purposes of the present application, the term “directly supplied” or “directly supplying” means that there is no intermediate unit or step changing the composition of a process stream. It is understood that the associated units, one providing a process stream and the other receiving the process stream, are in direct fluid communication. The term “indirectly supplied” or “indirectly supplying” means that there is an intermediate unit or step changing the composition of a process stream.

[0024] Other definitions are provided in connection with one or more of above or below embodiments.

[0025] In an embodiment, said step vii) is understood as directly supplying the main synthesis gas stream to the purification unit.

[0026] In an embodiment, in step vii) said supplying of the main synthesis gas stream to the purification unit is conducted without supplying the main synthesis gas stream to an additional hydrogen purification unit upstream the purification unit. Capital expenditures (CAPEX) are reduced, as the use of an additional hydrogen purification unit, such as an additional PSA unit, being supplied with the main synthesis gas stream is avoided.

[0027] In an embodiment, step iv) i.e. separating the CCh-depleted synthesis gas stream into at least a first and second portion, is conducted immediately downstream the CCh-re- moval unit.

[0028] The term “immediately downstream” means at the outlet, here at the outlet of the CO2- removal unit. It is understood that there is no intermediate unit or step changing the composition of the CCh-depleted synthesis gas stream prior to said separation.

[0029] Hence, this separation, or interchangeably, split of the CCh-depleted synthesis gas, is taken right after the CCh-removal unit, at its outlet, where the CCh-depleted synthesis gas is at a temperature of about e.g. 40-50°C, which is suitable for the hydrogen purification unit, in particular where this unit is a PSA unt. The CCh-depleted synthesis gas corresponding to inlet gas to the purification unit, suitably a nitrogen wash unit (NWU), is not considered as the temperature here would be e.g. 5°C which, is low for PSA operation.

[0030] In an embodiment, between step v) and vi) the process comprises:

[0031] - passing the first off-gas stream from the hydrogen purification unit through a hydrogen purification unit off-gas compressor prior to combining with the second portion of the CC>2-depleted synthesis gas stream.

[0032] The off-gas stream from the hydrogen purification unit, herein referred to as first off-gas stream from the hydrogen purification unit, is withdrawn at a low pressure compared to the pressure of the CCh-depleted synthesis gas stream supplied thereto or the hydrogen product stream. The hydrogen purification unit off-gas compressor increases the pressure of the first off-gas to meet the pressure of the CCh-depleted synthesis gas stream, thus also adapts the pressure of the main synthesis gas stream to correspond to that of the CCh-depleted synthesis gas prior to the split. For instance, the pressure of the CC>2-depleted synthesis gas stream and the hydrogen product stream is 25-30 barg, while the pressure of the first off-gas being withdrawn is 0.1-1 barg. The first off- gas is thus compressed to 25-30 barg prior to combining with the second portion of the CC>2-depleted synthesis gas stream.

[0033] In an embodiment, a portion of the shifted synthesis gas stream in step iii) is supplied to the CC>2-removal unit, and another portion of the shifted synthesis gas stream is supplied to the second portion of the CCh-depleted synthesis gas stream, optionally to the main synthesis gas stream.

[0034] Hence, a bypass stream around the shifting step is provided, which reduces the capacity required in the CCh-removal unit. The bypass stream is denoted above as “another portion of the shifted synthesis gas stream”.

[0035] In an embodiment, in step iii) said at least a portion of the shifted synthesis gas stream is the entire portion of the shifted synthesis gas stream. Accordingly, there is no provision of the above bypass stream, as shown in the embodiment of appended Fig. 2.

[0036] In an embodiment, step iii) comprises:

[0037] - cooling the at least a portion of the shifted synthesis gas stream, and removing water therefrom, optionally by supplying the thus cooled shifted synthesis gas stream to a water separator, such as a process condensate (PC) unit.

[0038] The thereby water depleted shifted synthesis gas is then supplied to the CCh-removal unit.

[0039] In an embodiment, step ix) comprises:

[0040] - passing the second off-gas stream from the purification unit through a recycle off-gas compressor;

[0041] - diverting from the recycle off-gas compressor a portion of the second-off gas stream as fuel gas at a lower pressure than the pressure of the second off-gas stream to the reforming unit, and supplying the fuel gas to a fired heater.

[0042] This purification unit is understood as a syngas purification unit for removing impurities in the syngas such as any of CO, CO2, H2O, CH4, Ar, which are detrimental to the ammonia synthesis catalyst in the downstream ammonia synthesis unit. This purification unit serves also to provide an ammonia synthesis gas with the right ratio of hydrogen to nitrogen for conducting said ammonia synthesis. For instance, the content of hydrogen and nitrogen is, respectively, 75 vol.% and 25 vol.%, corresponding to a H2:N2 ratio of 3.

[0043] Suitably, a portion of the ammonia synthesis gas is diverted and provided as a recycle hydrogen stream to the reforming unit, such as to the hydrocarbon feed gas, preferably upstream a hydrogenator and sulfur absorber of said reforming unit.

[0044] This purification unit produces an off-gas stream, herein referred to as second off-gas stream from the purification unit, which is withdrawn at a lower pressure than the main synthesis gas stream being supplied to this unit. The recycle off-gas compressor increases the pressure of the second off-gas to meet the pressure of the reforming unit, while the portion being diverted as fuel gas is provided at a lower pressure and supplied to a fired heater of the process or plant. For instance, where the purification unit is a nitrogen wash unit, as it will also become apparent from one or more of below embodiments, the main synthesis gas may be supplied thereto at 25-30 barg, with the second off-gas stream being withdrawn at 1-5 barg. The recycle off-gas compressor increases this pressure to e.g. 30-50 barg, corresponding to the pressure of the reforming unit, while the diverted fuel gas is discharged at e.g. 2-10 barg.

[0045] In an embodiment, the purification unit is any of: a nitrogen wash unit (NWU), a methanation unit.

[0046] NWU and methanation units are well-known in the art of preparation of ammonia synthesis gas.

[0047] The NWU may comprise a drier and a cooler. The main syngas is first cooled down to cryogenic temperatures using liquid nitrogen. This causes the impurities to condense into a liquid state while the hydrogen and nitrogen remain in the gaseous state. The cooled main syngas is then passed through a wash column where the liquid impurities are separated from the gaseous components. The impurities are washed out of the gas stream and can be collected at the bottom of the column. The purified main syngas is then heated back to the required temperature for the ammonia synthesis process. This is typically done by heat exchange with the incoming syngas to improve energy efficiency. The purified syngas, now consisting mainly of hydrogen and nitrogen, is then used in the ammonia synthesis process.

[0048] A NWU provides the following benefits. The NWU is highly efficient and can achieve very high levels of purity. However, it is also energy-intensive due to the need for cooling and reheating the syngas. Therefore, it is typically used in situations where high purity is required and energy costs may not be of a major concern. A NWU may be integrated with other units of the process and plant. For instance, where the reforming unit comprises an autothermal reformer (ATR) along with an air separation unit (ASU) which produces the oxygen for the ATR, a nitrogen stream is also produced in the ASU which is then suitably supplied to the NWU.

[0049] A methanation unit is another route for purifying the main syngas for ammonia synthesis. Unlike nitrogen wash, which is a physical process, methanation is a chemical process that converts carbon monoxide (CO) and carbon dioxide (CO2) in the syngas into methane (CH4) and water (H2O).

[0050] A methanation unit provides the following benefits. If the catalyst used in the ammonia synthesis process is highly sensitive to CO and CO2, methanation may be preferred as it can reduce these components to very low levels. A methanation unit is generally less energy intensive than NWU because it does not require the main syngas to be cooled and reheated. If the main syngas contains a high proportion of CO and CO2, methanation may be more efficient at removing these components than a NWU. A methanation unit, like a NWU, may be integrated with other units of the process and plant, such as the reforming unit and steam reforming and WGS unit, which can improve overall process efficiency.

[0051] In an embodiment, the purification unit is a NWU and the process further comprises:

[0052] - supplying a nitrogen stream to the NWU; such as a nitrogen-rich stream, a liquid nitrogen stream, or both;

[0053] - withdrawing a hydrogen-fuel stream from the NWU;

[0054] - supplying the hydrogen-fuel stream to a fired heater; optionally, supplying the hydro- gen-fuel stream to an auxiliary steam boiler for steam production. While the NWU may at first glance appear as energy intensive, the present invention realizes that a reduction in energy consumption and thus increase in energy efficiency is nonetheless achieved, as a hydrogen-fuel stream, suitably containing 60-80 vol. % H2, is withdrawn from the NWU which can be used as hydrocarbon fuel in fired heaters of the process and plant, optionally as hydrocarbon fuel for an auxiliary boiler.

[0055] As less hydrogen is recycled via the recycle off-gas compressor, this further enables reducing the net recycle flow to the reforming unit and thereby reduce the required capacity of the reforming unit, WGS unit and CCh-removal unit. The available hydrogen is used as hydrocarbon fuel in fired heaters, thus reducing the need of using e.g. natural gas as hydrocarbon fuel for the combustion (burning) in the fired heaters. Lower energy consumption figures result along with a lower carbon footprint, thus a process and plant with lower carbon intensity (Cl), as the use of natural gas is reduced.

[0056] In an embodiment, the ammonia synthesis unit comprises an ammonia refrigeration unit under the production of a tail off-gas stream, and the process further comprises: withdrawing from the ammonia refrigeration unit said tail off-gas stream and:

[0057] - combining the tail off-gas stream with the second off-gas stream to the reforming unit, such as by combining the tail gas stream with the portion of the sec- ond-off gas stream which is diverted as fuel gas; and / or

[0058] - combining the tail off-gas stream with a hydrogen-fuel stream from the purification unit, in which the purification unit is a NWU; and / or

[0059] - supplying, preferably directly supplying, the tail off-gas stream to a fired heater.

[0060] It will be understood that the ammonia synthesis unit comprises an ammonia synthesis loop including an ammonia synthesis reactor i.e. an ammonia converter. From the ammonia synthesis loop, an ammonia product stream is withdrawn, for instance containing ammonia in a concentration of 99.9 vol.% or more. The ammonia synthesis unit may further comprise an ammonia refrigeration unit. The ammonia refrigeration unit suitably comprises a flash vessel arranged to receive the ammonia product stream and provide a bottom stream as the ammonia product stream, which may then be sent to storage outside battery limits of the process or plant. An ammonia vapor stream is withdrawn as the overhead stream and supplied to an ammonia compressor of the ammonia refrigeration unit. Accordingly, the ammonia refrigeration unit may further comprise an ammonia evaporator, i.e. chiller, being provided with a raw ammonia product withdrawn from the ammonia converter as heat exchanging medium, and / or one or more ammonia evaporators using the main synthesis gas as heat exchanging medium. The ammonia refrigeration unit further comprises said ammonia compressor, an ammonia condenser, an ammonia accumulator and a tail off-gas separator e.g. tail off-gas scrubber. From the ammonia synthesis loop, a purge gas is withdrawn from the gas being recycled to the ammonia converter, the purge gas suitably containing 60 vol.% or more of hydrogen. This purge gas is suitably indirectly supplied to the tail off-gas separator of the ammonia refrigeration unit. From the tail off-gas separator, ammonia water is removed as so is the tail off-gas stream, suitably containing 60 vol.% or more hydrogen. This tail off-gas stream is suitably withdrawn at a pressure of 5-20 barg and supplied, preferably directly supplied, to a fired heater of the process and plant, thus further reducing the need for natural gas being supplied thereto. Lower energy consumption figures result along with a lower carbon footprint, hence lower Cl, as the use of natural gas is further reduced.

[0061] In an embodiment, the fired heater is a fired heater associated with the reforming unit for preheating a hydrocarbon feed gas; and / or a fired steam superheater for producing a superheated steam.

[0062] The term “fired heater associated with the reforming unit” means that the fired heater serves to provide heat to one or more hydrocarbon feed gas streams of the reforming unit. The term “fired steam superheater” means that steam is supplied thereto and the thus produced superheated steam is advantageously applied for driving a turbine for producing electricity.

[0063] In an embodiment, only the diverted fuel gas from the purification unit is supplied to the fired steam superheater.

[0064] A fired heater is typically associated with a reforming unit; suitably, for preheating the hydrocarbon feed gas, such as natural gas, being supplied to a pre-reforming unit for preheating a pre-reformed hydrocarbon gas upstream an ATR or other reforming reactor.

[0065] Upstream the pre-reforming unit, a hydrogenator and sulfur absorber are suitably provided. The hydrocarbon feed gas is preheated in a fired heater prior to said hydrogenator and sulfur absorber, as well as to pre-heat pre-reformed syngas upstream the ATR, as described above.

[0066] In an embodiment, between step vi) and vii) the process further comprises:

[0067] - cooling the main synthesis gas stream, and removing water therefrom, optionally by supplying the thus cooled main synthesis gas stream to a water separator, such as a process condensate (PC) unit.

[0068] The first off-gas from the hydrogen purification unit, for instance PSA off-gas, is thus preferably indirectly supplied to the purification unit, preferably a NWU, due to high water content of the main syngas which increases the molecular sieve dryer size in the NWU. Also, the main syngas inlet temperature to the NWU increases to 10°C or higher, instead of, for instance 5°C, when providing the cooling step. NWU efficiency decreases with increased inlet temperature. It will be understood that the NWU may then not need to comprise a dryer and cooler, as cooling and water removal is already provided in the main synthesis gas upstream the NWU.

[0069] As recited above, for the purposes of the present application, the term “indirectly supplied” means that there is an intermediate unit or step changing the composition of a process stream. The term “directly supplied” means that there is no intermediate unit or step changing the composition of a process stream.

[0070] In an embodiment, in step vii) said supplying of the main synthesis gas stream to the purification unit is conducted without supplying the main synthesis gas stream to an additional hydrogen purification unit upstream the purification unit, and the process comprises: cooling the main synthesis gas stream and removing water therefrom, optionally by supplying the thus cooled main synthesis gas stream to a water separator, such as a process condensate unit (PC) unit. The first off-gas from the hydrogen purification unit, for instance PSA off-gas, is by the invention advantageously routed to mix with the second portion of the CCh-depleted syngas into the main syngas before said cooling, by compressing with the hydrogen purification unit off-gas compressor, e.g. PSA off-gas compressor. This further enables easier separation of the excess water in the water separator, e.g. PC unit and avoid a high gas inlet temperature to NWU, as explained above.

[0071] In an embodiment, said cooling is provided by an ammonia evaporator, i.e. a chiller, receiving an ammonia liquid stream as heat exchanging medium; suitably, the ammonia synthesis unit comprises an ammonia refrigeration unit, in which the ammonia liquid stream is withdrawn from the ammonia refrigeration unit.

[0072] Suitably, as already recited, the ammonia synthesis unit comprises an ammonia refrigeration unit, the ammonia refrigeration unit comprises an ammonia accumulator. Said ammonia liquid stream is thus advantageously at least a portion of an ammonia liquid stream withdrawn from said ammonia accumulator. The ammonia vapor is then supplied to the ammonia compressor of the ammonia refrigeration unit. A portion of the ammonia liquid stream withdrawn from said ammonia accumulator is supplied to the above-mentioned one or more ammonia evaporators, i.e. one or more chillers, being provided with a raw ammonia product withdrawn from the ammonia converter as heat exchanging medium.

[0073] High integration of process streams and associated units is thereby achieved. A process stream from the ammonia refrigeration unit, thus from the back end of the process and plant, is advantageously utilized to cool the main synthesis gas farther upstream and later used for producing the ammonia product.

[0074] In an embodiment, said at least first and second portion of the CCh-depleted synthesis gas stream have the same composition.

[0075] The term “same composition” means within 10% of the one or more components of the stream.

[0076] In an embodiment, - the reforming unit comprises any of: an autothermal reformer (ATR), a steam methane reformer (SMR), an electrically heated steam methane reformer (e-SMR), a convection heated reactor, and combinations thereof;

[0077] - the WGS unit comprises at least one of: a high temperature shift (HTS) reactor; a medium shift temperature (MTS) reactor, and a low temperature shift (LTS) reactor;

[0078] - the CC>2-removal unit is any of: an amine wash unit, a CO2 membrane separation unit, and a cryogenic separation unit, preferably an amine wash unit.

[0079] - the hydrogen purification unit is any of: a pressure swing adsorption (PSA) unit; a membrane unit.

[0080] It is understood that a given unit provides the associated process step.

[0081] For instance, there is reforming in the reforming unit; there is water gas shifting (shift) in the WGS unit; there is 002-removal in the 002-removal unit; there is hydrogen purification in the hydrogen purification unit.

[0082] As already recited, the term “unit” may be used interchangeably with the term “section” and means a physical section comprising one or more units. For instance, the term “reforming unit” means a “reforming section” which may comprise a hydrogenator, sulfur absorber, pre-reforming unit, and ATR.

[0083] In an embodiment, the reforming unit comprises an ATR and optionally also a pre-re- forming unit, yet there is no steam methane reforming (SMR) unit, i.e. the use of a conventional SMR, also referred to in the art as “radiant furnace” or “tubular reformer”, is omitted.

[0084] The ATR, SMR, e-SMR and convection heated reactor (convection reformer), are well- known in the art. In a convection reformer, preferably comprising one or more bayonet reforming tubes such as an HTCR reformer i.e. Topsoe bayonet reformer, where the heat for reforming is transferred by convection along with radiation. In an SMR, the heat for reforming is transferred chiefly by radiation in a radiant furnace; in an autothermal reformer (ATR), there is a partial oxidation of the hydrocarbon feed with oxygen and steam followed by catalytic reforming; in an electrically heated steam methane reformer (e-SMR), electrical resistance is used for generating the heat for catalytic reforming. Combinations of these reactors is also envisaged. In particular, when using e-SMR, electricity from green resources may be utilized, such as from electricity produced by wind power, hydropower, and solar sources, thereby further minimizing the carbon dioxide footprint.

[0085] For more information on these reformers, details are herein provided by direct reference to Applicant’s patents and / or literature. For instance, for tubular and autothermal reforming an overview is presented in “Tubular reforming and autothermal reforming of natural gas - an overview of available processes”, lb Dybkjaer, Fuel Processing Technology 42 (1995) 85-107; and EP 0535505 for a description of HTCR. For a description of ATR and / or SMR for large scale hydrogen production, see e.g. the article “Large- scale Hydrogen Production”, Jens R. Rostrup-Nielsen and Thomas Rostrup-Nielsen”: https: / / www.topsoe.com / sites / default / files / topsoe_large_scale_hydrogen_produc.pdf For a description of e-SMR which is a more recent technology, reference is given to in particular applicant’s WO 2019 / 228797 A1.

[0086] The WGS unit comprises at least one of: a high temperature shift (HTS) reactor; a medium shift temperature (MTS) reactor, and a low temperature shift (LTS) reactor. These shift reactors are well-known in the art.

[0087] The CO2-removal unit is any of: an amine wash unit, a CO2 membrane i.e. CO2 membrane separation unit, and a cryogenic separation unit, preferably an amine wash unit. These units are also well-known in the art.

[0088] The hydrogen purification unit is any of: a pressure swing adsorption (PSA) unit; a membrane unit, such as a Pd-membrane unit. These units are also well-known in the art.

[0089] In an embodiment, the reforming unit further comprises pre-reforming in a pre-reforming unit, such as an adiabatic pre-reforming unit. In an embodiment, the prereforming is conducted in one or more adiabatic prereforming stages with interstage preheating, i.e. with heating in between prereforiming stages. In another embodiment, a single pre-re- forming unit, such as a single adiabatic pre-reforming unit is provided. Suitably, a prereforming unit is provided the reforming unit and upstream e.g. the ATR. In the prereforming unit all higher hydrocarbons can be converted to carbon oxides and methane, but the prereforming unit is also advantageous for light hydrocarbons. Providing the prereforming unit, hence prereforming step, may have several advantages including reducing the required O2 consumption in the ATR and allowing higher inlet temperatures to the ATR since cracking risk by preheating is minimized. Furthermore, the prereforming unit may provide an efficient sulfur guard resulting in a practically sulfur free feed gas entering the ATR and the downstream system. The prereforming step may be carried out at temperatures between 300-650°C, preferably 390-480°C. Preferably, the prereforming is conducted in one or more adiabatic prereforming stages with interstage preheating, i.e. with heating in between prereforiming stages.

[0090] In an embodiment,

[0091] - the reforming unit comprises a pre-reforming unit, preferably a single pre-reforming unit, such as a single adiabatic pre-reforming unit, together with an ATR, i.e. a standalone ATR;

[0092] - the WGS unit is a HTS reactor together with a downstream LTS reactor;

[0093] - the CC>2-removal unit is an amine wash unit;

[0094] - the hydrogen purification unit is a PSA unit.

[0095] For the purposes of the present application, the provision of a pre-reforming unit, in particular a single pre-reforming unit, together with an ATR, is referred to as “standalone ATR”. It will be understood that no other reformers are included, such as a primary reformer e.g. an SMR.

[0096] In an embodiment, said at least a portion of the second off-gas stream being supplied to the reforming unit, is added to the hydrocarbon feed gas to the reforming unit.

[0097] This second off-gas stream has a significant methane concentration and is thus advantageously combined with the hydrocarbon feed gas, this being for instance natural gas or pre-reformed natural gas.

[0098] In an embodiment, the reforming unit comprises a pre-reforming unit for producing a pre-reformed hydrocarbon feed and an autothermal reformer (ATR), and said at least a portion of the second off-gas stream being supplied to the reforming unit, is added to a point in between the pre-reforming unit and the ATR, i.e. to the pre-reformed hydrocarbon feed to the ATR.

[0099] In an embodiment, said at least a portion of the second off-gas stream being supplied to the reforming unit is added to a point immediately upstream the ATR after preheating the pre-reformed hydrocarbon feed to the ATR inlet temperature.

[0100] The term “immediately upstream” means at the inlet, here at the inlet of the ATR. It will be understood that in connection with a given unit, here the ATR, the unit comprises an inlet and an outlet.

[0101] In an embodiment, the purification unit is a nitrogen wash unit (NWU) and the process further comprises:

[0102] - supplying air to an Air Separation Unit (ASU) and withdrawing from the ASU: an oxy- gen-rich stream and a nitrogen-rich stream;

[0103] - supplying at least a portion of said oxygen-rich stream, optionally under the addition of steam, to the ATR;

[0104] - supplying at least a portion of said nitrogen-rich stream to the nitrogen wash unit (NWU).

[0105] Thereby, high integration of process streams is achieved, as nitrogen is already available in connection with the ASU associated with the ATR.

[0106] In an embodiment, the at least a portion of the second off-gas stream being supplied to the reforming unit is at least 60 vol.%, such as at least: 65, 70, 75, 80, 85, 90 vol.% of the second off-gas stream being withdrawn from the purification unit.

[0107] Since there is a significant content of methane, e.g. 20-40 vol%, in this second off-gas stream being supplied via the recycle off-gas compressor to the reforming unit, less hydrogen is recycled and thus the required capacity of the reforming, water gas shift and CO2 removal unit, is reduced. In a second aspect of the invention, there is also provided a plant, i.e. a process plant, for carrying out the process according to any of the above embodiments. The plant comprises:

[0108] - a reforming unit arranged to receive a hydrocarbon feed gas and provide a raw synthesis gas;

[0109] - a water gas shift (WGS) unit arranged to receive at least a portion of the raw synthesis gas and provide a shifted synthesis gas;

[0110] - a CC>2-removal unit arranged to receive at least a portion of the shifted synthesis gas and provide a CCh-rich stream and a CCh-depleted synthesis gas;

[0111] - a splitting point, such a juncture, arranged to receive the CCh-depleted synthesis gas stream and separate the CCh-depleted synthesis gas stream into at least a first and second portion;

[0112] - a hydrogen purification unit arranged to receive the first portion of the CCh-depleted synthesis gas stream and provide a hydrogen product stream and a first off-gas stream;

[0113] - a mixing point, such as mixing unit or juncture, arranged to combine at least a portion of the first off-gas stream with the second portion of the CCh-depleted synthesis gas stream into a main synthesis gas stream;

[0114] - a purification unit arranged to receive the main synthesis gas stream, optionally the purification unit being in direct fluid communication with said mixing point, and provide an ammonia synthesis gas stream and a second off-gas stream;

[0115] - an ammonia synthesis unit arranged to receive the ammonia synthesis gas stream and provide an ammonia product stream;

[0116] - a conduit for supplying at least a portion of the second off-gas stream to the reforming unit.

[0117] It will be understood that the term “conduit” means a process line, such as pipe, carrying a given process stream.

[0118] Any of the embodiments and associated benefits in connection with the first aspect (process) of the invention may be used in connection with the second aspect (plant) of the invention, or vice versa. Fig. 1 shows a process and plant layout for producing hydrogen an ammonia according to the prior art, specifically with respect to the feature of the off-gas from a hydrogen purification unit being combined with off-gas from downstream purification unit and recycled to the reforming unit.

[0119] Fig. 2 shows a process and plant layout for producing hydrogen an ammonia according to an embodiment of the invention, specifically with respect to the feature of the off-gas from a hydrogen purification unit being returned to main syngas stream instead of being combined with the off-gas stream from downstream purification.

[0120] For comparison purposes, the other features recited are common in both Fig. 1 and Fig. 2. Hence, features of Fig. 2 are included as part of Fig. 1 to enable proper comparison. Associated benefits of the embodiment according to the invention (Fig. 2) are provided in the corresponding example farther below.

[0121] With reference to Fig. 1 , which is in accordance with the prior art specifically with respect to the feature of the off-gas from the hydrogen purification unit (PSA unit) being combined with off-gas from downstream purification unit and recycled to the reforming unit, a process and plant 100 is shown. A hydrocarbon feed gas 101 such as natural gas is supplied to reforming unit 110, suitably comprising a hydrogenator, sulfur absorber, pre-reforming unit and ATR, as well as associated fired heater for preheating the hydrocarbon feed gas (none of these are shown). A raw synthesis gas 103, i.e. raw syngas 103, is produced and supplied to water gas shift (WGS) unit 112, suitably comprising a high temperature shift (HTS) reactor together with a downstream low temperature shift (LTS) reactor (not shown). A shifted syngas 107 is withdrawn and supplied to a CC>2-removal unit 114, suitably an amine absorber, from which a CCh-rich stream 109 is withdrawn, as so is a CCh-depleted syngas 111. The CCh-depleted syngas 111 is separated into at least a first 11 T and second portion 111”. The first portion 11 T is supplied to a hydrogen purification unit 116, suitably a PSA unit, from which a hydrogen product 113 suitable for export is withdrawn, as well as a first off-gas 115. This first off-gas 115 is combined with second off-gas 105 from downstream purification unit 120. The second portion 111” of the CCh-depleted syngas 111 represents a main syngas which is cooled in ammonia evaporator 118, i.e. a chiller, receiving an ammonia liquid stream 117 from ammonia refrigeration unit (not shown) of downstream ammonia synthesis unit 124 as heat exchanging medium. The ammonia vapor stream 117’ is directed back to the ammonia compressor of said ammonia refrigeration unit. The thus cooled main syngas 11 T” is supplied to the purification unit 120, suitably a nitrogen wash unit (NWU), which is being supplied with a nitrogen stream 121. From the purification unit 120, a hydrogen-fuel 123 is withdrawn, which is used in a fired heater of the process and plant, in particular the fired heater associated with the reforming unit 110. The hydrogen-fuel 123 may also be supplied to an auxiliary steam boiler for steam production, suitably outside the battery limits of the plant. From the purification unit 120 an ammonia syngas 119 and the second off-gas 105 are also withdrawn. The ammonia syngas 119 is supplied to an ammonia synthesis unit 124 from which an ammonia product 125 is withdrawn. Suitably, the ammonia synthesis unit 124 comprises also said ammonia refrigeration unit (not shown). The first 115 and second off-gas 105 are combined into off-gas stream 129 and supplied to the reforming unit 110 as compressed stream 129’ via a recycle off-gas compressor 122. A portion 129” is diverted as fuel gas at a lower pressure and supplied to a fired heater.

[0122] With reference to Fig. 2, which is in accordance with an embodiment of the present invention, a process and plant 200 is shown. A hydrocarbon feed gas 201 such as natural gas is supplied to reforming unit 210, suitably comprising a hydrogenator, sulfur absorber, pre-reforming unit and ATR, as well as associated fired heater for preheating the hydrocarbon feed gas (none of these are shown). A raw synthesis gas 203, i.e. raw syngas 203, is produced and supplied to water gas shift (WGS) unit 212, suitably comprising a high temperature shift (HTS) reactor together with a downstream low temperature shift (LTS) reactor (not shown). A shifted syngas 207 is withdrawn and supplied to a CC>2-removal unit 214, suitably an amine absorber, from which a CCh-rich stream 209 is withdrawn, as so is a CCh-depleted syngas 211 . The CCh-depleted syngas 211 is separated into at least a first 21 T and second portion 211”. The first portion 21 T is supplied to a hydrogen purification unit 216, suitably a PSA unit, from which a hydrogen product 213 suitable for export is withdrawn, as well as a first off-gas 215. At least a portion of the first off-gas 215 is passed through a hydrogen purification unit off-gas compressor 226 into compressed first off-gas 215’ and combined with the second portion 211” of the CC>2-depleted syngas 211 via a mixing point such as a juncture into a main syngas 227. The main syngas 227 is advantageously cooled in ammonia evaporator 218, i.e. a chiller, receiving an ammonia liquid stream 217 from ammonia refrigeration unit (not shown) of downstream ammonia synthesis unit 224 as heat exchanging medium. The ammonia vapor stream 217’ is directed back to the ammonia compressor of said ammonia refrigeration unit. The thus cooled main syngas 227’ is supplied to a purification unit 220, suitably a nitrogen wash unit (NWU) which is being supplied with a nitrogen stream 221. As shown in the figure, preferably the main syngas 227 or cooled main syngas 227’ is supplied directly to the purification unit 220, e.g. without supplying the main synthesis gas stream 227, 227’ to an additional hydrogen purification unit upstream the purification unit 220. In other words, there is no e.g. additional hydrogen purification unit arranged to receive the main synthesis gas stream upstream the purification unit 220. Accordingly, the purification unit is in direct fluid communication with said mixing point. In another embodiment, there is no e.g. additional hydrogen purification unit arranged to receive the main synthesis gas stream 227, 227’ upstream the purification unit 220, but water is removed from the main synthesis gas stream 227’ in a water separator, such as a process condensate (PC) unit (not shown), upstream the purification unit 220. From the purification unit 220, a hydrogen-fuel 223 is withdrawn, which is used in a fired heater of the process and plant, in particular the fired heater associated with the reforming unit 210. The hydrogen-fuel 223 may also be supplied to an auxiliary steam boiler for steam production, suitably outside the battery limits of the plant. From the purification unit 220 an ammonia syngas 219 and a second off-gas 205 are also withdrawn. The ammonia syngas 219 is supplied to an ammonia synthesis unit 224 from which an ammonia product 225 is withdrawn. Suitably, the ammonia synthesis unit 224 comprises also said ammonia refrigeration unit (not shown). At least a portion of the second off-gas 205 is supplied to the reforming unit 210 as compressed stream 205’ via a recycle off-gas compressor 222. A portion 205” of the second-off gas 205 is diverted as fuel gas at a lower pressure and supplied to a fired heater. Suitably, the fired the fired heater is a fired heater associated with the reforming unit 210 for preheating a hydrocarbon feed gas, as mentioned above. Suitably, the fired heater is a fired steam superheater for producing a superheated steam. The superheated steam is advantageously applied for driving a turbine for producing electricity. EXAMPLE

[0123] The following table shows a comparison between the prior art as in Fig. 1 with respect to an embodiment of the invention as in Fig. 2:

[0124] Hence, the invention enables at least higher production of ammonia, while at the same time having a lower specific energy consumption as well as a decrease of the required capacity of the reforming unit, WGS unit and CCh-removal unit by reducing the net recycle flow.

Claims

CLAIMS1. Process for producing hydrogen and ammonia, comprising: i) supplying a hydrocarbon feed gas to a reforming unit and withdrawing from the reforming unit a raw synthesis gas stream; ii) supplying at least a portion of the raw synthesis gas stream to a water gas shift (WGS) unit and withdrawing from the WGS unit a shifted synthesis gas stream; iii) supplying at least a portion of the shifted synthesis gas stream to a CO2-removal unit and withdrawing from the CO2-removal unit: a CO2-rich stream and a CO2-depleted synthesis gas stream; iv) separating the CO2-depleted synthesis gas stream into at least a first and second portion; v) supplying the first portion of the CO2-depleted synthesis gas stream to a hydrogen purification unit and withdrawing from the hydrogen purification unit: a hydrogen product stream and a first off-gas stream; vi) combining at least a portion of the first off-gas stream with the second portion of the CO2-depleted synthesis gas stream into a main synthesis gas stream; vii) supplying the main synthesis gas stream to a purification unit and withdrawing from the purification unit: an ammonia synthesis gas stream and a second off-gas stream; viii) supplying the ammonia synthesis gas stream to an ammonia synthesis unit and withdrawing from the ammonia synthesis unit an ammonia product stream; ix) supplying at least a portion of the second off-gas stream to the reforming unit.

2. Process according to claim 1 , wherein: step vii) is directly supplying the main synthesis gas stream to the purification unit.

3. Process according to any of claims 1-2, wherein between step v) and vi) the process comprises:- passing the first off-gas stream from the hydrogen purification unit through a hydrogen purification unit off-gas compressor prior to combining with the second portion of the CO2-depleted synthesis gas stream.

4. Process according to any of claims 1-3, wherein step ix) comprises:- passing the second off-gas stream from the purification unit through a recycle off-gas compressor;- diverting from the recycle off-gas compressor a portion of the second-off gas stream as fuel gas at a lower pressure than the pressure of the second off-gas stream to the reforming unit, and supplying the fuel gas to a fired heater.

5. Process according to any of claims 1-4, wherein the purification unit is any of: a nitrogen wash unit (NWU), a methanation unit.

6. Process according to claim 5, wherein the purification unit is a NWU and the process further comprises:- supplying a nitrogen stream to the NWU;- withdrawing a hydrogen-fuel stream from the NWU;- supplying the hydrogen-fuel stream to a fired heater; optionally, supplying the hydro- gen-fuel stream to an auxiliary steam boiler for steam production.

7. Process according to any of claims 1-6, wherein ammonia synthesis unit comprises an ammonia refrigeration unit under the production of a tail off-gas stream, and the process further comprises: withdrawing from the ammonia refrigeration unit said tail off-gas stream and:- combining the tail off-gas stream with the second off-gas stream to the reforming unit, such as by combining the tail gas stream with the portion of the sec- ond-off gas stream which is diverted as fuel gas; and / or- combining the tail off-gas stream with a hydrogen-fuel stream from the purification unit, in which the purification unit is a NWU; and / or- supplying, preferably directly supplying, the tail off-gas stream to a fired heater.

8. Process according to any of claims 4-7, wherein the fired heater is a fired heater associated with the reforming unit for preheating a hydrocarbon feed gas; and / or a fired steam superheater for producing a superheated steam.

9. Process according to any of claims 1 and-3-8, wherein between step vi) and vii) the process further comprises:- cooling the main synthesis gas stream, and removing water therefrom, optionally by supplying the thus cooled main synthesis gas stream to a water separator, such as a process condensate (PC) unit.

10. Process according to claim 9, wherein said cooling is provided by an ammonia evaporator receiving an ammonia liquid stream as heat exchanging medium.11 . Process according to any of claims 1-10, wherein said at least first and second portion of the CC>2-depleted synthesis gas stream have the same composition.

12. Process according to any of claims 1-11 , wherein:- the reforming unit comprises any of: an autothermal reformer (ATR), a steam methane reformer (SMR), an electrically heated steam methane reformer (e-SMR), a convection heated reactor, and combinations thereof;- the WGS unit comprises at least one of: a high temperature shift (HTS) reactor; a medium shift temperature (MTS) reactor, and a low temperature shift (LTS) reactor;- the CC>2-removal unit is any of: an amine wash unit, a CO2 membrane separation unit, and a cryogenic separation unit, preferably an amine wash unit.- the hydrogen purification unit is any of: a pressure swing adsorption (PSA) unit; a membrane unit.

13. Process according to claim 12, wherein the reforming unit further comprises prereforming in a pre-reforming unit, such as an adiabatic pre-reforming unit.

14. Process according to any of claims 1-13, wherein:- the reforming unit comprises a pre-reforming unit, preferably an adiabatic pre-reforming unit, and an ATR;- the WGS unit is a HTS reactor together with a downstream LTS reactor;- the 002-removal unit is an amine wash unit;- the hydrogen purification unit is a PSA unit.

15. Process according to any of claims 1-14, wherein said at least a portion of the second off-gas stream being supplied to the reforming unit, is added to the hydrocarbon feed gas to the reforming unit.

16. Process according to claim 15, wherein the reforming unit comprises a pre-reforming unit for producing a pre-reformed hydrocarbon feed and an autothermal reformer (ATR), and said at least a portion of the second off-gas stream being supplied to the reforming unit, is added to a point in between the pre-reforming unit and the ATR.

17. Process according to claim 16, wherein said at least a portion of the second off-gas stream being supplied to the reforming unit is added to a point immediately upstream the ATR after preheating the pre-reformed hydrocarbon feed to the ATR inlet temperature.

18. Process according to any of claims 5-17, wherein the purification unit is a nitrogen wash unit (NWU) and the process further comprises:- supplying air to an Air Separation Unit (ASU) and withdrawing from the ASU: an oxy- gen-rich stream and a nitrogen-rich stream;- supplying at least a portion of said oxygen-rich stream, optionally under the addition of steam, to the ATR;- supplying at least a portion of said nitrogen-rich stream to the nitrogen wash unit (NWU).

19. Process according to any of claims 1-18, wherein the at least a portion of the second off-gas stream being supplied to the reforming unit is at least 60 vol.%, such as at least: 65, 70, 75, 80, 85, 90 vol.% of the second off-gas stream being withdrawn from the purification unit.

20. Plant for carrying out a process according to any claims 1-19, the plant comprising:- a reforming unit arranged to receive a hydrocarbon feed gas and provide a raw synthesis gas;- a water gas shift (WGS) unit arranged to receive at least a portion of the raw synthesis gas and provide a shifted synthesis gas;- a CC>2-removal unit arranged to receive at least a portion of the shifted synthesis gas and provide a CCh-rich stream and a CCh-depleted synthesis gas;- a splitting point, such a juncture, arranged to receive the CCh-depleted synthesis gas stream and separate the CCh-depleted synthesis gas stream into at least a first and second portion;- a hydrogen purification unit arranged to receive the first portion of the CCh-depleted synthesis gas stream and provide a hydrogen product stream and a first off-gas stream;- a mixing point, such as mixing unit or juncture, arranged to combine at least a portion of the first off-gas stream with the second portion of the CCh-depleted synthesis gas stream into a main synthesis gas stream;- a purification unit arranged to receive the main synthesis gas stream, optionally the purification unit being in direct fluid communication with said mixing point, and provide an ammonia synthesis gas stream and a second off-gas stream;- an ammonia synthesis unit arranged to receive the ammonia synthesis gas stream and provide an ammonia product stream;- a conduit for supplying at least a portion of the second off-gas stream to the reforming unit.

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