Method and APPARATUS for hydrogen production with low carbon dioxide levels

Abstract

 The invention relates to a method and an apparatus for producing a carbon-monoxide-free make-up gas (15) consisting largely of hydrogen and nitrogen for ammonia synthesis, a hydrocarbon-containing feed (1) being converted, by a reforming step (B) and a water-gas shift (G), into a synthesis gas (6) consisting largely of hydrogen and carbon dioxide and containing carbon monoxide, from which synthesis gas raw hydrogen (8) is obtained by separating carbon dioxide (7), at least a portion of which raw hydrogen is conditioned to form a feed (cryogenic feed) (10) for cryogenic gas separation (K), in which a hydrogen-rich, carbon-monoxide-free first gas fraction (11), which is used as make-up gas or supplemented with nitrogen (14) to form make-up gas (15), and a hydrogen-rich, low-carbon second gas fraction, which is used as fuel gas (13) for generating process heat, are produced from the cryogenic feed (10). The method and apparatus are characterized in that, during cryogenic gas separation (K), the hydrogen-rich, low-carbon second gas fraction (13) is obtained with a carbon content that is lower than that of the cryogenic feed (10) and higher than that of the hydrogen-rich, carbon-monoxide-free first gas fraction (11).Fig. 01

Description

Full specificationMETHOD AND APPARATUS FOR HYDROGEN PRODUCTION WITH LOW CARBON DIOXIDE LEVELS technical feildThe invention relates to a method for producing a carbon-monoxide-free make-up gas consisting largely of hydrogen and nitrogen for ammonia synthesis, a hydrocarbon-containing feed being converted, by a reforming step and a water-gas shift, into a synthesis gas consisting largely of hydrogen and carbon dioxide and containing carbon monoxide, from which synthesis gas raw hydrogen is obtained by separating carbon dioxide, at least a portion of which raw hydrogen is conditioned to form a feed (cryogenic feed) for cryogenic gas separation, in which a hydrogen-rich, carbon-monoxide-free first gas fraction, which is used as make-up gas or supplemented with nitrogen to form make-up gas, and a hydrogen-rich, low-carbon second gas fraction, which is used as fuel gas for generating process heat, are produced from the cryogenic feed. The invention also relates to an apparatus suitable for carrying out the method according to the invention.BACKGROUND OF THE INVENTION Ammonia is one of the most widely produced chemicals in the world. It is primarily used as a raw material for the production of fertilizers, but is also becoming increasingly important as an energy source and hydrogen storage means. On an industrial scale, it is synthesized almost exclusively from nitrogen and hydrogen using the Haber-Bosch process. Although hydrogen and nitrogen are still mainly produced today with the formation and release of large amounts of climate-damaging carbon dioxide, there are increasing efforts to not release the resulting carbon dioxide into the atmosphere, but to dispose of it through sequestration or material use. In the Haber-Bosch process, an ammonia synthesis gas, consisting mainly of hydrogen and nitrogen in which the two substances are present in a stoichiometric ratio of 3:1 for the synthesis of ammonia, is supplied to an ammonia synthesis process at a pressure of between 100 and 200 bar in order to be converted exothermically to ammonia with catalytic support in an ammonia reactor. However, thermodynamic limitations render the conversion incomplete resulting in a gas mixture that contains considerable amounts of hydrogen and nitrogen in addition to ammonia. This gas mixture leaves the ammonia reactor at a temperature between 400 and 450°C, and is subsequently cooled in a series of heat exchangers to separate ammonia by condensation and obtain a recycle gas consisting largely of hydrogen and nitrogen and containing residues of non-separated ammonia, which is recycled to increase the ammonia yield in a synthesis cycle to the ammonia reactor, and in the process is mixed with a make-up gas to form the ammonia synthesis gas. According to the prior art, in order to produce a make-up gas for ammonia synthesis, a hydrocarbon-containing feed, such as natural gas, is optionally desulfurized and then reformed, for example by partial oxidation, autothermal reforming, or steam reforming, to form a raw synthesis gas that consists largely of hydrogen, carbon monoxide, and carbon dioxide and, particularly when the reforming step is carried out using atmospheric oxygen, also contains significant amounts of argon. The raw synthesis gas is then subjected to a water-gas shift in order to react the carbon monoxide it contains with water to form hydrogen and carbon dioxide, and to obtain a synthesis gas. Carbon dioxide is typically separated from the synthesis gas by acid gas scrubbing, obtaining a carbon dioxide fraction with a purity sufficient for its sequestration or material use, as well as a hydrogen fraction known as raw hydrogen, which, in addition to hydrogen, also includes residues of carbon monoxide, carbon dioxide, methane, and possibly also argon. To prevent carbon monoxide from entering the ammonia reactor via the make-up gas and poisoning the catalyst material used therein, the raw hydrogen is treated according to prior art by methanation, pressure swing adsorption, or a cryogenic process, carbon monoxide being converted or separated and a hydrogen fraction being produced that, due to its low carbon monoxide content, typically below 10 ppmv, is referred to as carbon-monoxide-free, and at least a portion of which is passed on as make-up gas or supplemented with nitrogen to form make-up gas. Acid gas scrubbing, as used to separate carbon dioxide from the synthesis gas, utilizes the ability of liquids to selectively absorb carbon dioxide and other acid gases from a gas mixture and keep them in solution. The higher the pressure at which the scrubbing is carried out, the better the acid gases are absorbed and separated from the gas mixture to be cleaned. Since the synthesis gas in the water-gas shift is produced at a pressure level typically between 25 and 45 bar(a), the volume flows to be treated are small, so that the carbon dioxide can be separated from the synthesis gas by an acid gas scrub with comparatively low investment and operating costs. Carbon dioxide is also present in flue gases produced during synthesis gas production in reformers and in furnaces used to heat feedstocks, both of which are typically heated by burning natural gas and / or carbon-containing residual gases. Unlike the synthesis gas, these flue gases are without pressure. Since they can therefore only be treated by acid gas scrubbing or other methods for carbon dioxide separation, which is very costly and laborious, they are usually released into the atmosphere together with the carbon dioxide produced during combustion. Therefore, using the methods described, no more than 90% of the carbon used to produce the make-up gas can be separated in the form of carbon dioxide and disposed of by sequestration or sent to a material use. Depending on legal requirements and penalties for carbon dioxide emissions, it may be advantageous to achieve higher carbon capture rates. One possibility for increasing the carbon capture rate is disclosed in patent application EP4328176A1, which proposes conditioning the raw hydrogen to form a hydrogen-rich, carbon-monoxide-free gas fraction by means of cryogenic gas separation, a first portion of which is used as make-up gas or supplemented with nitrogen to form make-up gas, and a second portion of which is used as fuel gas, the combustion of which does not produce carbon dioxide. However, a disadvantage of this is that a portion of the hydrogen-rich, carbon-monoxide-free gas fraction produced at considerable expense cannot be converted to ammonia.SUMMARY The object of the invention is to provide a method of the generic type and an apparatus for carrying out said method, which allow the production of a make-up gas for ammonia synthesis with reduced carbon dioxide release, but without the described disadvantages of the prior art. The object is achieved according to the invention by means of a method in which, during cryogenic gas separation, the hydrogen-rich, low-carbon second gas fraction is obtained with a carbon content that is lower than that of the cryogenic feed and higher than that of the hydrogen-rich, carbon-monoxide-free first gas fraction. Preferably, the cryogenic gas separation comprises nitrogen scrubbing, in which at least a portion of the cryogenic feed cooled by heat exchange with process streams to be heated is scrubbed in a column using imported high-pressure nitrogen, which is obtained, for example, in gaseous form from an air separator and is likewise cooled and liquefied by heat exchange with process streams to be heated, in order to separate carbon monoxide in particular, but also other substances undesirable in the make-up gas, such as carbon dioxide, methane, and argon. The cryogenic feed, or a separated portion of the cryogenic feed, is introduced in gaseous form into the lower region of the scrubbing column, where, as it travels upward across mass transfer elements, it is brought into intensive contact with the nitrogen supplied in liquid form at the top of the scrubbing column. As the nitrogen flows downward, it becomes enriched with undesirable substances, while the content of undesirable substances in the countercurrently flowing gas phase decreases continuously, such that the gas fraction withdrawn from the top of the scrubbing column consists primarily of hydrogen and nitrogen and contains only a non-critical amount of carbon monoxide, which is harmful to the catalyst of ammonia synthesis. This head gas, which is therefore referred to as carbon-monoxide-free, is heated by heat exchange with process streams to be cooled and, optionally, enriched only with nitrogen to adjust the composition required for the make-up gas. In a preferred variant of the method according to the invention, it is provided that, in addition to the hydrogen-rich, carbon-monoxide-free head gas, a hydrogen-rich, low-carbon gas fraction having a carbon content that is lower than that of the cryogenic feed and higher than that of the head gas is withdrawn from the scrubbing column via a side draw and, after being heated by heat exchange with process streams to be cooled, is used as fuel gas. Unlike in EP4328176A1, only a portion of the cryogenic feed fed to the scrubbing column is subjected to complete scrubbing with liquid nitrogen, while the gas fraction provided as fuel gas is obtained only by partial scrubbing. Nitrogen scrubbing can therefore be carried out with a smaller amount of scrubbing agent compared to the prior art. In particular, when the hydrogen-rich, low-carbon gas fraction is withdrawn below the uppermost mass transfer element of the scrubbing column, the resulting reduction in the amount of nitrogen required leads to a significant improvement in efficiency. Selecting the position of the side draw makes it possible to adjust the carbon content of the fuel gas so that a predetermined carbon capture rate is achieved with the lowest possible investment and operating costs. During nitrogen scrubbing, in addition to the undesirable substances in the make-up gas, some of the hydrogen contained in the cryogenic feed is inevitably co-absorbed. To recover this hydrogen, at least a portion of the nitrogen laden with the components separated from the cryogenic feed can be withdrawn from the bottom of the scrubbing column and expanded into a separator referred to as a column bottom separator, where it is separated into a hydrogen-rich, low-carbon gas fraction and a liquid fraction containing the majority of the carbon separated from the cryogenic feed. One embodiment of the method according to the invention provides for the use of at least a portion of the hydrogen-rich, low-carbon gas fraction obtained in the column bottom separator as fuel gas. In particular, if the cryogenic feed contains methane, the invention provides for cooling the cryogenic feed during cryogenic gas separation to such an extent that the majority of the methane it contains condenses out. The resulting two-phase mixture is separated in a separator arranged upstream of the nitrogen scrubbing process and referred to as a column feed separator, producing a methane-rich liquid fraction and a hydrogen-rich, low-carbon gas fraction, which contains a portion of the carbon monoxide present in the cryogenic feed and possibly argon. A first portion of the hydrogen-rich gas fraction, which is depleted in carbon content relative to the cryogenic feed, is withdrawn from the column feed separator and subjected to scrubbing with liquid nitrogen, while a second portion of the hydrogen-rich, low-carbon gas fraction is used as fuel gas after being heated by heat exchange with process streams to be cooled. Each of the fuel gases produced according to the invention can be combusted either alone or together with one or more other hydrogen-rich, low-carbon gas fractions obtained in cryogenic gas separation to generate process heat. To avoid blockages caused by freezing water during cryogenic gas separation, the conditioning of the raw hydrogen to form cryogenic feed comprises a drying step in which water is removed from the optionally pretreated raw hydrogen, preferably by adsorption. The adsorbers loaded with water in the drying step are regenerated using a water-free regeneration gas. According to the invention, a water-free gas from the cryogenic gas separation is used as the regeneration gas, which water-free gas is obtained by vaporizing and heating by heat exchange with process streams to be cooled at least a portion of the bottom product formed during the nitrogen scrubbing and / or at least a portion of the liquid phases obtained in the column bottom separator and / or in the column feed separator. Since each of these gases contains carbon, at least portions thereof can, in order to improve the carbon capture rate of the method, be recycled after use as regeneration gas, or recycled directly, and used in the reforming step. While it is being conditioned to form the cryogenic feed, the raw hydrogen can be subjected to methanation upstream of the drying step, carbon monoxide and carbon dioxide contained in the raw hydrogen being reacted with hydrogen to form water and methane. Preferably, the fuel gas produced according to the invention is used for preheating a feedstock for the reforming step and / or for providing heat for the reforming reaction and / or for producing or superheating process steam. The method according to the invention can be used to particular advantage in the production of "blue" ammonia, whereby an attempt is made to dispose of the carbon dioxide obtained during the production of the make-up gas as completely as possible by sequestration or material utilization. The invention further relates to an apparatus for producing a carbon-monoxide-free make-up gas consisting largely of hydrogen and nitrogen for ammonia synthesis, comprising a reforming unit in which a hydrocarbon-containing feed can be reacted to obtain a raw synthesis gas containing hydrogen, carbon monoxide, and carbon dioxide, a water-gas shift and carbon dioxide separation by means of which a hydrogen-rich fraction comprising residual carbon monoxide and referred to as raw hydrogen can be produced from the raw synthesis gas, a treatment device by means of which at least a portion of the raw hydrogen can be conditioned to form a hydrogen-rich feed gas (cryogenic feed) containing carbon compounds for a cryogenic gas separator, and a cryogenic gas separator by means of which a hydrogen-rich, carbon-monoxide-free first gas fraction and a hydrogen-rich, low-carbon second gas fraction can be formed from the cryogenic feed, it being possible to pass the first gas fraction onward as the make-up gas or to supplement it with nitrogen to form the make-up gas, and it being possible to use the second gas fraction as fuel gas for generating process heat. With respect to the apparatus, the object is achieved in that the cryogenic gas separator is configured to produce a hydrogen-rich, low-carbon second gas fraction as fuel gas, the carbon content of which is lower than that of the cryogenic feed and higher than that of the hydrogen-rich, carbon-monoxide-free first gas fraction. In a preferred embodiment of the apparatus according to the invention, the cryogenic gas separator comprises a scrubbing column having mass transfer elements in which at least a portion of the cryogenic feed can be subjected to scrubbing with liquid nitrogen for the separation of carbon monoxide, it being possible to withdraw the hydrogen-rich, carbon-monoxide-free first gas fraction from the top of the scrubbing column. The scrubbing column is particularly preferably designed with a side draw through which a hydrogen-rich, low-carbon second gas fraction can be withdrawn as fuel gas. The side draw is expediently arranged between the lowermost and the uppermost mass transfer element of the scrubbing column. In a further embodiment of the apparatus according to the invention, the cryogenic gas separator comprises a separator connected to the scrubbing column and referred to as a column bottom separator, into which at least a portion of the nitrogen laden with separated components in the scrubbing column can be expanded in order to be separated into a liquid phase enriched with carbon compounds and a hydrogen-rich, low-carbon gas phase, at least a portion of which can be withdrawn as fuel gas. In another embodiment of the apparatus according to the invention, the cryogenic gas separator is configured to form a two-phase mixture from at least a portion of the cryogenic feed by cooling and, in a separator connected to the scrubbing column and referred to as a column feed separator, to separate it into a liquid phase enriched with carbon compounds and a hydrogen-rich, low-carbon gas phase, a first portion of which can be fed to the scrubbing column for scrubbing with liquid nitrogen and a second portion of which can be withdrawn as fuel gas. Preferably, the treatment device of the apparatus according to the invention comprises an adsorption gas dryer with which water can be separated from the optionally pretreated raw hydrogen in order to obtain a dry cryogenic feed. Advantageously, the adsorption gas dryer is connected to the cryogenic gas separator in such a way that a water-free gas obtainable in the cryogenic gas separator can be supplied to the adsorption gas dryer as regeneration gas. Preferably, the connection is designed such that at least a portion of the bottom product obtained during the nitrogen scrubbing and / or at least a portion of the liquid phases obtainable in the column bottom separator and / or in the column feed separator can be used as regeneration gas after being heated and vaporized by heat exchange with process streams to be cooled. Since each of these gases contains carbon, the apparatus according to the invention advantageously has a connection through which at least a portion of one of these gases can be recycled after use as regeneration gas, or recycled directly, and used in the reforming step. The treatment device can further comprise a methanation reactor arranged upstream of the gas dryer, by means of which the raw hydrogen can be subjected to methanation. Embodiments of the apparatus according to the invention further provide a burner-fired furnace designed for preheating a feedstock for reforming, or for providing heat for the reforming reaction, or for producing or superheating process steam, in which furnace a fuel gas that can be produced according to the invention can be combusted.Brief description of the drawings The invention shall be explained in more detail below using two embodiments schematically illustrated in Fig. 1 and 2. Fig. 1 shows a preferred variant of a device according to the invention for generating a make-up gas for ammonia synthesis. Fig. 2 shows a cryogenic gas separation, which can be used particularly advantageously for the production of the make-up gas according to the invention.Detailed description In Fig. 1, a hydrocarbon-containing feed 1, which is natural gas, for example, is fed to a desulfurization device A in order to obtain a desulfurized hydrocarbon-containing feed 2, which is converted in the reforming device B using oxygen 3, which is obtained from air 4 in an air separator F, for example by partial oxidation or autothermal reforming, to a raw synthesis gas 5 containing hydrogen, carbon monoxide, carbon dioxide, methane, and argon. To increase the hydrogen content, the raw synthesis gas 5 is subjected to a water-gas conversion G, in which carbon monoxide reacts with water to form hydrogen and carbon dioxide, producing a low-carbon monoxide gas fraction 6, from which the majority of the carbon dioxide is removed in an acid gas scrubbing process H. The removed carbon dioxide 7 is supplied for sequestration or material use, while the remaining hydrogen-rich gas fraction 8, which contains carbon monoxide and is referred to as raw hydrogen, is passed on to the treatment device M, where it is used to produce the hydrogen-rich cryogenic feed 10 comprising methane and argon using a gas dryer L, preferably an adsorption dryer. Optionally, the raw hydrogen 8 can be subjected to methanation J in the treatment device M, during which carbon monoxide and carbon dioxide are reacted with hydrogen to form methane, and a largely carbon monoxide and carbon-dioxide-free gas mixture 9 is obtained for onward passage to the gas dryer L. In the cryogenic gas separator K, at least one hydrogen-rich, carbon-monoxide-free gas fraction 11, one methane-rich, low-argon, and low-nitrogen residual gas 12, and one hydrogen-rich, low-carbon gas fraction 13 are obtained from the cryogenic feed 10, the carbon content of which is lower than that of the cryogenic feed 10 and higher than that of the hydrogen-rich, carbon-monoxide-free gas fraction 11. While the hydrogen-rich, carbon-monoxide-free gas fraction 11 is supplemented with nitrogen 14 to form the make-up gas 15 for ammonia synthesis (not shown) and the methane-rich, low-argon, and low-nitrogen residual gas 12 is recycled via the compressor P to a point upstream of the reforming device B, the hydrogen-rich, low-carbon gas fraction 13 is supplied as fuel gas to a furnace O used in the reforming device B to heat the hydrocarbon-containing feed 2 and is combusted to form a largely carbon-dioxide-free flue gas 16. In the cryogenic gas separator K of Fig. 2, which is arranged in a cold box C, the cryogenic feed 10 is cooled in the heat exchangers E1 and E2 to such an extent that the methane it contains condenses out almost completely. The two-phase mixture 20 formed during cooling is separated in the column feed separator D1 into a methane-rich liquid phase 21 and a hydrogen-rich, low-carbon gas phase 22, which also contains the majority of the argon and carbon monoxide present in the cryogenic feed 10. The majority of the hydrogen-rich, low-carbon gas phase 22 is introduced into the lower region of the scrubbing column W via line 23. Nitrogen, which is obtained in gaseous form at increased pressure from the air separator F, is introduced via line 26 into the cold box C, where the liquid nitrogen stream 27 is produced by cooling in the heat exchangers E1 and E2, the majority of which is expanded via the valve b as scrubbing nitrogen 28 to the top of the scrubbing column W. The hydrogen-rich gas phase 23 is led upwards in the scrubbing column W and in the process is brought into intensive contact with the scrubbing nitrogen 28, in particular any carbon monoxide contained therein as well as methane residues and a large proportion of the argon present in the cryogenic feed 10 being scrubbed out and passing into the bottom fraction. The overhead stream 11 of the scrubbing column W is a hydrogen-rich, carbon-monoxide-free gas fraction that contains nitrogen and is free of methane and argon. Although said gas fraction meets the purity requirements for the make-up gas for ammonia synthesis, it has an insufficient nitrogen content. Therefore, after a first warming step in the heat exchanger E2, pre-cooled nitrogen 31 is added to the head stream 11 via the valve c. After a second heating step in the heat exchanger E1, a further amount of nitrogen 32 is supplied via the valve d in order to obtain the make-up gas 15 with the required hydrogen / nitrogen ratio. The sump fraction is withdrawn from the scrubbing column W via line 29 and expanded via the throttling device a into the column bottom separator D2, where it is separated into a liquid phase 40 consisting largely of nitrogen, carbon compounds, and argon and a hydrogen-rich, low-carbon gas phase 41. The gas phase 41, together with the hydrogen-rich, low-carbon gas fraction 42 withdrawn from the scrubbing column W via a side draw and obtained by incomplete nitrogen scrubbing, is passed onward through line 13 and, after being heated in the heat exchangers E1 and E2 by heat exchange with process streams to be cooled, is delivered as fuel gas. In addition, a portion 24 of the hydrogen-rich, low-carbon gas phase 22 from the column feed separator D1 can be metered into the fuel gas 13 via the valve h. The methane-rich liquid phase 21 present at the pressure of the cryogenic feed 10 is withdrawn from the column feed separator D1 via the valve e and is combined with a first portion 33 of the liquid phase 40 obtained in the column bottom separator D2, the first portion being fed via the valve f, to form the material stream 34, which, after evaporation and heating in the heat exchangers E2 and E1, forms the methane-rich, nitrogen-poor residual gas 12, which is fed back before the reforming step R. In order to prevent argon enrichment in the reformer circuit, a second portion 35 of the liquid phase 40 is withdrawn via the valve g and, after evaporation and heating in the heat exchangers E2 and E1, is discharged from the process as purge gas 43. In order to compensate for refrigeration losses and to maintain the refrigeration balance, liquid nitrogen 36 is introduced into the cold box C and supplemented by a portion 37 of the liquefied nitrogen 27 supplied via the valve h to form the nitrogen stream 38, which, after evaporation and heating in the heat exchangers E2 and E1, is disposed of via line 39 or fed for further use.

Claims

1. A method for producing a carbon-monoxide-free make-up gas (15) consisting largely of hydrogen and nitrogen for ammonia synthesis, a hydrocarbon-containing feed (1) being converted, by a reforming step (B) and a water-gas shift (G), into a synthesis gas (6) consisting largely of hydrogen and carbon dioxide and containing carbon monoxide, from which synthesis gas raw hydrogen (8) is obtained by separating carbon dioxide (7), at least a portion of which raw hydrogen is conditioned to form a feed (cryogenic feed) (10) for cryogenic gas separation (K), in which a hydrogen-rich, carbon-monoxide-free first gas fraction (11), which is used as make-up gas or supplemented with nitrogen (14) to form make-up gas (15), and a hydrogen-rich, low-carbon second gas fraction, which is used as fuel gas (13) for generating process heat, are produced from the cryogenic feed (10), characterized in that, during cryogenic gas separation (K), the hydrogen-rich, low-carbon second gas fraction (13) is obtained with a carbon content that is lower than that of the cryogenic feed (10) and higher than that of the hydrogen-rich, carbon-monoxide-free first gas fraction (11). 2. The method according to claim 1, characterized in that, during cryogenic gas separation (K), at least a portion (23) of the cryogenic feed (10), for the separation of carbon compounds, is scrubbed with liquid nitrogen (28) in a scrubbing column (W), the hydrogen-rich, carbon-monoxide-free gas fraction (30) being withdrawn overhead, and a hydrogen-rich, low-carbon gas fraction (42) being withdrawn from the scrubbing column (W) as fuel gas (13) via a side draw. 3. The method according to claim 2, characterized in that a nitrogen-rich, carbon compound-containing bottom fraction (29) obtainable in the scrubbing column (W) is separated into a nitrogen-rich liquid phase (40) enriched with carbon compounds and a hydrogen-rich, low-carbon gas phase (41), from which at least a portion is withdrawn as fuel gas (13). 4. The method according to claim 2 or 3, characterized in that, during cryogenic gas separation (K), a two-phase mixture (20) is formed from at least a portion of the cryogenic feed (10) by cooling, which two-phase mixture is separated into a liquid phase (21) enriched with carbon compounds and a hydrogen-rich, low-carbon gas phase (22), of which a first portion (23) is subjected to scrubbing with liquid nitrogen (28), while a second portion (24) is withdrawn as fuel gas (13). 5. The method according to any of claims 1 to 4, characterized in that the raw hydrogen (8) for the formation of the cryogenic feed (10) is subjected to methanation (J) in order to react carbon monoxide and carbon dioxide with hydrogen to form methane and water. 6. The method according to any of claims 1 to 5, characterized in that at least one methane-rich liquid phase (34) obtained during cryogenic gas separation (K) is fed wholly or partially to the reforming device (B) as a feed after evaporation and heating. 7. The method according to any of claims 1 to 6, characterized in that the fuel gas (13) obtained during cryogenic gas separation (K) is combusted to heat a feedstock (2) intended for the reforming step (B). 8. An apparatus for producing a carbon-monoxide-free make-up gas (15) consisting largely of hydrogen and nitrogen for ammonia synthesis, comprising a reforming unit (B) in which a hydrocarbon-containing feed (1) can be reacted to obtain a raw synthesis gas (5) containing hydrogen, carbon monoxide, and carbon dioxide, a water-gas shift (G) and carbon dioxide separation (H) by means of which a hydrogen-rich fraction comprising residual carbon monoxide and referred to as raw hydrogen (8) can be produced from the raw synthesis gas (5), a treatment device (M) by means of which at least a portion of the raw hydrogen (8) can be conditioned to form a hydrogen-rich feed gas (cryogenic feed) (10) containing carbon compounds for a cryogenic gas separator (K), and a cryogenic gas separator (K) by means of which a hydrogen-rich, carbon-monoxide-free first gas fraction (11) and a hydrogen-rich, low-carbon second gas fraction (13) can be formed from the cryogenic feed (10), it being possible to pass the first gas fraction (11) onward as the make-up gas or to supplement it with nitrogen (14) to form the make-up gas (15), and it being possible to use the second gas fraction (13) as fuel gas for generating process heat, characterized in that the cryogenic gas separator (K) is configured to produce a hydrogen-rich, low-carbon second gas fraction (13) as fuel gas, the carbon content of which is lower than that of the cryogenic feed (10) and higher than that of the hydrogen-rich, carbon-monoxide-free first gas fraction (11). 9. The apparatus according to claim 8, characterized in that the cryogenic gas separator (K) comprises a scrubbing column in which at least a portion (23) of the cryogenic feed (10) can be subjected to scrubbing with liquid nitrogen (28) for the separation of carbon compounds, it being possible to withdraw the hydrogen-rich, carbon-monoxide-free first gas fraction (30) overhead, and to withdraw a hydrogen-rich, low-carbon second gas fraction (42) from the scrubbing column (W) as fuel gas (13) via a side draw. 10. The apparatus according to claim 9, characterized in that the cryogenic gas separator (K) comprises a separator (D2) connected to the scrubbing column (W) and referred to as a column bottom separator, into which at least a portion of the nitrogen (29) laden in the scrubbing column (W) with separated components can be expanded in order to be separated into a liquid phase (40) enriched with carbon compounds and a hydrogen-rich, low-carbon gas phase (41), at least a portion of which can be withdrawn as fuel gas (13). 11. The apparatus according to any of claims 9 or 10, characterized in that a two-phase mixture (20) can be formed from at least a portion of the cryogenic feed (10) in the cryogenic gas separator (K) by cooling, which two-phase mixture, in a separator (20) arranged in the cryogenic gas separator (K) and referred to as a column feed separator, can be separated into a liquid phase (21) enriched with carbon compounds and a hydrogen-rich, low-carbon gas phase (22), a first portion (23) of which can be fed to the scrubbing column (W) for scrubbing with liquid nitrogen (28) and a second portion (24) of which can be withdrawn as fuel gas (13). 12. The apparatus according to any of claims 8 to 11, characterized in that the treatment device (M) comprises a methanation reactor (J) with which carbon dioxide and carbon monoxide present in the raw hydrogen (8) can be reacted with hydrogen to form methane and water. 13. The apparatus according to any of claims 8 to 12, characterized in that the cryogenic gas separator (K) is connected to the reforming device (B) in such a way that a liquid phase (21, 40) comprising carbon compounds and produced in the cryogenic gas separator (K) can be supplied wholly or partially to the reforming device (B) as a feed after evaporation and heating. 14. The apparatus according to any of claims 8 to 13, characterized in that it comprises a furnace (O) in which a fuel gas (13) obtainable in the cryogenic gas separator (K) can be combusted to heat a feedstock (2) that can be supplied to the reforming device (B).

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