Process for producing ammonia

Abstract

A process for producing ammonia comprises a first operation mode and a second operation mode; the first operation mode comprises the steps of: producing a gaseous CO stream by electrolysis in electrolytic cells supplied with energy and a CO2 stream; liquefying the gaseous CO stream obtaining a liquid CO stream that is stored in a CO storage unit (5); sending a high pressure CO stream, taken from the CO storage unit (5), to a CO conversion unit (13) together with a liquid water stream to conduct a water-gas shift reaction and produce H2 to use in the ammonia synthesis reaction; the second operation mode comprises the steps of: operating at least a part of the electrolytic cells in reverse mode, i.e., as fuel cells, where CO and O2 are fed and CO2 is produced with generation of electricity; the cells operating as fuel cells being supplied with CO taken and pumped from the CO storage unit (5) and stored in the first operation mode, and with O2 produced in an air distillation step.

Description

[0001] PROCESS FOR PRODUCING AMMONIA

[0002] Cross-Reference to Related Applications

[0003] This Patent Application claims priority from Italian Patent Application No.

[0004] 102024000029730 filed on December 23, 2024, the entire disclosure of which is incorporated herein by reference.

[0005] Technical Field

[0006] The present invention relates to a process for producing ammonia with energy storage.

[0007] Background

[0008] Processes for producing ammonia on an industrial scale can be divided into two main categories: conventional processes, studied at length and proposed in various versions, including the Haber Bosch process; and modern processes in which hydrogen is produced by electrolysis of water.

[0009] The Haber Bosch process essentially consists of the following operations:

[0010] 1) Generation of syngas, predominantly composed of H2, CO and N2, from hydrocarbons (for example methane). Preparation of syngas from methane is conducted in two steps, respectively called primary and secondary reforming. The feed to the primary reforming is composed of hydrocarbons and steam, taken to high temperature by supplying heat from outside the reforming reactor. The thermal energy required by the process is supplied by burning a certain fuel. The reaction between hydrocarbons and steam leads to the formation of a mixture predominantly composed of unreacted CO, H2, CH4 and H2O; at the outlet of the primary reforming section, air is added to this mixture, forming the feed to the secondary reforming. The secondary reforming process consists in partially burning the mixture obtained in the primary reforming to eliminate as much 02 and CH4 as possible, which form the inert materials in the ammonia production process; what is obtained, in practice, with secondary reforming, is a gas mainly composed of CO, H2O, N2, CO2 and smaller amounts ofCH4 and 02;

[0011] 2) Water-gas shift (WGS) reaction to produce hydrogen, using steam in a series of reactors, drastically reducing the CO and H2O content in the gas and producing a further amount of hydrogen. Overall, water-gas shift reaction produces a mixture composed of H2, N2, CO2, H2O and unreacted CO, smaller amounts of inert materials, such as Ar (introduced with the combustion air in the secondary reforming), CH4 and oxygen;

[0012] 3) Separation of CO2, for example by means of an amine unit, thereby obtaining gaspredominantly containing H2, N2, CO and modest amounts of inert materials;

[0013] 4) Methanation: the CO still present in the gas after separation of the CO2 is sent to methanation where, using the hydrogen that forms a prevalent part of the mixture, it is converted into methane and water; the latter is eliminated by condensation and / or adsorption; 5) Compression of the feed to be sent to the ammonia synthesis reactor: necessary as the Haber Bosch process is conducted at a higher pressure than the pressure of all other conventional processes (around 200 barg with respect to pressures lower than 100 barg); 6) Ammonia synthesis: in the ammonia synthesis reactor, NH3 is formed through the reaction between N2 and H2; as the reaction, in the operating conditions applied, has an equilibrium shifted toward the reagents, part of the products (ammonia) must be removed by condensation, while the residual part of the remaining gas, mainly composed of N2, H2, NH3 and inert substances, is recovered at the inlet of the ammonia synthesis reactor;

[0014] 7) Purging of part of the recovered gas: to avoid the accumulation of inert materials, part of the gas is not recirculated to the reactor.

[0015] Instead, with regard to modern processes for producing “green” ammonia, the hydrogen required for the ammonia synthesis reaction is produced by water electrolysis and steps 1-4 indicated above are thus replaced by the production of hydrogen by electrolysis from water and electricity, and by the separation of nitrogen from air, for example using membranes or cryogenic distillation. Steps 5-7 described with reference to the Haber Bosch process remain applicable.

[0016] Conventional ammonia synthesis processes are energy-intensive and require the combustion of fossil fuels, resulting in the emission of large amounts of CO2. Above all, when the object of ammonia production is the synthesis of synthetic fuels, such as ammonia or a mixture of ammonia and ammonium nitrate, in order to prevent the emission of carbon dioxide into the atmosphere, it is clear how conventional processes become lose meaning.

[0017] On the other hand, processes for producing “green” ammonia, which normally use renewable energy sources, have in common the defect that the availability of renewable energy is inconsistent: the production of hydrogen and, albeit to a lesser extent, nitrogen, will in turn be inconsistent. This has important implications that reflect on the ammonia synthesis process, which has limited flexibility with regard to substantial variations both in the reactor feed and in the stoichiometric ratio between hydrogen and nitrogen.

[0018] Possible mitigations of these problems consist in storing hydrogen and nitrogen, or possiblestrategies for recycling ammonia to the synthesis reactor, to limit hydrogen conversion when there is a shortage.

[0019] However, the accumulation of hydrogen is complicated and energy -intensive; in fact, large scale accumulations of gaseous hydrogen requires the use of numerous high pressure tanks, while storage in liquid form is both complicated in the process, which requires the use of a large number of machines and catalysts for allotropic transformation of the hydrogen from ortho to para, and energy-intensive: in fact, hydrogen is liquid, at atmospheric pressure, at around -253 °C.

[0020] An object of the present invention is to overcome the aforesaid drawbacks, providing a process for producing ammonia that can operate without being overly affected by fluctuations in energy availability, as occurs, for example, with renewable energy sources.

[0021] In particular, an object of the invention is to provide a process for producing ammonia that uses the energy resources available, in particular from renewable resources, in a fully efficient way, compensating for any shortages of available energy with energy accumulated in the form of chemical energy.

[0022] Summary

[0023] The present invention relates to a process for producing ammonia as defined in the appended claim 1 and, for its preferred auxiliary features, in the dependent claims.

[0024] In substance, to overcome energy fluctuations, which can occur in particular when renewable energy sources are used (but also in general on any electrical grid), instead of the accumulation of hydrogen as occurs in certain prior art solutions, the invention uses the accumulation of chemical energy which is then used for the generation of hydrogen.

[0025] In particular, hydrogen is produced by conducting a so called water-gas shift (WGS) reaction, i.e., the chemical reaction between carbon monoxide and water to give hydrogen and carbon dioxide:

[0026]

[0027] CO + H2O CO2 + H2

[0028] In turn, CO is advantageously produced, in gaseous form, by electrolysis of CO2, preferably via solid oxide cells (SOCs) supplied, for example, by renewable energies.Instead of nitrogen (required together with hydrogen for ammonia synthesis) it is advantageously obtained continuously from the distillation of liquid air previously liquefied and accumulated, when a large amount of energy is available.

[0029] By distilling the air at low pressure with a heat pump, liquid nitrogen is obtained, which can also be pumped at high pressure, as well as a gas rich in oxygen, which can be used, together with a further portion of CO, to supply the solid oxide cells in reverse mode, i.e., operating as fuel cells (instead of as electrolytic cells), obtaining electricity. The cells used are thus reversible cells, which can operate alternatively as electrolytic cells (if supplied with CO2 and energy to form CO and 02) and as fuel cells (if supplied with CO and 02 to produce C02 and energy).

[0030] The C02 developed, both by the cells operating as fuel cells and by the water-gas shift reaction, is recondensed and made available to be electrolyzed once again.

[0031] The C02, through cyclic conversion to CO and back to C02, acts as an accumulator of chemical energy, which can be used in a closed loop.

[0032] The present invention thus provides for two operation modes, conducted alternatively when large amounts of electricity are available and when little or no energy is available and conducted in succession and repeatedly one after the other.

[0033] The first operation mode (in which large amounts of electricity are available) is essentially conducted as follows:

[0034] A. in the electrolysis unit, a stream composed of a mixture of CO and C02 and an oxygen stream are obtained from a C02 stream;

[0035] B. the mixture of CO and C02 is sent to the CO / CO2 separation unit from which a C02 stream and a CO stream are obtained;

[0036] C. the CO stream is sent to the liquefaction unit and then, in liquid form, to the storage unit; according to a preferred aspect of the invention, liquefaction of the gaseous CO stream is conducted via heat exchanges also with a high pressure liquid CO stream;

[0037] D. the high pressure CO stream, heated after having given up heat to the gaseous CO stream in the liquefaction unit, is sent, in gaseous form, to the conversion unit together with a liquid water stream for the water-gas shift reaction; the water-gas shift reaction converts gaseous CO into a stream containing H2, C02, CO and H20; in accordance with a preferred aspect ofthe invention, the conversion unit operates at high pressure, indicatively around 200 barg, and thus differs from conventional water-gas shift reactors, typically operating within 100 barg; therefore, the use of saturators of the gaseous CO stream is required;

[0038] E. the dehydrated and cooled stream of H2, CO2 and CO is sent to the CO2 separation unit; the stream rich in CO2 is at high pressure and is taken to the storage pressure of the CO2, separating and recompressing the gases that develop at lower pressure, rich in CO and H2; CO2 separation takes place simply by cooling and forms a further aspect of the invention; F. the gas composed mainly of H2 and CO is sent to the methanation unit, to significantly reduce its CO content, and a gas predominantly composed of H2, CH4 and H2O is delivered; G. the water is separated, obtaining a gas composed only of H2 and smaller amounts of CH4; H. the dehydrated gas is sent to the ammonia synthesis unit together with an N2 stream;

[0039] I. the N2 stream sent to the ammonia synthesis unit is obtained by compressing and cooling an atmospheric air stream, obtaining its liquefaction; a further aspect of the invention is the heat exchange between the air stream and a high pressure liquid nitrogen stream to recover cooling energy;

[0040] J. the liquid air is accumulated and stored;

[0041] K. a liquid air stream is taken and transferred at a higher pressure to cryogenic distillation with heat pump obtaining the stream di liquid nitrogen pumped at high pressure.

[0042] The second operation mode (in which little or no energy is available) is conducted by modifying steps A, B, C essentially as follows:

[0043] A', at least a part of the electrolytic cells of the electrolysis unit operate in reverse mode, i.e., as fuel cells, where CO is converted to CO2 with generation of electricity to supply to the plant. As the conversion of CO to CO2 is not complete, the resulting CO / CO2 mixture is separated. A stream rich in oxygen, which is generated by distillation of the air, is fed to the other electrode;

[0044] B'. the mixture of CO and CO2 produced in the electrolysis unit (in the cells operating as fuel cells) is sent to the CO2 separation unit, from which a liquid CO2 stream, and a CO stream that is recirculated to the cells operating as fuel cells, are obtained;

[0045] C. in this operation mode there is no net production of CO, so that the CO liquefaction unit is not operational and the CO required to supply the cells operating as fuel cells to produce H2 is taken and pumped from the storage unit. The high pressure CO stream, together with the high pressure nitrogen stream, contributes to cooling of the dehydrated stream composed of H2, CO2 and CO; in the process, the high pressure CO stream is heated and is subject to possible further heating.If in the second operation mode only some of the cells of the electrolysis unit operate in reverse mode as fuel cells, the remaining cells of the electrolysis unit can continue to operate in the first operation mode, or be idle, depending on the overall requirements.

[0046] With respect to the first operation mode, the N2 stream sent to the ammonia synthesis unit is obtained, instead of by compressing and cooling an atmospheric air stream obtaining its liquefaction (step I of the first operation mode), in this second operation mode it is instead taken and transferred at higher pressure from the storage unit.

[0047] The advantages of the present invention are apparent from the above.

[0048] The CO liquefaction point, at atmospheric pressure, is around -190 °C, comparable to that of liquid air, so that the energy cost for the liquefaction of one mole of CO, from which one mole of hydrogen can be obtained, is significantly lower. In fact, hydrogen liquefaction typically requires an energy cost equal to 30% of the energy required to obtain it from water.

[0049] The use of solid oxide cells for the production of CO from CO2 allows the electrolytic production of large volumes of hydrogen or carbon monoxide, with a limited energy cost.

[0050] Moreover, among the advantages of CO storage, it must be considered that the liquefaction system is much simpler.

[0051] Further, the fact of having liquid CO and air available allows them to be taken to the pressure required by the ammonia synthesis process (around 200 barg) with a limited energy cost, thus at least partly offsetting the energy spent in liquefaction.

[0052] A further advantage of the invention is that of minimizing the plant components that need to be shut down due to lack of energy, reducing these to the ones that best support cyclic operation and / or shutdown.

[0053] Ultimately, the invention allows the continuous production of ammonia from water, air and energy available in a fluctuating / discontinuous manner, in particular safeguarding the continuity of use of critical parts of the plant such as the chemical reactors; the process of the invention is more energy efficient with respect to other “green ammonia” production processes involving the storage of liquid hydrogen.The invention is also suitable to be integrated in various applications: for example, by adding a urea production section, the invention can be used to convert CO2 to urea, rather than produce it from natural gas.

[0054] Brief Description of the Drawings

[0055] The invention is further described in the following non-limiting examples of embodiment, with reference to the figures of the accompanying drawings, wherein:

[0056] - Fig. 1 is a schematic view of an ammonia production plant operating in implementation of a first operation mode of the process of the invention;

[0057] - Fig. 2 is a schematic view of the plant of Fig. 1, operating in implementation of a second operation mode of the process of the invention;

[0058] - Figs. 3 and 4 are schematic views, with parts removed for clarity, of a preferred embodiment of a plant operating in implementation of the first and, respectively, of the second operation mode.

[0059] Description of Embodiments

[0060] Figs. 1 and 2 illustrate a plant 1 for implementing the process for producing ammonia in accordance with the invention: Fig. 1 shows the plant 1 in a first configuration for implementation of a first operation mode of the process for producing ammonia in accordance with the invention; Fig. 2 shows the plant 1 in a second configuration for implementation of a second operation mode of the process for producing ammonia of the invention. For simplicity of representation, each of the Figs. 1 and 2 shows the components of the plant 1 required for implementation of the respective operation mode, but it is understood that the plant 1 comprises all the components shown in both Figs. 1 and 2.

[0061] Therefore, with reference to Figs. 1 and 2, the plant comprises an electrolysis unit 2, a CO / CO2 separation unit 3, a CO liquefaction unit 4 and a liquid CO storage unit 5, all connected in series along a first branch 6 by respective lines 6a, 6b, 6c.

[0062] The electrolysis unit 2 comprises reversible cells, in particular solid oxide cells (SOC), which can operate alternatively as electrolytic cells (if supplied with CO2 and energy to form CO and 02) and as fuel cells (if supplied with CO and 02 to produce C02 and energy).

[0063] The electrolysis unit 2 has a C02 feed inlet 7 to supply a C02 stream to the electrolysis unit 2.The electrolysis unit 2 is connected to the CO / CO2 separation unit 3 via a CO2 recirculation line 8 (Fig. 1) to return a CO2 stream from the CO / CO2 separation unit 3 to the electrolysis unit 2; and via a CO recirculation line 9 (Fig. 2) to return a CO stream from the CO / CO2 separation unit 3 to the electrolysis unit 2. The electrolysis unit 2 is also connected to the liquid CO storage unit 5 via a CO return line 10 (Fig. 2), to supply a liquid CO stream to the electrolysis unit 2.

[0064] The liquid CO storage unit 5 is connected to the CO liquefaction unit 4 also by a high pressure liquid CO line 11 (Fig. 1).

[0065] The plant 1 further comprises a CO conversion unit 13, a first dehydration unit 14, a first refrigeration unit 15, a CO2 separation unit 16, a methanation unit 17, a second dehydration unit 18, an ammonia synthesis unit 19, all connected in series along a second branch 20 by respective lines 20a-20f.

[0066] The CO conversion unit 13 is provided with a water supply inlet 21, for supplying liquid water to the CO conversion unit 13, and is connected to the CO liquefaction unit 4 via a gas CO supply line 22, for supplying a high pressure gaseous CO stream to the CO conversion unit 13.

[0067] The first refrigeration unit 15 is connected: to the liquid CO storage unit 5 by a high pressure liquid CO line 23 CO (Fig. 2), which takes a high pressure liquid CO stream from the liquid CO storage unit 5 to the refrigeration unit 15; to the CO conversion unit 13 by a high pressure gas CO line 24 (Fig. 2), that takes a high pressure gas CO stream to the CO conversion unit 13; to the ammonia synthesis unit 19 by a high pressure gas N2 line 25 (Fig. 2), that supplies a high pressure gaseous nitrogen stream to the ammonia synthesis unit 19.

[0068] The ammonia synthesis unit 19 is connected to a second refrigeration unit 27 via a delivery line 28a and a return line 28b; the second refrigeration unit 27 is connected to a liquid ammonia storage unit 29 by an ammonia line 30.

[0069] The plant 1 further comprises an air liquefaction unit 33, a liquid air storage unit 34, and an air distillation unit 35, connected in series by respective lines 36a, 36b in which liquid air produced in the air liquefaction unit 33 circulates.

[0070] The air liquefaction unit 33 has an air inlet 37 (Fig. 1), to supply an air stream to the airliquefaction unit 33, and is connected to the ammonia synthesis unit 19 by a high pressure gas N2 line 38 (Fig. 1) to supply a high pressure gaseous nitrogen stream, obtained in the air liquefaction unit 33, to the ammonia synthesis unit 19.

[0071] The air distillation unit 35 is connected to the air liquefaction unit 33 by an N2 recirculation line 39 (Fig. 1), which returns a high purity and high pressure nitrogen stream, produced in the air distillation unit 35, to the air liquefaction unit 33.

[0072] The air distillation unit 35 is further connected to the first refrigeration unit 15 by a nitrogen line 42 (Fig. 2), to take a high purity and high pressure nitrogen stream to the refrigeration unit 15; and to the electrolysis unit 2 by an oxygen line 43, which takes a gaseous stream rich in oxygen, produced in the air distillation unit 35, to the electrolysis unit 2 (Fig. 2).

[0073] The process according to the present invention is implemented, for example using the plant of Figs. 1 and 2, in two operation modes: in a first mode (Fig. 1), a large amount of electricity is available, while in the second mode little or no energy is available (Fig. 2); the first and the second operation mode are conducted alternatively to each other and in succession and repeatedly one after the other.

[0074] With reference to Fig. 1, the first operation mode (implemented when a large amount of energy is available), is conducted as follows.

[0075] The electrolysis unit 2 is supplied with electricity (widely available in this operation mode) and its cells thus operate as electrolytic cells.

[0076] The electrolysis unit 2 is thus supplied with a CO2 stream through the inlet 7 and produces a mixture of CO and CO2 (in variable proportions as a function of the electrolysis technology applied, but typically composed of around 50% of CO, the remaining part being unreacted CO2), and a stream of oxygen, possibly diluted in an stream of air, or steam.

[0077] Advantageously, the dilution of oxygen with air or steam serves to flush the oxygen generating electrode of the cells of the electrolysis unit 2, so as to reduce the potential difference required at the electrodes of the cells of the electrolysis unit 2.

[0078] The electrolysis unit 2 preferably comprises one or more solid oxide electrolytic cells (SOECs), operating at a temperature of around 800 °C and at a pressure typically below 10barg.

[0079] The electrolysis unit 2 also comprises heat exchangers for pre-heating the reagents and recovering heat from the products, and a possible compressor of the CO / CO2 mixture.

[0080] The oxygen generated by electrolysis in the electrolysis unit 2 is taken from the electrolysis unit 2 through an oxygen outlet 44 and can be dispersed into the atmosphere or sent to possible external users.

[0081] The mixture of CO and CO2 produced in the electrolysis unit 2 is sent, via the line 6a, to the CO / CO2 separation unit 3, from which a CO2 stream and a CO stream are obtained.

[0082] Separation of CO2 from the CO / CO2 mixture can be conducted with known methods; for example, the CO / CO2 separation unit 3 comprises a two column system with a column for absorption of the CO2 by means of a solvent, and a column for regeneration of the solvent; in turn, the solvent can be a physical solvent (for example methanol) or a chemical solvent (for example an amine solution).

[0083] During the solvent regeneration step, the CO2 captured is released and, possibly via a compressor, is recirculated to the electrolysis unit 2 via the CO2 recirculation line 8.

[0084] The gaseous CO stream remaining after absorption of the CO2 is sent, via the line 6b, to the CO liquefaction unit 4, where it is subjected to the liquefaction process, and is then sent in liquid form, via the line 6c, to the liquid CO storage unit 5.

[0085] CO liquefaction can take place, for example, through refrigeration cycles; in accordance with the present invention, the liquid CO storage unit 5 is provided with pumping devices so as to provide a high pressure liquid CO stream (preferably at least 210 barg), to be used in the series of heat exchanges that lead, in the CO liquefaction unit 4, to liquefaction of the gaseous CO stream. The heat recovery thus performed allows energy to be saved during liquefaction, as well as heating of the liquid CO stream.

[0086] After passing through the CO liquefaction unit 4, the heated and high pressure liquid CO stream is sent, via the CO gas feed line 22, to the CO conversion unit 13, for example a water-gas shift unit, where, together with a liquid water stream supplied through the inlet 21, it is converted into a stream containing H2, CO2, CO and H2O.The CO conversion unit 13 operates at a pressure of at least 180 barg, preferably around 200 barg, and thus differs from conventional water-gas shift reactors that operate at a pressure below 100 barg; therefore, the CO conversion unit 13 comprises saturators that add water to the gaseous CO stream (unlike conventional reactors in which mixing with steam occurs).

[0087] The mixture of H2, CO2, CO and H2O delivered from the CO conversion unit 13 is sent, via the line 20a, to the first dehydration unit 14, which removes water from the mixture by applying conventional methods, such as refrigeration and subsequent contact with molecular sieves, producing a dehydrated mixture of H2, CO2 and CO, which is sent via the line 20b to the subsequent refrigeration unit 15.

[0088] A reduction of the temperature of the dehydrated mixture of H2, CO2 and CO is obtained in the refrigeration unit 15.

[0089] The dehydrated and cooled mixture of H2, CO2 and CO is then sent, via the line 20c, to the CO2 separation unit 16, where a liquid stream rich in CO2 is separated. As this stream rich in CO2 is at high pressure, it is taken to the CO2 storage pressure (preferably around 7 barg), separating and recompressing the gases that develop at lower pressure, rich in CO and H2.

[0090] In accordance with the invention, separation of the CO2 is thus obtained simply by cooling.

[0091] The remaining liquid phase, taken from the CO2 separation unit 16 through a liquid CO2 outlet 45, has a degree of purity of the CO2 sufficient for storage and for subsequent re-use.

[0092] Instead, the gaseous phases are treated in the CO2 separation unit 16, at a pressure of around 200 barg, to separate H2 from CO2, for example by absorption of the CO2 through methanol and regeneration in a methanol stripping column.

[0093] The operations carried out in the CO2 separation unit 16 produce a gas mainly composed of H2 and CO, which is sent via the line 20d to the methanation unit 17, to significantly reduce its CO content, by means of the same hydrogen forming the mixture; methane and water are formed in the process, so that a gas predominantly composed of H2, CH4 and H2O is obtained from the methanation unit 17.

[0094] Since, besides CO, H2O also poisons the usual ammonia synthesis catalysts, the gas formedin the methanation unit 17 is sent, via the line 20e, to the second dehydration unit 18 where the water is separated and a gas formed only of H2 and smaller amounts of CH4 is obtained and sent, via the line 20f, to the ammonia synthesis unit 19, together with a high pressure gaseous nitrogen stream coming from the air liquefaction unit 33 via the line 38.

[0095] The ammonia synthesis unit 19 partially converts H2 and N2 to ammonia, which is partially condensed through cooling by a refrigeration cycle in the refrigeration unit 27; any purge gases are removed from the ammonia synthesis unit 19 via a purge outlet.

[0096] The liquid ammonia is sent to the liquid ammonia storage unit 29, while the gas remaining after partial condensation of the ammonia, mainly composed of H2, N2 and NH3, is recycled to the ammonia synthesis unit 19.

[0097] The high pressure gaseous nitrogen stream supplied to the ammonia synthesis unit 19 is obtained as follows.

[0098] An atmospheric air stream is supplied (preferably after having been purified) via the air inlet 37 to the air liquefaction unit 33, where the air is liquefied by compression and cooling.

[0099] Preferably, the air is compressed at a pressure between 2 and 200 barg, preferably between 20 and 100 barg.

[0100] In accordance with the present invention, heat exchange between the atmospheric air stream supplied to the air liquefaction unit 33 and a high pressure liquid nitrogen stream (greater than 20 barg, preferably around 200 barg) takes place in the air liquefaction unit 33, in order to recover frigories that minimizes the energy used for liquefaction of the air.

[0101] The liquid air thus obtained is sent via the line 36a to the liquid air storage unit 34, where it is accumulated.

[0102] A liquid air stream is taken from the liquid air storage unit 34 via the line 36b (optionally via a pump, not illustrated) and transferred at higher pressure (for example a pressure between 1 and 20 barg, in particular between around 2 and 2.5 barg) to the air distillation unit 35, where distillation of the liquid air stream transferred from the liquid air storage unit 34 is conducted, for example by a cryogenic distillation column with heat pump.Thanks to heat transfer from the product at the top of the cryogenic distillation column to the related reboiler, liquid nitrogen, at the top, and a stream rich in 02 (containing around 60% of 02) are obtained.

[0103] The liquid nitrogen stream obtained is pumped (via a pump, not illustrated) at high pressure (of at least 80 barg or greater, for example around 200 barg) and forms the high pressure nitrogen stream that is supplied via the line 39 to the air liquefaction unit 33 and from here, after heat exchange with the air, to the ammonia synthesis unit 19 via the line 38.

[0104] With reference to Fig. 2, the second operation mode (implemented when little or no energy is available), is conducted as follows.

[0105] At least a part of the cells of the electrolysis unit 2 operate in reverse mode, as fuel cells, where CO is converted to CO2 with generation of electricity (if only some cells of the electrolysis unit 2 operate in reverse mode as fuel cells, the remaining cells of the electrolysis unit 2 can continue to operate in the first operation mode described above, or remain idle).

[0106] The cells operating as fuel cells are supplied (at the respective electrodes) with a liquid CO stream taken from the liquid CO storage unit 5 and supplied to the electrolysis unit 2 via the CO return line 10, and with a gaseous stream rich in oxygen, produced in the air distillation unit 35 and supplied to the electrolysis unit 2 via the oxygen line 43.

[0107] The electricity produced by the cells of the electrolysis unit 2 operating as fuel cells can be used both to make the other cells of the electrolysis unit 2 operate as electrolytic cells, and to supply electricity to the plant 1.

[0108] As the conversion of CO to CO2 is not complete, a CO / CO2 mixture is obtained from the cells operating as fuel cells of the electrolysis unit 2 and is sent, via the line 6a, to the CO / CO2 separation unit 3, from which a liquid CO2 stream, which is recovered through a liquid CO2 outlet 46, and a recycle CO stream, which is sent, via the CO recirculation line 9, to the cells of the electrolysis unit 2 operating as fuel cells, are obtained.

[0109] The CO / CO2 separation unit 3 operates as described with reference to the first operation mode. However, in this case the separated CO is recirculated to the electrolysis unit 2 (precisely to its cells operating as fuel cells) and there is no net production of CO, so that the CO liquefaction unit 4 is not in operation.The CO required to supply the electrolysis unit 2 and to produce hydrogen is taken from the liquid CO storage unit 5.

[0110] In particular, a first liquid CO stream is sent to the cells operating as fuel cells of the electrolysis unit 2, via the CO return line 10; and a second high pressure liquid CO stream is sent, preferably at around 210 barg, to the refrigeration unit 15 via the high pressure liquid CO line 23.

[0111] In the refrigeration unit 15 the high pressure liquid CO stream, together with a high pressure nitrogen stream coming from the air distillation unit 35 via the nitrogen line 42, contributes to cooling of the mixture of H2, CO2 and CO dehydrated in the first dehydration unit 14; the high-pressure liquid CO stream gives up heat and is heated passing (possibly after further heating) to gaseous phase, to then be sent to the CO conversion unit 13 via the line 24.

[0112] The high pressure and heated gaseous CO stream sent to the CO conversion unit 13, together with a liquid water stream supplied by the inlet 21, is converted to a mixture of H2, CO2, CO and H2O. This mixture is sent, via the line 20a, to the dehydration unit 14, which removes water from the mixture producing a dehydrated mixture of H2, CO2 and CO, which is sent to the subsequent refrigeration unit 15 via the line 20b.

[0113] A reduction of the temperature of the dehydrated mixture of H2, CO2 and CO is obtained in the refrigeration unit 15, by means of heat exchange with the cold streams of high pressure CO and high pressure N2 supplied through the respective lines 23, 42.

[0114] The dehydrated and cooled mixture of H2, CO2 and CO is sent via the line 20c to the CO2 separation unit 16, where a liquid stream rich in CO2 is separated.

[0115] As this stream rich in CO2 is at high pressure, it is taken to the storage pressure of the CO2 (preferably 7 barg), separating and recompressing gases that develop at lower pressure, rich in CO and H2.

[0116] Also in this case, as described previously, separation of a significant amount of CO2 takes place simply by cooling.

[0117] The remaining liquid phase, taken from the outlet 45, has a degree of purity of the CO2sufficient for storage and subsequent re-use.

[0118] Also in this second operation mode, the gaseous phases are treated, at a pressure of around 200 barg, to separate H2 from CO2, for example through CO2 absorption by methanol and regeneration in the methanol stripping column.

[0119] As described with reference to the first operation mode, the operations carried out in the CO2 separation unit 16 produce a gas mainly composed of H2 and CO, which is sent via the line 20d to the methanation unit 17, to significantly reduce its CO content, by means of the same hydrogen forming the mixture; methane and water are formed in the process, so that a gas predominantly composed of H2, CH4 and H2O is obtained from the methanation unit 17.

[0120] Just as before, to prevent poisoning of the ammonia synthesis catalyst, the gas formed in the methanation unit 17 is sent, via the line 20e, to the second dehydration unit 18, where the water is separated and a gas composed only of H2 and smaller amounts of CH4 is obtained and sent, via the line 20f, to the ammonia synthesis unit 19, together with a high pressure gaseous nitrogen stream coming from the refrigeration unit 15 via the line 25.

[0121] The ammonia synthesis unit 19 operates substantially as described previously, partially converting H2 and N2 to ammonia, which is partially condensed by cooling through a refrigeration cycle in the refrigeration unit 27.

[0122] Also in this case, the liquid ammonia is sent to the liquid ammonia storage unit 29, while the gas remaining after partial condensation of the ammonia, mainly composed of H2, N2 and NH3, is recycled to the ammonia synthesis unit 19.

[0123] The high pressure gaseous nitrogen stream used in the ammonia synthesis unit 19 is obtained as follows.

[0124] A liquid air stream, accumulated in the first operation mode in the liquid air storage unit 34, is taken from the liquid air storage unit 34 and transferred, at an increased pressure (for example at around 2 barg), to the air distillation unit 35, via the line 36b.

[0125] The air distillation unit 35 operates as described previously: the liquid air is distilled, for example by a cryogenic distillation column with heat pump and, thanks to the transfer of heat from the product at the top of the cryogenic distillation column to the related reboiler, aliquid nitrogen, at the top, and a stream rich in 02 (containing around 60% of 02) are obtained.

[0126] In this operation mode, the stream rich in 02 produced in the air distillation unit 35 is supplied to the cells operating as fuel cells of the electrolysis unit 2, via the line 43.

[0127] The liquid nitrogen stream obtained is pumped at high pressure (around 200 barg) and forms the high pressure nitrogen stream that is supplied, via the line 42, to the refrigeration unit 15.

[0128] When the plant 1 operates in the first operation mode, both liquid air and CO are produced and simultaneously consumed; however, as the amounts of liquid air and of CO produced are greater than those consumed, in the first operation mode reserves of liquid air and of liquid CO accumulate, and hence chemical energy to use in the second operation mode accumulates.

[0129] A concrete embodiment of the plant 1 for implementation of the process of the present invention is described below, with reference to Figs. 3 and 4. Fig. 3 shows the plant 1 configured for implementation of the first operation mode of the process for producing ammonia in accordance with the invention; Fig. 4 shows the plant 1 configured for implementation of the second operation mode of the process for producing ammonia of the invention. For simplicity of representation, each of Figs. 3 and 4 shows the components of the plant 1 required to implement the respective operation mode, but it is understood that the plant 1 comprises all the components shown in both Figs. 3 and 4.

[0130] With reference to Fig. 3, the first operation mode is carried out as follows.

[0131] 1) A purified and compressed atmospheric air stream LI is liquefied in the air liquefaction unit 33.

[0132] 2) In the air liquefaction unit 33, the air is liquefied both by a refrigeration cycle and by heat exchange with a high pressure nitrogen stream n7, obtaining a liquid air stream L2.

[0133] 3) The liquid air stream L2 is sent to a tank T1 of the liquid air storage unit 34, where liquid air is stored.

[0134] 4) A stream L3 is taken from the tank T1 of the liquid air storage unit 34 and compressed,possible by means of a pump Pl, to form a stream L4 at a higher pressure (for example 2.5 barg) sent to the air distillation unit 35, in particular to a distillation column Cl.

[0135] 5) A nitrogen stream nl is obtained from the top of the column Cl and is condensed, at least partially, in a heat exchanger El (where it gives up heat to a liquid refrigerant stream rl), obtaining an at least partially condensed nitrogen stream n2.

[0136] 6) The stream n2 is sent to a tank V 1.

[0137] 7) A liquid nitrogen stream n3 is taken from the tank VI and divided into two streams n4, n6.

[0138] 8) The stream n4 is supplied at the top of the column Cl, for example by means of a pump P2 that supplies the column Cl with a reflux stream n5.

[0139] 9) The stream n6 is sent to a pump P3, which takes it to high pressure (for example 200 barg), and supplies the stream n7 to the air liquefaction unit 33.

[0140] 10) The air liquefaction unit 33 produces a high pressure gaseous nitrogen stream n8.

[0141] 11) A stream 01 rich in oxygen is obtained from the bottom of the column Cl and sent to a reboiler E2, where it is partially vaporized, obtaining a stream o2 containing oxygen.

[0142] 12) The stream o2 is sent to a separator V2, where a gaseous stream o3 rich in oxygen and a liquid stream o4 rich in oxygen are separated.

[0143] 13) The stream o3 is supplied to the bottom of the column Cl, where it acts as stripping gas.

[0144] 14) The stream o4 is heated in a heat exchanger E3, where it receives heat from a fluid refrigerant stream rlO, obtaining the stream o5, at least partially vaporized, which is removed from the plant 1.

[0145] 15) The liquid refrigerant stream rl (for example composed of nitrogen) receives heat, in the heat exchanger El, from the gaseous nitrogen stream nl and, following this vaporizes, giving rise to the gaseous refrigerant stream r2.

[0146] 16) The stream r2 is sent to a compressor KI, which increases its pressure (for example to 7barg), providing a high pressure gaseous refrigerant stream r3.

[0147] 17) The refrigerant stream r3 is combined with a second high pressure gaseous refrigerant stream r9, to form a high pressure gaseous refrigerant stream r4.

[0148] 18) The stream r4 is divided into the stream rlO and into a stream r4’.

[0149] 19) The stream r4’ gives up heat, in the heat exchanger E2, to the stream ol, condensing in a liquid stream r5.

[0150] 20) The stream rlO gives up heat, in the heat exchanger E3, to the stream o4, condensing in a liquid stream rll.

[0151] 21) The high pressure liquid refrigerant streams r5 and rl 1 are combined in a stream r6.

[0152] 22) The stream r6 is taken to lower pressure (for example 1 barg), for example by means of a throttle valve; this operation causes partial vaporization of the stream r6, providing a partially vaporized and low pressure refrigerant stream r7.

[0153] 23) The stream r7 is sent to a separator V3, where a low pressure gaseous refrigerant stream r8 and the low pressure liquid refrigerant stream rl are separated.

[0154] 24) The low pressure gaseous refrigerant stream r8 is sent to a compressor K2, which increases its pressure (for example 7 barg), giving rise to the second high pressure gaseous refrigerant stream r9.

[0155] 25) A stream fl mainly composed of CO2 is sent to the electrolysis unit 2 from a CO2 reserve and, by means of electricity, the CO2 is split into CO and 02; the electrolysis unit 2 preferably comprises solid oxide electrolytic cells. The electrolytic cells generally do not allow full conversion of the reagents but, typically, provide mixtures composed 50% of unchanged reagents and the remaining percentage of reaction products (for example: 50% CO2, 50% CO). Therefore, a stream f2 composed of a mixture of CO and CO2 will be obtained from the electrolysis unit 2.

[0156] 26) The stream f2 is sent to the CO / CO2 separation unit 3, where CO2 and CO are separated into two streams, respectively f3 and f4. The CO / CO2 separation unit 3 comprises, forexample, an absorption system by physical solvents (for example methanol) or chemical solvents (for example amine); its working pressure is, for example, is around 40 barg.

[0157] 27) The stream f3, predominantly composed of CO2, is supplied to the electrolysis unit 2 together with the stream fl to be subjected to electrolysis.

[0158] 28) Instead, the stream f4 is sent to the CO liquefaction unit 4 where, by means of a refrigeration cycle and of heat exchange with a high pressure cold stream c2, it is condensed into a liquid CO stream f5.

[0159] 29) The stream f5 is sent to a tank T2 of the liquid CO storage unit 5.

[0160] 30) A liquid CO stream cl is pumped from the liquid CO storage unit 5, by a pump P4, at high pressure (for example 210 barg), obtaining the high pressure cold stream c2.

[0161] 31) The stream c2 is heated in the CO liquefaction unit 4 through heat exchange with the hot stream f4, obtaining a gaseous CO stream c4.

[0162] 32) The stream c4 is heated further in a heat exchanger E5, for example by means of steam or electricity, obtaining a further heated gaseous CO stream c5.

[0163] 33) The stream c5 enters a saturator V4 where it comes into contact with a hot water stream wl4, which partially evaporates, and the steam mixes with the CO forming the stream c5, obtaining a stream c6.

[0164] 34) The stream c6 enters a reactor R1 of the CO conversion unit 13; for example, the reactor R1 is composed of a heat exchanger between reagents and products and by three catalyst beds in series.

[0165] The object of the heat exchanger is to heat the stream c6, through heat recovery from the reaction products, to the temperature required for the water-gas shift reaction (preferably 300 °C).

[0166] Each catalyst bed promotes the conversion of CO and H2O to CO2 and H2, with the development of heat; to facilitate formation of the products, the temperature at the outlet of each catalyst bed should not exceed 550 °C and at the inlet of the subsequent catalyst bedshould be around 300 °C.

[0167] This is obtained through injection, between the catalyst beds, of hot water streams w9 and wlO, which are vaporized by the hot gases delivered from each bed; the steam thus added then also becomes part of the reagents.

[0168] 35) A stream c7 is delivered from the reactor R1 (for example at a temperature of 440 °C), preferably containing no more than 3% of CO (mol / mol), the remaining portion being composed of CO2, H2 and H2O.

[0169] 36) The stream c7 is cooled in a heat exchanger E6, where it gives up heat to a high pressure liquid water stream w7 obtaining a stream c8 that is sent to a saturator V5.

[0170] 37) A liquid water stream wl2 is injected into the saturator V5, where it evaporates, at least partially, thus providing further steam, which is mixed with the gaseous stream c8. A gaseous stream c9 at a temperature preferably not exceeding 230 °C is obtained.

[0171] 38) The stream c9 enters a second reactor R2 of the CO conversion unit 13, where CO and H2O react further to form H2 and CO2; a stream clO is delivered from the reactor R2, at a temperature preferably not exceeding 250 °C.

[0172] 39) The stream clO gives up heat in a heat exchanger E7, where it heats a liquid water stream w2, and forms a stream cl 1.

[0173] 40) The stream cl 1 is sent to a saturator V6, where a liquid water stream wl3 is injected and evaporates, at least partially, thus providing further steam which mixes with the gaseous stream cl 1. A gaseous stream cl2 is obtained, at a temperature preferably of 180 °C.

[0174] 41) The stream cl2 enters a further reactor R3 of the CO conversion unit 13, where CO and H2O react further to form H2 and CO2; the stream cl3, containing not more than 0.2% of CO (mol / mol) is delivered from the reactor R3.

[0175] 42) The stream cl 3 gives up heat in a heat exchanger E8, where it heats a liquid water stream wl, and becomes a stream cl4.

[0176] 43) The stream c!4 is cooled further in a heat exchanger E9, by means of a refrigerant fluid,for example air or water, to a temperature around room temperature, causing condensation of a part of the water contained in the stream cl4. A mixed phase stream cl 5 is obtained.

[0177] 44) The stream cl 5 is sent to a separator V7, where a liquid water stream dl and a gaseous stream cl 6 are separated.

[0178] 45) The gaseous stream cl6 is dehydrated in a dehydrator Dl of the dehydration unit 14 which reduces its humidity to a content preferably below 500 ppm mol forming a dehydrated stream cl 7.

[0179] 46) The dehydrated gaseous stream cl7 is cooled in a heat exchanger E20 of the refrigeration unit 15, for example by means of a refrigeration cycle, preferably to a temperature of around -45 °C, obtaining a mixed phase stream cl 8.

[0180] 47) The stream cl 8 is sent to the CO2 separation unit 16, in particular to a separator V8, which separates a gaseous stream cl 9 rich in hydrogen from a liquid stream c27 predominantly composed of CO2.

[0181] 48) The stream c27 is taken to lower pressure, by means of an expander or a throttle valve, obtaining a mixed phase stream c28.

[0182] 49) The stream c28 is sent to a separator V9, which separates a gaseous stream c29 rich in hydrogen from a liquid stream c33 predominantly composed of CO2.

[0183] 50) The stream c33 is taken to lower pressure, by means of an expander or a throttle valve, obtaining a mixed phase stream c34.

[0184] 51) The stream c34 is sent to a separator V10, together with a stream c38, where a gaseous stream c35 rich in hydrogen is separated from a liquid stream c39 predominantly composed of CO2. The purity, in CO2, of the stream c39 is sufficient for it to be sent to the CO2 reserve from which the stream fl is taken.

[0185] 52) The stream c35 is recompressed by a compressor K4 to higher pressure (preferably 205 barg), obtaining a stream c36.

[0186] 53) The stream c29 is recompressed by a compressor K3 to higher pressure (preferably 205barg), obtaining a stream c30.

[0187] 54) The streams c30 and c36 are combined in a stream c31, which is refrigerated in a heat exchanger E10 by means of a refrigeration cycle; due to lowering of the temperature, the stream c31 partially condenses, obtaining a mixed phase stream c31’ .

[0188] 55) The stream c31’ is sent to a separator Vll, where a gaseous stream c32 rich in hydrogen is separated from a liquid stream c37 predominantly composed of CO2.

[0189] 56) The stream c37 is taken to lower pressure, by means of an expander or a throttle valve, obtaining a mixed phase stream c38, which is sent to the separator VI 0.

[0190] 57) The stream c32 is combined with the stream cl9 to form a stream c20.

[0191] 58) The gaseous stream c20 is sent to a C2 absorption column of the CO2 separation unit 16, where, by means of a solvent (for example methanol), a stream c21, with a CO2 content preferably below 0.15% (mol / mol), is obtained.

[0192] 59) The stream c21 is heated in a heat exchanger El 6 by the heat given up by the stream c23; a heated stream c22 having a temperature preferably of 200 °C is obtained.

[0193] 60) The stream c22 is sent to a methanation reactor R4 of the methanation unit 17, where the hydrogenation reaction of CO, CO2 and any methanol takes place to form CH4 and H2O. A stream c23 is obtained from methanation.

[0194] 61) The stream c23 is cooled in a heat exchanger E16 giving up thermal energy to the stream c21; the cooled stream c24 can contain liquid water, which is separated from the gaseous phase in a separator VI 3; the separated water stream d2 is removed from the plant 1, while a gaseous stream c25 remains.

[0195] 62) The stream c25 still contains H2O in gas phase, which is removed in a dehydrator D2 of the dehydration unit 18, obtaining a dehydrated stream c26, preferably containing less than 10 ppm of H2O.

[0196] 63) The stream c26 is combined with the stream n8 and with a stream al 1 (described below) to form a stream al.64) The stream al is heated in a heat exchanger El 7 by a stream a4, obtaining a heated stream a2.

[0197] 65) The stream a2 is sent to an ammonia synthesis reactor R5 of the ammonia synthesis unit 19, where N2 and H2 present in the feed are partially converted to NH3; a stream a3 rich in NH3 is obtained from the reactor R5.

[0198] 66) The stream a3 contains the reaction heat and has a temperature preferably of 550 °C.

[0199] 67) The stream a3 is sent to the refrigeration unit 27 for heat recovery, where it cools giving up heat to a working fluid (for example water), obtaining a cooled stream a4.

[0200] The refrigeration unit 27 is connected to a steam turbine unit (STU), which produces energy and supplies steam for the plant users, such as the heat exchangers E5 and E14, which have the function of heating some plant fluid.

[0201] 68) The stream a4, containing N2, H2 and NH3, is cooled further in a heat exchanger E17, where it heats the stream al and from which a further cooled stream is obtained a5.

[0202] 69) The stream a5 is cooled even further in another heat exchanger E18, where thermal energy is given up to a stream alO, obtaining a stream a6.

[0203] 70) By a refrigeration cycle, the stream a6 is cooled to a temperature, for example, of -38 °C, obtaining a mixed phase stream a7, which contains liquid ammonia.

[0204] 71) The stream a7 is sent to a separator V14, which separates a liquid ammonia stream al2 from a gaseous stream a8, mainly composed of N2, H2 and NH3, accompanied by smaller amounts of inert substances, such as CH4 and Ar.

[0205] 72) The stream al2 is taken to lower pressure, for example by a throttle valve, obtaining a mixed phase stream al 3.

[0206] 73) The stream al3 is sent to a separator V15, where a liquid stream al4, almost exclusively composed of liquid NH3, and a gaseous stream al 5, rich in NH3, H2 and CH4, are separated; the liquid ammonia stream al4 is sent to the storage (i.e., to the liquid ammonia storage unit29), while the gaseous stream al 5 is sent to further treatment units (known and not described for simplicity).

[0207] 74) A purge stream a8’, the purpose of which is to remove the inert substances (mainly CH4 and Ar) from the ammonia production cycle, preventing them from accumulating, is drawn off from the stream a8, and a gaseous stream a9 remains.

[0208] 75) The stream a9 is compressed, by a recycle compressor K7, obtaining the stream alO.

[0209] 76) The stream alO is heated, in a heat exchanger E18, through heat exchange with the stream a5, and the stream al 1 is obtained.

[0210] 77) With regard to the cycle of the solvent used in the absorption column C2, a stream ml of said solvent is supplied to the top of the column C2 at a temperature preferably of -45 °C.

[0211] 78) A stream m2 of CO2-laden solvent, the pressure of which is reduced, for example through the use of an expander K5, is obtained from the bottom of the column C2, obtaining a mixed phase stream m3.

[0212] 79) The stream m3 is sent to a separator V12, where a liquid stream m4 and a gaseous stream c42, mainly composed of CO2 and H2, are separated.

[0213] 80) The stream c42 is recompressed by a compressor K6, obtaining a stream c43.

[0214] 81) The stream c43 is cooled in a heat exchanger Ell, for example with water or air and / or another refrigerant fluid, obtaining a stream c44, which is fed to the absorption column C2.

[0215] 82) The stream m4 is separated into two streams m5, m7.

[0216] 83) The stream m5 is heated, by a heat exchanger E12, through heat exchange with a hot stream m8, obtaining a heated stream m6.

[0217] 84) The stream m6 is supplied to the solvent regeneration column C3.

[0218] 85) The stream m7 is supplied to the top of the column C3, where it serves as reflux.86) The stream m8 is obtained from the bottom of the column C3 and is cooled in the heat exchanger E12, obtaining a cooled regenerated solvent stream m9.

[0219] 87) The stream m9 is pumped at a pressure suitable to be supplied to the head of the column C2 by a pump P5, obtaining a stream mlO.

[0220] 88) The stream mlO is cooled, by means of a refrigeration cycle, in a heat exchanger E13, to a temperature preferably of -45 °C.

[0221] 89) A stream mil is extracted from the bottom of the column C3 and is at least partially vaporized in a heat exchanger E14 (which forms the reboiler of the column C3), for example by steam or electricity, obtaining a stream ml 2, at least partially vaporized, which enters the bottom of the column C3.

[0222] With reference to Fig. 4, the second operation mode is carried out as follows:

[0223] 1) The stream L3 is taken from the tank T1 of the liquid air storage unit 34 and compressed, possibly by means of the pump Pl, to form the stream L4 at a higher pressure (for example 2.5 barg) which is sent to the distillation column Cl of the air distillation unit 35.

[0224] 2) The nitrogen stream nl is obtained from the top of the column Cl and is condensed, at least partially, in the heat exchanger El (where it gives up heat to the liquid refrigerant stream rl), obtaining the at least partially condensed stream n2.

[0225] 3) The stream n2 is sent to the tank V 1.

[0226] 4) The liquid nitrogen stream n3 is taken from the tank VI and divided into the streams n4, n6.

[0227] 5) The stream n4 is supplied to the top of the column Cl, for example by means of the pump P2 that supplies the reflux stream n5 to the column Cl.

[0228] 6) The stream n6 is sent to the pump P3 that takes it to high pressure (for example 200 barg) and supplies a stream n7 to a heat exchanger E4.

[0229] 7) In the heat exchanger E4, the stream n7 of liquid nitrogen is heated by the stream cl 7, becoming a gaseous nitrogen stream n8.8) The stream ol rich in oxygen is obtained from the bottom of the column Cl and sent to the reboiler E2, where it is partially vaporized, obtaining the stream o2.

[0230] 9) The stream o2 is sent to the separator V2, where the gaseous stream o3 rich in oxygen and the liquid stream o4 rich in oxygen are separated.

[0231] 10) The stream o3 is supplied to the bottom of the column Cl where it acts as stripping gas.

[0232] 11) The stream o4 is heated in the heat exchanger E3, where it receives heat from the fluid refrigerant stream rlO, obtaining the stream o5, at least partially vaporized, which is sent to the heat exchanger E4.

[0233] 12) In the heat exchanger E4, the stream o5 is heated through heat exchange with the stream cl7, and becomes the gaseous stream 06.

[0234] 13) The stream 06 preferably contains at least 60% (mol / mol) of 02, the remaining part being mainly N2, and is sent to the electrolysis unit 2, to be supplied to one of the electrodes of the cells that, in this operation mode, act as fuel cells, producing electricity.

[0235] 14) The liquid refrigerant stream rl (for example composed of nitrogen) receives heat, in the heat exchanger El, from the gaseous nitrogen stream nl and consequently vaporizes, giving rise to the gaseous refrigerant stream r2.

[0236] 15) The stream r2 is sent to the compressor KI, which increases its pressure, preferably to 7 barg, giving rise to the high pressure gaseous refrigerant stream r3.

[0237] 16) The refrigerant stream r3 is combined with a second high pressure gaseous refrigerant stream r9, to form the high pressure gaseous refrigerant stream r4.

[0238] 17) The stream r4 is divided into the streams r4’ and rlO.

[0239] 18) The stream r4’ gives up heat, in the heat exchanger E2, to the stream ol, condensing in the liquid stream r5.

[0240] 19) The stream rlO gives up heat, in the heat exchanger E3, to the stream o4, condensing inthe liquid stream rl 1.

[0241] 20) The high pressure liquid refrigerant streams r5 and rl 1 are combined in the stream r6.

[0242] 21) The stream r6 is taken to lower pressure (for example 1 barg), for example by a throttle valve; this operation causes partial vaporization of the stream r6, giving rise to the partially vaporized low pressure refrigerant stream r7.

[0243] 22) The stream r7 is sent to the separator V3, where the low pressure gaseous refrigerant stream r8 and the low pressure liquid refrigerant stream rl are separated.

[0244] 23) The low pressure gaseous refrigerant stream r8 is sent to the compressor K2 which increases its pressure (for example to 7 barg), giving rise to the second high pressure gaseous refrigerant stream r9.

[0245] 24) A stream cl is taken from the tank T2 of the liquid CO storage unit 5 and is pumped by the pump P4 in a high pressure cold stream c2, which is sent to the heat exchanger E4, where it is vaporized by the heat received from the stream cl7, and becomes the gaseous CO stream c3.

[0246] 25) The stream c3 is divided into the streams c3’ and c4.

[0247] 26) The stream c3’ is sent to the electrolysis unit 2, where it supplies the cells that, in this operation mode, act as fuel cells and produce electricity. The reversible cells of the electrolysis unit 2 and the related accessory systems, such as the separation and recovery systems of the unreacted CO delivered from the cells, are known and therefore not described in detail for simplicity.

[0248] 27) The stream c4 is heated further in the heat exchanger E5, for example by means of steam or electricity, obtaining the further heated gaseous CO stream c5.

[0249] 28) The stream c5 enter the saturator V4 where it comes into contact with the hot water stream wl4, which partially evaporates, and the steam mixes with the CO forming the stream c5, obtaining the stream c6.

[0250] 29) The stream c6 enters the reactor R1 of the CO conversion unit 13, formed by a heatexchanger between reagents and products and by three catalyst beds in series. The object of the heat exchanger is to heat the stream c6, through recovery of heat from the reaction products, to the temperature required for the water-gas shift reaction (preferably 300 °C). Each catalyst bed promotes the conversion of CO and H2O to CO2 and H2, with the development of heat; to facilitate formation of the products, the temperature at the outlet of each catalyst bed should not exceed 550 °C and at the inlet of the subsequent catalyst bed should be around 300 °C. This is obtained through injection, between the catalyst beds, of the hot water streams w9 and wlO, which are vaporized by the hot gases delivered from each bed; the steam thus added then also becomes part of the reagents.

[0251] 30) The stream c7 is delivered from the reactor Rl, preferably at a temperature of 440 °C, preferably containing no more than 3% of CO (mol / mol). The remaining portion is composed of CO2, H2 and H2O.

[0252] 31) The stream c7 is cooled in the heat exchanger E6, where it gives up heat to the high pressure liquid water stream w7. The stream c8 thus obtained is sent to the saturator V5.

[0253] 32) The liquid water stream wl2 is injected into the saturator V5 and evaporates, at least partially, thus supplying further steam which mixes with the gaseous stream c8. The gaseous stream c9 is obtained, preferably at a temperature not exceeding 230 °C.

[0254] 33) The stream c9 enters the second reactor R2 of the CO conversion unit 13, where CO and H2O react further to form H2 and CO2; the stream clO is delivered from the reactor R2, preferably at a temperature not exceeding 250 °C.

[0255] 34) The stream clO gives up heat in the heat exchanger E7, where it heats the liquid water stream w2, and becomes the stream cl 1.

[0256] 35) The stream cl 1 is sent to the saturator V6, where the liquid water stream wl3 is injected and evaporates, at least partially, thus supplying further steam that mixes with the gaseous stream cl 1. The gaseous stream cl2 is obtained, preferably at a temperature of 180 °C.

[0257] 36) The stream cl2 enters the third reactor R3 of the CO conversion unit 13, where CO and H2O react further to form H2 and CO2; the stream cl3, containing no more than 0.2% of CO (mol / mol), is delivered from the reactor.37) The stream cl 3 gives up heat in the heat exchanger E8, where it heats the liquid water stream wl, and becomes the stream cl 4.

[0258] 38) The stream cl4 is cooled further in the heat exchanger E9, by means of a refrigerant fluid, for example air or water, to a temperature close to room temperature, causing condensation of a part of the water contained in cl4. The mixed phase stream cl 5 is obtained.

[0259] 39) The stream cl 5 is sent to the separator V7, where the liquid water stream dl and the gaseous stream cl 6 are separated.

[0260] 40) The gaseous stream cl6 is dehydrated in the dehydrator Dl of the dehydration unit 14 which reduces its humidity to a content preferably below 500 ppm mol, forming the dehydrated stream cl 7.

[0261] 41) The dehydrated gaseous stream cl7 is cooled in the heat exchanger E4 through heat exchange with the streams o5, n7, c2, obtaining a mixed phase stream cl 8.

[0262] 42) The stream cl8 is sent to the separator V8 of the CO2 separation unit 16, which separates the gaseous stream cl 9 rich in hydrogen from the liquid stream c27 predominantly composed ofCO2.

[0263] 43) The stream c27 is taken to lower pressure, by means of an expander or a throttle valve, obtaining the mixed phase stream c28.

[0264] 44) The stream c28 is sent to the separator V9, which separates the gaseous stream c29 rich in hydrogen from the liquid stream c33 predominantly composed of CO2.

[0265] 45) The stream c33 is taken to lower pressure, by means of an expander or a throttle valve, obtaining the mixed phase stream c34.

[0266] 46) The stream c34 is sent to the separator VI 0, together with the stream c38, where the gaseous stream c35 rich in hydrogen is separated from the liquid stream c39 predominantly composed of CO2. The purity, in CO2, of the stream c39 is sufficient for it to be sent to the CO2 reserve from which the stream fl is taken.

[0267] 47) The stream c35 is recompressed by the compressor K4 to higher pressure (preferably 205barg), obtaining the stream c36.

[0268] 48) The stream c29 is recompressed by the compressor K3 to higher pressure (preferably 205 barg), obtaining the stream c30.

[0269] 49) The streams c30 and c36 are combined in the stream c31, which is refrigerated in the heat exchanger E10 by means of a refrigeration cycle; due to lowering of the temperature, the stream c31 partially condenses, obtaining the mixed phase stream c31’ .

[0270] 50) The stream c31’ is sent to the separator VI 1, where the gaseous stream c32 rich in hydrogen is separated from the liquid stream c37 predominantly composed of CO2.

[0271] 51) The stream c37 is taken to lower pressure, by means of an expander or a throttle valve, obtaining the mixed phase stream c38 which is sent to the separator VI 0.

[0272] 52) The stream c32 is combined with the stream cl9 to form the stream c20.

[0273] 53) The gaseous stream c20 is sent to the absorption column C2 where, by means of a solvent (for example methanol), the stream c21 is obtained, with a CO2 content preferably below 0.15% (mol / mol).

[0274] 54) The stream c21 is heated in the heat exchanger El 6 by the heat given up by the stream c23; the heated stream c22 obtained preferably has a temperature of 200 °C.

[0275] 55) The stream c22 is sent to the methanation reactor R4 of the methanation unit 17, where the hydrogenation reaction of CO, CO2 and any methanol takes place to form CH4 and H2O. The stream c23 is obtained.

[0276] 56) The stream c23 is cooled in the heat exchanger E16 giving up thermal energy to the stream c21; the cooled stream is c24 and can contain liquid water, which is separated from the gaseous phase in the separator VI 3; the separated water stream d2 is removed from the plant 1, while the gaseous stream c25 remains.

[0277] 57) The stream c25 still contains gaseous phase H2O, which is removed in the dehydrator D2 of the dehydration unit 18, obtaining the dehydrated stream c26, preferably containing less than 10 ppm of H2O.58) The stream c26 is combined with the streams n8, al 1 to form the stream al.

[0278] 59) The stream al is heated in the heat exchanger El 7 by the stream a4, obtaining the heated stream a2.

[0279] 60) The stream a2 is sent to the ammonia synthesis reactor R5 of the ammonia synthesis unit 19, where N2 and H2 present in the feed are partially converted to NH3; the stream a3 rich in NH3 is obtained from the reactor R5.

[0280] 61) The stream a3 contains the reaction heat and preferably has a temperature of 550 °C.

[0281] 62) The stream a3 is sent to the refrigeration unit 27 for heat recovery, where it cools giving up heat to a working fluid (for example water), obtaining the cooled stream a4.

[0282] The refrigeration unit 27 is connected to the steam turbine unit STU, which produces energy and supplies steam for the plant users, such as the heat exchangers E5, E14, which have the function of heating some plant fluid.

[0283] 63) The stream a4, containing N2, H2 and NH3, is cooled further in the heat exchanger E17, where it heats the stream al and from which a further cooled stream a5 is obtained.

[0284] 64) The stream a5 is cooled even further in the heat exchanger El 8, where it gives up thermal energy to the stream alO, obtaining the stream a6.

[0285] 65) Through a refrigeration cycle, the stream a6 is cooled to a temperature, for example, of -38 °C, obtaining the mixed phase stream a7, which contains liquid ammonia.

[0286] 66) The stream a7 is sent to the separator V14, which separates the liquid ammonia stream al2 from the gaseous stream a8, mainly composed of N2, H2 and NH3, accompanied by smaller amounts of inert substances such as CH4 and Ar.

[0287] 67) The stream al2 is taken to lower pressure, for example by a throttle valve, obtaining the mixed phase stream al 3.

[0288] 68) The stream a!3 is sent to the separator V15, where the liquid stream al4, almostexclusively composed of liquid NH3, is separated from the gaseous stream al5, rich in NH3, H2 and CH4; the liquid ammonia stream al4 is sent to storage (i.e. to the liquid ammonia storage unit 29), while the gaseous stream al 5 is sent for further treatment (known).

[0289] 69) The purge stream a8’, the purpose of which is to remove the inert substances (mainly CH4 and Ar) from the ammonia production cycle, preventing them from accumulating, is drawn off from the stream a8, and the gaseous stream a9 remains.

[0290] 70) The stream a9 is compressed by the recycle compressor K7, obtaining the stream alO.

[0291] 71) The stream alO is heated, in the heat exchanger E18, through heat exchange with the stream a5, and the stream al 1 is obtained.

[0292] 72) The cycle of the solvent used in the absorption column C2 is identical to the one described with reference to the first operation mode: the stream ml of solvent is supplied to the top of the column C2, preferably at a temperature of -45 °C.

[0293] 73) The stream m2 of CO2-laden solvent, the pressure of which is reduced, for example, through the use of the expander K5, is obtained from the bottom of the column C2, obtaining the mixed phase stream m3.

[0294] 74) The stream m3 is sent to the separator V12, where the liquid stream m4 is separated from the gaseous stream c42, mainly composed of CO2 and H2.

[0295] 75) The stream c42 is recompressed by the compressor K6, obtaining the stream c43.

[0296] 76) The stream c43 is cooled in the heat exchanger Ell, for example with water or air and / or another refrigerant fluid, obtaining the stream c44, which is fed to the absorption column C2.

[0297] 77) The stream m4 is separated into the streams m5, m7.

[0298] 78) The stream m5 is heated, by the heat exchanger E12, through heat exchange with the hot stream m8, obtaining the heated stream m6.

[0299] 79) The stream m6 is supplied to the solvent regeneration column C3.80) The stream m7 is supplied to the top of the column C3, where it serves as reflux.

[0300] 81) The stream m8 is obtained from the bottom of the column and is cooled in the heat exchanger E12, obtaining the cooled regenerated solvent stream m9.

[0301] 82) The stream m9 is pumped at a pressure suitable to be supplied to the top of the column C2 by the pump P5, obtaining the stream mlO.

[0302] 83) The stream mlO is cooled, by means of a refrigeration cycle, in the heat exchanger E13, preferably to a temperature of -45 °C.

[0303] 84) The stream mil obtained from the bottom of the column C3 is at least partially vaporized in the heat exchanger E14 (the reboiler of the column C3), for example by steam or electricity, obtaining the stream ml 2, at least partially vaporized, which enters the bottom of the column C3.

[0304] Finally, it is understood that further modifications and variations can be made to the process for producing ammonia described herein without departing from the scope of the appended claims.

Claims

CLAIMS1. A process for producing ammonia comprising a step of producing ammonia via an ammonia synthesis reaction from N2 and H2, wherein N2 is produced via liquefaction and distillation of an air stream and H2 is produced via a water-gas shift reaction; the process comprising a first operation mode comprising the steps of:- producing a gaseous CO stream by electrolysis in electrolytic cells of an electrolysis unit (2) supplied with energy and a CO2 stream;- liquefying the gaseous CO stream in a CO liquefaction unit (4) to obtain a liquid CO stream which is stored in a liquid CO storage unit (5);- supplying a high pressure CO stream, taken from the liquid CO storage unit (5), to a CO conversion unit (13) together with a liquid water stream to conduct said water-gas shift reaction, where CO is converted to CO2 and H2 is produced for use in said ammonia synthesis reaction;the process also comprising a second operation mode, conducted alternatively and subsequently to the first operation mode and comprising the steps of:- operating at least part of the electrolytic cells of the electrolysis unit (2) in reverse mode, i.e. as fuel cells, where CO and 02 are supplied and CO2 is produced with generation of electricity; the cells operating as fuel cells being supplied with CO taken and pumped from the liquid CO storage unit (5) and accumulated in the first operation mode, and with 02 produced in the air distillation step;the first and the second operation modes being conducted alternatively to each other and in succession and repeatedly one after the other.

2. The process according to claim 1, wherein the step of liquefying the gaseous CO stream is conducted by thermal exchange of said gaseous CO stream with a high pressure CO stream taken in liquid form from the liquid CO storage unit (5) and brought to a pressure greater than 5 barg required by the subsequent ammonia synthesis reaction; and wherein the high pressure CO stream, heated and in gaseous form after having given up heat to the gaseous CO stream in the CO liquefaction unit (4), is sent to the CO conversion unit (13).

3. The process according to claim 1 or 2, wherein in the first operation mode the ammonia synthesis reaction is supplied with an N2 stream obtained by compressing and cooling an atmospheric air stream which is thus liquefied; while in the second operation mode the ammonia synthesis reaction is supplied with an N2 stream which is taken and transferred at a higher pressure from a liquid air storage unit (34).

4. The process according to any one of the preceding claims, wherein the CO gaseous stream produced in the electrolytic cells contains unreacted CO2; and the first operation mode comprises a step of separating CO from CO2 in a CO / CO2 separation unit (3) obtaining a gaseous stream of CO2, which is recirculated to the electrolytic cells, and a gaseous current of CO, which is sent to said CO liquefaction unit (4).

5. The process according to any one of the preceding claims, wherein a gaseous stream containing H2, CO2, CO and H2O is obtained from the water-gas shift reaction; and the process comprises a step of dehydrating said gaseous stream containing H2, CO2, CO and H2O to remove water therefrom; and a step of separating, in a CO2 separation unit (16), a liquid stream rich in CO2 from a gaseous stream of CO and H2, wherein the separation of CO2 occurs by cooling only.

6. The process according to claim 5, comprising a step of storing the liquid stream rich in CO2 for subsequent use.

7. The process according to claim 5 or 6, comprising the steps of sending the gaseous mixture of CO and H2 to a methanation unit (17) to conduct a methanation reaction with formation of a gaseous stream of H2, CH4 and H2O; and dehydrating the gaseous stream of H2, CH4 and H2O to obtain a dehydrated gaseous stream of H2 and CH4 to be sent to the ammonia synthesis reaction.

8. The process according to any one of the preceding claims, wherein the CO conversion unit (13) operates at a pressure greater than 5 barg.

9. The process according to any one of the preceding claims, wherein the conversion of CO to CO2 in the cells operating as fuel cells is not complete, and the second operation mode comprises a step of separating the CO / CO2 mixture produced in said cells operating as fuel cells into a CO stream, which is recirculated to the cells operating as fuel cells, and a CO2 stream.

10. The process according to any one of the preceding claims, wherein the second operation mode comprises the step of using a high pressure CO stream taken from the CO storage unit (5), and a high pressure nitrogen stream produced from the air distillation step, to cool a dehydrated stream of H2, CO2 and CO produced in the water-gas shift step.

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