Process for producing hydrogen

Abstract

A process for producing hydrogen comprises a first operation mode and a second operation mode; the first operation mode comprises the steps of: producing CO in gaseous form by electrolysis in electrolytic cells supplied with energy and CO2; using a portion of the CO produced by electrolysis in a CO conversion step together with H2O, for example in a conversion step by water-gas shift reaction or an electrolysis conversion step of CO and H2O, where CO is converted to CO2 and H2 is produced; storing the remaining portion of the CO in a storage, optionally in liquid form after a liquefaction step; the second operation mode comprises the steps of taking CO, stored in the first operation mode, from the storage and using it, together with H2O, in the CO conversion step, for example in a conversion step by water- gas shift reaction or an electrolysis conversion step of CO and H2O, where CO is converted to CO2 and H2 is produced.

Description

[0001] PROCESS FOR PRODUCING HYDROGEN

[0002] Cross-Reference to Related Applications

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

[0004] 102024000029655 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 hydrogen or syngas.

[0007] In particular, the present invention concerns a process suitable to guarantee a continuity of hydrogen or syngas (CO / H2 mixture) production from water, CO2 and renewable energies (or excess grid electricity). In the preferred case in which the energy used in the process is of renewable or nuclear origin, the hydrogen or the syngas produced in accordance with the invention can be considered “green”.

[0008] Background

[0009] Processes for producing hydrogen and / or syngas on an industrial scale can be divided into two main categories:

[0010] - conventional processes, in which a hydrogen rich gas or syngas is produced from coal or hydrocarbons, and then refined, through suitable purification operations, to obtain hydrogen and / or syngas of the desired purity and composition (in the case of syngas, H2 / CO molar ratio); this type of process produces CO2, which is usually released into the atmosphere, contributing to the phenomenon of global warming. In this case, this is referred to as the production of “grey” hydrogen and syngas; instead, if the carbon dioxide is captured and sequestered, the hydrogen and the syngas produced are referred to as “blue”;

[0011] - modem processes, in which hydrogen, CO and syngas are produced by electrolysis of water and of CO2 and hence, CO2 is not produced, at least directly; if the electricity used also derives from renewable sources, this is referred to as “green” hydrogen and syngas.

[0012] In general, conventional processes for the production of hydrogen and of syngas consume large amounts of fossil fuels (generally coal, methane and / or hydrocarbons with higher molecular weight) resulting in the release of large amounts of CO2 and consumption of non-renewableresources.

[0013] On the other hand, processes for producing green hydrogen and syngas have in common the defect that the availability of renewable energy is inconsistent, so that the production of hydrogen and syngas will in turn fluctuate. This has important implications that reflect on all those synthesis processes that make use of it and that, usually, have limited flexibility with regard to substantial variations both in the reactor load and in the stoichiometric ratio between hydrogen and CO and other reagents. Moreover, the fluctuations in energy availability could have significant repercussions if hydrogen were to replace methane in distribution networks such as gas pipelines.

[0014] One possible way to mitigate this problem consists in storing hydrogen and syngas, in gaseous or liquid form (the latter is more suitable for large scale storage). However, hydrogen storage is complicated and energy -intensive; in fact, large-scale storage of gaseous hydrogen or syngas requires the use of numerous high pressure tanks, while storage in liquid form, mainly for hydrogen, is both complicated in the liquefaction 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: hydrogen is liquid, at atmospheric pressure, at around -253 °C.

[0015] An object of the present invention is to overcome the aforesaid drawbacks, providing a process for producing hydrogen (syngas production also being intended as included in the definition of hydrogen production) that can operate without being overly affected by fluctuations in energy availability, as occurs, for example, with renewable energy sources.

[0016] In particular, an object of the invention is to provide a process for producing hydrogen (or syngas) 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 stored in the form of chemical energy.

[0017] Summary

[0018] The present invention relates to a process for producing hydrogen (or syngas) as defined in the appended claim 1 and, for its preferred auxiliary features, in the dependent claims.Here and hereunder, process for producing hydrogen is also meant as a process that leads to the production of syngas (H2 and CO mixtures) or in any case mixtures containing H2 as main component (i.e., present in an amount exceeding 50% by volume with respect to the total volume of the mixture).

[0019] In accordance with the invention, to overcome energy fluctuations, which can occur in particular when renewable energy sources are used (but also in general on any electrical grid), hydrogen is stored that is produced, rather than by electrolysis, through a water-gas shift (WGS) reaction, i.e., a chemical reaction between carbon monoxide and water to give hydrogen and carbon dioxide:

[0020]

[0021] CO + H2O CO2 + H2

[0022] 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.

[0023] Therefore, instead of hydrogen, CO2 is converted to CO using electrolysis and the latter is then stored, preferably in liquid form, obtaining a chemical energy storage; the CO produced and stored is thus available to be used to generate hydrogen, in particular through the water-gas shift reaction (with methods that are substantially known or adapted to be integrated into a particular plant making use of hydrogen). The CO2 released in the purification process of the hydrogen produced by water-gas shift reaction is reused for the electrolytic production of CO, closing the cycle: CO2 electrolysis CO production

[0024]

[0025] CO conversion to H2 and CO2.

[0026] The present invention provides for two operation modes, conducted alternatively when large amounts of electrical energy are available and when little or no energy is available and conducted in succession and repeatedly one after the other.

[0027] In the first operation mode (in which a large amount of electrical energy is available), CO2 is taken from a storage (or from an available supply source) and is sent to an electrolysis unit for the conversion of CO2 to CO. A CO stream, which is sent to CO separation unit, is obtained; the CO is then liquefied and stored in liquid form. A liquid CO stream is taken and sent to a pumping unit, obtaining a higher pressure stream which, through a heat exchange unit, returns to gaseous phase and is sent, together with a stream predominantly composed of H2O, to a unit for converting CO by water-gas shift reaction. A stream predominantly composed of H2 and CO2 is delivered and is sent to a purification unit, the purpose of which is to reduce the CO2and H2O content; a hydrogen-rich stream, available for users, and a stream predominantly composed of CO2, which is recycled to the electrolysis unit or sent to the storage, are obtained.

[0028] The second operation mode (in which little or no energy is available) is conducted in the same way as the first operation mode, with the exception of the fact that liquid CO stored in the first operation mode is used directly.

[0029] The advantages of the present invention are apparent from the above.

[0030] The CO liquefaction point, at atmospheric pressure, is of around -190 °C, comparable to that of liquid air, so that the energy cost for liquefaction of one mole of CO, from which one mole of hydrogen can be obtained, is significantly lower with respect to the liquefaction of one mole of hydrogen. In fact, hydrogen liquefaction typically requires an energy cost equal to 30% of the energy required to produce it from water.

[0031] The production of CO from CO2 by electrolysis, using in particular solid oxide cells, which are currently the most promising technology for the electrolytic production of large volumes of hydrogen or carbon monoxide, requires an energy cost, per mole of hydrogen obtainable (by water-gas shift reaction), comparable to the energy cost for producing one mole of hydrogen from water (which entails the difficulty of recovering heat in the presence of steam / liquid water phase transitions).

[0032] Moreover, among the advantages of CO storage, it must be considered that the liquefaction system is much simpler.

[0033] Further, the fact of having liquid CO available allows high pressure hydrogen to be generated with a limited energy cost, thus at least partly offsetting the energy spent in liquefaction, and eliminating the use of hydrogen compressors.

[0034] Also, CO storage allows electrical energy to be obtained if it is consumed in a fuel cell, which could be the same cell used for electrolysis, only operating in reverse mode.

[0035] Finally, 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 cyclicoperation and / or shutdown.

[0036] Brief Description of the Drawings

[0037] The invention is further described in the following non-limiting examples of embodiment, with reference to the figures of the accompanying drawings, wherein:

[0038] - Fig. 1 is a schematic view of a hydrogen production plant operating in implementation of a first operation mode of the process of the invention;

[0039] - Fig. 2 is a schematic view of the plant of Fig. 1, with parts removed for clarity, operating in implementation of a second operation mode of the process of the invention; - Figs. 3 and 4 are schematic views of a variant of the plant of Figs. 1 and 2 in implementation respectively of a first and a second operation mode of a different embodiment of the process of the invention;

[0040] - Figs. 5 and 6 are schematic views of a further variant of the plant of Figs. 1 and 2 in implementation respectively of a first and a second operation mode of a different embodiment of the process of the invention;

[0041] - Figs. 7A and 7B are schematic views of an electrolytic cell that can be used in the plant of Figs. 5 and 6 in implementation respectively of a first and of a second operation mode of a further embodiment of the process of the invention.

[0042] Description of Embodiments

[0043] Figs. 1 and 2 illustrate a plant 1 for implementing the process for producing hydrogen 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 hydrogen 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 hydrogen of the invention. For simplicity of representation, Fig. 2 shows only the components of the plant 1 required for implementation of the respective operation mode.

[0044] With reference to Fig. 1, the plant 1 comprises a CO2 feed unit 2 (which can be a CO2 storage unit, and / or a unit for connection to an external CO2 source), a CO2 pumping / compression unit 3, an electrolysis unit 4, a CO purification unit 5, a CO liquefaction and storage unit 6, a CO pumping / compression unit 7, an energy recovery unit 8, a CO conversion unit 9, a CO purification unit 10, all connected in series by respective lines 1 la-1 Ih.A CO2 recirculation line 12 connects the CO purification unit 5 to the electrolysis unit 4 to send a flow of CO2 from the CO purification unit 5 back to the electrolysis unit 4.

[0045] A further CO2 recirculation line 13, which branches into two lines 13a, 13b, connects the CO purification unit 10 respectively with the CO2 feed unit 2 and the electrolysis unit 4, to send respective flows of CO2 from the CO purification unit 10 back to the CO2 feed unit 2 and to the electrolysis unit 4.

[0046] 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 electrical energy is available, while in the second mode (Fig. 2) there is little or no energy available; the first and the second operation mode are conducted alternatively to each other and in succession, and repeated one after the other.

[0047] In the first operation mode, where a large amount of energy is available, CO is stored and is then used in the second operation mode, when less energy is available.

[0048] With reference to Fig. 1, the first operation mode (implemented when a large amount of energy is available), is conducted as follows.

[0049] A CO2 stream is taken from the CO2 feed unit 2 through the line Ila and sent to the CO2 pumping / compression unit 3, which increases its pressure obtaining a high pressure CO2 stream, i.e., greater than the pressure of the initial CO2 stream (in particular greater than 1 barg, preferably between 4 and 80 barg).

[0050] The high pressure CO2 stream is sent via the line 1 lb to the electrolysis unit 4, preferably comprising one or more solid oxide electrolysis cells (SOECs), for example operating at a temperature of around 800°C and at a pressure typically below 10 barg, and related auxiliary systems, such as heat exchangers to carry out heat recovery and the pre-heating respectively of the reacted gases and of the reagents, electrical systems, and the like.

[0051] Following the electrochemical reactions that take place in the electrolysis unit 4 a streampredominantly composed of a CO / CO2 mixture (typically containing 50%vol of CO) and an oxygen-rich stream are obtained.

[0052] The oxygen generated by electrolysis in the electrolysis unit 4 is removed through an oxygen outlet 14 and can be dispersed into the atmosphere or sent to possible external users.

[0053] The stream containing the CO / CO2 mixtures is sent via the line 11c to the CO purification unit 5, where a CO gas stream, predominantly composed of CO and with a water and CO2 content preferably below 100 ppm each, and even more preferably below 50 ppm each, is separated; a CO2 stream, predominantly composed of CO2 is obtained as by-product of the CO purification process and is recycled to the electrolysis unit 4 via the line 12.

[0054] The CO stream obtained in the CO purification unit 5 is sent via the line lid to the CO liquefaction and storage unit 6 where, preferably but optionally, the CO is liquefied; regardless of the optional liquefaction process, the CO liquefaction and storage unit 6 has the purpose of storing the CO contained, preferably but not necessarily in liquid form, or in the form of compressed, or cryo-compressed, gas.

[0055] A CO stream is taken from the CO liquefaction and storage unit 6 and sent via the line 1 le to the CO pumping or compression unit 7. The CO pumping or compression unit 7 produce a high pressure CO stream, indicatively greater than 5 barg and preferably between 20 and 80 barg, i.e., at a pressure greater than the pressure of the stream supplied to the CO pumping or compression unit 7.

[0056] In this first operation mode, in which a large amount of energy is available, CO can be produced in an amount greater than the amount required for the production of hydrogen and therefore CO is stored.

[0057] The high pressure CO stream obtained in the CO pumping or compression unit 7 typically has a temperature lower than -100 °C and is thus advantageously sent, via the line 1 If, to the heat energy recovery unit 8, for recovery of cooling energy from the high pressure CO stream through heat exchange with one or more internal or external process streams, as a function of the specific integration between the plant 1 described herein and any other chemical units towhich hydrogen is supplied; in all cases, a high pressure and high temperature CO stream, i.e. at a temperature greater than the temperature of the high pressure CO stream supplied to the energy recovery unit 8, is obtained.

[0058] The high pressure and high temperature CO stream delivered from the energy recovery unit 8 is sent via the line 11g to the CO conversion unit 9, which also receives a water stream supplied via an H20 feed line 15 and where a conversion of CO to CO2 takes place by a water-gas shift (WGS) reaction.

[0059] A converted gas stream, predominantly composed of H2 and CO2 and also containing CO and H2O, is delivered from the CO conversion unit 9; the converted gas stream can have a variable amount of CO based on the requirements of the processes to which the present invention intends to supply hydrogen.

[0060] The converted gas stream produced in the CO conversion unit 9 is sent via the line 1 Ih to the CO purification unit 10, where the CO2 and H2O content of the converted gas stream is reduced; a hydrogen-rich stream, predominantly composed of H2 and CO, is obtained and sent via an outlet line 16 to users requiring hydrogen; the concentration of CO can be adjusted in the CO purification unit 10 according to the particular requirements of the users.

[0061] A recycle CO2 stream, predominantly composed of CO2, is obtained as by-product from the CO purification unit 10 and is recycled to the electrolysis unit 4 via the line 13b, and / or sent to the CO2 feed unit 2 via the line 13a.

[0062] With reference to Fig. 2, the second operation mode (implemented when little or no energy is available), is conducted as follows.

[0063] A CO stream (preferably in liquid form) is taken from the CO liquefaction and storage unit 6, where in the previous operation mode the CO was stored, preferably in liquid form; the CO stream is sent via the line lie to the CO pumping / compression unit 7. The CO pumping / compression unit 7 produces a high pressure CO stream, indicatively greater than 5 barg and preferably between 20 and 250 barg, i.e., at a pressure greater than the pressure of the stream supplied to the CO pumping / compression unit 7.Also in this case, as described previously with reference to the first operation mode, as the high pressure CO stream obtained in the CO pumping / compression unit 7 typically has a temperature lower than -100 °C, it is sent via the line Ilf to the energy recovery unit 8, obtaining a high temperature CO stream, i.e., at a temperature greater than the temperature of the high pressure CO stream supplied to the energy recovery unit 8.

[0064] Just as in the first operation mode, the high pressure and high temperature CO stream delivered from the energy recovery unit 8 is sent via the line 11g to the CO conversion unit 9, which also receives a water stream supplied via the H2O feed line 15 and where a conversion of CO to CO2 takes place by a water-gas shift (WGS) reaction.

[0065] The converted gas stream delivered from the CO conversion unit 9, predominantly composed of H2 and CO2 and also containing CO and H2O and with a variable amount of CO based on the needs of the processes to which hydrogen is to be suppled, is sent via the line 1 Ih to the CO purification unit 10, where the CO2 and H2O content of the converted gas stream is reduced; a hydrogen-rich stream, predominantly composed of H2 and CO is obtained and is sent via the outlet line 16 to users requiring hydrogen; the CO concentration can be adjusted in the CO purification unit 10 according to the particular requirements of users.

[0066] A recycle CO2 stream, predominantly composed of CO2, is obtained as by-product from the CO purification unit 10 and in this case is entirely recycled to the CO2 feed unit 2 via the lines 13, 13a, to be stored.

[0067] The present invention finds particularly advantageous application in cases in which the production of hydrogen takes place, as well as by WGS reaction, also in combination with other technologies, in particular intended for the production of green hydrogen.

[0068] In the embodiment of Figs. 3-4, part of the hydrogen is produced, in the first operation mode (when there is a large amount of energy), as well as by WGS reaction also by a water splitting reaction, conducted in a water splitting unit where, by supplying electrical energy and / or light energy and / or thermal energy (or combinations thereof), water is split into 02 and H2.

[0069] The global hydrogen flow produced, obtained by combining production by water splitting andby CO2 electrolysis followed by WGS reaction, can be stabilized in various ways, thanks to the intrinsic flexibility of the process according to the present invention.

[0070] Small and / or short variations of electrical current can be levelled by the use of electric batteries; larger variations can be compensated for by reducing the stream available to the electrolysis unit 4, which will produce less CO, but this shortage will be offset by the storage present in the plant 1, in gaseous and / or liquid form.

[0071] Even greater variations of the supply of electrical energy, which require a reduction of the electrolytic hydrogen produced by the water, can be dealt with by increasing the flow rate of CO to the reactors of the CO conversion unit 9 in which the WGS reaction takes place; in turn, in this case, the use of gaseous CO is more suitable to compensate for short and limited load variations in the production of electrolytic hydrogen, while the use of liquid CO can better offset variations of larger size and longer duration.

[0072] Fig. 3 illustrates a plant la, for the remaining part analogous to the one described with reference to Fig. 1, further comprising a CO compression and storage unit 7 and a water splitting unit 21, comprising at least one water splitter in which, by supplying electrical energy and / or light energy and / or thermal energy (or combinations thereof), water is split into 02 and H2.

[0073] The CO compression and storage unit 7 is connected by a branch line 22 to the line lid and, hence, to the CO purification unit 5, to receive a CO stream from it; and by a CO feed line 23 to the line 11g (optionally, after passing, not illustrated for simplicity, through the energy recovery unit 8) and then to the CO conversion unit 9.

[0074] The water splitting unit 21 has a water inlet 24 for feeding water to the water splitting unit 21 and an oxygen outlet 25 from which oxygen is removed. The water splitting unit 21 is connected by a hydrogen line 26 to an H2 compression and storage unit 27, in turn connected to users requiring hydrogen by a hydrogen outlet line 28 which is connected, for example to the outlet line 16. Optionally, a bypass line 29 branches from the line 26 and connects to the line 28.

[0075] In the first operation mode (Fig. 3), the plant la operates as follows.Similarly to what was described previously with reference to Fig. 1, a CO2 stream is taken from the CO2 feed unit 2 through the line Ila and sent to the CO2 pumping / compression unit 3, which increases its pressure obtaining a high pressure CO2 stream, i.e. greater than the pressure of the initial CO2 stream (in particular greater than 1 barg, preferably between 4 and 80 barg).

[0076] The high pressure CO2 stream is sent via the line 1 lb to the electrolysis unit 4, from which a stream predominantly composed of a CO / CO2 mixture (typically containing 50%vol of CO), which is sent to the CO purification unit 5 through the line 11c, and an oxygen-rich stream, which is removed through the oxygen outlet 14, are obtained.

[0077] A CO gas stream, predominantly composed of CO and with a water and CO2 content preferably below 100 ppm each, and even more preferably below 50 ppm each, is separated in the CO purification unit 5; a CO2 stream predominantly composed of CO2 is obtained as by-product of the CO purification process and is recycled to the electrolysis unit 4 via the line 12.

[0078] The CO gas stream is partly sent through the line 1 Id to the CO liquefaction and storage unit 6, in which the CO is liquefied and stored, while the remaining portion is sent via the line 22 to the CO compression and storage unit 7, where the CO is compressed (in particular at a pressure greater than 5 barg, preferably between 40 and 500 barg), possibly also refrigerated, and stored.

[0079] A CO stream is taken from the CO liquefaction and storage unit 6 and sent via the line 1 le to the CO pumping / compression unit 7. The CO pumping / compression unit 7 produces a high pressure CO stream, indicatively greater than 5 barg and preferably between 20 and 250 barg, i.e., at a pressure greater than the pressure of the stream supplied to the CO pumping / compression unit 7.

[0080] The high pressure CO stream obtained in the CO pumping / compression unit 7 typically has a temperature lower than -100 °C and is therefore advantageously sent, via the line Ilf, to the energy recovery unit 8, for recovery of the cooling energy from the high pressure CO stream by heat exchange with one or more internal or external process streams, as a function of thespecific integration between the plant la described herein and any other chemical units to which hydrogen is supplied; in all cases, a high pressure and high temperature CO stream is obtained, i.e. at a temperature greater than the temperature of the high pressure CO stream supplied to the energy recovery unit 8.

[0081] A supplementary CO stream is taken from the CO compression and storage unit 7 via the line 23 and is combined with the high pressure and high temperature CO stream circulating in the line 11g and sent together with it to the CO conversion unit 9, after possible recovery of cooling energy in the energy recovery unit 8 (not illustrated in Fig. 3 for simplicity).

[0082] The conversion of CO to CO2 by water-gas shift (WGS) reaction takes place in the CO conversion unit 9, also supplied with water from the H2O feed line 15, as described previously.

[0083] A converted gas stream, predominantly composed of H2 and CO2 and also containing CO and H2O, is delivered from of the CO conversion unit 9; the converted gas stream can have a variable amount of CO based on the requirements of the processes to which the present invention intends to supply hydrogen.

[0084] The converted gas stream produced in the CO conversion unit 9 is sent via the line 1 Ih to the CO purification unit 10, where the CO2 and H2O content of the converted gas stream is reduced; a hydrogen-rich stream, predominantly composed of H2 and CO, is obtained and is sent via an outlet line 16 to users requiring hydrogen; the CO concentration can be adjusted in the CO purification unit 10 according to the particular requirements of users.

[0085] A recycle CO2 stream, predominantly composed of CO2 is obtained from the CO purification unit 10 as by-product and is recycled to the electrolysis unit 4 via the line 13b, and / or sent to the CO2 feed unit 2 via the line 13a.

[0086] An 02 stream, which is removed through the oxygen outlet 25, and an H2 stream are obtained from the water splitting unit 21, supplied with water through the water inlet 24 and with external energy (electrical energy, light energy, thermal energy, or combinations thereof).

[0087] The hydrogen delivered from the water splitting unit 21 is sent to users requiring hydrogen viathe line 29, and / or via the line 26 to the H2 compression and storage unit 27, where the gaseous hydrogen is compressed, possibly also refrigerated, and stored, for delayed delivery to users via the line 28.

[0088] In the second operation mode (Fig. 4), the plant la operates as follows.

[0089] A CO stream (preferably in liquid form) is taken from the CO liquefaction and storage unit 6, where in the previous operation mode CO was stored, preferably in liquid form, and is sent via the line lie to the CO pumping / compression unit 7. The CO pumping / compression unit 7 produces a high pressure CO stream, indicatively greater than 5 barg and preferably between 20 and 250 barg, i.e., at a pressure greater than the pressure of the stream supplied to the CO pumping / compression unit 7.

[0090] Also in this second operation mode, as described previously with reference to the first operation mode, as the high pressure CO stream obtained in the CO pumping / compression unit 7 typically has a temperature lower than -100 °C, it is sent via the line 1 If to the energy recovery unit 8, obtaining a high temperature CO stream, i.e., at a temperature greater than the temperature of the high pressure CO stream supplied to the energy recovery unit 8.

[0091] Just as in the first operation mode, the high pressure and high temperature CO stream delivered from the energy recovery unit 8 is sent via the line 11g to the CO conversion unit 9.

[0092] A gaseous CO stream is taken from the CO compression and storage unit 7 and sent via the CO feed line 23 to the CO conversion unit 9, together with the high pressure and high temperature CO stream delivered from the energy recovery unit 8, after possible recovery of cooling energy in the energy recovery unit 8 (not shown in Fig. 4).

[0093] As described previously, the conversion unit 9 also receives a water stream supplied via the H2O feed line 15, for conversion of CO to CO2 by water-gas shift (WGS) reaction.

[0094] The converted gas stream delivered from the CO conversion unit 9, predominantly composed of H2 and CO2 and also containing CO and H2O and with a variable amount of CO based on the requirements of the processes to which hydrogen is to be supplied, is sent via the line 1 Ihto the CO purification unit 10, where the CO2 and H2O content of the converted gas stream is reduced; a hydrogen-rich stream, predominantly composed of H2 and CO, is obtained and is sent via the outlet line 16 to users requiring hydrogen; the CO concentration can be adjusted in the CO purification unit 10 according to the particular requirements of users.

[0095] Also in this case, a CO2 recycle stream, predominantly composed of CO2 is obtained as byproduct from the CO purification unit 10 and in this case is entirely recycled to the CO2 feed unit 2 via the lines 13, 13a, to be stored.

[0096] Moreover, from the H2 compression and storage unit 27, in which gaseous hydrogen was stored in the first operation mode, a further hydrogen stream is taken and also sent to users, via the line 28.

[0097] In the further embodiment shown in Figs. 5 and 6, the electrolysis unit 4 described previously comprises, instead of or together with the electrolytic cells described previously, at least one co-electrolyzer, composed of one or more electrolytic cells capable of producing a mixture of CO and H2, of variable composition, from CO2 and H2O.

[0098] This embodiment is more suitable for those chemical processes requiring a mixture of CO and H2 to perform subsequent chemical reactions, for example to produce methanol, or hydrocarbons.

[0099] Moreover, given the present of water and hydrogen in the electrolytic cell, the accumulation speed of carbon deposits on the electrodes is reduced, thereby increasing the useful life of the electrolytic cell.

[0100] By making use of cryogenic techniques it is possible to separate the liquid CO, storable and usable in the production of hydrogen, or as component of a mixture for chemical use, and a gas rich in hydrogen and, to a lesser extent, CO, which can be supplied to a chemical reactor.

[0101] Fig. 5 illustrates a plant lb that, with respect to the embodiment of Fig. 3, is distinguished by the fact that the electrolysis unit 4 comprises at least one co-electrolyzer, composed of one or more electrolytic cells capable of producing a mixture of CO and H2, with a variable composition, from CO2 and H2O.In the first operation mode (Fig. 5), the plant lb operates as follows.

[0102] Similarly to what was described previously, a high pressure CO2 stream, coming from the CO2 feed unit 2 and passing through the CO2 pumping / compression unit 3, is supplied to the electrolysis unit 4; in this case, the electrolysis unit 4 is also supplied with an H2O stream, preferably in the form of steam, supplied via a water feed line 30.

[0103] The electrolysis unit 4 is in this case a co-electrolysis unit, capable of producing a mixture of CO and H2, of variable composition, from CO2 and H2O, performing both electrolysis of H2O to give H2 and 02, and electrolysis of CO2 to form CO and 02.

[0104] The electrolysis unit 4 preferably comprises solid oxide electrolysis cells (SOEC) and related auxiliary systems.

[0105] Following the electrochemical reactions that take place in the electrolysis unit 4, a stream predominantly composed of a CO / CO2 / H2 / H2O mixture and an oxygen-rich stream, are obtained.

[0106] The oxygen generated by electrolysis in the electrolysis unit 4 is removed through the oxygen outlet 14.

[0107] The stream containing the CO / CO2 / H2 / H2O mixture is sent via a line 11c to a purification unit 5b capable of separating CO and H2 from CO2 and H2O; a mixture predominantly composed of H2 and CO, with a water and CO2 content preferably below 100 ppm each, and even more preferably below 50 ppm each, is obtained.

[0108] A stream predominantly composed of CO2, which is recycled to the electrolysis unit 4 via the line 12, and a water stream, which is discharged from a water outlet 31, are obtained as byproducts of the purification process.

[0109] The mixture of H2 and CO obtained in the purification unit 5b is taken via a line lid and then divided into respective lines 32a, 32b, 32c.A first portion of the H2 and CO mixture obtained in the purification unit 5b is sent via the line 32a to the CO liquefaction and storage unit 6, where the H2 and CO mixture is partially condensed and divided into a liquid fraction, predominantly composed of CO, which is stored, and a hydrogen-rich gaseous fraction, also comprising CO, which is sent via an outlet line 33 to users requiring hydrogen.

[0110] A second portion of the H2 and CO mixture obtained in the purification unit 5b is sent via the line 32c to a compression and storage unit 20b, where the gas is compressed, possibly also refrigerated, and stored.

[0111] A third portion of the H2 and CO mixture obtained in the purification unit 5b is sent via the line 32b directly to users requiring H2 / CO mixtures.

[0112] Similarly to what was described previously, a CO stream is taken from the CO liquefaction and storage unit 6 and sent via the line lie to the CO pumping or compression unit 7, from which a high pressure CO stream is delivered and sent to the energy recovery unit 8, obtaining a high pressure and high temperature CO stream that is sent to the CO conversion unit 9, which also receives a water stream supplied via the H2O feed line 15 and where the conversion of CO to CO2 takes place by a water-gas shift (WGS) reaction.

[0113] Optionally, a fraction of the high pressure and high temperature CO stream delivered from the energy recovery unit 8 is taken from the line 11g to be sent to possible CO users, via a CO outlet line 34.

[0114] The converted gas stream delivered from the CO conversion unit 9, predominantly composed of H2 and CO2 and also containing CO and H2O, and with a variable amount of CO based on the requirements of the processes to which hydrogen is to be supplied, is sent via the line 1 Ih to the CO purification unit 10, where the CO2 and H2O content of the converted gas stream is reduced; a hydrogen-rich stream, predominantly composed of H2 and CO, is obtained and sent via the outlet line 16 to users requiring hydrogen; the CO concentration can be adjusted in the CO purification unit 10 according to the particular requirements of users.

[0115] A recycle C02 stream, predominantly composed of CO2, is obtained from the CO purificationunit 10 as by-product and is recycled to the electrolysis unit 4 via the line 13b, and / or sent to the CO2 feed unit 2 via the line 13a.

[0116] As described previously with reference to Fig. 3, also in this case an 02 stream, which is removed via the oxygen outlet 25, and an H2 stream, which is sent to the H2 compression and storage unit 27, are obtained from the water splitting unit 21 supplied with water via the water inlet 24 and with external energy (electrical energy, light energy, thermal energy, or combinations thereof).

[0117] The hydrogen delivered from the water splitting unit 21 is sent to users requiring hydrogen via the line 29, and / or via the line 26 to the H2 compression and storage unit 27, where the gaseous hydrogen is compressed, possibly also refrigerated, and stored, for delayed delivery to users via the line 28.

[0118] In the second operation mode (Fig. 6), the plant lb operates as follows.

[0119] Similarly to what was described with reference to Fig. 4, a CO stream (preferably in liquid form) is taken from the CO liquefaction and storage unit 6, where in the previous operation mode the CO was stored, preferably in liquid form, and sent via the line lie to the CO pumping / compression unit 7, where a high pressure CO stream is produced and sent via the line 1 If to the energy recovery unit 8, obtaining a high temperature CO stream.

[0120] A stream, essentially composed of a mixture of CO and H2, is taken from the compression and storage unit 20b and, after possible recovery of cooling energy in the energy recovery unit 8 (not shown in Fig. 6 for simplicity) is sent via a syngas line 35 to users requiring syngas.

[0121] A portion of the high temperature CO stream delivered from the energy recovery unit 8 can be sent to users requiring CO, via the line 34.

[0122] The remaining portion of the high temperature CO stream delivered from the energy recovery unit 8 is sent to the CO conversion unit 9, together with water supplied via the H2O feed line 15.

[0123] After the water-gas shift reaction, the converted gas stream delivered from the CO conversionunit 9, predominantly composed of H2 and CO2 and also containing CO and H2O, and with a variable amount of CO based on the requirements of the processes to which hydrogen is to be supplied, is sent via the line llhtothe CO purification unit 10, where the CO2 andH2O content of the converted gas stream is reduced; a hydrogen-rich stream, predominantly composed of H2 and CO, is obtained and sent via the outlet line 16 to users requiring hydrogen; the concentration of CO can be adjusted in the CO purification unit 10 according to the particular requirements of users.

[0124] A recycle CO2 stream, predominantly composed of CO2, is obtained from the CO purification unit 10 as by-product and is recycled to the CO2 feed unit 2 via the lines 13, 13a, to be stored. Moreover, a further hydrogen stream is taken from the H2 compression and storage unit 27, where in the first operation mode gaseous hydrogen was stored, and is also sent to users, via the line 28.

[0125] In further variants, the conversion step of CO to CO2 with production of H2 conducted in the CO conversion unit 9 can be conducted by electrolysis, for example in fuel cells supplied with CO, which is converted to CO2, and with H2O, which produces H2 and 02.

[0126] The fuel cells used for CO conversion can be the same electrolytic cells (or a portion thereof) used in the electrolysis unit 4 to produce CO from CO2 and used operating in reverse mode: when the cells are supplied with energy and CO2, they operate as electrolytic cells and produce CO, while when they are supplied with CO and 02 they produce CO2 with generation of electrical energy.

[0127] Therefore, the CO conversion unit 9 can also be integrated with the electrolysis unit 4.

[0128] It is also possible to use CH4 to reduce the electrolytic potential of the electrolysis reactions of H2O and CO2, which leads to significant energy savings.

[0129] For this purpose, the electrolysis unit 4 (more precisely, co-electrolysis unit) of the plant lb described with reference to Fig. 5 comprises at least one electrolytic cell 40 illustrated in Figs.

[0130] 7 A, 7B.The cell 40, preferably a solid oxide cell, comprises an anode channel 41 provided with at least one anode 42, and a cathode channel 43 provided with at least one cathode 44.

[0131] In the non-limiting example illustrated, but not necessarily, the anode 42 has an Ni-ScSZ active layer (Ni and scandia-stabilized zirconia), for example with a thickness of 10 pm, placed on an Ni-YSZ (Ni and yttria-stabilized zirconia) support layer, for example with a thickness of 695 pm; and the cathode 44 has an LSM-ScSZ (lanthanum strontium manganite; scandia- stabilized zirconia) active layer, for example with a thickness of 15 pm; an electrolyte layer 45, for example in ScSZ (scandia-stabilized zirconia), for example with a thickness of 10 pm, is placed between the active layer of the anode and active layer of the cathode.

[0132] As shown in Fig. 7A, in the first operation mode (when a large amount of energy is available), the anode channel 41 is supplied with a CH4 and H2O stream and the following reactions occur at the anode 42:

[0133] CH4+ H2O 3H2+ CO

[0134] H2+ y2O22' H2O + 2e"

[0135]

[0136] CO + y2O22' CO2+ 2e"

[0137] so that a stream essentially composed of H2, CO, H2O, CO2 is delivered from the anode channel 41.

[0138] The cathode channel 43 is supplied with an H2O and CO2 stream and the following reactions occur at the cathode 44:

[0139]

[0140] H2O + 2e H2+ C2O22'

[0141] CO2+ 2e" -A CO ' / 2O22'

[0142] so that a stream mainly composed of H2 and CO (and containing unreacted H2O and CO2) is delivered from the cathode channel 43.

[0143] As shown in Fig. 7B, in the second operation mode (when little or no energy is available), the anode channel 41 is supplied with a CO stream stored during the first operation mode and the following reaction occurs at the anode 42:

[0144] C

[0145]

[0146] O + ’ / 2O22' CO2+ 2e"

[0147] resulting in the production of a CO2 stream.The cathode channel 43 is supplied with H2O and the following reaction occurs at the cathode: H2O + 2e" -> H2 + ' / 2O2’

[0148] hence with production of H2 and 02.

[0149] In these conditions, the cell also produces electrical energy, operating as fuel cell, operating in reverse mode with respect to the first operation mode, in which it operates as electrolytic cell.

[0150] By adding CO taken from the storage to the hydrogen produced in the cell 40, it is possible to produce syngas.

[0151] Clearly, all the variants and embodiments described previously, and / or their various parts, can be combined with one another.

[0152] Finally, it is understood that further modifications and variations can be made to the process for producing hydrogen described herein without departing from the scope of the appended claims.

Claims

CLAIMS1. A process for producing hydrogen comprising a first operation mode comprising the steps of:- producing CO in gaseous form by electrolysis in electrolytic cells supplied with energy and CO2;- optionally purifying the CO, produced by electrolysis, to remove unreacted CO2 from the CO;- using a part of the CO, produced by electrolysis, in a CO conversion step together with H2O, for example in a conversion step by water-gas shift reaction or an electrolysis conversion step of CO and H2O, where CO is converted to CO2 and H2 is produced;- storing the remaining part of the CO in a storage, optionally in liquid form after a liquefaction step;the process further comprising a second operation mode, conducted alternatively and subsequently to the first operation mode and comprising the steps of:- taking CO, stored in the first operation mode, from the storage and using said CO, together with H2O, in the CO conversion step, for example in a conversion step by water-gas shift reaction or an electrolysis conversion step of CO and H2O, where CO is converted to CO2 and H2 is produced.

2. The process according to claim 1, comprising a step of purifying the CO produced by electrolysis to remove from the CO unreacted CO2, which is recirculated to the step of producing CO by electrolysis.

3. The process according to claim 1 or 2, wherein the CO conversion step comprises a water-gas shift reaction between CO and H2O to produce CO2 and H2.

4. The process according to claim 3, wherein the water-gas shift reaction produces a gas stream containing H2, CO2, CO, H2O; and the process comprises a purification step of said gas stream containing H2, CO2, CO, H2O to separate a CO2-rich stream, which is recirculated to the step of producing CO by electrolysis, from an H2-rich stream.

5. The process according to claim 1 or 2, wherein the CO conversion stepcomprises a conversion step by electrolysis of CO and H2O, where CO is converted to CO2 and H2 is produced.

6. The process according to any one of the preceding claims, wherein the electrolytic cells are reversible cells, particularly solid oxide cells (SOC), which can operate alternately as electrolytic cells when fed with CO2 and energy to form CO and 02, and as fuel cells when fed with CO and 02 to produce CO2 and energy.

7. The process according to claim 6, wherein the electrolytic cells are fed, in the first operation mode, at the anode with CH4 and H2O to conduct the reactions:CH4+ H2O 3H2+ COH2+ y2O22' H2O + 2e"CO + y2O22' CO2+ 2e"and at the cathode with H2O and CO2 to conduct the reactions:H2O + 2e H2+ C2O22'CO2+ 2e" -> CO ' / 2O22' .

8. The process according to claim 6 or 7, wherein the electrolytic cells are fed, in the second operation mode in which they operate as fuel cells, at the anode with CO stored during the first operation mode to conduct the reaction:CO + ’ / 2O22' CO2+ 2e"and at the cathode with H2O to conduct the reaction:H2O + 2e" H2 + ' / 2O2’ .

9. The process according to any one of the preceding claims, comprising a water splitting step, in which electrical energy and / or light energy and / or thermal energy or combinations thereof are supplied to split H2O into 02 and H2; and wherein the hydrogen produced in the water splitting step is sent to users requiring hydrogen or is stored, optionally compressed and refrigerated, for delayed delivery to users.

10. The process according to any one of the preceding claims, wherein an additional portion of the CO produced by electrolysis is sent to a CO compression and storage step to accumulate high-pressure gaseous CO, particularly at a pressure greater than 5 barg, preferably between 100 and 500 barg; and wherein the accumulated high-pressure gaseous CO is sent tothe CO conversion step in the first operation mode and / or in the second operation mode to supplement the CO supply to said CO conversion step.

11. The process according to any one of the preceding claims, wherein the step of producing CO by electrolysis is carried out in electrolytic cells capable of producing a mixture of CO and H2 from CO2 and H2O, performing both the electrolysis of H2O to give H2 and 02, and the electrolysis of C02 to form CO and 02.

12. The process according to claim 11, wherein said electrolytic cells produce an oxygen-rich stream which is removed, and a mixture of CO, C02, H2, H20 which is subjected to a purification step to remove C02 and H20 from a mixture of H2 and CO; and the process comprises a step of recirculating C02 separated in said purification step to the step of producing CO by electrolysis.

13. The process according to claim 11 or 12, wherein a first portion of the H2 and CO mixture obtained in the purification step is sent to a CO liquefaction and storage step, where the H2 and CO mixture is partially condensed and divided into a liquid fraction, predominantly composed of CO, which is stored, and a hydrogen-rich gaseous fraction, which is made available to users requiring hydrogen; a second portion of the H2 and CO mixture obtained in the purification step is sent to a compression and storage step, where it is compressed, possibly also refrigerated, and stored in gaseous form; and a third portion of the H2 and CO mixture obtained in the purification step is sent to users requiring H2 and CO mixtures.

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