Process for producing syngas to be used in methanol or GTL hydrocarbons synthesis

Abstract

A process for producing synthesis gas (syngas), to be used in particular for the synthesis of methanol or GTL hydrocarbons, comprises a first operation mode and a second operation mode; the first operation mode comprises the steps of: producing H2 by a reaction of splitting H2O into H2 and O2; producing CO by a reaction of splitting CO2 into CO and O2; using a first part of the CO obtained from the splitting of CO2 to form, together with H2 produced by water splitting, syngas; using a second part of the CO obtained from the splitting of CO2 in a CO conversion step together with H20, in which a water-gas shift reaction is conducted to convert CO into CO2 and produce H2, resulting in syngas; storing a third part of the CO in a storage; in the second operation mode, conducted alternatively to the first operation mode, CO stored in the first operation mode is taken from the storage and used, together with H2O, in a CO conversion step, in which the water-gas shift reaction is conducted to convert CO to CO2 and produce H2, resulting in syngas.

Description

[0001] PROCESS FOR PRODUCING SYNGAS TO BE USED IN METHANOL OR GTL HYDROCARBONS SYNTHESIS

[0002] Cross-Reference to Related Applications

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

[0004] 102024000029745 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 synthesis gas (syngas), to be used in particular for the synthesis of methanol or GTL hydrocarbons (i.e. hydrocarbons produced by gas-to-liquids, GTL, processes), using water, CO2 and electricity.

[0007] In particular, the present invention relates to a process adapted to guarantee continuous production of syngas and consequently of methanol and GTL hydrocarbons using electricity that is obtained from renewable sources or is excess grid electricity.

[0008] Background

[0009] Most of the industrial production of methanol takes place starting from synthesis gas (syngas), in turn produced starting from liquid or gaseous fossil fuels, by which is meant mainly coal and hydrocarbons.

[0010] Most industrial processes in the field of GTL (gas-to-liquids processes for converting gaseous hydrocarbons into longer-chain hydrocarbons in liquid form) also take place by applying the Fischer-Tropsch process to syngas, in turn produced starting from liquid or gaseous fossil fuels, by which is meant mainly coal and hydrocarbons.

[0011] In both cases, therefore, the preliminary production of syngas is required.

[0012] The production process of syngas on an industrial scale involves the conversion of coal or hydrocarbons, via steam, or oxygen and possibly carbon dioxide, into a mixture consisting of H2, CO, CO2 and H2O.

[0013] In particular, it is known to produce methanol by combining a natural gas reforming process with a subsequent methanol synthesis reaction.

[0014] In the natural gas reforming step, which for the sake of simplicity can be considered as consisting exclusively of methane, the following main chemical reactions take place:CH4 + H20 -> CO + 3 H2

[0015]

[0016] CO + H2O CO2 + H2.

[0017] Subsequently, the syngas obtained is used to produce methanol, by the methanol synthesis reaction:

[0018] CO + 2 H2 -> CH30H.

[0019] In general, traditional syngas production processes are energy-intensive and consume large amounts of fossil fuels (generally coal, methane and / or hydrocarbons with higher molecular weight) with the consequent release of large amounts of CO2 and consumption of non-renewable resources.

[0020] On the other hand, the production processes of green syngas (which use renewable energy sources) share the defect that the availability of renewable energy is fluctuating, whereby the production of syngas will in turn be inconstant. This has important implications which reverberate on all those synthesis processes which use it and which, usually, are not very flexible in relation to important variations in both the load to the reactors and in the stoichiometric ratio between hydrogen and CO and other reagents.

[0021] A possible mitigation of this problem consists in storing syngas or hydrogen, but this in turn entails significant energy costs.

[0022] An object of the present invention is to overcome the drawbacks highlighted above, by providing a process for producing syngas, in particular to be used for the subsequent synthesis of methanol or in GTL processes for the production of hydrocarbons, which can operate without being excessively affected by fluctuations in the availability of energy, as occurs for example with renewable energy sources.

[0023] In particular, it is an object of the invention to provide a process for producing syngas which uses the available energy resources in a fully efficient manner, in particular from renewable sources, compensating for any deficiencies in energy availability with energy stored in the form of chemical energy.

[0024] Summary

[0025] The present invention relates to a process for producing synthesis gas (syngas), to be used in particular for the synthesis of methanol or GTL hydrocarbons, as defined in the appended claim 1 and, for its preferred auxiliary characteristics, in the dependent claims.

[0026] In accordance with the invention, the syngas to be used in the methanol synthesisprocesses or GTL processes, instead of being produced from raw fossil materials such as coal and hydrocarbons, is obtained starting from water and CO2.

[0027] In order to guarantee a continuous flow of reagents to the synthesis reactors (of methanol or GTL hydrocarbons), even in circumstances with highly variable energy supplies, the invention proposes generating the syngas, in the absence of sufficient energy, starting from previously stored carbon monoxide, through the water-gas shift (WGS) reaction:

[0028]

[0029] CO + H2O CO2 + H2.

[0030] In accordance with the invention, therefore, CO2 is converted into CO, in particular by electrolysis, preferably via solid oxide cells (SOC), fed for example from renewable energies; and the CO is stored, preferably in liquid form, resulting in a storage of chemical energy to be used for the generation of syngas according to a water-gas shift process.

[0031] Since the stoichiometric hydrogen required for the synthesis of methanol is generated by the reaction

[0032]

[0033] CO + H2O CO2 + H2

[0034] the CO2 generated is suitably separated from the syngas and recycled.

[0035] The invention thus achieves some important advantages.

[0036] Since the liquefaction point of CO, at atmospheric pressure, is about -190°C, comparable to that of liquid air, the energy expenditure for the liquefaction of a mole of CO, from which a mole of hydrogen can be obtained, is significantly lower with respect to the liquefaction of a mole of hydrogen. In fact, the liquefaction of hydrogen typically requires an energy expenditure equal to 30% of the energy needed to obtain it from water. CO storage is therefore much more advantageous than hydrogen storage.

[0037] It should then be considered that, by producing 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, there is an energy expenditure, per mole of hydrogen obtainable (via water-gas shift), comparable to the energy expenditure of producing a mole of hydrogen from water; this is due to the difficulty of carrying out thermal recoveries in the presence of steam / liquid water phase transitions.

[0038] Furthermore, among the advantages of CO storage, it must be considered that theliquefaction system is much simpler.

[0039] The fact of having liquid CO, then, allows generating hydrogen at high pressure even with a limited energy expenditure, thus at least partially repaying the energy spent on liquefaction, and eliminating the use of hydrogen compressors.

[0040] Furthermore, the storage of CO allows obtaining electricity if it is consumed in a fuel cell, which could be the same cell used for electrolysis, only operating in reverse operation.

[0041] Finally, a further advantage of the invention is that of minimizing the plant components which must be stopped due to lack of energy, reducing them to those which best withstand cycling and / or stopping, i.e. refrigeration cycles for the production of liquefied gases (CO and possibly air).

[0042] Brief Description of the Drawings

[0043] The invention is further described in the following non-limiting example embodiments, with reference to the figures of the accompanying drawings, wherein:

[0044] - Figure 1 is a schematic view of a plant for producing syngas and the subsequent synthesis of methanol or GTL hydrocarbons, implementing a first operation mode of the process of the invention;

[0045] - Figure 2 is a schematic view of the plant of Figure 1, with parts removed for the sake of clarity, implementing a second operation mode of the process of the invention;

[0046] - Figures 3 and 4 are schematic views of a variant of the plant of Figures 1 and 2, respectively implementing a first and a second operation mode of a different embodiment of the process of the invention.

[0047] Description of Embodiments

[0048] In Figures 1 and 2, a plant 1 is illustrated for implementing the process for producing syngas, to be used for the synthesis of methanol or GTL hydrocarbons (i.e. hydrocarbons obtained by gas-to-liquids, GTL, processes), in accordance with the invention: Figure 1 shows the plant 1 in a first configuration for implementing a first operation mode of the process of the invention; Figure 2 shows the plant 1 in a second configuration for implementing a second operation mode of the process. For simplicity of representation, only the components of the plant 1 required to implement the respective operation mode are shown in Figure 2.

[0049] The plant 1 comprises:a water splitting unit 2 (water splitter), connected to a water supply and configured to conduct the splitting of water into 02 and H2 by the chemical reaction H20 - H2 + / i 02; for example, the water splitting unit 2 operates by electrolysis, or via a set of photochemical reactions;

[0050] a C02 feeding unit 3 (which can be a C02 storage unit and / or a unit connecting to an external source of C02);

[0051] a co-splitting unit 4 (co-splitter), connected to a water supply and to the C02 feeding unit 3 and configured to conduct the chemical reactions H20 - H2 + / i 02 and C02 CO + / i 02; preferably, the co-splitting unit 4 operates by electrolysis, or via a set of photochemical reactions;

[0052] a cryogenic separator 5, connected to the co-splitting unit 4 and configured to separate via cooling and consequent partial condensation a liquid stream consisting mainly of CO and a gaseous stream consisting of a H2 / C0 mixture (syngas);

[0053] a C02 splitting unit 6 (C02-splitter), connected to the C02 feeding unit 3 and configured to conduct the chemical reaction C02

[0054]

[0055] CO + Yi 02; for example, the C02 splitting unit 6 operates by electrolysis, or via a set of photochemical reactions;

[0056] a CO purification and liquefaction unit 7, connected to the C02 splitting unit 6; a CO storage unit 8, connected to the CO purification and liquefaction unit 7; a saturator 11, connected to the CO storage unit 8;

[0057] a WGS unit 12, i.e. a water-gas shift unit, connected to the saturator 11 and comprising one or more CO shift reactors 13 and a residual heat recovery boiler 14; a compression unit 15, connected to the WGS unit 12;

[0058] a dehydration unit 16, connected to the compression unit 15;

[0059] a cryogenic C02 separator 17 connected to the dehydration unit 16 and configured to separate condensed (liquid) C02 from a residual gaseous flow;

[0060] a C02 removal unit 18, connected to the cryogenic C02 separator 17 and configured to remove C02 from a syngas stream and operating for example by solvent absorption, for example with methanol or amine solutions;

[0061] a thermal recovery unit 20, interposed between the CO storage unit 8 and the saturator 11, between the dehydration unit 16 and the cryogenic C02 separator 17, and further connected to the C02 removal unit 18 to conduct thermal exchanges between the flows coming from the CO storage unit 8, the dehydration unit 16 and the C02 removalunit 18;

[0062] a synthesis unit 21, connected to the water splitting unit 2 and the CO2 removal unit 18 and comprising one or more reactors 22, in particular methanol synthesis reactors, where the syngas is converted into crude methanol (i.e. a mixture of methanol and water), or Fischer-Tropsch reactors, where the syngas is subjected to a Fischer-Tropsch process for the synthesis of GTL hydrocarbons;

[0063] a synthesis products storage unit 23, connected to the synthesis unit 21 to receive the synthesis products thereof (methanol or GTL hydrocarbons);

[0064] a fuel gas unit 24, connected to the synthesis unit 21 to receive methanol synthesis or GTL process exhaust gas therefrom;

[0065] a combustion unit 25, connected to the fuel gas unit 24 and configured to burn fuel gases with oxygen;

[0066] a steam turbine unit 26, comprising one or more steam turbines and an electric generator and connected to the combustion unit 25 for producing energy by exploiting steam generated in the combustion unit 25.

[0067] The process according to the present invention is implemented, for example by using the plant of Figures 1 and 2, in two operation modes conducted alternatively to each other and in succession and repeatedly one after the other: in a first mode (Figure 1), there is abundant electricity, while in the second mode (Figure 2) there is little or no energy availability.

[0068] In the first operation mode, where there is ample energy availability, CO is stored, which is then used in the second operation mode, when the energy availability is lower.

[0069] With reference to Figure 1, the first operation mode (implemented when there is a high energy availability), is conducted as follows.

[0070] The water splitting unit 2 is fed with a stream fl of water which is split into gaseous hydrogen and oxygen by the reaction H2O - H2 + / i 02, for example by electrolysis or via photochemical reactions. A gaseous stream of 02, which is recovered from an oxygen outlet for any other uses, and a gaseous stream f2 of H2, which is sent to the synthesis unit 21, are obtained from the water splitting unit 2.

[0071] A gaseous stream f4 of C02 is taken from the C02 feeding unit 3, which is optionally divided into several streams, according to the operating needs and opportunities of the plant 1; in particular, the stream f4 is divided into a stream flO whichis sent to the CO2 splitting unit 6, and into a stream f5 which is sent to the co-splitting unit 4, and optionally into a stream f5’ which is sent to the synthesis unit 21.

[0072] The reaction CO2

[0073]

[0074] CO + i 02 takes place in the C02 splitting unit 6, for example by electrolysis or via photochemical reactions, resulting in an 02 stream, which is recovered from an oxygen outlet for any other uses, and a stream fl 1 consisting of a C02 / C0 mixture.

[0075] The stream fl 1 is sent to the CO purification and liquefaction unit 7 to separate CO from C02, for example by solvent absorption, such as methanol or amine solutions, resulting in a stream fl2 of CO, preferably containing less than 100 ppm (vol) CO2 and even more preferably less than 50 ppm CO2, and a stream fl 3 mainly consisting of CO2, which is recirculated to the CO2 splitting unit 6. The stream fl2 of CO can also be liquefied, in whole or in part, in the CO purification and liquefaction unit 7 and is then sent to the CO storage unit 8. The part of the stream fl2 of CO that is not liquefied is sent as stream fl4 to the synthesis unit 21.

[0076] The stream f5 is sent to the co-splitting unit 4, which is also fed with a stream f6 of water; the reactions H2O - H2 + i 02 and

[0077]

[0078] CO2 CO + i 02 take place in the cosplitting unit 4, for example by electrolysis or via photochemical reactions, resulting in an 02 stream, which is recovered from an oxygen outlet for any other uses, and a stream f7 consisting of a C0 / H2 mixture; optionally, the co-splitting unit 4 also includes separators for removing H2O and CO2 from the stream f7 (C0 / H2 mixture).

[0079] The stream f7 is sent to the cryogenic separator 5 from which are obtained, for example by partial condensation, a liquid stream f9, consisting mainly of CO, and a gaseous stream f8, consisting of a H2 / C0 mixture (syngas).

[0080] The stream f9 is sent to the CO storage unit, while the stream f8 is combined with the stream fl4 coming from the CO purification and liquefaction unit 7, with the possible stream f5’ of CO2 coming from the CO2 feeding unit 3, and with the stream f2 of H2 exiting from the water splitting unit 2, forming a syngas stream fl 5 that is fed to the synthesis unit 21.

[0081] A CO stream fl 6 is taken from the CO storage unit 8, which passes in the thermal recovery unit 20, where the stream fl 6 is heated by exchanging heat with a multiplicity of other hotter streams, as described in the following, and forming a heated stream fl 7.

[0082] The stream fl 7 is further heated and is added with water by means of the saturator11, resulting in a saturated stream fl 8.

[0083] The stream fl 8 is sent to the WGS unit 12, where it is used for the CO shift reaction in the reactors 13, together with a stream w9 of steam exiting from the steam turbine unit 26, and cooled in the boiler 14 by a stream w6 of liquid water becoming a stream w7 of steam, producing a stream fl 9 of reaction products, consisting of a CO / CO2 / H2 / H2O mixture.

[0084] Within the WGS unit 12, the reaction products exiting from the various reactors 13 can be sent to the boiler 14 joined in a single stream fl9’, or even separately in respective streams f 19’ ; the thermal recoveries carried out in the boiler 14 will be adapted accordingly.

[0085] In all cases, the reaction products release heat (reaction heat developed in the reactors 13) to the stream w6 of liquid water with consequent removal of part of the water from the reaction products in the stream fl 9 exiting from the WGS unit 12.

[0086] The stream fl 9 can optionally be compressed in the compression unit 15, resulting in the stream f20, which is further dehydrated in the dehydration unit 16 to decrease the water content thereof to less than 100 ppm and, preferably, to less than 50 ppm, resulting in a dehydrated stream f21 mainly consisting of CO, CO2, H2.

[0087] The dehydrated stream f21 is cooled in the thermal recovery unit 20, releasing heat to the stream fl 6, with consequent partial condensation of the CO2 and formation of a mixed-phase stream f22.

[0088] The stream f22 is sent to the cryogenic CO2 separator 17 where the previously condensed liquid CO2 is separated, together with a further portion of condensed CO2 via refrigeration, possibly recovering further cold energy from the thermal recovery unit 20. A liquid CO2 stream f22’ and a residual gaseous stream f23 are obtained.

[0089] The stream f23 is sent to the CO2 removal unit 18 where a further portion of CO2 is removed, for example by solvent absorption, for example with methanol or amine solutions, resulting in a gaseous stream f24 of CO2 and a syngas stream f25.

[0090] The stream f24 is liquefied in the thermal recovery unit 20, resulting in a stream f24’ consisting mainly of liquid CO2.

[0091] The syngas stream f25 is fed to the reactors 22 of the synthesis unit 21, which are also fed with the stream fl 5; depending on the type of reactors 22, the conversion into crude methanol i.e. a mixture of methanol and water, or into a mixture of hydrocarbonsobtained by a GTL process, in particular the Fischer-Tropsch process, takes place in the reactors 22; the synthesis products form a stream f26 which is stored in the synthesis products storage unit 23.

[0092] The reaction heat developed in the methanol synthesis or Fischer-Tropsch process (or other GTL process) can be removed by thermal exchange with a stream wl of liquid water transforming into a stream w2 of steam.

[0093] A gaseous stream f27, mainly consisting of H2, CO, CO2 and (depending on the synthesis conducted) methanol or light hydrocarbons, is released from the reactors 22 in varying proportions based on the proportions of H2 and CO in the reagents (i.e. in the streams fl 5 and f25); the stream f27 is sent to the fuel gas unit 24, along with any additional stream f29 that may consist of H2, CO, or variable mixtures of H2 and CO.

[0094] A stream f28 of fuel gases is obtained from the fuel gas unit 24, which is sent to the combustion unit 25 where it is burned thanks to the oxygen contained in a stream al, which could consist of air, oxygen with purity greater than 95 mol%, and / or oxygen coming from the water splitting unit 2, the CO2 splitting unit 6 and / or the co-splitting unit 4. The combustion generates heat and a stream a2 of gas, mainly consisting of CO2, H2O and possibly nitrogen and which is removed through a gas outlet. From the stream a2, it is possible to separate and recover CO2, and this is particularly convenient when the stream al consists of 02 with purity greater than 95 mol%.

[0095] The heat produced in the combustion of the stream f28 heats the streams w2, w7 and a possible stream w4 of liquid water, which become respective streams w3, w8, w5 of superheated steam.

[0096] The streams w3, w5 and w8 are sent to the steam turbine unit 26 to actuate the steam turbines and produce energy; the steam exits the turbines in the stream w9, which as already described is fed to the reactors 13, and in a possible further stream wlO of steam to be used for other purposes.

[0097] In this first operation mode, where there is ample energy availability, CO can be produced in greater amounts than that required for producing the syngas necessary for the synthesis of methanol or GTL processes, whereby CO is stored.

[0098] With reference to Figure 2, the second operation mode (implemented when there is a low or no energy availability, alternatively and subsequently to the first operation mode), is conducted as follows.With respect to the first operation mode, the water splitting unit 2, the CO2 splitting unit 6, the CO purification and liquefaction unit 7, the co-splitting unit 4 and the cryogenic separator 5 are not active in the second operation mode. The synthesis unit 21 is fed with syngas produced using the CO stored in the first operation mode, which is treated in a manner similar to that described above with reference to the first operation mode.

[0099] From the CO storage unit 8, where CO was stored in the previous operation mode, a stream fl 6 is also taken in this case, which is sent to the thermal recovery unit 20, where the stream fl 6 is heated, exchanging heat with the hotter streams, as described above, and forming a heated stream fl 7.

[0100] Optionally, a stream fl 7’ is taken from the stream fl 7, which is sent to the reactors 22 of the synthesis unit 21, also fed with the stream f25 coming from the CO2 removal unit 18.

[0101] As in the first operation mode, the stream fl 7 is further heated and is added with water by means of the saturator 11, resulting in the saturated stream fl 8.

[0102] The stream fl 8 is sent to the WGS unit 12, where it is used for the CO shift reaction in the reactors 13, together with a stream w9 of steam exiting from the steam turbine unit 26, and cooled in the boiler 14 by a stream w6 of liquid water becoming a stream w7 of steam; a stream fl 9 of reaction products is obtained, consisting of a CO / CO2 / H2 / H2O mixture.

[0103] Also in this operation mode, the thermal exchanges inside the WGS unit 12, symbolically represented by the stream fl 9’ (or by several streams fl 9’ exiting from respective reactors) cause the removal of part of the water from the reaction products.

[0104] The stream fl 9 can optionally be compressed in the compression unit 15, resulting in the stream f20, which is further dehydrated in the dehydration unit 16 to decrease the water content thereof to less than 100 ppm and, preferably, to less than 50 ppm, resulting in a dehydrated stream f21 mainly consisting of CO, CO2, H2.

[0105] The dehydrated stream f21 is cooled in the thermal recovery unit 20, causing the partial condensation of the CO2 and formation of a mixed-phase stream f22.

[0106] The stream f22 is sent to the cryogenic CO2 separator 17 where the previously condensed liquid CO2 is separated, together with a further portion of condensed CO2 via refrigeration, possibly recovering further cold energy from the thermal recovery unit 20.A liquid C02 stream f22’ and a residual gaseous stream f23 are obtained.

[0107] The stream £23 is sent to the CO2 removal unit 18 where a further portion of CO2 is removed, for example by solvent absorption, for example with methanol or amine solutions, resulting in a gaseous stream f24 of CO2 and a syngas stream f25.

[0108] The stream f24 is liquefied in the thermal recovery unit 20, resulting in a stream f24’ consisting mainly of liquid CO2.

[0109] The syngas stream f25 is fed to the reactors 22 of the synthesis unit 21, which are also fed with the stream fl 7’ and optionally with a stream f5’ of CO2 taken from the CO2 feeding unit 3.

[0110] The conversion into crude methanol takes place in the reactors 22, i.e. the conversion into a mixture of methanol and water, or into light hydrocarbons; the reaction products are taken in a stream f26.

[0111] The reaction heat developed in the synthesis of methanol or GTL hydrocarbons can be removed by thermal exchange with a stream wl of liquid water, which transforms into a stream w2 of steam.

[0112] A gaseous stream f27, mainly consisting of H2, CO, CO2 and (depending on the synthesis conducted) methanol or light hydrocarbons, is released from the reactors 22 in varying proportions based on the proportions of H2 and CO in the reagents (i.e. in the streams fl 5 and £25); the stream f27 is sent to the fuel gas unit 24, along with any additional stream f29 that may consist of H2, CO, or variable mixtures of H2 and CO.

[0113] A stream f28 of fuel gases is obtained from the fuel gas unit 24, which is sent to the combustion unit 25 where it is burned thanks to the oxygen contained in a stream al, which could consist of air, oxygen with purity greater than 95 mol%, and / or oxygen previously produced by the water splitting unit 2, the CO2 splitting unit 6 and / or the cosplitting unit 4. The combustion generates heat and a gas stream a2, mainly consisting of CO2, H2O and possibly nitrogen. From the stream a2, it is possible to separate and recover CO2, and this is particularly convenient when the stream al consists of 02 with purity greater than 95 mol%.

[0114] The heat produced in the combustion of the stream f28 heats the streams w2, w7 and a possible stream w4 of liquid water, which become respective streams w3, w8, w5 of superheated steam.

[0115] The streams w3, w5 and w8 are sent to the steam turbine unit 26 to actuate thesteam turbines and produce energy; the steam exits the turbines in the stream w9, which as already described is fed to the reactors 13, and in a possible further stream wlO of steam.

[0116] In the embodiment illustrated in Figures 1 and 2, the frigories of the stored CO are used for the removal of CO2; however, the same purpose can be pursued by other means, for example washing with solvents, such as amine solutions, and subsequent capture and sequestration of the CO2 released during solvent regeneration. In doing so, frigories are made available for refrigerating some other stream (for example, natural gas to be liquefied). In this case, it is advisable to anticipate the CO2 removal with respect to compression to decrease the required compression energy.

[0117] An embodiment of this type is illustrated in Figures 3 and 4.

[0118] Figure 3 shows a plant la substantially similar to what is described with reference to Figure 1, but in which the CO2 removal unit 18 is placed at the outlet of the WGS unit 12 and upstream of the compression unit 15, and the dehydration unit 16 is directly connected to the synthesis unit 21, so that the stream f25 exiting from the dehydration unit 16 does not pass in the thermal recovery unit 20; therefore, the cryogenic CO2 separator 17 is not present.

[0119] Similarly to what is described with reference to the plant 1 of Figure 1, also in this embodiment, when the plant la operates in the first operation mode (Figure 3), the water splitting unit 2 is fed with a stream fl of water, resulting in a gaseous stream of 02, which is recovered from an oxygen outlet for any other uses, and a gaseous stream f2 of H2, which is sent to the synthesis unit 21.

[0120] A gaseous stream f4 of CO2 is taken from the CO2 feeding unit 3, which is optionally divided into several streams, according to the operating needs and opportunities of the plant la; in particular, the stream f4 is divided into a stream flO which is sent to the CO2 splitting unit 6, into a stream f5 which is sent to the co-splitting unit 4, and optionally into a stream f5’ which is sent to the synthesis unit 21.

[0121] A stream of 02, which is recovered from an oxygen outlet for any other uses, and a stream fl 1 (CO2 / CO mixture) are obtained from the C02 splitting unit 6.

[0122] The stream fl 1 is sent to the CO purification and liquefaction unit 7, resulting in a stream fl2 of CO, preferably containing less than 100 ppm (vol) CO2 and even more preferably less than 50 ppm CO2, and a stream f!3 mainly consisting of CO2, which isrecirculated to the CO2 splitting unit 6. Also in this case, the stream fl2 of CO can be liquefied, in whole or in part, in the CO purification and liquefaction unit 7 and is then sent to the CO storage unit 8. The part of the stream fl2 of CO that is not liquefied is sent as stream fl4 to the synthesis unit 21.

[0123] The stream f5 is sent to the co-splitting unit 4, which is also fed with a stream f6 of water; an 02 stream is obtained, which is recovered from an oxygen outlet for any other uses, and a stream f7 consisting of a C0 / H2 mixture; optionally, the co-splitting unit 4 also includes separators for removing H20 and C02 from the stream f7 (C0 / H2 mixture).

[0124] The stream f7 is sent to the cryogenic separator 5 from which are obtained, for example by partial condensation, a liquid stream f9, consisting mainly of CO, and a gaseous stream f8, consisting of a H2 / C0 mixture (syngas).

[0125] The stream f is sent to the CO storage unit 8, while the stream f8 is combined with the stream fl4 coming from the CO purification and liquefaction unit 7, with the possible stream f5’ of C02 coming from the C02 feeding unit 3, and with the stream f2 of H2 exiting from the water splitting unit 2, forming a syngas stream fl 5 that is fed to the synthesis unit 21.

[0126] A stream fl6 of CO is taken from the CO storage unit 8, which passes in the thermal recovery unit 20, where the stream fl 6 is heated by exchanging heat with other hotter streams and forming a heated stream fl 7.

[0127] The stream fl 7 is further heated and is added with water by means of the saturator 11, resulting in a saturated stream fl 8.

[0128] The stream fl 8 is sent to the WGS unit 12, where it is used for the CO shift reaction in the reactors 13, together with a stream w9 of steam exiting from the steam turbine unit 26, and cooled in the boiler 14 by a stream w6 of liquid water, which becomes a stream w7 of steam, producing a stream fl9 of reaction products, consisting of a CO / CO2 / H2 / H2O mixture.

[0129] Within the WGS unit 12, the reaction products exiting from the various reactors 13 can be sent to the boiler 14 joined in a single stream fl9’, or even separately in respective streams fl 9’; the thermal recoveries carried out in the boiler will be adapted accordingly.

[0130] In all cases, the reaction products release heat (reaction heat developed in thereactors 13) to the stream w6 of liquid water with consequent removal of part of the water from the reaction products in the stream fl 9 exiting from the WGS unit 12.

[0131] The stream fl9 is sent to the CO2 removal unit 18, where CO2 is removed, for example by solvent absorption, for example with methanol or amine solutions, resulting in a stream f22, mainly formed by CO2 gas, and a stream f20, mainly consisting of H2, CO and CO2 in a ratio preferably less than 10%.

[0132] The stream f20 can optionally be compressed in the compression unit 15, resulting in the stream f21 , which in turn can be further dehydrated in the dehydration unit 16, from which a syngas stream f25 is obtained.

[0133] The syngas stream f25 is fed to the reactors 22 of the synthesis unit 21, which are also fed with the stream fl 5; depending on the type of reactors 22, the conversion into crude methanol i.e. a mixture of methanol and water, or into a mixture of hydrocarbons obtained by a GTL process, in particular the Fischer-Tropsch process, takes place in the reactors 22; the synthesis products form a stream f26 which is stored in the synthesis products storage unit 23.

[0134] The reaction heat developed in the methanol synthesis or Fischer-Tropsch process (or other GTL process) can be removed by thermal exchange with a stream wl of liquid water, which transforms into a stream w2 of steam.

[0135] A gaseous stream f27, mainly consisting of H2, CO, CO2 and (depending on the synthesis conducted) methanol or light hydrocarbons, is released from the reactors 22 in varying proportions based on the proportions of H2 and CO in the reagents (i.e. in the streams fl 5 and f25); the stream f27 is sent to the fuel gas unit 24, along with any additional stream f29 that may consist of H2, CO, or variable mixtures of H2 and CO.

[0136] A stream f28 of fuel gases is obtained from the fuel gas unit 24, which is sent to the combustion unit 25 where it is burned thanks to the oxygen contained in a stream al, which could consist of air, oxygen with purity greater than 95 mol%, and / or oxygen coming from the water splitting unit 2, the CO2 splitting unit 6 and / or the co-splitting unit 4. The combustion generates heat and a gas stream a2, mainly consisting of CO2, H2O and possibly nitrogen and which is removed through a gas outlet. From the stream a2, it is possible to separate and recover CO2, and this is particularly convenient when the stream al consists of 02 with purity greater than 95 mol% .

[0137] The heat produced in the combustion of the stream f28 heats the streams w2, w7and a possible stream w4 of liquid water, which become respective streams w3, w8, w5 of superheated steam.

[0138] The streams w3, w5 and w8 are sent to the steam turbine unit 26 to actuate the steam turbines and produce energy; the steam exits the turbines in the stream w9, which as already described is fed to the reactors 13, and in a possible further stream wlO of steam to be used for other purposes.

[0139] One or more hot streams f23, for example methane which, condensing, becomes LNG or nitrogen which becomes liquid nitrogen, are sent to the thermal recovery unit 20 to be cooled via thermal exchange with the stream fl 6, resulting in one or more corresponding cold streams f24.

[0140] In the second operation mode (Figure 4), the plant la operates as follows.

[0141] Also in this embodiment, the water splitting unit 2, the CO2 splitting unit 6, the CO purification and liquefaction unit 7, the co-splitting unit 4 and the cryogenic separator 5 are not active in the second operation mode. The synthesis unit 21 is fed with syngas produced using the CO stored in the first operation mode, which is treated in a manner similar to that described above with reference to the first operation mode.

[0142] From the CO storage unit 8, where CO was stored in the previous operation mode, a stream fl 6 is also taken in this case, which is sent to the thermal recovery unit 20, where the stream fl 6 is heated, exchanging heat with the stream f23 (or a multiplicity of streams f23) and forming a heated stream fl 7.

[0143] Optionally, a stream fl7 ’is taken from the stream fl7 and sent to the reactors 22 of the synthesis unit 21.

[0144] As in the first operation mode, the stream fl 7 is further heated and is added with water by means of the saturator 11, resulting in the saturated stream fl 8.

[0145] The stream fl 8 is sent to the WGS unit 12, where it is used for the CO shift reaction in the reactors 13, together with a stream w9 of steam exiting from the steam turbine unit 26, and cooled in the boiler 14 by a stream w6 of liquid water, which becomes a stream w7 of steam; a stream fl9 of reaction products is obtained, consisting of a CO / CO2 / H2 / H2O mixture.

[0146] Also in this operation mode, the thermal exchanges inside the WGS unit 12, symbolically represented by the stream fl 9’ (or by several streams fl 9’ exiting from respective reactors) cause the removal of part of the water from the reaction products.The stream fl9 is sent to the CO2 removal unit 18, where CO2 is removed, for example by solvent absorption, for example with methanol or amine solutions, resulting in a stream f22, mainly formed by CO2 gas, and a stream f20, mainly consisting of H2, CO and CO2 in a ratio preferably less than 10%.

[0147] The stream f20 can optionally be compressed in the compression unit 15, resulting in the stream f21 , which in turn can be further dehydrated in the dehydration unit 16, from which a syngas stream f25 is obtained.

[0148] The syngas stream f25 is fed to the reactors 22 of the synthesis unit 21, which are also fed with the stream fl 7’ and optionally with a stream f5’ of CO2 taken from the CO2 feeding unit 3.

[0149] The conversion into crude methanol, i.e. a mixture of methanol and water, or into light hydrocarbons takes place in the reactors 22; the reaction products are taken in a stream f26.

[0150] The reaction heat developed in the synthesis of methanol or GTL hydrocarbons can be removed by thermal exchange with a stream wl of liquid water, which transforms into a stream w2 of steam.

[0151] A gaseous stream f27, mainly consisting of H2, CO, CO2 and (depending on the synthesis conducted) methanol or light hydrocarbons, is released from the reactors 22 in varying proportions based on the proportions of H2 and CO in the reagents (i.e. in the streams fl 5 and f25); the stream f27 is sent to the fuel gas unit 24, along with any additional stream f29 that may consist of H2, CO, or variable mixtures of H2 and CO.

[0152] A stream f28 of fuel gases is obtained from the fuel gas unit 24, which is sent to the combustion unit 25 where it is burned thanks to the oxygen contained in a stream al, which could consist of air, oxygen with purity greater than 95 mol%, and / or oxygen previously produced by the water splitting unit 2, the CO2 splitting unit 6 and / or the cosplitting unit 4. The combustion generates heat and a stream a2 of gas, mainly consisting of CO2, H2O and possibly nitrogen. From the stream a2, it is possible to separate and recover CO2, and this is particularly convenient when the stream al consists of 02 with purity greater than 95 mol% .

[0153] The heat produced in the combustion of the stream f28 heats the streams w2, w7 and a possible stream w4 of liquid water, which become respective streams w3, w8, w5 of superheated steam.The streams w3, w5 and w8 are sent to the steam turbine unit 26 to actuate the steam turbines and produce energy; the steam exits the turbines in the stream w9, which as already described is fed to the shift reactors, and in a possible further stream wlO of steam.

[0154] One or more hot streams f23, for example methane which, condensing, becomes LNG or nitrogen which becomes liquid nitrogen, are sent to the thermal recovery unit 20 to be cooled via heat exchange with the stream fl 6, resulting in one or more corresponding cold streams f24.

[0155] Also in this case, a stream f5’ taken from the CO2 feeding unit can be optionally fed to the synthesis reactors 22.

[0156] Finally, it is understood that the process described and illustrated herein can be subject to further modifications and variations that do not depart from the scope of the appended claims.

Claims

1. CLAIMS1. A process for producing synthesis gas (syngas), to be used in particular for the synthesis of methanol or GTL hydrocarbons, comprising a first operation mode comprising the steps of:- producing H2 by a reaction of splitting H2O into H2 and 02: H2O - H2 +x / i 02,for example via electrolytic or photochemical reactions;- producing CO by a reaction of splitting CO2 into CO and 02: CO2CO +x / i 02,for example via electrolytic or photochemical reactions;- using a first part of the CO obtained from the splitting of CO2 to form, together with H2 produced by water splitting, syngas, i.e. a H2 / C0 mixture, to be used for the synthesis of methanol or GTL hydrocarbons;- using a second part of the CO obtained from the splitting of CO2 in a CO conversion step together with H2O, in which a water-gas shift reaction is conducted: CO+ H2O CO2 + H2to convert CO into CO2 and produce H2, resulting in syngas, to be used for the synthesis of methanol or GTL hydrocarbons;- storing a third part of the CO obtained from the splitting of CO2 in a storage, optionally after a purification step from residual CO2 and 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 a CO conversion step, in which a water-gas shift reaction is conducted:CO + H2O CO2 + H2,to convert CO to CO2 and produce H2, resulting in syngas, to be used for the synthesis of methanol or GTL hydrocarbons.

2. The process according to claim 1, wherein the syngas produced is used in a methanol synthesis reaction or in a Fischer-Tropsch process for the synthesis of GTL hydrocarbons.

3. The process according to claim 1 or 2, wherein in the first operation mode a first part of H2 is produced by a water splitting reaction in a water splitting unit (2), which is fed with H2O and where the reaction of splitting water into H2 and 02 takes place; and a second part of H2 is produced in a co-splitting unit (4), which is fed with H20 and C02 and where both the reaction of splitting H20 into H2 and 02 and the reaction of splitting C02 into CO and 02 take place.

4. The process according to claim 3, wherein in the first operation mode a first part of CO is produced by the reaction of splitting C02 into CO and 02 in said cosplitting unit (4), and a second part of CO is produced by the reaction of splitting C02 into CO and 02 in a C02 splitting unit (6), fed with C02.

5. The process according to claim 4, wherein CO produced in the C02 splitting unit (6) is subjected to a purification step to separate CO from C02, resulting in a stream (fl 3) predominantly consisting of C02, which is recirculated to the C02 splitting unit (6), and a stream (fl 2) of CO, preferably containing less than 100 ppm (vol) C02 and even more preferably less than 50 ppm CO2, which is sent to storage.

6. The process according to claim 5, wherein at least part of the purified CO is subjected to a liquefaction step, to be stored in liquid form.

7. The process according to any one of claims 3 to 6, wherein said first part of CO produced in the co-splitting unit (4) is subjected to a cryogenic separation step to separate, for example by partial condensation, a liquid stream (f9) consisting mainly of CO, which is sent to storage, and a gaseous stream (f8), consisting of syngas.

8. The process according to any one of the preceding claims, wherein in either operation mode CO taken from storage is sent to a thermal recovery unit (20) where it is heated by exchanging heat with at least one hotter stream from the CO conversion step, and is added with water by means of a saturator (11) before being fed to the CO conversion step.

9. The process according to claim 8, wherein in the CO conversion step, the CO shift reaction is conducted using a stream (w9) of steam exiting from a steam turbine unit (26), and a stream (fl 9) of reaction products, comprising a CO / CO2 / H2 / H2O mixture, is produced, which is cooled by a stream (w6) of liquid water becoming a stream (w7) of steam.

10. The process according to claim 8 or 9, wherein the reaction products of theshift reaction are subjected to dehydration to remove water and decrease the water content to less than 100 ppm and, preferably, to less than 50 ppm.

11. The process according to any one of claims 8 to 10, wherein the reaction products of the shift reaction are subjected to a CO2 removal step, for example by solvent absorption, for example with methanol or amine solutions, and a dehydration step, to form a syngas stream to be used for the synthesis of methanol or GTL hydrocarbons.

12. The process according to any one of claims 8 to 11, wherein the reaction products of the shift reaction are sent to the thermal recovery unit (20) to release heat to the CO stream taken from the storage, resulting in partial condensation of CO2 and formation of a mixed-phase stream (f22), which is subjected to cryogenic separation to remove liquid CO2 from a residual gaseous stream (f23) which is subjected to said CO2 removal step.

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