Production of synthesis fuels from co2 with conversion of by-products into synthesis gas and separation of co2

Abstract

The invention relates to a method for converting CO2, comprising: treating the CO2 (1) with H2 (2) by means of RWGS (3) in order to produce a first synthesis gas (4); separating the first synthesis gas in order to produce a CO2-depleted gaseous effluent (8) and CO2 (7) which is recycled to the RWGS unit (3); converting the CO2-depleted gaseous effluent by means of Fischer-Tropsch (10) in order to produce a hydrocarbon effluent (11) and a first gaseous FT effluent (13); treating the hydrocarbon effluent by means of hydroconversion (19) in order to produce at least one first C8-C22 hydrocarbon cut (20) and a second C4-C8 hydrocarbon cut (23); and converting the first gaseous FT effluent, at least in part, and the second hydrocarbon cut, at least in part, into synthesis gas (24) in a reaction unit for the conversion of by-products, in order to produce a second synthesis gas (25) which is sent for separation with the first synthesis gas.

Description

Technical field The present invention relates to the production of synthetic fuels, namely gasoline, kerosene, gas oil, and / or other hydrocarbon products, such as naphtha, or lubricant bases, of very high quality (essentially devoid of sulfur, of aromatics, of nitrogen). More particularly, an object of the present invention is to produce synthetic fuels from carbon dioxide (CO2) and water. The conversion of carbon dioxide into a fuel base according to the invention relates in particular to the following stages: the conversion of carbon dioxide and hydrogen into a first synthesis gas composed predominantly of carbon monoxide (CO) and of H2, the separation of the synthesis gases into a CO2-rich stream and a CO2-poor stream, the conversion of the CO2-poor synthesis gas into synthetic hydrocarbons by the Fischer-Tropsch (FT) process, the hydroconversion of the hydrocarbon effluent from the FT process and conversion of products from the FT process. Prior art The use of the reverse water-gas shift (RWGS) process to convert a mixture of carbon dioxide and of hydrogen into synthesis gas CO + H2 has been known to a person skilled in the art for a very long time. The same is true for the Fischer-Tropsch synthesis process, which makes it possible to convert said synthesis gas into a mixture of paraffins, and / or of olefins and / or of oxygen-based compounds, depending on the catalyst and the operating conditions. In the case where paraffins are produced, it is preferable to improve certain properties thereof in order to enable them to be used for transportation applications. Sequences of unit operations have formed the object of patent applications, these sequences of unit operations aiming to convert carbon dioxide into a base for fuels, often known under the term of e-fuels (or electrofuels). Mention may be made, for example, of the patent application US2010 / 0280135 A1, which describes a renewable Fischer-Tropsch synthesis process which makes it possible to produce hydrocarbons and alcohols from wind energy, residual carbon dioxide and water. The process comprises the following unit operations: electrolysis of water to produce hydrogen and oxygen, an RWGS reactor for the production of synthesis gas, a Fischer-Tropsch synthesis in a high-temperature multitubular reactor. Various recycling options are described (e.g. recycling after separation of unconverted carbon dioxide from the RWGS, recycling of ex-FT carbon dioxide to RWGS, recycling of ex-FT unconverted H2 and CO to FT). The patent application US2012 / 0079767 A1 describes a process and system for producing synthesis gas by combining hydrogen and carbon monoxide originating from separate sources while controlling the molar ratio (H2 / CO) of the synthesis gas produced. The hydrogen is produced by electrolysis of water. The carbon monoxide is produced by reacting carbon dioxide captured from the exhaust gases of stationary combustion engines with hydrogen in an RWGS reactor. Hydrocarbon fuels are produced from the synthesis gas by the Fischer-Tropsch synthesis. The patent application US2007 / 0142481 A1 describes a process for the synthesis of hydrocarbons comprising the introduction of hydrogen and of carbon monoxide into a first Fischer-Tropsch reaction stage, making it possible for the hydrogen and the carbon monoxide to partially react catalytically to form hydrocarbons. At least a part of a tail gas which comprises unreacted hydrogen and carbon monoxide, obtained from the first reaction stage, is introduced into a second Fischer-Tropsch reaction stage which is a two-phase high-temperature catalytic Fischer-Tropsch reaction stage. The hydrogen and the carbon monoxide can at least partially react catalytically in the second reaction stage to form gaseous hydrocarbons. This patent application is characterized by the presence of two Fischer-Tropsch reactors in series; the second one treats the unconverted synthesis gas from the first one. There is no recycling of carbon dioxide or recycling of water. The patent application WO 2022 / 232 936 A1 describes a process which makes it possible to increase the production of carbon monoxide and to recycle carbon dioxide during the treatment of synthesis gas using a unit for the conversion of carbon dioxide into carbon monoxide, such as a Reverse Water Gas Shift (RWGS) reactor, converting the excess CO2 from the synthesis gas produced into additional CO, while using an external source of green, renewable or low-carbon-intensity hydrogen. Thus, the analysis of the prior art demonstrates that the sequence of RWGS and Fischer-Tropsch unit operations makes it possible to produce synthetic bases for fuels from carbon dioxide and hydrogen, it being possible in some cases for said hydrogen to be produced by electrolysis of water with a source of electricity such as solar power or wind power. On the other hand, the prior art does not mention the possibility of integrating additional units making it possible to increase the conversion of carbon dioxide into paraffins having a carbon number of between 8 and 22, and preferably essentially between 9 and 16 carbon atoms per molecule. Summary of the invention The invention relates to the conversion of carbon dioxide so as to produce a first synthesis gas rich in CO and H2, the separation of the CO2 present in the synthesis gases, the conversion of the CO2-depleted synthesis gas into synthetic hydrocarbons by the Fischer-Tropsch (FT) reaction, the hydroconversion of the hydrocarbon effluent from the FT process and the conversion of byproducts into a second synthesis gas rich in CO and H2. Specifically, the present invention relates to a device and to a process for the production of synthetic fuels from carbon dioxide and hydrogen, making possible improved production of paraffins having from 8 to 22 carbon atoms per molecule, indeed even essentially paraffins having from 9 to 16 carbon atoms per molecule. The present invention is based on the presence of a reverse water-gas shift RWGS reaction unit, fed with carbon dioxide and H2 and which produces a first synthesis gas rich in CO and H2. The invention is also based on the presence of a unit for the separation of the carbon dioxide contained in the synthesis gases upstream of the Fischer-Tropsch reaction unit. The carbon dioxide separated from the synthesis gases is recycled at the inlet of the RWGS reaction unit. Thus, the synthesis gas sent to the Fischer-Tropsch reaction unit no longer substantially contains carbon dioxide. The invention is also based on the presence of a hydroconversion unit (upgrading unit; known to a person skilled in the art), fed with the hydrocarbon effluent from the FT process and which produces synthetic fuels which are compatible with use for terrestrial, aviation and maritime fuels. The invention is finally based on the presence of a unit for the conversion of byproducts into synthesis gas fed with byproducts from the FT reaction unit and from the hydroconversion unit, to produce a second synthesis gas rich in H2 and CO. Advantageously, the second synthesis gas is returned at the inlet of the unit for the separation of carbon dioxide, making it possible to recycle, to the RWGS unit, the CO2 resulting from the RWGS unit and from the unit for the conversion of byproducts into synthesis gas and to upgrade additional CO in the Fischer-Tropsch reaction unit. The unit for the separation of carbon dioxide is thus fed with the first synthesis gas resulting from the RWGS unit and with the second synthesis gas resulting from the unit for the conversion of byproducts into synthesis gas. Preferably, the hydrogen needed in the process is entirely provided by a water electrolysis unit. Advantageously, hydrogen produced by electrolysis of water can be used for the conversion of carbon dioxide, the Fischer-Tropsch synthesis and the hydroconversion. Thus, the process according to the invention does not require an external supply of hydrogen, for example produced by steam reforming of natural gas. The electrolyzer preferably operates with low-carbon electricity, which contributes to the renewable nature of the fuels and gases produced. In addition, the water used for the production of hydrogen can originate, at least in part, from the recycling of the water produced in the various stages of the process, which exhibits the advantage of limiting the external supply of water. Advantageously, the oxygen produced by the electrolysis of water can also feed the unit for the conversion of byproducts into synthesis gas. According to a first aspect, the abovementioned objects, and also other advantages, are obtained by a process for the conversion of a feedstock containing carbon dioxide, comprising the following stages: - treating a feedstock rich in carbon dioxide with a first hydrogen source in a reverse watergas shift RWGS reaction unit to produce a first synthesis gas rich in carbon monoxide and H2 and a first water effluent; - treating the first synthesis gas in a unit for the separation of carbon dioxide to produce a gaseous effluent depleted in carbon dioxide and a gaseous effluent rich in carbon dioxide; - recycling the gaseous effluent rich in carbon dioxide at the inlet of the RWGS reaction unit; - converting the gaseous effluent depleted in carbon dioxide in a Fischer-Tropsch reaction unit to produce a hydrocarbon effluent, a second water effluent, a first gaseous FT effluent rich in CO, H2, CO2 and hydrocarbons having from 1 to 5 carbon atoms per molecule, and optionally a second gaseous FT effluent rich in hydrocarbons having from 1 to 5 carbon atoms per molecule; - treating the hydrocarbon effluent in a hydroconversion reaction unit (hydrotreating and / or hydrocracking and / or hydroisomerization unit) with a third hydrogen source to produce at least a first hydrocarbon cut comprising paraffins having a carbon number of between 8 and 22, and preferably essentially from 9 to 16 carbon atoms per molecule, for example to the specifications for transportation applications, a third gaseous effluent comprising hydrocarbons having from 1 to 5 carbon atoms per molecule, and a second hydrocarbon cut comprising paraffins having from 4 to 8 carbon atoms per molecule; - converting at least a part of the first gaseous FT effluent and at least a part of the second hydrocarbon cut, in a reaction unit for the conversion of byproducts into synthesis gas, to produce a second synthesis gas rich in carbon monoxide and hydrogen; and - sending the second synthesis gas into the unit for the separation of carbon dioxide. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas is fed with, at least in part, the second gaseous FT effluent. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas is fed with, at least in part, the third gaseous effluent. According to one or more embodiments, the feedstock rich in carbon dioxide and / or the gaseous effluent rich in carbon dioxide are purified, separately or after mixing, before being introduced into the RWGS reaction unit. According to one or more embodiments, the gaseous effluent depleted in carbon dioxide is purified before being introduced into the Fischer-Tropsch reaction unit. The stages of purification of the effluents are targeted at removing, at least partially, at least one of the following compounds: organic compounds, ionic entities, sulfur compounds, nitrogen compounds, halogenated compounds, metals (e.g. heavy metals) and transition metals. According to one or more embodiments, the RWGS reaction unit comprises at least one reactor used under at least one of the following operating conditions: - temperature of between 700°C and 1200°C, preferentially of between 750°C and 1100°C and more preferentially still of between 780°C and 1050°C; - pressure of between 0.1 MPa and 10 MPa, preferentially of between 0.1 MPa and 5 MPa and more preferentially of between 0.1 MPa and 3.5 MPa; - space velocity of the gas at the inlet of the reactor of between 2000 Sl / kgcata / h and 40 000 Sl / kgcata / h; - catalyst comprising a metal or a combination of metals chosen from the group consisting of the elements Ni, Cu, Fe, Co, Pt, Pd, Ru, Ag and Au. According to one or more embodiments, the catalyst for the RWGS reaction comprises a support, for example based on alumina, on silica, on silica-alumina or on siliceous alumina. According to one or more embodiments, the unit for the separation of carbon dioxide employs a separation by chemical solvent and / or physical solvent and / or membrane and / or adsorption on a solid. According to one or more embodiments, the Fischer-Tropsch reaction unit comprises at least one reactor used under at least one of the following operating conditions: - temperature of between 170°C and 280°C, preferably between 190°C and 260°C and preferentially between 210°C and 240°C; - absolute pressure of between 1.0 MPa and 6.0 MPa, preferably between 1.5 MPa and 3.5 MPa and preferentially between 2.0 MPa and 3.0 MPa; - catalyst comprising cobalt or iron, preferably cobalt, the catalyst optionally comprising a support, for example based on alumina, on silica, on silica-alumina, on siliceous alumina or on titanium. According to one or more embodiments, the process comprises feeding the Fischer-Tropsch reaction unit with a second hydrogen source. According to one or more embodiments, the hydroconversion reaction unit comprises at least one reactor used under at least one of the following operating conditions: - temperature of between 200°C and 500°C, more preferentially between 280°C and 450°C and more preferentially still between 300°C and 420°C; - pressure of between 0.2 MPa and 15 MPa, preferably between 0.5 MPa and 12 MPa and more preferably between 1 MPa and 10 MPa; - space velocity, defined as being the ratio of the flow rate by volume of the feedstock at ambient temperature and ambient pressure to the volume of the catalyst, of between 0.1 h-1 and 10 h-1, preferably between 0.2 h-1 and 7 h-1 and more preferentially between 0.5 h-1 and 5 h-1; - hydrogen flow rate of between 70 and 2000 standard liters of hydrogen per liter of feedstock per hour and preferably between 150 and 1500 standard liters of hydrogen per liter of feedstock and more preferentially between 300 and 1500 standard liters of hydrogen per liter of feedstock; - hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprising at least one hydrogenating-dehydrogenating metal chosen from the group consisting of the metals from group VIB and group VIIIB of the Periodic Table and / or at least one solid which is a Bronsted acid, that is to say a solid which can release one or more protons, and optionally a binder. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas comprises at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 9 MPa; - temperature of between 600°C and 2000°C. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas comprises a catalytic or noncatalytic partial oxidation unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the partial oxidation unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 9 MPa and preferably between 0.1 MPa and 4 MPa; - temperature of between 600°C and 2000°C, preferably between 800°C and 1700°C, preferably between 900°C and 1500°C or between 1100°C and 1500°C; - presence of oxygen used for the combustion, with an oxygenation rate of between 0.2 and 0.6, preferably between 0.25 and 0.5, in order to promote the formation of carbon monoxide; - presence of a catalyst containing a noble metal, such as rhodium, or nickel or a nickel alloy, when the partial oxidation unit is catalytic. According to one or more embodiments, a part of the first gaseous FT effluent and / or at least a part of the second gaseous FT effluent are, at least in part, upgraded to give synthetic methane (e-methane), synthetic natural gas (e-SNG) or synthetic liquefied petroleum gas (e-LPG). According to one or more embodiments, a part of the first gaseous FT effluent is bled off. According to a second aspect, the abovementioned objects, and also other advantages, are obtained by a device for the conversion of a feedstock containing carbon dioxide, comprising the following units: - a reverse water-gas shift RWGS reaction unit suitable for treating a feedstock rich in carbon dioxide with a first hydrogen source and producing a first synthesis gas rich in carbon monoxide and H2 and a first water effluent; - a unit for the separation of carbon dioxide suitable for treating the first synthesis gas, producing a gaseous effluent depleted in carbon dioxide and a gaseous effluent rich in carbon dioxide, and recycling the gaseous effluent rich in carbon dioxide at the inlet of the RWGS reaction unit; - a Fischer-Tropsch reaction unit suitable for converting the gaseous effluent depleted in carbon dioxide, and producing a hydrocarbon effluent, a second water effluent, a first gaseous FT effluent rich in CO, H2, CO2 and hydrocarbons having from 1 to 5 carbon atoms per molecule, and optionally a second gaseous FT effluent rich in hydrocarbons having from 1 to 5 carbon atoms per molecule; - a hydroconversion reaction unit (hydrotreating and / or hydrocracking and / or hydroisomerization unit) suitable for treating the hydrocarbon effluent with a third hydrogen source and producing at least a first hydrocarbon cut comprising paraffins having a carbon number of between 8 and 22, and preferably essentially from 9 to 16 carbon atoms per molecule, for example to the specifications for transportation applications, a third gaseous effluent comprising hydrocarbons having from 1 to 5 carbon atoms per molecule, and a second hydrocarbon cut comprising paraffins having from 4 to 8 carbon atoms per molecule; and - a reaction unit for the conversion of byproducts into synthesis gas suitable for converting at least a part of the first gaseous FT effluent, at least a part of the second hydrocarbon cut, optionally a gas stream comprising, at least in part, the second gaseous FT effluent and / or, at least in part, the third gaseous effluent, and producing a second synthesis gas rich in carbon monoxide and hydrogen which is sent to the unit for the separation of carbon dioxide. Embodiments of the device and of the process according to the abovementioned aspects, and also other characteristics and advantages, will become apparent on reading the description which will follow, given purely by way of nonlimiting illustration, and with reference to the following drawing. List of the figures Figure 1 shows a diagrammatic representation of a process according to the present invention in which the reaction unit for the conversion of byproducts into synthesis gas is fed with at least one gaseous FT effluent and at least one hydrocarbon cut comprising paraffins having from 4 to 8 carbon atoms per molecule resulting from the hydroconversion unit. Description of the embodiments Embodiments of the process and of the device according to the invention will now be described in detail. In the following detailed description, many specific details are disclosed in order to provide a deeper understanding of the process and of the device according to the invention. However, it will be apparent to a person skilled in the art that the process according to the invention can be implemented and the device according to the invention can be employed without these specific details. In other cases, well-known features have not been described in detail in order to avoid unnecessarily complicating the description. In the present description, the term “to comprise” is synonymous with (means the same thing as) “to include” and “to contain”, and is inclusive or open and does not exclude other elements not stated. It is understood that the term “to comprise” includes the exclusive and closed term “to consist of”. In addition, in the present description, an effluent essentially, substantially or solely comprising a compound A corresponds to an effluent comprising at least 95% by weight, preferably at least 98% by weight, very preferably at least 99% by weight, indeed even 100%, of compound A. Finally, in the present description, an effluent which is rich in a compound A corresponds to an effluent comprising at least 10% by weight or by volume, preferably at least 30% by weight or by volume, preferably at least 50% by weight or by volume, preferably at least 70% by weight or by volume, preferably at least 90% by weight or by volume, preferably at least 95% by weight or by volume, of compound A, such as at least between 50% by weight or by volume and 99% by weight or by volume of compound A. In the present description, an effluent which is rich in a compound A can correspond to an effluent consisting of compound A (e.g. impurities of less than 1000 ppm, indeed even of less than 100 ppm). In the present description, the term "physical solvent" is synonymous with (means the same thing as) solvent forming weak bonds (e.g. hydrogen bond, van der Waals bond) with the solute, the solvent not forming a strong bond (e.g. covalent bond, ionic bond) with the solute. The present invention can be defined as a process and a device comprising a sequence of unit operations making it possible to produce synthetic hydrocarbons, such as synthetic fuels, for example gasoline, kerosene, gas oil and / or naphtha, or lubricant bases, preferably of very high quality, from carbon dioxide. The process and the device according to the invention are in particular characterized in that they use and comprise units for the reverse water-gas shift (RWGS), for the separation of carbon dioxide from a gaseous effluent, for the Fischer-Tropsch (FT) synthesis, for the hydroconversion (hydrotreating and / or hydrocracking and / or hydroisomerization) of hydrocarbon cuts resulting from the Fischer-Tropsch reaction unit, and for the conversion of hydrocarbon byproducts from the process (RWGS and / or Fischer-Tropsch synthesis and / or hydroconversion) into synthesis gas. One of the characteristics of the present invention can be epitomized by the use of carbon dioxide for the production of synthetic fuels, gasoline, kerosene, gas oil and / or naphtha, or lubricant bases of very high quality. The present invention is notably based on the presence of a carbon dioxide separation unit to extract the carbon dioxide from the other gaseous compounds of the process and recycle it. The present invention is also based on a reaction unit for the conversion of byproducts into synthesis gas suitable for treating hydrocarbon byproducts to produce a synthesis gas rich in carbon monoxide and hydrogen in order to improve the production of products of interest, in particular a cut comprising paraffins having from 9 to 16 carbon atoms per molecule. Thus, the combination of the units for the conversion of carbon dioxide, for the separation of carbon dioxide, for FT synthesis, for hydroconversion and for the conversion of byproducts into synthesis gas makes it possible to produce bases for fuels, and in particular for fuel for the aviation sector, with a high yield. With reference to figure 1, the process / device for the conversion of carbon dioxide into liquid hydrocarbons uses / comprises: - an RWGS reaction unit 3 suitable for converting, at least partially, the carbon dioxide of a feedstock rich in carbon dioxide 1 with a first hydrogen source 2 and producing a first synthesis gas 4 rich in CO and H2 and a first water effluent 5 produced from the RWGS reaction; - a unit for the separation of carbon dioxide 6 suitable for treating the first synthesis gas 4 and producing a gaseous effluent depleted in carbon dioxide 8 and a gaseous effluent rich in carbon dioxide 7, and recycling the gaseous effluent rich in carbon dioxide 7 at the inlet of the RWGS reaction unit 3; - a Fischer-Tropsch reaction unit 10 suitable for converting the gaseous effluent depleted in carbon dioxide 8, optionally with a second hydrogen source 9, and producing at least a hydrocarbon effluent 11, a first gaseous FT effluent 13 rich in CO, H2, CO2 and hydrocarbons having from 1 to 5 carbon atoms per molecule, optionally a second gaseous FT effluent 16 rich in            hydrocarbons            having            from            1            to 5 carbon atoms per molecule, and a second water effluent 12 produced from the Fischer-Tropsch synthesis resulting from the condensation of water gaseous under the operating conditions of the Fischer-Tropsch reaction; - a hydroconversion reaction unit 19 (hydrotreating and / or hydrocracking and / or hydroisomerization unit) suitable for treating the hydrocarbon effluent 11 with a third hydrogen source 18 and producing at least a third gaseous effluent 22 comprising hydrocarbons having from 1 to 5 carbon atoms per molecule, a second hydrocarbon cut 23 comprising paraffins having from 4 to 8 carbon atoms per molecule, and a first hydrocarbon cut 20 comprising paraffins having from 8 to 22 carbon atoms per molecule, and preferably essentially paraffins having from 9 to 16 carbon atoms per molecule; - a reaction unit for the conversion of byproducts into synthesis gas 24 suitable for converting at least a part 15 of the first gaseous FT effluent 13, the second hydrocarbon cut 23 and optionally a gas stream 17 comprising, at least in part, the second gaseous FT effluent 16 and / or, at least in part, the third gaseous effluent 22, producing a second synthesis gas 25 rich in carbon monoxide and hydrogen, and sending the second synthesis gas 25 to the unit for the separation of carbon dioxide 6. Feedstocks of the process According to one or more embodiments, the feedstock rich in carbon dioxide 1 can originate from a carbon dioxide capture unit fed with gaseous effluents resulting from a refinery, an incinerator, a petrochemical unit, a chemical unit, a thermal power plant, a paper mill, an ethanol plant, a sugar refinery, a cement plant, a lime production unit or blast furnaces. According to one or more embodiments, the feedstock rich in carbon dioxide 1 comprises at least 50% by weight of CO2, preferably at least 70% by weight of CO2, at least 90% by weight of CO2, at least 95% by weight of CO2, such as at least between 50% by weight and 99% by weight of CO2. In practice, the feedstock rich in carbon dioxide 1 can consist of CO2 (e.g. impurities of less than 1000 ppm, indeed even of less than 100 ppm). According to one or more embodiments, the feedstock 1 can originate from a carbon dioxide capture unit fed with biogas, natural gas, synthesis gas produced by gasification of biomass or other fuels, refinery gas, biomass fermentation gas, cement plant gas and blast furnace gas. The carbon dioxide of the feedstock 1 can also be carbon dioxide present in the air. According to one or more embodiments, the feedstock rich in carbon dioxide 1 can originate from a direct air capture (DAC) device and / or from a direct ocean capture (DOC) device. The hydrogen (2, 9, 18) required for the process can be produced by a water electrolysis unit. The electrolyzer technologies used for the water electrolysis are, for example: alkaline electrolysis, proton exchange membrane (PEM) electrolysis, solid oxide electrolysis (SOE) or anion exchange membrane (AEM) electrolysis. The operating conditions (temperature, pressure, nature of the electrolyte, electrodes and diaphragm / membrane) are then specific to each technology: Alkaline electrolyzer: - temperature of between 60°C and 90°C, - pressure of between 0.1 MPa and 20 MPa, preferably between 0.1 MPa and 4 MPa, -     electrolyte comprising KOH, -     electrodes comprising a metal alloy, - diaphragm comprising asbestos, polytetrafluoroethylene and / or nickel oxide; Electrolyzer of proton exchange membrane (PEM) type: -    temperature of between 50°C and 80°C, -    pressure of between 0.1 MPa and 20 MPa, preferably between 1.8 MPa and 5.5 MPa, - electrolyte comprising a polymer membrane, - electrodes comprising a metal alloy; Solid oxide electrolyzer (SOE): - temperature of between 800°C and 900°C, - pressure of between 0.1 MPa and 2 MPa, preferably between 0.1 MPa and 0.5 MPa, - electrolyte comprising a ceramic (e.g. perovskite type) membrane, - electrodes comprising a metal alloy; Electrolyzer of anion exchange membrane (AEM) type: -    temperature of between 50°C and 70°C, -    pressure of between 0.1 MPa and 20 MPa, preferably between 0.1 MPa and 3.5 MPa, - electrolyte comprising a polymer membrane, - electrodes comprising a metal alloy. Reverse water-gas shift (RWGS) reaction unit The RWGS reaction unit 3 produces the first synthesis gas 4, rich in CO, preferably rich in hydrogen, and containing unconverted carbon dioxide, and produces the first water effluent 5. The first hydrogen source 2 required for the RWGS reaction can, for example, originate from a water electrolysis unit. According to one or more embodiments, the RWGS reaction unit 3 comprises at least one reactor used under at least one of the following operating conditions: - temperature of between 700°C and 1200°C, preferentially of between 750°C and 1100°C and more preferentially still of between 780°C and 1050°C; - pressure of between 0.1 MPa and 10 MPa, preferentially of between 0.1 MPa and 5 MPa and more preferentially of between 0.1 MPa and 3.5 MPa; - space velocity of the gas at the inlet of the reactor of between 2000 Sl / kgcata / h and 40 000 Sl / kgcata / h; - catalysts based on the elements Ni, Cu, Fe, Co or precious metals, such as Pt, Pd, Ru, Ag and Au. According to one or more embodiments, the catalyst for the RWGS reaction comprises a support, for example based on alumina, on silica, on silica-alumina or on siliceous alumina. According to one or more embodiments, the amount of hydrogen at the inlet of the RWGS reaction unit is adjusted so that the H2 / CO molar ratio in the first synthesis gas 4 is compatible with the requirement of the Fischer-Tropsch unit, that is to say of between 0.5 and 4, preferably between 1 and 3, more preferably between 1.5 and 2.5. According to one or more embodiments, the temperature of the effluent leaving the RWGS reactor(s) is at least 700°C, preferably at least 750°C, very preferably at least 780°C. According to one or more embodiments, the first synthesis gas 4 comprises less than 2 mol% of water. The first synthesis gas 4 is sent to the unit for the separation of carbon dioxide 6. According to one or more embodiments, the first synthesis gas 4 can be compressed and then optionally cooled before being sent to the unit for the separation of carbon dioxide 6. Unit for the separation of carbon dioxide In the unit for the separation of carbon dioxide 6, the first synthesis gas 4 and the second synthesis gas 25 are treated to produce: - the gaseous effluent depleted in carbon dioxide 8; and - the gaseous effluent rich in carbon dioxide 7, with respect to the carbon dioxide content of the combined synthesis gases 4 and 25. For the separation of the CO2, several agents can be used, such as solvents and solids. Preferably, the unit for the separation of carbon dioxide 6 employs at least one chemical solvent based on amines, and / or at least one physical solvent, such as, for example, based on polyethylene glycol dialkyl ether or on methanol, and / or an item of temperature swing physical adsorption equipment, and / or a membrane. Advantageously, the separation of the carbon dioxide makes it possible to limit the carbon dioxide content in the feedstock of the Fischer-Tropsch reaction unit 10. One widespread technology for the separation of carbon dioxide is based on the phenomenon of absorption, namely the passage of a chemical entity from a gas to a liquid. The gas containing the impurities, or the entity to be separated, is sent to a column where it is brought into contact with a liquid solvent, it being possible for the two streams to be employed according to various hydrodynamic configurations (cocurrent, crosscurrent or countercurrent, the latter solution being preferred for reasons of favorable thermodynamic equilibrium). This absorption is carried out using an absorbent solution, comprising a chemical solvent or a physical solvent, this distinction being linked to whether or not there is a chemical reaction between the component absorbed and the solvent. Physical absorption is preferred with a view to minimizing the energy cost of the process; this is particularly suitable in the case of high partial pressure of the entity to be separated. Chemical absorption is, for its part, preferred in the case of high dilution and low partial pressure of the entity to be separated and / or in the case where a high rate of recovery of this entity is desired or finally if it is desired to achieve a strict specification with regard to the maximum allowable concentration of this entity in the gas stream, once washed. Thus, for the separation of carbon dioxide with a high rate of recovery in order to limit the carbon dioxide content in the Fischer-Tropsch reaction unit 10, and with gaseous effluents having a low value of carbon dioxide concentration, typically of between 3% and 15% by volume, washing by chemical absorption, for example using an amine-comprising solvent, for example an amine of alkanolamine type, is highly suitable. The absorbent solutions commonly used today are aqueous solutions comprising one or more compounds which are reactive or which exhibit a physicochemical affinity with the acid compounds. The reactive compounds can, for example and nonlimitingly, be amines (primary, secondary, tertiary, cyclic or noncyclic, aromatic or nonaromatic, saturated or unsaturated), alkanolamines, polyamines, amino acids, alkali metal salts of amino acids, amides, ureas, phosphates, carbonates or borates of alkali metals. According to one or more embodiments, the absorbent solution is an aqueous solution comprising one or more reactive compounds with an amine function and the structure of which is described from page 6, line 1, to page 7, line 3, of the patent application WO 2007 / 104 856. According to one or more embodiments, the reactive compounds represent from 10% by weight to 90% by weight, preferably between 20% by weight and 50% by weight, very preferably between 25% by weight and 40% by weight, of the total weight of the absorbent solution. Chemical absorption with amine-comprising solvents is based on acid-base equilibria, low temperature favoring the reaction between the basic amine and the acidic carbon dioxide, high temperature favoring the reverse reaction. Thus, processes involving amines, using, for example, an aqueous phase having 20-50% by weight of amine, employ two columns (not represented) in which the solvent circulates from the one to the other. In the first column, called the absorber, the stream to be washed (i.e. the combined synthesis gases 4 and 25) is brought into contact with the amine-comprising solvent at low temperature. The amine-comprising solvent flows in the column and captures the carbon dioxide. At the column bottom, the amine-comprising solvent (“rich” solvent) reaches a predetermined loading rate, the ratio between the number of moles of carbon dioxide captured to the number of moles of amines; at the column top, the gas stream exits, for its part, at predetermined specifications, namely a carbon dioxide content for example close to 10 to 100 times lower than the initial content in the stream to be washed. The rich solvent is sent to the second column, called the regenerator, the operation of which is similar to that of a distillation column, operating at high temperature. The regenerated amine-comprising solvent (“poor” solvent) can, for its part, be sent back to the absorber. The amine-comprising solvent thus circulates continuously in a closed loop from one column to the other, preferably passing through a feedstock / effluent heat exchanger making it possible to cool the poor solvent and to preheat the rich solvent while saving energy at the scale of the process. According to one or more embodiments, the regenerator operates at high temperature of between 90°C and 250°C, preferably between 110°C and 240°C, very preferably between 120°C and 200°C, at the column bottom. The gaseous effluent rich in carbon dioxide 7 released from the regenerator is then sent to the RWGS reaction unit 3. According to one or more embodiments, the gaseous effluent rich in carbon dioxide 7 comprises at least 90 vol% of carbon dioxide, preferably at least 95 vol% of carbon dioxide, very preferably at least 98 vol% of carbon dioxide. According to one or more embodiments, the gaseous effluent rich in carbon dioxide 7 exhibits a temperature of between 20°C and 250°C, preferably between 30°C and 200°C, very preferably between 40°C and 150°C, on leaving the unit for the separation of carbon dioxide 6. According to one or more embodiments, the gaseous effluent rich in carbon dioxide 7 exhibits a pressure of between 0.1 MPa and 4 MPa, preferably between 0.1 MPa and 3.5 MPa, very preferably between 0.1 MPa and 3 MPa, on leaving the unit for the separation of carbon dioxide 6. An essential aspect of the separation of carbon dioxide by solvent is the stage of regeneration of the separating agent. Depending on the type of absorption (physical and / or chemical), regeneration by expansion and / or by distillation and / or by entrainment by a vaporized gas, known as "stripping gas", is generally envisaged. One of the main limitations of the solvents commonly used today is the need to employ high flow rates of absorbent solution, which leads to a high energy consumption for the regeneration of the solvent, but also to large sizes of items of equipment (columns, pumps, and the like). This is particularly true in the case where the carbon dioxide partial pressure is low. Such an energy consumption represents a considerable operating cost for the process for the separation of carbon dioxide. The regeneration energy depends on the nature of the amines and on the carbon dioxide partial pressure and is typically of between 1 GJ / t and 4 GJ / t of carbon dioxide separated. Another possible implementation is based on the principle of adsorption by means of a solid adsorbent exhibiting a strong chemical affinity for carbon dioxide. To ensure a continuous mode of operation, the processes operate with several reactors in parallel. The carbon dioxide is adsorbed on the solid adsorbent and the stream to be treated (i.e. the synthesis gases 4 + 25) becomes depleted as it progresses through the solid bed, and, at the outlet, the stream contains little or no carbon dioxide. However, the solid adsorbent gradually becomes saturated and can no longer adsorb the carbon dioxide. The stream to be treated is then sent to another reactor containing a solid adsorbent which is not saturated with carbon dioxide and the capturing operation continues. In parallel, the reactors saturated with carbon dioxide form the subject of an operation of regeneration: - by a rise in temperature; the reference is then to temperature swing adsorption (TSA); and - by placing under partial vacuum; the reference is then to pressure swing adsorption (or VPSA for vacuum pressure swing adsorption, or simply VSA or PSA, optionally in the presence of a gas which promotes desorption). The barrier to TSA processes is the large amount of heat required for the regeneration. According to one or more embodiments, the solid adsorbent for the capturing of carbon dioxide is chosen from the following compounds: activated carbon, zeolites, aluminas, silicas, synthetic fibers with or without impregnated amines, solids of metal-organic framework (MOF) type, or supported alkaline carbonates. These solid adsorbents are increasingly used in the case of the capturing of carbon dioxide from the air. The energy for regeneration of the adsorbents with physisorption in these cases, for example on zeolites, is of the order of 0.6 to 0.9 GJ / t of carbon dioxide. For solid-supported amines, the regeneration energy is of between 5.4 and 7.2 GJ / t of carbon dioxide. Another possible implementation for the separation of carbon dioxide is membrane separation. Membrane separation processes were initially not widely recommended for post-combustion separation of carbon dioxide, processes of gas-liquid absorption in a chemical solvent being regarded as the most mature and most suitable technology for carrying out this operation. However, the latest technologies make it possible to separate carbon dioxide economically with membranes (dense polymers, inorganic materials, hybrid matrices, liquid membranes). Reference may be made to the review article: Oil Gas Sci. Technol. - Rev. IFP Energies nouvelles, Volume 69, Number 6, November-December 2014. The main performance quality is a rate of capture and a purity of the carbon dioxide which are greater than 90%. The gaseous effluent depleted in carbon dioxide 8 is sent to the Fischer-Tropsch reaction unit 10. The gaseous effluent depleted in carbon dioxide 8 can be compressed before being sent to the Fischer-Tropsch reaction unit 10. The gaseous effluent rich in carbon dioxide 7 is sent to the RWGS reaction unit 3. The gaseous effluent rich in carbon dioxide 7 can be compressed before being sent to the RWGS reaction unit 3. Fischer-Tropsch synthesis reaction unit According to the invention, in the Fischer-Tropsch reaction unit 10, the carbon monoxide and the hydrogen which are present in the gaseous effluent depleted in carbon dioxide 8 react to produce the hydrocarbon effluent 11, the first gaseous FT effluent 13 rich in CO, H2, CO2 and hydrocarbons having from 1 to 5 carbon atoms per molecule, optionally the second gaseous FT effluent 16 rich in hydrocarbons having from 1 to 5 carbon atoms per molecule, and a second water effluent 12. According to one or more embodiments, the gaseous effluent depleted in carbon dioxide 8 sent to the Fischer-Tropsch reaction unit 10 comprises carbon monoxide and hydrogen with an H2 / CO molar ratio of between 0.5 and 4, preferably between 1 and 3, more preferably between 1.5 and 2.5. According to one or more embodiments, the amount of hydrogen upstream (e.g. at the inlet) of the Fischer-Tropsch reaction unit 10 is adjusted, for example by means of the second hydrogen source 9, so that the H2 / CO molar ratio is as defined above. The Fischer-Tropsch reaction unit 10 employs one or more suitable reactors, the technology of which is known to a person skilled in the art. It can concern, for example, one or more multitubular fixed bed reactors, or one or more reactors of slurry bubble column type, or one or more microchannel reactors. According to one or more embodiments, the Fischer-Tropsch reaction unit employs one or more reactors of bubble column type. As the synthesis is highly exothermic, this embodiment makes it possible, inter alia, to improve the thermal control of the reactor and to create only a small pressure drop. The catalyst employed in this Fischer-Tropsch synthesis is generally any catalytic solid known to a person skilled in the art which makes it possible to carry out the Fischer-Tropsch synthesis. According to one or more embodiments, the catalyst employed in the Fischer-Tropsch synthesis comprises cobalt or iron, preferably cobalt. The catalyst employed is generally a supported catalyst. The support can, by way of example, be based on alumina, on silica, on silica-alumina, on siliceous alumina or on titanium. According to one or more embodiments, the Fischer-Tropsch reaction unit 10 comprises at least one reactor used under at least one of the following operating conditions: - temperature of between 170°C and 280°C, preferably between 190°C and 260°C and preferentially between 210°C and 240°C, - absolute pressure of between 1.0 MPa and 6.0 MPa, preferably between 1.5 MPa and 3.5 MPa and preferentially between 2.0 MPa and 3.0 MPa. According to one or more embodiments, the Fischer-Tropsch reaction unit 10 is suitable for producing steam by the vaporization of water (not represented) in a heat exchanger located inside the Fischer-Tropsch reaction unit 10, making it possible to remove the heat from the Fischer-Tropsch reaction, which is an exothermic reaction. According to one or more embodiments, an FT effluent obtained at the outlet of the FT reactor is separated in a first stage, e.g. vapor-liquid-liquid separation, to produce the first gaseous FT effluent 13 and the second water effluent 12 and also a liquid effluent, the latter being separated in a second stage, e.g. by distillation, to produce the hydrocarbon effluent 11 and the second gaseous FT effluent 16. According to one or more embodiments, an FT effluent obtained at the outlet of the FT reactor is separated in one stage to produce the first gaseous FT effluent 13, the second water effluent 12 and also the hydrocarbon effluent 11. According to one or more embodiments, the hydrocarbon effluent 11 comprises: n-paraffins, olefins and oxygen-based compounds resulting from the condensation of hydrocarbons which are gaseous under the operating conditions of the Fischer-Tropsch reaction. According to one or more embodiments, the hydrocarbon effluent 11 comprises hydrocarbon compounds comprising at least 5 carbon atoms per molecule, preferably at least 6 carbon atoms. According to one or more embodiments, the hydrocarbon effluent 11 comprises less than 5% by weight of water, preferably less than 2% by weight of water, very preferably less than 1% by weight of water. According to one or more embodiments, the second water effluent 12 is produced by the Fischer-Tropsch synthesis and results from the condensation of water which is gaseous under the operating conditions of the Fischer-Tropsch reaction. According to one or more embodiments, the first gaseous FT effluent 13 comprises unconverted synthesis gas, carbon dioxide and gaseous hydrocarbons, such as C1 to C5 paraffins (predominant - e.g. at least 50% by weight of the gaseous hydrocarbons), C2 to C5 olefins and C1 to C5 oxygen-based compounds. At least a part 15 of the first gaseous FT effluent 13 is sent to the reaction unit for the conversion of byproducts into synthesis gas 24. According to one or more embodiments, a part 14 of the first gaseous FT effluent 13 is bled off to avoid the accumulation of inert compounds, such as nitrogen, for example, in the process. According to one or more embodiments, at least a part of the second gaseous FT effluent 16 is sent to the reaction unit for the conversion of byproducts into synthesis gas 24, via the gas stream 17. According to one or more embodiments, the gas stream 17 predominantly comprises (i.e., to at least 50% by weight) hydrocarbons, such as paraffins having from 1 to 5 carbon atoms per molecule. According to one or more embodiments, at least a part of the second gaseous FT effluent 16 is extracted from the process, for example to produce energy. Hydroconversion reaction unit (hydrotreating and / or hydrocracking and / or hydroisomerization unit) In the hydroconversion reaction unit 19, the hydrocarbon effluent 11 and the third hydrogen source 18 react to produce at least: - the third gaseous effluent 22 comprising hydrocarbons having from 1 to 5 carbon atoms per molecule, - the second hydrocarbon cut 23 comprising paraffins having from 4 to 8 carbon atoms per molecule, - the first hydrocarbon cut 20 comprising paraffins having from 8 to 22 carbon atoms per molecule, and preferably essentially paraffins having from 9 to 16 carbon atoms per molecule, and - optionally a third water effluent 21. The hydrocarbon effluent 11 is sent to the hydroconversion reaction unit 19 to undergo a hydrotreating and / or hydrocracking and / or hydroisomerization reaction. On leaving the hydroconversion reaction unit 19, the first hydrocarbon cut 20 can be upgraded, in particular to give synthetic fuels, namely gasoline, kerosene, gas oil and / or other hydrocarbon products, such as naphtha, or lubricant bases of very high quality (essentially devoid of sulfur, of aromatics, of nitrogen). One possible option is the production of paraffinic cuts, base products for petrochemical processes, for example the production of a C10-C13 cut intended for the production of linear alkyl benzene (LAB), or also waxes for various industrial applications. According to one or more embodiments, the first hydrocarbon cut 20 predominantly comprises hydrocarbons having from 8 to 22 carbon atoms per molecule (e.g., at least 50% by weight, preferably at least 70% by weight, very preferably at least 90% by weight), such as C9 to C16 paraffins, which are linear and / or branched. According to one or more embodiments, the third gaseous effluent 22 predominantly comprises hydrocarbons having from 1 to 5 carbon atoms per molecule (e.g., at least 50% by weight, preferably at least 70% by weight, very preferably at least 90% by weight), such as C1 to C5 paraffins, which are linear and / or branched. According to one or more embodiments, at least a part of the third gaseous effluent 22 is sent to the reaction unit for the conversion of byproducts into synthesis gas 24 via the gas stream 17. According to one or more embodiments, at least a part of the third gaseous effluent 22 is extracted from the process, for example to produce energy. According to one or more embodiments, the second hydrocarbon cut 23 predominantly comprises hydrocarbons having from 4 to 8 carbon atoms per molecule (e.g., at least 50% by weight, preferably at least 70% by weight, very preferably at least 90% by weight), such as C4 to C8 paraffins, which are linear and / or branched. At least a part of the second hydrocarbon cut 23 is sent to the reaction unit for the conversion of byproducts into synthesis gas 24. According to one or more embodiments, the hydroconversion reaction unit 19 comprises at least one reactor used under at least one of the following operating conditions: - temperature of between 200°C and 500°C, more preferentially between 280°C and 450°C and more preferentially still between 300°C and 420°C; - pressure of between 0.2 MPa and 15 MPa, preferably between 0.5 MPa and 12 MPa and more preferably between 1 MPa and 10 MPa; - space velocity, defined as being the ratio of the flow rate by volume of the feedstock at ambient temperature and ambient pressure to the volume of the catalyst, of between 0.1 h-1 and 10 h-1, preferably between 0.2 h-1 and 7 h-1 and more preferentially between 0.5 h-1 and 5 h-1; - hydrogen flow rate of between 70 and 2000 standard liters of hydrogen per liter of feedstock per hour and preferably between 150 and 1500 standard liters of hydrogen per liter of feedstock and more preferentially between 300 and 1500 standard liters of hydrogen per liter of feedstock. According to one or more embodiments, the hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises at least one hydrogenating-dehydrogenating metal chosen from the group consisting of the metals from group VIB and group VIIIB of the Periodic Table and / or at least one solid which is a Bronsted acid, that is to say a solid which can release one or more protons, and optionally a binder. In the present description, the groups of chemical elements are given, by default, according to the CAS classification (CRC Handbook of Chemistry and Physics, published by CRC Press, Editor-in-Chief D.R. Lide, 81st edition, 2000-2001). For example, group VIIIB according to the CAS classification corresponds to the metals from columns 8, 9 and 10 according to the new IUPAC classification; group VIB according to the CAS classification corresponds to the metals from column 6 according to the new IUPAC classification. According to one or more embodiments, the hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises at least one noble metal from group VIIIB chosen from ruthenium, rhodium, palladium, osmium, iridium and platinum, taken alone or as a mixture, and preferably from platinum and palladium, taken alone or as a mixture, and preferably used in their reduced form. According to one or more embodiments, the hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises: at least one metal chosen from nickel, molybdenum, tungsten, cobalt, ruthenium, indium, palladium and platinum; at least one support chosen from aluminas, boron oxides, magnesias, zirconias, titanium oxides and clays. According to one or more embodiments, the support is chosen from an alumina, silica-alumina, siliceous alumina, silica and zeolite. According to one or more embodiments, the hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises at least one metal from group VIIIB chosen from nickel and cobalt and / or at least one metal from group VIB chosen from molybdenum and tungsten, and preferably used in their sulfide form. According to one or more embodiments, in the case where said hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises at least one noble metal from group VIIIB, the content of noble metal of said catalyst, expressed as element, is of between 0.01% and 5% by weight, with respect to the finished catalyst, preferably between 0.05% and 4% by weight and very preferentially between 0.10% and 2% by weight. According to one or more embodiments, in the case where said hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises at least one metal from group VIB and / or at least one non-noble metal from group VIIIB chosen from nickel and cobalt, the content of metal from group VIB of said catalyst is, as oxide equivalent, of between 5% and 40% by weight, with respect to the finished catalyst, preferably between 10% and 35% by weight, and the content of metal from group VIIIB in said catalyst is, as oxide equivalent, of between 0.5% and 15% by weight, with respect to the finished catalyst, preferably between 1% and 10% by weight, preferably between 1% and 8% by weight and very preferentially between 1.5% and 6% by weight. According to one or more embodiments, the hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprises or consists of at least one noble metal and a support comprising or consisting of at least one zeolite and at least one binder. According to one or more embodiments, the zeolite-based hydrotreating and / or hydrocracking and / or hydroisomerization catalyst is advantageously of bifunctional type, that is to say that it possesses a hydrogenating-dehydrogenating function and a hydroisomerizing function. Reaction unit for the conversion of byproducts into synthesis gas According to the invention, at least a part 15 of the gaseous effluent 13 from the Fischer-Tropsch reaction unit and of the second hydrocarbon cut 23 resulting from the hydroconversion are sent to the reaction unit for the conversion of byproducts into synthesis gas 24, in which the hydrocarbon compounds (i.e. paraffins, olefins, oxygen-based compounds) are converted at least partially, to produce the second synthesis gas 25 comprising (preferably predominantly, indeed even essentially) carbon monoxide and hydrogen. According to one or more embodiments, a fourth water effluent (not represented in the figures) is produced by the reaction unit for the conversion of byproducts into synthesis gas 24. Various catalytic and noncatalytic processes, known to a person skilled in the art, can be used to convert the byproducts present in the first gaseous FT effluent 13 and the second hydrocarbon cut 23 and to produce the second synthesis gas 25: -    steam methane reforming (SMR); -     partial oxidation (POX), which can be catalytic or thermal; -    autothermal reforming (ATR); -    combined reforming (CR); and - dry reforming (Dry Reforming of Methane or DRM). Steam methane reforming (SMR) Steam methane reforming (SMR) consists in reacting a hydrocarbon feedstock (gas, naphtha) over a catalyst in the presence of excess steam to obtain a synthesis gas rich in CO and H2. This reaction is endothermic and equilibrated and can be summarized as 1 CH4 and 1 H2O producing 1 CO and 3 H2 (in the case of methane, for example). Steam methane reforming is typically carried out by circulating the feedstock, to which steam has been added, through tubes filled with catalyst, generally a nickel catalyst, for example comprising from 5% to 30% by weight of nickel deposited on a support comprising mainly (i.e. at least 50% by weight) alumina, or a mixture of alumina and of one or more other refractory compounds. The tubes are typically heated by radiation in tubular furnaces. Excess steam is used to limit carbon deposits and to increase the conversion. This process is highly suitable for gaseous feedstocks, in particular for natural gas, but also for naphtha. The steam methane reforming reaction is promoted by high temperatures and is generally carried out in a furnace. The heat required for the reaction is produced by the combustion with air of a fuel (for example natural gas or the hydrocarbon feedstock) in the steam methane reforming furnace. The synthesis gas obtained by the steam methane reforming reaction contains mainly (i.e. at least 50% by weight) hydrogen, carbon monoxide, carbon dioxide, and also steam and unconverted hydrocarbon feedstock. Steam methane reforming is typically carried out at a temperature of between 600°C and 900°C and at a pressure which can range up to 2.5 MPa but can also be carried out at a temperature of greater than 900°C. The synthesis gas exiting the steam methane reforming furnace is subsequently cooled and the water condensed is separated from the synthesis gas. The cooling of the synthesis gas exiting the steam methane reforming furnace can be carried out in an exchanger which produces steam. Other technical details relating to the steam methane reforming process can be found in the reference work: “Conversion Processes”, P. Leprince, Editions Technip, 2001, Paris 15th arrondissement, pages 455-495. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a steam methane reforming unit and a section for cooling the synthesis gas and for separation of condensed water, the steam methane reforming unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 4 MPa and preferably between 1 MPa and 3 MPa; - temperature of between 600°C and 1500°C, preferably between 700°C and 900°C; and - presence of a catalyst comprising nickel. Partial oxidation (POX) Partial oxidation (POX), sometimes referred to as partial oxidation gasification, is a form of combustion with lower amounts of oxygen than the stoichiometric amount to form carbon dioxide and steam. Partial oxidation is slightly exothermic and can be used as an alternative to steam methane reforming to produce synthesis gas. Partial oxidation can treat light hydrocarbon feedstocks, heavy hydrocarbon feedstocks, asphalts and petroleum coke but also coal and biomass (e.g. wood, green waste, and the like). Partial oxidation is preferably carried out at high temperature (e.g. between 1100°C and 1500°C), pressure between 1 MPa and 9 MPa or more, in the presence of virtually pure oxygen (purity of greater than 98% by volume). The partial oxidation reaction proper corresponds to the reaction between 1 CH4 and 0.5 O2 to produce 1 CO and 2 H2 (in the case of methane, for example). Partial oxidation is a reaction which brings the gas mixture to a temperature of between 1100°C and 1500°C, with preheating beforehand of the feedstock and of the oxygen, for example to 250°C. A high temperature makes it possible to convert virtually all the feedstock and prevents the formation of soot. The amount of oxygen typically used corresponds to 35% of the stoichiometric oxygen required for complete oxidation to CO2 and water. To control the adiabatic flame temperature, which can rise to 1900°C with air and to 2800°C with 95% O2, an inert gas can be used, such as, for example, steam or carbon dioxide. The addition of steam will increase the H2 to CO molar ratio in the synthesis gas produced, whereas the addition of carbon dioxide will reduce the H2 to CO molar ratio in the synthesis gas produced. The synthesis gas exiting the partial oxidation reactor is subsequently cooled and the condensed water is separated from the synthesis gas. The cooling of the synthesis gas exiting the partial oxidation reactor can be carried out in an exchanger which produces steam. The industrial partial oxidation process is well known to a person skilled in the art, which process is marketed by Shell, Texaco, BASF-Lurgi, Air Liquide, and like. See, for example, the chapters: Hydrogen, 2. Production (p. 249, Vol. 18) and Carbon Monoxide (p. 679, Vol. 6), of the encyclopedia: Ullmann’s Encyclopedia of Industrial Chemistry. Other technical details relating to the partial oxidation process can be found in the reference work: “Conversion Processes”, P. Leprince, Editions Technip, 2001, Paris 15th arrondissement, pages 455-495. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a partial oxidation unit preferably operated at the oxygen stoichiometry so as to obtain the oxidation of the mixture of hydrocarbons and so as to have a maximum concentration of carbon monoxide in the second synthesis gas 25. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a partial oxidation unit operated with an oxygen flow rate adjusted to have the desired temperature in the partial oxidation chamber and to minimize the content of light hydrocarbons in the second synthesis gas 25. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a noncatalytic partial oxidation unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the partial oxidation unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 9 MPa and preferably between 1 MPa and 4 MPa; - temperature of between 600°C and 2000°C, preferably between 800°C and 1700°C and preferably between 1100°C and 1500°C; and - presence of oxygen used for the combustion, with an oxygenation rate of between 0.2 and 0.6, preferably between 0.25 and 0.5, in order to promote the formation of carbon monoxide. The oxygenation rate is defined as the ratio of the molar flow rate of injected oxygen with respect to the theoretical oxygen flow rate for complete oxidation of all the hydrocarbons. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a catalytic partial oxidation unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the partial oxidation unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 9 MPa and preferably between 0.1 MPa and 4 MPa; - temperature of between 600°C and 2000°C, preferably between 800°C and 1700°C and preferably between 900°C and 1500°C; and - presence of oxygen used for the combustion, with an oxygenation rate of between 0.2 and 0.6, preferably between 0.25 and 0.5, in order to promote the formation of carbon monoxide. The oxygenation rate is defined as the ratio of the molar flow rate of injected oxygen with respect to the theoretical oxygen flow rate for complete oxidation of all the hydrocarbons; - presence of a catalyst containing a noble metal, such as rhodium, or nickel or a nickel alloy. Autothermal Reforming (ATR) Autothermal reforming (ATR), also known as catalytic partial oxidation, consists of a partial oxidation immediately followed by catalytic steam methane reforming under adiabatic conditions at high temperatures, for example in the outlet temperature range 900°C-1000°C. Autothermal reforming can strictly control the final composition of the synthesis gas by combining a noncatalytic partial oxidation with a catalytic steam methane reforming in a single reactor. It consumes less oxygen than the partial oxidation but requires a catalytic bed. As with steam methane reforming, autothermal reforming can only treat light feedstocks (gas, naphtha). The autothermal reforming reaction corresponds to the reaction between 1 CH4, 0.25 O2 and 0.5 H2O to produce 1 CO and 2.5 H2 (in the case of methane, for example). In the autothermal reforming process, the hydrocarbon feedstock is mixed with steam and then optionally heated in a furnace before being introduced into an autothermal reactor. Oxygen is also introduced into the autothermal reactor. The heat required for the reaction is supplied by the partial combustion of the feedstock with oxygen. The feedstock arrives in the reactor at a pressure typically of between 3 MPa and 10 MPa. The autothermal reforming reactor comprises two successive zones: a first zone corresponding to a combustion chamber and a second zone with a catalytic bed. In the combustion chamber, the partial oxidation of the hydrocarbon feedstock takes place with oxygen burners. In the second zone, the synthesis gas produced in the combustion chamber passes through a catalytic bed in which the steam methane reforming reaction takes place. The synthesis gas exiting the autothermal reforming reactor is subsequently cooled and the condensed water is separated from the synthesis gas. The cooling of the synthesis gas exiting the autothermal reforming reactor can be carried out in an exchanger which produces steam. Other technical details relating to the autothermal reforming processes can be found in the reference work: “Conversion Processes”, P. Leprince, Editions Technip, 2001, Paris 15th arrondissement, pages 455-495. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises an autothermal reforming unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the autothermal reforming unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 10 MPa and preferably between 1 MPa and 4 MPa; - temperature of between 800°C and 1100°C, preferably between 900°C and 1000°C; and - presence of a catalyst based on a noble metal, nickel or nickel alloy. Combined Reforming (CR) Combined reforming (CR), or two-stage reforming, comprises a first tubular steam reformer (steam methane reforming furnace) combined in series with a second autothermal reformer (reforming reactor) into which oxygen is added. A fraction of the feedstock can be sent directly to the second autothermal reformer. As with steam reforming, the feedstock is mixed with steam and then preheated to approximately 500°C before entering the steam methane reforming furnace. The synthesis gas exiting the reforming furnace, which is rich in unconverted feedstock, is mixed with oxygen and sent to the autothermal reforming reactor. The synthesis gas exiting the autothermal reforming reactor has a temperature of between 900°C and 1000°C. The synthesis gas exiting the autothermal reforming reactor is subsequently cooled and the condensed water is separated from the synthesis gas. The cooling of the synthesis gas exiting the autothermal reforming reactor can be carried out in an exchanger which produces steam. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a combined reforming unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the combined reforming unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 10 MPa and preferably between 1 MPa and 4 MPa; - temperature of between 800°C and 1100°C, preferably between 900°C and 1000°C; and - presence of a catalyst based on a noble metal, nickel or nickel alloy. Dry Reforming (DRM) Dry reforming, also known as carbon dioxide reforming, corresponds to the reaction between 1 CH4 and 1 CO2 to produce 2 CO and 2 H2 (in the case of methane, for example). The term “carbon dioxide reforming” originates from the fact that carbon dioxide replaces the steam of conventional steam methane reforming. Dry reforming is particularly advantageous if the targeted synthesis gas is one with a low H2 to CO molar ratio, of approximately 1. Dry reforming operates at a temperature generally of between 900°C and 1000°C and at an operating pressure of between 0.1 MPa and 4 MPa. The catalysts used are catalysts based on a noble metal, nickel or nickel alloy. The synthesis gas exiting the dry reforming reactor is subsequently cooled and the condensed water is separated from the synthesis gas. The cooling of the synthesis gas exiting the dry reforming reactor can be carried out in an exchanger which produces steam. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 comprises a dry reforming unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the dry reforming unit comprising at least one reactor used under at least one of the following operating conditions: - absolute pressure of between 0.1 MPa and 10 MPa and preferably between 0.1 MPa and 4 MPa; - temperature of between 800°C and 1100°C, preferably between 900°C and 1000°C; and - presence of a catalyst based on a noble metal, nickel or nickel alloy. According to one or more embodiments, the second synthesis gas 25 produced in the reaction unit for the conversion of byproducts into synthesis gas 24 exhibits an absolute pressure of between 0.1 MPa and 9 MPa, preferably between 0.1 MPa and 4 MPa. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 supplies the required heat to the RWGS reaction unit 3 and / or to the unit for the separation of carbon dioxide 6 via one or more heat exchanges. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 is suitable for producing steam by (indirect) heat exchange between water (not represented) and an effluent exiting a reactor for the conversion of byproducts into synthesis gas, to optionally supply thermal energy to the unit for the separation of carbon dioxide 6. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 is fed with a stream of oxygen, for example obtained by electrolysis of water or separation from the air. According to one or more embodiments, the reaction unit for the conversion of byproducts into synthesis gas 24 is fed with steam and / or carbon dioxide. Advantageously, the reaction unit for the conversion of byproducts into synthesis gas 24 makes it possible to convert substantially all the hydrocarbon byproducts of the process into CO and thus to upgrade them in the form of the desired products. Thus, the yield of desired products of the process according to the invention is improved. The second synthesis gas 25, on leaving the reaction unit for the conversion of byproducts into synthesis gas 24, is sent to the unit for the separation of carbon dioxide 6. The second synthesis gas 25 can be compressed and then optionally cooled before being sent to the unit for the separation of carbon dioxide 6. Units for the purification of the effluents (optional) According to one or more embodiments, the feedstock rich in carbon dioxide 1 and / or the gaseous effluent rich in carbon dioxide 7 are purified, separately or after mixing, before being introduced into the RWGS reaction unit 3. According to one or more embodiments, the gaseous effluent depleted in carbon dioxide 8 is purified before being introduced into the Fischer-Tropsch reaction unit 10. The stages of purification of the effluents are targeted at removing, at least partially, the sulfur compounds, the nitrogen compounds, the halogens, the heavy metals and the transition metals. The main technologies for purification of the gases are: adsorption, absorption and catalytic reactions. Examples The various examples relate to sequences, in accordance with or not in accordance with the invention, the objective of which is to produce paraffins having from 9 to 16 carbon atoms per molecule starting from a carbon dioxide feedstock, the flow rate of which is 37.3 tonnes per hour. Example 1: not in accordance with the invention Example 1 is not in accordance with the invention according to figure 1. Example 1 illustrates a device for the conversion of CO2 comprising: - an RWGS reaction unit 3, fed with CO2 1 and the first hydrogen source 2, and which produces the first synthesis gas 4 rich in CO and H2 and the first water effluent 5; - a unit for the separation of carbon dioxide 6 which produces the gaseous effluent depleted in CO2 8 and the gaseous effluent rich in CO2 7 starting from the synthesis gases 4 and 25; the gaseous effluent rich in CO2 7 is recycled at the inlet of the RWGS reaction unit 3; - a Fischer-Tropsch reaction unit 10 fed with the gaseous effluent depleted in CO2 8 and the second hydrogen source 9, and which produces the hydrocarbon effluent 11, the second water effluent 12, the first gaseous FT effluent 13 rich in CO, H2, CO2 and hydrocarbons and the second gaseous FT effluent 16 rich in hydrocarbons; - a hydroconversion reaction unit 19 (hydrotreating and / or hydrocracking and / or hydroisomerization unit) fed with the hydrocarbon effluent 11 and the third hydrogen source 18, and which produces the first hydrocarbon cut 20, the third water effluent 21, the third gaseous effluent 22 and the second hydrocarbon cut 23; - a reaction unit for the conversion of byproducts into synthesis gas 24 of partial oxidation type fed with oxygen, with the part 15 of the first gaseous FT effluent 13 and with the gas stream 17 comprising the mixture of the gaseous effluents 16 and 22, and which produces the second synthesis gas 25 which is sent to the inlet of the unit for the separation of carbon dioxide 6. In example 1, the second hydrocarbon cut 23 comprising paraffins having from 4 to 8 carbon atoms per molecule is not sent to the reaction unit for the conversion of byproducts into synthesis gas 24 of partial oxidation type. Table 1 summarizes the inlet and outlet throughputs of the overall process. 5 Table 1 Throughputs (tonnes / h) Feedstocks Products CO2 1 37.3 H2 2 5.5 H2 9 0.5 H2 18 0.1 O2 (not represented in the figure) 6.7 First water effluent 5 16.2 Second water effluent 12 20.4 Third water effluent 21 0.1 Water effluent produced by 24 (not represented in the figure) 1.5 Second hydrocarbon cut 23 (paraffins having from 4 to 8 carbon atoms per molecule) 3.0 First hydrocarbon cut 20 (paraffins having from 9 to 16 carbon atoms per molecule) 8.9 The throughput of paraffins having from 9 to 16 carbon atoms per molecule produced in example 1 is 8.9 tonnes / h, i.e. a yield of 24% by weight with respect to the CO2 feedstock. Example 2: in accordance with the invention Example 2 is in accordance with the invention according to figure 1. Example 2 illustrates a 10 device for the conversion of CO2 comprising: - an RWGS reaction unit 3, fed with CO2 1 and the first hydrogen source 2, and which produces the first synthesis gas 4 rich in CO and H2 and the first water effluent 5; - a unit for the separation of carbon dioxide 6 which produces the gaseous effluent depleted in CO2 8 and the gaseous effluent rich in CO2 7 starting from the synthesis gases 4 15 and 25; the gaseous effluent rich in CO2 7 is recycled at the inlet of the RWGS reaction unit 3; - a Fischer-Tropsch reaction unit 10 fed with the gaseous effluent depleted in CO2 8 and the second hydrogen source 9, and which produces the hydrocarbon effluent 11, the second water effluent 12, the first gaseous FT effluent 13 rich in CO, H2, CO2 and hydrocarbons and the second gaseous FT effluent 16 rich in hydrocarbons; - a hydroconversion reaction unit 19 (hydrotreating and / or hydrocracking and / or hydroisomerization unit) fed with the hydrocarbon effluent 11 and the third hydrogen source 18, and which produces the first hydrocarbon cut 20, the third water effluent 21, the third gaseous effluent 22 and the second hydrocarbon cut 23; 5   - a reaction unit for the conversion of byproducts into synthesis gas 24 of partial oxidation type fed with oxygen, with the part 15 of the first gaseous FT effluent 13, with the gas stream 17 comprising the mixture of the gaseous effluents 16 and 22, and with the second hydrocarbon cut 23, and which produces the second synthesis gas 25 which is sent to the inlet of the unit for the separation of carbon dioxide 6. 10 Unlike example 1, in example 2, the second hydrocarbon cut 23 is sent to the reaction unit for the conversion of byproducts into synthesis gas 24 of partial oxidation type to be converted therein into CO and H2, preferably predominantly into CO and H2, to produce a second synthesis gas 25 comprising at least 50% by weight of CO and H2. Table 2 summarizes the inlet and outlet throughputs of the overall process. 15 Table 2 Throughputs (tonnes / h) Feedstocks Products CO2 1 37.3 H2 2 5.6 H2 9 1.2 H2 18 0.2 O2 (not represented in the figure) 13.2 First water effluent 5 16.6 Second water effluent 12 27.0 Third water effluent 21 0.1 Water effluent produced by 24 (not represented in the figure) 2.0 First hydrocarbon cut 20 (paraffins having from 9 to 16 carbon atoms per molecule) 11.8 The throughput of paraffins having from 9 to 16 carbon atoms per molecule produced in example 2 according to the invention is 11.8 tonnes / h, i.e. an increase of 33% with respect to example 1 and a yield of 32% by weight with respect to the CO2 feedstock.

Claims

1. A process for the conversion of a feedstock containing carbon dioxide, comprising the following stages:- treating a feedstock rich in carbon dioxide (1) with a first hydrogen source (2) in a reverse water-gas shift RWGS reaction unit (3) to produce a first synthesis gas (4) rich in carbon monoxide and H2 and a first water effluent (5);- treating the first synthesis gas (4) in a unit for the separation of carbon dioxide (6) to produce a gaseous effluent depleted in carbon dioxide (8) and a gaseous effluent rich in carbon dioxide (7);- recycling the gaseous effluent rich in carbon dioxide (7) at the inlet of the RWGS reaction unit (3);- converting the gaseous effluent depleted in carbon dioxide (8) in a Fischer-Tropsch reaction unit (10) to produce a hydrocarbon effluent (11), a second water effluent (12), a first gaseous FT effluent (13) rich in CO, H2, CO2 and hydrocarbons having from 1 to 5 carbon atoms per molecule, and optionally a second gaseous FT effluent (16) rich in hydrocarbons having from 1 to 5 carbon atoms per molecule;- treating the hydrocarbon effluent (11) in a hydroconversion reaction unit (19) with a third hydrogen source (18) to produce at least a first hydrocarbon cut (20) comprising paraffins having a carbon number of between 8 and 22, and preferably essentially from 9 to 16 carbon atoms per molecule, a third gaseous effluent (22) comprising hydrocarbons having from 1 to 5 carbon atoms per molecule, and a second hydrocarbon cut (23) comprising paraffins having from 4 to 8 carbon atoms per molecule;- converting at least a part (15) of the first gaseous FT effluent (13) and at least a part of the second hydrocarbon cut (23), in a reaction unit for the conversion of byproducts into synthesis gas (24), to produce a second synthesis gas (25) rich in carbon monoxide and hydrogen; and- sending the second synthesis gas (25) into the unit for the separation of carbon dioxide (6).

2. The process as claimed in claim 1, in which the reaction unit for the conversion of byproducts into synthesis gas (24) is fed with, at least in part, the second gaseous FT effluent (16).

3. The process as claimed in claim 1 or claim 2, in which the reaction unit for the conversion of byproducts into synthesis gas (24) is fed with, at least in part, the third gaseous effluent (22).

4. The process as claimed in any one of the preceding claims, in which the feedstock rich in carbon dioxide (1) and / or the gaseous effluent rich in carbon dioxide (7) are purified, separately or after mixing, before being introduced into the RWGS reaction unit (3).

5. The process as claimed in any one of the preceding claims, in which the gaseous effluent depleted in carbon dioxide (8) is purified before being introduced into the Fischer-Tropsch reaction unit (10).

6. The process as claimed in any one of the preceding claims, in which the RWGS reaction unit (3) comprises at least one reactor used under at least one of the following operating conditions:- temperature of between 700°C and 1200°C, preferentially of between 750°C and 1100°C and more preferentially still of between 780°C and 1050°C;- pressure of between 0.1 MPa and 10 MPa, preferentially of between 0.1 MPa and 5 MPa and more preferentially of between 0.1 MPa and 3.5 MPa;- space velocity of the gas at the inlet of the reactor of between 2000 Sl / kgcata / h and 40 000 Sl / kgcata / h;- catalyst comprising a metal or a combination of metals chosen from the group consisting of the elements Ni, Cu, Fe, Co, Pt, Pd, Ru, Ag and Au.

7. The process as claimed in any one of the preceding claims, in which the unit for the separation of carbon dioxide (6) employs a separation by chemical solvent and / or physical solvent and / or membrane and / or adsorption on a solid.

8. The process as claimed in any one of the preceding claims, in which the Fischer-Tropsch reaction unit (10) comprises at least one reactor used under at least one of the following operating conditions:- temperature of between 170°C and 280°C, preferably between 190°C and 260°C and preferentially between 210°C and 240°C;- absolute pressure of between 1.0 MPa and 6.0 MPa, preferably between 1.5 MPa and 3.5 MPa and preferentially between 2.0 MPa and 3.0 MPa;- catalyst comprising cobalt or iron, preferably cobalt, the catalyst optionally comprising a support, for example based on alumina, on silica, on silica-alumina, on siliceous alumina or on titanium.

9. The process as claimed in any one of the preceding claims, in which the process comprises feeding the Fischer-Tropsch reaction unit (10) with a second hydrogen source (9).

10. The process as claimed in any one of the preceding claims, in which the hydroconversion reaction unit (19) comprises at least one reactor used under at least one of the following operating conditions:- temperature of between 200°C and 500°C, more preferentially between 280°C and 450°C and more preferentially still between 300°C and 420°C;- pressure of between 0.2 MPa and 15 MPa, preferably between 0.5 MPa and 12 MPa and more preferably between 1 MPa and 10 MPa;- space velocity, defined as being the ratio of the flow rate by volume of the feedstock at ambient temperature and ambient pressure to the volume of the catalyst, of between 0.1 h-1 and 10 h-1, preferably between 0.2 h-1 and 7 h-1 and more preferentially between 0.5 h-1 and 5 h-1;- hydrogen flow rate of between 70 and 2000 standard liters of hydrogen per liter of feedstock per hour and preferably between 150 and 1500 standard liters of hydrogen per liter of feedstock and more preferentially between 300 and 1500 standard liters of hydrogen per liter of feedstock;- hydrotreating and / or hydrocracking and / or hydroisomerization catalyst comprising at least one hydrogenating-dehydrogenating metal chosen from the group consisting of the metals from group VIB and group VIIIB of the Periodic Table and / or at least one solid which is a Bronsted acid, that is to say a solid which can release one or more protons, and optionally a binder.

11. The process as claimed in any one of the preceding claims, in which the reaction unit for the conversion of byproducts into synthesis gas (24) comprises at least one reactor used under at least one of the following operating conditions:- absolute pressure of between 0.1 MPa and 9 MPa;- temperature of between 600°C and 2000°C.

12. The process as claimed in any one of the preceding claims, in which the reaction unit for the conversion of byproducts into synthesis gas (24) comprises a catalytic or noncatalytic partial oxidation unit followed by a section for cooling the synthesis gas and for separation of the condensed water, the partial oxidation unit comprising at least one reactor used under at least one of the following operating conditions:- absolute pressure of between 0.1 MPa and 9 MPa and preferably between 0.1 MPa and 4 MPa;- temperature of between 600°C and 2000°C, preferably between 800°C and 1700°C, preferably between 900°C and 1500°C or between 1100°C and 1500°C;- presence of oxygen used for the combustion, with an oxygenation rate of between 0.2 and 0.6, preferably between 0.25 and 0.5, in order to promote the formation of carbon monoxide;- presence of a catalyst containing a noble metal, such as rhodium, or nickel or a nickel alloy, when the partial oxidation unit is catalytic.

13. The process as claimed in any one of the preceding claims, in which a part of the first gaseous FT effluent (13) and / or at least a part of the second gaseous FT effluent (16) are, at least in part, upgraded to give synthetic methane, synthetic natural gas or synthetic liquefied petroleum gas.

14. The process as claimed in any one of the preceding claims, in which a part (14) of the first gaseous FT effluent (13) is bled off.

15. A device for the conversion of a feedstock containing carbon dioxide, comprising the following units:- a reverse water-gas shift RWGS reaction unit (3) suitable for treating a feedstock rich in carbon dioxide (1) with a first hydrogen source (2) and producing a first synthesis gas (4) rich in carbon monoxide and H2 and a first water effluent (5);- a unit for the separation of carbon dioxide (6) suitable for treating the first synthesis gas (4), producing a gaseous effluent depleted in carbon dioxide (8) and a gaseous effluent rich in carbon dioxide (7), and recycling the gaseous effluent rich in carbon dioxide (7) at the inlet of the RWGS reaction unit (3);- a Fischer-Tropsch reaction unit (10) suitable for converting the gaseous effluent depleted in carbon dioxide (8), and producing a hydrocarbon effluent (11), a second water effluent (12), a first gaseous FT effluent (13) rich in CO, H2, CO2 and hydrocarbons having from 1 to 5 carbon atoms per molecule, and optionally a secondgaseous FT effluent (16) rich in hydrocarbons having from 1 to 5 carbon atoms per molecule;- a hydroconversion reaction unit (19) suitable for treating the hydrocarbon effluent (11) with a third hydrogen source (18) and producing at least a first hydrocarbon cut5            (20) comprising paraffins having a carbon number of between 8 and 22, andpreferably essentially from 9 to 16 carbon atoms per molecule, for example to the specifications for transportation applications, a third gaseous effluent (22) comprising hydrocarbons having from 1 to 5 carbon atoms per molecule, and a second hydrocarbon cut (23) comprising paraffins having from 4 to 8 carbon atoms per10            molecule; and- a reaction unit for the conversion of byproducts into synthesis gas (24) suitable for converting at least a part (15) of the first gaseous FT effluent (13), at least a part of the second hydrocarbon cut (23), optionally a gas stream (17) comprising, at least in part, the second gaseous FT effluent (16) and / or, at least in part, the third gaseous15            effluent (22), and producing a second synthesis gas (25) rich in carbon monoxideand hydrogen which is sent to the unit for the separation of carbon dioxide (6).

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