Pretreatment of ft off-gas for recirculation into a conversion reactor

Abstract

The present invention relates to processes for off-gas scrubbing in Fischer-Tropsch syntheses comprising hydrocarbons with a chain length of C12 to C20, preferably C14 to C16, as absorbents.

Description

[0001] D / INEDUE-021-PC November 19, 2025

[0002] Pretreatment of FT exhaust gas for recirculation into a conversion reactor

[0003] All documents cited in the present application are incorporated in their entirety by reference.

[0004] The present invention relates to processes for exhaust gas scrubbing in Fischer-Tropsch syntheses comprising hydrocarbons of chain length C12 to C20 as absorbents.

[0005] State of the art:

[0006] Various processes are used to produce liquid synthetic fuels from renewable electricity, such as Fischer-Tropsch synthesis or methanol synthesis. In Fischer-Tropsch synthesis (FT synthesis), synthesis gas, a mixture of carbon monoxide and hydrogen, is required to induce the chain growth of hydrocarbons, i.e., the formation of carbon-carbon (CC) links. To classify the resulting liquid synthetic fuels as CO2-neutral, it is necessary to use CO2 as the source of carbon in the molecules.The conversion of CO2 to its building block carbon monoxide (CO) can occur either electrochemically in an electrolysis cell or by splitting CO2 into CO and 0.5 O2 in a plasma; alternatively, some of the hydrogen produced by electrolysis can be used to reduce CO2 to CO with the formation of water, according to a conversion reaction known as reverse water gas shift (rWGS). Corresponding processes are described, for example, in US 7,420,004 B2, US 2014 / 0288195 Al, WO 2016 / 162022 Al, or EP 2 049 232 Bl (=WO 2008 / 014854 Al). This process combination is also described in EP 3 678 767 Al, including the recycling of the CO2 gas not converted in the rWGS after the Fischer-Tropsch synthesis with the residual CO components contained therein (not converted in the Fischer-Tropsch synthesis), as well as the gaseous hydrocarbons produced such as methane, ethane, etc.into the conversion reactor to use it as a reformer. EP 3 678 767 Al further describes the use of steam and various other substances (D / INEDUE-021-PC, November 19, 2025).

[0007] High-temperature materials and coating methods for reactor construction are described to reduce both steel carburization and carbon formation. In an alternative process configuration, residual CO from the Fischer-Tropsch synthesis is first converted to CO2 via a reverse reaction (water-gas shift, or WGS) at low temperature (200-300°C) to prevent carbon formation through CO decomposition reactions and the Boudouard reaction. EP 3 678 767 Al also describes the use of flow heaters, which can be used for effective gas preheating and the introduction of the necessary heat of reaction for the conversion of hydrocarbons and CO2. The integrated heat exchange between product gas and reactant gases within the reactor is also described therein.

[0008] In WO 2019 / 175476 Al, the addition of oxygen to the reactant mixture of the rWGS reactor is described. This is also a way to prevent carbon formation, for example through the oxidative conversion of CO and the hydrocarbons contained in the Fisch-Tropsch exhaust gas.

[0009] Literature on ethene production indicates that short-chain unsaturated compounds, in particular, can lead to carbon formation and that these must be removed from recycling streams as completely as possible. For example, F. Joensen & JR Nielsen, Journal of Power Sources 105 (2002), 195-201, report that ethene has the highest tendency to form carbon among a number of other alkenes, alkanes, and isomers, and that ethene can occur as a decomposition product of longer-chain alkanes and alkenes when these are, for example, reacted with steam to produce synthesis gas.

[0010] Further state of the art concerning Fischer-Tropsch syntheses is described, for example, in EP 3 031 884 Bl; US 2023 / 382820 Al; WO 2008 / 115933 Al;

[0011] WO 2018 / 112654 Al; WO 2022 / 079098 Al; WO 2022 / 171643 Al; WO 2022 / 171906 A2; WO 2022 / 225938 A3; WO 2023 / 139258 Al; WO 2024 / 056870 Al; WO 2024 / 110349 Al; WO 2024 / 110371 Al; WO 2024 / 110372 Al; WO 2024 / 110373 Al; WO 2024 / 110402 Al or

[0012] US 2011 / 0306682 Al stands out.

[0013] Finally, WO 2008 / 115933 Al can be cited as a further example of the prior art, which discloses the absorption of methane simultaneously with CO2 or in multi-stage absorption processes. D / INEDUE-021-PC 19 November 2025

[0014] Generally speaking, hydrocarbons can be converted via a steam reforming process. However, this process also requires a high temperature, so problems due to carbon formation on hot surfaces can occur when the gas is heated. It is also known that nickel-containing catalysts, which are also used in the conversion reaction, can catalyze a steam reforming process. Therefore, concepts with electrical heating in the reactor (see EP 3 678 767 Al or WO 2021 / 063795 Al) may be affected by carbon formation from hydrocarbons during prolonged operation. The insulating effect of ceramic components can be reduced by carbon layers.

[0015] From a thermodynamic perspective, hydrocarbon components ranging from CH4 and Cz hydrocarbons to approximately Cio hydrocarbons can be present in the FT exhaust gas even at separation temperatures around 0°C. These hydrocarbons tend to undergo thermally induced carbon formation mechanisms on hot surfaces, similar to pyrolysis. The gas-liquid mixture produced in the FT process is typically cooled through two to three temperature stages, and the generated hydrocarbons are condensed out along with the product water. Due to the residual partial pressure of water and the varying chain lengths of the hydrocarbons, solid residue is to be expected if the temperature drops below the freezing point of water. Therefore, cooling the process product below 0°C and thus achieving complete separation of the hydrocarbons is not feasible.

[0016] Little can currently be calculated in the literature regarding the kinetics and thermodynamics of the decomposition of longer-chain hydrocarbons. However, it is to be expected that hydrocarbons with a chain length of four carbon atoms or more, in particular, may be prone to thermally induced decomposition, especially on hot surfaces.

[0017] Table 1 below shows typical hydrocarbon concentrations expected in the FT exhaust gas at a process pressure of 20-30 bar, a conversion rate of 40-80% in the FT synthesis, and a condensation temperature of 5-10°C, assuming a chain length growth probability of 0.8-0.95 due to vapor pressures. Even with a total product flow rate of approximately 40 kg / h from the synthesis process, a mass flow rate of more than 1 kg / h of C4+ hydrocarbons can result. D / INEDUE-021-PC November 19, 2025

[0018] Table 1:

[0019] Therefore, based on the known state of the art, there is still a considerable need to improve the current state of the art.

[0020] Task:

[0021] The object of the present invention was therefore to overcome the disadvantages of the prior art described above and to provide devices and methods which no longer exhibit these problems, or at least only to a significantly lesser extent, and in particular to find ways to prevent or at least reduce carbon formation in Fischer-Tropsch cycle systems with rWGS or co-electrolysis.

[0022] Further tasks arise for the expert when considering the requirements and from the following description.

[0023] Solution:

[0024] These and other problems that arise for the person skilled in the art from the present description are solved by the items described in the independent claims.

[0025] Preferred and particularly advantageous embodiments are described in the dependent claims and the following description.

[0026] Detailed description of the invention:

[0027] Within the scope of the present invention, all quantities are to be understood as weights unless otherwise specified. D / INEDUE-021-PC November 19, 2025

[0028] Within the scope of the present invention, the term "ambient temperature" means a temperature of 20°C. Unless otherwise specified, temperature readings are in degrees Celsius (°C).

[0029] Unless otherwise stated, the reactions or process steps listed are carried out at ambient pressure (=normal pressure / atmospheric pressure), i.e. at 1013 mbar.

[0030] Pressure specifications within the scope of the present invention, unless otherwise stated, mean relative pressure specifications compared to standard conditions of 1013 mbar, i.e., x bar means x bar gauge and not absolute (bar). a ).

[0031] The first essential object of the present invention is a process for producing synthetic fuels by means of a Fischer-Tropsch synthesis, in particular a Fischer-Tropsch synthesis whose exhaust gases are recycled in a cycle of rWGS or co-electrolysis, comprising a washing step for removing unwanted hydrocarbons from the exhaust gas, in which the exhaust gases of the Fischer-Tropsch synthesis remaining after thermal separation of Fischer-Tropsch products from the product stream of the Fischer-Tropsch synthesis are brought into contact with an absorbent.

[0032] The second essential object of the present invention is a method for reducing carbon formation in the production of synthetic fuels by means of Fischer-Tropsch synthesis, the exhaust gases of which are recycled in a cycle, preferably a rWGS or a co-electrolysis, comprising a washing step for removing unwanted hydrocarbons from the exhaust gas, in which the exhaust gases of the Fischer-Tropsch synthesis remaining after thermal separation of Fischer-Tropsch products from the product stream of the Fischer-Tropsch synthesis are brought into contact with an absorbent.

[0033] Both the first and the second essential subject matter of the present invention, i.e., the two methods mentioned, are characterized in that a hydrocarbon of chain length Ci2bis C is used as the absorbent. 20preferably Ci4 to Ci6, particularly preferably Ci6, especially a mixture of Ci6 isomers, or a mixture of such hydrocarbons. D / INEDUE-021-PC 19 November 2025

[0034] In preferred embodiments of the present invention, the hydrocarbons have a chain length of Ci2bis C 20 , preferably Ci4bis Ci6, branched or linear hydrocarbons are, preferably branched hydrocarbons.

[0035] Examples of hydrocarbons that can be used according to the invention are tetradecane, 2-methyltridecane, 3-methyltridecane, 2-ethyldodecane, 3-ethyldodecane, mixtures thereof, as well as mixtures of these and / or other Ci4 hydrocarbon isomers and Ci6 isomer mixtures.

[0036] The hydrocarbon used is preferably a branched hydrocarbon isomer, but it can also be a linear hydrocarbon or a mixture of isomers and linear alkanes. Branched substances are preferred because the absorption properties for hydrocarbons increase and the pour point and viscosity of the liquid detergent are reduced, which has a positive effect on the mass transfer between the gas and the detergent and the absorption.

[0037] In some preferred embodiments of the present invention, the hydrocarbons used are linear or branched aliphatic hydrocarbons of chain lengths Ci2 to C 20 , preferably Ci4bis Ci6, Ci6 isomer mixtures or mixtures thereof.

[0038] In some preferred embodiments of the present invention, the hydrocarbons used are a mixture of Ci6 isomers.

[0039] In further preferred embodiments of the present invention, the washing step is carried out at a pressure of 5 bar to 30 bar, preferably 7 bar to 20 bar.

[0040] In further preferred embodiments of the present invention, the washing step is carried out at a temperature of 1°C to 10°C, preferably 2°C to 8°C, and in particular 3°C to 6°C.

[0041] In further preferred embodiments of the present invention, the exhaust gas and the absorbent are conducted in countercurrent flow during the washing step. D / INEDUE-021-PC November 19, 2025

[0042] In further preferred embodiments of the present invention, the mass flow ratio (especially in kg / h) of absorbent to hydrogen (i.e. the hydrogen introduced into the synthesis) is between 5 and 75, preferably between 10 and 50, particularly preferably between 20 and 30.

[0043] In particularly preferred embodiments of the first and second essential features of the present invention, the washing step is carried out at a pressure of 5 bar to 30 bar, preferably 7 bar to 20 bar; the washing step is carried out at a temperature of 1°C to 10°C, preferably 2°C to 8°C, particularly 3°C to 6°C; in the washing step, the exhaust gas and the absorbent are passed in countercurrent flow; the mass flow ratio [kg / h] of absorbent to hydrogen is between 10 and 75, preferably between 15 and 50, particularly preferably between 20 and 30; linear or branched aliphatic hydrocarbons of chain lengths C12 to C1 are used as the washing agent (absorbent). 20 , preferably Ci4bis Ci6, Ci6 isomer mixtures or mixtures are used.

[0044] In further particularly preferred embodiments of the first and second essential features of the present invention, the washing step is carried out at an absorption pressure of 5 bar to 9 bar, in particular 7 bar; the washing step is carried out at a temperature of 3°C to 5°C, in particular 5°C; the exhaust gas and the absorbent are carried out in countercurrent flow during the washing step; the mass flow ratio [kg / h] of absorbent to hydrogen is between 20 and 30, in particular 25; a Ci6 isomer mixture is used as the washing agent (absorbent).

[0045] Another essential aspect of the present invention is the use of a hydrocarbon of chain length Ci2bis C 20 , preferably Ci4bis Ci6 or a mixture of such hydrocarbons as an absorbent in a washing step according to D / INEDUE-021-PC 19 November 2025

[0046] Removal of unwanted hydrocarbons from the exhaust gases remaining after thermal separation of a Fischer-Tropsch product from a Fischer-Tropsch synthesis, in particular exhaust gases that are recycled in a circuit to rWGS or co-electrolysis.

[0047] In some configurations, a Ci6 isomer mixture is also preferred.

[0048] In preferred embodiments of the present invention, the washing process is carried out at a pressure of 5 bar to 30 bar, preferably 7 bar to 20 bar, particularly 10 bar to 15 bar, and temperatures of 1°C to 10°C, preferably 2°C to 8°C, particularly 3°C to 6°C, and with a mass flow ratio of absorbent to hydrogen between 10 and 75, preferably between 15 and 50, particularly preferably between 20 and 30, particularly at 25, and the exhaust gas and absorbent are passed in countercurrent flow.

[0049] In further particularly preferred embodiments of the invention, the washing step is carried out at an absorption pressure of 5 bar to 9 bar, in particular 7 bar; the washing step is carried out at a temperature of 3°C to 5°C, in particular 5°C; the exhaust gas and the absorbent are carried in countercurrent flow during the washing step; the mass flow ratio [kg / h] of absorbent to hydrogen is between 20 and 30, in particular 25.

[0050] The solution according to the present invention is therefore essentially based on the insertion of a washing process after the thermal separation of the FT product, wherein a hydrocarbon of chain length C12 to C20, preferably C14 to Ci6, a Ci6 isomer mixture or a mixture of such hydrocarbons is used.

[0051] Furthermore, the hydrocarbon absorption process preferably uses a pressure between that of the FT synthesis and that of the conversion step. Therefore, the pressure is between 5 and 30 bar, preferably between 7 and 20 bar. D / INEDUE-021-PC November 19, 2025

[0052] The pressure is preferably limited upwards within the scope of the present invention, even though a high pressure is desirable for the absorption of hydrocarbons. This is because the absorbent (detergent) also absorbs CO2, which is still contained in the exhaust gas and should be returned to the conversion reactor as completely as possible as an unconverted feedstock.

[0053] In contrast, lowering the pressure below the conversion stage would require further compression.

[0054] Furthermore, carrying out the absorption at 1°C to 10°C is one of the preferred process parameters within the scope of the present invention. Higher temperatures reduce absorption; below 1°C, the detergent pour point and freezing point become critical, and the effort required for refrigeration also increases significantly.

[0055] In some preferred embodiments, the detergent is also a hydrocarbon with a specific number of carbons in the molecule (for example, only Ci4 chain, only Cis chain, or only Ci6 chain); or a hydrocarbon mixture of hydrocarbons with only one specific number of carbons in the molecule (for example, a mixture of only Ci4 isomers, only Cis isomers, or only Ci6 isomers); or a hydrocarbon mixture with a very limited variance in the number of carbons in the molecule, for example, a mixture in which only hydrocarbons with a number of carbons in the molecule from C12 to Ci4, or with a number of carbons in the molecule from C12 to Ci4, or with a number of carbons in the molecule from Ci4 to Cie are present; or a Ci6 isomer mixture.

[0056] This can also be explained by the definition of the process parameters with regard to absorption temperature as well as the regeneration procedure. Thus, after contact with the FT exhaust gas, the detergent is directed into a desorption apparatus, where a temperature increase combined with a pressure reduction leads to the regeneration of the detergent and the release of the hydrocarbons and the also absorbed CO2. By concentrating the detergent into a small C- D / INEDUE-021-PC 19 November 2025

[0057] Within this range, the desorption temperature and pressure can be precisely controlled, minimizing the loss of detergent into the gas phase.

[0058] Furthermore, the countercurrent flow of the detergent with FT exhaust gas is preferred, as is the countercurrent flow of an optional inert gas such as N2, argon, or steam in the desorption process to increase hydrocarbon desorption. However, this increases the effort required for potential CO2 recovery. Therefore, the use of steam near atmospheric conditions would be a preferred stripping agent, as it condenses easily and can thus be separated from the CO2. According to the invention, desorption preferably takes place at a pressure between 0.5 and 5 bar. g preferably between 1 and 3 bar g , and at temperatures between 40°C and 140°C, preferably between 60°C and 120°C.

[0059] According to the invention, both absorption and desorption preferably take place in a bubble column or a packed column in countercurrent flow. In this way, more than one theoretical separation stage is possible, and the absorption rate (amount of absorbed species divided by the amount of species originally present in the gas stream) can be further increased. From the perspective of the increasing CO2 loss compared to the increased separation efficiency, two or even just one theoretical separation stage are preferred according to the invention.

[0060] According to the invention, the mass flow ratio of detergent (absorbent) to hydrogen is preferably between 10 and 75, more preferably between 15 and 50, particularly preferably between 20 and 30, and especially at 25. In this way, a CO2 loss of less than 10 wt.% (based on the amount of CO2 used in the overall process) can be maintained. With a mass flow rate of 500 kg / h of detergent in the cycle, the loss of detergent (absorbent) is preferably in the single digits in g / h. The choice of desorption conditions is crucial for this and can be primarily influenced by the choice of desorption temperature. At higher pressures, the loss can be limited; however, the regeneration of the detergent is then also limited, and thus so is its absorption capacity. D / INEDUE-021-PC November 19, 2025

[0061] In particularly preferred embodiments of the present invention, the absorption for removing unwanted hydrocarbons from the exhaust gas of the Fischer-Tropsch synthesis after the thermal separation of Fischer-Tropsch products is a single-stage absorption; i.e., the processes according to the invention require and comprise only a single absorption stage for removing unwanted hydrocarbons from the exhaust gas of the Fischer-Tropsch synthesis after the thermal separation of Fischer-Tropsch products.

[0062] In particularly preferred embodiments of the present invention, no CC separation is carried out by means of amine absorption.

[0063] The absorbent according to the invention, i.e., a hydrocarbon of chain length C12 to C20, preferably C14 to C16, or a mixture of such hydrocarbons, is used particularly alone and without the addition of other substances. It should be taken into account that technically induced impurities, preferably less than 1 wt.%, particularly preferably less than 0.5 wt.%, may be present, and particularly preferably such impurities are below the detection limit (with commonly used laboratory equipment, not always cutting-edge instruments). Therefore, according to the invention, it is particularly preferred if the absorbent according to the invention, or to be used according to the invention, is not used together with CC absorbents, and in particular not together with amine-containing ones.

[0064] It should be noted again that, within the scope of the present invention, it is preferred if: in the processes according to the invention, the FT exhaust gases are recycled, in particular in a circuit, preferably a rWGS or a co-electrolysis;

[0065] Amine-containing absorbents for CC separation are absent;

[0066] CO2 losses are avoided; on the contrary, the present invention aims not to remove CO2, but to preserve it.

[0067] An important preferred aspect of the present invention is not to remove CO2, but to leave it in the exhaust gas. D / INEDUE-021-PC November 19, 2025

[0068] Another important preferred aspect of the present invention is that the exhaust gases from the Fischer-Tropsch synthesis are each returned to the rWGS or a co-electrolysis.

[0069] The devices and methods according to the invention offer several significant advantages over the prior art, some of which, but not all, are as follows: the carbon-promoting byproducts of Fischer-Tropsch synthesis in the hot temperature zones of the rWGS can be removed from the cycle to a very high extent; the absorption and subsequent desorption of carbon-promoting byproducts present in FT exhaust gas can be carried out very simply and with high efficiency; the methods according to the invention are easy to control and monitor; the inventive approach is superior to systems based on membrane systems, because membrane systems are not applicable and lead to greater losses of valuable substances such as H2, CO, CO2 due to the molecular sizes of the hydrocarbons;The inventive method is superior to systems based on solid adsorbents because solid adsorbents are more difficult and energy-intensive to regenerate, and competitive adsorption of, for example, CO2 or CO occurs, with CO being the more valuable product; in the present invention, the competitive adsorption of CO2 can also be kept constant during load changes of the system (which preferably operates with fluctuating renewable energy for hydrogen production) by adjusting the volume flows.

[0070] Where, in the description of the methods or uses according to the invention, steps or the methods as a whole are characterized as "consisting of," this is to be understood as referring to the essential steps mentioned. Self-evident or inherent steps not explicitly mentioned, such as measuring quantities, parameter measurements / monitoring, heating, cooling, etc., are not thereby excluded. D / INEDUE-021-PC November 19, 2025

[0071] The various embodiments of the present invention, e.g. - but not exclusively - those of the various dependent claims, can be combined with each other in any way, provided that such combinations do not contradict each other.

[0072] Character description:

[0073] The present invention is explained in more detail below with reference to the drawings. The drawings are not to be interpreted as limiting and are not to scale. They are schematic and do not include all the features found in conventional devices, but are reduced to those essential for understanding the present invention. For example, screws, connections, etc., are not shown or not shown in detail. The same reference numerals in the figures, the description, and the claims denote the same features. Explanatory passages that do not refer directly to a specific figure are accordingly not limited to that figure, but should be understood within the context of the entire disclosure.

[0074] Fig. 1 shows a process scheme of the inventive method for producing synthetic fuels by means of a Fischer-Tropsch synthesis.

[0075] Hydrogen (H2), carbon dioxide (CO2), and water are introduced into a rWGS device rW from the left side. The rWGS product is then passed on, and the introduced water (H2O) as well as the water formed by the reaction is removed. The water-depleted product stream (gaseous) is then compressed in a compression stage K and subsequently directed into a Fischer-Tropsch synthesis unit FT, where Fischer-Tropsch synthesis takes place. The synthesis product is then discharged, and the desired target product is condensed in a condensation stage PK. The remaining exhaust gas still contains undesired byproducts (in particular C2 to Cio hydrocarbons); therefore, according to the present invention, the exhaust gas is scrubbed in a scrubbing device W with an absorbent (Ci2-C). 20-hydrocarbon(s)) is brought into contact (scrubbed), which removes these unwanted byproducts from the exhaust gas. The exhaust gas freed of unwanted byproducts is then returned to the rWGS device rW via a recycling line R (long dashed arrow). The absorbent loaded with unwanted byproducts is fed into a regeneration / desorption device D (dashed arrow), D / INEDUE-021-PC November 19, 2025, where it is freed from the byproducts; this can be done using steam (H₂O (g)), as illustrated here, but also with other means or combinations of steam and other means. The absorbent is also recirculated and, after desorption of the byproducts, returned to the scrubbing device W (the absorbent cycle is shown by the dashed arrows).The figure illustrates that water can be removed from the byproduct stream exiting the regeneration / desorption unit D (for example, by condensation), after which the water-depleted byproduct stream is directed to an optional recovery unit O-RG. In this unit, C2-C10 hydrocarbons can be separated from CO2, the latter of which can be returned to the rWGS (regenerative water system). This unit could, for example, be an amine-based CO2 adsorption or absorption device, the design of which is not described further as it is sufficiently known to those skilled in the art. Whether this recovery is required and implemented depends on the size of the plant and the cost of supplying CO2 to the rWGS.

[0076] Reference symbol list:

[0077] C2-C10 hydrocarbons with 2 to 10 carbon atoms in the molecule (undesirable byproducts)

[0078] C12-C20 hydrocarbons with 12 to 20 carbon atoms in the molecule (absorbents)

[0079] D Regeneration of the absorbent / desorption step

[0080] FT Fischer-Tropsch synthesis

[0081] K Compression (of the gases originating from the rWGS and possibly dehydrated)

[0082] O-RG device for CO₂ recovery / separation

[0083] Hydrocarbons (optional)

[0084] PK Condensation of products from Fischer-Tropsch synthesis rW rWGS = reverse water-gas shift reaction

[0085] R Recycling / Recirculation of exhaust gases into the rWGS

[0086] W Washing step / Absorption step D / INEDUE-021-PC November 19, 2025

[0087] The invention will now be further explained with reference to the following non-limiting example.

[0088] An FT process was performed as follows: pressure 20.5 bar gCO conversion was 67% at an average temperature of 209°C. The separation of FT products in the two successive separation stages took place at 178°C and 6°C.

[0089] The following table shows the separation efficiency of the respective species achieved in this example at an absorption pressure of 7 bar and a temperature of 5°C, as well as a desorption temperature of 80°C at 2 bar in a separation stage (concerning absorption and desorption) with a Ci6 isomer mixture.

[0090] Table 2:

[0091] Above 120°C and below 2 bar, the loss of detergent (absorbent) became significant. At higher absorption pressures, CO2 loss increased.

[0092] The detergent flow (absorbent flow) in this example was

[0093] 500 kg / h with an H2 feed stream of 20 kg / h into the power-to-liquid process.

[0094] The mass flow ratio of absorbent to hydrogen was therefore 25.

[0095] In this way, CO2 loss was kept below 10%. The loss of detergent (D / INEDUE-021-PC, November 19, 2025) was 8 g / h with a mass flow rate of 500 kg / h of detergent in the cycle.

[0096] It is evident that the inventive method achieves considerable technical advantages; in particular, the byproducts of Fischer-Tropsch synthesis that promote carbon formation were removed from the cycle to a very high extent, thereby largely preventing carbon formation in the rWGS reactor.

Claims

D / INEDUE-021-PC November 19, 2025 1. A process for producing synthetic fuels by means of a Fischer-Tropsch synthesis comprising a washing step for removing unwanted hydrocarbons from the exhaust gas, in which the exhaust gases of the Fischer-Tropsch synthesis remaining after thermal separation of Fischer-Tropsch products from the product stream of the Fischer-Tropsch synthesis are brought into contact with an absorbent, characterized in that a hydrocarbon of chain length C12 to C20, preferably C14 to C16, or a mixture of such hydrocarbons is used as the absorbent.

2. A method for reducing carbon formation in the production of synthetic fuels by Fischer-Tropsch synthesis, the exhaust gases of which are recycled in a cycle, comprising a washing step for removing unwanted hydrocarbons from the exhaust gas, in which the exhaust gases of the Fischer-Tropsch synthesis remaining after thermal separation of Fischer-Tropsch products from the product stream of the Fischer-Tropsch synthesis are brought into contact with an absorbent, characterized in that a hydrocarbon of chain length C12 to C20, preferably C14 to C16, or a mixture of such hydrocarbons is used as the absorbent.

3. Method according to claim 1 or 2, characterized in that the hydrocarbons of chain length C12 to C20, preferably C14 to C16, are branched or linear hydrocarbons, preferably branched hydrocarbons.

4. Method according to one of claims 1 to 3, characterized in that the washing step is carried out at a pressure of 5 bar to 30 bar, preferably 7 bar to 20 bar. D / INEDUE-021-PC November 19, 2025 5. Method according to one of claims 1 to 4, characterized in that the washing step is carried out at a temperature of 1°C to 10°C, preferably 2°C to 8°C, in particular 3°C to 6°C.

6. Method according to one of claims 1 to 5, characterized in that in the washing step the exhaust gas and the absorbent are carried in countercurrent flow.

7. Method according to any one of claims 1 to 6, characterized in that the mass flow ratio of absorbent to hydrogen is between 5 and 75, preferably between 10 and 50, particularly preferably between 20 and 30, especially at 25.

8. Method according to any one of claims 1 to 7, characterized in that the regeneration of the absorbent takes place at a pressure between 0.5 and 5 bar. g preferably between 1 and 3 bar g , and takes place at temperatures between 40°C and 140°C, preferably between 60°C and 120°C.

9. Use of a hydrocarbon of chain length C12 to C20, preferably Ci4 to Cie, or a mixture of such hydrocarbons as an absorbent, alone and especially not in combination with a CO2 absorbent, in a washing step for the removal of unwanted hydrocarbons from the exhaust gases remaining after thermal separation of a Fischer-Tropsch product of a Fischer-Tropsch synthesis.

10. Use according to claim 9, characterized in that the washing step is carried out at a pressure of 5 bar to 30 bar and temperatures of 1°C to 10°C, as well as a mass flow ratio of absorbent to hydrogen between 10 and 75, and exhaust gas and absorbent are carried out in countercurrent flow, and, optionally, regeneration of the absorbent at a pressure between 0.5 and 5 bar g preferably between 1 and 3 bar g , and takes place at temperatures between 40°C and 140°C, preferably between 60°C and 120°C.

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