Method for producing butanol from ethanol

Abstract

The invention relates to a method for producing butanol from ethanol, said method comprising the following steps: (i) reacting aqueous ethanol to form acetaldehyde in the presence of a ruthenium oxide catalyst, (ii) reacting the formed acetaldehyde to form a crotonaldehyde, (iii) reacting the formed crotonaldehyde to form butanol. In each of the steps (i), (ii) and (iii), an aqueous phase is present.

Description

[0001] Process for producing butanol from ethanol

[0002] AREA OF INVENTION

[0003] The present invention relates to a process for producing butanol, in particular 1-butanol, from ethanol, for example bioethanol.

[0004] BACKGROUND

[0005] A fundamental process for converting ethanol to butanol is known as Guerbet condensation. It is based on a three-step reaction sequence of ethanol to 1-butanol via acetaldehyde and crotonaldehyde. The individual steps of this three-step reaction sequence are typically carried out in separate reactors, thus requiring three reactors. Guerbet condensation performed in this way achieves only a low selectivity on the order of 60% and an effective yield of only 40%.

[0006] Another process, published by Dow Chemical, directly converts ethanol into butanol. However, this is a highly endothermic process, which is disadvantageous from an energetic point of view.

[0007] There may therefore be a need to optimize the production of butanol from ethanol, particularly with regard to reducing the number of reactors required while simultaneously optimizing resources. It may also be desirable to use bioethanol in the butanol production process.

[0008] SUMMARY OF THE INVENTION The inventors of the present invention have discovered that by adjusting various process parameters, such as temperature, pressure, phase control, gas separation, and / or heat recovery, the three process steps of a conventional Guerbet condensation no longer need to be carried out completely separately from one another. Instead, at least two process steps can be performed in the same reactor, resulting not only in simplified equipment but also in energy savings. In particular, it has proven extremely advantageous for each of the three process steps to contain an aqueous or liquid phase, especially when the reaction conditions (such as pressure and temperature, but also phase control and gas separation) are selected in each process step to ensure the presence of an aqueous or liquid phase.Furthermore, this allows for a special stability of the process, which makes it possible to use bioethanol (which typically has a lower purity than synthetic ethanol) as a starting material.

[0009] The present invention relates accordingly to a process for the production of butanol, in particular 1-butanol, from ethanol, the process comprising the following steps: (i) reaction (in particular oxidation) of aqueous ethanol (mixture of water and ethanol) (with oxygen) to (gaseous) acetaldehyde in the presence of a ruthenium oxide catalyst (ruthenium(IV) oxide, RuOz), (ii) reaction (in particular condensation) of the acetaldehyde formed to a crotonaldehyde (in particular trans-crotonaldehyde, dissolved in the aqueous phase), (iii) reaction (in particular reduction) of the crotonaldehyde formed (with hydrogen) to butanol. In each of steps (i), (ii), and (iii), an aqueous (liquid) phase is present. In particular, in each of steps (i), (ii), and (iii), the reaction conditions (such as pressure and temperature) can be selected such that an aqueous (liquid) phase is present.Further tasks and advantages of embodiments of the present one.

[0010] The invention will become apparent from the following detailed description.

[0011] DETAILED DESCRIPTION OF THE INVENTION

[0012] Further details of the present invention and other embodiments thereof are described below. However, the present invention is not limited to the following detailed description, which merely serves to illustrate the teachings of the invention.

[0013] It should be noted that features described in connection with one exemplary embodiment can be combined with any other exemplary embodiment. In particular, features described in connection with one exemplary embodiment of a method according to the invention can be combined with any other exemplary embodiment of a method according to the invention, and vice versa, unless expressly stated otherwise.

[0014] When a term is used with an indefinite or definite article, such as "ein," "eine," "eines," "der," "die," and "das" in the singular, this also includes the plural form, and vice versa, unless the context clearly indicates otherwise. The expressions "aufweisen" and "umfassen," as used here, do not only include the meaning of "contain" or "include," but can also mean "consist of" and "essentially consist of."

[0015] Unless explicitly stated otherwise, the terms "at least partially" or "at least a part" as used here can mean at least 1% of it, at least 2% of it, at least 5% of it, at least 10% of it, at least 15% of it, at least 20% of it, at least 25% of it, at least 30% of it, at least 35% of it, at least 40% of it, at least 45% of it, at least 50% of it, at least 55% of it, at least 60% of it, at least 65% of it, at least 70% of it, at least 75% of it, at least 80% of it, at least 85% of it, at least 90% of it, at least 95% of it, at least 98% of it, at least 99% of it, and can also mean 100% of it.

[0016] In a first aspect, the present invention relates to a process for producing butanol, in particular 1-butanol, from ethanol. The process comprises three steps:

[0017] (i) The oxidation of aqueous ethanol (with oxygen) to (gaseous) acetaldehyde in the presence of a ruthenium oxide catalyst.

[0018] (ii) The condensation of the acetaldehyde formed in step (i) to crotonaldehyde. Acetic acid formed as a byproduct in step (i) can act as a catalyst. The reaction product acetaldehyde obtained in step (i) therefore does not need to be purified before further use in step (ii); rather, it can even be advantageous if the acetic acid formed as a byproduct remains in the mass stream and catalyzes the condensation of acetaldehyde to crotonaldehyde in step (ii). The crotonaldehyde formed dissolves in the aqueous phase, which has the advantage of largely preventing further condensation to C6 compounds such as hexanoic acid.

[0019] (iii) The reduction of the crotonaldehyde formed (with hydrogen) to butanol. This reaction step, a hydrogenation, can also be carried out using a catalyst, for example a platinum and / or palladium catalyst.

[0020] In each of steps (i), (ii), and (iii), an aqueous or liquid phase is present. This can be achieved, in particular, by a suitable choice of reaction conditions (such as pressure and temperature), for example, by increasing the pressure and / or decreasing the temperature. Phase control and / or gas separation can also contribute to the presence of an aqueous phase. Without wishing to be bound to any theory, the inventors currently assume that

[0021] - the presence of an aqueous phase in step (i) can in particular contribute to the fact that the oxidation of ethanol occurs specifically to acetaldehyde and only to a small extent further to acetic acid;

[0022] - the presence of an aqueous phase in step (ii) can in particular help to prevent further condensation of the crotonaldehyde to C6 compounds such as hexanol to a large extent;

[0023] - the presence of an aqueous phase in step (iii) can counteract in particular the formation of oxyhydrogen gas (explosive gas mixture of hydrogen and oxygen).

[0024] According to an exemplary embodiment, in step (i) the aqueous ethanol is a mixture of ethanol and water with an ethanol content of at least 5 wt.%, in particular at least 10 wt.%, in particular at least 15 wt.%, and in particular at least 20 wt.%. Preferably, the ethanol content in the mixture of ethanol and water is at most 95 wt.%, in particular at most 90 wt.%, in particular at most 85 wt.%, and in particular at most 80 wt.%. The presence of the ethanol to be reacted as aqueous ethanol, in particular in the amounts mentioned above, contributes (together with the process parameters pressure and temperature discussed below) to the fact that in step (i) the ethanol is predominantly oxidized to acetaldehyde and only reacts further to acetic acid to a small extent.However, a certain amount of acetic acid that may be formed during the reaction in step (i) is quite advantageous, since acetic acid can be used as a catalyst for the condensation of the acetaldehyde in step (ii).

[0025] According to an exemplary embodiment, in step (i) the reaction is carried out at a (mean) reaction temperature of less than 290 °C, in particular less than 260 °C, preferably less than 220 °C. Preferably the reaction temperature in step (i) is at least 150 °C.

[0026] According to an exemplary embodiment, in step (i) the reaction is carried out at a pressure above 14 bar, in particular above 17 bar, preferably above 21 bar. However, the pressure in step (i) can also be below 20 bar.

[0027] To prevent the ethanol from being extensively oxidized to acetic acid or even CO2 in step (i), it must be present in aqueous solution. According to the invention, this can be achieved (in contrast to the prior art) by selecting a relatively high pressure and a relatively low temperature for this step. Energy efficiency aspects also play a role, as optimal resource consumption is crucial, especially for a large-scale industrial process. Surprisingly, particularly good yields were achieved when the reaction temperature and pressure were maintained as specified above. These process conditions ensure that an essentially permanent aqueous phase is present.

[0028] According to one exemplary embodiment, the ruthenium oxide catalyst has a porous support. In other words, the ruthenium oxide can be applied to a porous support. The porous support can, for example, comprise titanium oxide (titanium dioxide, TiC₂). It has proven advantageous for the ruthenium oxide catalyst to have a specific surface area in the range of 10 to 30 m². 2 / g (surface area formed per gram of ruthenium oxide), preferably 15 to 20 m² 2 / g, has. The specific surface area can be determined by gas adsorption according to the BET method ISO 9277:2022.

[0029] According to an exemplary embodiment, acetic acid is additionally produced in step (i) during the reaction. By appropriately selecting the process parameters (temperature, pressure, as described above) in step (i), it can be ensured that only a very small amount of acetic acid is formed. This can then be used as a catalyst in the subsequent step (ii).

[0030] According to an exemplary embodiment, in step (i) the reaction is further carried out in the presence of an oxygen promoter, in particular selected from the group consisting of vanadium oxide (V₂O₅), tungsten oxide (WO₃), molybdenum oxide (MoCh), and combinations thereof. The addition of an oxygen promoter in step (i) can support the reaction process, thereby improving the quality and / or the yield.

[0031] According to an exemplary embodiment, in step (ii) the reaction is carried out at a temperature in the range of 280 °C to 420 °C, in particular from 300 °C to 400 °C, and / or at a pressure above 25 bar. These process conditions contribute to a reduction in the formation of C6 compounds, which is particularly advantageous with regard to the production of 1-butanol instead of 2-butanol.

[0032] According to an exemplary embodiment, in step (ii) the reaction is carried out in the presence of a catalyst. The catalyst for the reaction in step (ii) can be selected, in particular, from the group consisting of zeolites, aluminum oxide (Al₂O₃), magnesium oxide (MgO), acetic acid, and combinations thereof. Advantageously, such a catalyst can be used in conjunction with the aforementioned process conditions (temperature, pressure) for step (ii), which can lead to particularly good results. Surprisingly, it has been found that magnesium oxide (MgO) is particularly well suited as a catalyst for the reaction in step (ii). This is remarkable because it is a basic oxide, yet surprisingly, it can very effectively catalyze the condensation of acetaldehyde to crotonaldehyde.According to an exemplary embodiment, in step (ii) the residence time of the mass flow is over 1 minute, in particular over 2 minutes, preferably over 4 minutes. To achieve the highest possible process yield, a sufficiently long residence time of the mass flow in step (ii) is advantageous. On the other hand, a long residence time reduces the throughput. Good results have been achieved when the residence time (i.e., the reaction time in the reaction environment under the defined process conditions) of the mass flow is over 1 minute, in particular over 2 minutes, preferably over 4 minutes.

[0033] According to an exemplary embodiment, in step (ii) the reaction products formed in addition to crotonaldehyde are less than 5000 ppm, in particular less than 3000 ppm, preferably less than 2000 ppm, of C6 compounds (especially hexanol). This can be achieved, for example, by conducting the reaction in step (ii) at a temperature in the range of 280 °C to 420 °C and / or at a pressure above 25 bar, and by the presence of an aqueous phase in which the formed crotonaldehyde can dissolve. The reduction or

[0034] Suppression of the formation of C6 compounds in step (ii) is in turn advantageous with regard to the production of (desired) 1-butanol instead of (undesired) 2-butanol.

[0035] According to an exemplary embodiment, after step (ii) and before step (iii), the oxygen content (in the reaction atmosphere and / or in the aqueous phase) is reduced. Additionally or alternatively, the crotonaldehyde formed in step (ii) is predominantly present in the aqueous phase. This can reduce or completely eliminate the risk of explosion in the subsequent step (iii). Firstly, reducing the oxygen content counteracts the formation of oxyhydrogen gas. Secondly, further protection against excessively rapid or localized overheating (crotonaldehyde is pyrophoric above 200 °C) is achieved because the crotonaldehyde is predominantly present in the aqueous phase, and water has a high heat capacity and a high enthalpy of vaporization.

[0036] According to an exemplary embodiment, in step (iii) the proportion of gaseous oxygen is below 2 vol.%, in particular below 1 vol.%, preferably below 500 ppm, and particularly below 50 ppm. By removing oxygen before step (iii), it can be ensured that the proportion of gaseous oxygen during the reduction of crotonaldehyde to butanol is below 2 vol.%, in particular below 1 vol.%, and preferably below 500 ppm. At such low oxygen concentrations, the formation of oxyhydrogen gas can be largely avoided.

[0037] According to an exemplary embodiment, the reaction in step (iii) is carried out electrocatalytically. In particular, the hydrogen required for the reduction of crotonaldehyde to butanol can be generated electrocatalytically (at the cathode). Additionally, the oxygen required for the oxidation of ethanol in step (i) can also be generated electrocatalytically (at the anode). Thus, an electrocatalytic implementation of step (iii) represents an advantageous alternative to the conventional method of passing hydrogen gas (from a gas cylinder) through the aqueous phase.

[0038] According to an exemplary embodiment, in step (iii) the reaction is carried out in the presence of a catalyst. For this purpose, a platinum catalyst and / or a palladium catalyst can be used in particular, as is common for hydrogenation.

[0039] According to an exemplary embodiment, in step (iii) (in the aqueous phase) there is a water content of more than 35 wt.%, in particular more than 42 wt.%, preferably more than 50 wt.%. For reasons of yield and safety, it is particularly advantageous if a water content of more than 35%, in particular more than 42%, preferably more than 50% is present in step (iii). Here, the stabilizing effect of the high heat capacity of water also comes into play.

[0040] According to an exemplary embodiment, the butanol formed in step (iii) has a 2-butanol content of less than 12 wt.%, in particular less than 8 wt.%, preferably less than 5 wt.%, based on the total butanol formed. With the process according to the invention, particularly by one or more of the exemplary embodiments mentioned above, it can be achieved that the proportion of 2-butanol (compared to the desired 1-butanol) in the resulting butanol is less than 12%, in particular less than 8%, preferably less than 5%. A high yield of 1-butanol is generally desired because

[0041] - 1-Butanol is less toxic than 2-Butanol

[0042] - 1-Butanol is significantly less water-soluble than 2-butanol, thus making it easier to separate from the aqueous phase.

[0043] 2-Butanol has a boiling point approximately the same as that of water, namely 99.5 °C (making distillation difficult or even impossible). 1-Butanol has a boiling point almost 20 °C higher than water, but does not need to be isolated by distillation because it is practically miscible with water (meaning phase separation can be used for isolation).

[0044] According to an exemplary embodiment, the concentration of heavy metals, in particular iron, chromium, and / or their salts or derivatives, in the aqueous phase in each of steps (i), (ii), and (iii) is below 5 ppm, particularly below 3 ppm, preferably below 0.5 ppm. The relatively low temperature in step (i) according to the invention reduces the formation of acids and thus their effects (for example, the leaching of heavy metals from reactor components). This allows for a reduced concentration of heavy metals in the reactor fluid compared to the prior art. According to an exemplary embodiment, the concentrations of heavy metals in steps (i),

[0045] (ii) and (iii) byproducts formed in the mass stream until phase separation of butanol. In other words, no purification, filtration, isolation, etc. of the intermediates or the final product is necessary, which represents a tremendous advantage for industrial production. As mentioned above, a byproduct that is not removed (such as acetic acid in step (i)) can even be advantageously utilized in a subsequent step (for example, the aforementioned acetic acid as a catalyst in step (ii)).

[0046] According to an exemplary embodiment, steps (i), (ii) and

[0047] (iii) carried out in a maximum of two reactors. By adjusting various process parameters, such as temperature, pressure, phase control, and gas separation (as exemplified above), two or more of the process steps (i), (ii), and (iii) can also be carried out in the same reactor, making it possible to perform all steps (i), (ii), and (iii) in a maximum of two reactors or even in just one reactor. This allows for considerable simplification of the equipment and also results in energy savings.

[0048] According to an exemplary embodiment, steps (i) and (ii) are carried out in the same reactor. In particular, the reactor has at least two zones that allow for different reaction conditions. Step (i) can be carried out in one zone, and step (ii) can be carried out in another zone. A different catalyst can be placed in each zone to catalyze the respective reaction. During the transition from the zone in which step (i) is carried out to the zone in which step (ii) is carried out, the temperature can be increased while the pressure can remain approximately constant.

[0049] According to an exemplary embodiment, steps (ii) and (iii) are carried out in the same reactor. In particular, the reactor has at least two zones that allow for different reaction conditions. Step (ii) can be carried out in one zone, and step (iii) can be carried out in another zone. Each zone can contain a different catalyst that catalyzes the respective reaction. During the transition from the zone in which step (ii) is carried out to the zone in which step (iii) is carried out, oxygen can be removed and hydrogen introduced, and the temperature can also be reduced.

[0050] According to one exemplary embodiment, at least one of the reactors has at least two (reaction) zones configured to allow different reaction conditions (compared to each other). For example, the zones can have different temperatures and / or pressures, or different gas atmospheres. Furthermore, different catalysts can be housed in the zones.

[0051] According to an exemplary embodiment, at least one of the reactors comprises a tube, in particular a tube bundle. Such a tube bundle reactor can advantageously be used, in particular, for step (i). The tube (or tubes of the tube bundle) preferably has an outer diameter of less than 100 mm, in particular less than 50 mm, and preferably less than 25 mm. It has been shown that a tube or a tube bundle is suitable as a reaction environment. To ensure sufficiently good coupling to the thermal environment provided outside the tube (e.g., cooling water for cooling or fuel gases for temperature increase), such a tube should not exceed a maximum diameter. Otherwise, the temperature gradient across the cross-section of the tube is so high that the process according to the invention can no longer be implemented stably or with sufficient yield.Good results were achieved when a pipe had an outside diameter of less than 100 mm, in particular less than 50 mm, preferably less than 25 mm.

[0052] According to an exemplary embodiment, the pressure drop across the pipe (along the pipe) is less than 7 bar, in particular less than 5 bar, preferably less than 3 bar. This allows for good process control. The pressure drop can be controlled, for example, by the filling, such as with a catalyst (bed).

[0053] According to an exemplary embodiment, at least one of the reactors is at least partially loaded with a catalyst. In particular, the catalyst material can be granular. The grain diameter can preferably be between 0.3 mm and 3 mm, more particularly between 0.6 mm and 1.2 mm, and preferably between 0.8 mm and 1 mm. This can, for example, facilitate the replacement of the catalyst material (for example, in the case of catalyst contamination due to the addition of insufficiently pure ethanol).

[0054] According to an exemplary embodiment, at least one of the reactors is at least partially loaded with a catalyst having a flow-resistance-reducing shape and / or surface. Advantageously, the catalyst can be substantially spherical. This can facilitate the flow of a mass flow, particularly when a tube in a reactor is at least partially loaded with the catalyst.

[0055] According to an exemplary embodiment, at least a portion of the ethanol is bioethanol. In particular, this can be bioethanol with a methanol content of less than 10 g / l, particularly less than 5 g / l, preferably less than 2 g / l, and / or with a sulfur content of less than 10 ppm, particularly less than 6 ppm, preferably less than 2 ppm. Due to the exceptional stability of the process according to the invention, bioethanol can also be processed. Normally, sulfur can deactivate a catalyst, and methanol can be converted to formic acid, which is highly corrosive to plant components (and bioethanols, in particular, have a higher proportion of these contaminants due to their production by fermentation). The special feature of the process according to the invention is that, in process step (i), even the unavoidable impurities of bioethanol do not produce excessive amounts of formic acid.In addition, the levels of these two impurities can be further reduced using by-processes that are obvious to those skilled in the art. These measures make it possible to process bioethanol without problems if it has a methanol content of less than 10 g / l, in particular less than 5 g / l, preferably less than 2 g / l, and / or a sulfur content of less than 10 ppm, in particular less than 6 ppm, preferably less than 2 ppm.

[0056] According to an exemplary embodiment, the yield of butanol, in particular of 1-butanol, is over 50%, in particular over 70%, preferably over 80%. The process according to the invention can thus even achieve a higher yield than a conventional Guerbet condensation despite the lower equipment requirements and the associated lower costs, which makes the process according to the invention particularly interesting for industrial use in the production of butanol from ethanol (even from bioethanol).

[0057] The present invention has been described with reference to specific embodiments and examples. However, the invention is not limited to these, and various modifications are possible without departing from the scope of the present invention.

Claims

REQUIREMENTS 1. A process for producing butanol from ethanol, comprising the following steps: (i) Reaction of aqueous ethanol to acetaldehyde in the presence of a ruthenium oxide catalyst; (ii) Reaction of the acetaldehyde formed to a crotonaldehyde; (iii) Reaction of the formed crotonaldehyde to butanol, characterized in that an aqueous phase is present in each of steps (i), (ii) and (iii).

2. The method according to claim 1, wherein in step (i) the aqueous ethanol is a mixture of ethanol and water with an ethanol content of at least 5 wt.%, in particular at least 10 wt.%.

3. Method according to claim 1 or 2, wherein in step (i) the reaction is carried out at a reaction temperature of less than 290 °C, in particular less than 260 °C, preferably less than 220 °C.

4. Method according to one of the preceding claims, wherein in step (i) the reaction is carried out at a pressure above 14 bar, in particular above 17 bar, preferably above 21 bar.

5. A method according to any of the preceding claims, wherein the ruthenium oxide catalyst has a porous support, for example comprising titanium oxide, in particular wherein the ruthenium oxide catalyst has a specific surface area in the range of 10 to 30 m². 2 / g, preferably 15 to 20 m 2 / g has, determined by gas adsorption according to BET method ISO 9277:2022.

6. Method according to one of the preceding claims, wherein in step (i) acetic acid is additionally formed during the reaction.

7. Method according to any one of the preceding claims, wherein in step (i) the reaction is further carried out in the presence of an oxygen promoter, in particular selected from the group consisting of vanadium oxide (V2O5), tungsten oxide (WO3), molybdenum oxide (MoCh) and combinations thereof.

8. Method according to any one of the preceding claims, wherein in step (ii) the reaction is carried out at a temperature in the range of 280 °C to 420 °C, in particular from 300 °C to 400 °C, and / or at a pressure above 25 bar.

9. A method according to any of the preceding claims, wherein in step (ii) the reaction is carried out in the presence of a catalyst, in particular selected from the group consisting of zeolites, aluminium oxide (Al2O3), magnesium oxide (MgO), acetic acid and combinations thereof.

10. Method according to one of the preceding claims, wherein in step (ii) the residence time of the mass flow is over 1 minute, in particular over 2 minutes, preferably over 4 minutes.

11. Method according to one of the preceding claims, wherein in step (ii) the reaction products are less than 5000 ppm, in particular less than 3000 ppm, preferably less than 2000 ppm, of C6 compounds, in particular hexanol, in addition to crotonaldehyde.

12. Method according to one of the preceding claims, wherein after step (ii) and before step (iii) the oxygen content is reduced and / or the crotonaldehyde formed in step (ii) is predominantly present in the aqueous phase.

13. Method according to one of the preceding claims, wherein in step (iii) the proportion of gaseous oxygen is below 2 vol.%, in particular below 1 vol.%, preferably below 500 ppm, in particular below 50 ppm.

14. Method according to one of the preceding claims, wherein in step (iii) the reaction is carried out electrocatalytically.

15. Method according to any of the preceding claims, wherein in step (iii) the reaction is carried out in the presence of a catalyst, in particular selected from the group consisting of a platinum catalyst and / or a palladium catalyst.

16. Method according to one of the preceding claims, wherein in step (iii) a water content of more than 35 wt.%, in particular more than 42 wt.%, preferably more than 50 wt.%, is present.

17. Method according to any of the preceding claims, wherein the butanol formed in step (iii) has a 2-butanol content of less than 12%, in particular less than 8%, preferably less than 5%.

18. Method according to one of the preceding claims, wherein in the aqueous phase in each of steps (i), (ii) and (iii) the content of heavy metals, in particular iron, chromium and / or their salts or derivatives, is below 5 ppm, in particular below 3 ppm, preferably below 0.5 ppm.

19. Method according to any of the preceding claims, wherein by-products formed in steps (i), (ii) and (iii) remain in the mass stream until phase separation of butanol.

20. Method according to any of the preceding claims, wherein steps (i), (ii) and (iii) are carried out in a maximum of two reactors.

21. The method of claim 20, wherein steps (i) and (ii) are carried out in the same reactor.

22. The method of claim 20 or claim 21, wherein steps (ii) and (iii) are carried out in the same reactor.

23. A method according to any one of claims 20 to 22, wherein at least one of the reactors has at least two zones configured to allow different reaction conditions.

24. Method according to one of claims 20 to 23, wherein at least one of the reactors comprises a tube, in particular a tube bundle, wherein the tube has an outer diameter of less than 100 mm, in particular less than 50 mm, preferably less than 25 mm.

25. Method according to claim 24, wherein the pressure drop across the pipe is less than 7 bar, in particular less than 5 bar, preferably less than 3 bar.

26. Method according to one of claims 20 to 25, wherein at least one of the reactors is at least partially loaded with a catalyst, in particular wherein the catalyst material is granular, preferably wherein the grain diameter is between 0.3 mm and 3 mm, in particular between 0.6 mm and 1.2 mm, preferably between 0.8 mm and 1 mm.

27. Method according to any one of claims 20 to 26, wherein at least one of the reactors is at least partially loaded with a catalyst having a flow resistance-reducing shape and / or surface, in particular being substantially spherical.

28. Method according to any of the preceding claims, wherein at least part of the ethanol is bioethanol, in particular bioethanol with a methanol content of less than 10 g / l, in particular less than 5 g / l, preferably less than 2 g / l and / or with a sulfur content of less than 10 ppm, in particular less than 6 ppm, preferably less than 2 ppm.

29. A method according to any of the preceding claims, wherein the yield of butanol is greater than 50%, in particular greater than 70%, preferably greater than 80%.