Separation of acrylic acid using membrane contacts

Description

[0001] Acrylic acid is a basic chemical that is produced industrially on a large scale. Among other things, it serves as a monomer for the production of acrylates.

[0002] An overview of the production of acrylic acid and its derived esters is provided by: Ohara, T., Sato, T., Shimizu, N., Prescher, G., Schwind, H., Weiberg, O., Marten, K., Greim, H., Shaffer, TD and Nandi, P. (2021). Acrylic Acid and Derivatives. In Ullmann's Encyclopedia of Industrial Chemistry. https: / / doi.org / 10.1002 / 14356007.a01_161.pub4.

[0003] An industrially important synthesis route involves oxidizing propene to acrolein in a first step and then to acrylic acid in a second step. The oxidation steps can be carried out in separate reactors using different catalysts or integrated in a single reactor using a single catalyst. The reaction typically occurs in the gas phase. The resulting reaction gas contains not only the desired acrylic acid but also many other byproducts. Since these byproducts significantly impair the usability of acrylic acid as a monomer for the production of high-quality acrylates, a key technical development goal in acrylic acid production is to separate the acrylic acid from the reaction gas with the highest possible purity.

[0004] In most cases, the reaction gas is contacted with a liquid absorber (usually water or an organic solvent) so that the acrylic acid is present in solution along with some byproducts. The solution is then distilled in several stages to obtain concentrated acrylic acid (crude acrylic acid - AA). This is then further purified by crystallization processes, resulting in highly pure acrylic acid (glacial acrylic acid - GAA).

[0005] This process has proven itself in decades of industrial practice. An inherent disadvantage is that acrylic acid is extremely reactive and even prone to self-polymerization. Therefore, considerable effort must be made to prevent the acrylic acid from reacting with itself during its purification. This is achieved, among other things, by adding inhibitors that suppress the reaction. However, this is not always completely successful, so polyacrylates can accumulate in the distillation columns over extended periods of operation. To maintain operational reliability, the acrylic acid distillation must be regularly interrupted, the columns emptied, and the polyacrylates manually removed. This is a very labor-intensive process that significantly increases the costs of acrylic acid purification.Furthermore, the distillation of acrylic acid requires a lot of heating energy, which increases production costs and the CO2 footprint of acrylic acid and its derivatives.

[0006] Document DE 2364679 discloses an extraction process for separating acrylic acid from an aqueous medium using a selective membrane. Direct contact between the aqueous medium and the other phase, in which the acrylic acid diffuses, is prevented by the selective membrane. The acrylic acid first diffuses from the aqueous medium into the selective membrane and then into the other phase. The diffusion resistance is relatively high, and diffusion is slow and energy-inefficient. Theoretically, the membrane area could be increased to improve diffusion, but this would result in increased material requirements, higher costs, and a more complex membrane apparatus design.

[0007] There is therefore great interest in making the purification of acrylic acid less complex and at the same time more energy-efficient.

[0008] One way to reduce heating energy consumption is through the use of extraction processes. In extraction processes, an aqueous absorber containing acrylic acid is contacted with an organic extraction medium. This concentrates the acrylic acid in the organic solvent. The organic phase then needs to be separated from the aqueous phase, and finally, the organic phase is distilled to obtain the acrylic acid. An advantage of the described extraction process compared to simple distillation is that less heating energy is required. This is because the solvent typically has a lower enthalpy of vaporization than water. Therefore, the distillative separation of acrylic acid from the solvent is more energy-efficient than separating acrylic acid from water.

[0009] A process for extracting acrylic acid with organic solvents is described in EP1466885A2. A mixture of isopropyl acetate and toluene is used as the extraction medium, and a KARR column is mentioned as the contactor. In KARR columns, the organic and aqueous phases are dispersed by rotating actuators to achieve a large contact area at the phase boundaries. A fundamental disadvantage of KARR columns is the mechanical energy required to rotate the actuators. Furthermore, the moving parts of the KARR column are particularly susceptible to polymers; therefore, KARR columns have relatively frequent shutdown intervals when used for acrylic acid extraction. In addition, a KARR column requires phase separation after extraction.

[0010] Membrane separation is another possible method for separating acrylic acid from aqueous streams. US5635071A describes the separation of carboxylic acids from aqueous streams using a nanofiltration membrane. The process is pressure-driven, preferably at 300 psig transmembrane pressure, and is carried out at a temperature of 40 to 50 °C. Specifically, a mixture of acetic acid and formic acid is separated through the membrane. The membrane material is not specified; only the suitability of a desalination membrane from Desalination Systems Inc. of California, available under the trademark DS-5-DK8040, is mentioned. What this specific membrane material is remains undisclosed. The selection of the membrane material is of crucial interest to a person skilled in the art of membrane technology. In this respect, the disclosure of US5635071A is incomplete.Furthermore, it is unclear whether the desalination membrane is also suitable for the separation of acrylic acid, as this was not investigated in the document. Finally, the temperature at which the nanofiltration is carried out is still quite high, so an increased tendency for polymerization is to be expected. This effect would likely be further amplified if the nanofiltration membrane were impermeable to the inhibitor. In that case, autopolymerization of the acrylic acid on the permeate side is certainly to be expected. As a result, it is doubtful whether the technical teaching disclosed in US5635071A is sufficient to achieve a membrane-based separation of acrylic acid from aqueous solutions.

[0011] In view of this prior art, the invention is based on the objective of providing a method for extracting acrylic acid from aqueous streams which requires little energy and allows for longer operating times.

[0012] This problem is solved by bringing the aqueous stream containing the acrylic acid into contact with a membrane, the side of which facing away from the aqueous stream is exposed to an organic solvent, the membrane being a membrane contactor.

[0013] Such a method is a first subject matter of the invention.

[0014] The invention is based on the fundamental idea that a so-called membrane contactor can be used for the extraction of acrylic acid. A membrane contactor is an apparatus divided into two compartments by a porous membrane: the so-called shell and the so-called lumen. The shell is located on this side of the membrane, while the lumen is located on the other side. Due to the material properties of the membrane, it exhibits different wettability for organic and aqueous media. When the two compartments are exposed to the aqueous and organic media, respectively, the pores of the membrane fill with the phase for which the membrane material has a higher wettability. Due to the surface tension between the two media at the interface, no mixing occurs: the surface tension between the two media prevents any separation.The contact area required for extraction between the two phases is defined by the pores of the membrane. As a result, the membrane contactor enables liquid-to-liquid extraction from the aqueous phase into the organic phase at a defined contact area without mixing the phases.

[0015] The advantage of the membrane contactor is therefore that extraction can be carried out without having to perform a subsequent phase separation.

[0016] Various membrane devices are known, differing in their mass transport mechanism and the membrane used, and can thus be divided into the following three groups: One group utilizes diffuse mass transport through a non-porous membrane to separate substances. This group includes gas permeation, pervaporation, and reverse osmosis. Another group is based on the principle of convective mass transport through a porous membrane, which is used in ultrafiltration and microfiltration. In contrast, membrane contactors represent a third group, based on the principle of diffuse mass transport through a porous membrane. Here, one phase wets the membrane contactor and fills its pores. This brings both phases into direct contact, allowing the substance to be separated to diffuse directly through the pores of the membrane contactor from one phase to the other, without first diffusing into the membrane.This reduces the diffusion resistance, allowing the substance to be separated to diffuse more quickly, or requiring a smaller contact area for separation. The membrane contactor itself serves only to stabilize the phase boundary. Compared to other membrane devices, the membrane contactor does not utilize any specific separation properties of the membrane, such as selectivity or cut-off. Further information on the various devices, and especially on membrane contactors, can be found in: T. Melin, R. Rautenbach: "Membrane Processes - Fundamentals of Module and System Design", 3rd edition, Springer-Verlag Berlin Heidelberg, 2007.

[0017] Suitable membrane contactors are known from WO 2008088293 A1 or EP 3444021 B1 and are also commercially available.

[0018] Preferably, a membrane containing or consisting of porous polytetrafluoroethylene (PTFE) is used. The PTFE may be hydrophilic, but this is not a requirement. PTFE is typically hydrophobic. Generally speaking, it is not important whether a hydrophilic or non-hydrophilic PTFE membrane is used. The only crucial factor in selecting the membrane material is that of the two phases to be brought into contact with the membrane, only one wets the membrane surface. If the two media also possess a surface tension with each other—as is the case with an organic medium on the one hand and an aqueous medium on the other—this prevents breakthrough through the membrane. Breakthrough here refers to the organic phase flowing through the pores into the aqueous phase. This is prevented by the higher pressure in the water and the surface tension.

[0019] Furthermore, it is generally advantageous to select a membrane material that is wettable with the medium in which the diffusion coefficient of the component to be replaced is higher.

[0020] However, the porosity of the membrane material is important. Preferably, the separating material has a porosity of 25% to 75%. Preferably, the porosity is around 50%. The pores should have a diameter between 0.2 µm and 0.4 µm. Preferably, the membrane consists entirely of appropriately porous PTFE.

[0021] Preferably, the membrane is configured as a hollow fiber module. Hollow fiber modules comprise a bundle of multiple hollow fiber membranes connected in parallel. A hollow fiber membrane has the shape of a pipe, with one medium flowing on the lumen side (inner side) and another medium flowing on the outer side (shell). The shell sides of all hollow fibers form a common space, while each hollow fiber has an individual lumen. Preferably, the aqueous flow is located on the lumen side, while the organic solvent flows on the shell side. The reverse is also possible. Preferably, the organic solvent and aqueous flow are directed in opposite directions through the hollow fiber module (countercurrent operation). This improves the maintenance of the concentration gradient.

[0022] One advantage of the extraction process is that it can be carried out at ambient temperature. This saves heating energy compared to distillation. Preferably, the extraction is performed at a temperature of 20°C to 30°C, as heating energy is generally not required at this temperature. Furthermore, the polymerization tendency of acrylic acid is lower at such low temperatures, which increases operational reliability, extends shutdown intervals, and reduces the need for inhibitors. Alternatively, the process can also be carried out at a temperature of 20°C to 50°C.

[0023] An organic solvent is used that readily absorbs acrylic acid but does not attack the membrane. Suitable organic solvents include toluene, n-heptane, isobutyl acetate, n-propyl acetate, isopropyl acetate, 2-pentanone, and methyl isobutyl ketone. Mixtures of these substances can also be used as solvents.

[0024] Preferably, toluene is used as the organic solvent. Toluene fulfills the requirements very well and is readily available as a bulk chemical. Mixing with other substances is therefore unnecessary. Thus, the fresh organic solvent preferably consists of 95% to 100% by weight toluene. The remainder can be impurities.

[0025] It should be clarified that the membrane used in the present process is stationary. This means that no mechanical power is required to move the membrane relative to its surroundings. In this respect, it offers an advantage over a KARR column, which includes moving parts. Furthermore, a stationary membrane is less susceptible to blockages caused by unwanted polymerization. The mechanical power required to circulate the aqueous medium through the membrane is negligible because the aqueous flow must be pumped in any case during a continuous process.

[0026] According to a preferred embodiment of the process, the organic solvent is circulated in a closed loop, the loop comprising the application of the organic solvent to the membrane and the distillation-based separation of the organic solvent from the acrylic acid. The closed-loop toluene recirculation is particularly well-suited for integration into a continuous process for acrylic acid or acrolein production. The toluene is not lost from the system, making the process especially sustainable. This process is particularly suitable for purifying the valuable material acrylic acid from toluene.

[0027] There are several criteria for deciding which medium is routed to the lumen side and which to the shell side. One factor to consider is the interface. The hollow fibers have a larger diameter on the outside than on the inside. Therefore, if the interface is on the outside (wetting medium routed to the lumen side), the interface area is somewhat larger than vice versa. Another aspect is fluid dynamics. The flow is more linear in the lumen than on the shell. For this reason, it makes sense to route the extraction stream through the lumen to prevent backmixing caused by turbulence in the flow with less depleted product before exiting the contactor. In light of these considerations, the organic solvent is preferably routed to the shell side, while the aqueous stream is routed to the lumen side.

[0028] If a hydrophobic membrane is used, it is necessary to force the aqueous flow through the membrane module at a higher pressure than the organic solvent. Specifically, it is preferred to operate the process with a pressure gradient between the aqueous and organic sides of the membrane, where the pressure on the aqueous side is between 1000 Pa and 10000 Pa higher than the pressure on the organic side.

[0029] Such a slight overpressure on the side of the non-wetting medium prevents the wetting liquid from breaking through. Therefore, with a non-hydrophilic membrane, the pressure should always be higher on the aqueous phase side than on the wetting (organic) phase side. When starting the process, one must begin on the aqueous phase side. If one were to start with the wetting phase, it would flow through the pores to the other side of the membrane because there is no counter-pressure. Accordingly, it is irrelevant whether the higher pressure is on the shell side or the lumen side. The only important thing is that the pressure is higher on the non-wetting liquid side than on the wetting side. Incidentally, the pressure gradient is not the driving force. The driving force is the concentration gradient.

[0030] A preferred embodiment of the method according to the invention therefore provides for a pressure gradient between the two sides of the non-hydrophilic membrane, wherein the pressure on the (organic) side facing away from the aqueous flow is between 1000 Pa and 10000 Pa lower than the pressure on the (aqueous) side facing the aqueous flow. If a hydrophilic membrane is used, the pressure gradient must be reversed accordingly.

[0031] An alternative embodiment of the inventive method therefore provides for a pressure gradient that prevails between the two sides of a hydrophilized membrane, wherein the pressure on the (organic) side facing away from the aqueous flow is between 1000 Pa and 10000 Pa higher than the pressure on the (aqueous) side facing the aqueous flow.

[0032] To prevent the undesired polymerization of acrylic acid during extraction, it is advisable to carry out the extraction in the presence of an inhibitor. The inhibitor must be present on both sides of the membrane. Due to the aqueous environment on one side and the organic environment on the other, it is possible to use different inhibitors that are correspondingly more stable in aqueous or organic environments. Accordingly, a further development of the process involves the aqueous stream containing a first inhibitor and the organic solvent containing a second inhibitor, where the first and second inhibitors are either the same or different.

[0033] Suitable first inhibitors in the aqueous stream include hydroquinone (1,4-dihydroxybenzene) and / or hydroquinone monomethyl ether (MEHQ), while 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (4-HT) can be used as a second inhibitor in the organic solvent.

[0034] The extraction method described here can also be combined with classical extraction methods including phase separation, for example, when acrylic acid needs to be separated in quantities for which the performance of the membrane contactors is insufficient. In this case, extraction would first be carried out classically with phase separation, and then, in a second step according to the invention, with a membrane contactor and without phase separation. The second extraction step according to the invention would then represent fine purification. Ultimately, the choice of the appropriate setup is a question of economic efficiency.

[0035] The extraction method described here also works in reverse, namely to extract acrylic acid from an organic stream into an aqueous stream. Surprisingly, the reverse process even works without adjusting the membrane's wettability. It seems that the wettability on each side of the membrane is irrelevant; what matters is that one side of the membrane has a higher wettability for water than for the organic solvent.

[0036] A second object of the invention is therefore a method for extracting acrylic acid from organic streams, characterized in that an organic stream containing acrylic acid is brought into contact with a membrane, the side of which facing away from the organic stream is exposed to an aqueous solvent, characterized in that the membrane is a membrane contactor.

[0037] In the present invention, the membrane is a membrane contactor.

[0038] According to the operating principle of a membrane contactor, the aqueous flow is in direct contact with the organic solvent, and vice versa, within the membrane contactor. Mass transfer therefore occurs directly between the organic phase and the aqueous phase without the intermediary of the membrane material.

[0039] Preferably, the extraction process according to the invention is used in the industrial production of acrylic acid.

[0040] Another object of the invention is therefore a process for the production of acrylic acid, comprising a first reaction step in which propene is reacted with oxygen to form acrolein, a second reaction step in which acrolein is reacted with oxygen to form acrylic acid, an absorption step in which acrylic acid is absorbed into an aqueous stream, and an extraction step in which the acrylic acid is extracted from the aqueous stream into an organic solvent in the manner described herein.

[0041] Specifically, the membrane contactor can be used instead of a KARR column. Alternatively, a combination of an extraction column without energy input and a membrane contactor can be used in series. Furthermore, the membrane contactor can be used in the wastewater streams of acrylic acid production to recover residual acrylic acid using an organic solvent.

[0042] The extraction process according to the invention can also be used in the industrial production of acrolein. A further object of the invention is therefore a process for the production of acrolein, comprising a reaction step in which propene is reacted with oxygen to form acrolein and acrylic acid, an absorption step in which acrylic acid is absorbed into an aqueous stream, and an extraction step in which the acrylic acid is extracted from the aqueous stream into an organic solvent in the manner described herein.

[0043] The membrane contactor can be used particularly in the wastewater streams of acrolein production to recover residual acrylic acid using a solvent.

[0044] Similarly, the process according to the invention can be used in the production of acrylates, in particular butyl acrylate, because aqueous streams containing acrylic acid are also produced in such processes. The acrylic acid can be recovered using the extraction process according to the invention.

[0045] The invention will now be explained in more detail using an exemplary embodiment. For this purpose, we will show: Figure 1: Simplified flow diagram of the experimental setup; Figure 2: Loading profile of acrylic acid in aqueous and organic phases; Figure 3: Height of the sedimentation line versus time after dispersion in the shaking tests (not according to the invention).

[0046] In the application case considered here, acrylic acid is to be extracted from an aqueous stream.

[0047] The extraction of acrylic acid from water using toluene on a PTFE membrane was examined as an example. Specifically, a membrane contactor in the form of a hollow fiber module, obtained from Memo3 GmbH, CH-4313 Möhlin, was used. The reverse application (extraction of acrylic acid from a PTFE-wetting stream using water) is also conceivable. Using a hydrophilized PTFE membrane is also a possibility. The extraction of 60% acrylic acid in water with toluene was successful in the experiments. In these experiments, the two solutions were repeatedly circulated from their reservoir over the contactor at room temperature and then collected back in the same reservoir.

[0048] A simplified process flow diagram of the experimental setup is shown in Figure 1 depicted.

[0049] The central element is the investigated membrane contactor 0. The membrane contactor 0 has no moving parts. A first container 1 holds water with the respective proportion of acrylic acid. A second container 2 contains toluene as an organic solvent. A first pump 3 pumps an aqueous stream containing acrylic acid from the first container 1 into the lumen of the membrane contactor 0. A second pump 4 pumps the organic solvent (toluene) from the second container 2 into the shell of the membrane contactor 0, thus creating an organic stream.

[0050] A first thermostat 5 with a connected heat exchanger sets the temperature of the aqueous stream in the lumen of the membrane contactor 0. A second thermostat 6 with a connected heat exchanger sets the temperature of the organic solvent in the shell of the membrane contactor 0. A third thermostat 7 sets the temperature of the membrane contactor 7 and the two containers 1 and 2. The measuring and control lines of the temperature control are in Figure 1 shown as dotted lines.

[0051] In addition to temperature control using the three thermostats 5, 6, 7, the experimental setup has a flow control system that regulates the pressures and volume flows of the aqueous and organic streams. This is achieved via the respective pumps 3, 4 and numerous valves located in Figure 1The components are shown but not numbered. The associated controllers and sensors are also shown but not individually numbered. The flow control measuring and control lines are in Figure 1 Shown as dashed lines.

[0052] The conduits through which the aqueous or organic current flows are in Figure 1 The aqueous stream containing acrylic acid and the organic solvent flow countercurrently through the membrane contactor 0 and are recycled into their respective containers 1, 2.

[0053] In membrane contactor 0, the acrylic acid is extracted from the aqueous stream into the organic solvent. The acrylic acid migrates from the lumen side to the shell side of the membrane. Acrylic acid is depleted in the water and enriched in the toluene.

[0054] In the first container 1, approximately 1 kg of 60% acrylic acid in water, stabilized with 500 ppm 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (4-HT), was placed and circulated over contactor 0. The aqueous phase was circulated on the lumen side of the contactor, which was designed as a hollow fiber module. On the shell side, toluene, also stabilized with 500 ppm 4-HT, was circulated. The toluene was placed in the second container 2 with approximately 2 L and circulated over the contactor. In membrane contactor 0, the aqueous and organic phases were contacted countercurrently. The experiment was conducted at 25°C. A pressure of approximately 65 mbar (6500 Pa above the pressure on the shell side) was set on the lumen side to prevent the toluene from breaking through. Both containers 1 and 2 were sampled regularly to determine the acrylic acid content in the two solutions.

[0055] In the Figure 2The diagram shows the experimentally determined loadings of the two phases. The mass loading is plotted over the duration of the experiment. The square dots (□) represent the aqueous phase, and the round dots (•) represent the organic phase. The loading for the aqueous phase is shown as a dashed line, calculated using computational interpolation. The loading for the organic phase is shown as a dash-dotted line, calculated using computational interpolation. The calculated equilibrium line for the aqueous phase is shown as a solid line.

[0056] The experiment demonstrated that the reduction of acrylic acid from the water using toluene as the extraction solvent via the membrane contactor occurred within 11 hours, almost reaching the first equilibrium point. During these 11 hours, no decrease in separation efficiency was observed, for example, due to polymerization and a possible associated decrease in membrane porosity. No swelling of the membrane was detected optically. No polymerization occurred. This can be explained by the fact that the experiment was conducted at room temperature and the inhibitors in both solutions prevented polymerization.

[0057] This experiment demonstrated the feasibility of extracting acrylic acid from water using toluene via a membrane contactor under 4-HT inhibition. A mass transfer coefficient was determined. kat a level of 2.4*10⁻⁶ < m / s, whose theoretical concentration profiles of acrylic acid in water and toluene agree with the experimental data only with minor deviations. The theoretical curves determined using the mass transfer coefficient are in Figure 2 The dashed lines represent the values. The solid line shows the determined equilibrium loading in the aqueous phase for each time point.

[0058] Tables 1a to 1d present the most important experimental data. Table 1a shows the setups used in the experiment, Table 1b shows the flow rates during the experiment, and Table 1c contains the general procedural instructions. The experimental results are summarized in Table 1d. Table 1a: Approaches to conducting the experiment toluene AA Water 4HT aqueous phase 60% 40% 500ppm Organic matter 100% 500ppm Table 1b: Flow rates during the experiment flow rate Should kg / h Weight of aqueous phase: 998 g (Target = 1 kg) Lumen 5 Weighing out organic phase 2135 g (Target = 2.1 kg) Shell 5 Table 1c: Procedural rules Test temperature: 25°C Trial duration: 24h Sample interval every 2 hours Sample quantity 10mL for aqueous phase 10mL for organic phase Remark: Have 1L of stopper solution (water with 500ppm 4-HT) ready. Table 1d: Experimental results Trial period PIR301 PIR302 PIR303 Differential pressure Status Status FIR 202 FIR 204 lumen Lumens out Shell one Lumen Shell Density lumens Dense shell hh:mm mbar mbar mbar cm cm g / mL g / mL 1,070 1,01 1,024 0,046 5 15 1,043 0,861 1 1,070 1,01 1,018 0,052 5,3 14,6 1,042 0,862 2 1,055 1,01 1,016 0,039 6,2 14 1,041 0,863 3 1,064 1,01 1,016 0,048 7,2 13,6 1,039 0,864 4 1,061 1,01 1,018 0,043 7,9 13,5 1,039 0,868 5 1,060 1,01 1,018 0,042 8,4 13 1,039 0,870 6 1,064 1,01 1,019 0,045 9 12,8 1,039 0,872 7 1,064 1,01 1,019 0,045 9,3 12,5 1,038 0,873 8 1,060 1,01 1,019 0,041 10 12,5 1,037 0,874 9 1,062 1,01 1,0172 0,045 10,5 12,5 1,03544 0,875 10 1,062 1,01 1,0167 0,045 11 12,5 1,0344 0,876 11 1,060 1,01 1,0169 0,043 11,5 12,5 1,03336 0,877

[0059] For comparison, extraction without a membrane was carried out using shaking experiments. These were performed with different concentrations of acrylic acid in water and toluene as the extraction solvent in a 2:1 ratio (water : toluene). With the same energy input, the two-phase solutions of acrylic acid, water, and toluene (stabilized with 4-HT) were dispersed, and the settling behavior was subsequently observed.

[0060] In the Figure 3The diagram shows the height at which the phase boundary was located plotted against the time after dispersion. The height is relative to the total fill height and is therefore dimensionless. Time is given in seconds. The round dots (∘) refer to the measurement points with 1% acrylic acid (AA), the square dots (□) refer to the measurement points with 10% acrylic acid (AA), and the crossed dots (x) refer to the measurement points with 40% acrylic acid (AA). The respective points form a sedimentation line (not shown).

[0061] The sedimentation rate quadruples when the concentration of acrylic acid used in water is reduced from 40% (0.6 mm / s) to 1% (2.4 mm / s). The corresponding data are shown in Table 2.

[0062] Normally, a significant amount of energy is required to produce droplets in a water-toluene system due to the high interfacial tension. Therefore, KARR columns were used for this extraction task in the prior art – with the disadvantages described earlier. The present shaking experiments have shown that the presence of a high concentration of acrylic acid demonstrably reduces the interfacial tension, allowing droplets to be produced with less energy input. Consequently, the energy input required for dispersion is considerably higher for 1% acrylic acid in water with toluene than for 40% acrylic acid in water with toluene.

[0063] Therefore, a combination of column and membrane contactor can be advantageous, depending on the required energy input. The optimum is determined by an economic analysis. Table 2: Sedimentation rate as a function of the concentration of acrylic acid used in aqueous phase in a phase ratio of aqueous phase to toluene. Density difference aqueous phase - organic phase Sediment velocity [mm / s] 1% acrylic acid 133 2,4 10% acrylic acid 129 1,6 40% acrylic acid 117 0,6 Reference sign

[0064] 0 Membrane contactor 1 First container (Lumen - water / acrylic acid) 2 Second container (Shell - toluene) 3 First pump 4 Second pump 5 First thermostat with heat exchanger 6 Second thermostat with heat exchanger 7 Third thermostat

Claims

1. A method for extracting acrylic acid from aqueous flows, in which an aqueous flow containing acrylic acid is brought into contact with a membrane, the side of which facing away from the aqueous flow is subjected to an organic solvent, characterised in that the membrane is a membrane contactor.

2. The method according to claim 1, characterised in that the membrane contains porous polytetrafluoroethylene or in that the membrane consists of porous polytetrafluoroethylene.

3. The method according to claim 1 or 2, characterised in that the membrane is provided in the form of a hollow fibre module, which is operated in counter flow.

4. The method according to claim 1, 2 or 3, characterised in that it is carried out at a temperature of 20°C to 30°C or of 20°C to 50°C.

5. The method according to any of claim 1 to 4, characterised in that the organic solvent contains at least one substance selected from the group consisting of toluene, n-heptane, isobutyl acetate, n-propyl acetate, isopropyl acetate, 2-pentanone, and methylisobutyl ketone.

6. The method according to claim 5, characterised in that toluene is used as an organic solvent.

7. The method according to one of the preceding claims, characterised in that the membrane is unmoving.

8. The method according to one of the preceding claims, characterised in that the organic solvent is circulated in the cycle, wherein the cycle comprises the application of the organic solvent to the membrane and distillative separation of the organic solvent from the acrylic acid.

9. The method according to one of the preceding claims, characterised in that the organic solvent is moved on the shell side, while the aqueous flow is moved on the lumen side.

10. The method according to one of claims 1 to 9, carried out using a hydrophobic membrane, characterised by a pressure gradient which prevails between the one and the other side of the membrane, wherein the pressure on the side facing away from the aqueous flow is between 1000 Pa and 10 000 Pa lower than the pressure on the side facing toward the aqueous flow.

11. The method according to one of claims 1 to 9, carried out using a hydrophilized membrane, characterised by a pressure gradient which prevails between the one and the other side of the membrane, wherein the pressure on the side facing away from the aqueous flow is between 1000 Pa and 10 000 Pa higher than the pressure on the side facing toward the aqueous flow.

12. The method according to one of the preceding claims; characterised in that the aqueous flow contains a first inhibitor and in that the organic solvent contains a second inhibitor, wherein the first inhibitor and the second inhibitor are the same of different.

13. The method according to claim 1, characterised in that there is at least one direct contact between the aqueous flow and the organic solvent inside the membrane contactor.

14. A method for extracting acrylic acid from organic flows, in which an organic flow containing acrylic acid is brought into contact with a membrane, the side of which facing away from the organic flow is subjected to an aqueous solvent, characterised in that the membrane is a membrane contactor.

15. The method according to claim 14, characterised in that there is at least one direct contact between the organic flow and the aqueous solvent inside the membrane contactor.

16. A method for producing acrylic acid, comprising a first reaction step in which propene is reacted with oxygen to form acrolein, a second reaction step in which acrolein is reacted with oxygen to form acrylic acid, and an absorption step in which acrylic acid is absorbed in an aqueous flow, characterised by an extraction step in which acrylic acid is at least partially extracted from the aqueous flow with a method according to one of claims 1 to 1317. A method for producing acrolein, comprising a reaction step in which propene is reacted with oxygen to from acrolein and acrylic acid and furthermore comprising an absorption step in which acrylic acid is absorbed in an aqueous flow, characterised by an extraction step in which acrylic acid is at least partially extracted from the aqueous flow with a method according to one of claims 1 to 13.