Method for purifying acrylic acid, the method involving the parallel use of an extraction unit and a splitting column

Abstract

The invention relates to a method for purifying acrylic acid, the method involving the parallel use of an extraction unit (ExE) and a splitting column (S-Kol), wherein the method has the steps a) to j). Each of the steps a) to j) is precisely described hereinafter in the description. In summary, however, it can be stated that in step a), acrylic acid, as a component of a product gas mixture (PGM), is separated into a crude acrylic acid stream (RAS), a liquid phase stream (FPS) containing acrylic acid, and a secondary component stream (NKS) in a separation column (T-Kol). Then, according to steps b) to f), the liquid phase stream (FPS) is further processed in the extraction unit (ExE), and acrylic acid, as a component of a first circulating gas stream (KGS1), is fed back into the separation column (T-Kol). According to steps b) to f), the secondary component stream (NKS) is further processed in the splitting column (S-Kol), and acrylic acid, as a component of a second circulating gas stream (KGS2), is fed back into the separation column (T-Kol). The actual target-product acrylic acid is a component of the crude acrylic acid stream (RAS), which can optionally be further processed.

Description

[0001] BASF SE 240362W001

[0002] Process for the purification of acrylic acid using a parallel extraction unit and a cracking column

[0003] Description

[0004] The present invention relates to a process for the purification of acrylic acid using an extraction unit (ExE) and a cracking column (S-Kol) in parallel, comprising steps a) to j). The exact definition of each step a) to j) is given below in the description.

[0005] In summary, acrylic acid, as a component of a product gas mixture (PGM), is separated in step a) in a separation column (T-col) into a crude acrylic acid stream (RAS), a liquid phase stream (FPS) containing acrylic acid, and a minor component stream (NCS). Subsequently, according to steps b) to f), the liquid phase stream (FPS) is further processed in the extraction unit (ExE), and acrylic acid is returned to the separation column (T-col) as a component of a first recirculating gas stream (KGS1). According to steps b) to f), the minor component stream (NCS) is further processed in the cracking column (S-col), and acrylic acid is returned to the separation column (T-col) as a component of a second recirculating gas stream (KGS2). The actual target product, acrylic acid, is a component of the crude acrylic acid stream (RAS), which can be further processed if necessary.

[0006] Acrylic acid is an important basic chemical. Due to its highly reactive double bond and acidic properties, it is particularly suitable, especially as a monomer, for the production of polymers. Of the acrylic acid monomers produced, for example, the majority is esterified before polymerization (e.g., to form adhesives, dispersions, or varnishes). Only a smaller portion is polymerized directly (e.g., to form superabsorbents). While high-purity monomers are generally required for the direct polymerization of acrylic acid, the purity requirements are not as stringent when the acrylic acid is esterified prior to polymerization.

[0007] Acrylic acid is obtainable, among other methods, by heterogeneously catalyzed gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen over solid-state catalysts at elevated temperatures. The term "C3 precursors" encompasses chemical compounds that are formally obtainable by the reduction of acrylic acid. In their production, these C3 precursors are diluted in the gaseous state, typically with inert gases such as nitrogen, CO2, saturated hydrocarbons, and / or water vapor, and reacted with molecular oxygen at elevated temperatures and, if necessary, elevated pressure over transition-metal mixed oxide catalysts. EB24-0362PC March 20, 2026 BASF SE 240362W001

[0008] 2

[0009] The product gas mixture is directed and oxidatively converted into an acrylic acid mixture and minor components such as furfurals, benzaldehyde and maleic anhydride, from which the acrylic acid must be separated.

[0010] The acrylic acid obtained is therefore not a pure product, but, as mentioned above, a mixture which, in addition to acrylic acid (usually > 90% or > 95% of the total weight), also contains typical byproducts of gas-phase oxidation such as water, lower aldehydes (e.g. furfurals, acrolein or methacrolein, benzaldehyde), lower carboxylic acids (e.g. acetic acid, propionic acid) etc., as well as oligomers of acrylic acid.

[0011] The formation of acrylic acid oligomers is caused by the fact that acrylic acid in the condensed phase forms acrylic acid oligomers (Michael adducts) through reversible Michael addition to itself and to the resulting dimer, as well as through oligomers formed by radical polymerization. The presence of water, the unavoidable byproduct of gas-phase catalytic oxidative synthesis of acrylic acid, and elevated temperatures promote the formation of acrylic acid oligomers.

[0012] Since the respective oligomers have a higher boiling point than acrylic acid, they accumulate in the high-boiling range (e.g. in the bottom liquid) both during distillative separation of acrylic acid and during fractional condensation of the product gas mixture in a gas-phase catalytic oxidative production.

[0013] In DE 10 2014 114 193 A1 and in the granted EP patent EP-B 3 201 167, a similar process for the production of acrylic acid is described, in which a temperature-controlled stream of mother acid is taken from a crystallization apparatus for obtaining purified acrylic acid and directed towards an absorption column and a cracking column. Additionally, a stream of minor components containing oligomeric acrylic acid is fed to the cracking column, yielding monomeric acrylic acid. Using a stripping gas stream, monomeric acrylic acid, as a component of a recirculating gas stream, is introduced into a quench apparatus, mixed there with a product gas stream, and the resulting mixture is then introduced into the bottom of the absorption column. However, DE 10 2014 114 193 A1 does not disclose a process for the purification of acrylic acid using an extraction unit (ExE) and a cracking column (S-Kol) in parallel.It is also not disclosed there that at least part of a recirculated gas stream taken from a cracking column and containing monomeric acrylic acid is fed into a separation column, wherein at least part of the recirculated gas stream is introduced into the separation column above the point where a product gas mixture is fed into the separation column.

[0014] EB24-0362PCBASF SE 240362W001

[0015] 3

[0016] WO 2008 / 090190 relates to a process for the production of acrylic acid, in which a product gas mixture containing acrylic acid, water vapor and impurities is generated by heterogeneously catalyzed gas-phase partial oxidation of at least one C3 precursor of acrylic acid with molecular oxygen over catalysts in the solid state at elevated temperature, the temperature of the product gas mixture containing acrylic acid, water vapor and impurities is optionally reduced by direct and / or indirect cooling, and the product gas mixture containing acrylic acid, water vapor and impurities is then fed into a condensation column equipped with separation-effective internals, is allowed to rise within the condensation column and thereby fractionally condenses, and is then passed via a first [unclear] located above the feed point of the product gas mixture into the condensation column.The condensing column discharges a side outlet containing water and impurities, depleted (depleted) crude acrylic acid as the target product, and via a second liquid phase outlet (preferably a side outlet) located above the first side outlet, acidic water still containing acrylic acid and impurities, and at the top of the condensing column a residual gas mixture containing impurities that boil at a lower temperature (relative to atmospheric pressure) than water, as well as from the bottom of the condensing column a bottom liquid containing acrylic acid and derivative products and impurities that boil at a higher temperature (relative to atmospheric pressure).A portion of the extracted acidic water is returned to the condensation column as such and / or, after cooling, as a return liquid, and the crude acrylic acid is optionally subjected to at least one further thermal separation process for further purification. However, WO 2008 / 090190 does not disclose a process in which at least a portion of the first recirculating gas stream (KGS1) is introduced into the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col). The corresponding return lines in the process according to WO 2008 / 090190 are generally directed into the sump of the condensation column used there.

[0017] PCT / EP2024 / 077646 relates to a process for the purification of acrylic acid by recirculating gas, comprising steps a) to f). Initially, acrylic acid is a component of a product gas mixture (PGM) which also contains impurities and, optionally, water. In step a), the product gas mixture (PGM) is separated in a separation column (T-col) into a crude acrylic acid stream (RAS) and an impurity stream (NKS). Subsequently, according to step b), the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col). According to step c), at least a portion of the impurity stream (NKS) is fed from the bottom of the column. EB24-0362PCBASF SE 240362W001

[0018] 4

[0019] The process involves transferring the product gas from a separation column (T-col) to a cracking column (S-col). In the cracking column (S-col), according to step d), at least a portion of the by-product stream (NCS) is cracked to obtain monomeric acrylic acid. According to step e), the monomeric acrylic acid is withdrawn as part of a recirculated gas stream (CGS) at the top of the cracking column (S-col). In step f), at least a portion of the recirculated gas stream (CGS) is fed into the separation column (T-col), with at least a portion of the recirculated gas stream (CGS) being introduced into the separation column (T-col) above the point where the product gas mixture (PGM) is fed into the separation column (T-col). However, PCT / EP2024 / 077646 does not disclose a process for the purification of acrylic acid using an extraction unit (ExE) and a cracking column (S-col) in parallel.

[0020] The state-of-the-art processes are advantageous in themselves and lead to corresponding yields of the valuable product, i.e., acrylic acid. However, these increased yields are only achieved through significant equipment and energy input.

[0021] Against this background, the object of the present invention is to specify a process for the purification of acrylic acid and to provide a corresponding plant which will further increase the purity of the valuable product compared to the prior art, but at the same time enable more efficient process control with regard to the equipment and energy expenditure.

[0022] The above problem is solved in a first aspect of the present invention by a process for purifying acrylic acid, comprising steps a) to j):

[0023] a) Separation of a product gas mixture (PGM) containing acrylic acid and by-products in a separation column (T-col) into a crude acrylic acid stream (RAS), a liquid phase stream (FPS) containing acrylic acid and a by-product stream (NKS),

[0024] b) Withdrawal of the crude acrylic acid stream (RAS) and the liquid phase stream (FPS) from the separation column (T Kol), wherein the withdrawal point of the liquid phase stream (FPS) is located above the withdrawal point of the crude acrylic acid stream (RAS) in the separation column (T Kol),

[0025] c) Feeding at least a portion of the liquid phase stream (FPS) containing acrylic acid and a first stripping gas stream (SGS1) into the bottom of an extraction unit (ExE),

[0026] EB24-0362PCBASF SE 240362W001

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[0028] d) Extraction of at least a part of the acrylic acid contained in the liquid phase stream (FPS) in the extraction unit (ExE) to obtain a first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid,

[0029] e) Extraction of the first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid at the head of the extraction unit (ExE),

[0030] f) Feeding at least part of the first recirculating gas stream (KGS1) taken in step e) into the separation column (T-col), wherein at least part of the first recirculating gas stream (KGS1) is introduced into the separation column (T-col) above the point where the raw _, acrylic acid stream (RAS) is taken from the separation column (T-col),

[0031] g) Feeding at least part of the by-product stream (VPC) from the bottom of the separation column (T-col) into a cracking column (S-col),

[0032] h) Splitting at least part of the by-product stream (NCS) in the cracking column (S-col) to obtain monomeric acrylic acid,

[0033] i) Supplying a second stripping gas stream (SGS2) at the bottom of the cracking column (S-col) and withdrawing a second recirculating gas stream (KGS2), containing the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col),

[0034] j) Feeding the second recirculating gas stream (KGS2) taken in step i) into the separation column (T-col), wherein the second recirculating gas stream (KGS2) may, if necessary, first be brought into contact completely or at least partially in a device (V2) with at least one other stream before being fed into the separation column (T-col).

[0035] The aforementioned problem is solved in a second aspect of the present invention by a system for the purification of acrylic acid, comprising

[0036] a separation column (T-col)

[0037] a splitting column (S-col)

[0038] an extraction unit (ExE)

[0039] a first line (L1) connecting the bottom of the separation column (T-col) with the cracking column (S-col)

[0040] a second line (L2) extending from the head of the splitting column (S-col), wherein the second line (L2) splits into two lines (L2-a and L2-b), wherein the line (L2b) is connected to the separation column (T-col) EB24-0362PCBASF SE 240362W001

[0041] 6

[0042] a third line (L3) connecting the separation column (T-col) and the bottom of the extraction unit (ExE) is connected and preferably opens into the separation column (T-col) above the feed point for the product gas mixture (PGM), wherein the third line (L3) opens into the separation column (T-col) above the withdrawal point of the crude acrylic acid stream (RAS),

[0043] A fourth line (L4) extends from the head of the extraction unit (ExE), wherein the fourth line (L4) enters the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is drawn off, and wherein the third line (L3) preferably enters the separation column (T-col) above the point where the fourth line (L4) enters the separation column (T-col).

[0044] The present invention has the advantage, due to the parallel use of an extraction unit and a cracking column, that in the process according to the invention for the purification of acrylic acid, acrylic acid, i.e. valuable product, can be recovered simultaneously in steps c) to e) or in steps g) to i).

[0045] This has the further advantage that the process according to the invention can still be carried out if, for example, the section using the cracked column cannot be used because this section is undergoing maintenance, so that purification / recovery of acrylic acid by the section using the extraction unit is still possible. The same applies, of course, to the reverse case where the extraction unit cannot be used and only the cracked column is functioning.

[0046] Another advantage of the method according to the invention is that, due to the parallel use of an extraction unit and a stripping column, the gas flows used, in particular the stripping gas flows, have more capacity to absorb acrylic acid compared to a serial arrangement of these device elements.

[0047] Furthermore, the acrylic acid contained in the crude acrylic acid stream (RAS) can be obtained in a higher purity, so that further processing steps are no longer strictly necessary. This effect is achieved in particular by introducing at least a portion of the first recirculating gas stream (KGS1) into the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col).

[0048] The present invention has the further advantage that in embodiments in which, in step j), at least a part of the second EB24-0362PCBASF SE 240362W001 extracted in step i)

[0049] 7

[0050] By introducing the recirculating gas stream (KGS2) below the point in the separation column (T-Kol) where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-Kol), the proportion of impurities with compounds that have a higher boiling point than the acrylic acid, and in particular the proportion of benzaldehyde and maleic anhydride in the crude acrylic acid stream (RAS) and thus also in any purified acrylic acid (AAS) obtained, can be significantly reduced.

[0051] A further advantage of the process according to the invention is that, if at least part of the mother acid (MS) or optionally the mother acid stream (MS-a) is recycled into the separation column (T-Kol), above the point in the separation column (T-Kol) from which the crude acrylic acid stream (RAS) is taken, the by-products such as low-boiling acetic acid or other optionally present low-boiling by-products can escape more readily towards the top of the separation column (T-Kol), and as a result, the discharge of the crude acrylic acid stream (RAS) towards the device (V1) contains less acetic acid or other low-boiling by-products as impurities.This also results in less acetic acid and low-boiling by-products and more acrylic acid being present in the apparatus (V1) and in the purified acrylic acid (AAS) compared to a prior art process in which the feed of the mother acid (MS) into the separation column (T-col) takes place below the point in the separation column (T-col) from which the crude acrylic acid stream (RAS) is taken.

[0052] A further significant advantage of the present invention is therefore that, with the inventive process and the inventive apparatus, by-products, in particular oligomeric acrylic acid, which arise during the production of acrylic acid, can be broken down and recycled into the production process with improved efficiency, thereby increasing the yield of the valuable product, i.e., acrylic acid, and its purity.

[0053] The present invention is described in detail below.

[0054] If process features are mentioned in the following description in connection with the system according to the invention, these preferably refer to the process according to the invention. Likewise, material features listed in connection with the process according to the invention preferably refer to the system according to the invention.

[0055] In a first aspect of the present invention, the aforementioned problem is solved by a process for purifying acrylic acid, comprising steps a) to j):

[0056] EB24-0362PCBASF SE 240362W001

[0057] 8

[0058] a) Separation of a product gas mixture (PGM) containing acrylic acid and by-products in a separation column (T-col) into a crude acrylic acid stream (RAS), a liquid phase stream (FPS) containing acrylic acid and a by-product stream (NKS),

[0059] b) Withdrawal of the crude acrylic acid stream (RAS) and the liquid phase stream (FPS) from the separation column (T Kol), wherein the withdrawal point of the liquid phase stream (FPS) is located above the withdrawal point of the crude acrylic acid stream (RAS) in the separation column (T Kol),

[0060] c) Feeding at least a portion of the liquid phase stream (FPS) containing acrylic acid and a first stripping gas stream (SGS1) into the bottom of an extraction unit (ExE),

[0061] d) Extraction of at least a part of the acrylic acid contained in the liquid phase stream (FPS) in the extraction unit (ExE) to obtain a first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid,

[0062] e) Extraction of the first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid at the head of the extraction unit (ExE),

[0063] f) Feeding at least part of the first recirculating gas stream (KGS1) withdrawn in step e) into the separation column (T-col), wherein at least part of the first recirculating gas stream (KGS1) is introduced into the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col),

[0064] g) Feeding at least part of the by-product stream (VPC) from the bottom of the separation column (T-col) into a cracking column (S-col),

[0065] h) Splitting at least part of the by-product stream (NCS) in the cracking column (S-col) to obtain monomeric acrylic acid,

[0066] i) Supplying a second stripping gas stream (SGS2) at the bottom of the cracking column (S-col) and withdrawing a second recirculating gas stream (KGS2), containing the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col),

[0067] j) Feeding the second recirculating gas stream (KGS2) taken in step i) into the separation column (T-col), wherein the second recirculating gas stream (KGS2) EB24-0362PCBASF SE 240362W001

[0068] 9

[0069] If necessary, before being fed into the separation column (T-col), it is first brought into contact completely or at least partially in a device (V2) with at least one other current.

[0070] The method according to the invention comprises steps a) to j) and is described below.

[0071] Figure 1 shows the process according to the invention in its basic form, wherein in step a) a product gas mixture (PGM) containing acrylic acid and impurities, and optionally water, is introduced into a separation column (T-col) and separated into a crude acrylic acid stream (RAS), a liquid phase stream (FPS) containing acrylic acid, and an impurity component stream (NKS). The crude acrylic acid stream (RAS) is entirely depleted of impurities and optionally water, whereas the impurity component stream (NKS) contains impurities with higher boiling points than acrylic acid.

[0072] The liquid phase stream (FPS) contains, in addition to acrylic acid, water and optionally other minor components. These minor components of the liquid phase stream (FPS) are preferably compounds with lower boiling points than acrylic acid. Preferably, the liquid phase stream (FPS) contains at least 60 wt% water, more preferably at least 80 wt% water, 2 wt% to 10 wt% acrylic acid, 2 wt% to 10 wt% acetic acid, and 0 wt% to 10 wt% formaldehyde, as well as optionally 0 wt% to 10 wt% other compounds.

[0073] Typically, the liquid phase stream (FPS) contains less acrylic acid (in wt%) relative to the other components of the respective stream compared to the crude acrylic acid stream (RAS). Preferably, the ratio of acrylic acid in the liquid phase stream (FPS) to acrylic acid in the crude acrylic acid stream (RAS) is 2 to 10 to 90 to 99 [wt% / wt%].

[0074] Product gas mixtures from acrylic acid production are known to those skilled in the art. Typically, such product gas mixtures contain acrylic acid, diacrylic acid, and polyacrylic acid, as well as, for example, acetic acid, water, nitrogen, and oxygen.

[0075] In the context of the present invention, the product gas mixture (PGM) preferably contains acrylic acid and accessory components, and optionally water, wherein accessory components include, for example, diacrylic acid, polyacrylic acid, nitrogen, maleic anhydride and oxygen, as well as formaldehyde, acrolein, formic acid, acetic acid, propionic acid, furfural, 2-furfural, benzaldehyde,

[0076] EB24-0362PCBASF SE 240362W001

[0077] 10

[0078] 4-Methoxyphenol, benzoic acid, phthalic anhydride, phenothiazine, propene, propane, carbon dioxide and carbon monoxide.

[0079] If the product gas mixture (PGM) is processed before being introduced into the separation column (T-col) (see also below in connection with device (V2)), preferably by quenching with a portion of the by-product stream (NKS) and / or by mixing with a portion of the second recirculating gas stream (KGS2), the product gas mixture (PGM) after processing (and before being introduced into the separation column) preferably contains 35 wt% to 50 wt% acrylic acid, 10 wt% to 25 wt% diacrylic acid, 2 wt% to 8 wt% polyacrylic acid, 20 wt% to 40 wt% nitrogen, 2 wt% to 7 wt% maleic anhydride, 0.2 wt% to 2 wt% benzoic acid, 1 wt% to 5 wt% water, 0.5 wt% to 2 wt% oxygen, and 0.5 wt% to 2 wt% carbon dioxide. 0.2 wt% to 2 wt% 4-methoxyphenol, 0.2 wt% to 2 wt% acetic acid, and also small amounts (each < 0.5 wt%) of formaldehyde, acrolein, formic acid, propionic acid, furfural, 2-furfural, benzaldehyde, phthalic anhydride,Phenothiazine, propene, propane and carbon monoxide.

[0080] If a product gas mixture (PGM) originating from acrylic acid production without further processing is introduced into the separation column (T-col), the product gas mixture (PGM) preferably contains 10 wt.% to 13 wt.% acrylic acid, 76 wt.% nitrogen, 4.5 wt.% water, 1.5 wt.% to 2.5 wt.% carbon dioxide, 2.5 wt.% to 3.5 wt.% oxygen, 0.6 wt.% carbon monoxide, and also small amounts (each < 0.5 wt.%) of formaldehyde, acrolein, formic acid, acetic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, benzoic acid, phthalic anhydride, propene, propane, and diacrylic acid.

[0081] Preferably, the product gas mixture (PGM) is fed into the separation column (T-col) at a temperature of 100 to 180 °C. Furthermore, it is preferred that the product gas mixture (PGM) is fed into the lower part of the separation column (T-col), more preferably into the bottom of the separation column (T-col).

[0082] Separation columns and separation-effective internals as such are known to those skilled in the art and can be found in textbooks on process engineering, in particular on thermal separation (for example, M. Baern et al., Technische Chemie, 2006, WILEY-VCH, Weinheim).

[0083] In the context of the present invention, a separation column is preferably understood to be an apparatus for the thermal separation of mixtures. Examples include rectification columns, distillation columns, and condensation columns. The column body is regularly considered to be a component of EB24-0362PCBASF SE 240362W001

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[0085] To prevent heat loss, the column is designed with an insulated, cylindrical tube. For evaporating the mixture to be separated, or a portion thereof, the lower end of the column can include an evaporator, or an evaporator can be implemented as an external unit alongside the column. Almost any conceivable heat source can be used for evaporation, such as hot water, electricity, microwaves, or waste heat from other processes or other process steps within the present process. For condensing the mixture to be separated, or a portion thereof, the column can include a condenser at the top, or the condenser can be implemented as an external unit alongside the column. Various operating media can be used for cooling in the condenser, such as water, refrigerants, air, or even media from other processes or other parts of the present process that need to be heated.To improve heat and mass transfer in the column, the column body can contain internals such as screen, bubble-cap, or valve trays, packing materials such as Raschig rings, or structured packings. Those skilled in the art know that horizontal separation-effective internals in such columns are also referred to as trays. In principle, columns with rotating inserts, so-called rotary columns, can also be used, which spray the reflux liquid in droplets.

[0086] In the context of the present invention, dual-flow trays and cross-flow trays are used in particular. Dual-flow trays as such are known to those skilled in the art. In the context of the present invention, these are horizontal internals that are installed in the column at specific intervals and have openings through which vapor and liquids can pass in counterflow. These openings can be, for example, holes or slots, the opening ratio being adjustable via the number of openings. Typically, dual-flow trays do not have a drain pipe connecting them to the next tray. Naturally, each dual-flow tray can be flush with the walls of the rectification column, but can also be connected to it via webs.

[0087] Crossflow trays are also known to those skilled in the art. In the context of the present invention, these are horizontal internals installed at specific intervals in the column, in which liquid is fed onto the tray at one side, flows over the tray, and comes into contact with the rising gas. The liquid then flows to a discharge chute, is collected there, and fed to the tray below. Crossflow trays can, for example, contain sieve holes, movable valves, bells, or tunnels.

[0088] EB24-0362PCBASF SE 240362W001

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[0090] According to the invention, it is preferred that dual-flow trays and / or cross-flow trays are used as separation-effective internals in the separation column (T-col). Furthermore, the separation column (T-col) can include additional inlets and / or outlets, for example at the top, to, for example, withdraw lighter-boiling components and / or recycle components. For example, a residual gas mixture containing by-products with lower boiling points than water can be withdrawn from the top of the separation column (T-col).

[0091] The operating pressure prevailing in the separation column (T-col) is preferably 0 to 5 bar, more preferably 0 to 3 bar and even more preferably 0 to 1.6 bar.

[0092] Crude acrylic streams, obtained during the purification of acrylic acid, are known to those skilled in the art. It is also known to those skilled in the art that such a crude acrylic acid stream typically does not represent a pure product, but rather contains acrylic acid as its largest component, as well as small amounts of other components, such as water, lower aldehydes (e.g., furfurals, acrolein, benzaldehyde), lower carboxylic acids (e.g., acetic acid, propionic acid), and diacrylic acid.

[0093] In the context of the present invention, the crude acrylic acid stream (RAS) contains acrylic acid and minor components, and optionally water, wherein minor components are understood to be, for example, acrolein, formic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, diacrylic acid, 4-methoxyphenol, and phenothiazine. Within the scope of the present invention, the crude acrylic acid stream (RAS) contains, in particular, 96 wt% to 98 wt% acrylic acid, 0.4 wt% to 1.0 wt% acetic acid, 0.2 wt% to 2 wt% water, and also small amounts (each < 0.5 wt%) of acrolein, formic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, diacrylic acid, 4-methoxyphenol, and phenothiazine.

[0094] By-products from acrylic acid production and / or purification are known to those skilled in the art. It is also known to those skilled in the art that such a stream of by-products typically contains acrylic acid and oligomeric acrylic acid as well as polyacrylic acid as its largest components, but also high-boiling substances such as benzaldehyde, furfural, and maleic acid.

[0095] In the context of the present invention, the minor component stream (MCS) contains acrylic acid and minor components, and optionally water, wherein minor components are understood to be, for example, diacrylic acid, oligomeric acrylic acid and / or polyacrylic acid, as well as proportions of maleic acid, benzoic acid, benzaldehyde, and furfurals. Within the scope of the present invention, the minor component stream (MCS) contains, in particular, 50 wt.% to 65 wt.% acrylic acid, EB24-0362PCBASF SE 240362W001

[0096] 13

[0097] 15 wt.% to 30 wt.% diacrylic acid and 5 wt.% to 10 wt.% polyacrylic acid, as well as 4 wt.% to 9 wt.% maleic anhydride, 0.5 wt.% to 2 wt.% benzoic acid, 0.5 wt.% to 1.5 wt.% water and 0.5 wt.% to 1 wt.% 4-methoxyphenol, in addition small amounts (each < 0.5 wt.%) of acrolein, formic acid, acetic acid, propionic acid, furfural, benzaldehyde, phthalic anhydride and phenothiazine.

[0098] Within the scope of the present invention, the by-product stream (NPS) preferably has a temperature of 100 °C to 130 °C, in particular 105 °C to 115 °C, in order to achieve, firstly, sufficient pre-thickening of the bottom liquid of the separation column (T-col) before transfer to the cracking column (S-col) and, secondly, to limit dimer formation in the bottom region of the separation column (T-col).

[0099] Subsequently, in step b), the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col). As shown below, after withdrawal from the separation column (T-col), the crude acrylic acid stream (RAS) is preferably further processed, for example by introducing it into the apparatus (V1).

[0100] Secondly, in step b) the liquid phase stream (FPS) is taken from the separation column (T-col) and further processed according to the following steps.

[0101] In the process according to the invention, step c) involves feeding at least a portion of the liquid phase stream (FPS) containing acrylic acid and a first stripping gas stream (SGS1) into the bottom of an extraction unit (ExE). Portions of this liquid phase stream (FPS) containing acrylic acid can optionally be recycled to the separation column (T-col) beforehand. This can be done in any desired quantity; for example, 10-50% by weight of the liquid phase stream (FPS) containing acrylic acid can be recycled to the separation column (T-col) beforehand. In one embodiment of the present invention, the entire liquid phase stream (FPS) containing acrylic acid is introduced into the bottom of an extraction unit (ExE) in step c).

[0102] The first stripping gas stream (SGS1) introduced into the bottom of the extraction unit (ExE) in step c) of the process according to the invention is known to those skilled in the art. Here, the first stripping gas stream (SGS1) is preferably directed towards the liquid surface of the sump liquid. The stripping gas stream is used in particular as a recirculating gas. For the purposes of the present invention, "recirculating gas" is understood to mean a gas which, in gas-phase oxidation, serves to dilute the reactants and to absorb heat of reaction.

[0103] EB24-0362PCBASF SE 240362W001

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[0105] The first stripping gas stream (SGS1) preferably contains nitrogen (for example, over 90 vol%) and, in the concentration range of a total of < 10 vol%, preferably < 5 vol%, oxygen, water vapor, carbon oxides and mixtures thereof and / or small amounts (< 0.8 vol%) of ethylene, ethane, propene, propane, acrolein, acrylic acid and / or acetic acid.

[0106] The first stripping gas stream (SGS1) and the second stripping gas stream (SGS2) described below can be the same or different. Preferably, both the first stripping gas stream (SGS1) and the second stripping gas stream (SGS2) have the same composition and / or both streams originate from the same source and are separated before use in the respective step of the process according to the invention.

[0107] In this extraction unit (ExE) according to step d) of the invention, at least a part of the acrylic acid contained in the liquid phase stream (FPS) is extracted to obtain a first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid.

[0108] Subsequently, according to step e) of the invention, the first circulating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid is extracted at the head of the extraction unit (ExE).

[0109] An extraction unit as such is known to those skilled in the art. Examples of suitable extraction units that can be used in the process according to the invention, particularly in steps c) to e), are disclosed, for example, in WO 2008 / 090190. For example, in at least a subset of liquid phase stream (FPS) not previously recycled into the separation column (T-col), the acrylic acid contained therein is separated from the water contained in the liquid phase stream (FPS) by extraction with an organic solvent, forming an organic extract containing acrylic acid, and incorporated into the organic solvent. Subsequently, the acrylic acid is separated from the organic extract using at least one thermal separation process, and at least some of the acrylic acid separated from the extract is recycled back into the separation column (T-col).At least some of the acrylic acid separated from the extract can be added to the further purification of the crude acrylic acid and / or incorporated into the aqueous solution of a metal hydroxide.

[0110] For example, the extraction unit is a device comprising two interconnected columns. According to step c), the aqueous liquid phase stream (FPS) containing acrylic acid is fed into the bottom of the first column, and a gas, i.e., the first stripping gas stream (SGS1), is fed into the bottom of the second column. In the first column, the acrylic acid, which is in the water, is completely or partially extracted. EB24-0362PCBASF SE 240362W001

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[0112] at least partially absorbed in an organic solvent supplied from above. This stream, containing the organic solvent and the acrylic acid absorbed therein, exits the first column at the bottom and enters the second column at the top.

[0113] In this second column, the acrylic acid is stripped from the organic solvent by the gas, i.e., the first stripping gas stream (SGS1). The organic solvent, from which the acrylic acid is drawn off, exits the second column at the bottom and enters the first column at the top, thus creating a solvent cycle.

[0114] In the process according to the invention, it is preferred, particularly in connection with steps c) to e) according to the invention, that the extraction unit (ExE) comprises an extraction column (E-col) and a second separation column (2TK), wherein in the extraction column (E-col) acrylic acid is depleted from the liquid phase stream (FPS) by extraction with an organic solvent and the organic solvent enriched with acrylic acid is brought into contact with the first stripping gas stream (SGS1) in the second separation column (2TK) to obtain a first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid.

[0115] Any extraction column (E-col) and / or second separation column (2TK) known to those skilled in the art can be used, as illustrated by example in connection with the separation column (T-col) of the present invention.

[0116] The organic solvent is, for example, at least one ester or diester of an aliphatic or aromatic monocarboxylic or dicarboxylic acid containing 5 to 20 carbon atoms and an alcohol containing 1 to 8 carbon atoms. Preferably, the organic solvent is selected from dimethyl phthalate, diethyl phthalate, dimethyl isophthalate, diethyl isophthalate, dimethyl terephthalate, and / or diethyl terephthalate. Optionally, mixtures of the aforementioned organic solvents may also be used.

[0117] In the process according to the invention, according to step f), at least a part of the first circulating gas stream (KGS1) withdrawn in step e) is fed into the separation column (T-col), wherein at least a part of the first circulating gas stream (KGS1) is introduced into the separation column (T-col) above the point at which the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col).

[0118] In one embodiment of the present invention, the first circulating gas stream (KGS1) extracted in step e) is completely fed into the EB24-0362PCBASF SE 240362W001 according to step f).

[0119] 16

[0120] The gas is introduced into the separation column (T-col). However, as described below, it is preferred within the scope of the present invention that the first recirculating gas stream (KGS1) withdrawn in step e) is separated into at least two parts according to step f). The separation of the first recirculating gas stream (KGS1) withdrawn in step e) can take place according to step f) inside or, preferably, outside the extraction unit (ExE).

[0121] Preferably, a first portion of the first recirculating gas stream (KGS1) withdrawn in step e) is introduced into the separation column (T-col) according to step f), and a second portion of the first recirculating gas stream (KGS1) is combined with the second stripping gas stream (SGS2) as described in detail below and introduced into the bottom of the cracking column (S-col) according to step i). The first and second portions of the first recirculating gas stream (KGS1) withdrawn in step e) can be in any ratio (in vol.%) to each other. Preferably, the ratio of the first to the second portion of the first recirculating gas stream (KGS1) is in the range of 40 to 100 vol.%, particularly 100 vol.%.

[0122] Furthermore, in the process according to the invention, it is preferred that, according to step f), at least a portion of the first circulating gas stream (KGS1) is fed into the separation column (T-col) below the point at which the liquid phase stream (FPS) is withdrawn from the separation column (T-col). The term "below the point" according to step f) means that, between the withdrawal point according to step b) and the feed point according to step f), there is generally at least one (separation) tray, preferably at least five (separation) trays, more preferably at least ten (separation) trays.

[0123] In the process according to the invention, in step g) at least a part of the by-product stream (NPS) is fed from the bottom of the separation column (T-col) into a cracking column (S-col).

[0124] Cracking columns as such are known to those skilled in the art. In the context of the present invention, a cracking column is understood to be a column in which oligomeric acrylic acid can be cracked into monomeric acrylic acid. According to the invention, the cracking column is typically equipped with separation-effective internals. Within the scope of the present invention, what has already been said in connection with the separation column (T-col) applies mutatis mutandis to the separation-effective internals. Typically, the by-product stream (NPS) is introduced into a middle tray of the cracking column (S-col). The aforementioned middle tray is, in particular, a tray in tray regions 8 and 10.

[0125] If necessary, the by-product stream (LCS) can be completely introduced into the cracking column (S-col). Within the scope of the present invention, EB24-0362PCBASF SE 240362W001

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[0127] However, it is preferred that the by-product stream (VPC) be separated into at least two parts. The separation of the by-product stream (VPC) can take place inside or, preferably, outside the separation column (T-col).

[0128] Preferably, a first portion of the by-product stream (NPS) according to step g) is introduced into the cracking column (S-col), and a second portion of the by-product stream (NPS) is returned directly and / or indirectly to the separation column (T-col). In the latter case, the second portion of the by-product stream (NPS) is preferably used as a quench liquid (see also below). The first and second portions of the by-product stream (NPS) can be in any ratio (in ol%). Preferably, the ratio of the first to the second portion of the by-product stream (NPS) is in the range of 0.1 to 50 vol%, particularly 0.3 to 1.5 vol%.

[0129] In the cracking column (S-col), at least a portion of the by-product stream (BPC) is cracked in step h) according to the invention, yielding monomeric acrylic acid. Preferably, in step h) in the cracking column (S-col), at least a portion of the oligomeric acrylic acid contained in the by-product stream (BPC) is cracked to yield monomeric acrylic acid. This typically occurs thermally at temperatures > 150 °C. The cracking can be accelerated by adding small amounts of amines to the bottom of the cracking column. Preferably, step h) is carried out as countercurrent rectification.

[0130] As mentioned above, it is known to those skilled in the art that the by-product stream (BPC) typically contains oligomeric acrylic acid, for example, diacrylic acid. To prevent polymerization after the cleavage of oligomeric acrylic acid into monomeric acrylic acid, the cleavage column (S-col) is preferably operated with polymerization inhibition. In principle, all polymerization inhibitors known to those skilled in the art can be used for this purpose. Typically, phenothiazine, p-methoxyphenol, or a mixture of both compounds are used as polymerization inhibitors.

[0131] For the efficiency of the process according to the invention, and particularly for the yield of the valuable product, it has proven advantageous if, in step h), 60% to 95%, and especially 85% to 90%, of the cleavable components of the by-product stream are cleaved. Typically, the di- and trimers of acrylic acid are cleaved. With regard to the yields of the valuable product, higher cleavage rates of 95% are advantageous, but technically difficult to control, since at cleavage yields > 95% the remaining product tends to form massive solids and is therefore very difficult to handle.

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[0134] The sump of the cracking column (S-col) contains in particular the high-boiling fractions, which are drawn off and disposed of.

[0135] Furthermore, according to the invention, it is preferred that dual-flow trays and / or cross-flow trays are used as separating internals in the cracked column (S-col); dual-flow trays are more preferred.

[0136] Dual-flow floors and cross-flow floors as such are known to those skilled in the art and have already been described in more detail above in the context of the present application.

[0137] Preferably, the cracking column (S-col) comprises 45 trays as separation-effective internals, which are preferably designed as dual-flow trays.

[0138] Furthermore, the method according to the invention includes step i):

[0139] i) Supplying a second stripping gas stream (SGS2) at the bottom of the cracking column (S-col) and withdrawing a second recirculating gas stream (KGS2), containing the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col).

[0140] Step i) is preferably carried out simultaneously with step h) in the method according to the invention.

[0141] The second stripping gas stream (SGS2) introduced into the bottom of the cracking column (S-col) in step i) of the process according to the invention is known to those skilled in the art. Here, the second stripping gas stream (SGS2) is preferably directed towards the liquid surface of the bottom liquid. As mentioned above, the stripping gas stream is used in particular as a recirculating gas.

[0142] The second stripping gas stream (SGS2) preferably contains nitrogen (for example, over 90 vol%) and, in the concentration range of a total of < 10 vol%, preferably < 5 vol%, oxygen, water vapor, carbon oxides and mixtures thereof and / or small amounts (< 0.8 vol%) of ethylene, ethane, propene, propane, acrolein, acrylic acid and / or acetic acid.

[0143] If steps h) and i) are carried out simultaneously, this is preferably done by countercurrent rectification of the by-product stream (LCS) in the cracking column (S-col) to separate the by-products contained in the by-product stream (LCS). The resulting monomeric acrylic acid is typically removed from the top of the cracking column (S-col) as a gas mixture without condensation, together with the supplied second stripping gas stream (SGS2), in the form EB24-0362PCBASF SE 240362W001

[0144] 19

[0145] of the second circulating gas stream (KGS2). The preferably selective stripping of the monomeric acrylic acid is based on its high vapor pressure.

[0146] In the context of the present invention, the second recirculating gas stream (KGS2) upon exiting the cracking column (S-col) preferably contains nitrogen, acrylic acid, water, and oxygen, as well as proportions of carbon dioxide and acetic acid, in particular 50 wt.% to 70 wt.% nitrogen, 20 wt.% to 40 wt.% acrylic acid, 3 wt.% to 5 wt.% water, 2 wt.% to 3 wt.% oxygen, 1 wt.% to 2 wt.% carbon dioxide, and 0 wt.% to 2 wt.% acetic acid, as well as small amounts (each < 0.7 wt.%) of carbon monoxide, acrolein, formic acid, propionic acid, furfural, maleic anhydride, benzaldehyde, propene, and propane. The second recirculating gas stream (KGS2) preferably has a temperature of 80 °C to 100 °C.

[0147] In the process according to the invention, in step j) the second circulating gas stream (KGS2) taken from step i) is fed into the separation column (T-col), wherein the second circulating gas stream (KGS2) is optionally brought into contact with at least one further stream completely or at least partially in a device (V2) before being fed into the separation column (T-col).

[0148] Figure 1 illustrates the two aforementioned options, with the second option shown in dashed lines. In this option, in step j), the second recirculating gas stream (KGS2) extracted in step i) is introduced into the separation column (T-col) in such a way that the second recirculating gas stream (KGS2) is first brought into contact, either completely or at least partially, with at least one other stream in a device (V2). This option is discussed in more detail below; see also Figures 3-5.

[0149] In one embodiment of the present invention, as shown for example in Figure 2, in step i) at least a part of the second recirculating gas stream (KGS2) withdrawn in step j), containing the second stripping gas stream (SGS2) and monomeric acrylic acid, is fed into the upper part of the separation column (T-col), more preferably into the head of the separation column (T-col).

[0150] In this embodiment, it is particularly preferred that in step j) at least a part of the second circulating gas stream (KGS2) taken in step i) is introduced into the separation column (T-col) above the point where the product gas mixture (PGM) is fed into the separation column (T-col).

[0151] However, according to the invention, it is preferred that in step j) the feeding of the second circulating gas stream (KGS2) withdrawn in step i) into the separation column (T-col) is carried out in such a way that the second circulating gas stream (KGS2) is initially completely or at least partially in contact with at least one further stream in a device (V2). EB24-0362PCBASF SE 240362W001

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[0153] is brought. It is preferred that the second recirculating gas stream (KGS2) taken in step i) is completely fed to the device (V2) according to step j), wherein in the device (V2) the recirculating gas stream (KGS) is preferably combined with quenched product gas mixture (PGM) and the mixture obtained is then fed to the separation column (T-col). This embodiment of the present invention in conjunction with step j) is discussed in detail below; see also Figures 3-5.

[0154] Furthermore, according to the invention, it is preferred that at least a portion of the first recirculated gas stream (KGS1) withdrawn in step e) is combined with the second stripping gas stream (SGS2) and introduced into the bottom of the cracking column (S-col) according to step i). This embodiment of the process according to the invention is additionally illustrated, for example, in Figure 2, whereby what has already been said for Figure 1 applies.

[0155] Furthermore, within the scope of the present invention, it is preferred that step a) is preceded by a further step a1):

[0156] a1) Feeding a product gas mixture (PGM) into a device (V2), quenching the product gas mixture (PGM) in the device (V2), removing the quenched product gas mixture (PGM) from the device (V2) and feeding the quenched product gas mixture (PGM) into the separation column (T Kol).

[0157] Preferably, a portion of the by-product stream (NCS) from step a) is introduced into the device (V2) as a quench liquid and / or at least a portion of the second recirculating gas stream (CGS2) withdrawn in step i) is introduced into the device (V2) as a quench medium to obtain the quenched product gas mixture (PGM). Optionally, the second recirculating gas stream (CGS2) can be liquefied before being introduced into the device (V2) so that it can be used there not as a gas, but as a quench liquid.

[0158] In particular, a portion of the by-product stream (NCS) from step a) is introduced into the device (V2) as quench fluid to obtain the quenched product gas mixture (PGM).

[0159] In this context, reference is also made to Figure 3 and, in conjunction with the part of the secondary component flow (SCF), to Figure 5.

[0160] Figure 3 shows the process according to the invention in its aforementioned further development, wherein initially everything already stated for Figures 1 and 2 applies, and wherein, in addition, the product gas mixture (PGM) is first passed through a device (V2) before being introduced into the separation column (T-col). This option EB24-0362PCBASF SE 240362W001

[0161] 21

[0162] However, this requires that at least a portion of the second recirculating gas stream (KGS2), containing the second stripping gas stream (SGS2) and monomeric acrylic acid, is separated before being fed into the separation column (T-col) according to step j). Preferably, at least 20 ol%, more preferably at least 40 vol%, of the second recirculating gas stream (KGS2) is separated from the second recirculating gas stream (KGS2) according to step i) before step j) is carried out in order to feed this separated portion to the apparatus (V2).

[0163] Furthermore, as shown in Figure 5, a portion of the by-product stream (NPS) is separated and introduced into the device (V2) as quench fluid. The quenched product mixture (PGM) discharged from the device (V2) thus additionally comprises the portion of the by-product stream (NPS) used as quench fluid.

[0164] The temperature of the gas mixture obtained during acrylic acid production is typically between 150 and 350 °C, usually between 200 and 300 °C. Acrylic acid production will be discussed in more detail later. Before the resulting gas mixture can be fed into the separation column (T-col), it must be cooled. This cooling typically occurs to between 100 and 180 °C.

[0165] Within the scope of the present invention, the gas mixture obtained in the production of acrylic acid, here referred to as the "product gas mixture (PGM)", is therefore preferably first fed to a device (V2). Typically, the device (V2) is a quench device.

[0166] Quench devices as such are known to those skilled in the art, as is the execution of a quench step. Any liquid can be used as the quench fluid; preferably, a portion of the by-product stream (LCS) originating from step a) is used as the quench fluid. Optionally, a portion of the second recirculating gas stream (CCS2) originating from step i) can also be used as the quench fluid, either additionally or alternatively.

[0167] In the context of acrylic acid production, and particularly in the present invention, a quench device is understood to be a device in which a hot product gas mixture containing acrylic acid is cooled, thereby preventing possible reactions of its components and, in particular, polymerization. All devices known in the prior art for this purpose (e.g., spray scrubbers, Venturi scrubbers, bubble columns, or other apparatus with sprayed surfaces) can be used, with Venturi scrubbers or spray coolers being preferred according to the invention.

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[0170] If required, a phenothiazine compound can be added to the quench device for stabilization or polymerization inhibition. Suitable phenothiazine compounds include, for example, phenothiazine itself, bis-(alpha-methylbenzyl)phenothiazine, 3,7-dioctylphenothiazine, and bis-(alpha-dimethylbenzyl)phenothiazine, of which phenothiazine is preferred.

[0171] Furthermore, according to the invention, it is preferred that the second recirculated gas stream (KGS2) taken in step i) is completely fed to the device (V2), wherein in the device (V2) the recirculated gas stream (KGS) is preferably combined with quenched product gas mixture (PGM) and the mixture obtained is then fed to the separation column (T-col).

[0172] The resulting mixture is preferably fed into the lower part of the separation column (T-col), in particular into the bottom of the separation column (T-col). Optionally, the second recirculating gas stream (KGS2) can also be used as a quench liquid for the product gas mixture (PGM).

[0173] Preferably, in the inventive process an additional step k) is carried out, wherein according to step k) the crude acrylic acid stream (RAS) from step b) is supplied to a device (V1), wherein further purification of the crude acrylic acid stream (RAS) is carried out in the device (V1) to obtain a purified acrylic acid (AAS) and a mother acid (MS).

[0174] Within the scope of the present invention, the device (V1) is preferably a crystallization device. Crystallization devices as such are known to those skilled in the art. The embodiment of the process according to the invention is not limited to a specific crystallization method, whereby acrylic acid is purified by partial crystallization, separation of the frozen acrylic acid from the liquid containing the impurities (mother acid), and by melting the separated pure acrylic acid crystals. For example, falling film crystallization or suspension crystallization as a combination of cooling disk crystallizers and washing columns can be used, with the latter process variant being preferred.

[0175] If necessary, water can be added to the crude acrylic acid stream (RAS) to be purified by crystallization before crystallization (up to 10 wt% or more, preferably up to 5 wt%). This facilitates the separation of acetic acid contained in the crude acrylic acid and also reduces the tendency to form crusts. If the aldehyde or other impurity content is elevated, water addition can be omitted, as the aldehydes can then perform the function of the water.

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[0178] After separation and washing of the crystals, they are usually placed in a container that advantageously already contains a quantity of purified acrylic acid crystals. If necessary, a polymerization inhibitor, such as phenothiazine, can be added. However, the residual polymerization inhibitor remaining in the crystals, which was added in previous process steps (discussed later), is generally sufficient. For a more detailed explanation of the crystallizative separation, see, for example, EP 0616998 A or DE 10223058 A.

[0179] Purified acrylic acid from processes for acrylic acid production and / or processes for acrylic acid purification is known to those skilled in the art. Typically, purified acrylic acid (AAS) contains acrylic acid and optionally small amounts of other components. Within the scope of the present invention, the purified acrylic acid (AAS) preferably contains > 99.6 wt% acrylic acid and, in addition, small amounts (each < 0.3 wt%) of acetic acid, water, propionic acid, and 4-methoxyphenol.

[0180] In the context of the present invention, the term "mother acid" (possibly also referred to as "mother liquor" in the prior art) refers to a solution of acrylic acid which, after separation of the pure product in a crystallization apparatus, contains the impurities separated in the crystallization apparatus, wherein the acrylic acid preferably constitutes a weight fraction of > 80 wt.% in the mother acid.

[0181] Furthermore, within the scope of the present invention, it is preferred that the method according to the invention (in addition to step k)) further comprises the following steps I) and m):

[0182] l) Extraction of the mother acid (MS) from the device (V1) and, if necessary, separation of the mother acid (MS) into two mother acid streams (MS-a and MS-b),

[0183] m) Recycling the mother acid (MS) or optionally the mother acid stream (MS-a) into the separation column (T Kol), wherein at least a part of the mother acid (MS) or optionally the mother acid stream (MS-a) is fed into the separation column (T-Kol) above or below, preferably above, the point from which the crude acrylic acid stream (RAS) is taken in step b).

[0184] In optional step I), the mother acid (MS) obtained in step k) can be withdrawn from the device (V1) and optionally split into two mother acid streams. Within the scope of the present invention, it is preferred that the mother acid (MS) according to step I) is split into two mother acid streams. The first obtained in this way is EB24-0362PCBASF SE 240362W001

[0185] 24

[0186] The first mother acid stream is referred to as "mother acid stream (MS-a)" and the second mother acid stream obtained as "mother acid stream (MS-b)".

[0187] According to the invention, step I) can be carried out without necessarily performing the subsequent optional step m). In this context, it is possible that the mother acid (MS) taken from the apparatus (V1), or optionally a partial stream thereof, is clamped into the separation column (T-col) at a point below the point from which the crude acrylic acid stream (RAS) is taken in step b). However, it is preferred according to the invention that the subsequently defined step m) is carried out after step I) according to the invention.

[0188] The mother acid (MS) extracted from the apparatus (V1) typically contains acrylic acid and optionally water and acetic acid. Within the scope of the present invention, the extracted mother acid contains in particular 90 wt.% to 95 wt.% acrylic acid, 3 wt.% to 6 wt.% water, 0.5 wt.% to 2 wt.% acetic acid, and 0.2 wt.% to 0.7 wt.% diacrylic acid, as well as small amounts (each < 0.5 wt.%) of formaldehyde, acrolein, formic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, 4-methoxyphenol, and phenothiazine.

[0189] In optional step m), the mother acid (MS) or, optionally, the mother acid stream (MS-a) is recycled into the separation column (T-col), wherein at least a portion of the mother acid (MS) or, optionally, the mother acid stream (MS-a) is preferably fed into the separation column (T-col) above the point from which the crude acrylic acid stream (RAS) is taken in step b). According to the invention, it is preferred that at least 80 wt% of the mother acid (MS) or, optionally, the mother acid stream (MS-a) is recycled into the separation column (T-col).

[0190] If the mother acid (MS) is not divided in step I), it is preferred according to the invention that the mother acid (MS) is fed completely into the separation column (T-col) above the point from which the crude acrylic acid stream (RAS) is taken in step b).

[0191] The term “above the point” according to step m) means that between the extraction point according to step b) and the injection point according to step m) there is usually at least one (separation) floor, preferably at least five (separation) floors, more preferably at least ten (separation) floors.

[0192] According to the invention, it is also preferred that the mother acid (MS) is completely recycled, or, if it is separated into two parts in step I), the mother acid stream (MS-a) is recycled between 10 and 50 trays, preferably 20 to 50 trays, into the separation column (T-col) above the outlet for the crude acrylic acid stream (RAS). EB24-0362PCBASF SE 240362W001

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[0194] In one embodiment of the present invention, it is also preferred that the mother acid (MS) is divided into two streams and that at least 80 wt.% of the mother acid (MS) is recycled in the mother acid stream (MS-a) to the separation column (T-col).

[0195] Furthermore, the mother acid (MS) is typically drawn off from the apparatus (V1) at a temperature just above the crystallization temperature of acrylic acid (15 °C to 20 °C) and, optionally, after splitting into two mother acid streams (MS-a and MS-b), the mother acid stream (MS-a) is tempered before being returned to the separation column (T-col). Preferably, the mother acid (MS) or, optionally, the mother acid stream (MS-a) is preheated to the corresponding thermodynamic equilibrium temperature at the feed point of the separation column (T-col) to ensure the most effective separation possible. "Tempered" in the context of the present invention means that the mother acid or the mother acid partial streams are heated after being drawn off from the apparatus (V1) to a temperature of preferably 50 °C to 100 °C, more preferably 70 °C to 100 °C, and particularly 80 °C to 95 °C.

[0196] Within the scope of the present invention, it is conceivable that the return of the mother acid (MS) or, optionally, the mother acid stream (MS-a) to the separation column (T-col) is carried out directly or indirectly. "Direct" here means that the device (V1) and the separation column (T-col) are directly connected to each other by a line, whereas "indirect" means that the mother acid (MS) or, optionally, the mother acid stream (MS-a) is first passed through at least one further device, such as a heating or cooling device, or a tank.

[0197] Figure 4 shows the inventive process in its aforementioned further developments, with the same principles initially applied as described in Figures 1-3. Additionally, according to step k), the crude acrylic acid stream (RAS) from step b) is fed to a device (V1), where further purification of the crude acrylic acid stream (RAS) takes place in the device (V1) to obtain a purified acrylic acid (AAS) and a mother acid (MS). Likewise, step I) is implemented in Figure 4, in which the mother acid (MS) is removed from the device (V1) and, if necessary, separated into two mother acid streams (MS-a and MS-b).Furthermore, step m) is implemented in Figure 4, according to which the mother acid (MS) or, if applicable, the mother acid stream (MS-a) is recycled into the separation column (T-Kol), wherein at least a part of the mother acid (MS) or, if applicable, the mother acid stream (MS-a) is fed into the separation column (T-Kol) above the point from which the crude acrylic acid stream (RAS) is taken in step b).

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[0200] Furthermore, according to the invention, it is preferred that in step I) the mother acid (MS) is extracted from the device (V1) and the mother acid (MS) is separated into two mother acid streams (MS-a and MS-b), and that the method further comprises step n):

[0201] n) Feeding the mother acid stream (MS-b) into the cracking column (S-col).

[0202] Figure 4 further shows that, if necessary, in step n) the second part of the mother acid stream (MS-b) is recycled into the splitting column (S-col) after the division of the mother acid (MS) preferably carried out in step I).

[0203] If the mother acid stream (MS-b) is fed into the cracked column (S-col) according to step n), the feed preferably takes place at the top of the cracked column (S-col), wherein the mother acid stream (MS-b) preferably serves as a reflux within the cracked column (S-col).

[0204] According to the invention, it is further preferred that the separation column (T-col) be used as

[0205] i) Rectification column,

[0206] ii) Distillation column, and / or

[0207] iii) Condensation column

[0208] is designed.

[0209] Rectification columns, distillation columns, and condensation columns are known to those skilled in the art. Reference is first made to the above description of a separation column for thermal separation in general. As already mentioned above, rectification is understood by those skilled in the art to be a thermal separation by multi-stage distillation. It is known to those skilled in the art that distillation is a thermal separation process and a distillation column is, accordingly, an apparatus for carrying out this thermal separation process, whereby volatile liquids are obtained or volatile solvents are separated from substances that are difficult to evaporate and can subsequently be collected by condensation.Accordingly, a condenser can be located at the top of such a separation column or as a separate apparatus, in which a vaporous substance can be converted into a liquid substance by cooling and thereby separated from a mixture of substances.

[0210] Furthermore, it is preferred according to the invention that in the inventive method in

[0211] i) the separation column (T-col) and / or

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[0214] ii) the fission column (S-col)

[0215] Dual-flow floors and / or cross-flow floors are used as separation-effective installations.

[0216] Furthermore, it is advantageous according to the invention that the process for purifying acrylic acid is preceded by the production of acrylic acid, in which a product gas mixture (PGM) containing acrylic acid, water vapor and impurities is produced by heterogeneously catalyzed gas-phase partial oxidation of at least one C3 precursor of acrylic acid with molecular oxygen on catalysts in the solid state at elevated temperature.

[0217] C3 precursors of acrylic acid are known to those skilled in the art. In principle, any C3 precursor of acrylic acid suitable for the production of acrylic acid by reaction with molecular oxygen over a solid catalyst can be used in the present invention. Known C3 precursors of acrylic acid include, for example, propane, propene, acrolein, propionaldehyde, and propionic acid.

[0218] According to the invention, it is preferred that the C3 precursor of the acrylic acid is propene and / or acrolein.

[0219] The gas-phase partial oxidation itself can be carried out as described in the prior art. Starting from propylene, the gas-phase partial oxidation can, for example, be carried out in two successive oxidation stages, as described in EP 0700714 A and EP 0700893 A. Likewise, the gas-phase partial oxidations cited in DE 19740253 A or DE 19740252 A can be used. In principle, however, the gas-phase partial oxidation can also be carried out according to other methods not explicitly listed here, which are known to those skilled in the art.

[0220] Furthermore, it has proven advantageous that the

[0221] i) the crude acrylic acid stream (RAS) according to step b) contains at least 90 wt% acrylic acid as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural, and / or

[0222] ii) the mother acid (MS) according to step h) contains at least 90 wt% acrylic acid as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural.

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[0225] In a second aspect of the present invention, the method according to the invention can advantageously be carried out by means of a system according to the invention, which is described below in its basic form.

[0226] Another object of the present invention is therefore a plant for the purification of acrylic acid comprising

[0227] a separation column (T-col)

[0228] a splitting column (S-col)

[0229] an extraction unit (ExE)

[0230] a first line (L1) connecting the sump of the separation column (T-col) with the splitting column (S-col)

[0231] a second line (L2) extending from the top of the splitting column (S-col), wherein the second line (L2) splits into two lines (L2-a and L2-b), wherein the line (L2b) is connected to the separation column (T-col) and preferably opens into the separation column (T-col) above the feed point for the product gas mixture (PGM).

[0232] a third line (L3) connecting the separation column (T-col) and the bottom of the extraction unit (ExE), wherein the third line (L3) enters the separation column (T-col) above the point of withdrawal of the crude acrylic acid stream (RAS),

[0233] A fourth line (L4) extends from the head of the extraction unit (ExE), wherein the fourth line (L4) enters the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is drawn off, and wherein the third line (L3) preferably enters the separation column (T-col) above the point where the fourth line (L4) enters the separation column (T-col).

[0234] The corresponding definitions of the elements of the system and the terms used have already been given above with regard to the method according to the invention and also apply to this system. Such a system is illustrated, for example, by Figure 1.

[0235] The expert also knows that at a point where a pipe is split into two parts, for example, there is usually a device, such as a valve, through which the flow of materials can be regulated.

[0236] The advantages of the system according to the invention are essentially the same as for the method according to the invention described above.

[0237] The system according to the invention is particularly advantageous when integrated into a complete system for acrylic acid production. As described above,

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[0240] The system according to the invention can increase the overall efficiency of acrylic acid production while simultaneously reducing the equipment required.

[0241] Furthermore, the inventive method can advantageously be carried out in a further development using a further developed system, which is illustrated, for example, by Figure 2. This further developed system additionally comprises a line (L4b) leading from the head of the extraction unit (ExE), wherein the line (L4b) opens into the cracking column (S-col).

[0242] Furthermore, the inventive method can advantageously be carried out in a further development using a further developed system, which is illustrated, for example, by Figure 3. This further developed system additionally comprises

[0243] a device (V2),

[0244] a fifth line (L5) connecting the device (V2) to the separation column (T-col), wherein the fifth line (L5) is used for feeding in the product gas mixture ((PGM),

[0245] the line (L2a) enters the device (V2),

[0246] a branch from the first line (L1) exiting the separation column (T-col), wherein this branch leads into the device (V2).

[0247] Furthermore, the inventive method can advantageously be carried out in a further development using a further developed system, which is illustrated, for example, by Figure 4. This further developed system additionally comprises

[0248] a device (V1)

[0249] a sixth line (L6) connecting the separation column (T-col) and the device (V1),

[0250] a seventh line (L7) leading from the device (V1), which splits into two lines (L7-a and L7-b), wherein the line (L7-a) is connected to the separation column (T-col) and opens into the separation column above or below, preferably above, the sixth line (L6).

[0251] Particularly preferred according to the invention is a system in which the extraction unit (ExE) comprises an extraction column (E-col) and a second separation column (2TK), wherein the third line (L3) opens into the bottom of the extraction column (E-col), the extraction column (E-col) and the second EB24-0362PCBASF SE 240362W001

[0252] 30

[0253] The separation column (2TK) is connected to each other by two lines, and the fourth line (L4) leads from the head of the second separation column (2TK).

[0254]

[0255] the Fi 1 to 5:

[0256] T-Kol separation column (T-Kol)

[0257] ExE Extraction Unit (ExE)

[0258] V1 Device (V1)

[0259] PGM product gas mixture (PGM)

[0260] RAS Crude Acrylic Acid Stream (RAS)

[0261] FPS Liquid Phase Stream (FPS)

[0262] MS mother acid

[0263] MS-a first part of the mother acid stream (MS-a)

[0264] MS-b second part of the mother acid stream (MS-b)

[0265] NKS secondary component current (NKS)

[0266] AAS Purified Acrylic Acid (AAS)

[0267] S-Col splitting column (S-Col)

[0268] SGS1 first strip gas stream (SGS1)

[0269] KGS1 first closed-loop gas flow (KGS1)

[0270] SGS2 second strip gas stream (SGS2)

[0271] KGS2 second closed-loop gas flow (KGS2)

[0272] V2 Device V2

[0273] The present invention is described in detail below with reference to examples, which, however, do not limit the invention. All features described and / or illustrated, individually or in any combination, constitute the subject matter of the invention, even independently of their compilation in the claims or their cross-reference. The examples described below are simulations based on mass and energy balances as well as thermodynamic phase equilibrium models.

[0274] It is important to note that the measured values ​​described in the examples all represent the steady state. Those skilled in the art know that in the steady state, the various described steps, such as the different inflows and outflows to products and auxiliary components, occur simultaneously and continuously.

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[0277] Example according to the invention 11

[0278] A specific example of the purification of acrylic acid is described here using Example 11. The calculation was performed using the "Chemasim" software from BASF SE.

[0279] The present example 11 according to the invention can be illustrated in particular with reference to Figure 1. For a description of the central elements of Figure 1, reference is made to the corresponding explanations above.

[0280] A hot product gas mixture (PGM) at a temperature of approximately 270 °C is fed from a section of the plant (not shown) where heterogeneously catalyzed gas-phase partial oxidation is carried out. This product gas mixture (PGM), whose composition is shown in Table 1, is introduced into the bottom section of a separation column (T-col).

[0281] Table 1. Main composition of the product gas mixture (PGM) for 11.

[0282] Components PGM wt%

[0283] Nitrogen 76.1

[0284] Acrylic acid 12.2

[0285] Water 5.37

[0286] Oxygen 2.34

[0287] CO / CO2 2.90

[0288] Acetic acid 0.28

[0289] Maleic anhydride 0.10

[0290]

[0291] In this separation column (T-col), the acrylic acid formed is separated from the product mixture by thermal separation and discharged from the column. The discharged acrylic acid stream, also referred to here as crude acrylic acid stream (RAS), whose main composition is shown in Table 2, is highly concentrated (>95 wt% acrylic acid) and has a temperature of approximately 100 °C.

[0292] Table 2. Main composition of the crude acrylic acid stream (RAS) for 11.

[0293] Components RAS Weight %

[0294] Acrylic acid 96.3

[0295] Acetic acid 0.496

[0296] Propionic acid 0.108

[0297] Furfural 0.276

[0298] Benzaldehyde 0.037

[0299] Maleic anhydride 0.030

[0300]

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[0302] A first stripping gas stream (SGS1), originating as recycled exhaust gas from an unspecified part of the plant where a portion of the exhaust gas is recycled and compressed, is fed to the extraction unit (ExE). This first stripping gas stream (SGS1), whose main composition is shown in Table 3, has a temperature of approximately 160 °C. A liquid phase stream (FPS), containing acrylic acid, water, and other light components and having a temperature of approximately 70 °C, is fed to the extraction unit (ExE) from the upper section of the separation column (T-col).

[0303] Table 3. Main composition of the first stripping gas stream (SGS1) for 11.

[0304] Components SGS1 wt%

[0305] Nitrogen 90.4

[0306] Acrylic acid 0.170

[0307] Water 2.44

[0308] Oxygen 2.78

[0309] CO / CO2 3.45

[0310] Acetic acid 0.124

[0311]

[0312] In this process, the second stripping gas stream (SGS2) has the same composition as the first stripping gas stream (SGS1).

[0313] The liquid phase stream (FPS) also contains light components such as acetic acid, formaldehyde, and formaldehyde oligomers. However, the main components of the liquid phase stream (FPS) are water and acrylic acid, as shown in Table 4.

[0314] Table 4. Main composition of the liquid phase stream (FPS) for 11.

[0315] Components FPS Weight %

[0316] Water 79.1

[0317] Acrylic acid 9.57

[0318] Acetic acid 6.03

[0319] Formaldehyde and its oligomers 4.64

[0320] Formic acid 0.64

[0321]

[0322] The extraction unit (ExE), as described in W02008 / 090190, uses an organic solvent to extract acrylic acid from the liquid phase stream (FPS). The acrylic acid is then recovered from the organic solvent using a thermal separation process. The aqueous phase, which mainly consists of water, remains after extraction using a

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[0325] organic solvent, especially in the extraction unit (ExE), is removed and disposed of.

[0326] A second stripping gas stream (SGS2), originating as recycled gas from an unspecified part of the plant, is introduced into the cracking column (S-col) below the bottom tray (separation-effective internals). This second stripping gas stream (SGS2) has a temperature of approximately 160 °C. A by-product stream (NKS), containing oligomeric acrylic acid and having a temperature of approximately 110 °C, is fed from the bottom of the separation column (T-col) onto a middle tray of the cracking column (S-col).

[0327] The minor component stream (MCS) also contains high-boiling substances such as benzaldehyde, furfural, and maleic anhydride. However, the main components of the minor component stream (MCS) are acrylic acid, its oligomers, and polyacrylic acid.

[0328] In the cracking column (S-col), these by-components, in particular the oligomers of acrylic acid, are re-split and discharged as the lighter-boiling fraction over the top of the cracking column (S-col) together with the second stripping gas stream (SGS2) as the second recirculating gas stream (KGS2), the main composition of which is shown in Table 6, and fed to the apparatus (V2) or the separation column (T-col). The heavier-boiling fractions remain, in particular, in the bottom of the cracking column (S-col) and are removed and disposed of.

[0329] In the present embodiment, the yield of acrylic acid as a valuable product is significantly increased and the content of acetic acid as a byproduct is reduced by recirculating (or introducing) the first recirculating gas stream (KGS1), the main composition of which is shown in Table 5, into the separation column (T-col), wherein the entire first recirculating gas stream (KGS1) is introduced above the bottom of the separation column (T-col) at which the product gas mixture (PGM) is introduced into the separation column (T-col); and by recirculating (or introducing) the second recirculating gas stream (KGS2) into the separation column (T-col), wherein the entire second recirculating gas stream (KGS2) is introduced at the bottom of the separation column (T-col) at which the product gas mixture (PGM) is introduced into the separation column (T-col). With the process and plant according to the invention, it is possible to produce 27 t / h of crude acrylic acid (RAS), wherein the acetic acid content is < 0.5 wt.%.

[0330] At the outlet of the separation column (T-col), a higher acrylic acid content of 96.3 wt.% was obtained in the crude acrylic acid stream (RAS), with a lower acetic acid content of 0.496 wt.%, a lower benzaldehyde content of 0.037 wt.%, and a lower maleic anhydride content of 0.030 wt.%.

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[0332] Table 5. Main composition of the first recirculating gas stream 1 (KGS1) for 11.

[0333] Components KGS1 wt%

[0334] Nitrogen 81.0

[0335] Acrylic acid 5.56

[0336] Water 5.21

[0337] Oxygen 2.49

[0338] CO2 2.36

[0339] Acetic acid 1.79

[0340]

[0341] Table 6. Main composition of the second closed-loop gas stream 2 (KGS2) for 11.

[0342] Components KGS2 wt%

[0343] Nitrogen 67.2

[0344] Acrylic acid 24.4

[0345] Water 2.9

[0346] Oxygen 2.1

[0347] CO2 2.0

[0348] Acetic acid 0.25

[0349] Maleic anhydride 0.003

[0350] Benzaldehyde 0.005

[0351]

[0352] Inventive example I2

[0353] The present example I2 according to the invention can be illustrated in particular with reference to Figure 3. For a description of the central elements of Figure 3, reference is made to the corresponding explanations above.

[0354] A hot product gas mixture (PGM) at a temperature of approximately 270 °C is supplied from a section of the plant (not shown) where heterogeneously catalyzed gas-phase partial oxidation is carried out. This mixture is fed to the device (V2), which is designed as a quench device to prevent possible reactions of the components of the product gas mixture (PGM) and to lower its temperature.

[0355] The gases and liquids supplied to the apparatus (V2) are introduced into the sump of the separation column (T-col) after exiting. In this separation column (T-col), the acrylic acid formed is separated from the product mixture by thermal separation and discharged. The discharged acrylic acid stream, also referred to here as the crude acrylic acid stream (RAS), is highly concentrated (>95 wt% acrylic acid) and has a temperature of approximately 100 °C.

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[0358] A first stripping gas stream (SGS1), originating as recycled gas from an unspecified part of the plant, is fed to the extraction unit (ExE). This first stripping gas stream (SGS1) has a temperature of approximately 160 °C. A liquid phase stream (FPS), containing acrylic acid, water, and other light components and having a temperature of approximately 70 °C, is fed to the extraction unit (ExE) from the upper section of the separation column (T-col).

[0359] The liquid phase stream (FPS) also contains lighter components such as acetic acid, formaldehyde, and formaldehyde oligomers. However, the main components of the liquid phase stream (FPS) are water and acrylic acid.

[0360] The extraction unit (ExE), as described in W02008 / 090190, uses an organic solvent to extract acrylic acid from the liquid phase stream (FPS). The acrylic acid recovered from the organic solvent is then processed using a thermal separation method. The aqueous phase, which mainly consists of water, remains in the extraction unit (ExE) after extraction and is removed and disposed of.

[0361] A second stripping gas stream (SGS2), originating as recycled gas from an unspecified part of the plant, is introduced into the cracking column (S-col) below the bottom tray (separation-effective internals). This second stripping gas stream (SGS2) has a temperature of approximately 160 °C. A by-product stream (NKS), containing oligomeric acrylic acid, is fed from the bottom of the separation column (T-col) onto a middle tray of the cracking column (S-col) at a temperature of approximately 110 °C.

[0362] The minor component stream (MC) also contains high-boiling substances such as benzaldehyde, furfural, and maleic anhydride. However, the largest component of the minor component stream (MC) is acrylic acid, its oligomers, and polyacrylic acid.

[0363] In the cracking column (S-col), these by-components, especially the oligomers of acrylic acid, are split back and discharged as the lighter-boiling fraction over the top of the cracking column (S-col) together with the second stripping gas stream (SGS2) as the second recirculating gas stream (KGS2) and fed to the apparatus (V2) or the separation column (T-col). The heavier-boiling fractions remain, in particular, in the bottom of the cracking column (S-col) and are removed and disposed of.

[0364] In the present embodiment, the first circulating gas stream (KGS1) is recirculated (or introduced) into the separation column (T-col) and into the cracking column (S-col), with a portion of the first circulating gas stream (KGS1) being above the EB24-0362PCBASF SE 240362W001

[0365] 36

[0366] By introducing a portion of the product gas mixture (PGM) at the bottom of the separation column (T-col) – at the point where the product gas mixture (PGM) is fed into the separation column (T-col) – and the remaining portion at the bottom of the cracking column (S-col) – at the point where the second stripping gas stream (SGS2) is fed into the cracking column (S-col), the yield of acrylic acid as a valuable product is significantly increased, and the content of acetic acid as a byproduct is reduced. With the process and plant according to the invention, it is possible to produce 27 t / h of crude acrylic acid (RAS) with an acetic acid content of < 0.5 wt%.

[0367] Inventive example I3

[0368] The present example I3 according to the invention can be illustrated in particular with reference to Figure 5. For a description of the central elements of Figure 5, reference is made to the corresponding explanations above.

[0369] A hot product gas mixture (PGM) at a temperature of approximately 270 °C is fed from a section of the plant (not shown) where heterogeneously catalyzed gas-phase partial oxidation is carried out. This mixture is fed to the device (V2), which is designed as a quench unit to prevent potential reactions of the components of the product gas mixture (PGM) and to lower its temperature. A portion of the by-product stream (CCS) described below is used as the quench fluid.

[0370] The gases and liquids supplied to the apparatus (V2) are introduced into the bottom section of the separation column (T-col) after exiting. In this separation column (T-col), the acrylic acid formed is separated from the product mixture by thermal separation and discharged. The discharged acrylic acid stream, also referred to here as crude acrylic acid stream (RAS), is highly concentrated (>95 wt% acrylic acid) and has a temperature of approximately 100 °C.

[0371] This crude acrylic acid stream (RAS) is fed to the apparatus (V1), which is designed as a crystallization apparatus as described in WO2025 / 078224, where crystallization purification is carried out. In addition to purified acrylic acid (AAS), so-called mother acid (MS) is obtained in the crystallization apparatus. The purified acrylic acid (AAS) and the mother acid (MS) are then discharged from the crystallization apparatus as the target product and mother acid stream, respectively, with the latter being recycled back into the process.

[0372] The temperature of the purified acrylic acid (AAS) withdrawn from the device (V1) is 17 °C and the calculated composition is shown in Table 7.

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[0375] The mother acid stream (MS) is fed directly to a lower section of the separation column (T-col). In embodiment I3, the mother acid stream (MS) is first split into a first and second partial stream (MS-a and MS-b). While the second part (MS-b) is fed to the cracking column (S-col), the first part of the mother acid (MS-a) is returned to the separation column (T-col). Specifically, the return occurs to a tray of the separation column (T-col) located above the tray from which the crude acrylic acid stream (RAS) is drawn towards the device (V1).

[0376] The reverse flow of mother acid (MS) discharged from the device (V1) is heated to 80 °C.

[0377] A first stripping gas stream (SGS1), originating as recycled gas from an unspecified part of the plant, is fed to the extraction unit (ExE). This first stripping gas stream (SGS1) has a temperature of approximately 160 °C. A liquid phase stream (FPS), containing acrylic acid, water, and other light components and having a temperature of approximately 70 °C, is fed to the extraction unit (ExE) from the upper section of the separation column (T-col).

[0378] The liquid phase stream (FPS) also contains lighter components such as acetic acid, formaldehyde, and formaldehyde oligomers. However, the main components of the liquid phase stream (FPS) are water and acrylic acid.

[0379] The extraction unit (ExE), as described in W02008 / 090190, uses an organic solvent to extract acrylic acid from the liquid phase stream (FPS). The acrylic acid recovered from the organic solvent is then processed using a thermal separation method. The aqueous phase, which mainly consists of water, remains in the extraction unit (ExE) after extraction and is removed and disposed of.

[0380] A second stripping gas stream (SGS2), originating as recycled gas from an unspecified part of the plant, is introduced into the cracking column (S-col) below the bottom tray (separation-effective internals). This second stripping gas stream (SGS2) has a temperature of approximately 160 °C. A by-product stream (NKS), containing oligomeric acrylic acid, is fed from the bottom of the separation column (T-col) onto a middle tray of the cracking column (S-col) at a temperature of approximately 110 °C.

[0381] The minor component stream (MCS) also contains high-boiling substances such as benzaldehyde, furfural, and maleic anhydride. The largest component of the

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[0384] However, the secondary component stream (SCS) is acrylic acid, its oligomers and polyacrylic acid.

[0385] In the cracking column (S-col), these by-components, especially the oligomers of acrylic acid, are split back and discharged as the lighter-boiling fraction over the top of the cracking column (S-col) together with the second stripping gas stream (SGS2) as the second recirculating gas stream (KGS2) and fed to the apparatus (V2) or the separation column (T-col). The heavier-boiling fractions remain, in particular, in the bottom of the cracking column (S-col) and are removed and disposed of.

[0386] In the present embodiment, the yield of acrylic acid as a valuable product is significantly increased and the content of acetic acid as a byproduct is reduced by recirculating (or introducing) the first recirculating gas stream (KGS1) into the separation column (T-col) and into the cracking column (S-col), wherein a portion of the first recirculating gas stream (KGS1) is introduced above the bottom of the separation column (T-col) – at the point where the product gas mixture (PGM) is fed into the separation column (T-col) – and the remaining portion is introduced at the bottom of the cracking column (S-col) – at the point where the second stripping gas stream (SGS2) is fed into the cracking column (S-col). With the process and plant according to the invention, it is possible to produce 26 t / h of crude acrylic acid (RAS) with an acetic acid content of < 0.15 wt%.

[0387] The purified acrylic acid (AAS) at the outlet of the device (V1) has a higher acrylic acid content of 99.80 wt.% and a lower acetic acid content of 0.133 wt.%, as shown in Table 7.

[0388] The reverse flow of mother acid (MS) discharged from the device (V1) has the calculated composition shown in Table 8.

[0389] Table 7. Main composition of purified acrylic acid (AAS) for I3.

[0390] Components AAS wt%

[0391] Acrylic acid 99,800

[0392] Acetic acid 0.133

[0393]

[0394] Table 8. Main composition of the mother acid (MS) for I3.

[0395] Components MS Weight %

[0396] Acrylic acid 92.7

[0397] Acetic acid 0.78

[0398] Benzaldehyde 0.050

[0399] Maleic anhydride 0.041

[0400]

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[0403] Comparison example C1

[0404] Comparative example C1 is carried out like example 11 according to the invention, with the difference that the first recirculating gas stream (KGS1) withdrawn from the extraction unit (ExE) is completely introduced into the stripping column (S-Col) and mixed there with the second stripping gas stream (SGS2); the resulting mixture is introduced into the bottom of the stripping column (S-Col). In this way, the crude acrylic acid stream (RAS) withdrawn from the separation column (T-Col) has a temperature of approximately 100 °C. The relevant calculated compositions are shown in Tables 9 to 13.

[0405] At the outlet of the separation column (T-col) a lower acrylic acid content of 95.4 wt.% was obtained with a higher acetic acid content of 0.665 wt.%, a higher benzaldehyde content of 0.284 wt.% and a higher maleic anhydride content of 0.196 wt.%.

[0406] Table 9. Main composition of the crude acrylic acid stream (RAS) for C1.

[0407] Components RAS Weight %

[0408] Acrylic acid 95.4

[0409] Acetic acid 0.665

[0410] Propionic acid 0.109

[0411] Furfural 0.485

[0412] Benzaldehyde 0.284

[0413] Maleic anhydride 0.196

[0414]

[0415] Table 10. Main composition of the first stripping gas stream (SGS1) for C1.

[0416] Components SGS1 wt%

[0417] Nitrogen 90.3

[0418] Acrylic acid 0.233

[0419] Water 2.42

[0420] Oxygen 2.78

[0421] CO / CO2 3.45

[0422] Acetic acid 0.127

[0423]

[0424] Table 11. Main composition of the liquid phase stream (FPS) for C1.

[0425] Components FPS Weight %

[0426] Water 76.2

[0427] Acrylic acid 12.70

[0428] Acetic acid 5.90

[0429] Formaldehyde and its oligomers 4.48

[0430] Formic acid 0.61

[0431]

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[0434] Table 12. Main composition of the first recirculating gas stream 1 (KGS1) for O1. Components KGS1 wt%

[0435] Nitrogen 83.0

[0436] Acrylic acid 4.88

[0437] Water 4.42

[0438] Oxygen 2.56

[0439] CO2 2.42

[0440] Acetic acid 1.18

[0441]

[0442] Table 13. Main composition of the second closed-loop gas stream 2 (KGS2) for O1. Components KGS2 wt%

[0443] Nitrogen 71.1

[0444] Acrylic acid 17.5

[0445] Water 4.6

[0446] Oxygen 2.2

[0447] 002 2.1

[0448] Acetic acid 1.03

[0449] Maleic anhydride 0.012

[0450] Benzaldehyde 0.030

[0451]

[0452] Comparison example C2

[0453] Comparative Example 02 is carried out like Example I3 according to the invention, with the difference that the first recirculated gas stream (KGS1) withdrawn from the extraction unit (ExE) is completely introduced into the cracking column (S-Col) and mixed there with the second stripping gas stream (SGS2); the resulting mixture is introduced into the bottom of the cracking column (S-Col). Furthermore, the crude acrylic acid stream (RAS) withdrawn from the separation column (T-Col) is fed to the apparatus (V1), in which further purification is carried out. The purified acrylic acid (AAS) as the target product and mother acid (MS) are obtained.

[0454] In this way, the AAS stream drawn from the device (V1) has a temperature of approximately 17 °C, and the calculated composition is shown in Table 14 for AAS and in Table 15 for MS. At the outlet of the device (V1), a lower acrylic acid content of 99.789 wt% and a higher acetic acid content of 0.150 wt% were obtained.

[0455] Table 14. Main composition of purified acrylic acid (AAS) for 02.

[0456] Components AAS wt%

[0457] Acrylic acid 99,789

[0458] Acetic acid 0.150

[0459]

[0460] EB24-0362PCBASF SE 240362W001

[0461] 41

[0462] Table 15. Main composition of the mother acid (MS) for C2. Components MS wt%

[0463] Acrylic acid 91.6

[0464] Acetic acid 1.00

[0465] Benzaldehyde 0.386 Maleic anhydride 0.266

[0466]

[0467] EB24-0362PC

Claims

BASF SE 240362W001 42 Claims 1. Method for the purification of acrylic acid, comprising steps a) to j): a) Separation of a product gas mixture (PGM) containing acrylic acid and by-products in a separation column (T-col) into a crude acrylic acid stream (RAS), a liquid phase stream (FPS) containing acrylic acid and a by-product stream (NKS), b) Withdrawal of the crude acrylic acid stream (RAS) and the liquid phase stream (FPS) from the separation column (T-col), wherein the withdrawal point of the liquid phase stream (FPS) is located above the withdrawal point of the crude acrylic acid stream (RAS) in the separation column (T-col), c) Feeding at least a portion of the liquid phase stream (FPS) containing acrylic acid and a first stripping gas stream (SGS1) into the bottom of an extraction unit (ExE), d) Extraction of at least a part of the acrylic acid contained in the liquid phase stream (FPS) in the extraction unit (ExE) to obtain a first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid, e) Extraction of the first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid at the head of the extraction unit (ExE), f) Feeding at least part of the first recirculating gas stream (KGS1) withdrawn in step e) into the separation column (T-col), wherein at least part of the first recirculating gas stream (KGS1) is introduced into the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col), g) Feeding at least part of the by-product stream (VPC) from the bottom of the separation column (T-col) into a cracking column (S-col), h) Splitting at least part of the by-product stream (NCS) in the cracking column (S-col) to obtain monomeric acrylic acid, EB24-0362PCBASF SE 240362W001 43 i) Supplying a second stripping gas stream (SGS2) at the bottom of the cracking column (S-col) and withdrawing a second recirculating gas stream (KGS2), containing the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col), j) Feeding the second recirculating gas stream (KGS2) taken in step i) into the separation column (T-col), wherein the second recirculating gas stream (KGS2) may, if necessary, first be brought into contact completely or at least partially in a device (V2) with at least one other stream before being fed into the separation column (T-col).

2. Method according to claim 1, characterized in that in step j) at least a part of the second circulating gas stream (KGS2) withdrawn in step i) is introduced into the separation column (T-col) above the point where the product gas mixture (PGM) is fed into the separation column (T-col).

3. Method according to claim 1 or 2, characterized in that at least a part of the first recirculating gas stream (KGS1) withdrawn in step e) is combined with the second stripping gas stream (SGS2) and is introduced into the bottom of the stripping column (S-col) according to step i).

4. Method according to one of claims 1 to 3, characterized in that the supply of at least a part of the first circulating gas stream (KGS1) according to step f) takes place below the point in the separation column (T-col) where the liquid phase stream (FPS) is withdrawn from the separation column (T-col).

5. Method according to any one of claims 1 to 4, comprising the further step a1), which precedes step a): a1) Feeding a product gas mixture (PGM) into the device (V2), quenching the product gas mixture (PGM) in the device (V2), removing the quenched product gas mixture (PGM) from the device (V2) and feeding the quenched product gas mixture (PGM) into the separation column (T-col), Preferably, a portion of the by-product stream (NKS) from step a) and / or at least a portion of the second recirculating gas stream (KGS2) extracted in step i) is introduced into the device (V2) as quench liquid to obtain the quenched product gas mixture (PGM), EB24-0362PCBASF SE 240362W001 44 In particular, a portion of the by-product stream (NCS) from step a) is introduced into the device (V2) as quench fluid to obtain the quenched product gas mixture (PGM).

6. A method according to any one of claims 1 to 5, wherein the crude acrylic acid stream (RAS) from step b) is supplied to a device (V1) and the method further comprises step k): k) further purification of the crude acrylic acid stream (RAS) in the apparatus (V1) to obtain a purified acrylic acid (AAS) and a mother acid (MS).

7. Method according to claim 6, further comprising steps I) and m): l) Extraction of the mother acid (MS) from the device (V1) and, if necessary, separation of the mother acid (MS) into two mother acid streams (MS-a and MS-b), m) Recycling the mother acid (MS) or optionally the mother acid stream (MS-a) into the separation column (T-col), wherein at least a part of the mother acid (MS) or optionally the mother acid stream (MS-a) is fed into the separation column (T-col) above or below, preferably above, the point from which the crude acrylic acid stream (RAS) is taken in step b).

8. The method according to claim 7, wherein in step m) the mother acid (MS) is separated into two mother acid streams (MS-a and MS-b) and the method further comprises step n): n) Feeding the mother acid stream (MS-b) into the cracking column (S-col).

9. Method according to one of claims 1 to 8, characterized in that the second recirculated gas stream (KGS2) taken in step i) is completely fed to the device (V2) according to step j), wherein in the device (V2) the recirculated gas stream (KGS) is preferably combined with quenched product gas mixture (PGM) and the mixture obtained is subsequently fed to the separation column (T-col).

10. A process according to any one of claims 1 to 9, characterized in that the extraction unit (ExE) comprises an extraction column (E-col) and a second separation column (2TK), wherein in the extraction column (E-col) acrylic acid is extracted from the liquid phase by extraction with an organic solvent. EB24-0362PCBASF SE 240362W001 45 The first stripping gas stream (SGS1) is depleted from the FPS stream and the acrylic acid-enriched organic solvent is brought into contact with the first stripping gas stream (SGS1) in the second separation column (2TK), obtaining a first recirculating gas stream (KGS1) containing the first stripping gas stream (SGS1) and acrylic acid.

11. Method according to any one of claims 1 to 10, characterized in that the separation column (T-col) is configured as i) Rectification column, ii) Distillation column, and / or iii) Condensation column is designed.

12. Method according to one of claims 1 to 11, characterized in that in i) the separation column (T-col) and / or ii) the fission column (S-col) Dual-flow floors and / or cross-flow floors are used as separation-effective installations.

13. A method according to any one of claims 1 to 12, wherein the method for purifying acrylic acid is preceded by the production of acrylic acid, in which a product gas mixture (PGM) containing acrylic acid, water vapor and impurities is produced by heterogeneously catalyzed gas-phase partial oxidation of at least one C3 precursor of acrylic acid with molecular oxygen on catalysts in the solid state at elevated temperature.

14. Method according to claim 13, characterized in that the C3 precursor of acrylic acid is propene and / or acrolein.

15. Method according to any one of claims 1 to 14, characterized in that i) the crude acrylic acid stream (RAS) according to step b) contains at least 90 wt% acrylic acid as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural, and / or EB24-0362PCBASF SE 240362W001 46 ii) the mother acid (MS) according to step I) contains at least 90 wt% acrylic acid as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural.

16. Plant for the purification of acrylic acid according to any one of claims 1 to 15, comprising a separation column (T-col) a splitting column (S-col) an extraction unit (ExE) a first line (L1) connecting the sump of the separation column (T-col) with the splitting column (S-col) a second line (L2) leading from the top of the splitting column (S-col), wherein the second line (L2) splits into two lines (L2-a and L2-b), wherein the line (L2b) is connected to the separation column (T-col) and preferably opens into the separation column (T-col) above the feed point for the product gas mixture (PGM); a third line (L3) connecting the separation column (T-col) and the bottom of the extraction unit (ExE), wherein the third line (L3) opens into the separation column (T-col) above the withdrawal point of the crude acrylic acid stream (RAS). A fourth line (L4) extends from the head of the extraction unit (ExE), wherein the fourth line (L4) enters the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is drawn off, and wherein the third line (L3) preferably enters the separation column (T-col) above the point where the fourth line (L4) enters the separation column (T-col).

17. System according to claim 16, further comprising a device (V2), a fifth line (L5) connecting the device (V2) to the separation column (T-col), wherein the fifth line (L5) is used for feeding in the product gas mixture ((PGM), the line (L2a) enters the device (V2), a branch from the first line (L1) exiting the separation column (T-col), wherein this branch leads into the device (V2).

18. System according to claim 17, further comprising a device (V1) EB24-0362PCBASF SE 240362W001 47 a sixth line (L6) connecting the separation column (T-col) and the device (V1), a seventh line (L7) leading from the device (V1), which splits into two lines (L7-a and L7-b), wherein the line (L7-a) is connected to the separation column (T-col) and opens into the separation column above or below, preferably above, the sixth line (L6).

19. Plant according to one of claims 16 to 18, characterized in that the extraction unit (ExE) comprises an extraction column (E-col) and a second separation column (2TK), wherein the third line (L3) opens into the bottom of the extraction column (E-col), the extraction column (E-col) and the second separation column (2TK) are connected to each other by two lines and the fourth line (L4) extends from the top of the second separation column (2TK). EB24-0362PC

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