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

Abstract

The invention relates to a method for purifying acrylic acid, the method involving the serial 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 introduced into a splitting column (S-Kol). According to steps g) to j), 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), wherein at least a part of the second circulating gas stream (KGS2) is introduced into the separation column (T-Kol) above the location at which the product gas mixture (PGM) is fed 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] Process for the purification of acrylic acid using a serial extraction unit and a cracking column

[0002] Description

[0003] The present invention relates to a process for the purification of acrylic acid using a serial extraction unit (ExE) and a cracked column (S-Col) comprising steps a) to j). The exact definition of each step a) to j) is given below in the description.

[0004] 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, as a component of a first recirculating gas stream (KGS1), is introduced into a cracking column (S-col). According to steps g) to j), the by-product 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 (CGS2), whereby at least a part of the second recirculating gas stream (CGS2) 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).The actual target product, acrylic acid, is part of the crude acrylic acid stream (RAS), which may require further processing.

[0005] 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.

[0006] 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 the production process, these C3 precursors, in a gaseous state, are typically diluted with inert gases such as nitrogen, CO2, saturated hydrocarbons, and / or water vapor. EB25-0415PC March 20, 2026, in a mixture with molecular oxygen, are passed over transition-metal mixed oxide catalysts at elevated temperatures and, if necessary, elevated pressure. This oxidative conversion results in a product gas mixture containing acrylic acid and minor components such as furfurals, benzaldehyde, and maleic anhydride, from which the acrylic acid must be separated.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] In German patent 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.DE 102014 114193 A1 does not, however, disclose that at least a 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 a 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.

[0011] EB25-0415PCWO 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 separating internals, is allowed to rise within the condensation column and thereby fractionally condenses, and is discharged 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 reflux liquid, and the crude acrylic acid is optionally subjected to at least one further thermal separation process for the purpose of further purification. However, WO 2008 / 090190 does not disclose a process in which at least a portion of a recirculated gas stream containing monomeric acrylic acid, extracted from a cracking column, is fed into a separation column, wherein at least a portion 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.

[0012] 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 separation column (T-col) into a cracking column (S-col). In the cracking column (S-col), according to step d), at least a portion of the impurity stream (NKS) is cracked to obtain monomeric acrylic acid.The monomeric acrylic acid is withdrawn as part of a recirculated gas stream (CGS) at the top of the cracking column (S-col) according to step e). 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 series.

[0013] 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.

[0014] 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.

[0015] 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):

[0016] 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),

[0017] 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),

[0018] 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),

[0019] EB25-0415PCd) Extraction of at least a portion 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,

[0020] 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),

[0021] f) 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),

[0022] g) Splitting at least part of the by-product stream (NCS) in the cracking column (S-col) to obtain monomeric acrylic acid,

[0023] h) Feeding at least part of the first recirculating gas stream (KGS1) taken in step e) to the bottom of the cracking column (S-col),

[0024] 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 at least a part of the first recirculating gas stream (KGS1) withdrawn in step e), the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col),

[0025] j) Feeding at least part of the second recirculating gas stream (KGS2) taken in step i) into the separation column (T-col), wherein at least part of the second recirculating gas stream (KGS2) 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).

[0026] The aforementioned problem is solved in a second aspect of the present invention by a system for the purification of acrylic acid, comprising

[0027] a separation column (T-col)

[0028] a splitting column (S-col)

[0029] an extraction unit (ExE)

[0030] a first line (L1) connecting the sump of the separation column (T-col) with the splitting column (S-col)

[0031] a second line (L2) leading from the top of the cracking column (S-col), wherein the second line (L2) is connected to the separation column (T-col) and enters the separation column (T-col) above the feed point for the product gas mixture (PGM).

[0032] EB25-0415PC 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),

[0033] a fourth line (L4) leading from the head of the extraction unit (ExE), wherein the fourth line (L4) leads into the cracking column (S-col).

[0034] The present invention has the advantage that at least a part of the second recirculating 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), in particular above the point where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col), so that the proportion of impurities with compounds having a lower boiling point than the acrylic acid, and in particular the proportion of acetic acid in the crude acrylic acid stream (RAS) and thus also in the purified acrylic acid (AAS) obtained, can be significantly reduced.

[0035] This advantage is further enhanced in the process according to the invention if, in the recycling of at least a part of the mother acid (MS) or optionally the mother acid stream (MS-a) 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 thereby contain less acetic acid or other low-boiling by-products as impurities in the discharge of the crude acrylic acid stream (RAS) towards the device (V1).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.

[0036] A further advantage of the inventive process when using the splitting column and extraction unit in series is that, compared to a parallel arrangement of these device elements, the electricity consumption is lower. This is preferably due to the fact that compressors have to pump less gas, in particular stripping gas and / or recirculating gas, when these device elements are arranged in series.

[0037] EB25-0415PCS If, within the scope of the present invention, a portion of the first recirculated gas stream (KGS1) withdrawn in step e) is additionally introduced directly into the separation column (T-col), preferably at least a portion of the first recirculated gas stream (KGS1) being 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), further advantages are associated. The process according to the invention can then be carried out both in parallel and in series with respect to the use of an extraction unit and a cracking column. The parallel use of an extraction unit and a cracking column has the additional advantage that, in the process according to the invention for the purification of acrylic acid, acrylic acid, i.e., the valuable product, can be recovered simultaneously in steps c) to e) and in steps g) to i).Furthermore, it is possible that the required stripping gas flows (SGS1) and (SGS2) can be used at lower temperatures in the respective process steps compared to performing the respective steps individually.

[0038] 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.

[0039] 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).

[0040] The present invention is described in detail below.

[0041] 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.

[0042] EB25-0415PC 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):

[0043] 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),

[0044] 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),

[0045] 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),

[0046] 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,

[0047] 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),

[0048] f) Feeding at least a part of the by-product stream (VPC) from the bottom of the separation column (T-col) into a cracking column (S-col),

[0049] g) Splitting at least part of the by-product stream (NCS) in the cracking column (S-col) to obtain monomeric acrylic acid,

[0050] h) Feeding at least part of the first recirculating gas stream (KGS1) taken in step e) to the bottom of the cracking column (S-col),

[0051] 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 at least a part of the first recirculating gas stream (KGS1) withdrawn in step e), the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col),

[0052] EB25-0415PCj) Feeding at least part of the second recirculating gas stream (KGS2) taken in step i) into the separation column (T-col), wherein at least part of the second recirculating gas stream (KGS2) 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).

[0053] The method according to the invention comprises steps a) to j) and is described below.

[0054] 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 completely depleted of impurities and optionally water, whereas the impurity component stream (NKS) contains impurities that boil at higher calculi than acrylic acid.

[0055] 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.

[0056] Typically, the liquid phase stream (FPS) contains less acrylic acid (in weight percent) 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%].

[0057] 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.

[0058] 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 are understood to be, for example, diacrylic acid, polyacrylic acid, nitrogen, maleic anhydride and oxygen, as well as formaldehyde, acrolein, EB25-0415PCaformic acid, acetic acid, propionic acid, furfural, 2-furfural, benzaldehyde, 4-methoxyphenol, benzoic acid, phthalic anhydride, phenothiazine, propene, propane, carbon dioxide and carbon monoxide.

[0059] 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.

[0060] 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.

[0061] 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).

[0062] 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).

[0063] 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 typically designed as a cylindrical tube insulated to prevent heat loss. For the evaporation of 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 apparatus adjacent to the column. Almost any conceivable heat source for evaporation can be used, such as hot water, electricity, microwaves, or waste heat from other processes or other process steps of the present process.For the condensation of 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 adjacent to 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 within the column, the column body can contain internals such as screens, bubble-cap trays, 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.

[0064] 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.

[0065] 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.

[0066] According to the invention (EB25-0415PC), 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 minor components with lower boiling points than water can be withdrawn from the top of the separation column (T-col).

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] In the context of the present invention, the secondary component stream (SCS) contains acrylic acid and secondary components and optionally water, wherein secondary 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 secondary component stream (SCS) contains in particular 50 wt.% to 65 wt.% acrylic acid, EB25-0415PC15 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.

[0072] 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).

[0073] 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).

[0074] 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.

[0075] 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).

[0076] 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.

[0077] EB25-0415PCThe 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.

[0078] 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.

[0079] 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.

[0080] 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).

[0081] 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 to 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 to 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.

[0082] 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 dissolved in water, is completely or at least partially absorbed in an organic solvent supplied from above. This stream, containing the organic solvent and the absorbed acrylic acid, exits the first column at the bottom and enters the second column at the top.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] The organic solvent is, for example, at least one ester or diester consisting 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.

[0087] In the process according to the invention, in step f) 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).

[0088] 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, the information already provided regarding the separation-effective internals applies analogously to the separation-effective internals already mentioned in connection with the separation column (T-col). 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.

[0089] Optionally, the by-product stream (BPC) can be completely introduced into the splitting column (S-col). However, within the scope of the present invention, it is preferred that the by-product stream (BPC) be separated into at least two parts. The separation of the by-product stream (BPC) can take place inside or, preferably, outside the splitting column (T-col).

[0090] Preferably, a first portion of the by-product stream (NPS) according to step f) 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%.

[0091] In the cracking column (S-col), at least a portion of the by-product stream (BPC) is cracked in step g) according to the invention, yielding monomeric acrylic acid. Preferably, in step h), at least a portion of the oligomeric acrylic acid contained in the by-product stream (BPC) is cracked in the cracking column (S-col), yielding monomeric acrylic acid. This is typically done 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 g) is carried out as countercurrent rectification.

[0092] 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.

[0093] For the efficiency of the process according to the invention, and in particular for the yield of the valuable product, it has proven advantageous if, in step g), 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.

[0094] The sump of the cracking column (S-col) contains in particular the high-boiling fractions, which are drawn off and disposed of.

[0095] 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.

[0096] 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.

[0097] Preferably, the cracking column (S-col) comprises 45 trays as separation-effective internals, which are preferably designed as dual-flow trays.

[0098] Furthermore, the inventive method comprises step h):

[0099] h) Feeding at least part of the first recirculating gas stream (KGS1) taken in step e) to the bottom of the cracking column (S-col).

[0100] In one embodiment of the present invention, the first recirculating gas stream (KGS1) withdrawn in step e) is completely introduced into the cracking column (S-col) according to step h). 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 h). The separation of the first recirculating gas stream (KGS1) withdrawn in step e) can take place according to step h) inside or, preferably, outside the extraction unit (ExE).

[0101] Preferably, a first part of the first recirculating gas stream (KGS1) withdrawn in step e) is introduced into the separation column (T-col) according to step f) as described in detail below, and a second part of the first recirculating gas stream (KGS1) is introduced into the tray of the cracking column (S-col) according to the present step h). The first part and the second part of the first recirculating gas stream (KGS1) withdrawn in step e) can be in any ratio (in vol.%) to each other.

[0102] EB25-0415PC Preferably, the ratio of the first part to the second part of the first circulating gas stream (KGS1) is in the range of 40 to 100 vol.%, particularly 100 vol.%.

[0103] Furthermore, the method according to the invention includes step i):

[0104] 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 at least part of the first recirculating gas stream (KGS1) withdrawn in step e), the second stripping gas stream (SGS2) and monomeric acrylic acid, at the top of the cracking column (S-col).

[0105] Step i) is preferably carried out simultaneously with step h) in the method according to the invention.

[0106] The two streams introduced into the bottom of the cracking column (S-col) in steps h) and i) of the process according to the invention (second stripping gas stream (SGS2) and at least a part of the first recirculating gas stream (KGS1) withdrawn in step e) can be introduced into the bottom of the cracking column (S-col) at the same or different locations. Optionally, the two aforementioned streams can also be mixed together before being introduced into the bottom of the cracking column (S-col).

[0107] 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 sump liquid. As mentioned above, the stripping gas stream is used in particular as a recirculating gas.

[0108] 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.

[0109] If steps h) and i) are carried out simultaneously, this is preferably done by countercurrent rectification of the by-product stream (NPS) in the cracking column (S-col) to separate the by-products contained in the NPS. The resulting monomeric acrylic acid is typically discharged from the top of the cracking column (S-col) as a gas mixture without condensation, together with the supplied second stripping gas stream (SGS2) and at least a portion of the first recirculating gas stream (KGS1) withdrawn in step e), in the form of the second recirculating gas stream (KGS2). The preferably selective stripping of the monomeric acrylic acid is based on its high vapor pressure.

[0110] 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 65 wt.% nitrogen, 30 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 1 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, propene, and propane. The second recirculating gas stream (KGS2) preferably has a temperature of 80 °C to 100 °C.

[0111] In the process according to the invention, in step j) at least a portion of the second recirculating gas stream (KGS2) withdrawn in step i) is fed into the separation column (T-col), wherein at least a portion of the second recirculating gas stream (KGS2) 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). As already mentioned, the second recirculating gas stream (KGS2) contains at least a portion of the first recirculating gas stream (KGS1) withdrawn in step e), the second stripping gas stream (SGS2), and monomeric acrylic acid.

[0112] Furthermore, according to the invention, it is preferred that in step j) at least a portion of the second recirculating gas stream (KGS2) withdrawn in step i) is introduced into the separation column (T-col) above the point where, according to step b), the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col). Preferably, this is done at least 10 trays, and even more preferably at least 30 trays, above the withdrawal point of the crude acrylic acid stream (RAS).

[0113] According to the invention, it is even more preferred that in step j) at least a portion of the second recirculating gas stream (KGS2) withdrawn in step i) is introduced into the separation column (T-col) above the point where, according to (the optionally defined step m), the mother acid (MS) or, optionally, the mother acid stream (MS-a) is recycled back into the separation column (T-col). It is particularly preferred that at least a portion of the mother acid (MS) or, optionally, the mother acid stream (MS-a) is fed into the separation column (T-col) above the point from which the crude acrylic acid stream (RAS) is withdrawn in step b). Preferably, this occurs at least 5 trays, and even more preferably at least 10 trays, above the feed point of the mother acid (MS) or, optionally, the mother acid stream (MS-a) into the separation column (T-col).

[0114] EB25-0415PC As shown below, at least a part of the second recirculating gas stream (KGS2) taken in step i), containing at least a part of the first recirculating gas stream (KGS1) taken in step e), the second stripping gas stream (SGS2) and monomeric acrylic acid, can be separated before being fed into the separation column (T-col) and, for example, mixed with another stream, in particular with the product gas mixture (PGM) and / or fed into the lower part of the separation column (T-col).

[0115] Preferably, at least 60 ol%, more preferably at least 80 ol%, of the second recirculating gas stream (KGS2), containing at least a part of the first recirculating gas stream (KGS1) withdrawn in step e), the second stripping gas stream (SGS2) and monomeric acrylic acid, are introduced into the separation column (T-col) above the point where the product gas mixture (PGM) is fed into the separation column (T-col).

[0116] In a preferred embodiment of the present invention, the complete 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).

[0117] In one embodiment of the process according to the invention, it is preferred that at least a portion of the first recirculated gas stream (KGS1) withdrawn in step e) is introduced directly into the separation column (T-col), wherein preferably at least a portion of the first recirculated 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). 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.

[0118] This option has already been mentioned above in connection with process step h) according to the invention. In other words, this means that a (further) portion of the first recirculating gas stream (KGS1) withdrawn in step e) is not introduced into the bottom of the stripping column (S-col) according to process step h). Instead, said (further) portion of the first recirculating gas stream (KGS1) withdrawn in step e) is introduced directly into the separation column (T-col). The term “directly” thus means that said (further) portion of the first recirculating gas stream (KGS1) withdrawn in step e) is introduced into the separation column (T-col) after withdrawal at the top of the extraction unit (ExE) without any intermediate stages, in particular without contact with a further / second stripping gas stream (SGS2).

[0119] EB25-0415PC In one embodiment of the present invention, a portion of the first recirculating gas stream (KGS1) withdrawn in step e) and at least a portion of the second recirculating gas stream (KGS2) withdrawn in step i) are introduced into the separation column (T-col) at the same point above the point where the product gas mixture (PGM) is fed into the separation column (T-col).

[0120] Furthermore, it is preferred that at least a part of the first recirculating gas stream (KGS1) taken in step e) and at least a part of the second recirculating gas stream (KGS2) taken in step i) are mixed together before being introduced into the separation column (T-col).

[0121] In another embodiment of the present invention, a portion of the first recirculating gas stream (KGS1) withdrawn in step e) and at least a portion of the second recirculating gas stream (KGS2) withdrawn in step i) are each introduced into the separation column (T-col) at different points above the point where the product gas mixture (PGM) is fed into the separation column (T-col). Preferably, in this embodiment of the present invention, at least a portion of the second recirculating gas stream (KGS2) withdrawn in step i) is introduced into the separation column (T-col) above the point where the liquid phase stream (FPS) is withdrawn from the separation column (T-col). Conversely, at least a portion of the first recirculating gas stream (KGS1) is introduced into the separation column (T-col) below the point where the liquid phase stream (FPS) is withdrawn from the separation column (T-col).

[0122] It is also preferred that the introduction of at least a part of the second recirculating gas stream (KGS2) withdrawn in step i) and / or at least a part of the first recirculating gas stream (KGS1) 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).

[0123] Furthermore, within the scope of the present invention, it is preferred that step a) is preceded by a further step a1):

[0124] 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).

[0125] 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 (KGS2) withdrawn in step i) is introduced into the device (V2) EB25-0415PC as a quench medium to obtain the quenched product gas mixture (PGM). Optionally, the second recirculating gas stream (KGS2) 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.

[0126] 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).

[0127] In this context, reference is also made to Figure 3 and, in conjunction with the portion of the by-product stream (NPS), to Figure 5. The mixture obtained is preferably fed into the lower part of the separation column (T-col), in particular into the bottom of the separation column (T-col).

[0128] Figure 3 shows the inventive process in its aforementioned further development, wherein the provisions already stated for Figures 1 and 2 apply, 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, however, requires that at least a portion of the second recirculating gas stream (KGS2), containing at least a portion of the first recirculating gas stream (KGS1) withdrawn in step e), 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 vol%, 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 supply this separated portion to the device (V2).

[0129] 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.

[0130] 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.

[0131] Therefore, 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", is preferably used.

[0132] The material, designated EB25-0415PC(PGM), is first fed to a device (V2). Typically, the device (V2) is a quench device.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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).

[0137] 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.

[0138] EB25-0415PC If necessary, water can be added to the crude acrylic acid stream (RAS) to be purified by crystallization prior to 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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):

[0143] 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),

[0144] m) Recycling of 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) EB25-0415PC 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).

[0145] 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 mother acid stream obtained in this way is hereinafter referred to as "mother acid stream (MS-a)" and the second mother acid stream obtained as "mother acid stream (MS-b)".

[0146] 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.

[0147] 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.

[0148] 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).

[0149] 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).

[0150] EB25-0415PCThe 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.

[0151] According to the invention, it is also preferred that the mother acid (MS) is completely or, if it is separated into two parts in step I), the mother acid stream (MS-a) is returned between 10 and 50 trays, preferably 20 to 50 trays, above the withdrawal point for the crude acrylic acid stream (RAS) to the separation column (T-col).

[0152] 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).

[0153] 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.

[0154] 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.

[0155] Figure 4 shows the inventive process in its aforementioned further developments, with the provisions already stated for Figures 1-3 initially applying. Additionally, according to step k), the crude acrylic acid stream (RAS) from step b) is fed to a device (V1), whereby 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, according to which the mother acid (MS) is removed from the device (V1) and optionally 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).

[0156] 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):

[0157] n) Feeding the mother acid stream (MS-b) into the cracking column (S-col).

[0158] 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).

[0159] 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).

[0160] According to the invention, it is further preferred that the separation column (T-col) be used as

[0161] i) Rectification column,

[0162] ii) Distillation column, and / or

[0163] iii) Condensation column

[0164] is designed.

[0165] 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 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.

[0166] Furthermore, it is preferred according to the invention that in the inventive method in

[0167] i) the separation column (T-col) and / or

[0168] ii) the fission column (S-col)

[0169] Dual-flow floors and / or cross-flow floors are used as separation-effective installations.

[0170] 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.

[0171] 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.

[0172] According to the invention, it is preferred that the C3 precursor of the acrylic acid is propene and / or acrolein.

[0173] 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.

[0174] Furthermore, it has proven advantageous that the

[0175] EB25-0415PCi) 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

[0176] 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.

[0177] 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.

[0178] Another object of the present invention is therefore a plant for the purification of acrylic acid comprising

[0179] a separation column (T-col)

[0180] a splitting column (S-col)

[0181] an extraction unit (ExE)

[0182] a first line (L1) connecting the sump of the separation column (T-col) with the splitting column (S-col)

[0183] a second line (L2) leading from the top of the cracking column (S-col), wherein the second line (L2) is connected to the separation column (T-col) and enters the separation column (T-col) above the feed point for the product gas mixture (PGM).

[0184] 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),

[0185] a fourth line (L4) leading from the head of the extraction unit (ExE), wherein the fourth line (L4) leads into the cracking column (S-col).

[0186] 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.

[0187] 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.

[0188] The advantages of the system according to the invention are essentially the same as for the method according to the invention described above.

[0189] EB25-0415PC The system according to the invention is particularly advantageous when integrated into a complete system for acrylic acid production. As described above, the system according to the invention can increase the overall efficiency of acrylic acid production while simultaneously reducing the equipment requirements.

[0190] Furthermore, the inventive process can be advantageously implemented in a further development using a further developed system, which is illustrated, for example, by Figure 2. This further developed system additionally comprises a fifth line (L5) connecting the separation column (T-col) and the head of the extraction unit (ExE), wherein the fifth line (L5) preferably opens into the separation column (T-col) above the point where the crude acrylic acid stream (RAS) is drawn off, and more preferably, the third line (L3) opens into the separation column (T-col) above the point where the fifth line (L5) enters the separation column (T-col).

[0191] 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

[0192] a device (V1)

[0193] a sixth line (L6) connecting the separation column (T-col) and the device (V1),

[0194] 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).

[0195] 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 5. This further developed system additionally comprises

[0196] a device (V2),

[0197] an eighth line (L8) connecting the device (V2) to the separation column (T-col), wherein the eighth line (L8) is used for feeding in the product gas mixture ((PGM),

[0198] 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) EB25-0415PC and opens into the separation column (T-col) above the feed point for the product gas mixture (PGM) and the line (L2a) opens into the device (V2),

[0199] a branch from the first line (L1) exiting the separation column (T-col), wherein this branch leads into the device (V2).

[0200] In one embodiment of the present invention, the second line (L2) or the line (L2b) opens into the separation column (T-col) at the same point above the feed point for the product gas mixture (PGM) as the fifth line (L5).

[0201] In another embodiment of the present invention, the second line (L2) or line (L2b) opens into the separation column (T-col) at different points above the feed point for the product gas mixture (PGM) than the fifth line (L5). Preferably, in this embodiment of the present invention, the second line (L2) or line (L2b) opens into the separation column (T-col) above the third line (L3), and the third line (L3) in turn opens into the separation column (T-col) above the point where the fifth line (L5) opens.

[0202] Particularly preferred according to the invention is a plant 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 separation column (2TK) are connected to each other by two lines and the fourth line (L4) leads from the top of the second separation column (2TK).

[0203] Reference symbol list for figures 1 to 5:

[0204] T-Kol separation column (T-Kol)

[0205] ExE Extraction Unit (ExE)

[0206] V1 Device (V1)

[0207] PGM product gas mixture (PGM)

[0208] RAS Crude Acrylic Acid Stream (RAS)

[0209] FPS Liquid Phase Stream (FPS)

[0210] MS mother acid

[0211] MS-a first part of the mother acid stream (MS-a)

[0212] MS-b second part of the mother acid stream (MS-b)

[0213] NKS secondary component current (NKS)

[0214] AAS Purified Acrylic Acid (AAS)

[0215] S-Col splitting column (S-Col)

[0216] SGS1 first strip gas stream (SGS1)

[0217] EB25-0415PCKGS1 first circuit gas flow (KGS1)

[0218] SGS2 second strip gas stream (SGS2)

[0219] KGS2 second closed-loop gas flow (KGS2)

[0220] V2 Device V2

[0221] 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.

[0222] 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.

[0223] Example according to the invention 11

[0224] 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.

[0225] The present inventive example 11 can be illustrated in particular with reference to Figure 1. For a description of the central elements in Figure 1, reference is made to the corresponding explanations above.

[0226] A hot product gas mixture (PGM) at a temperature of approximately 260 °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), composed as shown in Table 1, is fed into the bottom of a separation column (T-col).

[0227] Table 1. Main composition of the product gas mixture (PGM)

[0228] Components PGM wt%

[0229] Nitrogen 75.3

[0230] Acrylic acid 11.8

[0231] Water 5.8

[0232] Oxygen 3.2

[0233]

[0234] EB25-0415PCCO I CO2 2.8

[0235] Acetic acid 0.24

[0236] Maleic anhydride 0.10

[0237]

[0238] In this separation column (T-col), the produced acrylic acid is separated from the product gas mixture (PGM) by thermal separation and discharged from the column. The discharged acrylic acid stream, also known 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.

[0239] Table 2. Main composition of the crude acrylic acid stream (RAS)

[0240] Components RAS Weight %

[0241] Acrylic acid 97.0

[0242] Acetic acid 0.570

[0243] Propionic acid 0.099

[0244] Furfural 0.087

[0245]

[0246] A first stripping gas stream (SGS1), originating as recycled gas from a plant section not described in detail, where a portion of the exhaust gas is recycled and compressed, is fed to the extraction unit (ExE). This first stripping gas stream (SGS1), which has the main composition according to Table 3, has a temperature of approximately 160 °C. A liquid phase stream (FPS) containing acrylic acid, water, and other light components at a temperature of approximately 60 °C is fed from the upper section of the separation column (T-col) to the extraction unit (ExE).

[0247] Table 3. Main composition of the first stripping gas stream (SGS1)

[0248] Components SGS1 wt%

[0249] Nitrogen 90.3

[0250] Acrylic acid 0.15

[0251] Water 1.76

[0252] Oxygen 3.88

[0253] CO / CO2 3.36

[0254] Acetic acid 0.07

[0255]

[0256] SGS2 has the same composition as SGS1.

[0257] 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, as shown in Table 4.

[0258] EB25-0415PC Table 4. Main composition of the liquid phase stream (FPS)

[0259] Components FPS Weight %

[0260] Water 79.6

[0261] Acrylic acid 11.9

[0262] Acetic acid 4.89

[0263] Formaldehyde and its 3.08

[0264] Oligomers

[0265] Formic acid 0.49

[0266]

[0267] 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 with the organic solvent, particularly within the extraction unit (ExE), and is removed and disposed of.

[0268] A second stripping gas stream (SGS2), originating as recycled gas from an unspecified section of the plant, is directed onto a tray in the lower section of a cracking column (S-col). This second stripping gas stream (SGS2) has a temperature of approximately 160 °C. A by-product stream (NKS), containing oligomeric acrylic acid, is directed 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.

[0269] 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.

[0270] In the cracking column (S-col), these by-components, especially the acrylic acid oligomers, are back-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), which has the main composition shown in Table 6, and fed to the separation column (T-col) above the position where the crude acrylic acid stream (RAS) is withdrawn from the separation column (T-col). The high-boiling fractions, in particular, remain in the bottom of the cracking column (S-col) and are removed and disposed of.

[0271] In the present example, a first recirculating gas stream (KGS1), which has the main composition as shown in Table 5, is fed into the lower section of the cracking column (S-col) by recycling (or injection), with the entire first recirculating gas stream (KGS1) being introduced into the lower section of the cracking column (S-col) at the point where the second stripping gas stream (SGS2) is injected into the cracking column (S-col); and the second recirculating gas stream (KGS2) is recycled (or injection) into the separation column (T-col), with the entire second recirculating gas stream (KGS2) being introduced onto an EB25-0415PC bottom of the separation column (T-col) located above the product gas mixture (PGM) feed line into the separation column (T-col); the yield of acrylic acid as a valuable product is significantly increased, and the content of acetic acid as a byproduct is reduced.Using the process and plant according to the present example, it is possible to produce 27 t / h of crude acrylic acid (RAS) with an acetic acid content of < 0.5 wt%.

[0272] As a result, the crude acrylic acid stream (RAS) at the outlet of the separation column (T-col) has a high acrylic acid purity of 97.0 wt%, with a low acetic acid content of 0.570 wt% and a low furfural content of 0.087 wt%.

[0273] Table 5. Main composition of the first recirculating gas stream (KGS1)

[0274] Components KGS1 wt%

[0275] Nitrogen 81.0

[0276] Acrylic acid 5.56

[0277] Water 5.21

[0278] Oxygen 2.49

[0279] CO2 2.36

[0280] Acetic acid 1.79

[0281]

[0282] Table 6. Main composition of the second closed-loop gas stream (KGS2)

[0283] Components KGS2 wt%

[0284] Nitrogen 67.2

[0285] Acrylic acid 24.4

[0286] Water 2.9

[0287] Oxygen 2.1

[0288] CO2 2.0

[0289] Acetic acid 0.25

[0290] Maleic anhydride 0.003

[0291] Benzaldehyde 0.005

[0292]

[0293] Comparison example C1

[0294] Comparative Example C1 is carried out like inventive Example 11, except that the second recirculating gas stream (KGS2), which is taken from the top of the cracking column (S-col), is fed completely into the bottom of the separation column (T-col) together with the product gas mixture (PGM). Furthermore, the feed point for the two combined streams is located below the point where the crude acrylic acid stream (RAS) is taken from the separation column (T-col). The main composition of the relevant streams is shown in Tables 7 to 11.

[0295] EB25-0415PC In this way, the crude acrylic acid stream (RAS) has a temperature of approximately 100 °C when it is drawn from the separation column (T-col), and the calculated composition is shown in Table 7. As a result, the crude acrylic acid stream (RAS) at the outlet of the separation column (T-col) has a lower acrylic acid purity of 96.5 wt% with a higher acetic acid content of 0.78 wt% and a higher furfural content of 0.106 wt%.

[0296] Table 7. Main composition of the crude acrylic acid stream (RAS)

[0297] Components RAS Weight %

[0298] Acrylic acid 96.5

[0299] Acetic acid 0.780

[0300] Propionic acid 0.102

[0301] Furfural 0.106

[0302]

[0303] Table 8. Main composition of the first stripping gas stream (SGS1)

[0304] Components SGS1 wt%

[0305] Nitrogen 90.7

[0306] Acrylic acid 0.10

[0307] Water 1.38

[0308] Oxygen 3.89

[0309] CO / CO2 3.38

[0310] Acetic acid 0.05

[0311]

[0312] Table 9. Main composition of the liquid phase stream (FPS)

[0313] Components FPS Weight %

[0314] Water 81.6

[0315] Acrylic acid 10.0

[0316] Acetic acid 4.74

[0317] Formaldehyde and its 3.11

[0318] Oligomers

[0319] Formic acid 0.49

[0320]

[0321] Table 10. Main composition of the first recirculating gas stream (KGS1)

[0322] Components KGS1 wt%

[0323] Nitrogen 83.0

[0324] Acrylic acid 4.88

[0325] Water 4.42

[0326] Oxygen 2.56

[0327] CO2 2.42

[0328] Acetic acid 1.18

[0329]

[0330] EB25-0415PC Table 11. Main composition of the second closed-loop gas stream (KGS2) Components KGS2 wt%

[0331] Nitrogen 71.1

[0332] Acrylic acid 17.5

[0333] Water 4.6

[0334] Oxygen 2.2

[0335] CO2 2.1

[0336] Acetic acid 1.03

[0337] Maleic anhydride 0.012

[0338] Benzaldehyde 0.030

[0339]

[0340] EB25-0415PC

Claims

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 a part of the by-product stream (VPC) from the bottom of the separation column (T-col) into a cracking column (S-col), g) Splitting at least part of the by-product stream (NCS) in the cracking column (S-col) to obtain monomeric acrylic acid, h) Feeding at least part of the first recirculating gas stream (KGS1) taken in step e) to the bottom of the cracking column (S-col), 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 at least a portion of the first gas stream withdrawn in step e). EB25-0415PC Circulating gas stream (KGS1), the second stripping gas stream (SGS2) and monomeric acrylic acid, at the head of the cracking column (S-col), j) Feeding at least part of the second recirculating gas stream (KGS2) taken in step i) into the separation column (T-col), wherein at least part of the second recirculating gas stream (KGS2) 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).

2. Method according to claim 1, characterized in that i) at least a part of the first recirculating gas stream (KGS1) withdrawn in step e) is introduced directly into the separation column (T-col), preferably at least a part of the first recirculating gas stream (KGS1) being 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), and / or ii) at least a part of the first recirculating gas stream (KGS1) taken in step e) and at least a part of the second recirculating gas stream (KGS2) taken in step i) are mixed together before being introduced into the separation column (T-col).

3. Method according to claim 2, characterized in that the feeding of at least a part of the second circulating gas stream (KGS2) withdrawn in step i) and / or at least a part of the first circulating gas stream (KGS1) 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).

4. A method according to any one of claims 1 to 3, 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 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.

5. Method according to any one of claims 1 to 4, comprising the further step a1), which precedes step a): EB25-0415PCa1) 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-col), Preferably, a portion of the by-product stream (NKS) from step a) and / or a portion of the second recirculating gas stream (KGS2) taken in step i) is introduced into the device (V2) as quench liquid to obtain the quenched product gas mixture (PGM), 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). EB25-0415PC9. Method according to one of claims 1 to 8, characterized in that the second recirculating gas stream (KGS2) withdrawn in step i) is completely fed into the separation column (T-col) according to step j), wherein the second recirculating gas stream (KGS2) is completely introduced into the separation column (T-col) above the point where the product gas mixture (PGM) enters the separation column (T-col).

10. Method according to any one of claims 1 to 9, characterized in that the separation column (T-col) is configured as i) Rectification column, ii) Distillation column, and / or iii) Condensation column is designed.

11. Method according to one of claims 1 to 10, 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.

12. A process according to any one of claims 1 to 11, wherein 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.

13. Method according to claim 12, characterized in that the C3 precursor of acrylic acid is propene and / or acrolein.

14. Method according to any one of claims 1 to 13, 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 EB25-0415PCii) 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.

15. Plant for the purification of acrylic acid according to any one of claims 1 to 14, 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 cracking column (S-col), wherein the second line (L2) is connected to the separation column (T-col) and enters 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) enters the separation column (T-col) above the point of withdrawal of the crude acrylic acid stream (RAS), a fourth line (L4) leading from the head of the extraction unit (ExE), wherein the fourth line (L4) leads into the cracking column (S-col).

16. System according to claim 15, further comprising a fifth line (L5) connecting the separation column (T-Kol) and the head of the extraction unit (ExE), wherein the fifth line (L5) preferably enters the separation column (T-Kol) above the point of withdrawal of the crude acrylic acid stream (RAS), wherein more preferably the third line (L3) enters the separation column (T-Kol) above the point of entry of the fifth line (L5).

17. System according to claim 16, further comprising a device (V1) a sixth line (L6) connecting the separation column (T-col) and the device (V1), a seventh line (L7) extending 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 is located above or below, EB25-0415PC preferably above the sixth line (L6) into the separation column.

18. Plant according to one of claims 15 to 17, 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). EB25-0415PC

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