Process for continuously producing methacrylic acid by hydrolysing alkyl methacrylates

Abstract

The present invention relates to a process for producing methacrylic acid by reacting a C1-C3 alkyl methacrylate with water. In this process, the reaction mixture obtained during the reaction is separated in a first rectification column (A) and the top stream (12) obtained is separated via a phase separator (a) to obtain an organic phase (5) and an aqueous phase (4). The resulting bottom stream (6) is separated in a second rectification column (C) to obtain a top stream (9) containing the methacrylic acid.

Description

[0001] Process for the continuous production of methacrylic acid by hydrolysis of alkyl methacrylates

[0002] Description

[0003] Field of the invention

[0004] The present invention relates to a process for producing methacrylic acid by reacting a Ci-Cs-alkyl methacrylate with water. In this process, the reaction mixture obtained in the reaction is separated in a first rectification column (A), and the resulting overhead stream (12) is separated via a phase separator (a) to obtain an organic phase (5) and an aqueous phase (4). The resulting bottoms stream (6) is separated in a second rectification column (C) to obtain an overhead stream (9) containing the methacrylic acid.

[0005] State of the art

[0006] Various processes for the production of methacrylic acid are known from the state of the art.

[0007] A common process involves the controlled oxidation of hydrocarbon gases, such as butylene. The disadvantage of these processes is the low yields obtained.

[0008] Furthermore, methacrylic acid can be obtained by reacting methacrylamide with water. This process is described, for example, in US Pat. No. 7,253,307. According to this document, the reaction of methacrylamide with water can be carried out in a stirred-tank reactor or a tubular reactor. The reaction is preferably carried out at a pressure in the range of 3.65 bar absolute to 7.7 bar absolute and a temperature in the range of 50 °C to 210 °C.

[0009] The processes for the production of methacrylic acid described in US 7,253,307 already lead to good yields with high purity.

[0010] Methacrylic acid is an important product in the chemical industry, serving as a starting material for many important products. Maximum yield with exceptionally high purity and low production costs are therefore particularly desirable. In particular, plant downtime and energy consumption should be kept as low as possible.

[0011] Methacrylamide is typically produced using the acetone cyanohydrin (ACH) sulfo process based on hydrogen cyanide and acetone in the presence of a large excess of sulfuric acid. This process produces large quantities of waste sulfuric acid or organically contaminated ammonium hydrogen sulfate solution. These can only be processed into sulfuric acid with considerable energy expenditure.

[0012] Furthermore, α-hydroxyisobutyric acid (2-HIBS) can serve as a starting material for the production of methacrylic acid. Such a process is described, for example, in US Pat. No. 3,487,101. This discloses the production of various methacrylic acid derivatives, in particular methacrylic acid and methacrylic acid esters, starting from 2-HIBS in the liquid phase. The conversion of 2-HIBS to methacrylic acid takes place in the presence of a dissolved basic catalyst at temperatures in the range of 180 °C to 320 °C in the presence of high-boiling esters (e.g., dimethyl phthalate) and cyclic anhydrides (e.g., phthalic anhydride). At 2-HIBS conversions of > 90%, methacrylic acid selectivities of approximately 98% are achieved. No information is provided on the long-term stability of the liquid catalyst solution or the stability of the anhydride used under reaction conditions.

[0013] RU 89631 also relates to a process for the preparation of methacrylic acid starting from 2-HIBS by dehydration in the liquid phase, the reaction being carried out in the absence of a catalyst with an aqueous solution of 2-HIBS (up to 62 wt% 2-HIBS in water) under pressure at high temperatures between 200 °C and 240 °C.

[0014] Furthermore, the use of propene as a basic raw material has been intensively investigated, producing methacrylic acid in moderate yields via hydrocarbonylation to isobutyric acid and dehydrogenative oxidation. This process involves the reaction of propene with carbon monoxide, hydrofluoric acid, or concentrated sulfuric acid, followed by hydrolysis of the intermediates in the presence of water. This process is not used in production.

[0015] Furthermore, it is known to use propanal, which is accessible in industrial processes starting from ethylene and C1 building blocks such as carbon monoxide, as a basic raw material. In these processes, the ß-hydroxycarbonyl compound formed in situ is converted into the corresponding α,β-unsaturated compound, methacrolein, in an aldolizing reaction with formaldehyde under dehydration.

[0016] An overview of the common processes for the production of methacrylic acid and its esters can be found in the literature, such as Weissermel, Arpe “Industrial Organic Chemistry”, VCH, Weinheim 1994, 4th edition, p. 305 ff or Kirk Othmer “Encyclopedia of Chemical Technology”, 3rd edition, Vol. 15, page 357.

[0017] EP 0487 853 describes the preparation of methacrylic acid starting from acetone cyanohydrin (ACH). In a first step, ACH is reacted with water at moderate temperatures in the presence of a heterogeneous hydrolysis catalyst. In a second step, a-hydroxyisobutyric acid amide is reacted with methyl formate or methanol / carbon monoxide to form formamide and methyl hydroxyisobutyric acid ester (HIBSM). In a third step, HIBSM is saponified with water in the presence of a heterogeneous ion exchanger to form HIBS. In a fourth step, HIBS is dehydrated by reacting in the liquid phase at high temperatures in the presence of a soluble alkali salt. Methacrylic acid preparation from HIBS is described at high conversions of around 99% with more or less quantitative selectivities.The multitude of required reaction steps and the necessity of intermediate isolation, especially the conduction of individual process steps at elevated pressure, make the process complicated and, especially in view of the high energy requirements, ultimately uneconomical. Furthermore, formamide is inevitably produced, a compound often considered an undesirable byproduct that requires expensive disposal. In another variant, formamide can be used to produce HCN. Ultimately, however, the disadvantages of the multi-stage reaction with complex cycles remain, which in turn leads to high energy expenditure and, in particular, to high specific steam consumption.

[0018] Finally, EP 2 714 640 discloses a process in which methacrylic acid is obtained by hydrolysis of ACH-based methyl methacrylate (MMA). The ACH is first converted to methacrylamide, which is then esterified with methanol to form MMA. Thus, unlike other processes, this MMA production process does not allow for the removal of methacrylic acid as an intermediate. Furthermore, the feed water must be heated. This is particularly disadvantageous from an energy perspective due to the high heat capacity of the water and the associated significant energy input.

[0019] However, such hydrolysis is only known for this specific ACH process. More efficient alternatives for producing MMA exist today that do not produce methacrylic acid as a by-product. For example, WO 2014 / 170223 describes a highly efficient process in which propionaldehyde is obtained from C2 fractions in a first step, which is then reacted with formaldehyde in a second step to form methacrolein (MAL). This MAL can then be oxidatively esterified to MMA in the presence of special metal or metal oxide catalysts and methanol. This process is distinguished from all other processes by its particularly high yield and particularly good selectivities. However, it has the disadvantage that methacrylic acid is not obtained as a by-product, or only in very small quantities.

[0020] A transfer of the hydrolysis process known from EP 2 714 640 to this MMA production is not known, especially since this hydrolysis process describes the use of purified MMA and not of intermediate product mixtures containing MMA in addition to the by-products specifically produced in this process.

[0021] It is also important in this context that different problematic by-products arise depending on the technology used and the raw material basis, so that there is a big difference whether the MMA is produced from an ACH-sulfo process or, for example, based on a C2 raw material (ethylene) as described in WO 2014 / 170223. A simple integration of an MMA to methacrylic acid (MAS) hydrolysis, as described in EP 2 714 640, into an MMA production process starting from methacrolein would mean that many additional separation steps would have to be incorporated, which in turn would incur high additional investment costs and overall would not show any synergy effects between the two processes.

[0022] This is also not easy to carry out, as the C2-based MMA according to WO 2014 / 170223 has a completely different by-product spectrum, particularly in the downstream processing steps, which must be taken into account when considering high yields and selectivities. It is known that MMA produced according to WO 2014 / 170223 can tend to yellow despite purification. Furthermore, this crude MMA contains particularly high concentrations of methacrylic acid, 1,1-dimethoxyisobutene, or Michael products of MAL and dimeric MAL, as well as their derivatives from oxidative esterification, such as the corresponding acids and methyl esters. Dimeric MAL, in particular, thus leads to significant amounts of methyl ester and / or acid in the MMA. If these undesired by-products are to be avoided, it is necessary to purify the MMA prior to hydrolysis. This is primarily achieved by distillation.However, the methacrylic acid, which also occurs as a by-product of the oxidative esterification, is separated and is thus lost from the yield.

[0023] Finally, EP 0 092 097 discloses a C2-based process via the intermediate methacrolein (MAL). Here, methacrylic acid is produced directly in a gas-phase step following the MAL synthesis and can optionally be discharged, worked up, and isolated. The actual goal of the process is esterification with methanol to MMA in a third step. Thus, although this process is suitable for producing methacrylic acid in addition to MMA, the achievable yields are limited by the unsatisfactory step from MAL to MAS in the gas phase, which ultimately has a negative impact on the economic viability of the process. A particular disadvantage of this process, which is efficient in terms of the raw material basis, is the subsequent conversion in the gas phase on a heteropolyacid. In this case, only partial conversions of MAL are achieved, and even according to the patent literature, the yield of methacrylic acid is a maximum of 80 to 85%.Even in the gas phase, the presence of by-products from MAL production from propionaldehyde, such as dimeric methacrolein and pentenals, has negative effects, so these by-products must be strictly limited.

[0024] US 8,791,296 B2 relates to a process for producing methacrylic acid in which a methacrylic acid ester is reacted with water. The resulting product streams are recycled multiple times, making the process energy-intensive. Furthermore, a portion of the methacrylic acid ester, particularly MMA, is distilled off as an azeotrope with the alcohol formed during hydrolysis, particularly methanol, which leads to yield losses. The process described in US 8,791,296 B2 comprises three distillation columns in series. In a first column, the methanol-methyl methacrylate azeotrope is separated; in a second column, methyl methacrylate and water are separated from methacrylic acid. The methacrylic acid is finally obtained as a distillate in a third column.

[0025] WO 2022 / 194590 A1 describes another process for producing methacrylic acid by hydrolysis of MMA. In this process, purification is carried out in a single distillation column. The reaction mixture is processed, for example, in a complex distillation column divided into four sections: a head section, an upper middle section, a lower middle section, and a bottom section. An MMA-methanol azeotrope is obtained in the head section. MMA and water, which are recycled, are obtained in the upper middle section. High-purity methacrylic acid is obtained in the lower middle section, and high-boiling by-products and an inert boiling oil are obtained in the bottom section. WO 2022 / 194590 A1 further discloses an embodiment in which the distillation column with four sections is divided into two distillation columns, each with two sections.The boiling oil used must be fed into the process and then removed again to remove high-boiling byproducts. Furthermore, the use of boiling oil is extremely energy-intensive, as it requires additional heating.

[0026] Many of the processes listed, especially those for hydrolysis, also have the disadvantage that water must be introduced into the system. However, this water must be heated to the reaction temperature before addition or in the reactor.

[0027] In particular, the processes according to US Pat. No. 8,791,296 B2 and WO 2022 / 194590 A1 are particularly steam-intensive and thus energy-intensive, as the workup is carried out exclusively by distillation. Furthermore, unreacted methyl methacrylate is removed from the processes together with methanol as an azeotrope. This reduces the molar yield of methacrylic acid based on the methyl methacrylate used.

[0028] In summary, it should be noted that to date there is no economically and technically obvious or technically simple solution to produce MMA alongside methacrylic acid, for example based on methacrolein as starting material and raw material, and in particular to produce the product ratio of the two valuable substances in a broad ratio with good overall yields and ultimately in an optimally economical manner.

[0029] Task

[0030] The object underlying the present invention was therefore to provide a novel, efficient, and economically attractive process for the production of methacrylic acid. The process should be as energy-efficient as possible and be capable of integration into larger plants for the production of C1-C3 alkyl methacrylates.

[0031] Solution This object was achieved by a process for the preparation of methacrylic acid comprising the following steps a) to e): a) Synthesis of a Ci-Cs-alkyl methacrylate in a first reactor (I), b) Reaction of at least a portion of the Ci-Cs-alkyl methacrylate synthesized in step a) with water in the presence of a first catalyst to form methacrylic acid in a second reactor (II) to obtain a reaction mixture (3) which contains methacrylic acid, water, a first alcohol, oligomers and the Ci-Cs-alkyl methacrylate, c) Separation of the reaction mixture (3) obtained in step b) in a first rectification column (A) and obtaining

[0032] • a top stream (12) containing water, the first alcohol and the Ci-Cs-alkyl methacrylate, and

[0033] • a bottom stream (6) containing methacrylic acid and the oligomers, d) separating the top stream (12) obtained in step c) in a phase separator (a) and obtaining

[0034] • an organic phase (5) containing the Ci-Cs-alkyl methacrylate, and

[0035] • an aqueous phase (4) containing water and the first alcohol, e) separating the bottom stream (6) obtained in step c) in a second rectification column (C) and obtaining

[0036] • a head stream (9) containing the methacrylic acid, and

[0037] • a bottom stream (10) containing the oligomers and residues of methacrylic acid.

[0038] Furthermore, this object was achieved by a process for the preparation of methacrylic acid comprising the following steps a) to e): a) synthesis of a Ci-Cs-alkyl methacrylate in a first reactor (I), b) reaction of at least a portion of the Ci-Cs-alkyl methacrylate synthesized in step a) with water in the presence of a first catalyst to form methacrylic acid in a second reactor (II) to obtain a reaction mixture (3) which contains methacrylic acid, water, a first alcohol, oligomers and the Ci-Cs-alkyl methacrylate, c) separation of the reaction mixture (3) obtained in step b) in a first rectification column (A) and obtaining

[0039] • a top stream (12) containing water, the first alcohol and the Ci-Cs-alkyl methacrylate, and

[0040] • a bottom stream (6) containing methacrylic acid and the oligomers, d) separating the top stream (12) obtained in step c) in a phase separator (a) and obtaining

[0041] • an organic phase (5) containing the Ci-Cs-alkyl methacrylate, and

[0042] • an aqueous phase (4) containing water and the first alcohol, e) separating the bottom stream (6) obtained in step c) in a second rectification column (C) and obtaining a top stream (9) containing the methacrylic acid and a bottom stream (10) containing the oligomers and residues of the methacrylic acid, wherein the aqueous phase (4) is separated in a third rectification column (B) to obtain

[0043] • a top stream (7) containing the first alcohol and a bottom stream (8) containing water.

[0044] The method according to the invention can in particular be adapted to varying market needs or

[0045] It takes into account market demands for methacrylic acid and MMA. It allows both products to be produced in freely selectable proportions. The process allows the production of methacrylic acid in addition to or as an alternative to MMA, particularly in MMA production processes where methacrylic acid is not an intermediate.

[0046] Compared to existing processes using strong acids, the preferential use of a strongly acidic ion exchanger as a heterogeneous catalyst allows standard material, related to corrosion resistance, to be used for construction.

[0047] The process according to the invention also has lower energy consumption, particularly due to reduced steam requirements for rectification through the use of a phase separator. At the same time, it enables higher overall yields of methacrylic acid. Furthermore, it allows the use of known, cost-effective equipment.

[0048] The low energy consumption results in particular from the fact that the overhead stream (12) obtained in step c) can be separated in the phase separator (a) in an energy-neutral manner into the organic phase (5) and the aqueous phase (4). The first alcohol, in particular methanol, as a by-product of the reaction in step b) can be removed from the process via the aqueous phase (4). In a preferred embodiment, the organic phase (5) is partly recycled to the first rectification column (organic phase (5a) recycled to the first rectification column) and partly recycled to the second reactor (II) (organic phase (5b) recycled to the second reactor). This makes it possible to increase the efficiency of the process according to the invention. Surprisingly, recirculation of the organic phase (5) to the second reactor (II) is possible, even though the organic phase (5) can contain up to 5% by weight.-% of the first alcohol, based on the total weight of the organic phase (5). Despite the presence of the first alcohol, the reaction in step b) is not disturbed.

[0049] In one embodiment of the invention, the aqueous phase (4) is separated in a third rectification column (B). The bottom stream (8) contains water, which is preferably recycled to the second reactor (II). This recycling is particularly advantageous since the bottom stream (8) has already been heated in the third rectification column (B) and thus no longer needs to be heated, or only slightly, in the second reactor (II). This also ensures energy savings in the present invention. Furthermore, since the stream fed to the third rectification column (B) is substantially smaller than that fed to the first rectification column (A) and the second rectification column (C), the third rectification column (B) can be significantly smaller, which further increases the cost-effectiveness of the process according to the invention. Another advantage of step d) is that only small amounts of C1-C8-alkyl methacrylate are present in the aqueous phase (4).This significantly increases the yield of methacrylic acid based on C1-C3 alkyl methacrylate compared to state-of-the-art processes.

[0050] The process according to the invention does not require any boiling oil, and therefore preferably does not use any boiling oil. This is advantageous because it reduces the risk of contamination of the methacrylic acid. Furthermore, disposal of the boiling oil is eliminated, and additional heating of the boiling oil, and thus additional energy consumption, is not necessary.

[0051] Methacrylic acid produced by the process according to the invention usually has low color numbers.

[0052] Another particularly advantageous feature of the process according to the invention is that water streams within the overall process, including the process for producing the C1-C8 alkyl methacrylate, can be designed to be particularly energy-efficient. Furthermore, byproduct formation in individual process steps can be controlled so that they do not interfere with each other. Furthermore, the water used in the process can be completely incorporated and converted within the process, thus avoiding the formation of an aqueous waste stream, which could be costly to dispose of.

[0053] By preferably using a first rectification column comprising a stripping section and a rectification section, a particularly gentle separation with little or even no polymerization is achieved within the first rectification column.

[0054] The present invention is described in more detail below.

[0055] The specified steps a) to e) can be performed in any order or even simultaneously. Typically, steps a), b), and c) are performed sequentially; steps d) and e) can be performed sequentially or simultaneously. Preferably, steps d) and e) are performed simultaneously.

[0056] In step a) of the process according to the invention, a Ci-Cs-alkyl methacrylate is synthesized in a first reactor (I).

[0057] In the context of the present invention, "a Ci-Cs-alkyl methacrylate" means both precisely one Ci-Cs-alkyl methacrylate and a mixture of two or more Ci-Cs-alkyl methacrylates. Precisely one Ci-Cs-alkyl methacrylate is preferred.

[0058] For the purposes of the present invention, a "Ci-Cs-alkyl methacrylate" refers to alkyl esters of methacrylic acid having 1 to 3 carbon atoms in the alkyl radical. The alkyl radical can be linear or branched. The alkyl radical can also contain heteroatoms within the alkyl radical and / or be substituted by heteroatoms, as is the case, for example, with hydroxyethyl methacrylate. For example, Ci-Cs-alkyl methacrylates according to the invention are selected from the group consisting of methyl methacrylate, ethyl methacrylate, 1-methylethyl methacrylate, and propyl methacrylate, with methyl methacrylate being particularly preferred.

[0059] A process in which the Ci-Cs-alkyl methacrylate comprises methyl methacrylate is therefore also preferred.

[0060] As the first reactor (I), all reactors known to the person skilled in the art which are suitable for the synthesis of C1-C3 alkyl methacrylate are suitable.

[0061] The synthesis of Ci-Cs-alkyl methacrylates is known to the person skilled in the art and is described, for example, in US 5,969,178, US 4,529,816 and WO 2014 / 170223.

[0062] Preferably, the synthesis in step a) comprises an oxidative esterification of methacrolein with an alcohol in the presence of a heterogeneous noble metal-containing catalyst. The alcohol preferably corresponds to the first alcohol obtained in step b) in the reaction mixture (3). Further preferably, methacrolein is previously prepared from propionaldehyde and formaldehyde via a Mannich condensation or by oxidation of isobutene. Consequently, in this variant, methacrolein can have been prepared based on C2 building blocks or based on C4 building blocks. In particular, the process according to the invention can be applied to a combination of a C2-based process for preparing methacrolein and a subsequent oxidative esterification to a C1-C2 alkyl methacrylate. This process is described, for example, in DE 3 213 681, US 4,408,079, CN 103 846 104 and in WO 2016 / 042000.

[0063] Therefore, a process is also preferred in which the synthesis in step a) comprises an oxidative esterification of methacrolein with an alcohol in the presence of a noble metal-containing catalyst.

[0064] Preference is also given to a process in which methacrolein is obtained from propionaldehyde and formaldehyde via a Mannich condensation or by oxidation of isobutene.

[0065] Furthermore, it is possible that the first reactor (I) comprises the final stage of an ACH process. In this stage, the sulfate of the methacrylamide is reacted with an alcohol, especially methanol, to form a C1-C5 alkyl methacrylate, especially MMA.

[0066] Therefore, a process is also preferred in which the synthesis in step a) comprises reacting the sulfate of the methacrylic acid amide with an alcohol to form the C1-C8 alkyl methacrylate. It is also possible for the synthesis in step a) to comprise the so-called alpha process, in which ethylene is converted to methyl propionate, which is then reacted with formaldehyde to form the C1-C3 alkyl methacrylate.

[0067] Therefore, a process is also preferred in which the synthesis in step a) comprises a conversion of ethylene to methyl propionate followed by a conversion with formaldehyde to the Ci-Cs-alkyl methacrylate.

[0068] Furthermore, it is possible that the synthesis in step a) comprises a reaction of acetone with hydrogen cyanide to form acetone cyanohydrin and subsequent, optionally multi-stage, reaction to the C1-C3 alkyl methacrylate.

[0069] Therefore, a process is also preferred in which the synthesis in step a) comprises a reaction of acetone with hydrocyanic acid to form acetone cyanohydrin and subsequent conversion to the Ci-Cs-alkyl methacrylate via the intermediates hydroxyisobutyric acid amide and methyl hydroxyisobutyrate.

[0070] Preferably, step a) comprises a purification of the C1-C8-alkyl methacrylate. Additionally or alternatively, the C1-C8-alkyl methacrylate obtained in step a) can be purified after step a) and before step b).

[0071] Therefore, a process is also preferred in which step a) comprises a purification of the C1-C3 alkyl methacrylate and / or in which the C1-C3 alkyl methacrylate is purified after step a) and before step b).

[0072] The purification of the C1-C8-alkyl methacrylate can be carried out using methods known to those skilled in the art. Preferably, the purification of the C1-C8-alkyl methacrylate is carried out by distillation. Furthermore, the purification of the C1-C8-alkyl methacrylate preferably comprises several distillations. The purification process itself is known and described, for example, in US 2016 / 068464 A1 and US 2021 / 047261 A1.

[0073] The purified Ci-Cs-alkyl methacrylate is preferably substantially free of components having boiling points in the range from 100°C to 165°C, with the exception of the Ci-Cs-alkyl methacrylate and methacrylic acid. "Substantially free" in this context means that it preferably contains less than 500 ppm by weight, more preferably less than 300 ppm by weight, and especially preferably less than 200 ppm by weight of components having boiling points in the range from 100°C to 165°C, with the exception of the Ci-Cs-alkyl methacrylate and methacrylic acid, based on the total weight of the purified Ci-Cs-alkyl methacrylate. The purified Ci-Cs-alkyl methacrylate is therefore preferably substantially free of components other than the Ci-Cs-alkyl methacrylate or methacrylic acid and which have a boiling point in the range from 100°C to 165°C. Furthermore, the purified Ci-Cs-alkyl methacrylate preferably contains less than 500 ppm by weight, particularly preferably less than 300 ppm by weight.ppm and particularly preferably less than 200 ppm by weight of aldehydes and / or ketones, based on the total weight of the purified C1-C8-alkyl methacrylate. In step b) of the process according to the invention, at least a portion of the C1-C8-alkyl methacrylate synthesized in step a) is reacted with water in the presence of a first catalyst to form methacrylic acid in a second reactor (II). This results in a reaction mixture (3). The reaction mixture (3) contains methacrylic acid, water, a first alcohol, oligomers, and the C1-C3-alkyl methacrylate.

[0074] The reaction in step a) is also called saponification or hydrolysis.

[0075] The molar ratio of Ci-Cs-alkyl methacrylate to water in step b) is, for example, in the range from 0.5 to 5.0, preferably in the range from 1.5 to 3.0.

[0076] The water in step b) can, for example, come from the first reactor (I) in step a). Additionally or alternatively, water can be recycled from other reaction steps as described below.

[0077] In the context of the present invention, “a first catalyst” means both exactly one first catalyst and a mixture of two or more first catalysts.

[0078] Suitable first catalysts are catalysts known to those skilled in the art for the reaction of C1-C5-alkyl methacrylates with water. The first catalyst preferably comprises at least one Brønsted acid. The first catalyst can be a homogeneous or heterogeneous catalyst.

[0079] Suitable homogeneous catalysts are, for example, selected from the group consisting of mineral and / or organic acids, such as sulfuric acid, methanesulfonic acid and / or toluenesulfonic acid.

[0080] The first catalyst is preferably a heterogeneous catalyst. For example, the first catalyst is selected from the group consisting of zeolites, ion exchange resins, and amorphous acid catalysts; ion exchange resins are particularly preferred.

[0081] Strongly acidic ion exchange resins are particularly preferred as ion exchange resins. Suitable ion exchange resins are known to the person skilled in the art and are selected, for example, from the group consisting of Lewatit K1221 from Lanxess AG, Lewatit K2629 from Lanxess AG, Lewatit K2431 from Lanxess AG, Dowex CM-4 from Dow Chemical, Dowex M-31 from Dow Chemical, Dowex M-3 MS from Dow Chemical, Amberlyst 39 Wet from Rohm & Haas, Amberlyst CSP2 from Rohm & Haas, Amberlyst CSP3 from Rohm & Haas, DIAION PK208 from Mitsubishi Chemicals, DIAION PK216 from Mitsubishi Chemicals, and DIAION PK228 from Mitsubishi Chemicals.

[0082] Zeolites suitable as the first catalyst are known as such.

[0083] Suitable amorphous acid catalysts are known to the person skilled in the art.

[0084] If the first catalyst is an acidic ion exchanger, a zeolite, or an amorphous acid catalyst, the first catalyst is preferably configured as a catalyst bed. It is further preferred that the catalyst bed, and thus the second reactor (II), be flowed through from top to bottom. In a further embodiment, it is preferred according to the invention that the catalyst bed, and thus the second reactor (II), be flowed through from bottom to top.

[0085] Reactors known to those skilled in the art are suitable as the second reactor (II). The second reactor (II) can also comprise, for example, sieves and / or inclined clarifiers and / or be constructed as a reactor cascade, reactor battery, or reactors connected in series or parallel. This is advantageous because the first catalyst remains in the second reactor (II), and the reaction mixture (3) is thus essentially free of the first catalyst.

[0086] The temperature during the reaction in step b) is, for example, in the range from 50 °C to 200 °C, preferably in the range from 90 °C to 120 °C, and particularly preferably in the range from 100 °C to 110 °C. If a homogeneous catalyst is used as the first catalyst, the temperature is, for example, in the range from 50 °C to 200 °C, preferably in the range from 90 °C to 170 °C.

[0087] The pressure during the reaction in step b) is, for example, in the range from 1.1 bar absolute to 10 bar absolute, preferably in the range from 1.5 bar absolute to 6 bar absolute.

[0088] Therefore, a process is also preferred in which the first catalyst in step b) is selected from the group consisting of homogeneous catalysts, zeolites, ion exchange resins and amorphous acid catalysts and / or in which the reaction in step b) takes place at a temperature in the range from 50 °C to 200 °C and a pressure in the range from 1.1 bar absolute to 10 bar absolute.

[0089] Particularly preferred is a process in which the reaction in step b) takes place in the presence of an ion exchange resin as the first catalyst at a temperature in the range from 90 °C to 120 °C and a pressure in the range from 1.5 bar absolute to 6 bar absolute.

[0090] The pressure in step b) refers to the pressure measured at the outlet of the second reactor (II). The conversion of C1-C8 alkyl methacrylate in step b) is, for example, at least 10%, preferably at least 15%, and particularly preferably at least 20%. The conversion of C1-C3 alkyl methacrylate in step b) is, for example, at most 60%, preferably at most 55%, and in particular at most 50%.

[0091] Therefore, a process in which the conversion of Ci-Cs-alkyl methacrylate in step b) is at least 10% is also preferred.

[0092] The reaction in step b) can additionally be carried out in the presence of a polymerization inhibitor. Suitable polymerization inhibitors are known as such and are described, for example, in Römpp-Lexikon Chemie; Editors: J. Falbe, M. Regitz; Stuttgart, New York; 10th edition (1996); keyword "antioxidants." For example, polymerization inhibitors are selected from the group consisting of hydroquinones, hydroquinone ethers, di-tert-butylcatechol, phenothiazine, N,N'-(diphenyl)-p-phenylenediamine, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, p-phenylenediamine, and methylene blue. Preferred polymerization inhibitors are selected from the group consisting of di-tert-butylcatechol, phenothiazine, N,N'-(diphenyl)-p-phenylenediamine, p-phenylenediamine, and methylene blue.

[0093] Methacrylic acid is formed when the Ci-Cs-alkyl methacrylate is reacted with water in step b).

[0094] In step b), a reaction mixture (3) is therefore obtained. The reaction mixture (3) contains methacrylic acid, water, a first alcohol, oligomers, and the Ci-Cs-alkyl methacrylate.

[0095] The first alcohol is a C1-C8 alcohol. The C1-C8 alcohol is the alcohol of the alkyl radical of the C1-C3 alkyl methacrylate. Accordingly, the first alcohol is preferably selected from the group consisting of methanol, ethanol, propanol, and iso-propanol. The first alcohol is particularly preferably methanol.

[0096] In the context of the present invention, "oligomers" are understood to mean oligomers of methacrylate, such as dimers, trimers, and tetramers of methacrylate. These are formed during the reaction of the C1-C8-alkyl methacrylate with water in step b). This reaction is known to the person skilled in the art. In this context, "oligomers of methacrylate" are understood to mean oligomers of C1-C8-alkyl methacrylate with itself, oligomers of methacrylic acid with itself, and oligomers of C1-C8-alkyl methacrylate with methacrylic acid.

[0097] The Ci-Cs-alkyl methacrylate is the same Ci-Cs-alkyl methacrylate that was synthesized in step a).

[0098] For example, the reaction mixture (3) contains in the range of 10 to 30 wt.%, preferably in the range of 15 to 25 wt.% methacrylic acid, based on the total weight of the reaction mixture (3). For example, the reaction mixture (3) contains in the range of 5 to 20 wt.%, preferably in the range of 10 to 15 wt.% water, based on the total weight of the reaction mixture (3).

[0099] For example, the reaction mixture (3) contains in the range of 5 to 20 wt.%, preferably in the range of 10 to 15 wt.% of first alcohol, based on the total weight of the reaction mixture (3).

[0100] For example, the reaction mixture (3) contains in the range of 55 to 80 wt.%, preferably in the range of 60 to 65 wt.% Ci-Cs-alkyl methacrylate, based on the total weight of the reaction mixture (3).

[0101] For example, the reaction mixture (3) contains at most 0.02 wt.% oligomers, based on the total weight of the reaction mixture (3).

[0102] Furthermore, the reaction mixture (3) may contain the first catalyst. In particular, the reaction mixture (3) contains the first catalyst if the first catalyst is a homogeneous catalyst. Preferably, the reaction mixture (3) is substantially free of the first catalyst. For example, the reaction mixture (3) contains at most 5.0 wt. %, preferably at most 2.0 wt. % of the first catalyst.

[0103] If the reaction in step b) takes place in the presence of a polymerization inhibitor, the reaction mixture (3) typically additionally contains the polymerization inhibitor. For example, the reaction mixture contains in the range of 0.001 wt.% to 0.1 wt.%, preferably in the range of 0.002 wt.% to 0.01 wt.%, of polymerization inhibitor.

[0104] The reaction mixture (3) may contain further components such as isobutyric acid, dimethyl ether, 3-hydroxyisobutyric acid and / or 3-hydroxyisobutyric acid methyl ester.

[0105] In step c) of the process according to the invention, the reaction mixture (3) obtained in step b) is separated in a first rectification column (A). This produces a top stream (12) containing water, the first alcohol, and the C1-C5-alkyl methacrylate, as well as a bottom stream (6) containing methacrylic acid and the oligomers.

[0106] Rectification columns known to those skilled in the art are suitable as the first rectification column (A). The first rectification column (A) preferably comprises a stripping section and a rectification section. The stripping section is also referred to as the stripping section.

[0107] Therefore, a process in which the first rectification column (A) comprises a rectification zone and a stripping zone is also preferred. Preferably, the reaction mixture (3) is transferred directly from the second reactor (II) to the first rectification column (A). However, it is also possible for the reaction mixture (3) to be temporarily stored, for example, in an intermediate container before being transferred to the first rectification column (A).

[0108] It is further preferred that the reaction mixture (3) be introduced centrally into the first rectification column (A). This is particularly advantageous if the first rectification column (A) comprises a stripping zone and a rectification zone.

[0109] Preferably, the reaction mixture (3) has a temperature in the range from 70 °C to 120 °C, particularly preferably in the range from 90 °C to 110 °C, when introduced into the first rectification column (A).

[0110] For example, the first rectification column (A) at the top of the first rectification column (A) has a temperature in the range from 10 to 20 °C, preferably in the range from 14 to 17 °C.

[0111] For example, the first rectification column (A) at the bottom of the first rectification column (A) has a temperature in the range from 80 to 100 °C, preferably in the range from 85 to 95 °C.

[0112] In step c), a top stream (12) containing water, the first alcohol and the Ci-Cs-alkyl methacrylate is obtained.

[0113] For example, the top stream (12) contains in the range of 4 to 12 wt.%, preferably in the range of 7.5 to 8.5 wt.%, of water, based on the total weight of the top stream (12).

[0114] For example, the top stream (12) contains in the range of 8 to 13 wt.%, preferably in the range of 10 to 11 wt.%, of first alcohol, based on the total weight of the top stream (12).

[0115] For example, the top stream (12) contains in the range of 60 to 90 wt.%, preferably in the range of 80 to 82 wt.%, of Ci-Cs-alkyl methacrylate, based on the total weight of the top stream (12).

[0116] If the reaction mixture (3) contained further components, the overhead stream (12) may also contain further components. Typically, the overhead stream (12) contains those further components that have the same boiling point as or a lower boiling point than the Ci-Cs-alkyl methacrylate. Therefore, the further components that the overhead stream (12) may contain are also referred to as low boilers.

[0117] Low boilers include, for example, dimethyl ether. The top stream (12) therefore contains, for example, dimethyl ether as a further component. For example, the top stream (12) contains in the range from 50 ppm by weight to 300 ppm by weight, preferably in the range from 100 ppm by weight to 250 ppm by weight, of dimethyl ether, based on the total weight of the top stream (12). In step c), a bottom stream (6) comprising methacrylic acid and the oligomers is also obtained. If the reaction mixture (3) additionally comprised the first catalyst, then the bottom stream (6) typically also comprises the first catalyst. If the reaction mixture (3) additionally comprised a polymerization inhibitor, then the bottom stream (6) typically additionally comprises the polymerization inhibitor.

[0118] If the reaction mixture contained other components, the bottom stream (6) may also contain other components. The bottom stream (6) typically contains those other components that have a higher boiling point than the C1-C8-alkyl methacrylate. Therefore, the other components that the bottom stream (6) may contain are also referred to as high boilers.

[0119] High boilers include, for example, isobutyric acid, 3-hydroxyisobutyric acid, and / or methyl 3-hydroxyisobutyrate. For example, the bottom stream (6) contains 30 ppm by weight to 100 ppm by weight of 3-hydroxyisobutyric acid, based on the total weight of the bottom stream (6). For example, the bottom stream (6) contains 30 ppm by weight to 100 ppm by weight of methyl 3-hydroxyisobutyrate, based on the total weight of the bottom stream (6).

[0120] For example, the bottom stream (6) contains 30 ppm by weight to 300 ppm by weight of isobutyric acid, based on the total weight of the bottom stream (6).

[0121] If the first catalyst is a heterogeneous catalyst, the bottom stream (6) contains, for example, in the range from 95 to 99.99 wt.%, preferably in the range from 99.5 to 99.9 wt.% of methacrylic acid, based on the total weight of the bottom stream (6).

[0122] For example, the bottom stream (6) then contains 0 to 5 wt.%, preferably 0.1 to 0.5 wt.% of oligomers, based on the total weight of the bottom stream (6).

[0123] For example, the bottom stream (6) then contains in the range from 0.005 to 0.5 wt.%, preferably in the range from 0.01 to 0.1 wt.% of polymerization inhibitor, based on the total weight of the bottom stream (6).

[0124] If the first catalyst is a homogeneous catalyst, the bottom stream (6) contains, for example, in the range from 75 to 99.9 wt.% of methacrylic acid, based on the total weight of the bottom stream (6).

[0125] For example, the bottom stream (6) then contains in the range from 5 to 25 wt.%, preferably in the range from 2 to 10 wt.% of first catalyst, based on the total weight of the bottom stream (6).

[0126] For example, the bottom stream (6) then contains in the range of 0.005 to 0.5 wt. %, preferably in the range of 0.01 to 0.1 wt. %, of polymerization inhibitor, based on the total weight of the bottom stream (6). In step d), the top stream (12) obtained in step c) is separated in a phase separator (a). This yields an organic phase (5) and an aqueous phase (4).

[0127] Phase separators known to the person skilled in the art are suitable as phase separators (a).

[0128] In one embodiment, extraction water (11) is fed to the phase separator (a). The use of extraction water is known per se. Extraction water is typically demineralized water (DE water). In one embodiment, the bottom stream (8) or parts thereof can be used as extraction water.

[0129] In step d), an organic phase (5) is obtained. The organic phase (5) contains the C1-C3 alkyl methacrylate.

[0130] For example, the organic phase (5) contains up to 99 wt.%, preferably up to 95 wt.% of C1-C3 alkyl methacrylate, based on the total weight of the organic phase (5).

[0131] For example, the organic phase (5) contains up to 5 wt.%, preferably up to 3.5 wt.% of the first alcohol, based on the total weight of the organic phase (5).

[0132] For example, the organic phase (5) contains up to 3 wt.%, preferably up to 1.5 wt.% of water, based on the total weight of the organic phase (5).

[0133] The resulting organic phase (5) can be at least partially recycled to the second reactor (II). This allows unreacted C1-C5 alkyl methacrylate to be converted again to methacrylic acid according to step b).

[0134] For example, in the range of 25 to 75% of the organic phase (5) can be recycled to the second reactor (II).

[0135] At least a portion of the organic phase (5) can be recycled to the first rectification column (A). For example, 75 to 25% of the organic phase (5) can be recycled to the first rectification column (A).

[0136] The aqueous phase (4) obtained in step d) contains water and the first alcohol.

[0137] The aqueous phase (4) can, if appropriate, also be recycled to the second reactor (II) after purification. Therefore, a process is also preferred in which the organic phase (5) obtained in step d) is at least partially recycled to the second reactor (II) and / or in which the aqueous phase (4) obtained in step d) is recycled to the second reactor (II), if appropriate after purification.

[0138] Preferably, the resulting aqueous phase (4) is transferred to a third rectification column (B) and separated therein into a first top stream (7) containing the first alcohol and a bottom stream (8) containing water.

[0139] Therefore, a process is also preferred in which the aqueous phase (4) is separated in a third rectification column (B) to obtain

[0140] • a head stream (7) containing the first alcohol, and

[0141] • a sump stream (8) containing water.

[0142] Rectification columns known to those skilled in the art are suitable as the third rectification column (B). The third rectification column (B) preferably comprises a stripping section and a rectification section.

[0143] Preferably, the aqueous phase (4) from the phase separator (a) is transferred directly to the third rectification column (B). However, it is also possible for the aqueous phase (4) to be temporarily stored, for example, in an intermediate container before being transferred to the third rectification column (B).

[0144] It is further preferred that the aqueous phase (4) be introduced centrally into the third rectification column (B). This is particularly advantageous if the third rectification column (B) comprises a stripping zone and a rectification zone.

[0145] For example, the third rectification column (B) at the top of the third rectification column (B) has a temperature in the range from 65 to 70 °C, preferably in the range from 68 to 70 °C.

[0146] For example, the third rectification column (B) at the bottom of the third rectification column (B) has a temperature in the range from 97 to 100.5 °C, preferably in the range from 99.5 to 100.5 °C.

[0147] The top stream (7) obtained in the third rectification column (B) can, for example, be recycled to the first reactor (I).

[0148] Therefore, a process in which the top stream (7) is recycled to the first reactor (I) is also preferred. It is also possible to recycle the bottom stream (8) obtained in the third rectification column (B) to the second reactor (II).

[0149] If the resulting bottom stream (8) is recycled to the second reactor, it is preferred according to the invention that water (2) is first mixed with the bottom stream (8). Preferably, the C1-C8-alkyl methacrylate stream (1) is then also mixed with the organic phase (5, 5b) recycled to the second reactor. The two resulting mixtures are then mixed together and fed to the second reactor (II).

[0150] In step e) of the process according to the invention, the bottom stream (6) obtained in step c) is separated in a second rectification column (C) into an overhead stream (9) and a bottom stream (10). The overhead stream (9) contains methacrylic acid, while the bottom stream (10) contains the oligomers and residues of the methacrylic acid.

[0151] Suitable second rectification columns (C) are rectification columns known to those skilled in the art that are suitable for purifying methacrylic acid. The second rectification column (C) preferably comprises a rectification zone.

[0152] It is further preferred that the bottom stream (6) obtained in step c) is introduced into the second rectification column (C) in the lower region of the latter.

[0153] For example, the temperature at the top of the second rectification column (C) is in the range of 80 to 90 °C, preferably in the range of 82 to 84 °C.

[0154] For example, the temperature at the bottom of the second rectification column (C) is in the range from 82 to 90 °C, preferably in the range from 85 to 87 °C.

[0155] In step e), a top stream (9) comprising methacrylic acid is obtained. The top stream (9) typically contains at least 99.50% by weight, preferably at least 99.98% by weight, of methacrylic acid, based on the total weight of the top stream (9). Furthermore, the top stream (9) may contain additional components, such as water and / or isobutyric acid. Preferably, the top stream (9) contains at most 200 ppm by weight of water. Further preferably, the top stream (9) contains at most 20 ppm of isobutyric acid.

[0156] Furthermore, in step e), a bottom stream (10) is obtained which contains oligomers and residues of methacrylic acid. For the purposes of the present invention, “residue of methacrylic acid” is understood to mean an amount of methacrylic acid which allows the oligomers and optionally further components present in the bottom stream (10) to be conveyed and to keep them flowable and thus conveyable by pumps. “Remainder of methacrylic acid” is understood to mean, for example, in the range from 3 to 15%, preferably in the range from 3 to 10%, of the methacrylic acid present in the bottom stream (6). If the bottom stream (6) contains high boilers, the bottom stream (10) typically also contains high boilers. If the bottom stream (6) contains the first catalyst and / or polymerization inhibitors, the bottom stream (10) typically also contains the first catalyst and / or polymerization inhibitors.

[0157] The bottom stream (10) obtained in step e) can be at least partially recycled to the first reactor (I). This allows the methacrylic acid residues to be converted back into C1-C5-alkyl methacrylate. This recirculation makes the process according to the invention particularly economical.

[0158] List of reference symbols

[0159] In the figures, the reference symbols have the following meaning

[0160] A first rectification column a phase separator

[0161] B third rectification column b distillate vessel of the third rectification column

[0162] C second rectification column c distillate vessel of the second rectification column

[0163] I first reactor la Ci-C3-alkyl methacrylate purification

[0164] II second reactor

[0165] (1) Ci-C3 alkyl methacrylate stream

[0166] (2) Water

[0167] (3) Reaction mixture

[0168] (4) aqueous phase

[0169] (5) organic phase

[0170] (5a) organic phase recycled to the first rectification column

[0171] (5b) organic phase recycled to second reactor

[0172] (6) Swamp stream

[0173] (7) Head current

[0174] (8) Swamp stream

[0175] (9) Head current

[0176] (10) Swamp stream

[0177] (11) Extraction water

[0178] (12) Headstream figures

[0179] Figure 1 shows an exemplary embodiment of the process according to the invention. According to step a), a C1-C8-alkyl methacrylate is prepared in a first reactor I. This is purified in the C1-C3-alkyl methacrylate purification stage 1a to obtain a C1-C8-alkyl methacrylate stream 1. This stream is mixed with a bottom stream containing B and transferred to the second reactor II. In this reactor, the C1-C8-alkyl methacrylate stream 1 is reacted with water 2 in the presence of a first catalyst to form methacrylic acid to obtain a reaction mixture 3 comprising methacrylic acid, water, a first alcohol, oligomers, and the C1-C8-alkyl methacrylate. The reaction mixture 3 is transferred centrally into a first rectification column A. Therein, the reaction mixture 3 is separated into a top stream 12 containing water, the first alcohol and the Ci-Cs-alkyl methacrylate, and a bottom stream 6 containing methacrylic acid and the oligomers.

[0180] The overhead stream 12 is transferred to a phase separator a, where it is separated into an organic phase 5 containing the C1-C8-alkyl methacrylate and an aqueous phase 4 containing water and the first alcohol. Extraction water 11 can optionally also be added to the phase separator a. The organic phase 5 can be separated into an organic phase 5a recycled to the first rectification column A and an organic phase 5b recycled to the second reactor II. In the embodiment shown, the organic phase 5a recycled to the first rectification column A is recycled at the top of the first rectification column A.

[0181] The aqueous phase 4 is transferred centrally into a third rectification column B and therein separated into a bottom stream 8, which contains water and is recycled to the second reactor II, and a top stream 7, which contains the first alcohol and which can also be recycled or, for example, thermally utilized.

[0182] The bottom stream 6 is transferred to the second rectification column C and introduced into it at the bottom of the second rectification column C. It is separated into a bottom stream 10, which contains the oligomers and residues of methacrylic acid, and a top stream 10, which contains the methacrylic acid as the end product.

[0183] Examples

[0184] Example 1 Reactor optimization: Variation of residence times and molar ratios in a fixed bed

[0185] In a setup according to Figure 1, a methyl methacrylate stream (1) with polymerization inhibitors and a water stream (2) are mixed, heated to 110°C and transferred to a second reactor (II) filled with a strongly acidic ion exchange resin (Lanxess Lewatit K2431 AG). The individual streams have a pressure in the range of 3 to 4 bar absolute. The methyl methacrylate stream (1) and the water stream (2) are adjusted such that the desired molar ratio (2 mol / mol, 1 mol / mol or 0.5 mol / mol) of methyl methacrylate to water is established at the desired residence time (based on the empty volume of the second reactor (II) completely filled with catalyst) of 1 h (hours), 1.5 h or 2.5 h. The reaction mixture (3) contains methanol, water, methyl methacrylate, oligomers, and methacrylic acid. Based on its composition, the conversion of methyl methacrylate can be determined. The results can be found in Table 1.

[0186] Table 1

[0187] Example 2 Indirect Split: (MOIMMA / MOIH2O ​​= 1), conversion X(MMA) = 27%, 1.5 h residence time

[0188] In a setup according to Figure 1, a methyl methacrylate stream (1) and a water stream (2) are mixed with the organic recycling stream (5b), containing methanol, methyl methacrylate, and traces of water, as well as with the aqueous recycling stream (8). The individual streams have a pressure of 5 bar absolute. The methyl methacrylate feed (1) is 656.6 g / h, the water feed (2) is 118.9 g / h, the recycling stream (5b) is 1537 g / h, and the recycling stream (8) is 227.8 g / h. The streams are adjusted so that the mixed overall stream has a molar ratio of 1:1 MMA to water. The stream is heated to the reaction temperature of 110 °C. In the second reactor (II), a residence time of 90 min results, and an MMA conversion of 27%. The reaction mixture (3) containing methanol, water, methyl methacrylate and methacrylic acid is fed into the first rectification column (A) centrally, between the upper and lower packing sections, using the column feed.

[0189] The first rectification column (A) is designed as a DN50 glass column and equipped with laboratory packing. A condenser is mounted at the top of the column, achieving a condensate outlet temperature of 11 °C. An inhibitor is sprayed onto the condenser to prevent polymerization and is fed into the column via a reflux. The inhibitor flow is 43.0 g / h and consists of 2500 ppm MEHQ solution in methyl methacrylate. The column top pressure is set to 40 mbar, resulting in a top temperature of 17.6 °C and a bottom temperature of 91.1 °C. Crude methacrylic acid is obtained at the bottom of the column. The distillate, consisting of methyl methacrylate, water, and MeOH, is obtained at the top.This distillate separates at 11 °C in a phase separator into an organic phase containing mainly methyl methacrylate and methanol, with a small amount of water, and an aqueous phase consisting mainly of water and methanol, with a small amount of methyl methacrylate. The organic phase (5) is used as reflux (5a) to the first rectification column (A), with a reflux ratio set to 1 / 1, and is fed to reactor (II) as recycle stream (5b) at 1537 g / h. The aqueous phase (4) is fed to the third rectification column (B) as a stream at 492.8 g / h.

[0190] The third rectification column (B) is designed as a DN50 glass column and equipped with laboratory packing. The aqueous phase (4) is fed into the center of the third rectification column (B). A condenser is mounted at the top of the column, which achieves a condensate outlet temperature of 40 °C. An inhibitor is sprayed onto the condenser to prevent polymerization of the traces of methyl methacrylate and is fed into the column via a reflux. The inhibitor flow is 10.0 g / h and consists of 2500 ppm MEHQ solution in methanol. The column top pressure is operated below atmospheric pressure, resulting in a top temperature of 66.1 °C and a bottom temperature of 99.6 °C. At the top, 272.9 g / h of distillate (7) is obtained, consisting of methanol, traces of methyl methacrylate, and water. The reflux ratio is set to 2 / 1.The bottom stream (8) 227.8 g / h, consisting of water, is used as a recycle stream to the second reactor (II).

[0191] The crude methacrylic acid stream (bottom stream (6)) at 527.8 g / h is fed to the bottom of the second rectification column (C). This column is designed as a DN50 glass column and equipped with laboratory packing. A condenser is mounted at the top of the column, achieving a condensate outlet temperature of 45°C. An inhibitor is sprayed onto the condenser to prevent polymerization and is fed into the column via a reflux. The inhibitor stream is 13.4 g / h and consists of 3000 ppm MEHQ solution in methacrylic acid. The second rectification column (C) is operated at a top pressure of 40 mbar, resulting in a top temperature of 83.3 °C and a bottom temperature of 88.9 °C. The reflux ratio is set to 0.6 / 1. 513.3 g / h of methacrylic acid (top stream (9)) are obtained as pure product at the top. In the bottoms, a bottoms stream (10) of 27.3 g / h, consisting of methacrylic acid, high boilers and inhibitors, is separated.Table 2 lists the material composition of the obtained mass flows.

[0192] Table 2 Residues are further substances in the trace range and include, among other things, isobutyric acid and its methyl esters, as well as dimethyl ether, hydroxyisobutyric acid and its methyl esters and the polymerization inhibitors MEHQ and PTZ as well as their derivatives that can be formed under reaction conditions.

[0193] Overview of steam and brine consumption in various methacrylic acid hydrolysis processes

[0194] Table 3 compares the consumption of steam and cooling brine for the process according to the invention with the processes as described in the prior art (US 8,791,296 B2 and WO 2022 / 194590 A1).

[0195] Steam and cooling brine consumption were calculated using commercially available process simulation software. The molar yield of methacrylic acid per mole of methyl methacrylate used is also given.

[0196] It is clearly evident that the steam requirement of the process according to the invention is lower than that of the prior art processes. Significantly less cooling brine is also required. The molar yield of methacrylic acid is significantly higher than in the prior art processes.

[0197] Table 3

Claims

Claims 1 . A process for the preparation of methacrylic acid comprising the following steps a) to e): a) synthesis of a Ci-Cs-alkyl methacrylate in a first reactor (I), b) reaction of at least a portion of the Ci-Cs-alkyl methacrylate synthesized in step a) with water in the presence of a first catalyst to form methacrylic acid in a second reactor (II) to obtain a reaction mixture (3) which contains methacrylic acid, water, a first alcohol, oligomers and the Ci-Cs-alkyl methacrylate, c) separation of the reaction mixture (3) obtained in step b) in a first rectification column (A) and obtaining • a top stream (12) containing water, the first alcohol and the Ci-Cs-alkyl methacrylate, and • a bottom stream (6) containing methacrylic acid and the oligomers, d) separating the top stream (12) obtained in step c) in a phase separator (a) and obtaining • an organic phase (5) containing the Ci-Cs-alkyl methacrylate, and • an aqueous phase (4) containing water and the first alcohol, e) separating the bottom stream (6) obtained in step c) in a second rectification column (C) and obtaining • a head stream (9) containing the methacrylic acid, and • a bottom stream (10) containing the oligomers and residues of methacrylic acid, wherein the aqueous phase (4) is separated in a third rectification column (B) to obtain • a head stream (7) containing the first alcohol, and • a sump stream (8) containing water.

2. Process according to claim 1, characterized in that the organic phase (5) obtained in step d) is at least partially recycled to the second reactor (II) and / or that the aqueous phase (4) obtained in step d) is recycled to the second reactor (II) after purification.

3. Process according to claim 1 or 2, characterized in that the head stream (7) is recycled to the first reactor (I).

4. The process according to any one of claims 1 to 3, characterized in that the Ci-Cs-alkyl methacrylate comprises methyl methacrylate.

5. Process according to one of claims 1 to 4, characterized in that step a) comprises a purification of the Ci-Cs-alkyl methacrylate and / or that the C1-C3 alkyl methacrylate is purified after step a) and before step b).

6. Process according to one of claims 1 to 5, characterized in that the conversion of Ci-Cs-alkyl methacrylate in step b) is at least 10%.

7. The process according to any one of claims 1 to 6, characterized in that the first catalyst in step b) is selected from the group consisting of homogeneous catalysts, zeolites, ion exchange resins and amorphous acid catalysts and / or that the reaction in step b) takes place at a temperature in the range from 50 °C to 200 °C and a pressure in the range from 1.1 bar absolute to 10 bar absolute.

8. Process according to one of claims 1 to 7, characterized in that the reaction in step b) takes place in the presence of an ion exchange resin as the first catalyst at a temperature in the range from 90 °C to 120 °C and a pressure in the range from 1.5 bar absolute to 6 bar absolute.

9. Process according to one of claims 1 to 8, characterized in that the first rectification column (A) comprises a rectification zone and a stripping zone.

10. Process according to one of claims 1 to 9, characterized in that the synthesis in step a) comprises an oxidative esterification of methacrolein with an alcohol in the presence of a noble metal-containing catalyst.

11. A process according to claim 10, characterized in that methacrolein is obtained from propionaldehyde and formaldehyde via a Mannich condensation or by oxidation of isobutene.

12. Process according to one of claims 1 to 9, characterized in that the synthesis in step a) comprises a reaction of the sulfate of the methacrylic acid amide with an alcohol to form the C1-C3 alkyl methacrylate.

13. The process according to any one of claims 1 to 9, characterized in that the synthesis in step a) comprises a conversion of ethylene to methyl propionate followed by reaction with formaldehyde to form the Ci-Cs-alkyl methacrylate.

14. The process according to any one of claims 1 to 9, characterized in that the synthesis in step a) comprises a reaction of acetone with hydrocyanic acid to form acetone cyanohydrin and subsequent reaction to form the Ci-Cs-alkyl methacrylate via the intermediates hydroxyisobutyric acid amide and methyl hydroxyisobutyric acid ester.

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