EP4308538A1 - Novel method for continuously producing methacrylic acid by catalytically hydrolyzing methyl methacrylate - Google Patents

Abstract

The invention relates to a continuous method for producing methacrylic acid by catalytically hydrolyzing methyl methacrylate, which has been produced on the basis of C-2, C-3, or C-4 raw materials. In the process, methyl methacrylate with a high degree of purity is reacted with water in the presence of a Brönstedt catalyzer in order to form a reaction mixture containing reactants and products and is processed in a distillation column, at the head of which a condensate containing an azeotrope of MMA with methanol is generated, in the central part of which a vapor condensate is generated which contains methacrylic acid with a high degree of purity, and in the sump of which a material mixture is accumulated which contains byproducts with a high boiling temperature and little methacrylic acid.

Description

New process for the continuous production of methacrylic acid by catalytic

Hydrolysis of methyl methacrylate

field of invention

The present invention describes a continuous process for the production of methacrylic acid by catalytic hydrolysis of methyl methacrylate, which has been produced starting from C-2, C-3 or C-4 raw materials. Here, methyl methacrylate of high purity is used

Water is reacted in the presence of a Brönstedt catalyst to form a reaction mixture containing starting materials and products and is worked up in a distillation column, at the top of which a condensate containing an azeotrope of MMA with methanol is produced, a vapor condensate containing methacrylic acid of high purity is produced in the middle part of the column and a mixture of substances is obtained in the bottom of the column which contains high-boiling by-products and little methacrylic acid. A further aspect of the present invention is the separation of an MMA-water azeotrope from the desired product, methacrylic acid, and its recycling into the reaction.

Furthermore, various embodiments for the efficient continuous production of methacrylic acid are presented, in particular the integration of azeotrope processing of the MMA-MeOH and MMA-water mixtures from methacrylic acid production with processing sections of the MMA production process, with optional starting materials such as MMA and / or water in can be recycled to the methacrylic acid production process and/or one or more of these azeotropes can optionally be used in the production of MMA.

Compared to the methods according to the prior art, the present invention makes it possible to save on apparatus and thus reduces the investment costs when constructing a new plant. In addition, the present process allows an increase in the product yields and, associated therewith, a reduction in the by-products and the associated disposal costs, as well as a reduction in the specific energy consumption.

PRIOR ART The present invention describes a new, continuous process for the production of methacrylic acid (MAS) based on the hydrolysis of methyl methacrylate (MMA) or other methacrylate esters.

Methacrylic acid is used in large amounts for the production of polymers or together with other copolymerizable compounds in copolymers. For example, is Methacrylic acid Component of solvent-resistant gloves, can be used in the manufacture of dimensionally stable foams and carbon fibers, is the basis in formulations for concrete superplasticizers (PCEs) and a large number of other polymers in which MAS creates specific properties. Furthermore, methacrylic acid is the starting material for special esters, which are produced by esterification with the corresponding alcohols. Methacrylic acid is also used for the production of hydroxy esters, which are components of lacquer and paint formulations.

This results in a great interest in manufacturing processes for this important chemical product that are as simple, economical and environmentally friendly as possible.

The production of MAS is based on three possible raw material groups, based on C3, C4 or C2 building blocks.

The first commercially important group is represented by C3 building blocks. In this case, MAS is produced mainly from hydrocyanic acid and acetone via the resulting acetone cyanohydrin (ACH) as the central intermediate. This process has the disadvantage that very large amounts of ammonium sulfate are obtained, the processing of which is associated with very high costs. Other C3-based processes that use a raw material base other than ACH have been described in the relevant patent literature and have now been implemented on a production scale, but they have similar problems.

In addition, processes for the production of MAS starting from methacrylic acid amide (MASA) are known. In this case, the ACH is usually first reacted with sulfuric acid, with a sulfuric acid solution of MASA being formed after going through a multi-stage reaction sequence. This mixture of substances is reacted with water, whereby MASA is hydrolyzed to MAS and the resulting ammonia is obtained as ammonium hydrogen sulphate. A variety of such methods are described in the prior art, for example in U.S. Pat. 7,253,307, according to which MASA is reacted with water at moderate pressures and temperatures between 50° and 210° in the presence of superstoichiometric amounts of sulfuric acid to form methacrylic acid.

The process according to US Pat. No. 7,253,307 provides good yields per se and allows the production of methacrylic acid of high purity quality, although large amounts of ammonium sulfate-containing sulfuric acid waste are produced, which have to be thermally regenerated to form fresh sulfuric acid or can also be disposed of. The effort involved in separating and isolating the methacrylic acid is correspondingly complex and usually includes a phase separation and at least two separation steps by distillation. In the last separation step, methacrylic acid is obtained as a top condensate in commercial purity, and the by-products of the reaction are obtained at the bottom of the column, often undefined dimers and oligomers, the formation of which cannot be prevented by this process.

Not only methods for the representation of MAS are known, which start from MASA.

In an alternative process, hydroxyisobutyric acid amide (HIBS) is used as starting material. Such a process is described in US Pat. No. 3,487,101, methacrylic acid itself and methacrylic acid esters derived therefrom becoming accessible. In this case, HIBS is mixed in the liquid phase in the presence of homogeneous basic catalysts with, for example, caustic soda, with HIBS forming intermediate salts from which water can be eliminated at temperatures of up to 320 °C and MAS is formed. The MAS can then be removed overhead. In the embodiment described here, high-boiling esters, for example dimethyl phthalate or phthalic anhydride, are used as dehydrating agents which also serve as solvents in the reaction matrix. Very good selectivities of about 98% are described at high conversions. In view of the complicated and multi-stage process to produce HIBS, it is understandable that this process is not considered in the industry as

manufacturing method prevailed. HIBS can be obtained in two stages by multiple hydrolysis of acetone cyanohydrin, with hydroxyisobutyric amide being formed as an intermediate product, which in turn can react further to form the acid. Here too, when using sulfuric acid as a reagent, ammonium sulphate-containing sulfuric acid solutions are formed, the regeneration of which is arbitrarily complex.

Further embodiments and optimizations for performing the HIBS conversion to MAS are disclosed in DE191367. Here, zinc bromide and lithium bromide are used as catalysts, which leads to a highly selective conversion. However, the halide-containing catalysts and the high temperatures are extreme due to the high level of corrosiveness

Demands on the materials of the plant and the formation of halogenated by-products, which occur with methacrylic acid in the distillate and would have to be laboriously separated, make the process less attractive. EP 04 87853 describes a complex, multi-stage process starting from acetone cyanohydrin, with hydroxyisobutyric acid also being used here as a starting material for the central step in the production of MA. ACH is catalytically hydrolyzed in a first reaction step, for example in the presence of heterogeneous manganese dioxide catalysts. Hydroxyisobutyric acid amide (HIBA) is formed in high yield. In the next step, HIBA is reacted with methyl formate or mixtures of methanol/carbon monoxide, with complex

Product mixtures containing methyl hydroxyisobutyrate (MHIB) and formamide occur. Formamide is dehydrated to hydrocyanic acid in a separate reaction stage, and HCN can then be reacted with acetone to form ACH. MHIB is hydrolyzed with water in the presence of a heterogeneous acidic ion exchanger to form HIBS, which is then reacted catalytically with basic alkali metal salts to form methacrylic acid with elimination of water. The large number of necessary reaction steps make the process unattractive, in particular with regard to the significant investment costs for the construction of a plant of this complexity. With growing importance today, isobutylene or tert-butanol are used as C-4-based raw materials as educts in MAS production. These are converted into MAS over several process steps. Methyl tert-butyl ether (MTBE), which is converted to isobutene by methanol elimination, can also be used as a third alternative starting material. In these production methods, isobutylene or tert-butanol is oxidized to form methacrolein in a first stage, which is then reacted with oxygen to form methacrylic acid. The resulting MAS is either isolated and purified or converted to MMA and other esters. More details on this method are, inter alia, in Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Methacrylic Acid and Derivatives, DOI: 10.1002/14356007. a16_441 pub2 and set out in Trends and Future of Monomer-MMA Technologies, SUMITOMO KAGAKU 2004-II. Further details on MMA and methacrylic acid production processes in general and specifically on the multi-stage gas-phase process starting from C4 building blocks are given in Krill and Rühling et. al. "Many paths lead to methacrylic acid methyl ester", WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, doi.org/10.1002/ciuz.201900869.

As a rule, the C4 route is started from the steamer field product IBEN or alternatively TBA, which is oxidized to methacrolein (MAL) in the first step by means of gas-phase oxidation. In a second gas-phase oxidation stage, the MAL obtained as an intermediate is oxidized to MAS. The gaseous reaction products are cooled in a subsequent quenching step and largely condensed. It is characteristic of the process that the second reaction stage does not show complete conversion in terms of MAL and unreacted MAL is recovered in an absorption and desorption unit (recycle MAL) in order to then feed it back to the second reaction stage.

Isobutylene or tert-butanol can be reacted with atmospheric oxygen in the gas phase at the heterogeneous catalyst to form MAL and then by means of an oxidative esterification reaction of MAL using methanol to form MMA. This method is described inter alia in US 5,969,178 and US 7,012,039. The disadvantages of this process relate in particular to a high energy requirement, which is due, among other things, to the pressureless mode of operation. In this process, the problem of evaporation of MAL is avoided, since the process takes place in the liquid phase, so MAL does not have to be converted into the gaseous state and the problem of mixing with critical oxygen-containing gases is avoided in this way. A solution for optimizing the two-stage isobutene gas phase process with MAS as an intermediate stage cannot be derived from this.

Another problem with all of these processes is the relatively unsatisfactory yield, in particular due to high losses in the oxidation steps and the associated formation of CO2. In addition, the formation of by-products, which continues to accompany this and requires complex process steps for isolating the product, be advised. All processes that start from isobutylene or equivalent C-4-based raw materials such as TBA or MTBE achieve selectivities of 80% to 90% per process stage in gas-phase oxidation over a heterogeneous catalyst system. Thus, based on the C3 or C4 starting component, an overall yield of a maximum of 65% to 70% is achieved. Naturally, the gas phase process runs at moderate pressures of between 1 and 2 bar absolute and generates a process gas in which the product component is only contained at around 4 to 6% by volume. The isolation of the product of value from the inert gas ballast is correspondingly energy-intensive and consumes large amounts of cooling energy and steam for multi-stage distillative work-up steps. What these processes also have in common is that they are usually carried out in the gas phase in the presence of heterogeneous catalysts. As a result, the separation effort is also considerable, in particular due to the need to separate the MAS, also with regard to the gas ballast.

Ethylene as a C2 building block can also be used as a raw material for MAS production. Propionaldehyde (PA) or, as a secondary product of PA, propionic acid can be produced and isolated by reacting ethylene with carbon monoxide or synthesis gas. The unsaturated carbonyl compounds can be efficiently prepared from these primary intermediates with formalin or formaldehyde in the sense of an aldolization. Methacrolein is obtained from PA and MAS is obtained directly from propionic acid. Methacrolein, in turn, can be further oxidized to MAS catalytically.

Both processes have not caught on technically or commercially, partly because the catalyst systems used do not have adequate long-term stability. In the propionic acid reaction with formaldehyde in the gas phase, the activity of the gas phase catalysts used drops drastically after just a few 100 h. It can be speculated that this is due to significant coking and deposits of non-volatile substances on the catalyst surface. On the other hand, the oxidation of methacrolein in the presence of water and solvents in the presence of special noble metal catalysts leads to slow dissolution of the support components, so that the activity of the catalysts cannot be maintained here either.

US Pat. No. 8,791,296 describes a process for preparing methacrylic acid based on the hydrolysis of methacrylic esters, which comprises the following process steps: preparation of acetone cyanohydrin, conversion of acetone cyanohydrin to methacrylamide, esterification of methacrylamide in the presence of alkanols to the corresponding methacrylic ester and hydrolysis of the methacrylic ester to methacrylic acid. With this process it is possible to produce methacrylic acid in high purities of ^ 99.5%, but the process is limited to the use of acetone cyanohydrin-based MMA production processes and a total of four process steps are necessary for the further process control for the isolation of methacrylic acid, which one require increased energy. The first step in the process includes Hydrolysis of methyl methacrylate to methacrylic acid. Three distillation steps at different pressures are then necessary.

Another feature of the process is that large circulating streams from two work-up apparatuses have to be returned to the actual reaction stage, with the ratio of the circulating streams being at least five times the amount relative to the feed stream into the reactor. The process comprises a reactor, a rectification column for separating off methanol, the top condensate of which is returned to the reactor, and a further rectification column operated in vacuo for separating off low boilers. Methacrylic acid is isolated in pure form and of high quality as the top product, more precisely as the condensate of the top stream, of a third column operated under vacuum. Due to the large number of apparatuses and the high recycling rate of condensates, some of which inhibit the equilibrium reaction, the process is disadvantageous in terms of energy and the operating and investment costs are high. In summary, a large number of processes for the production of MA are known, which start either from acetone (C3), propylene (C3), ethylene (C2) or isobutene (C4). Here, ACH, isobutyric acid or hydroxyisobutyric acid are produced and isolated as central intermediates. The MAS processes, which start from ACH and isobutene, the so-called C-3 or C-4-based processes, have become established industrially. The established methods are clearly summarized in the literature and discussed, for example, in Weissermel, Arpe "Industrielle OrganoChemie" "Industrial Organic Chemistry', VCH, Weinheim 1994, 4th edition, p. 305 ff. or in Kirk Othmer 'Encyclopaedia of Chemical Technology', 3rd edition, Vol.

15, page 357.

Task: Against the background of the prior art discussed, the task arose of providing a new process for the preparation of methacrylic acid which does not have the disadvantages of the prior art or only to a reduced extent.

In particular, it was the object of the present invention to provide a novel method that can be implemented with as little equipment as possible and therefore low investment costs. At the same time, the general operating costs and the specific energy consumption in continuous operation should be kept as low as possible. A special aspect to be solved was also the avoidance or reduction of operational disruptions due to polymer deposits in the continuous operation of said plant.

A further task was the simplification of product processing in order to achieve a specification-compliant (meth)acrylic acid quality as well as the optimal separation and recycling or material recycling of the resulting MMA-containing azeotropes.

Furthermore, it was an object of the invention to provide by-products of the MMA hydrolysis in a form that allows simple work-up and sensible recycling. This also includes the separation of the azeotrope from methanol and methyl (meth)acrylate, as well as the integration into a process for the production of MMA.

Other objects that are not explicitly mentioned can result from the following description, the claims, the examples or from the overall context of the present invention.

solution

The stated problems were solved by providing a novel process for the continuous production of (meth)acrylic acid. This novel process, which can be carried out continuously, is based on the reaction of (meth)acrylic acid esters, in particular methyl methacrylate, with water in the presence of an acidic catalyst in the form of a catalytic hydrolysis.

The method according to the invention has the following method steps (a) and (b):

(a) In a reactor I, a (meth)acrylic ester and water are reacted in the presence of a Bronsted acid. This takes place to give a mixture which contains at least one (meth)acrylic ester, water and an alcohol corresponding to the (meth)acrylic ester and an unsaturated acid.

This is in step

(b) separating this mixture in a rectification column with an upper, middle and lower section.

This rectification column - hereinafter also simply referred to as column - is characterized by the following features: (i) In the upper region of the column, at the top of the column, a mixture consisting of the alcohol and (meth)acrylic ester is separated off.

(ii) In a side draw S1 of the column, which can be located, for example, in the middle region of the column, a mixture of (meth)acrylic ester and water is separated off and thus removed.

(iii) In the middle region, in a side draw S2 of the column, which can be located, for example, in the middle region of the column, (meth)acrylic acid is separated off and withdrawn.

(iv) In the lower region, in the bottom of the column, a mixture of substances which contains components with higher boiling points than (meth)acrylic acid is removed.

The (meth)acrylic ester is preferably MMA, the alcohol is correspondingly methanol and the (meth)acrylic acid formed is methacrylic acid.

The term (meth)acrylic acids is known in the art, acrylic acid and methacrylic acid being understood here. The term (meth)acrylic ester is known in the art, which is to be understood as meaning acrylic ester and methacrylic ester.

However, the process can also be applied to other alkyl (meth)acrylates, such as in particular butyl (meth)acrylate or ethylhexyl methacrylate, with slight modifications which can be easily and specifically deduced by a person skilled in the art. It is even possible to apply the process to functionalized (meth)acrylates such as hydroxyethyl methacrylate.

Reactor I

As described, the process has a reactor I in which at least one catalyst is preferably provided. This reactor I does not necessarily have to be a reactor operated in isolation. Rather, reactor I can also be in the form of a reaction area. Reactor I can be inside and/or outside of the rectification column. However, this reactor is preferably implemented outside the rectification column in a separate area, which is shown in more detail in FIGS. 1, 2 and 3 for preferred embodiments. Flow tube reactors have proven to be particularly favorable for such a separate reactor I.

The following process parameters are particularly favorable for the reaction in reactor I:

The reaction is generally preferably carried out at temperatures in the range from 20 to 200.degree. C., particularly preferably at 40 to 150.degree. C., in particular at 60 to 110.degree. The reaction temperature depends on the set system pressure. In the production of methacrylic acid from methyl methacrylate and water, the reaction temperature is preferably from 60 to 130.degree. C., particularly preferably from 70 to 120.degree. C. and very particularly preferably from 80 to 110.degree.

With regard to the operating pressure, which indirectly also determines the reaction temperature, a distinction is made between the precise implementation of the present invention. When the reactor is arranged within the column, the reaction is preferably carried out in the pressure range from 5 to 200 mbar, in particular at 10 to 100 mbar and particularly preferably at 20 to 50 mbar.

If the reactor is located outside the column, different pressure and temperature conditions can be selected there than in the column. This has the advantage that the reaction parameters of the reactor can be set independently of the operating conditions in the column. When the reactor is outside the column, the reaction is preferably carried out at pressures in the range 0.5 to 20 bar, more preferably 1 to 10 bar, most preferably 3 to 5 bar.

All pressures given are absolute pressures.

The reaction time of the reaction depends on the reaction temperature; the residence time in the reactor for a single pass is preferably 0.5 to 15 minutes and more preferably 1 to 5 minutes.

The process according to the invention is preferably carried out in such a way that reactor I is fed continuously with a reactant mixture of (meth)acrylic ester and water in a molar ratio of between 1:20 and 20:1.

In the actual production of methacrylic acid from methyl methacrylate and water, the molar feed ratio of water to methyl methacrylate is preferably 0.5 to 20:1, particularly preferably 0.5 to 10:1 and very particularly preferably 1.0 to 4:1.

In addition to the reactants, the reaction mixture may comprise other components such as, for example, solvents, catalysts and polymerization inhibitors.

The rectification column

With the aid of the column used according to the invention with the separation sections described, the methacrylic acid is surprisingly removed in a very simple manner and with little effort in high purity in the central part of the column. The rectification column can be made from any material suitable for this purpose. These include, but are not limited to, stainless steel and other suitable inert materials. Preference is given to an embodiment of the present invention in which the starting materials (meth)acrylic ester and water present in the side draw S1 are recycled to the reaction region of the reactor I. There these starting materials are reacted together with fresh water and (meth)acrylic acid ester. The side draw stream is optionally subjected to a phase separation before the at least partial recirculation in the reaction.

It is particularly advantageous if the column used according to the invention is designed in such a way that the (meth)acrylic acid is removed in a purity of more than 95% by weight via side draw S2. The side draw S2 is generally located in the column below the side draw S1 and the feed stream

The pressure at the top of the reaction column used according to the present invention is preferably from 5 to 1200 mbar, particularly preferably from 20 to 1100 mbar and very particularly preferably from 50 to 500 mbar. The top stream obtained is preferably subjected to a further material separation after removal from the column in order to obtain the remaining methyl (meth)acrylate and the corresponding alcohol, in particular methyl methacrylate and methanol, separately from one another. In this way, purified methyl (meth)acrylate that has not been completely converted can be returned to the process to increase the yield.

High boilers such as added inhibitors can be discharged from the bottom of the rectification column used according to the present invention by customary methods. This can be done, for example, using a thin-film evaporator or a corresponding alternative device. The isolated, evaporating substances are particularly preferably returned to the rectification column and non-evaporating high boilers are discharged.

Any rectification column which has 5 to 20 plates each in the upper, middle and lower region can be used for the reaction according to the present invention. The number of plates in the upper area is particularly preferably 5 to 15 and in the middle and lower area each 5 to 15. In the present invention, the number of plates in a plate column is multiplied by the plate efficiency or the number of theoretical T as the number of plates stages in the case of a packed column or a column with random packings.

Examples of a rectification column with trays include such as bubble-cap trays, sieve trays, tunnel trays, valve trays, slot trays, sieve-slot trays, perforated bubble-cap trays, nozzle trays, centrifugal trays, for a rectification column with packings, packings corresponding to the state of the art and industrially available are suitable. Examples are the Raschig Super Ring or the Sulzer NeXRing. Industrially available packings are suitable as structured packing metallic structured packings such as MellapakPlus (Sulzer) or the RMP structured packing from RVT. Structured packings with catalyst pockets, for example Katapak (Sulzer), can also be used.

A rectification column with combinations of sections of trays, sections of packing and/or sections of structured packing can also be used.

A rectification column with random packings and/or structured packings for the 3 regions is preferably used. Particular preference is given to using internals which lead to low pressure losses during operation according to the invention.

Depending on the design and operating parameters, there are a large number of industrially used collector and distributor types. For example, chimney tray collectors are particularly suitable for the complete removal of liquid side streams. For example, manifolds enable high distribution densities and can reduce uneven liquid distributions, thereby reducing the risk of polymerization. An exemplary embodiment looks as follows: Preference is given to the

Feed streams of the fresh reactants are fed into reactor I with the recycle stream, which consists predominantly of unreacted reactants and was obtained from the column. An inert boiling oil can be present at the bottom of the column in order to avoid long residence times for the (meth)acrylic acid target product. (Meth)acrylic acid is preferably drawn off in gaseous form between the middle and lower region, while at the top of the column the methanol formed is drawn off as an azeotrope with methyl (meth)acrylate and traces of water as the lowest-boiling reaction component. Unreacted starting materials are returned to the reaction area, for example by means of a pump.

The Catalyst

In reactor I, preference is given to using heterogeneous catalysts, including in one embodiment of a reaction region within the column. Acidic fixed bed catalysts, in particular acidic ion exchangers, are particularly suitable as heterogeneous catalysts.

The particularly suitable acidic ion exchangers include, in particular, cation exchange resins such as styrene-divinylbenzene polymers containing sulfonic acid groups. Suitable cation exchange resins can be obtained commercially under the trade name Amberlyst®, under the trade name Dowex® and under the trade name Lewatit®.

A heterogeneous fixed bed catalyst can be used in any section of the rectification column. This is preferably used in the middle region of the column. The amount of catalyst in liters is preferably 1/10 to 10 times, particularly preferably 1/5 to 5 times, the amount of newly formed (meth)acrylic acid to be produced in L/h.

The amount of catalyst in the feed to reactor I, specified in liters, is in a ratio of 1:10 to 10:1, preferably between 1:5 and 5, to the amount of (meth)acrylic acid measured in liters and taken off via the side draw S2 of the column 1.

Furthermore, the catalyst can be provided in a separate area of the reactor I, this area being connected to the other areas of the apparatus. This separate arrangement of the catalyst area is preferred, it being possible for the educts to be continuously passed through the catalyst area. This continuously produces (meth)acrylic acid and newly formed methanol.

An alternative embodiment is the use of a homogeneous catalyst, such as sulfuric acid. Disadvantages of such an embodiment are the high material requirements with regard to corrosion resistance and the effort involved in separating for the recovery and recycling of the homogeneous catalyst.

excipients

It has also proven to be advantageous if the bottom of the column contains an inert boiling oil which does not take part in the reaction. In the context of the present invention, high-boiling, inert substances with long-term thermal stability are referred to as boiling oils. These substances have a boiling point that is higher than the boiling points of the components involved in the reaction. A boiling oil is preferably used in order to ensure that the (meth)acrylic acid formed is removed by distillation without polymerization. However, the boiling point of the boiling oil should not be too high either, in order to reduce the thermal stress on the (meth)acrylic acid formed. The boiling point of the optionally used boiling oil is particularly preferably from 170 to 400.degree. C., in particular from 240 to 290.degree. C., at normal pressure (1013 mbar).

Suitable boiling oils include higher-chain, unbranched paraffins having 12 to 20 carbon atoms, aromatic compounds such as alkyl-substituted phenols or naphthalene compounds, sulfolane (tetrahydro-thiophene-1,1-dioxide) or mixtures of these. 2,6-di-tert-butyl-para-cresol, 2,6-di-tert-butyl phenol, sulfolane, diphyl or mixtures of these substances have proved to be particularly suitable boiling oils. The optionally, but preferably, used boiling oil is very particularly preferably sulfolane.

Diphyl is a eutectic mixture of 75% by weight biphenyl oxide and 25% by weight biphenyl. It has also proved advantageous to use polymerization inhibitors. The polymerization inhibitors that can be used with preference include octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, phenothiazine, hydroquinone, hydroquinone monomethyl ether, 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl ( TEMPOL), 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, para-substituted phenylenediamines such as e.g. B. N,N'-diphenyl-p-phenylenediamine, 1,4-benzoquinone, 2,6-di-tert-butyl-alpha-(dimethylamino)-p-cresol, 2,5-di-tert-butylhydroquinone or mixtures of two or more of these stabilizers. Phenothiazine and/or hydroquinone monomethyl ether are very particularly preferred. The inhibitor can be metered into the feed before the reactor and/or after the reactor and/or into the rectification column, preferably at the top thereof.

Special embodiments

In a particular embodiment of the present invention, the stream of substance separation containing a mixture of MMA and methanol can be processed in a further separate plant in such a way that methanol and MMA are separated from one another. A pressure swing distillation, for example, is suitable for this. The separated MMA can be fed back into the process according to the invention. The methanol can be used in a separate plant to produce MMA.

In a particular embodiment of the present invention, MMA and methanol can be separated by extraction in a separate plant. This is particularly preferably done by extracting the stream obtained via the side draw S1 with the top stream of the rectification column. At least one of the phases formed can be used again in a chemical reaction. This reaction can also involve the production of further alkyl (meth)acrylates, in particular MMA.

Alternatively, it is possible that these separate plants are part of a production plant for the production of MMA or already exist in these production plants in another function. This production plant can be chosen relatively freely on a C2, C3 or C4

procedures are based. In C2 and C3 processes, a suitable point of introduction of the azeotrope of MMA and methanol is before the phase separation step of the organic and aqueous phases. In the case of C4 processes, initiation prior to the esterification reaction is suitable. The person skilled in the art is aware of other suitable points of introduction in the processes described which allow the azeotrope to be used or worked up. By skilfully coupling the process according to the invention with processes for the production of MMA, the number of apparatuses, the need for auxiliary materials and the amount of waste can be reduced while the yield is increased at the same time. Preferred Embodiments of the Invention

Three embodiments of the method according to the invention are conceivable for the method. These are shown in Figures 1 to 3. The process according to FIG. 1 is the preferred embodiment, since the yield and the specific steam consumption can be particularly optimized according to this embodiment.

In this exemplary embodiment, the feed streams methyl methacrylate (1) and water (2) are mixed with the recycle stream (15) and fed to the preheater (H) and heated to the reaction temperature. The recycle stream consists mainly of the unreacted educts methyl methacrylate and water, as well as portions of methanol and methacrylic acid.

In this embodiment, the reactants (3) heated to reaction temperature are fed to reactor I (A). The hydrolysis reactor is operated in a temperature range between 80 °C and 110 °C and in a pressure range between 3 bar and 5 bar. The molar feed ratio of water to methyl methacrylate is between 1.5 and 4. The hydrolysis reactor is preferably designed as a flow tube reactor and is equipped with an acidic fixed bed catalyst.

The reactor product stream (4) is reduced to the column operating pressure by means of an expansion valve (B) and fed into the distillation column (C), preferably below the first side draw S1 below the upper separating region (C1) of the column. The pressure at the top of the column is between 0.05 and 1 bar. In addition to the product methacrylic acid, the reactor product stream also contains the reaction by-product methanol and the unreacted starting materials methyl methacrylate and water. The distillation column (C) consists of three separation sections: the upper separation section (C1), the middle separation section (C2) and the lower separation section (C3).

In an alternative embodiment of the present invention, an external phase separator (E) can optionally be installed between the upper and middle separation section according to the first exemplary embodiment described above. This variant is shown as embodiment II in FIG.

In a third alternative variant, the phase separator (E) can also be installed inside the column between the two separating sections (C1) and (C2). This variant is in turn shown as embodiment III in FIG. 3 by way of example.

In the first separation section (C1), the low-boiling reaction by-product methanol is separated from the intermediate boilers, the educts water and methacrylic acid, and drawn off at the top. Due to the azeotrope between methanol and methyl methacrylate, it is not possible to separate pure methanol. The top product (8) therefore usually contains methyl methacrylate in addition to methanol. Condensation takes place via the condenser (D). The top product (8) can be separated into the pure substances methanol and methyl methacrylate using a treatment method suitable for azeotropes. For example, methyl methacrylate can be returned to the process according to the invention. Alternatively, the skilful introduction of the top product (8) in processes for the production of methyl methacrylate enables the almost complete recovery of the unreacted methyl methacrylate and the reuse of the by-product methanol. In C2-based processes, the preferred infeed of the azeotrope mixture is in the extraction stage. In C3-based processes, it should be introduced into the distillation column to remove low boilers. In C4-based processes, injection upstream of the esterification reactors is suitable.

In embodiment I - as shown in Figure 1 - the liquid effluent from the upper separation section (C1) is collected in a collector. Some or all of this liquid stream is passed out of the column as a side draw S1 (13). The second part is conducted as a liquid reflux stream via a distributor to the middle separation section (C2). The side offtake (13) is fed to the pump (G).

Alternatively, in exemplary embodiment II according to FIG. 2, the liquid waste stream (13) is fed to the phase separator (E). The organic phase (140) is completely separated off and fed to the pump (G) as a recycle stream (14). Depending on the selected operating parameters, the aqueous phase (14W) is fed partially or completely to the pump (G) as a recycle stream (14). The second part, as a liquid reflux stream, after being mixed with the reactor product (5), is passed as a liquid stream via a distributor to the middle separation section (C2). In this embodiment, the phase separator (E) is outside of the column (C). Optionally, the phase separator (E) within the column, as in an example

Embodiment III shown in Figure 3, are installed. The liquid stream (13) collected in the collector is fed to the phase separator (E). The organic phase (140) is separated off completely and fed to the pump (G) as recycle stream (14). Depending on the mode of operation, some or all of the aqueous phase (14W) is fed to the pump (G) as a recycle stream (14). The second portion is sent as a liquid recycle stream to a header above the middle separation section (C2).

In embodiment I, the methacrylic acid product is purified from the educts water and methyl methacrylate and any remaining traces of methanol in the middle separation section. If the process is operated according to embodiment II or III, the product methacrylic acid is purified from methyl methacrylate and possibly remaining traces of methanol and water in the separation section (C2). The liquid phase of the middle separation section is collected in a collector and sprayed onto the lower separation section (C3) by means of a distributor. The gas stream rising from the lower separating section (C3) is partially passed out of the column as a side draw S2 (10) through suitable internals. This side stream S2 (10) contains the pure product (methacrylic acid). In the lower separation section (C3), methacrylic acid is separated from the boiling oil contained in the bottom. Stream (11) is the boiling oil inflow. This is preferably injected via a suitable distribution device in the upper third of the separation section. The high boilers are drawn off via the bottom outlet (12). The high boilers are discharged by suitable design of the evaporator (F), such as a thin-film evaporator. Substances with a boiling temperature at normal pressure (1013 mbar) of between 200 and 400° C., in particular between 240 and 290° C., are suitable as boiling oil. Suitable boiling oils are described above.

To avoid polymerization, a polymerization inhibitor, which can also simply be called a stabilizer, (6) is preferably introduced at the top of the column. A more detailed description of this can also be found above.

The side draw S1 (14) is brought to the reactor operating pressure by the pump (G) and then mixed with the fresh starting materials (1) and (2) as a recycle stream (15).

The usual process control of the prior art according to US Pat. No. 8,791,296 comprises 3 distillation columns in series. The azeotrope of methanol and methyl methacrylate is separated off in the first column. In the second column, methyl methacrylate and water are separated from methacrylic acid, the variants with or without phase separation representing possible embodiments at the top. In the third column, methacrylic acid is obtained as a distillate. High-boiling by-products are drawn off as the bottom product. In one embodiment according to Figure 4, the separation of the azeotrope methanol and methyl methacrylate (8) described in separation section (C1) takes place in a dedicated distillation column (I) first and then in a second distillation column (L) the separation of methyl methacrylate and water (17) from methacrylic acid. Methacrylic acid is taken off as a side draw (10) and a boiling oil (11) can optionally be used in the bottom to remove high boilers and reduce the bottom temperature. Two variants are possible at the top of this second distillation column (L). In a first variant, there is no phase separation and the condensate is divided accordingly into reflux and distillate. The second variant has a phase separation at the top. The aqueous phase is used as reflux and partly discharged as distillate or from the process. The organic phase is recycled into the reactor as distillate. In a further connection variant (see FIG. 5), the azeotrope of methanol and methyl methacrylate is separated off as distillate (8) in the azeotrope column (I). In the same column (I), a mixture of water and methyl methacrylate is removed as a side draw (19) and methacrylic acid and high boilers are obtained at the bottom (16). In the MAS column (L), pure methacrylic acid is obtained as the top product (10) and high boilers are removed as the bottom product (18). The second column can optionally be operated with boiling oil (11) in order to reduce the bottom temperature.

The methods according to the invention and its alternative embodiments share the feature that three separation steps have to be carried out, with two azeotropes having to be separated from the target product. To reduce the number of devices required, one or more separation steps are integrated in one device. The target product is preferably drawn off as a side stream. What these processes also have in common is that a heterogeneous catalyst is used for the hydrolysis.

In addition to the process according to the invention, a plant for the production of methacrylic acid is also part of the present invention. This novel system is characterized in that a heterogeneous catalyst for the hydrolysis of methyl methacrylate with water to form methacrylic acid and methanol is present in a reactor I, and that the system for processing azeotropes formed from methyl methacrylate and water as well as from

Methyl methacrylate and methanol has a rectification column with three separation zones, from which methacrylic acid is drawn off in high purity from a side stream draw.

LIST OF REFERENCE NUMERALS Figure 1 represents an embodiment without phase separation

Figure 2 shows an embodiment with an external phase separator

Figure 3 shows an embodiment with an internal phase separator

FIG. 4 represents an alternative embodiment with two distillation columns connected in series

FIG. 5 represents a further alternative embodiment with two distillation columns connected in series streams

(1) Methyl methacrylate lead

(2) Water supply line

(3) Reactor feed line

(4) Reactor Product

(5) Distillation column inlet

(6) Stabilizer addition

(7) Exhaust

(8) Distillate (MEOH, MMA)

(9) Distillation column reflux

(10) Side draw S2 MAS Product stream from separation section C2 (11) Boil feed stream (12) Bottom stream

(13) Side draw S1 from separation section C1

(14) Recycle stream at column operating pressure

14W water phase of the side draw after phase separation

140 Organic phase of the side draw after phase separation

(15) Recycle stream under pressure

(16) Bottom stream azeotrope column

(17) Distillate MAS column

(18) Bottom stream MAS column (19) Side draw (MMA & H20) Azeotrope column apparatus

(A) Hydrolysis reactor

(B) Relief valve

(C) Distillation column

(i) Azeotrope Separation Section

(ii) Separation Section MMA vs. MAS

(iii) MAS vs boiling oil separation section

(D) condenser

(E) Decanter

(F) evaporator

(G) pump

(H) Preheater

(I) Azeotrope column

(J) Azeotrope column condenser (K) Azeotrope column evaporator

(L) MAS column

(M) Condenser MAS column

(N) Evaporator MAS column

EXAMPLES Example 1 In a setup according to the embodiment without phase separation according to FIG. 1, a methyl methacrylate feed stream (1) and a water feed stream (2) are mixed with the recycle stream (15) containing methanol, water, methyl methacrylate and methacrylic acid. The individual streams have a pressure of 4 bar. The temperature of the streams is 22 °C. The methyl methacrylate feed (1) is 500 g/h and the recycle (15) is 1539 g/h. The water inflow (2) is adjusted in such a way that a molar ratio of 2:1 water to MMA is established in the mixed total flow. The stream is heated to the reaction temperature of 110 °C by means of a preheater (H). In the reactor (A) there is a residence time of 60 min, a space-time yield based on methacrylic acid of 200 kg/(h*m ³ ) and an MMA conversion of 30%. The reactor product stream (4) is expanded to 200 mbar using the expansion valve (B) and fed to the column feed (5) into the distillation column (C). the

Distillation column is designed as a DN50 glass column. There are 3 packing sections installed. The top packing section (C1) and the middle packing section (C2) each have 2 m Sulzer DX laboratory packing, the bottom packing section (C3) has 1 m Sulzer DX laboratory packing. A collector is installed between the upper and middle packing section, via which the entire liquid phase of the upper section is drawn off as a side stream (13). Below this collector, the column feed (5) is fed via a distributor to the middle packing section (C2). A collector is installed below the middle packing section, with the help of which the liquid phase is collected from the middle packing section and fed into a distributor. Between the collector and the distributor there is a nozzle for drawing off the gaseous product stream of methacrylic acid (10). The liquid from the collector is fed to the lower packing section (C3) via the distributor. A condenser (D) is installed at the top of the column, which achieves a condensate outlet temperature of 7°C. Evaporator (F) is designed as a thin film evaporator. The pressure at the top of the column is set at 100 mbar. Stabilizer is sprayed onto the condenser via line (6) to prevent polymerization and is fed into the column via reflux (9). The stabilizer flow is 10 g/h and consists of 2% MEHQ solution in methyl methacrylate. A reflux ratio of R/D=12 is set at the top of the column. The top temperature of the column is 15.1°C. The upper packing section (C1) serves to separate the azeotrope methanol and methyl methacrylate from excess methyl methacrylate, water and methacrylic acid. 195 g/h of distillate (8) containing methanol and methyl methacrylate are drawn off. At the end of the upper packing section (C1) 1539 g/h recycle stream (13) are drawn off as a liquid side stream containing methanol, water, methyl methacrylate and methacrylic acid and fed to the pump (G) and compressed to 4 bar. In the middle packing section (C2), 386 g/h of methacrylic acid are separated from the lower-boiling components methyl methacrylate and water and drawn off as a gaseous side stream (10). In the middle of the lower packing section (C3), 10 g/h of sulfolane (11) is used

Boiling oil added to the column. This ensures that methacrylic acid is not exposed to temperatures higher than 95 °C, which reduces the risk of polymerisation. At the same time, high boilers formed are discharged via the thin-film evaporator (F) as a bottom stream (12). The sump temperature is 198 °C. In addition, an almost methacrylic acid-free bottom is produced, which minimizes methacrylic acid losses. In

Table 1 lists the mass flows observed and the material composition of the individual flows.

Table 1: Mass flows and the material composition

Table 2 shows the specific excipient consumption achieved with the process.

A molar yield of 0.90 moles of MAS per mole of MMA used is achieved. example 2

In a setup according to the embodiment with external phase separation according to FIG. 2, a methyl methacrylate feed stream (1) and a water feed stream (2) are mixed with the recycle stream (15) containing methanol, water, methyl methacrylate and methacrylic acid. The individual streams have a pressure of 4 bar. The temperature of the streams is 22 °C. The methyl methacrylate feed (1) is 500 g/h and the recycle (15) is 1353 g/h. The water feed (2) is adjusted in such a way that a molar ratio of 2:1 water to methyl methacrylate is established in the mixed total stream. By means of preheater (H), the current is on the

Reaction temperature of 110 °C heated. In the reactor (A) there is a residence time of 60 min, a space-time yield based on methacrylic acid of 200 kg/(h*m ³ ) and an MMA conversion of 30%. The reactor product stream (4) is expanded to 200 mbar using the expansion valve (B) and fed to the column feed (5) into the distillation column (C). The distillation column is designed as a DN50 glass column. There are 3 packing sections installed. The top packing section (C1) and the middle packing section (C2) each have 2 m Sulzer DX laboratory packing, the bottom packing section (C3) has 1 m Sulzer DX laboratory packing. A collector is installed between the upper and middle packing section, via which the entire liquid phase of the upper packing section (C1) is drawn off as a side stream (13). The column feed (5) is fed via a distributor to the middle packing section (C2). A collector is installed below the middle packing section, with the help of which the liquid phase is collected from the middle packing section and fed into a distributor. Between the collector and the distributor there is a nozzle for drawing off the gaseous product stream of methacrylic acid (10). The liquid from the collector is fed to the lower packing section (C3) via the distributor. A condenser (D) is installed at the head of the column, which achieves a condensate outlet temperature of 7 °C. Evaporator (F) is designed as a thin film evaporator. The pressure at the top of the column is set at 100 mbar. Stabilizer is sprayed onto the condenser via line (6) to prevent polymerization and is fed into the column via reflux (9). The stabilizer flow is 10 g/h and consists of 2% MEHQ solution in methyl methacrylate. A reflux ratio of R/D=12 is set at the top of the column. The top temperature of the column is 15.9°C. The upper packing section (C1) serves to separate the azeotrope methanol and methyl methacrylate from excess methyl methacrylate, water and methacrylic acid. 219 g/h of distillate (8) containing methanol and methyl methacrylate are drawn off. At the end of the upper packing section (C1), 1776 g/h of liquid phase are drawn off as a side stream (13) containing methanol, water, methyl methacrylate and methacrylic acid and fed to the phase separator (E). 846 g/h of aqueous phase (14W) and 930 g/h of organic phase (140) are formed. The aqueous phase (14W) is divided in a ratio of 1:1, the first part being mixed with the column feed (5) and being fed via a distributor to the middle packing section (C2). The second part is mixed with the organic phase (140) and gives the recycle stream (14) at 1353 g/h. This stream is raised by means of pump (G). 4 bar compressed. In the middle packing section (C2), 358 g/h of methacrylic acid are separated from the lower-boiling components methyl methacrylate and water and drawn off as a gaseous side stream (10) under the collector under the middle packing section (C2). In the middle of the lower packing section (C3), 10 g/h of sulfolane (11) are added to the column as boiling oil. This ensures that methacrylic acid is not exposed to temperatures higher than 95 °C, which reduces the risk of polymerisation. At the same time, high boilers formed are discharged via the thin-film evaporator (F) as a bottom stream (12). The sump temperature is 198 °C. In addition, an almost methacrylic acid-free bottom is produced, which minimizes methacrylic acid losses. Table 3 lists the observed mass flows and the material composition of the individual flows.

Table 3: Mass flows and material composition

Table 4 shows the specific excipient consumption achieved with the process.

A molar yield of 0.83 moles of MAS per mole of MMA used is achieved. Comparative example 3

A structure according to the disclosure of US Pat. fed inflow of 82 g/h. These reactant streams are mixed with the recycle stream (1471 g/h), which is the top product of the second distillation column, and form the reactor feed stream at 2052 g/h. The water inflow was adjusted in such a way that a molar ratio of 2:1 water to methyl methacrylate was established in the reactor inflow stream. The individual streams have a pressure of 4 bar. The reactor feed stream is heated to the reaction temperature of 110 °C with the aid of a preheater. In the reactor (A) there is a residence time of 60 min, a space-time yield based on methacrylic acid of 200 kg/(h*m ³ ) and an MMA conversion of 30%. The reactor product stream contains methanol, water, methyl methacrylate and methacrylic acid and is sent to the bottom of the first distillation column. At a top pressure of 1000 mbar, a top temperature of 64.3 °C and a bottom temperature of 83.3 °C are established. The reflux ratio is set to 12. The azeotrope (196 g/h) consisting of MEOH and MMA is drawn off at the top. The bottom stream is 1840 g/h and mainly contains water, methyl methacrylate and methacrylic acid and a little methanol.

This stream is fed into the middle of a downstream second distillation column which is operated at a top pressure of 100 mbar. A top temperature of 38.9 °C and a bottom temperature of 93.2 °C are established. The reflux ratio is 0.7. The overhead product (1471 g/h) consists of the azeotrope of water and MMA. The bottom product contains MA, traces of high boilers and stabilizers and is 385 g/h.

The bottom product from the second distillation column is fed into the middle of the third distillation column for fine purification of the methacrylic acid. The third column is operated at a top pressure of 100 mbar and a top temperature of 93.2° C. and a bottom temperature of 98.6° C. are established. The reflux ratio is set to 2. 380 g/h of methacrylic acid are obtained as a pure product at the top. In the bottom, 5 g/h of high boilers and stabilizers are removed via a thin film evaporator.

Each of the three distillation columns has a stabilizer addition at the condenser to prevent polymerization, and this stabilizer reaches the columns via the reflux. The stabilizer stream is 10 g/h for each column and consists of 2% strength MEHQ solution in methyl methacrylate. Table 5 lists the observed mass flows and the material composition of the individual flows. Table 5: Mass flows and material composition

Table 6 shows the specific excipient consumption achieved with the process. A molar yield of 0.88 moles of MAS per mole of MMA used is achieved.

Claims

Expectations

1. A process for the continuous production of (meth)acrylic acid by reacting (meth)acrylic esters with water, characterized in that

(A) in a reactor I, a (meth) acrylic acid ester and water are reacted in the presence of a Bronsted acid, with a mixture containing a

(Meth)acrylic acid ester, water, and an alcohol corresponding to the (meth)acrylic acid ester and an unsaturated acid are obtained,

(b) and this mixture is separated in a rectification column with an upper, middle and lower area, so that (i) in the upper area of the column, a mixture consisting of the alcohol and (meth)acrylic acid ester is separated off as the distillate of the column,

(ii) a mixture of (meth)acrylic acid ester and water is separated off in a side draw S1 of the column,

(iii) (meth)acrylic acid is separated off in a side draw S2 of the column, and (iv) in the lower region, at the bottom of the column, a mixture of substances which contains higher-boiling components than (meth)acrylic acid is removed.

2. The method according to claim 1, characterized in that the starting materials (meth)acrylic ester and water contained in the side draw S1 are returned to the reaction region of the reactor I, where they are reacted together with fresh water and (meth)acrylic ester and optionally the side draw stream is subjected to a phase separation before at least partial recycling into the reaction.

3. The process as claimed in claim 1 or 2, characterized in that the (meth)acrylic acid is removed in a purity greater than 95% by weight via side draw S2 and side draw S2 is located in the column below side draw S1.

4. The method according to at least one of claims 1 to 3, characterized in that the bottom of the column contains an inert boiling oil which does not take part in the reaction.

5. The process as claimed in at least one of claims 1 to 4, characterized in that a reactant mixture of (meth)acrylic acid ester and water in a molar ratio of between 1:20 and 20:1 is fed continuously to reactor I.

6. The process as claimed in at least one of claims 1 to 5, characterized in that the Bronsted acid in reactor I is a heterogeneous, acidic fixed-bed catalyst. 7 The method according to claim 6, characterized in that an acidic cation exchanger is used as a catalyst.

8. The method according to at least one of claims 1 to 7, characterized in that the reactor I is located outside the column. 9. The method according to any one of claims 1 to 8, characterized in that the (meth)acrylic acid is methacrylic acid, the (meth)acrylic ester is methyl methacrylate and the alcohol is methanol.

10. The method according to claim 4, characterized in that a high-boiling, inert substance having a boiling point which is higher than the boiling points of the components involved in the reaction is used as the boiling oil.

11. The method according to claim 10, characterized in that 2,6-di-tert-butyl-para-cresol, 2,6-di-tert-butyl-phenol, sulfolane, diphyl or mixtures of these substances, preferably sulfolane, are used as boiling oil will.

12. The method according to at least one of claims 1 to 11, characterized in that high-boiling components are discharged from the bottom of the column and evaporating substances are returned to the column.

14. The method according to at least one of claims 9 to 12, characterized in that the top stream of the rectification column is subjected to a further material separation in order to obtain methyl methacrylate and methanol. 15. The method according to claim 14, characterized in that the stream of substance separation comprising a mixture of MMA and methanol is processed in a further separate plant in such a way that methanol and MMA are separated from one another, and that this separate plant is one plant for the production of MMA based on a C2, C3 or C4 process, the isolated methanol being converted at least partially into further MMA in this plant.

16. The method according to claim 14, characterized in that the substances are separated from MMA and methanol by extraction, in particular by extracting the stream obtained via the side draw S1 with the top stream of the rectification column, at least one of the phases formed being used again in a chemical reaction. 17. Plant for the production of methacrylic acid, characterized in that in a

Reactor I contains a heterogeneous catalyst for the hydrolysis of methyl methacrylate with water to form methacrylic acid and methanol, and that the plant for working up the azeotropes formed from methyl methacrylate and water and from methyl methacrylate and methanol has a rectification column with three separation zones from which methacrylic acid is separated from a side stream draw into high purity is drawn off.