WO2023117754A1

WO2023117754A1 - Process for producing alkyl methacrylates with higher yields and reduced emissions of volatile organic compounds

Info

Publication number: WO2023117754A1
Application number: PCT/EP2022/086343
Authority: WO
Inventors: Patrick WINGS; Florian Klasovsky; Steffen Krill
Original assignee: Röhm Gmbh
Priority date: 2021-12-23
Filing date: 2022-12-16
Publication date: 2023-06-29
Also published as: TW202340134A

Abstract

The present invention relates to an environmentally friendly process for producing alkyl methacrylate, more particularly methyl methacrylate (MMA), the process comprising the production of an alkyl methacrylate precursor; esterification; processing of the raw product and storing the pure product, the alkyl methacrylate being recovered from the gaseous outlet streams of the process by way of at least one absorption step and being suitably recirculated to the process. The alkyl methacrylate precursor is particularly an MMA precursor, such as, for example, methacrylic acid amide (MASA) or its hydrogen sulfate (MASA·H2SO4).

Description

Process for the production of alkyl methacrylates with improved yield and reduced emissions of volatile organic compounds

Description

The present invention relates to an environmentally friendly process for the production of alkyl methacrylate, in particular methyl methacrylate (MMA), comprising preparing an alkyl methacrylate precursor; esterification; Processing of the crude product and storage of the pure product, wherein alkyl methacrylate is recovered from the gaseous outlet streams of the process by at least one absorption step and can be recycled to the process in a suitable manner. In particular, the alkyl methacrylate precursor is an MMA precursor, such as methacrylic acid amide (MASA) or its hydrogen sulfate (MASA H2SO4). With the aid of the process according to the invention, high yields of alkyl methacrylate, in particular MMA, can be achieved and at the same time the release (emission) of volatile organic compounds into the environment can be reduced.

In particular, the invention includes a method and a device for separately or jointly carrying out at least one absorption step, in which the gaseous outlet streams (e.g. exhaust air and exhaust gases), which lead gaseous by-products and co-products as well as auxiliary materials, by means of suitable receiver phases (absorbents) and under Setting optimal process engineering parameters are dealt with. In this way, in particular, preliminary and intermediate products, which can be converted into the target product MMA, or MMA itself, are depleted in the gaseous outlet streams and returned to the process.

State of the art

Methyl methacrylate (MMA) is used in large quantities to produce polymers and copolymers with other polymerizable compounds. Furthermore, methyl methacrylate is an important building block for various special esters based on the chemical base methacrylic acid (MAA), which can be produced by transesterification of MMA with the corresponding alcohol or are accessible by condensation of methylacrylic acid and an alcohol. The methacrylic acid required for this can be obtained from MMA by hydrolysis with water. This results in a great interest in manufacturing processes for this starting material that are as simple, economical and environmentally friendly as possible. A number of commercial processes are known for producing alkyl methacrylates, in particular methyl methacrylate (MMA), on an industrial scale, starting from C2, C3 or C4 building blocks. The general state of the art is essentially described in current reviews and overview articles, e.g. K. Nagai, T. Ui (“Trends and Future of Monomer-MMA Technologies”, Sumitomo Chemical Co. Ltd.; Basic Chemicals Research Laboratory), “, S. Krill (“Many roads lead to methyl methacrylate”, Chem. Our time, 2019, 53; 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).

Typically, the production processes for MMA involve the conversion of C2, C3 or C4 building blocks into a reactive MMA precursor compound, which is then reacted with methanol and optionally water to give a crude MMA-containing product (see Figure 1). Examples of typical reactive MMA precursor compounds are methacrolein (MAL) for C2-based processes, derivatives of methacrylic acid amide (MASA) or 2-hydroxyisobutyric acid (2-HIBS) for C3-based processes and methacrylic acid (MAS) for C4-based processes . After the MMA-containing crude product has been worked up by a suitable sequence of thermal separation steps, a pure MMA product is typically obtained, which is often stored and/or transported until it is used further. In order to prevent the formation of polymeric deposits and other by-products, suitable stabilizers are typically added to MMA during storage.

C2 building blocks as a basis for the production of MMA

Although ethylene is readily available as a C2 building block, MMA is only produced on this basis to a small extent. All commercially relevant C2-based processes have in common that methanol is used both for the formation of the methyl ester function and for the production of the intermediate product formaldehyde (FA). Alongside ethylene, methanol is therefore the most important raw material in C2-based manufacturing processes.

In the production of MMA from ethylene using the BASF process, the propionaldehyde (PA) obtained by hydroformylation (e.g. according to C. He, F. You, Ind. Eng. Chem. Res. 2014, 53, 11442-11459) is first in a first reaction step with formaldehyde (FA) to form methacrolein (MAL) (eg DE 3220858 A1). In a second reaction step, the MAL is selectively converted to methacrylic acid (MAA), as a reactive MMA precursor compound, on thermostable heteropolyacids in the gas phase in the presence of atmospheric oxygen. MAS is then converted to MMA by catalytic esterification with methanol. In the so-called ALPHA process, ethylene is converted by methoxycarbonylation into methyl propionate (MP), as a reactive MMA precursor compound (eg WO 9619434 A1 or EP 3272733 A1). In a subsequent reaction step, formalin obtained by oxidation of methanol is dewatered to form anhydrous formaldehyde (FA) and then reacted in the gas phase over doped catalysts based on a zirconium-silicon mixed oxide with the previously formed MP to form MMA. This process often has selectivities > 94%.

Like the BASF process, the so-called LiMA process converts ethylene with synthesis gas to propionaldehyde (PA) in the first stage, with rhodium-based phosphine or phosphite ligands advantageously being used as catalysts. By reaction with formalin, MAL is then obtained in almost quantitative yields as a reactive MMA precursor compound (e.g. WO 2016/042000 A1). The MAL is then converted directly to MMA in the presence of methanol in a direct oxidative esterification over a noble metal-based heterogeneous catalyst (e.g. WO 2014/170223), with high conversions and selectivities being able to be achieved.

C3 building blocks as a basis for the production of MMA

A commercial process that is widespread worldwide is based on the C3 building block acetone as the basic material and is usually referred to as the C3 process or ACH-Sulfo process. Here, acetone is first reacted with hydrocyanic acid (HON) to form the central intermediate product acetone cyanohydrin (ACH). This intermediate product is isolated and used for the following process steps to produce methacrylic acid (MAS) and MMA. A typical process for producing MMA starting from ACH by the sulfuric acid-based ACH-Sulfo process is described in US Pat. No. 4,529,816, for example.

For example, DE 102006 058250 describes a process for preparing alkyl esters of methacrylic acid comprising preparing ACH from HCN and acetone, purifying the ACH, preparing methacrylic acid amide from ACH using sulfuric acid, esterifying the methacrylic acid amide with an alkyl alcohol in the presence of a mixture from water and sulfuric acid to methacrylic acid ester and the final purification of the ester.

In the ACH sulfo process, ACH is hydrolyzed using sulfuric acid to form a-hydroxyisobutyric acid amide (2-HIBA) and its sulfate ester (2-SIBA). This Partial step is referred to in the relevant literature as amidation or hydrolysis, since the nitrile group of the ACH is formally converted into an amide function. 2-HIBA, 2-SIBA and methacrylic acid amide sulfate (MASA) already formed by elimination in the amidation are then thermally converted into MASA and to a lesser extent into MAS in the sulfuric acid reaction mixture, with the conversion of SIBA to MASA generally being easier than that Conversion of HIBA to MASA. This sub-step is often referred to as conversion. The reaction conditions, such as heat supply and residence time, must generally be optimized to the effect that, in particular, the desired main reaction of the conversion, namely the conversion

MASA, as well as the also desired conversion of the by-product HIBA to MASA. The gaseous by-products formed during conversion, such as carbon monoxide or carbon dioxide, are typically discharged from the process.

The sulfuric acid MASA solution obtained after conversion (reactive MMA precursor compound) can then be esterified with methanol and water to form MMA (esterification). As an alternative to this, sulfuric acid MASA solution can be converted with water to methacrylic acid (MAS) after conversion (hydrolysis). Residual 2-HIBA is typically partly converted to 2-hydroxyisobutyric acid methyl ester (2-HIBSM), but partly also hydrolyzed to 2-hydroxyisobutyric acid (2-HIBS). The thermal decomposition of yield-relevant intermediate stages can occur in particular during the conversion but also in the esterification step. Together with other components, there is therefore also a need to remove gaseous secondary components in the esterification step.

In addition to the ACH sulfo process, other C3-based process variants are described in the prior art, which manage without the use of sulfuric acid. Thus, the problems associated with the use of sulfuric acid, the sub-plant for its recovery (Sulfuric Acid Recovery, SAR) and the formation of ammonium sulfate as a by-product are avoided. For example, the multi-stage Mitsubishi Gas Chemical (MGC) process was developed in the mid-1980s and implemented in the 1990s. In the first stage of the MGC process, hydrocyanic acid is added to acetone to form the ACH. The ACH is then hydrated to form hydroxyisobutyric acid amide (2-HIBA) as a reactive MMA precursor compound using a manganese dioxide catalyst (eg EP 0487853). In the following step, methanol is first reacted homogeneously with CO under pressure in the presence of a strong catalyst base (e.g. sodium methoxide) to form methyl formate (e.g. according to DE 3436608). Methyl formate as the activated ester then reacted in a transamidation reaction with hydroxyisobutyric acid amide to formamide and hydroxyisobutyric acid methyl ester HIBSM. Typically, the conversion is about 85% with selectivities of 98%. (e.g. US 5312966A). These two products are then mostly processed separately. For example, formamide can be split into hydrocyanic acid and water at low pressure and 450 - 580 °C in a heterogeneous contact based on iron or aluminum phosphate with various dopings. The yield to HCN is usually about 94%. Both HCN and the water are typically recirculated in the MMA process, which significantly reduces the need for hydrogen cyanide. As described in US Pat. No. 5,075,493, US Pat. No. 5,371,273 or US Pat. No. 5,739,379, 2-HIBSM is dehydrated separately in the gas phase on a heterogeneous zeolite catalyst in the presence of methanol to give MMA, with yields of >90% often being achieved. Methacrylic acid is formed as a by-product in this step.

The use of sulfuric acid is also avoided in the C3-based Aveneer process from Evonik Röhm GmbH. Here, hydrocyanic acid, e.g. obtained by the Andrussow, BMA or BF process, is first converted to ACH. In the first step, the ACH is hydrated with water, usually on a modified manganese dioxide contact (e.g. DE 102008044218) in almost quantitative yields to form 2-HIBA, as a reactive MMA precursor compound. 2-HIBA is then reacted with methanol to give methyl 2-hydroxyisobutyrate (2-HIBSM) (e.g. EP 2043994). The ammonia that is released is usually recovered and recycled as a starting material in an HCN process. Another key step is the catalytic cross-transesterification of methacrylic acid and 2-HIBSM to form MMA and 2-hydroxyisobutyric acid (2-HIBS), e.g. in DE 102005023975 or DE 102005023976. The methacrylic acid itself is produced by catalytic dehydration of the resulting hydroxyisobutyric acid.

C4 building blocks as a basis for the production of MMA

MMA manufacturing processes based on the C4 building blocks isobutene, methyl tert-butyl ether or tert-butanol include their conversion into the reactive MMA precursor compounds methacrolein or methacrylic acid. In particular, the following three C4-based methods are well known:

The so-called tandem C4 direct oxidation process (Sumitomo process) typically takes place without intermediate isolation of methacrolein. Here is a first step MAL is produced from isobutene, which is oxidized to MAS in a second process step before MAS is esterified with methanol to form MMA in a third process step. This process is also referred to in the literature as a "tandem process" since the process gas of the first stage is oxidized directly to MAS without isolating the intermediate product MAL.

In the so-called separate C4 direct oxidation (Mitsubishi process), similar to the tandem process, MAL is produced from isobutene in a first process step. MAL is then typically isolated and purified in liquid form in a separate second process step before being vaporized and oxidized to MAS in a third process step. The esterification with methanol to form MMA then takes place in a fourth process step.

Asahi's direct metha process is often also described as a direct oxidative esterification process. Here too, in a first process step, MAL is produced from isobutene in the gas phase at a first contact and isolated and intermediately purified in a second process step. In the third process step, MAL is directly oxidatively esterified to MMA, with this step typically being carried out in the liquid phase on a suspended catalyst. The main difference between this process and the other two C4 processes is the combination of a process step in the gas phase and an oxidative step in the liquid phase.

The C4-based processes mentioned are described in the prior art, e.g. S. Nakamura, H. Ichihashi (Vapor Phase Catalytic Oxidation of Isobutene to Methacrylic Acid, Stud. Surf. Sci. Catal. 1981, 7, 755-767).

Storage and stabilization of MMA

When storing alkyl methacrylates such as MMA, suitable stabilizers usually have to be added to prevent the formation of oligomeric and polymeric secondary products. In addition to loss and contamination of the product, the reaction enthalpy released and the risk of an uncontrolled exothermic reaction represent a safety-relevant aspect (e.g. Methacrylate Esters Safe Handling Manual, Methacrylate Producers Association and Methacrylates Sector Group of the European Chemical Industry Council, 2008). Often phenolic compounds, phenylenediamine compounds, quinones or catechols or amine-N-oxides are used as stabilizers. Substances that only develop their function in the presence of small amounts of free oxygen are often used as stabilizers. Dissolved oxygen is fundamentally required for the effective functioning of these stabilizers, since it acts as an initial, efficient radical scavenger. Alkyl methacrylates such as MMA should therefore never be handled in an oxygen-free atmosphere. Typically, an oxygen-containing atmosphere is provided above the liquid surface in the storage tank to ensure the effectiveness of the stabilizer. Oxygen is normally slowly consumed here as part of the radical scavenging mechanism. It is therefore usually necessary to continuously and/or periodically flow through the storage tanks with a small amount of an oxygen-containing gas mixture, for example air (the so-called stabilizer air) or a nitrogen/oxygen gas mixture with 5-21% by volume of oxygen. Since some alkyl methacrylate vapors, such as MMA vapors, form flammable mixtures with air at room temperature, it may be desirable to use a gas mixture, e.g. a nitrogen/oxygen gas mixture, containing less than 21% by volume oxygen. to reduce flammability. However, the gas mixture (atmosphere) above the alkyl methacrylate should contain at least 5% oxygen to ensure the effectiveness of the stabilizer.

The exhaust air resulting from the storage of alkyl methacrylate is usually loaded with alkyl methacrylate up to the saturation limit and thus discharges a considerable part of the product of value from the process. For example, a typical gas flow of 150 m ³ /h corresponding to the MMA vapor pressure at a storage temperature of 25 °C can lead to an annual loss of 18.51 MMA.

Emissions and how to avoid them

The production of alkyl methacrylates, such as MMA, and derived products such as MAS and other alkyl methacrylates, are operated in plant complexes that often produce up to several 100,000 tons of product per year. In this respect, even minor improvements in yield can lead to significant savings. In particular, the waste gas and waste air streams, which contain alkyl methacrylate, for example MMA, and methanol, represent yield-relevant losses which should be converted back into the target product by suitable measures. Likewise, for ecological reasons, there is a great need to reduce the gaseous emissions of volatile organic compounds (VOCs). The minimization of MMA emissions via gaseous exhaust streams is also strictly regulated by law. Some methacrylates are classified as volatile organic compounds (VOC) under the US Clean Air Act. MMA is listed by the US Environmental Protection Agency (EPA) as a hazardous air pollutant under the Clean Air Act Amendment of 1990. Within the EU, the handling and storage of MMA falls under the 96-61-EC directive on the control and prevention of emissions and under the REACH regulation (EC No. 1907/2006), which requires direct emissions into the atmosphere to be monitored and permitted .

A number of precautions for preventing or minimizing VOC emissions during the manufacture and storage of MMA are described in the prior art. In addition to purely physical processes such as absorption, adsorption and condensation, combined physical-chemical processes are also used in which MMA is usually irreversibly converted into substances that can be separated from gas streams more easily than MMA due to their higher boiling point and/or which be released into the atmosphere without further treatment due to their lower reactivity and/or toxicity.

Those skilled in the art know that vapors of organic compounds can be condensed out of gas mixtures either by increasing the pressure or, depending on the temperature dependence of their vapor pressure, by targeted cooling. For example, the vapor pressure of MMA and methanol is typically reduced by a factor of 10 by lowering the temperature by 40 K.

Various condensation processes, for example, are described in the prior art. The organic vapors can be cooled indirectly, for example by heat exchangers, or by direct contact, for example by sprinkling the gas stream in a tray or packed column, by atomizing the coolant in the gas stream (with a subsequent demister) or in a venturi nozzle operated with the coolant (Jet), can be accomplished. If the partial pressure of the organic substance to be condensed is greatly reduced by a high proportion of inert gas, only a small part can be condensed out at a given coolant temperature. This proves to be disadvantageous for gaseous outlet streams loaded with MMA and/or methanol at room temperature, since the high inert gas content, in particular nitrogen content, often requires a very low coolant temperature and thus a high use of cooling energy. Alternatively, the proportion of condensed organic substance by increasing the pressure in the condensation system, which in turn requires compression energy.

CN 104815514 A describes a recycling of polymethyl methacrylate (PMMA) by depolymerization, the resultant M MA-containing vapors first being subjected to multi-stage pre-cooling and then to a likewise multi-stage combination of compression and cooling. The disadvantage here is that multi-stage cooling and compression and thus a considerable amount of energy are required. If the evaporation enthalpy released during the condensation of the organic vapors is to be used in the process (energy integration), this requires a high level of equipment complexity.

Common methods for the adsorptive depletion of organic compounds in gaseous outlet streams are described, for example, in F. Woodard, Industrial Waste Treatment Handbook, Butterworth-Heinemann, 2001. The removal of organic compounds by adsorption on solid granules, such as activated carbon, is often used for removal of volatile organic compounds (VOCs). Activated carbon treatment systems (adsorbers) for the treatment of gas flows are configured, for example, as cylindrical containers with activated carbon granulate beds, whereby several adsorbers often have to be connected in parallel and/or sequentially in order to enable the exchange of used beds during operation (e.g. CN 109045940 A) .

The disadvantage of adsorptive separation using activated carbon is often that the loaded adsorbent has to be disposed of as waste or can only be regenerated with great effort. In the case of adsorption of MMA, sufficient stabilization must also be ensured to prevent uncontrolled polymerisation (Methacrylate Esters Safe Handling Manual, Methacrylate Producers Association and Methacrylates Sector Group of the European Chemical Industry Council, 2008). Activated carbon beds should therefore only be used for MMA depleted exhaust gases. In addition, to prevent flashback into the storage tank concerned, activated carbon beds must always be isolated from the storage tank by a flame suppression system (i.e., flame arrestor, scrubber, etc.). Furthermore, other adsorbents such as resins, activated alumina, silica gel and molecular sieves are known.

Common methods for absorptive depletion (absorption) of organic compounds in gaseous outlet streams are, for example, in NP Cheremisinoff, Handbook of Air Pollution Prevention and Control, Chapter 7 - Prevention and Control Hardware, Butterworth-Heinemann, 2002, pages 389-497. Absorptive processes are widely used in the treatment, separation and purification of gas streams containing high concentrations of organic substances, e.g. B. in natural gas cleaning. In general, during absorption, organic substances in the gas stream are dissolved in a liquid solvent (receiver phase or absorbent). The contact between the absorbing liquid and the exhaust gas takes place, for example, in countercurrent spray towers, scrubbers or packing or tray columns. The availability of a suitable absorbent often limits the use of absorptive depletion. In addition, a suitable absorbent should have high solubility for the vapor or gas, low vapor pressure, low viscosity and low price.

Commonly used absorbents include water, mineral oils, or other non-volatile oils. Water is used to absorb VOCs with relatively high water solubilities. Amphiphilic block copolymers added to water can increase the solubility of hydrophobic VOCs in water, but remain in the loaded absorbent and make recycling more difficult. Another consideration in employing absorptive processes is the treatment or disposal of the absorbate-laden waste streams.

WO 9627634 A1 describes the recovery of unreacted monomers in olefin polymerization using an inert solvent as absorbent. The waste gas from the polymerization process is passed through a scrubber and the unreacted monomer contained in the waste gas and the gaseous by-products of the polymerization are taken up in the inert solvent. The loaded absorbent is then treated in several distillation steps and partially recycled. A disadvantage of the method described is the costly and energy-intensive recovery of the absorbent.

DE 2023205 A1 describes a process for recovering volatile, unreacted monomers which are formed during the polymerization of monomer mixtures which contain acrylonitrile and unsaturated monomers such as MMA and which do not require any further purification before they are returned to subsequent polymerization reactions. The monomer-loaded vapor mixtures are cooled to around 15°C and dried and then brought into contact with a liquid consisting essentially of acrylonitrile. The remaining vapor mixtures are then brought into contact with an aqueous phase, with the acrylonitrile passing through the aqueous phase is absorbed. A disadvantage of the process described is that the monomer-loaded vapor mixtures must be subjected to cooling and drying, which requires a high level of equipment complexity and high operating costs for generating the cold.

EP 2083020 A1 describes a process for the absorptive recovery of unreacted monomers without prior cooling. Here 15-80% of the ethylene unreacted in the production of polyolefins can be recovered by cooled absorption with liquid long-chain hydrocarbons, e.g. hexane. The hydrocarbons used for absorption are circulated for renewed absorption after subsequent desorption of the ethylene taken up. A disadvantage of the method described is the use of an additional substance as an absorbent, which means the risk of further contamination.

Established procedures for minimizing MMA emissions in process and storage-related waste air and waste gas streams based on physical-chemical methods are described, for example, in Methacrylate Esters Safe Handling Manual, Methacrylate Producers Association and Methacrylates Sector Group of the European Chemical Industry Council, 2008 Accordingly, a reactive scrubbing is initially recommended for the treatment of exhaust air from MMA storage tanks. An activated charcoal cartridge can be used for the final exhaust gas cleaning if it is protected against high organic pollution.

A corresponding technical method is disclosed in CN 212733497 U, for example. The depletion of MMA in a tank farm exhaust air from 1000 - 2500 mg/m ³ to 50 mg/m ³ is achieved in that the exhaust air is directed by means of a blower through a washing tower operated with alkaline lye, in which the MMA is first physically dissolved and is immediately hydrolyzed by the action of the base to form methanol and water-soluble alkali metal methacrylate salts. The depleted exhaust gas is passed through an activated carbon bed. The disadvantage of this process is that the separated MMA cannot be recovered or cannot be recovered economically.

It is also possible to convert organic substances in a gas flow to harmless substances such as CO2 by oxidation. For example, CN 211216181 U describes how an exhaust gas containing butyl acetate, cyclohexanone and MMA is passed through a mixture which, in addition to hydrogen peroxide, has extremely finely divided air bubbles. A depletion of the organic components by > 90% is achieved. The disadvantage here is that MMA is not available for further processing and the provision of the finely distributed bubbles requires a great deal of energy.

CN 111359600 A describes an unpressurized variant for the oxidative degradation of organic vapors in outlet streams using a fixed bed catalyst that contains nanostructured titanium dioxide, a photosensitizer and an active redox system. This process can also be used to remove MMA from correspondingly contaminated exhaust air and exhaust gases, but it cannot be recovered for further use.

The document CN 112 691 513 A describes a process for cleaning exhaust gases containing VOC, which comprises a two-stage absorption. For example, any VOC-containing exhaust gas is cooled here and treated with a water-soluble organic solvent, e.g. methanol, in a first stage and with water in a second stage.

Furthermore, the prior art describes methods in which some of the unit operations described above are combined to achieve the goals of recovering MMA and obtaining an MMA-free gaseous outlet stream. CN 109621676 A describes a process for recovering MMA, in which MMA vapors from the depolymerization of PMMA are first partially condensed out in a heat exchanger operated with cooling brine at -15 to 18°C. The remaining depleted gas stream is then heated, treated in a scrubber operated with aqueous alkali and finally fed to a total oxidation. The process described here requires a lot of energy for the MMA recovery in the cooling step and additional costs for apparatus and auxiliary materials such as alkali or support gas for waste gas incineration.

In summary, none of the processes described allow the almost complete recycling of the recyclables MMA and methanol from correspondingly charged gaseous outlet streams from MMA production processes, and their loss-free conversion into the target product. In principle, with the purely physical processes described, only an insufficient proportion of the valuable materials can be recovered from the gaseous outlet streams with justifiable effort. In the physical-chemical processes described, the separated MMA is often either irreversibly bound or converted into other compounds by chemical reaction, so it cannot be returned to the manufacturing process and is lost as a valuable product. There is thus still a great need for a commercial process for the production of alkyl methacrylates, such as MMA, with almost complete recycling of alkyl methacrylates, such as MMA, contained in gaseous outlet streams, eg waste air and waste gas streams. In particular, alkyl methacrylates such as MMA shall be recovered from the effluent streams of the preparation of the reactive MMA precursor compound, the esterification with methanol and the storage of the pure alkyl methacrylate product.

Task

One object of the present invention is to provide a process for the preparation of alkyl methacrylate, preferably MMA, which overcomes the disadvantages of the prior art described with little investment outlay and a consistently high degree of operational reliability. In particular, the yield of a low-emission process for the production of alkyl methacrylate, preferably MMA, should be sustainably increased. In particular, it is the object of the present invention to reduce the release of the target product MMA during production using a C3 process, in particular using the ACH-sulfo process. In principle, the treatment of gaseous outlet streams and the recovery of alkyl methacrylate, preferably MMA, by the process according to the invention should be possible in all conventional and above-described production processes for alkyl methacrylate, preferably MMA.

Solution

It was surprisingly found that the above-mentioned objects are achieved in that the recovery of alkyl methacrylate, preferably MMA, by the optimized absorptive treatment of gaseous outlet streams (e.g. exhaust gas, exhaust air) and the advantageous recycling of the resulting laden absorbent in the process according to the invention is significantly improved. This increases the overall process yield of the manufacturing process and minimizes the emission of volatile organic compounds (VOCs). In particular, it has been found that a single-stage or multi-stage absorption (scrubbing) with a combination of an alcohol-containing, preferably methanol-containing, absorbent and an aqueous absorbent can provide MMA-loaded absorbent streams which increase the overall yield can be returned to the manufacturing process. These loaded absorbent streams supplement or substitute in an advantageous manner the supply of fresh alcohol, in particular fresh methanol, and fresh water in the esterification step. With the aid of the method according to the invention, the combustion of organically loaded gaseous outlet streams with the formation of CO2 can be avoided. In addition, the supply of non-process substances is avoided and the absorbents used can be used directly as educt streams in the process.

The process is also simple, robust, cost-effective and combines the advantages of reducing VOC emissions and increasing MMA production yields.

The invention and its characteristic features are described in detail below.

Description of the invention

The present invention relates to a process for producing alkyl methacrylate, preferably methyl methacrylate (MMA), comprising the process steps: a. Production of at least one alkyl methacrylate precursor compound comprising the reaction of acetone cyanohydrin and sulfuric acid in a first reaction stage (amidation) to form a first reaction mixture and conversion in a second reaction stage, comprising heating the first reaction mixture, preferably to a temperature in the range from 130 to 200° C., whereby a second reaction mixture containing the alkyl methacrylate precursor compound and sulfuric acid is obtained; and b. Reacting the second reaction mixture with water and alcohol, preferably methanol, in a third reaction stage (esterification), a third reaction mixture containing alkyl methacrylate, preferably methyl methacrylate, being obtained as the crude alkyl methacrylate product; and c. Separation of alkyl methacrylate, preferably MMA, from the third reaction mixture in a work-up section, comprising at least two distillation steps, with low boilers being separated off from the alkyl methacrylate crude product in one distillation step and high boilers being separated off from the alkyl methacrylate crude product in another distillation step, and a pure alkyl methacrylate product, preferably a pure MMA product, being obtained as the top fraction of the last distillation step; and i.e. Storage of the pure alkyl methacrylate product obtained in process step (c) in at least one storage device and optionally intermediate storage of the third reaction mixture obtained in process step b (crude alkyl methacrylate product) in at least one intermediate storage device; and e. Absorption of a gaseous outlet stream GS, with at least one gaseous outlet stream obtained in process step b, c and/or d being treated with at least two liquid absorbents, with at least one alkyl methacrylate-loaded absorbent, preferably at least one MMA-loaded absorbent , is obtained, and wherein the liquid absorbents comprise at least one alcohol-containing, preferably methanol-containing, absorbent and at least one aqueous absorbent.

In particular, the present invention relates to a process for the production of alkyl methacrylate, preferably methyl methacrylate (MMA), comprising the process steps (a) to (e) as described above, wherein the at least one alkyl methacrylate-loaded absorbent, which after the treatment of the gaseous outlet stream GS is obtained in process step (e) is at least partially supplied in liquid form to process step b.

For the purposes of the present invention, the term “ppm” means ppm by weight (e.g. mg/kg) without further details.

The term stream, phase or fraction containing an educt, product and/or by-product is to be understood in the context of the invention as meaning that the compound(s) mentioned is (are) contained in the respective stream, for example is the predominant proportion of the reactant, product and/or by-product in the corresponding stream. In principle, other components can be present in addition to the compounds mentioned. The naming of the components often serves to clarify the respective process step.

For the purposes of the invention, the expression “vapor” or “vapor stream” refers to a gaseous process stream, for example a gaseous overhead stream of a distillation column. For the purposes of the invention, the expression “low boilers” designates chemical compounds which have a lower boiling point than the corresponding alkyl methacrylate, for example MMA. For the purposes of the invention, the expression “high boilers” designates chemical compounds which have a higher boiling point than the corresponding alkyl methacrylate, for example MMA. For the purposes of the invention, the expression “separated off in a distillative step” means that the compound(s) mentioned are/are depleted in the corresponding mixture or in the corresponding stream in a distillative step.

In particular, the present invention relates to a process for the production of alkyl methacrylate, in particular MMA, by the ACH-sulfo process. In principle, it is possible to use the absorptive post-treatment described according to the invention (absorption step (e)) of the gaseous outlet streams in another process for the production of alkyl methacrylate, in particular MMA. For example, the absorption described according to the invention can be used in a C3-based process described above, in which no sulfuric acid is used as starting material. For example, the absorption described according to the invention can be applied to the production of MMA by the Mitsubishi Gas Chemical Process (MGC), e.g. according to EP 0487853 or by the Aveneer process, e.g. according to DE 102008044218, EP 2043994, DE 102005023975, DE 102005023976 ), whereby 2-HIBA, as a reactive MMA precursor compound, is obtained and reacted with methanol.

Furthermore, the absorption described according to the invention can be applied to the production of alkyl methacrylate, in particular MMA, according to a C2-based process described above, for example according to the BASF process (e.g. according to C. He, F. You, Ind. Eng. Chem. Res 2014, 53, 11442-11459), with methacrylic acid (MAS) in particular, as a reactive MMA precursor compound, being converted to MMA by catalytic esterification with methanol. Likewise, the absorption described according to the invention can be applied to the production of alkyl methacrylate, in particular MMA, according to the LiMA process described above (e.g. according to WO 2016/042000 A1 and WO 2014/170223), in particular MAL, as a reactive MMA precursor compound, in the presence of methanol in a direct oxidative esterification over a heterogeneous catalyst.

Furthermore, the absorption described according to the invention can be attributed to the production of alkyl methacrylate, in particular MMA, according to a C4-based process described above Methods are used, in particular methacrylic acid (MAA), as a reactive MMA precursor compound, is converted to MMA by esterification with methanol, for example by means of tandem C4 direct oxidation according to the Sumitomo method, separate C4 direct oxidation according to the Mitsubishi method and direct metha methods according to Asahi.

Alkyl methacrylate-containing, in particular MMA-containing, gaseous outlet streams which are obtained during storage of the alkyl methacrylate product, in particular the MMA product, can preferably be treated in an absorption described according to the invention. Particularly preferably, gaseous outlet streams obtained during storage of the pure alkyl methacrylate product, in particular the pure MMA product, for example before bottling and/or transport of the end product, can be treated in an absorption described according to the invention. Such gaseous outlet streams typically arise during storage due to the provision of an oxygen-containing atmosphere, which is often necessary for the activation of the stabilizer.

Preferably, the gaseous outlet stream GS, which is treated in the absorption in process step (e), comprises at least two gaseous outlet streams selected from

GS1 a gaseous outlet stream GS1 obtained in process step (a) which contains carbon monoxide and sulfur dioxide;

GS2 a gaseous outlet stream GS2 obtained in process step (b) which contains alkyl methacrylate, preferably MMA, particularly preferably 1.0 to 5.0% by volume, alkyl methacrylate, preferably MMA; and contains at most 10% by volume, preferably from 0.1 to 10% by volume, of oxygen, in each case based on the total volume of the outlet stream GS2;

GS3 a gaseous outlet stream GS3 obtained in process step (c) which contains alkyl methacrylate, preferably MMA, particularly preferably 1.0 to 5.0% by volume, alkyl methacrylate, preferably MMA; and contains at most 10% by volume, preferably from 0.1 to 10% by volume, of oxygen, based in each case on the total volume of the outlet stream GS3; and

GS4 a gaseous outlet stream GS4 obtained in process step (d), which alkyl methacrylate, preferably MMA, particularly preferably 0.1 to 5.0 % by volume, alkyl methacrylate, preferably MMA, and at least 10% by volume, preferably from 10 to 20% by volume, oxygen, based in each case on the total volume of the outlet stream GS4.

In a further preferred embodiment, the gaseous outlet stream GS, which is treated in the absorption in process step (e), comprises at least two gaseous outlet streams selected from the gaseous outlet streams GS2, GS3 and GS4; the gaseous outlet stream GS particularly preferably comprises all outlet streams GS2, GS3 and GS4. The gaseous outlet streams GS2, GS3 and GS4 can in particular be conducted partially or completely, individually or in combined form as a gaseous outlet stream GS (e.g. via (5)) into the absorption in process step (e).

Advantageously, one or more stabilizers are added to various streams of process steps (b), (c) and/or (d) in order to prevent or reduce polymerization of the alkyl methacrylate, preferably of the methyl methacrylate. For example, a stabilizer can be added to the third reaction mixture obtained after esterification. For example, one or more stabilizers can be added during storage and/or the optional intermediate storage according to process step (d).

In particular, stabilizers are used which generally develop their effect only in the presence of gaseous oxygen. After the oxygen has reacted, an oxygen-depleted air remains, which typically occurs as waste gas in process steps (b), (c) and/or (d), and which preferably occurs as gaseous outlet stream GS2, GS3 and/or GS4 for the purpose Recovery of the alkyl methacrylate can be performed in process step (e) (e.g. via (5)). An oxygen-containing gas mixture, preferably air, preferably flows through at least parts of the apparatus used for the work-up according to process step (c) and/or the storage devices according to process step (d) in order to provide the oxygen.

The amount of the oxygen-containing gas mixture is preferably small compared to the other streams of the process. Typical amounts of the oxygen-containing gas mixture are often 50 to 1500 Nm ³ /h, preferably 100 to 500 Nm ³ /h.

In the esterification in process step (b) and/or the work-up in process step (c) and/or the storage in process step (d), preference is given to phenolic ones Compounds, phenylenediamine-based compounds, quinones and / or catechols used. In addition, amine-N-oxides such as TEMPOL or combinations of the stabilizers mentioned can be used. Typical stabilizers include, but are not limited to, hydroquinone monomethyl ether (MEHQ, CAS 150-76-5), hydroquinone (HQ, CAS 123-31-9), 2,4-dimethyl-6-tert-butylphenol (BDMP, DMTBP or Topanol-A® / AO30® / IONOL K78®, CAS 1879-09-0), 2,6,-di-tert-butyl-4-methylphenol (BHT/Topanol-O®, CAS 128-37-0 ), Compounds or mixtures comprising phenylenediamine compounds, for example N-(1,4-dimethylpentyl)-N'-phenyl-p-phenylenediamine (CAS 3081-01-4), N,N'-diisopropyl-p-phenylenediamine (CAS 4251-01-8), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (CAS 793-24-8), such as Naugard® 1-4701 or Santoflex™ 434PD.

It is also possible to add one or more stabilizers to the first and/or second reaction mixture obtained in step (a), in particular during or after the first reaction stage (amidation) and/or the second reaction stage (conversion). A stabilizer can preferably be added to the cooled second reaction mixture. Phenothiazine and other adequately acting stabilizers are preferably used in the amidation and/or conversion in process step (a).

In a preferred embodiment, the pure alkyl methacrylate product obtained in process step (c) and/or the third reaction mixture obtained as crude alkyl methacrylate product in process step (b) contains at least one stabilizer; which is typically activated by molecular oxygen; and the storage device and optionally the intermediate storage device in process step (d) are flowed through with an oxygen-containing gas mixture, at least one gaseous outlet stream GS4 being obtained in process step (d) which contains alkyl methacrylate, preferably methyl methacrylate, and at least 10% by volume , preferably 10 to 20% by volume, particularly preferably 10 to 15% by volume, oxygen, based on the total outlet stream GS4, this gaseous outlet stream GS4 being fed into the absorption in process step (e).

Amidation and conversion in step (a)

Process step (a) of the process according to the invention comprises the amidation in the first reaction stage and the conversion in the second reaction stage. Typically, in process step (a), a gaseous outlet stream GS1 is obtained, which in particular comprises the exhaust gases from the first and/or second reaction stage of process step (a). The gaseous outlet stream GS1 preferably contains predominantly carbon monoxide and sulfur dioxide, and the gaseous outlet stream GS1 particularly preferably contains 70 to 99% by volume of carbon monoxide and 1 to 20% by volume of sulfur dioxide, based in each case on the total volume of the outlet stream GS1.

This gaseous outlet stream GS1 can be at least partially treated in the absorption according to process step (e). In particular, the gaseous outlet stream GS1 can be treated together with at least one further gaseous outlet stream selected from GS2, GS3 and GS4 in the absorption according to method step (e). In one embodiment, all gaseous outlet streams GS1, GS2, GS3 and GS4 are at least partially, preferably completely, treated in the absorption according to method step (e).

The gaseous outlet stream GS1 is preferably partially or completely discharged from the process (e.g. via (1b) and (12)). In process step (a), a gaseous outlet stream GS1 is particularly preferably obtained which contains carbon monoxide and sulfur dioxide, this gaseous outlet stream GS1 being separated from the gaseous outlet streams which are present in process steps (b), (c) and /or (d) are obtained, is discharged from the process. Typically, this discharged gaseous outlet stream GS1 can be fed to an incinerator, for example in a connected plant for regenerating the sulfuric acid. Optionally, the discharged gaseous outlet flow GS1 can be subjected to an exhaust gas scrubbing and/or partial condensation. In a preferred embodiment, part or all of the gaseous outlet stream GS1 from process step (a) is fed into process step (b) as stripping medium (e.g. via (1b)).

First reaction step in step (a) (amidation)

In process step (a), the process according to the invention comprises the reaction of acetone cyanohydrin (ACH) and sulfuric acid, typically in one or more reactors, in a first reaction stage (amidation), preferably at a temperature in the range from 70 to 130° C., particularly preferably 70 to 120°C, obtaining a first reaction mixture typically containing sulfoxyisobutyric acid amide (SIBA) and methacrylic acid amide (MASA). The sulfuric acid used in the first reaction stage preferably has a concentration in the range from 98.0% by weight to 100.5% by weight, preferably 98.0% by weight to 100.0% by weight, preferably from 99 .0% to 99.9% by weight. In particular, the stated concentration of the sulfuric acid used relates to the total mass of the sulfuric acid feed stream to the first reaction stage.

The ACH used can be produced by means of known technical processes (see, for example, Ullmann's Encyclopedia of Industrial Chemistry, 4th edition, volume 7). Typically, hydrocyanic acid and acetone are converted to ACH in an exothermic reaction in the presence of a basic catalyst, e.g., an amine. Such a process stage is described in DE 10 2006 058 250 and DE 10 2006 059 511, for example.

Typically, the amidation of ACH and sulfuric acid produces a-hydroxyisobutyric acid amide (HIBA) or its hydrogen sulfate (HIBA H2SO4), sulfuric acid ester of HIBA (a-sulfoxyisobutyric acid amide, SIBA) or its hydrogen sulfate (SIBA H2SO4) and methacrylic acid amide hydrogen sulfate (MASA) as the main products H2SO4) as a solution in excess sulfuric acid. It is known to those skilled in the art that the proportions of the components mentioned in the reaction mixture are variable and depend on the reaction conditions.

The first reaction stage is preferably operated with an excess of sulfuric acid. The excess sulfuric acid can serve in particular to keep the viscosity of the reaction mixture low, which means that heat of reaction can be removed more quickly and the temperature of the reaction mixture can be lowered.

Typically, the ACH feed stream contains from 98.0 to 99.8% by weight, preferably from 98.3 to 99.3% by weight, of acetone cyanohydrin; 0.1 to 1.5% by weight, preferably 0.2 to 1% by weight, acetone, and 0.1 to 1.5% by weight, preferably 0.3 to 1% by weight, water , related to the ACH material flow.

Preference is given to sulfuric acid and ACH in the first reaction stage in process step (a) in a molar ratio of sulfuric acid to ACH in the range from 1.2 to 2; preferably 1.25 to 1.6; particularly preferably from 1.4 to 1.45.

In principle, anyone skilled in the art for carrying out hydrolysis reactions can carry out the amidation (hydrolysis) in the first reaction stage Reactor are used, for example stirred tank reactors and loop reactors or combinations of said reactors. Alternatively, several reactors connected in parallel, if necessary, but preferably in series, can be used. In one possible embodiment, 1 to 5 reactors are connected in series; a sequential arrangement of 2-3 reactors is preferably used.

The reaction of acetone cyanohydrin with sulfuric acid in the first reaction stage is exothermic. It is therefore advantageous to dissipate most or at least some of the heat of reaction produced, for example with the aid of suitable heat exchangers, in order to obtain an improved yield. However, excessive cooling is generally to be avoided in order not to increase the viscosity of the reaction mixture too much and to prevent the crystallization of ingredients and thus disruptive deposits, e.g. on heat exchangers. The cooling medium, in particular the cooling water, typically has a temperature in the range from 20 to 90.degree. C., preferably from 50 to 90.degree. C. and particularly preferably from 60 to 70.degree. As a rule, temperature differences in the product inlet/outlet of the apparatus of about 1 to 20° C., in particular 2 to 7° C., are preferred.

The reaction of ACH and sulfuric acid in one or more reactors in a first reaction stage (amidation) takes place at a temperature in the range from 70 to 130.degree. C., preferably from 70 to 120.degree. C., particularly preferably from 85 to 110.degree.

Typically, the first reaction stage (amidation) can be carried out batchwise and/or continuously. The first reaction stage is preferably carried out continuously, for example in one or more loop reactors. Suitable reactors and processes are described, for example, in WO 2013/143812. The first reaction stage can advantageously be carried out in a cascade of two or more, preferably two, loop reactors. Alternatively, stirred or pumped-through continuous stirred tank (CSTR) reactors can be used, or a combination of reactors.

The residence time in the amidation step is designed to provide sufficient time to maximize the yield of HIBA, SIBA, MAS and MASA. Typically, the static residence time in the reactors, especially in the loop reactors, is in the range from 5 to 35 minutes, preferably from 8 to 20 minutes.

The reactors, for example the loop reactors, for the first reaction stage (amidation) preferably each comprise at least one gas separator (gas outlet). Typically gaseous by-products are separated and discharged here. During the amidation, the main gaseous by-product typically formed is carbon monoxide, which is preferably discharged from the process in the form of or as part of a waste gas stream GS1.

It is also possible to combine the gaseous outlet stream obtained in the first reaction stage with a gaseous outlet stream obtained in the second reaction stage (converting). In a preferred embodiment, the gaseous outlet stream obtained in the first and/or second reaction stage is discharged directly from the process without being fed to the absorption in process step (e).

Second reaction step in step (a) (conversion)

In process step (a), the process according to the invention also comprises converting the first reaction mixture, comprising heating the first reaction mixture, preferably to a temperature in the range from 130 to 200° C., preferably 130 to 170° C., particularly preferably 140 to 170° C , more preferably 140 to 165 °C. Typically, the conversion takes place in one or more reactors, for example heat conversion apparatus or conversion reactors, a second reaction mixture containing predominantly methacrylic acid amide (MASA) and sulfuric acid being obtained.

Typically, during conversion, when the first reaction mixture is heated to a temperature in the range of 130 to 200 °C, the amount of MASA or MASA H2SO4 is increased by dehydration of HIBA or by sulfuric acid elimination from SIBA, with the first reaction mixture being a sulfuric acid solution containing SIBA, HIBA and MASA, each predominantly in the form of the hydrogen sulfates.

In principle, the conversion can be carried out in known reactors which make it possible to achieve the stated temperatures in the stated periods of time. The energy can be supplied in a known manner, for example by means of steam, hot water, suitable heat carriers, electrical energy or electromagnetic radiation, such as microwave radiation. The conversion in the second reaction stage is preferably carried out in one or more heat conversion apparatus. Any heating medium known to those skilled in the art, for example a thermally loadable oil, a salt bath, electromagnetic radiation, electrical heating, superheated water or steam can be used as the heating medium. Saturated steam is preferably used as the heating medium.

The residence time of the reaction mixture in the second reaction stage (conversion) is often in the range from 2 to 15 min, preferably from 2 to 10 min. The second reaction stage preferably comprises heating the first reaction mixture in so-called preheater segments of the conversion reactor and subsequent residence the reaction mixture in so-called residence time segments, which in particular are operated adiabatically. In the conversion process step, it is advantageous to heat the reaction mixture as quickly as possible and within a first defined time to the required temperature (preheating, preheater segment) and then leave it at this temperature for a second defined time (dwell, dwell time segment) .

Like the preheater segment, the dwell time segment can consist of one or more tubes, which, unlike the preheater segment, are usually unheated. Preferred configurations of the second reaction stage (converting) and suitable devices, comprising preheater segments and dwell time segments, are described in the international patent application PCT/EP2021/077640.

The heat conversion apparatus used for the conversion can also preferably be combined with one or more gas separators. For example, it is possible to pass the reaction mixture through a gas separator after leaving the preheater segment and/or after leaving the residence time segment of the heat conversion apparatus. In particular, gaseous by-products can be separated off from the reaction mixture in the form of the gaseous outlet stream GS1.

In a preferred embodiment, a gaseous outlet stream GS1 containing carbon monoxide and sulfur dioxide is obtained in process step (a). This gaseous outlet stream GS1 is formed in particular by one or more gaseous outlet streams from the first and/or second reaction stage and/or a subsequent process step, for example cooling and/or intermediate storage. In a preferred embodiment, the second reaction mixture obtained in the second reaction stage, containing predominantly methacrylic acid amide and sulfuric acid, is cooled after the conversion to a temperature below 120° C., preferably to a temperature in the range from 90 to 120° C., for example in a Cooler with a cooling medium with a temperature in the range from 60 to 100°C. During this cooling of the second reaction mixture, gaseous by-products are preferably at least partially separated off from the second reaction mixture in the form of the gas stream GS 1 containing carbon monoxide and sulfur dioxide.

In one embodiment, the second reaction mixture, in particular the cooled second reaction mixture, is stored in a storage device, for example a storage tank (buffer container), before it is fed as a stream to the reaction in process step (b) (esterification). A uniform inflow to the subsequent process step (b) (esterification) can thus advantageously be ensured. On the other hand, the intermediate storage allows in particular a further degassing of the cooled second reaction mixture and thus acts as a gas separator. In principle, known and suitable cooling media can be used to cool the second reaction mixture. The use of cooling water is advantageous. Typically, the cooling medium has a temperature in the range from 30 to 120°C.

A gaseous discharge stream GS1, which is obtained in the cooling step (gas separator after conversion) and/or intermediate storage after conversion, is preferably completely or partially discharged from the process (e.g. as stream 1c).

Furthermore, the gaseous discharge stream GS1 can be carried out completely or partially in process step (b) (esterification) (e.g. as stream 1b). Typically, all of the degassed second reaction mixture is fed into process step (b) (esterification).

Third reaction stage in step (b) (esterification)

In process step (b), the process according to the invention comprises reacting the second reaction mixture, typically containing predominantly MASA, with water and alcohol in a third reaction stage (esterification), a third reaction mixture containing alkyl methacrylate, preferably methyl methacrylate, being obtained as the crude alkyl methacrylate product ; and

In a preferred embodiment, the second reaction mixture, containing predominantly MASA, is reacted (esterified) with water and methanol, with a third reaction mixture containing M ethyl methacrylate is obtained. The conditions for the esterification on an industrial scale are known to those skilled in the art and are described, for example, in US Pat. No. 5,393,918.

A gaseous outlet stream GS2 is preferably obtained in process step (b), the outlet stream GS2 preferably containing from 1.0 to 5.0% by volume, preferably from 3.0 to 5.0% by volume, of alkyl methacrylate Methyl methacrylate, and at most 10% by volume, preferably from 0.1 to 10% by volume, oxygen, in each case based on the total volume of the outlet stream GS2.

In a preferred embodiment, this gaseous outlet stream GS2 from process step (b) is at least partially treated in the absorption according to process step (e). The gaseous outlet stream GS2 is particularly preferably treated together with at least one further gaseous outlet stream selected from GS3 and GS4 in the absorption according to process step (e). The gaseous outlet stream GS2 can be partially discharged from the process (e.g. via (2b) and (12)).

The reaction in the third reaction stage in process step (b) (esterification) is preferably carried out in one or more suitable reactors, for example in heated vessels. In particular, steam-heated boilers can be used. In a preferred embodiment, the esterification takes place in two or more, for example three or four, successive tanks (tank cascade).

Typically, the esterification is carried out at temperatures in the range from 100 to 180°C, preferably from 100 to 150°C, at pressures of up to 7 bara, preferably at pressures up to 2 bara, and using sulfuric acid as catalyst.

The second reaction mixture is preferably reacted with an excess of alcohol, preferably methanol, and water. The addition of the second reaction mixture, containing predominantly methacrylamide, and the addition of alcohol are preferably carried out in such a way that a molar ratio of methacrylamide to alcohol in the range from 1:1.0 to 1:1.6 results.

The alcohol fed into the third reaction stage (esterification) is preferably composed of alcohol freshly fed into the process (fresh alcohol) and/or alcohol contained in recirculated streams (recycled streams) of the process according to the invention. Preference is given to at least part of the in the third reaction stage alcohol used, particularly preferably all of the alcohol used in the third reaction stage, in the form of the alkyl methacrylate-loaded absorbent which is obtained in process step (e), provided (eg 11). Typically, the alkyl methacrylate-loaded absorbent is obtained as a liquid phase from a waste gas scrubber.

Typically, the addition of water in the third reaction stage takes place in such a way that the concentration of water is in the range from 10 to 30% by weight, preferably 15 to 25% by weight, based in each case on the entire reaction mixture. In principle, the water fed into the third reaction stage (esterification) can come from any source and contain various organic compounds, provided that no compounds are present which adversely affect the esterification or the subsequent process stages. The water fed into the third reaction stage preferably comes from recirculated streams (recycling streams) of the process according to the invention, for example from the purification of the alkyl methacrylate. It is also possible, if necessary, to feed fresh water, in particular deionized water or well water, into the third reaction stage (esterification).

Particularly preferably, at least part of the water used in the third reaction stage, particularly preferably the majority of the water used in the third reaction stage, is provided in the form of the alkyl methacrylate-loaded absorbent which is obtained in process step (e) (e.g. 11). Typically, the alkyl methacrylate-loaded absorbent is obtained as an aqueous liquid phase from a waste gas scrubber.

Esterification with methanol typically yields a third reaction mixture containing alkyl methacrylate, preferably MMA, alkyl hydroxyisobutyrate, especially methyl hydroxyisobutyrate (HIBSM) and other by-products, as well as significant amounts of water and unreacted alcohol, e.g., methanol.

In the esterification in process step (b), the vaporizable part of the third reaction mixture is preferably removed from the reactors in gaseous form (vapours) and fed to further work-up, for example a distillation step. If preference is given to using a cascade of a plurality of reactors, for example a plurality of tanks, the vaporizable portion of the reaction mixture formed can be removed as a vapor stream in each tank and passed to further work-up. Preference is only in the The vaporizable part of the reaction mixture produced in the last two boilers is removed as a vapor stream and fed into further work-up.

In a preferred embodiment, this vaporizable part from process step (b) is at least partially treated as gaseous outlet stream GS2, which contains alkyl methacrylate, preferably methyl methacrylate, and at most 10% by volume oxygen, in the absorption according to process step (e).

Workup in step (c)

In process step (c), the process according to the invention comprises the removal of alkyl methacrylate, preferably MMA, from the third reaction mixture (obtained after esterification) in a work-up section, comprising at least two distillation steps, with low boilers being separated off from the alkyl methacrylate crude product in a distillation step and in another distillation step, high boilers are removed from the alkyl methacrylate crude product, and a pure alkyl methacrylate product, preferably a pure MMA product, is obtained as the top fraction of the last distillation step;

A gaseous outlet stream GS3 is typically obtained in process step (c), the outlet stream GS3 preferably containing 1.0 to 5.0% by volume, preferably 3.0 to 5.0% by volume, of alkyl methacrylate MMA, and at most 10% by volume, preferably from 0.1 to 10% by volume, oxygen, based on the total volume of the outlet stream GS3.

In a preferred embodiment, this gaseous outlet stream GS3 from process step (c) is at least partially treated in the absorption according to process step (e). The gaseous outlet stream GS3 is particularly preferably treated together with at least one further gaseous outlet stream selected from GS2 and GS4 in the absorption according to process step (e). The gaseous outlet stream GS3 can be partially discharged from the process (e.g. via (3b) and (12)).

In the work-up part according to process step (c), in particular the separation steps known to those skilled in the art can be used, in particular rectification, extraction, stripping and/or phase separation steps. The removal of methyl methacrylate from the third reaction mixture preferably comprises at least two distillation steps, at least one phase separation step and at least one extraction step. Preferred versions of the work-up part are described, for example, in the international application PCT/EP2021/077488.

The work-up preferably comprises the pre-purification of the third reaction mixture which is obtained in the esterification. In particular, the pre-cleaning comprises at least one distillation step, at least one phase separation step and at least one extraction step. The pre-purification preferably comprises at least one, preferably at least two, distillation steps. In particular, an alkyl methacrylate product, preferably an MMA product, which has a purity in the range of at least 85% by weight is obtained in the pre-purification.

The work-up preferably comprises the fine purification of the alkyl methacrylate product, which has a purity in the range of at least 85% by weight, the fine purification comprising at least one, preferably at least two, distillation steps. In particular, fine purification gives an alkyl methacrylate product, preferably an MMA product, which has a purity of at least 99.0% by weight.

The alkyl methacrylate pure product, which is obtained as the top fraction of the last distillation step in process step (c), and which is typically removed from the process after storage in process step (d) as a product stream (e.g. (14)), can, in particular, be of the desired specification can be obtained. In a preferred embodiment, the pure alkyl methacrylate product obtained in process step (c) and/or (d) contains at least 99.9% by weight, preferably at least 99.95% by weight, based on the total alkyl methacrylate Pure product, alkyl methacrylate, especially MMA. Typically 10 to 300 ppm methacrylonitrile (MAN) and/or less than or equal to 10 ppm acetone, based in each case on the total alkyl methacrylate pure product, can be present as by-products.

One or more stabilizers are advantageously added to various streams of process step (c) in order to prevent or reduce polymerization of the methyl methacrylate. At least one stabilizer which is activated by molecular oxygen is preferably used. Small amounts of an oxygen-containing gas mixture, preferably air, are preferably passed through at least parts of the apparatus used for the work-up in process step (c) in order to provide the oxygen for activating the stabilizer. After the oxygen has reacted, there remains an oxygen-depleted gas mixture which emerges as a waste gas from process step (c) and which typically contains alkyl methacrylate product, in particular MMA. In a preferred embodiment, this offgas from process step (c) is at least partially treated as gaseous outlet stream GS3, which contains alkyl methacrylate, preferably methyl methacrylate, and at most 10% by volume oxygen in the absorption according to process step (e). The gaseous outlet stream GS3 is particularly preferably treated together with at least one further gaseous outlet stream selected from GS2 and GS4 as gaseous outlet stream GS in the absorption according to method step (e).

Typically, process step (b) (esterification) produces a liquid waste stream consisting essentially of dilute sulfuric acid. This waste stream is typically discharged from the process. This waste stream is preferably fed, in particular together with one or more aqueous waste streams from the process according to the invention, to a process for regenerating sulfuric acid or to a process for obtaining ammonium sulfate.

Storage and optional intermediate storage in step (d)

In process step (d), the process according to the invention comprises the storage of the pure alkyl methacrylate product obtained in process step (c) in at least one storage device and optionally the intermediate storage of the crude alkyl methacrylate product obtained in process step b (third reaction mixture) in at least one intermediate storage device.

The storage or the storage device and optionally the intermediate storage or the intermediate storage device preferably comprise a container (e.g. a storage tank d) for receiving the product (e.g. the alkyl methacrylate pure product and/or the alkyl methacrylate crude product), at least one inlet for the product, at least one outlet for the product and at least one inlet for an oxygen-containing gas mixture and at least one outlet for an oxygen-depleted gas mixture, the stored product containing at least one stabilizer which is activated by molecular oxygen.

Typically, the amount of the oxygen-containing gas mixture used in storage and optionally intermediate storage is in the range from 50 to 1500 Nm ³ /h, preferably 100 to 500 Nm 3 /h. As a rule, the oxygen-depleted gas mixture contains alkyl methacrylate, in particular MMA, and is treated as gaseous outlet stream GS4 in the absorption according to process step (e).

The pure alkyl methacrylate product stored according to process step (d) is preferably discharged via the at least one outlet into a transport device. For example, the pure alkyl methacrylate product can be filled into a transport container and/or fed to downstream processing via one or more pipelines.

Typically, during the storage of the pure alkyl methacrylate product, preferably the pure MMA product, in process step (d) an alkyl methacrylate-containing offgas, preferably an MMA-containing offgas, is obtained. In a preferred embodiment, this exhaust gas from process step (d) is at least partially treated as gaseous outlet stream GS4 in the absorption according to process step (e). The gaseous outlet stream GS4 is particularly preferably treated together with at least one further gaseous outlet stream selected from GS2 and GS3 as gaseous outlet stream GS (e.g. (5)) in the absorption according to process step (e). The gaseous outlet stream GS4 can be partially discharged from the process (e.g. via (4b) and (12)).

The gaseous outlet stream GS4 preferably contains alkyl methacrylate, preferably methyl methacrylate, and at least 10% by volume of oxygen, based on the total volume of the outlet stream GS4. A gaseous outlet stream GS4 is particularly preferably obtained in process step (d) which contains 0.1 to 5.0% by volume, preferably 0.2 to 2.0% by volume, alkyl methacrylate, preferably MMA, and at least 10 % by volume, preferably from 10 to 20% by volume, particularly preferably from 10 to 15% by volume, of oxygen, in each case based on the total volume of the outlet stream GS4.

In particular, the volume fraction of oxygen; alkyl methacrylate, preferably methyl methacrylate; and optionally methanol in the gaseous outlet stream GS4 below the lower explosive limit (LEL), the LEL being determined by ignition tests in the autoclave as described in DIN EN 1839:2017-04.

The pure alkyl methacrylate product stored in process step (d) typically contains at least one stabilizer, in particular at least one stabilizer which is activated by molecular oxygen. If necessary, such a stabilizer can be added again in process step (d). The storage device and optionally the intermediate storage device in process step (d) are preferably provided with a dry, oxygen-containing atmosphere which covers the surface of the stored alkyl methacrylate in order to provide the oxygen for activating the stabilizer. For example, an oxygen-containing gas mixture, for example air, can flow through the storage device and optionally the intermediate storage device. After the oxygen has reacted, an oxygen-depleted gas mixture remains, which exits as waste gas from process step (d) and which typically contains alkyl methacrylate, in particular MMA.

The storage and the optional intermediate storage according to process step (d) within the meaning of the invention can take place in any storage devices known to those skilled in the art, such as storage tanks. The capacity of the storage device and the optional intermediate storage device can usually be freely selected. It has proven useful to adapt the capacity to the quantity and frequency of the expected loading and unloading. A minimum capacity of 1.5 times the volume of an expected load is preferred for uninterrupted operation.

The structural design of the storage device and the optional intermediate storage device can be, for example, an above-ground tank comprising a vertical shell, a flat bottom and a conical top. In particular, the chosen design should enable the contents to be mixed evenly during loading operations. The storage device and the optional intermediate storage device can be set up on a concrete base with a concrete dike with sufficient capacity to avoid damage and the risk of product spills. For safety reasons, the storage device and/or intermediate storage device and any conveyor devices connected to it should always be set up in a diked and/or walled-in area. A tank shaft with a floor drain line through the tank base is preferred for complete emptying of the storage device and/or intermediate storage device. The storage device and/or intermediate storage device are preferably constructed predominantly from steel or stainless steel. Polyethylene, polypropylene or fluoropolymers are also suitable and can be used in particular for seals and fittings.

The storage device and/or intermediate storage device preferably have suitable

Safety devices, such as a heat-resistant paint and / or thermal insulation to minimize the absorption of thermal energy from the environment; electrical grounding to avoid energy input from sparks; temperature monitoring devices (eg thermocouples) to detect possible polymerization events; Measuring devices for determining the fill level to avoid overfilling the storage facility; a high level switch to close the tank supply line. A differential pressure transmitter made of steel or stainless steel flushed with a dry, oxygen-containing gas is suitable as a level measuring device, for example.

The storage or optional intermediate storage according to process step (d) should in principle take place at a pressure which is higher than the vapor pressure of the corresponding alkyl methacrylate, for example MMA, at the storage temperature. The storage device and/or intermediate storage device can typically have a device for pressure relief, in particular to prevent damage as a result of a polymerization reaction occurring in an uncontrolled manner. Typically, such pressure relief devices may include predetermined breaking points in the form of seams and/or roof constructions. Alternatively or in addition, the storage device and/or intermediate storage device can be equipped with weighted manhole covers or bursting discs with a low response pressure. Often the storage device and/or intermediate storage device may include a pressure equalization valve and/or overflow device, for example weighted pallet or breathing valves with flexible diaphragms, sealing pots with glycol.

The storage and optional intermediate storage in process step (d) preferably takes place at temperatures which are suitable for minimizing the evaporation of alkyl methacrylate, for example MMA, but preferably do not necessitate separate cooling. The storage and optional intermediate storage in process step (d) preferably takes place at ambient temperature.

In particular, the storage or optionally intermediate storage according to process step (d) comprises at least one venting device, in particular to prevent corrosion and polymerization.

A desiccant can be used to ensure a low water vapor concentration in the gas space of the storage device and/or intermediate storage device. Suitable desiccants include, for example, alumina, molecular sieves, and calcium chloride. The desiccant can, for example, on the vent line, the Filling gas line for introducing the oxygen-containing gas mixture or the inlet line of the stabilizer can be installed. The desiccant is preferably refreshed at appropriate time intervals, typically every 3 to 6 months, to ensure effectiveness and prevent polymer blocking.

Absorption of the gaseous outlet stream GS in step (e)

In process step (e), the process according to the invention comprises the absorption of a gaseous outlet stream GS, with at least one gaseous outlet stream obtained in process step b, c and/or d being treated with at least two liquid absorbents, with at least one alkyl methacrylate loaded absorbent is obtained, and wherein the liquid absorbent comprises at least one alcohol-containing, preferably methanol-containing, absorbent and at least one aqueous absorbent.

In a preferred embodiment, in process step (e) an alkyl methacrylate-depleted gaseous outlet stream is obtained after treatment with the at least two liquid absorbents. Typically, this alkyl methacrylate-depleted gaseous outlet stream is discharged from the process, optionally after thermal treatment (e.g. stream (9)). This discharged exhaust gas stream preferably contains less than 5 g of alkyl methacrylate, in particular MMA, per cubic meter of exhaust gas.

According to the invention, the at least one alkyl methacrylate-loaded absorbent, which is obtained after the treatment of the gaseous outlet stream GS in process step (e), is in liquid form at least partially, preferably completely, in process step (b) (esterification) and optionally partially in supplied to process step c. The alkyl methacrylate-loaded absorption medium is particularly preferably a mixture of the alcohol-containing absorption medium and the aqueous absorption medium, each in alkyl methacrylate-loaded form.

The liberated in the process steps described above Alkylmethacrylat- containing, eg MMA-containing, gaseous outlet streams are in process step (e) with a liquid phase as an absorbent in contact, which preferably has a high capacity for absorbing alkyl methacrylate vapors, eg MMA vapors (pickup phase). As a rule, the alkyl methacrylate, for example MMA, is dissolved in the liquid phase by mass transfer and at the same time is depleted in the gas phase. If the gas phase contains only alkyl methacrylate, such as MMA, the gas phase can through the absorption process disappear completely. If, as in the present process, the gas phase contains, in addition to alkyl methacrylate, eg MMA, other gaseous components which are less soluble in the absorbent than alkyl methacrylate, eg MMA, these remain in the gas phase and leave absorption step (e) as alkyl methacrylate -depleted, eg MMA-depleted, gaseous outlet stream. In principle, absorption can be brought about or promoted by physical interactions (physical absorption) between the absorbent and the substance/substances to be absorbed as well as by chemical reaction (chemical absorption). In process step (e) for separating off alkyl methacrylate, for example MMA, with the aid of alcohol and water, for example methanol and water, the intramolecular interactions described are utilized in favor of the absorption process.

In principle, preference is given to absorbents which have a high solubility for the respective alkyl methacrylate, e.g. MMA, and which have no miscibility gap with it. Absorbents are particularly preferably selected which can be introduced unchanged into the preceding process steps and contribute to the formation of the alkyl methacrylate target product, e.g. MMA, therein.

According to the invention, at least one alcohol-containing, preferably methanol-containing, absorbent is used in process step (e). This alcohol-containing absorbent particularly preferably contains at least 50% by weight, preferably at least 80% by weight, particularly preferably at least 95% by weight, based on the total alcohol-containing absorbent, alcohol, preferably methanol. In a preferred embodiment, the alcohol-containing absorbent contains at least 99% by weight of methanol.

A corresponding alcohol is preferably used as an absorbent for the preparation of other alkyl methacrylates. The absorbent loaded with alkyl methacrylate and containing alcohol can advantageously be introduced directly into process step (b) of the esterification and reacted there with the reactive MMA precursor compound, preferably MASA, to form alkyl methacrylate, for example MMA.

According to the invention, process step (e) also includes treating the gaseous outlet stream GS with at least one aqueous absorbent.

Typically, this aqueous absorbent contains at least 50% by weight, preferably at least 80% by weight, particularly preferably at least 95% by weight, based on the total aqueous absorbent, water. In particular, deionized water (DI water) or tap water can be used as an aqueous absorbent.

In a preferred embodiment, the absorption in process step (e) comprises: at least one first absorption step (e1), in which the gaseous outlet stream GS is mixed with a first liquid, alcohol-containing absorbent containing at least 50% by weight alcohol, preferably methanol , is treated to obtain an alkyl methacrylate-loaded absorbent and an alcohol-enriched gas stream; and at least one second absorption step (e2) in which the alcohol-enriched gas stream (from (e1)) is treated with a second, aqueous absorbent containing at least 50% by weight water, an alcohol-loaded aqueous absorbent being obtained .

As a rule, the alcohol, in particular the methanol, from the alcohol-enriched gas stream, which is obtained in the first absorption step (e1), is to be converted into the aqueous phase (receiver phase) by the second absorption step (e2) with the second aqueous absorption medium . The alcohol-loaded absorbent can preferably be fed into the esterification in process step (b).

Preferably, the alcohol-loaded aqueous absorbent obtained in the second absorption step (e2) and the alkyl methacrylate-loaded absorbent obtained in the first absorption step (e2) are at least partly in the process step (b) (esterification) and optionally partly supplied to process step (c). In this case, the two loaded absorbents from (e1) and (e2) can be recycled separately or combined. In a preferred embodiment, both loaded absorbents are completely returned to the esterification in process step (b). In particular, the amount of fresh alcohol and fresh water required in the esterification can thus be reduced.

The first absorption step (e1) and the second absorption step (e2) can be carried out in at least two different, separate absorption devices (washers) or in one absorption device. In a preferred embodiment, the absorption in process step (e) comprises the following steps:

(e1) treating the gaseous outlet stream GS, preferably one or more gaseous outlet streams obtained in process step b, c and/or d, in a first absorption device with a liquid, alcohol-containing absorbent containing at least 50% by weight. % alcohol, preferably methanol; whereby an alcohol-enriched gas stream is obtained, and

(e2) treating the alcohol-enriched gas stream obtained in process step (e1) in a second absorption device with an aqueous absorbent containing at least 50% by weight water.

In another preferred embodiment, the absorption in process step (e) is carried out in a single absorption device, with one or more gaseous outlet streams obtained in process step b, c and/or d being fed to the lower part of the absorption device, and the at least two liquid absorbents are added in the upper part of the apparatus, preferably at different points. In this case, the gaseous outlet stream and the at least two absorbents are preferably conducted in countercurrent.

In a further preferred embodiment, the absorption in process step (e) is carried out in a single absorption device, whereby one or more gaseous outlet streams obtained in process step b, c and/or d are fed in the lower part of the absorption device; in a first absorption step (e1) at least one liquid, alcohol-containing absorbent containing at least 50% by weight alcohol, preferably methanol; is fed above the feed of the gaseous outlet streams; in a second absorption step (e2), at least one aqueous absorbent containing at least 50% by weight of water is fed in above the inlet of the alcohol-containing absorbent; and wherein the gaseous outlet stream and the at least two absorbents are passed countercurrently. In particular, the only absorption device here is a separating apparatus which comprises at least two separate packings, in particular packings for absorbing MMA with methanol and packings for absorbing methanol with water.

It is also possible for the at least one alcohol-containing, preferably methanol-containing, absorbent and the at least one aqueous absorbent to be added together as a mixture in the absorption in process step (e), e.g. in the single absorption device.

In the absorption in process step (e), preference is given to the gaseous outlet stream GS with 0.1 to 5.0 kg of liquid absorbents per m ³ gaseous outlet stream GS (based on the sum of the absorbents) and at a temperature of 5 to treated at 40 °C. Preferably, the at least two absorbents used for the absorption in process step (e) have a temperature in the range from 5 to 30° C., and a temperature in the range from 5 to 40° C. occurs in the absorption operation in process step (e). °C on.

Conventional absorption devices known to those skilled in the art can be used in the absorption in process step (e). In particular, gas-liquid contact apparatus (scrubbers) known to the person skilled in the art, which are used, for example, in air pollution control, can be used. These typically come in a variety of types and sizes for removing one or more gaseous components from a gas stream by mass transfer (diffusion) and converting it to a liquid phase. The gas phase and the liquid phase can be brought into contact with one another in various ways in the scrubbers used according to the invention. In a first possible constructive embodiment, the gas is dispersed into the continuous liquid receiver phase (absorbent), e.g. in a column absorber with exchangeable trays, a bubble column, a vessel absorber or a dispersing agitator. In another possible embodiment, the liquid receiver phase (absorbent) can be sprayed into the continuous gas, e.g. in a free-space scrubber without internals such as venturi, injector, spray, annular gap, radial flow scrubber or a scrubber with rotating internals such as rotary -, cross veil, dishwasher.

Gas-liquid contact apparatuses which are customary and known to those skilled in the art are preferably used as the absorption device in the absorption in process step (e). in which a continuous gas phase and a film of the continuous receiver phase (absorbent) are brought into contact, the film being spread out on a stationary or moving support, for example. Examples of this are column absorbers with random packing, fluidized bed absorbers, falling film absorbers or surface absorbers. This absorption device is typically characterized by a small pressure drop of about 2 to 5 mbar/m, allows a variable gas loading and, like tray columns, allows the realization of a large number of theoretical separation stages.

In a preferred embodiment, packed towers conventionally used in gas absorption are used as the absorption device in process step (e). These consist, for example, of cylindrical columns provided with a gas inlet and a distribution space at the bottom, a liquid inlet and distribution space at the top, and liquid and gas outlets at the bottom and at the top, respectively. Typically, the column contains plates, screens, trays, random or structured packing, typically of a chemically inert, solid material.

For example, the absorption in process step (e) can be carried out in an irrigated packed column, the liquid absorbent (receiver phase) running from above over the packing and uniformly wetting it downwards. The gaseous outlet streams normally enter the column from the bottom, are distributed over the tray and flow upwards through the interstices of the packing countercurrent to the liquid absorbent. External cooling of the column can be provided in order to further increase the degree of absorption. The column packing can be any material which is chemically inert to the liquids used and preferably has a low bulk density. Suitable materials are, for example, steel, stainless steel, porcelain, glass, ceramics or PTFE as well as GRP materials lined with PTFE/PFA/PVDF. Beds of regularly shaped units, such as rings, grids, spirals and comminuted solids, can advantageously be used.

In principle, the absorption in process step (e) can be carried out in countercurrent, in cocurrent or in a combination of cocurrent and countercurrent devices. Typically, cocurrent absorption can be realized through the use of jet pumps or nozzles. Here, the exhaust gas to be treated is brought into contact with a finely divided, liquid absorbent in order to create the largest possible surface area for mass transfer. Description of the figures

FIG. 1 describes the highly generalized reaction network for the formation of methyl methacrylate from reactive MMA precursor compounds, which can be generated from either the C2 building block ethylene, the C3 building block acetone or the C4 building block isobutene.

Starting from ethylene, this is first hydroformylated in the BASF process and the resulting propionaldehyde is condensed with formaldehyde formed by oxidation of methanol to give methacrolein. After gas-phase oxidation, methacrylic acid results as a reactive MMA precursor compound that can be directly esterified with methanol to give MMA. In the alpha process, the reactive MMA precursor compound methyl propionate is formed directly from ethylene by methoxycarbonylation, which is condensed with formaldehyde formed by oxidation of methanol to give MMA.

In the Li MA process, the methacrolein (reactive MMA precursor compound) formed analogously to the BASF process by hydroformylation and subsequent condensation with formaldehyde from ethylene is directly oxidatively esterified with methanol to form MMA.

In the C3-based process, starting with acetone and hydrocyanic acid, acetone cyanohydrin (ACH) is first produced with the addition of a basic catalyst (e.g. diethylamine Et2NH or alkali hydroxides). The acetone cyanohydrin is reacted with sulfuric acid in several reaction steps in the ACH sulfo process to form the reactive MMA precursor compound methacrylamide hydrogen sulfate (MASA H2SO4), which is then hydrolyzed to form methacrylic acid (MAS) or esterified with methanol (MeOH) to form methyl methacrylate (MMA). being transformed. Alternatively, the Aveneer process and the Mitsubishi Gas Chemical process lead via the reactive MMA precursor compound 2-HIBA, which is derived from ACH by hydrolysis.

Starting from the C4 building block isobutene, this can first be converted in a gas-phase oxidation to methacrolein, which is oxidized in a further gas-phase reaction to form the reactive MMA precursor compound methacrylic acid, analogously to the third stage of the BASF process. Subsequent esterification with methanol leads in turn to MMA. MMA can also be transesterified with other alcohols to form alkyl methacrylates with the release of methanol. Depending on the process variant selected, it can also be economical to also produce methacrylic acid from MMA by hydrolysis.

FIG. 2 (scheme for comparative examples V1 and V2) shows a flow chart for the preparation of MMA, comprising the process steps a preparation of a reactive MMA precursor compound, b reaction of the reactive MMA precursor compound with methanol to form an MMA crude product, c thermal purification of the MMA Crude product to form a pure MMA product and d storage of the pure MMA product and f treatment of the gaseous outlet streams resulting from the process steps mentioned in a condensation f.

Process step a comprises the production of a mixture of methacrylamide (MASA), methacrylic acid (MAS) and hydroxyisobutyric acid amide (HIBA) as MMA precursor compound by reacting acetone cyanohydrin 16 (ACH) with sulfuric acid 17 by means of amidation and conversion in reactors a. A gas stream GS1 containing carbon monoxide and sulfur dioxide is discharged from process step a. The gas stream GS1 can either be fed to the esterification in process step b via 1b and/or discharged from the process via 1c and 12 and/or fed to the condensation step f via 1a and 5.

The second reaction mixture obtained is then reacted with methanol 6 and water 8 by esterification in reactors b (process step b), a crude MMA product (third reaction mixture) being obtained. An MMA-containing gas stream GS2, which preferably contains at most 10% by volume of oxygen, is discharged from process step b. The gas stream GS2 can either be discharged from the process via 2b and 12 and/or fed to the condensation step f via 2a and 5.

The MMA crude product is thermally processed in the devices c (process step c), with MMA being separated from the MMA crude product, with low boilers being separated from the MMA crude product in a first distillation step and high boilers being separated from the MMA crude product in a second distillation step be, with a pure MMA product 13, is obtained as overhead fraction. An MMA-containing gas stream GS3, which preferably contains at most 10% by volume of oxygen, is discharged from process step c. The gas stream GS3 can either be discharged from the process via 3b and 12 and/or fed to the condensation step f via 3a and 5. The pure MMA product 13 is then fed into a storage tank d (method step d), through which an oxygen-containing gas mixture 15 flows. An M MA-containing gas stream GS4, which preferably contains at least 10% by volume of oxygen and whose content of combustible substances is preferably below the lower explosion limit (LEL), is discharged from the storage tank. The gas stream GS4 can either be discharged from the process via 4b and 12 and/or fed to the condensation step f via 4a and 5. The product stream 14 of the pure alkyl methacrylate product is discharged from the storage tank d.

The gaseous outlet streams 5 resulting from process steps a, b, c and/or d, in particular formed by one or more of the streams GS1/1a, GS2/2a, GS3/3a and/or GS4/4a, are fed into the condensation device f , wherein the condensate obtained in process step f is returned via 11a to process step b and optionally c and is optionally partially discharged via 11b.

FIG. 3 (diagram for example B1 according to the invention) shows a flow chart of a preferred embodiment of the method according to the invention.

Process steps a, b, c and d with the associated material flows correspond to the description of FIG. 2 above.

The gaseous outlet streams resulting from process steps a, b, c and/or d are formed as a gaseous outlet stream 5, in particular by one or more of the streams GS1/1a, GS2/2a, GS3/3a and/or GS4/ 4a into the absorption step e.

In the embodiment according to FIG. 3, the gaseous outlet stream 5 is fed into the first absorption step e1 (methanol scrubbing) and treated there with fresh methanol 6 . This results in a methanol-enriched and MMA-depleted gas stream 7, which is fed into the second absorption step e2 (water scrubbing), and an MMA-loaded absorbent 11, which is fed back via 11a to process step b and optionally partially to process step c and is optionally discharged partially from the process via 11b.

In the second absorption step e2, the MMA-depleted gas stream 7 is treated with fresh water 8, with an MMA and methanol-depleted waste gas 9 being obtained, which is discharged from the process and, for example, is fed to an incinerator. In addition, a methanol-loaded aqueous absorbent 10 is obtained, which is returned to the first absorption step e1; is optionally partially returned to process step b via 10a and 11 and optionally partially discharged from the process via 10b.

Figure 4 (scheme for example B2 according to the invention) shows a flow chart of a further preferred embodiment of the method according to the invention for the production of MMA, in particular the treatment of the gaseous outlet stream 5, in particular formed by GS2, GS3 and GS4, by absorption, the absorptions of MMA with methanol and the absorption of methanol with water can be carried out in an apparatus e. In the absorption column e, fresh methanol 6 is added above the addition of the gaseous outlet stream 5 and fresh water 8 is added above the addition of fresh methanol 6. The gas stream 5 and the liquid absorbents 6 and 8 are preferably carried out in countercurrent. The MMA and methanol-depleted waste gas 9 is discharged from the process. The absorbent 11 loaded with MMA and methanol is returned to process step b.

Reference List

The abbreviations in Figure 1 and the description of Figure 1 have the following

meanings:

ACH acetone cyanohydrin;

FA formaldehyde

2-HIBA alpha-hydroxyisobutyric acid amide; alpha-hydroxyisobutyramide;

HIBA H2SO4 alpha-hydroxyisobutyric acid amide hydrogen sulfate, alpha-hydroxyisobutyramide hydrogen sulfate;

HIBS alpha-hydroxyisobutyric acid;

HIBSM alpha-hydroxyisobutyric acid methyl ester

MAL methacrolein

MAN methacrylonitrile;

MAS methacrylic acid;

MASA methacrylic acid amide / methacrylamide;

MASA- H2SO4 Methacrylic acid amide hydrogen sulfate;

MMA methyl methacrylate;

MP methyl propionate

PA Propionaldehyde 2-SIBA alpha-sulfoxyisobutyric acid amide, also referred to as sulfoxyisobutyric acid amide or sulfoxyisobutyramide;

SIBA H2SO4 alpha-Sulfoxyisobutyric acid amide hydrogen sulfate, also known as

denotes sulfoxyisobutyric acid amide hydrogen sulfate;

SIBN alpha-sulfoxyisobutyronitrile, also referred to as sulfoxyisobutyronitrile;

In FIGS. 2 to 4, the reference symbols have the following meaning:

apparatuses:

(a) Amidation and Conversion

(b) esterification

(c) Thermal processing part

(d) Storage / storage tank

(e1) First absorption step (methanol scrubbing)

(e2) Second absorption step (water washing)

(e) Absorption / combined methanol-water scrubbing

(f) partial condensation

Material flows:

(GS1) Gas stream containing carbon monoxide and sulfur dioxide from process step a

(GS2) MMA-containing gas stream from process step b (preferably with an oxygen concentration of <10% by volume)

(GS3) MMA-containing gas stream from process step c (preferably with an oxygen concentration of <10% by volume)

(GS4) Gas stream containing MMA from storage d (preferably below the lower explosion limit LEL)

(1a,b,c) exhaust gas amidation/conversion (preferably <10Vol% O2)

(2a, b) Exhaust gas esterification (containing MMA; preferably <10% by volume O2)

(3a, b) Waste gas thermal treatment (containing MMA; preferably <10% by volume O2)

(4a, b) Tank farm waste gas (containing MMA; preferably >10% by volume O2)

(5) Gaseous outlet stream GS for aftertreatment

(6) fresh methanol (educt and optional washing liquid)

(7) Methanol-enriched gas stream from e1 (MMA-depleted)

(8) Fresh water (educt and optional washing liquid)

(9) Exhaust gas (MMA and methanol depleted) (10, 10a,b) Methanol-loaded aqueous absorbent

(11 , 11 a,b) MMA-loaded absorbent (containing water, MMA and methanol)

(12) Exhaust gas for thermal aftertreatment

(13) MMA pure product

(14) Product stream of pure MMA product from storage

(15) Feed oxygen-containing gas mixture

(16) ACH feed

(17) Sulfuric Acid Feed

examples

Comparative example V1: Condensation of the combined gaseous outlet streams GS1, GS2, GS3 and GS4 for the recovery of MMA

Comparative example C1 relates to a process for the production of methyl methacrylate (MMA) by the ACH-Sulfo process based on the flow chart according to FIG.

(a) Amidation/Conversion

13847 kg/h of acetone cyanohydrin (ACH) 16 with a composition of 99.0% acetone cyanohydrin, 0.3% acetone, 0.5% water and free sulfuric acid 17 were used in a mass ratio of 1.63/1 [kg H2SO4/kg ACH]. 99.7% sulfuric acid fed to an amidation/conversion in process step a. The amidation was operated at 98° C., the conversion at 158° C. and a pressure of 980 mbara. Furthermore, the exhaust gases GS1 formed by side reactions of the amidation/conversion were separated from the liquid reaction mixture and fed to a condensation f in the form of stream 1a. The composition of the exhaust gas GS1 or 1a is shown in Table 1.

(b) esterification

The second reaction mixture obtained in the course of amidation/conversion a, which essentially consisted of methacrylamide (MASA), methacrylic acid (MAS) and hydroxyisobutyric acid amide (HIBA), dissolved in sulfuric acid, was then fed to esterification b. The liquid second reaction mixture was fed without using recycle stream 11, feeding 4850 kg/h of methanol as stream 6; 2000 kg/h of water as stream 8 and 3220 kg/h of direct steam are esterified at approx. 100-140 °C and a slight overpressure of 50-150 mbar(g).

The MMA crude product formed in the esterification b was then distilled off and, together with the recycle stream 11a from the condensation f, fed to the thermal work-up c for separating off MMA. The dilute waste acid remaining in process step b was discharged from process (b).

To inhibit polymer formation, a solution of a phenylenediamine-based stabilizer was used at various points in process step b, which requires oxygen to function optimally. In process step b, air containing 21% by volume of O 2 was fed in as the oxygen-containing gas mixture 15 at several points for this purpose.

In the esterification b, in particular the distillation of the MMA crude product in b, a gaseous outlet stream GS2 was obtained.

(c) Thermal processing

The separation of MMA from the MMA crude product comprised several thermal separation steps c and at least two distillation stages, with a low boiler fraction being separated off in the first stage and a high boiler fraction being separated from the MMA crude product in the second stage and the pure MMA product 13 as the top fraction of the last distillation step was obtained.

In the thermal processing c, as in process step b, air containing 21% by volume of O 2 was added as a co-agent of the stabilizer in the form of the oxygen-containing gas mixture 15 . A gaseous outlet stream GS3 was obtained. The amount and composition of GS2 and GS3 are summarized in Table 1. The resulting exhaust gas quantities are shown in Table 1.

The purified MMA pure product 13 obtained in several thermal process steps c was then cooled to 8° C. and fed to a storage tank d.

(d) Storage of the MMA pure product

For the optimal function of the stabilizer in the tank farm d, air with a content of 21% by volume O2 was also fed into the tank farm d as an oxygen-containing gas mixture 15 in a controlled manner. There was a gaseous effluent stream GS4 containing MMA obtained according to Table 1. In addition, further M MA-saturated exhaust gas quantities, which continuously arose as a result of filling and emptying processes, were added to the outlet flow GS4. The outlet stream GS4 was also fed to the partial condensation f for the purpose of MMA recovery. The amount and composition of GS4 are summarized in Table 1.

(f) Condensation of the gaseous outlet streams

The gaseous outlet streams GS1, GS2, GS3 and GS4 were combined to form a combined process waste gas 5 and fed to a condensation device f. Condensation f was operated at a process-side temperature level of approx. 12 °C. Cold water with a flow temperature of approx. 1 °C was used as the cooling medium. The gas cooling and partial condensation of MMA from 5 took place in the condensation f in countercurrent in a vertically arranged tube bundle apparatus at a slight vacuum of 990 mbara.

The condensed MMA 11a was fed continuously to thermal processing c, while the depleted exhaust gas 9 was fed to post-combustion.

Comparative example V2: Condensation of the combined gaseous outlet streams GS2, GS3 and GS4 for the recovery of MMA

Comparative example V2 relates to a process for the production of methyl methacrylate (MMA) by the ACH-Sulfo process based on the flow chart according to FIG.

Comparative example V2 was carried out in accordance with comparative example V1 as described above. In contrast to comparative example C1, only the gaseous outlet streams GS2, GS3 and GS4 were combined to form gas stream 5 and fed into condensation f to recover MMA.

The exhaust gases GS1 formed by side reactions in the amidation/conversion to a were separated from the liquid reaction mixture and discharged from the process in the form of stream 1c and fed to incineration. The composition of the exhaust gas GS1 or 1c is shown in Table 1. Table 1: Comparison of comparative examples C1 and C2

Monomer includes MMA and MAS (included as a by-product in small amounts) Example B1 according to the invention: absorption of the combined gaseous outlet streams

GS1, GS2, GS3 and GS4 in scrubbers e1 and e2 for the recovery of MMA

Example B1 according to the invention relates to a process for the production of methyl methacrylate (MMA) using the ACH-Sulfo process based on the flow chart according to FIG. and d (storage of the pure MMA product) took place as described in Comparative Example C1. The results obtained are shown in Table 2.

In contrast to comparative example C1, only the gaseous outlet streams GS2, GS3 and GS4 were combined to form gas stream 5 and led to absorption e for the recovery of MMA. The waste gases formed in the amidation/conversion a by side reactions (gaseous outlet stream GS1) were separated from the liquid reaction mixture and discharged from the process in the form of streams 1c and 12 and fed to incineration.

The amide mixture fed to process step b (second reaction mixture) was recirculated using recycle stream 11a, which represents the MMA-loaded absorbent from absorption e or absorption e1 and e2, with the supply of 3220 kg/h of direct steam at approx. 100 - 140 ° C and slight excess pressure of 50-150 mbar (g).

(e) Absorption of the gaseous outlet streams in e1 and e2

The gaseous outlet streams GS2, GS3 and GS4 were combined to form a combined gaseous outlet stream 5 and fed into a first countercurrent packed column e1 for scrubbing (absorption), which was operated at 20° C. and 10 mbarg.

The first absorption column e1 was operated with feed stream 10 from the second absorption column e2 and with feed stream 6 of 4850 kg/h of methanol. In this first absorption step ei, the MMA contained in the combined outlet stream 5 at least partially passed into the methanol-containing liquid phase, which was carried out as MMA-loaded absorption medium 11 from column e1 and completely recycled to process step b. The methanol-enriched gas stream 7 leaving the absorption column e1, which was almost saturated with methanol and depleted in MMA, was then fed to a second absorption step e2 in a second countercurrent packed column e2, which was operated at 20° C. and 10 mbarg with the feed of 2000 kg / h water was operated as inflow stream 8.

In this second absorption step e2, at least part of the methanol contained in the gas stream 7 passed into the aqueous liquid phase, with a methanol-loaded aqueous absorbent 10 being obtained, which was carried out from the column e2 and completely recycled into process step e1.

In addition, in the second absorption step e2, a methanol-depleted gas stream almost saturated with water was obtained at the top of column e2, which gas stream was discharged from the process as waste gas 9 .

The washing liquids (absorbents) used in the absorption columns e1 and e2, methanol as fresh methanol 6 and water as fresh water 8, were used in the form of MMA-loaded absorption medium 11 (MMA-loaded alcohol and water-containing wash solution) as reactants for the esterification in process step b and partially replaced the supply of fresh methanol or water (see Table 2).

Example B2 according to the invention: absorption of the combined gaseous outlet streams GS1, GS2, GS3 and GS4 in a scrubber e for the recovery of MMA

Example B2 according to the invention relates to a process for the production of methyl methacrylate (MMA) by the ACH-Sulfo process based on the flow chart according to FIGS. 3 and 4.

Example B2 was carried out according to example B1, as described above. In contrast to example B1, the combined gas streams GS2, GS3 and GS4 were treated in a single absorption device e (scrubber) according to FIG. 4, as described below.

(e) absorption of the gaseous outlet streams in e

The gaseous outlet streams GS2, GS3 and GS4 were combined into a combined gaseous outlet stream 5 and subjected to scrubbing (absorption) from below or fed near the bottom into a countercurrent packed column e (according to FIG. 4), which was operated at 20° C. and 10 mbarg. The absorption column e had two packing units arranged one above the other.

The bottom packing unit of e was operated feeding 4850 kg/h of methanol as feed stream 6, with the first absorbent 6 being added at the top of the bottom packing unit. The upper packing unit is operated with the supply of 2000 kg/h of deionized water as the feed stream 8, with the second absorbent 8 being added at the upper part of the upper packing unit.

In this absorption step e, the MMA contained in the combined outlet stream 5 went at least partially into the liquid phase (absorbent), which contains methanol and water, and which is carried out as MMA-loaded absorbent 11 from column e and completely recycled to process step b became.

In addition, in absorption step e at the top of column e, a gas stream depleted in methanol and MMA and almost saturated with water was obtained, which was discharged from the process as waste gas 9 .

The washing liquids (absorbent) used in the absorption columns e1 and e2, methanol 6 and water 8, were fed in the form of the MMA-loaded absorbent 11 as reactants to the esterification in process step b, where they replaced the supply of fresh methanol or water (cf. table 2).

Table 2: Comparison of examples B1 and B2 with comparative example V2

Monomer includes MMA and MAS (included as a by-product in small amounts) Explanations of examples V1, V2, B1 and B2

It can be seen from Comparative Examples V1 and V2 that a considerable amount of MMA is contained in the outlet stream GS4 of the method described, this outlet stream being caused by the air introduced for stabilization. Although this outlet stream GS4 has a comparatively low concentration of MMA, it is larger than the other gaseous outlet streams GS2 and GS3. The high proportion of inert gas in the air fed in leads to a reduction in the partial pressure of the MMA in the outlet stream, which makes it more difficult to recover MMA by cooling and partial condensation of the outlet stream.

In comparative example V1, all the gaseous outlet streams of process steps a, b, c and d are interconnected and a downstream condensation at low temperatures is used, which corresponds to the process used as standard in the prior art. If, as in Comparative Example C2, only the MMA-rich gaseous outlet streams GS2, GS3 and GS4 from process steps b, c and d are used for recovery, the amount of MMA recovered can go from 69 t/h to 198 t/h be increased, which corresponds to an increase in yield of 0.1%. However, this increase is significantly lower than the increase in the amount of MMA recovered in Examples B1 and B2 according to the invention.

From Examples B1 and B2 according to the invention it is evident that significantly larger amounts of MMA can be recovered from the gaseous outlet streams of the C3 process through the use of absorption, operated with methanol and water, than is the case according to the prior art by means of partial condensation is. In the process according to the invention it is possible to achieve a 0.2% higher MMA yield with a simultaneously higher methanol yield without the use of additional educts or the generation of additional waste streams.

As can be seen from Table 2, a partial stream of the reactants used in the esterification, methanol and deionized water, is used as an absorbent in an absorption device e or in two separate absorption devices e1 and e2 and, loaded with MMA, is returned to the esterification as reactants.

The example B2 according to the invention also shows that the two-stage absorption with methanol and water can be carried out in a separation device if the packed column used in this case feed options for the at several points have appropriate absorbents. In this way, the absorptive recovery of MMA is significantly simplified and does not represent a greater outlay in terms of equipment than a partial condensation, with a simultaneous increase in the recycled amount of MMA.

Claims

patent claims . Process for the production of alkyl methacrylate, preferably methyl methacrylate, comprising the process steps: a. Production of at least one alkyl methacrylate precursor compound comprising the reaction of acetone cyanohydrin and sulfuric acid in a first reaction stage (amidation) to form a first reaction mixture and conversion in a second reaction stage comprising heating the first reaction mixture, with a second reaction mixture containing the alkyl methacrylate precursor compound and sulfuric acid is obtained; and b. Reacting the second reaction mixture with water and alcohol, preferably methanol, in a third reaction stage (esterification), a third reaction mixture containing alkyl methacrylate, preferably methyl methacrylate, being obtained as the crude alkyl methacrylate product; and c. Separation of alkyl methacrylate, preferably MMA, from the third reaction mixture in a work-up section, comprising at least two distillation steps, with low boilers being separated off from the alkyl methacrylate crude product in one distillation step and high boilers being separated off from the alkyl methacrylate crude product in another distillation step, and a pure alkyl methacrylate product, preferably a pure MMA product, being obtained as the top fraction of the last distillation step; and d. Storage of the pure alkyl methacrylate product obtained in process step c in at least one storage device and optionally intermediate storage of the third reaction mixture obtained in process step b (crude alkyl methacrylate product) in at least one intermediate storage device; and e. Absorption of a gaseous outlet stream GS, with at least one gaseous outlet stream obtained in process step b, c and/or d being treated with at least two liquid absorbents, with at least one alkyl methacrylate-loaded absorbent being obtained, and with the liquid absorbent at least one alcohol-containing, preferably methanol-containing, absorbent and at least one aqueous

Absorbents comprise, wherein the at least one alkyl methacrylate-loaded absorbent, which is obtained after the treatment of the gaseous outlet stream GS in process step e, is at least partially fed into process step b in liquid form. Process according to Claim 1, characterized in that the pure alkyl methacrylate product obtained in process step c and/or the third reaction mixture obtained as crude alkyl methacrylate product in process step b contains at least one stabilizer; and the storage device and optionally the intermediate storage device are flowed through with an oxygen-containing gas mixture in process step d, at least one gaseous outlet stream GS4 being obtained in process step d, which contains alkyl methacrylate, preferably methyl methacrylate, and at least 10% by volume, based on the Total volume of the outlet stream GS4, containing oxygen, this gaseous outlet stream GS4 being fed into the absorption in process step e. Process according to claim 1 or 2, characterized in that the gaseous outlet stream GS treated in the absorption in process step e comprises at least two gaseous outlet streams selected from

GS1 a gaseous outlet stream GS1 obtained in process step a, which contains carbon monoxide and sulfur dioxide;

GS2 a gaseous outlet stream GS2 obtained in process step b, which contains alkyl methacrylate, preferably methyl methacrylate, and at most 10% by volume, based on the total volume of the outlet stream GS2, oxygen;

GS3 a gaseous outlet stream GS3 obtained in process step c, which contains alkyl methacrylate, preferably methyl methacrylate, and at most 10% by volume, based on the total volume of the outlet stream GS3, oxygen; and

GS4 a gaseous outlet stream GS4 obtained in process step d, which contains alkyl methacrylate, preferably methyl methacrylate, and at least 10% by volume, based on the total volume of the outlet stream GS4, oxygen. Process according to Claim 3, characterized in that the gaseous outlet stream GS which is treated in the absorption in process step e comprises at least two gaseous outlet streams selected from the gaseous outlet streams GS2, GS3 and GS4, particularly preferably the gaseous outlet stream GS all outlet streams GS2, GS3 and GS4. The method according to any one of claims 1 to 4, characterized in that the absorption in process step e at least a first absorption step e1, in which the gaseous outlet stream GS with a first liquid, alcohol-containing absorbent containing at least 50 wt .-% alcohol , preferably methanol, to obtain an alkyl methacrylate-loaded absorbent and an alcohol-enriched gas stream; and at least one second absorption step e2, in which the alcohol-enriched gas stream is treated with a second, aqueous absorbent containing at least 50% by weight of water, an alcohol-loaded aqueous absorbent being obtained; includes. Process according to one of Claims 1 to 5, characterized in that an alkyl methacrylate-depleted gaseous outlet stream, which is obtained in process step e after treatment with the at least two liquid absorbents, is optionally discharged from the process after thermal treatment. The method according to any one of claims 1 to 6, characterized in that in process step a a gaseous outlet stream GS1 is obtained which contains carbon monoxide and sulfur dioxide, and wherein this gaseous outlet stream GS1 is separated from the gaseous outlet streams which in the process steps b, c and / or d are obtained, is discharged from the process.

8. The process as claimed in any of claims 1 to 7, characterized in that in process step a a gaseous outlet stream GS1 is obtained which contains 70 to 99% by volume of carbon monoxide and 1 to 20% by volume of sulfur dioxide.

9. The method according to any one of claims 1 to 8, characterized in that in process step b a gaseous outlet stream GS2 is obtained and in process step c a gaseous outlet stream GS3 is obtained, the streams GS2 and GS3 being 1.0 to 5 0% by volume, preferably 3.0 to 5.0% by volume, of alkyl methacrylate, preferably methyl methacrylate, and at most 10% by volume, preferably from 0.1 to 10% by volume, of oxygen, based on the total volume of outlet stream GS2 or GS3.

10. The method according to any one of claims 1 to 9, characterized in that in process step d a gaseous outlet stream GS4 is obtained, which 0.1 to 5.0 vol .-%, preferably 0.2 to 2.0 vol. -%, alkyl methacrylate, preferably methyl methacrylate, and at least 10% by volume, preferably 10 to 20% by volume, particularly preferably from 10 to 15% by volume, oxygen.

11. The method according to any one of claims 1 to 10, characterized in that in the absorption in process step (e) the gaseous outlet stream GS with 0.1 to 5.0 kg of liquid absorbents per m ³ gaseous outlet stream GS (related to the sum of the absorbents) and treated at a temperature of 5 to 40 °C.

12. The method according to any one of claims 1 to 11, characterized in that the absorption in method step (e) comprises the following steps e1. treating the gaseous outlet stream GS in a first absorption device with a liquid alcohol-containing absorbent containing at least 50% by weight alcohol, preferably methanol; whereby an alcohol-enriched gas stream is obtained, and e2. Treating the alcohol-enriched gas stream, which is obtained in process step e1, in a second absorption device with an aqueous absorbent containing at least 50% by weight of water. Process according to any one of claims 1 to 12, characterized in that the absorption in step e is carried out in a single absorption device, with one or more gaseous outlet streams obtained in step b, c and/or d in the lower part of the absorption device are supplied, and the at least two liquid absorbents are added in the upper part of the apparatus, preferably at different points. Process according to any one of claims 1 to 13, characterized in that the absorption in step e is carried out in a single absorption device, with one or more gaseous outlet streams obtained in step b, c and/or d in the lower part of the absorption device be fed; in a first absorption step (e1) at least one liquid, alcohol-containing absorbent containing at least 50% by weight alcohol, preferably methanol; is fed above the feed of the gaseous outlet streams; in a second absorption step (e2), at least one aqueous absorbent containing at least 50% by weight of water is fed in above the inlet of the alcohol-containing absorbent; and wherein the gaseous outlet stream and the at least two absorbents are passed countercurrently.