Field of the invention

The invention relates to the field of discontinuous (trans)esterification of (meth)acrylate compounds with an alcohol to produce a target product accompanied by the separation of a side product.

Background of the invention

In the production of methacrylates, the reaction process can be divided into several steps. Roughly simplified, these consist of a heating phase, a reaction phase and, as a rule, a post-degassing phase in which excess raw materials are separated from the product under reduced pressure.

Furthermore, a final temperature (Tend) is reached at the end of the reaction, which depends on the product and the reaction mixture. Tend is significantly higher than the initial reaction temperature (Tstart) at the beginning of the reaction, since the (meth)acrylic acid derivative used (e.g. methyl methacrylate (MMA), or also an esterification with methacrylic acid (MAS)) determines the reactor temperature by its boiling point. In the course of the reaction, the composition of the reaction mixture changes as a result of product formation. The presence of the product, a further high- boiling component, raises the boiling point of the mixture and thus also the overall reactor temperature.

Unfortunately, the reaction rate in the reactor is not optimal and there is a need to shorten the reaction time in order to make the production process faster, more efficient and more cost-effective.

One way to accelerate the reaction is to increase Tend. However, increasing Tend to accelerate the reaction, which can easily be achieved by e.g. increasing the pressure, has adverse effects due to several aspects. With increased temperature, the undesirable formation of side products increases, especially when using basic catalysts. These include in particular the Michael addition to the (meth)acrylic acid derivatives by the corresponding alcoholates as nucleophiles leading to an undesired side product. In addition, with elevated temperature there is always an increased risk of undesired polymerization. Thus, increasing pressure in order to accelerate reaction procedure is possible, but entails comparable considerable risks, as increased pressure leads to increased temperature. The background for the accelerated reaction rates are the physical principles as described in the Van-'t-Hoff equation, or more simplified and generalized under the reaction ratetemperature rule, i.e. as a rule of thumb, reaction rates double for every ten degrees Celsius increase in temperature. To avoid this problem, the production of (meth)acrylic acid esters is therefore normally carried out under isobaric conditions and, in particular, under atmospheric conditions. Another way to shorten the reaction time is to bring the reaction temperature to the Tend from the beginning, and to avoid or limit an increase of the temperature above a predetermined temperature by a constant linear pressure drop during the reaction time. However, this has the disadvantage of strongly scattering reaction results, since the reaction temperature of each batch is slightly different, even if an identical product is to be produced. The reaction rate varies, and thus a static ramp is sometimes more and sometimes less suitable for reaction control.

CA 2 841 384 A101 discloses a process for preparing (meth)acrylic esters of polyols by reaction of tripropylene glycol with acrylic acid and/or methacrylic acid in the presence of acidic esterification catalysts and in the presence of polymerization inhibitors and air, the resultant water of condensation distilled off, with operation taking place preferably in reactors equipped with dephlegmators or distillation columns. Therein (meth)acrylic acid is metered in in three or more portions, the reaction temperature is set to a level in the range from 70 to 150°C, and the water formed in the reaction is removed from the reaction space under reduced pressure, the reduced pressure being 0,6 bar or less.

US 2013/172598 A1 discloses a process for preparing ethylene glycol dimethacrylate, comprising transesterifying ethylene glycol with an ester of methacrylic acid a reaction mixture comprising lithium amide (LiNH2) and lithium chloride (LiC I) as catalyst and distilling the alcohol liberated from the ester of methacrylic acid to separate the alcohol from the reaction mixture. Additional methacrylate is added to the reaction mixture during the reaction. A polymerization inhibitor or oxygen can also be added.

In addition, it has been shown that every reduction in pressure results in a strong boiling of the reactor contents, since when the pressure is reduced, the reactor contents previously at the boiling point are inevitably warmer than the now newly established boiling point of the current (new) pressure. This is particularly critical, if the pressure reduction is not controlled properly because it can lead to overfilling of the reactor contents and consequently to a safety shutdown of the reactor. Furthermore, in reactions using distillation columns, a very high pressure difference occurs in the column as a result of the high liquid loading, which can cause flooding of the column packing and a safety shutdown of the reactor as well.

Therefore, the objective of the present invention is to provide a method that overcomes the aforementioned problems. To this end, the present invention regulates the reaction temperature in the reactor by repeatedly adjusting the pressure in the reaction chamber to the changing boiling temperature of the reaction mixture, e.g. via pressure stages, thereby aiming for a more or less isothermal temperature curve. The dynamic pressure control is linked, or adapted, to the reaction temperature in the reactor chamber in order to avoid strong boiling. Through repeatedly adjusting the pressure in this way, the reaction temperature in the reactor does not rise above a desired Tend and the reaction runs in an accelerated manner. Summary of the invention

The invention is directed to a method for preparing an alkyl (meth)acrylate product by a (trans)esterification reaction of a reaction mixture in a reactor system, the reactor system comprising a reboiler, a reaction chamber comprising the reaction mixture, a column with a column head, a vapor transfer line, a condenser, a reflux tank, a reflux line, a distillate take off line, and a receiver vessel, wherein the reaction mixture comprises a (meth)acrylate starting material and a first alcohol which are converted by the (trans)esterification reaction at a reaction temperature and a given pressure in the reaction chamber in the presence of a catalyst into the alkyl (meth)acrylate product and a side product, wherein during the (trans)esterification reaction at least one portion of the side product is continuously removed by distillate take off, and wherein the given pressure is repeatedly adjusted in order to maintain a range of the reaction temperature.

The present invention carries out the reaction under significant lower temperature. The reaction temperature in the reactor is kept constant by repeatingly adjusting the pressure in the reaction chamber to the changing boiling temperature of the reaction mixture. Running the reaction under vacuum or reduced pressure allows in addition the removal of the side product (e.g. methanol).

Furthermore, with sufficiently low temperature, there is no loss in activity of the strong basic catalysts as the catalysts are active at room temperature. The filtration of the process liquid is improved enormously, so that significantly fewer filters need to be replaced and cleaned. In addition, the color of the crude product, that is the product before filtration and distillation, improves from amber brown to light yellow, indicating an increased purity of the product. The distillation of the new process material is running much longer with less steam pressure increase on wiped film evaporators per same product amount since the wall and waste tubes are less plugged.

An additional advantage of the invention is that by regulating the temperature through pressure adjustment thereby avoiding a temperature threshold being exceeded in the reactor before the end of the reaction, less excess (meth)acrylate starting material is needed in the reaction chamber for reaction temperature control. Reducing excess amounts of the (meth)acrylate starting material allows the amount of the first alcohol to be increased instead. In this way, the reaction chamber volume can be used more efficiently with regard to the space-time yield. Description of the drawings

Figure 1 illustrates a reactor system according to the invention, the reactor system comprises a reboiler (12), a reaction chamber (1), a column (2) with a column head (3), a vapor transfer line (4), a condenser (5), a reflux tank (6), a reflux line (7), a distillate take off line (8) with a mass flow meter (10), a receiver vessel (9), and a recycle line (11 ).

Figure 2 illustrates a temperature curve of a transesterification reaction without pressure adjustment.

Figure 3 illustrates a temperature curve of a transesterification reaction with pressure adjustment.

Figure 4 illustrates an alternative reactor system comparable to the reactor system according Figure 1 .

Detailed description of the invention

Unless otherwise particularly defined herein, the related terms used in the present invention have the following definitions.

As used herein, the term "(meth)acrylate" refers to both (meth)acrylic acid and (meth)acrylic acid ester. Furthermore, it refers to both methacrylate and acrylate. Or it refers to methacrylate or acrylate depending on the respective context. It also refers to methacrylic acid and methacrylic acid ester. For example, the term "a (meth)acrylate compound" or "a (meth)acrylate product" refers to (meth)acrylic acid and a (meth)acrylic acid ester, e.g. an alkyl (meth)acrylate.

Preferred (meth)acrylate products that can be prepared according to the instant application are selected from the group consisting of C2-18 alkyl (meth)acrylates, wherein the alkyl group can be aromatic or non-aromatic, cyclic, linear or branched and may be substituted by one or more heteroatoms selected from N, O, S or P.

Suitable (meth )acry late products are selected from the group consisting of ethyl (meth)acrylate, n- propyl (meth)acrylate, iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, 1 ,4-butylene-di(meth)acrylate, 1 ,3-butylene-di(meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, iso-decyl (meth)acrylate, undecyl (meth)acrylate, 5- methylundecyl (meth)acrylate, dodecyl (meth)acrylate, 2-methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, octadecyl (meth)acrylate, lauryl methacrylate and stearyl methacrylate; cyclopentyl (meth)acrylate, cyclohexyl methacrylate, cycloheptyl (meth)acrylate and cyclooctyl (meth)acrylate; allyl (meth)acrylate; tetra hydrofurfuryl (meth)acrylate; benzyl (meth)acrylate; N-(2-methacryloyloxyethyl) ethylene urea; and ethyleneglycol dimethacrylate. As used herein, the term "(trans)esterification" or "(trans)esterification reaction" refers to both an esterification reaction and a transesterification reaction. Or it refers to either an esterification reaction or a transesterification reaction depending on the respective context.

As used herein, the term "reactor system" refers to a reactor in which a chemical reaction of a reaction mixture can take place, and wherein the contents of the reactor can be subjected to a distillation process before the course of a chemical reaction and/or during the course of a chemical reaction and/or after the course of a chemical reaction or independently of a chemical reaction. The advantage of such a reactor is that it is not necessary to provide several chambers, one for a reaction to take place and one for the substance or substances to be distilled, thus saving material, time for a transfer of the substances and costs. The reactor system comprises at least a reboiler, a reaction chamber, a column with a column head, a feed line to the reaction chamber or to the column, a vapor transfer line, a condenser, a reflux tank, a reflux line, a distillate take off line, and a receiver vessel.

As used herein, the term "reboiler" refers to an apparatus for heating a liquid and/or converting a liquid to its vapor or, in other words, gaseous state.

As used herein, the term "reaction chamber" refers to a container in which a chemical reaction is carried out. There are a wide variety of chambers being referred to as such. For example, the sizes of reaction chambers range from micro reaction chambers, which hold a few microliters, to reaction chambers for a few milliliters, to chambers with a volume of numerous cubic meters. The most important characteristic of each reaction chamber is its resistance to the reaction conditions.

As used herein, the term "reaction mixture" refers to a mixture of substances that participate in a reaction. Such a reaction mixture can comprise or consist of:

- the starting material(s),

- the starting material(s) and possible additive(s) such as auxiliary substances, which can be, for example, catalysts or other reaction accelerators,

- the starting material(s) and product(s) including possible side-product(s),

- the starting material(s), additive(s) and product(s) including possible side-product(s),

- the product(s) including possible side-product(s) and the additive(s),

- the product(s) alone and additive(s),

- the product(s) including possible side-product(s), or

- the product(s) alone.

As used herein, the term "column" refers to an apparatus for the thermal separation of mixtures. To avoid heat loss, the column can be an insulated, preferably cylindrical, tube, which can be made in particular of steel, high-alloy stainless steels, glass or plastic. The height of the column body can mainly be dictated by the required quality of separation; the diameter by the volume flow of the mixture to be separated. The column can be placed between the reaction chamber and the distillation head. The number of individual distillations required for the same separation performance can also be referred to as the "theoretical plate number". At the surface of the column, the equilibrium between the liquid and gaseous phases can constantly be re-established by condensation and evaporation. As a result, the proportion of the low-boiling component continues to increase towards the top, while the higher-boiling component flows back into the reaction chamber, the sump. The size of the surface area of the column can be greatly increased in various ways by the design of trays, as in the Vigreux column, or by filling with packing or structured packing.

As used herein, the term "condenser" refers to a unit in which the vapor produced during distillation, which can be composed of the various volatile components of the solution to be separated, can liquefy by cooling.

As used herein, the term "reflux tank" refers to a container into which condensed distillate flows, which then either flows back to the column and/or into the reaction chamber or is removed from the system.

As used herein, the term "reflux line" refers to a line which can feed distillate from the reflux tank back to the column or to the reaction chamber.

As used herein, the term "distillate take off line" refers to a line which can remove at least a portion of distillate from the reflux tank, or in general from the reactor system, or the distillation system.

As used herein, the term "feed line" refers to supply lines which feed substances or substance mixtures to e.g. the reaction chamber or to the column.

As used herein, the term "starting material" refers to raw material, initial material, educts, feedstock, reactants or initial reactants which can be used to undergo a chemical reaction, whereby the chemical reaction may result at least in a product or products, which can include side-products or by-products.

A further embodiment is directed to the method as outlined further above, wherein the at least one portion of the side product is continuously removed by distillate take off over a predominant amount of time during the (trans)esterification reaction, and wherein the given pressure is repeatedly adjusted over a predominant amount of time during the (trans)esterification reaction in order to maintain a range of the reaction temperature.

As used herein, the term "predominant amount of time" means more than 50% of the reaction time, more preferred than 60% and most preferred more than 75% of the reaction time. As herein the term "reaction time" is defined as the duration in which the side product is removed from the reaction mixture. The reaction time strongly depends on the first alcohol used in the transesterification reaction and can vary between 2 hours and up to 48 hours.

As used herein, the term "pressure stages" refers to pressure drop or pressure build-up at defined pressure values. The difference between two pressure stages (abs.) can be e.g. less than 1000 mbar, less than 500 mbar, less than 100 mbar, less than 50 mbar, or less than 10 mbar.

As used herein, the term "recipe control" refers to a control system which, after evaluation of certain data such as the temperature of the reaction chamber, can automatically control certain processes such as pressure adjustment.

As used herein, the term "wt%" refers to weight percentage.

The problem underlying the present invention is solved by a method for preparing an alkyl (meth)acrylate product by a (trans)esterification reaction of a reaction mixture in a reactor system, the reactor system comprises a reboiler, a reaction chamber comprising the reaction mixture, a column with a column head, a vapor transfer line, a condenser, a reflux tank, a reflux line, a distillate take off line, and a receiver vessel, the reaction mixture comprises a (meth)acrylate starting material and a first alcohol which are converted by the (trans)esterification reaction at a reaction temperature and a given pressure in the reaction chamber in the presence of a catalyst into the alkyl (meth)acrylate product and a side product, wherein the at least one portion of the side product is continuously removed by distillate take off, and wherein the given pressure is repeatedly adjusted in order to maintain a range of the reaction temperature. The reboiler can be part of larger heating and/or cooling system.

This has the advantage that the reaction runs in an accelerated manner, which furthermore leads to an increased space-time yield.

Preferably, the repeated adjustment of the given pressure is performed in pressure stages, preferably wherein the difference between two pressure stages (abs.) is less than 500 mbar, preferably less than 100 mbar, more preferably less than 50 mbar, and even more preferably less than 10 mbar.

This has the advantage that the pressure can be adjusted in steps which can gradually adjust the pressure. Furthermore, through pressure steps it is possible to adjust the pressure in reaction to the situation in the reactor. The lower the pressure stages are, the more sensitive the reaction can be.

A pressure drop is favorable for separation of a side product. Preferably, the (trans)esterification reaction is performed at a pressure (abs.) within the range of 0.1 bar to 5.0 bar, preferably 0.1 bar to 4.0 bar, more preferably 0.1 bar to 3.0 bar, even more preferably 0.1 bar to 2.0 bar, most preferably 0.5 bar to 1.5 bar, for example 1 bar, or wherein the (trans)esterification reaction is started at a pressure of above 1.0 bar and the reaction process ends at a pressure of below 1.0 bar, where in “above 1.0 bar” means a range of + 0.01 to 0.1 bar, preferably 0.01 bar to 0.5 bar.

If side-product formation is significantly increased at elevated temperature, it is advisable to allow the reaction to end at a pressure below normal pressure, i.e. normal pressure can be defined as 1 bar, in order to lower the reaction temperature above the boiling temperature of the reactants in a targeted manner.

Preferably, the repeated adjustment of the given pressure is performed continuously.

This has the advantage that the pressure can be controlled and adjusted when the reaction temperature is lower than the boiling temperature. Shutdowns of the reactor due to boiling of the reaction mixture can be prevented.

As the boiling temperature of the reaction mixture changes during the (trans)esterification reaction, preferably the (trans)esterification reaction is carried out while the pressure is repeatedly adjusted to the boiling temperature of the reaction mixture. The regulation of pressure stages can also be affected automatically by a recipe control.

This has the advantage that the reaction conditions remain more or less constant over a predominant amount of time or the whole time of the (trans)esterification reaction and that the reaction conditions remain at an optimal reaction temperature. This has the further advantage that the reaction is conducted at optimal speed and polymerization as well as further side effects as generation of undesirable side products including in particular the Michael addition to the (meth)acrylic acid derivatives by the corresponding alcoholates as nucleophiles are reduced to a minimum.

Within the present invention the given pressure is repeatedly adjusted in a way to maintain the reaction temperature typically in the range of from 70°C to not more than 130°C. Preferably, the given pressure is repeatedly adjusted in a way to maintain the reaction temperature in the range of from 80°C to not more than 125°C, more preferably in the range of from 90°C to not more than 120°C, for example in the range of from 105°C to 115°C.

The starting material comprises the alkyl (meth)acrylate and the first alcohol. The reaction mixture may comprise two or more starting materials and two or more products. Further preferably, the two or more starting materials are a first educt, a second educt, and, optionally, one or more further compounds, and wherein the two or more products are a first product, a second product, and, optionally, one or more further products.

The alkyl (meth)acrylate can be selected from the group consisting of C1-4 alkyl (meth)acrylates or (meth)acrylic acid. Preferably are used methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate and (meth)acrylic acid; especially preferred is methyl methacrylate.

The first alcohol can be selected from the group consisting of aromatic or non-aromatic, cyclic, linear or branched C2-18 alkyl alcohols which may optionally be substituted by one or more heteroatoms selected from N, O, S or P.

A suitable aromatic C2-18 alkyl alcohol is benzyl alcohol.

Suitable cyclic C2-18 alkyl alcohols are cyclopentyl alcohol, cyclohexyl alcohol, cycloheptyl alcohol, cyclooctyl alcohol.

Suitable substituted cyclic C2-18 alcohols are tetrahydrofurfuryl alcohol and N-(2- hydroxyethyl)ethylene urea.

Suitable linear or branched, saturated or unsaturated C2-18 alkyl alcohols are ethanol, propanol, iso-propanol, allyl alcohol, n-butanol, iso-butanol, 1 ,4 butane diol, 1 ,3 butane diol, pentanol, hexanol, 2-ethylhexyl alcohol, heptyl alcohol, octyl alcohol, nonyl alcohol, decyl alcohol, iso-decyl alcohol, undecyl alcohol, 5-methylundecyl alcohol, dodecyl alcohol, 2-methyldodecyl alcohol, tridecyl alcohol, 5-methyltridecyl alcohol, tetradecyl alcohol, pentadecyl alcohol, hexadecyl alcohol, and octadecyl alcohol.

The term "linear or branched C2-18 alkyl alcohols" encompasses individual alkyl alcohols of a particular length and likewise mixtures of alkanols with different lengths. Amongst the mixtures of alkanols with different lengths, particularly preferred are linear C12-14 alkanol mixtures, mixtures of linear and branched C12-15 alkanols, and mixtures of C16-18 alkanols.

Suitable substituted C2-18 alkanols are methoxy ethylene glycols in different chain length.

The reaction mixture can further comprise additives such as reaction accelerators, reaction inhibitors, buffer, solvents, stabilizer, water, viscosity index improvers, thickeners, antioxidants, corrosion inhibitors, dispersants, high pressure additives, defoamers, catalysts or enzymes.

Preferably the catalyst is selected from titanium(IV) alcoholates, zirconium acetylacetonate, or from strong basic compounds. Stabilizers comprise e.g. hydroquinone methyl ether (HQME), phenotiazin (PTZ), 2, 2,6,6- tetramethylpiperidinyloxyl (TEMPOL), or 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (hydroxy- TEMPOL).

Furthermore, at least an alcohol, e.g. a second alcohol, is formed as a side product. Further side products can be formed by the reaction, which then are also present in the reaction mixture.

The side product or second alcohol corresponds to the C1-4 alcohol released from the alkyl (meth )acry late used a s starting material during the transesterification reaction or can be water if methacrylic acid is used as starting material.

As mentioned above an additional advantage of the invention is that less excess (meth)acrylate starting material is needed in the reaction chamber for reaction temperature control. When performing the method according to the invention the ratio of the first alcohol to the (meth)acrylate starting material before the start of the reaction can be in the range of 1 :1 to 1 :5. It is preferred that the ratio is in the range of 1 :1 to 1 :2, more preferably in the range of 1 :1 .1 to 1 :1 .5, and most preferably in the range of 1 : 1.1 to 1 : 1.3, for example 1 :1.2.

The pressure change in the reaction chamber is performed by actively changing a gas space or liquid-filled space. The gas for pressure regulation can be oxygen, nitrogen, ethylene oxide or isobutene or mixtures thereof. Increasing the pressure by a liquid is also possible, but it limits the reactor volume and is therefore disadvantageous. The use of a gas has therefore proved to be preferred. Since air must always be supplied to the reaction for stabilization, this gas is most preferably an oxygen-containing gas such as air or diluted air. If the reactant is also a gas, it has proved particularly preferable to mix this reactant with oxygen or an oxygen-containing gas.

Preferably the reboiler is an internal or external heating coil, a heating jacket, an internal heat exchanger or an external heat exchanger.

The reactor system may further comprise one or more elements selected from the group of one or more mass flow meters, a recycle line, and a feed line to the reaction chamber or to the column, preferably wherein the feed line is a first feed line, and wherein the reactor system further comprises a second feed line to the reaction chamber or to the column, and, optionally, further feed lines to the reaction chamber or to the column.

Preferably, the feed line is a first feed line, and the reactor system comprises a second feed line to the reaction chamber or to the column, and, optionally, further feed lines to the reaction chamber or to the column. This has the advantage that substances and/or mixtures can be added to the reaction chamber and/or column via several feed lines. Substances and/or mixtures can be added simultaneously or successively and/or continuously or discontinuously via these feed lines. Different substances and/or mixtures can be added to the reaction chamber and/or column via the feed lines. Also, identical substances and/or mixtures can be added to the reaction chamber and/or column via the feed lines. Furthermore, identical substances and/or mixtures and different substances and/or mixtures can be added to the reaction chamber and/or column via the feed lines. These substances or mixtures can be, for example, the starting materials used for the reaction and/or additives such as reaction accelerators, reaction inhibitors, buffer, solvents, stabilizer, water, viscosity index improvers, thickeners, antioxidants, corrosion inhibitors, dispersants, high pressure additives, defoamers, catalysts or enzymes.

Through the reflux line distillate can flow back onto the column or into the reaction chamber. The reflux tank stores reflux of the distillate.

In the inventive method, the side product can also be removed in form of a mixture of the side product and the (meth)acrylate starting material.

When performing the method according to the invention, it is a preferred option to compensate at least a portion of the converted (meth )acry late starting material by adding a further amount of the (meth )acry late starting material to the reaction mixture, preferably via a feed line to the reaction chamber or to the column.

When performing the method according to the invention, it is a further preferred option that a filling level of the reaction chamber is kept constant by compensating at least a portion of the converted (meth)acrylate starting material and/or of the first alcohol by adding a further amount of the (meth)acrylate starting material and/or of the first alcohol to the reaction mixture in the reaction chamber, preferably via a feed line to the reaction chamber or to the column.

This has the advantage that available volume in the reactor chamber is used for the (trans)esterification reaction, which increases the space-time yield. Preferably, further (meth )acry late starting material is added to the maximal reactor filling level in order to use the maximal volume of the reactor for the (trans)esterification reaction and in order to even more increase the space-time yield. This has the further advantage that an optimal heat transfer from the outer wall into the reaction wall into the reaction medium is achieved as a maximum utilized heat transfer area is provided. Further preferably, the reactor filling level is maintained constant during the (trans)esterification process.

Preferably, the (trans)esterification reaction is carried out in the presence of a polymerization inhibitor.

This has the advantage that polymerization is inhibited, and the space-time yield is increased. Furthermore, the product can be purer. As an inhibitor, p-phenylenediamines, phenothiazine and hydroxylamines like N,N- diethylhydroxylamin (DEHA) and N,N-bis(2-hydroxypropyl)hydroxylamine (HPHA) can be used. Most preferred inhibitors are hydrochinone (HQ), its methyl ether HQME and 2, 2,6,6- tetramethylpiperidinyloxyl (TEMPO), or its derivatives like 4-hyd roxy-2, 2,6,6- tetramethylpiperidinyloxyl (TEMPOL) and any derivative of this functionalizing the hydroxy group like methacrylic ester of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl.

The polymerization inhibitor HQME can be present in the range of 1 ppm to 1000 ppm, preferably 10 ppm to 1000 ppm, more preferred 10 ppm to 500 ppm, and most preferred 50 ppm to 100 ppm, with ppm in weight. The polymerization inhibitor TEMPOL or any of its derivatives can be present in the range of 0.1 ppm to 1000 ppm, more preferred 1 ppm to 500 ppm, and most preferred 10 ppm to 100 ppm, with ppm in weight.

Preferably, the (trans)esterification reaction is carried out with introduction of oxygen into the reaction mixture.

Beside oxygen, also air can be introduced into the reaction to speed up the reaction process. The oxygen may be introduced continuously or discontinuously, preferably the oxygen is introduced continuously.

In the method according to the invention, the (meth)acrylate starting material may comprise methyl (meth )acry late and the side product may comprise methanol, or the (meth)acrylate starting material may comprise ethyl (meth)acrylate and the side product may comprise ethanol, or the (meth)acrylate starting material may comprise n-butyl (meth)acrylate and the side product may comprise butanol, or the (meth)acrylate starting material may comprise (meth)acrylic acid and the side product may comprise water, preferably the (meth)acrylate starting material comprises methyl (meth )acry late and the side product comprises methanol.

In the method according to the invention, the catalyst can be selected from titanium(IV) alcoholates, for example titanium(IV) tetraisopropanolate, titanium(IV) tetrabutanolate, titanium(IV) tetrakis(2-ethylhexanolate), or mixtures thereof; from zirconium acetylacetonate; or from strong basic compounds, for example alkali oxides, earth alkali oxides, alkali hydroxides, earth alkali hydroxides, alkali alkoxides, earth alkali alkoxides, alkali amides, and earth alkali amides, preferably wherein the strong basic catalyst comprises one or more compounds selected from the group consisting of calcium oxide (CaO), calcium hydroxide (Ca(OH)2), lithium hydroxide (LiOH), sodium methanolate (NaOMe), lithium methanolate (LiOMe), lithium tert-butoxide (LiOt-Bu), lithium iso-propoxide (LiOiPr), lithium amide (UNH2), or mixtures thereof. The method according to the invention has the advantage that catalysts previously unsuitable or poorly suited for the (trans)esterification reaction can now be used efficiently in the (trans)esterification process, which leads to an accelerated reaction speed and to an increased space-time yield.

Figure 1 illustrates a reaction system, comprising a stirred reactor with a reaction chamber (1 ), a column (2), vapor transfer line (4), a column condenser (5), a reflux tank (6), a reflux line (7), outtake line (8), a receiver vessel (9), at least one flow meter (10), a recycle line (11 ), a heating and/or cooling system (12, 12a), comprising a reboiler (12) and a heat exchanger (12a), a feed line (13) connected with the reactor (1 ) and/or a feed line (13a) connected to the column (2), a distillate take off line (15, 15a), and a reaction chamber take off line (14).

The column (2) is above and in connection with the reactor (1 ), preferably on top of the reactor (1 ). From the column head (3) steam is transferred via the vapor transfer line (4) into the column condenser (5) and condensate is directed to the connected reflux tank (6). The reflux line (7) leads to the top of the column (2). In the shown example, the reflux line (7) leads the from the reflux tank (6) to the top of the column (2). The pressure within the reaction system and/or the column (2) is controlled by (not shown) a infeed pump, a vacuum pump and/or a valve unit, comprising at least one valve and/or one throttle.

With reference to Figure 1 , it is illustrated that the amount of reflux (7) to the column head (3) may be controlled via the fill level in the reflux tank (6). If more distillate accumulates from the condenser system (5), the amount of reflux will increase if the fill level in the reflux tank (6) rises. If no distillate is withdrawn from the system via the distillate transfer line (8), all condensate obtained from the condenser system (5) is fed to column via a reflux tank (6) and a reflux line (7). In the process step of transesterification, it is preferred that a controller ensures that there is always a transfer of distillate via a transfer line (8) to a receiver vessel (9). This is done from the calculation of the reflux ratio (v = reflux stream in the reflux line (7) / distillate transfer in line (8)). The distillate transfer stream (8) is adjusted so that the reflux ratio (v) is always in a range of maximum 15 and minimum 0.33.

The transfer stream (8) can be freely selected between the specified limits, which are defined by the reflux ratio (v). However, this is preferably set in such a way that a constant column top concentration of methanol in the column head (3) can be maintained. The concentration at the top of the column (2) is determined using the flow meter (10) in stream (7) or (8).

The energy input into the reaction system and mainly in the rection chamber (1 ) is realized via the reboiler (12) of the heating system (12, 12a) of the reactor. The system can be heated e.g. via steam, thermal oil system, electrically, etc. The required heat output (energy input into the reactor) is typically regulated in such a way that certain parameters (total distillate occurring in the condenser system (5), pressure drop across column (2) and reactor temperature) are not exceeded during the process. The main control variable for the energy input is typically the amount of the distillate stream occurring in the condenser (5) or the reflux tank (6). This should be kept constant throughout the reaction.

The amount of distillate produced should be chosen such that for the column (2) an F factor in the range of 0.5 to 3.0 Pa ^A (1/2), preferably in the range of 1.0 to 2.0 Pa ^A (1/2) is achieved.

The reactor system shown in Figure 4 comprises a stirred reactor with a reaction chamber (1), a column (2), having a column head (3), a vapor transfer line (4), a column condenser (5), a reflux tank (6), a reflux line (7), outtake line (8), a receiver vessel (9), a flow meter (10), a recycle line (11 ) and a reboiler (12). The column (2) is above and in connection with the reactor (1), preferably on top of the reactor (1). The reflux line (7) leads to the top of the column (2), e.g. into the column head (3). In the shown example, the reflux line (7) leads the from the reflux tank (6) to the top of the column (2).

The following examples aim to explain and clarify to the skilled person the principles of the present invention and to show how the invention can be applied in practice.

Examples:

Example 1 (comparative example)

Methyl methacrylate (MMA) was used as starting material and MPEG 750 (methoxy polyethylene glycol with an average MW of 750 g/mol) was used as first alcohol

Figure 2 shows a temperature profile over time of a transesterification reaction, wherein the pressure and therefore the temperature of the reaction is not adjusted. The production of methacrylates, namely the reaction process, can be divided into several steps. Roughly simplified, these consist of a heating phase, a reaction phase and, as a rule, a post-degassing phase, vacuum phase, in which excess reactants are separated from the crude product under reduced pressure and isolated. Particularly due to the last step, plants for the production of methacrylate derivatives are designed for reaction control under vacuum, which is indirectly relevant to the invention.

Furthermore, when the reaction is conducted to the end of the reaction, a final temperature is established which depends on the product and the reaction mixture. This is significantly higher than the initial reaction temperature at the start of the reaction, since here the methacrylic acid derivative used determines the reactor temperature by its boiling point. In the course of the reaction, the composition of the reaction mixture changes due to product formation, so that the presence of a further high-boiling component increases the boiling point of the mixture and thus also the overall reactor temperature.

Example 2

Methyl methacrylate (MMA) was used as starting material and MPEG 750 (methoxy polyethylene glycol with an average MW of 750 g/mol) was used as first alcohol

Figure 3 shows a temperature profile over time of an embodiment of the invention, wherein the starting temperature is risen to the final temperature by continuous adjustment of the absolute pressure already from the beginning of the reaction. During the reaction this pressure is continuously lowered, so that the reactor temperature does not rise above the desired final temperature.

However, rising the starting temperature to the final temperature by continuous adjustment of the absolute pressure already from the beginning of the reaction is not necessary to provide a beneficial effect.

With the pressure controlled boiling point it was surprisingly found that a significant amount of (meth )acry late starting material could be removed from the reaction mixture, without impacting the reaction time. The amount of reactor volume saved by this was refilled with stoichiometric ratios of reactants and by this an improvement in space time yield was observed in the order of 20 to 45%, depending on the first alcohol employed in the reaction.