Process for producing (METH)acrylic esters

Abstract

The invention relates to a process for producing (meth)acrylic esters comprising: (i) reacting (meth)acrylic acid and an alkanol or isobutene, thereby obtaining a reaction mixture comprising (meth)acrylic ester and (meth)acrylic acid in at least one reactor (3, 23); (ii) working-up the reaction mixture by rectification in at least one column (27, 43; 93, 105), obtaining a product stream, a stream containing low boilers and a stream containing high boilers; (iii) optionally concentrating and cracking the high boilers from the stream containing high boilers in a cracking apparatus (35); each reactor (3, 23), each column (27, 43) and each cracking apparatus (35) being connected with at least one evaporator as a reboiler, in which (meth)acrylic acid and / or (meth)acrylic ester and / or high boilers comprising stream is evaporated and recycled into the respective apparatus, wherein each of the evaporators is designed in such a way that the specific heat load is 2 to 20 kW / m2.

Description

[0001] 230192W001

[0002] Process for producing (meth)acrylic esters

[0003] Description

[0004] The invention relates to a process for producing (meth)acrylic esters comprising:

[0005] (i) reacting (meth)acrylic acid and an alkanol, thereby obtaining a reaction mixture comprising (meth)acrylic ester and (meth)acrylic acid in at least one reactor;

[0006] (ii) working-up the reaction mixture by rectification in at least one column, obtaining a product stream, a stream containing low boilers and a stream containing high boilers;

[0007] (iii) optionally concentrating and cracking the high boilers from the stream containing high boilers in a cracking apparatus; each reactor, each column and each cracking apparatus being connected with at least one evaporator as a reboiler, in which a (meth)acrylic acid and / or (meth)acrylic ester and / or high boilers comprising stream is evaporated and recycled into the respective apparatus.

[0008] (Meth)acrylic esters, also termed as alkyl acrylates, are well known and used, for example, as monomers for the preparation of aqueous polymer dispersions, which may be used, for example, as adhesives, paints or textile, leather and paper auxiliaries. Particularly (meth)acrylic esters having long-chain ester groups, so-called high (meth)acrylic esters, are of particular interest.

[0009] (Meth)acrylic esters generally are produced by esterification of (meth)acrylic acid with an alkanol in the presence of a catalyst. Such processes are described for example in “Encyclopedia of Chemical Technology, Vol. 1 , pages 347 to 248. A further process for producing C6-C12 alkyl (meth)acrylic esters is described in WO-A 2023 / 094252.

[0010] For obtaining a pure (meth)acrylic ester as product, it is necessary to work-up the crude reaction product obtained in the esterification reactor. Usually, impurities are removed by distillation processes. For removing low boilers, reactive distillation may be used. The remaining reaction mixture, containing the reaction product, remainders of low boilers and high boilers is worked-up by rectification in at least one column, preferably in a cascade of at least two distillation columns.

[0011] For heating and evaporating the bottom streams of the distillation columns and the reactive distillation, evaporators are used. However, since several components in the reaction mixture, particularly the (meth)acrylic ester and (meth)acrylic acid tend to polymerize, fouling forms on surfaces of the apparatuses, particularly the evaporators, which requires cleaning of the apparatuses in regular intervals. 230192W001

[0012] 2

[0013] For reducing fouling, it is known for example from EP-A 0 728 831 to provide a coating on the inner walls of reactor tubes in processes for decomposition of hydrocarbons.

[0014] An evaporator having low susceptibility to malfunctions is described for example in DE-C 3626 359.

[0015] However using such evaporators still have no influence on the formation of polymers which may form fouling on hot surfaces in the evaporators.

[0016] To reduce fouling in distillations processes of mixtures containing (meth)acrylic acid, it is known from EP-B 1 043 050 to use specific linear velocities of vapor in connecting lines. However, this increased velocity may reduce fouling in connecting lines but still does not reduce the formation of polymers and fouling in the evaporators.

[0017] Therefore, it is an object of the present invention to provide a process for producing (meth)acrylic esters in which the formation of undesired by-products, particularly the formation of polymers is reduced and, thus, fouling on surfaces of apparatuses used in the process can be reduced or even avoided.

[0018] This object is achieved by a process for producing (meth)acrylic esters comprising:

[0019] (i) reacting (meth)acrylic acid and an alkanol or isobutene, thereby obtaining a reaction mixture comprising (meth)acrylic ester and (meth)acrylic acid in at least one reactor;

[0020] (ii) working-up the reaction mixture by rectification in at least one column, obtaining a product stream, a stream containing low boilers and a stream containing high boilers;

[0021] (iii) optionally concentrating and cracking the high boilers from the stream containing high boilers in a cracking apparatus; each reactor, each column and each cracking apparatus being connected with at least one evaporator as a reboiler, in which a (meth)acrylic acid and / or (meth)acrylic ester and / or high boilers comprising stream is evaporated and recycled into the respective apparatus, wherein each of the evaporators is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2.

[0022] Surprisingly, it has shown that evaporation of (meth)acrylic acid and / or (meth)acrylic ester and / or high boilers comprising streams result in a reduction of the formation of undesired byproducts and, particularly, in the reduction of fouling. For this reason, it is possible to extend the cleaning intervals. Since cleaning of apparatuses, particularly the removal of fouling requires long shutdown periods and as further the fouling may result in unplanned production outage, by specific heat loads being in a range from 2 to 20 kW / m2the yield of a plant for producing (meth)acrylic esters can be increased. A further advantage of larger intervals between cleaning 230192W001

[0023] 3 steps and formation of less fouling is that a smaller amount of cleaning agents is necessary. Since disposal of used cleaning agent and cleaning discharge may cause ecological damages, a reduced need of cleaning agents and a reduced cleaning discharge also reduces the possibility of ecological and environmental hazard.

[0024] As the formation of undesired by-products depends on the concentration of (meth)acrylic ester, it is preferred that each evaporator in which the concentration of (meth)acrylic ester is 90 wt-% or below, the specific heat load preferably is in a range from 2 to 18 kW / m2, more preferred in a range from 4 to 17 kW / m2, particularly in a range from 6 to 15 kW / m2. If the concentration of (meth)acrylic ester is above 90 wt-% the specific heat load preferably is in a range from 2 to 15 kW / m2, more preferred in a range from 4 to 12 kW / m2, particularly in a range from 6 to 10 kW / m2.

[0025] For producing (meth)acrylic esters, (meth)acrylic acid, an alkanol and a catalyst are fed into the at least one reactor, in which the (meth)acrylic acid and the alkanol are esterified, forming the respective (meth)acrylic ester. Generally, the reactor is a reactive distillation column comprising a reaction section and a distillation section and the (meth)acrylic acid and the catalyst are fed into the reaction section and the alkanol is fed into the distillation section.

[0026] The alcohol fed into the reactor depends on the (meth)acrylic ester to be produced. Suitable alcohols for example are selected from the group consisting of methanol, ethanol, n-butanol, and 2-ethyl hexanol. Thus, the respective (meth)acrylic ester produced in the reaction, for example, is methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, or 2-ethylhexyl (meth)acrylate. For the production of tert-butyl (meth)acrylate isobutene and acrylic acid are reacted instead of the corresponding alcohol.

[0027] Catalysts to be used in the reaction usually are acidic catalysts, particularly strongly acidic catalysts. Suitable catalysts for example are sulfuric acid, alkyl or aryl sulfonic acid, like methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, para-toluenesulfonic acid, dodecylbenzenesulfonic acid or a strongly acidic ion exchanger. The catalyst content preferably is 0.1 to 10 wt-% and particularly 0.3 to 5 wt-%, based on the total amount of the reaction mixture.

[0028] The reaction may be carried out in one reactor or in a cascade of two or more reactors, preferably in two to four reactors, particularly in two to three reactors. As an alternative to use a cascade of two or more reactors, it is also possible to use a reactor having at least two reaction regions which may be separated for example by separating walls. In this case, the number of reaction regions may be greater than 4.

[0029] If the reaction is carried out in a cascade of two or more reactors, at least one reactor, usually the first reactor, is a reactive distillation column. Depending on the (meth)acrylic ester produced, each reactor in the column may be a reactive distillation column or at least one reactor following the reactive distillation column may be any other type of reactor, particularly a tank reactor. 230192W001

[0030] 4

[0031] Preferably, the reaction is carried out at a temperature in a range between 70 and 150 °C, particularly in a range from 80 to 130 °C, and a pressure in a range from 100 mbar(abs) to ambient pressure, preferably in a range from 200 to 800 mbar(abs). If the reaction is carried out in a cascade of two or more reactors, the temperature preferably is set such that it rises along the cascade.

[0032] To reduce polymerization of the (meth)acrylic acid, the reaction preferably is carried out in the presence of at least one polymerization inhibitor. Suitable polymerization inhibitors are described for example in WO-A 2023 / 094252 and comprise for example are alkylphenols, hydroxyphenols, aminophenols, nitrosophenols, alkoxyphenols, tocophenols, N-oxyls, aromatic amines or phenylenediamines, hydroxylamines, phosphorus-containing compounds, sulfur-containing compounds.

[0033] For carrying out the distillation of low boilers in the reactive distillation, it is necessary to evaporate a part of the reaction mixture. For this purpose, a part of the reaction mixture is withdrawn from the reaction section and fed into at least one evaporator. In the at least one evaporator, the part of the reaction mixture evaporates and then is recycled into the bottom part of the distillation section of the reactive distillation column. The evaporated reaction mixture rises in the distillation part and partly condenses due to decreasing temperature from bottom to top of the distillation section. The condensed part flow back into the reaction section.

[0034] Even though generally polymerization inhibitors are added to the reaction mixture, the (meth)acrylic acid and the (meth)acrylic ester still contained in the reaction mixture tend to polymerize on hot surfaces in the at least one evaporator. To avoid this polymerization, the at least one evaporator for evaporating the part of the reaction mixture is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2, preferably in a range from 4 to 18 kW / m2and particularly in a range from 6 to 15 kW / m2.

[0035] Suitable evaporators which can be used for evaporating the part of the reaction mixture may be any kind of evaporator known to a skilled person, for example heat exchangers, preferably shell-and-tube heat exchangers, plate type heat exchangers or spiral plate heat exchangers, particularly shell-and-tube heat exchangers, operating as a natural circulation evaporator or a forced circulation evaporator, Robert evaporators or forced circulation flash evaporators. Particularly preferred are shell-and-tube heat exchangers operating as natural circulation evaporator or as a forced circulation evaporator.

[0036] The distillation section generally is designed like any common distillation column and has conventional internals like trays, structured packings or random packings. Suitable trays for example are bubble trays, sieve trays, valve trays, Thormann trays and / or dual flow trays. Suitable random packings comprise rings, coils, saddles, Raschig rings, Intos or Pall rings, barrel or Intalox saddles. Preferably, the distillation section comprises 5 to 20 theoretical plates to achieve a desirable degree of separation. 230192W001

[0037] 5

[0038] As the reaction mixture obtained in the reaction still contains non-reacted (meth)acrylic acid and alcohol, inhibitors, catalyst, and further low boilers and high boilers like polymers formed from the (meth)acrylic acid besides the (meth)acrylic ester, the reaction mixture obtained by the reaction in step (i) is worked-up to purify the (meth)acrylic ester. Generally, working-up of the reaction mixture is carried out by rectification in at least one column, by which a product stream, a stream containing low boilers and a stream containing high boilers are obtained.

[0039] Low boilers particularly are water and alcohol and high boilers are oligomers or polymers which may form by unintended polymerization of the (meth)acrylic acid, reaction products of contaminants that may be contained in the reactants, or reaction of the products with the alkanol.

[0040] In a first embodiment, rectification in step (ii) is carried out in at least two columns, comprising a first distillation column for separating the high boilers and a final distillation column for finishing distillation, wherein the stream containing high boilers is withdrawn at the bottom from the first distillation column, a stream containing the (meth)acrylic ester is withdrawn at the top of the first distillation column and the product stream is withdrawn as a side stream at the lower part of the final distillation column. Besides using at least two columns, it is also possible to combine two of the at least two columns to a divided wall column.

[0041] This way for working-up the reaction mixture for example is used for working-up reaction mixtures containing 2-ethylhexyl acrylate or tert-butyl acrylate or working-up n-butyl acrylate which is produced in an azeotropic process.

[0042] The first distillation column preferably is operated with a bottom temperature in a range from 80 to 150 °C, more preferred a bottom temperature in a range from 90 to 147 °C and particularly in a range from 100 to 145 °C, a top temperature preferably in a range from 55 to 140 °C, more preferred a top temperature in a range from 85 to 135 °C and particularly in a range from 92 to 125 °C and a pressure preferably in a range from 0.02 to 1 .2 bar(abs), more preferred in a range from 0.04 to 1 .1 bar(abs) and particularly in a range from 0.06 to 1 .06 bar(abs). Thereby, the temperature and the pressure at which the first distillation column is operated depends on the (meth)acrylic ester to be worked-up. If, for example, the (meth)acrylic ester is n-butyl acrylate, the first distillation preferably is operated with a bottom temperature in a range from 80 to 130 °C, more preferred a bottom temperature in a range from 90 to 120 °C and particularly in a range from 100 to 115 °C, a top temperature preferably in a range from 55 to 105 °C, more preferred in a range from 85 to 100 °C and particularly in a range from 92 to 98 °C, and a pressure preferably in a range from 0.07 to 1 .2 bar(abs), more preferred in a range from 0.9 to 1 .1 bar(abs) and particularly in a range from 1 .04 to 1 .06 bar(abs). If, for example, the (meth)acrylic ester is 2-ethylhexyl acrylate, the first distillation column is operated with a bottom temperature in a range from 125 to 150 °C, more preferred in a range from 130 to 147 °C and particularly in a range from 135 to 145 °C, a top temperature preferably in a range from 100 to 140 °C, more preferred in a range from 105 135 °C and particularly in a range from 110 to 125°C, and a pressure preferably in a range from 0.02 to 0.09 bar(abs), more preferred in a range from 0.04 to 0.08 bar(abs) and particularly in a range from 0.06 to 0.07 bar(abs). 230192W001

[0043] 6

[0044] The first distillation column may be any type of distillation column known to a skilled person and contains internals as usually used in distillation columns, for example trays, structured packings or random packings. Trays as internals are preferred. Particularly preferably, the trays are bubble trays, sieve trays, valve trays or Thormann trays or, particularly dual flow trays. For an optimum separation of high boilers from the reaction mixture, it is preferred that the first distillation column contains 1 to 20 theoretical plates, more preferred 2 to 154 theoretical plates and particularly 3 to 11 theoretical plates, wherein the number of theoretical plates depends on the (meth)acrylic ester to be worked-up. If, for example, the (meth)acrylic ester is n-butyl acrylate, the first distillation column preferably contains 5 to 20 theoretical plates, more preferred 7 to 15 theoretical plates and particularly 9 to 11 theoretical plates and if, for example, the (meth)acrylic ester is 2-ethylhexyl acrylate, the first distillation column preferably contains 1 to 9 theoretical plates, more preferred 2 to 7 theoretical plates and particularly 3 to 6 theoretical plates.

[0045] For operating the first distillation column, the stream containing high boilers is separated into two parts, one part is withdrawn from the first distillation column and the other part is fed into at least one evaporator, evaporated and then returned into the first distillation column. Due to the high boilers, (meth)acrylic acid and (meth)acrylic ester which are contained in the stream containing high boilers and which tend to polymerize and form fouling on the surfaces of the at least one evaporator, the at least one evaporator is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2, preferably in a range from 4 to 18 kW / m2and particularly in a range from 6 to 15 kW / m2. Suitable evaporators for example are heat exchangers, preferably shell-and-tube heat exchangers, plate type heat exchangers or spiral plate heat exchangers, particularly shell-and-tube heat exchangers, operating as a natural circulation evaporator or a forced circulation evaporator, Robert evaporators, or forced circulation flash evaporators. Preferably, the evaporator is a forced circulation evaporator, a Robert evaporator or a natural circulation evaporator.

[0046] The part of the stream containing high boilers, which is not fed into the at least one evaporator is withdrawn and preferably fed into a cracking apparatus and / or a heavies concentration. In the cracking apparatus polymers obtained from the polymerization of (meth)acrylic ester and (meth)acrylic acid are cracked and the obtained (meth)acrylic acid, (meth)acrylic ester and alkanol are recycled into the reaction. High boilers which cannot be cracked and / or which do not result from polymerization of (meth)acrylic acid or (meth)acrylic ester are concentrated and withdrawn from the process as a heavies residue. Cracking the high boilers from the stream containing high boilers, usually is carried out in a vessel in the presence of a catalyst. Reactants and (meth)acrylic ester obtained by cracking are withdrawn at the top of the vessel and a high boiling residue is disposed. Concentrating the high boilers may be carried out in an additional column upstream the vessel for cracking or in a column placed on top of the vessel. For concentrating, at least a part of the stream containing high boilers needs to be evaporated. The at least one evaporator used for evaporating the at least part of the stream containing high boilers also is an evaporator which is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2, preferably in a range from 4 to 15 kW / m2and particularly in a range from 6 230192W001

[0047] 7 to 10 kW / m2, to reduce or avoid (meth)acrylic acid and / or (meth)acrylic ester which is contained in the stream to be evaporated to polymerize on the surfaces of the at least one evaporator and form fouling.

[0048] The at least one evaporator used for evaporating the stream containing high boilers for cracking and concentrating the high boilers may be any suitable evaporator known to a skilled person, for example a heat exchanger, preferably a shell-and-tube heat exchanger, a plate type heat exchanger or a spiral plate heat exchanger, particularly a shell-and-tube heat exchanger, operating as a natural circulation evaporator or a forced circulation evaporator, a Robert evaporator, or a forced circulation flash evaporator, a forced circulation flash evaporator being preferred.

[0049] The stream containing the (meth)acrylic ester which is withdrawn at the top of the first distillation column usually still contains low boilers, for example (meth)acrylic acid, water and non-reacted alcohol, and remaining high boilers, which have to be removed to obtain purified (meth)acrylic ester as a product. For this purpose, the stream containing (meth)acrylic ester is fed into the final distillation, in which the remaining low boilers and high boilers are separated from the (meth)acrylic ester. The purified (meth)acrylic ester is obtained as a side outlet in the lower part of the final distillation column. Remaining high boilers collect at the bottom of the final distillation column and are recycled into the first distillation column. The remaining low boilers are withdrawn at the top of the final distillation column and recycled into the reaction.

[0050] The final distillation column preferably is operated with a bottom temperature in a range from 89 to 150 °C, more preferred a bottom temperature in a range from 100 to 150 °C and particularly in a range from 115 to 149 °C and a bottom pressure in a range from 0.06 to 0.48 bar(abs), more preferred in a range from 0.09 to 0.45 bar(abs) and particularly in a range from 0.12 to 0.43 bar(abs), a top temperature in a range from 65 to 125 °C, more preferred a top temperature in a range from 70 to 120 °C and particularly in a range from 71 to 119 °C and a top pressure in a range from 0.04 to 0.4 bar(abs), more preferred in a range from 0.05 to 0.33 bar(abs) and particularly in a range from 0.06 to 0.31 bar(abs), the temperatures and pressures each depending on the (meth)acrylic ester to be worked-up. If the (meth)acrylic ester, for example, is n- butyl acrylate, the final distillation column preferably is operated with a bottom temperature in a range from 89 to 125 °C, more preferred in a range from 100 to 121 °C and particularly in a range from 115 to 119 °C at a bottom pressure in a range from 0.22 to 0.48 bar(abs), more preferred in a range from 0.3 to 0.45 bar(abs) and particularly in a range from 0.38 to 0.43 bar(abs), top temperature in a range from 65 to 90 °C, more preferred in a range from 70 to 83 °C and particularly in a range from 71 to 81 °C and a top pressure in a range from 0.15 to 0.4 bar(abs), more preferred in a range from 0.2 to 0.33 bar(abs) and particularly in a range from 0.23 to 0.31 bar(abs). If the (meth)acrylic ester, for example) is 2-ethylhexyl acrylate, the final distillation column preferably is operated with a bottom temperature in a range from 125 to 150 °C, more preferred in a range from 130 to 150 °C and particularly in a range from 142 to 149 °C, a bottom pressure in a range from 0.06 to 0.16 bar(abs), more preferred in a range from 0.09 to 0.15 bar(abs) and particularly in a range from 0.12 to 0.14 bar(abs), a top temperature in a range from 100 to 125 °C, more preferred in a range from 105 to 120 °C and particularly in a 230192W001

[0051] 8 range from 109 to 119 °C and a top pressure in a range from 0.04 to 0.14 bar(abs), more preferred in a range from 0.05 to 0.12 bar(abs) and particularly in a range from 0.06 to 0.1 bar(abs).

[0052] The final distillation column may be any type of distillation column known to a skilled person and contains internals as usually used in distillation columns, for example trays, structured packings or random packings. Trays as internals are preferred. Particularly preferably, the trays are bubble trays, sieve trays, valve trays or Thormann trays or dual flow trays, particularly dual flow trays. Further, the internals used in the final distillation column also may be structured packings or random packings. For an optimum operation of the final distillation column, it is preferred that the final distillation column contains 7 to 24 theoretical plates, more preferred 8 to 23 theoretical plates and particularly 9 to 22 theoretical plates, wherein the number of theoretical plates depends on the (meth)acrylic ester to be worked-up. If the (meth)acrylic ester, for example, is n- butyl acrylate, the final distillation column preferably contains 14 to 24 theoretical plates, more preferred 16 to 23 theoretical plates and particularly 18 to 22 theoretical plates, and if the (meth)acrylic ester, for example, is 2-ethylhexyl acrylate, the final distillation column preferably contains 8 to 17 theoretical plates, more preferred 8 to 15 theoretical plates and particularly 9 to 13 theoretical plates.

[0053] For operating the final distillation column, heat must be supplied. For this purpose, a part of the bottom stream is fed into at least one evaporator, at least partly evaporated and recycled into the final distillation column. To avoid or at least reduce fouling, particularly from polymerization of (meth)acrylic ester or (meth)acrylic acid, the at least one evaporator is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2, preferably in a range from 4 to 15 kW / m2and particularly in a range from 6 to 10 kW / m2. Suitable evaporators for example are heat exchangers, preferably shell-and-tube heat exchangers, plate type heat exchangers or spiral plate heat exchangers, particularly shell-and-tube heat exchangers, operating as a natural circulation evaporator or a forced circulation evaporator, Robert evaporators, or forced circulation flash evaporators. Preferably, the evaporator is a shell-and-tube heat exchanger, operating as a natural circulation evaporator or a forced circulation evaporator, or a Robert evaporator.

[0054] Depending on the purity of the (meth)acrylic ester obtained as product, it may be necessary to use further distillation columns for removing low boilers and high boilers from the (meth)acrylic ester.

[0055] In a second embodiment, rectification in step (ii) is carried out in at least two columns, comprising a first distillation column for separating low boilers and a final distillation column for separating high boilers, wherein the stream containing low boilers is withdrawn at the top of the first distillation column and a stream containing the (meth)acrylic ester and high boilers is withdrawn at the bottom of the first distillation column and the product stream is withdrawn at the top of the final distillation column and the stream containing high boilers is withdrawn at the bottom of the final distillation column. Besides using at least two columns, it is also possible to combine two of the at least two columns to a divided wall column. 230192W001

[0056] 9

[0057] This process, in which the high boilers are withdrawn at the bottom of the final distillation column and the low boilers are withdrawn at the top of the first distillation column is used, for example, for the production of n-butyl acrylate in an extractive process. In this process, the reaction is carried out in at least one reactive distillation column, preferably in three reactive distillation columns and the reaction is followed by catalyst extraction with water. The catalyst preferably is an alkyl sulfonic acid, particularly butyl sulfonic acid. The water with the catalyst is recycled into the reaction and the reaction mixture depleted in catalyst obtained by the extraction, is neutralized, thereby obtaining a neutralized reaction mixture and washing the neutralized reaction mixture. The thus obtained neutralized and washed reaction mixture is fed into the first distillation column, in which low boilers are removed. A high boiling fraction containing the (meth)acrylic ester and high boilers collect at the bottom of the first distillation column.

[0058] A stream containing the (meth)acrylic ester and high boilers is withdrawn from the bottom of the first distillation column, a part of the stream containing the (meth)acrylic ester and high boilers is fed into at least one evaporator to supply heat for the distillation in the first distillation column and the other part of the stream containing the (meth)acrylic ester and high boilers is fed into the final distillation column. Due to the high boilers, (meth)acrylic acid and (meth)acrylic ester which are contained in the stream containing (meth)acrylic ester and high boilers and which tend to polymerize and form fouling on the surfaces of the at least one evaporator, the at least one evaporator is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2, preferably in a range from 4 to 15 kW / m2and particularly in a range from 6 to 10 kW / m2. Suitable evaporators for example are heat exchangers, preferably shell-and-tube heat exchangers, plate type heat exchangers or spiral plate heat exchangers, particularly shell-and-tube heat exchangers, operating as a natural circulation evaporator or a forced circulation evaporator, Robert evaporators, or forced circulation flash evaporators. Preferably, the evaporator is a shell- and-tube heat exchanger, operating as a natural circulation evaporator or a forced circulation evaporator, or a Robert evaporator.

[0059] It the (meth)acrylic ester, for example, is n-butyl acrylate, the first distillation column preferably is operated with a bottom temperature in a range from 95 to 120 °C, more preferred a bottom temperature in a range from 100 to 115 °C and particularly in a range from 104 to 110 °C, a bottom pressure in a range from 0.15 to 0.36 bar(abs), more preferred in a range from 0.22 to 0.34, and particularly in a range from 0.25 to 0.31 bar(abs), a top temperature in a range from 56 to 70 °C, more preferred a top temperature in a range from 60 to 68 °C and particularly in a range from 63 to 66 °C and a top pressure in a range from 0.1 to 0.25 bar(abs), more preferred in a range from 0.11 to 0.2 bar(abs) and particularly in a range from 0.12 to 0.16 bar(abs).

[0060] The first distillation column may be any type of distillation column known to a skilled person and contains internals as usually used in distillation columns, for example trays, structured packings or random packings. Trays as internals are preferred. Particularly preferably, the trays are bubble trays, sieve trays, valve trays or Thormann trays or dual flow trays and particularly dual flow trays. Besides trays, in the first distillation column alternatively structured packings or random 230192W001

[0061] 10 packings are preferred. For an optimum separation of low boilers from the reaction mixture, it is preferred that the first distillation column contains 16 to 29 theoretical plates, more preferred 18 to 27 theoretical plates and particularly 20 to 25 theoretical plates, if, for example, the (meth)acrylic ester is n-butyl acrylate.

[0062] To further purify the stream containing (meth)acrylic ester and high boilers, this stream is fed into the final distillation column. In the final distillation column, purified (meth)acrylic ester as product stream is obtained at the top of the final distillation column and high boilers collect at the bottom of the final distillation column.

[0063] For supplying the heat necessary for the distillation, a part of the stream containing high boilers is fed into at least one evaporator, at least partly evaporated in the at least one evaporator and then recycled into the final distillation column. To reduce or avoid fouling by polymerizing (meth)acrylic acid and / or (meth)acrylic ester, the at least one evaporator is designed in such a way that the specific heat load is in a range from 2 to 20 kW / m2, preferably in a range from 4 to 15 kW / m2and particularly in a range from 6 to 10 kW / m2. Suitable evaporators for example are heat exchangers, preferably shell-and-tube heat exchangers, plate type heat exchangers or spiral plate heat exchangers, particularly shell-and-tube heat exchangers, operating as a natural circulation evaporator or a forced circulation evaporator, Robert evaporators, or forced circulation flash evaporators. Due to the high boilers contained in the stream, the evaporator preferably is a shell-and-tube heat exchanger, operating as a natural circulation evaporator or a forced circulation evaporator, or a Robert evaporator.

[0064] If the (meth)acrylic ester, for example, is n-butyl acrylate, the final distillation column preferably is operated with a bottom temperature in a range from 100 to 120 °C, more preferred a bottom temperature in a range from 103 to 118 °C and particularly in a range from 106 to 116 °C, a bottom pressure in a range from 0.12 to 0.23 bar(abs), more preferred in a range from 0.14 to 0.21 bar(abs) and particularly in a range from 0.16 to 0.19 bar(abs), a top temperature in a range from 75 to 90 °C, more preferred a top temperature in a range from 80 to 86 °C and particularly in a range from 82 to 84 °C and a top pressure in a range from 0.09 to 0.16 bar(abs), more preferred in a range from 0.1 to 0.14 bar(abs) and particularly in a range from 0.11 to 0.12 bar(abs).

[0065] The final distillation column may be any type of distillation column known to a skilled person and contains internals as usually used in distillation columns, for example trays, structured packings or random packings. Trays as internals are preferred. Particularly preferably, the trays are bubble trays, sieve trays, valve trays or Thormann trays or dual flow trays, with dual flow trays being particularly preferred. For an optimum operation of the final distillation column, it is preferred that the final distillation column contains 7 to 20 theoretical plates, more preferred 8 to 14 theoretical plates and particularly 9 to 11 theoretical plates, if the (meth)acrylic ester is n-butyl acrylate. 230192W001

[0066] 11

[0067] Depending on the purity of the purified (meth)acrylic ester to be obtained, it may be necessary to provide additional distillation steps. Each distillation step will be carried out in a distillation column as described above. As long as the stream, which is evaporated for heat supply to the distillation column contains (meth)acrylic acid, (meth)acrylic ester and / or high boilers, at least one evaporator is used as described above in connection with the first and final distillation columns.

[0068] The stream containing high boilers withdrawn from the final distillation column may be further worked-up by concentrating and cracking the high boilers. Concentrating and cracking the high boilers preferably is carried out as described above in connection with the first embodiment.

[0069] Independent of whether the distillation is carried out according to the first embodiment or to the second embodiment, low boilers containing streams may be further worked up. This working-up may be operated as in present known processes.

[0070] Examples of the invention are shown in the figures and described in more detail in the following description.

[0071] In the figures:

[0072] Figure 1 shows a schematic flow chart of a process for producing 2-ethylhexyl acrylate;

[0073] Figure 2 shows a schematic flow chart of a process for producing n-butyl acrylate in an azeotropic process

[0074] Figure 3 shows a schematic flow chart of a process for producing n-butyl acrylate in an extractive process

[0075] Figure 1 shows a schematic flow chart of a process for producing 2-ethylhexyl acrylate.

[0076] For producing 2-ethylhexyl acrylate, acrylic acid, catalyst, and 2-ethylhexanol are fed into a first reactor 1 . The first reactor 1 is a reactive distillation column, comprising a reaction section 3 and a distillation section 5. The acrylic acid is fed into the reaction section 3 via a first inlet 7 and the catalyst is fed into the reaction section 3 via a second inlet 9. The 2-ethylhexanol is fed into the distillation section 5 via a side feed 11 at the top of the distillation section 5.

[0077] In the reaction section 3, the acrylic acid and the 2-ethylhexanol react, thereby forming a reaction mixture containing 2-ethyl hexyl acrylate. To operate the distillation in the reaction section, the reaction section 3 comprises at least one evaporator in which a part of the reaction mixture is evaporated and recycled into the reaction section. The vapors obtained by the evaporation rise into the distillation section 5 and due to a lower temperature at the top of the distillation section 5, a part of the vapor condenses and flows back into the reaction section 3. Low boilers and 230192W001

[0078] 12 water which did not condense are withdrawn at the top of the distillation section 5 via a top outlet 13 and fed into a phase separator 15. In the phase separator 15, water and low boilers are separated and a part of the low boilers is recycled into the distillation section 5 via a recycle line 17 and another part of the low boilers is withdrawn from the process by a withdrawal line 19. The water is withdrawn from the process via waste water line 21.

[0079] The reaction mixture obtained in the reaction section 3 is transferred into a second reactor 23, in which the reaction is completed. The second reactor 23 also is equipped with at least one evaporator for evaporating at least a part of the reaction mixture. The evaporated part of the reaction mixture is recycled into the distillation section 5 of the first reactor 1 via a recycle line 25.

[0080] The reaction mixture containing the 2-ethyl hexyl acrylate is transferred into a first distillation column 27 for removing high boilers via a side feed 29. At the top of the first distillation column a 2-ethyl hexyl acrylate comprising stream is withdrawn via a head outlet 31 and a high boilers comprising stream is withdrawn from the first distillation column 27 via a bottom outlet 33.

[0081] A part of the high boilers containing stream is recycled into the reaction section 3 of the first reactor 1 via a line 37 and the remaining high boilers containing stream is fed into an apparatus 35 for concentrating and cracking high boilers. The concentrated high boilers are withdrawn from the process via outlet 39 and the cracked high boilers, which particularly comprise acrylic acid and 2-ethyl hexyl acrylate are returned into the second reactor 23 via line 41 .

[0082] The 2-ethyl hexyl acrylate comprising stream which is withdrawn at the top of the first distillation column 27 is fed into a final distillation column 43 via a side feed 45. In the final distillation column 43 the 2-ethyl hexyl acrylate is separated from remaining low boilers, particularly acrylic acid and 2-ethylhexanol, and remaining high boilers to obtain purified 2-ethyl hexyl acrylate as product, which is withdrawn via a side outlet 46 in the bottom part of the final distillation column 43. The low boilers are withdrawn via a head outlet 47 and recycled into the reaction section 3 of the first reactor 1 .

[0083] As the bottom liquid in the final distillation column still may contain 2-ethyl hexyl acrylate, the bottom stream is returned into the first distillation column 27 via a return line 49.

[0084] According to the invention, each evaporator used with the first reactor 1 , the second reactor 23, the first distillation column 27, the final distillation column 43 and the apparatus 35 for concentrating and cracking high boilers is designed in such a way that the specific heat load is 2 to 20 kW / m2.

[0085] Figure 2 shows a schematic flow chart of a process for producing n-butyl acrylate in an azeotropic process.

[0086] The process for producing n-butyl acrylate in an azeotropic process in many parts correspond to the process for producing 2-ethyl hexyl acrylate as described above in connection with figure 1. 230192W001

[0087] 13

[0088] For producing n-butyl acrylate in an azeotropic process, acrylic acid and catalyst are fed into the reaction section 3 of the first reactor 1 via the first inlet 7 and the second inlet 9. n-butanol is fed at the top of the distillation section 5 of the first reactor 1 via the side feed 11 . For finishing the reaction, the reaction mixture is fed into the second reactor 23.

[0089] Low boilers including water are withdrawn from the distillation section 5 via the top outlet 13 and fed into the phase separator 15. In difference to the process for producing 2-ethyl hexyl acrylate, here the low boilers still contain a significant amount of water. Therefore, the low boilers comprising phase is fed into a column 51 for concentrating the low boilers. The column 51 particularly is a distillation column. Since the amount of monomers like acrylic acid and acrylic ester in the low boilers comprising phase is below 10 ppm, a common evaporator can be used for operating the column 51. The low boilers obtained as the higher boiling phase at the bottom of the column 51 , particularly n-butanol, are returned into the distillation section 5 of the first reactor 1 via return line 53. The low boilers which are withdrawn at the top of the column 51 are fed into a second phase separator 55 via line 57. In the second phase separator 55, a low boilers comprising organic phase and an aqueous phase are obtained. The low boilers comprising organic phase is withdrawn via the withdrawal line 19. The aqueous phase is fed into a stripping column 59. In the stripping column 59, butanol is stripped from the aqueous phase with steam 61 , the steam being fed into the stripping column 59 in countercurrent. The butanol containing steam obtained at the top of the stripping column 59 is recycled into the phase separator 15.

[0090] In the phase separator 15, besides the phase containing the low boilers, an aqueous phase is obtained. The aqueous phase is fed into a third phase separator 63, into which also the n-butyl acrylate comprising stream is fed which is withdrawn via the head outlet 31 of the first distillation column 27. In the third phase separator, an aqueous phase and an organic phase containing the n-butyl acrylate are obtained, a part of the organic phase being fed into the third distillation column via the side feed 45 and another part of the organic phase is recycled into the first distillation column 23 via side feed 65. A part of the aqueous phase also is fed into the first distillation column 23 via a side feed 67 and another part of the aqueous phase is fed into the stripping column 59.

[0091] The high boilers obtained in the first distillation column 23 and being withdrawn from the first distillation column via the bottom outlet 33 are fed into the apparatus 35 for concentrating and cracking high boilers. Concentrating and cracking the high boilers thereby is carried out as described above for the process for producing 2-ethyl hexyl acrylate.

[0092] By the additional columns and phase separators in the process for producing n-butyl acrylate compared to the process for producing 2-ethyl hexyl acrylate, it is possible to regain most of the butanol used in the process. The additional process steps for regaining the butanol are necessary due to the lower boiling point of n-butanol compared to 2-ethylhexanoL 230192W001

[0093] 14

[0094] As in the process for producing 2-ethyl hexyl acrylate, also in the process for producing n-butyl acrylate, the reaction section 3 of the first reactor 1 , the second reactor 23, the first distillation column 23, the second distillation column 43 and the apparatus 35 for concentrating and cracking high boilers are equipped with evaporators, which, according to the invention, are designed in such a way that the specific heat load is 2 to 20 kW / m2.

[0095] Besides the azeotropic process as described above in connection with figure 2, n-butyl acrylate also may be produced in an extractive process.

[0096] For carrying out the extractive process, acrylic acid is fed into the reaction section 3 of the first reactor 1 via the first inlet 7 and catalyst is fed into the reaction section 3 of the first reactor 1 via the second inlet 9. n-butanol is fed into the distillation section 5 of the first reactor 1 via the side feed 11 , which in difference to the azeotropic process is at the bottom of the distillation section 5. The reaction mixture of the first reactor 1 is fed into a reaction zone 69 of the second reactor 23 and from the reaction zone 69 of the second reactor into a reaction zone 71 of a third reactor 73. In difference to the processes shown in figures 1 and 2, here the second reactor 23 and also the third reactor 73 are designed as reactive distillation columns.

[0097] The low boilers obtained in the distillation section 5 of the first reactor 1 are fed into the phase separator 15 in which the low boilers are separated into an aqueous phase and an organic phase containing butanol. The organic phase is returned into the distillation section 5 of the first reactor 1 and the aqueous phase is fed into the stripping column 59, in which butanol is stripped from the aqueous phase with steam 61.

[0098] The low boiling streams obtained in the distillation sections 75, 77 of the second reactor 23 and the third reactor 73 are fed into a second phase separator 79. In the second phase separator 79 an aqueous phase and an organic phase are obtained. The organic phase is returned into the distillation sections 75, 77 of the second reactor 23 and the third reactor 73 and the aqueous phase is fed into the stripping column 59.

[0099] The reaction mixture obtained in the reaction section 71 of the third reactor 73 is fed into an extractor 81 . In the extractor 81 , the catalyst is extracted from the reaction mixture with water. The water preferably is a part of the water obtained as aqueous phases in the phase separator 15 and the second phase separator 79. The water enriched with the catalyst, preferably an alkyl sulfonic acid, preferably butyl sulfonic acid, is recycled into the distillation section 5 of the first reactor 1 via return line 83. The extraction is followed by neutralization and washing 85. The neutralization is carried out with an aqueous caustic solution, preferably an aqueous alkali hydroxide solution and particularly with aqueous sodium hydroxide solution. After neutralization the reaction mixture is washed with water. By neutralization, a neutralized reaction mixture and a stream containing acrylic acid are obtained. The stream containing acrylic acid is fed into an acrylic acid extraction 87 via line 89. The water used for washing the reaction mixture is fed into the stripping column 59 via line 91. 230192W001

[0100] 15

[0101] The neutralized and washed reaction mixture is fed into a first distillation column 93 for removing low boilers. In the first distillation column 93 a top stream is obtained containing low boilers and a bottom stream containing the n-butyl acrylate and high boilers.

[0102] The low boilers are fed into a column 95 for concentrating the low boilers. The concentrated low boilers are withdrawn at the head of the column 95 and fed into a butanol extraction 97. From the butanol extraction 97, low boilers are removed via withdrawal line 19.

[0103] The higher boiling components obtained at the bottom of the column 95 for concentrating the low boilers are fed into the acrylic acid extraction 87. Into the stream containing the higher boiling components additional n-butanol 101 is fed before feeding the stream into the acrylic acid extraction 87.

[0104] For purifying the acrylic acid, water 103 is fed into the acrylic acid extraction 87. The purified acrylic acid is recycled into the first reactor 1 and the water containing the extracted components is fed into the stripping column 59. In the stripping column, butanol is stripped with steam from the water containing streams fed into the stripping column 59. The butanol containing steam is fed into the acrylic acid extraction 87 and a part of the water depleted in butanol is withdrawn from the process via waste water line 21 . Another part of the water depleted in butanol is used as extractant in the butanol extraction 97, in which butanol is extracted with the water from the low boilers. The water and butanol containing stream obtained in the butanol extraction 87 is fed into the stripping column 59 via line 99.

[0105] The n-butyl acrylate and high boilers containing stream obtained at the bottom of the first distillation column 93 is fed into a final distillation column 105.

[0106] In the final distillation column 105, purified n-butyl acrylate is obtained at the head and withdrawn as product stream via a head outlet 107.

[0107] The high boilers collect at the bottom of the final distillation column 105 and are withdrawn via a bottom outlet 109 and fed into the apparatus 35 for concentrating and cracking high boilers, which operates in the same way as described above for the process for producing 2-ethyl hexyl acrylate and the azeotropic process for producing n-butyl acrylate. The components obtained by cracking the high boilers, particularly acrylic acid and n-butyl acrylate are returned into the first reactor 1 via line 41 and the concentrated high boilers are removed from the process via the outlet 39.

[0108] According to the invention, each evaporator used with the first reactor 1 , the second reactor 23, the third reactor 73, the first distillation column 93, the final distillation column 105 and the apparatus 35 for concentrating and cracking high boilers is designed in such a way that the specific heat load is 2 to 20 kW / m2.

Claims

230192W00116Claims1 . A process for producing (meth)acrylic esters comprising:(i) reacting (meth)acrylic acid and an alkanol or isobutene, thereby obtaining a reaction mixture comprising (meth)acrylic ester and (meth)acrylic acid in at least one reactor (3, 23);(ii) working-up the reaction mixture by rectification in at least one column (27, 43; 93, 105), obtaining a product stream, a stream containing low boilers and a stream containing high boilers;(iii) optionally concentrating and cracking the high boilers from the stream containing high boilers in a cracking apparatus (35); each reactor (3, 23), each column (27, 43) and each cracking apparatus (35) being connected with at least one evaporator as a reboiler, in which (meth)acrylic acid and / or (meth)acrylic ester and / or high boilers comprising stream is evaporated and recycled into the respective apparatus, wherein each of the evaporators is designed in such a way that the specific heat load is in a range from 2 to 20 kW / (m2).

2. The process according to claim 1 , wherein each evaporator independently is a shell-and- tube heat exchanger, a plate type heat exchanger or a spiral plate heat exchanger operating as a natural circulation evaporator or a forced circulation evaporator, a Robert evaporator or a forced circulation flash evaporator.

3. The process according to claim 1 or 2, wherein each evaporator connected to a heavies concentration or a cracking apparatus (35) is a forced circulation flash evaporator.

4. The process according to any of claims 1 to 3, wherein the alkanol is selected from the group consisting of methanol, ethanol, n-butanol, and 2-ethyl hexanol.

5. The process according to any of claims 1 to 4, wherein rectification in step (ii) is carried out in at least two columns, comprising a first distillation column (27) for separating high boilers and a final distillation column (43) for finishing distillation, wherein the stream containing high boilers is withdrawn at the bottom of the first distillation column (27), a stream containing the (meth)acrylic ester is withdrawn at the top of the first distillation column (27) and the product stream is withdrawn as a side stream at the lower part of the final distillation column (43).230192W001176. The process according to claim 5, wherein the at least one evaporator connected to the bottom of the first distillation column (27) is a heat exchanger operating as a forced circulation flash evaporator or a natural circulation evaporator, or a Robert evaporator.

7. The process according to any of claims 1 to 4, wherein rectification in step (ii) is carried out in at least two columns, comprising a first distillation column (93) for separating low boilers and a final distillation column (105) for separating high boilers, wherein the stream containing low boilers is withdrawn at the top of the first distillation column (93) and a stream containing the (meth)acrylic ester and high boilers is withdrawn at the bottom of the first distillation column (93) and the product stream is withdrawn at the top of the final distillation column (105) and the stream containing high boilers is withdrawn at the bottom of the final distillation column (105).

8. The process according to claim 7, wherein the at least one evaporator connected to the bottom of the final distillation column (105) is a heat exchanger operating as a forced circulation flash evaporator or a natural circulation evaporator, or a Robert evaporator.

9. The process according to any of claims 1 to 8, wherein at least one reactor (1) is a reactive distillation column, comprising a reaction section and a distillation section, in which low boilers are removed from the reaction mixture.

10. The process according to any of claims 1 to 9, wherein each evaporator in which the concentration of (meth)acrylic ester is 90 wt-% or below, the specific heat load is in a range from 2 to 18 kW / m2and each evaporator in which the concentration of (meth)acrylic ester is above 90 wt-%, the specific heat load is in a range from 2 to 15 kW / m2.

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