Improved process for manufacturing alkyl (METH)acrylate

Abstract

The present invention relates to the manufacturing of alkyl (meth)acrylate by transesterification catalysed by one or more chemical species that precipitate upon contact with water. More specifically, it relates to an improved process for manufacturing alkyl (meth)acrylate, leading to a high yield of a product that meets purity standards, and comprising a step of reusing the heavy by-products generated during this manufacture owing to a step of thermal and catalytic cracking, generating recycled reagents in the process as well as methane and mineral salts obtained by hydrothermal gasification of this cracking residue alone, mixed with the residues of other alkyl esters or with alcohols.

Description

[0001] IMPROVED MANUFACTURING PROCESS

[0002] ALKYL (MET)ACRYLATE

[0003] TECHNICAL FIELD

[0004] The present invention relates to the manufacture of alkyl (meth)acrylate by transesterification catalyzed by one or more chemical species that precipitate upon contact with water. More specifically, it relates to an improved process for manufacturing alkyl (meth)acrylate, leading to high productivity of a product meeting purity standards, and comprising a step of valorizing the heavy by-products generated during this manufacture through a thermal and catalytic cracking step, generating reagents recycled in the process as well as methane and mineral salts obtained by hydrothermal gasification of this cracking residue alone, in mixture with residues of other alkyl esters or with alcohols.

[0005] PREVIOUS ART AND TECHNICAL PROBLEM

[0006] Alkyl (meth)acrylates corresponding to formula (I): in which:

[0007] - Ri is a hydrogen atom or a methyl radical,

[0008] - R2 is an alkyl radical, linear or branched, containing 4 to 12 carbon atoms, and may contain a nitrogen atom, are obtained by transesterification reaction between a heavy alcohol of formula R2-OH (II), in which R2 is as defined above, and a light alkyl (meth)acrylate of formula (III): in which Ri is as defined above and R3 represents the methyl or ethyl radical.

[0009] The reaction is an equilibrium reaction that is generally carried out in a stirred reactor, in the presence of a transesterification catalyst and at least one polymerization inhibitor. The azeotropic mixture of light (III) methacrylate and light alcohol R3OH generated during transesterification is continuously withdrawn during the reaction in order to shift the equilibrium towards the production of alkyl (meth)acrylate.

[0010] The economic and ecological balance of industrial manufacturing processes for (meth)acrylic derivatives depends on the recycling of fractions generated during the purification process of raw products, these fractions being likely to contain unreacted reactants, valuable secondary products and / or the desired compound, in non-negligible quantities, as well as the reaction catalyst.

[0011] The transesterification reaction synthesizing (meth)acrylic derivatives also leads to the formation of impurities, usually in the form of "heavy" compounds (i.e., having a molecular mass greater than 233 u, such as Michael adducts resulting from Michael addition reactions of an alcohol molecule of formula R2OH or R3OH to the double bond of a (meth)acrylic ester of formula (I) or (III), or oligomers or polymers.

[0012] Thus, Michael adducts of formula (IV), (V), (VI) and (VII) may be present in the crude reaction and generally exhibit a higher boiling point than the desired product (I).

[0013] These by-products are generally concentrated in a so-called heavy fraction, separated during the purification process of crude alkyl (meth)acrylate. This heavy fraction may include, within the meaning of the present invention, not only Michael adducts (IV), (V), (VI) and (VII), but also generally the transesterification catalyst, polymerization inhibitors, as well as a minor fraction of residual reagents and / or the desired product (I).

[0014] The process for manufacturing alkyl acrylate by transesterification described in document WO 2014 / 096648 does not envisage the recovery of this heavy fraction, which was intended for destruction.

[0015] The valorization of the heavy fraction was considered by the applicant in document WO 2018 / 104677 describing a process for manufacturing dimethylaminoalkyl acrylate of formula (la). The crude product purification section consists of successive distillations including a heavy fraction separation step, called tailing, followed by a removal of unreacted reagents, called topping, and finally a rectification of the product to obtain a high-purity alkyl ester. The heavy fraction obtained at the bottom of the tailing column is partially recycled to the reactor and partially sent to a film evaporator, which allows the recycling of the lighter compounds in the reaction. The heavy residue is sent to a valorization unit called a cracker (or possibly partially recycled to the reactor).The process thus involves the recovery of Michael adducts, which can generate new reactants through cracking, and partial recycling of the catalyst. However, cracking always generates a final residue containing catalyst, which must be incinerated.

[0016] Documents WO 2013 / 045786 and WO 2016 / 124837 highlight the use of antifouling additives and / or fluxing agents (viscosity reducers) in the cracking process. The final residue obtained by this thermal and / or catalytic cracking method must be treated with additives to prevent fouling from the process due to the presence of mineral salts, and more specifically, a portion of the catalyst. This residue is recovered through incineration.

[0017] The oxidation of organic matter (incineration) into carbon dioxide and water is a long-established process, frequently used to treat organic waste and produce heating steam. In the conventional steam energy process, rapid oxidation of organic fuels is often used to generate heat, which is then transferred to a fluid such as water in a heat exchanger. A heat loss of 10-15% is expected due to losses that inevitably occur in the exhaust column of conventional boilers.

[0018] In addition to possible blockages due to solids feeding the boiler, hot spots due to salt deposits on boiler tubes or ash deposits on tube faces exposed to flame or hot gases reduce proper heat transmission and consequently heat transfer efficiency, and can even lead to very costly downtime losses due to tube wall rupture.

[0019] According to O. Boutin and JC. Ruiz (Techniques de l'ingénieur J7010, 2021 edition), hydrothermal gasification can be implemented to treat organic effluents on a pilot scale. This conversion technology makes it possible to convert wet biomass (>70% moisture) into synthesis gas (a mixture of methane, hydrogen, and carbon dioxide) and to separate the mineral salts present in the input. It has been implemented, for example, in the gasification of algae for hydrogen production (FR 3030562), the treatment of primary sludge from a wastewater treatment plant, and the catalytic gasification of pig manure, possibly mixed with eucalyptus wood.

[0020] In applications WO 2024 / 084149, WO 2024 / 069082 and PCT / FR2024 / 050667, the applicant described the valorization of the final residues from the manufacture of acrylic acid or acrylic esters obtained by direct esterification from acrylic acid, using hydrothermal gasification. In these documents, water is injected simultaneously with the residues to be treated in the salt separation device, without this causing a precipitation problem.

[0021] However, in the case of the transesterification process for manufacturing alkyl (meth)acrylate, the residues to be treated by hydrothermal gasification contain up to 10% by weight of transesterification catalyst. Thus, document WO 2014 / 096648 describes an alkyl titanate, preferably ethyl titanium and / or 2-octyl titanate in 2-octanol solution, as the transesterification catalyst. These salts precipitate upon contact with water (equation 1) and form TiO₂, which is insoluble in water.

[0022] Ti(OEt)4 + 2H2O → TiO2? + 4 EtOH (equation 1)

[0023] Implementing hydrothermal gasification of the organic fraction of the residues would not be possible in this scenario due to the precipitation of metallic salts in the presence of water, which would clog the equipment. In contrast, the recovery of the organic fraction is achieved by transforming it into a gaseous mixture of methane, hydrogen, and carbon dioxide, which at least meets the energy requirements of the process and can be used in the gas network of an industrial site or exported.

[0024] Thus, in the case of a process for manufacturing alkyl (meth)acrylate by transesterification catalyzed by one or more chemical species which precipitate on contact with water, it remains to determine the conditions of this hydrothermal gasification for this ultimate residue, alone or in mixture with other residues from the manufacture of (meth)acrylic acid and (meth)acrylic esters.

[0025] Consequently, there is a need to eliminate heavy by-products and to valorize the ultimate residues from the manufacture of alkyl (meth)acrylates.

[0026] One of the objectives of the present invention is therefore to provide an improved process for the production of alkyl (meth)acrylates, enabling the optimization of the valorization of the various resulting residues.

[0027] SUMMARY OF THE INVENTION

[0028] The present invention describes a process for manufacturing alkyl (meth)acrylate having purification steps generating a so-called heavy fraction, consisting among other things of Michael adducts, polymerization inhibitors and transesterification catalyst, and involving the cracking of this heavy fraction by means of a cracking unit as described in documents WO 2013 / 045786 and WO 2016 / 124837.

[0029] It complements the scheme for the purification and valorization of secondary products by cracking described in these documents, by associating it with hydrothermal gasification which makes it possible to transform the residue of the cracking into valuable gas (methane, hydrogen, CO2), which at a minimum makes it possible to provide energy to the process or valorize in a gas network on the one hand and into valuable mineral salts on the other.

[0030] The invention relates to a process for manufacturing alkyl (meth)acrylate by transesterification of a light (meth)acrylate with a so-called heavy alcohol in the presence of a transesterification catalyst which precipitates in the presence of water and polymerization inhibitor(s), leading to the obtaining of a crude reaction mixture, said process comprising at least the following steps: i) tailing said crude reaction mixture in a first distillation column allowing to obtain: - at the top, a stream composed of the (meth)acrylate and the unreacted reactants;

[0031] - at the bottom, a stream comprising heavy by-products, polymerization inhibitors and said catalyst; ii) the upper stream of said first distillation column is subjected to a topping column enabling separation:

[0032] - at the top, the unreacted reagents which are recycled to a transesterification section;

[0033] - at the bottom, a stream containing (meth)acrylate, which is conveyed to a rectification column for purification; iii) the bottom stream of said first distillation column is subjected to thermal cracking to obtain:

[0034] - at the top, high-quality products such as alkyl (meth)acrylate, which are recycled to feed the topping column,

[0035] - at the base, an ultimate residue, iv) said ultimate residue is subjected to hydrothermal gasification in a hydrothermal gasification unit comprising:

[0036] - a separator allowing mineral salts to be obtained at the bottom in the form of a solid residue;

[0037] - a gasification reactor comprising a catalyst which allows the conversion of organic products into gases such as methane, hydrogen and carbon dioxide, obtained at the top, and to obtain at the bottom, water free of carbon residue which is recycled at the inlet of the separator.

[0038] The present invention overcomes the drawbacks of the prior art. More particularly, it provides a process for obtaining a high-purity alkyl (meth)acrylate with a mass purity of ester greater than 99.5%, enabling the cracking of Michael adducts, thereby increasing the productivity of the process and improving the energy balance, by recovering the residue to be eliminated in the form of valuable mineral salts and gases.

[0039] Other features and advantages of the invention will become clearer from the detailed description that follows, with reference to the attached Figure 1.

[0040] Figure 1 shows an overall diagram of the process for synthesizing alkyl (meth)acrylate according to the invention, involving stepwise distillation with the combination of a thermal and catalytic cracker and hydrothermal gasification equipment. DETAILED DESCRIPTION OF THE INVENTION

[0041] The invention relates to a process for manufacturing alkyl (meth)acrylate by transesterification of a light (meth)acrylate with a heavy alcohol, in the presence of a transesterification catalyst that precipitates in the presence of water. For the sake of simplicity, said transesterification manufacturing process is exemplified by the following characteristics.

[0042] The term "alkyl (meth)acrylate" refers to alkyl T-acrylate or alkyl methacrylate.

[0043] According to one embodiment, the invention relates to a process for manufacturing alkyl acrylate.

[0044] According to one embodiment, the invention relates to a process for manufacturing alkyl methacrylate.

[0045] The transesterification reaction leads to the obtaining of a crude reaction mixture containing residual alkyl (meth)acrylate, light (meth)acrylate and heavy alcohol, catalyst and impurities resulting from side reactions.

[0046] According to one embodiment, heavy alcohol is a linear or branched, primary or secondary alcohol, comprising between 4 and 12 carbon atoms, which may include at least one nitrogen atom.

[0047] According to one embodiment of the invention, the heavy alcohol is an amino alcohol of formula (Ilb):

[0048] HO-AN (R'2)(R'3) (Ilb) in which:

[0049] - A is an alkylene radical, linear or branched, in C1-C5

[0050] - R'2 and R'3, identical or different from each other, each represent a C1-C4 alkyl radical.

[0051] Heavy alcohols can be, for example, N,N-dimethyl aminoethanol (DMAE), N,N-diethyl aminoethanol, N,N-dimethyl aminopropanol.

[0052] According to a preferred embodiment of the invention, the amino alcohol is N,N-dimethyl aminoethanol (DMAE), and Tester (meth)acrylic is N,N-dimethyl aminoethyl acrylate (ADAME).

[0053] According to one embodiment of the invention, the heavy alcohol is an alcohol of formula R2OH in which R2 represents a linear or branched alkyl chain in the C4-C12 group, preferably in the C5-C12 group. The heavy alcohol may be primary or secondary. Examples of heavy alcohols include 2-ethyl hexanol, 2-octanol, and 2-propyl heptanol.

[0054] According to one embodiment, the light alkyl (meth)acrylate is selected from methyl (meth)acrylate and ethyl (meth)acrylate.

[0055] According to one embodiment, the light alkyl acrylate is methyl acrylate or ethyl acrylate.

[0056] According to one embodiment, the light alkyl methacrylate is methyl methacrylate or ethyl methacrylate.

[0057] The light (meth)acrylate / light alcohol azeotrope is withdrawn to shift the transesterification reaction equilibrium towards the production of the desired alkyl (meth)acrylate. The azeotrope is advantageously recycled back to the light (meth)acrylate manufacturing unit since it is free of troublesome impurities that could form (meth)acrylic byproducts.

[0058] After the transesterification step, the process according to the invention includes purification steps to provide an ester corresponding to the aforementioned purity standards.

[0059] According to various embodiments, the process comprises the following characteristics, possibly combined. The indicated contents are expressed by weight, unless otherwise stated. The indicated ranges of values ​​include the limits.

[0060] With reference to Figure 1, the crude reaction mixture (1) obtained in the transesterification section is subjected to the steps described below.

[0061] The crude reaction mixture (1) is subjected to de-encapping in a first distillation column, also called a de-encapping column, to obtain:

[0062] - at the top, a stream (2) composed of the desired product (ester), light acrylate and unreacted heavy alcohol,

[0063] - at the bottom, a stream (3) containing Michael adducts, polymerization inhibitors and transesterification catalyst.

[0064] The head flow (2) of the tailing column is subjected to a tailing step to obtain:

[0065] - at the top, a stream (4) containing the unreacted light acrylate and heavy alcohol which are recycled to the transesterification reaction,

[0066] - at the bottom, a stream (5) containing the desired product (ester).

[0067] The bottom stream (5) from the topping column containing the desired product is rectified in a purification (rectification) column to obtain:

[0068] - at the top, the high purity ester (6), - at the bottom, a stream (7) containing mostly ester which is recycled at the tailing column.

[0069] The foot flow (3) of the tailing column is distributed as follows:

[0070] - part of the flux (3) is returned to the transesterification reaction in order to recycle part of the transesterification catalyst,

[0071] - part of the stream (3) is sent to a film evaporator allowing to obtain at the top, a stream (8) containing ester and unreacted reactants which are directed to the tailing column, and at the bottom a stream (9) containing the heavy products corresponding to the Michael adducts, the transesterification catalyst and the polymerization inhibitors, which is directed to the cracking unit.

[0072] A final portion of the stream (3) is sent directly to the cracking unit. Streams (3) and (9) feed the cracker. In one embodiment, the cracker is a double-jacketed reactor or a reboiler topped with a distillation column.

[0073] Cracking can be thermal and / or catalytic; it can be carried out in batch or continuous.

[0074] The cracking stage yields:

[0075] - at the top, a stream (10) containing the valuable products from cracking (reagents and desired product), which are recycled to feed the topping column or the transesterification reactor;

[0076] - at the bottom, the ultimate cracking residue (11).

[0077] Typically, the ultimate residue (11) is treated by hydrothermal gasification to obtain mineral salts, methane, hydrogen and carbon dioxide type gases, and water.

[0078] The hydrothermal gasification unit includes:

[0079] - a salt separator, which is a reactor allowing the separation at the bottom of it of the solid residues (12) comprising mineral salts in the form of solid residues, and of the solution comprising water and organic products at the top;

[0080] - a gasification reactor, which transforms these into gaseous products of the methane, hydrogen and carbon dioxide type (13), and to obtain at the bottom water free of carbon residues (14) which can be cleverly recycled at the inlet of the salt separator.

[0081] In one embodiment, in the hydrothermal gasification unit, the cracking residue (11) and water (stream 14) are injected via two circuits using high-pressure pumps into a separator at a temperature range of 350°C to 450°C and a pressure of 25 MPa. The salt separator separates the salt (stream 12) at its base from the water and organic solution at its head. A gasification reactor comprising a catalyst completes the conversion of the organic products into gas.

[0082] According to one embodiment, the hydrothermal gasification unit includes a liquid / gas separator which allows the recovery at the bottom of an aqueous phase (14) which can be recycled at the inlet of the separator, and a gaseous phase (flow 13) rich in methane which can be used to produce electric current which can provide the energy necessary for the operation of the gasification but also for those of the reaction and the purification train of this process or be exported elsewhere.

[0083] This hydrothermal gasification can be carried out in batch or continuous mode.

[0084] In one embodiment, the catalyst for the transesterification reaction is an organometallic catalyst derived from titanium, such as tetraalkyl titanates, or from tin, such as dibutyltin oxide; a zirconium derivative, such as zirconium(IV) tetra(acetylacetonate); a zinc derivative, such as zinc(II) bis(acetylacetonate) or basic zinc carbonate; or an alkali or alkaline earth metal alkoxide, such as lithium, sodium, or magnesium alkoxides. These catalysts have an aqueous solubility of less than 5 mg / L, which implies salt precipitation upon contact between the thermal cracking residues and water.

[0085] In one embodiment, the polymerization inhibitor is selected from hydroquinone, phenothiazine, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) derivatives such as 4-OH-TEMPO, and di-tert-butyl catechol, alone or in mixtures. The residence time in the reactor is generally between 1 and 8 hours, preferably between 3 and 5 hours. Advantageously, 50 to 5000 ppm of polymerization inhibitor are introduced into the various distillation columns.

[0086] According to one embodiment, the tailing and topping steps can be carried out in a single column having an internal wall and called a partition column allowing the lateral withdrawal of the desired alkyl (meth)acrylate and obtaining at the top of the column the unreacted reactants, which are recycled to the transesterification section, and at the bottom of the column the heavy fraction containing the Michael adducts and the transesterification catalyst.

[0087] In one embodiment, thermal cracking can be catalytic and carried out using a transesterification catalyst. In another embodiment, the streams (3) and (9) feeding the cracker can be supplemented with a fluxing agent or anti-deposition agent such as a phosphoric ester or an alkyl phthalate. The optional addition of fluxing agent or anti-deposition agent corresponds to a mass concentration of 0.001% to 1%, preferably 0.1% to 0.5%.

[0088] According to one embodiment, cracking is carried out at a temperature of 100°C to 250°C, preferably from 150°C to 200°C.

[0089] According to one embodiment, the mass composition of the main compounds of the residue at the bottom of the cracker (flux 11) is as follows:

[0090] - Michael's by-products, ranging from 5% to 20%,

[0091] - Heavy alcohol content of 0 to 10%,

[0092] - (meth)acrylate sought at 0 to 5%,

[0093] - Transesterification catalyst of 20% to 45%,

[0094] - Phenothiazine: 1-5%,

[0095] - heavy (Molecular mass > 233) qsp 100%.

[0096] Depending on the embodiment, hydrothermal gasification is carried out at a temperature of 350-450°C and a pressure of 25 MPa.

[0097] Depending on the embodiment, hydrothermal gasification includes a gasifier allowing the salt to be separated at the bottom of the gasifier and a gas and liquid mixture to be obtained at the top, followed by a gas-liquid separator.

[0098] In one embodiment, the hydrothermal gasification system includes a separator for separating salt under supercritical water conditions, a gasifier, and a gas-liquid separator. In one embodiment, the concentration of organic product / water + organic product in the salt separator is between 10 g / L and 400 g / L.

[0099] According to one embodiment, the cracking residue can be diluted with an anhydrous alkyl radical alcohol comprising 1 to 12 carbon atoms, linear or branched, to carry out hydrothermal gasification.

[0100] According to one embodiment, the cracking residue is mixed with a phosphoric ester or alkyl phthalate derivative, and in all proportions, to carry out hydrothermal gasification, in order to reduce the viscosity and / or the salt concentration.

[0101] In one embodiment, the cracking residue is mixed with another cracking residue from a different (meth)acrylic ester and / or (meth)acrylic acid production unit, provided there is chemical compatibility and no water present, and in any proportion, to carry out hydrothermal gasification. In one embodiment, the hydrothermal gasification process is purely thermal and non-catalytic.

[0102] According to one embodiment, the cracking residue is injected into the hydrothermal gasification unit separately but simultaneously with biological sludge from wastewater treatment.

[0103] According to one embodiment, the water used to carry out the hydrothermal gasification is water from condensate of the acrylic ester manufacturing processes.

[0104] According to one embodiment, the water used to carry out hydrothermal gasification is water containing organic products which is usually intended to be treated in a biological industrial wastewater treatment plant.

[0105] According to one embodiment, the water used to carry out hydrothermal gasification is demineralized water, well water, or low-mineralized water.

[0106] According to one embodiment, the water exiting the gasifier (14), free of organic compounds, is advantageously recycled to feed the salt separator.

[0107] According to one embodiment, the salts obtained and separated (12) are used for the manufacture of new mineral or organometallic compounds.

[0108] According to one embodiment, 94% to 99% of the carbon introduced into the gasification unit is recovered in the form of gas (13).

[0109] According to one embodiment, the gas (13) from the hydrothermal gasification unit is composed (by volume) of 40-70% methane, 5-20% hydrogen and 20-40% carbon dioxide.

[0110] According to one embodiment, the gases from the hydrothermal gasification unit can be further fractionated to isolate methane from other compounds.

[0111] The following examples illustrate the present invention without however limiting its scope.

[0112] EXPERIMENTAL SECTION

[0113] In the examples, percentages are given by weight unless otherwise stated.

[0114] Example of hydrothermal gasification of heavy ADAME (N,N-dimethylaminoethyl acrylate)

[0115] The ADAME heavy mixture (formula 1a) is composed of:

[0116] - N,N-Dimethylaminoethanol < 0.1% - N,N-Dimethylaminoethyl acrylate (0-5%)

[0117] - N,N-dimethylethoxypropionate of N,N-dimethylaminoethyl 1-15%

[0118] - N,N-dimethylaminoethanol 5-25%

[0119] - Phenothiazine: 1-5%.

[0120] - heavy + catalyst: qsp 100%

[0121] Such that the proportion of carbon atoms per unit mass is 59%, and the mass concentration of salts is 7.5%.

[0122] A mixture of heavy ADAME diluted in anhydrous methanol (one part by volume to one part by volume) and 970 g / h of water is introduced through two separate pipes into a separator and a catalytic reactor, both operating at 400°C and 25 MPa. After 6 hours of testing under stabilized conditions, the heavy ADAME is transformed into a salt identified as being predominantly titanium oxide with a purity level ranging from 78% to 87% and a salt recovery rate of 100%, and a gaseous mixture with the following volume composition: 56% CEE; 27% CO2 and 16% H2.

[0123] Example of hydrothermal gasification of heavy ADAME and heavy butyl acrylate

[0124] Heavy ADAME and heavy residue Butyl Acrylate are mixed at a mass ratio of 1:2, such that the proportion of carbon atoms per unit mass is 61%, and the mass concentration of salts is 3.5%.

[0125] The mixture of heavy ADAME and heavy butyl acrylate was introduced at a rate of 1 g / h, along with 699 g / h of water, through two separate pipes into a separator and a catalytic reactor, both operating at 400°C and 25 MPa. After a 1-hour test, the mixture of heavy ADAME and butyl acrylate was transformed into a salt identified as being predominantly titanium dioxide with a purity of 66% and a salt recovery rate of 100%, and a gaseous mixture with the following volumetric composition: 63% CEE, 29% CO2, and 8% H2. The amount of TOC (total organic carbon) present in the residual water was less than 2 mg / L. Counter-example of hydrothermal gasification of heavy ADAME

[0126] The mixing of heavy ADAME upstream of the salt separator with water at pressure and ambient temperature resulted in an opaque inhomogeneous mixture containing precipitated salts causing the clogging of the hydrothermal gasification unit, in particular the salt separator.

Claims

DEMANDS 1. A process for manufacturing alkyl (meth)acrylate by transesterification of a light (meth)acrylate with a so-called heavy alcohol in the presence of a transesterification catalyst which precipitates in the presence of water and polymerization inhibitor(s), leading to the obtaining of a crude reaction mixture, said process comprising at least the following steps: i) stemming of said crude reaction mixture in a first distillation column enabling the obtaining of: - at the top, a stream composed of (meth)acrylate and unreacted reagents; - at the bottom, a stream comprising heavy by-products, polymerization inhibitors and said catalyst; ii) the upper stream of said first distillation column is subjected to a topping column enabling separation: - at the top, the unreacted reagents which are recycled to a transesterification section; - at the bottom, a stream containing (meth)acrylate, which is conveyed to a rectification column for purification; iii) the bottom stream of said first distillation column is subjected to thermal cracking to obtain: - at the top, high-quality products such as alkyl (meth)acrylate, which are recycled to feed the topping column, - at the base, an ultimate residue, iv) said ultimate residue is subjected to hydrothermal gasification in a hydrothermal gasification unit comprising: - a separator allowing mineral salts to be obtained at the bottom in the form of a solid residue; - a gasification reactor comprising a catalyst which allows the conversion of organic products into gases such as methane, hydrogen and carbon dioxide, obtained at the top, and to obtain at the bottom, water free of carbon residue which is recycled at the inlet of the separator.

2. A process according to claim 1, wherein the catalyst for the transesterification reaction is an organometallic type catalyst derived from titanium, such as tetraalkyl titanates, or derived from tin such as dibutyl tin oxide, a zirconium derivative such as zirconium (IV) tetra(acetylacetonate), a zinc derivative such as zinc (II) bis(acetylacetonate) or basic zinc carbonate, or an alkali metal or alkaline earth alkoxide such as lithium, sodium or magnesium alkoxides.

3. A process according to any one of claims 1 or 2, wherein the polymerization inhibitor is selected from hydroquinone, phenothiazine, TEMPO derivatives (2,2,6,6-tetramethyl-l-piperidinyloxy) such as 4-OH-TEMPO, di-tert-butylcatechol, alone or in mixture.

4. A process according to claims 1 to 3, wherein the cracking is carried out at a temperature of 100°C to 250°C, preferably from 150°C to 200°C.

5. A method according to claims 1 to 4, wherein the mass composition of the main components of the residue at the bottom of the cracker (flow 11) is as follows: - Michael's revenues of 5% to 20%, - heavy alcohol content of 0 to 10%, - (meth)acrylate sought at 0 to 5%, - transesterification catalyst of 20% to 45%, - phenothiazine: 1-5%, - heavy (Molecular mass > 233) qsp 100%.

6. A process according to one of claims 1 to 5, wherein hydrothermal gasification is carried out at a temperature of 350-450°C and a pressure of 25 MPa.

7. A process according to one of claims 1 to 6, wherein the cracking residue is mixed with another cracking residue from another (meth)acrylic ester and / or (meth)acrylic acid production unit, subject to compatibility chemical and absence of water, and in all proportions, to achieve hydrothermal gasification.

8. A process according to any one of claims 1 to 6, wherein the cracking residue is mixed with an anhydrous alcohol or a phosphoric ester or alkyl phthalate derivative, and in any proportions, to carry out hydrothermal gasification, in order to reduce the viscosity and / or the salt concentration.

9. A process according to any one of claims 1 to 7, wherein 94% to 99% of the carbon introduced into the gasification unit is recovered in the form of gas.

10. A process according to claim 1 to 8, wherein the residue / water + residue concentration in the hydrothermal gasification feed is between 10 g / L and 400 g / L.

11. A process according to any one of claims 1 to 9, wherein the gasification generates a gas (flow 13) composed by volume ratio of 40 to 70% methane, 5-20% hydrogen and 20-40% carbon dioxide.

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