Method for producing (meth)acrylic acid

Abstract

The present invention relates to the production of (meth)acrylic acid in a process based on the use of two distillation columns (dehydration and finishing) in the absence of external organic solvents. The invention more particularly relates to the implementation of thermal decomposition preceded by evaporation, hydrolysis at the bottom of the finishing column, followed by recycling of the gas phase of the cracker at the bottom of the dehydration column, and upgrading of the residue of this decomposition by hydrothermal gasification. The process according to the invention makes it possible to improve the energy balance of the process while improving the material balance.

Description

[Technical Field]

[0001] The present invention relates to the production of (meth)acrylic acid in a process based on the use of two distillation columns (dehydration and finishing) in the absence of external organic solvents. The invention more particularly relates to the implementation of thermal decomposition preceded by evaporation, hydrolysis at the bottom of the finishing column, followed by recycling of the gas phase of the cracker at the bottom of the dehydration column, and upgrading of the residue of this decomposition by hydrothermal gasification. The process according to the invention makes it possible to improve the energy balance of the process while improving the material balance. [Background technology]

[0002] The synthesis of acrylic acid used on a large industrial scale employs a reaction for the catalytic oxidation of propylene in the presence of oxygen.

[0003] This reaction is usually carried out in the gas phase and is usually carried out in two steps: the first step carries out the substantially quantitative oxidation of propylene to an acrolein-rich mixture, and then the second step carries out the selective oxidation of acrolein to acrylic acid.

[0004] The gas mixture resulting from the second step consists, apart from acrylic acid, of unconverted compounds resulting from the reactants involved or impurities generated during at least one of the following reaction steps. - light compounds which are non-condensable under the temperature and pressure conditions normally used, i.e. essentially propylene, propane, nitrogen, unconverted oxygen, carbon monoxide and carbon dioxide, which are formed in small amounts by final oxidation; light compounds that are condensable, i.e. essentially water, unconverted acrolein, light aldehydes such as formaldehyde, glyoxal and acetaldehyde, formic acid, acetic acid or propenoic acid, - Compounds with a boiling point slightly higher than that of acrylic acid, namely furfuraldehyde, benzaldehyde, maleic acid and maleic anhydride, benzoic acid, 2-butenoic acid, phenol, protoanemonin, - Finally, heavy compounds derived from the addition of compounds with nucleophilic character onto the double bond of unsaturated carbonyl monomers by Michael reaction.

[0005] The complexity of the gas mixture obtained in this process makes it necessary to carry out a series of operations to recover the acrylic acid contained in this gas effluent and convert it into a grade of acrylic acid suitable for its end use, such as the synthesis of acrylic esters or the production of polymers of acrylic acid and / or acrylic esters.

[0006] Document EP 2066613 describes a process for recovering acrylic acid (AA) without the use of external water or azeotropic solvents, based on the "solvent-free" technology. The process uses only two distillation columns to purify the cooled gas reaction mixture: a dehydration column and a finishing column (or purification column) to which a portion of the bottoms stream from the dehydration column is fed.

[0007] According to this method, the cooled gaseous reaction stream is subjected to dehydration in a first column, the gaseous stream distilled at the top of the column is sent to a condenser, where light compounds are partially condensed and returned to the dehydration column in the form of liquid reflux to absorb acrylic acid, and the non-condensed gaseous effluent is at least partially returned to the reaction, with the remainder being removed.

[0008] The bottoms stream from the dehydration tower is sent to a second tower called the finishing tower. During the purification / finishing step, a stream rich in heavy compounds is removed at the bottom, and a distillate containing water and light by-products is recovered at the top, condensed, and then recycled to the bottom of the first dehydration tower, thus forming a recycle loop.

[0009] A purified acrylic acid stream is recovered in liquid or vapor form by a side draw from the finishing column. The resulting acrylic acid generally has a purity greater than 98.5 wt.% and contains less than 0.5 wt.% water and less than 0.4 wt.% acetic acid.

[0010] The temperature and pressure operating conditions of the finishing column are not critical in this process and can be determined according to the distillation methods known from the prior art. However, preferably, the purification column is operated at a pressure below atmospheric pressure, thus preventing the polymerization of the unsaturated products present and minimizing the formation of heavy by-products. These compounds are heavy products that consume acrylic acid monomer and thereby reduce the recovery efficiency. In the case of the AA production unit, these are essentially as follows: - derivatives of the addition of acrylic acid to the double bond of another acrylic acid molecule: 3-acryloxypropionic acid, also known as "acrylic acid dimer" or "AA dimer"; - derivatives resulting from the addition of acrylic acid to the double bond on an AA dimer molecule to form an "AA trimer" and other oligomers formed by successive additions of acrylic acid to the double bond of a preceding AA oligomer; - Derivatives of the addition of carboxylic acids or water formed as by-products of acrylic acid to the double bonds of AA or the above mentioned oligomers.

[0011] Similar to radical polymerization, this covalent reaction for the formation of Michael derivatives is strongly promoted by temperature. Therefore, the installation of a column with a large number of baffles to meet the quality requirements of acrylic acid brings about disadvantages in terms of product losses, which can only be compensated for by additional high-temperature decomposition treatment of the Michael derivative to regenerate acrylic acid monomer or by recycling this stream from the bottom of the finishing column, optionally to the ester unit.

[0012] Recovery of upgradeable monomers from heavy Michael derivative compounds is difficult in the case of heavy products derived from AA production units. Specifically, during the thermal cracking process to regenerate acrylic acid (which is distilled and upgraded), residues are left behind, the viscosity of which increases significantly to the point where it cannot be extracted from the cracking reactor if high cracking efficiencies are required.

[0013] The main factor limiting the efficiency of the recovery of compounds derived from the Michael reaction contained in the heavy stream from an AA plant is the increase in viscosity of the heavy residue obtained at the bottom of the cracker when the acrylic monomer-rich fraction is vaporized, as described in document FR 2727964. The evaporation of light compounds during cracking leads to the concentration of heavy products in the residue stream and an increase in the viscosity of this stream. However, the residue after cooling must remain sufficiently fluid to be transported and then treated for its destruction. The viscosity of the residue obtained at the end of cracking increases with the residence time of the treated mixture at high temperature and with the amount of light monomers recovered by distillation. To obtain a viscosity of the residue compatible with normal transport conditions and to reduce fouling phenomena, these two parameters must be limited, thereby having the effect of reducing the cracking yield.

[0014] Application FR2206330 accommodates this improvement in regeneration yield in a continuous process without a significant increase in dynamic viscosity close to 1 Pa.s by carrying out the hydrolysis of the heavy by-products at a water:heavy acrylic acid compound weight ratio in the range of 0.1 to 1.3 before carrying out the pyrolysis.

[0015] When light esters (methyl acrylate (MA) or ethyl acrylate (EA)) are produced near an AA production unit, co-cracking of the respective heavy products can improve the situation and make the cracking residue more fluid. The proposed solution allows for the recovery of the maximum amount of AA per cracking operation while managing the viscosity of the resulting residue. Thus, document EP 717031 showed that if cracking is carried out using a mixture of heavy products from an AA production unit and an acrylic ester (EA) production unit, it is possible to improve the recovery efficiency of these upgradeable valuable products compared to the individual cracking of the heavy streams from these units. The effect of adding the heavy product from the ester unit (EAHP) to the heavy product from the AA unit (HAA) is to reduce the viscosity of the final residue. The cracking reaction is carried out at temperatures between 180 °C and 220 °C, at atmospheric pressure, with a residence time of 0.5 to 3 hours, using a mixture with a 9 / 1 to 1 / 9 AA / ester heavy product ratio.

[0016] Document FR 3 110 571 proposes combining partial condensation with a cracking reactor, thereby making it possible to increase the cracking yield without significantly affecting the viscosity of the residue.

[0017] When treating heavy acrylic acid compounds (HAA) alone, it was also envisaged that a solvent be added to the residue.

[0018] Document EP 3255030 teaches the addition of a higher alcohol during the cleavage of the residues, whereby the maleic anhydride present in the residues is converted into maleic esters which are less susceptible to polymerization.

[0019] US6414183 teaches that the discharged residue is diluted with a solvent such as acetic acid, water, and methanol, in the bottom of a distillation column or evaporator at a ratio of 0.1 to 5 times the bottom product before combustion.

[0020] Application WO2021 / 224044 describes a method for decomposing Michael adducts of acrylic acid by diluting them in a solvent having a boiling point at 1013 hPa of at least 170°C and a solubility in water at 25°C of at least 20 g per 100 g of water, the solvent being selected from alcohols such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol and 2-ethoxyethanol, carboxamides such as N,N-dimethylacetamide, N-methylacetamide and N,N-dimethylformamide, sulfoxides such as dimethyl sulfoxide, and sulfones such as sulfolane.

[0021] However, these solutions have several drawbacks, such as the generation of waste materials to be burned, the provision of additional facilities for mixing, or the energy consumption if water is chosen as the solvent to be evaporated. Furthermore, most of these solvents produce nitrogen- or sulfur-containing derivatives when burned.

[0022] Oxidation of organic matter to carbon dioxide and water (incineration) is often used to treat organic residues and generate heating steam. Traditional methods use rapid oxidation of organic fuels to generate heat, which is then transferred to a fluid such as water in a heat exchanger. Heat losses of 10-15% are expected as a result of the losses that inevitably occur in the exhaust stack of a conventional boiler. In addition to possible blockages due to solids fed into the boiler, hot spots due to salt deposits on boiler tubes or ash deposits on tube surfaces exposed to the flame or hot gases can reduce good heat transfer and consequent heat transfer efficiency, or even result in very costly time losses due to tube wall failure.

[0023] First, there is still a need for a solvent-free purification line for the heavy products from the bottom of the finishing tower that is independent of the operation of the (meth)acrylic acid ester unit. Furthermore, it would be desirable to be able to upgrade the final residue from the production of (meth)acrylic acid to transportable methane gas instead of converting it to CO by combustion. [Prior art documents] [Patent documents]

[0024] [Patent Document 1] European Patent Application Publication No. 2066613 [Patent Document 2] French Patent Invention No. 2727964 [Patent Document 3] French Patent Invention No. 2206330 [Patent Document 4] European Patent Application Publication No. 717031 [Patent Document 5] French Patent Invention No. 3110571 [Patent Document 6] European Patent Application Publication No. 3255030 [Patent Document 7] U.S. Patent No. 6,414,183 [Patent Document 8] International Publication No. 2021 / 224044 Summary of the Invention

[0025] The present invention makes it possible to meet the above-mentioned needs. More specifically, the present invention provides an improved method for producing (meth)acrylic acid, which makes it possible to better upgrade the end product, which is usually sent for incineration, by regenerating the starting materials through decomposition and by converting the residue into a combustible gas. The method according to the present invention makes it possible to improve the energy balance of the method while improving the material balance.

[0026] This result is obtained by carrying out thermal decomposition prior to evaporation, hydrolysis at the bottom of the finishing tower, followed by recycling the gas phase of the cracker at the bottom of the dehydration tower, and upgrading the residue of this decomposition by hydrothermal gasification.

[0027] The subject of the present invention is a process for producing technical grade (meth)acrylic acid in the absence of organic solvents from a gaseous reaction mixture comprising (meth)acrylic acid obtained by gas phase oxidation of a precursor of (meth)acrylic acid, comprising the following steps: a) dehydrating said gaseous reaction mixture in a first distillation column, called the dehydration column, to obtain an overhead stream, at least a portion of which is condensed and returned to the dehydration column in the form of reflux, and a bottom stream, at least a portion of which is returned to the bottom of the dehydration column to form a recycle loop; b) distilling at least a portion of said bottom stream from the dehydration column in a second distillation column, called the finishing column, making it possible to separate a bottom stream containing heavy compounds, an overhead stream containing light compounds, at least a portion of which is returned to the dehydration column, and a side stream withdrawn of technical-grade (meth)acrylic acid, c) concentrating the bottoms stream from the finishing tower in an evaporator to obtain a bottoms stream enriched in Michael adduct and a top stream comprising (meth)acrylic acid which is returned to the finishing tower; d) hydrolyzing the evaporator bottoms stream in the presence of water in a hydrolysis unit to produce a hydrolysis product stream; e) pyrolyzing said stream of hydrolysis product in a cracker to produce an overhead stream that is recycled to the dehydration tower, and a bottoms residue; and f) hydrothermal treatment of said residue in the presence of water in a hydrothermal gasifier, producing methane, hydrogen and CO2 type gases at the top and a solid residue and water at the bottom; The present invention provides a method comprising:

[0028] The present invention makes it possible to overcome the drawbacks of the prior art. More specifically, the present invention provides a method for obtaining high-purity technical-grade (meth)acrylic acid having a (meth)acrylic acid purity of more than 98.5% as a specification, and for integrating a process for upgrading the Michael adduct into a reactant that is recycled to the process, thus increasing the productivity of the process and improving the energy balance by upgrading the residue to be removed.

[0029] Other features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying FIG. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is an overall diagram of an acrylic acid purification process including a combination cracker and hydrothermal gasifier preceding an evaporator and hydrolysis unit at the bottom of the finishing tower. DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention provides an improved method for obtaining high purity, technical grade (meth)acrylic acid.

[0032] According to various implementations, the method includes the following features, in appropriate combinations:

[0033] According to one embodiment, the process according to the present invention is a process for producing high purity, technical grade acrylic acid.

[0034] According to one embodiment, the method according to the present invention is a method for producing high purity technical grade methacrylic acid.

[0035] The present invention will be explained below by taking the example of a method for producing acrylic acid. The method according to the present invention may also include other preliminary, intermediate or subsequent steps, provided that they do not adversely affect the production of purified acrylic acid.

[0036] According to one embodiment of the present invention, the acrylic acid precursor is acrolein.

[0037] According to one embodiment of the present invention, acrolein is obtained by oxidation of propylene or oxydehydrogenation of propane.

[0038] According to one embodiment of the present invention, the gaseous reaction mixture comprising acrylic acid obtained by oxidation in the gas phase of an acrylic acid precursor comprises carbon of renewable origin.

[0039] According to one embodiment of the present invention, the acrylic acid precursor is derived from glycerol, 3-hydroxypropionic acid or 2-hydroxypropionic acid (lactic acid).

[0040] According to a preferred embodiment of the present invention, the gaseous reaction mixture comprises acrylic acid derived from propylene obtained by a two-stage oxidation process.

[0041] According to one embodiment, the finishing column is a conventional distillation column.

[0042] According to one embodiment, the finishing column is a dividing wall column.

[0043] According to one embodiment, the finishing tower operates under a reduced pressure of 5 to 60 kPa.

[0044] According to one embodiment, an aldehyde reducing chemical may be injected into the feed of the finishing tower.

[0045] According to one embodiment, the evaporator located at the bottom of the finishing tower is a thin-film evaporator operating under reduced pressure of 0.5 to 100 kPa.

[0046] According to one embodiment, the product from the top of the evaporator is recycled to the side draw line of the finishing column.

[0047] According to one embodiment, the product from the top of the evaporator is recycled to the bottom of the finishing tower below the side draw line.

[0048] According to one embodiment, the pressure in the hydrolysis unit varies between 0.1 and 2 MPa, preferably between 0.5 and 1.5 MPa.

[0049] According to one embodiment, the water / adduct weight ratio in the hydrolysis unit is in the range of 0.1 to 1.3, limits included.

[0050] According to one embodiment, the temperature in the hydrolysis unit varies between 80°C and 200°C, preferably between 150°C and 200°C.

[0051] According to one embodiment, the pyrolysis reaction occurs in the absence of a catalyst.

[0052] According to one embodiment, the decomposition temperature is between 140°C and 260°C, preferably between 160°C and 210°C.

[0053] According to one embodiment, the pyrolysis is carried out on an adduct of acrylic acid.

[0054] According to one embodiment, the pyrolysis is carried out on a mixture of an acrylic acid adduct and an ester adduct.

[0055] According to one embodiment, the residence time of the reaction mixture in the decomposition reactor is between 0.5 hours and 10 hours, preferably between 4 hours and 10 hours.

[0056] According to one embodiment, the pyrolysis reaction takes place under atmospheric pressure or slight pressure (up to 0.2 MPa).

[0057] According to one embodiment, the cracker overhead product is recycled to the dehydration tower boiler.

[0058] According to one embodiment, the product from the top of the cracker is mixed with the product from the top of the finishing tower.

[0059] According to one embodiment, the bottom stream (residue) from the reactor obtained at the end of the pyrolysis operation has a dynamic viscosity of less than 1 Pa.s, preferably less than 10 Pa.s, measured at a temperature of 100°C, for example using a Brookfield "CAP 1000" viscometer of the cone / plate type.

[0060] According to one embodiment, a polymerization inhibitor is used in at least one step of the process according to the invention. The polymerization inhibitor can be added at different points together with the introduction of the reactants or at the top of the distillation column, exchanger and condenser.

[0061] Polymerization inhibitors that may be used include, for example, phenothiazine (PTZ), hydroquinone (HQ), hydroquinone monomethyl ether (HQME), di-tert-butyl para-cresol (BHT), para-phenylenediamine, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), di-tert-butylcatechol or TEMPO derivatives such as OH-TEMPO, manganese acetate, alone or in any proportion of their mixtures, at a content in the reaction medium that may be between 50 ppm and 5000 ppm, generally between 150 ppm and 1000 ppm, optionally in the presence of depleted air.

[0062] To make the inhibitor more effective, it is advisable to inject oxygen, air, or "depleted" air containing 7% O2 into the bottom of the column. Preferably, the amount of oxygen injected corresponds to a content of 0.2% to 0.5% of the amount of organic vapor in the column.

[0063] According to one embodiment, the hydrothermal gasifier includes a first reactor, a second reactor, and a liquid gas separator.

[0064] According to one embodiment, the residue is injected directly into the gasification, and the water required for the hydrothermal treatment is injected elsewhere.

[0065] According to one embodiment, the residue is mixed with the water necessary for the hydrothermal treatment before being introduced into the gasification.

[0066] According to the embodiment, the hydrothermal gasification is carried out at a temperature of 350 to 450° C. and a pressure of 25 MPa.

[0067] According to said embodiment, the hydrothermal gasification comprises a gasifier for separating salt at the bottom and a mixture of gas and liquid at the top.

[0068] According to one embodiment, the hydrothermal gasification includes a separator for separating salts under critical conditions, a gasifier, and a liquid-gas separator.

[0069] According to one embodiment, the hydrothermal gasification includes a salt separator, a gasifier including a catalyst, and a liquid gas separator.

[0070] According to one embodiment, the concentration of residue / water+residue in the salt separator is between 10 g / l and 400 g / l.

[0071] According to one embodiment, the water used to carry out the hydrothermal gasification may be demineralized water, water from drilling, or weak mineral water.

[0072] According to one embodiment, the water leaving the gasifier free of organic compounds can advantageously be recycled to the separator feed or to the hydrolyzer feed.

[0073] According to one embodiment, the obtained and separated salt can be upgraded as a fertilizer.

[0074] According to one embodiment, a proportion of 94% to 99% of the carbon introduced into the gasification is upgraded in the form of gas.

[0075] According to one embodiment, the gas resulting from the gasification is composed of 40-70% methane, 5-20% hydrogen and 20-40% carbon dioxide.

[0076] According to one embodiment, the gas can be further fractionated to isolate methane from other compounds.

[0077] According to the process shown in Figure 1, a gaseous reaction mixture 1 comprising acrylic acid obtained by gas-phase oxidation of a precursor of acrylic acid is fed to a first distillation column 10. The gaseous reaction mixture, which generally contains a water / acrylic acid weight ratio between 0.3 and 2, preferably between 0.3 and 1.2, can be pre-cooled before being subjected to dehydration in the dehydration column 10.

[0078] In addition to water and acrylic acid, the reaction mixture contains non-condensable light products such as nitrogen, oxygen, carbon monoxide and carbon dioxide, as well as various light or heavy by-products of different chemical nature, which may be acrolein, light aldehydes such as formaldehyde or acetaldehyde, heavy aldehydes such as furfuraldehyde or benzaldehyde, light acids such as formic acid, acetic acid or propionic acid, heavy acids such as maleic acid, benzoic acid or 2-butenoic acid, and protoanemonin, a heavy compound of the lactone type.

[0079] The dehydration column produces an overhead stream 2, at least a part of which is condensed in a condenser 13 and returned to the dehydration column in the form of reflux 7 to absorb acrylic acid, and the other part (stream 14) comprising non-condensable light compounds is generally sent partially or completely to a purification device or partially recycled to another step of the process for producing acrylic acid, preferably a step located upstream of the reactor for producing reaction mixture 1.

[0080] The purpose of the dehydration step is to remove in the overhead stream not only most of the water present in the reaction mixture, but also non-condensable and condensable light compounds. The dehydration step produces an overhead stream 2 containing most of the water and light compounds, with very small amounts of acrylic acid and heavy compounds, and a bottoms stream 15 depleted in light compounds, containing most of the acrylic acid together with heavy by-products, the weight content of water being generally less than 10%, preferably less than 7%.

[0081] A typical weight composition of the bottoms stream 15 from the dehydration column essentially comprises acrylic acid (70-90%), acetic acid (2-20%), water (2-15%) and heavy by-products.

[0082] The dehydration tower generally contains 5 to 50 theoretical plates, preferably 20 to 30 theoretical plates.

[0083] Advantageously, the dehydration column is operated at atmospheric pressure or slightly higher, 1.5×10 5 It operates at absolute pressures up to Pa.

[0084] Advantageously, the temperature at the top of the dehydration tower is at least 40° C., preferably between 40° C. and 80° C. The temperature of the bottom stream from the dehydration tower preferably does not exceed 120° C.

[0085] The bottoms stream 15 from the dehydration tower is sent at least in part (stream 3) to the top of a second distillation tower 16, called a refining or finishing tower, where an overhead stream 8 and a bottoms stream 9 are separated.

[0086] A portion 20 of the bottoms liquid stream 15 from the dehydration tower is sent to heat exchanger 12, which may be a heater or a cooler, and is reinjected into the dehydration tower to form a bottoms recycle loop. Preferably, a portion 11 from the bottoms loop is reinjected between the gaseous reaction mixture feed and the top of the dehydration tower.

[0087] The remainder of liquid stream 15 (stream 3) is sent as feed to finishing tower 16.

[0088] Finishing column 16 is a conventional distillation column typically containing 5 to 30 theoretical plates, preferably 8 to 20. This distillation column is combined at the bottom with at least one reboiler 17 and at the top with a condenser 19.

[0089] The temperature and pressure in column 16 are not critical and can be determined according to distillation methods known from the prior art. Preferably, however, finishing column 16 operates at a pressure below atmospheric pressure, allowing it to operate at a relatively low temperature, thus preventing polymerization of any unsaturated products present and minimizing the formation of heavy by-products.

[0090] Advantageously, the finishing column operates under an absolute pressure in the range of 5 kPa to about 60 kPa, with the temperature of the top stream advantageously being between 40°C and about 90°C and the temperature of the bottom stream being between 60°C and 120°C.

[0091] The overhead gas stream 8 from the finishing tower is sent to a condenser 19, and the exiting liquid stream 4 is returned to the dehydration tower and combined with the stream from the bottom loop of the dehydration tower. The overhead stream 8 contains water and condensable light by-products.

[0092] The side draw 5, located in the bottom third of the finishing tower, preferably above the theoretical plate 3 starting from the bottom of the tower, contains technical-grade acrylic acid having a purity of more than 98.5%.

[0093] Stream 9, separated at the bottom of the finishing column, also contains most of the heavy by-products, particularly Michael addition products such as 3-acryloyloxypropionic acid, maleic anhydride / acid, benzoic acid, and polymerization inhibitors. A typical weight composition of bottoms stream 9 essentially consists of acrylic acid (70-90%), polymerization inhibitors (0.5-2%), and heavy by-products (5-30%).

[0094] This stream 9 can be partially recycled to the bottom of the finishing tower or sent via line 6 to a falling film evaporator.

[0095] The evaporator 21 operates under reduced pressure between 0.5 kPa and 60 kPa and in the temperature range between 50° C. and 150° C. After condensation and addition of stabilizers, the gas stream 22 essentially comprising acrylic acid is returned to the column one theoretical plate below the side draw.

[0096] Preferentially, residue 25 also contains an acrylic acid content greater than 10% and less than 40% in order to limit its viscosity. This stream 25 and water 34 are introduced under pressure into a reactor and allowed to hydrolyze for a time ranging from 1 hour to 5 hours at a temperature ranging from 100°C to 170°C and an autogenous pressure of 1 atmosphere to 2 MPa. This reactor can be a fully stirred reactor, a reactor with an external recirculation loop and exchanger, or a piston reactor. Stream 27 is then fed to the cracker.

[0097] The cracker 28 is equipped with a liquid-gas separator and an external recirculation loop fed by a tubular exchanger heated by steam at a pressure between 1.5 and 3 MPa. The cracking time is between 1 and 10 hours at near-atmospheric pressure. After complete condensation and inhibitor addition, the overhead stream 32 is mixed with the stream 4 returning to the dehydration tower.

[0098] The residue 30 and water 37 are injected by two circuits via high-pressure pumps into a hydrothermal gasifier 33 at a temperature range between 350°C and 450°C and a pressure of 25 MPa. The apparatus includes: - a first reactor for separating the salt at the bottom of the reactor from the water and the organic solution; - a second gasification reactor containing a catalyst, which allows the conversion of the organic products into gas to be completed; - a liquid-gas separator that makes it possible, at the bottom, to recover an aqueous phase 35 that can be recycled to the inlet of the separator or to the inlet of the hydrolyzer, and a gas phase 36 rich in methane that can be upgraded to produce electricity, providing the energy necessary for the operation of the gasification, as well as the energy of the reaction and purification lines of the method, or that can be carried away elsewhere.

[0099] This hydrothermal gasification can be carried out in batch mode or, preferably, in continuous mode.

[0100] The following examples illustrate the present invention without, however, limiting its scope.

[0101] <Experimental section> In the examples, unless otherwise indicated, percentages are given by weight and the following abbreviations are used: PTZ: Phenothiazine AA: acrylic acid MA: Maleic acid H2O: Water DiAA: acrylic acid dimer AA3: Acrylic acid trimer Heavy compounds: Oligomers with a weight greater than AA3 HQ: Hydroquinone ACOH: acetic acid HAA: Heavy Acrylic Acid Compounds [Example]

[0102] <Solvent-free pilot purification test> The characteristics of the solvent-free process are as follows: Dehydration tower: 300mm in diameter Theoretical plate number: 22 Finishing tower: diameter 300mm Theoretical plate number: 17 Side draw: 14

[0103] The compositions obtained at various points in the process are shown in Table 1.

[0104] [Table 1]

[0105] The bottoms of the column contain about 11% DiAA and are concentrated in a thin film evaporator.

[0106] <Concentration at the bottom of the column using a thin-film evaporator> The concentration at the bottom of the column was simulated by Aspen.

[0107] The operating conditions and compositions are shown in Table 2 below.

[0108] [Table 2]

[0109] Concentration of the bottom product by evaporation is efficient. In fact, the AA content in the bottom is reduced. Conversely, this evaporation produces acrylic acid with a purity of 98% at the top, but this product is not efficient enough to be mixed with the technical-grade acrylic acid obtained by withdrawing the side stream, and therefore must be recycled to the column.

[0110] <Pyrolysis with or without prior hydrolysis> This example corresponds to examples 1 and 2 of application FR 2206330 and shows the advantage of treating the evaporator bottoms by pyrolysis prior to hydrolysis in terms of the efficiency of this treatment. The results are shown in Table 3.

[0111] It should also be noted that the overhead product therefore consists of approximately 70% AA and 30% water, which is very close to the composition at the top of the finishing tower, making mixing of these two fluids very easy.

[0112] [Table 3]

[0113] <Hydrothermal gasification> Hydrothermal gasification is illustrated by the very similar case of a Michael adduct, namely the Michael adduct of a heavy butyl acrylate compound.

[0114] The heavy ABU compound mixture consists of: - Butanol <0.1% - Butyl acrylate (5-10%) - Butyl hydroxypropionate (BHP): 1-3% - Butyl butoxypropionate (BPB) 70-80% - Butyl acryloxypropionate (AA / ABU) 4-6% - Dibutyl maleate: 2~5% - Phenothiazines: 1-3%.

[0115] 33 g / h of heavy ABU compounds and 970 g / h of water are introduced into the separator and catalytic reactor via two different pipes, both operating at 400 °C and 25 MPa. After a 6-hour test under stabilization conditions, the heavy ABU compounds are converted into a gas mixture with the following volumetric composition: 51% CH4, 34% CO2 and 19% H2. The energy content of this gas corresponds to 7096 kWh / ton ABU. The amount of TOC (total organic carbon) is less than 1 mg / l.

Claims

1. A method for producing technical grade (meth)acrylic acid in the absence of organic solvents from a gaseous reaction mixture containing (meth)acrylic acid obtained by gas phase oxidation of a precursor of (meth)acrylic acid, the method comprising the steps of: a) dehydrating said gaseous reaction mixture in a first distillation column, called the dehydration column, to obtain an overhead stream, at least a portion of which is condensed and returned to the dehydration column in the form of reflux, and a bottom stream, at least a portion of which is returned to the bottom of the dehydration column to form a recycle loop; b) distilling at least a portion of said bottom stream from the dehydration column in a second distillation column, called the finishing column, making it possible to separate a bottom stream containing heavy compounds, an overhead stream containing light compounds, at least a portion of which is returned to the dehydration column, and a side stream withdrawn of technical-grade (meth)acrylic acid; c) concentrating the bottoms stream from the finishing tower in an evaporator to obtain a bottoms stream enriched in Michael adduct and a top stream comprising (meth)acrylic acid which is returned to the finishing tower; d) hydrolyzing the evaporator bottoms stream in the presence of water in a hydrolysis unit to produce a hydrolysis product stream; e) pyrolyzing said stream of hydrolysis product in a cracker to produce an overhead stream that is recycled to the dehydration tower, and a bottoms residue; and f) In a hydrothermal gasifier, the residue is subjected to hydrothermal treatment in the presence of water, producing methane, hydrogen and CO ２ a step in which a gas of the type is produced and a solid residue and water are produced at the bottom; A method comprising:

2. 2. The method of claim 1, wherein the evaporator located at the bottom of the finishing tower is a thin-film evaporator operating under a reduced pressure of 0.5 to 100 kPa.

3. 3. The process of claim 1 or 2, wherein the product from the top of the evaporator is recycled to the bottom of the finishing column below the side draw line.

4. The process according to any one of claims 1 to 3, wherein the temperature of the hydrolysis unit varies between 80°C and 200°C.

5. 5. The process according to claim 1, wherein the weight ratio of water / adduct in the hydrolysis unit is in the range of 0.1 to 1.3, limits included.

6. 6. The method according to claim 1, wherein the pyrolysis is carried out on an adduct of acrylic acid.

7. 6. The method according to claim 1, wherein the pyrolysis is carried out on a mixture of an adduct of acrylic acid and an adduct of an acrylic ester.

8. 8. The process of any one of claims 1 to 7, wherein the product from the top of the cracker is mixed with the product from the top of the finishing tower.

9. The method according to any one of claims 1 to 8, wherein the hydrothermal gasification comprises a separator for separating salts under supercritical conditions, a gasifier, and a liquid-gas separator.

10. 10. The method of claim 9, wherein the residue / water+residue concentration in the separator is between 10 g / l and 400 g / l.

11. 11. The method of any one of claims 1 to 10, wherein the gasification produces a gas composed of 40% to 70% methane, 5 to 20% hydrogen and 20 to 40% carbon dioxide.

12. The method according to any one of claims 1 to 11, wherein the water leaving the gasifier, free from organic compounds, is recycled to the separator feed or to the hydrolyzer feed.

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