Method for producing (METH)acrylic acid

Abstract

The present invention relates to the production of (meth)acrylic acid by means of a method based on the use of two distillation columns (a dehydration column and a finishing column) in the absence of an external organic solvent. The invention relates more particularly to the implementation of thermal cracking preceded by evaporation, hydrolysis of the bottom of the finishing column followed by recycling of the gaseous phase of the cracker at the bottom of the dehydration column, and utilization of the cracking residue by way of hydrothermal gasification. The method according to the invention makes it possible to improve the energy balance of the process while at the same time improving its material balance.

Description

[0001] DESCRIPTION

[0002] Title: PROCESS FOR THE MANUFACTURE OF (METH)ACRYLIC ACID

[0003] TECHNICAL FIELD

[0004] The present invention relates to the production of (meth)acrylic acid in a process based on the use of two distillation columns (a dehydration column and a finishing column) in the absence of an external organic solvent. It more particularly relates to the use of thermal cracking preceded by evaporation, hydrolysis of the bottom of the finishing column followed by recycling of the gaseous phase from the cracker to the bottom of the dehydration column and recovery of the residue from this cracking 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.

[0005] TECHNICAL BACKGROUND AND TECHNICAL PROBLEM

[0006] The acrylic acid synthesis process, used on a large industrial scale, involves a catalytic oxidation reaction of propylene in the presence of oxygen.

[0007] This reaction is generally carried out in the gas phase and most often in two stages: the first stage carries out the substantially quantitative oxidation of propylene into a mixture rich in acrolein, then the second stage carries out the selective oxidation of acrolein into acrylic acid.

[0008] The gas mixture resulting from the second stage consists, apart from acrylic acid, of untransformed compounds resulting from the reagents involved or of impurities generated during at least one of the reaction stages, namely:

[0009] - light compounds which are incondensable under the temperature and pressure conditions usually used, namely essentially: propylene, propane, nitrogen, unconverted oxygen, carbon monoxide and dioxide formed in small quantities by ultimate oxidation;

[0010] - light condensable compounds, essentially: water, unconverted acrolein, light aldehydes such as formaldehyde, glyoxal and acetaldehyde, formic acid, acetic acid or propionic acid;

[0011] - compounds with a boiling point slightly higher than that of acrylic acid: furfuraldehyde, benzaldehyde, maleic acid and anhydride, benzoic acid, 2-butenoic acid, phenol, protoanemonin; - finally, heavy compounds derived from the addition of compounds with nucleophilic properties to the double bond of unsaturated carbonyl monomers, by Michael reaction.

[0012] The complexity of the gas mixture obtained in this process requires a set of operations to be carried out to recover the acrylic acid contained in this gaseous effluent and transform it into a grade of acrylic acid compatible with its final use, for example the synthesis of acrylic esters or the production of acrylic acid polymers and / or acrylic esters.

[0013] Document EP 2 066 613, based on so-called "solvent-free" technology, describes a process for recovering acrylic acid (AA) without using external water or azeotropic solvent. This process uses only two distillation columns to purify the cooled gaseous reaction mixture: a dehydration column, and a finishing column (or purification column) fed by part of the bottom flow of the dehydration column.

[0014] 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 in which the light compounds are partly condensed and returned to the dehydration column in the form of liquid reflux to absorb the acrylic acid, the uncondensed gaseous effluent being returned at least partly to the reaction and the remainder being eliminated.

[0015] The bottom flow of the dehydration column is sent to a second column called the finishing column. During the purification / finishing step, a flow rich in heavy compounds is removed at the bottom and a distillate comprising water and light by-products is recovered at the top which is condensed and then recycled to the bottom of the first dehydration column, thus forming a recirculation loop.

[0016] A stream of purified acrylic acid is recovered in liquid or vapor form by side draw from the finishing column. The acrylic acid obtained is generally of purity greater than 98.5% by mass and contains less than 0.5% by mass of water and less than 0.4% by mass of acetic acid.

[0017] The operating conditions in temperature and pressure for the finishing column are not critical in this process, and can be determined in accordance with known state-of-the-art distillation methods. However, preferably, the purification column is operated at a pressure below atmospheric pressure, thus avoiding the polymerization of the unsaturated products present, minimizing the formation of heavy by-products. These compounds are heavy products that reduce the recovery efficiency by consuming the monomeric acrylic acid. In the case of an AA production unit, these are essentially:

[0018] - addition derivatives of acrylic acid on the double bond of another acrylic acid molecule: 3-acryloxypropionic acid also called "dimeric acrylic acid" or "AA dimer";

[0019] - of acrylic acid addition derivatives on the double bond on an AA dimer molecule, to form the "AA trimer" and other oligomers formed by successive additions of acrylic acid on the double bonds of the preceding AA oligomers,

[0020] - carboxylic acid addition derivatives formed as by-products of acrylic acid or water on the double bond of AA or the oligomers mentioned above.

[0021] Like free radical polymerization, this covalent reaction of Michael derivative formation is strongly favored by temperature. Therefore, the installation of columns with a high number of rectification plates to meet the quality requirements of acrylic acid leads to disadvantages in terms of product loss, which can only be compensated by an additional high-temperature cracking treatment of the Michael derivatives to regenerate the acrylic acid monomer or by recycling this stream from the finishing column bottom, possibly to an ester unit.

[0022] The recovery of valuable monomers from heavy Michael-derived compounds is difficult in the case of heavy compounds from an AA production unit. Indeed, during the thermal cracking process that regenerates acrylic acid, which is distilled and recovered, a residue remains whose viscosity increases sharply when high cracking yields are sought, until it can no longer be extracted from the cracking reactor.

[0023] The main limiting factor in the efficiency of regeneration of compounds derived from the Michael reaction contained in heavy streams from AA workshops is the increase in the viscosity of the heavy residue obtained at the bottom of the cracker, when the fraction rich in acrylic monomers has been vaporized, as described in document FR2727964. The vaporization of light compounds during cracking leads to a concentration of heavy products in the residue stream and an increase in the viscosity of this stream. However, the residue must remain sufficiently fluid after cooling to be transported and then treated for destruction. The viscosity of the residue obtained at the end of cracking increases with the residence time at high temperature of the mixture to be treated and with the quantity of light monomers recovered by distillation.To obtain a residue viscosity compatible with normal transfer conditions and to reduce fouling phenomena, it is necessary to limit these two parameters, which has the effect of reducing the cracking efficiency. Application FR 2206330 satisfies this improvement in regeneration efficiency in a continuous process, without a significant increase in dynamic viscosity close to 1 Pa.s, by implementing hydrolysis of the heavy by-products with a water:heavy acrylic acid mass ratio ranging from 0.1 to 1.3 before implementing thermal cracking.

[0024] In the case where there is a production of light esters (methyl acrylate (MA) or ethyl acrylate (EA)) close to the AA production unit, co-cracking of the respective heavy products can improve the situation, by making the cracking residue more fluid. The proposed solution makes it possible to recover the maximum amount of AA per cracking operation while managing the viscosity of the residue formed. Thus, in document EP 717 031 it has been shown that it is possible to improve the efficiency of the recovery of these valuable noble products, if the cracking is carried out from a mixture of heavy products coming from an AA production unit and an acrylic ester (EA) production unit, compared to the individual cracking of the heavy streams from these units. The effect of adding heavy products coming from the ester units (LEA) to the heavy products coming from an AA unit (LAA) is to reduce the viscosity of the final residue.The cracking reaction is carried out from mixtures with a ratio of heavy AA / heavy ester of 9 / 1 to 1 / 9, at a temperature of 180 to 220°C, under atmospheric pressure, during a residence time of 0.5 to 3 hours.

[0025] Document FR3110571 proposes to associate partial condensation with the cracking reactor, which makes it possible to increase the cracking yield without any significant effect on the viscosity of the residue.

[0026] In the case of treatment of heavy acrylic acid (LAA) alone, it has also been considered to add a solvent to the residue.

[0027] EP 3255030 teaches the addition of higher alcohols during cleavage of the residue, with maleic anhydride present in the residue being converted to maleic acid esters which are less susceptible to polymerization.

[0028] US 6414183 teaches the dilution of the rejected residue with solvents such as acetic acid, water and methanol. This dissolution is carried out on a distillation column or evaporator foot in a proportion of 0.1 to 5 times relative to the foot product before combustion treatment.

[0029] Application WO 2021 / 224044 describes a process for decomposing Michael adducts of acrylic acid, by dilution 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, said 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.

[0030] However, these solutions have several disadvantages, such as the generation of waste to be burned, the provision of additional equipment for the preparation of the mixture or the energy consumption if we assume that water is chosen as the solvent to be vaporized. In addition, most of these solvents generate nitrogen or sulfur derivatives during burning.

[0031] The oxidation of organic matter (incineration) to carbon dioxide and water is often used to treat organic residues and produce heating steam. In the conventional process, rapid oxidation of organic fuels is used to produce heat, which is then transferred in a heat exchanger to a fluid such as water. A heat loss of 10-15% is expected due to the losses necessarily occurring in the exhaust stack of conventional boilers. In addition to possible blockage due to solids feeding the boiler, hot spots due to salt deposits on the boiler tubes, or ash deposits on the tube faces exposed to the flame or hot gases, reduce good heat transmission and therefore the heat transfer efficiency, or even cause very costly time losses due to rupture of the tube walls.

[0032] On the one hand, there remains a need for a solvent-free purification train for heavy products from the bottom of the finishing column, which is independent of the operation of the (meth)acrylic ester units. On the other hand, it is desirable to be able to recover the final residue from the production of (meth)acrylic acid into exportable methane gas, instead of converting it into CO2 by combustion.

[0033] SUMMARY OF THE INVENTION

[0034] The present invention makes it possible to meet the above-mentioned needs. More particularly, the invention provides an improved process for the production of (meth)acrylic acid which makes it possible to better recover the final products usually sent to incineration, by regenerating the raw materials by cracking and transforming the residue into combustible gas. The process according to the invention makes it possible to improve the energy balance of the process while improving the material balance.

[0035] This result is obtained by implementing thermal cracking preceded by evaporation, hydrolysis of the bottom of a finishing column followed by recycling of the gas phase from the cracker to the bottom of the dehydration column and recovery of the residue from this cracking by hydrothermal gasification.

[0036] The present invention relates to a process for manufacturing technical (meth)acrylic acid, in the absence of organic solvent, 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) a step of dehydrating said gaseous reaction mixture in a first distillation column called a dehydration column, leading to a top stream of which at least part is condensed and returned to the dehydration column in the form of reflux, and to a bottom stream of which at least part is returned to the lower part of the dehydration column to form a recirculation loop;b) a step of distilling at least a portion of said bottom stream of the dehydration column in a second distillation column called a finishing column, making it possible to separate a bottom stream containing heavy compounds, a top stream containing light compounds, at least a portion of which is returned to the dehydration column, and a side draw-off stream of technical (meth)acrylic acid; c) a step of concentrating said bottom stream of the finishing column in an evaporator leading to a bottom stream concentrating the Michael adducts, and a top stream comprising (meth)acrylic acid which is returned to the finishing column; d) a step of hydrolyzing said bottom stream of the evaporator, in the presence of water, in a hydrolyzer, leading to obtaining a stream of hydrolyzed products;e) a step of thermal cracking of said stream of hydrolyzed products, in a cracker, leading to the production of a top stream which is recycled to the dehydration column and a bottom residue; and f) a step of hydrothermal treatment of said residue, in the presence of water, carried out in hydrothermal gasification equipment leading to the production of methane, hydrogen and CO2 type gases at the top, and solid residues and water at the bottom.;

[0037] The present invention overcomes the disadvantages of the state of the art. More particularly, it provides a process for obtaining a high-purity technical (meth)acrylic acid having as a specification a (meth)acrylic acid purity greater than 98.5%, integrating a process for recovering the Michael adducts into recycled reagents in the process, thus increasing the productivity of the process and improving the energy balance by recovering the residue to be eliminated. Other characteristics and advantages of the invention will become more apparent upon reading the detailed description which follows, with reference to the attached Figure 1.

[0038] Fig.l: overall diagram of the acrylic acid purification process with the association of an evaporator at the bottom of the finishing column, a thermal cracker preceded by a hydrolyzer with hydrothermal gasification equipment.

[0039] DETAILED DESCRIPTION OF THE INVENTION

[0040] The invention provides an improved process for obtaining high purity technical (meth)acrylic acid.

[0041] According to various embodiments, said method comprises the following characteristics, possibly combined.

[0042] According to one embodiment, the method according to the invention is a method for manufacturing high-purity technical acrylic acid.

[0043] According to one embodiment, the method according to the invention is a method for manufacturing high purity technical methacrylic acid.

[0044] The invention is described below using the example of the process for manufacturing acrylic acid. The process according to the invention may further comprise other preliminary, intermediate or subsequent steps provided that they do not adversely affect the production of purified acrylic acid.

[0045] According to one embodiment of the invention, the precursor of acrylic acid is acrolein.

[0046] According to one embodiment of the invention, acrolein is obtained by oxidation of propylene or by oxy dehydrogenation of propane.

[0047] According to one embodiment of the invention, the gaseous reaction mixture comprising acrylic acid obtained by gas-phase oxidation of an acrylic acid precursor comprises carbon of renewable origin.

[0048] According to one embodiment of the invention, the precursor of acrylic acid is derived from glycerol, 3-hydroxypropionic acid or 2-hydroxypropionic acid (lactic acid).

[0049] According to a preferred embodiment of the invention, the gaseous reaction mixture comprises acrylic acid derived from propylene obtained according to a two-step oxidation process. According to one embodiment, the finishing column is a conventional distillation column.

[0050] According to one embodiment, the finishing column is a column with a separating wall.

[0051] According to one embodiment, the finishing column operates under a reduced pressure of 5 to 60 kPa.

[0052] According to one embodiment, an aldehyde reducing chemical agent may be injected into the feed of the finishing column.

[0053] According to one embodiment, the evaporator placed at the bottom of the finishing column is a film evaporator working under reduced pressure of 0.5 to 100 kPa.

[0054] According to one embodiment, the overhead product from the evaporator is recycled into the side draw line of the finishing column.

[0055] According to one embodiment, the head product of the evaporator is recycled to the bottom of the finishing column, below the lateral withdrawal line.

[0056] According to one embodiment, the pressure in the hydrolyser varies between 0.1 and 2 MPa, preferably between 0.5 and 1.5 MPa.

[0057] According to one embodiment, the water / adduct mass ratio in the hydrolyser varies from 0.1 to 1.3 inclusive.

[0058] According to one embodiment, the temperature in the hydrolyser varies between 80 and 200°C, preferably between 150 and 200°C.

[0059] According to one embodiment, the thermal cracking reaction takes place in the absence of a catalyst.

[0060] According to one embodiment, the cracking temperature is between 140 and 260°C, preferably between 160 and 210°C.

[0061] According to one embodiment, the thermal cracking is carried out on acrylic acid adducts.

[0062] According to one embodiment, the thermal cracking is carried out on a mixture of acrylic acid adducts and esters.

[0063] According to one embodiment, the residence time of the reaction mixture in the cracking reactor is between 0.5 h and 10 h, preferably between 4 h and 10 h.

[0064] According to one embodiment, the thermal cracking reaction takes place at atmospheric pressure or under slight pressure (maximum 0.2 MPa). According to one embodiment, the product at the top of the cracker is recycled to the boiler of the dehydration column.

[0065] According to one embodiment, the overhead product of the cracker is mixed with the overhead product of the finishing column.

[0066] According to one embodiment, the reactor bottom flow (residue) obtained at the end of the thermal cracking 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+" cone-plate type viscometer.

[0067] According to one embodiment, polymerization inhibitors are used in at least one of the steps of the manufacturing process according to the invention. The addition of the polymerization inhibitors can be done at different locations, with the introduction of the reactants or at the top of the distillation column, exchangers and condensers.

[0068] Examples of polymerization inhibitors that may be used include phenothiazine (PTZ), hydroquinone (HQ), hydroquinone monomethyl ether (EMHQ), di-tert-butyl para-cresol (BHT), paraphenylenediamine, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), di-tert-butylcatechol, or TEMPO derivatives, such as OH-TEMPO, manganese acetate alone or their mixtures in all proportions, at contents in the reaction medium that may be between 50 ppm and 5000 ppm, optionally in the presence of depleted air, but generally at contents between 150 ppm and 1000 ppm.

[0069] To make the inhibitors more effective, oxygen, air or air said to be depleted to 7% O2 should be injected at the bottom of the column. Preferably, the quantity of oxygen injected corresponds to a content of 0.2% to 0.5% relative to the quantity of organic vapor in the column.

[0070] According to one embodiment, said hydrothermal gasification equipment comprises a first reactor, a second reactor and a gas-liquid separator.

[0071] According to one embodiment, the residue is injected as is into the gasification and the water necessary for the hydrothermal treatment is injected elsewhere.

[0072] According to one embodiment, the residue is mixed with the water necessary for the hydrothermal treatment before introduction into the gasification.

[0073] According to the embodiment, hydrothermal gasification is carried out at a temperature of 350-450°C and a pressure of 25 MPa. According to the embodiment, hydrothermal gasification comprises a gasifier for separating the salt at the bottom, and a gas and liquid mixture at the top.

[0074] According to one embodiment, the hydrothermal gasification comprises a separator for separating the salt under critical conditions, a gasifier and a gas-liquid separator.

[0075] According to one embodiment, the hydrothermal gasification comprises a salt separator, a gasifier comprising a catalyst, and a gas-liquid separator.

[0076] According to one embodiment, the concentration of residue / water + residue in the salt separator is between 10g / l and 400g / l.

[0077] According to one embodiment, the water used to carry out the hydrothermal gasification may be demineralized water, water from drilling, or weakly mineralized water.

[0078] According to one embodiment, the water leaving the gasifier, free of organic compounds, can advantageously be recycled to the feed of the separator or to the feed of the hydrolyser.

[0079] According to one embodiment, the salts obtained and separated can be used as fertilizers.

[0080] According to one embodiment, a proportion of 94% to 99% of the carbon introduced into the gasification is recovered in the form of gas.

[0081] According to one embodiment, the gas resulting from gasification is composed of 40-70% methane, 5-20% hydrogen and 20-40% carbon dioxide.

[0082] In one embodiment, the gases may be further fractionated to isolate methane from other compounds.

[0083] According to the method shown in Figure 1, a gaseous reaction mixture 1 comprising acrylic acid obtained by gas-phase oxidation of an acrylic acid precursor feeds a first distillation column 10. The gaseous reaction mixture comprising a water / acrylic acid mass ratio generally between 0.3 and 2, preferably between 0.3 and 1.2, may be pre-cooled before being subjected to dehydration in the dehydration column 10.

[0084] The reaction mixture comprises, in addition to water and acrylic acid, light incondensable products such as nitrogen, oxygen, carbon monoxide and dioxide, as well as various light or heavy by-products of different chemical nature, which may be light aldehydes such as acrolein, 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 lactone compound.

[0085] The dehydration column leads to a head stream 2 of which at least a part is condensed in a condenser 13 and returned to the dehydration column in the form of reflux 7 to absorb the acrylic acid, the other part (stream 14) comprising the incondensable light compounds generally being sent partially or totally to a purification device or recycled in part to other stages of the acrylic acid production process, preferably in a stage located upstream of the reactor for producing the reaction mixture 1.

[0086] The purpose of the dehydration step is to eliminate in a head stream most of the water present in the reaction mixture, but also the incondensable light compounds and the condensable light compounds. It generates a head stream 2 comprising most of the water and light compounds, with acrylic acid and heavy compounds in very small quantities, and a bottom stream 15 depleted in light compounds comprising most of the acrylic acid with heavy by-products, and a mass content of water generally less than 10%, preferably less than 7%.

[0087] A typical mass composition of the bottom stream 15 from the dehydration column consists mainly of acrylic acid (70-90%), acetic acid (2-20%), water (2-15%), and heavy by-products.

[0088] The dehydration column generally comprises 5 to 50 theoretical plates, preferably 20 to 30 theoretical plates.

[0089] Advantageously, the dehydration column operates at atmospheric pressure or slightly higher, up to an absolute pressure of 1.5 x 10 5 Pa.

[0090] Advantageously, the temperature in the upper part of the dehydration column is at least 40°C, preferably between 40°C and 80°C. The temperature of the bottom stream of the dehydration column preferably does not exceed 120°C.

[0091] The bottom flow 15 of the dehydration column is sent at least in part (flow 3), to the top of a second distillation column 16, called the purification column or finishing column, in which a top flow 8 and a bottom flow 9 are separated.

[0092] A portion 20 of the liquid flow 15 from the bottom of the dehydration column is sent to a heat exchanger 12 which may be a heater or a cooler and reinjected into the dehydration column, so as to constitute a recirculation loop at the bottom. Preferably, the portion 11 of the bottom loop is reinjected between the feed of the reaction gas mixture and the top of the dehydration column.

[0093] The remainder (stream 3) of the liquid stream 15 is sent to feed the finishing column 16.

[0094] The finishing column 16 is generally a conventional distillation column comprising from 5 to 30 theoretical plates, preferably from 8 to 20 theoretical plates. This distillation column is associated at the bottom with at least one reboiler 17 and at the top with a condenser 19.

[0095] The temperature and pressure in column 16 are not critical, and can be determined in accordance with known distillation methods of the state of the art. However, preferably, finishing column 16 operates at a pressure below atmospheric pressure, allowing operation at relatively low temperatures, thereby avoiding polymerization of unsaturated products present, and minimizing the formation of heavy by-products.

[0096] Advantageously, the finishing column operates under an absolute pressure ranging from 5 kPa to approximately 60 kPa, the temperature of the head stream being advantageously between 40°C and approximately 90°C, and the temperature of the bottom stream being between 60°C and 120°C.

[0097] The overhead gas stream 8 from the finishing column is sent to the condenser 19, and the outgoing liquid stream 4 is returned to the dehydration column, mixed with the stream from the bottom loop of the dehydration column. The overhead stream 8 includes water and the light condensable by-products.

[0098] The side draw-off stream 5 located in the first third of the bottom of the finishing column, preferably above the theoretical plate 3 from the bottom of the column, comprises technical acrylic acid with a purity of >98.5%.

[0099] Stream 9 separated at the bottom of the finishing column comprises most of the heavy by-products, including Michael adducts such as 3-acryloxypropionic acid, maleic anhydride / acid, benzoic acid, and polymerization inhibitors. A typical mass composition of bottom stream 9 comprises mainly acrylic acid (70-90%), polymerization inhibitors (0.5-2%), and heavy by-products (5-30%).

[0100] This stream 9 can be partly recycled to the bottom of the finishing column, or sent to a falling film evaporator via line 6. The evaporator 21 operates under a reduced pressure of 0.5 kPa to 60 kPa and in a temperature range of 50°C to 150°C. After condensation and addition of stabilizer, the gas stream 22 comprising essentially acrylic acid is returned to the column one theoretical stage below the side draw-off.

[0101] Preferably, the residue 25 still comprises an acrylic acid content > 10% and <40% in order to limit the viscosity of the latter. This flow 25 as well as water 34 are introduced under pressure into a reactor allowing hydrolysis for a time between 1 h and 5 h under an autogenous pressure of atmospheric pressure to 2 MPa in a temperature range from 100°C to 170°C. This reactor can be a perfectly stirred reactor, a reactor equipped with an external recirculation loop and an exchanger or a piston reactor. The flow 27 then feeds a thermal cracker.

[0102] This cracker 28 comprises a gas-liquid separator, an external recirculation loop fed by a tubular exchanger heated by steam having a pressure between 1.5 and 3 MPa. The cracking duration is between 1 h and 10 h under a pressure close to atmospheric pressure. After total condensation and addition of inhibitor, the overhead stream 32 is mixed with stream 4 which returns to the dehydration column.

[0103] The residue 30 is injected and water 37 is injected by two circuits via high pressure pumps into a hydrothermal gasification equipment 33 in a temperature range between 350°C and 450°C and a pressure of 25 MPa. This equipment comprises:

[0104] - a first reactor allowing the salt at the bottom to be separated from the water and organic solution,

[0105] - a second gasification reactor comprising a catalyst, which allows the conversion of organic products into gas to be completed, and

[0106] - a gas-liquid separator which allows the recovery at the bottom of an aqueous phase 35 which can be recycled at the inlet of the separator or at the inlet of the hydrolyser, and a gaseous phase 36 rich in methane which can be used to produce current, which can provide the energy necessary for the operation of the gasification but also for that of the reaction and the purification train of this process or be exported elsewhere.

[0107] This hydrothermal gasification can be carried out in batch mode, or preferably in continuous mode. The examples below illustrate the present invention without, however, limiting its scope.

[0108] EXPERIMENTAL PART

[0109] In the examples, percentages are given by weight unless otherwise stated and the following abbreviations have been used:

[0110] PTZ: Phenothiazine

[0111] AA: Acrylic acid

[0112] MA: Maleic Acid

[0113] H20: Water

[0114] DiAA: Dimer of acrylic acid

[0115] AA3: Trimer of acrylic acid

[0116] Heavy: Oligomers with a mass greater than AA3

[0117] HQ: Hydroquinone

[0118] ACOH: Acetic Acid

[0119] LAA: heavy acrylic acid

[0120] Solvent-free nutrient pilot test

[0121] The characteristics of the solvent-free process are as follows:

[0122] Dehydration column: diameter 300mm

[0123] Number of theoretical floors: 22

[0124] Finishing Column: diameter 300mm

[0125] Number of theoretical floors: 17

[0126] Side draw: 14

[0127] The compositions obtained at the different points of the process are indicated in Table 1: [Table 1]

[0128] The column base contains approximately 11% DiAA which will be concentrated on a film evaporator.

[0129] Concentration of the column foot on a film evaporator

[0130] The column foot concentration was simulated on Aspen.

[0131] The operating conditions and compositions are shown in Table 2 below:

[0132] [Table 2]

[0133] Concentrating the bottom product by evaporation is effective. In fact, there is a reduction in the AA content in the bottom. However, this evaporation, which generates acrylic acid at the top with a purity of 98%, is not effective enough for this product to be mixed with the technical acrylic acid obtained from the side draw-off and must therefore be recycled into the column. Thermal cracking with or without prior hydrolysis

[0134] This example corresponds to examples 1 and 2 of application FR 2206330 and shows the advantage of treating the bottom of the evaporator by cracking preceded by hydrolysis on the effectiveness of this treatment. The results are presented in Table 3.

[0135] It should also be noted that the top product will therefore be composed of approximately 70% AA and 30% water, which is very close to the composition of the top of the finishing column and makes mixing these two fluids very easy.

[0136] [Table 3]

[0137] Hydrothermal gasification

[0138] Hydrothermal gasification will be illustrated by a very similar case of Michael adducts, namely those of heavy butyl acrylate.

[0139] The ABU heavy mixture is composed of

[0140] - Butanol < 0.1%

[0141] - Butyl acrylate (5-10%)

[0142] - Butyl hydroxypropionate (HPB): 1-3%

[0143] - Butyl butoxypropionate (BPB) 70-80%

[0144] - Butyl acryloxypropionate (AA / ABU) 4-6%

[0145] - Dibutylmaleate: 2-5%

[0146] - Phenothiazine: 1-3%.

[0147] A 33g / h ratio of heavy ABU and 970g / h of water are introduced through two different 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 ABU are transformed into a gaseous mixture having the following volume composition: 51% CH4; 34% CO2 and 19% H2. The energy content of this gas corresponds to 7096 kWh / Tonne ABU. The TOC (total organic carbon) content is < 1mg / l.

Claims

CLAIMS Process for the manufacture of technical (meth)acrylic acid, in the absence of organic solvent, 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. a step of dehydration of said gaseous reaction mixture in a first distillation column called a dehydration column, leading to a top stream of which at least part is condensed and returned to the dehydration column in the form of reflux, and to a bottom stream of which at least part is returned to the lower part of the dehydration column to form a recirculation loop; b.a step of distilling at least a portion of said bottom stream of the dehydration column in a second distillation column called a finishing column, making it possible to separate a bottom stream containing heavy compounds, a top stream containing light compounds, at least a portion of which is returned to the dehydration column, and a side draw-off stream of technical (meth)acrylic acid; c. a step of concentrating said bottom stream of the finishing column in an evaporator leading to a bottom stream concentrating the Michael adducts, and a top stream comprising (meth)acrylic acid which is returned to the finishing column; d. a step of hydrolyzing said bottom stream of the evaporator, in the presence of water, in a hydrolyzer, leading to obtaining a stream of hydrolyzed products; e.a step of thermal cracking of said stream of hydrolyzed products, in a cracker, leading to the production of a top stream which is recycled to the dehydration column and a bottom residue; and f. a step of hydrothermal treatment of said residue, in the presence of water, carried out in hydrothermal gasification equipment leading to the production of methane, hydrogen and CO2 type gases at the top, and solid residues and water at the bottom. Process according to claim 1 wherein the evaporator placed at the bottom of the finishing column is a film evaporator working under reduced pressure of 0.5 to 100 kPa.

3. Method according to one of claims 1 or 2, in which the top product of the evaporator is recycled to the bottom of the finishing column below the lateral withdrawal line.

4. Method according to one of claims 1 to 3, in which the temperature of the hydrolyser varies between 80 and 200°C.

5. Method according to one of claims 1 to 4, in which the water / adduct mass ratio in the hydrolyser varies from 0.1 to 1.3 limits inclusive.

6. Process according to one of claims 1 to 5, in which the thermal cracking is carried out on acrylic acid adducts.

7. Process according to one of claims 1 to 5, in which the thermal cracking is carried out on a mixture of acrylic acid adducts and acrylic esters.

8. Method according to one of claims 1 to 7, in which the overhead product of the cracker is mixed with the overhead product of the finishing column.

9. Method according to one of claims 1 to 8, in which the hydrothermal gasification comprises a separator for separating the salt under critical conditions, a gasifier and a gas-liquid separator.

10. Method according to claim 9, wherein the concentration of residue / water + residue in said separator is between 10g / l and 400g / l.

11. Method according to one of claims 1 to 10, in which the gasification generates a gas composed of 40 to 70% methane, 5-20% hydrogen and 20-40% carbon dioxide.

12. Method according to one of claims 1 to 11, in which the water leaving the gasifier, free of organic compounds, is recycled to the feed of the separator or to the feed of the hydrolyser.

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