Improved process for producing high purity butyl acrylate

Abstract

The invention relates to the production of butyl acrylate by direct esterification of acrylic acid with butanol, the reaction being catalyzed by sulfuric acid. More specifically, the invention relates to an improved method for preparing butyl acrylate, which can obtain high-purity butyl acrylate. The Michael adducts formed in the process are upgraded in the form of reactants and esters by thermal and catalytic cracking and in the form of methane and mineral salts by hydrothermal gasification in the form of final residues.

Description

Technical Field

[0001] The present invention relates to the preparation of butyl acrylate by direct esterification of acrylic acid with butanol, which reaction is catalyzed by sulfuric acid. More specifically, the subject of the present invention is an improved process for the preparation of butyl acrylate, such that high purity butyl acrylate can be obtained by upgrading, by thermal and catalytic cracking, the Michael adducts formed during the process, in the form of esters and reactants which can be recycled in the process, and by hydrothermal gasification, in the form of methane and mineral salts, also upgrading the final residues. Background Art

[0002] The esterification of acrylic acid is an equilibrium reaction that produces water, and the water must be removed during the reaction to shift the equilibrium towards the production of the acrylate.

[0003] Problems generally associated with the preparation of butyl acrylate by direct esterification of acrylic acid in the presence of sulfuric acid as a catalyst are usually related to the complexity of the purification steps required to obtain a high purity product after the reaction step, which is not conducive to the productivity of the process.

[0004] As described in applicant's patent EP 0 609 127, the industrial process involves esterifying acrylic acid (AA) with an excess of butanol in the presence of sulfuric acid. The reaction mixture at the end of the reaction contains butyl acrylate, residual acrylic acid, butyl hydrogen sulfate, traces of sulfuric acid and various impurities resulting from side reactions. Subsequently, the reaction mixture is subjected to a step of neutralization and washing with water, the purpose of which is to remove the "acidic" impurities: residual sulfuric acid, butyl hydrogen sulfate and acrylic acid. The mixture free of acidic impurities is subjected to various purification steps, which results in the recovery of purified butyl acrylate. One of the "topping" steps consists in particular of distilling off the butanol and the light by-products. The butanol can thus be recycled to the esterification reaction.

[0005] The last step of purifying the butyl acrylate consists in feeding the mixture containing the ester free of light products into a final distillation column, from the top of which the heavy by-products are removed by distillation and purified, and for its part, the heavy by-products are found at the bottom of the distillation column and are subsequently concentrated in an evaporator. The top product from this evaporator, i.e. butyl acrylate (BuA), is returned to the bottom of the rectification column, which makes it possible on the one hand to recover it as a finished product, but also to keep the temperature at the bottom of the column low enough to avoid fouling problems related to the thermosensitive nature of the monomer.

[0006] In addition to Michael derivatives and a few percent of free monomers, the residue from the evaporator also contains a few percent of polymerization inhibitors that accumulate during all purification steps, such as mainly phenothiazine in free form or its adduct forms with AA or BuA, as well as heavy compounds of polymeric nature that are more or less soluble in the medium. Usually, this residue is removed by incineration, which results in a significant loss of yield.

[0007] Among the by-products resulting from side reactions, mention may be made of light products such as butyl acetate, butyl propionate, dibutyl ether or isobutyl acrylate, or heavy products such as dibutyl maleate.

[0008] The "heavy" compounds resulting from the Michael addition reaction spontaneously form units for the production of butyl acrylate. These parasitic reactions are promoted by the high temperatures encountered especially at the bottom of the distillation columns of these units. Thus, the addition of acrylic acid, unreacted butanol or water of reaction to the double bond of butyl acrylate mainly forms:

[0009] - Butyl acryloyloxypropionate (BAP) by adding acrylic acid (AA) to butyl acrylate (BuA);

[0010] - Butyl hydroxypropionate (BHP) by adding water to butyl acrylate;

[0011] - Butyl butoxypropionate (BBP) by adding butanol to butyl acrylate.

[0012] The formation of poly(pluri)addition or mixed compounds is also possible.

[0013] One of the characteristics of heavy by-products is that their boiling points are higher than those of acrylic acid, butanol and butyl acrylate. Due to their low volatility, they accumulate at the bottom of the last distillation column, at the bottom of the evaporator used to concentrate this residue.

[0014] Various solutions have been proposed for upgrading these heavy by-products that still contain polymerization inhibitors.

[0015] Document CN 1063678 proposes a method for treating oxygen-containing esters formed during the synthesis of butyl acrylate using a protonic acid catalyst such as sulfuric acid or p-toluenesulfonic acid. Compounds such as phthalates can also be added, as described in document US 4293 347.

[0016] The drawback of these cracking methods is that the residual product is viscous and contains solids. US 6,617,470 proposes using arylsulfonic acids such as dodecylsulfonic acid as a catalyst, which inhibits the formation of solids in the bottom residue. US 2011 / 0230675 proposes continuously adding water when cracking by acid catalysis to avoid the formation of solid deposits. CN 102173990 proposes adding a copper salt to the feed of the cracker to facilitate the subsequent treatment of the final cracking residue.

[0017] The applicant is not bound by any one interpretation and believes that the interaction between phenothiazine and the acid catalyst during catalytic and thermal cracking is the main cause of the formation of solids in the final residue of the cracker.

[0018] Application FR 2,108,885 uses a thermal cracker without a catalyst, which makes it possible to successfully inhibit this interaction. However, the cracking yield is still lower than the yield obtained during thermal and catalytic cracking.

[0019] Application FR 2,111,319 finally provides a simplified method that enables thermal and catalytic cracking while maintaining the integrity of the equipment by pre-treating the stream from the bottom of the last distillation column to evaporation and the stream from the top of the last distillation column to two successive condensations. Then, the bottom of this evaporator contains heavy residue and inhibitor as well as heavy products of more or less soluble polymeric nature. The residue from the cracker will consist essentially of unreacted Michael adduct and the acid catalyst used during cracking.

[0020] Although the formation of solid deposits in the cracker has been inhibited, it has been found that the treatment step of the residue, which is usually carried out by incinerating the residue in the form of heated vapors, still has problems.

[0021] In fact, two solutions can be chosen: the two residues from the bottom of the evaporator and from the bottom of the cracker can be treated separately, which results in two operations, or the two residue streams can be mixed, and in fact, on the one hand phenothiazine, and on the other hand the catalyst used during cracking will be together, resulting in the formation of solids.

[0022] The oxidation (combustion) of organic matter to obtain carbon dioxide and water has been known for a long time and is very often used to treat organic residues and produce heating steam. In a conventional steam form energy process, the rapid oxidation of an organic fuel is typically used to generate heat, which is then transferred to a fluid stream, such as water, in a heat exchanger. Due to the losses that necessarily occur in the exhaust stack of a conventional boiler (boiler), heat losses of 10% to 15% are expected. In addition to possible blockages due to the feeding of solids into the boiler, hot spots due to salt deposition on the boiler tubes or ash deposition on the tube surfaces exposed to the flame or hot gases reduce good heat transfer and thus the heat transfer efficiency, and actually even result in very costly downtime due to tube wall rupture.

[0023] Since 1981, the document FR 2 481 949 has described oxidation in a supercritical medium, in which oxygen is additionally supplied in a mixture composed of water and an organic input to convert an aqueous stream with a low organic feed into CO 2 and water, thereby producing a first incinerator under supercritical conditions, which is more efficient because the combustion energy is transported by water at the pressure and critical temperature, without the need for an exchanger as in a conventional boiler.

[0024] The document Gazéification de biomasse en eau supercritique [Gasification of biomass in supercritique] by O. Boutin and J.C. Ruiz appeared in Techniques de l′ingénieur [Techniques of the Engineer] J7010 on May 10, 2013, and describes the implementation of hydrothermal gasification on a pilot scale to treat certain organic effluents. This conversion technology makes it possible to convert wet biomass (>70% moisture) into syngas (a mixture of methane, hydrogen, and carbon dioxide) and separate the mineral salts present in the input. For example, it has been used for the gasification of algae to produce hydrogen, the treatment of primary sludge from wastewater treatment plants, or the catalytic gasification of pig manure possibly mixed with eucalyptus wood.

[0025] There is a need to have a method that enables the energy upgrading of final waste while preventing the formation of solids in the residues that block the incinerator.

[0026] It has now been found that by replacing the evaporator at the bottom of the distillation column and the distillation under reduced pressure and in an inert atmosphere with a single evaporator and its stepwise condensation system, in a process for producing high-purity butyl acrylate, the formation of solids in the cracker residue is significantly reduced while simplifying the equipment used. In addition, the combination of the hydrothermal gasification method with the said evaporator enables upgrading of the final residue in the form of a gas with a high calorific value, in particular providing methane gas that can be output, rather than converting it to CO 2 . SUMMARY OF THE INVENTION

[0027] The present invention enables the above requirements to be met. More specifically, the present invention provides an improved process for producing butyl acrylate by direct esterification of acrylic acid with butanol, which enables better upgrading of the final product that is usually sent for incineration by cracking and regenerating the starting materials and converting the residue into fuel gas. The process according to the present invention enables improvement of the energy balance of the process while improving the material balance.

[0028] The present invention consists in implementing hydrothermal gasification in combination with a thermal and catalytic cracker for upgrading Michael adducts. The subject of the present invention is to upgrade these adducts as much as possible in the presence or absence of solid products, obtaining on the one hand starting materials that can be recycled and upgraded in the distillation line, and on the other hand producing a gas phase composed of methane, hydrogen and carbon dioxide in the presence of solids and salts, which enables the energy requirements of the process to be minimized and which can be upgraded or output in the natural gas system of an industrial site.

[0029] The present invention is also applicable to organic products that are free of moisture and have not hitherto been upgraded by hydrothermal gasification.

[0030] The present invention describes an evaporation system that enables the temperature at the bottom of the distillation column compatible with the thermosensitive properties of butyl acrylate to be maintained by recycling butyl acrylate and sending the Michael adduct to a thermal and catalytic cracker, while discharging very heavy compounds and polymerization inhibitors at the bottom of the evaporator for a process for obtaining high-purity butyl acrylate, as described for the reaction section in patent EP 0 609 127.

[0031] It also supplements the purification scheme described in this patent in the following way: on the one hand, the evaporator placed at the bottom of the column for purifying butyl acrylate is combined with its stepwise condensation system, and the recycling of the top product resulting from the cracking in the process is described, and on the other hand, hydrothermal gasification, which enables the residue from the cracking to be converted into an upgradable gas (methane, hydrogen, CO 2 ), which enables the energy supply to the process to be minimized or upgraded in the gas system.

[0032] The subject of the present invention is a process for the preparation of butyl acrylate by direct esterification of acrylic acid with an excess of butanol in the presence of sulfuric acid as catalyst and at least one polymerization inhibitor, leading to a crude reaction mixture comprising butyl acrylate, residual acrylic acid, residual butanol, butyl hydrogen sulfate, traces of sulfuric acid and impurities resulting from side reactions, said process comprising a neutralization step and a step of washing with water, leading to a reaction mixture free of "acidic" impurities, characterized in that the reaction mixture from which the acidic impurities have been washed out is subjected to at least the following steps:

[0033] a) A topping step in a first distillation column called a topping column, such that:

[0034] - At the top, a stream consisting essentially of unreacted reactants;

[0035] - At the bottom, a stream containing the desired ester and heavy by-products;

[0036] b) A rectification step by tailing the bottom stream from the topping column in a second distillation column, such that:

[0037] - At the top, the purified desired ester;

[0038] - At the bottom, a stream containing heavy by-products;

[0039] c) A step of concentrating the bottom stream from the rectification column in an evaporator, obtaining a top stream, cooled in two steps in order to recycle the light compounds present to the rectification column and concentrate the Michael adduct;

[0040] d) A step of cracking the Michael adduct concentrate in a thermal cracker, such that:

[0041] - At the top, the noble products resulting from the cracking, which are recycled to the feed of the topping column;

[0042] - At the bottom, the final residue,

[0043] e) A step of hydrothermally treating the residue in a hydrothermal gasification device in the presence of water, leading to the obtaining of methane, hydrogen and CO 2 type gases at the top and solid residues and water at the bottom.

[0044] The present invention makes it possible to overcome the drawbacks of the prior art. More specifically, the present invention provides a method that makes it possible to obtain butyl acrylate with a high purity, having an ester purity greater than 99.5% as per the specifications, which method combines an optimized method for removing polymerization inhibitors, making it possible to crack the Michael adduct to obtain the reactants (acrylic acid and alcohol) and obtain the final product, thus increasing the yield of the method and improving the energy balance by upgrading the residues to be removed.

[0045] Reference appendix Figure 1 , by reading the following detailed description, other features and advantages of the present invention will become more apparent.

[0046] Figure 1 : General diagram of the method for synthesizing butyl acrylate according to the present invention, which has a combination of items of a thermal and catalytic cracker and a hydrothermal gasification device. Detailed description

[0047] The subject of the present invention is a method for preparing butyl acrylate by direct esterification of acrylic acid with an excess of butanol in the presence of sulfuric acid as a catalyst and at least one polymerization inhibitor, resulting in a crude reaction mixture that contains butyl acrylate, residual acrylic acid, residual butanol, butyl hydrogen sulfate, traces of sulfuric acid, and impurities resulting from side reactions.

[0048] According to various embodiments, the method includes the following features, combined if appropriate.

[0049] After the esterification step, the method according to the present invention includes a step of neutralization and washing with water, obtaining a reaction mixture free of "acidic" impurities (sulfuric acid, butyl hydrogen sulfate, acrylic acid dimer, and residual acrylic acid).

[0050] Characterized in that the reaction mixture washed free of acidic impurities as described above is subjected to at least the following steps:

[0051] a) A topping step in a first distillation column called a topping column, making it possible to obtain:

[0052] - At the top, a stream consisting essentially of unreacted reactants;

[0053] - At the bottom, a stream containing the desired ester and heavy by-products;

[0054] b) Passing the bottom stream from the topping column through a trailing fertilization distillation column, making it possible to separate:

[0055] - At the top, the purified desired ester;

[0056] - At the bottom, a stream containing heavy by-products;

[0057] ​c) The step of concentrating the bottoms stream from the rectification column in an evaporator, particularly a thin-film evaporator, to obtain a tops stream, which is cooled in two steps in order to recycle the light compounds present to the rectification column and to concentrate the Michael adduct with a very low inhibitor content;

[0058] d) The step of cracking the tops stream from the evaporator in a thermal cracker such that:

[0059] - At the top, the valuable products resulting from the cracking, which are recycled to the feed of the debutanizer;

[0060] - At the bottom, the final residue,

[0061] e) The step of hydrothermally treating the said residue in a hydrogasification unit in the presence of water, resulting in obtaining, at the top, methane, hydrogen and CO 2 type gases and, at the bottom, solid residues and water.

[0062] According to one embodiment, the thermal cracking may or may not be catalytic.

[0063] According to one embodiment, the two residues are treated separately: the residue from the bottom of the evaporator and the residue from the cracker.

[0064] According to one embodiment, the two residues (the residue from the bottom of the evaporator and the residue from the cracker) are mixed and treated in the same gasification operation.

[0065] According to one embodiment, the hydrogasification unit comprises a first reactor, a second reactor and a gas-liquid separator.

[0066] According to one embodiment, the residue is injected as such into the gasification and water required for the hydrothermal treatment is further injected.

[0067] According to one embodiment, the residue is mixed with the water required for the hydrothermal treatment before being introduced into the gasification.

[0068] According to this embodiment, the hydrogasification is carried out at a temperature of 350 °C to 450 °C and a pressure of 25 MPa.

[0069] According to this embodiment, the hydrogasification comprises a gasifier such that salts can be separated at the bottom and a gas and liquid mixture can be separated at the top.

[0070] According to one embodiment, the hydrogasification comprises a separator, a gasifier and a gas-liquid separator such that salts can be separated under critical conditions.

[0071] According to one embodiment, the concentration of residue / water + residue in the salt separator is from 10 g / l to 400 g / l.

[0072] According to one embodiment, the water used for hydrothermal gasification can be demineralized water, water produced by drilling, or weakly mineralized water.

[0073] According to one embodiment, the water free of organic compounds at the outlet of the gasifier can be advantageously recycled to the feed of the separator.

[0074] According to one embodiment, the obtained and separated salts can be upgraded to fertilizers.

[0075] According to one embodiment, 94% to 99% of the carbon introduced into the gasification is upgraded in gaseous form.

[0076] According to one embodiment, the gas produced by gasification consists of 40% to 70% methane, 5% to 20% hydrogen, and 20% to 40% carbon dioxide.

[0077] According to one embodiment, the gas can be further fractionated to isolate methane from other compounds.

[0078] According to one embodiment, the mass composition of the main compounds of the residue from the bottom of the cracker is as follows:

[0079] - Butyl butoxypropionate (BBP): 65 - 90%

[0080] - p - Toluenesulfonic acid (PTSA): 0.1 - 4%

[0081] - Phenothiazine: 100 - 1000 ppm

[0082] - Heavy products (molecular weight > 264 g / mol): > 1%.

[0083] According to one embodiment, the mass composition of the bottom product from the evaporator is as follows:

[0084] - Butyl butoxypropionate (BBP): 65% - 85%

[0085] - p - Toluenesulfonic acid: 0%

[0086] - Phenothiazine: 1 - 10%

[0087] - Heavy products (molecular weight > 264 g / mol): > 10%.

[0088] Reference Figure 1, which represents a preferred mode of the present invention. The topping section comprises a distillation column having an equivalent of 10 to 30 theoretical plates, preferably 10 to 15 theoretical plates. The internals for the column can be valve trays or perforated trays with weirs, crossflow trays, such as two - flow trays, ripple trays or shell turbogrid trays, or stacked packings, such as structured packings, such as Mellapack 250X from Sulzer.

[0089] The topping column is fed in the upper third of the column, preferably between theoretical plates 3 to 10 counted from the top of the column. The overhead stream of the column essentially contains unreacted reactants. This upgradable stream is recycled to the reaction.

[0090] The column operates at a reflux ratio (flow rate of condensed liquid returned to the column / flow rate recycled to the reaction) of 4 / 1 to 1 / 1, preferably 3 / 1. Advantageously, a polymerization inhibitor from 50 to 5000 ppm is introduced into the purification system of the process according to the present invention.

[0091] As polymerization inhibitors that can be used, mention may be made, for example, of phenothiazine (PTZ), hydroquinone (HQ), hydroquinone monomethyl ether (HQME), bis(tert - butyl) - p - cresol (BHT), p - phenylenediamine, TEMPO (2,2,6,6 - tetramethyl - 1 - piperidinyloxy), bis(tert - butyl)catechol or TEMPO derivatives such as OH - TEMPO, alone or in mixtures in all proportions. The content in the reaction medium can be between 50 ppm and 5000 ppm, optionally in the presence of lean air, but usually the content is between 150 ppm and 1000 ppm. The addition of the polymerization inhibitor can be carried out at different locations, either along with the introduction of the reactants or at the top of the distillation column.

[0092] To make the polymerization inhibitor more effective, it is recommended to inject oxygen, air or "lean" air having 7% O 2 at the bottom of the column. Preferably, the amount of oxygen injected corresponds to a content of 0.2% to 0.5% relative to the amount of organic vapors in the column.

[0093] The column can be operated under vacuum to minimize the thermal exposure of heat - sensitive compounds within the column. Advantageously, the topping column operates under a vacuum in the range of 1000 Pa to 30000 Pa.

[0094] The bottom stream is preferably fed to a column such that a purified ester can be obtained at the bottom of the column between theoretical plates 6 to 9.

[0095] The distillation column for the pure product contains 2 to 15 theoretical plates, preferably an equivalent of 6 to 12 theoretical plates. The internals for the column can be valve trays or perforated trays with weirs, cross-flow trays, such as two-flow trays, corrugated trays or shell-and-tube grid trays, or stacked packings, such as structured packings, such as Mellapack 250X from Sulzer.

[0096] The top stream from the column consists of high-purity butyl acrylate, which has an ester purity of greater than 99.5% according to the specification.

[0097] The column operates at a reflux ratio (flow rate of the condensed liquid returned to the column / flow rate of the pure product) of 1 / 8 to 1 / 1, preferably 1 / 4. Like the topping column, it is stabilized and air or lean air (7% O 2 ) is injected at the bottom of the column. The column can be operated under vacuum to minimize the thermal exposure of heat-sensitive compounds in the column. Advantageously, the pure product column operates under a vacuum in the range of 1000 Pascals to 20,000 Pascals.

[0098] Advantageously, the operating temperature is between 50°C and 160°C.

[0099] The bottom stream is concentrated on a thin-film evaporator, which is very suitable for viscous fouling products and contaminated liquids. The evaporation is carried out in the temperature range of 80°C to 100°C, more particularly 90°C to 100°C, and in the pressure range of 800 Pa to 2000 Pa. The top stream from this evaporator is cooled in two successive steps:

[0100] - Partially condensed in the temperature range of 50°C to 80°C, more particularly 60°C to 70°C, at the same pressure as the reactor pressure, so as to obtain on the one hand a liquid stream of the Michael adduct that will be fed to the cracker and on the other hand a vapor stream,

[0101] - The vapor stream is condensed in the temperature range of 20°C to 40°C and more particularly 20°C to 30°C, and then the liquid stream is pumped and mixed with the bottom product from the topping column for feeding to the rectification column.

[0102] The bottom residue is subjected to hydrothermal gasification either alone or as a mixture.

[0103] The top adduct stream and the p-toluenesulfonic acid catalyst (PTSA) in aqueous solution or pre-dissolved in the Michael adduct are continuously introduced into a forced recycle cracker equipped with an external exchanger, in the temperature range of 160°C to 210°C, with a residence time of the order of 2 to 10 hours and a pressure of 200 kPa to atmospheric pressure.

[0104] According to one embodiment, the cracking is carried out at a temperature of 160°C to 180°C at atmospheric pressure.

[0105] The catalyst content is from 0.5% to 3% relative to the amount of the adduct fed to the cracker, as in FR 2 901272 for example. The reactants resulting from this cracking, mainly butanol and butyl acrylate, are returned to the purification line of the process. For its part, the residue is treated by hydrothermal gasification alone or preferably as a mixture.

[0106] The residue and water are injected via a high-pressure pump into a hydrothermal gasification device at a temperature in the range of 350 °C to 450 °C and a pressure of 25 MPa through two circuits.

[0107] The first reactor makes it possible to separate the salts at its bottom from the aqueous organic solution.

[0108] The second gasification reactor comprises a catalyst which makes it possible to complete the conversion of the organic products to produce gases.

[0109] The gas-liquid separator makes it possible to collect at the bottom the aqueous phase and the methane-rich gas phase. The aqueous phase can be recycled to the inlet of the separator, the gas phase can be upgraded to produce an electric current which can supply the energy required for the gasification operation and the energy required for the operation of the reaction and purification lines of the process or can be further output. This hydrothermal gasification can be carried out in batch mode or preferably in continuous mode.

[0110] The following examples illustrate the invention without limiting its scope.

[0111] Experimental part

[0112] In the examples, unless otherwise indicated, the percentages are shown by weight and the following abbreviations are used:

[0113] PTZ: phenothiazine

[0114] BuA: butyl acrylate

[0115] BuOH: butanol

[0116] BBP: butyl butoxypropionate

[0117] DBE: dibutyl ether

[0118] BAP: butyl acryloyloxypropionate

[0119] Heavy product: molecular weight > 264 g / mol

[0120] PTSA: p-toluenesulfonic acid

[0121] BPTS: butyl p-toluenesulfonate

[0122] Example 1. Obtaining the residue produced by the thin-film evaporator

[0123] A commercially available DV210 / 1 m2 thin-film evaporator operating at a pressure of 130 Pa and heated by 0.4 MPa steam feeds a mixture at a rate of 50 kg / h: BuA: 9%; BuOH: 3.5%; BBP: 67%; BAP: 5.7%; PTZ: 1.8%; balance to 100%: heavy residue.

[0124] The top product (40 kg / h) is condensed at 20 °C in a 4 m 2 tubular exchanger. It is stabilized by adding a 250 g / h solution of butyl acrylate containing 2% phenothiazine. The top product fed to the cracker contains 12.5% BuA, 75% BBP, 3% butanol and 900 ppm phenothiazine.

[0125] The bottom product from the evaporator contains PTZ (6%), about 10% of the heavy product and BBP as the balance to 100%.

[0126] Example 2. Thermal and catalytic cracking tests

[0127] A forced recirculation boiling vessel with a volume of 40 L is used. A heavy BuA blend placed on a balance is continuously fed using a diaphragm pump, and 1 mass% of p-toluenesulfonic acid is added thereto. The feed flow rate is measured using a mass flow meter placed on the feed line and also by the change in mass indicated by the balance over time. This operation is carried out at a regulated pressure so as not to evaporate butyl butoxypropionate. The temperature of the reaction medium and the temperatures at the inlet and outlet of the heat exchanger are continuously measured. The heat transfer fluid used to bring heat to the exchanger is sourced from an oil heater. The heating power is fixed so as to keep the test temperature fixed.

[0128] The heavy BuA blend has been pre-distilled under vacuum and contains approximately 600 ppm of phenothiazine.

[0129] After cracking at 180 °C at a pressure of 40 kPa (300 mmHg) and a residence time of 9 h (defined as the volume of the reactor relative to the feed flow rate in terms of the mass of the Michael adduct), the top flow rate / feed flow rate ratio is 76%, and the top composition contains 48% butyl acrylate and 15% butanol.

[0130] The residue itself contains in particular 0.6% PTSA, 2.5% BPTS, 80% BBP and 10% of the heavy product.

[0131] Example 3. Hydrothermal gasification

[0132] The mixture of the heavy BuA blend consists of:

[0133] - Butanol: <0.1%

[0134] - Butyl acrylate: 5 - 10%

[0135] - Butyl hydroxypropionate (BHP): 1 - 3%

[0136] - Butyl butoxypropionate (BBP): 70 - 80%

[0137] - Butyl acryloyloxypropionate (BAP): 4 - 6%

[0138] - Dibutyl maleate: 2 - 5%

[0139] - Phenothiazine: 1 - 3%.

[0140] A heavy BuA blend at 33 g / h and 970 g / h of water are introduced via two different pipes into a separator and a catalytic reactor, both operating at 400 °C and 250 bar. After 6 hours of testing under steady conditions, the heavy BuA blend is converted into a gas mixture with the following volume components: 51% CH 4 , 34% CO 2 and 19% H 2 . The amount of energy of this gas corresponds to 7096 kWh / ton of BuA. The amount of TOC (total organic carbon) < 1 mg / l.

Claims

1. A process for the preparation of butyl acrylate by direct esterification of acrylic acid with butanol in the presence of sulfuric acid as a catalyst and at least one polymerization inhibitor, leading to a crude reaction mixture comprising butyl acrylate, residual acrylic acid, residual butanol, butyl hydrogen sulfate, trace amounts of sulfuric acid and impurities resulting from side reactions, said process comprising a neutralization step and a step of washing with water, leading to a reaction mixture free of acidic impurities, characterized in that the reaction mixture from which the acidic impurities have been washed off is subjected to the following steps: a) a topping step in a first distillation column called a topping column, such that there is obtained: - at the top, a stream consisting essentially of unreacted reactants; - at the bottom, a stream containing the desired ester and heavy by-products; b) a rectification step by tailing the bottom stream from said topping column in a second distillation column, such that there can be separated: - at the top, the purified desired ester; - at the bottom, a stream containing heavy by-products; c) a step of concentrating the bottom stream from said rectification column in an evaporator, obtaining a top stream, cooled in two steps in order to recycle the light compounds present to the rectification column and concentrate the Michael adduct; d) a step of cracking said Michael adduct concentrate in a thermal cracker, such that there is obtained: - at the top, the valuable product resulting from the cracking, which is recycled to the feed of the topping column; - at the bottom, the final residue, e) In the presence of water, the hydrothermal treatment step of the residue is carried out in a hydrothermal gasification device, resulting in obtaining methane, hydrogen and CO gas at the top, and obtaining solid residue and water at the bottom. 2 ​ 2. The process according to claim 1, wherein the acidic impurities are sulfuric acid, butyl hydrogen sulfate, acrylic acid dimer and residual acrylic acid.

3. The process according to any one of claims 1 and 2, wherein the polymerization inhibitor is selected from: phenothiazine (PTZ), hydroquinone (HQ), hydroquinone monomethyl ether (HQME), di(tert-butyl)-p-cresol (BHT), p-phenylenediamine, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), di(tert-butyl)catechol, OH-TEMPO, or mixtures of all of them in all proportions.

4. The process according to any one of claims 1 to 3, wherein the polymerization inhibitor is used in the reaction medium or in the purification step in an amount between 50 ppm and 5000 ppm, preferably between 150 ppm and 1000 ppm.

5. The process according to any one of claims 1 to 4, wherein the polymerization inhibitor is added at different locations to the reactants or at the top of the distillation column.

6. The process according to any one of claims 1 to 5, wherein the evaporation is carried out in a temperature range of 80 °C to 100 °C, preferably 90 °C to 100 °C and at a pressure of 800 Pa to 2000 Pa.

7. The process according to any one of claims 1 to 6, wherein the top stream from the evaporator is cooled in two successive steps: a. Partial condensation is carried out at a temperature in the range of 50°C to 80°C, and more particularly 60°C to 70°C, at the same pressure as that of the evaporator, so as to obtain, on the one hand, a liquid stream of Michael adduct having an inhibitor content of less than 1000 ppm for feeding the cracking unit, and on the other hand, a vapor stream, and b. Before the liquid stream is pumped and fed to the distillation column, the vapor stream is condensed at a temperature in the range of 20°C to 40°C, and more particularly 20°C to 30°C.

8. The method according to any one of claims 1 to 6, wherein the cracking is carried out at a temperature of 160°C to 180°C under atmospheric pressure.

9. The method according to any one of claims 1 to 8, wherein the obtained butyl acrylate has a purity greater than 99.5%.

10. The method according to any one of claims 1 to 9, wherein the hydrothermal gasification includes a separator, a gasifier and a gas-liquid separator capable of separating salts under critical conditions.

11. The method according to claim 10, wherein the concentration of residue / water + residue in the separator is 10 g / l to 400 g / l.

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

13. The method according to any one of claims 10 to 12, wherein the water free of organic compounds at the outlet of the gasifier is recycled to the feed of the separator.

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