Improved method for producing alpha-beta-unsaturated carboxylic acids from poly(3-hydroxyalkanoate)

Abstract

The present invention relates to a method for producing α-β-unsaturated carboxylic acids by thermolysis of poly(3-hydroxyalkanoate), while limiting clogging associated with the unintentional condensation of hot α-β-unsaturated carboxylic acid vapours generated on walls in the method and with the subsequent formation of solids via a radical polymerisation reaction. The invention is based on the introduction of a radical polymerisation inhibitor into the thermolysis reactor and on the use of specific thermolysis conditions that make this inhibitor partially volatile during the thermolysis of the poly(3-hydroxyalkanoate).

Description

[0001] IMPROVED PROCESS FOR THE PRODUCTION OF a-P UNSATURATED CARBOXYLIC ACIDS FROM POLY(3-HYDROXYALKANOATE)

[0002] Domain

[0003] The present invention relates to a process for the production of unsaturated α-P carboxylic acids by thermolysis of poly(3-hydroxyalkanoate), while limiting fouling phenomena linked to accidental condensation of hot vapors of unsaturated α-P carboxylic acids generated on process walls and to the subsequent formation of solids by a radical polymerization reaction. The invention is based on the introduction into the thermolysis reactor of a radical polymerization inhibitor and the use of particular thermolysis conditions making this inhibitor partially volatile during the thermolysis of the poly(3-hydroxyalkanoate). Thus, in the event of accidental condensation of hot vapors of unsaturated α-P carboxylic acids generated on process walls, the radical polymerization inhibitor also condenses and protects the liquid phase formed against a radical polymerization reaction.

[0004] Prior art and

[0005] The industrial production of unsaturated α-P carboxylic acids is currently mainly carried out from raw materials of fossil origin. For example, acrylic acid is obtained by oxidation of propylene, or methacrylic acid can be obtained by oxidation of isobutylene.

[0006] There is a strong market demand for these ap-unsaturated carboxylic acids, used as monomers in many applications, to be obtained from bio-based raw materials. These bio-based raw materials are derived from renewable organic matter (biomass) of biological origin (microorganisms, plants or animals).

[0007] One possible way to obtain these unsaturated ap carboxylic acids is the thermolysis at temperatures of 150 to 300°C of corresponding poly(3-hydroxyalkanoates), according to the following reaction:

[0008] Ri= H or alkyl and R2= H or alkyl; n is a number greater than 30

[0009] If RI=R2= H:

[0010] Poly(3-hydroxyalkanoate) = poly(3-hydroxypropionate) (P3HP);

[0011] Unsaturated carboxylic acid a~P = propenoic acid (acrylic acid).

[0012] If Ri= methyl and R2= H:

[0013] Poly(3-hydroxyalkanoate) = poly(3-hydroxyisobutyrate) (P3HiB);

[0014] Unsaturated carboxylic acid a~P = isobutenoic acid (methacrylic acid).

[0015] If Ri= H and R2= methyl:

[0016] Poly(3-hydroxyalkanoate) = poly(3-hydroxybutyrate) (P3HB);

[0017] Unsaturated carboxylic acid a~P = 2-butenoic acid (crotonic acid).

[0018] If R1=H and R2 = ethyl:

[0019] - Poly(3-hydroxyalkanoate) is poly(3-hydroxy valerate) (P3HV);

[0020] - Unsaturated carboxylic acid a~P = pent-2-enoic acid

[0021] These poly(3-hydroxyalkanoates) can themselves be previously obtained by chemical transformations of raw materials of fossil origin, but also by fermentation of biomass.

[0022] A potential problem with any process for manufacturing ap-unsaturated carboxylic acids is that these compounds can readily polymerize radically when they are hot and in the liquid phase. This is true in deliberately formed liquid phases, such as the liquid phases present in a distillation column, in a reactor or in a condenser, but it can also happen in liquid phases formed accidentally, such as those formed during the accidental condensation of hot vapors on a wall with a cold spot. The usual consequence of this process is the deposition of solid polymers in the plant equipment, which eventually cause blockages and require a plant shutdown for cleaning, which is difficult and costly in terms of non-productive downtime.

[0023] To reduce these disadvantages, radical polymerization inhibitors are conventionally added at all stages of the production process, i.e. at the synthesis stage and at the purification stages.

[0024] For example, during the industrial production of acrylic acid (AA), radical polymerization inhibitors are added to the absorption column in water of AA vapors from the catalytic oxidation of propylene, then to each distillation column and finally to the finished product.

[0025] The radical polymerization inhibitors conventionally used in these manufacturing processes are phenolic derivatives such as hydroquinone (HQ) and its derivatives such as hydroquinone methyl ether (EMHQ), 2,6-di-terbutyl-4-methyl phenol (BHT) or 2,4-dimethyl-6-terbutyl phenol (Topanol A); phenothiazine and its derivatives; nitroxide compounds such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-l-oxyl (4-OH-TEMPO); or amine compounds such as paraphenylenediamine derivatives.

[0026] A disadvantage of these inhibitors is that they are usually considered non-volatile under the conditions of production of unsaturated carboxylic acid. In order to be present in all liquid phases containing unsaturated carboxylic acid, they must therefore be injected into the reaction but also into the feeds, boilers, condensers and reflux of the purification equipment. Sprays of inhibitors in solution can also be used to protect all surfaces on which hot vapors of unsaturated carboxylic acid are likely to condense accidentally.This problem is well known to those skilled in the art; for example, during the industrial purification of acrylic acid by distillation, polymerization inhibitors are added to the feed, condenser and reflux of the distillation column but also frequently sprayed in the form of sprays to protect the dome, the swan neck, the manholes, or any other element of the column where acrylic acid vapors are likely to condense.

[0027] Document EP 2398832 describes another solution aimed at preventing polymerization, including in the event of accidental fortuitous condensation of AA vapors. It uses a second type of inhibitor, called fugitive, i.e. with a volatility, under the operating conditions of production, close to that of unsaturated ap acid, here acrylic acid. This inhibitor is then present in the gas phase and condenses at the same time as the acrylic acid vapors during accidental condensation. However, these polymerization inhibitors, which are nitrosobenzene derivatives, have the disadvantage of being toxic.

[0028] The problem of polymerization during the production of unsaturated α-P carboxylic acids is also present when these are produced by thermolysis of the corresponding poly(3-hydroxyalkanoate).

[0029] US 2568636 describes the thermolysis of poly(3-hydroxypropionate) (P3HP) to form acrylic acid (AA) at temperatures between 130 and 300 °C and the use of triarylphosphates to limit the polymerization of AA in the thermolysis reactor. US 3002017 describes a similar thermolysis in which AA vapors are absorbed into cold AA to limit polymerization during the condensation step.

[0030] US 9115070 describes the thermolysis of P3HP to form AA using a tertiary amine catalyst to reduce the reaction temperature. Conventional polymerization inhibitors, such as phenothiazine (PTZ), can potentially be used in the reaction medium at 10 to 1000 ppm by weight relative to P3HP to reduce the polymerization phenomena of AA formed in the thermolysis medium.

[0031] US 10065914 describes the thermolysis of P3HP to form AA at temperatures of 100 to 300 °C, using a sodium acrylate catalyst to reduce the reaction temperature and thus limit the risks of polymerization of the AA formed in the thermolysis medium. The use of polymerization inhibitors, such as PTZ and EMHQ, in the liquid phase of the thermolysis reactor as well as their deliberate introduction into the liquid phases of a distillation or a condenser also makes it possible to reduce polymerization phenomena.

[0032] The prior art methods, however, have a major drawback. They describe how to reduce the risks associated with the polymerization of liquid phases of unsaturated a~P acids by injecting a polymerization inhibitor into these deliberately formed liquid phases (thermolysis medium, condenser, liquid phase of a distillation column, etc.), but do not provide a solution in the case where the liquid phase is formed accidentally, for example during the unwanted condensation of unsaturated ap acid on a "cold spot" of an installation. The use of sprays to spray these inhibitors on all the walls of an industrial system is certainly possible but complex to implement. The use of non-conventional inhibitors, such as nitrosobenzene derivatives, is also complex to implement industrially.

[0033] The inventors have now surprisingly discovered that it is possible to drastically reduce polymerization phenomena linked to the fortuitous condensation of hot vapors in a process for the synthesis of unsaturated ap carboxylic acid from poly(3-hydroxyalkanoate) without using a non-conventional inhibitor. The parameters of the thermolysis reaction of poly(3-hydroxyalkanoate) to unsaturated ap carboxylic acid can in fact be adjusted in order to obtain a significant volatility of certain conventional polymerization inhibitors. These inhibitors are then present in the gas phase and condense at the same time as the unsaturated ap carboxylic acid during fortuitous condensation on a cold point, instantly protecting the liquid phase formed. There is no need for a complex system for distributing the inhibitor at several points in the process or for expensive and toxic inhibitors.

[0034] Accordingly, the invention proposes to provide a simple and easy-to-implement solution for reducing fouling phenomena and thus maintaining high reliability and high productivity in processes for manufacturing unsaturated α-P carboxylic acids from poly(3-hydroxypropionate).

[0035] Summary of the invention

[0036] The present invention relates to a process for the manufacture of unsaturated carboxylic acids a~P by thermolysis of poly(3-hydroxyalkanoate) carried out in a thermolysis reactor from which the generated carboxylic acid vapors reach a condenser, in the presence of one or more polymerization inhibitors, characterized in that the pressure in the reactor is adjusted so that it is less than twice the vapor pressure of at least one of the inhibitors at the temperature at which the thermolysis is carried out.

[0037] According to the invention, the thermolysis conditions used (pressure and temperature) allow one of the polymerization inhibitors to be significantly volatile and therefore to obtain the desired effect. According to various embodiments, said method comprises the following characteristics, possibly combined. The contents indicated are expressed by weight, unless otherwise indicated. Within the ranges of values ​​indicated, the limits are included.

[0038] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermoly process comprises a single type of 3-hydroxyalkanoate units and the product formed is therefore composed of a single unsaturated carboxylic acid.

[0039] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermoly process comprises several different 3-hydroxyalkanoate units and the product formed is therefore composed of a mixture of different ap-unsaturated carboxylic acids. Examples of P3HA copolymers are poly-3-hydroxybutyrate-co-3-hydroxypropionate (poly-3HB-co-3HP) or poly-3-hydroxybutyrate-co-3-hydroxyvalerate (poly-3HB-co-3HV).

[0040] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermolysis process is obtained from raw materials of fossil origin.

[0041] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermolysis process is obtained from raw materials of renewable origin or at least partly of renewable origin. According to this embodiment, the poly(3-hydroxyalkanoate) is more than 50% by weight, preferably more than 80% by weight, advantageously 100% by weight of renewable origin.

[0042] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermolysis process is obtained by chemical reaction, for example obtaining P3HP by polymerization of P-propiolactone itself obtained from ethylene oxide and carbon monoxide.

[0043] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermolysis process is obtained by biological reaction, in particular by fermentation.

[0044] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermolysis process is purified prior to the thermolysis reaction.

[0045] According to one embodiment, the poly(3-hydroxyalkanoate) used in the thermolysis process is used without prior purification, in particular without separation from the cell membrane, if it has been obtained by fermentation. According to one embodiment, the poly(3-hydroxyalkanoate) is obtained inside a cell by a fermentation reaction, the biomass is washed and dried but the poly(3-hydroxyalkanoate) is not separated from the cell membrane before the thermolysis step.

[0046] According to one embodiment, the poly(3-hydroxyalkanoate) is obtained inside a cell by a fermentation reaction, the biomass is washed and dried and the poly(3-hydroxyalkanoate) is separated from the cell membrane before the thermolysis step, for example by extraction.

[0047] According to one embodiment, the thermolysis reaction of the poly(3-hydroxyalkanoate) takes place in the absence of solvent, the product then being in solid form or in the molten state.

[0048] According to one embodiment, the thermolysis reaction of the poly(3-hydroxyalkanoate) takes place in solution.

[0049] According to one embodiment, the thermolysis reaction of the poly(3-hydroxyalkanoate) takes place in suspension.

[0050] According to one embodiment, the thermolysis reaction of the poly(3-hydroxyalkanoate) takes place in batch.

[0051] According to one embodiment, the thermolysis reaction of the poly(3-hydroxyalkanoate) takes place continuously.

[0052] According to one embodiment, the thermolysis reaction of the poly(3-hydroxyalkanoate) takes place in the absence of a catalyst.

[0053] The polymerization inhibitors used in the process according to the invention are chosen from inhibitors conventionally used in existing industrial processes for the production of unsaturated α-P carboxylic acids. These include phenolic derivatives such as hydroquinone (HQ) and its derivatives; phenothiazine and its derivatives; nitroxide compounds; and amino compounds such as paraphenylenediamine derivatives.

[0054] According to one embodiment, the poly(3-hydroxyalkanoate) contains the 3-hydroxypropionate unit and at least one of the unsaturated α-P carboxylic acids produced is acrylic acid.

[0055] In one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxypropionate) and the produced a~P unsaturated carboxylic acid is acrylic acid. In one embodiment, the poly(3-hydroxyalkanoate) contains the 3-hydroxybutyrate unit and at least one of the produced a~P unsaturated carboxylic acids is crotonic acid.

[0056] According to one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxybutyrate) and the unsaturated α-P carboxylic acid produced is crotonic acid.

[0057] According to one embodiment, the poly(3-hydroxyalkanoate) contains the 3-hydroxyisobutyrate unit and at least one of the unsaturated α~P carboxylic acids produced is methacrylic acid.

[0058] According to one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxyisobutyrate) and the unsaturated α-P carboxylic acid produced is methacrylic acid.

[0059] Another subject of the invention relates to a process for purifying the unsaturated carboxylic acid(s) a~P obtained by the process of thermolysis of poly(3-hydroxyalkanoate) operated at a pressure less than twice the vapor pressure of at least one polymerization inhibitor at the thermolysis temperature, characterized in that it comprises a step of condensation of the vapors of the unsaturated carboxylic acid(s) ap thus obtained, followed by one or more purification steps.

[0060] The present invention meets the need expressed in the state of the art. It makes it possible to prevent the risks of fouling due to the accidental condensation of vapors of unsaturated a~P carboxylic acids on cold points in the case of the generation of unsaturated a~P carboxylic acids by thermolysis of poly(3-hydroxypropionate). The invention makes it possible in particular to protect the zone located between the thermolysis reactor and the condenser. Thanks to the polymerization inhibitor made volatile in the thermolysis medium, it will be present in the gas phase in the entire part of the installation where the unsaturated ap carboxylic acid(s) are in the gas phase. The invention also makes it possible to avoid the formation of polymers in the reaction medium.

[0061] The invention will now be described in more detail in the following description. Detailed description of the invention

[0062] The invention aims to produce unsaturated ap carboxylic acids on an industrial scale by thermolysis of poly(3-hydroxy alkanoate), without being confronted with the problem of fouling of the installations used, due to the polymerization of the vapors of unsaturated ap carboxylic acids when they condense on cold points of the installation.

[0063] The invention proposes to provide a method for reducing or eliminating this risk of fouling. The invention is based on the addition of a polymerization inhibitor and the choice of pressure and temperature conditions in the poly(3-hydroxyalkanoate) thermolysis reactor, so that the inhibitor has significant volatility under the reaction conditions. Typically, the pressure in the reactor is adjusted so that it is less than twice the vapor pressure of the inhibitor at the thermolysis temperature.

[0064] The term "thermolysis" of poly(3-hydroxyalkanoate) means its chemical decomposition into unsaturated carboxylic acid a~P obtained under the effect of temperature. This term is synonymous with pyrolysis.

[0065] According to IUPAC, "saturating vapor pressure" is the pressure exerted by a pure substance (at a given temperature) in a system containing only the vapor and the condensed phase (liquid or solid) of the substance. (Pure and Applied Chemistry, 1990, Volume 62, No. 11, pp. 2167-2219 & Glossary of atmospheric chemistry terms (Recommendations 1990), page 2212).

[0066] In the description of the invention, the term "vapor pressure" will be used in the same sense as the term "saturating vapor pressure". Also, the term "vapor pressure" is synonymous with "vapor pressure".

[0067] In the process for thermolysis of poly(3-hydroxyalkanoate) according to the invention, the poly(3-hydroxyalkanoate) is heated to a temperature of 130 to 300°C, preferably 170 to 230°C.

[0068] The reaction medium in the thermolysis reactor comprises at least one polymerization inhibitor, in particular in an amount of 50 ppm to 5% by weight, in particular 0.01% to 3% by weight, relative to the weight of the poly(3-hydroxyalkanoate). When there are two or more inhibitors, their overall content does not exceed 5% by weight. The polymerization inhibitors are chosen from the inhibitors conventionally used in existing industrial processes for the production of unsaturated carboxylic acids. These include phenolic derivatives such as hydroquinone (HQ) and its derivatives such as hydroquinone methyl ether (EMHQ), 2,6-di-terbutyl-4-methylphenol (BHT) or 2,4-dimethyl-6-terbutylphenol (Topanol A); phenothiazine and its derivatives; nitroxide compounds such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-l-oxyl (4-OH-TEMPO); amine compounds such as paraphenylenediamine derivatives.

[0069] In the process for thermolysis of poly(3-hydroxyalkanoate) according to the invention, the temperature and pressure conditions in the thermolysis reactor are chosen so that the ap-unsaturated carboxylic acid(s) formed are in the form of vapors and at least one of the inhibitors is volatile. This is achieved when the pressure in the reactor is less than twice the vapor pressure of one of the inhibitors at the thermolysis temperature.

[0070] According to one embodiment, at least one of said polymerization inhibitors is hydroquinone methyl ether (HMEQ).

[0071] For example, EMHQ has a vapor pressure of:

[0072] - 20 kPa at 190°C; the pressure in the reactor is adjusted below 40 kPa for thermolysis at 190°C;

[0073] - 28.5 kPa at 200°C; the pressure in the reactor is adjusted below 57 kPa for thermolysis at 200°C;

[0074] - 39 kPa at 210°C; the pressure in the reactor is adjusted below 78 kPa for thermolysis at 210°C.

[0075] The method according to the invention thus makes it possible to adopt specific pressure conditions to obtain volatility of the inhibitor and thus protect the operation in the event of unwanted condensation of hot vapors of unsaturated carboxylic acids on a cold point of one of the walls of the equipment.

[0076] According to one embodiment of the invention, the thermolysis reaction is carried out in the presence of a solvent, either in solution or in suspension. In order to limit the vaporization of the solvent with the vapors of ap-unsaturated carboxylic acids generated during the thermolysis of the poly(3-hydroxyalkanoate), the solvent is chosen so that its vapor pressure at the thermolysis temperature of the poly(3-hydroxyalkanoate) is less than three-quarters of the pressure at which the thermolysis is carried out.

[0077] According to one embodiment, for operating conditions of 200°C and 20 kPa, the solvent must have a vapor pressure at 200°C of less than 15 kPa and can therefore be chosen from:

[0078] - higher alkanes containing more than 14 carbon atoms; for example, the vapor pressure of n-hexadecane (Cl 6) at 200°C is 10 kPa. When the solvent is an alkane, the thermolysis reaction takes place in suspension.

[0079] - fatty acids containing more than 8 carbon atoms; for example, the vapor pressure of capric acid (CIO) at 200 °C is 11.2 kPa. When the solvent is a fatty acid, the thermolysis reaction takes place in suspension.

[0080] - Polyglycol dimethyl ethers (glymes) from tetraglyme; for example, the vapor pressure of tetraglyme at 200°C is 10.2 kPa. When the solvent is a glyme, the thermolysis reaction takes place in solution.

[0081] - sulfolane, which has a vapor pressure of 10.3 kPa at 200°C. When the solvent is sulfolane, the thermolysis reaction takes place in solution.

[0082] According to one embodiment, when the thermolysis reaction of the poly(3-hydroxyalkanoate) is carried out in suspension or solution in a solvent, the operating pressure is between 1.5 times the vapor pressure of the solvent at the thermolysis temperature and twice the vapor pressure of at least one inhibitor at the thermolysis temperature.

[0083] The preferred operating pressure for the thermolysis reaction of poly(3-hydroxyalkanoate) is just below the vapor pressure of the inhibitor.

[0084] According to a preferred embodiment, the polymerization inhibitor is hydroquinone methyl ether and the operation is carried out in solution in a solvent such as sulfolane or tetraglyme.

[0085] According to one embodiment, the thermolysis of the poly(3-hydroxyalkanoate) takes place in the absence of a catalyst. The use of catalysts makes it possible to accelerate the kinetics of thermolysis and / or to reduce its temperature. However, the use of a catalyst makes the process more complex and more difficult to implement on an industrial scale.

[0086] The invention also relates to a process for purifying the unsaturated ap carboxylic acid(s) obtained by the poly(3-hydroxyalkanoate) thermolysis process carried out at a pressure less than twice the vapor pressure of at least one polymerization inhibitor at the thermolysis temperature, characterized in that it comprises a step of condensing the vapors of the unsaturated ap carboxylic acid(s) thus obtained, followed by one or more purification steps. The purification operations may generally comprise distillations, liquid / liquid extractions, separations using a film evaporator, or crystallizations, or a combination of these techniques.

[0087] The following examples illustrate the present invention without, however, limiting its scope.

[0088] EXPERIMENTAL PART

[0089] Thermolysis tests of poly(3-hydroxypropionate) (P3HP) to generate acrylic acid (AA) are carried out in a laboratory setup. 2 g of pure P3HP are introduced into a 25 mL two-necked flask.

[0090] An inhibitor (PTZ or EMHQ) may be added at a rate of 20 mg.

[0091] Optionally, a solvent is added at a rate of 10 g.

[0092] The side neck of the flask is equipped with a thermometer to monitor the reaction temperature. The upper neck of the flask is equipped with a separation bridge leading to a water-cooled side condenser, which in turn leads to a recipe consisting of a second 25 mL flask. A connection between the condenser and the recipe allows for reduced pressure to be established in the assembly.

[0093] At the beginning of the experiment, the system is placed under the desired pressure and then the flask containing the P3HP, and possibly the inhibitor and / or the solvent, is placed in a heating system to establish the desired thermolysis temperature (oil bath or electric heating mantle). The recipe is cooled by an ice bath. The separation bridge between the thermolysis flask and the lateral condenser is left uninsulated in order to simulate the existence of cold spots.

[0094] As soon as the thermolysis reactor reaches more than 170 °C, the formation of AA vapors is observed, which condense mainly in the lateral condenser but also on the cold spots of the separation bridge. After 4 h of heating, the formation of AA vapors in the thermolysis reactor diminishes and the experiment is then stopped. The fouling state of the separation bridge, representing the zone of involuntary condensation of AA on cold spots in an industrial installation, is then judged visually. The AA recovered in the recipe is also analyzed by gas chromatography in order to verify the presence or absence of the inhibitor possibly introduced into the thermolysis reactor, testifying to its volatility or not in the experimental conditions tested.

[0095] The main results obtained are presented in Table 1.

[0096] Comparative tests 1, 2, 8, 9, 15 and 16, carried out without any polymerization inhibitor, show a strong fouling of the separation bridge where hot AA vapors condense on cold points, and this is carried out without solvent (1, 2), in suspension (8, 9) or in solution (15, 16) as well as at atmospheric pressure (1, 8, 15) or under a reduced pressure of 20 kPa (2, 9, 16).

[0097] Comparative tests 3, 6, 7, 10, 13, 14, 17, 20 and 21, carried out in the presence of a polymerization inhibitor but under an operating pressure in the assembly, greater than twice the vapor pressure of the inhibitor at the thermolysis temperature, also show a strong fouling of the separation bridge where hot AA vapors condense on cold points, and this is carried out without solvent (3, 6, 7), in suspension (10, 13, 14) or in solution (17, 20, 21). It is also noted that no traces of the inhibitor are found in the AA recovered in the recipe, a sign that it was not volatile under the operating conditions of the thermolysis.

[0098] Tests 4, 5, 11, 12, 18 and 19, according to the invention, carried out in the presence of a polymerization inhibitor under an operating pressure in the assembly, less than twice the vapor pressure of the inhibitor, show a notable reduction in the fouling of the separation bridge where hot AA vapors condense on cold points, and this whether carried out in mass (4, 5), in suspension (11, 12) or in solution (18, 19). The presence of inhibitor in the AA recovered in the recipe is also noted, at least in trace amounts, a sign that it was volatile under the operating conditions of the thermolysis.The reduction in fouling, as well as the presence of the inhibitor in the formed AA, is small but significant when the operating pressure is just below twice the vapor pressure of the inhibitor at the thermolysis temperature (4, 11, 18), and more marked when the operating pressure is below the vapor pressure of the inhibitor at the thermolysis temperature (5, 12, 19). [Table 1]. Additional tests 22 to 31 (Table 2), carried out in solvent at 200°C and 20 kPa in the presence of EMHQ, i.e. under the conditions of the invention, show an absence of fouling of the separation bridge where hot AA vapors condense on cold points. The presence of the inhibitor in the AA recovered in the recipe is also noted, a sign that it is volatile under the operating conditions of the thermolysis. They make it possible to demonstrate the importance to be given to the choice of solvent during a thermolysis of poly(3-hydroxyalkanoate) carried out according to the invention in a solvent medium. Thus, if the vapor pressure of the solvent at the thermolysis temperature (here 200°C) is not less than three quarters of the operating pressure (here 20 kPa, i.e. a vapor pressure of the solvent at 200°C less than 15 kPa), we observe a strong pollution of the AA recovered in the recipe by the solvent used (22, 26, 29).This phenomenon is drastically limited when using a solvent whose vapor pressure at the thermolysis temperature is less than three-quarters of the operating pressure (23, 24, 25, 27, 28, 30, 31). C14 = n-tetradecane; C16 = n-hexadecane; C18 = n-octadecane; C20 = n-eicosane.

[0099] C8 acid = octanoic acid; C10 acid = decanoic acid; C12 acid = dodecanoic acid

[0100] [Table 2]

Claims

CLAIMS Process for the production of unsaturated carboxylic acids by thermolysis of poly(3-hydroxyalkanoate) carried out in the presence of at least one polymerization inhibitor, characterized in that the operating pressure in the reactor is less than twice the vapor pressure of one of the inhibitors at the thermolysis temperature. Process according to claim 1, wherein the thermolysis temperature is between 130 and 300°C, preferably between 170 and 230°C. Process according to one of claims 1 and 2, wherein the inhibitor content is from 50 ppm to 5% by weight, in particular from 0.01% to 3% by weight, relative to the weight of the poly(3-hydroxyalkanoate). Process according to one of claims 1 to 3 wherein said at least one of the polymerization inhibitors is a compound chosen from phenolic derivatives, phenothiazine derivatives, nitroxide derivatives or paraphenylenediamine derivatives.Process according to any one of the preceding claims wherein said at least one of the polymerization inhibitors is hydroquinone methyl ether (HMEQ). Process according to any one of the preceding claims wherein the poly(3-hydroxyalkanoate) is of petrochemical origin or at least partly of renewable origin. Process according to any one of claims 1 to 5 wherein the poly(3-hydroxyalkanoate) is more than 50% by weight, preferably more than 80% by weight, advantageously 100% by weight of renewable origin.

8. Process according to any one of the preceding claims in which the poly(3-hydroxyalkanoate) is obtained by chemical reaction.

9. Process according to claim 7 in which the poly(3-hydroxyalkanoate) is obtained by fermentation.

10. Method according to claim 9 in which the poly(3-hydroxyalkanoate) is separated from the biological medium prior to the thermolysis step, for example by extraction.

11. Process according to any one of the preceding claims in which the thermolysis reaction is carried out in the absence of solvent.

12. Process according to any one of claims 1 to 10 in which the thermolysis reaction is carried out in suspension in a solvent.

13. Process according to any one of claims 1 to 10 in which the thermolysis reaction is carried out in solution in a solvent.

14. Process according to any one of claims 12 or 13 in which the solvent has a vapor pressure, at the temperature of the thermolysis reaction, less than three-quarters of the operating pressure.

15. The method of claim 13 wherein the solvent is sulfolane.

16. The method of claim 15 wherein at least one of the polymerization inhibitors is hydroquinone methyl ether.

17. Process according to any one of the preceding claims in which the thermolysis of the poly(3-hydroxyalkanoate) is carried out in batch.

18. Process according to any one of claims 1 to 16 in which the thermolysis of the poly(3-hydroxyalkanoate) is carried out continuously.

19. A method according to any one of the preceding claims wherein the poly(3-hydroxyalkanoate) contains the 3-hydroxypropionate unit and at least one of the a~P unsaturated carboxylic acids produced is acrylic acid.

20. A method according to any one of the preceding claims wherein the poly(3-hydroxyalkanoate) is poly(3-hydroxypropionate) and the unsaturated α-P carboxylic acid produced is acrylic acid.

21. A method according to any one of claims 1 to 18 wherein the poly(3-hydroxyalkanoate) contains the 3-hydroxybutyrate unit and at least one of the a~P unsaturated carboxylic acids produced is crotonic acid.

22. A method according to any one of claims 1 to 18 and 21, wherein the poly(3-hydroxyalkanoate) is poly(3-hydroxybutyrate) and the unsaturated α-P carboxylic acid produced is crotonic acid.

23. A method according to any one of claims 1 to 18 wherein the poly(3-hydroxyalkanoate) contains the 3-hydroxyisobutyrate unit and at least one of the a~P unsaturated carboxylic acids produced is methacrylic acid.

24. A process according to any one of claims 1 to 18 and 23, wherein the poly(3-hydroxy alkanoate) is poly(3-hydroxyisobutyrate) and the unsaturated carboxylic acid produced is methacrylic acid.

25. Process for the manufacture of unsaturated carboxylic acids obtained by the process according to any one of the preceding claims, characterized in that it comprises a step of condensation of the products thus obtained, followed by one or more purification steps chosen from distillation, liquid / liquid extraction, separation using a film evaporator, or crystallization or a combination of these techniques.

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