Improved process for producing alpha, beta-unsaturated carboxylic acids from poly (3-hydroxyalkanoates)

Abstract

The present invention relates to a process for the production of alpha-beta-unsaturated carboxylic acids by the pyrolysis of poly (3-hydroxyalkanoates), while limiting the fouling phenomena associated with the unexpected condensation of the hot vapors of the generated alpha, beta-unsaturated carboxylic acids on the walls of the process and with the subsequent formation of solids by free-radical polymerization. The invention is based on the introduction of a free radical polymerization inhibitor into a pyrolysis reactor and the use of specific pyrolysis conditions to partially volatilize the inhibitor during the pyrolysis of the poly (3-hydroxyalkanoate).

Description

Technical Field

[0001] The present invention relates to a process for producing α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxyalkanoates) while limiting fouling phenomena associated with the accidental condensation of the hot α,β-unsaturated carboxylic acid vapors produced on the walls of the process and with the subsequent formation of solids by free-radical polymerization reactions. The invention is based on the introduction of a free-radical polymerization inhibitor in the pyrolysis reactor and the use of specific pyrolysis conditions so that the inhibitor partially volatilizes during the pyrolysis of the poly(3-hydroxyalkanoates). Thus, in the event of an accidental condensation of the hot vapors of the produced α,β-unsaturated carboxylic acids on the walls of the process, the free-radical polymerization inhibitor also condenses and protects the formed liquid phase from free-radical polymerization reactions. Background Art

[0002] At present, α,β-unsaturated carboxylic acids are mainly produced industrially from fossil raw materials. For example, acrylic acid is obtained by oxidation of propylene or methacrylic acid by oxidation of isobutylene.

[0003] There is a high demand for these α,β-unsaturated carboxylic acids as monomers for a variety of applications and they are required to be obtained from bio-based raw materials, which are derived from renewable organic matter (biomass) such as microorganisms, plants or animals.

[0004] One possible route to obtain these α,β-unsaturated carboxylic acids is to pyrolyze the corresponding poly(3-hydroxyalkanoate) at temperatures between 150°C and 300°C as follows:

[0005]

[0006] R1=H or alkyl, R2=H or alkyl; n is a number greater than 30

[0007] If R1=R2=H:

[0008] - poly(3-hydroxyalkanoate) = poly(3-hydroxypropionate) (P3HP);

[0009] -α,β-unsaturated carboxylic acid = acrylic acid (propenoic acid) (acrylic acid, acrylic acid).

[0010] If R1 = methyl, R2 = H:

[0011] - poly(3-hydroxyalkanoate)=poly(3-hydroxyisobutyrate) (P3HiB);

[0012] -α,β-unsaturated carboxylic acid = methacrylic acid (methacrylic acid).

[0013] If R1=H, R2=methyl:

[0014] - poly(3-hydroxyalkanoate) = poly(3-hydroxybutyrate) (P3HB);

[0015] -α,β-unsaturated carboxylic acid = but-2-enoic acid (crotonic acid).

[0016] If R1=H, R2=ethyl:

[0017] - the poly(3-hydroxyalkanoate) is poly(3-hydroxyvalerate) (P3HV);

[0018] -α,β-unsaturated carboxylic acid = pent-2-enoic acid.

[0019] These poly(3-hydroxyalkanoates) can be obtained by chemical conversion of fossil raw materials or by fermentation of biomass.

[0020] A potential problem in any process for producing α,β-unsaturated carboxylic acids is the tendency of these compounds to undergo free radical polymerization at high temperatures and in the liquid phase. This occurs not only in liquid phases that are formed intentionally, such as in distillation columns, reactors or condensers, but can also occur in liquid phases that are formed accidentally, such as when hot vapor unexpectedly condenses on the walls of cold spots. The usual consequence is the deposition of solid polymer in the equipment, which eventually leads to blockages and requires downtime for cleaning, resulting in the difficulty and cost of non-productive downtime.

[0021] In order to reduce these disadvantages, free radical polymerization inhibitors are usually added at various stages of the production process, namely the synthesis stage and the purification stage.

[0022] For example, in the industrial production of acrylic acid (AA), a free radical polymerization inhibitor is added to an absorption column for absorbing AA vapor produced by catalytic oxidation of propylene into water, then added to each distillation column and finally added to the final product.

[0023] Free radical polymerization inhibitors commonly used in these production processes include phenolic derivatives, such as hydroquinone (HQ) and its derivatives (such as hydroquinone methyl ether (HQME), 2,6-di-tert-butyl-4-methylphenol (BHT) or 2,4-dimethyl-6-tert-butylphenol (Topanol A)); phenothiazine and its derivatives; nitroxide compounds, such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO); and amino compounds, such as p-phenylenediamine derivatives.

[0024] A disadvantage of these inhibitors is that they are generally considered to be non-volatile under the production conditions of α,β-unsaturated carboxylic acids. In order to be present in all liquid phases containing α,β-unsaturated acids, they must be injected into the reaction, but also into the feed, boilers, condensers and reflux of the purification equipment. Spraying of solutions of the inhibitors can also be used to protect all surfaces on which hot α,β-unsaturated acid vapors could accidentally condense. This problem is well known to those skilled in the art; for example, during the industrial purification of acrylic acid, polymerization inhibitors are added to the feed, condensers and reflux of distillation columns, but are also often applied in the form of sprays to protect domes, goosenecks, manholes or any other column components on which acrylic acid vapors could condense.

[0025] Document EP 2398832 describes another solution aimed at preventing polymerization, including in the event of accidental condensation of AA vapors. It uses a second type of inhibitor, called volatile inhibitors, i.e. inhibitors that have a volatility similar to that of the α,β-unsaturated acid (here acrylic acid) under production operating conditions. This inhibitor is then present in the gas phase and condenses simultaneously with the acrylic acid vapors in the event of accidental condensation. However, these polymerization inhibitors (nitrobenzene derivatives) are toxic.

[0026] The polymerization problem also exists in the production of α,β-unsaturated carboxylic acids by pyrolysis of the corresponding poly(3-hydroxyalkanoates).

[0027] US2568636 describes the thermal decomposition of poly(3-hydroxypropionate) (P3HP) at temperatures between 130°C and 300°C to form acrylic acid (AA), and uses triaryl phosphates to limit the polymerization of AA in the pyrolysis reactor. Patent US 3002017 describes a similar pyrolysis process in which AA vapor is absorbed by cold AA to limit polymerization in the condensation stage.

[0028] US 9115070 describes the process of pyrolyzing P3HP to form AA using a tertiary amine catalyst to reduce the reaction temperature. Conventional polymerization inhibitors, such as phenothiazine (PTZ), can be used in the reaction medium at a weight ratio of 10 to 1000 ppm to reduce the polymerization of AA formed in the pyrolysis medium.

[0029] US10065914 describes the pyrolysis of P3HP to form AA at temperatures between 100°C and 300°C, using a sodium acrylate catalyst to reduce the reaction temperature, thereby limiting the risk of polymerization of the AA formed in the pyrolysis medium. The deliberate introduction of polymerization inhibitors (such as PTZ and HQME) in the liquid phase of the pyrolysis reactor and in the liquid phase of the distillation or condenser can also reduce polymerization phenomena.

[0030] However, the processes of the prior art have a major drawback. They describe how to reduce the risks associated with the polymerization of α,β-unsaturated acids in liquid phases by injecting polymerization inhibitors into these intentionally formed liquid phases (pyrolysis medium, condensers, liquid phases of distillation columns, etc.), but do not provide a solution in case of accidental formation of liquid phases (for example when condensation of α,β-unsaturated acids is not desired at "cold spots" of the equipment). The use of sprays allowing these inhibitors to be sprayed onto all the walls of industrial systems is possible, but is complicated to implement. The use of unconventional inhibitors (such as nitrobenzene derivatives) is also complicated to implement industrially.

[0031] The inventors have now surprisingly found that polymerization phenomena associated with accidental condensation of hot vapors can be significantly reduced during the synthesis of α,β-unsaturated carboxylic acids from poly(3-hydroxyalkanoates) without the use of unconventional inhibitors. In particular, the parameters of the thermolysis reaction of poly(3-hydroxyalkanoates) to α,β-unsaturated carboxylic acids can be adjusted to obtain a significant volatility of some conventional polymerization inhibitors. These inhibitors are then present in the gas phase and condense simultaneously when the α,β-unsaturated carboxylic acid accidentally condenses on a cold spot, immediately protecting the formed liquid phase. There is no need for complex systems for dispensing inhibitors at multiple points in the process, nor for expensive and toxic inhibitors.

[0032] The present invention thus proposes a simple and easy to implement solution to reduce fouling phenomena, thereby maintaining high reliability and increasing productivity during the production of α,β-unsaturated carboxylic acids from poly(3-hydroxypropionate). Summary of the invention

[0033] The present invention relates to a process for producing α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxyalkanoates), carried out in a pyrolysis reactor, from which the carboxylic acid vapors generated enter a condenser, in the presence of one or more polymerization inhibitors, characterized in that the pressure in the reactor is adjusted to less than twice the vapor pressure of at least one inhibitor at the temperature at which the pyrolysis is achieved.

[0034] According to the present invention, the pyrolysis conditions (pressure and temperature) used are such that a polymerization inhibitor has a significant volatility, thereby achieving the desired effect.

[0035] According to various embodiments, the process includes the following features, which can be used in combination as appropriate. Unless otherwise specified, the contents shown are by weight. Within the numerical range shown, the limits are included.

[0036] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process comprises a single type of 3-hydroxyalkanoate unit, and the product thus formed consists of a single α,β-unsaturated carboxylic acid.

[0037] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process comprises several different 3-hydroxyalkanoate units, so that the formed product consists of a mixture of different α,β-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).

[0038] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is obtained from raw materials of fossil origin.

[0039] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is obtained from raw materials of renewable origin or at least partially from raw materials of renewable origin. According to this embodiment, the poly(3-hydroxyalkanoate) has more than 50% by weight, preferably more than 80% by weight, more preferably 100% by weight of renewable origin.

[0040] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is obtained by chemical reaction, for example P3HP is obtained by polymerization of β-propiolactone, which itself is obtained from ethylene oxide and carbon monoxide.

[0041] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is obtained by biological reaction, in particular by fermentation.

[0042] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is purified prior to the pyrolysis reaction.

[0043] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is not purified before use, in particular without isolating the cell membranes, if it is obtained by fermentation.

[0044] According to one embodiment, the poly(3-hydroxyalkanoate) is obtained intracellularly by a fermentation reaction, and the biomass is washed and dried, but the poly(3-hydroxyalkanoate) is not separated from the cell membrane before the pyrolysis step.

[0045] According to one embodiment, the poly(3-hydroxyalkanoate) is obtained intracellularly by a fermentation reaction, the biomass is washed and dried, and the poly(3-hydroxyalkanoate) is separated from the cell membranes, for example by extraction, prior to the pyrolysis step.

[0046] According to one embodiment, the pyrolysis reaction of poly(3-hydroxyalkanoate) is carried out in the absence of a solvent, and the product is thus in a solid or molten state.

[0047] According to one embodiment, the thermolysis reaction of poly(3-hydroxyalkanoate) is carried out in solution.

[0048] According to one embodiment, the pyrolysis reaction of poly(3-hydroxyalkanoate) is carried out in suspension.

[0049] According to one embodiment, the thermolysis reaction of poly(3-hydroxyalkanoate) is carried out in a batch mode.

[0050] According to one embodiment, the pyrolysis reaction of poly(3-hydroxyalkanoate) is performed continuously.

[0051] According to one embodiment, the pyrolysis reaction of poly(3-hydroxyalkanoate) is carried out in the absence of a catalyst.

[0052] The polymerization inhibitor used in the process according to the invention is selected from the inhibitors conventionally used in existing industrial processes for producing α,β-unsaturated carboxylic acids. These include phenolic derivatives, such as hydroquinone (HQ) and its derivatives; phenothiazine and its derivatives; nitroxide compounds; and amino compounds, such as p-phenylenediamine derivatives.

[0053] According to one embodiment, the poly(3-hydroxyalkanoate) contains 3-hydroxypropionate units, and at least one of the generated α,β-unsaturated carboxylic acids is acrylic acid.

[0054] According to one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxypropionate), and the generated α,β-unsaturated carboxylic acid is acrylic acid.

[0055] According to one embodiment, the poly(3-hydroxyalkanoate) contains 3-hydroxybutyrate units, and at least one of the generated α,β-unsaturated carboxylic acids is crotonic acid.

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

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

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

[0059] Another subject of the invention relates to a process for purifying α,β-unsaturated carboxylic acids obtained by a pyrolysis process of poly(3-hydroxyalkanoates), the pyrolysis process being carried out at a pressure less than twice the vapor pressure of at least one inhibitor at the pyrolysis temperature, characterized in that it comprises a step of condensing the vapor of the α,β-unsaturated carboxylic acid or cars thus obtained, followed by one or more purification steps.

[0060] The present invention meets the needs expressed in the prior art. It makes it possible to prevent the risk of fouling due to the accidental condensation of α,β-unsaturated carboxylic acid vapors on cold spots when generating α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxypropionate). In particular, the present invention makes it possible to protect the area between the pyrolysis reactor and the condenser. By making the polymerization inhibitor volatile in the pyrolysis medium, it will be present in the gas phase throughout the part of the plant where the α,β-unsaturated carboxylic acid or carboxylic acids are in the gas phase. The present invention also makes it possible to avoid the formation of polymers in the reaction medium.

[0061] The present invention will be described in more detail in the following description. DETAILED DESCRIPTION

[0062] The present invention aims to produce α,β-unsaturated carboxylic acids on an industrial scale by pyrolysis of poly(3-hydroxyalkanoates) without facing the problem of equipment fouling due to polymerization of the α,β-unsaturated carboxylic acid vapor when it condenses on cold spots of the equipment.

[0063] The present invention proposes a process capable of reducing or eliminating this risk of fouling. The invention is based on the addition of a polymerization inhibitor and the selection of pressure and temperature conditions in the pyrolysis reactor of poly(3-hydroxyalkanoate) such that the inhibitor has a 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 pyrolysis temperature.

[0064] The term "thermolysis" of poly(3-hydroxyalkanoate) refers to its chemical decomposition into α,β-unsaturated carboxylic acids under the action of temperature. This term is synonymous with pyrolysis.

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

[0066] In the description of the present invention, the term "vapor pressure" will be used synonymously with the term "saturated vapor pressure." In addition, the term "vapor pressure" is synonymous with "vapor tension."

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

[0068] The reaction medium in the pyrolysis reactor contains at least one polymerization inhibitor in a proportion 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 two or more inhibitors are present, their total content does not exceed 5% by weight.

[0069] The polymerization inhibitor is selected from inhibitors conventionally used in existing industrial processes for producing α,β-unsaturated carboxylic acids. These include phenolic derivatives such as hydroquinone (HQ) and its derivatives (such as hydroquinone methyl ether (HQME), 2,6-di-tert-butyl-4-methylphenol (BHT) or 2,4-dimethyl-6-tert-butylphenol (Topanol A)); phenothiazine and its derivatives; nitroxide compounds such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO); amino compounds such as p-phenylenediamine derivatives.

[0070] In the pyrolysis process of poly(3-hydroxyalkanoate) according to the invention, the temperature and pressure conditions in the pyrolysis reactor are selected so that the formed α,β-unsaturated carboxylic acid is in the vapor state and at least one inhibitor is volatile. This is achieved when the pressure in the reactor is less than twice the vapor pressure of one inhibitor at the pyrolysis temperature.

[0071] According to one embodiment, at least one of the polymerization inhibitors is hydroquinone methyl ether (HQME).

[0072] For example, the vapor pressure of HQME is:

[0073] 20 kPa at -190 °C; the pressure in the reactor was adjusted to less than 40 kPa to perform pyrolysis at 190 °C;

[0074] 28.5 kPa at -200 °C; the pressure in the reactor was adjusted to less than 57 kPa to carry out pyrolysis at 200 °C;

[0075] 39 kPa at -210°C; the pressure in the reactor was adjusted to less than 78 kPa to carry out pyrolysis at 210°C.

[0076] The process according to the invention thus makes it possible to use specific pressure conditions in order to obtain a volatility of the inhibitor so as to protect the operation when the hot vapor of the α,β-unsaturated carboxylic acid undesirably condenses on a cold spot of one of the walls of the apparatus.

[0077] According to one embodiment of the present invention, the pyrolysis reaction is carried out in the presence of a solvent, whether in solution or in suspension. In order to limit the volatilization of the solvent and the α,β-unsaturated carboxylic acid vapor generated during the pyrolysis of the poly(3-hydroxyalkanoate), the solvent is selected so that its vapor pressure at the pyrolysis temperature of the poly(3-hydroxyalkanoate) is less than three quarters of the pressure at which the pyrolysis is carried out.

[0078] According to one embodiment, for operating conditions of 200°C and 20 kPa, the solvent should have a vapor pressure of less than 15 kPa at 200°C and can therefore be selected from:

[0079] - Higher alkanes containing more than 14 carbon atoms; for example, n-hexadecane (C16) has a vapor pressure of 10 kPa at 200° C. When the solvent is an alkane, the pyrolysis reaction is carried out as a suspension.

[0080] - fatty acids containing more than 8 carbon atoms; for example, capric acid (C10) has a vapor pressure of 11.2 kPa at 200° C. When the solvent is a fatty acid, the pyrolysis reaction is carried out in suspension.

[0081] - Polyethylene glycol dimethyl ether (glyme) starting from tetraethylene glycol dimethyl ether; for example, the vapor pressure of tetraethylene glycol dimethyl ether at 200° C. is 10.2 kPa. When the solvent is polyethylene glycol dimethyl ether, the thermolysis reaction is carried out as a suspension.

[0082] - Sulfolane, which has a vapor pressure of 10.3 kPa at 200° C. When the solvent is sulfolane, the thermal decomposition reaction is carried out in solution.

[0083] According to one embodiment, when the pyrolysis reaction of poly(3-hydroxyalkanoate) is carried out in a solvent as a suspension or solution, the operating pressure is between 1.5 times the vapor pressure of the solvent at the pyrolysis temperature and twice the vapor pressure of at least one inhibitor at the pyrolysis temperature.

[0084] The preferred operating pressure for the pyrolysis reaction of poly(3-hydroxyalkanoate) is slightly below the vapor pressure of the inhibitor.

[0085] 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 tetrahydrothiophene sulfone or tetraethylene glycol dimethyl ether.

[0086] According to one embodiment, the pyrolysis of poly(3-hydroxyalkanoate) is carried out in the absence of a catalyst. The use of a catalyst can accelerate the kinetics of the pyrolysis and / or reduce its temperature. However, the use of a catalyst makes the process more complicated and more difficult to implement on an industrial scale.

[0087] The invention also relates to a process for purifying one or more α,β-unsaturated carboxylic acids obtained by a pyrolysis process of poly(3-hydroxyalkanoates), the pyrolysis process being carried out at a pressure less than twice the vapor pressure of at least one inhibitor at the pyrolysis temperature, characterized in that it comprises a step of condensing the vapor of the one or more α,β-unsaturated carboxylic acids thus obtained, followed by one or more purification steps. The purification operation may generally comprise distillation, liquid / liquid extraction, separation using a membrane evaporator, or crystallization, or a combination of these techniques.

[0088] The following examples illustrate the present invention but do not limit its scope.

[0089] Experimental Section

[0090] The pyrolysis of poly(3-hydroxypropionate) (P3HP) to produce acrylic acid (AA) was tested in a laboratory apparatus. 2 grams of pure P3HP were introduced into a 25 ml two-necked flask.

[0091] Optionally, 20 mg of inhibitor (PTZ or HQME) was added.

[0092] Optionally, 10 grams of solvent were added.

[0093] The side neck of the flask was equipped with a thermometer to monitor the reaction temperature. The upper neck of the flask was equipped with a split bridge connected to a water-cooled side condenser, which in turn was connected to a receiver consisting of a second 25 ml flask. A valve between the condenser and the receiver allowed the establishment of reduced pressure in the assembly.

[0094] At the beginning of the experiment, the system is placed at the desired pressure, and the flask containing P3HP (and optional inhibitor and / or solvent) is placed in the heating system to establish the desired pyrolysis temperature (oil bath or electric heating mantle). The receiver is cooled with an ice bath. The separation bridge between the pyrolysis flask and the side condenser is not insulated to simulate the presence of a cold spot.

[0095] When the pyrolysis reactor temperature was greater than 170° C., the formation of AA vapor was observed, which condensed mainly in the side condensers, but also on the cold spots of the separation bridge. After 4 hours of heating, the formation of AA vapor in the pyrolysis reactor gradually decreased, and the experiment was stopped.

[0096] The fouling state of the separation bridge was then judged visually, representing an area of ​​unexpected condensation of AA on cold spots in an industrial unit. The AA collected in the receiver was also analyzed by gas chromatography to verify the presence of inhibitors introduced in the pyrolysis reactor, demonstrating their volatility or non-volatility in the experimental conditions tested.

[0097] The main results obtained are shown in Table 1.

[0098] Comparative tests 1, 2, 8, 9, 15 and 16, carried out in the absence of any polymerization inhibitor, show severe fouling of the separation bridge where the hot AA vapor condenses on the cold spot, and this is true whether carried out without solvent (1, 2), as a suspension (8, 9) or as a solution (15, 16) and at atmospheric pressure (1, 8, 15) or under reduced pressure of 20 kPa (2, 9, 16).

[0099] Comparative tests 3, 6, 7, 10, 13, 14, 17, 20 and 21, performed in the presence of polymerization inhibitors but at an operating pressure in the assembly that was greater than twice the vapor pressure of the inhibitor at the pyrolysis temperature, also showed severe fouling of the separation bridge where the hot AA vapor condensed on the cold spot, and this was true whether performed without solvent (3, 6, 7), in suspension (10, 13, 14) or in solution (17, 20, 21). It was also noted that there was no trace of the inhibitor in the AA recovered in the receiver, indicating that it was not volatile under the pyrolysis operating conditions.

[0100] Tests 4, 5, 11, 12, 18 and 19 according to the invention, carried out in the presence of a polymerization inhibitor and at an operating pressure in the assembly that was less than twice the vapor pressure of the inhibitor, showed a significant reduction in fouling of the separation bridge where the hot AA vapor condensed on the cold spot, whether carried out in masse (4, 5), in suspension (11, 12) or in solution (18, 19). It was also noted that the inhibitor was present in the AA collected in the receiver, at least at trace levels, indicating that it is volatile under the pyrolysis operating conditions. The reduction in fouling and the presence of the inhibitor in the formed AA was small but significant when the operating pressure was slightly less than twice the vapor pressure of the inhibitor at the pyrolysis temperature (4, 11, 18); the effect was more pronounced when the operating pressure was below the vapor pressure of the inhibitor at the pyrolysis temperature (5, 12, 19).

[0101] [Table 1]

[0102]

[0103]

[0104] Supplementary tests 22 to 31 (Table 2) were carried out in the presence of HQME at 200°C and 20 kPa in the solvent, i.e. under the conditions of the invention, showing no fouling of the separation bridge where the hot AA vapor condenses on the cold spot. The presence of inhibitors in the AA recovered in the receiver was also noted, indicating its volatility under the pyrolysis operating conditions.

[0105] They show the importance of the choice of solvent when the thermolysis of poly(3-hydroxyalkanoates) is 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. the vapor pressure of the solvent at 200° C. is less than 15 kPa), it is observed that the AA collected in the receiver is severely contaminated with the solvent used (22, 26, 29). This phenomenon is very limited when a solvent whose vapor pressure at the thermolysis temperature is less than three quarters of the operating pressure is used (23, 24, 25, 27, 28, 30, 31).

[0106] C14 = n-tetradecane; C16 = n-hexadecane; C18 = n-octadecane; C20 = n-eicosane C8 acid = octanoic acid; C10 acid = decanoic acid; C12 acid = dodecanoic acid

[0107] [Table 2]

[0108]

[0109]

Claims

1. A process for producing α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxyalkanoates), said pyrolysis being 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 inhibitor at the pyrolysis temperature.

2. The process according to claim 1, wherein the pyrolysis temperature is between 130°C and 300°C, preferably between 170°C and 230°C.

3. The process according to 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).

4. The process according to any one of claims 1 to 3, wherein the at least one polymerization inhibitor is a compound selected from phenol derivatives, phenothiazine derivatives, nitroxide derivatives or p-phenylenediamine derivatives.

5. The process according to any one of the preceding claims, wherein the at least one polymerization inhibitor is methyl ether hydroquinone (MEHQ).

6. The 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.

7. Process according to any one of claims 1 to 5, wherein the poly(3-hydroxyalkanoate) is of more than 50% by weight, preferably more than 80% by weight, advantageously 100% by weight, of renewable origin.

8. The process according to any one of the preceding claims, wherein the poly(3-hydroxyalkanoate) is obtained by chemical reaction.

9. The process of claim 7, wherein the poly(3-hydroxyalkanoate) is obtained by fermentation.

10. A process according to claim 9, wherein the poly(3-hydroxyalkanoate) is separated from the biological medium prior to the pyrolysis step, for example by extraction.

11. A process according to any one of the preceding claims, wherein the thermolysis reaction is carried out in the absence of a solvent.

12. The process according to any one of claims 1 to 10, wherein the thermolysis reaction is carried out as a suspension in a solvent.

13. The process according to any one of claims 1 to 10, wherein the thermolysis reaction is carried out as a solution in a solvent.

14. A process according to any one of claims 12 and 13, wherein the solvent has a vapour pressure at the temperature of the pyrolysis reaction which is less than three quarters of the operating pressure.

15. The process according to claim 13, wherein the solvent is sulfolane.

16. The process of claim 15, wherein at least one polymerization inhibitor is hydroquinone methyl ether.

17. A process according to any one of the preceding claims, wherein the pyrolysis of poly(3-hydroxyalkanoate) is carried out in batch mode.

18. The process of any one of claims 1 to 16, wherein the pyrolysis of the poly(3-hydroxyalkanoate) is performed continuously.

19. A process according to any one of the preceding claims, wherein the poly(3-hydroxyalkanoate) contains 3-hydroxypropionate units and at least one of the α,β-unsaturated carboxylic acids produced is acrylic acid.

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

21. The process of any one of claims 1 to 18, wherein the poly(3-hydroxyalkanoate) contains 3-hydroxybutyrate units and at least one of the produced α,β-unsaturated carboxylic acids is crotonic acid.

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

23. The process of any one of claims 1 to 18, wherein the poly(3-hydroxyalkanoate) contains 3-hydroxyisobutyrate units and at least one of the α,β-unsaturated carboxylic acids produced is methacrylic acid.

24. The process of any one of claims 1 to 18 and 23, wherein the poly(3-hydroxyalkanoate) is poly(3-hydroxyisobutyrate) and the α,β-unsaturated carboxylic acid produced is methacrylic acid.

25. Process for the production of α,β-unsaturated carboxylic acids obtained by the process according to any one of the preceding claims, characterized in that The production process comprises a step of condensing the product thus obtained, followed by one or more purification steps chosen from distillation, liquid / liquid extraction, separation using a membrane evaporator, or crystallization, or a combination of these techniques.

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