Process for the continuous upgrading of heavy by-products from acrylic acid production

Abstract

The present invention relates to a method for recovering, by thermal cracking, acrylic acid (AA) from heavy by-products (residues called LAA) from an AA production unit, for recycling in the acrylic acid production facility. Said method consists of two continuous steps while improving the current performance of cracking plants.

Description

[0001] CONTINUOUS RECYCLING PROCESS FOR HEAVY BY-PRODUCTS FROM

[0002] THE MANUFACTURE OF ACRYLIC ACID

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a process for the regeneration, by thermal cracking, of acrylic acid (AA), from heavy by-products (residues called LAA) from an AA production unit, with a view to their recycling in the acrylic acid production workshop. This process consists of two stages operating continuously while improving the current performance of cracking installations.

[0005] TECHNICAL BACKGROUND

[0006] Under the influence of temperature during the distillation stages, the production of acrylic acid is accompanied by the formation of heavy compounds, derived from the addition of compounds with nucleophilic properties to the double bond of unsaturated carbonyl monomers, by Michael reaction. "Heavy" compounds are those compounds whose boiling point is higher than that of the acrylic monomer produced.

[0007] In the case of an AA production unit, this essentially involves:

[0008] - addition derivatives of acrylic acid on the double bond of another acrylic acid molecule: 3-acryloxypropionic acid also called "dimeric acrylic acid" or "AA dimer";

[0009] - of acrylic acid addition derivatives on the double bond on an AA dimer molecule, to form the "AA trimer" and other oligomers formed by successive additions of acrylic acid on the double bonds of the preceding AA oligomers, and

[0010] - carboxylic acid addition derivatives formed as by-products of acrylic acid or water on the double bond of AA or the oligomers mentioned above.

[0011] The recovery of recoverable monomers from heavy Michael-derived compounds is difficult in the case of heavy compounds originating from an AA production unit. Indeed, during the thermal cracking process that regenerates acrylic acid, which is distilled and recovered, a residue remains whose viscosity increases sharply when high cracking yields are sought, until it can no longer be extracted from the cracking reactor. The main limiting factor in the efficiency of the regeneration of compounds derived from the Michael reaction contained in the heavy streams of AA workshops is the increase in the viscosity of the heavy residue obtained at the bottom of the cracker, when the fraction rich in acrylic monomers has been vaporized.

[0012] The vaporization of light compounds during cracking results in a concentration of heavy products in the residue stream, and an increase in the viscosity of this stream. However, the residue must remain sufficiently fluid after cooling to be transported and then treated for destruction.

[0013] In the case where there is a production of light esters (methyl acrylate (MA) or ethyl acrylate (EA)) close to the AA production unit, co-cracking of the respective heavy esters can improve the situation, by making the cracking residue more fluid. The proposed solution allows to recover the maximum amount of AA per cracking operation while managing the viscosity of the residue formed without being dependent on another production unit.

[0014] Thus, in document EP 717031, it has been shown that it is possible to improve the efficiency of the recovery of these valuable noble products, if the cracking is carried out from a mixture of heavy products coming from an AA production unit and an acrylic ester (EA) production unit, compared to the individual cracking of the heavy streams from these units. The effect of the addition of heavy products coming from the ester units (LEA) to the heavy products coming from an AA unit (LAA) is to reduce the viscosity of the final residue. The cracking reaction is carried out from mixtures with a ratio of heavy AA / heavy ester of 9 / 1 to 1 / 9, at a temperature of 180 to 220°C, under atmospheric pressure, during a residence time of 0.5 to 3 hours. In this process, the cracking and vaporization of the generated light compounds are carried out in a reactor, then the generated gas stream is sent to a distillation column and finally the bottom stream of the distillation column is recycled into the reactor.On the other hand, since the light fraction obtained by cracking is mainly made up of AA and ester acrylic monomers, which are particularly sensitive to polymerization, the distillation step must necessarily be carried out under reduced pressure, so as to reduce the temperature, to avoid the formation of polymer in the column. Furthermore, the rectification plates of the distillation column cause the effective separation of the polymerization inhibitors entrained in the gas mixture, which flow back to the bottom of the column, and consequently, it is necessary to introduce fresh polymerization inhibitors at the top of the column, to avoid the formation of polymers in the upper part of the column. Therefore, the reaction steps, carried out under higher pressure, and the distillation steps, carried out under reduced pressure, must be separated.The installation to carry out the process must therefore be equipped with a reactor and a condenser at the head, operated at the same pressure, and a distillation column operated at reduced pressure, fed by the condensed product, and comprising a boiler at the bottom, and at the head a condenser, reflux equipment and an inhibitor supply. This device is complicated and expensive.

[0015] Furthermore, co-cracking heavy AAs mixed with heavy EAs leads to operational constraints. When the ester unit is shut down, the cracking operation must be stopped. This results in economic losses.

[0016] In other cases, AA heavys are thermally cracked discontinuously without the addition of ester heavys and generate an extremely viscous residue, which limits the performance of this cracking and causes problems with the storage and transfer of the residues.

[0017] To overcome the viscosity problem, it is also known to add a solvent to the cracking residue of heavy AA.

[0018] EP 3255030 teaches the addition of higher alcohols during cleavage of the residue, with the maleic anhydride present in the residue being converted to maleic acid esters which are less susceptible to polymerization.

[0019] US 6414183 teaches diluting the discharged residue with solvents such as acetic acid, water, and methanol.

[0020] WO 2021 / 224044 describes a process for decomposing Michael adducts of acrylic acid, by dilution in a solvent 1 having a boiling point at 1013 hPa of at least 170°C and a solubility in water at 25°C of at least 20 g per 100 g of water, said solvent being selected from alcohols such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol and 2-ethoxyethanol, carboxamides such as N,N-dimethylacetamide, N-methylacetamide and N,N-dimethylformamide, sulfoxides such as dimethyl sulfoxide, and sulfones such as sulfolane.

[0021] However, this solution has several disadvantages, such as the generation of waste to be burned, if it is not an internal flow, or the provision of additional equipment for the production of the mixture. In addition, most of these solvents generate nitrogen or sulfur derivatives during burning. As a result, there is a need to improve the regeneration efficiency, by thermal cracking, of heavy compounds originating solely from AA units.

[0022] SUMMARY OF THE INVENTION

[0023] The invention relates to a process for regenerating a mixture of heavy by-products from an acrylic acid (LAA) production unit, said process comprising the following steps: i. introducing said heavy by-products with water into a hydrolyser and subjecting them to continuous hydrolysis, the water:LAA mass ratio ranging from 0.1 to 1.3 (inclusive) for a period of 1 to 10 h, preferably from 1 to 5 h, resulting in a mixture of hydrolysed products being obtained, ii. continuously injecting said mixture of hydrolysed products into a reactor and subjecting it to thermal cracking producing a gaseous overhead stream containing acrylic acid and water and a bottom stream (residue) concentrated in heavy products, iii. recovering a lighter fraction rich in AA and water which can be recycled to different points in the process, iv. recovering said residue for disposal treatment.

[0024] According to various embodiments, said method comprises the following characteristics, possibly combined.

[0025] According to one embodiment, the pressure in the hydrolyser varies between 0.1 and 2 MPa, preferably between 0.5 and 1.5 MPa.

[0026] According to one embodiment, the temperature in the hydrolyser varies between 80 and 200°C, preferably between 150 and 200°C.

[0027] According to one embodiment, the cracking temperature is between 140 and 260°C, preferably between 160 and 210°C.

[0028] According to one embodiment, the residence time of the reaction mixture in the cracking reactor is between 0.5 h and 10 h, preferably between 4 h and 10 h.

[0029] According to one embodiment, the thermal cracking reaction takes place at atmospheric pressure or under slight pressure (maximum 0.2 MPa).

[0030] According to one embodiment, the reactor bottom flow (residue) obtained at the end of the thermal cracking operation has a dynamic viscosity of less than 1 Pa.s, measured at a temperature of 100°C, for example using a Brookfield "CAP 1000+" cone-plate type viscometer.

[0031] According to one embodiment, said gaseous overhead stream containing acrylic acid and water is injected into a condenser.

[0032] According to one embodiment, the thermal cracking reaction takes place in the absence of a catalyst.

[0033] The present invention overcomes the disadvantages of the prior art. It allows the maximum amount of AA to be recovered per cracking operation, while managing the viscosity of the residue formed without being dependent on another production unit. This is achieved by combining a step of hydrolysis of the heavy by-products from an acrylic acid production unit, and a step of thermal cracking of the hydrolyzed products.

[0034] The main advantages of the method according to the invention are:

[0035] • A simple and inexpensive process in terms of investment since it requires practically only one additional piece of equipment (the hydrolyser) compared to the cracking stage alone.

[0036] • A process to reduce discharges by reducing the quantity of cracking residue.

[0037] • A lower cracking temperature compared to cracking without a hydrolysis step, thus allowing energy savings.

[0038] • A less viscous residue compared to cracking without a hydrolysis step.

[0039] • The cracking step does not depend on other units (esters).

[0040] • Hydrolysis also makes it possible to avoid special treatment of the residues to ensure their removal, such as the addition of solvent, for example, to make them more fluid.

[0041] • An increase in the recovery yield of noble products from the heavy Michael derivatives present in the heavy streams of AA workshops by pushing the cracking reaction further, the viscosity limit of the residue being reached later compared to a solution without hydrolysis.

[0042] LIGURES

[0043] Figure 1 schematically represents an embodiment of an installation according to the invention. DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0044] The invention is now described in more detail and in a non-limiting manner in the following description.

[0045] The term “heavy by-products from an acrylic acid production unit” includes:

[0046] • addition derivatives of acrylic acid on the double bond of another acrylic acid molecule: 3-acryloxypropionic acid also called “dimeric acrylic acid” or “AA dimer”;

[0047] • addition derivatives of acrylic acid to the double bond on an AA dimer molecule, to form the “AA trimer” and other oligomers formed by successive additions of acrylic acid to the double bonds of the preceding AA oligomers;

[0048] • carboxylic acid addition derivatives formed as by-products of acrylic acid (e.g. acetic acid) or water on the double bond of AA or the oligomers mentioned above.

[0049] The term "hydrolyzer" refers to a reactor in which the hydrolysis reaction of the mixture of water and heavy AA can take place. This reactor can be heated and maintain pressure. It can be a conventional stirred reactor, a plug-flow reactor with or without an internal premixer, or in a heat exchanger.

[0050] The invention is based on a thermal cracking process, coupled with a hydrolysis operation carried out beforehand on the heavy by-products from an AA production unit.

[0051] Acrylic monomers involved in Michael addition derivatives can be regenerated by hydrolyzing the oligomers prior to the heat treatment step. This hydrolysis reaction forms hydroxypropionic acid (HPA), which can be thermally cracked into acrylic acid. Hydrolysis reduces the oligomer chains, making the residue less viscous.

[0052] Continuous hydrolysis is carried out under pressure ranging from 0.1 to 2 MPa.

[0053] The regeneration efficiency (expressed as the cracking yield) depends essentially on: a / the parameters for hydrolysis: temperature and pressure, hydrolysis residence time and the Water / Heavy AA ratio, and

[0054] - b / parameters for cracking: the temperature and the residence time of the heat treatment. Increasing these last two parameters (b / ) tends to improve the regeneration efficiency, but this is done at the expense of an increase in the viscosity of the cracking residue.

[0055] Cracking performance is characterized by two values: the TRU or useful recovery rate: this is the quantity of acrylic acid recovered after cracking reduced to the quantity of heavy AA which feeds the cracker:

[0056] TRU = mass AA recovered / Heavy AA cracker feed the cracking rate or yield: this is the quantity of acrylic acid recovered after cracking reduced to the sum of the compounds that can be recovered in the cracker feed (Acrylic acid (AA), acrylic acid dimers (AA2) and hydroxypropionic acid (AHP)).

[0057] According to the embodiment of the process shown in Figure 1, the stream containing said heavy by-products from acrylic acid production workshop (LAA) and water are introduced together or separately in continuous mode into the RI reactor. The LAA stream is rich in heavy compounds derived from Michael addition generated during the synthesis and purification steps of acrylic acid, and also contains other heavy compounds accumulated during the synthesis and purification processes, in particular polymerization inhibitors.

[0058] The mixture (1) containing the heavy acrylic acid compounds and water is heated to the temperature required to carry out the hydrolysis of the Michael addition derivatives into lighter compounds. The stream (2) leaving the hydrolyser RI is then continuously introduced into a second reactor R2 where it is heated to the temperature required to carry out the cracking of the Michael addition derivatives into lighter compounds which are extracted in the form of a gaseous mixture (3) at the top of the reactor.

[0059] This vapor stream rich in acrylic acid and containing some heavy compounds including inhibitors in low concentration, is advantageously recycled in the acrylic acid production process, either directly in vapor form, or after total condensation in an El flux condenser (4).

[0060] According to one embodiment, at least one polymerization inhibitor is introduced at the condenser EL. These inhibitors are chosen from polymerization inhibitors known to those skilled in the art: phenolic derivatives such as hydroquinone and its derivatives such as hydroquinone methyl ether, 2,6-di-terbutyl-4-methyl phenol (BHT), and 2,4-dimethyl-6-terbutyl phenol (Topanol A), phenothiazine and its derivatives, manganese salts, such as manganese acetate, thiocarbamic or dithiocarbamic acid salts, such as metal thiocarbamates and dithiocarbamates, such as copper di-n-butyldithiocarbamate, N-oxyl compounds, such as 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (4-OH-TEMPO), compounds with groups nitroso, such as N-nitroso phenyl hydroxylamine and its ammonium salts, amine compounds such as paraphenylenediamine derivatives or a mixture of these inhibitors.

[0061] The residue flow recovered at the bottom of the reactor (5) is cooled, then removed in the form of a liquid of moderate viscosity, so that it can be transported without difficulty by pump, for example to storage or an incineration unit.

[0062] EXAMPLES

[0063] The following examples illustrate the invention without limiting it.

[0064] Example 1: Continuous hydrolysis and cracking (according to the invention)

[0065] The assembly consists of a piston reactor with a total volume of 16.3 cm 3 , placed in an oil bath heated to 150°C. Hydrolysis is carried out under pressure of 0.4 MPa, for a residence time of one hour. The hydrolyzed flow is continuously introduced into a glass reactor with a total volume of 500 cm 3, heated by oil recirculation through its double wall, equipped with a side outlet for the residue to exit, ensuring a constant useful volume of liquid contained in the reactor of 156.6 cm 3 .

[0066] The reactor is equipped with a stirrer, a temperature probe immersed in the liquid phase, a vertical pipe at the top for vapor extraction, and a condenser. The liquid (distillate) is collected in a receiving flask and analyzed.

[0067] Duration of the trial: 55 hours

[0068] Hydrolysis: T= 150°C, residence time: 1h, P = 0.4 MPa, water / heavy AA ratio = 0.5 Cracking: T= 173°C, atmospheric pressure, residence time: 10h, Water / heavy AA ratio = 0.5

[0069] TRU = 68.95%

[0070] Overall cracking rate = 94.85%

[0071] Viscosity 0.785 Pa.s. Example 2 (comparative): Cracking without prior hydrolysis

[0072] In this case, the flow of heavy AA is directly introduced continuously into a glass reactor with a total volume of 500 cm 3 , heated by oil recirculation through its double wall, equipped with a side outlet for the residue to exit, ensuring a constant useful volume of liquid contained in the reactor of 156.6 cm 3 .

[0073] The reactor is equipped with a stirrer, a temperature probe immersed in the liquid phase, a vertical pipe at the top for vapor extraction, and a condenser. The liquid (distillate) is collected in a receiving flask and analyzed.

[0074] Cracking: T= 183°C, atmospheric pressure, residence time: 100

[0075] TRU = 29.2%

[0076] Overall cracking rate = 45.1%

[0077] After 24 hours, the viscosity is 1.034 Pa.s.

[0078] The test was stopped due to a blockage of the residue outlet after 32 hours.

[0079] The results obtained are described in Table 1 below.

[0080] [Table 1]

[0081] These comparative examples show that the hydrolysis step allows for significantly improved performance at a lower temperature and the residue generated is less viscous. The addition of water allows for further cracking without encountering plugging problems.

[0082] The hydrolysis step generates a stream with efficient cracking (high TRU and low viscosity) despite a relatively long residence time.

Claims

DEMANDS 1. A process for regenerating heavy by-products from an acrylic acid (AAC) production unit, said process comprising the following steps: i. introducing said heavy by-products with water into a hydrolyzer and subjecting them to continuous hydrolysis, the water:AAC ratio ranging from 0.1 to 1.3 for a period of 1 to 10 h, preferably from 1 to 5 h, leading to the obtaining of a mixture of hydrolyzed products, ii. continuously injecting said mixture of hydrolyzed products into a reactor and subjecting it to thermal cracking producing a gaseous head stream containing acrylic acid and water and a bottom stream (residue) concentrated in heavy products, iii. recovering a lighter fraction rich in AA and water which can be recycled at various points in the process, iv. recovering said residue for disposal treatment.

2. A method according to claim 1, wherein step i) is carried out at a pressure between 0.1 and 2 MPa, preferably between 0.5 and 1.5 MPa.

3. A process according to any one of claims 1 and 2, wherein the hydrolysis temperature is between 80 and 200°C, preferably between 150 and 200°C.

4. A method according to any one of claims 1 to 3, wherein the cracking temperature is between 140 and 260°C, preferably between 160 and 210°C.

5. A method according to any one of claims 1 to 4, wherein the residence time of the reaction mixture in the cracking reactor is between 0.5h and 100, preferably between 4h and 100.

6. A process according to any one of claims 1 to 4, wherein the reactor bottom flow (residue) obtained at the end of the thermal cracking operation has a dynamic viscosity measured at 100°C of less than 1 Pa.s. A method according to any one of the preceding claims comprising a step of injecting said gaseous overhead stream containing acrylic acid and water into a condenser. A method according to claim 7 wherein at least one polymerization inhibitor is introduced into said condenser. A method according to any one of claims 1 to 8 wherein the thermal cracking reaction takes place in the absence of a catalyst.

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