Process for upgrading heavy by-products from acrylic acid production

Abstract

The present invention relates to a process for recovering acrylic acid (AA) from heavy by-products (residues, referred to as LAA) from AA production units by thermal cracking for recycling in acrylic acid production plants. The process consists of two steps: batch hydrolysis and continuous cracking and improves the existing performance of the cracking plant (plant).

Description

Technical Field

[0001] The present invention relates to a process for regenerating acrylic acid (AA) from heavy by-products (products) (residue, called AAHP) from AA production units by thermal cracking, with the view to recycling it in acrylic acid production facilities. The process consists of two steps: hydrolysis carried out in batches and cracking carried out continuously, and improves the existing performance of cracking facilities (plants). Background Art

[0002] Under the influence of the temperature during the distillation step, the production of acrylic acid is accompanied by the formation of heavy compounds (derivatives of compounds with nucleophilic properties added to the double bonds of unsaturated carbonyl-containing monomers by Michael reaction). Compounds with a boiling point higher than that of the acrylic monomer produced are called "heavy" compounds.

[0003] In the case of an AA production unit these are essentially:

[0004] - derivatives of acrylic acid added to the double bond of another acrylic acid molecule: 3-acryloxypropionic acid, also called "acrylic acid dimer" or "AA dimer";

[0005] - derivatives of acrylic acid added to the double bonds of the AA dimer molecule, resulting in the formation of "AA trimers", as well as other oligomers formed by successive additions of acrylic acid to the double bonds of the aforementioned AA oligomers; and

[0006] - Carboxylic acids formed as by-products of acrylic acid, or derivatives in which water is added to the double bonds of AA or the above oligomers.

[0007] In the case of heavy products originating from AA production units, it is difficult to recover upgradeable monomers from heavy Michael derivative compounds. In particular, during the thermal cracking process of regenerated acrylic acid (which is distilled and upgraded), a residue remains, the viscosity of which increases greatly when high cracking efficiency is sought, until it can no longer be extracted from the cracking reactor.

[0008] The main factor limiting the efficiency of the regeneration of the compounds derived from the Michael reaction contained in the heavy stream from the AA plant is the increase in viscosity of the heavy residue obtained at the bottom of the cracker when the fraction rich in acrylic monomers evaporates.

[0009] The evaporation of light compounds during cracking leads to a concentration of heavy products in the residue stream, and increases the viscosity of this stream. However, the residue must remain sufficiently fluid after cooling to be transported and then processed for the purpose of destroying it.

[0010] In case the production of light esters (methyl acrylate (MA) or ethyl acrylate (EA)) is close to the AA production unit, co-cracking of the corresponding heavy products can improve this situation, making the cracking residue more mobile. The proposed solution allows to recover the maximum amount of AA per cracking operation while managing the viscosity of the residue formed, without relying on additional production units.

[0011] Thus, in document EP 717 031, it has been shown that if a mixture of heavy products from an AA production unit and an acrylic ester (EA) production unit is used for cracking, the recovery efficiency of these upgradeable valuable heavy products can be improved compared to the heavy streams from these units that are cracked separately. The effect of adding the heavy products (EAHP) from the ester unit to the heavy products (AAHP) from the AA unit is to reduce the viscosity of the final residue. The cracking reaction is carried out with a mixture having a ratio of 9 / 1 to 1 / 9 of AA heavy products / ester heavy products, at a temperature of 180° C. to 220° C., at atmospheric pressure, for a residence time of 0.5 to 3 hours. In this process, cracking and evaporation of the generated light compounds are carried out in a reactor, and then the generated gas stream is sent to a distillation column, and finally the bottom stream from the distillation column is recycled to the reactor. On the other hand, since the light fraction obtained by cracking is mainly composed of acrylic monomers AA and esters that are particularly sensitive to polymerization, the distillation stage must be carried out under reduced pressure to reduce the temperature to prevent the formation of polymers in the column. Furthermore, the rectification trays of the distillation column lead to effective separation of polymerization inhibitors entrained in the gas mixture, which flow back to the column bottom and therefore fresh polymerization inhibitors have to be introduced at the top of the column to prevent polymer formation in the upper part of the column.

[0012] For this reason, it is necessary to separate the reaction phase carried out at high pressure from the distillation phase carried out at reduced pressure. Therefore, the plant for carrying out the process must be equipped with a reactor and a top condenser operating at the same pressure, and a distillation column operating at reduced pressure, fed with the condensed product and comprising a boiler at the bottom and a condenser, reflux equipment accessories and a feed for the inhibitor at the top. This arrangement is complex and expensive.

[0013] In addition, the co-cracking of the AA heavy products mixed with the EA heavy products leads to operational limitations. Specifically, when the ester unit is shut down, the cracking operation must be shut down. This leads to economic losses.

[0014] In other cases, AA heavies are thermally cracked batchwise without the addition of ester heavies and produce extremely viscous residues, which limits the performance of the cracking and creates problems with the storage and transfer of the residues.

[0015] In order to overcome viscosity related problems it is also known to add solvents to the AA heavy product cleavage residue.

[0016] Document EP 3255030 teaches the addition of higher alcohols during the cracking of the residue, the maleic anhydride present in the residue being converted into maleic esters which are less susceptible to polymerization.

[0017] Document US Pat. No. 6,414,183 teaches diluting the discharged residue with solvents such as acetic acid, water and methanol.

[0018] WO 2021 / 224044 describes a process for decomposing Michael adducts of acrylic acid by dilution in a solvent 1 having a boiling point of at least 170°C at 1013 hPa and a solubility in water of at least 20 g per 100 g of water at 25°C, the 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 cyclopentane).

[0019] However, this solution has several disadvantages, such as the generation of waste that needs to be burned if this is not an internal stream, or the provision of additional equipment for mixing. In addition, most of these solvents generate nitrogen- or sulfur-containing derivatives when burned.

[0020] Therefore, there is a need to improve the regeneration efficiency of heavy compounds derived solely from AA units by thermal cracking. Summary of the invention

[0021] The present invention relates to a process for regenerating a mixture of heavy by-products (AAHP) from an acrylic acid production unit, said process comprising the following steps:

[0022] i. introducing the heavy by-product with water into a hydrolyser and subjecting it to batch hydrolysis for a period of 1 to 10 h, preferably 1 to 5 h, with a water:AAHP ratio ranging from 0.1 to 1.3 (limits included), resulting in a mixture of hydrolyzate,

[0023] ii. injecting the mixture of hydrolyzate into a reactor and subjecting it to continuous thermal cracking to produce a gaseous overhead stream containing acrylic acid and water and a bottom stream (residue) concentrated with heavy products,

[0024] iii. Recovery of a lighter fraction rich in AA and water which can be recycled to various points in the process,

[0025] iv. collecting the residue for disposal purposes.

[0026] According to various embodiments, the process includes the following features if appropriately combined.

[0027] According to one embodiment, the pressure in the hydrolyser varies between 0.1 and 2 MPa, preferably between 0.5 and 1.5 MPa.

[0028] According to one embodiment, the temperature in the hydrolyser varies between 80°C and 200°C, preferably between 150°C and 200°C.

[0029] According to one embodiment, the cracking temperature is between 140°C and 260°C, preferably between 160°C and 210°C.

[0030] According to one embodiment, the residence time of the reaction mixture in the cleavage reactor is between 0.5 h and 10 h, preferably between 1 h and 2 h.

[0031] According to one embodiment, the thermal cracking reaction occurs at atmospheric pressure or micropressure (maximum 0.2 MPa).

[0032] According to one embodiment, the gaseous overhead stream comprising acrylic acid and water is injected into a condenser.

[0033] According to one embodiment, the bottom stream from the reactor (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. using, for example, a Brookfield “CAP 1000+” viscometer of cone / plate type.

[0034] According to one embodiment, the thermal cracking reaction is carried out in the absence of a catalyst.

[0035] The present invention makes it possible to overcome the disadvantages of the prior art. It makes it possible to recover the maximum amount of AA per cracking operation while managing the viscosity of the residue formed, without relying on additional production units. This is achieved by combining a step of hydrolysis of heavy by-products from an acrylic acid production unit with a step of thermal cracking of the hydrolyzate.

[0036] The main advantages of the process according to the invention are:

[0037] A simple process which is inexpensive in terms of investment, since it requires practically only one additional piece of equipment (hydrolyser) compared to a separate cleavage step;

[0038] The process allows for reduced emissions by reducing the amount of cracking residues;

[0039] The residue is less viscous compared to cleavage without a hydrolysis step;

[0040] The cleavage step is independent of other units (especially ester units);

[0041] The hydrolysis also makes it possible to avoid special treatments of the residue in order to ensure its discharge, such as the addition of solvents, for example to fluidize it;

[0042] - Improved efficiency in recovering valuable heavy products from heavy Michael derivative products present in the heavy product stream from the AA facility by further driving the cracking reaction, reaching the viscosity limit of the residue later than in a scenario without hydrolysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 An embodiment of the device according to the invention is shown diagrammatically. DETAILED DESCRIPTION

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

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

[0046] Derivatives of acrylic acid added to the double bond of another acrylic acid molecule: 3-acryloxypropionic acid, also known as "acrylic acid dimer" or "AA dimer";

[0047] Derivatives of acrylic acid added to the double bonds of AA dimer molecules, resulting in the formation of "AA trimers", as well as other oligomers formed by successive additions of acrylic acid to the double bonds of the aforementioned AA oligomers;

[0048] Carboxylic acids (eg acetic acid) formed as by-products of acrylic acid, or derivatives in which water is added to the double bonds of AA or the above oligomers.

[0049] The term "hydrolyzer" refers to a reactor in which the hydrolysis reaction of a mixture of water and AA heavy products can be carried out. The reactor can be heated and maintained at a certain pressure. The latter can be a conventional stirred reactor or can be in a heat exchanger.

[0050] The invention is based on a continuous thermal cracking process coupled with a previous batch hydrolysis operation of heavy by-products from an AA production unit.

[0051] The acrylic monomers involved in the Michael addition derivatization can be regenerated by hydrolyzing the oligomers before the heat treatment step. The hydrolysis reaction forms hydroxypropionic acid (HPA), which can be thermally cleaved to provide acrylic acid. The hydrolysis makes it possible to reduce the oligomer chains, which makes it possible to reduce the viscosity of the residue.

[0052] The batch mode hydrolysis was carried out at pressures ranging from 0.1 to 2 MPa.

[0053] The regeneration efficiency (expressed as cracking efficiency) depends mainly on:

[0054] -a / hydrolysis parameters: temperature and pressure, hydrolysis residence time and water / AA heavy product ratio, and

[0055] -b / Cracking parameters: temperature and residence time of thermal treatment.

[0056] An increase in these last two parameters (b / ) tends to improve the regeneration efficiency, but this occurs at the expense of an increase in the viscosity of the cleavage residue.

[0057] The cracking performance is characterized by two values:

[0058] - URR or Useful Recovery Ratio: This is the amount of acrylic acid recovered after cracking relative to the amount of AA heavies fed to the cracker:

[0059] URR = Mass of AA collected / Heavy AA product in cracker feed

[0060] - Crack rate or efficiency: This is the amount of acrylic acid recovered after cracking relative to the sum of the upgradeable compounds (acrylic acid (AA), acrylic acid dimer (AA2) and hydroxypropionic acid (HPA)) in the cracker feed.

[0061] according to Figure 1 In the embodiment of the process shown, a stream containing said heavy by-product (AAHP) from an acrylic acid production facility is introduced into the reactor R1 together with water or separately. The AAHP stream is rich in heavy Michael addition derivative compounds generated during the acrylic acid synthesis and purification steps and also contains other heavy compounds accumulated during the synthesis and purification process, in particular polymerization inhibitors.

[0062] The mixture (1) containing heavy acrylic acid compounds and water is heated to the temperature required for hydrolysis of the Michael addition derivatives to provide lighter compounds. The stream (2) is collected after the hydrolysis step is completed. It is then continuously introduced into the second reactor R2 where it is heated to the temperature required for cleavage of the Michael addition derivatives to provide lighter compounds which are extracted at the top of the reactor in the form of a gaseous mixture (3).

[0063] This vapour stream, which is rich in acrylic acid and contains some heavy compounds (including inhibitors in low concentrations), is advantageously recycled to the acrylic acid production process either directly in vapour form or after complete condensation in condenser E1 as stream (4).

[0064] According to one embodiment, at least one polymerization inhibitor is introduced into the condenser E1. These inhibitors are selected from polymerization inhibitors known to those skilled in the art: phenol derivatives such as hydroquinone and its derivatives (for example hydroquinone methyl ether), 2,6-di(tert-butyl)-4-methylphenol (BHT) and 2,4-dimethyl-6-(tert-butyl)phenol (Topanol A), phenothiazine and its derivatives, manganese salts such as manganese acetate, salts of thiocarbamic acid or dithiocarbamic acid such as metal thiocarbamates and metal dithiocarbamates (for example copper di(n-butyl)dithiocarbamate), N-oxyl compounds such as 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (4-OH-TEMPO), compounds with nitroso groups such as N-nitrosophenylhydroxylamine and its ammonium salts, amine compounds such as p-phenylenediamine derivatives, or mixtures of these inhibitors.

[0065] The residue stream collected at the bottom of the reactor (5) is cooled and then removed in the form of a medium-viscosity liquid so that it can be pumped without difficulty, for example as far as a storage tank or an incineration unit.

[0066] Example

[0067] The following examples illustrate the invention without limiting it.

[0068] The loss rate is defined by the ratio mass of distillate / mass of heavy products. In case of addition of water, the rate becomes the "corrected loss rate" by subtracting this mass of water from the amount of distillate.

[0069] Example 1: Batch hydrolysis and continuous lysis (according to the invention)

[0070] The assembly used for the hydrolysis operation consisted of an AmAr laboratory autoclave reactor made of HC 276 capable of maintaining a maximum pressure of 80 bar at 250°C and equipped with an internal stirrer, a pressure gauge, a nitrogen inlet, a temperature immersion probe and a regulated external electrical heating mantle. Its usable volume was 450 ml.

[0071] The mixture to be hydrolyzed is introduced into the reactor and the reactor is then closed with a jaw system to seal it. The nitrogen line connected to the reactor makes it possible to place it under a pressure of 6 bar, followed by increasing the temperature. The mixture is then heated to a temperature of 150° C. for 1 hour. The pressure read on the pressure gauge increases to 12 bar. Once the hydrolysis is complete, the mixture is discharged through the bottom valve after cooling to room temperature.

[0072] The assembly for the cracking operation consists of a 500 ml jacketed glass reactor equipped with a stirrer, a temperature probe immersed in the liquid phase, a vertical tube in the upper part for extracting steam, a side branch connection (which makes it possible to obtain a constant level and enables the cracking residue to be discharged) and a condenser. The previously hydrolyzed mixture is continuously introduced into the reactor and subsequently heated to the required temperature. The liquid (distillate) is collected in a receiving bottle and analyzed. The residue discharged by overflow is collected in a receiving bottle.

[0073] Duration of the test: 61h

[0074] Hydrolysis: T = 150 ° C, residence time: 1h, P = 1.2MPa, water / AA heavy product ratio = 0.5 Cracking: T = 168 ° C, atmospheric pressure, residence time: 4h, water / AA heavy product ratio = 0.5

[0075] -URR = 61%

[0076] -Corrected wear rate = 66.5%

[0077] -Viscosity = 0.437 Pa.s

[0078] Example 2: Continuous cleavage without hydrolysis (comparative)

[0079] Duration of the test: 58h

[0080] Cracking: T = 193 ° C, residence time: 4h, atmospheric pressure, water / AA heavy product ratio = 0.5

[0081] -URR=55.0%

[0082] - Wear rate = 62.15%

[0083] -Viscosity = 2.361 Pa.s

[0084] Comparison of Example 1 with the comparative example (Example 2) demonstrates that cleavage can be carried out at lower temperatures when the stream to be cleaved has previously been hydrolyzed. Furthermore, the viscosity of the residue is lower.

Claims

1. A process for regenerating a heavy by-product (AAHP) from an acrylic acid production unit, the process comprising the following steps: i. introducing the heavy by-product with water into a hydrolyser and subjecting it to batch hydrolysis for a period of 1 to 10 h, preferably 1 to 5 h, with a water:AAHP ratio ranging from 0.1 to 1.3, resulting in a mixture of hydrolyzate, ii. injecting the mixture of hydrolyzate into a reactor and subjecting it to continuous thermal cracking to produce a gaseous overhead stream containing acrylic acid and water and a bottom stream (residue) concentrated with heavy products, iii. Recovery of a lighter fraction rich in AA and water which can be recycled to various points in the process, iv. collecting the residue for disposal purposes.

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

4. The process of one of claims 1 to 3, wherein the pyrolysis temperature is between 140°C and 260°C, preferably between 160°C and 210°C.

5. The process as claimed in one of claims 1 to 4, wherein the residence time of the reaction mixture in the cleavage reactor is between 0.5 h and 10 h, preferably between 1 h and 2 h.

6. Process according to one of claims 1 to 4, wherein the bottom stream (residue) from the reactor obtained at the end of the thermal cracking operation has a dynamic viscosity measured at 100°C of less than 1 Pa.s.

7. The process according to one of the preceding claims, comprising the step of injecting the gaseous overhead stream comprising acrylic acid and water into a condenser.

8. The process of claim 7, wherein at least one polymerization inhibitor is introduced at the condenser.

9. The process of one of claims 1 to 7, wherein the thermal cracking reaction takes place in the absence of a catalyst.

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