EP1680390A2

EP1680390A2 - Method for purifying (meth)acrylic acid by oxidising a gaseous substrate

Info

Publication number: EP1680390A2
Application number: EP04791449A
Authority: EP
Inventors: Michel Fauconet; Jean-Marie Destenay
Original assignee: Arkema SA
Current assignee: Arkema France SA
Priority date: 2003-11-04
Filing date: 2004-10-01
Publication date: 2006-07-19
Also published as: CN100473636C; WO2005049541A2; US7253313B2; FR2861725A1; FR2861725B1; KR101092275B1; WO2005049541A3; JP4673854B2; JP2007510637A; KR20060124604A; CN1894194A; US20070106093A1

Abstract

The inventive method for purifying (meth)acrylic acid by oxidising a propane and/or propylene and/or acrolein (acrylic acid production) and isobutane and/or isobutene and/or tert-butyl alcohol and/or methacroleine (methacrylic acid production) gaseous substrate consists in sending a gaseous mixture of a reaction (1) to the base of an absorption column (C1) which is supplied by the top and in counter-current by at least one type of hydrophobe absorption heavy solvent, in sending a flow (2) from the base of the column (C1) to a light component separation column (C2) and in sending a flow (4) from the base of the column (C2) to a separation device which is provided with three sections i.e. exhaustion (S1), concentration (S2) and rectification (S3), respectively in order to extract a pure desired acid, extract and advantageously recycling one or several types of heavy solvent in the absorption column (C1) and to separate intermediate heavy stabiliser-containing components from the desired (meth)acrylic acid.

Description

PROCESS FOR THE PURIFICATION OF (METH) ACRYLIC ACID OBTAINED BY OXIDATION OF A GASEOUS SUBSTRATE

The present invention is part of a process for manufacturing (meth) acrylic acid according to which the oxidation of a gaseous substrate (propane and / or propylene and / or acrolein in the case of 1) is carried out. acrylic acid; isobutane and / or isobutene and / or tert-butyl alcohol and / or methacrolein in the case of methacrylic acid) by catalytic or redox route, and the (meth) acrylic acid is recovered , starting from the hot reaction gas mixture, by absorption against the current of a hydrophobic heavy solvent, said absorption being carried out in the presence of at least one polymerization inhibitor (also called stabilizer). The invention described here relates in particular to the substantially quantitative recovery, from a crude mixture of acrylic acid in the solvent previously rid of the light compounds: acrylic acid (recovery yield> 98.5%), the flux purified containing less than 0.5% heavy compounds; heavy hydrophobic solvent (recovery yield> 99.9%); stabilizers at boiling temperature <260 ° C under atmospheric pressure (recovery yield> 50%). The main process for the synthesis of acrylic acid uses a catalytic oxidation reaction of propylene using an oxygen-containing mixture.

This reaction is generally carried out in the vapor phase, most often in two stages, which can be carried out in two separate reactors or a single reactor: - the first stage carries out the substantially quantitative oxidation of propylene to a mixture rich in acrolein, in which acrylic acid is a minority; the second step completes the conversion of acrolein to acrylic acid. The gas mixture resulting from the second oxidation stage consists of: acrylic acid; light compounds which cannot be condensed under the temperature and pressure conditions usually used (nitrogen, oxygen and propylene not converted, propane present in reactive propylene, carbon monoxide and carbon dioxide formed in small quantities by ultimate oxidation); light condensable compounds, in particular water, generated by the oxidation reaction of propylene, unconverted acrolein, light aldehydes, such as formaldehyde and acetaldehyde, and acetic acid, the main impurity generated in. reaction section; - heavy compounds: furfuraldehyde, benzaldehyde, maleic anyhydride, etc. A synthesis process well described in the literature of methacrylic acid by oxidation is in principle identical to that of acrylic acid, except for the reactive substrate (which can be isobutene or tertiobutanol), intermediate oxidation product (methacrolein) and the nature of the condensable light compounds by-products (the reaction gas mixture contains inter alia acrylic acid in addition to the light present in the reaction gas of the acrylic acid synthesis process) . The second stage of manufacture consists in recovering acrylic acid from the hot gas mixture, previously cooled to a temperature of 150-200 ° C, by introducing this gas at the foot of an absorption column where it meets against -current solvent absorption, which we will designate below under the term "solvent", introduced at the top of the column, and inside which are simultaneously involved cooling, condensation, absorption and rectification processes. In most of the methods described, the solvent used in this column is water or a hydrophobic solvent with high boiling point. The use of a hydrophobic heavy solvent in this first absorption section, as described in French patents n ° 2 146 386 and German n ° 4 308 087, presents advantages compared to the use of the water, the main one being to make the separation of light compounds much easier, especially water and acetic acid which are difficult to remove in aqueous solution processes. At the end of the absorption step, the known methods of recovering (meth) acrylic acid from solutions of the monomer in the solvent involve a step of removing light compounds at the top of a column. stripping, intended to return most of these light compounds to the previous absorption column, where they will be eliminated in the overhead stream, as described in the two French and German patents mentioned above. For a complete elimination of the light compounds, it is necessary to bring into play a distillation column making it possible to eliminate the last traces of these volatile impurities at the head. Other alternatives have been proposed, such as that described in European patent n ° 706 986 which consists in making

1 elimination of the last traces of light impurities (topping) at the top of an upper section of the final distillation column of acrylic acid, the pure monomer being recovered in a stream withdrawn laterally. A process is proposed in the French patent application filed today in the name of the present applicant and having also for the title "Process for the purification of (meth) acrylic acid obtained by oxidation of a tra t ga zous" where the conditions of the absorption and stripping sections are chosen so as to obtain a flow of acid (meth) raw acrylic, freed of most of these light impurities in a single step, without additional column or topping section. The following stages of the purification process, in the case of using a hydrophobic heavy absorption solvent, will aim to quantitatively recover (meth) acrylic acid, separate the heavy impurities from the solvent and regenerate the solvent in order from its recycling to the absorption stage. Indeed, the need to recover and recycle the solvent in a sort of "loop" imposed for economic reasons and in order to avoid polluting discharges, it is then necessary to separate and eliminate the compounds heavier than the acid acrylic, in order to avoid their accumulation in this solvent loop. For this purpose, a sufficient quantity of these heavy compounds must be purged, at least equal to that which is supplied to the first absorption section. Among the heavier compounds than acrylic acid are the impurities generated in the reaction step, but also polymerization inhibitors. introduced at each stage of the purification process. Indeed, (meth) acrylic acid being a sensitive product which can easily lead to a polymerization process favored by temperature, polymerization inhibitors are introduced into the purification equipment so as to avoid this process. The polymerization inhibitors conventionally cited in the purification processes of (meth) acrylic acid are numerous. Mention may be made, among them, of phenolic compounds, such as hydroquinone or the methyl ether of hydroquinone,. phenothiazine and its derivatives, such as methylene blue, quinones such as benzoquinone, manganese salts, such as manganese acetate, metal thiocarbamates such as copper salts of dithiocarbamic acid, such as copper dibutyldithiocarbamate, N-oxyl compounds, including 4-hydroxy-2, 2, 6, β-tetramethyl piperidinoxyl, amino compounds, such as paraphenylene diamine derivatives, nitroso group compounds such as N-nitrosophenyl hydroxylamine, and salts of ammonium from N-nitrosophenyl hydroxylamine. The vast majority of polymerization inhibitors are heavier compounds than (meth) acrylic acid, which are therefore not entrained with (meth) acrylic acid during distillation processes. Also, these inhibitors are generally introduced at all points of the equipment (column heads, condensers, etc.) which can be the seat of a liquid-vapor equilibrium leading to the condensation of flows rich in (meth) acrylic acid. The effectiveness of these polymerization inhibitors, used alone or as a mixture, is generally increased when they are used in combination with the introduction of oxygen or of a gaseous flow containing oxygen at the bottom of the column. The polymerization inhibitors are introduced pure, when they are liquid, or in solution in a solvent, which will advantageously be chosen from the absorption solvent or (meth) acrylic acid. The heavy compounds accumulated in the process also consist of impurities originating from secondary reactions at the synthesis step • of acrylic acid by oxidation of propylene or propane or at the synthesis step of methacrylic acid by oxidation. isobutene or isobutane. Another category of heavy impurities is that which comes from degradation reactions during the purification stages. The use of hydrophobic heavy absorption solvents with boiling points significantly higher than that of (meth) acrylic acid forced to undergo significant temperature levels, particularly at the bottom of the column. These high thermal levels favor degradation reactions of the constituents present in the flow, for example of the solvent and of polymerization inhibitors, and therefore require significant energy consumption, to bring the flow to a boil. The thermal levels reached by these streams poor in light compounds can be limited to a certain extent by the operation of the equipment under reduced pressure, but the constraints of the condensing temperature at the head of the column which cannot be too low under penalty of generate excessive cooling, column size, condenser and vacuum generation costs, which increase proportionally with the vacuum level, causing temperatures above 180 ° C to be generally obtained in the hottest parts of the equipment. Consequently, it is preferable to limit the number of unit operations involving the boiling of streams rich in solvent. In the case of processes using heavy hydrophobic solvents, it is necessary to distinguish among the heavy compounds: the "intermediate" compounds, the boiling point of which is between that of (meth) acrylic acid and that solvent. Among these compounds, mention may be made of impurities generated during the reaction step, such as maleic anhydride, furfuraldehyde, benzaldehyde, etc. or polymerization inhibitors which are more volatile than the solvent; "heavy" compounds which have a boiling point higher than that of the solvent, among which mention may be made of impurities formed in the process, such as oligomers, derivatives of addition to the double bond of (meth) acrylic acid, polymers, degradation products of the solvent or inhibitors, etc., and the polymerization inhibitors that are less volatile than the solvent. At the end of the step of removing residual light compounds, (meth) acrylic acid is most often recovered, from its mixture with the heavy solvent containing the heavier impurities, at the head of a column. distillation, the compounds less volatile than the monomer being eliminated in the bottom stream of this column. The elimination of a sufficient quantity of compounds heavier than the solvent, so as to avoid their accumulation in the process, 'can be carried out by purging a stream enriched in these compounds, obtained at the bottom of a solvent regeneration section, consisting in evaporating or distilling all or part of the solvent flow circulating in a loop in the process. The stream enriched with compounds heavier than the solvent is then removed at the bottom of the evaporator or the distillation column. However, the removal of compounds heavier than the solvent is costly in terms of energy and is inevitably accompanied by an equally costly loss of solvent, poorly separated from the heavier compounds, and of the polymerization inhibitors. It is therefore economically advantageous to limit the quantity of flux concerned by the heavy impurity purge treatment. As described in the aforementioned French patent No. 2,146,386, the separation of the intermediate compounds can be envisaged at the bottom of a water washing column, or at the top of a distillation column. In the case of extraction using water, the method has the disadvantage of generating an additional aqueous flow laden with polluting compounds which it is expensive to treat before discharge, and furthermore, this method does not allow '' remove all impurities heavy especially those which are not very polar. The consequence is the progressive accumulation of these intermediate compounds in the solvent loop, which inevitably leads to their entrainment in distilled (meth) acrylic acid. In the other case of separation by distillation, the process also suffers from notable disadvantages. In the purification processes using absorption by hydrophobic heavy solvent, the concentration of (meth) acrylic acid in the raw topped mixture does not exceed 30%, the remainder consisting essentially of the solvent. As the concentrations of intermediate heavy compounds are in the minority relative to (meth) acrylic acid, these compounds are extremely diluted in the stream essentially consisting of solvent obtained at the bottom of a distillation column of (meth) acrylic acid. Consequently, the separation of small quantities of intermediate compounds in a distillation column fed by a bottom flow from the preceding distillation column, as described in French patent No. 2,146,386, requires treating a very large flow of very diluted mixture. The drawbacks of such a process are a significant loss of solvent entrained at the head of the column, costly energy consumption to bring to a boil a large quantity of heavy solvent and. A greatly increased size of equipment, essentially bottom of column and boiler, resulting in increased investment costs. In addition, such a process does not allow effective recovery for recycling on the columns located upstream of the process, stabilizers heavier or lighter than the solvent. An improvement in the process for removing heavy intermediate impurities has been described in French Patent No. 2,736,912, in which a purification section is claimed which consists of distilling pure acrylic acid at the top of a first column, leaving pass a little monomer at the bottom, then send the column bottom stream containing the heavy intermediate impurities to the feed of another column, to laterally withdraw a stream rich in intermediate compounds, and to recycle the top stream column rich in acrylic acid to the previous column. This method has the disadvantage of generating further loss of solvent and polymerization inhibitors. The Applicant Company sought to resolve the problems described above in the prior art in order to significantly improve the purification process of (meth) acrylic acid with absorption by a hydrophobic heavy solvent, on the following points : - recovery of the solvent, recovery of the stabilizers, reduction of the energy consumption and the size of the equipment, limitation of the losses of products by thermal degradation, while ensuring a particularly effective recovery of the (meth) acrylic acid. To this end, it is proposed to direct the flow obtained at the bottom of the absorption column to a separation device comprising a separation column eliminating the light compounds, then to three sections respectively of exhaustion, concentration and rectification. , making it possible to recover the desired pure acid, to recover and advantageously to recycle to the absorption column the hydrophobic heavy solvent (s), and to separate from the desired (meth) acrylic acid the heavy intermediate compounds including stabilizers, stream - weakly concentrated in desired (meth) acrylic acid - which can advantageously be treated in a downstream distillation column. In this small column, we realize elimination of the intermediate heavy compounds at the head of the column, in sufficient quantity to avoid their accumulation in the solvent loop, in a stream with a low concentration of solvent and stabilizer; and at the bottom of the column recovering most of the solvent and stabilizer initially present in the feed to the column, with a view to recycling them as a stabilizing stream at the top of the preceding columns of the recovery process. The subject of the present invention is therefore a process for the purification of (meth) acrylic acid obtained by oxidation by catalytic route or by redox route, of a gaseous substrate consisting of propane and / or propylene and / or acrolein in the case of the manufacture of acrylic acid, and with isobutane and / or isobutene and / or tert-butyl alcohol and / or methacrolein in the case of the manufacture of acid methacrylic, said gaseous mixture mainly consisting of: propane and / or propylene or isobutane and / or isobutene depending on whether the substrate contains it; ultimate oxidation products; the desired (meth) acrylic acid; - (meth) acrolein; tertiary butyl alcohol in the case of the production of methacrylic acid; water vapor; acetic acid with, in the case of the production of methacrylic acid, acrylic acid as a by-product; and heavy products of secondary reactions, according to which the gaseous reaction mixture is addressed at the bottom of an absorption column (Cl) which is fed at the head and against the current by at least one heavy solvent of hydrophobic absorption, absorption having place in the presence of at least one polymerization inhibitor in order to obtain: at the top of the column (Cl), a gas flow consisting of: - light compounds that cannot be condensed (that is to say, incondensable under normal pressure conditions and temperature) consisting of propane and / or propylene or isobutane and / or isobutene, depending on whether acrylic acid or methacrylic acid is produced, and the ultimate oxidation products of the mixture; major quantities of light condensable compounds consisting of water and acetic acid in the case of the manufacture of acrylic acid, or of water, acetic acid and acrylic acid in the case of making methacrylic acid; and (meth) acrolein; at the bottom of said column (Cl), a stream consisting of: (meth) acrylic acid; the heavy absorption solvent (s); heavy side reaction products; the polymerization inhibitor (s); and - minor quantities of said light condensable compounds, then the flow from the column (Cl) is sent to a separation column (C2) in which a separation is carried out to obtain: - at the top, a flow constituted by the light impurities that we refer to the lower part of the absorption column (Cl); and at the bottom, a flow consisting of: - (meth) acrylic acid in solution in the absorption solvent (s); a small proportion of said light condensable compounds; heavy products of side reactions; and the polymerization inhibitor (s), characterized in that the column bottom liquid flow (C2) is sent to the head feed of a first separation section (SI) capable of making it possible to obtain at the head, a gas flow; and - at the bottom, a stream essentially containing the absorption solvent (s) rid of the lighter compounds, said stream being recycled to the column (Cl) feed directly or after elimination of part of the heavy compounds contained in this stream; said gaseous flow obtained at the head of the first section (SI), or the liquid flow generated by the condensation of this gas, being sent to the bottom of a second separation section (S2) capable of concentrating the intermediate heavy compounds whose temperature of boiling is between that of the solvent (or of the solvent with the lowest boiling point in the case of a mixture of solvents) and that of (meth) acrylic acid, and making it possible to obtain: head, a gas flow; and - at the bottom, a liquid flow which is returned to the head of the first section (SI), said gaseous flow obtained at the head of the second section (S2), or the liquid flow generated by condensation of this gas, being sent at the lower part of a third separation section (S3) capable of making it possible to obtain: at the top, a gas flow which is condensed and partially recycles at the top of said section (S3), the rest being withdrawn in as pure (meth) acrylic acid free of heavy impurities; and - at the bottom, a liquid lux which is returned to the top of the second section (S2). We can qualify sections (SI), (S2) and (S3) respectively of lower exhaustion section, intermediate section of concentration of heavy intermediate compounds, and upper rectification section. The purification according to the present invention is therefore carried out in three successive sections S1, S2 and S3. These three sections have in common the fact that they are the seat of unit operations involving at least one liquid-gas separation stage. In these separation operations, a liquid mixture (vaporized) and / or a gas mixture (condensed) containing compounds of different volatilities generate a vapor richer in the most volatile compounds and a liquid richer in the less volatile compounds. These separation operations can be carried out in any assembly combining conventional distillation equipment: all types of distillation columns, all types of evaporators, boilers and condensers.

. In accordance with a first embodiment of the method according to the invention - which will be described in more detail below with reference to Figure 2 - the sections (SI), (S2) and (S3) are the lower, intermediate sections respectively and higher of the same column (C3), the bottom flow of the column (C2) being addressed to the column (C3) above the section (SI). In this case, the number of theoretical plates of the column (C3) is advantageously from 8 to 25, in particular from 10 to 20, the number of theoretical plates of each of the sections (SI), (S2) and (S3) of the column (C3) being advantageously respectively:

- from 1 to 5, in particular from 1 to 3;

- from 1 to 10, in particular from 1 to 5; and

- from 3 to 20, in particular from 5 to 15. The head pressure of the column (C3) is advantageously from 2.7 to 27 kPa (20 to 202 torr), preferably from 6.7 to 24 kPa (50 to 180 torr). The bottom temperature of the column (C3) is advantageously from 150 to 250 ° C, preferably from 180 to 230 ° C, and the head temperature of said column (C3) is advantageously from 40 to 110 ° C, preferably from 65 to 95 ° C. Preferably, column (C3) is a distillation column provided with a bottom boiler, a condenser at the head, with a reflux rate T _R imposed at the head of 0.5 / 1 to 4/1, preferably from 0.5 / 1 to 2/1, since obtaining a (meth) acrylic acid of sufficient purity requires a large number of separation stages. . According to a second embodiment of the method according to the invention - which will be described in more detail below with reference to Figure 3 - the sections (SI) and (S2) are respectively the lower and upper sections of the same column (C3ι), the bottom flow of the column (C2) being addressed to the column (C3ι) above the section (Si), and the section (S3) is the single section of a column (C3 ₂ ) fed in its lower part by the column head flow (C3χ). The head pressure of the column (C3ι) is advantageously from 2.7 to 27 kPa (20 to 202 torrs), preferably from 4 to 15 kPa (30 to 112 torrs), and the head pressure of the column (C3 ₂ ) is advantageously from 2.7 to 27 kPa (20 to 202 torrs), preferably from 6.7 to 24 kPa (50 to 180 torrs). The foot temperature of each column

(C3ι) and (C3 ₂ ) is advantageously from 150 to 250 ^C C, preferably from 170 to 210 ° C, and the head temperature of each of said columns (C3χ) and (C3 ₂ ) is advantageously from 40 to 110 ° C, preferably 60 to 90 ° C. According to a third embodiment of the method according to the invention - which will be described in more detail below with reference to Figure 4 - the sections (SI) and (S2) are each formed by at least one evaporator, the flow of the column bottom (C2) being addressed to the supply of the evaporator (El) or of a first evaporator (Eli) of several evaporators mounted in series in the section (SI), the stream containing the solvent (s) absorption freed from lighter compounds being obtained at the bottom of the evaporator (El) or the last evaporator (El ₂ ) of the series (Eli; El ₂ ) of section (SI), and section (S3) is the single section of a column (C3 ₃ ) fed in its lower part by the head flow of the evaporator (E2) or of the last evaporator (E2 ₂ ) of several evaporators mounted in series in section (S2). The head pressure of the column (C3 ₃ ) is advantageously from 2.7 to 27 kPa (20 to 202 torr), preferably from 6.7 to 24 kPa (50 to 180 torr). The bottom temperature of the column (C3 ₃ ) is advantageously from 150 to 250 ° C, preferably from 170 to 210 ° C, and the head temperature of said column (C3 ₃ ) is advantageously from 40 to 110 ° C, preferably from 60 to 90 ° C.

In accordance with a particular characteristic of the process according to the invention, the concentration of (meth) acrylic acid in the feed of the section (SI) is from 5 to 70% by weight, in particular from 10 to 30% by weight. In accordance with another particular characteristic of the process according to the invention, the flow of the heavy intermediate intermediate compounds of the section (S3) is addressed to a column (C4) adapted to allow the elimination, at the head, of at least one part of the heavy intermediate compounds, and the recovery at the bottom, of a flow of the heavy solvent (s) and of the inhibitor (s) polymerization initially present in the feed stream of the column (C4), said stream being advantageously recycled as a stabilizing stream at the top of the preceding columns or sections (Cl; C2; C3; C3χ; C3 ₂ ; C3 ₃ ). The column head pressure (C4) is advantageously from 2.7 to 40 kPa (20 mmHg to 300 mmHg), in particular from 9.3 to 20 kPa (70 mmHg to 150 mmHg). The bottom stream (9) of the section (SI) is advantageously recycled at the top of the absorption column (Cl), where appropriate after elimination of a stream of heavy impurities having a boiling point higher than that of the solvent or higher than the solvent with the highest boiling point. To compensate for the losses sustained during the purification chain, fresh solvent (s) can be introduced into the loops rich in solvent, in particular at the level of the bottom streams of the section (SI) and of the column (C4) recycled into column head (Cl). In the case where this solvent contains light impurities at a boiling point close to that of (meth) acrylic acid, it may prove to be particularly advantageous to supply the additional solvent at the level of the flow supplying the column (C4 ), so as to remove these light impurities which are difficult to separate from (meth) acrylic acid. According to the present invention, one or more heavy hydrophobic absorption solvents having an boiling point at atmospheric pressure greater than 200 ° C. are advantageously used, ditolyl ether being particularly preferred as a hydrophobic heavy solvent. The patent literature gives numerous examples of heavy hydrophobic solvents. The polymerization inhibitor (s) in the presence of which the absorption in column (Cl) and the separations in column (C2) are carried out and the sections (SI) to (S3) are chosen in particular from those indicated above.

Figure 1 of the accompanying drawing shows an overall diagram of the purification process of (meth) acrylic acid according to the invention, involving the three resections SI, S2 and S3 which have just been described, Figures 2 to 4 respectively illustrating the three particular embodiments described above by the more precise representation of the mounting of the sections SI, S2 and S3. If we refer to Figure 1, applied to the synthesis of acrylic acid, we can see that the reaction gas mixture resulting from the oxidation of propylene and acrolein, mainly consisting of: • a part of compounds which cannot be condensed under the conditions of column operating pressure: propylene, ultimate oxidation products (CO, C0 ₂ ), • on the other hand, condensable compounds: acrylic acid, acrolein, water, acetic acid, heavier side reactions in very small quantities, is sent (flow 1_) at the foot of an absorption column Cl fed at the head and against the current by a hydrophobic heavy solvent recovered in the subsequent stages of the process (flow 1_3: solvent at boiling point above 200 ° C at atmospheric pressure). Flow 2 obtained at the bottom of column Cl is mainly composed of acrylic acid and of the solvent, as well as small amounts of acetic acid, water and acrolein. This stream is then freed from these light impurities by sending it to a distillation column C2 in which they will be concentrated at the top, mixed with acrylic acid and traces of solvent. The gas flow obtained at the head of column C2 is condensed through an exchanger and returned to column C1 (flow 3). The stream 4_ obtained at the bottom of column C2 then consists mainly of acrylic acid in solution in the solvent, as well as heavy impurities, resulting from secondary reactions, present in small quantities in the stream of reaction gas. The gas flow 1_4 entrained at the head of column C1 contains the compounds' initially present in the reaction gas and not absorbed: products which cannot be condensed at the operating pressure of the column (propylene, CO, C0 ₂ ), water, acrolein, acetic acid . Most of this flow 1_4 will advantageously be recycled in the reaction step (flow 1_5), to convert the noble reagents it contains, and we can perform a low purge of this flow (flow 1_6) to avoid accumulation in the loop thus formed of the noncondensable compounds resulting from the ultimate oxidation of propylene (CO, C0 ₂ ) and of the nitrogen originating from the air introduced at the level of the reaction stage. According to a slightly different embodiment, the stream 1_4 can also be freed from part of the water and the impurities which it contains by cold condensation of the vapors, before being partially recycled in the reaction step and disposed of by incineration. The stream 4_, consisting of (meth) acrylic acid in solution in the solvent and heavier impurities than the acrylic monomer is then sent into a set of three sections, each element SI, S2, S3 of this set can constitute or not a separate element (column, evaporators or series of evaporators): • a lower exhaustion section SI whose role is to recover most of the solvent contained in stream 4_, free of (meth) acrylic acid and a quantity of impurities and stabilizers belonging to the group of intermediate heavy compounds, sufficient to prevent their accumulation in the loop formed by flows 2, _4 and 1_3_. This flow 4_ is introduced into the column C1 at a place situated above this section SI; • an intermediate section of concentration S2, in which the impurities and stabilizers belonging to the group of intermediate heavy compounds (at boiling temperature between that of the solvent and that of (meth) acrylic acid) are concentrated with a view to their elimination in a flow 5_ withdrawn laterally at a level located above the section S2; • an upper rectification section S3, at the head of which the (meth) acrylic acid, freed of most of the heavier impurities, is obtained (flow 6). The flow 5_ withdrawn from the bottom of section S3 is sent to the supply of a column C4 carrying out the recovery of the main part of the solvent and stabilizers belonging to the group of heavy intermediate compounds (flow 1_ withdrawn at the bottom of column C4) and the elimination of heavy intermediate compounds at the top of the column (flow 8_). According to a particularly advantageous embodiment, the fresh solvent introduced into the purification loop in order to compensate for the small losses suffered along the purification chain will be introduced into the supply of column C4 (flow 1_7), in order to rid this solvent of the possible presence of lighter impurities in stream 8_ withdrawn at the top of column C4. The stream 1_ rich in solvent and stabilizer can be advantageously recycled at one or more of the columns C1, C2 and of all of the sections SI to S3. Finally, part of the stream 9 drawn off at the bottom of the SI section (stream 1_0) could possibly, if necessary, be sent into an apparatus making it possible to remove heavy impurities having a boiling point higher than that of the solvent in a stream 1_1_ , and the regenerated solvent (flow 12_) can be returned, with the part of flow 9 untreated, to supply the absorption column (flow 1_3). This step of deconcentration of heavy impurities can be carried out in any device making it possible to carry out a coarse separation of compounds at a temperature much higher than that of the solvent, for example in a distillation column or more advantageously any type of evaporator. In FIG. 2, it can be seen that a column C3 has been shown which alone houses the sections SI, S2 and S3.

In Figure 3, we can see that there is shown a column C3ι which houses the sections SI and S2 and a column C3 ₂ which houses the section S3. The head flow from column C3ι feeds column C3 ₂ at the bottom. In Figure 4, we can see that the SI section includes two evaporators in series (Eli, El ₂ ), the flow 4 supplying the first evaporator Eli at the top, the bottom flow of the evaporator Eli supplying the second evaporator El ₂ at the head, and the bottom stream of the evaporator El ₂ constituting the stream 9; and the section ^' S2 includes two evaporators in series (E2ι, E2 ₂ ), the head flow of the Eli evaporator supplying the evaporator E2 ₂ at the head and the head flow of the evaporator E2ι supplying the evaporator E2 ₂ on your mind. The flu ^'head of the evaporator E2 ₂ feeds the C3 column ₃ which houses the section S3. The flow from the top of the column El ₂ and the flow from the bottom of the column E2ι are recycled to the feed (4) of the Eli evaporator. The bottom stream of the E2 ₂ evaporator is recycled to the supply of the E2χ evaporator. The distillation columns C1, C2, C3, C3ι,

C3 ₂ , C3 ₃ and C4 can be of any type (with trays perforated with or without overflows, with caps, structured packing or loose.). They can be fitted with all types of boilers (vertical or horizontal exchangers, thermosiphon or forced circulation, film evaporators of all types, etc.). Condensers of any type can be vertical or horizontal. The evaporators Eli, El ₂ , E2ι, E2 ₂ can be of any type: evaporators with ^' vertical or inclined tubes ^' , with plates, with forced circulation, with rotating film, with agitated film, with scraped film. Preferably, the column housing the section S3 (C3, C3 ₂ , C3 ₃ ) will be equipped with a stabilizer supply at the head and at the level of the gas inlet into the head condenser. The condensation of the C2 column head vapors can also, advantageously, be protected from polymerization phenomena by the introduction of stabilizers at the inlet of the condenser. The concentration of stabilizers in the solvent flow circulating in a loop in columns C1, C2 and all of the sections S1 to S3 of the purification process will be maintained at a sufficient value, if necessary by external addition of fresh stabilizer, so as to avoid polymerization phenomena. This addition can be made at any point in the process. To avoid the generation of polymers, we can make one. addition of oxygen, in pure or diluted form (air) at the bottom of the columns working under reduced pressure and containing (meth) acrylic acid. In a particularly advantageous manner, the introduction of oxygenated flows into the column C3 will be carried out at a level located between the bottom of the column and the lateral withdrawal, preferably between the boiler and the main supply. EXAMPLES

The examples described below illustrate the invention. The percentages are given in mass percentages. In these examples, the following abbreviations have been used: - AA: acrylic acid; - DTE: ditolylether; - EMHQ: methyl ether of hydroquinone

Example 1:

This example describes a possible operation of a single C3 column. The experimental setup consists of a distillation column equipped with a bottom boiler, a condenser at the top, and an outlet making it possible to laterally take off a part of the liquid passing through a plate situated between the supply plate and the column head. The distillation operation is carried out under reduced pressure 13,300 Pa (100 mm Hg). The liquid flow supplying the column in continuous mode is a synthetic mixture faithfully representing the composition of a flow of crude acrylic acid obtained at the end of the absorption-topping stage (column bottom C2) of a process ^' as shown in _. Figure 1 in appendix: - DTE: 81.74% - AA: 18.1% - maleic anhydride: 0.266 - acetic acid: 0.018% - furfuraldehyde: 0.005% - benzaldehyde: 0.003% - EMHQ: 0.127%. This column, 130 cm high, has a bottom section equipped with 4 weir trays with an internal diameter of 35 mm and an upper section with 16 25 mm internal diameter overflow trays. The set has an efficiency of 15 theoretical plates. The feed stream (500 g / h) is preheated to 115 ° C through an exchanger. The column is supplied between the 2 ^nd and the 3 ^rd plate of the lower section of the column, numbered from the bottom, and part of the descending liquid flow is taken laterally from the upper plate of the bottom section . To avoid polymerization phenomena, EMHQ is introduced at the top of the column, and air is injected, at a flow rate of 1 liter / h, at the level of the plate receiving the supply. The vapors are condensed at the top and part ¹ of the flow of the condensed liquid (180 g / h) is returned to the top of the column, while the distillate (96 g / h) is withdrawn. The pure product obtained essentially contains acrylic acid, the impurities being limited to 0.1% maleic anhydride, 0.1% acetic acid, and less than 0.01% furfuraldehyde, benzaldehyde and benzoic acid. The liquid stream withdrawn laterally (5.4 g / h), at a temperature of 142 ° C., is composed of: DTE: 58.3% EMHQ: 7.8% maleic anhydride: 20.5% - acrylic acid: 11 , 4% benzoic acid: 1.5% benzaldehyde: 0.4% furfuraldehyde: 0.1%. The column bottom flow is withdrawn at a temperature of 210 ° C. It titrates 0.01% of acrylic acid. The acrylic acid recovery yield is greater than 99%. Example 2:

This example illustrates the operation of the recovery column C4 as described in the diagram Figure 1 in the appendix. The column, fitted with a thermosyphon boiler at the bottom and a condenser at the head, is made up of 8 perforated trays with overflows with an internal diameter of 25 mm. The feed stream from the lateral withdrawal of a column C3 is sent at a flow rate of 125.5 g / h, in the boiler of column C4. It consists of: DTE: 65% maleic anhydride: 17% AA: 12.4% - EMHQ: 4.2% benzoic acid: 1.1% furfuraldehyde: 0.17% benzaldehyde: 0.13%. The operating pressure being 13,300 Pa (100 mm Hg), 37 g / h of condensed liquid flow are withdrawn at the head of the column, at a temperature of 107 ° C., by applying a reflux / pouring rate of 5/1. At the bottom of the column, 88.5 g / h of liquid flow are recovered at a temperature of

195 ° C. Recovery rates for noble products reach 97% for DTE, 78% for EMHQ and 6% for

1 'AA. The impurity removal yields are 89% for maleic anhydride, 94% for furfuraldehyde, 97% for benzaldehyde, 27% for benzoic acid. By taking into account the performance of each of the columns C3 and C4 described in the two previous examples, we can calculate the overall recovery yields of the entire purification section of the process described in Figure 1. Relative to the flow of AA crude from the heavy solvent absorption step, the flux feed of column C3, the recovery yields of the solvent and of the stabilizer reach respectively 99.98% and 85.6%, while the loss of AA in the flow at the head of column C4 remains limited to 0.6 % of the AA initially contained in the gross AA.

Example 3:

This example describes a possible operation of a separation assembly involving two columns C3ι and C3 ₂ in series (Figure 3). The assembly consists of a first C3ι column operating under a pressure of 11,700 Pa (80mm Hg) equipped with 4 perforated trays each provided with a weir (or 3 theoretical trays), a thermosyphon boiler at the bottom, a condenser at the head, supplied between the 2 ^nd and the 3 ^rd plate using a pump (490 g / h) by a mixture of composition characteristic of the medium (crude acrylic acid) obtained at the bottom of column C2 above: - DTE: 79.83% AA: 19.8% maleic anhydride: 0.172% furfuraldehyde: 0.022% EMHQ: 0.146% At the head of column C3ι, part of the condensed flow is returned to the level of the upper plate, with a reflux rate (flow of liquid returned / flow of withdrawn liquid) 0.2 / 1. The temperature measured at the boiler is 180 ° C, and the head temperature reaches 119 ° C. The flow obtained at the bottom of the column titers 0.082% of AA, ie a recovery rate of the monomer at the top of the column of 99.7%. The stream withdrawn at the top of column C3ι (120.7 g / h), mainly containing 19% of DTE, 80.15% of A, 0.63% of maleic anhydride, 0.064% of furfuraldehyde, 0.07% of HQME, is sent by a pump at the ^4th tray (counted from the bottom) of a second column C3 ₂ , equipped with 16 perforated trays provided with weirs (12 theoretical trays). This C3 ₂ column is equipped at the bottom with a thermosiphon boiler, at the head of a condenser, operates under a pressure of 22,600 Pa (170mm Hg) and receives at the head a mixture of stabilizer (EMHQ at 5% in AA ). The reflux rate applied at the top (flow rate of liquid discharged / flow rate of liquid withdrawn) is 1.5 / 1. The bottom temperature is 187 ° C, the top temperature is 93 ° C. The pure acrylic acid obtained at the head of the column has a 99.87% AA content and contains only 325 ppm of furfuraldehyde, 100 ppm of maleic anhydride, and no trace of DTE is detected. The foot flow contains, in addition to the DTE, 4.12% of A (overall AA recovery rate on all of the two columns: 98.7%), 2.62% of maleic anydride and 0 , 17% furfuraldehyde.

Claims

1 - Process for purifying (meth)acrylic acid obtained by catalytic or redox oxidation of a gaseous substrate consisting of propane and/or propylene and/or acrolein in the case of manufacturing of acrylic acid, and by

isobutane and/or isobutene and/or tert-butyl alcohol and/or methacrolein in the case of the manufacture of methacrylic acid, said gas mixture mainly consisting of: propane and/or propylene or isobutane and/or isobutene depending on whether the substrate contained it; ultimate oxidation products; - the desired (meth)acrylic acid; (meth)acrolein; tert-butyl alcohol in the case of manufacturing methacrylic acid; water vapor; - acetic acid with, in the case of the manufacture of methacrylic acid, acrylic acid as a by-product; and heavy secondary reaction products, according to which the gaseous reaction mixture is sent to the bottom of an absorption column (Cl) which is fed at the top and counter-current with at least one heavy hydrophobic absorption solvent, the absorption taking place in the presence of at least one polymerization inhibitor to obtain: - at the top of the column (Cl), a gas flow consisting of: light non-condensable compounds consisting of propane and/or propylene or isobutane and/or isobutene, depending on whether acrylic acid or methacrylic acid is produced, and the ultimate oxidation products of the mixture; major quantities of light condensable compounds consisting of water and acetic acid in the case of the manufacture of acrylic acid, or of water, acetic acid and acrylic acid in the case where methacrylic acid is manufactured; and (meth)acrolein; at the bottom of said column (Cl), a flow consisting of: - (meth)acrylic acid; the heavy absorption solvent(s); heavy side reaction products; the polymerization inhibitor(s); and minor quantities of said light condensable compounds, then the flow from the column (Cl) is sent to a separation column (C2) in which a separation is carried out to obtain: at the top, a flow consisting of the light impurities that the ' we refer to the lower part of the absorption column (Cl); and at the bottom, a flow consisting of: (meth)acrylic acid in solution in the absorption solvent(s); - a small proportion of said light condensable compounds; heavy products of side reactions; and the polymerization inhibitor(s), characterized in that the liquid flow (4) from the bottom of the column (C2) is sent to the top feed of a first separation section (SI) capable of making it possible to obtain at the top, a gas flow; and - at the bottom, a flow (9) essentially containing the absorption solvent(s) freed from the compounds lighter, said flow being recycled as a feed to the column (Cl) directly or after elimination of the heavy compounds contained in this flow; said gas flow obtained at the head of the first section (Si), or the liquid flow generated by the condensation of this gas, being sent to the bottom of a second separation section (S2) capable of concentrating the intermediate heavy compounds whose temperature boiling point is between that of the solvent (or of the solvent with the lowest boiling point in the case of a mixture of solvents) and that of the (meth)acrylic acid, and to make it possible to obtain: in head, a gas flow; and at the bottom, a liquid flow which is returned to the head of the first section (SI), said gas flow obtained at the head of the second section (S2), or the liquid flow generated by condensation of this gas, being sent to the lower part of a third separation section (S3) capable of making it possible to obtain: - at the head, a gas flow which is partly condensed and recycled at the head of said section (S3), the remainder being withdrawn in as pure (meth)acrylic acid free of heavy impurities; and at the bottom, a liquid flow which is returned to the head of the second section (S2).

2 - Method according to claim 1, characterized in that the sections (SI), (S2) and (S3) are the respectively lower, intermediate and upper sections of the same column (C3), the flow (4) of foot of the column (C2) being addressed to the column (C3) above the section (SI).

3 - Method according to claim 2, characterized in that the number of theoretical plates of the column (C3) is from 8 to 25, in particular from 10 to 20, the number of theoretical plates of each of the sections (SI), (S2) and (S3) of column (C3) being respectively: - from 1 to 5, in particular from 1 to 3; - from 1 to 10, in particular from 1 to 5; And

- from 3 to 20, in particular from 5 to 15. 4 - Process according to one of claims 2 and 3, characterized in that the head pressure of the column (C3) is 2.7 to 27 kPa, in particular from 6.7 to 24 kPa, the bottom temperature of the column (C3) is from 150 to 250°C, in particular from 180 to 230°C, and the head temperature of said column (C3) is from 40 to 110°C, in particular 65 to 95°C. 5 - Method according to one of claims 2 to 4, characterized in that the column (C3) is a distillation column provided with a boiler at the bottom, with a condenser at the top, with a reflux rate T _R imposed at the top from 0.5/1 to 4/1, preferably from 0.5/1 to 2/1. 6 - Method according to claim 1, characterized in that the sections (SI) and (S2) are the respectively lower and upper sections of the same column (C3ι), the flow (4) at the bottom of the column (C2 ) being addressed to the column (C3ι) above the section (Si), and the section (S3) is the single section of a column (C3 ₂ ) supplied in its lower part by the head flow of the column (C3ι). 7 - Method according to claim 6, characterized in that the head pressure of the column (C3ι) is 2.7 to 27 kPa, in particular 4 to 15 kPa and the head pressure of the column (C3 ₂ ) is 2.7 to 27 kPa, in particular 6.7 to 24 kPa, and that the bottom temperature of each of the columns (C3ι) and (C3 ₂ ) is 150 to 250°C, preferably 170 at 210°C, and the head temperature of each of the columns (C3 _X ) and (C3 ₂ ) is 40 to 110°C, preferably 60 to 90°C. 8 - Method according to claim 1, characterized in that the sections (SI) and (S2) are each formed by at least one evaporator, the flow (4) from the base of the column (C2) being sent as a supply to the ' evaporator (El) or a first evaporator (Eli) of several evaporators connected in series of the section (SI), the flow (9) containing the absorption solvent(s) freed from the lighter compounds being obtained at the bottom of the 1' evaporator (El) or the last evaporator (El ₂ ) of the series (Eli; El ₂ ) of the section (SI), and the section (S3) is the single section of a column (C3 ₃ ) supplied in its part lower by the head flow of the evaporator (E2) or of the last evaporator (E2 ₂ ) of several evaporators connected in series of the section (S2). 9 - Method according to claim 8, characterized in that the head pressure of the column (C3 ₃ ) is from 2.7 to 27 kPa, in particular from 6.7 to 24 kPa, and that the bottom temperature of the column (C3 ₃ ) is 150 to 250°C, in particular 170 to 210°C, and the head temperature of said column (C3 ₃ ) is 40 to 110°C, in particular 60 to 90°C vs. 10 - Method according to one of claims 1 to

9, characterized in that the concentration of (meth)acrylic acid in the feed of the section (SI) is

5 to 70% by weight, in particular 10 to 30% by weight. 11 - Method according to one of claims 1 to

10, characterized in that the flow (5) of the intermediate heavy compounds at the bottom of the section (S3) is sent to a column (C4) adapted to allow the elimination, at the top, of at least a portion (8) intermediate heavy compounds, and the recovery at the bottom, of a stream (7) of the heavy solvent(s) and of the polymerization inhibitor(s) initially present in the stream (5) feeding the column (C4) , said flow (7) being advantageously recycled as a stabilizing flow at the top of the previous columns or sections (Cl; C2; C3; C3ι; C3 ₂ ; C3 ₃ ). 12 - Method according to claim 11, characterized in that the column head pressure

(C4) is 2.7 to 40 kPa, in particular 9.3 to 20 kPa. 13 - Method according to one of claims 1 to 9, characterized in that the flow (9) at the bottom of the section (SI) is recycled at the top of the absorption column (Cl), if necessary after elimination of a stream (11) of heavy impurities having a boiling temperature higher than that of the solvent or higher than that of the solvent having the highest boiling point. 14 - Method according to one of claims 11 to 13, characterized in that, to compensate for the losses suffered during the purification chain, fresh solvent(s) are introduced into the solvent-rich loops, in particular at the level of the bottom flow of the section (SI) and the column (C4) (respectively 13 and 7) recycled to the top of the column Cl, and/or, in the case where this solvent contains light impurities at close to boiling point from that of (meth)acrylic acid, an additional solvent is supplied to the flow (5) supplying the column (C4). 15 - Method according to one of claims 11 to 14, characterized in that one or more heavy hydrophobic absorption solvents are used having a boiling point under atmospheric pressure greater than 200°C. 16 - Process according to claim 15, characterized in that ditolyl ether is used as a heavy hydrophobic solvent. 17 - Method according to one of claims 1 to 16, characterized in that the polymerization inhibitor(s) in the presence of which are carried out the absorption in the column (Cl) and the separations in the column (C2) and the sections (SI) to (S3) are chosen from phenolic compounds, such as hydroquinone or hydroquinone methyl ether, quinones such as benzoquinone, phenothiazine and its derivatives such as methylene blue, salts of manganese, such as manganese acetate, metal thiocarbamates such as copper salts of dithiocarbamic acid, such as copper dibutyldithiocarbamate, N-oxyl compounds, including 4-hydroxy-2, 2, 6, 6-tetramethyl piperidinoxyl, amine compounds, such as paraphenylene diamine derivatives, compounds with nitroso groups such as N- nitrosophenyl hydroxylamine, and the ammonium salts of N-nitrosophenyl hydroxylamine.