Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
  • C07C51/42—Separation; Purification; Stabilisation; Use of additives
  • C07C51/43—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation
  • C07C51/44—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation by distillation
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
  • C07C51/42—Separation; Purification; Stabilisation; Use of additives
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
  • C07C51/42—Separation; Purification; Stabilisation; Use of additives
  • C07C51/48—Separation; Purification; Stabilisation; Use of additives by liquid-liquid treatment
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C57/00—Unsaturated compounds having carboxyl groups bound to acyclic carbon atoms
  • C07C57/02—Unsaturated compounds having carboxyl groups bound to acyclic carbon atoms with only carbon-to-carbon double bonds as unsaturation
  • C07C57/03—Monocarboxylic acids
  • C07C57/04—Acrylic acid; Methacrylic acid

Definitions

• 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.
• 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 reactive substrate which can be isobutene or tertiobutanol
• intermediate oxidation product metalhacrolein
• 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.
• solvent introduced at the top of the column, and inside which are simultaneously involved cooling, condensation, absorption and rectification processes.
• the solvent used in this column is water or a hydrophobic solvent with high boiling point.
• (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, ⁇ -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.
• 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 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.
• 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.
• 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.
• (meth) acrylic acid is most often recovered, from its mixture with the heavy solvent containing the heavier impurities, at the head of a column.
• 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.
• 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.
• the process also suffers from notable disadvantages.
• the concentration of (meth) acrylic acid in the raw topped mixture does not exceed 30%, the remainder consisting essentially of the solvent.
• 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.
• 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.
• a separation device comprising a separation column eliminating the light compounds, then to three sections respectively of exhaustion, concentration and rectification.
• 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
• 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.
• 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).
• 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:
• 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.
• 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. .
• the sections (SI) and (S2) are respectively the lower and upper sections of the same column (C3 ⁇ ), the bottom flow of the column (C2) being addressed to the column (C3 ⁇ ) 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 (C3 ⁇ ).
• the head pressure of the column (C3 ⁇ ) 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).
• (C3 ⁇ ) 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 (C3 ⁇ ) and (C3 2 ) is advantageously from 40 to 110 ° C, preferably 60 to 90 ° C.
• 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.
• 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.
• 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; C3 ⁇ ; 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.
• 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).
• this solvent contains light impurities at a boiling point close to that of (meth) acrylic acid
• 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.
• 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.
• 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_).
• 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.
• part of the stream 9 drawn off at the bottom of the SI section 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.
• FIG. 2 it can be seen that a column C3 has been shown which alone houses the sections SI, S2 and S3.
• FIG 3 we can see that there is shown a column C3 ⁇ which houses the sections SI and S2 and a column C3 2 which houses the section S3.
• the head flow from column C3 ⁇ feeds column C3 2 at the bottom.
• 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 (E2 ⁇ , E2 2 ), the head flow of the Eli evaporator supplying the evaporator E2 2 at the head and the head flow of the evaporator E2 ⁇ 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 E2 ⁇ 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 E2 ⁇ evaporator.
• 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 , E2 ⁇ , 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.
• 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.
• 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 .
• 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 .
• 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 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.
• 88.5 g / h of liquid flow are recovered at a temperature of
• This example describes a possible operation of a separation assembly involving two columns C3 ⁇ and C3 2 in series ( Figure 3).
• the assembly consists of a first C3 ⁇ 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%
• 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 C3 ⁇ (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.

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