MXPA99000345A - Preparation of acrylic acid and metric acid - Google Patents

Abstract

A process for the preparation of acrylic acid or methacrylic acid comprises: (a) the preparation of a gaseous mixture of which have essentially the composition of a reaction mixture of the gas phase catalytic oxidation of C3-C4 alkanes, C3-alkenes C4, C3-C4 alkanols and / or C3-C4 alkanals and / or C3-C4 alkanals and / or precursors thereof for acrylic or methacrylic acid form, characterized by (b) the condensation of said product mixture, (C) the crystallization of the acrylic acid or the methacrylic acid from the solution obtained in step (b), and (d) the removal of the crystals resulting from the mad liquor

Description

PREPARATION OF ACRYLIC ACID ICO AND HET CALLIC ACID The present invention relates to a process for the preparation of acrylic acid and methacrylic acid, Acrylic acid and an essential basic chemical. Due to its highly reactive double bond and acid function, it is particularly useful as a monomer for the preparation of polymers. Most of the production of acrylic acid monomer is esterified every before the pavement, for example to make adhesives, dispersions or b in coatings. Only a fraction of the production of acrylic acid monomer is directly polymerized, for example, to make superabsorbents. While in general the direct polymerization of acrylic acid requires high purity monomer, the purity requirements of acrylic acid are not so high if acrylic acid is contaminated after polymerization. It is known that acrylic acid can be produced by gas phase nesting heterogeneously catalyzed by propene with molecular oxygen in solid catalysts at a temperature of 20 400 ° C in two stages through acralexna (for example DE ~ -A ~ -1 962 431 »DE-A - 2 943"7O7, DE-C-1 20 * 3 502, DE-A-195 08 558, EP-A-0 257 560, EP-A-0 253 409, DE-A-2 251 364, EP -A-0 117 146, TB-Bi 450 986 and EP-AO 293 224. The catalysts used are multi-component, nesting catalysts based, for example, on two of the molybdenum, bismuth and iron elements (in stage). 1) to maliphenidate and vanadium (in step 2), DE-C-2 136 396 presents the acid acrylation of the reaction gases obtained by the catalytic oxidation in the gas phase of prapene to acraheme by countercurrent absorption with a mixture of 75% by weight of diphenyl ether and 25% by weight of biphenol. Document DE-A-2 449 780 describes the cooling of the hot reaction gas by partial evaporation of the solvent in a direct condenser (quenching apparatus) before countercurrent absorption. The problem is that, as with additional processing steps, the production of solids in the apparatus, which reduces the availability of the plant »According to DE ~ -A ~~ 4 308 087, this production of solids can be reduced by adding a polar solder co or a pair of dimethyl phthalate in an amount of 0.1 to 25% by weight to a relatively nonpolar solvent mixture of diphenylether and biphenyl. In addition to the above-described absorption of the reaction product comprising acrylic acid in a high-boiling solvent mixture, other known processes involve a total condensation of the acrylic acid and the reaction water formed during the course of the reaction. the catalytic oxidation. This provides a solution of aqueous acrylic acid which can be prepared by adding distillation with an azea ropo (PF-C-429 391, JP-A-1 124 766, JP-A-7 118 766, JP-A-7). 318 966-R "and JP-A-7 241 885) or by means of an extraction process (DE-A-2 164 767, JP-A-5 81 40-039, JP-A-4 80 91 013). In EP-AO 551, the mixture of acrylic acid and byproducts produced by catalytic oxidation in gas phase comes into contact with water in an absorption tank and the resulting aqueous solution is distilled in the presence of a solvent which it forms an azeotrope with polar elements with low boiling point, or for example water or acetic acid. DE-C-2 323 328 discloses the removal of acrylic acid from an acidic butanal ester / acrylic acid esterification ichor residue, by extraction with a specific mixture of organic solvents. The processes described above have the disadvantage of using for extraction or absorption an organic solvent that must be removed again in an additional operation, such as rectification, ba or the application of an important thermal energy. This can cause the polymerization of acrylic acid xco. JP-A-7 082 210 describes a process for the purification of an acrylic acid comprising acetic acid, propionic acid, acrolema and furfural as well as acrylic acid. This process comprises the addition of water and then the crystallization ba under reduced pressure to obtain a purity of 99% for the acrylic acid crystals after removal and washing. Japanese patent 45-32417 presents a process wherein a methacrylic or aqueous acrylic acid solution further comprising acetic acid and prapionic acid is exuded from heptane to toluene and is removed from the water of the extract medium distillation. In the next step, the remaining extract is cooled to a temperature of -20 to -80 ° C to cause the crystallisation of acrylic acid or ethacrylate acid.The crystals are removed and the mother liquor is recycled in the process of extraction »According to this patent, it is necessary to use an organic solvent or an extracting substance, since the solution solidifies otherwise by cooling without forming crystals.This process has disadvantages insofar as, in addition to requiring After the addition of an organic solvent, a distillation process must be carried out to remove the water Canadian Patent 790 625 refers to an additional purification process for crude acrylic acid by fractional crystallization. propionic is the main impurity in the raw acrylic acid, the temperature na is reduced due to the peptide temperature of the acrylic acid / propionic acid system ico, while na is reduced below the eutectic temperature of the acrylic acid / acetic acid system when the acetic acid is the main substance. The acrylic acid used for crystallization in this process is prepared by conventional processes, for example, by gas phase oxidation from propene to acralema, and then subjected to a prepurification by known conventional processes, for example, by extraction . The crystallization of acrylic acid is preferably carried out in the substantial absence of water. EP-AO 616 998 describes a process for purifying acrylic water by a combination of static and dynamic crystallisation of a prepuplicated acrylic acid, for example, an acid Acrylic that has been pre-pumped by distillation. O ne to the processes described in the above documents is the fact that they require a (re) purification of the acrylic acid before crystallization. Since prepuphication generally employs organic solvents that have to be separated again, the conditions of strong application of heat, there is always the risk of premature polymerization of acidic acid » EP-AO 002 612, which refers to a process for purifying acrylic acid in an aqueous solution by fractional fractionation, presents the addition of salts to the acrylic acid solution in order to break the system eutectic water / acne acid. ico that exists in a content of acrylic acid of 63% by volume. EP-A-0 675 100 describes a process for preparing carboxylic acids (C3-C6) < to 5. { 3 unsaturated, for example, methacrylic acid. by means of the. oxidative dehydrogenation of the corresponding saturated carboxylic acid (C3-C6), followed by melt crystallization with subsequent fractional distillation or followed by fractional distillation with subsequent melt crystallization. It is an object of the present invention to provide a process for obtaining acrylic acid or acid etacrylic. ico with high purity without complex operations. We have found that this object is achieved, surprisingly, by a process in which the acrylic acid or the methacrylic acid coming from one. mixture ds produ.tos gas that has. been submitted to. condensation can crystallize directly from the. solution formed in the condensation. In particular, the process found requires neither additional purification step nor additional auxiliary. The present invention therefore offers a process for. the preparation of acrylic acid or methacrylic acid, which comprises; (to. preparation of a mixture. gaseous product having essentially the composition of a reaction mixture of the catalytic oxidation in gas phase of alkanes (C3-C4), alkenes (C3-C4). alkanols (C3-C4) and / or alkanals (C3-C4) and / or precursors thereof to form acrylic acid or methacrylic acid, (b) the condensation of said gas mixture of products, (c) the crystallization of acrylic acid or methacrylic acid from the solution obtained in step ib), and (d) la. removal of crystals resulting from the adre liquor »In a preferred embodiment, at least a part of the mother liquor is recycled to step (c) (step (e)). Additional preferred embodiments of the present invention will be apparent from the following description, from the subclaims, of. drawing and example. In the process of the present invention, the acrylic acid or methacrylic acid is directly and immediately crystallized without additional intermediate or purification steps and without the addition of auxiliaries from the solution formed in the course of the condensation of the. product mix »La. The product mixture essentially has the composition of a reaction product formed in a catalytic oxidation process in gas phase to form the acid. The single figure illustrates a preferred embodiment for carrying out the process of the present invention. Step (a) : Step i) produces a gaseous mixture of products having essentially the composition of a reaction mixture of the catalytic oxidation in the gas phase of C3 or C4 alkanes, the C3 to C quenas, C3 or C4 alkanols, and / or C3 alkanals. or C4 and / or precursors thereof to form ~ > c? da acri. l i.co to good methacrylic acid. It is particularly advantageous for the gas mixture of products to be prepared by catalytic oxxdaxon in the phase of μrane gas, acroleipate, tert-butanol, isobutene, xsobutane, xsobutxraldehyde, metacrolean, isabutipic acid or methyl tert-butyl ether. Suitable starting compounds include all the precursors of the compounds set forth above at p.rt.x.r of which the initial compound C3 / C4 is formed as an intermediate product during the gas phase oxidation. Examples which may be mentioned herein are methyl tert -butyl ether and isobutyl acid for the production of metacross acid. A particularly useful source of gaseous mixture of products to be used in the process of the present invention is the catalytic phase conversion of Propene gas and acryolsine in acrylic acid using molecular oxygen in accordance with known processes, especially with what is described in the references mentioned above. The. The reaction in the gas phase is preferably carried out at a temperature between 200 and 450 ° C with or without super-temperature pressure. The heterogeneous catalysts used are preferably catalysts of multiple oxidizing components based on molybdenum oxides, bismuth and iron in stage 1 (oxidation of acrolexin in prapene) - the malbony and vanadium oxides in stage 2 (oxidation of acrolein in acid acractical). When prapane is used as the starting material, it can be converted into a mixture of prapene / propane by: catalytic dropping in accordance with what is described, for example, in Catalysis Today 24 (1995), 307-313, or in the documen or US-A-5 510 558; by oxidation dragenation, in accordance with that described, for example, in CN-1 105 35 5 or by catalytic dehydration in accordance with that described, for example, in EP-AO 253 409, EP-A-293 224, DE-A-395 08 558 or EP-AO 117 146, When a mixture of propene / propane is used, prapana acts as a diluent. Additional suitable propene-propane mixtures are the refinery propene (70% prapene). and 30% of prapana) or the propene of thermal decomposition (95% of propene and 5 of propane). In principle mixtures of propene / propane such as those mentioned above can be oxidized with axigene or air or a mixture of oxygen and nitrogen of any composition in acrolein and acrylic acid. The conversion of prapene to acrylic acid is exothermic. The reaction gas, which includes the initial materials and products, preferably includes a gas l Inert diluent »eg recycle gas (see below), atmospheric nitrogen» one or more saturated (C1-C6) hydrocarbons, especially methane and / or propane, v / or water vapor, can therefore absorb only a small fraction of the heat of the reaction. Even though the type of reactor used is not a matter of any restriction per se, it is common to use heat exchangers of groups of tubes packed with the oxidation catalyst, so that with this type of apparatus the predominant fraction of the cal the reaction can be removed to the tube walls cooled by convection and radiation. The catalytic oxidation process in the gas phase produces pure acrylic acid, but a gaseous mixture which, as in the home of acrylic acid, can include essential elements. as secondary components acrolema and / or propene without conversion, water vapor, carbon monoxide carbon dioxide, nitrogen, propane, oxygen, acetic acid, prapionic acid, formaldehyde, other aldehydes and anhydride »The product mixture of the reaction it usually comprises (all percentages are based on the total reaction mixture) from 1 to 30% by weight of acrylic acid, from 0.05 to 1% by weight of propene and from 0. 5 1% by weight of acralein, from 0.05 to 10% by weight of oxygen, from 0.05 to 2% by weight of acetic acid, from 0.01 to 2% by weight of propionic acid, from 0.05 to 1% by weight of formaldehyde, 0.05 a. 2% in weight *) of aldehydes, from 0.01 to 0.5% by weight of maleic anhydride and from 20 to 98% by weight, preferably from 50 to 98% by weight, of inert dioxide gases. The inert dioxide gases include, in particular, saturated hydrocarbons (Cl-C6), for example from 0 to 90% by weight of methane and / or propane, and also from 1 to 30% by weight of water vapor, 0.05 to 15% by weight of carbon oxides and 0 to 90% by weight of nitrogen, all percentages are based on 100% by weight of diluent gas. Methacrylic acid can be prepared in a manner similar to acrylic acid by catalytic gas-phase reaction of C4 starting compounds with molecular oxygen. A particularly advantageous way of obtaining methacrylic acid is, for example, by the. Catalytic oxidation in isobutene gas phase. isobutane, tert-butanal. , isabuthyraldehyde, methacrolein or methyl tert-butyl ether. The catalysts used are again oxide catalysts mixed with transition metals (for example Mo, V, W and / or Fe). Particularly suitable processes for the production of methacrylic acid are the processes based on methacrolein, especially when methacrolein is produced by catalytic oxidation in the gas phase of tert-butanol, isobutane or isabutene, or by the. formaldehyde reaction with propionaldehyde in accordance 32 with that described in EP-B-0 092 097 or in document EP-B-0 058 927. In this way it is possible to prepare methacrylic acid in 2 stages ed ane (11 33 condensation of prapionaldehyde with farmaldehyde (in the presence of a secondary amine as a catalyst) to form methacrylic acid and (2) the subsequent oxidation of methacrylic acid in methacrylic acid, or in the case of the production of acrylic acid, the product is not pure methacrylic acid, but a gaseous mixture which, like methacrylic acid, can essentially include as secondary components non-converted methacrole and / or water vapor, carbon monoxide, carbon dioxide, nitrogen, oxygen, acetic acid, pro-ionic acid, additional aldehydes and male anhydride The process of the present invention is employed especially when the reaction mixture comprises from 0.02 to 2% by weight of methacrylic acid, based on the total reaction mixture, and comprises another part essentially the same corresponding constituents as in the case of the production of acnic acid. Paxßo (b) s Step (b) submits the reaction product of step (a) to a condensation, particularly to a partial to total condensation, to obtain a solution. The condensation can be carried out in one or several stages in accordance with usual processes, and the type of condensation does not present any special restriction. Condensation is advantageously carried out in a direct condenser where the previously produced condensate comes into contact with the hot gaseous product of the reaction. Suitable apparatuses for condensation are in particular dew purifiers, Ventup scrubbers, bubble columns or apparatuses having draining surfaces. The mixture obtained by partial condensation of the reaction product of step (a) comprises preferably from 60 to 99.5% by weight of acrylic or methacrylic acid, from 0.1 to 40% by weight of water, and also from 0.1 to 15% by weight of impurities, especially, each percentage based on 100% by weight of condensed, from 0.01 to 57. by weight of acrolema to metacralein, from 0.05 to 5% by weight of acetic acid, from 0.01 to 5% by weight of propionic acid, from 0"5% by weight of formaldehyde, of 0.01 to 5% by weight of additional aldehydes, and from 0% to 5% by weight of maleic acid »It is particularly preferable that the condensation offers a mixture comprising 93 to 98% by weight of acrylic or methacrylic acid, from 1 to 5% by weight of water, also from 0.5 to 5% by weight of impurities, esp especially, each percentage based on 100% by weight of condensate, 0.01 to 3% by weight of acralein or metae.ra lein, 0.1 to 3% by weight of acetic acid, 0.01 to 3% by weight of acid propionic, from 0.01 to 3% by weight of formaldehyde, from 0.01 to 3% by weight of additional aldehydes, and from 0.01 to 3% by weight of aleic acid. Step (c): In step (c) the solution obtained in step (b) is crystallized, which comprises acid acrxl cu or bxen methacrylic acid »Asx, the solution obtained in the condensation step is fed directly to the glass passage ation. No solvent is added, especially when organic solvents are not added. The process of Christianization used is not subject to any restriction »The crystallization can be carried out continuously or in batches, in one or more stages. The crystallization is preferably carried out in a single step. In another preferred embodiment of the invention, the crystallization is carried out as a fractional crystallization. In the technique of fractional crystallization, it is usual to call all stages that produce a purer crystallized than a solution of acrylic acid or methacrylic acid co-fed to purification stages and all other stages, separation stages, The multi-stage functionalization is advantageously employed in accordance with the countercurrent principle where the crystallization is separated from the mother liquor at each stage and fed to the particular stage which has the next highest degree of purity, while the crystallization residue is fed to the particular stage that has the next lowest degree of purity »The temperature of the solution during the crystallization is advantageously within the range of -5 ° C to -5-14 ° C, especially within the range of 12 * C to -S ^ C. The solids content in the crystallizer is preferably within the range of 80 g of solids / 100 g, preferably within the range of 15 to 35 g of solids / 100 g. In a preferred embodiment of the invention , the cooling is carried out by cooling the walls of the apparatus or by evaporating the solution under reduced pressure. In the case of crystallization by cooling, the heat is removed through scraper chillers connected to it. a stirred tank or a tank without agitator *. The circulation of the crystal suspension is ensured in this case by means of a pump »Alternatively, it is possible to remove * the heat through the walls of a stirred tank having a narrow clearance. A preferred additional embodiment of crystallization by cooling includes the use of manufactured cooling disk crystallizers, for example, by Gouda Pa ses bajos). In a further suitable variant for cooling crystallization, the heat is removed by conventional heat transfer elements (preferably heat transfer elements from plates or groups of tubes). These apparatuses, unlike scraped surface coolers, agitated tanaues having narrow gap agitators or b in cooling disc crystallizers, have no device to prevent glass layers on the heat transfer surfaces. In operation, if a state is reached in which the resistance to heat transfer has become too high due to the formation of a crispy layer, the operation passes to a second apparatus. During operation of the second apparatus, the first apparatus is regenerated * preferably by melting the glass layer or by rinsing the side with an unsaturated solution), Sx the resistance to heat transfer becomes too high in the second device, the operation goes back to the first device, etc. This variant can also be operated with more than 2 devices alternating between them. In addition, the crystallization can be carried out by conventional evaporation of the solution ba or reduced pressure. In a further advantageous embodiment of the invention, a crystallization is carried out in apparatuses in which the crystals grow on cooled surfaces within the crystallization apparatus, ie they are immobilized in the apparatus (for example, crystallization process in layer). of Sulzer Chemtech (Switzerland) or static process of BEFS PROI? M (France)) »Step (d)? Step (d) separates the acrylic acid or methacrylic acid crystals obtained in step c) from the mother liquor. For a layered crystallization or a static crystallization, the separation of the crystals from the mother liquor can be The crystals are immobilized in the apparatus so that the mother liquor can be removed from the apparatus by letting it flow outwards. The crystals are removed from the crystallization apparatus by means of the fusion of the crystals. after allowing the fusion to flow out »In the case of suspension crystallization, any solid-liquid separation is adequate. In a preferred embodiment of the intention, the crxstals are separated from the mother liquor by filtration and / or centrifugation. Advantageously, filtration or centrifugation is preferred by a "pre-thickening" of the suspension, for example, by means of one or more hydrazychones. The centrifugation can be carried out in any known centrifuge that works in batches or continuously. It is very useful to use centrifugal air blowers that can be operated in one or several stages »Screw screen centrifuges are also suitable as well as screw discharge centrifuges (decanters!) Filtration is done with the help of press filters can be operated in batch or in continuous, with or without agitator or by means of band filters »In general, the filtration can be carried out under superatmospheric pressure or under reduced pressure. or followed by additional processing steps to increase the purity of the crystals ob in the crystal cake In a particularly advantageous embodiment of the invention, the separation of the crxstals from the mother liquor is followed by a one-stage washing or well in multiple stages and / or exudate crystals or crystal cake »During washing, 3rd amount of washing liquor or is suitably within the range of 0 to 500 g of 3 washing icor / 100 g of crystallized, preferably within the range of 30 to 200 g of washing liquor / 100 g of crystallization. The washing liquor used is not subject to any restriction. However it is helpful to var with pure product, that is, with a liquid of acrylic acid or methacrylic acid whose purity is greater than the purity of the glass cake to be washed. A washing with water can also be carried out »The washing can be carried out in the usual apparatus for this purpose» It is useful to use ep washing columns where the removal of the mother liquor and the washing are carried out in the same apparatus, centrifuges that can be operated in one or several stages or either press filters or band filters »Washing can be carried out in centrifuges to band filters in one. to several stages. The wash liquor can be passed countercurrent in relation to the crystal cake. The exudate describes a local fusion of impure regions. The amount of exudate is preferably within the range of 100 g of crystallized melt / 100 g of crystallized before exudate, preferably within the range of 5 to 35 g of crystallized melt / 100 g of crystallized. It is especially preferable to carry out the exudate in centrifuges or band filters. A combined wash and exudate in an apparatus may also be suitable. Acrylic acid crystals and methacrylic acid crystals after solid-liquid separation and any further washing and / or exudate constitute the purified acid obtained from the process. The purity of the crystals obtained is generally within the range of 97 to 99.99% by weight of acrylic acid to methacrylic acid, especially within the range of 98.5 to 99.9% by weight of acrylic acid or methacrylic acid. The process of the present invention includes only very minimal amounts of impurities, such as for example acetic acid, maleic acid or aldehydes.

JO If desired, the purified acid can be prepared by known methods or b further purified by known methods. In a useful modality of the invention, step (d) is followed by the step described below »Step faith: Step e) relieves the mother liquor that remains after separation of the crystals, at least partially direct to the step of cri tization tc) »The proportion of recycled mother liquor is within the range of 0 to 100% by weight, preferably within the range of 20 to 80% by weight. The figure illustrates a preterm embodiment of the process of the present invention. Line 2 and compressor 3 supply air to synthesis reactors 4 and 5. In addition, reactor 4 is supplied through line 9 with recycled gas that has been compressed by the compressor & and consisting essentially of nitrogen, carbon oxides as well as unconverted initial materials, together with propene or issbutena of line 1 »Synthesis reactor 4 is where the first stage of gas phase oxidation is carried out in two stages, that is, the oxidation of propene or isobutene in the corresponding acralema. The acrolema is then oxidized in the corresponding acid in the synthesis reactor. This provides a gaseous mixture of products which comprises impurities mentioned above as the acid. This gas mixture of products is fed through line 7 to the condenser. The cooled part of the product mixture is removed through line 9, with a portion recycled as recycled gas, in accordance with what has been described above, in reactor 4. , while the other portion, preferably 50% of the total current of line 9, is removed from the plant as waste gas through line 10. The solution formed in condenser 8 is fed through the line 11 to crystallizer 12, where the crystallization is carried out »The mother liquor of the crystallization is fed together with the crystallisation through line 13 to a suitable apparatus 14 for solid = 3 preparation. liquid, the crystallizate is removed through line 15 and the mother liquor through line 16 »In a preferred embodiment, at least a certain part of the mother liquor is re-introduced through line 17 in the crystallization step (in line 11 and in crystallizer 12) »Thus, the purified crude acid is removed through line 15» The process of the present invention has the advantage in comparison with existing processes that, after 3a condensation of the mixture of products produced in the gas phase oxidation, the solution formed in the condensation provides a very high quality raw acid directly by crystallization. When a crystallization that has more than one purification step is used, it is possible to produce a pure acid directly, without carrying out a prepuphication which departs from the aforementioned references, Canadian Patent 790 625, JP- -7 082 21 and EP-AO 616 998 »It is an additional advantage It is an important object of the process of the present invention that the process is carried out at a relatively low temperature, ie the main stream of acrylic acid or methacrylic acid is directly removed as a product by condensation and crystallization. contrary to the prior art, no auxiliary is added and therefore no significant thermal pressure is required (especially in the case of high acidic contents) to remove this auxiliary, minimizing polymerization problems and the use of stabilizers of prior art process. In addition, fouling is avoided or said fouling is reduced as a result. It is surprising that it is possible to crystallize acrylic acid solutions or methacrylic acid solutions obtained by gas phase oxidation and condensation directly and that the products obtained have a very high purity. It is especially surprising that this is possible with water-based clogs.

The following Example, which exemplifies a preferred embodiment of the invention, illustrates said invention. Example 1 A stirred vessel of Z liter of glass (jacket, helical agitator) was charged with 2.3 I of the following mixtures 77.8% in weight of acrylic acid, »01% by weight of acetic acid, 1.07% by weight of malic acid, 3.66% by weight of farmaldehyde and 13» 46% by weight of water »This mixture corresponds in the drawing of the figure to the corrxent on line 11 coming from the condenser 8 »This solution was cooled from room temperature to -0.9ßC by cooling through the crystallizer jacket» The first acrylic acid crystals were formed at this temperature »Additional cooling down to -4 »4Í! C lasted 3 hours with 54 minutes. A solids content of 35.5 g of solids / 100 g of suspension was obtained at this temperature »218.7 g of the suspension were separated at -4.4 ° C in a centrifuge at 2,000 revolutions per minute during the time of the centrifugation. One-minute reflux in 77.76 g of crystals and 140.94 g of mother liquor. The crystals were then washed for 1 minute in 1A centrifuge with 40 g of pure acrylic acid with an acrylic acid content of more than 99.6%. Analysis of the crystals revealed the following composition; Acrylic acid 98.97% by weight Acetic acid 0.32.1% by weight Maleic acid 0.0204% by weight Formaldehyde 0.119% by weight 5 Water. .43% by weight The process of the invention thus provides an acrylic acid of high purity compared to the acrylic acid to the implant. fifteen twenty r ^ s

Claims (10)

1. CLAIMS 1, A process for preparing acrylic acid or methacrylic acid which comprises (a) the preparation of a gas mixture of products having essentially the composition of a reaction mixture of catalytic oxidation of gas phase of the gas (C3-C4) ), alkenes (C3-C4), alkanols (C3-C4) and / or alcanaies (C3-C4) and / or precursors thereof to form acrylic acid to either methacrylic acid, characterized by (b) the condensation of said gas mixture of products,
2. (c) the crystallization of acrylic acid or methacrylic acid from the solution obtained in step (b), and (d) the removal of the resulting crystals of the mother liquor »2» A process according to claim 1, which comprises an additional step (e) of recycling of al. minus one part of said mother liquor from step (d) to step (c).
3. 3, A process according to claim 1, or claim 2, wherein said gaseous mixture of products of step (a) is prepared by catalytic oxidation in gas phase of propene, acrylamine tert-butanal. isobutene, isabutane, isabuthyraldehyde, methacrolein, isobutyric acid or methyl tert-butyl ether.
4. A process according to any of claims 1 to 3, wherein said step condensation (b) is carried out in one or several stages as partial condensation or tota3.
5. 5. A process according to any of the preceding claims, wherein said crystallization of step (c) is carried out in one or more stages.
6. 6) A process of consistency with any of the preceding claims, wherein said step crystallization ( c) is carried out at a temperature of said solution within a range of -25 ** C to +14 < ! C.
7. 7, A process according to any of the preceding claims, wherein said step crystallization (c) is carried out by removing the heat by walls of cooling apparatus or by evaporating said solution under reduced pressure,
8. 8. A process according to any of the preceding claims, wherein said step crystals (d) are removed from said mother liquor by filtration / or centrifugation.
9. 9. A process according to any of the preceding claims, wherein said crystals removed in step (d) are subjected to ores at a washing and / or exudate step.
10. 10, A process of consistency with any of the - "?" 7

    previous claims, wherein said recycling of step (e) is carried out by means of recycling 20 a. 80% by weight of said mother liquor to step (c).