MXPA00002402A - Method for producing acrylic acid and methacrylic acid - Google Patents

Abstract

The invention relates to a method for producing acrylic acid and methacrylic acid by producing a gaseous product mixture which essentially has the composition of a reaction mixture of the catalytic gas phase oxidation of C3-/C4- alkanes, -alkenes, -alkanols or -alkanals or precursors thereof to form acrylic acid or methacrylic acid. The product is separated from the gaseous product mixture according to a method comprising the following steps:a) condensing the gaseous product mixture;b) crystallising the acrylic acid or methacrylic acid from the solution obtained in (a);c) separating the resulting crystals from the mother liquor and d) returning at least a part of the mother liquor from stage (c) to stage (a).

Description

METHOD FOR THE PRODUCTION OF ACRYLIC ACID AND METACRYL ACID The present invention relates to a process for the preparation of acrylic acid or methacrylic acid. Acrylic acid is an important base chemical. Based on its highly reactive double bond as well as based on its acid function, it is especially suitable as a monomer for the preparation of polymerized products. Of the processed amount of acrylic acid monomers, most are esterified before polymerization - for example in glues, dispersions or lacquers. Only the minor part of the acrylic acid monomers produced is directly polymerized - for example in "superabsorbent". While, in general, in the case of the direct polymerization of the acrylic acid monomers a high purity is required, the purity requirements of the acrylic acid are not so high when the acrylic acid is esterified before the polymerization. It is generally known that acrylic acid can be made from a heterogeneous catalyzed gas phase oxidation of propene with molecular oxygen in catalysts that are in a solid aggregate state at temperatures between 200 and 400 ° C in one stage or in two steps in acrolein (see, for example, DE-Al 962 431, DE-A-2 943 707, DE-C-1 205 502, DE-A-195 08 558, EP-AO 257 565, EP-AO 253 409, DE-A-2 251 364, EP-AO 117 146, GB-1- 450 986 and EP-AO 293 224). Catalysts of various oxidizing components are used, for example based on oxides of elements such as molybdenum, bismuth and iron (in stage 1) or molybdenum and vanadium (in stage 2). It is known from DE-C-2 136 396 to separate the acrylic acid from the reaction gases obtained from the catalytic oxidation of propene or acrolein by countercurrent absorption with a mixture of approximately 75% by weight of diphenylether and about 25% by weight of diphenyl. Furthermore, it is known from DE-A-2 449 780 to cool the hot reaction gases by partially cooling the solvent in a direct condenser (immediate cooling apparatus) before countercurrent absorption. The problem here as well as in additional steps of the procedure is the presence of solids in the devices, which reduces the availability of equipment. According to DE-A-4 308 087 this presence of solids can be reduced to the extent that a polar solvent such as, for example, dimethyl phthalate is added to the relatively non-polar solvent mixture of diphenylether and diphenyl. amount of 0.1 to 25% by weight. In addition to the absorption described above of the reaction product containing acrylic acid in a high-boiling solvent mixture, other known methods involve a total condensation of acrylic acid and the water of reaction that comes from the catalytic oxidation. In this way an aqueous solution of acrylic acid is obtained which can be further processed by distillation with an azeotropic medium (see DE-C-429 391, JP-A-1 124 766, JP-A-7 118 766, JP -A-7 118 966-R, JP-A-7 118 968-R, JP-A-7 241 885) or in an extraction process (see, DE-A-2 164 767, JP-A-5 81 40-039 and JP-A-4 80 91 013). In EP-AO 551 111 the mixture prepared through catalytic oxidation in the gas phase of acrylic acid and by-products is put in contact with water in an absorption tower and the aqueous solution obtained is distilled in the presence of a solvent that forms an azeotrope with low-boiling polar elements such as water or acetic acid. DE-C-2 323 328 discloses the separation of acrylic acid from an aqueous butanol-acrylic acid esterification liquor by extraction with a special mixture of organic solvents. The drawback in the case of the processes described above is that, for extraction and absorption, an organic solvent is used which must be separated again by an additional processing step such as, for example, rectification with a high thermal load. In this way the danger of a polymerization of acrylic acid arises. JP-A-07 082 210 describes a process for cleaning acrylic acid which also contains acrylic acid, acetic acid, propionic acid, acrolein and furfural. In this process, after the addition of water, a crystallization is carried out under vacuum, so that after purification and washing of the acrylic acid crystals a purity of 99.6% is obtained. Japanese Patent 45-32417 presents a process in which an aqueous solution of acrylic acid or an aqueous solution of methacrylic acid which further contains acetic acid and propionic acid, is extracted with heptane or toluene and finally the water is removed by distillation. of the extract. In the next stage the remaining extract is cooled to a temperature between -20 and -80 ° C, in order to cause the crystallization of acrylic acid or methacrylic acid. The crystals are separated, and the mother liquor is recycled to the extraction process. In accordance with this patent document the use of an extraction medium or organic solvent is necessary since otherwise the solution, when cooled, solidifies, without producing crystals. The drawback in this process is that apart from the addition of an organic solvent, a distillation step is required to separate the water. Canadian Patent 790 625 relates to an additional cleaning process for crude acrylic acid by fractional crystallization. In the case of propionic acid as the main impurity of the crude acrylic acid the temperature is not lowered to a level lower than the peritectic temperature of the acrylic acid-propionic acid system, while in the case of acetic acid as the main impurity, the temperature is not lowering to a level lower than the eutectic temperature of the acid, acrylic-acetic acid system. The acrylic acid used for the crystallization is prepared according to customary procedures, such as, for example, gas phase oxidation of propene or acrolein, and is finally subjected to a preliminary cleaning step by means of conventional known methods, for example extraction. In accordance with the data of the patent document, the crystallization of the acrylic acid is preferably carried out essentially in the absence of water. EP-A-0 616 998 discloses a process for cleaning acrylic acid through a combination of dynamic and static crystallization, where as an initial product, pre-purified acrylic acid, for example, acrylic acid, which is pre-purified by distillation, is used. The procedures described in the above documents have in common that they require a pre-cleaning of the acrylic acid before crystallization. Since the precleaning in general terms requires an organic solvent, which must then be separated again by means of an important thermal load, the problem of an undesired polymerization of the acrylic acid always arises. From EP-AO 002 612, a method of cleaning acrylic acid found in an aqueous solution by fractional crystallization, with addition of salts to the solution of acrylic acid, in order to break the water eutectic -acrylic acid, which is in a volumetric content of 63% acrylic acid. EP-AO 675 100 describes a process for the preparation of C3-C6 alpha, beta-unsaturated carboxylic acids such as, for example, methacrylic acid, by oxidative dehydration of the corresponding saturated C3-C6 carboxylic acid followed by melt crystallization. with subsequent fractional distillation or with fractional distillation with subsequent melting crystallization. The object of the present invention is to provide a process through which acrylic acid or methacrylic acid with high purity is obtained without requiring expensive process steps. This object is achieved through a process to separate acrylic acid or methacrylic acid from a gas mixture, which contains, in addition to acrylic acid or methacrylic acid, at least one additional component. The process according to the present invention for the separation of acrylic acid or methacrylic acid is characterized in that a) the gas mixture is condensed, b) the acrylic acid or methacrylic acid is crystallized from the solution obtained in step a ), c) the crystals obtained from the mother liquor of stage b) are separated and d) at least part of the mother liquor from stage c) is recycled to stage a). In accordance with the present invention there is also a process for preparing acrylic acid or methacrylic acid. This results from the catalytic gas phase oxidation of C3 / C4 alkanes, C3 / C4 alkenes, C3 / C4 alkanols, and / or C3 / C alkanals, and / or precursors with the formation of a crude product that sets apart acrylic acid and methacrylic acid also contains at least one component of the group of unreacted starting products and by-products. The preparation process according to the present invention is characterized in that the gaseous crude product after the process according to the present invention is treated for the separation of acrylic acid or methacrylic acid. It was found that acrylic acid or methacrylic acid from a gaseous mixture of products subjected to condensation can be crystallized directly from the solution obtained by condensation. The important thing in this case is that no additional cleaning stage is required and that no additive or auxiliary is required. In a preferred embodiment the condensation in step (a) is carried out in a column. Additional preferred embodiments of the present invention are apparent from the following description and examples. In the case of the process according to the present invention, the acrylic acid or the methacrylic acid is directly and immediately crystallized without additional intermediate steps or additional cleaning steps and without the addition of auxiliaries, from the solution coming from the condensation of the product mix This mixture of products essentially has the composition of a reaction product coming from the catalytic oxidation in the gas phase of the acid. The only figure shows a preferred embodiment for carrying out the method according to the present invention. Preparation of a gaseous mixture of products containing acrylic acid or methacrylic acid First a gas mixture of products is prepared having essentially the composition of a reaction mixture catalytic oxidation in gas phase of C3 or C alkanes, C3 or C3 alkenes , C3 or C4 alkanols and / or C3 or C4 alkanals and / or precursors thereof in acrylic acid or methacrylic acid. It is particularly advantageous for the gas mixture of products to be prepared by catalytic gas-phase oxidation of propene, propane, acrolein, tert-butanol, isobutene, isobutane, isobutyraldehyde, methacrolein, isobutyric acid or methyl tert-butyl ether. As starting compounds, all the precursors of the aforementioned C3 / C compounds in which an initial compound C3 / C is first formed as an intermediate product during gas phase oxidation can be used. Examples mentioned for the preparation of methacrylic acid are methyl tertiary butyl ether or isobutyric acid. Both acrylic acid and methacrylic acid can be made directly from propane or isobutane. A gaseous mixture of catalytic gas phase oxidation of C3-C alkanes, C3-C alkenes, C3-C alkanols and / or C3-C.1 alkanals and / or precursors of these in acrylic acid is preferably used as raw material. or methacrylic acid. Especially preferred is the catalytic gas phase reaction of propene and / or acrolein in acrylic acid with molecular oxygen in accordance with known procedures, especially in accordance with what is described in the documents mentioned above. Preferably, in this case, work is carried out at a temperature within a range of 200 to 450 ° C and possibly under high pressure. Preferably, as catalysts heterogeneous, catalysts of multiple oxidizing components are used based on molybdenum oxide, bismuth and iron in the first stage (oxidation of propene in acrolein) and molybdenum and vanadium oxide in the second stage (oxidation of acrolein in acrylic acid). These reactions are carried out, for example, in one stage or in two stages. If propane is used as an initial substance, then it can be transformed into a propene / propane mixture through catalytic oxidant dehydration in accordance with what is indicated, for example, in Today Catalysis 24 (1995), 307-313 or in the US document. -A-5 510 558; by homogeneous oxidant hydration as described for example in CN-A-1 105 352; or by catalytic dehydration, as described for example in EP-AO 253 409, DE-A-195 08 558, EP-AO 293 224 or EP-AO 117 146. Suitable propene / propane mixtures are also refinery propene ( 70% of propene and 30% of propane) or propene of reformation (95% of propene and 5% of propane). Basically propene / propane mixtures like the above can be oxidized with oxygen or air or a mixture of oxygen and nitrogen in combination with acrolein and acrylic acid. By using a mixture of propene / propane, propane serves as a dilution and / or reagent gas. A suitable process is also described in EP-B-0 608 838, wherein the propane as a reagent is directly transformed into acrylic acid. The transformation of propene into acrylic acid is strongly exothermic. The reaction gas which, apart from the educts and products, preferably contains an inert dilution gas, such as, for example, recycle gas (see below), nitrogen from the air, one or more saturated Ci-Cβ hydrocarbons, especially methane and / or or propane, and / or water vapor, can absorb only a small part of the heat of the reaction. Although the type of reactors used per se is not subject to any limitation, tube heat exchangers, filled with oxidation catalytic converter, are mainly used, since they can remove most of the heat produced by the reaction through convection and irradiation in the walls of the cooling tubes. In the case of catalytic oxidation in gas phase, pure acrylic acid is not obtained but a gaseous mixture, which apart from acrylic acid, as additional components, may also contain essentially unreacted acrolein and / or propene, water vapor, monoxide carbon, carbon dioxide, nitrogen, propane, oxygen, acetic acid, propionic acid, formaldehyde, additional aldehydes and maleic acid anhydrides. Usually, the reaction mixture contains, relative to the total reaction mixture, from 1 to 30% by weight of acrylic acid, from 0.05 to 1% by weight of propene and from 0.05 to 1% by weight of acrolein, from 0.05 to 10% by weight of oxygen, 0.05 to 2% by weight of acetic acid, 0.01 to 2% by weight of propionic acid, 0.05 to 1% by weight of formaldehyde, 0.05 to 2% by weight of aldehyde , from 0.01 to 0.5% by weight of maleic acid anhydride and from 20 to 98% by weight, preferably from 50 to 98% by weight, of inert diluting gases. Inert dilution gases include, in particular, saturated Ci-Cβ hydrocarbons, for example from 0 to 90% by weight of methane and / or propane, and in addition from 1 to 30% by weight of water vapor, from 0.05 to 15% by weight of carbon oxide and from 0 to 90% by weight of nitrogen, each time in relation to 100% by weight of dilution gas. Methacrylic acid can, analogously to acrylic acid, be prepared by the gas phase catalytic reaction of C4 starting compounds with molecular oxygen. Methacrylic acid can be obtained in a particularly advantageous manner, for example by catalytic gas phase oxidation of isobutene, isobutane, tert-butanol, isobutyraldehyde, ethacrolein, or methyl tert-butyl ether. Catalysts are also catalysts of mixed transition metal oxides (for example Mo, V, and / or Fe), and the reaction is carried out, for example, in one stage or in several stages. Especially suitable methods are the processes in which the preparation is carried out from methacrolein, especially when the methacrolein is made from the catalytic oxidation in gas phase of tert-butanol, isobutane, or isobutene or through of the reaction of formaldehyde with propionaldehyde according to EP-BO 092 097 or EP-BO 058 927. It is also possible to prepare methacrylic acid in two stages by (I) condensation of propionaldehyde with formaldehyde (in the presence of of a secondary amine as catalyst) in methacrolein and (II) finally the oxidation of methacrolein in methacrylic acid. A further suitable procedure is described in EP-B-O 608 838, in which isobutane as a reagent can be transformed directly into methacrylic acid. In the same way as in the case of the preparation of acrylic acid, pure methacrylic acid is not obtained but a gaseous mixture which may contain, in addition to methacrylic acid, as additional products, essentially unreacted methacrolein and / or water vapor, monoxide carbon, carbon dioxide, nitrogen, oxygen, acetic acid, propionic acid, additional aldehydes and maleic acid anhydride. The process according to the present invention is used especially when the reaction mixture contains from 0.02 to 2% by weight of methacrolein in relation to the total reaction mixture and in addition essentially the same components as in the case of the preparation of the reaction. acrylic acid Step (a): In step (s) the gaseous mixture of products containing acrylic acid or methacrylic acid is subjected to condensation, especially partial or total condensation, whereby a solution is obtained. The condensation is carried out preferably in a column. For this purpose, a column with separation elements, especially packed, fillers and / or floors, preferably bell floors, sifting floors, valve floors and / or double flow floors is used. In this way, the components that can be condensed from the gaseous mixture of products that were made are fractionated by condensation by cooling. Since the gas mixture, as a consequence of impurities as well as dilution gases, contains a heavy fraction of boiling product, an average fraction of boiling product and a light fraction of boiling product as well as components which can not be condensed, it can provide in the column in the corresponding places one or several lateral exits. Unlike conventional condensation, a condensation column already allows a separation into individual components. Suitable columns include at least one cooling installation, whereby all heat transfer or heat exchange devices in which the accumulated heat of condensation is indirectly removed (externally) are suitable. Preferred are tube heat exchangers, plate heat exchangers, as well as fans. Suitable cooling means are air in the case of corresponding air coolers and cooling liquids, especially water, in the case of other cooling devices. If a cooling installation is contemplated, then it is placed at the head of the column, where the light fraction of the boiling product condenses. Since the gaseous mixture containing acrylic acid or methacrylic acid contains several fractions, it is advantageous to integrate several cooling devices into several sections of the column, for example a cooling device in the lower section of the column for the condensation of the fraction heavy boiling product and a cooling installation at the head of the column for the condensation of the heavy fraction of boiling product. The fraction with acrylic acid or with methacrylic acid is obtained in the middle part of the column in one or several lateral outlets. The solution which crystallizes in step (b) is then removed by condensation as the average fraction of the boiling product. The pressure in the column depends on the quantity of non-condensable components and is preferably within a range of 0.5 to 5 bar of absolute pressure, especially within a range of 0.8 to 3 bar of absolute pressure. The exact operating conditions for the column, such as temperature and pressure, or connection and arrangement of the cooling system or cooling installations, arrangement of the lateral outlet or lateral outlets to obtain acrylic or methacrylic acid, choice of the height of the column and the diameter of the column, number and spacing of the sections / floors of separation in the column or type of attachment for separation of the column can be provided by the specialist based on usual tests in relationship to the separation technique. In a preferred embodiment the hot gas mixture is cooled directly or indirectly before condensation. It is preferred, in the case of direct cooling, that the gaseous mixture be cooled with the aid of the heavy fraction of condensed boiling product from the gas mixture. In other cases, an assistant is included in the procedure, but must be removed later. As for the apparatus, this pre-cooling can be carried out in a sump area in the column (with or without integration to the column) or separated from the column in an individual apparatus, such as a gas cooler, a device of rapid cooling or an instant pot. In a preferred installation, the condensation of the gas mixture of the reaction is carried out in a column in the following manner, where the column is divided into several sections, where the following operations are carried out with different technical procedures: sump: Cooling of the hot gas mixture The hot gas mixture is directed and cooled in the sump area. This can be carried out by indirect cooling, for example heat exchanger, or by direct cooling with the heavy fraction of boiling product condensed in the next column section as the cooling medium. First cooling section: Condensation of the heavy fraction of boiling product In the area of the first cooling section, the heat of condensation is removed externally through a heat exchanger with water, for example as a cooling medium, in the As the heavy fraction of the condensed boiling product is taken out of the column, it is cooled through the heat exchanger and a part of the cooled, condensed boiling product heavy fraction is recycled to the column, while the another part, usually less than 1% by weight in relation to 100% condensate is removed through the lateral intake. The heavy fraction of condensed, recycled boiling product is brought upstream against the rising gas. First cooling zone until lateral intake: Enrichment of the heavy fraction of boiling product Between the first cooling zone and the lateral intake, an enrichment is carried out by distillation and condensation of the heavy fraction of boiling product coming from the gas stream that It rises upstream. Lateral intake: Acid intake Acrylic acid or methacrylic acid is taken in the lateral collection. - From the lateral intake to the second cooling zone: Enrichment of the average fraction of boiling product In the area between the lateral intake and the second cooling zone the enrichment of the average fraction of boiling product is carried out of the gas stream that rises countercurrent, so that the average fraction of boiling product is enriched from the lateral intake. - Second cooling zone: Condensation of the light fraction of boiling product In the area of the second cooling zone the condensation of the light fraction of boiling product is carried out from the rising gas stream to countercurrent. The heat of condensation is removed externally in the second cooling zone through a heat exchanger cooled with water for example, as a cooling medium, wherein the light fraction of boiling product is removed, cooled and a part of the light fraction of boiling product, condensed, cooled is recycled to the column while the other part is removed. The components that were not condensed, among which are preferably nitrogen, carbon monoxide, carbon dioxide, oxygen, methane, propane and propene, are removed from the top of the column. Furthermore, the condensation can be carried out by means of conventional processes consisting of one or more stages, so that the condensation technique is not subject to any special limitation. Preferably the condensation is carried out in a direct condenser, whereby the condensate already obtained is brought into contact with the hot gaseous reaction product. As for devices for condensation, spray-type washing machines, venturi-type washing machines, blow columns or devices with irrigated surfaces are particularly suitable. The mixture obtained by partial or total condensation of the reaction product from the product gas mixture formed, especially the condensate of the average fraction of boiling product obtained by column condensation, preferably contains from 60 to 99.5. % by weight of acrylic acid or methacrylic acid, from 0.1 to 40% by weight of water, in addition to 0.1 to 15% by weight of impurities, especially, each time in relation to 100% by weight of condensate, from 0.01 to 5 % by weight of (meth) acrolein, from 0.05 to 5% by weight of acetic acid, from 0.01 to 5% by weight of propionic acid, from 0.01 to 5% by weight of formaldehyde, from 0.01 to 5% by weight of aldehydes and 0.01 to 5% by weight of maleic acid. Especially preferred is a mixture obtained by condensation containing from 93 to 98% by weight of acrylic acid or methacrylic acid, from 1 to 5% by weight of water, in addition to 0.5 to 5% by weight of impurities, especially, each time in relation to 100% by weight of condensate, from 0.01 to 3% by weight of acrolein or methacrolein, from 0.1 to 3% by weight of acetic acid, from 0.01 to 3% by weight of propionic acid, 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 maleic acid. Step (b): In step (b) the solution obtained in step (a) containing acrylic acid or methacrylic acid is crystallized. In this way the solution obtained in the condensation stage is taken directly to the crystallization. It works without addition of solvent, especially without the addition of organic solvent. The crystallization process employed does not present any limitation. The crystallization can be carried out continuously or discontinuously, in one stage or in several steps. Preferably the crystallization is carried out in one step. In another preferred embodiment of the present invention, the crystallization is carried out as fractional crystallization. Usually in the case of fractional crystallization all the steps that produce a purer crystallized than the aqueous acrylic acid solution provided or that the aqueous methacrylic acid solution provided are known as cleaning steps and all other steps as entrainment steps. . Conveniently, the process is carried out in several stages in accordance with the countercurrent principle, in which after crystallization in each stage the crystallization is separated from the mother liquor and this crystallized is fed to the corresponding stage with the following degree of higher purity, while the crystallization residue is fed to the corresponding stage with the next lower degree of purity. Preferably the temperature of the solution during crystallization is within a range of + 5 ° C to + 14 ° C, especially between 8 ° C and 12 ° C. The solids content in the crystallizer is preferably within a range of 0 to 80 g / 100 g, preferably 15 to 35 g of solid / 100 g. In a modality preferred embodiment of the present invention crystallization is carried out by cooling the walls of the apparatus or by vaporizing the solution under vacuum. By means of the crystallization by means of cooling the heat is brought to the grate cooler connected to a stirring vessel or to a vessel without stirring. The circulation of the crystalline suspension is ensured through a pump. There is also the possibility of stirring the heat through the wall of a stirring vessel with movable agitators. A preferred additional modality in the case of crystallization by cooling is the use of the cooling plate crystals method as, for example the one manufactured by Fa. Gouda (Holland). In an additional variation suitable for crystallization by cooling, the heat is removed through a usual heat transfer device (tube-type heat transfer devices or plates are preferred). Unlike grate coolers, stirring vessels with movable agitators or cooling glass sheets, these apparatuses have no device to prevent the formation of glass layers on the heat transfer surfaces. If during operation a state is reached in which the opposition to the heat transfer by the crystal layer formation reaches too high a value, it is switched to a second apparatus. During the operating time of the second apparatus, the first apparatus is regenerated (preferably by melting the crystal layer or by washing the apparatus with an unsaturated solution). When a too high resistance to heat transfer is reached in the second apparatus, the use of the first apparatus etc. is returned. This variant can also work with more than two devices in succession. Furthermore, the crystallization can be carried out by means of a usual vaporization of the solution under vacuum. In another additional embodiment of the present invention, the crystallization is carried out in apparatuses in which the crystals grow in the crystallization apparatus, on cooling surfaces, ie they are fixed in the apparatus (for example crystallization processes in layers). of the company Sulzer Chemtech (Switzerland)) or the static crystallization procedure of the company BEFS PROKEM (France)). Step (c): In step (c) the acrylic acid crystals obtained in step (b) or the methacrylic acid crystals obtained in step (b) are separated from the mother liquor. In the case of layer crystallization or static crystallization, the separation of the crystals from the mother liquor can be carried out in the crystallization apparatus itself, since the crystals are fixed in the apparatus and the mother liquor can be removed by its flow out of the apparatus. The removal of the crystals from the crystallization apparatus is carried out by melting the crystals and then by the melt flow. In the case of suspension crystallization, all solid-liquid separation processes are adequate. In a preferred embodiment of the present invention the crystals are separated by filtration and / or centrifugation of the mother liquor. Preferably, before filtration or centrifugation, the suspension is thickened, for example by hydrocyclone (s). For centrifugation all known centrifuges that operate discontinuously or continuously are suitable. The use of thrust centrifuges that can operate in one or more stages is especially preferred. In addition, snail screening centrifuges or snail production centrifuges (decanter) are also suitable. Filtration is advantageously carried out by discontinuous or continuous filtration with either no agitator or through a band filter. In general, the filtration can be carried out under pressure or under vacuum. During and / or after the separation of solids-liquids, the existence of additional process steps can be considered to raise the purity of the crystals or of the crystal cake. In a particularly advantageous embodiment of the present invention, after the separation of the crystals from the mother liquor, a washing and / or purification of the crystals or of the crystal cake is introduced in one or several steps. During washing, the quantity of washing liquid is suitably between 0 and 500 g of washing liquid / 100 g of crystallized, preferably between 30 and 200 g of washing liquid / 100 g of crystallized. Regarding the washing liquid there is no limitation. Profitably, however, it is washed with pure product, that is, with liquid containing acrylic acid or methacrylic acid whose purity is higher than the purity of the glass cake to be washed. In addition you can also apply a wash with water. The washing can be carried out in conventional apparatuses for this purpose. Advantageously, washing columns are used in which the separation of the mother liquor is carried out and the washing is carried out in an apparatus, centrifuges that can be used in one stage or in several stages, or filters or filters. band. The washing can be carried out in centrifuges or band filters in one or several stages. In this way, the washing liquid can be brought upstream against the crystal cake. In the case of liquefaction, it is a local fusion of a zone of impurities. Preferably the amount of liquefaction is between 0 and 100 g of melted crystals / 100 g of crystallized before liquefaction, preferably between 5 and 35 g of crystallized melt / 100 g of crystallized. Preferably the realization of the liquefaction is carried out in centrifuges or band filters. Likewise, the realization of a combination of washing and liquefaction in an apparatus may be adequate. The crystals of acrylic acid or crystals of methacrylic acid after the separation of solids-liquids and optionally after further washing and / or liquification represent the purified acid obtained from the process. The purity of the crystals obtained generally ranges from 97 to 99.99% by weight of acrylic acid or methacrylic acid, especially from 98.5 to 99.9% by weight of acrylic acid or methacrylic acid. According to the process according to the present invention, the obtained crystals contain small amounts of impurities such as, for example, acetic acid, maleic acid or aldehydes. If desired, the purified acid can be esterified according to known methods or said acid can be further purified according to known methods. Step (d): In step (d) the mother liquor remaining after the preparation of the crystals from step (c) is fed at least partially directly to the condensation step (a). The part of the recycled mother liquor is between 0 and 100% by weight, especially between 80 and 100% by weight, preferably 100% by weight. The figure shows a preferred embodiment for carrying out the method according to the present invention. The synthesis reactors 4 and 5 receive air through the pipe 2 and the compressor 3. In addition, reactor 4 is provided on line 9 from the compressor 6 compressed recycling gas consisting essentially of nitrogen, carbon oxide as well as unreacted educts, together with propene or isobutene from pipeline 1. In the synthesis reactor 4, the first stage of a two-stage gas oxidation is carried out, that is, the oxidation of propene or isobutene in the corresponding acrolein. In the synthesis reactor 5 the acrolein is then oxidized in the corresponding acid. In this way, a gaseous mixture of products is obtained, which, in addition to acid, also contains the aforementioned impurities. This mixture is carried through the pipe 7 to the condenser 8, where it is cooled and condensed. The condenser 8 is shown in the figure in the form of a column. The non-condensed part of the product mixture is removed through line 9, and a part is taken to reactor 4, as a recycling site, as described above, and the other part, preferably 50% of the stream total of pipe 9, is removed as exhaust gas from the facility through pipe 10. The heavy fraction of boiling product, condensed is removed through pipe 18, while the light fraction of boiling product, condensed, is removed through the pipe 19. The average fraction of condensed boiling product, which contains most of the acrylic acid or methacrylic acid, is fed through the pipe 11 (side intake) to the crystallization device 12, where crystallization is carried out. The mother liquor coming from the crystallization is fed along with the crystallized one through the pipe 13 to an appropriate apparatus 14 for the separation of solids-liquids, by which the crystallized is removed through the pipe 15 and the mother liquor It is removed through the pipe 16. At least a part of the mother liquor is carried through the pipe 17 to the condenser 8, preferably in the area of the lateral intake (pipe 11), and in this way is returned to the condensation In this way the purified crude acid is purged through the pipe 15. By recycling the mother liquor in the condensation stage it is possible to obtain a high yield of up to 99.5% by weight in accordance with the present invention. The process according to the present invention is especially suitable for the preparation of acrylic acid or methacrylic acid from gaseous reaction mixtures of a type containing a significant amount of water vapor. The process according to the present invention has additional advantages in comparison with procedures known to date, in that after the condensation of the mixture of products obtained by gas phase oxidation directly from the condensation of the solution obtained by crystallization, a crude acid of very good quality is obtained. By using a crystallization with more than one purification step, a pure acid can be obtained directly, so that no prior cleaning is required unlike the case of the aforementioned documents, Canadian Patent 790 625, JP-A- 0 07 082 210-A and EP-AO 616 998. A further important advantage of the process according to the present invention is that the process is carried out at relatively low temperature, ie the main stream of acrylic acid is carried directly after condensation and crystallization as a product outside the process. Since, unlike the state of the art, no auxiliary is added and consequently no significant thermal load is required (especially in the case of high acrylic acid contents) for the separation of this auxiliary, the problems related to the polymerization and with the use of process stabilizers as those observed in the state of the art. In addition, fouling is avoided or reduced in this way. It is surprising to observe that solutions of acrylic acid or methacrylic acid solutions can be directly crystallized by oxidation in the gas phase and condensation, and in this way products with high purity can be obtained. It is especially surprising that this can also be achieved in the case of aqueous condensates.

The invention will be explained and represented more precisely based on the following example which represents a preferred embodiment of this invention. Example The following gaseous product mixture with a temperature of 270 ° C is obtained through oxidation in the catalytic phase of propene. Table 1 Components concentration% by weight Water 4.358 Formaldehyde 0.20 Acetic acid 0.43 Acrylic acid 10.1 Maleic acid anhydride 0.07 Benzoic acid 0.02 acrolein 0.1 Phthalic acid anhydride 0.01 Propionic acid 0.002 Maleic acid 0 Allyl acrylate 0. 001 Benzaldehyde 0. 001 Furfural 0. 002 Phenothiazine 0 Nitrogen 76. 4 Oxygen 3. 6 Carbon dioxide 0.75 Carbon dioxide 2.62 Propene 0.52 Propane 0.73 The mixture (10931 g / h) was fed to the condensation stage (a). As a condensing device, a floor column with 27 bell floors was used. The temperature in the sump of the column reached 100 ° C. The heat of the condensation was removed through a heat transfer device on floors 1 and 27. At the top of the column, phenothiazine was added as a stabilizer. On the 27th floor, a current of 425 g / h of the following composition was removed: Table 2 Components concentration% by weight Water 89.47 Formaldehyde 0.125 Acetic acid 6.345 Acrylic acid 4.0 Maleic acid anhydride < 0.0001 Benzoic acid < 0.0001 acrolein 0.0541 Phthalic acid anhydride < 0.0001 Propionic acid < 0.0001 Maleic acid < 0.0001 Allyl acrylate 0.0012 Benzaldehyde < 0.0001 Furfural < 0.0001 Fenothiazolin < 0.0001 Nitrogen 0 Oxygen 0 Carbon oxide 0 Carbon dioxide 0 Propene 0 Propane 0 A stream of 2 g / h was removed from the following composition in the column sump: Table 3 Concentration components% by weight Water 1.21 Formaldehyde 0.0036 Acetic acid 0.879 Acrylic acid 39.45 Maleic acid anhydride 34.55 Benzoic acid 10.931 acrolein 0.0103 Phthalic acid anhydride 5.465 Propionic acid 0.0477 Maleic acid < 0.001 Allyl acrylate 0.0113 Benzaldehyde < 0.2673 Furfural < 0.3639 Phenothiazine 6.8039 Nitrogen 0 Oxygen 0 Carbon oxide 0 Carbon dioxide 0 Propene 0 Propane 0 The exhaust gas had the following composition: Table 4 Concentration components% by weight Water 0.982 Formaldehyde 0.239 Acid. acetic acid 0.0305 Acrylic acid 0.0103 Maleic acid anhydride < 0.0001 Benzoic acid < 0.0001 acrolein 0.1253 Phthalic acid anhydride < 0.0001 Propionic acid < 0.0001 Maleic acid < 0.0001 Allyl acrylate < 0.0001 Benzaldehyde < 0.0001 Furfural < 0.0001 Fenothiazolin < 0.0001 Nitrogen 89.054 Oxygen 4.1797 Carbon dioxide 0.873 Carbon dioxide 3.050 Propene 0.6054 Propane 0.850 The exhaust gas from the condensation column was reintroduced to the reaction (60% by weight) or was removed from the process (40% by weight) . On the 11th floor, a liquid stream of 4657 g / h at a temperature of 84.5 ° C was removed from the column, and then crystallized. This stream had the following composition: Table 5 Concentration components% by weight Water 2.52 Formaldehyde 0.0062 Acetic acid 5.899 Acrylic acid 90.972 Maleic acid anhydride 0.399 Benzoic acid < 0.0001 acrolein 0.0128 Phthalic acid anhydride < 0.0001 Propionic acid 0.0564 Maleic acid < 0.0001 Allyl acrylate 0.0548 Benzaldehyde 0.0006 Furfural 0.0492 Phenothiazine 0.0300 Nitrogen 0 Oxygen 0 Carbon oxide 0 Carbon dioxide 0 Propene 0 Propane 0 Finally, the mixture from the 11th floor was crystallized in a 10 L agitation vessel with a swirling stirrer. The heat of crystallization was removed through the double jacket of the container. The equilibrium temperature of the solution reached 4.8 ° C. The suspension created through crystallization (30 g of solid / 100 g of suspension) was separated in a centrifuge at a rate of 2000 revolutions per minute (diameter of centrifuge: 250 mm) and rotation time of 1 minute, through which crystals and mother liquor were obtained. The crystals (1281 g / h) were finally washed with melted crystallized (296 g / h) for 1 minute at 2000 revolutions per minute. The mother liquor (3376 g / h) was reintroduced together with the washing liquid to the condensation column on the 10th floor. Analysis of the crystals gave the following composition. Table 6 Components concentration% by weight Water 0.1066 Formaldehyde 0.0003 Acetic acid 0.9619 Acrylic acid 98.8816 Maleic acid anhydride 0.0225 Benzoic acid <; 0.0001 acrolein 0.0009 phthalic acid anhydride < 0.0001 Propionic acid 0.0162 Maleic acid < 0.0001 Allyl acrylate 0.0031 Benzaldehyde < 0.0001 Furfural 0.0028 Phenothiazine 0.0041 Nitrogen 0 Oxygen 0 Carbon oxide 0 Carbon dioxide 0 Propene 0 Propane 0 It is evident from table 6 that by using the method according to the present invention a high purity acrylic acid is obtained. 15 20 • 25

Claims (1)

1. CLAIMS A process for separating acrylic acid or methacrylic acid from a gaseous mixture, which in addition to acrylic acid or methacrylic acid contains at least one additional component, characterized in that a) the gas mixture is condensed, b) the acrylic acid is crystallized or either methacrylic acid obtained in step a) from the solution, c) the obtained crystals are separated from the mother liquor of stage b) and d) at least a part of the mother liquor from stage c) is reintroduced after from separation to stage a). A process according to claim 1, characterized in that the gaseous mixture is found as a raw material of the gas phase catalytic oxidation of alkanes, alkenes, alkanols and / or alkanals Cs / C and / or precursors of them for the production of acrylic acid or methacrylic acid. A process for the preparation of acrylic acid or methacrylic acid through the catalytic gas phase oxidation of alkanes, alkenes, alkanols and / or C3 / C alkanals and / or precursors thereof with the formation of a raw material, that in addition to acrylic acid or methacrylic acid contains at least one component selected from the group consisting of unreacted initial elements and by-products, characterized in that the gaseous raw material is prepared in accordance with the process indicated in claim 1. A process of according to one of claims 1 to 3, characterized in that the condensation in step (a) is carried out in a column with separation elements. A procedure in accordance with the claim 4, characterized in that, in the condensation, the solution that is crystallized in step (b) is removed as the average fraction of boiling product. A process according to one of claims 1 to 5, characterized in that the crystallization in step b) is carried out in one step or in several stages. A process according to one of claims 1 to 6, characterized in that the temperature of the solution during the crystallization in step (b) is within a range of + 5 ° C to + 14 ° C. A process according to one of claims 1 to 7, characterized in that the crystallization in step (b) removes the heat by cooling the walls of the apparatus or by evaporating the solution in vacuum. A process according to one of claims 1 to 8, characterized in that the crystals in step (c) are separated from the mother liquor by filtration and / or centrifugation. A method according to one of claims 1 to 9, characterized in that the crystals separated in step (c) are subjected to at least washing and / or liquefaction. A process according to one of claims 1 to 10, characterized in that in step (d) between 0 and 100% by weight, preferably 100% by weight of the mother liquor from stage (c) is reintroduced into the stage (a).