MXPA97000316A - Preparation of acrylic acid and this - Google Patents

Abstract

A process for preparing acrylic acid and / or esters comprises the steps of: (a) phase oxidation of propene catalytic gas and / or acrylic acid acrolein in order to obtain a gaseous reaction product comprising acrylic acid, (b) solvent absorption of the reaction product, (c) distillation of the solvent loaded with the acrylic acid to obtain a crude acrylic acid and the solvent, (d) purification of the crude acrylic acid by crystallization, and (e) optionally esterification of the acrylic acid

Description

"PREPARATION OF ACRYLIC ACID AND STERES" The present invention relates to a process and apparatus for preparing acrylic acid and esters. Acrylic acid is an important basic chemical. Due to its highly reactive double bond and acid function, it is particularly suitable for being used as a monomer for preparing polymers. From the produced amount of acrylic acid monomer, the main part is esterified before the polymerization, for example, to form adhesive dispersions or acrylate coatings. Only the smallest part of the acrylic acid monomer produced is directly polymerized, for example, to form superabsorbents. While in general, the direct polymerization of acrylic acid requires high purity monomer, the acrylic acid for acrylate conversion before polymerization does not have to be so pure. It is common knowledge that acrylic acid can be produced by gas phase oxidation heterogeneously catalyzed by propene with molecular oxygen through solid catalysts at temperatures between 200 ° C and 400 ° C in two stages through acrolein (see for example the inventions numbers DE-A 19 62 431, DE-A 29 43 707, DE-C 1 205 502, EP-A 257 565, EP-A 253 409, DE-B 22 51 364, EP-A 117 146, GB-C 1 450 986 and EP-A 293 224). The catalysts used are oxidic multicomponent catalysts, for example in oxides of the elements molybdenum, chromium, vanadium or tellurium. Patent Number DE-C 21 36 396 discloses removing the acrylic acid from the reaction gases obtained in the catalytic oxidation of propene or acrolein by countercurrent absorption with a mixture of 75 weight percent of diphenyl ether and 25% by weight. percent by weight of diphenyl. In addition, Patent Number DT-A 24 49 7800 discloses the cooling of the hot reaction gas by partially evaporating the solvent in a direct condenser (rapid cooling apparatus) in advance of the countercurrent absorption. The problem here and with additional process steps, is the production of solids in the apparatus, which reduces the availability of the plant. According to the Patent Number DE-A 43 08 087, the amount of these solids can be reduced by adding a polar solvent, such as dimethyl phthalate, in an amount of 0.1 to the mixture of the relatively apolar solvent of diphenyl ether and diphenyl. percent to 25 percent by weight. In addition to the above-described absorption of the reaction product comprising acrylic acid in a high boiling temperature solvent mixture, other existing processes involve a total condensation of the acrylic acid of the reaction water also formed in the course of the catalytic oxidation. The result is an aqueous solution of acrylic acid which can also be treated by distillation with an azeotropic agent (see, for example, Patent Numbers DE-C 34 29 391, JA 11 24 766, JA 71 18 766, JA 71 18 966-R , JA 71 18968-R, JA-72 41 885) or by means of an extraction process (see, for example, Patent Numbers DE-A 2 164 767, J5 81 40-039, JA 80 91 013). In EP-A 551 111, the mixture of acrylic acid and byproducts of the catalytic gas phase oxidation is contacted with water in an absorption tower and the resulting aqueous solution is distilled in the presence of a solvent forming a solvent. azeotrope with low polar boilers such as water or acetic acid. Patent Number DE-C 23 23 328 describes the removal of acrylic acid from a residual liquid of esterification of aqueous acrylic acid or an aqueous solution of acrylic acid as formed in the production of acrylic acid by oxidation of propene or acrolein by extraction with a specific mixture of organic solvents. Regardless of the type of acrylic acid production processes described above, the quality of the acrylic acid obtainable in this manner is generally not sufficient to be able to polymerize the salable products therefrom. Any further processing towards polyacrylic acid is prevented in particular by the by-products of phase oxidation of catalytic gas from acetic acid, propionic acid and aldehydes. It is known how to remove the aldehydes by chemical pretreatment with AGHC (aminoguanidine hydrogen carbonate) or NH2NH2 (see for example Patent Numbers EP-A 270 999, PT-78 759-A, PT 81 876-A, US- 37 25 208-S) and the additional steps of distillation treatment (for distillation of a low boiler and purification of acrylic acid). The acrylic acid purified in this way is suitable, for example, for preparing superabsorbents. It is also known how to remove acetic acid from acrylates after esterification by distillation. Even if the acrylic acid is not used as a direct starting material for the production of polyacrylic acid, but is esterified before further processing, which is the case of the volume of the world's crude acid production, the co-components previously mentioned such as acetic acid, propionic acid and aldehydes interfere. Therefore, in virtually all additional processing options for acrylic acid, the same co-components interfere, namely acetic acid, propionic acid and aldehydes. A distillation of acrylic acid is problematic due to the high temperature stress and the associated intense tendency for the acrylic acid to form dimers, oligomers or polymers. Neither can the co-components be separated with a propionic acid, especially at a similar boiling temperature, in the distillation of acrylic acid (boiling point of the acrylic acid of 141.6 ° C, boiling point of propionic acid of 140.9 ° C). An object of the present invention is to provide a process for preparing acrylic acid or esters by which the aforementioned interfering secondary components are removed and the acrylic acid of different qualities can be provided very flexibly as required. It has been found that this object is achieved when the crude acrylic acid obtained by catalytic gas phase oxidation of propene and / or acrolein, the subsequent absorption in a solvent and the distillation of the acrylic acid / solvent mixture is purified by dynamic crystallization. and fractional static. The present invention therefore provides a process for preparing acrylic acid and / or esters, comprising the steps of: (a) catalytic gas phase oxidation of propene and / or acrolein in acrylic acid to obtain a gaseous reaction product comprising acrylic acid, (b) solvent absorption of the reaction product, (c) distillation of the solvent charged with the reaction product in order to obtain a crude acrylic acid and the solvent, (d) purification of the crude acrylic acid by crystallization, and (e) optionally the esterification of the crystalline acrylic acid. The present invention also provides an apparatus for carrying out the process of the invention. The preferred embodiments of the invention are defined in the corresponding sub-clauses.

Step (a): According to the invention, the phase reaction of catalytic gas of propene and / or acrolein with molecular oxygen to form the acrylic acid can be carried out according to known processes, especially as described in the aforementioned references . The preferred reaction is carried out at a temperature of 200 ° C to 400 ° C. The heterogeneous catalysts used are preferably oxidic multiple component catalysts based on molybdenum, chromium, vanadium and / or tellurium oxides. The conversion of propene to acrylic acid is intensely exothermic. The reaction gas, which in addition to the starting materials and products advantageously comprises a dilution gas, for example recycle gas (see below), atmospheric nitrogen and / or water vapor can therefore absorb only a small part. of the heat of the reaction. Although the nature of the reactors used in principle is not subject to any restrictions, it is common to use tube heat exchangers packed with the oxidation catalyst, since with this type of equipment the predominant part of the heat produced in the reaction is It can stir towards the cooled tube walls by convection and radiation. However, step (a) provides non-pure acrylic acid but a gaseous mixture which in addition to acrylic acid can include essentially as unconverted acrolein and / or propene co-components, water vapor, carbon monoxide, carbon dioxide, nitrogen , oxygen, acetic acid, propionic acid, formaldehyde, additional aldehydes and maleic anhydride. The reaction product mixture usually comprises in each case, based on the total reaction mixture, 0.05 percent to 1 percent by weight of propene and 0.05 percent to 1 percent by weight of acrolein, of 0.01 percent to 2 percent by weight of propane, from 1 percent to 20 percent in water vapor, from 0.05 percent to 15 percent by weight of carbon oxides, from 10 percent to 90 percent by weight of nitrogen, from 0.05 percent to 5 percent by weight of oxygen, from 0.05 percent to 2 percent by weight of acetic acid, from 0.01 percent to 2 percent by weight of propionic acid, from 0.05 percent to 1 percent weight percent formaldehyde, from 0.05 percent to 2 percent by weight aldehydes and also from 0.01 percent to 0.5 percent by weight maleic anhydride.

Step (b) Step (b) comprises removing the acrylic acid and part of the co-components of the reaction gas by absorption with a solvent. According to the invention, suitable solvents include in particular all high boiling point solvents, preferably solvents having a boiling point higher than 160 ° C. of specific suitability is a mixture of diphenyl ether and biphenyl, especially the commercially obtainable mixture of 75 weight percent diphenyl ether and 25 weight percent biphenyl.

In the present, the terms boilers of high temperature, boilers of medium temperature and boilers of low temperature and the corresponding adjectives of boiling at high temperature, boiling at medium temperature and boiling at low temperature designate, respectively, the compounds with a boiling temperature higher than acrylic acid (high temperature boilers); compounds with approximately the same boiling temperature as acrylic acid (medium temperature boilers); and compounds with a lower boiling temperature than acrylic acid (low temperature boilers). Advantageously, the hot reaction gas obtained from step (a) is cooled in advance of the absorption by partially evaporating the solvent in an appropriate apparatus, for example a direct condenser or a rapid cooling apparatus. The appropriate equipment for this purpose includes venturi tube washers, bubble columns or spraying condensers. The high boiling temperature co-components of the reaction gas from step (a) are condensed in the non-vaporized solvent. further, the partial evaporation of the solvent is a cleaning step for the solvent. In a preferred embodiment of the invention, a purge stream of the non-vaporized solvent of preferably 1 percent to 10 percent of mass flow to the absorption column is removed and subjected to a solvent cleaning step. This involves distilling the solvent to leave a residue comprising the high boiling temperature co-components which, if necessary to be further thickened, can be discarded, for example, incinerated. This distillation with solvent serves to avoid an excessively high concentration of heavy boilers in the solvent stream. The absorption is carried out in a countercurrent absorption column which is preferably equipped with a double flow valve or plates and is subjected to a downward flow of the solvent (not vaporized). The gaseous reaction product and any vaporized solvent are introduced into the base of the column and then cooled to the absorption temperature. Cooling is advantageously carried out by cooling cycles; that is, the hot solvent is removed from the column, the heat exchangers are cooled and reintroduced into the column at a point above its point of withdrawal. In addition to acrylic acid, these solvent cooling cycles will condense the low, high and medium boiling temperature co-components and also the vaporized solvent. As soon as the reaction gas stream has cooled to the absorption temperature, the effective absorption is carried out by absorbing the remainder of the acrylic acid remaining in the reaction gas and also part of the low temperature co-components. of boiling. The non-absorbed reaction gas remaining from step (a) it is further cooled so that the condensable part of the low boiling temperature co-components thereof especially water, formaldehyde and acetic acid, can be separated by condensation. Next, this condensed material is known as acidic water. The remaining gas stream, which will then be referred to as recycle gas, consists predominantly of nitrogen, carbon oxides and unconverted starting materials. Preferably the recycle gas is partially recirculated to the reaction stages as a dilution gas. A solvent stream loaded with acrylic acid, the co-components of high and medium boiling temperature and also a small proportion of the low boiling temperature co-components is removed from the base of the column used in step (b) and, in a preferred embodiment of the invention, it is subjected to a desorption. Advantageously, this desorption is carried out in a column, which is preferably equipped with a double flow valve or plates but which can also be equipped with ordered or discarded packing, in the presence of a purification gas. The purification gas used can be any inert gas or gas mixture; Preference is given to the use of a mixture of air and nitrogen gases or recycle gas, since the latter is obtained in step (a) in the course of part of the solvent that is vaporizing. In the desorption step, part of the recycled gas withdrawn before step (a) purges the charged solvent from the volume of the boiling low boilers. Since the main amounts of acrylic acid are purified as well as in the course of desorption, it is economic sense not to discard this stream, which is then called the recycle gas purification but to recirculate it for example towards the stage where the partial vaporization of the solvent is carried out, or towards the absorption column. Since the scrubbing gas forms part of the recycle gas, it still contains significant quantities of low-boiling boilers. The operation of the desorption column can be improved by removing the low boiling temperature boilers from the scrubbing gas before it is introduced into the column. Technically this is advantageously carried out by cleaning the scrubbing gas in a countercurrent washing column with the solvent recovered in step (c) which will be described below. A solvent stream loaded with acrylic acid and virtually free of low boiling temperature boilers can then be removed from the bottom of the desorption column.

Step (c): In step (c), the acrylic acid is separated from the solvent together with the components of medium boiling temperature and the last remaining co-components of low boiling temperature by distillation for which in principle it can be use any distillation column. Preference is given to the use of a column having sieve plates for example double flow plates or cross flow screen plates made of metal. In the section of rectification of the column, the acrylic acid is distilled to be free of the solvent and the co-components of medium boiling temperature, such as maleic anhydride. In order to reduce the content of acrylic acid in boilers having a low boiling temperature, it is advantageous to lengthen the rectification section of the column and remove the acrylic acid from the column as a secondary stream. This acrylic acid will then be referred to as crude acrylic acid, regardless of its purity. At the top of the column, a stream rich in low boiling temperature boilers is then removed after partial condensation. However, since this stream still contains acrylic acid, it is advantageously not discarded, but rather recycled to the absorption step (b). At the base of the column, the solvent, which is free of low boiling temperature boilers and almost free of acrylic acid, is removed and preferably preferentially recycled to the countercurrent wash column where the scrubbing gas from the pass (b) it is cleaned, so that low boiling temperature boilers can be flushed out of the scrubbing gas. The solvent almost free of acrylic acid is then fed to the absorption column. In a preferred embodiment of the invention, the acidic water which may still contain acrylic acid in solution is treated extractive with a small stream of the solvent almost free of acrylic acid. In this extraction of acidic water, the acrylic acid is extracted into the solvent and thus recovered from the acidic water. In turn, the acidic water extracts the boiling polar components at medium temperature from the solvent stream and therefore avoids an accumulation of these components in the solvent cycle. The current of the acidic water resulting from the boilers of temperature and low and medium boiling can be concentrated, which is required in particular when there are environmental protection requirements. This makes it possible to fill even the most stringent requirements of the United States hazardous waste law. The crude acrylic acid obtained in step (c) comprises, in each case based on the crude acrylic acid, preferably from 98 percent to 99.8 percent by weight, particularly from 98.5 percent to 99.5 percent by weight of acrylic acid and from 0.2 percent to 2 percent by weight, in particular from 0.5 percent to 1.5 percent by weight of impurities, such as for example acetic acid, aldehydes and maleic anhydride. This acrylic acid can be used if desired for esterification, when the purity requirements are not too high.

Step (d): In step (d), the raw acrylic acid from step (c) is further purified by crystallization, preferably by means of fractional crystallization by the combination of dynamic and static crystallization. The type of dynamic or static crystallization used is not subject to any special restrictions. In static crystallization (for example, US Patent No. 3,957,164 and FR No. 2,668,946), the liquid phase moves only by free convection, while in dynamic crystallization, the liquid phase moves by forced convection. a forced flow in a completely filled apparatus (see for example, in the Patent Number DE 26 06 364) or the request for a film that falls or runs down to a cooled wall (Patent Numbers DT 1 769 123 and EP 218 545). According to the invention, preferably the crude acrylic acid to be purified is introduced into the crystallizer in the liquid phase and then into a solid phase which differs in composition from the liquid phase introduced by freezing on the cooled surfaces. a certain proportion of the acrylic acid used has been frozen (advantageously within the range of 50 percent to 80 percent, especially 60 percent). 70 percent) the remaining liquid phase is separated. This is advantageously carried out by pumping the residual phase leaving it or allowing it to flow away. The crystallization step can be followed by additional purification steps, such as the so-called washing of the crystal layer (see Patent Number DE 3,708,709) or so-called transpiration, that is, the partial melting of the contaminated glass areas. Advantageously, the crystallization step is followed by a transpiration step when the total purification effect of a step is to be improved. The dynamic and static crystallization can each be carried out in one or more stages. The multistage processes here are advantageously operated in accordance with the principle of countercurrent flow whereby after crystallization at each stage the crystalline product is separated from the residue and fed to any stage having the next highest degree of purity, while the crystallization residue is fed to any stage having the next lower degree of purity. In a customary way, all the stages that produce a crystalline product that is purer than the feed of the crude acrylic acid solution are called purification stages, while all the other stages are called purification stages. Advantageously, a plurality of stages are operated in sequence in the same crystallizer by intermediate storage of the individual stage fractions (crystallization product and residual melt or crystallization residue) in containers. To minimize the losses of the product, a high crystallization yield is sought, however, the maximum obtainable yield is limited by the position of the eutectic points between the acrylic acid and its co-components. If a eutectic composition is reached during crystallization, no further separation is effected between the acrylic acid and the corresponding impurity / co-component. The crystallization tests with acrylic acid of different purity showed that even at impurity concentrations lower than those of the eutectic composition, a dynamic crystallization will produce only poorly adherent glass layers. Therefore, it is proposed that the residual melt of the dynamic crystallization be further treated in a static crystallization as described in the Patent Number EP 0 616 998. Due to the very low layer formation, the static crystallization will produce a good adhesion of layers coupled with a good purification effect. The static stages are integrated in a total process in accordance with the principle of countercurrent flow described above; that is, the product of static crystallization of higher purity is fed to the stage where the dynamic crystallization product of lower purity is produced, and the residual melt of dynamic crystallization of lower purity is fed to the stage where the purest crystals are produced by static crystallization. Advantageously, the dynamic crystallization is carried out in a completely filled apparatus or by means of a falling or draining film. These two methods are particularly fast and efficient. The number of crystallization steps required depends on the degree of purity desired and is capable of being easily determined. Advantageously, a seed layer is frozen before the start of crystallization.

Step (e): If desired, the pure acrylic acid obtained in step (d) is esterified according to known methods: The process and an apparatus are described with reference to the drawing, which illustrates a preferred embodiment of the invention. In the drawing Figure 1 shows a functional diagram of the process according to the present invention, Figure 2 shows the interconnection of the apparatus according to the present invention.

Referring to Figure 1, the recycle gas consisting essentially of nitrogen, carbon oxides and unconverted starting materials is compressed and fed together with propene and air to a reactor where the heterogeneously catalyzed oxidation of propene is effected. in acrolein. The resulting intermediate reaction gas is supplied with additional air so that the heterogeneously catalyzed oxidation of the acrolein can be carried out in the second reactor. The resulting hot gaseous reaction product comprising the acrylic acid is cooled ahead of the absorption step by partial evaporation of the solvent in a direct condenser K9, the high-boiling co-components of the reaction product condensing to the non-vaporized solvent. A purge stream from the direct condenser K9 is subjected to a distillation with solvent in which the solvent is distilled and the high boiling temperature co-components remain therein. The latter can also be thickened and discarded, for example, by incineration. Column K10 is subjected to a downward flow of the solvent (not vaporized), while the vaporized solvent and the gaseous reaction product are introduced into the K10 column at the base thereof and then cooled to the absorption temperature. The cooling is effected by cooling cycles (not shown). Not only the vaporized solvent but also the acrylic acid and all the co-components of high and medium boiling temperature are condensed towards these cooling cycles. After the entire reaction gas stream has cooled to the absorption temperature, effective absorption is effected. The remaining acrylic acid remaining in the reaction gas and also part of the low boiling temperature co-components are absorbed. Then the remaining non-absorbed reaction gas is further cooled so that the condensable part of the low-boiling temperature co-components can be separated from the gas stream shown in Figure 1 as cooling with the acidic water. In what follows, therefore, this condensate will be referred to as acidic water. The remaining gas stream, the recycle gas can then be returned in part to the reaction steps of Figure 1 as a dilution gas. The solvent loaded with acrylic acid and the co-components are removed from the base of column K10 and fed to the K20 desorption column. There the charged solvent is purified by a part of the recycle gas that is taken from the oxidation stages of the larger part of the low boiling temperature heaters. Since the predominant amounts of the acrylic acid are also purified, this stream is recirculated, for example, back to the direct condenser K9. To improve the desorption performance of the K20 column, the low boiling temperature boilers present in the scrubbing gas are removed before entering the K20 column. Technically, this is advantageously carried out by cleaning the scrubbing gas in a countercurrent washing column K19 using the solvent recovered from the K13 column described below. In the next step of the process, a solvent stream loaded with acrylic acid and virtually free of low boiling temperature boilers is removed from the bottom of the desorption column K20 and fed to the distillation column K30 which is preferably a sieve plate column. The high boiling temperature solvent and the medium boiling temperature co-components, for example, the maleic anhydride condenses towards the base of the K30 column. Since the acrylic acid removed at the top of the K30 column still comprises significant amounts of low boiling point co-components, this content of the low boiling temperature boiler is advantageously reduced by further lengthening the rectification section of the column K30 and removing the acrylic acid from the column as a secondary stream. This acrylic acid is known as the raw acrylic acid. The stream rich in light material removed at the top of the distillation column K30 still contains acrylic acid and is therefore advantageously recycled back to the absorption column K10. The solvent removed from the bottom of the distillation column K30 which is free of boiling low boiling temperature and almost free of acrylic acid is fed mostly to the K19 column of the countercurrent wash so that as mentioned above, the low boiling temperature boilers can wash the purge gas stream leading to the desorption column K20. The solvent, which is almost free of acrylic acid, is then fed to the absorption column K10. A small purge stream of the solvent almost free of acrylic acid from the bottom of the distillation column K30 is used to subject the acidic water which still contains acrylic acid in solution to an extractive treatment. In this extraction of acidic water, part of the acrylic acid is recovered from the acidic water and, in the other direction, the acidic water draws all the polar components from the solvent purge stream. The resulting acidic water can be pre-evaporated and then incinerated. The crude acrylic acid obtained from the secondary stream of the distillation column K30 is then subjected to dynamic crystallization and static crystallization. Depending on the desired purity requirements of the acrylic acid, it may also be sufficient to carry out only a dynamic crystallization. The dynamic crystallization is carried out either with a completely filled tube or by means of a falling film. The number of crystallization steps varies as a function of the desired purity of the acrylic acid. Preferably, the dynamic crystallizer is constructed as a heat exchanger. If desired, the resulting or pure acrylic acid can then be esterified with alcohols to form the desired acrylates. The process of this invention, therefore, offers the following advantages: lower yield losses through the combined use of dynamic and static crystallization; better quality of acid or ester, since propionic acid can be separated by crystallization. The propionic acid would be esterified in unpleasant smell propannates at the stage of acrylic ester production and, therefore, would not be welcome in the final products in any case; there is no need for the addition of a chemical reagent to remove the aldehydes; there is no need for distillation of the acetate during further processing of the acid in esters; lower consumption of alcohol in the purification stage. The examples that will be given below illustrate the preferred embodiments of the invention. A miniplant produced 426 grams of acrylic acid per hour. The apparatus interconnection, the flow rates and the operating parameters employed can be seen in Figure 2. This Figure illustrates the separation steps of Figure 1 in the form of an apparatus bearing the same designations. Oxidation of propene with air through acrolein was carried out in two successive reaction tubes of 26 millimeters in diameter with a catalyst bed length of 2.5 meters. The first tube was packed with a catalyst impregnated on the surface as described in the Patent EP 575 897 and the second reaction tube contained a catalyst impregnated on the surface as described in Patent Number EP 609 750. The columns K10, K20 and K30 were specular finish or laboratory columns with a thermostat of 50 millimeters in diameter . The direct K9 condenser was a venturi tube washer. Columns K10 and K20 were packed with 5-millimeter metal spirals. The distillation column K30 was packed with sieve plates (double flow plates) made of metal. The perforations in the sieve plates were such that layers of the foam are formed.

Dynamic crystallization Dynamic crystallization was carried out in a crystallizer as described in DE-A 26 06 364 (BASF) using a fully filled tube. The data of the crystalliser were the following: two passes with a tube (internal diameter of 26 millimeters) per pass, tube length of 5 meters, primary circuit pump as the centrifugal pump with speed control, plant volume of 11 liters in the primary side, - freezing regime of approximately 45 percent (freezing regime = mass of the crystallized product / mass of the raw melt), container of four stages each with volume of 100 liters, temperature control of the plant by unit of refrigeration and steam of 4 bar through the heat exchanger. The plant was controlled through a process control system and the program sequence for one was as follows: 1. Filling the primary circuit. 2. Emptying the primary circuit and freezing the seed layer. 3. Temperature rise to approximately 2 ° C less than the melting temperature. 4. Fill the primary circuit until crystallization occurs. 5. Crystallize (temperature program). 6. Falling residual fusion of the pump when the crystallization has finished. 7. Raise the temperature to melt the glass layer. 8. Melted crystallized product descending from the pump. 9. A new stage is started. Temperatures, pressures, volume flows depend on the stage that will be operated.

Crystallization of static layer.

The plant used for this purpose was a tube crystallizer made of glass with an internal diameter of 80 millimeters and a length of 1 meter. The temperature of the crystallizer was controlled through a jacket made of glass. The filling level of the crystallizer varied from 2.0 to 5.0 liters (variable). The temperature of the plant was controlled through a programmed thermostat. The freezing regime (after transpiration) was approximately 50 percent. The sequence of Program for a stage was as follows: 1. Filling the crystallizer. 2. Adjust the temperature of the appliance with a content of approximately 1 K at the aforementioned melting temperature. 3. Crystallize (temperature program). 4. Download the residual fusion after the crystallization is finished. 5. Transpiration (temperature program). 6. Melt the crystallized product. 7. Start a new stage. The temperatures depend on the stage that will be operated.

Example 1 426 grams per hour of the crude acrylic acid from the secondary stream of the distillation column K30 was removed in an amount as disclosed below in Table 1.

Example 2 The crude acrylic acid mentioned in the Example 1 was purified in one of the dynamic crystallization stages described above. A pure acrylic acid was obtained in a purity of 99.95 weight percent with a propionic acid content of 51 percent parts per million and an acetic acid content of 345 weight percent parts per million. the crystallization residue of this purification step was further treated in three stages of dynamic crystallization and two stages of static crystallization. The purity of the pure acrylic acid obtained in this way is shown in the table below Picture Type of Acid Acid Acid acid acrylic propionic acetic acid in percentage in parts by weight per million per million Acrylic acid crude 99.7 203 2000 Pure acrylic acid 99.95 51 345 As can be seen in the table, the combined use of dynamic and static crystallization leads to a very pure acrylic acid. More specifically, the content of propionic acid (which would lead to malodorous propionates in the production of the acrylic ester) was reduced to a quarter of its original value, which would not have been possible by distillation, even at prohibitive cost.

Claims (9)

R E I V I N D I C A C I O N E S:

1. A process for preparing acrylic acid and / or esters comprising the steps of: (a) catalytic gas phase oxidation of propene and / or acrolein in acrylic acid to obtain a gaseous reaction product comprising acrylic acid, (b) absorption with solvent of the reaction product, (c) distillation of the solvent charged with the reaction product in a column to obtain a crude acrylic acid and the solvent, (d) purification of the crude acrylic acid by crystallization, and (e) optionally esterification of the Acrylic crystalline acid.

2. A process according to claim 1, wherein the high boiling temperature solvent, especially a mixture of diphenyl ester and biphenyl, is used in step (b).

3. A process according to claim 1 or 2, with one or more of the following features: the reaction product is cooled before step (b) by contacting the same with the solvent and evaporating part of the solvent, the absorption from step (b) is carried out as countercurrent absorption in an absorption column, the lower product of the absorption column is subjected to a desorption in advance of step (c) to remove the secondary components of low boiling temperature, the product obtained in the upper part of the distillation column in step (c) is recycled to step (d), - the product obtained in the base of the distillation column in step (c) is recycled to the absorption step (b), in step (c), the crude acrylic acid is removed from the distillation column as a secondary stream.

4. The process according to any of claims 1 to 3, wherein in step (d) fractional crystallization is employed by a combination of dynamic and static crystallization.

5. A process according to claim 4, wherein in step (d), the residue of the dynamic crystallization is subjected to a static crystallization and the static crystallization product is subjected to a dynamic crystallization.

6. A process according to claim 5, wherein the dynamic crystallization is carried out with the whole flow apparatus or by means of a falling film.

7. A process according to any of claims 4 to 6, wherein the dynamic and / or static crystallizations of step (d) are carried out as countercurrent crystallizations.

The apparatus for preparing acrylic acid, comprising in the following order: at least one reactor for a catalytic gas phase reaction, an absorption column, a distillation column, - at least one dynamic crystallizer and, at least one static crystallizer, and at least one intermediate storage container.

9. The apparatus according to claim 8, with one or more of the following features: a direct condenser for the partial evaporation of the solvent that is provided between the catalytic gas phase reactor and the absorption column, the absorption column is contruye as a column of countercurrent absorption, - a desorption column is provided between the absorption column and the distillation column, the distillation column is equipped with sieve plates, - the dynamic and / or static crystallizers are constructed as heat exchangers .