JP5489726B2 - Acrylic acid production method - Google Patents

Description

The present invention relates to a product gas mixture comprising acrylic acid, water vapor and secondary components by heterogeneously catalyzed gas phase partial oxidation of at least one C ₃ precursor of acrylic acid using molecular oxygen on a solid catalyst at elevated temperatures. The temperature of the product gas mixture comprising acrylic acid, water vapor and secondary components is optionally reduced by direct and / or indirect cooling, and then the product gas mixture comprising acrylic acid, water vapor and secondary components is obtained. The crude acrylic acid, which contains water and secondary components in a dilute form, is separated from the condensation column as a target product by pouring into a condensation column equipped with a separation internal structure and rising in the condensation column. Directing to the condensation column via a first side outlet located above the feed point of the product gas mixture and further adding acrylic acid and secondary components Acid water is guided through a second liquid phase outlet (preferably a side outlet) located above the first side outlet and has a lower boiling point than water ((based on atmospheric pressure As a result, the residual gas mixture containing secondary components (boiling at a lower temperature than water) is introduced to the top of the condensation column and has a higher boiling point than acrylic acid and subsequent products, and acrylic acid (based on atmospheric pressure). A bottom liquid further containing secondary products is derived from the bottom space of the condensation column and a portion of the recovered acidic water is returned to the condensation column as is and / or as a reflux after cooling and optionally further purification. It relates to a process for producing acrylic acid, wherein at least one further thermal separation method is applied to crude acrylic acid for the purpose of

  Acrylic acid is an important intermediate used, for example, in the preparation of polymer dispersions (optionally including alkanol and its ester forms) and superabsorbent polymers.

Acrylic acid is obtained, inter alia, by heterogeneously catalyzed gas phase partial oxidation of a C ₃ precursor of acrylic acid (C ₃ precursor compound) using molecular oxygen on a solid catalyst at high temperatures (this term is particularly The formula includes chemical compounds obtained by reduction of acrylic acid, and known C ₃ precursors of acrylic acid are, for example, propane, propene, acrolein, propionaldehyde, and propionic acid, the term of which Also included are precursor compounds, such as glycerol (starting from glycerol, for example, acrylic acid can be obtained by heterogeneous catalytic oxidative dehydration in the gas phase; eg EP-A 1710227, WO 06/114506 and WO 06/092722). reference).

In this production, the above starting gas, which is totally diluted with an inert gas, for example nitrogen, CO ₂ , saturated hydrocarbons and / or water vapor, is used in the form of a mixture with molecular oxygen at elevated temperature and optionally at elevated pressure ( Flow over a mixed oxide catalyst (eg transition metal) and contain acrylic acid, water, and undesirable byproducts (from which acrylic acid must be removed), such as furfural, benzaldehyde, acetone, formaldehyde and maleic anhydride, etc. Oxidative conversion to product gas mixture (the by-products other than water vapor and inert diluent gas shall be included in this document under the term “secondary component”; this term is typically added in the acrylic acid removal process. (Including polymerization inhibitors).

  Starting from propionaldehyde and / or propionic acid, the heterogeneously catalyzed gas phase partial oxidation with molecular oxygen is at least partially oxidative dehydration.

  As described at the beginning, the documents DE-A 199 45533, DE-A 1992 4532, WO 01/77056, DE-A 10156016, DE-A 102243625, DE-A 10223058, DE-A 10235847, WO 2004/0355514, WO 00/53560 and DE-A 10332758 include: A process for the production of acrylic acid is disclosed in which the basic removal of the crude acrylic acid is carried out by fractional agglomeration of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation. The term “crude acrylic acid” means that the acrylic acid recovered via the first side-flow outlet is not a pure product, but acrylic acid (generally 50 or 60% by weight of the total weight, usually 70 Or 80 wt% or more, often 90 wt% or more, often 95 wt% or more), and water and secondary components such as lower aldehydes (e.g., furfural, acrolein, benzaldehyde), lower carboxylic acids (e.g., (Acetic acid, propionic acid, formic acid) and the like. In each case, the total content of water and secondary components relative to the content of acrylic acid is lower in the crude acrylic acid than in the product gas mixture of the gas phase partial oxidation, since the crude acrylic acid is a component of these components. Is explained as an overall dilute form (in contrast, individual components may be present in the crude acrylic acid in relatively high concentrations).

  In some cases, the purity of the crude acrylic acid removed in this way is the potential end use of acrylic acid (for example, for its esterification purposes or to form polymers obtained by free radical polymerization). Is already enough. However, in many cases, a higher purity acrylic acid is obtained from the crude acrylic acid having the purity required for a particular end use (higher acrylic acid content in weight percent compared to the crude acrylic acid). For this purpose, the removed acrylic acid is subjected to at least one further thermal separation method.

  Thermal separation methods are those in which at least two-phase systems are physically obtained by (generally heat) energy supply or recovery, and heat and mass transfer occur due to temperature and quantitative gradients that exist between the phases. It is understood that this is the method that ultimately results in the desired separation and extraction.

  Often, thermal separation methods are performed in a separation column that includes a separation internal structure in which the at least two phases are generally directed countercurrent to each other. In many cases, one of the two phases is a gas (generally induced as a rising phase in a separation column) and the other is a liquid (generally induced as a falling phase in a separation column). Basically, however, the at least two layers may be liquid (eg in the case of an extract) or solid and liquid (eg in the case of crystals) or solid and gas (eg in the case of an adsorbate).

  Since one of the at least two phases is a liquid and the other is a gas, an example of the configuration of a thermal separation method that is an inherent element of the term “thermal separation method” used in this document is rectification (ascending The gas phase is induced countercurrently to the descending gas phase in the separation column) and desorption (reverse method of absorption; the gas dissolved in the liquid phase reduces the pressure on the liquid phase, thereby reducing the temperature of the liquid phase. Derived from the liquid phase by raising and / or inducing the gas phase through the liquid phase; when accompanied by induction of the gas phase, desorption is also referred to as stripping). However, fractional condensation of absorption (generally gas rising in the separation column is induced countercurrently with at least one absorbent descending in the separation column) and gas mixtures (examples of gas / liquid phases) are also “heat Part of the separation method. A particularly preferred thermal separation treatment for further purification of the crude acrylic acid is further crystallization purification (crystallization).

However, the disadvantages of known methods for the basic removal of crude acrylic acid by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation of at least one C ₃ precursor of acrylic acid are:
It is an additional generation of acidic water further containing acrylic acid and secondary components. The term “acidic water” refers first to the fact that acidic water is generally 50% by weight or more, often 60% by weight or more, often 70% by weight, often 80% by weight of water (generally reaction water , And diluting water (steam) used as an inert diluent gas in the process of gas phase partial oxidation).

  However, the term also means that acidic water has a pH of less than 7 because it contains, along with water, secondary acids such as propionic acid, acetic acid and formic acid, and acrylic acid (acrylic acid). The total content of carboxylic acids as secondary components other than is usually 10% by mass or less, and in some cases, 5% by mass or less based on the mass of acidic water.

  Usually, the acrylic acid content of acidic water is 4 or 5 to 15% by weight, often about 10% by weight. The disadvantages of the process for the basic removal of acrylic acid from the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation recommended in the prior art are that they further contain acrylic acid and are returned to the rectification column. All non-acidic water is sent to incineration (see in particular DE-A 10243625, WO 2004/035514 and DE-A 10332758).

  This is disadvantageous in that acid water incineration, for example, reduces the yield of the desired acrylic acid product.

  In view of the prior art described, the object of the present invention is to provide an improved process for the production of acrylic acid which is particularly notable in that it reliably increases the yield without significantly reducing the purity of the acrylic acid. Met.

Thus, a product gas mixture comprising acrylic acid, water vapor and secondary components is converted into a heterogeneous catalyst of at least one C ₃ precursor of acrylic acid (C ₃ precursor compound) using molecular oxygen on a solid catalyst at high temperature. Obtained by gas phase partial oxidation, the temperature of the product gas mixture comprising acrylic acid, water vapor and secondary components is optionally reduced by direct (by direct contact with the coolant) and / or indirect cooling and then acrylic The product gas mixture containing acid, water vapor and secondary components is poured into a condensing column with a separation internal structure and raised in the condensing column for fractional condensation, resulting in an overall lean water and secondary component. The crude acrylic acid in the form of a target product from the condensation column via a first side outlet located above the feed point of the product gas mixture Acid water containing acrylic acid and a secondary component, and a second liquid phase outlet located above the first side outlet (preferably side outlet: all in this document The description includes, in particular, secondary components that are induced via the acidic water side outlet and have a boiling point lower than water (boiling at a lower temperature than water (based on atmospheric pressure)). The residual gas mixture is guided to the top of the condensation column and the bottom liquid further containing acrylic acid and subsequent products and secondary products having a boiling point higher than acrylic acid is derived from the bottom space of the condensation column and recovered acidity. A portion of the water is returned to the condensation column as it is and / or after cooling as a reflux, optionally with at least one further thermal separation process for the purification of the crude acrylic acid. And the acrylic acid present in at least a part of the acidic water that has not been returned to the condensation column is absorbed into the organic solvent from the acidic water by extraction with an organic solvent, and an organic extract containing acrylic acid is prepared, The acrylic acid is then removed from the organic extract using at least one thermal separation method, the acrylic acid removed from the extract is returned to the condensation column, or sent for further purification of the crude acrylic acid, and / or metal A method for producing acrylic acid to be absorbed in an aqueous hydroxide solution has been found.

  Advantageously, according to the invention, at least 25%, better at least 50%, even better still at least 75%, preferably all of the acidic water that has not been returned to the condensation column is extracted, Further processing according to the invention.

  In principle, all known extraction devices for liquid-liquid extracts are useful for carrying out the extraction required for the present invention. This device should ensure as much as possible a large phase interfacial area and a dense distribution of these droplets in the continuous phase and a rapid and substantially complete phase separation upon completion of mass transfer. .

  In the simplest case, the extraction device used may be a mixer (eg a stirred vessel or a fixed mixer) with a separator (eg a settling device). The agitator used can basically be all common agitators. Examples include disk stirrers, impeller stirrers, cross beam stirrers, grid stirrers, blade stirrers, anchor stirrers, paddle stirrers, propeller stirrers, spiral stirrers, and multi-layer repulsive countercurrent stirrers. Can be mentioned. The stirrer may be multi-stage, that is, a plurality of stirrers that are coaxially stacked. It is preferred to use a two-stage impeller agitator. Useful settling machines are basically all common vessels. It is preferred to use a horizontal container. The extraction device can be operated continuously or batchwise, the settling is not required for batch operation, and the phase separation takes place in a stirred vessel. In the mixer, the substance to be extracted (a valuable material) moves to the absorbent phase and in the separator the two phases are separated (by gravity). In continuous operation, the internal structure intersecting the flow direction can improve the separation of the feed into the heavy and light phases in the settler. Useful internal structures basically include all common internal structures such as perforated sheets, plates, structural fillers and / or messy fillers. Among the messy fillers, rings, spirals, saddles, Raschigs, Intos or Pall rings, barrels or interlox saddles, top-packs etc. or fillers including blades are preferred. A combination of a perforated sheet and a messy filler is particularly preferred. The residence time in the settling machine is typically 0.05 to 2 hours.

  A cascade is mentioned when connecting a plurality of mixer-sedimentation apparatus in cascade. The devices can be cocurrent or countercurrent or AC connected.

  Where appropriate from the point of view of the application, the extraction carried out according to the present invention will be carried out in an extraction column containing a separation internal structure. The phase with the higher specific gravity enters the top of the column and the phase with the lower specific gravity enters the bottom. In the column, the two phases move in countercurrent. Basically, the extraction column is equivalent to a countercurrent distillation (rectification) column. In other words, suitable extraction columns for the inventive extraction method are of essentially known design and can have custom built internal structures.

  Possible column types include extraction columns, both columns where energy is input and columns where energy is not input. An extraction column containing internal structures in the form of structural fillers and / or messy fillers may be operated with or without input of energy. Among the messy fillers, fillers including rings, spirals, saddles, Raschigs, Intos or Pall rings, barrels or interlox saddles, top-packs and the like are preferred. A particularly suitable structural filler for the extraction column used according to the invention is, for example, the structural filler of Julius Montz GmbH in D-40705 Hilden, for example the Montz-Pak B1-350 structural filler. It is preferred to use a perforated structure filler composed of a stainless steel sheet. Packed columns containing structural packing are known per se to those skilled in the art, see, for example, Chem. -Ing. Tech. 58 (1986) 1, p. 19-31 and Gebrüder Sulzer Aktiengesellschaft, Technischen Rundschau Sulzer [Sulzer Technical Review], 2/1979, Winterhur, Switzerland. 49ff. In addition, an extraction column containing an internal structure in the form of a tray is also suitable, where a rhythmic sieve tray column and a countercurrent sieve tray column must be distinguished. In a rhythmic sieve tray column, the two phases are induced through flow orifices in the sieve tray (typically holes or circular flow orifices). In the upward stroke of the rhythm, the lighter phase is pushed up through the holes in the sieve tray and the heavier phase is lowered in the downward process accordingly. The curl column works in the same way, except that the phase is not a rhythmic liquid here and the sieve tray moves up and down. A rhythmic sieve tray column is preferred. In contrast, using a counter-current sieve tray column, due to density differences, the continuous phase descends from one tray to the next through a downcomer, and only the dispersed phase passes through the sieve tray holes (mass The terminology used in this document for moving trays is in accordance with the terminology of DE-A 1032758).

  One to ten theoretical plates are sufficient for the extraction of acidic water according to the invention. The theoretical stage (or theoretical separation stage) is very commonly referred to in this document as the spatial unit of a separation column used in a thermal separation method that includes separation internals and provides concentration corresponding to thermodynamic equilibrium. Point to. In other words, the term “theoretical plate” is applicable to extraction columns that include mass transfer trays as well as extraction columns that include structural and / or messy packing.

  When an extraction column is used in the method according to the invention, the phase having the higher specific gravity of the two phases is introduced into the upper part of the column by a distribution channel from the viewpoint of the application, and the distribution of the cross section is very uniform. Become. The phase with the lower specific gravity enters the bottom of the column via the distribution channel as well, from an application point of view as appropriate. Thus, in the column, the lighter phase rises and the heavy phase falls. If the lighter of the two phases is dispersed, i.e. present in the form of droplets, phase separation occurs at the top of the column. In the opposite case where the heavy phase is dispersed, phase separation occurs at the bottom of the column. In any case, the preferred droplet size has a diameter (longitudinal dimension) in the range of 1 to 10 mm, preferably in the range of 2 to 5 mm.

  Advantageously, according to the invention, the extractant generally has a boiling point (in each case based on atmospheric pressure) than acrylic acid because it facilitates subsequent removal of acrylic acid from the organic extract. Is high).

  In the extraction carried out according to the invention, the situation is that the organic solvent used as the extractant has a higher probability of having a significantly higher viscosity than water. In this case, according to the present invention, it is advantageous if the organic extract entering the extraction column is present as the dispersed phase and the acidic water is present as the continuous phase (this accelerates the movement of the material between the two layers, for example). And ultimately allowing shorter columns to give the same separation results; a continuous aqueous phase also wets extraction columns made from stainless steel and their internal structures more fully; In addition, the latter is stabilized (the coalescence tendency is reduced) by the substance extracted being moved from the continuous phase to the dispersed phase. When an organic extractant having a mass density larger than that of acidic water is used, the extractant is introduced and dispersed in the upper part of the column, and the resulting extractant droplets descend in the column. In the opposite case, i.e., when using an extractant having a mass density lower than that of acidic water, the extractant is dispersed at the bottom of the column and the resulting extractant droplets rise in the column. In an extraction column of the type already described, which has an internal structure in the form of structural packing, messy packing and / or trays, the droplets of the dispersed phase usually creep entirely along the internal structure in another way. Therefore, the dispersed continuous phase needs to efficiently wet the selected internal structure.

  In the simplest method, the organic extractant typically has a round channel orifice (hole), is placed in the column cross section, and is usually a tube (usually extending along a specific cross section length of a cylindrical extraction column). Having the same cross-section; including tube downcomers). The circular channel orifice faces down when the organic extractant is introduced into the top of the column and up when the extractant is introduced into the bottom of the column. The diameter (longest dimension) of the channel orifice is typically 1 mm to 10 mm, preferably 3 mm to 6 mm, and in many cases 2 to 5 mm. The extractant can simply be poured into the distribution tube and back through the flow path orifice.

  In principle, it is also possible to use a stirred column or a centrifugal extractor to carry out the acidic water extraction according to the invention. A stirred column improves the contact between the two phases. All the stirrers of the column are on a common axis from the point of view of application as appropriate. The column tube appropriately includes a stator ring on the wall. The generally centrally arranged shaft typically has a stirrer device so that in each case the stirrer rotates between two stator rings. Examples of stirring columns include RDC (rotating disk contact) columns, ARD (asymmetric rotating disk) columns, Kuhni columns (stirring columns according to Kuhni design) or QVF stirred cell extractors. Centrifugal extractors use centrifugal force to mix and separate two phases induced in countercurrent. Centrifugal force also results in good retentate / extract separation when the two layers form a stable emulsion. As an example, a Podovir Niak extractor or a Westferia extractor is given.

  The extraction device for the process according to the invention is preferably made from material 1.4571. This also applies to other devices that can be used to remove acrylic acid from the product gas mixture of heterogeneously catalyzed gas phase partial oxidation.

The driving force for separating the extract from the extracted residue is the difference in mass density (g / cm ³ ) between the two layers. A large mass density difference between the two liquid phases facilitates phase separation and reduces emulsion formation.

Therefore, advantageously, the mass density of kg / m ³ units, mass density of water and (Similarly kg / m ³ units) (the temperature employed in the the pressure and extracted employed extraction as the base) 25 kg Organic solvents with a difference of greater than / m ³ , preferably greater than 50 kg / m ³ are used for the extraction carried out according to the invention. However, in general, the mass density difference is 250 kg / m ³ or less, generally 150 kg / m ³ or less.

  In addition, when the kinematic viscosity of the organic extract under the extraction conditions is 100 mPa · s or less, preferably 50 mPa · s or less, it is suitable for the method according to the present invention. However, in general, the kinematic viscosity is 1 mPa · s or more. Particularly preferred kinematic viscosities according to the invention are kinematic viscosities in the range of 2 to 10 mPa · s.

  A relatively large interfacial tension between the two fluid phases is advantageous for the method according to the invention. Against the background already described, suitable extractants according to the present invention for acidic water extraction include organic liquids whose boiling point at standard pressure (1 atm) exceeds about 150 ° C. or about 160 ° C. Examples include an intermediate oil fraction by paraffin distillation, diphenyl ether, diphenyl, or a mixture of said liquids, for example a mixture of about 70 to 75% by weight diphenyl ether and 25 to 30% by weight diphenyl. Further, it is preferable to use a mixture of 70 to 75% by mass of diphenyl ether and 25 to 30% by mass of diphenyl, and further 0.1 to 25% by mass of o-dimethyl phthalate based on the mixture. is there.

Particularly preferred organic solvents according to the present invention for acidic water extraction are aliphatic or aromatic monohydric compounds whose alcohol component contains 1 to 8 carbon atoms and whose carboxylic acid component contains 5 to 20 carbon atoms. Or an ester of a dicarboxylic acid (especially when both carboxyl groups are esterified). The alcohol component preferably has only two hydroxyl groups or just one hydroxyl group prior to esterification. The alcohol component preferably comprises a monovalent (one OH group) or a divalent (two OH groups) alkanol. Advantageously, the number of carbon atoms of the alcohol component (especially in the case of monovalent or divalent alkanols) is 1 to 6, more preferably 1 to 4, most preferably 1 or 2. Aliphatic or aromatic mono- or dicarboxylic acids are advantageously 5 to 15 carbon atoms, preferably 5 to 10 carbon atoms, more preferably 6 to 8 carbon atoms (especially 1 to 4 alkanols). Or each esterification having 1 to 2 carbon atoms (the same applies in the case of diesters). Dicarboxylic acid is more suitable than monocarboxylic acid as the acid component of the corresponding ester (particularly when both carboxyl groups are esterified). Phthalic acid, isophthalic acid and terephthalic acid and furthermore adipic acid are particularly preferred acid components of the corresponding esters according to the invention. The latter are dialkyl esters (C ₁ -to C ₈ -alkyl, preferably C ₁ -to C ₆ -alkyl, particularly preferably C ₁ -to C ₄ -alkyl, better still C ₁ -or C _2- This is especially true in the case of alkyl). In other words, particularly suitable extractants for the process according to the invention are dimethyl phthalate, diethyl phthalate (for example Palatinol® from BASF Aktiengesellschaft), dimethyl isophthalate, diethyl isophthalate, dimethyl terephthalate, terephthalic acid Diethyl, dimethyl adipate and diethyl adipate. Further esters suitable for the acidic water extraction of the invention are triesters of phosphoric acid, such as tributyl phosphate or tricresyl phosphate. Useful cresyl radicals include ortho-cresyl, meta-cresyl and para-cresyl. Branched or linear monovalent C ₆ -to C ₁₂ -alkanol esters (eg 2-propylheptyl acrylate or 2-ethylhexyl acrylate) with acrylic acid and maleic acid with monovalent C ₄ - mono alkanols - - C ₁₀ from or diester is also useful as an extractant for the acid water extraction of the present invention. At the same time, according to the invention, among all the above extractants, the boiling point at standard pressure is above 150 ° C, or above 160 ° C, or above 170 ° C, or above 180 ° C, or above 190 ° C. Those are preferred.

  In general, the extracted acidic water will contain acetic acid (generally the component with the third largest mass% based on the total amount of acidic water) as an additional component along with acrylic acid and water. Depending on the method of carrying out the partial oxidation (the catalyst selected, the water content of the reaction mixture, the temperature of the partial oxidation), the acidic water can be up to 10% by weight or up to 5% by weight (often 2 to 8% by weight) %) Or higher amounts of acetic acid. Often, acidic water contains acrylic acid in a mass ratio of about twice that of acetic acid. The content of other possible acidic secondary components is usually significantly less. Therefore, according to the present invention, an extractant that preferentially absorbs acrylic acid compared to acetic acid is preferred. These extractants include in particular diethyl phthalate.

  The present invention is also advantageous when the extractant does not react with water under the extraction conditions and is only slightly soluble in water. For example, diethyl phthalate is particularly hydrolytically stable. A further advantage of diethyl phthalate is that it has a relatively high boiling point at standard pressure (1 atm), which is advantageously greater than 200 ° C. for the extractant (organic solvent) used according to the invention, and more It is preferably 225 ° C. or higher, and more preferably 250 ° C. or higher.

  In addition, the solubility in water is relatively low (this also reduces extraction loss). In general, acidic water is obtained by fractional condensation of the product gas mixture carried out according to the invention at a temperature of 50 to 80 ° C., preferably 60 to 70 ° C. In other words, it is usually recovered at this temperature via a second liquid phase outlet (preferably a side outlet) (the lower the temperature, the less the polymerization inhibitor is required; when appropriate) , Acidic water, extractant, extract residue and / or extract need not be added separately). Therefore, extraction is also performed at a temperature within this range as appropriate from the viewpoint of use. In other words, advantageously according to the invention, the extractant is essentially an extraction device, preferably an extraction column (more preferably a column containing a structural filler, advantageously Montz-Pak B1-350) at said temperature. Will be guided to. Advantageously, it is fed to the extraction column from the bottom and an extractant with a higher specific gravity (preferably diethyl phthalate) is introduced from the top. Typically, the temperature of the extractant introduced is not significantly different from the temperature of the supplied acidic water. Typically, the magnitude of this temperature difference is from 0 ° C. to 20 ° C., preferably from 0 ° C. to 15 ° C., and in many cases from 0 ° C. to 10 ° C. The pressure of the acidic water recovered from the condensation column is typically from just over 1 bar to 1.5 bar, often 2 bar, at the recovery point, according to the invention. The recovered acidic water is guided to the extraction column by a pump. The transport pressure can be, for example, 2 to 6 bar. The working pressure in the extraction column is selected according to the present invention so that no additional pump is required to transport the organic extract to the first stripping column. However, in principle, acidic water extraction can also be carried out at higher or lower temperatures and at higher or lower pressures. When the extraction column is operated, the procedure is as follows from an application point of view, where the extraction column is first filled with acidic water and then, as described above, advantageously in the form of droplets at the top of the extraction column. Is introduced. Acidic water (preferably a continuous phase) can basically be fed directly through a suitable feed nozzle. However, basically, acidic water can also be supplied via a supply tube having one (or more) flow orifices on its wall (the diameter of the flow orifice is typically between 5 and 10 mm). is there).

  As already mentioned, the extraction column is manufactured from stainless steel as appropriate from the viewpoint of application. Typical wall thickness is 5 to 20 mm. The extraction column is usually insulated on the outside by conventional methods.

  In the method of the present invention, the ratio V between the flow rate of the organic extractant (E; kg / h unit) supplied to the extraction column and the flow rate of acidic water (S; kg / h unit), that is, E: S is 0. It may be from 05 to 20, preferably from 0.1 to 10, better from 0.8 to 1.2, more preferably 1: 1.

  Acidic water that is dilute (extracted) with acrylic acid is usually sent for disposal (eg, incinerated or directed to a water treatment plant). Typically according to the present invention, acidic water is drained from the top of the extraction column (as a retentate) and organic extracts containing acrylic acid are typically drained from the bottom of the extraction column.

  The removal of acrylic acid from the organic extract, whose recovery temperature from the extraction column substantially corresponds to the supply temperature of the acidic water to the extraction column, basically using a different thermal separation method or said thermal separation It can be implemented using a combination of methods.

  A preferred variant removal is crystal removal. Possible crystallization methods include all those recommended in DE-A 198 38 845 and DE-A 102005015637. The reason why crystal removal is preferred is that crystallization is a distinct separation method as opposed to other thermal separation methods. In other words, the composition of acrylic acid crystals formed is almost independent of the liquid phase, ie the composition of the liquid extract (in the ideal case it is completely independent). In contrast, indistinguishable thermal separation methods form when the separation method is employed, and the composition of the phase containing the target product at a high concentration is highly dependent on the composition of the mixture being separated. In other words, a distinctional separation method is sufficient in thermodynamic theory if an equilibrium is established once to obtain a pure target product, whereas a non-distinguishable separation method has this purpose. Therefore, the thermodynamic equilibrium must be established continuously and repeatedly. According to the invention, crystallization methods that are particularly suitable for the purpose of removing acrylic acid from the extract are suspension crystallization and layer crystallization.

(Especially if the extract used has a boiling point (at standard pressure) of at least 60 ° C., preferably at least 80 ° C., most preferably at least 100 ° C.) higher than the corresponding boiling point of acrylic acid (= 141 ° C.) Particularly advantageously according to the invention, the removal of acrylic acid from the extract will be carried out by desorption. The desorption device used is typically a column having a separation internal structure. Useful such column internal structures basically include all known separation internal structures. These include in particular trays, structural fillers and / or messy fillers. Among the mass transfer trays, bubble cap trays, sieve trays (eg, forced sieve trays or trickle sieve trays (dual flow trays)), valve trays and / or Thormann trays (ie Thormann® trays) are preferred. Among the messy fillers, those containing rings, spirals, saddles, Raschigs, Intos or Pall rings, barrels or interlox saddles, top-packs, etc. or blades are preferred. Double flow trays and / or Thormann trays are particularly preferred. Of the Thormann trays, a single-flow Thommann tray is preferred. When dual flow trays and Thormann trays are used, it is most preferred that the double flow tray is used at the bottom of the desorption device and the Thormann tray is used at the top of the desorption device. In general, it is sufficient if there are 5 to 10 theoretical plates in the desorption device. Basically, desorption can be caused solely by lowering the pressure. Particularly advantageous from an application point of view, it is non-condensable under desorption conditions (the boiling point of the stripping gas at 1 atm is preferably at least 50 ° C. below the boiling point of water), (with respect to chemical changes) By diluting with an essentially inert gas, the partial pressure of desorbed acrylic acid is reduced. In other words, the removal of acrylic acid from the extraction phase of the acidic water extraction is most preferably performed by gas-based stripping (gas stripping). In other words, when a gas is passed through the extract, acrylic acid is absorbed by the gas and is derived from the extract. The driving force behind this method is the transfer of acrylic acid from the extract to the gas due to the vapor pressure of acrylic acid being removed from the extract being greater than its partial pressure in the gas. Advantageously, the extract is directed against the flow direction of the stripping gas (ie counter-current) for the purpose of stripping. In other words, the extract is advantageously introduced at the top of the stripping column and the stripping gas is directed to the bottom of the stripping column. Particularly advantageously, the stripping gas used is a residual gas (or residual gas mixture) derived from the top of the rectification column or a part of this residual gas having the same or different composition as the overall residual gas. Or a gas whose composition corresponds to one or more inert components (eg, water vapor, N ₂ , CO ₂ and / or air) of the residual gas. The preferred use according to the invention of the residual gas mixture as stripping gas is, firstly, without the need to supply further stripping gas (which must be disposed of later) and secondly the product of partial oxidation The use of a stripping gas that does not contain components unrelated to the overall method of removing acrylic acid from the gas mixture. In general, the residual gas is primarily an inert diluent gas used for partial oxidation of the C ₃ precursor of acrylic acid (which does not substantially change chemically with partial oxidation), and more typically as a by-product in partial oxidation. Consisting of water vapor that may be formed and added as a diluent gas, and carbon monoxide formed by undesired complete oxidation as a side reaction (often a portion of the residual gas having a residual gas mixture composition, This is returned to the partial oxidation as a dilution gas; this is referred to in this document as a partial oxidation circulation gas or circulation gas). It is a small amount of molecular oxygen that was not consumed in the partial oxidation (for example, residual oxygen, which is advantageous to avoid the formation of undesirable polymers) and / or a small amount of unconverted C ₃ precursor and / or unreacted. Further included are intermediates for conversion. Inert diluent gases used for partial oxidation (eg N ₂ , CO ₂ , H ₂ O and / or saturated hydrocarbons, etc.) first serve to absorb the heat of reaction released in the partial oxidation. Secondly, by maintaining the reaction mixture explosion range, or in the region of the safety control can explosion range, totally secure processing of heterogeneously catalyzed gas phase partial oxidation of C ₃ precursors (See, for example, DE-A 19740253, DE-A 19740252, DE-A 10243625, DE-A 10332758 and WO 2004/035514).

  When a double flow tray is used as a separation internal structure in a desorption column (stripping column), the total diameter (diameter of the channel orifice in the tray) is generally 8 to 50 mm, preferably 10 to 35 mm. . The total diameter is preferably larger towards the bottom. The tray spacing (tray is typically arranged at regular intervals) is often 300 to 800 mm, often 400 to 600 mm, often 500 mm. As already mentioned, the desorption device is typically made of austenitic steel, preferably of material 1.4571 (DIN EN10020).

  The column feed of extract to be stripped is present in the upper region of the desorber, preferably in its uppermost tray (theoretical stage). From an application point of view, the extract is preheated (typically to a temperature of 80 to 120 ° C., often about 100 ° C.). In order to additionally supply heat to the bottom of the stripping column (desorption column), it is advantageous to use conventional design internal and / or external indirect heat exchangers (eg Robert evaporators, forced circulation tube bundles) Heat exchangers, forced circulation tube bundle flash heat exchangers, plate heat exchangers etc .; see eg EP-A 854129) and / or heat exchangers via jacket heating (the heat medium used is preferably partially oxidized) Water vapor obtained using the waste heat of It is preferably carried out via an external circulation evaporator with natural or forced circulation. More preferably, an external circulation evaporator with forced circulation (especially flash circulation) is used. It is possible to use multiple evaporators connected in cascade or in parallel (typical bottom temperatures are 145 to 165 ° C, often about 155 ° C).

  An inert stripping gas that does not condense under the desorption conditions employed is advantageously introduced directly into the bottom of the desorption apparatus. The supply from the external heat transfer (the necessary heat is supplied to the pre-recovered bottom liquid) is advantageously one of the lower column trays (theoretical plates) (preferably the 2nd to 4th from the bottom) In the region of the theoretical plate) or below. The substantially acrylic acid-free extractant obtained at the bottom of the desorption column is advantageously returned to the top of the acidic water extraction.

  Because it usually has a higher temperature compared to the extract obtained in the extraction device, the extract that is derived from the extraction to the stripping column to heat the extract and cool the extractant, and stripping Extractant returned to the extraction from the bottom of the column (required extraction and prevention of active polymerization of the stripping column will be carried out by return, advantageously in the bottom of the stripping column; for example If the stripping gas contains molecular oxygen, use MEHQ and / or PTZ) in an indirect heat exchange to a conventionally designed indirect heat exchanger (tube bundle heat exchanger or plate heat exchanger) It is reasonable. Subsequently, the extractant is brought to the working temperature in acid water extraction in an indirect heat exchanger, usually incidentally, for example by water cooling. With respect to the extractant feed stream returned to the extraction from the bottom of the stripping column, 0.01 to 1% by weight of bottom liquid of the stripping column is usually discharged concomitantly (purification feed stream) (the water cooling In addition, an appropriate feed stream of fresh extractant is usually replenished to the extractant return feed stream). This purified feed stream can be disposed of (eg, incinerated with residual gas) or processed by rectification to convert to a new extractant.

The stripping gas is preferably directed to the stripping column at a temperature that generally corresponds at least to the temperature of the bottom liquid in the stripping column (in general, the difference between the two temperatures does not exceed 30 ° C., better Not exceeding 20 ° C.). If the residual gas is used as a stripping gas, it can be brought to said temperature, for example by compressing it to a high pressure. Advantageously, this compression can be carried out with the circulating gas (typically using a centrifugal compressor) to a pressure that is also suitable for the return of residual gas to C ₃ precursor partial oxidation. . Typically, the pressure is 2 to 4 bar (typically greater than 1 bar and at a residual gas pressure of 1.5 bar or less), often 2.5 to 3.5 bar.

For a 1 kg extract, the amount of stripping gas required to strip until no acrylic acid is present is typically 1.5 to 2.5 Nm ³ . A typical loading of stripping gas containing acrylic acid (at the top of the stripping column) is 2 to 6% by weight.

  Basically, the amount of heat input to the bottom of the stripping column can be reduced by increasing the amount of stripping gas used. One of the reasons why it is preferable to remove acrylic acid from the extract by stripping with an inert stripping gas that typically does not condense under desorption conditions is that it is from an energy and equipment standpoint. This is because it is a particularly efficient removal method with relatively loose requirements. In addition, in contrast to, for example, crystal or rectification removal from the extract, it has the advantage that it contains a high concentration of acrylic acid and does not produce additional liquid phases that require further significant polymerization prevention costs and labor.

  The above facts, among other things, allow the acrylic acid-containing stripping gas to be directly returned to the condensation column (used for fractional condensation of the product gas mixture of partial oxidation), especially when residual gas is used for stripping. Due to the fact that. Optionally, the return is performed under a first sidestream outlet to the condensation column. Advantageously, the return is carried out directly in the bottom space of the condensation column. In principle, it can be carried out directly in the bottom liquid.

  Thus, the present application is present in particular in at least a part (advantageously at least 25% by weight, better at least 50% by weight, preferably at least 75% by weight or all) of acid water that has not been returned to the condensation column Acrylic acid is absorbed from acidic water to the organic solvent by extraction with an organic solvent to form an organic extract containing acrylic acid and then stripped with a stripping gas to remove the acrylic acid from the organic extract. And removing the acrylic acid-containing stripping gas back to the condensation column. Basically, it is also possible to carry out the return of acrylic acid present in the stripping gas under load to the condensation column in a condensed form. However, it is also possible to send the condensed acrylic acid together with the crude acrylic acid recovered from the condensation column for further purification.

However, particularly advantageously, in the process according to the invention, the bottom liquid is still present in the bottom liquid recovered from the condensation column in the second stripping column (and later condensed) before the bottom liquid is sent for disposal. A loaded stripping gas (or at least a part thereof) will be used to strip the free acrylic acid (which will be returned to the column) as well.
Advantageously according to the invention, the stripping gas in the load state of this second stripping is also subjected to (countercurrent) rectification, as already described in WO 2004/035514 and DE-A 10332758 (provided that , The residual gas is used as is as stripping gas) (and later it is returned to the condensation column).

  Bottom space in the condensing column used in the process according to the invention (or very generally a column with a separating internal structure), the lowest separating internal structure in the condensing column (eg induced in countercurrent in the separating column) The space under the trays, structural fillers and messy fillers) that enlarge the interface between the phases to be produced.

  In particular, a so-called bottom liquid containing constituents whose boiling point at standard pressure (1 atm) exceeds that of acrylic acid collects therein. These include, firstly, the fractional condensation of the product gas feed stream of the partial oxidation as well as secondary products such as by-products having a higher boiling point than acrylic acid, such as maleic anhydride, phenothiazine added as a polymerization inhibitor. A conversion product that is simply formed from the constituents in the process. These conversion products include, in particular, free radical polymers of acrylic acid that form in an undesirable manner regardless of polymerization prevention. Also included are high molecular weight compounds formed by condensation reactants of different components of the product gas mixture of partial oxidation. They include, inter alia, the acrylic acid itself and the Michael adduct formed by the reversible Michael addition of acrylic acid to the acrylic acid dimer ("diacrylic acid") or oligomer thus formed (hence the book In the literature, the term “acrylic acid oligomer” always refers to the corresponding Michael adduct, not the acrylic acid oligomer formed by free radical polymerization). The high temperature action (eg, preferably by “stripping” with a gas that is preferably (chemically) inert and does not condense under appropriate conditions) by simultaneously removing the formed acrylic acid is Reverse the addition. However, monomeric acrylic acid itself is a non-negligible component of the bottom liquid in the condensation column, which means it is worth stripping completely. However, the bottom liquid of the condensation column also contains by-products (eg, benzaldehyde, furfural and maleic anhydride) that boil at normal pressure and at a temperature just above the boiling point of acrylic acid, so this second stripping of For loaded stripping gas (e.g., before it is returned to the condensation column), the purity of the second stripping gas containing acrylic acid (in the same column) is increased (countercurrent). Distillation (ie countercurrent of the reflux liquid) is further applied.

  The return of the first loaded stripping gas and / or the second loaded stripping gas to the condensation column does not necessarily have to be performed by a direct path.

  Instead, the first loaded stripping gas and / or the second loaded stripping gas can be mixed at least partially or completely with the product gas mixture and the resulting gas mixture is Can be directed to a condensation column.

  Mixing can be carried out before, during and / or after direct and / or indirect cooling of the product gas mixture of the gas phase partial oxidation.

  Hereinafter, for differentiation purposes, the acrylic acid-containing stripping gas from the extract stripping will be referred to as the “first loaded gas” and the (condensation column) acrylic acid-containing (preferably purified) from the bottom stripping. The stripping gas is referred to as “second loaded gas” (and accordingly, the extraction stripping is referred to as “first stripping” and the bottom stripping is referred to as “second stripping”. Etc.). Generally, the first loaded gas exits the extract stripping at a pressure of 1.5 to 3.5 bar, often 2 to 3 bar, for example 2.5 bar. The temperature is typically about 10 ° C. below the temperature at which the extract is introduced into the first stripping column. Typical temperatures for the first loaded gas are 65 to 95 ° C, often 75 to 95 ° C, or 85 ° C.

  In order to perform a second stripping (including stripping gas rectification), useful columns are basically all columns that contain separate internal structures, possible internal structures being, for example, Structural fillers, messy fillers and / or trays. In principle, it is also possible to use a column containing a rotating internal structure, known as a rotating column that sprays the reflux liquid onto the droplets. According to the invention, it is preferred to use a column containing only trays and / or structural fillers as the second stripping column. The tray used is preferably a double flow tray, and the stripping column particularly advantageously comprises a double flow tray exclusively as a separating internal structure.

  In this document, a double flow tray is understood to refer to a plate having a simple flow path (holes, grooves, etc.). The gas rising in the second stripping column and the reflux liquid descending in the second stripping column flow in the opposite direction in the same flow path. The cross section of the flow path is adjusted to the filling amount of the second stripping column in a manner known per se. If it is too small, the ascending gas flows through the flow path at such a high speed that the reflux liquid descending in the stripping column is confined without substantially undergoing the separation action. If the cross section of the flow path is too large, there is a risk that the ascending gas and the descending reflux pass each other without substantial exchange and the tray is depleted. Typically, dual flow trays do not have a drain tube connecting them to the next tray. Of course, each double-flow tray can be in direct contact with the wall of the second stripping column. However, it can also be brought into contact with the wall via a connecting element. Unlike the hydraulic seal alternating current tray, the double flow tray is depleted as the fill of the second stripping column increases.

  Optionally, according to the present invention, a dual flow tray stripping column that can be used for the second stripping (including designated rectification) can contain up to 60 double flow trays. Advantageously, they are formed from an orifice ratio of 10 to 20%, preferably 10 to 15% (ratio of tray area (D) permeable to cracked gas and total area (U) of the tray) D: U ratio).

The flow path of the double flow tray is preferably a circular hole and the diameter of the circle is uniform within the tray. The diameter of the circle is about 10 to 30 mm. The diameter of the circle is preferably 10 to 20 mm or 10 to 15 mm at the top of the column and preferably 20 to 30 mm at the bottom of the column. Furthermore, the circular holes are preferably arranged uniformly on the individual double-flow tray with a precise triangular pitch (cf. DE-A 10302219). Furthermore, the perforation burr of the channel orifice opened in the double flow tray in the second stripping column is preferably facing down. Usually, the double flow trays in the second stripping column are arranged at equal intervals. Typically, the tray spacing is 300 mm to 500 mm. According to the invention, a tray spacing of 400 mm is also suitable. If appropriate, the bottom liquid to be stripped (which may be used in advance for direct cooling of the product gas mixture of the partial oxidation and / or is supplemented with the high-boiling fraction recovered in the lower region of the condensation column. May be fed to the 4th to 10th double flow trays (calculated from below) of the second stripping column. The bottom temperature in the second stripping column is advantageously maintained between 150 and 190 ° C, preferably between 160 and 180 ° C. The working pressure in the second stripping column is usually from 1 to 3 bar, often from 1.5 to 2.5 bar.
The energy input required in the second stripping column is advantageously supplied utilizing an external forced circulation tube bundle flash evaporator, whereas the bottom liquid recovered from the second stripping column is Is supplied for the purpose of superheating it, and then superheated and returned to the second stripping column (see DE-A 1032758). Basically, at this point it is also possible to use a pure forced circulation evaporator or a natural circulation evaporator, for example a Robert evaporator, which can also be integrated into the second stripping column.

  The second stripping column itself is thermally insulated from the environment from the point of view of the application as appropriate (similar to the condensation column and the first stripping column).

  The reflux liquid can be produced by direct and / or indirect cooling. According to the invention, a direct cooling method is advantageously employed. To this end, in the simplest way, the gas flowing in the last tray (theoretical stage) is separated from the separation internal structure of the second stripping column, for example, by the stack tray (attached to the separation part) ) To a quenching device that can be integrated into a second stripping column. However, basically, the quenching device can also be arranged in the space outside the second stripping column. Useful quenching devices include all devices known in the prior art for this purpose (eg, spray scrubbers, venturi scrubbers, bubble columns, or other devices having a spray surface), and venturi scrubbers. Preferably, a steam or spray cooler is used. Advantageously, a countercurrent device (for example with an impingement plate nozzle) is used. In the case of indirect cooling of the quench liquid, it is typically induced through a (indirect) heat transfer or heat exchanger. In this respect, suitable devices are any common heat exchanger or heat exchanger. Suitable devices include tube bundle heat exchangers, plate heat exchangers and air coolers. A suitable refrigerant is air in a suitable air cooler and in other cooling devices is a coolant, in particular water (eg surface water). From the viewpoint of use as appropriate, the quenching liquid used (hereinafter also referred to as “quenching liquid 0”) is a part of the condensate formed by quenching. The other part of the condensate formed by quenching is returned to the uppermost tray (theoretical stage) of the second stripping column as a substantially reflux liquid (a small part of the condensate is split at this point). Can also be fed to and mixed with the bottom liquid recovered from the condensation column; a portion of the resulting mixture can be used as a quench liquid 1 that will be further introduced in further processing of this document. Another part of this mixture forms the feed to the second stripping column, and the (relatively less) remaining amount of this mixture is mixed with the remaining part of the condensate, and then in this mixing, First, the reflux liquid to the uppermost tray (theoretical stage) of the second stripping column is formed, and secondly, the quench liquid 0 is formed). Typically, the temperature of the quench liquid 0 just prior to use for quenching is about 40 ° C., whereas the reflux liquid is typically returned at about 80 ° C. The mass ratio of the reflux liquid returned to the bottom liquid (and possibly high boiling liquid) supplied to the second stripping column is typically 2 or higher. It is often 2 to 10, preferably 4 to 8.

  Of course, the second stripping column (as well as all other equipment in which a liquid phase having a significant acrylic acid content is derived) must be operated while preventing polymerization. The polymerization inhibitors used for this purpose are basically all polymerization inhibitors known in the prior art.

  Examples thereof include phenothiazine (PTZ) and p-methoxyphenol (MEHQ). Often these two polymerization inhibitors are used in combination. If appropriate for this purpose, they are added and dissolved in pure acrylic acid. MEHQ is preferably measured as a melt.

  However, by adding a portion of the bottom liquid (and optionally the high boiling fraction) that has been polymerized, that is, contains a polymerization inhibitor and is derived from the condensation column, to the quench circuit 0, the quench circuit 0 and the second In particular, the stripping column can be prevented in a particularly simple manner. Thus, typically, this quench liquid 0 contains, for example, 0.01 to 0.1 wt% MEHQ and 0.01 to 0.5 wt% PTZ. Typically, molecular oxygen present in the first loaded gas (which functions as a stripping gas for the second stripping column) facilitates polymerization prevention.

  Part of the bottom liquid produced at the bottom of the second stripping column is continuously discharged and removed. If necessary, an organic solvent such as methanol is added to maintain the least volatile residue in a fluid state. Instead of methanol, it is also possible to use other hydrophilic organic liquids such as ethanol or those recommended in WO 2004/035514 (or mixtures of such liquids). The exhaust stream is about 10 or 20 to 30% by weight relative to the bottom liquid feed stream fed from the condensation column. Where appropriate, from an application point of view, the energy is supplied to the second stripping column by diverging from the feed stream discharged from the forced circulation flash evaporator in a superheated form. It is degassed, diluted with, for example, methanol and sent to, for example, residue incineration.

A second loaded gas (typically 70 ° C. to 90 ° C.) discharged from the quench circuit 0 in a cooled form can then be returned to the condensation column as is (at this point). , For 1 kg of bottom liquid supplied from the condensing column, a first loaded gas of 1 to 10 Nm ³ is used as the stripping gas in the second stripping column; Is present in a boiled state at the bottom of the second stripping column). Its acrylic acid content is typically 15 to 20%. In principle, it is also possible to carry out the return of the acrylic acid present therein to the condensation column in a condensed form. However, the acrylic acid condensate can also be fed for further purification along with the crude acrylic acid recovered from the condensation column. According to the invention, it is preferred to return acrylic acid to the condensing column as it is, i.e. in gaseous form, as a constituent of the gas in the second load state (basically, the return is partially condensed). Can also be implemented).

  The return of the second loaded gas to the condensation column (all statements apply equally to the return of the first loaded gas to the condensation column as it is) is preferably the first (Basically it can also be sent directly back to the cooling circuit of the gas phase partial oxidation product gas mixture). From an application point of view, it is returned to the bottom space of the condensation column. It can be carried out immersed in the bottom liquid or above the level of the bottom liquid and under the first tray (theoretical stage) of the condensation column. The bottom space of the condensation column advantageously includes a droplet separator (eg, a centrifugal droplet separator) to suppress entrainment of the bottom liquid droplets by the rising gas. Furthermore, the bottom space can be separated from the lowest separated internal structure by the first exhaust stack.

  The residence time of the bottom liquid (optionally including the recovered high-boiling fraction) fed from the condensation column to the second stripping column in the second stripping device is typically (finally drained) Should be 0.5 to 4 hours). Of course, a re-cracking catalyst for the re-cracking of acrylic acid oligomers can be measured at the bottom of the second stripping unit, for example as recommended in WO 2004/035514.

  For the bottom of the second stripping column, adjuvants and / or dispersants such as Komad® 313 from Mol (Hungary) (eg EP-A 1062197 and / or US-A 3,271,296) For example, tertiary amines (eg, trimethylamine, triethylamine, N, N, N ′, N′-tetra) that reduce contamination of indirect heat exchangers to provide the required energy and improve acrylic acid yield. It is also possible to add methyl-1,6-hexanediamine and pentamethyldiethylenetriamine). The addition amount may be 0.1 to 10% by weight with respect to the bottom of the second stripping column. They are advantageously already high-boiling substances or they are not stripped at the same time because these additives form these substances with acrylic acid.

  One of the reasons why the described inventive procedure is advantageous is that the yield of acrylic acid can be improved without significantly reducing the purity of the crude acrylic acid recovered via the first side-flow outlet. It is. Even if the residual gas from the condensation column is used as described for the purpose of stripping, this relates in particular to the described variant stripping.

In this case, the purity of both the first loaded gas and the second loaded gas with respect to the secondary component that splits when the acrylic acid is later used (eg, to produce a super water supply polymer) is already The acrylic acid present therein is removed from the gas phase of the first or second loaded gas (in each case the first and / or second loaded gas as a whole or a part thereof) To) aqueous solutions of metal hydroxides (eg, alkali metal hydroxides and / or alkaline earth metal hydroxides, eg, NaOH, KOH, Ca (OH) ₂ and / or Mg (OH) ₂ ) For example, acrylic acid obtained when absorbed in aqueous sodium hydroxide solution, in a manner that corresponds completely to the procedure described in WO2003 / 095410. An aqueous solution of thorium, for example, (see also WO2003 / 014172 with respect to this matter) which can be directly used in the production of ultra-water polymer by suitable free radical polymerization.

  This is especially because by-products such as formaldehyde and formic acid usually preferentially remain in the aqueous phase of acidic water. For example, the first and second acrylic acid by countercurrent scrubbing with an aqueous sodium hydroxide solution (or any of the aqueous metal hydroxide solutions) in a column containing separated internal structures (tray, messy packing and / or structural packing) The second loaded gas feed can later be returned to extract stripping and / or sent to incineration.

Typically, the acrylic acid-containing product gas mixture of a heterogeneously catalyzed gas phase partial oxidation of a C ₃ precursor of acrylic acid with molecular oxygen on a solid catalyst is, for example, (especially the C ₃ precursor used). When the body is propylene) it can have the following contents:

1 to 30% by weight acrylic acid 0.05 to 10% by weight molecular oxygen 1 to 30% by weight water 0 to 5% by weight acetic acid 0 to 3% by weight propionic acid 0 to 1% by weight malein Acid and / or maleic anhydride 0 to 2% by weight acrolein 0 to 1% by weight formaldehyde 0 to 1% by weight furfural 0 to 0.5% by weight benzaldehyde 0 to 1% by weight propylene, and other As a content, a substantially inert gas such as nitrogen, carbon monoxide, carbon dioxide, methane and / or propane Typically, the product gas mixture is 0.005% relative to the acrylic acid present. More than mol%, often 0.03 mol% or more of furfural. However, in general, the furfural content on this basis is 3 mol% or less.

  The gas phase partial oxidation itself can be carried out as described in the prior art. Starting from propylene, the gas phase partial oxidation can be carried out in two successive oxidation stages as described, for example, in EP-A 700714 and EP-A 700893. However, it will be understood that gas phase partial oxidation cited in DE-A 1974253 and DE-A 1740252 can also be used.

For the purpose of forming small amounts of secondary components, propylene gas phase partial oxidation is preferably carried out as described in DE-A 10148586. For this purpose, the propylene source used may be polymer grade propylene or chemical grade propylene as described in DE-A 10232748. If C ₃ precursor used is propane can be carried out as described partial oxidation DE-A10245585.

  However, gas phase partial oxidation is basically performed in US 2006/0161019, WO 2006/094101, WO 2006/002703, WO 2006/002713, WO 2005/113127, DE-A102004021773, EP-A1611076, WO2005 / 108342, EP-A16565635, EP-A16556335, EP-A 1682478, EP-A 1682477, German applications 1020060542214. and 102006024901.1, EP-A161080, EP-A1734030, German applications 102006000996.7, 102002006206.4, 102006206200.8, international application PCT / EP2006 / 0665416, PCT / EP2006 / 066784 and PCT / E It can be carried out as described in 2006/067080 literature.

  Often, the temperature of the product gas mixture after undergoing gas phase partial oxidation is from 150 to 350 ° C, in many cases from 200 to 300 ° C, and sometimes up to 500 ° C.

  If appropriate from an application point of view, the hot product mixture is then cooled in a quenching device by direct cooling, generally from 100 to 180 ° C., and advantageously from the application point of view, together with the quench liquid 1 used, For the purpose of fractional condensation, it is preferably poured into the lower part (preferably the bottom part, for example the bottom space) of the condensation column containing the separation internal structure.

  Suitable condensation column internal structures are basically all common internal structures, in particular trays, structural packings and / or messy packings. Among the trays, a bubble cap tray, a sieve tray, a valve tray and / or a double flow tray are preferable. Typically, the total number of separation trays in the tray column is 20 to 100, often 20 to 80, preferably 50 to 80.

  Preferably according to the invention, the condensing column is, as a separating internal structure, in order from the bottom, the first double-flow tray, then the hydraulically sealed AC tray recommended for DE-A10243625, DE-A19924532 and DE-A10243625. (E.g. Thormann (R) tray). The number of double flow trays may be 5 to 60, often 25 to 45, and the number of hydraulic seal AC trays may similarly be 5 to 60, often 30 to 50. In the acidic water formation region suitable according to the present invention (the acrylic acid content of the reflux liquid from the bottom to the top is 15% by mass or less, and in some cases 10% by mass or less), the useful separation internal structure is preferably Are the valve trays described in DE-A 1992 4532 and DE-A 10243625. In principle, however, it is also possible to use other common separation internal structures (individual regions in the condensation column are of course also very generally as cascades of correspondingly small columns, ( Can be configured in exactly the same way (rather than stacked on top of each other), which is the term “condensation column” specifically used in this document, and the “column” generally used in this document. This is why the term includes the corresponding cascade of smaller columns).

  The quenching device 1 used may be any known device corresponding to this purpose in the prior art (eg spray scrubber, venturi scrubber, bubble column or other device having a spray surface). Preferably, a steam or spray cooler is used.

  The indirect cooling or heating of the quench liquid 1 is carried out in particular during the start-up process, but is preferably not necessarily carried out via a heat exchanger or heat exchanger. In this respect, all common heat exchangers or heat exchangers are suitable. Bundle heat exchangers, plate heat exchangers and air coolers are preferred. Suitable refrigerants are air in the corresponding air cooler, and coolant in other cooling devices, especially water.

  The quench liquid 1 used is, for example, via a bottom liquid recovered from the bottom of the condensation column (possibly mixed with a condensate derived from the quench circuit 0) or via a side outlet close to the bottom. The recovered bottom liquid, high-boiling fraction, or a mixture of the bottom liquid and high-boiling fraction (especially the bottom space and the lowest tray (theoretical stage (lowest separation internal structure))) If separated). In some cases, only a portion of the quench liquid 1 recovered from the bottom of the condensation column is guided through the heat exchanger. The temperature of the quench liquid 1 entering the quench apparatus 1 is generally 90 ° C. to 120 ° C. as appropriate.

  A quenched (or otherwise cooled or uncooled) product mixture of catalytic gas phase partial oxidation (according to the invention, the quench liquid 1 preferably used for direct cooling as described, preferably In the form of a mixture) in the condensing column advantageously comprises a centralized centrifugal droplet separator, this column being generally separated from the lowest separating internal structure by a first stack tray In the bottom space (if appropriate from an application point of view, in this case, the high-boiling fraction is continuously poured into the bottom of the condensation column via a connecting line or overflow). In an exemplary and preferred embodiment (discussed hereinafter without limiting the overall functionality), this bottommost separation internal structure comprises a first series of suitably equally spaced double flow trays. A first double flow tray. The exhaust tube tray functions simultaneously with the recovery tray from which the condensate (high-boiling fraction) is continuously recovered and guided to the quenching device 1 or the bottom space as part of the quenching liquid 1. The first series of double flow trays terminates in a second exhaust stack tray (collection tray). From this second collection tray, crude acrylic acid, preferably having a purity of 90 or 95% by weight, is continuously collected as a medium boiling fraction at the first side outlet.

  The crude acrylic acid is sent to a further distillation (rectification) and / or further crystallization purification stage, and at least one of the bottom liquid and / or mother liquor obtained during the distillation (rectification) and / or crystallization process. Will be returned to the condensation column located below the first side outlet and above the first collection tray. This return is preferably performed while adjusting the heat. In other words, the returned cold mother liquor is recovered from the condensing column and is passed through one side to cool the crude acrylic acid further purified by crystallization induced on the other side of the heat exchanger. Or it is induced through a plurality of cascaded indirect heat exchangers (eg, helical heat exchangers). At the same time, this heats the mother liquor. For this purpose, it is preferred to use two plate heat exchangers connected in cascade.

  If appropriate, the recovered crude acrylic acid (as a medium boiling fraction) will be sent for crystallization for further purification. The crystallization process used is basically limited. Crystallization can be carried out continuously or batchwise in one or more stages until the desired purity is achieved.

  If necessary, it is advantageously possible to add water to the crude acrylic acid to be purified before crystallization (generally this is 20% by weight, based on the amount of acrylic acid present). Or up to 10% by weight, usually up to 5% by weight water). When the content of the aldehyde or other secondary component is large, the aldehyde in this case can perform the function of water, so that the addition of water can be omitted. Particularly advantageously according to the invention, the water is added in the form of acidic water. This improves the yield of glacial acrylic acid.

  Even when acidic water is added to the crude acrylic acid as described above (this measure also leads to an improvement in the yield of acrylic acid), the most stringent requirements (purity of 98% by mass or more) are satisfied (for example, acrylic acid). It is surprising that esterified grade acrylic acid (to produce n-butyl, 2-ethylhexyl acrylate, methyl acrylate and ethyl acrylate) can be obtained by a single crystallization step. If appropriate, this crystallization stage is carried out as described in DE-A 19924532, column 10 or DE-A 10223058, Example 1 (for example in the cooling-disk crystallizer described in WO 2006/111565). Performed as suspension crystallization. The acrylic acid crystal formed by suspension crystallization has a cubic or cubic outer shape. The length (L) to thickness (D) ratio is typically in the range of L: D = 1: 1 to L: D = 6: 1, preferably in the range of 1: 1 to 4: 1. Preferably it is in the range of 1.5: 1 to 3.5: 1. The crystal thickness D is typically in the range of 20 to 600 μm, often 50 to 300 μm. The length L of the crystal is typically in the range of 50 to 1500 μm, often 200 to 800 μm. In the case of acrylic acid suitable for esterification, suspended crystals can be removed from the residual mother liquor on a centrifuge (eg a 2 or 3 stage pusher centrifuge), in which case the removed crystals are advantageously Is washed by centrifugation with molten pure crystals. Suspended crystals are produced in a washing column, for example a melt washing column (for example a column according to WO 01/77056 or DE-A 10156016 or DE-A 10223058, or WO 2006/111565, WO 04/35514, WO 03/41833, WO 02/09833, WO 03/41832, DE-A10036881, WO02 / 55469 and WO03 / 78378), when separated from the remaining mother liquor, superabsorbent grade acrylic acid (purity 99.7-99.9% by weight or more) by a single crystallization stage. It is also possible to obtain acrylic acid suitable for producing superabsorbent or other polyacrylates. In this case, all of the mother liquor to be removed is appropriately returned to the condensation column.

  However, the crystallization can also be carried out as a fractional falling film crystallization as recommended in EP-A 616998. This can include, for example, two or more (eg 2 to 4) purification steps (a suitable falling film crystallizer in this situation is, for example, 10 to 15 m long) 1000 to 1400 crystallization tubes having an outer diameter of 50 to 100 mm can be included, and the mother liquor removed in the higher purification stage can be returned to one of the previous purification stages. The mother liquor removed in the purification step is advantageously completely returned to the condensation column, instead of returning to one of the previous purification steps, all the mother liquor from the individual purification steps is returned to the condensation column. The pure product of the penultimate purification stage can be fed completely or partially to the last purification stage, and if a partial feed is performed, the remaining amount is taken from the last purification stage. Pure product Universally mixed later will get immediate final product that can be used.

  As appropriate, according to the present invention, a portion of the crude acrylic acid recovered through the first side flow outlet is supplied to the double flow tray located below the associated recovery tray. In some cases, the mother liquor returned to the condensation is also generally supplied to this tray. Prior to feeding, the mother liquor will generally be heated while adjusting the heat to a temperature substantially corresponding to the recovery temperature of the crude acrylic acid, as already described.

  Another portion of the crude acrylic acid recovered via the first side outlet is advantageously heated by indirect heat exchange by 10 to 15 ° C. and above the recovery point, preferably the first subsequent It will be returned to the agglomeration column just below the double flow tray. This measure has a positive effect on the acetic acid content of the recovered crude acrylic acid.

  Above the second collection tray is a second series of equally spaced double flow trays followed by hydraulically sealed alternating current mass transfer trays (e.g., a Thormann tray, Or a modified Thormann tray as described in DE-A 10243625. The top dual flow tray is optionally modified as a distribution tray. In other words, it has an overflow channel including, for example, a sawtooth overflow.

  The first Thormann tray from the bottom is a tray in which the liquid flowing from the tray flows through six downcomers configured as tubes from the viewpoint of application as appropriate. These tubes are hydraulically sealed from the gas space of the second double flow tray from the bottom. The height of the weirs of the six downcomers is reduced in the flow direction of the AC tray as appropriate from the viewpoint of application. Advantageously, the hydraulic seal has an empty orifice with the impingement plate. The outflow tube is preferably distributed evenly over the latter half of the tray cross section (opposite to the side fed to the tray), more preferably the last third part.

  The hydraulic seal is cup-shaped with an inclined overflow dam (45 ° C.). The AC mass transfer tray ends with a third exhaust stack tray (collection tray).

  A double flow valve tray is preferably disposed above the third collection tray. Valve trays and valve trays that can be used in accordance with the present invention include, for example, Technology Fortschrittsberichte [Technical progre sport rations. Volume 61; 96 to 138 can be found. They are characterized by providing a flow orifice that substantially corresponds to a particular load for steam flowing over a wide load range. According to the invention it is preferred to use a ballast tray. In other words, a cage whose orifice is closed by mass is placed at the orifice of the tray. According to the invention, the VV12 valve of Stahl (Viernheim, Germany) is particularly preferred. In effect, water and components that are less volatile than water condense in the valve tray space. The resulting condensate is acidic water.

  Acidic water is continuously collected from the third collection tray at the second side outlet. A portion of the recovered acidic water is recovered in the condensation column up to the uppermost tray of the cross-flow mass transfer tray. Another portion of the recovered acidic water is cooled by indirect heat exchange, divided in an appropriate manner, and returned to the condensation column as well. The cooled portion is returned to the condensation column to the topmost valve tray (at a temperature of 15 to 25 ° C., preferably 20 to 25 ° C.), the other cooled portion is connected to the third collection tray and the topmost tray. Are returned (at a temperature of 20 to 35 ° C., preferably 25 to 30 ° C.) to the valve trays arranged at approximately equal intervals between the two valve trays. According to the present invention, the amount of acrylic acid present can be removed from the remaining amount of acidic water recovered.

In order to convert a C ₃ precursor stored in liquid form, such as propylene, into the gas phase for heterogeneous catalytic gas phase oxidation, an appropriate portion of acidic water is used in an evaporator of the C ₃ precursor (eg, By being guided through a propylene evaporator, some kind of cooling (which can be performed by one or more indirect heat exchangers connected in cascade) occurs.

  Components that are more volatile than water are drawn into the top of the condensing column in the form of a gas as a residual gas (or residual gas mixture) and are usually at least partly returned to the gas phase partial oxidation as a dilution gas (circulating gas). Is done. In order to prevent condensation in the circulating gas compressor, the residual gas mixture is preheated by indirect heat exchange. The portion of the residual gas mixture that is not circulated is usually sent to incineration. A portion of the residual gas mixture (preferably compressed) is advantageously used as a stripping gas for removing acrylic acid from the extract and the bottom liquid of the condensation column, as already described. Advantageously, the gas phase partial oxidation is carried out with excess molecular oxygen, so that when the residual gas mixture is used as the stripping gas, the residual gas mixture and the first and second stripping gases Contains molecular oxygen.

  For the purpose of polymerization prevention, a p-methoxyphenol (= MEHQ) solution of acrylic acid or (preferably according to the invention) a MEHQ melt and (in both cases) further a phenothiazine solution of acrylic acid are hydraulically sealed. It is supplied to the uppermost tray of the AC mass transfer tray. The acrylic acid used is preferably pure acrylic acid obtained by further purification of the recovered crude acrylic acid. For example, glacial acrylic acid (pure product) obtained in the course of further crystallization purification can be used. This solution can also be used as appropriate for the stabilization of pure products.

  In addition, a pure product phenothiazine (= PTZ) solution is fed approximately in the middle of the column cross-section containing the hydraulically sealed mass transfer tray.

  Basically, the formation of acidic water can be carried out, for example, other than in the first condensation column (see DE-A 10235847). In this case, the downstream space (second column) substantially free of internal structures (this space can also be integrated into the condensation column for the purpose of acid water formation; Usually separated from the uppermost separated internal structure in the condensing column by, for example, an exhaust stack) or in a space containing the internal structure, by performing direct cooling appropriately with quench liquid 2 later, the first Water is substantially condensed from the low boiling gas feed stream exiting from the top of the condensation column. The resulting condensate is consequently acidic water. A portion of the acidic water is then preferably returned to improve the separation performance at the top of the first condensation column. A further part of the acid water is indirectly cooled with an external heat exchanger and used as quench liquid 2, and then acrylic acid can be extracted from the remaining acid water according to the invention. The constituents of the low-boiling feed stream that are more volatile than water are then usually returned at least partially as a recycle gas to the gas phase partial oxidation, or form a residual gas that is used as a stripping gas.

  Optionally, the double flow tray in a preferred variant of the process according to the invention extends in the condensation column to near the cross section of the condensation column, from which the acrylic acid content of the reflux towards the top of the column is the mass of the reflux It is 90 mass% or less with respect to.

  The number of double flow trays is generally 25 to 45, as already mentioned, in the preferred variant of the fractional condensation described. Their orifice ratio is about 12 to 25%. The double flow tray preferably has a circular hole with a uniform circular diameter as a flow path. The latter is about 10 to 20 mm. If necessary, the diameter of the holes in the condensation column can be reduced or increased from top to bottom and / or the number of holes can be reduced or increased (eg, the hole diameter can be a constant 14 mm). And the orifice ratio can be increased from 17.4% to 18.3% from top to bottom). However, the number of holes may be constant for all dual flow trays. In addition, the circular holes are preferably arranged uniformly with a strict triangular pitch relative to the individual double-flow tray (see DE-A 10302219).

  Further, in the condensation column, the perforation burr of the flow path orifices opened in the double flow tray is preferably facing down (this reduces undesirable polymer formation).

  According to the present invention, it is desirable if the number of double flow trays used in the condensation column corresponds to about 10 to 15 theoretical plates.

  The number of hydraulically sealed alternating mass transfer trays following a double flow tray in a condensation column suitable according to the invention is generally 30 to 50 as already described. Their orifice ratio is about 5 to 25%, preferably 10 to 20% (orifice ratio very generally represents the fraction of the channel cross section in the entire cross section; the AC mass transfer tray used suitably Then, it is generally within the above range as appropriate).

  According to the present invention, it is preferred to use a single flow alternating mass transfer tray.

  In general, the number of suitable alternative hydraulically sealed AC trays for fractionated product gas mixture condensation will be about 10 to 30, often corresponding to 25 theoretical stages.

  The hydraulic seal cross-flow tray and any valve tray used will have at least one downcomer. They can have a single flow or multiple flow, for example a double flow configuration. Even in a single flow configuration, it can have more than one downcomer. In general, the supply shaft of the valve tray is also hydraulically sealed.

  The polymerization prevention of the quench system 1 for the product gas mixture of partial gas phase oxidation is used with the polymerization inhibitor present in the bottom liquid (from the condensation column) used for quenching or for quenching ( It can be carried out with a polymerization inhibitor present in the high-boiling fraction (from the condensation column).

  Again, the reason why the process according to the invention is advantageous is that the yield of crude acrylic acid of substantially the same purity can be increased. All the contents described in this document apply in particular to product gas mixtures obtained by heterogeneous partial oxidation of propylene to acrylic acid (preferably two steps). The preferred variants according to the invention do not limit the overall performance.

  Finally, it should also be emphasized that the first stripping gas and the second stripping gas advantageously contain molecular oxygen.

In addition, in the process according to the present invention, optionally, the boiling point T _s at a pressure of 1 atm is the boiling point of water T _w or more at 1 atm (non-reflux liquid) Another liquid absorbent, a first side It should also be added that it is fed to the concentration column via a feed point located between the flow outlet and its second side flow outlet. Basically, it can be carried out as described, for example, in EP-A 1818324. Useful absorbents may be, for example, relatively high boiling organic liquids. For example, organic liquids recommended as absorbents in DE-A 10336386 and the prior art cited in this document are suitable. In this respect it is understood that it is also possible to use absorbents recommended in EP-A 1818324.

  For example, suitable such absorbents are the high boiling (inert) liquid hydrophobic organic liquids described in EP-A 722926, DE-A 4436243 and DE-A 10336386 (for example dimethyl phthalate, diethyl phthalate and / or Diphyl). ). For example, the boiling point at standard pressure (1 atm) is higher than the boiling point of acrylic acid and does not contain an externally active polar group. Therefore, for example, at least about 70% by mass that cannot form a hydrogen bond. A liquid consisting of molecules. Examples of such absorbents include mixtures of diphenyl ether (70 to 75% by weight) and diphenyl (25 to 30% by weight) called Diphyl®, and 70 to 75% by weight of diphenyl ether and 25 to 30% by weight. And a mixture of 0.1 to 25% by weight of dimethyl phthalate with respect to the mixture.

  It will be appreciated that the absorbent used may be an aqueous liquid. For example, useful aqueous liquids are those recommended in EP-A 1818324. In addition, water is included in these absorbents. The aqueous absorbent used may be the aqueous phase remaining in the extracted form in the acidic water extraction carried out according to the invention.

  Typically, the addition of the absorbent serves the purpose of controlling, in a controlled manner, the proportion of certain secondary components present in the crude acrylic acid derived from the condensation column via the first side-flow outlet. It will be. Thus, the absorbent itself used is advantageous if it contains (or preferably does not contain) these secondary components in a very low mass ratio.

  In the case of using an aqueous absorbent, for example, it is suitable when the individual mass ratio of the secondary component is smaller than the corresponding mass ratio in acidic water.

  In particular, a particularly advantageous method for reducing the content of aldehydes (especially the content of furfural) having the same boiling point as acrylic acid in the crude acrylic acid derived from the condensation column via the first side outlet. It is preferable to supply an aqueous absorbent (for example, water). Basically, the absorbent used may be a mixture of aqueous and organic liquids. Basically, the mixture may be multiphase.

  The absorbent can advantageously be directed to the condensation column at the same height as the reflux. Both can also be guided to the condensation column in the form of a mixture.

  The temperature of the absorbent as it is fed to the condensation column can vary significantly. It may be higher or lower than the reflux temperature. Often, the temperature of the absorbent will be in the range of the reflux liquid temperature ± 20 ° C. The temperature of the reflux liquid and the absorbent is preferably the same.

  When the crude acrylic acid recovered via the first side-flow outlet contains additional parts of absorbent that are additionally used, they are generally those of the crude acrylic acid described in this document. It is removed therefrom by further crystallization purification. Alternatively, the additionally used absorbent is usually obtained in the bottom liquid derived from the agglomeration column and can be reused as the absorbent after being removed therefrom as described above.

  As appropriate, according to the present invention, the mass flow rate of the absorbent guided to the condensation column is 0 relative to the mass flow rate of acrylic acid induced to the condensation column as a component of the product gas mixture fed to the condensation column. To 30%.

  Based on the longitudinal section of the condensation column disposed between the column from the first side outlet and the second side outlet, the supply of absorbent to the condensation column is It is guided to the one-third portion from the top of the longitudinal section.

Instead of applying further crystallization purification of the crude acrylic acid described in this document to the crude acrylic acid obtained according to the present invention, the procedure described in DE-A 10336386 (in particular from claim 16 of this document). 20 can be applied to crude acrylic acid obtained by further crystallization purification as described in No. 20. The further crystallization purification can also be applied to any desired mixture of the two crude acrylic acids. The procedure can be used, for example, to remove acrylic acid from the product gas mixture of a heterogeneously catalyzed partial gas phase oxidation (optionally performed in parallel) of a C ₃ precursor of acrylic acid and It will be used when the removal method according to DE-A 10336386 is carried out in parallel. The implementation of the inventive fractional condensation for acrylic acid removal must be temporarily stopped, but if the crude acrylic acid obtained according to DE-A 10336386 has to be fed to this further crystallization purification, Partial feed stream of mother liquor obtained from removal of suspended crystals from acrylic acid crystal suspension (5 to 30%, typically based on the mass flow rate of crude acrylic acid fed to further crystallization purification 10-20% mass flow (removed crystals must meet specifications); the residual feed stream of the mother liquor is usually crystalline (along with the mass flow of crude acrylic acid (in the form of a mixture)) Is stored in the storage tank in the middle (the supply to the storage tank may be continuous or periodic) and after the fractional condensation has been resumed, By To a fractional condensation in the form of a mixture with the mother liquor (which later becomes the mass flow rate of the mother liquor obtained according to the invention and is from 0 to 20%, preferably from 0 to 10% with respect to the mass flow rate returned to the fractional condensation. Will be returned gradually (with a mass flow of%). For this purpose, the recovery from the storage tank may be continuous or periodic. It is usually recovered so that the crystals obtained meet specifications (the crystallization design capability generally exceeds the design capability of crude acrylic acid production; however, in short, the crystallization design Further crystallization purification beyond capacity is possible).

  Accordingly, the present invention particularly includes the following embodiments.

1. A product gas mixture comprising acrylic acid, water vapor and secondary components is obtained by heterogeneously catalyzed gas phase partial oxidation of a C ₃ precursor of at least one acrylic acid with molecular oxygen on a solid catalyst at high temperature to produce acrylic The temperature of the product gas mixture containing acid, water vapor and secondary components is optionally reduced by direct and / or indirect cooling, and then the product gas mixture containing acrylic acid, water vapor and secondary components is separated by internal structure Into a condensing column equipped with a product, and fractionally condensed by raising in the condensing column, and water and water through a first side outlet located above the feed point of the product gas mixture to the condensing column A crude acrylic acid containing a secondary component in a diluted form is derived as a target product, and is obtained via a second liquid phase outlet located above the first side outlet. Deriving acidic water further containing phosphoric acid and secondary components, deriving a residual gas mixture containing secondary components having a lower boiling point than water at the top of the condensing column, from acrylic acid and acrylic acid from the bottom space of the condensing column A bottom liquid further comprising subsequent products and secondary products having a high boiling point is withdrawn from the condensing column and a portion of the recovered acidic water is returned to the condensing column as is and / or as a reflux after cooling; In a process for producing acrylic acid, optionally in which at least one further thermal separation process is applied to the crude acrylic acid for the purpose of further purification thereof, the acrylic present in it in at least a partial amount of acidic water not returned to the condensation column. The acid is absorbed into the organic solvent from the acidic water by extraction with an organic solvent to form an organic extract containing acrylic acid, and then Removing acrylic acid from the organic extract using at least one thermal separation method, returning the acrylic acid removed from the extract to the condensation column, or sending it to further purification of the crude acrylic acid, and / or metal A method for producing acrylic acid, comprising absorbing an aqueous solution of hydroxide.

2. The method according to embodiment 1, characterized in that the C ₃ precursor is propylene or acrolein or a mixture of propylene and acrolein.

  3. At least 25% by mass of the acidic water that has not been returned to the condensing column is absorbed into the organic solvent from the acidic water by extraction with an organic solvent to extract an organic extract containing acrylic acid. Forming and then removing the acrylic acid from the organic extract using at least one thermal separation method, returning the acrylic acid removed from the extract to a condensation column, or sending it to further purification of the crude acrylic acid, and The method according to Embodiment 1 or 2, wherein the method is absorbed in an aqueous solution of metal hydroxide.

  4). 4. A process according to any of embodiments 1 to 3, characterized in that the extraction of acrylic acid from acidic water is carried out in an extraction column comprising structural fillers and / or sieve trays as separated internal structures.

  5. Introduce organic solvent to the top of the extraction column, introduce acidic water to the bottom region of the extraction column, raise the organic solvent as a dispersed phase in the continuous acidic water phase, or introduce acidic water to the top of the extraction column And the organic solvent is introduced into the bottom region of the extraction column and the organic solvent is raised as a dispersed phase in a continuous acidic water phase.

  6). Embodiment 1, characterized in that the organic solvent comprises at least one ester of an aliphatic or aromatic monocarboxylic acid containing 5 to 20 carbon atoms and an alcohol having 1 to 8 carbon atoms. 6. The method according to any one of 5 to 5.

  7). From embodiment 1, characterized in that the organic solvent comprises at least one diester of an aliphatic or aromatic dicarboxylic acid containing 5 to 20 carbon atoms and an alcohol having 1 to 8 carbon atoms The method according to any one of 5 to 5.

  8). Method according to embodiment 1 to 5 or embodiment 7, characterized in that the organic solvent is dimethyl phthalate, diethyl phthalate, dimethyl isophthalate, diethyl isophthalate, dimethyl terephthalate and / or diethyl terephthalate .

  9. The method according to any one of Embodiments 1 to 8, wherein the boiling point of the organic solvent at atmospheric pressure is 200 ° C. or higher.

  10. Acrylic acid is removed from the organic extract by stripping with a first stripping gas, the first stripping gas loaded with acrylic acid is returned to the condensation column and / or the first loaded 10. The method according to any of embodiments 1 to 9, characterized in that acrylic acid present in the stripping gas is absorbed in an aqueous solution of metal hydroxide.

  11. Acrylic acid is removed from the organic extract by stripping using a first stripping gas, and the resulting first stripping gas loaded with acrylic acid is derived from the condensation column as a second stripping gas. The acrylic acid still present in the bottom liquid is stripped off and the resulting second stripping gas loaded with acrylic acid is returned to the condensation column and / or into the second stripping gas. 10. The method according to any of embodiments 1 to 9, characterized in that the acrylic acid present is absorbed in an aqueous solution of metal hydroxide.

  12 Embodiment, characterized in that stripping of the bottom liquid derived from the condensation column is carried out in a stripping column with a separation internal structure, the temperature of the bottom of the stripping column being 150 to 190 ° C 11. The method according to 11.

  13. A second stripping gas loaded with acrylic acid is applied to the countercurrent rectification and returned to the condensation column before loading with acrylic acid and / or the acrylic acid present therein is converted to an aqueous solution of metal hydroxide. The method according to embodiment 11 or 12, characterized in that it is absorbed.

14 The first stripping gas is air, characterized in that it is a N _2, CO ₂ and / or water vapor, the method according to any of embodiments 10 to 13 to be used.

  15. 14. A method according to any of embodiments 10 to 13, characterized in that the first stripping gas used is a residual gas mixture.

  16. Embodiment 16. The process according to any of embodiments 1 to 15, characterized in that the crude acrylic acid is further purified by crystallization.

  17. Embodiment 17. The method according to embodiment 16, characterized in that a portion of the acidic water that has not been returned to the condensation column is added to the crude acrylic acid before further crystallization purification.

  18. 18. A process according to embodiment 16 or 17, characterized in that further crystallization purification of the crude acrylic acid or its mixture with acidic water is carried out by suspension crystallization.

  19. The method according to embodiment 18, further comprising using a wash column to separate the mother liquor remaining in suspension crystallization from the formed suspension crystals.

  20. Embodiment 20. The method according to any of embodiments 17 to 19, characterized in that it is followed by a method of free radical polymerization that polymerizes molten acrylic acid crystals and / or metal salts thereof.

  21. Embodiment 10, 11, characterized in that it is followed by a method of free radical polymerization of polymerizing acrylic acid absorbed in the aqueous solution of metal hydroxide from the loaded first and / or second stripping gas. 14. The method according to any one of 13 to 13.

  22. The method according to any of embodiments 1 to 21, characterized in that a partial amount of the residual gas mixture is returned to the gas phase partial oxidation as a circulating gas.

23. Embodiment 23. The method according to any of embodiments 1 to 22, characterized in that the organic solvent has a mass density that differs from the mass density of water by 25 kg / m ³ or more under extraction conditions.

24. The method according to any of embodiments 1 to 23, characterized in that the aqueous solution of metal hydroxide comprises NaOH, KOH, Ca (OH) ₂ and / or Mg (OH) ₂ in dissolved form. .

25.1 Supply a liquid absorbent whose boiling point T _s at 1 atm is greater than or equal to the boiling point T _w of water at 1 atm, located between its first side outlet and its second side outlet 25. Method according to any of embodiments 1 to 24, characterized in that it is fed to the condensation column via a point.

  US Provisional Patent Application No. 60 / 886,671 filed Jan. 26, 2007 and No. 60/987619 filed Nov. 16, 2007 are hereby incorporated by reference in the present application. Many variations from the present invention are possible with respect to the above teachings. Thus, it is possible to assume that the invention can be practiced otherwise than as described herein within the scope of the appended claims.

Examples and Comparative Examples Example 1 (to explain steady state)
Two-stage heterogeneous catalytic gas phase partial oxidation of propylene of chemical grade purity in three parallel tandem reactor series yields a product gas mixture having a temperature of 270 ° C. and a pressure of 1.5 bar, and the following composition: .

10.141% by weight acrylic acid,
0.2609% by weight of acetic acid,
4.6513% by weight of water,
0.0251% by weight of formic acid,
0.0851% by weight formaldehyde,
0.1052 mass% acrolein,
0.0024% by weight of propionic acid,
0.0028 mass% furfural,
0.0012% by weight of allyl acrylate,
0.0013 wt% allyl formate,
0.0032% by weight of benzaldehyde,
0.01151% by weight of maleic anhydride,
0.0096% by weight benzoic acid,
0.0126% by weight of phthalic anhydride,
2.0334 mass% CO ₂ ,
0.6604 mass% CO,
0.6259% by weight of propane,
0.1459% by weight of propylene,
2.3772 mass% of O _2, and 78.5670 wt% of N _2.

  The reaction gas mixture fed in each case to the three tandem reactor series is in each case a mixture of circulating gas and chemical grade propylene, into which the primary air is metered later. Mixing is carried out in each case with a stationary mixer.

  In each case, secondary air is also fed between the first stage reactor and the second stage reactor.

  When the quantity ratio is adjusted in accordance with the operating state of a specific reactor series, the following is performed upstream of the first reactor.

A) Circulating gas = 25339 kg / h
Chemical grade propylene = 3481 kg / h
Air = 17791 kg / h
B) Circulating gas = 38341 kg / h
Chemical grade propylene = 4035 kg / h
Air = 19561 kg / h
C) Circulating gas = 30874 kg / h
Chemical grade propylene = 4689 kg / h
Air = 23936kg / h
The main contents of the three reaction gas mixture feed streams obtained are as follows.

A) 10.2909 mass% O ₂ ,
1.2926 mass% CO ₂ ,
0.4136 mass% CO,
0.6533% by weight of propane,
7.2976 mass% propylene,
1.5383 wt% H ₂ O and 78.2693 wt% N ₂ ;
B) 8.9782% by weight O ₂ ,
1.4660% by weight CO ₂ ,
0.4710 mass% CO,
0.6744% by weight propane,
6.3907% by weight of propylene,
1.6626% by weight H ₂ O, and 80.789% by weight N ₂ ; and C) 10.6981% by weight O ₂ ,
1.2357% by weight CO ₂ ,
0.3948 mass% CO,
0.6500 mass% propane,
7.6922% by weight of propylene,
1.4968% by weight of H ₂ O and 77.5992% by weight of N ₂ .

  The product gas mixture (177184 kg / h) is cooled by direct cooling to a temperature of 107.3 ° C. with a spray cooler operating in parallel flow (quenching 1).

  The liquid used to directly cool the product gas mixture (quenching liquid 1) is a bottom liquid recovered from the bottom of the concentration column described below and a small amount (251 kg / h) recovered from the quench circuit 0. Part of the mixture 1 with the condensate.

  The contents of this mixture 1 (temperature = 104.9 ° C.) are as follows.

64.6170% by weight of acrylic acid,
0.3883 wt% acetic acid,
1.0300% by weight of water,
0.0132% by weight formic acid,
0.0011% by weight formaldehyde,
0.0083 mass% acrolein,
0.0384% by weight of propionic acid,
0.3331 wt% furfural,
0.0016 wt% allyl acrylate,
0.0008% by weight of allyl formate,
0.1609% by weight of benzaldehyde,
4.401 mass% maleic anhydride,
0.3976% by weight of benzoic acid,
0.5228% by weight of phthalic anhydride,
12.8970 mass% diacrylic acid,
13.9178% by weight of polyacrylic acid 0.1258% by weight of phenothiazine,
0.4408 wt% MEHQ,
0.7052% by weight of other high-boiling components, and 0.0002% by weight of oxygen.
(The diacrylic acid and polyacrylic acid are Michael adducts)
In order to directly cool the product gas mixture, an amount of only 458 m ³ / h is fed to the spray cooler of the quench circuit 1 at this temperature. By supplying 3137 kg / h to the second stripping column and 1195 kg / h to the quenching circuit 0, it is desirable that the quenching liquid 0 and the refluxing liquid be guided to the uppermost tray of the second stripping column. No polymerisation.

  A mixture of the product gas mixture cooled to 107.3 ° C. and the non-evaporable quench liquid 1 that provides direct cooling is introduced directly to the bottom of the condensation column. The pressure in the bottom space and quench 1 is 1.50 bar.

  The inner diameter of the condensation column in the area of the Thromann tray is 6.5 m or 6.0 m.

  3137 kg of mixture 1 is fed as feed to a second stripping column containing 50 double-flow trays as the separation internal structure. Just like the condensation column, the second stripping column is insulated from the environment. The inner diameter of the second stripping column is a constant 2.4 m across all double flow trays. Double flow trays are placed in a second stripping column at regular intervals (400 mm). Their orifice ratio is a constant 12%. From bottom to top, the hole diameter of the first eight double-flow trays is a constant 14 mm (hole diameter corresponding to the exact triangular pitch; the distance from the hole center to the hole center is equal to 26 mm ), The hole diameter of all subsequent dual-flow trays is a constant 14 mm (also corresponding to the exact triangular pitch; the distance from the hole center to the hole center is equal to 25 mm).

  3137 kg / h of mixture 1 is fed to the eighth double-flow tray (from the bottom) at a temperature of 105.2 ° C.

  Energy is supplied to the second stripping column using an external forced circulation triple flow tube bundle flash evaporator (Grundeperation chemister Verhahntechnik, 4th edition, Steinkoffff Verlag Dressen, 1974, p. 434). A 271650 kg / h bottom liquid, recovered from the bottom of the second stripping column and having a temperature of 151.7 ° C. and a pressure of 1.655 bar, is fed thereto and has the following contents.

2.0713 wt% acrylic acid,
0.2201 wt% acetic acid,
0.2608% by weight of water,
0.0094% by weight of formic acid,
0.0001% by weight formaldehyde,
0.0036 mass% acrolein,
0.0017% by weight of propionic acid,
0.0001 mass% furfural,
0.0008% by weight of allyl formate,
0.5731% by weight of diethyl phthalate,
0.7638% by weight of benzaldehyde,
23.2670% by weight maleic anhydride,
2.2693% by weight benzoic acid,
2.9848% by weight of phthalic anhydride,
38.3947 mass% diacrylic acid,
21.8422% by weight polyacrylic acid 0.7185% by weight phenothiazine,
2.5151% by weight of MEHQ,
4.6615% by weight of other high-boiling components, and 0.0001% by weight of oxygen.
(The diacrylic acid and polyacrylic acid are Michael adducts)
The heat medium guided through the space surrounding the heat exchanger tube (guided by meandering with a suitable deflection plate) is water vapor (pressure = 16 bar). As it flows through the heat exchanger tube, the bottom liquid is heated and the entire amount of 270900 kg / h induced through the heat exchanger is returned to the bottom of the second stripping column at a temperature of 158.6 ° C. The total 750 kg / h of bottom liquid guided through the heat exchanger is branched, degassed, diluted with methanol and sent to residue incineration.

  In addition, the first loaded gas of 17424 kg / h derived from the top of the first stripping column is in the second stripping column at a temperature of 82.2 ° C. and a pressure of about 2.50 bar. Supplied at the bottom and has the following contents.

3. 3997 mass% acrylic acid,
1.1198% by weight acetic acid,
4. 1030% by weight of water,
0.0622% by weight of formic acid,
0.0321 mass% formaldehyde,
0.1181% by weight of acrolein,
0.0030% by weight of propionic acid,
0.0006 mass% furfural,
0.0139% by weight of allyl formate,
0.0247% by weight diethyl phthalate,
2.5667 wt% O ₂ ,
2. 1951 mass% CO ₂ ,
0.7129 mass% CO,
0.6756% by weight propane,
0.1575% by weight propylene, and 84.8133% by weight N ₂ .

  In a spray cooler operating in parallel flow, the gas in the second loading state is guided from the top of the second stripping column in an amount of 25931 kg / h (temperature = 96.7 ° C., pressure = 1.58 bar). Is cooled to a temperature of 80.9 ° C. by direct cooling (quenching 0) and partially condensed.

  The gas mixture remaining in direct cooling is returned to the bottom space of the condensation column (not immersed) at a pressure of 1.58 bar in an amount of 20755 kg / h and has the following contents.

18.5793% by weight acrylic acid,
0.9849% by weight acetic acid,
3.6185% water by weight,
0.0571% by weight of formic acid,
0.0271% by weight formaldehyde,
0.1006% by weight of acrolein,
0.0099% by weight of propionic acid,
0.0436 mass% furfural,
0.0003 wt% allyl acrylate,
0.0117% by weight of allyl formate,
0.0053 mass% benzaldehyde,
0.0642% by weight of maleic anhydride,
0.0001% by weight benzoic acid,
0.0001% by weight diacrylic acid,
0.0001 mass% MEHQ,
2.1548 mass% of O _2,
1.8428 wt% CO ₂ ,
0.5985% CO by weight,
0.5672% by weight of propane,
0.1322 wt% of propylene, and 71.2018 wt% of N _2.

  The quench liquid 0 used is a 32956 kg / h mixture of 1195 kg / h of mixture 1 and 37761 kg / h of condensate formed by direct cooling of quench 0 (for this purpose, this part of the mixture is indirect heat exchange Cool from 80.9 ° C. to 40.1 ° C. (type: spiral heat exchanger; heat exchange for water). 6020 kg / h of this mixture having a temperature of 80.9 ° C. is led as reflux to the uppermost tray of the second stripping column.

  The mixture has the following contents.

87.9166 mass% acrylic acid,
2.3313 wt% acetic acid,
2.6130 wt% H ₂ O,
0.0688% by weight of formic acid,
0.0010% by weight formaldehyde,
0.0151% by weight of acrolein,
0.0480% by weight of propionic acid,
0.2980% by weight of furfural,
0.0008% by weight of allyl acrylate,
0.0096 wt% allyl formate,
0.0568% by weight of benzaldehyde,
1. 1099% by weight of maleic anhydride,
0.0761% by weight of benzoic acid,
0.1000% by weight of phthalic anhydride,
2. 4592% by weight of diacrylic acid,
2.6522% by weight polyacrylic acid 0.0240% by weight phenothiazine,
0.0847 mass% MEHQ,
0.1345% by weight of other high-boiling components, and 0.0004% by weight of oxygen.
(The diacrylic acid and polyacrylic acid are Michael adducts)
A centrifugal drop separator is integrated into the bottom space of the condensation column to prevent bottom liquid droplets from being carried upwards from the bottom space.

  The bottom space of the condensing column is, as already mentioned, the first collection tray (about 16 distributed roofs) at a column height of 7.80 m (calculated from the column bottom as well as all heights). An exhaust tube tray having an attached exhaust tube; an exhaust tube diameter: 600 mm; an exhaust tube height: 1 m) is coupled at a column height of 7.80 m.

  The collection tray has a double wall configuration with a 2 ° inward tilt and a central draw cup and draw nozzles (DN-200). The free gas cross section is about 30%.

  A high boiling fraction of 88579 kg / h is guided from this first collection tray to the bottom space located below the first collection tray.

  The high-boiling fraction has the following contents at a temperature of 99.8 ° C. and a pressure of about 1.50 bar.

94.6665% by weight acrylic acid,
0.5402 wt% acetic acid,
1.3577 mass% water,
0.0160% by weight of formic acid,
0.0014% by weight formaldehyde,
0.0071 mass% acrolein,
0.0577% by weight of propionic acid,
0.3814 mass% furfural,
0.0023% by weight of allyl acrylate,
0.0010% by weight of allyl formate,
0.1279% by weight of benzaldehyde,
2. 1925% by weight maleic anhydride,
0.0051 mass% benzoic acid,
0.0046% by weight of phthalic anhydride,
0.6008% by weight of diacrylic acid,
0.0061% by weight of phenothiazine,
0.0314 wt% of MEHQ, and 0.0002 wt% O _2.

  The bottom temperature is 104.9 ° C. and the bottom pressure (at the liquid level) is 1.51 bar.

  First, the first tray of the 15 double-flow trays is disposed 2.0 m above the first collection tray. These double flow trays (with a constant number of holes of 33678) are mounted at equal intervals with a tray interval of 380 mm. The flow path orifice consists of a circular orifice with a constant diameter of 14 mm and the perforated burr is pointing down in the separation column. The arrangement of the centers of the circular channels follows a strict triangular pitch. The shortest distance between the two circles is 24.5 mm.

  The 15 double flow trays function as distribution trays. For this purpose, the column wall consists of two insertion tubes (45 mm diameter) per insertion tube (diameter: 15 mm) between the second collection tray and the 15th double flow tray. DN˜150).

  The crude acrylic acid and mother liquor are returned to the condensation column via the insertion tube.

  The first series of double-flow trays is a second collection tray (16 stacks with 16 nearly evenly distributed roof stacks) mounted 1.50 m above the last double-flow tray. Tray; stack height is about 1.70 m, central draw cup with draw nozzle (DN-250), free gas cross section is 30%).

  From the second collection tray, crude acrylic acid at a temperature of 97.1 ° C. is continuously collected as a first side outlet at 1.48 bar and has the following contents.

96.7716% by weight acrylic acid,
0.8253 wt% acetic acid,
1.6640% water by weight,
0.0213% by weight of formic acid,
0.0018% by weight formaldehyde,
0.0070 mass% acrolein,
0.0681% by weight of propionic acid,
0.1642 mass% furfural,
0.0027% by weight allyl acrylate,
0.0012% by weight of allyl formate,
0.0164% by weight of benzaldehyde,
0.1052% by weight of maleic anhydride,
0.3278% by weight of diacrylic acid,
0.0050% by weight of phenothiazine,
0.0180 wt% MEHQ, and 0.0002 wt% oxygen.

  The recovered acrylic acid and water vapor obtained by further crystallization purification of the recovered crude acrylic acid from 18474 kg / h of the crude acrylic acid recovered from the second recovery tray is used as the heating medium (72716 kg / h). The mother liquor heated to 90 ° C. by indirect heat exchange is returned to the double-flow tray located under the second collection tray of the aggregation column and the condensation column via the insertion tray directly below the second collection tray. .

  When 89.303 kg / h of crude acrylic acid recovered from the second recovery tray is cooled to 29 ° C. by multistage indirect heat exchange (especially while adjusting heat to the mother liquor returned to the condensing column), To buffer the tank farm as it is. Then, 1204 kg / h of acidic water recovered from the second side outlet of the condensation column is added to the cooled crude acrylic acid.

  Acidic water has the following contents.

10.7677% by weight of acrylic acid,
6.4390% by weight acetic acid,
79.5610% by weight of water,
0.7038% by weight of formic acid,
2.4712 mass% formaldehyde,
0.0132 mass% acrolein,
0.0082% by weight of propionic acid,
0.0013 mass% furfural,
0.0331 wt% allyl formate,
0.0001 wt% MEHQ, and 0.0013 wt% oxygen.

The resulting mixture is cooled to 16 ° C. by another indirect heat exchange (heat exchange to cooling brine (water / glycol mixture; 25-35 wt% glycol and 65-75 wt% water)) and then in parallel Distribute to 2 to 3 cooling disk crystallizers operating (see WO 2006/111565). Each of these is a series of 24 wiped circular cooling plates (with refrigerant (a mixture of water and glycol; glycol content = 25 to 35% by mass) circulated inside each of them at equal intervals of 30 ± 1 cm) Including troughs arranged in a suspended manner (plate diameter = 3.3 m). A specific refrigerant is passed from one cooling disk to the next cooling disk, countercurrently to the crystallization mixture, and flowing to a specific crystallizer. In other words, a particular refrigerant is divided into two parallel flow forms and guided through a particular crystallizer cooling plate. One flow passes through even numbered cooling plates and the other flow passes through odd numbered cooling plates (cooling disk numbers start at 1 depending on the flow direction). The specific amount of refrigerant per crystallizer is 180-220 t / h (metric tons) in total, ie 90-110 t / h for one flow. The pressure drop per cooling disk is 60 to 100 mbar. The inlet temperature of the refrigerant (brine) is +2.5 to + 3 ° C. The outlet temperature is 2.5 ° C. higher than that. The thickness of the cooling surface manufactured from stainless steel is 4 mm. The heat transfer coefficient on the brine side is about 1500 to 2500 W / (m ² · K). The heat transfer coefficient is usually 380 to 420 W / (m ² · K). The specific cooling performance is 1.5 ± 0.2 kW per 1 m ² of the cooling surface. The wiping of the cooling plate suppresses the formation of the crystal layer. Crude acrylic acid with a high water content is continuously guided from the rear to the front (through a pump or under overflow control) through a specific crystallizer. At the same time, the single-phase crude acrylic acid with a high water content is concentrated (residence time 2.5 hours), and is converted into a solid phase having a temperature of 7 to 8.5 ° C. and a solid content at the outlet of about 25% by mass. A two-phase suspension containing acid crystals results. The mass density of the suspension is typically 1110 to 1115 kg / m ³ . The wiper speed is 5 to 6 revolutions per minute. The wiper is driven and the axis passing through the center of the cooling disk is sealed with a flushed stuffing box filler (Teflon or graphite filler thread, wash rate = several to tens of liters per hour per seal).

  A hollow profile (eg, a tube in the simplest embodiment) is fitted (eg, welded) around the non-wiping cooling disk, and (crystallized) by a second heating medium (eg, a water / glycol mixture as well). Heating to a temperature above the crystallization temperature; usually from 8 to 20 ° C., preferably from 10 to 14 ° C.). A second heat medium flows in parallel to these peripheral heaters.

  In addition, the wiper is preferably divided radially (in four sections). The specific pressure of the installed wiper perpendicular to the cooling surface is 3 to 5 N per cm length of the edge of the active wiper. The wiper material used is high molecular weight polyethylene or ultra high molecular weight polyethylene, for example Multilene® PE1000. In addition to the wiper, the shaft is a paddle that provides improved mixing (between two cooling disks, two appropriately arranged in front of the first and last cooling disks, respectively).

  In order to remove the mother liquor from the suspended crystals at the last part of the particular crystallizer in the direction of suspension transport (preferably above the last cooling disk), EP-A1272453, EP-A1448283, WO03 / 041833, As described in EP-A1305097, DE-A10156016, DE-A102005018702 and DE-A10223058, the suspension is mounted by dipping the tube (appropriately immersed; Through the weir, from which it can flow to a stirred recovery vessel filled with the wash column) to the hydraulic melt wash column. The diameter of the washing column is 1.4 m. The washing column is filled with the crystal suspension using a centrifugal pump (channel wheel type) and the flow rate is preferably controlled by adjusting the pump speed. The controlled feed pump is likewise configured as a centrifugal pump with a regulating valve. Typically, the feed flow control flow employed to condition the wash column is 5 to 60 t / h, usually 8 to 30 t / h. In some cases, it is possible to operate a particular wash column without using a controlled feed stream when the amount of liquid supplied with the suspension is already sufficient to transport the crystal layer. Typical ratios of effective transport differential pressure and effective cleaning differential pressure are 1.1 to 3, usually 1.2 to 1.8. The rotation speed is usually a value of 5 to 10 per minute. The temperature of the melt circuit is usually 13 to 16 ° C. The filtration front is monitored according to DE-A 102005018702, using two pressure drop measurements for different layer lengths set in relation to each other. The cleaning front is controlled by measuring the temperature of the crystal layer.

For control purposes, the total height of the crystal layer is 250 to 1500 mm, usually 600 to 1100 mm. The cleaning front is typically 100 to 200 meters above the blade. A suitable melt circuit pump is a centrifugal pump with a shaft seal (slip-ring seal; double design, septum medium cooled to 15-30 ° C. (water / glycol mixture)) flushed on the product side, or This is a magnet-coupled pump with enhanced sliding bearing flushing. The circulation rate in a specific melt circuit is 10 to 15 m ³ / h per ton of purified crystals scraped from the blade. 200 to 300 ppm by weight MEHQ, or 40 to 70 ppm by weight MEHQ, or 100 to 300 ppm by weight PTZ, to stabilize the melt circuit for later use in a column specific manner . In addition, air, or lean air (nitrogen-air mixture containing up to 6% by volume of oxygen) is introduced into the melt circuit, and the excess amount of the melt before the wash melt enters the wash column (the wash melt into the melt). The fraction not melted) is removed by a gas separator. Thereby, the content of dissolved oxygen in the molten pure product is 5 to 40 ppm by mass.

  [A] To produce esterified grade acrylic acid, it is sufficient to remove suspended crystals using a centrifuge (eg, a 2 or 3 stage pusher centrifuge) instead of a melt wash column. The preferred mesh size is 150 to 300 μm, the available centrifugal acceleration is 500 to 900 g, usually 600 to 800 g, and the preferred stroke speed is 40 to 80 strokes / minute.

  The crystals removed in the second or third stage of centrifugation are preferably washed with 0.15 to 0.3 kg of washing solution per kg of crystals. The temperature of the cleaning liquid is 15 to 30 ° C, preferably 20 to 30 ° C. In order to avoid deposition, the centrifuge solid discharge chute is flushed with a flushing liquid whose temperature is adjusted to 15 to 30 ° C. The flush liquid or the washing liquid is preferably a molten crystal that has been removed and washed using a centrifuge. It is reasonable to maintain the centrifuge casing, suspension supply tube and cleaning liquid supply tube at a temperature of 15 ° C. or higher and 40 ° C. or lower in order to prevent accumulation and incula station. The production system space of the centrifuge is suitably deactivated with nitrogen or a mixture of air and nitrogen. The shaft seal is purged with a gas (eg, nitrogen or a mixture of air and nitrogen) or water.

b) Layer crystallization of two or more (eg 2 to 4) purification stages instead of suspension crystallization (eg falling film crystallization as described in EP-A 616998, or fully internal flow tube) It is also possible to employ a falling film type crystallization). Instead of returning the mother liquor from the next purification step to the previous purification step, it can also be returned to the condensation column together. ]
17894 kg / h of glacial acrylic acid (temperature = 14 ° C., pressure = 1.5 bar) was recovered from the melt circuit and 3 kg / h of 101 kg / h glacial acetic acid (25 ° C.) recovered from the melt circuit. When stabilized by adding a total 104 kg / h solution of MEHQ (temperature = 25 ° C., pressure = 1.1 bar), it has the following contents:

99.7334 mass% acrylic acid,
0.2091% by weight acetic acid,
0.0180% by weight of water,
0.0230% by weight of propionic acid,
0.0001 mass% furfural,
Less than 0.0001% by weight of benzaldehyde,
0.0001% by weight maleic anhydride,
0.0002% by weight diacrylic acid,
0.0150 wt% of MEHQ, and 0.001 wt% O _2.

  It is very suitable for producing superabsorbents based on sodium polyacrylate.

  5 kg / h of PTZ is dissolved in 352 kg / h of the heated glacial acrylic acid to produce an inhibitor solution 1 at 25 ° C. 19 kg / h MEHQ is dissolved in 30 kg / h inhibitor solution 1 to form an inhibitor solution 2 at 25 ° C. in the same manner.

  17439 kg / h of MEHQ stabilized glacial acrylic acid (25 ° C., 1.5 bar) is continuously fed to the storage tank.

  The mother liquor removed in the washing column is first guided to a heatable collection vessel and from there to a tank. From this tank, heat-adjusted (as already mentioned) to 90 ° C. and with the 18474 kg / h crude acrylic acid recovered in the second recovery tray in the amount of 72716 kg / h (from the bottom of the condensation column). Return to the 15th double-flow tray. The composition of the returned mother liquor is as follows.

94.6188% by weight acrylic acid,
1.0690% by weight acetic acid,
3. 3562 mass% water,
0.0378% by weight of formic acid,
0.0431% by weight formaldehyde,
0.0088% by weight of acrolein,
0.0782% by weight of propionic acid,
0.2016% by weight of furfural,
0.0034% by weight allyl acrylate,
0.0021% by weight allyl formate,
0.0202% by weight of benzaldehyde,
0.1292% by weight of maleic anhydride,
0.4025% by weight of diacrylic acid,
0.0061% by weight of phenothiazine,
0.0227 wt% MEHQ, and 0.0003 wt% oxygen.

  The first tray of 21 additional double-flow trays of the type already described, arranged 2.9 m above the second recovery tray in the condensation column, again at regular intervals with a tray spacing of 380 mm. (Also, the hole diameter is constant 14 mm, but the number of holes is constant 32020, and the shortest distance between the centers of the two circular channels is 24.5 mm).

  From 800 mm above the last double-flow tray, the condensing column begins to conically expand. Above 500 mm above the last double-flow tray, this spread ends with a column diameter of 6.50 m.

  From this height, i.e. 1.50 m above the last double-flow tray, an equidistant arrangement (tray spacing = 500 mm) of 28 conventional single-flow Thommann trays begins. The first tray from the bottom of the Thormann tray is a tray through which liquid is discharged from the tray discharge port through six downcomers configured as tubes. These tubes are hydraulically sealed down from the gas space of the next double flow tray. The six drainage tube weir heights are lowered in the flow direction of the AC tray. The hydraulic seal has an empty orifice with the impingement plate. The drain tube is evenly distributed over the last third of the tray cross section (opposite to the side fed to the tray). The hydraulic seal is performed in a cup with an inclined overflow dam (45 °).

  Alternatively, the Thormann tray is configured such that liquid flow directions opposite to each other are obtained in a continuous flow path through the arrangement of circulation grooves in the hood of the Thormann tray.

  The Tormann tray has an orifice ratio of 14%. The ratio of the exhaust tube area to the groove outlet area is 0.8. The height of the exhaust pipe and the height of the drainage weir are 40 mm. The tray interval of the bubble cap (the distance between the lower edge of the groove and the tray) is 10 mm. The groove height is 15 mm. The angle between the obliquely inclined groove and the longitudinal edge of the hood is 30 degrees. The length of the longitudinal edge of the hood is a maximum of 800 mm. In the edge region of the column, the hood length is shortened to 200 mm in order to adjust to the roundness of the column. The distance in the cross flow direction between the two hoods on one line is 66 mm. The drainage area of the downcomer is 1.5% with respect to the cross-sectional area of the tray. The width between the two longitudinal lower edges of the hood is 64 mm.

  From the height of the topmost Thormann tray, the separation column begins to narrow again into a conical shape. This narrowing is completed 700 mm above the uppermost Thommann tray, reducing the column inner diameter to 6.00 m.

  A third collection tray (16 exhaust stacks with substantially evenly distributed exhaust stacks; exhaust stack height = 1.50 m) is placed 1.70 m above the topmost Thommann tray.

  535506 kg / h of acidic water having a temperature of 65.1 ° C. and a pressure of about 1.24 bar is collected from the third collection tray as the second side outlet.

  As already described, the acidic water has the following contents.

10.7677% by weight of acrylic acid,
6.4390% by weight acetic acid,
79.5610% by weight of water,
0.7038% by weight of formic acid,
2.4712 mass% formaldehyde,
0.0132 mass% acrolein,
0.0082% by weight of propionic acid,
0.0013 mass% furfural,
0.0331 wt% allyl formate, and 0.0001 wt% MEHQ, and 0.0013 wt% oxygen.

  The collected 25537 kg / h acidic water (65.1 ° C.) is returned to the uppermost Thurmann tray together with the inhibitor solution 2.

  Return 329 kg / h of inhibitor solution 1 (at a temperature of 25 ° C.) (counting from the bottom) to the 19th Thormann tray.

  The collected 316 kg / h acidic water is sent to incineration.

The collected 310 m ³ / h acidic water is returned to the sixth tray (counted from the bottom) of the valve trays described below at a temperature of 29.1 ° C. (cooling is performed by multistage indirect heat exchange) To do).

  The recovered 1,94011 kg / h of acidic water is returned to the uppermost tray of the valve tray as described below at a temperature of 23 ° C. (cooling is carried out with said amount of acidic water by multistage indirect heat exchange; 29.1 The final cooling stage from 0 ° C. to 23 ° C. is carried out with thermal and heat regulation (liquid chemical grade propylene is used as refrigerant and evaporates at the same time; the resulting gaseous propylene is later gas phase partial oxidation Used in the construction of reaction gas mixture for))).

  The recovered 1204 kg / h of acidic water is added to the crude acrylic acid that is further purified by crystallization as previously described.

  Thereafter, the recovered acidic water of 6010 kg / h is fed to the extraction column for further inventive extraction.

  Eleven double flow valve trays are mounted at equal intervals (tray spacing = 500 mm) 2300 mm above the third collection tray in the condensation column. The height of the overflow weir is 18 to 35 mm (the height of the upper tray is higher than the height of the lower tray). The orifice ratio (specific hole area) is 14.8%, and the sum of the drain area of the two continuous valve tray downcomers is about 10% of the column cross-sectional area. The valve used was a VV12 valve from Stahl (Viermheim, Germany).

The pressure at the top of the column is 1.17 bar.
At the top of the column, 170121 kg / h residual gas having a temperature of 31 ° C. and the following contents are discharged from the separation column.

0.1946% by weight acrylic acid,
0.1246% by weight acetic acid,
2.3031 mass% water,
0.0062% by weight of formic acid,
0.1212% by weight of acrolein,
0.0002% by weight of propionic acid,
0.0001 mass% furfural,
0.0027% by weight of allyl formate,
2.3427 mass% of CO _2,
0.7609 mass% CO,
0.721 mass% propane,
0.1681% by weight of propylene,
2.6387% by weight of O ₂ ,
90.5158% by weight of N ₂ .

  In an indirect heat exchanger, the temperature is raised to about 160 ° C. by heating the residual gas to 38 ° C. and then compressing 110880 kg / h of this residual gas with a circulating gas compressor to a pressure of 2.9 bar. 94553 kg / h of compressed circulating gas is returned to the gas phase partial oxidation as circulating gas. For the purpose of stripping the extract by acidic water extraction, 16327 kg / h of compressed circulation gas is fed to the first stripping column and 59241 kg / h of residual gas is sent to incineration.

  The extraction column for the acidic water extraction is mounted as a separation internal structure so that it flows at the edge (height of the filler material: 200 mm), and the effective total height of 10 m is the Montz-Pak B1-350 which is stacked on top of each other. Includes a perforated structure filler composed of a stainless steel sheet of mold (material 1.4571).

  The inner diameter of the extraction column for all packing materials is a constant 800 mm. Their height is 14 m. The extractant used is Palatinol® A. In order to improve the phase separation at the bottom and reduce the entrainment of extractant at the top of the column, the diameter of the bottom and top vessel of the column is increased to 1100 mm. In addition, messy plastic packing (eg, polyethylene or Teflon) is introduced at the top of the column as a fusion aid.

  Extracted 6010 kg / h of acidic water (temperature = 65.1 ° C.) below the bottom packing of the extraction column via a tubular distribution channel with a suitable flow orifice (8 mm diameter hole) To supply. Above the top packing of the extraction column, about 25 kg / h of new Palatinol® A) and 5987 kg / h of extractant returned from the first stripping column and pre-stripped Are introduced (temperature = 50 ° C.).

  The returned extractant has the following contents.

0.5% by mass or less of acrylic acid,
0.03 mass% or less acetic acid,
0.02 mass% or less of water,
0.001% by weight or less of formic acid,
0.0035% by mass or less of acrolein,
0.0005% by weight or less of propionic acid,
0.0001 mass% or less furfural,
0.001 wt% allyl formate,
MEHQ of 0.03 mass% or less,
0.0001 wt% oxygen, and 99.5 wt% Palatinol® A.

The specific mass of acidic water is 967.5 kg / m ³ . The extractant is likewise introduced via a tubular distribution channel with a suitable channel orifice (4 mm diameter hole).

  Acidic water forms a continuous phase, and the extractant forms a dispersed phase in the form of droplets (droplet diameters range from 2 to 5 mm) descending in the aqueous phase.

  At the top of the extraction column, a 4930 kg / h extraction residue (temperature about 57.6 ° C.) with the following contents is recovered.

1.7618% by weight acrylic acid,
4. 3046% by weight acetic acid,
90.1197 mass% water,
0.6446% by weight of formic acid,
2. 8993 wt% formaldehyde, and 0.27 wt% Palatinol® A.

  It is sent to incineration along with the residual gas to be incinerated. 7090 kg / h extract containing the following contents is recovered from the bottom of the extraction column (temperature about 64.5 ° C.).

8.1556% acrylic acid,
2.448% by weight acetic acid,
4.7901% by weight water,
0.1490% by weight of formic acid,
0.0788% by weight formaldehyde,
0.0140 mass% acrolein,
0.0073% by weight of propionic acid,
0.0014 mass% furfural,
0.0282 wt% allyl formate, and 0.0192 wt% MEHQ,
84.2726% by weight Palatinol® A.

  The entire extract is directed to the top of the first stripping column. The extract is heated to 95 ° C. by indirect heat exchange in a plate heat exchanger. The heat medium used is 5987 kg / h bottom liquid recovered in the first stripping column. The first stripping column contains 5 double flow trays and 15 Thormann trays as the separation internal structure. The first stripping column is insulated from the environment just like the extraction column. The inner diameter of the first stripping column for all trays is a constant 1.5 m.

  Its height is 14.5m. The lowest five trays are configured as double flow trays and are arranged at equal intervals (500 mm) in the first stripping column. Their orifice ratio is a constant 18%. The hole diameter of the double flow tray is a constant 14 mm (the hole arrangement corresponds to the exact triangular pitch). Above the uppermost double-flow tray, 15 single-flow Thommann trays (500 mm) spaced equally are placed. The Thormann tray is configured such that liquid flow directions opposite to each other are obtained in a continuous flow path through the arrangement of circulation grooves in the hood of the Thormann tray. The orifice ratio (gas flow path area based on the cross section) is 14%.

  A layer as a droplet trap (height 400 mm, metal pole ring, 25 × 25) is also placed above the last tray.

Below the lowest double-flow tray, 13000 Nm ³ / h of compressed residual gas (pressure about 2.9 bar, temperature about 160 ° C.) is directed to the first stripping column and the extraction descends in the stripping column Raise by interacting with things.

  A first loaded gas (temperature = 82.2 ° C.) of 17424 kg / h is induced from the top of the first stripping column and supplied to the second stripping column. The temperature at the bottom of the first stripping column is about 155 ° C. 49311 kg / h of bottom liquid is continuously recovered from the bottom of the first stripping column. The 5987 kg / h bottom liquid recovered from the first stripping column is cooled to 50 ° C. by two-stage indirect heat exchange (first stage in a plate heat exchanger with heat adjustment to the extract) Return to top. The 43324 kg / h bottom liquid recovered from the first stripping column is heated to 160 ° C. in an external forced circulation tube bundle flash evaporator and returned to the bottom of the first stripping column.

Comparative Example The procedure is the same as in the example except that the acidic water extracted in the example is not extracted and incinerated as in the prior art.

  Do not use an extraction column and a first stripping column. In order to strip the bottom liquid recovered from the condensation column, a corresponding amount of compressed residual gas is used. The crude acrylic acid feed stream recovered via the first side outlet is 87307 kg / h and contains 96.863 mass% acrylic acid. The amount of glacial acrylic acid supplied to the storage tank is 16994 kg / h. Its purity is 99.736% by mass for acrylic acid.

Example 2
498 g of acidic water which is not mixed with the inhibitor and has a temperature of 50 ° C. is stirred with a two-stage three-blade stirrer with an internal volume of 1.3 l at 50 ° C. (conditioned with water). Into a jacketed stirred vessel. The acidic water had the following contents.

2. 19% by weight formaldehyde,
82.00% by weight of water,
4.01% by weight acetic acid,
11.09% by weight acrylic acid,
0.69% by weight formic acid and 0.01% by weight diacrylic acid.

  Subsequently, 499 g of dimethyl phthalate at 50 ° C. was similarly added with stirring. At a constant temperature of 50 ° C., the resulting mixture was stirred at a rate of 250 revolutions / minute for 10 minutes. When the stirrer was switched off and the mixture was left at 50 ° C., the phases were separated in a short time. The mass of the organic phase was 565 g. The mass of the aqueous phase was 432 g. It was further revealed by gas chromatographic analysis of the aqueous phase to contain 20.26 g acrylic acid and 13.0 g acetic acid.

Example 3
A glass extraction column was used to extract acrylic acid from acidic water. The acidic water that was not mixed with the inhibitor had the following contents.

2.36% by weight formaldehyde,
83.12% by weight of water,
3.98% by weight acetic acid,
9.70% by weight acrylic acid,
0.68% by weight formic acid and 0.01% by weight diacrylic acid.

  The extraction column could be conditioned via a jacket (with 60 ° C. water). The separation internal structure present in the extraction column was a structural stainless steel sheet metal filler (1.4404 type stainless steel, B1-350 type Montz perforated filler). The inner diameter of the extraction column in the packed part was 40 mm. A 10 kg / h aqueous solution (temperature = 60 ° C.) was directed to the bottom of the column. To the top of the extraction column, 10 kg / h diethyl phthalate (temperature = 60 ° C.) was fed in countercurrent as a dispersed phase (droplet size: 4 to 5 mm). The aqueous extract from the top of the column further contained 0.8 wt% acrylic acid and 2.9 wt% acetic acid. Therefore, it was possible to reduce the content of acrylic acid in acidic water (relative to the initial mass content) by 93.3% by mass and the content of acetic acid by 41% by mass. 0.2% by mass of diethyl phthalate was dissolved in the extraction residue.

Example 4
The procedure was similar to Example 3. As extractant and dispersed phase, 27.5 kg / h dimethyl phthalate (T = 60 ° C.) was introduced at the top of the extraction column (droplet size: 4 to 5 mm). At a flow rate of 27.5 kg / h, the acidic water fed to the bottom of the column at a temperature of 60 ° C. as a continuous phase likewise had the following contents:

2.77% by weight formaldehyde,
82.82% by weight of water,
4.02% by weight acetic acid,
9.53 mass% acrylic acid,
0.63 wt% formic acid and 0.05 wt% diacrylic acid.

  The aqueous extractant descending at the top of the extraction column further contained 0.2% by weight acrylic acid and 2.4% by weight acetic acid. Therefore, it was possible to reduce the acrylic acid content in acidic water (relative to the initial mass content) by 98.3% by mass and the acetic acid content by 51.5% by mass. 0.9% by mass of dimethyl phthalate was dissolved in the extraction residue.

Example 5
The procedure is the same as in Example 1. In addition to the reflux liquid (acidic water), 1000 kg / h of water (further absorbent) is introduced into the condensation column with it at the same temperature. As a result, the furfural content of the crude acrylic acid derived from the condensation column as the first side outlet from the second collection tray was 0.1642% by mass (value in Example 1) to 0.1225% by mass. To decrease.

Example 6
The procedure is the same as in Example 1. In addition to the reflux liquid (acidic water), 5000 kg / h of water (further absorbent) is introduced into the condensation column with it at the same temperature. As a result, the furfural content of the crude acrylic acid derived from the condensation column as the first side outlet from the second collection tray was 0.1642% by mass (value in Example 1) to 0.1125% by mass. To decrease.

Claims (25)

A product gas mixture comprising acrylic acid, water vapor and secondary components is obtained by heterogeneously catalyzed gas phase partial oxidation of a C ₃ precursor of at least one acrylic acid with molecular oxygen on a solid catalyst at high temperature to produce acrylic The temperature of the product gas mixture containing acid, water vapor and secondary components is optionally reduced by direct and / or indirect cooling, and then the product gas mixture containing acrylic acid, water vapor and secondary components is separated by internal structure Into a condensing column equipped with a product, and fractionally condensed by raising in the condensing column, and water and water through a first side outlet located above the feed point of the product gas mixture to the condensing column A crude acrylic acid containing a secondary component in a diluted form is derived as a target product, and is obtained via a second liquid phase outlet located above the first side outlet. Deriving acidic water further containing phosphoric acid and secondary components, deriving a residual gas mixture containing secondary components having a lower boiling point than water at the top of the condensing column, from acrylic acid and acrylic acid from the bottom space of the condensing column A bottom liquid further comprising subsequent products and secondary products having a high boiling point is withdrawn from the condensing column and a portion of the recovered acidic water is returned to the condensing column as is and / or as a reflux after cooling; In a process for producing acrylic acid, optionally in which at least one further thermal separation process is applied to the crude acrylic acid for the purpose of further purification thereof, the acrylic present in it in at least a partial amount of acidic water not returned to the condensation column. The acid is absorbed into the organic solvent from the acidic water by extraction with an organic solvent to form an organic extract containing acrylic acid, and then Removing acrylic acid from the organic extract using at least one thermal separation method, returning the acrylic acid removed from the extract to the condensation column, or sending it to further purification of the crude acrylic acid, and / or metal A method for producing acrylic acid, comprising absorbing an aqueous solution of hydroxide. C ₃ precursor, characterized in that it is a propylene or acrolein, or a mixture of propylene and acrolein, The method of claim 1.   At least 25% by mass of the acidic water that has not been returned to the condensing column is absorbed into the organic solvent from the acidic water by extraction with an organic solvent to extract an organic extract containing acrylic acid. Forming and then removing the acrylic acid from the organic extract using at least one thermal separation method, returning the acrylic acid removed from the extract to a condensation column, or sending it to further purification of the crude acrylic acid, and The method according to claim 1, wherein the method is absorbed in an aqueous solution of metal hydroxide.   4. Extraction of acrylic acid from acidic water is carried out in an extraction column comprising structural fillers and / or sieve trays as separate internal structures, according to any one of claims 1-3. Method.   Introduce organic solvent to the top of the extraction column, introduce acidic water to the bottom region of the extraction column, raise the organic solvent as a dispersed phase in the continuous acidic water phase, or introduce acidic water to the top of the extraction column The process according to claim 4, characterized in that the organic solvent is introduced into the bottom region of the extraction column and the organic solvent is raised as a dispersed phase in a continuous acidic water phase.   The organic solvent comprises at least one ester of an aliphatic or aromatic monocarboxylic acid containing 5 to 20 carbon atoms and an alcohol having 1 to 8 carbon atoms. 6. The method according to any one of items 1 to 5.   2. The organic solvent comprises at least one diester of an aliphatic or aromatic dicarboxylic acid containing 5 to 20 carbon atoms and an alcohol having 1 to 8 carbon atoms. The method according to any one of 5 to 5.   The method according to claim 1, wherein the organic solvent is dimethyl phthalate, diethyl phthalate, dimethyl isophthalate, diethyl isophthalate, dimethyl terephthalate and / or diethyl terephthalate. .   The method according to claim 1, wherein the boiling point of the organic solvent at atmospheric pressure is 200 ° C. or more.   Acrylic acid is removed from the organic extract by stripping with a first stripping gas, the first stripping gas loaded with acrylic acid is returned to the condensation column and / or the first loaded The method according to claim 1, wherein acrylic acid present in the stripping gas is absorbed in an aqueous solution of metal hydroxide.   Acrylic acid is removed from the organic extract by stripping using a first stripping gas, and the resulting first stripping gas loaded with acrylic acid is derived from the condensation column as a second stripping gas. The acrylic acid still present in the bottom liquid is stripped off and the resulting second stripping gas loaded with acrylic acid is returned to the condensation column and / or into the second stripping gas. 10. A process according to claim 1, wherein the acrylic acid present is absorbed in an aqueous solution of metal hydroxide.   The stripping of the bottom liquid derived from the condensation column is carried out in a stripping column with a separating internal structure, the temperature of the bottom of the stripping column being 150 to 190 ° C. 11. The method according to 11.   A second stripping gas loaded with acrylic acid is applied to the countercurrent rectification and returned to the condensation column before loading with acrylic acid and / or the acrylic acid present therein is converted to an aqueous solution of metal hydroxide. 13. A method according to claim 11 or 12, characterized in that it is absorbed. The first stripping gas, air, N _2, and wherein the CO is ₂ and / or water vapor, the method according to any one of claims 10 to 13 to be used.   14. A method according to any one of claims 10 to 13, characterized in that the first stripping gas used is a residual gas mixture.   The process according to any one of claims 1 to 15, characterized in that the crude acrylic acid is further purified by crystallization.   The process according to claim 16, characterized in that a part of the acidic water not returned to the condensation column is added to the crude acrylic acid before further crystallization purification.   18. Process according to claim 16 or 17, characterized in that further crystallization purification of the crude acrylic acid or its mixture with acidic water is carried out by suspension crystallization.   19. The method according to claim 18, further comprising using a wash column to separate the mother liquor remaining in suspension crystallization from the formed suspension crystals.   20. A process according to any one of claims 17 to 19, characterized in that the process of free radical polymerization for polymerizing molten acrylic acid crystals and / or metal salts thereof follows. 12. A method of free radical polymerization of polymerizing acrylic acid absorbed into the aqueous solution of metal hydroxide from the loaded first and / or second stripping gas, followed by , or 1 3 the method according to any one of.   The method according to any one of claims 1 to 21, characterized in that a partial amount of the residual gas mixture is returned to the gas phase partial oxidation as a circulating gas. The method according to any one of claims 1 to 22, characterized in that the organic solvent has a mass density which, under extraction conditions, differs from the mass density of water by more than 25 kg / m ³ . The aqueous solution of metal hydroxide contains NaOH, KOH, Ca (OH) ₂ and / or Mg (OH) ₂ in a dissolved form, 24. the method of. A liquid absorbent having a boiling point T _s at one atmospheric pressure _{equal to} or higher than a boiling point T _w of water at one atmospheric pressure is provided with a supply point located between the first side outlet and the second side outlet. The process according to claim 1, wherein the process is fed to a condensation column.