WO2016162175A1 - Method for the dehydration of 3-hydroxypropanoic acid to form acrylic acid - Google Patents

Method for the dehydration of 3-hydroxypropanoic acid to form acrylic acid Download PDF

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Publication number
WO2016162175A1
WO2016162175A1 PCT/EP2016/055465 EP2016055465W WO2016162175A1 WO 2016162175 A1 WO2016162175 A1 WO 2016162175A1 EP 2016055465 W EP2016055465 W EP 2016055465W WO 2016162175 A1 WO2016162175 A1 WO 2016162175A1
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acrylic acid
preferably
acid
aqueous
wt
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PCT/EP2016/055465
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German (de)
French (fr)
Inventor
Tim BLASCHKE
Ortmund Lang
Stefan Koch
Marco Hartmann
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Basf Se
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Priority to EP15162598 priority
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Publication of WO2016162175A1 publication Critical patent/WO2016162175A1/en

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    • CCHEMISTRY; METALLURGY
    • C12BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12PFERMENTATION OR ENZYME-USING PROCESSES TO SYNTHESISE A DESIRED CHEMICAL COMPOUND OR COMPOSITION OR TO SEPARATE OPTICAL ISOMERS FROM A RACEMIC MIXTURE
    • C12P7/00Preparation of oxygen-containing organic compounds
    • C12P7/40Preparation of oxygen-containing organic compounds containing a carboxyl group including Peroxycarboxylic acids
    • C12P7/42Hydroxy-carboxylic acids
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/347Preparation of carboxylic acids or their salts, halides or anhydrides by reactions not involving formation of carboxyl groups
    • C07C51/377Preparation of carboxylic acids or their salts, halides or anhydrides by reactions not involving formation of carboxyl groups by splitting-off hydrogen or functional groups; by hydrogenolysis of functional groups
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42Separation; Purification; Stabilisation; Use of additives
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42Separation; Purification; Stabilisation; Use of additives
    • C07C51/43Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42Separation; Purification; Stabilisation; Use of additives
    • C07C51/43Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation
    • C07C51/44Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation by distillation
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42Separation; Purification; Stabilisation; Use of additives
    • C07C51/487Separation; Purification; Stabilisation; Use of additives by treatment giving rise to chemical modification
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C57/00Unsaturated compounds having carboxyl groups bound to acyclic carbon atoms
    • C07C57/02Unsaturated compounds having carboxyl groups bound to acyclic carbon atoms with only carbon-to-carbon double bonds as unsaturation
    • C07C57/03Monocarboxylic acids
    • C07C57/04Acrylic acid; Methacrylic acid
    • CCHEMISTRY; METALLURGY
    • C12BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12PFERMENTATION OR ENZYME-USING PROCESSES TO SYNTHESISE A DESIRED CHEMICAL COMPOUND OR COMPOSITION OR TO SEPARATE OPTICAL ISOMERS FROM A RACEMIC MIXTURE
    • C12P7/00Preparation of oxygen-containing organic compounds
    • C12P7/40Preparation of oxygen-containing organic compounds containing a carboxyl group including Peroxycarboxylic acids

Abstract

The invention relates to a method for the dehydration of aqueous 3-hydroxypropanoic acid to form acrylic acid. Said 3-hydroxypropanoic acid is produced by fermentation, the 3-hydroxypropanoic acid is separated from the fermentation broth, the content of aldehydes in the aqueous 3-hydroxypropanoic acid is reduced to below 0.02 wt.%, the dehydration is carried out in the liquid phase and aqueous acrylic acid is continually stripped from the liquid phase.

Description

A process for the dehydration of 3-hydroxypropionic acid to acrylic acid Description

The invention relates to a process for the dehydration of aqueous 3-hydroxypropionic acid to acrylic acid, wherein the 3-hydroxypropionic acid is produced by fermentation, the 3-hydroxypropionic acid is separated from the fermentation broth, the content of aldehydes in the aqueous 3-hydroxypropionic acid to 0.02 wt .-% is lowered, the dehydration is carried out in liquid phase and aqueous acrylic acid is continuously distilled off from the liquid phase.

Acrylic acid is a valuable monomer for preparing polymers, including water-absorbing polymer particles, a binder for aqueous emulsion paints and adhesives dispersed in aqueous solvent because of its very reactive double bond and their carboxylic acid group.

Water-absorbing polymeric particles are used to produce diapers, tampons,

Sanitary napkins and other hygiene articles, but also used as water-retaining agents in market gardening. The water-absorbing polymer particles are also referred to as superabsorbents.

The production of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT. Graham, Wiley-VCH, 1998, pages 71 to 103. described.

Acrylic acid is industrially produced exclusively from fossil raw materials. This is considered by consumers of hygiene products to be disadvantageous. There is therefore a need for the water-absorbing polymeric particles used in hygiene articles from

to produce renewable raw materials.

One possible way is the fermentative production of 3-hydroxypropionic acid and their conversion to acrylic acid. The preparation of 3-hydroxypropionic acid by fermentation is described for example in WO 2012/074818 A2.

Dehydration of 3-hydroxy propionic acid in the gas phase is mentioned in US 7,538,247.

The dehydration of 3-hydroxypropionic acid in the liquid phase, for example, in WO 2006/092271 A2, WO 2008/023039 A1, JP 2010-180171, EP 2 565 21 1 A1 and

EP 2565212 A1. The object of the present invention to provide an improved process for the production of acrylic acid based on renewable raw materials.

The object is achieved by a process for dehydration of aqueous 3-hydr- oxypropionsaure to acrylic acid in the liquid phase, said aqueous acrylic acid is continuously distilled off from the liquid phase, characterized in that the 3-Hydroxypropion- acid is produced by fermentation, the 3 hydroxypropionic from the

Fermentation broth is separated and the total amount of 2-furfural, glyoxal,

Benzaldehyde and crotonaldehyde in the aqueous 3-hydroxypropionic before

Dehydration is reduced below 0.02 wt .-%.

The total amount of 2-furfural, glyoxal, benzaldehyde, and crotonaldehyde in the aqueous 3-hydroxypropionic acid is prior to dehydration preferably less than 0.015 wt .-%, more preferably below 0.01 wt .-%, most preferably less than 0.005 wt .-% lowered.

The total amount of 2-furfural, glyoxal, benzaldehyde, and crotonaldehyde in the aqueous 3-hydroxypropionic acid is at preferably at least 25%, more preferably at least 50%, even more preferably, reduced by at least 75%.

The content of 2-furfural in the aqueous 3-hydroxypropionic acid is prior to dehydration preferably less than 0.015 wt .-%, more preferably below 0.01 wt .-%, most preferably less than 0.005 wt .-%, reduced. The amount of glyoxal in the aqueous 3-hydroxypropionic acid is prior to dehydration preferably below 0.01 wt .-%, more preferably less than 0.005 wt .-%, most preferably less than 0.001 wt .-%, reduced.

The content of benzaldehyde in the aqueous 3-hydroxypropionic acid is in front of the

Dehydration preferably below 0.01 wt .-%, more preferably less than 0.005 wt .-%, most preferably less than 0.001 wt .-%, reduced.

The content of crotonaldehyde in the aqueous 3-hydroxypropionic acid is in front of the

Dehydration preferably below 0.01 wt .-%, more preferably less than 0.005 wt .-%, most preferably less than 0.001 wt .-%, reduced.

The method for reducing the presence of aldehydes in the aqueous 3-hydroxypropionic is not limited. The aqueous 3-hydroxypropionic acid can be purified for example by crystallization, distillation or stripping with an inert gas. In a preferred embodiment of the present invention, the content is at

reducing aldehydes in the aqueous 3-hydroxypropionic acid by means of a chemical treatment. In the chemical treatment, the aldehydes contained in the aqueous 3-hydroxypropionic be selectively reacted with a suitable reagent. suitable

Reagents are for example 2,4,6-trihydroxypyrimidine (barbituric acid)

Adipic, phenylenediamine and Aminoguanidindihydrazid. The chemical treatment is most preferably carried out at a temperature of preferably at least 30 ° C, more preferably at least 40 ° C, at least 50 ° C.

The duration he chemical treatment is preferably 5 to 120 minutes, particularly preferably 10 to 90 minutes, most preferably 20 to 60 minutes.

The present invention is based on the recognition that the obtained by fermentation and treated aqueous 3-hydroxypropionic acid, surprisingly

may contain aldehyde impurities, in particular 2-furfural and glyoxal. The aldehydic contaminants may also occur during one of the processing of the wäsrigen 3-hydroxypropionic acid from the fermentation broth. It was further found that these aldehydic impurities are the cause of the formation of polymer deposits in the dehydration of the aqueous 3-hydroxypropionic acid. Furthermore, it was found that the aldehydic contaminants can be separated by means of a chemical treatment prior to dehydration easily.

The liquid phase preferably contains from 5 to 95 wt .-%, particularly preferably from 10 to 90 wt .-%, most preferably from 20 to 80 wt .-% of an organic

Solvent. The boiling point of the organic solvent is in the range of 1013 mbar, preferably 200 to 350 ° C, more preferably from 250 to 320 ° C, very particularly preferably of 280 to 300 ° C. Suitable organic solvent are, for example

Phthalates such as dimethyl phthalate and diethyl phthalate, isophthalic how

Dimethyl and diethyl, terephthalic, such as dimethyl and diethyl, alkanoic acids such as nonanoic and decanoic acid, biphenyl and / or diphenyl ether.

In a preferred embodiment of the present invention, an aprotic polar solvent is used. Aprotic polar solvents contain no ionizable proton in the molecule and are generally known, Ionizable protons for example, contain molecules with OH, SH, and NH groups. The preferred aproptisch-polar solvent contain bonded hydrogen atoms only to carbon atoms. The dipole moment of the aprotic polar solvent is preferably from 10 to

30 x 10 "30 cm, more preferably from 12 to 25 x 10" 30 cm, most preferably from

14 to 20 x 10- 30 cm. Suitable aprotic polar solvents are ketones, lactones, lactams, nitro compounds, tertiary carboxylic acid amides, urea derivatives, sulfoxides and sulfones. Sulfolane, ethylene carbonate, propylene carbonate and gamma-valerolactone be used to advantage. Sulfolane is particularly preferred. Aprotic polar solvents additionally prevent unwanted wall deposits in the reactor and heat exchangers.

The aqueous acrylic acid is advantageously by means of a rectification column 2 from the

Reaction mixture separated. About the selection of plates and

Reflux ratio, the content of 3-hydroxypropionic acid in the distillate can be kept low.

The aqueous acrylic acid is preferably separated by means of a rectification column 3 in an acrylic acid-rich phase and a water rich phase. an entraining agent is particularly preferred in the rectification column 3 is used.

In a preferred embodiment of the present invention, the separation of the aqueous acrylic acid and separation of the aqueous acrylic acid in an acrylic acid-rich phase and a water-rich phase is carried out in a rectification column 4, wherein the separation of the aqueous acrylic acid from the liquid phase below a

Side draw is carried out in the rectification column 4, takes place, the separation of the aqueous acrylic acid in an acrylic acid-rich phase and a water rich phase above the side draw and the acrylic acid-rich phase is removed in liquid form at the side offtake. an entraining agent is particularly preferably used in the rectification column 4.

In a particularly preferred embodiment of the present invention is the

Rectification column 4 is a dividing wall column, wherein the feed to the

Rectification column 4 and the side offtake of the rectification column 4 are on different sides of the partition.

The acrylic acid-rich phase obtained is preferably purified by crystallization.

the mother liquor of crystallization below the side draw in the rectifying column 4 is particularly preferably recycled.

In the following, the inventive method will be described: Preparation of 3-hydroxypropionic acid

In the process of this invention produced by fermentation of aqueous 3-hydroxypropionic acid is preferably used. Such a process is disclosed for example in WO 02/090312 A1.

Production of acrylic acid from the aqueous 3-hydroxypropionic acid, in an optional step i) a portion of the

Water are distilled off, wherein a portion of the monomeric 3-hydroxypropionic acid with elimination of water are reacted to oligomeric 3-hydroxypropionic acid. The chemical treatment to reduce the levels of aldehydes can be carried out in step i). Oligomers 3-hydroxypropionic acid is the product of at least two molecules of

3-hydroxy propionic acid. The molecules are connected to one another by esterification of the carboxyl group of one molecule with the hydroxyl group of the other molecule. Oligomeric acrylic acid is the product of at least two molecules of acrylic acid. The molecules are interconnected to each other by Michael addition of the carboxyl group of one molecule with the ethylenic double bond of the other molecule.

The temperature during the reaction is preferably less than 100 ° C, more preferably less than 90 ° C, very particularly preferably less than 80 ° C is performed. Excessively high temperatures favor the undesired in step i) dehydration of

monomeric 3-hydroxypropionic acid to acrylic acid.

The pressure in the reaction is preferably from 5 to 300 mbar, particularly preferably from 15 to 200 mbar, most preferably from 30 to 150 mbar. Lower pressures in step i) permit gentle removal of water from the liquid phase. At low pressures are uneconomical. The pressure is the pressure in the reactor or in a distillation, the pressure in the distillation bottoms. The heat can be supplied via internal and / or external heat exchanger of conventional design and / or via jacketed heating (as heat carrier is advantageously

Steam used). Preferably, it takes place via external circulation evaporator with natural or forced circulation. Particularly preferred external circulation evaporators are used with forced circulation. Such evaporators are described in EP 0854129 A1. The use of a plurality of evaporators connected in series or parallel, is also possible. The aqueous mixture obtained in step i) of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid preferably contains from 5 to 50 wt .-% water, more preferably from 10 to 40 wt .-% water, most preferably from 15 to 35 weight .-% Water.

The aqueous mixture obtained in step i) of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid contains preferably from 10 to 60 wt .-% of monomers 3-hydroxypropionic acid, particularly preferably from 20 to 50 wt .-% of monomers 3-hydroxypropionic acid, particularly acid preferably from 25 to 45 wt .-% of monomers 3-Hydroxypropion-.

The water content is in step i) is preferably wt .-%, reduced by at least 5 wt .-%, more preferably at least 10 wt .-%, most preferably by at least 15 °. The value by which the water content was lowered, the difference from the

Water content of the used aqueous 3-hydroxypropionic acid (starting material) and the

Water content of the obtained aqueous mixture of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid (product).

The water content can be determined by the usual methods, for example by Karl Fischer titration.

The content of monomeric 3-hydroxypropionic acid is preferably used in step i) by

at least 5 wt .-%, more preferably at least 15 wt .-%, most preferably at least 25 wt .-%, reduced. The value by which the content of monomeric 3-hydroxypropionic acid was lowered, the difference from the content of the monomeric 3- hydroxypropionic the used aqueous 3-hydroxypropionic acid (starting material) and the content of monomeric 3-hydroxypropionic the resulting aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid (product). The content of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid can be determined by HPLC. To determine the oligomeric 3-hydroxypropionic the signals of the first four oligomers, ie, evaluated up to pentamers using the calibration factor of the monomeric 3-hydroxypropionic acid and the sum formed. Monomers, acrylic acid and oligomeric acrylic acid can be determined analogously.

The water is separated in step i) advantageously by means of a rectification column. 1 The rectification column 1 is of conventional design and has the customary internals. Suitable column internals are in principle all common internals, for example trays, packings and / or dumped. Among the trays are preferably bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays, under the packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, Berl or Intalox saddles, or preferred braids.

The feed into the rectification column 1 is expediently in its lower region. The inlet temperature is preferably from 20 to 100 ° C, more preferably from 30 to 80 ° C, very particularly preferably from 40 to 60 ° C. Particularly preferred dual-flow trays below the feed (the stripping section) and Thormann trays above the feed

(Reinforcing member). Usually 2 to 5 theoretical plates below the feed, and 2 to 15 theoretical trays above the feed of the rectification column 1 are sufficient. The rectification is typically performed in such that the required for the implementation of bottom pressure is established. The head pressure results from the bottom pressure, the number and the type of column internals and the fluidynamischen requirements of rectification.

A portion of the bottoms liquid can be conveyed together with the inlet in the lower region of Rektifikationskolone. 1 Characterized is circulated a portion of the bottom liquid on the trays below the feed (the stripping section).

The rectification column 1 is usually made of austenitic steel, preferably made of the material 1.4571 (according to DIN EN 10020).

The cooling removed at the top of the rectification column 1 water can indirectly, for example by heat exchangers which are known to those skilled in the known and are not particularly limited, or directly, for example by a quench, take place. For this purpose, the condensed water is already being cooled by means of a suitable heat exchanger and the cooled liquid sprayed above the take-off point in the vapor. This spraying can be done in a separate apparatus or in the rectification itself. When sprayed in the rectification is the tapping point

advantageously formed as a collecting tray. By installations that improve the mixing of the chilled water to the vapor, the effect of direct cooling can be increased. For this principle all common internals,

for example trays, packings and / or dumped. Among the floors

Cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. Among the random packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, Berl or Intalox saddles, or braids. Particularly preferred dual-flow trays. As a rule, here 2 to 5 theoretical plates

sufficient. These floors are not included in the information given hitherto to the number of theoretical plates of the Rektifikationskolone first Direct condensation of the water can be carried out in several stages, with an upwardly decreasing temperature. Preferably, the cooling is carried out but by indirect cooling. The aqueous mixture obtained in step i) of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid is continuously removed from the bottom of the distillation and reacted in step ii) to acrylic acid. The reaction of the aqueous 3-hydroxypropionic acid or aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid from step i) to acrylic acid is used in step ii) in a liquid phase at a temperature of preferably from 140 to 240 ° C, more preferably from 160 to 230 ° C, very particularly preferably from 180 to 220 ° C is performed. The pressure is preferably 25-750 mbar, particularly preferably from 50 to 500 mbar, most preferably from 100 to 300 mbar. At lower pressure, the liquid phase contains less monomeric acrylic acid, whereby the danger of a free-radical polymerization decreases. In the case of distillative separation of the resulting acrylic acid, the undesired formation of oligomeric acrylic acid is suppressed in the condensate by the lower temperature of the condensate. The pressure is the pressure in the reactor or in a distillation, the pressure in the distillation bottoms.

The used in step ii) aqueous 3-hydroxypropionic acid or aqueous mixture of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid from step i) preferably contains from 5 to 50 wt .-% water, more preferably from 10 to 40 wt .-% water , very particularly preferably from 15 to 35 wt .-% water.

The used in step ii) aqueous 3-hydroxypropionic acid or aqueous mixture of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid from step i) preferably contains from 10 to 60 wt .-% of monomers 3-hydroxypropionic acid, particularly preferably from 20 to 50 wt. - monomeric% 3-hydroxypropionic acid, very particularly preferably from 25 to 45 wt .-% of monomers 3-hydroxypropionic acid.

The heat can be supplied via internal and / or external heat exchanger of conventional design and / or via jacketed heating (as heat carrier is advantageously

Steam used). Preferably, it takes place via external circulation evaporator with natural or forced circulation. Particularly preferred external circulation evaporators are used with forced circulation. Very particular preference

Forced circulation flash evaporators used. in a

Forced circulation flash evaporator does not find the evaporation on a hot

Surface instead, but by depressurization. Such evaporators are described in EP 0854129 A1. The use of a plurality of evaporators connected in series or parallel, is also possible.

The liquid phase preferably contains a polymerization inhibitor. 1 suitable

1 polymerization inhibitors are phenothiazine, hydroquinone, hydroquinone monomethyl ether, copper salts and / or manganese salts. Most particularly preferred are phenothiazine and

Hydroquinone. The liquid phase preferably contains from 0.001 to 5% by weight, particularly preferably from 0.01 to 2 wt .-%, most preferably from 0.1 to 1% by weight of the polymerization inhibitor. 1 an oxygen-containing gas is additionally used for polymerization inhibition advantageous. Particularly suitable for this purpose are air / nitrogen mixture with an oxygen content of 6 vol .-% (lean air). Is an oxygen-containing gas used for inhibiting polymerization, it is preferably supplied below the evaporator.

The liquid phase preferably contains from 15 to 90 wt .-%, particularly preferably from 20 to 85 wt .-%, most preferably from 30 to 80 wt .-% of the aprotic polar solvent.

The reaction in step ii) is preferably carried out in the absence of a catalyst. But the reaction in step ii) can also be catalyzed basic or acidic. Suitable basic catalysts are high boiling tertiary amines such as pentamethyldiethylenetriamine. Suitable acidic catalysts are high-boiling inorganic or organic acids, such as phosphoric acid and dodecylbenzenesulfonic acid. High boiling herein means a boiling point at 1013 mbar, preferably of at least 160 ° C, particularly preferably at least 180 ° C, most preferably at least 190 ° C. When a catalyst is used, the amount of catalyst in the liquid phase, preferably from 1 to 60 wt .-%, more preferably from 2 to 40 wt .-%, most preferably from 5 to 20 wt .-%.

The formed during the reaction in step ii) aqueous acrylic acid is separated advantageously by means of a rectification column (rectifying column 2) from the reaction mixture.

When using a rectification column 2, the reaction of the aqueous 3-hydroxypropionic acid or aqueous mixture of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid from step i) is to acrylic acid in the bottom of

The rectification column 2 instead and the aqueous 3-hydroxypropionic acid or the aqueous

Mixture of monomeric and oligomeric 3-hydroxy propionic acid 3-hydroxypropionic acid from step i) is the inlet of the rectification column. 2

When using a rectification column 2, the polymerization inhibitor is 1 at least partially metered in via the return line.

The rectification column 2 is of conventional design and has the customary internals. Suitable column internals are in principle all common internals, for example trays, packings and / or dumped. Among the floors

Bubble cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred, among the packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, Berl or Intalox saddles, or braids. Particularly preferred dual-flow trays.

In general, 3 to 10 theoretical trays in the rectification column are sufficient. 2 The rectification is usually carried out at reduced pressure. The head pressure is preferably from 50 to 900 mbar, particularly preferably from 100 to 500 mbar, most preferably from 150 to 300 mbar. With too high a head pressure, the aqueous is

Acrylic acid unnecessary thermal stress and by very low head pressure, the method is technically expensive. In addition, the concentration of acrylic acid at a lower pressure is lower. The bottom pressure resulting from the pressure in the head, the number and type of

Column internals and the fluidynamischen requirements of rectification.

The rectification column 2 is usually made of austenitic steel, preferably made of the material 1.4571 (according to DIN EN 10020).

The cooling of the severed head of the rectification column 2 aqueous acrylic acid may be indirect, for example by heat exchangers which are known to those skilled in the known and are not particularly limited, or directly, for example by a quench, take place. Preferably, it is carried out by direct cooling. By condensed aqueous acrylic acid is already being cooled by means of a suitable heat exchanger and the cooled liquid sprayed above the take-off point in the vapor. This spraying can be done in a separate apparatus or in the rectification itself. When spraying in the rectification unit, the extraction point of the aqueous acrylic acid is advantageously embodied as a collecting tray. By internals, which enhance the mixing of the cooled aqueous acrylic acid with the vapors, the effect of direct cooling can be increased. For this principle all common internals, for example trays, packings and / or dumped. Among the trays are bubble trays, sieve trays,

Valve trays, Thormann trays and / or dual-flow trays are preferred. Among the random packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, Berl or Intalox saddles, or braids. Particularly preferred dual-flow trays. Usually 2 to 5 theoretical plates are sufficient. These floors are not included in the information given hitherto to the number of theoretical plates of the Rektifikationskolone second Direct condensation of the aqueous acrylic acid may be carried out in several stages, with an upwardly decreasing temperature. Preferably, the cooling is effected by direct cooling.

A portion of the withdrawn at the top of the rectification column 2 the aqueous acrylic acid, preferably 10 to 40 wt .-% based on the related total amount of distillate is used as reflux for the rectification column 2, the remainder of the aqueous acrylic acid is

discharged. When using an aprotic polar solvent having low solubility in water, the condensed distillate of the rectification column 2 by means of a phase separator can be separated. The organic phase can be recycled to the rectification column 2, for example in the sump of the rectification column 2. The aqueous phase can also be partly recycled to the rectification column 2, for example as reflux, and for direct cooling of the vapor.

A portion of the bottoms (residue) of the rectification column 2 can be discharged and a distillation 1 (distillation residue) can be supplied. The residue is preferably guided through a solids separator (cyclone) and possibly supplemented by fresh aprotic polar solvent.

The aqueous acrylic acid may be obtained by distillation (in an optional step iii) in an acrylic acid-rich phase crude acrylic acid) and a water-rich phase (acid water) to be separated.

The heat input in step iii) may be via internal and / or external heat exchanger of conventional design and / or via jacketed heating (as heat carrier is advantageously steam used). Preferably, it is carried out via external

Circulation evaporator with natural or forced circulation. Particularly preferred external circulation evaporators are used with forced circulation. Such evaporators are described in EP 0854129 A1. The use of a plurality of evaporators connected in series or parallel, is also possible. The aqueous acrylic acid preferably contains a polymerization inhibitor 2. Suitable polymerization inhibitors 2 are phenothiazine, hydroquinone, and / or

Hydroquinone. Most particularly preferred are phenothiazine and

Hydroquinone. The liquid phase preferably contains from 0.001 to 5% by weight, particularly preferably from 0.01 to 2 wt .-%, most preferably from 0.1 to 1% by weight of the polymerization inhibitor 2. Advantageously, in addition, an oxygen containing gas to

Polymerization inhibition used. Particularly suitable for this purpose are air / nitrogen mixture with an oxygen content of 6 vol .-% (lean air). Is an oxygen-containing gas used for inhibiting polymerization, it is preferably supplied below the evaporator.

the separated acrylic acid rich phase (crude acrylic acid) is an advantageous

added polymerization inhibitor. 3 Suitable polymerization inhibitors are phenothiazine, 3, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, hydroquinone, and / or

Hydroquinone. Most particularly preferred are phenothiazine and

Hydroquinone. To assist the separation of the aqueous acrylic acid in an acrylic acid-rich phase (crude acrylic acid) and a water-rich phase (acid water) in step iii) may be a

Entraining agents are added. Suitable entraining agents are low-boiling hydrophobic organic solvent having a solubility in water at 23 ° C of preferably less than 5 g per 100 ml of water, more preferably less than 1 g per 100 ml of water, very particularly preferably of less than 0.2 per g 100 ml of water, and a boiling point at 1013 mbar in the range of preferably 60 to 160 ° C, particularly preferably from 70 to 130 ° C, very particularly preferably 75 to 1 15 ° C. Suitable hydrophobic organic solvents are for example aliphatic hydrocarbons such as hexane, heptane, dodecane, cyclohexane, methylcyclohexane, isooctane and hydrogenated triisobutylene, aromatic hydrocarbons such as benzene, toluene, xylene and ethylbenzene, ketones such as methyl isobutyl ketone, ethers such as methyl tert-butyl ether or mixtures thereof.

For the fractional distillation of the aqueous acrylic acid is an acrylic acid-rich phase

(Crude acrylic acid) and a water-rich phase (acid water) in step iii) is preferably a rectification column 3 are used.

When using a rectification column 3, the polymerization inhibitor 2 is at least partially metered in via the return line.

The rectification column 3 is of conventional design and has the customary internals. Suitable column internals are in principle all common internals, for example trays, packings and / or dumped. Among the floors

Bubble cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred, among the packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, Berl or Intalox saddles, or braids. Particularly preferred dual-flow trays.

As a rule, 10 to 30 theoretical trays in the rectification column sufficiently. 3 The rectification is usually carried out at reduced pressure. The head pressure is preferably from 50 to 600 mbar, particularly preferably from 150 to 400 mbar, most preferably of 200 to 300 mbar. With too high a head pressure, the aqueous acrylic acid becomes unnecessary thermal stress and by very low head pressure, the method is technically expensive. In addition, the concentration of acrylic acid at a lower pressure is lower. The bottom pressure resulting from the pressure in the head, the number and type of

Column internals and the fluidynamischen requirements of rectification.

The rectification column 3 is usually made of austenitic steel, preferably made of the material 1 .4571 (according to DIN EN 10020).

The cooling of the severed head of the rectification column 3 water-rich phase (acid water) may be indirect, for example by heat exchangers which are known to those skilled in the known and are not particularly limited, or directly, for example by a quench, take place. Preferably, it is carried out by direct cooling. By condensed water-rich phase (acid water) is already using a suitable

The heat exchanger is cooled and the cooled liquid sprayed above the take-off point in the vapor. This spraying may, in a separate apparatus or in the

carried out rectification itself. When spraying in the rectification unit, the extraction point of the water-rich phase (acid water) is advantageously embodied as a collecting tray. By internals, which enhance the mixing of the cooled water-rich phase (acid water) with the vapors, the effect of direct cooling can be increased. For this principle all common internals, for example trays, packings and / or dumped. Among the trays are bubble trays, sieve trays,

Valve trays, Thormann trays and / or dual-flow trays are preferred. Among the random packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, Berl or Intalox saddles, or braids. Particularly preferred dual-flow trays. Usually 2 to 5 theoretical plates are sufficient. These floors are not included in the information given hitherto to the number of theoretical plates of the Rektifikationskolone third Direct condensation of the water-rich phase (acid water) may be carried out in several stages, with an upwardly decreasing temperature. Preferably, the cooling is effected by direct cooling.

A portion of the condensed at the head of the rectification column 3 water-rich phase

(Acid water) can be used as reflux, the remainder of the water-rich phase (acid water) is discharged and for the recovery of acrylic acid of a

Acid water extraction are fed.

When using a hydrophobic organic solvent, the condensed distillate from the rectification column 3 is separated by means of a phase separator. The organic phase can be fed back into the rectification column 3, for example as reflux. The removed from the bottom of the rectification column 3 acrylic acid-rich phase

(Crude acrylic acid) can be used directly for the preparation of water-absorbing polymer particles. Preferably, the acrylic acid-rich phase (crude acrylic acid) is further purified by crystallization. The obtained in the crystallization mother liquor may be in the

The rectification column 3 can be returned, preferably below the removal point for the acrylic acid-rich phase (crude acrylic acid).

The acrylic acid-rich phase (crude acrylic acid) can be prepared by crystallization layer, such as

described for example in EP 0616998 A1, or be purified by suspension crystallization, as described in DE 100 39 025 A1. Suspension crystallization is preferred. The combination of a suspension Krista neutralization with a Waschkolone as described in WO

described 2003/041832 A1, is particularly preferred. In a particularly preferred embodiment of the present invention, the separation of the aqueous acrylic acid from the liquid phase and the separation of the aqueous acrylic acid in an acrylic acid-rich phase (crude acrylic acid) and a water-rich phase (acid water) by means of a rectification column with a side draw are

(Rectification column 4) is performed. The rectification column 4 combines the functions of rectification columns 2 and 3 in a single rectification column. Here, the portion below the side take-off of the rectification column 2 and the portion above the side draw of the rectification column corresponds 3. When using a rectification column 4, the reaction of the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid to acrylic acid in the bottoms of the rectification column 4 takes place, and the aqueous mixture of monomeric and oligomeric 3- hydroxypropionic acid 3-hydroxypropionic acid is the feed to the

Rectification column. 4

The heat supply in the sump of the rectification column 4 via internal and / or external heat exchanger (heat transfer is again, preferably, water vapor) of conventional design and / or double-wall heating. Preferably, it takes place via external circulation evaporator with natural or forced circulation. Particularly preferred external circulation evaporators with forced circulation. Very particular preference forced circulation flash evaporators are used. in a

Forced circulation flash evaporator does not find the evaporation on a hot surface instead, but by depressurization. Such evaporators are described in EP 0854129 A1. The use of a plurality of evaporators connected in series or parallel, is also possible. Preferably 2 to 4 evaporators are operated in parallel.

The feed into the rectification column 4 is expediently carried out in its lower region.

Preferably, it is carried below the first tray of the rectification column 4 and / or in the circulation of the heat exchanger. The inlet temperature is preferably at least 50 ° C, particularly preferably at least 100 ° C, very particularly preferably of at least 150 ° C.

Is an oxygen-containing gas used for inhibiting polymerization, it is preferably supplied below the lowermost tray.

The bottom residue of the rectification column 4 can be discharged and a

Residue distillation or residue cleavage are supplied. The bottom residue is preferably passed through a solids separator (cyclone) and optionally supplemented with fresh high boiling organic solvent.

Via the side offtake of the rectification column 4 is the acrylic acid-rich phase

(Crude acrylic acid) removed. The removal of the acrylic acid-rich phase (crude acrylic acid) is carried out in a conventional manner and is not limited. Suitable is the removal of a collecting tray, wherein the total return is collected and a portion is discharged and the other part is used as reflux below the collecting tray, or through a floor with an integrated removal means, preferably via a dual-flow tray with integrated removal means.

The extracted acrylic acid rich phase (Rohacrylsaure) is cooled by a heat exchanger (as the coolant, for example, surface water are suitable). The use of multiple

Heat exchanger, in series or in parallel, is also possible. The heat exchangers are known to those skilled in the art and are not particularly limited.

The extracted acrylic acid rich phase (crude acrylic acid) is discharged and partly used as solvent for the polymerization. 2 The cooling of the severed head of the rectification column 4 water-rich phase (acid water) may be indirect, for example by heat exchangers which are known to those skilled in the known and are not particularly limited, or directly, for example by a quench, take place. Preferably, it is carried out by direct cooling. By condensed water-rich phase (acid water) is already using a suitable

The heat exchanger is cooled and the cooled liquid sprayed above the take-off point in the vapor. This spraying may, in a separate apparatus or in the

carried out rectification itself. When spraying in the rectification unit, the extraction point of the water-rich phase (acid water) is advantageously embodied as a collecting tray. By internals, which enhance the mixing of the cooled water-rich phase (acid water) with the vapors, the effect of direct cooling can be increased. For this principle all common internals, for example trays, packings and / or dumped. Among the trays are bubble trays, sieve trays,

Valve trays, Thormann trays and / or dual-flow trays are preferred. Among the random packings are preferably those with rings, coils, saddle Ikörpern, Raschig, Intos or Pall rings, Berl or Intalox saddles, or braids. Particularly preferred dual-flow trays. Usually 2 to 5 theoretical plates are sufficient. These floors are not included in the information given hitherto to the number of theoretical plates of the Rektifikationskolone fourth Direct condensation of the water-rich phase (acid water) may be carried out in several stages, with an upwardly decreasing temperature. Preferably, the cooling is effected by direct cooling.

A portion of the condensed at the head of the rectification column 4 water-rich phase

(Acid water) can be used as reflux, the remainder of the water-rich phase (acid water) is discharged and for the recovery of acrylic acid of a

Acid water extraction are fed. When using a hydrophobic organic solvent, the condensed distillate of the rectification column 4 is separated by means of a phase separator. The organic phase can be recycled to the rectification column 4, for example as reflux. In a preferred embodiment of the present invention, a dividing wall column is employed as a rectification column. 4 A dividing wall column having a vertical partition which divides the cross-section of a portion of the column into two sections. The return flow is distributed to the two column sections. The feed and the side draw of the

Partition column are on different sides of the partition.

The crude acrylic acid withdrawn the rectification column 4 may be used directly for the preparation of water-absorbing polymer particles. Preferably, the

Crude acrylic acid further purified by crystallization. The obtained in the crystallization mother liquor can be recycled to the rectification column 4, preferably below the removal point for the crude acrylic acid. Preferably, the recycled mother liquor to cool the crude acrylic acid and discharged from the rectification column 4 crude acrylic acid to be used for heating the mother liquor (heat system).

The crude acrylic acid may be as described in DE 100 39 025 A1 by layer crystallization, as described for example in EP 0616998 A1, or by suspension crystallization, chromatography. Suspension crystallization is preferred. The combination of a

Suspensionsknstalisation as described in WO 2003/041832 A1 with a scrubbing column, is particularly preferred. The acrylic acid thus produced can be used directly as a monomer for the production of homopolymers or copolymers, especially acrylic acid homopolymers, acrylic acid / maleic anhydride copolymers, acrylic acid / maleic acid copolymers and acrylic acid / methacrylic acid copolymers, but also for the production of water-absorbing polymer particles and Acrylsäureestern, including are methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate, as well as the homopolymers and copolymers thereof may be used.

Producing water-absorbing polymer particles water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated acid-functional monomer

at least can be partially neutralized, in particular neutralized acrylic acid b) at least one crosslinker,

c) at least one initiator, optionally one or more copolymerizable with the above-mentioned under a) monomers ethylenically unsaturated monomers and

optionally one or more water-soluble polymers, prepared and are usually insoluble in water.

The monomers a) are preferably water-soluble, that is, the solubility in water at 23 ° C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 100 g water.

Suitable monomers a) are for example ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Further suitable monomers a) are

for example, ethylenically unsaturated sulfonic acids such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

The proportion of acrylic acid and / or salts thereof to the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, most preferably at least 95 mol%.

Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are for example ethylenically unsaturated groups which can be free-radically interpolymerized into the polymer chain and functional groups which can form a) covalent bonds with the acid groups of the monomers. Also suitable are polyvalent metal salts which can form coordinate bonds with at least two acid groups of the monomer a), suitable crosslinkers b).

Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be free-radically interpolymerized into the polymer network. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate,

Polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine,

Tetraallylammonium chloride, tetraallyloxyethane as described in EP 0530438 A1, di- and triacrylates as described in EP 0547847 A1, EP 0559476 A1, EP 0632068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, as well as acrylate containing further ethylenically unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or

Crosslinker, such as in DE 195 43 368 A1, DE 196 46 484 A1, WO

90/15830 A1 and WO 2002/032962 A2. Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane,

Methylenebismethacrylamide, 15-tuply ethoxylated trimethylolpropane triacrylate,

Polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine. Very particularly preferred crosslinkers b) are the acrylic acid or methacrylic acid to di- or triacrylates esterified multiply ethoxylated and / or propoxylated glycerols as described for example in WO 2003/104301 A1. Particularly advantageous are di- and / or triacrylates of 3- to 10-tuply ethoxylated glycerol. Very particular preference is given to di- or triacrylates of 1 - to 5-tuply ethoxylated and / or propoxylated glycerol. Most preferred are the triacrylates of 3- to 5-tuply ethoxylated and / or propoxylated glycerol, especially the triacrylate of 3-tuply ethoxylated glycerol. The amount of crosslinker b) is preferably 0.05 to 1, 5 wt .-%, particularly preferably 0.1 to 1 wt .-%, most preferably 0.2 to 0.5 wt .-%, each based on

Monomer a). With increasing crosslinker content, the centrifuge retention capacity (CRC) and absorbance drops below a pressure of 21, 0 g / cm 2 passes through a maximum. The initiators c) all can radicals under the polymerization

generating compounds can be used, for example, thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are

Sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid,

Sodium / sodium and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal initiators and redox initiators are used as

Sodium / hydrogen peroxide / ascorbic acid. The reducing component but a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite is preferably used.

Such mixtures are as Brüggolite® FF6 and FF7 Brüggolite® available (Bruggemann Chemicals; Germany; Heilbronn).

With the ethylenically unsaturated, acid groups-bearing monomers a) copolymerizable ethylenically unsaturated monomers d) are for example acrylamide, methacrylamide,

Hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl

Dimethylaminoethyl, dimethylaminopropyl, diethylaminopropyl,

Dimethylaminoethyl methacrylate, diethylaminoethyl.

As water-soluble polymers e) include polyvinyl alcohol, polyvinylpyrrolidone, starch,

Starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified

Cellulose, are used.

Typically, an aqueous monomer solution is used. The water content of

Monomer is preferably from 40 to 75 wt .-%, more preferably from 45 to 70 wt .-%, most preferably from 50 to 65 wt .-%. It is also possible

Monomersuspensionen, ie, monomer solutions with excess monomer a),

for example sodium use. With increasing water content, the energy consumption increases in the subsequent drying and, with falling water content, the heat of polymerization can only be removed inadequately.

The polymerization inhibitors commonly used in acrylic acid require dissolved oxygen for optimum effect. Therefore, the monomer solution before the polymerization by inertization, ie flowing through with an inert gas, preferably nitrogen or carbon dioxide, be freed of dissolved oxygen. Preferably, the oxygen content of the monomer before the polymerization to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, very particularly preferably lowered to less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading or belt reactors. In the kneader, the resulting in the polymerization of an aqueous monomer solution or suspension polymer gel is comminuted continuously by, for example, in opposite stirring shafts, as described in WO 2001/038402 A1. The polymerization on the belt is described for example in DE 38 25 366 A1 and US 6,241, 928th In the polymerization in a belt reactor a polymer gel which has to be comminuted in a further process step, for example in an extruder or kneader is produced. For improving the drying properties of the crushed polymer gel obtained by means of a kneader, can also be extruded.

but it is also possible to aqueous monomer to dropletize and to polymerize the droplets obtained in a heated carrier gas stream. In this case, the

Process steps of polymerization and drying are summarized as described in WO 2008/040715 A2, WO 2008/052971 A1 and WO 1/026876 201 A1.

The acid groups of the polymer gels are typically partly neutralized. The neutralization is preferably carried out at the stage of the monomers. This is usually done by mixing in the neutralizing agent as an aqueous solution or else preferably as a solid. The degree of neutralization is preferably from 25 to 95 mol%, particularly preferably from 30 to 80 mol%, very particularly preferably from 40 to 75 mol%, for which the customary neutralizing agents can be used, preferably

Alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or

Alkali metal hydrogencarbonates and mixtures thereof. Instead of alkali metal salts and ammonium salts can be used. Sodium and potassium are particularly preferred as alkali metals, but very particular preference is given to sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof. However, it is also possible for the neutralization after the polymerization carried out at the stage of forming in the polymerization the polymer gel. Furthermore, it is possible up to 40 mol%, preferably 10 to 30 mol%, particularly preferably 15 to 25 mol%, to neutralize the acid groups before polymerization by already added to a portion of the neutralizing agent to the monomer solution and the desired final degree of neutralization only after the polymerization at the stage of the polymer gel is set. When the polymer gel at least partially neutralized after the polymerization, the polymer gel is preferably comminuted mechanically, for example by means of an extruder, wherein the neutralizing agent is sprayed on, sprinkled or poured on and then be carefully mixed. The gel mass obtained can be repeatedly extruded for homogenization.

The polymer gel is then preferably dried with a band dryer to the

Residual moisture content is preferably 0.5 to 15 wt .-%, particularly preferably 1 to 10 wt .-%, most preferably 2 to 8 wt .-%, by weight, wherein the residual moisture content according to the EDANA recommended test method No.. WSP 230.2- 05 "Mass loss Upon Heating" is determined. At too high a residual moisture content, the dried polymer gel has too low a glass transition temperature T g and is difficult to process further. At too low a residual moisture content of the dried polymer gel is too brittle, and in the subsequent comminution steps, undesirably large amounts of polymer particles with too small a particle size ( "fines") in. The solids content of the gel before the drying is preferably from 25 to 90 wt .-% , more preferably from 35 to 70 wt .-%, most preferably from 40 to 60 wt .-%. Alternatively, a fluidized bed dryer or a paddle dryer for the drying may be used.

The dried polymer is then ground and classified, useful grinding apparatus typically single- or multi-stage rolling mills, preferably two or three stage

Roll mills, pin mills, hammer mills or vibratory mills are employed.

The average particle size of the separated fraction as a product polymer particles is preferably at least 200 μηη, particularly preferably μηη from 250 to 600, especially from 300 to 500 μηη. The average particle size of the product fraction can be found in "Particle Size Distribution" by the EDANA recommended test method No. WSP 220.2-05., The mass of the screen fractions are plotted in cumulated form and the mean particle size is determined graphically. The mean particle size here is the value of the mesh size which gives rise to a cumulative 50 wt .-%.

The proportion of particles having a particle size of greater than 150 μηη is preferably at least 90 wt .-%, particularly preferably at least 95 wt .-%, most preferably at least 98 wt .-%.

Polymer particles with too small a particle size lower the permeability (SFC). The proportion of excessively small polymer particles ( "fines") should be low.

Excessively small polymer particles are therefore typically removed and recycled into the process. This is preferably done before, during or immediately after polymerization, ie, before the drying of the polymer gel. The small polymer particles can be wetted with water and / or aqueous surfactant before or during the recirculation. It is also possible in later process steps to separate excessively small polymer particles, for example, after the surface or another coating step. In this case, the recycled are surface to small polymer particles or otherwise coated, for example with fumed silica. When a kneading reactor used for polymerization, the small polymer particles are preferably added during the last third of the polymerization.

If the added very late excessively small polymer particles, for example, only in a downstream of the polymerization reactor, for example, an extruder, then the excessively small polymer particles can become difficult to incorporate into the resulting polymer. Insufficiently incorporated, excessively small polymer particles dissolve during the grinding process again, are from the dried polymer in classifying therefore removed again and increase the amount of excessively small polymer particles. The proportion of particles μηη with a particle size of at most 850, is preferably at least 90 wt .-%, particularly preferably at least 95 wt .-%, most preferably at least 98 wt .-%.

The proportion of particles μηη with a particle size of at most 600, is preferably at least 90 wt .-%, particularly preferably at least 95 wt .-%, most preferably at least 98 wt .-%.

Polymer particles with too large particle size lower the swell rate. The proportion of excessively large polymer particles should also be low.

Excessively large polymer particles are therefore typically removed and recycled into the grinding of the dried polymer gel.

The polymer particles can be used to further improve the properties

be surface. Suitable surface postcrosslinkers are compounds containing groups which can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0083022 A2, EP 0543303 A1 and EP 0937736 A2, di- or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 described 23 617 A1 and EP 0450922 A2, or .beta.-hydroxyalkylamides such as in DE 102 04 938 A1 and US 6,239,230 described. Further, in DE 40 20 780 C1 cyclic carbonates, by DE 198 07 502 A1 2- oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 992 C1 bis- and poly-2-oxazolidinones, in DE 198 54 573 A1 2-oxotetrahydro-1, 3-oxazine and its derivatives, by DE 198 54 574 A1 N-acyl-2-oxazolidinones, 102 04 937 A1, in DE cyclical

Ureas, in DE 103 34 584 A1 bicyclic Amidoacetale, in EP 1199327 A2 oxetanes and cyclic ureas and 2003/031482 A1 morpholine-2,3-dione and its derivatives as described in WO suitable surface postcrosslinkers.

Preferred surface are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1, 4-butanediol.

Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1, 3-propanediol.

Furthermore, surface postcrosslinkers which comprise additional polymerizable ethylenically unsaturated groups as described in DE 37 13 601 A1 The amount of surface postcrosslinker is preferably from 0.001 to 5 wt .-%, particularly preferably 0.02 to 2 wt .-%, most more preferably 0.05 to 1 wt .-%, each based on the polymer particles.

In a preferred embodiment of the present invention before, during or after the surface applied in addition to the Oberflächennachvernetzern polyvalent cations on the particle surface.

The usable in the inventive method polyvalent cations include for example divalent cations such as the cations of zinc, magnesium, calcium, iron and strontium, trivalent cations such as the cations of aluminum, iron, chromium, rare earths and

Manganese, tetravalent cations such as the cations of titanium and zirconium. Possible counterions are hydroxide, chloride, bromide, sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate, citrate and lactate. There are also possible salts having different counter ions, for example, basic aluminum salts such as aluminum mono Aluminiummonoacetat or lactate.

Aluminum sulfate, and aluminum lactate Aluminiummonoacetat are preferred. Except

Metal salts can also be used polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, 0.001 to 1 wt .-%, preferably 0.005 to 0.5 wt .-%, particularly preferably 0.02 to 0.2 wt .-%. based in each case on the polymer particles. The surface postcrosslinking is typically performed such that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. in the

After the spraying, the polymer particles are coated with surface thermal drying, wherein the surface postcrosslinking can take place both before and during drying.

The spraying of a solution of the surface postcrosslinker is preferably performed in mixers with moving mixing tools, such as screw mixers, disk mixers, and

Paddle mixer, performed. Particular preference is given to horizontal mixers such as

Paddle mixers, very particular preference to vertical mixers. The distinction between horizontal mixers and vertical mixers is via the bearing of the mixing shaft, ie

Horizontal mixer having a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers horizontal Ploughshare® mixers include for example (Gebr Lödige Maschinenbau GmbH;. Paderborn, Germany), Vrieco-Nauta

Continuous Mixer (Hosokawa Micron BV, Doetinchem, Netherlands), Processall® Mixmill Mixer (Processall® Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands). but it is also possible the surface postcrosslinker spraying in a fluidized bed. The surface postcrosslinkers are typically used as an aqueous solution. the penetration depth of the surface postcrosslinker can be adjusted in the polymer particles on the content of nonaqueous solvent and total amount of solvent.

If only water is used as solvent, a surfactant is advantageously added. Characterized the wetting behavior is improved and reduces the tendency to agglomerate. Preferably, however, solvent mixtures are used, for example, isopropanol / water, 1, 3-propanediol / water and propylene glycol / water, the

Mixture mass ratio is preferably from 20:80 to 40:60. The thermal drying is preferably in contact dryers, more preferably paddle dryers, most preferably disk dryers. Suitable dryers are, for example, Hosokawa Bepex® horizontal paddle driers (Hosokawa Micron GmbH; Leingarten, Germany), Hosokawa Bepex® Disc driers (Hosokawa Micron GmbH; Leingarten, Germany), Holo-Flite® dryers (Metso Minerals Industries Inc .; Danville, USA ) and Nara paddle Dryer (NARA Machinery Europe; Frechen; Germany). Moreover, fluidized bed dryers can be used.

Drying may take place in the mixer itself, by heating the jacket or blowing in warm air. Equally suitable is a downstream dryer, for example a tray dryer, a rotary tube oven or a heatable screw. Particularly advantageous is mixed in a fluid bed dryer and dried. Preferred drying temperatures are in the range 100 to 250 ° C, preferably 120 to 220 ° C, particularly preferably 130 to 210 ° C, most preferably 150 to 200 ° C. The preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes and usually at most 60 minutes.

In a preferred embodiment of the present invention, the

water-absorbing polymer particles cooled after the thermal drying. The cooling is preferably in contact coolers, more preferably paddle coolers, disk coolers very particularly preferably carried out. Suitable coolers include, for example Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH; Leingarten, Germany), Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH; Leingarten, Germany), holograms Flite® coolers (Metso Minerals Industries Inc .; Danville, USA ) and Nara paddle cooler (NARA Machinery Europe; Frechen; Germany). Moreover, fluidized bed coolers can be used.

In the cooler, the water absorbing polymer particles to 20 to 150 ° C, preferably 30 to 120 ° C, particularly preferably 40 to 100 ° C, most preferably 50 to 80 ° C cooled.

Subsequently, the surface polymer particles can be classified again, be excessively small and / or separated into large polymer particles and recycled to the process. The surface postcrosslinked polymer particles can be coated or to further improve the properties moistened.

The subsequent moistening is preferably carried out at 30 to 80 ° C, particularly preferably at 35 to 70 ° C, very particularly preferably at 40 to 60 ° C. At excessively low temperatures, the water-absorbing polymer particles tend to agglomerate, and at higher

Temperatures, water already evaporates noticeably. The amount of water used for remoisturizing is preferably from 1 to 10 wt .-%, more preferably from 2 to 8 wt .-%, most preferably from 3 to 5 wt .-%, each based on the

water-absorbing polymer particles. Through the subsequent moistening the mechanical stability of the polymer particles is increased, and reduces their tendency to static charge. the subsequent moistening is advantageously carried out in the cooler by thermal drying.

Suitable coatings for improving the swell rate and the

Permeability (SFC) are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent

Metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings against the undesired caking tendency of the polymer particles include for example fumed silica such as Aerosil® 200, and surfactants, such as Span® 20th

methods

Determination of the average molar mass of the oligomers

The average molar mass of the oligomers is determined medium gel permeation chromatography. The sample is dissolved in hexafluoro-2-propanol and filtered through a header 0,2μηη PTFE syringe (PTFE syringe filter) filtered. The concentration of the solution should be about 1, 5 mg / ml.

It is a combination of the following series-connected separation columns used: - HFIP-LG guard column, 8.0 mm x 50 mm (Waters, Eschborn, Germany)

- PL-HFIPgel column, 7.5 mm x 300 mm (Agilent Technologies, Waldbronn, Germany)

- PL-HFIPgel column, 7.5 mm x 300 mm (Agilent Technologies, Waldbronn, Germany)

As eluent hexafluoro-2-propanol is used with 0.05 wt .-% potassium trifluoroacetate. The flow rate is 1, 00 ml / min. The temperature of the columns is 35 ° C. The injected volume is 50μΙ. The running time is 45 minutes.

As a detector, a refractive index detector of the type DRI Agilent 1100 Refractive Index Detector (Agilent Technologies, Waldbronn, Germany). The system is a commercially available narrow PMMA standards having a molecular weight from 800 to 1,820,000 g / mol calibrated (PSS Polymer Standards Service GmbH, Mainz, Germany).

The average molar mass of the oligomers is the weight average of the oligomers with a molar mass of at least 400 g / mol.

Determining the content of 2-furfural, glyoxal, benzaldehyde and crotonaldehyde

The contents of 2-furfural, glyoxal, benzaldehyde and crotonaldehyde are determined by HPLC after reaction with 2,4-dinitrophenylhydrazine.

For the reagent solution 4 g of 2,4-dinitrophenylhydrazine are weighed and added with 800 ml of water and 200 ml concentrated hydrochloric acid. The mixture is stirred until a homogeneous solution is formed. To Sample preparation About 100 mg of sample are weighed into a 5 ml volumetric flask, mixed with 2 ml of the reagent solution and heated in a water bath for 5 minutes on 60 ° C. After cooling% eluent A and 50 wt .-% eluent B is filled with a mixture of 50 wt .-. Eluent A is water and eluent B is acetonitrile.

To calibrate the 2-furfural, glyoxal, benzaldehyde and crotonaldehyde at least four known weights to the same procedure are subjected.

For HPLC a separation column of type symmetry shield RP18 5μηη, 150x2,1 mm (Waters Corporation, Milford, USA) is used. The temperature is 45 ° C, the injection volume is 5 μΙ, the flow rate is 0.4 ml / min and the run time is 45 minutes. The UV detector is set at 370 nm. is for the chromatographic separation following

Gradient used: Starting with a mobile phase composition of 60 wt .-% eluent A and 40 wt .-% eluent B is increased within 25 minutes the proportion of eluent B linear up to a composition of 30 wt .-% eluent A and 70 wt .-% eluant B. then one lowers within one minute the proportion of eluent B linear up to a composition of 60 wt .-% eluent a and 40 wt .-% eluent B. After 45 minutes the analysis run ends.

Determination of 3-Hydroxoypropionsäure and acrylic acid

The levels of 3-Hydroxoypropionsäure and acrylic acid are

Reversed phase chromatography with ultraviolet detection determined.

To Sample Preparation about 100 to 300 mg of sample are weighed into a 50ml volumetric flask and filled up with mobile phase A. Eluent A is a mixture of 1000 ml of water and 1 ml of 0.5 molar sulfuric acid.

For calibration of the 3-Hydroxypropionäure four weights (280 mg, 180 mg, 90 mg and 60 mg) are used, wherein prior to filling the 50ml volumetric flask with about 100 μΙ 25gew.-% sulfuric acid to a pH value of is acidified 3 to 4 (if necessary post-acidification). The calibration range is 0.1 to 280 mg / 50ml.

For calibration of the acrylic acid comprises at least two weights are diluted at least six concentrations. The calibration range is 0.01 to 0.9 mg / 50ml.

To reverse phase chromatography is a separation column of type Prontosil 120-3-C18 AQ 3μηη, 150x4, 6mm (BISCHOFF analysis technology and equipment GmbH, Leonberg, Germany). The temperature is 25 ° C, the injection volume is 50 μΙ, the flow rate is 1, 5 ml / min and the run time is 15 minutes. The UV detector is set at 205 nm. From the beginning to 8 minutes is 100 wt .-% eluent A, from 8 to 1 1, 5 minutes a mixture of 40 wt .-% eluent A and 60 wt .-% eluent B, 1 1, 5 minutes to end 100 is used wt .-% A eluent. Eluent B is acetonitrile. Determining the content of oligomeric 3-Hydroxoypropionsäure and oligomeric acrylic acid

The contents of oligomeric 3-Hydroxoypropionsäure and oligomeric acrylic acid are determined by ion exclusion chromatography with refractive index detection.

For sample preparation, the components to be analyzed by means of a solid phase extraction (solid phase extraction) separated from the sample matrix. For this, a SPE cartridge type Baker band SiOH 6 mL, 1000 mg (JT Baker, Avantor Performance Materials, Inc., Center Valley, PA, USA) is used. The SPE cartridge is activated with 6 ml of methanol, and rinsed twice with 6 ml of eluent. The SPE cartridge must never run dry. The sample is then pipetted onto the SPE cartridge and ten times flushed into a 10 ml volumetric flask with 1 ml of eluent. The amount of sample employed is at bottom samples μΙ 65, wherein head 85 μΙ samples and extract samples 75 μΙ. Provided that the samples do not contain a hydrophobic solvent (high boiling organic solvent entrainer), these samples can be injected without extraction, to 85 μΙ be directly dissolved in 10 ml of eluent. The eluent is 0.1 vol .-% aqueous phosphoric acid is used.

Ion exclusion chromatography for the two separation columns of type Shodex RSpak KC 81 1, 300x8mm (Showa Denko KK Shodex (Separation & HPLC) Group, Kawasaki, Japan) may be used in series. The temperature is 40 ° C, the injection volume is 100 μΙ, the flow rate is 1, 0 ml / min and the run time is 45 minutes. As eluent 0.1 wt .-% strength aqueous phosphoric acid is used. The autosampler was cooled to 15 ° C.

To evaluate a blank withdrawal is made before the integration. For this purpose, the eluent is injected and the chromatogram thus obtained subtracted from the sample chromatogram. The evaluation is carried out on area percent, wherein by means of the following formula is converted in weight percent:

% By weight (oligomer) = ^ ew ^ is ^ {monomer) x Fiä c fo en% (Qiig 0mer ^

Area% (monomer)

the contents of dimer, trimer, tetramer and pentamer (ie n = 2 to 5) respectively added to the evaluation of the oligomers. The retention times are controlled by injection of 3-hydroxypropionic acid and diacrylic acid. Examples

Example 1 (not of the invention) The dehydration of aqueous 3-hydroxypropionic acid to acrylic acid was carried out in a reactor with forced circulation-decompression evaporator and attached rectification column.

As a reactor, a 6 I glass container with jacket was used. The amount of liquid in the reactor was about 2000 g. The reactor is also the bottom of the rectification.

The forced-circulation flash evaporator consisted of a pump, a heat exchanger and a pressure holding valve. The reactor contents were by means of the pump via the

Heat exchanger and the pressure holding valve conveyed in a circle. The heat exchanger was heated by means of thermal oil. The temperature in the reactor was about the temperature of the

Thermal oil regulated.

The rectification column was electrically trace heated an inner diameter of 50 mm and was. The rectification column had an expanded metal packs with a length of 50cm (Montz Pak Type BSH-750; Julius Montz GmbH, Hilden, Germany).

The feed 250 g / hr aqueous 3-hydroxypropionic acid and 40 g / h of aqueous sulfolane (50wt .-%) are fed into the reactor. The aqueous 3-hydroxypropionic had the following composition:

20.1 wt .-% water,

2.2 wt .-% acrylic acid,

1 .3 wt .-% oligomeric acrylic acid,

52.6 wt .-% 3-hydroxypropionic acid,

21, 5 wt .-% oligomeric 3-hydroxypropionic acid,

0.0220 wt .-% of 2-furfural,

0.0031 wt .-% glyoxal,

0.0015 wt .-% benzaldehyde and

0.0021 wt .-% crotonaldehyde

The aqueous sulfolane contained for inhibiting polymerization in addition 0.1 wt .-%

Phenothiazine and 0.5 wt .-% hydroquinone monomethyl ether.

By the forced circulation flash evaporator, the reactor contents were promoted in a circle. Before the pressure-holding valve, the pressure 1, was 4 bar. The temperature in the reactor was 180 ° C. The pressure at the top of the rectification column was 150 mbar. The vapor was condensed by a cooler and partly recycled as reflux in the rectification column and partly discharged. There were discharged 296 g / h of condensate. The condensate had the following composition:

37.1 wt .-% water,

62.6 wt .-% acrylic acid,

<0.0001 wt .-% 3-hydroxypropionic acid and

<0.1 wt .-% sulfolane

Below and above the metal mesh packing, respectively 12 g / h of a 5 wt .-% aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl were dosed.

Furthermore, 15 g / h of a solution of phenothiazine and hydroquinone monomethyl ether in acrylic acid was metered into the condensate. The solution contained 2 wt .-% phenothiazine and 4 wt .-% hydroquinone monomethyl ether.

25 g / h of residue were discharged from the reactor. After 126 h of operation strong polymer formation was observed in the package in the lower part of the column.

Example 2 (Inventive) The aqueous 3-hydroxypropionic acid was chemically treated prior to dehydration. The aqueous 3-hydroxypropionic acid used had prior to the chemical treatment of the following composition:

20.1 wt .-% water,

2.2 wt .-% acrylic acid,

1, 3 wt .-% oligomeric acrylic acid,

52.6 wt .-% 3-hydroxypropionic acid,

21, 5 wt .-% oligomeric 3-hydroxypropionic acid,

0.0220 wt .-% of 2-furfural,

0.0031 wt .-% glyoxal,

0.0015 wt .-% benzaldehyde and

0.0021 wt .-% crotonaldehyde

The aqueous 3-hydroxypropionic acid was treated with 0.05 wt .-% 2,4,6-trihydroxypyrimidine (barbituric acid), based on 3-hydroxypropionic acid, and stirred at 45 ° C for five hours. After chemical treatment, the aqueous 3-hydroxypropionic acid contained 0.0026 wt .-% of 2-furfural, less than 0.0005 wt .-% glyoxal, less than 0.0005 wt .-% benzaldehyde and contains less than 0.0005. -% crotonaldehyde. The subsequent dehydration of 3-hydroxypropionic acid to acrylic acid was carried out in a reactor with forced circulation-decompression evaporator and attached rectification column.

As a reactor, a 6 I glass container with jacket was used. The amount of liquid in the reactor was about 2000 g. The reactor is also the bottom of the rectification.

The forced-circulation flash evaporator consisted of a pump, a heat exchanger and a pressure holding valve. The reactor contents were by means of the pump via the

Heat exchanger and the pressure holding valve conveyed in a circle. The heat exchanger was heated by means of thermal oil. The temperature in the reactor was about the temperature of the

Thermal oil regulated.

The rectification column was electrically trace heated an inner diameter of 50 mm and was. The rectification column had an expanded metal packs with a length of 50cm (Montz Pak Type BSH-750; Julius Montz GmbH, Hilden, Germany).

The feed 250 g / h of the chemically treated aqueous 3-hydroxypropionic acid and 40 g / h of aqueous sulfolane were added (50wt .-%) are fed into the reactor. The aqueous sulfolane contained for inhibiting polymerization in addition 0.1 wt .-%

Phenothiazine and 0.5 wt .-% hydroquinone monomethyl ether.

By the forced circulation flash evaporator, the reactor contents were promoted in a circle. Before the pressure-holding valve, the pressure 1, was 4 bar. The temperature in the reactor was 180 ° C.

The pressure at the top of the rectification column was 150 mbar. The vapor was condensed by a cooler and partly recycled as reflux in the rectification column and partly discharged. There were discharged 293 g / h of condensate. The condensate had the following composition:

36.1 wt .-% water,

62.9 wt .-% acrylic acid,

<0.0001 wt .-% 3-hydroxypropionic acid and

<0.1 wt .-% sulfolane

Below and above the metal mesh packing, respectively 12 g / h of a 5 wt .-% aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl were dosed. Furthermore, 15 g / h of a solution of phenothiazine and hydroquinone monomethyl ether in acrylic acid was metered into the condensate. The solution contained 2 wt .-% phenothiazine and 4 wt .-% hydroquinone monomethyl ether.

26 g / h of residue were discharged from the reactor. After 450 hours of operation no polymer formation was observed in the package.

Claims

claims
1 . A process for dehydration of aqueous 3-hydroxypropionic acid to acrylic acid in the liquid phase, said aqueous acrylic acid is continuously distilled off from the liquid phase, characterized in that the 3-hydroxypropionic acid is produced by fermentation, the 3-hydroxypropionic acid is separated from the fermentation broth and the total amount of 2-furfural, glyoxal, benzaldehyde, and
Crotonaldehyde is lowered in the aqueous 3-hydroxypropionic acid prior to dehydration under 0.02 wt .-%.
2. The method according to claim 1, characterized in that the total amount of
2-furfural, glyoxal, benzaldehyde and crotonaldehyde is lowered in the aqueous 3-hydroxypropionic acid by at least 25%.
3. The method according to claim 1 or 2, characterized in that the content of
2-furfural is reduced in the aqueous 3-hydroxypropionic acid prior to dehydration under 0.005 wt .-%.
4. The method according to any of claims 1 to 3, characterized in that the content of glyoxal is lowered in the aqueous 3-hydroxypropionic acid prior to dehydration under 0.001 wt .-%.
5. The method according to any one of claims 1 to 4, characterized in that the content of aldehydes is reduced by a chemical treatment.
6. The method according to claim 5, characterized in that the chemical treatment is carried out at a temperature of at least 30 ° C.
7. The method according to claim 5 or 6, characterized in that the chemical
Treatment for a period is carried out from 20 to 60 minutes.
8. The method according to any one of claims 5 to 7, characterized in that, for
chemical treatment 2,4,6-trihydroxypyrimidine is used.
9. The method according to any one of claims 1 to 8, characterized in that
Dehydration is carried out continuously.
10. The method according to any one of claims 1 to 9, characterized in that
containing liquid phase from 5 to 95 wt .-% of an organic solvent.
1. 1 A method according to claim 10, characterized in that the organic
Solvent is an aprotic polar solvent.
12. The method according to claim 1 1, characterized in that the aprotic polar solvent is sulfolane.
13. The method according to any of claims 1 to 12, characterized in that the aqueous acrylic acid is separated by means of a rectification column. 2
14. A method according to any one of claims 1 to 13, characterized in that the aqueous acrylic acid is separated by means of a rectification column 3 in an acrylic acid-rich phase and a water rich phase.
15. The method according to claim 14, characterized in that in the
Rectifying column 3 an entrainer is used.
16. The method according to claim 14 or 15, characterized in that the separation of the aqueous acrylic acid and separation of the aqueous acrylic acid in an acrylic acid-rich phase and a water-rich phase is carried out in a rectification column 4, wherein the separation of the aqueous acrylic acid from the liquid
phase is carried out below a side draw in the rectifying column 4, takes place, the separation of the aqueous acrylic acid in an acrylic acid-rich phase and a water rich phase above the side draw and the acrylic acid-rich phase is removed in liquid form at the side offtake.
17. The method according to claim 16, characterized in that
Rectification column 4 is a dividing wall column, wherein the feed to the rectification column 4 and the side offtake of the rectification column 4 located on different sides of the partition.
18. The method according to any one of claims 14 to 17, characterized in that the acrylic acid-rich phase obtained is purified by crystallization.
A method according to claim 18, characterized in that the mother liquor of
Figure imgf000034_0001
Crystallization below the side take-off is recycled into the rectification column. 4
PCT/EP2016/055465 2015-04-07 2016-03-14 Method for the dehydration of 3-hydroxypropanoic acid to form acrylic acid WO2016162175A1 (en)

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