WO2016037775A1

WO2016037775A1 - Method for the continuous dehydration of 3-hydroxypropanoic acid to form acrylic acid

Info

Publication number: WO2016037775A1
Application number: PCT/EP2015/068122
Authority: WO
Inventors: Tim BLASCHKE; Ortmund Lang; Marta ZAJACZKOWSKI-FISCHER; Marco Hartmann
Original assignee: Basf Se
Priority date: 2014-09-09
Filing date: 2015-08-05
Publication date: 2016-03-17

Abstract

The invention relates to a method for the continuous dehydration of aqueous 3-hydroxypropanoic acid to form acrylic acid in a liquid phase, wherein the liquid phase has a temperature of 120 to 250°C, aqueous acrylic acid is continuously isolated from the liquid phase, and the liquid phase contains 5 to 95 wt.-% of a polar aprotic solvent.

Description

Process for the continuous dehydration of 3-hydroxypropionic acid to acrylic acid

description

The invention relates to a process for the continuous dehydration of aqueous 3-hydroxypropionic acid to acrylic acid in the liquid phase, wherein the liquid phase has a temperature of 120 to 250 ° C, aqueous acrylic acid is continuously separated from the liquid phase and the liquid phase from 5 to 95 Wt .-% of an aprotic polar solvent contains.

Acrylic acid, because of its very reactive double bond as well as its carboxylic acid group, is a valuable monomer for making polymers, e.g. water-absorbing polymer particles, binders for aqueous emulsion paints and adhesives dispersed in aqueous solvent.

Water-absorbing polymer particles are used in the manufacture of diapers, tampons, sanitary napkins and other sanitary articles, but also as water-retaining agents in agricultural horticulture. The water-absorbing polymer particles are also referred to as superabsorbers.

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

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

One possible route is the fermentative production of 3-hydroxypropionic acid and its conversion to acrylic acid. The production of 3-hydroxypropionic acid by fermentation is described, for example, in WO 2012/074818 A2. The dehydration of 3-hydroxypropionic acid in the gas phase is mentioned in US 7,538,247.

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

EP 2 565 212 A1.

The object of the present invention was to provide an improved process for the production of acrylic acid based on renewable raw materials. The object was achieved by a process for the continuous dehydration of aqueous 3-hydroxypropionic acid to acrylic acid in the liquid phase, wherein the liquid phase has a temperature of 120 to 250 ° C and aqueous acrylic acid is continuously separated from the liquid phase, thereby in that the liquid phase contains from 5 to 95 wt .-% of an aprotic solvent and the aprotic solvent has a dipole moment of at least 10 x 10 ^{"having 30} cm.

A list of the dipole moments of some selected inventive and non-inventive solvents is shown in the following table:

Aprotic solvents contain no ionizable proton in the molecule and are generally known, for example, ionizable protons contain molecules with OH, SH and NH groups. The preferred aproptic solvents contain exclusively hydrogen atoms bonded to carbon atoms.

The dipole moment of the employed according to the invention aprotic solvent is preferably from 12 to 25 x 10 ^"30 cm, more preferably from 14 to 20 x ^{10" 30} cm, most preferably from 15 to 18 x 10 ^"30 cm.

The aprotic solvents have a boiling point at 1013 mbar of preferably 200 to 350 ° C, more preferably from 250 to 320 ° C, most preferably from 280 to 300 ° C, on.

Suitable aprotic solvents are ketones, lactones, lactams, nitro compounds, tertiary carboxylic acid amides, urea derivatives, sulfoxides and sulfones. Sulfolane, and gamma-valerolactone are advantageously used. Sulfolane is particularly preferred. The present invention is based on the finding that polar aprotic solvents can prevent unwanted wall deposits in the reactor and heat exchanger.

The aqueous 3-hydroxypropionic acid used preferably contains from 5 to 50% by weight of water, particularly preferably from 10 to 40% by weight of water, very particularly preferably from 15 to 35% by weight of water.

The aqueous acrylic acid is advantageously separated off from the reaction mixture by means of a rectification column 2. By selecting the separation stages and the reflux ratio, the content of 3-hydroxypropionic acid and aprotic solvent in the distillate can be kept low.

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

In a preferred embodiment of the present invention, the separation of the aqueous acrylic acid and the separation of the aqueous acrylic acid in a high-acrylic phase and a high-water phase in a rectification column 4 is carried out, wherein the separation of the aqueous acrylic acid from the liquid phase below a side draw in the rectification column 4 takes place, the separation of the aqueous acrylic acid in a high acrylic acid phase and a high-water phase above the side take off and the acrylic acid-rich phase is removed liquid at the side offtake. Particularly preferably, an entraining agent is used in the rectification column 4.

In a particularly preferred embodiment of the present invention, the rectification column 4 is a dividing wall column, wherein the feed to the rectification column 4 and the side draw of the rectification column 4 are located on different sides of the dividing wall.

The obtained acrylic acid-rich phase is preferably purified by crystallization. Particularly preferably, the mother liquor of the crystallization is recycled below the side draw into the rectification column 4.

The method according to the invention is described below: Preparation of 3-hydroxypropionic acid

In the process according to the invention, preference is given to using ge-3-hydroxypropionic acid produced by fermentation. Such a method is described, for example, in

WO 02/090312 A1 discloses.

Preparation of acrylic acid In an optional step i), part of the water can be distilled off from the aqueous 3-hydroxypropionic acid, some of the monomeric 3-hydroxypropionic acid reacting with elimination of water to give oligomeric 3-hydroxypropionic acid.

Oligomeric 3-hydroxypropionic acid is the product of at least two molecules

3-hydroxy propionic acid. The molecules are interconnected 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 by Michael addition of the carboxyl group of one molecule with the ethylenic double bond of the other molecule.

The temperature in the reaction is preferably less than 100 ° C, more preferably less than 90 ° C, most preferably less than 80 ° C performed. Too high temperatures favor the undesirable 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, very particularly preferably from 30 to 150 mbar. Lower pressures in step i) allow gentle removal of the water from the liquid phase. Too 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 double wall heating (as a heat carrier water vapor is advantageously used). Preferably, it takes place via external circulation evaporator with natural or forced circulation. External circulation evaporators with forced circulation are particularly preferably used. Such evaporators are described in EP 0 854 129 A1. The use of several evaporators, connected in series or in parallel, is possible. The pH during distillation is preferably at least 1.5, more preferably at least 1.8, most preferably at least 2.0. Too low pH increases oligomer formation and shifts the ratio of monomeric 3-hydroxypropionic acid to oligomeric 3-hydroxypropionic acid. Too high a proportion of long-chain oligomers increases the viscosity and deteriorates the heat transfer. Thus, usually no compounds catalyzing the oligomerization, in particular no strong acids, such as sulfuric acid, organic sulfonic acids and phosphoric acid added.

The aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid obtained in step i) preferably contains from 5 to 50% by weight of water, more preferably from 10 to 40% by weight of water, most preferably from 15 to 35% by weight of water.

The aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid obtained in step i) preferably contains from 10 to 60% by weight of monomeric 3-hydroxypropionic acid, more preferably from 20 to 50% by weight of monomeric 3-hydroxypropionic acid, most preferably from 25 to 45% by weight of monomeric 3-hydroxypropionic acid. The water content in step i) is preferably reduced by at least 5% by weight, more preferably by at least 10% by weight, most preferably by at least 15% by weight. The value by which the water content was lowered is the difference between the water content of the aqueous 3-hydroxypropionic acid used (educt) and the water content of the resulting aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 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 in step i) is preferably reduced by at least 5% by weight, more preferably by at least 15% by weight, most preferably by at least 25% by weight. The value by which the content of monomeric 3-hydroxypropionic acid was lowered is the difference between the content of monomeric 3-hydroxypropionic acid of the aqueous 3-hydroxypropionic acid used (educt) and the content of monomeric 3-hydroxypropionic acid of the obtained aqueous mixture of monomeric 3 Hydroxypropionic acid and oligomeric 3-hydroxypropionic acid (product).

The content of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid can be determined by means of HPLC. To determine the oligomeric 3-hydroxypropionic acid, the signals of the first four oligomers, ie to the pentamer, are evaluated using the calibration factor of the monomeric 3-hydroxypropionic acid and the sum is formed. Monomeric acrylic acid and oligomeric acrylic acid can be determined analogously.

The water is advantageously separated off in step i) by means of a rectification column 1. The rectification column 1 is of a known type and has the usual installations. In principle, all standard installations are suitable as column internals, for example trays, packings and / or fillings. Among the soils, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred, among the trays are those with rings, spirals, calipers, Raschig, Intos or Pall rings, Berl or Intalox saddles or Braided preferred.

The feed into the rectification column 1 is expediently carried out in its lower region. The feed temperature is preferably from 20 to 100 ° C, more preferably from 30 to 80 ° C, most preferably from 40 to 60 ° C. Particularly preferred are dual-flow trays below the inlet (stripping section) and Thormannböden above the inlet (reinforcing section). As a rule, 2 to 5 theoretical plates below the feed and 2 to 15 theoretical plates above the feed of the rectification column 1 are sufficient. The rectification is usually carried out in such a way that the bottom pressure required for the reaction sets in. The top pressure results from the sump pressure, the number and type of column internals and the fluid dynamic requirements of the rectification.

To increase the residence time and thus to increase the conversion of monomeric 3-hydroxypropionic acid to oligomeric 3-hydroxypropionic acid, it is advantageous to promote part of the bottoms liquid together with the feed to the lower region of the rectification column 1. As a result, part of the sump liquid is conveyed in a circle via the trays below the inlet (stripping section).

The rectification column 1 is usually made of austenitic steel, preferably of the material 1.4571 (according to DIN EN 10020). The cooling of the water separated off at the top of the rectification column 1 can be effected indirectly, for example by heat exchangers which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. For this purpose, already condensed water is cooled by means of a suitable heat exchanger and sprayed the cooled liquid above the sampling point in the vapor. This spraying may be done in a separate apparatus or in the rectification unit itself. When spraying in the rectification unit, the extraction point of the water is advantageously designed as a catch bottom. Built-in components that improve the mixing of the cooled water with the vapor can increase the effect of direct cooling. In principle, all common installations are suitable for this, for example floors, packings and / or fillings. Among the trays, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. Among the fillings are those with rin gen, coils, calipers, Raschig, Intos or Pall rings, Berl or Intalox saddles or braids preferred. Particularly preferred are dual-flow trays. As a rule, 2 to 5 theoretical plates are sufficient here. These soils are not taken into account in the previous information on the number of theoretical plates of the rectification column 1. The direct condensation of the water can also be carried out in several stages, with the temperature rising to the top. Preferably, however, the cooling takes place by indirect cooling.

The aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid thus obtained in step i) is withdrawn continuously from the bottom of the distillation and reacted in step ii) to give 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 carried out in step ii) in the liquid phase at a temperature of preferably from 140 to 240 ° C., more preferably from 160 to 230 ° C, most preferably from 180 to 220 ° C carried out. The pressure is preferably from 25 to 750 mbar, particularly preferably from 50 to 500 mbar, very particularly preferably from 100 to 300 mbar. At lower pressure, the liquid phase contains less monomeric acrylic acid and in the case of distillative removal of the resulting acrylic acid, the undesired formation of oligomeric acrylic acid in the condensate is suppressed. The pressure is the pressure in the reactor or in a distillation, the pressure in the distillation bottoms.

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

The aqueous 3-hydroxypropionic acid or aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid from step i) used in step ii) preferably contains from 10 to 60% by weight of monomeric 3-hydroxypropionic acid, more preferably from 20 to 50 % By weight of monomeric 3-hydroxypropionic acid, most preferably from 25 to 45% by weight of monomeric 3-hydroxypropionic acid.

The heat can be supplied via internal and / or external heat exchanger of conventional design and / or double wall heating (as a heat carrier water vapor is advantageously used). Preferably, it takes place via external circulation evaporator with natural or forced circulation. External circulation evaporators with forced circulation are particularly preferably used. Forced circulation expansion evaporators are very particularly preferably used. In a forced circulation flash evaporator evaporation does not take place on a hot surface, but by pressure release. Such evaporators are described in EP 0 854 129 A1. The use of several evaporators, connected in series or in parallel, is possible.

The liquid phase preferably contains a polymerization inhibitor 1. Suitable polymerization inhibitors 1 are phenothiazine, hydroquinone, hydroquinone monomethyl ether, copper salts and / or manganese salts. Very particular preference is given to phenothiazine and hydroquinone monomethyl ether. The liquid phase preferably contains from 0.001 to 5% by weight, particularly preferably from 0.01 to 2% by weight, very particularly preferably from 0.1 to 1% by weight, of the polymerization inhibitor 1. Advantageously, an oxygen-containing gas is additionally used for polymerization inhibition. Particularly suitable for this purpose are air / nitrogen mixtures having an oxygen content of 6% by volume (lean air). If an oxygen-containing gas is used for polymerization inhibition, this is preferably supplied below the evaporator.

The liquid phase preferably contains from 10 to 95% by weight, more preferably from 20 to 85% by weight, most preferably from 30 to 80% by weight of the aprotic solvent.

The reaction in step ii) is preferably carried out in the absence of a catalyst. 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 means here a boiling point at 1013 mbar of preferably at least 160 ° C, more preferably at least 180 ° C, most preferably at least 190 ° C.

If a catalyst is used, the amount of catalyst in the liquid phase is preferably from 1 to 60 wt .-%, particularly preferably from 2 to 40 wt .-%, most preferably from 5 to 20 wt .-%. The aqueous acrylic acid formed during the reaction in step ii) is advantageously separated off from the reaction mixture by means of a rectification column (rectification column 2).

When using a rectification column 2, the reaction of the aqueous 3-hydroxypropionic acid or the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid from step i) to acrylic acid takes place in the bottom of the rectification column 2 and the aqueous 3-hydroxypropionic acid or the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid from step i) is the feed of the rectification column 2. When using a rectification column 2, the polymerization inhibitor 1 is metered at least partially via the reflux. The rectification column 2 is of a known type and has the usual installations. In principle, all standard installations are suitable as column internals, for example trays, packings and / or fillings. Among the soils, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. Among the trays are those with rings, spirals, calipers, Raschig, Intos or Pall rings, Berl or Intalox Saddling or braiding preferred. Particularly preferred are dual-flow trays.

In general, 3 to 10 theoretical plates in the rectification column 2 are sufficient. The rectification is usually carried out at reduced pressure. The top pressure is preferably from 50 to 900 mbar, particularly preferably from 100 to 500 mbar, very particularly preferably from 150 to 300 mbar. If the top pressure is too high, the aqueous acrylic acid is unnecessarily thermally stressed, and if the top pressure is too low, the process becomes technically too expensive. In addition, the concentration of acrylic acid is lower at lower pressure. The bottom pressure results from the top pressure, the number and type of column internals and the fluid dynamic requirements of the rectification.

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

The cooling of the aqueous acrylic acid separated off at the top of the rectification column 2 can be effected indirectly, for example by heat exchangers which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. Preferably, it is done by direct cooling. For this purpose, already condensed aqueous acrylic acid is cooled by means of a suitable heat exchanger and the cooled liquid is sprayed in the vapor above the take-off point. This spraying may be done in a separate apparatus or in the rectification unit itself. When spraying in the rectification unit, the removal point of the aqueous acrylic acid is advantageously designed as a capture bottom. By means of internals, which improve the mixing of the cooled aqueous acrylic acid with the vapor, the effect of direct cooling can be increased. In principle, all common installations are suitable for this, for example floors, packings and / or fillings. Among the trays, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. Among the beds, those with rings, coils, calipers, Raschig, Intos or Pall rings, Berl or Intalox saddles or braids are preferred. Particularly preferred are dual-flow trays. As a rule, 2 to 5 theoretical plates are sufficient here. These soils are not taken into account in the previous information on the number of theoretical plates of the rectification column 2. The direct condensation of the aqueous acrylic acid can also be carried out in several stages, with upward decreasing temperature. Preferably, the cooling is carried out by direct cooling. A portion of the aqueous acrylic acid withdrawn at the top of the rectification column 2, preferably from 10 to 40% by weight, based on the total amount of distillate, is used as reflux for the rectification column 2, the remainder of the aqueous acrylic acid being discharged. When using an aprotic solvent with low solubility in water, the condensed distillate of the rectification column 2 can be separated by means of a phase separator. The organic phase can be recycled to the rectification column 2, for example into the bottom of the rectification column 2. The aqueous phase can also be partially recycled to the rectification column 2, for example as reflux and for the direct cooling of the vapor.

The bottom residue of the rectification column 2 can be discharged and fed to a residue distillation or a residue splitting. The bottom residue is preferably passed through a solids separator (cyclone) and optionally supplemented by fresh aprotic solvent.

The resulting aqueous acrylic acid can be separated by distillation into an acrylic-acid-rich phase (crude acrylic acid) and a water-rich phase (acid water) in an optional step iii).

The heat supply in step iii) can be done via internal and / or external heat exchanger conventional design and / or double wall heating (as a heat carrier is advantageously used water vapor). Preferably, it takes place via external circulation evaporator with natural or forced circulation. External circulation evaporators with forced circulation are particularly preferably used. Such evaporators are described in EP 0 854 129 A1. The use of several evaporators, connected in series or in parallel, is possible.

The aqueous acrylic acid preferably contains a polymerization inhibitor 2. Suitable polymerization inhibitors 2 are phenothiazine, hydroquinone and / or hydroquinone monomethyl ether. Very particular preference is given to phenothiazine and hydroquinone monomethyl ether. The liquid phase preferably contains from 0.001 to 5% by weight, particularly preferably from 0.01 to 2% by weight, very particularly preferably from 0.1 to 1% by weight, of the polymerization inhibitor 2. containing gas used for polymerization inhibition. Particularly suitable for this purpose are air / nitrogen mixtures having an oxygen content of 6% by volume (lean air). If an oxygen-containing gas is used for polymerization inhibition, this is preferably fed below the evaporator.

The separated acrylic acid-rich phase (crude acrylic acid) is advantageously added to a polymerization inhibitor 3. Suitable polymerization inhibitors 3 are phenothiazine, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, hydroquinone and / or hydroquinone monomethyl ether. Very particular preference is given to phenothiazine and hydroquinone monomethyl ether. To assist in the separation of the aqueous acrylic acid into a high-acrylic acid phase (crude acrylic acid) and a water-rich phase (acid water) in step iii), an entraining agent may be added. Suitable entraining agents are low boiling hydrophobic organic solvents having a solubility in water at 23 ° C of preferably less than 5 grams per 100 milliliters of water, more preferably less than 1 gram per 100 milliliters of water, most preferably less than 0.2 grams per gram 100 ml of water, and a boiling point at 1013 mbar in the range of preferably 60 to 160 ° C, more preferably from 70 to 130 ° C, most preferably from 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 distillative separation of the aqueous acrylic acid, a high-acrylic acid phase (crude acrylic acid) and a water-rich phase (acid water) in step iii), a rectification column 3 is preferably used.

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

The rectification column 3 is of a known type and has the usual installations. In principle, all standard installations are suitable as column internals, for example trays, packings and / or fillings. Among the trays, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. The trays include those with rings, spirals, saddles, Raschig, Intos or Pall rings, Berl or Intalox rings. Saddling or braiding preferred. Particularly preferred are dual-flow trays.

In general, 10 to 30 theoretical plates in the rectification column 3 are sufficient. The rectification is usually carried out at reduced pressure. The top pressure is preferably from 50 to 600 mbar, more preferably from 150 to 400 mbar, most preferably from 200 to 300 mbar. If the top pressure is too high, the aqueous acrylic acid is unnecessarily thermally stressed, and if the top pressure is too low, the process becomes technically too expensive. In addition, the concentration of acrylic acid is lower at lower pressure. The bottom pressure results from the top pressure, the number and type of column internals as well as the fluid dynamic requirements of the rectification.

The rectification column 3 is usually made of austenitic steel, preferably of the material 1 .4571 (according to DIN EN 10020). The cooling of the separated at the top of the rectification column 3 water-rich phase

(Acid water) can indirectly, for example by heat exchangers, the expert in itself are known and are not subject to any particular restriction, or directly, for example by a quench done. Preferably, it is done by direct cooling. For this purpose, already condensed water-rich phase (sour water) is cooled by means of a suitable heat exchanger and sprayed the cooled liquid above the sampling point in the vapor. This spraying may be done in a separate apparatus or in the rectification unit itself. When spraying in the rectification unit, the removal point of the water-rich phase (sour water) is advantageously designed as a catch bottom. By means of internals, which improve the mixing of the cooled water-rich phase (sour water) with the vapor, the effect of direct cooling can be increased. In principle, all common installations are suitable for this, for example floors, packings and / or fillings. Among the trays, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. Among the beds, those with rings, coils, calipers, Raschig, Intos or Pall rings, Berl or Intalox saddles or braids are preferred. Particularly preferred are dual-flow trays. As a rule, 2 to 5 theoretical plates are sufficient here. These soils are not taken into account in the previous information on the number of theoretical plates of the rectification column 3. The direct condensation of the water-rich phase (sour water) can also be carried out in several stages, with upward decreasing temperature. Preferably, the cooling is carried out by direct cooling. A portion of the water-rich phase (acid water) condensed at the top of the rectification column 3 can be used as reflux, the remainder of the water-rich phase (sour water) is discharged and fed to the extraction of acidic acid from an acid water extraction. When using a hydrophobic organic solvent, the condensed distillate of the rectification column 3 is separated by means of a phase separator. The organic phase can be recycled to the rectification column 3, for example as reflux.

The acrylic acid-rich phase (crude acrylic acid) withdrawn from the bottom of the rectification column 3 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 mother liquor obtained in the crystallization can be recycled to the rectification column 3, preferably below the take-off point for the high-acrylic acid phase (crude acrylic acid).

The acrylic acid-rich phase (crude acrylic acid) can be purified by layer crystallization, as described, for example, in EP 0 616 998 A1, or by suspension crystallization, as described in DE 100 39 025 A1. The suspension crystallization is preferred. The combination of a suspension crystallization with a washing column, as described in WO 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 into a high-acrylic acid phase (crude acrylic acid) and a high-acid phase (acid water) are carried out by means of a rectification column with a side draw (rectification column 4). The rectification column 4 combines the tasks of the rectification columns 2 and 3 in a single rectification column. 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 takes place in the bottom of the rectification column 4 and the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is the feed of the rectification column 4.

The heat in the bottom 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 are external circulation evaporator with forced circulation. Forced circulation expansion evaporators are very particularly preferably used. In a forced circulation flash evaporator evaporation does not take place on a hot surface, but by pressure release. Such evaporators are described in EP 0 854 129 A1. The use of several evaporators, connected in series or in parallel, is 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 takes place below the first bottom of the rectification column 4 and / or into the circulation of the heat exchanger. The feed temperature is preferably at least 50 ° C, more preferably at least 100 ° C, most preferably at least 150 ° C.

If an oxygen-containing gas is used for polymerization inhibition, this is preferably supplied below the lowest soil. The bottom residue of the rectification column 4 can be discharged and fed to a residue distillation or a residue splitting. The bottom residue is preferably passed through a solids separator (cyclone) and optionally supplemented by fresh high-boiling organic solvent. About the side of the rectification rectification column 4, the acrylic acid-rich phase (crude acrylic acid) is removed. The removal of the acrylic acid-rich phase (crude acrylic acid) takes place usual way and is subject to no restriction. The extraction is suitable via a collecting base, wherein the entire return is collected and one part is discharged and the other part is used as a return below the catch bottom, or via a floor with integrated extraction facility, preferably via a dual-flow floor with integrated drainage. zugsmöglichkeit.

The withdrawn acrylic acid-rich phase (crude acrylic acid) is cooled by means of a heat exchanger (for example, surface waters are suitable as the coolant). The use of several heat exchangers, connected in series or in parallel, is possible. The heat exchangers are known per se to the person skilled in the art and are not subject to any particular restriction.

The withdrawn acrylic acid-rich phase (crude acrylic acid) is discharged and partially used as a solvent for the polymerization inhibitor 2. The cooling of the water-rich phase (acid water) separated off at the top of the rectification column 4 can be effected indirectly, for example by heat exchangers which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. Preferably, it is done by direct cooling. For this purpose, already condensed water-rich phase (sour water) is cooled by means of a suitable heat exchanger and the cooled liquid is sprayed in the vapor above the take-off point. This spraying may be done in a separate apparatus or in the rectification unit itself. When spraying in the rectification unit, the removal point of the water-rich phase (sour water) is advantageously designed as a catch bottom. By means of internals, which improve the mixing of the cooled water-rich phase (acid water) with the vapor, the effect of direct cooling can be increased. In principle, all common installations are suitable for this, for example floors, packings and / or fillings. Among the trays, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred. Among the beds, those with rings, coils, calipers, Raschig, Intos or Pall rings, Berl or Intalox saddles or braids are preferred. Particularly preferred are dual-flow trays. As a rule, 2 to 5 theoretical plates are sufficient here. These soils are not taken into account in the previous information on the number of theoretical plates of the rectification column 4. The direct condensation of the water-rich phase (sour water) can also be carried out in several stages, with upward decreasing temperature. Preferably, the cooling is carried out by direct cooling.

A part of the condensed at the top of the rectification column 4 water-rich phase (sour water) can be used as reflux, the remainder of the water-rich phase (acid water) is discharged and fed to the recovery of acrylic acid to a sour water extraction. 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 used as the rectification column 4. A dividing wall column has a vertical dividing wall which divides the cross section of a part of the column into two sections. The reflux is distributed to the two column sections. The inlet and the side outlet of the dividing wall column are located on different sides of the partition wall.

The crude acrylic acid removed from the rectification column 4 can be used directly for the preparation of water-absorbing polymer particles. Preferably, the crude acrylic acid is further purified by crystallization. The mother liquor obtained in the crystallization can be recycled to the rectification column 4, preferably below the take-off point for the crude acrylic acid. Preference is given to using the recycled mother liquor for cooling the crude acrylic acid and the crude acrylic acid discharged from the rectification column 4 to heat the mother liquor (heat bond).

The crude acrylic acid can be purified by layer crystallization, as described, for example, in EP 0 616 998 A1, or by suspension crystallization, as described in DE 100 39 025 A1. The suspension crystallization is preferred. The combination of a suspension crystallization with a wash column, as described in WO 2003/041832 A1, is particularly preferred. The acrylic acid thus prepared can be used directly as a monomer for the preparation of homopolymers or copolymers, in particular acrylic acid homopolymers, acrylic acid / maleic anhydride copolymers, acrylic acid / maleic acid copolymers and acrylic acid / methacrylic acid copolymers, but also for the preparation of water-absorbing polymer particles and Ac - Rylsäureestern, eg Methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate and their homo- and Copolymerimeren thereof are used.

Preparation of Water-Absorbent Polymer Particles Water-absorbing polymer particles are prepared by polymerizing a monomer solution or suspension containing a) at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized, in particular partially neutralized acrylic acid

b) at least one crosslinker,

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

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

The monomers a) are preferably water-soluble, i. the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 100 g of 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 in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly 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 radically copolymerized into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b).

Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be incorporated in the polymer network in free-radically polymerized form. 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 0 530 438 A1, di- and triacrylates, as in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 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, in addition to acrylate groups, contain further ethylenically unsaturated Groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, as described, for example, 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 polyethyleneglyoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form diioder triacrylates, as described, for example, in WO 2003/104301 A1. Particularly advantageous are di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol. Very particular preference is given to diacrylates or triacrylates of 1 to 5 times ethoxylated and / or propoxylated glycerol. Most preferred are the triacrylates of 3 to 5 times ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-times ethoxylated glycerol.

The amount of crosslinker b) is preferably from 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) decreases and the absorption under a pressure of 21.0 g / cm ² passes through a maximum.

As initiators c) it is possible to use all compounds which generate free radicals under the polymerization conditions, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal initiators and redox initiators are used, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. However, the reducing component used is preferably a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals, Heilbronn, Germany).

For example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, are ethylenically unsaturated monomers d) which are copolymerizable with the ethylenically unsaturated acid group-carrying monomers a). As water-soluble polymers e) it is possible to use 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. Usually, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, more preferably from 45 to 70 wt .-%, most preferably from 50 to 65 wt .-%. It is also possible to use monomer suspensions, ie monomer solutions with excess monomer a), for example sodium acrylate. With increasing water content, the energy expenditure increases during the subsequent drying and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

The polymerization inhibitors customarily used in acrylic acid require dissolved oxygen for optimum action. Therefore, the monomer solution may be polymerized prior to polymerization by inerting, i. Flow through with an inert gas, preferably nitrogen or carbon dioxide, are freed of dissolved oxygen. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, most preferably less than 0.1 ppm by weight. Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel resulting from the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating 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 Pat. No. 6,241,928. During the polymerization in a belt reactor, a polymer gel is formed, which must be comminuted in a further process step, for example in an extruder or kneader.

In order to improve the drying properties, the comminuted polymer gel obtained by means of a kneader may additionally be extruded.

However, it is also possible to drip an aqueous monomer solution and to polymerize the drops produced in a heated carrier gas stream. Here, the process steps polymerization and drying can be summarized, as described in WO 2008/040715 A2, WO 2008/052971 A1 and WO 201 1/026876 A1.

The acid groups of the polymer gels obtained are usually partially neutralized. The neutralization is preferably carried out at the stage of the monomers. This is usually done by mixing the neutralizing agent as an aqueous solution or 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%, wherein the customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or Alkalimetallhydrogenkarbonate and mixtures thereof. Instead of alkali metal salts and ammonium salts can be used. Sodium and potassium are particularly preferred as alkali metals, but most preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof. But it is also possible to carry out the neutralization after the polymerization at the stage of Polymergeis formed during the polymerization. Furthermore, it is possible to neutralize up to 40 mol%, preferably 10 to 30 mol%, particularly preferably 15 to 25 mol%, of the acid groups before the polymerization by adding a part of the neutralizing agent already to the monomer solution and the desired final degree of neutralization only after the polymerization is adjusted at the stage of Polymergeis. If the polymer gel is 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 can be sprayed, sprinkled or poured on and then thoroughly mixed in. For this purpose, the gel mass obtained can be extruded several times for homogenization.

The polymer gel is then preferably dried with a belt dryer until the residual moisture content is preferably 0.5 to 15 wt .-%, particularly preferably 1 to 10 wt .-%, most preferably 2 to 8 wt .-%, wherein the residual moisture content according to EDA-NA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel has too low a glass transition temperature T _g and is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle and in the subsequent comminution steps undesirably large quantities of polymer particles with too small particle size ("fines") are produced. , particularly preferably from 35 to 70% by weight, very particularly preferably from 40 to 60% by weight, but optionally also a fluidized-bed dryer or a paddle dryer can be used for the drying. The dried polymer gel is then ground and classified, for grinding usually single or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills, can be used.

The mean particle size of the polymer particles separated off as product fraction is preferably at least 200 μm, more preferably from 250 to 600 μm, very particularly from 300 to 500 μm. The mean particle size of the product fraction can be determined by means of the EDANA recommended test method No. WSP 220.2-05 "Particle Size Distribution", in which the mass fractions of the sieve fractions are cumulatively applied and the average particle size is determined graphically. The average particle size here is the value of the mesh size, which results for a cumulative 50% by weight.

The proportion of particles having a particle size of greater than 150 μm is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight. Polymer particles with too small particle size lower the permeability (SFC). Therefore, the proportion of too small polymer particles ("fines") should be low.

Polymer particles which are too small are therefore usually separated off and returned to the process. This is preferably done before, during or immediately after the polymerization, i. before drying the polymer gel. The too small polymer particles can be moistened with water and / or aqueous surfactant before or during the recycling.

It is also possible to separate off small polymer particles in later process steps, for example after surface postcrosslinking or another coating step. In this case, the recycled too small polymer particles are surface postcrosslinked or otherwise coated, for example with fumed silica.

If a kneading reactor is used for the polymerization, the too small polymer particles are preferably added during the last third of the polymerization.

If the polymer particles which are too small are added very late, for example only in an apparatus downstream of the polymerization reactor, for example an extruder, then the polymer particles which are too small can only be incorporated into the resulting polymer gel with difficulty. Insufficiently incorporated too small polymer particles, however, dissolve again during the grinding of the dried polymer gel, are therefore separated again during classification and increase the amount of recycled too small polymer particles.

The proportion of particles having a particle size of at most 850 μm, is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight.

The proportion of particles having a particle size of at most 600 μm, is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.

Polymer particles with too large particle size reduce the swelling rate. Therefore, the proportion of polymer particles too large should also be low. Too large polymer particles are therefore usually separated and recycled to the grinding of the dried Polymergeis.

The polymer particles can be surface postcrosslinked to further improve the properties. Suitable surface postcrosslinkers are compounds containing groups that 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 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230.

Furthermore, in DE 40 20 780 C1 cyclic carbonates, in 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-oxazolidinone, in DE 198 54 573 A1 2-Oxotetrahydro-1,3-oxazine and its derivatives, in DE 198 54 574 A1 N-acyl-2-oxazolidinones, in DE 102 04 937 A1 cyclic ureas, in DE 103 34 584 A1 bicyclic Amidoacetals, described in EP 1 199 327 A2 oxetanes and cyclic ureas and in WO 2003/031482 A1 morpholine-2,3-dione and its derivatives as suitable surface postcrosslinkers.

Preferred surface postcrosslinkers 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, it is also possible to use surface postcrosslinkers which contain additional polymerizable ethylenically unsaturated groups, as described in DE 37 13 601 A1

The amount of surface postcrosslinker is preferably 0.001 to 5 wt .-%, more preferably 0.02 to 2 wt .-%, most preferably 0.05 to 1 wt .-%, each based on the polymer particles.

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

The polyvalent cations which can be used in the process according to the invention are, 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 Cations of titanium and zirconium. As a counterion, hydroxide, chloride, bromide, sulfate, hydrogen sulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate, citrate and lactate, are possible. It is also possible to use salts with different counterions, for example basic aluminum salts, such as aluminum monoacetate or aluminum monolactate. Aluminum sulfate, aluminum monoacetate and aluminum lactate are preferred. Apart from metal salts, polyamines can also be used as polyvalent cations. The amount of polyvalent cation used is, for example, from 0.001 to 1% by weight, preferably from 0.005 to 0.5% by weight, more preferably from 0.02 to 0.2% by weight. in each case based on the polymer particles. The surface postcrosslinking is usually carried out so that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. Subsequent to the spraying, the polymer coated with surface postcrosslinker are thermally dried, whereby the surface postcrosslinking reaction can take place both before and during drying.

The spraying of a solution of the surface postcrosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Particularly preferred are horizontal mixers, such as paddle mixers, very particularly preferred are vertical mixers. The distinction between horizontal mixer and vertical mixer is made by the storage of the mixing shaft, i. Horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers are, for example, Horizontal Pflugschar® mixers (Gebr. Lödige Maschinenbau GmbH, Paderborn, Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV, Doetinchem, the Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati, USA) and Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, The Netherlands). However, it is also possible to spray the surface postcrosslinker solution in a fluidized bed.

The surface postcrosslinkers are typically used as an aqueous solution. The amount of non-aqueous solvent or total solvent can be used to adjust the penetration depth of the surface postcrosslinker into the polymer particles.

If only water is used as the solvent, it is advantageous to add a surfactant. As a result, the wetting behavior is improved and the tendency to clog is reduced. However, preference is given to using solvent mixtures, for example isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the mixing mass ratio preferably being from 20:80 to 40:60.

The thermal drying is preferably carried out in contact dryers, more preferably paddle dryers, very particularly preferably disk dryers. Suitable dryers are, for example, Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH;

Leingarten; Germany), Hosokawa Bepex® Disc Dryer (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 also be used. The drying can take place in the mixer itself, by heating the jacket or blowing hot air. Also suitable is a downstream dryer, such as a hopper dryer, a rotary kiln or a heatable screw. Particularly advantageous is mixed and dried in a fluidized bed dryer.

Preferred drying temperatures are in the range 100 to 250 ° C, preferably 120 to 220 ° C, more 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 are cooled after the thermal drying. The cooling is preferably carried out in contact coolers, particularly preferably blade coolers, very particularly preferably disk coolers. Suitable coolers are, for example, Hosokawa Bepex® Horizontal Paddle Coolers (Hosokawa Micron GmbH, Leingarten, Germany), Hosokawa Bepex® Disc Coolers (Hosokawa Micron GmbH, Leingarten, Germany), Holo-Flite® coolers (Metso Minerals Industries, Inc., Danville, USA ) and Nara Paddle Cooler (NARA Machinery Europe, Frechen, Germany). Moreover, fluidized bed coolers can also be used.

In the cooler, the water-absorbing polymer particles to 20 to 150 ° C, preferably 30 to 120 ° C, more preferably 40 to 100 ° C, most preferably 50 to 80 ° C, cooled. Subsequently, the surface-postcrosslinked polymer particles can be classified again, wherein too small and / or too large polymer particles are separated and recycled to the process.

The surface-postcrosslinked polymer particles can be coated or rehydrated to further improve the properties.

The post-wetting is preferably carried out at 30 to 80 ° C, more preferably at 35 to 70 ° C, most preferably at 40 to 60 ° C. If the temperatures are too low, the water-absorbing polymer particles tend to agglomerate and water evaporates appreciably at higher temperatures. The amount of water used for the rewetting is preferably from 1 to 10 wt .-%, particularly preferably from 2 to 8 wt .-%, most preferably from 3 to 5 wt .-%, each based on the water-absorbing polymer particles. By rewetting the mechanical stability of the polymer particles is increased and their tendency to static charge reduced. The post-humidification in the cooler is advantageously carried out after the thermal drying. Suitable coatings for improving the swelling 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 unwanted caking tendency of the polymer particles are, for example, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20.

Methods Determination of the content of 3-hydroxypropionic acid and acrylic acid

The contents of 3-hydroxypropionic acid and acrylic acid are determined by reversed-phase chromatography with ultraviolet detection. For sample preparation approx. 100 to 300 mg sample are weighed into a 50 ml volumetric flask and filled up with eluent A. Eluent A is a mixture of 1000 ml of water and 1 ml

0.5 molar sulfuric acid.

For the calibration of the 3-hydroxypropionic acid, four initial weights (about 280 mg, 180 mg, 90 mg and 60 mg) are used, with about 100 μΙ 25gew.% sulfuric acid to a pH of before filling the 50 ml volumetric flask 3 to 4 is acidified (possibly acidify). The calibration range is 0.1 to 280 mg / 50ml.

To calibrate the acrylic acid, at least two initial weights are diluted to at least six concentrations. The calibration range is 0.01 to 0.9 mg / 50ml.

For reversed-phase chromatography, a separation column of the type Prontosil 120-3-C18 AQ 3μηη, 150 × 4, 6 mm (BISCHOFF Analysentechnik GmbH, Leonberg, Germany) is used. The temperature is 25 ° C, the injection volume is 50 μΙ, the flow rate is 1, 5 ml / min and the duration is 15 minutes. The UV detector is set to 205 nm. From beginning to 8 minutes, 100% by weight of eluent A, from 8 to 11.5 minutes, a mixture of 40% by weight of eluent A and 60% by weight of eluent B, of 11.5 minutes to the end 100% by weight of eluent A is used. Eluent B is acetonitrile. Determination of the content of oligomeric 3-hydroxypropionic acid and oligomeric acrylic acid

The contents of oligomeric 3-hydroxypropionic acid and oligomeric acrylic acid are determined by ion exclusion chromatography with refractive index detection. For sample preparation, the components to be analyzed are separated from the sample matrix by means of solid phase extraction. For this purpose, an SPE Cartridge type bakerband SiOH 6 ml, 1000 mg (JTBaker, Avantor Performance Materials, Inc., Center Valley, PA, USA). The SPE cartridge is activated with 6 ml of methanol and rinsed twice with 6 ml of eluent each time. The SPE cartridge should never run dry. The sample is then pipetted onto the SPE cartridge and rinsed ten times with 1 ml of eluent in a 10 ml volumetric flask. The amount of sample used in sump samples is 65 μΙ, in head samples 85 μΙ and in extract samples 75 μΙ. If the samples do not contain a hydrophobic solvent (high-boiling organic solvent, entrainment agent), these samples can be sprayed on without extraction by dissolving 85 μΙ directly in 10 ml of eluent. The eluant used is 0.1% by volume aqueous phosphoric acid.

For ion-exclusion chromatography, two separation columns of the type Shodex RSpak KC-81 1, 300x8 mm (SHOWA DENKO K.K. Shodex (Separation & HPLC) Group, Kawasaki, Japan) are used in series. The temperature is 40 ° C, the injection volume is 100 μΙ, the flow rate is 1, 0 ml / min and the running time is 45 minutes. The eluent used is 0.1% strength by weight aqueous phosphoric acid. The autosampler is cooled to 15 ° C.

For evaluation, a blank value deduction is made before integrating. For this purpose, eluent is injected and the chromatogram thus obtained is subtracted from the sample chromatogram. The evaluation is carried out using area percent, whereby the following formula converts it into percent by weight:

% By weight (oligomer) = ^ ^ew ^ ^is ^ {monomer) _x Fiä _c fo _en% (Qiig _0mer ^

Area% (monomer) To evaluate the oligomers, the contents of the dimers, trimers, tetramers and pentamers (i.e., n = 2 to 5) are added together. Retention times are controlled by injection of 3-hydroxypropionic acid and diacrylic acid.

Determination of pH

To determine the pH, 1.0 g of sample is dissolved or suspended in 10 ml of demineralized water at 25.degree. After 10 minutes, the pH of the aqueous phase is measured at 25 ° C by means of a pH electrode.

Examples

Example 1 (comparison)

The dehydration of 3-hydroxypropionic acid to acrylic acid was carried out in a forced circulation flash evaporator reactor with rectification column attached. The reactor used was a glass container with a double jacket. The amount of liquid in the reactor was about 2500 g. The reactor is at the same time the bottom of the rectification column.

The forced circulation flash evaporator consisted of a pump, a heat exchanger and a pressure holding valve. The contents of the reactor were circulated by the pump via the heat exchanger and the pressure-maintaining valve. The heat exchanger was heated by means of heat transfer oil. The liquid level in the reactor was controlled by the temperature of the heat transfer oil. The rectification column had an inner diameter of 50 mm and was heated electrically accompanied. The rectification column had expanded metal packages of 50 cm in length (MONTZ Pak type BSH-750, Julius Montz GmbH, Hilden, Germany).

As feed, 250 g / h of aqueous 3-hydroxypropionic acid and 10 g / h of diethyl phthalate were fed into the reactor. The aqueous 3-hydroxypropionic acid had the following composition:

39.1% by weight of water,

0.8% by weight of acrylic acid,

1.0% by weight of oligomeric acrylic acid,

42.1 wt .-% of 3-hydroxypropionic acid and

14.0% by weight of oligomeric 3-hydroxypropionic acid

The diethyl phthalate additionally contained 0.1% by weight of phenothiazine and 0.5% by weight of hydroquinone monomethyl ether for polymerization inhibition.

The forced circulation flash evaporator conveyed the reactor contents in a circle. Before the pressure relief valve the pressure was 1, 4 bar and the temperature 170 ° C.

The pressure at the top of the rectification column was 150 mbar. The vapor was condensed by means of a cooler and partly recycled as reflux into the rectification column and partially discharged. 284 g / h of condensate were discharged. The condensate had the following composition:

49.5% by weight of water,

50.7% by weight of acrylic acid,

<0.0001 wt .-% of 3-hydroxypropionic acid and

<0.1% by weight of diethyl phthalate

Below and above the expanded metal packing each 12 g / h of a 5 wt .-% aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl were metered. Furthermore, 20 g / h of a solution of phenothiazine and hydroquinone monomethyl ether in acrylic acid were metered into the condensate. The solution contained 2% by weight of phenothiazine and 4% by weight of hydroquinone monomethyl ether. 13 g / h of residue were discharged from the reactor.

After a running time of 180 hours, the temperature of the heat transfer oil had risen from 190 ° C to 252 ° C. There were significant deposits in the heat exchanger and precipitated solid found in the solution.

Example 2

The dehydration of 3-hydroxypropionic acid to acrylic acid was carried out in a forced circulation flash evaporator reactor with rectification column attached.

The reactor used was a glass container with a double jacket. The amount of liquid in the reactor was about 2500 g. The reactor is at the same time the bottom of the rectification column. The forced circulation flash evaporator consisted of a pump, a heat exchanger and a pressure holding valve. The contents of the reactor were circulated by the pump via the heat exchanger and the pressure-maintaining valve. The heat exchanger was heated by means of heat transfer oil. The liquid level in the reactor was controlled by the temperature of the heat transfer oil.

The rectification column had an inner diameter of 50 mm and was heated electrically accompanied. The rectification column had expanded metal packages 50 cm in length (MONTZ Pak type BSH-750, Julius Montz GmbH, Hilden, Germany) as feed were 250 g / h of aqueous 3-hydroxypropionic acid and 40 g / h of aqueous sulfolane

(50gew .-%) conveyed into the reactor. The aqueous 3-hydroxypropionic acid had the following composition:

40.5% by weight of water,

I, 6% by weight of acrylic acid,

1.7% by weight of oligomeric acrylic acid,

41.6 wt .-% of 3-hydroxypropionic acid and

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

The aqueous sulfolane additionally contained 0.1% by weight of phenothiazine and 0.5% by weight of hydroquinone monomethyl ether for polymerization inhibition. The forced circulation flash evaporator conveyed the reactor contents in a circle. Before the pressure relief valve, the pressure was 1, 4 bar and the temperature 200 ° C.

The pressure at the top of the rectification column was 150 mbar. The vapor was condensed by means of a cooler and partly recycled as reflux into the rectification column and partially discharged. 296 g / h of condensate were discharged. The condensate had the following composition: 57.3% by weight of water,

42.2% by weight of acrylic acid,

<0.0001 wt .-% of 3-hydroxypropionic acid and

<0.1% by weight of sulfolane 12 g / h of a 5% strength by weight aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl were metered in underneath and above the expanded metal packing.

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

From the reactor 25 g / h of residue were discharged.

After a running time of 207 hours, the temperature of the heat transfer oil had risen from 222 ° C to 230 ° C. There were almost no deposits found in the heat exchanger.

Example 3 (Comparative) The dehydration of 3-hydroxypropionic acid to acrylic acid was carried out in a forced circulation flash evaporator reactor with rectification column attached.

The reactor used was a glass container with a double jacket. The amount of liquid in the reactor was about 2500 g. The reactor is at the same time the bottom of the rectification column.

The forced circulation flash evaporator consisted of a pump, a heat exchanger and a pressure holding valve. The contents of the reactor were circulated by the pump via the heat exchanger and the pressure-maintaining valve. The heat exchanger was heated by means of heat transfer oil. The liquid level in the reactor was controlled by the temperature of the heat transfer oil. The rectification column had an inner diameter of 50 mm and was heated electrically accompanied. The rectification column had expanded metal packages 50 cm in length (MONTZ Pak type BSH-750, Julius Montz GmbH, Hilden, Germany) 250 g / h aqueous 3-hydroxypropionic acid and 20 g / h diethyl phthalate were fed into the reactor as feed. The aqueous 3-hydroxypropionic acid had the following composition:

40.5% by weight of water,

I, 6% by weight of acrylic acid,

1.7% by weight of oligomeric acrylic acid,

41.6 wt .-% of 3-hydroxypropionic acid and

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

The forced circulation flash evaporator conveyed the reactor contents in a circle. Before the pressure relief valve, the pressure was 1, 4 bar and the temperature 200 ° C. The pressure at the top of the rectification column was 150 mbar. The vapor was condensed by means of a cooler and partly recycled as reflux into the rectification column and partially discharged. 266 g / h of condensate were discharged. The condensate had the following composition: 62.8% by weight of water,

36.7% by weight of acrylic acid,

<0.0001 wt .-% of 3-hydroxypropionic acid and

<0.01% by weight of diethyl phthalate 12 g / h of a 5% strength by weight aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl were metered in underneath and above the expanded metal packing.

45 g / h of residue were discharged from the reactor.

After a running time of 16 hours, the temperature of the heat transfer oil had risen from 220 ° C to 240 ° C. There were significant deposits in the heat exchanger and precipitated solid found in the solution. An analysis showed that it was phthalic anhydride. Example 4 Dehydration of 3-hydroxypropionic acid acrylic acid was carried out in a reactor

Forced circulation evaporator and attached rectification carried out.

The forced circulation flash evaporator consisted of a pump, a heat exchanger and a pressure holding valve. The contents of the reactor were circulated by the pump via the heat exchanger and the pressure-maintaining valve. The heat exchanger was heated by means of heat transfer oil. The liquid level in the reactor was controlled by the temperature of the heat transfer oil.

The rectification column had an inner diameter of 50 mm and was heated electrically accompanied. The rectification column had expanded metal packages of 50 cm in length (MONTZ Pak type BSH-750, Julius Montz GmbH, Hilden, Germany).

As feed, 175 g / h of aqueous 3-hydroxypropionic acid and 40 g / h of aqueous sulfolane (50% by weight) were fed into the reactor. The aqueous 3-hydroxypropionic acid had the following composition: 18.9% by weight of water,

1 .6% by weight of acrylic acid,

2.7% by weight of oligomeric acrylic acid,

54.8 wt .-% of 3-hydroxypropionic acid and

18.0% by weight of oligomeric 3-hydroxypropionic acid

The aqueous sulfolane additionally contained 0.1% by weight of phenothiazine and 0.5% by weight of hydroquinone monomethyl ether for polymerization inhibition.

The forced circulation flash evaporator conveyed the reactor contents in a circle. Before the pressure relief valve, the pressure was 1, 4 bar and the temperature 200 ° C.

The pressure at the top of the rectification column was 150 mbar. The vapor was condensed by means of a cooler and partly recycled as reflux into the rectification column and partially discharged. 223 g / h of condensate were discharged. The condensate had the following composition: 45.5% by weight of water,

54.8% by weight of acrylic acid,

<0.0001 wt .-% of 3-hydroxypropionic acid and

<0.01% by weight sulfolane

Below and above the expanded metal packing each 12 g / h of a 5 wt .-% aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl were metered.

From the reactor 25 g / h of residue were discharged. After a running time of 280 hours, the temperature of the heat transfer oil had risen from 224 ° C to 227 ° C. There were almost no deposits found in the heat exchanger.

Example 5

The reaction of an aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid to acrylic acid is carried out in a reactor with forced circulation desulphurisation apparatus and attached rectification column 4. An entrainer is added at the top of the rectification column.

The reactor used is a 51 glass container with a double jacket. The amount of liquid in the reactor is about 4000 g. The temperature in the reactor is 196 ° C. The pressure in the reactor is 230 mbar. The reactor is at the same time the bottom of the rectification column 4. The forced circulation expansion evaporator consists of a pump, a heat exchanger and a pressure-holding valve. The contents of the reactor are circulated by means of the pump via the heat exchanger and the pressure maintenance valve. The feed of aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid and high-boiling solvent is metered into the circuit before the heat exchanger. Below the heat exchanger, 13 l / h of an air / nitrogen mixture with an oxygen content of 6% by volume (lean air) are metered into the circuit.

250 g / h of aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid and 43 g / h of high-boiling solvent are used as feed. The aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid has the following composition: 20.1% by weight of water,

1.5% by weight of acrylic acid,

1, 5 wt .-% oligomeric acrylic acid

45.0% by weight of 3-hydroxypropionic acid.

29.5% by weight of oligomeric 3-hydroxypropionic acid,

0.4% by weight of 2-hydroxypropionic acid,

0.4% by weight of succinic acid,

0.0015% by weight of formaldehyde,

0.001% by weight of acetaldehyde,

0.012% by weight of arabitol,

0.041% by weight of erythritol,

0.7% by weight of 2-hydroxy (iso) valeric acid,

Glucose, glucose esters and other high boilers

The high-boiling solvent has the following composition:

70.0% by weight of sulfolane

30.0% by weight of water

39 g / h of residue are taken from the reactor

The rectification column 4 has an inner diameter of 50 mm and is electrically accompanied by heating. The rectification column has a total of 7 expanded metal packings each 50 cm in length (MONTZ Pak type BSH-750, Julius Montz GmbH, Hilden, Germany) and a side take-off with the side take-off between the 3rd and 4th expanded metal packs (counted from below). ,

The side trigger is a catch bottom. There, the liquid is completely removed as crude acrylic acid. Of these, 450 g / h are recycled as reflux below the catch tray in the rectification column 4 and the remaining crude acrylic acid is conveyed by a pump through a heat exchanger, thereby cooled to 15 ° C and converted into a suspension crystallization. There are obtained 1 102 g / h of crude acrylic acid having the following composition: 1, 2 wt .-% water,

98.8% by weight of acrylic acid,

<0.001% by weight of oligomeric acrylic acid 0.002% by weight of 3-hydroxypropionic acid,

<0.001% by weight of oligomeric 3-hydroxypropionic acid

0.03 wt.% Sulfolane,

0.007% by weight of phenothiazine

0.036% by weight of 2-hydroxypropionic acid, <0.001% by weight of succinic acid,

<0.001% by weight of formaldehyde,

<0.001% by weight of acetaldehyde,

0.006 wt .-% 2-hydroxy (iso) valeric acid and

0.01% by weight of toluene

For suspension crystallization, a glass container with a helical stirrer is used. The heat of crystallization is dissipated via a cooled double jacket. The temperature of the crystal suspension in the glass container is 13.8 ° C. The crystal suspension produced during the crystallization is separated discontinuously into crystals and mother liquor by means of a centrifuge at 2000 rpm and a spin time of 1 min. The crystals are washed with pure acrylic acid and melted. The mother liquor is heated to 50 ° C and completely dosed below the catch tray in the rectification column 4. The pure acrylic acid obtained is admixed with 1.6 g / h of a 2% strength by weight solution of hydroquinone monomethyl ether in pure acrylic acid. A total of 164 g / h of pure acrylic acid are obtained.

As buffer between the continuous rectification and the discontinuous suspension crystallization, two 51 glass containers with double jacket are used.

3.9 g / h of a 2% strength by weight solution of phenothiazine in crude acrylic acid are metered onto the top of the rectification column 4. The vapor obtained at the top of the rectification column 4 is cooled to 25 ° C. in a heat exchanger, condensed and collected in a phase separator. As entrainer toluene is used. 0.4 g / h of toluene are metered into the organic phase to compensate for losses of entrainer. The separated in the phase separator entraining agent is recycled to the top of the rectification column 4.

From the aqueous phase 1 180 g / h are sprayed in the vapor before the heat exchanger. The remaining aqueous phase (sour water) is discharged. The sour water has the following composition: 97.7% by weight of water,

2.2% by weight of acrylic acid,

0.003% by weight of formaldehyde,

0.002 wt .-% acetaldehyde and

0.07 wt .-% toluene

The exhaust gas is passed through another heat exchanger (aftercooler).

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Claims

claims

1 . Process for the continuous dehydration of aqueous 3-hydroxypropionic acid to acrylic acid in the liquid phase, wherein the liquid phase has a temperature of 120 to 250 ° C and aqueous acrylic acid is continuously separated from the liquid phase, characterized in that the liquid phase of 5 to 95 wt .-% of an aprotic solvent and the aprotic solvent containing a dipole moment of at least 10 x 10 ^"30 cm. 2. the method according to claim 1, characterized in that the aprotic solvent has a dipole moment 15 to 20 x ^10" having ³⁰ Cm ,

3. The method according to claim 1 or 2, characterized in that the aprotic solvent has a boiling point at 1013 mbar from 200 to 350 ° C.

4. The method according to any one of claims 1 to 3, characterized in that the aprotic solvent has a boiling point at 1013 mbar from 280 to 300 ° C.

5. The method according to any one of claims 1 to 4, characterized in that the apostatic solvent is selected from sulfolane, ethylene carbonate, propylene carbonate and gamma-valerolactone.

6. The method according to any one of claims 1 to 5, characterized in that the aprotic solvent is sulfolane.

7. The method according to any one of claims 1 to 6, characterized in that the used aqueous 3-hydroxypropionic acid of 15 to 35 wt .-% water.

8. The method according to any one of claims 1 to 7, characterized in that the aqueous acrylic acid is separated by means of a rectification column 2.

9. The method according to any one of claims 1 to 8, characterized in that the aqueous acrylic acid is separated by means of a rectification column 3 in a high-acrylic phase and a water-rich phase.

10. The method according to claim 9, characterized in that in the rectification column 3, an entraining agent is used.

1 1. A method according to claim 9 or 10, characterized in that the separation of the aqueous acrylic acid and the separation of the aqueous acrylic acid into an acrylic acid rich phase and a water-rich phase in a rectification column 4 is carried out, wherein the separation of the aqueous acrylic acid from the liquid phase takes place below a side draw in the rectification column 4, the separation of the aqueous acrylic acid in a high-acrylic phase and a high-water phase above the side of take off and the acrylic acid-rich phase is taken off liquid on the side take-off.

2. The method according to claim 1 1, characterized in that the rectification column 4 is a dividing wall column, wherein the feed to the rectification column 4 and the side draw of the rectification column 4 are located on different sides of the partition wall.

3. The method according to any one of claims 9 to 12, characterized in that the obtained high-acrylic phase is purified by crystallization.

4. The method according to claim 13, characterized in that the mother liquor of the crystallization is recycled below the side draw in the rectification column 4.