WO2015177026A1

WO2015177026A1 - Method for the continuous dehydration of 3-hydroxypropanoic acid to form acrylic acid

Info

Publication number: WO2015177026A1
Application number: PCT/EP2015/060635
Authority: WO
Inventors: Tim BLASCHKE; Ortmund Lang; Nicolai Tonio Woerz; Marco Hartmann
Original assignee: Basf Se
Priority date: 2014-05-19
Filing date: 2015-05-13
Publication date: 2015-11-26

Abstract

The invention relates to a method for dehydrating aqueous 3-hydroxypropionic acid to form acrylic acid. In a first step of said method, a low-water mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is produced, and in a second step, the low-water mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is reacted in a liquid phase to form aqueous acrylic acid.

Description

Process for the continuous 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 in a first step, a low-water mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is produced and in a second step, the low-water mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is reacted in the liquid phase to aqueous acrylic acid.

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 on an industrial scale almost 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.

DE 1 668 609 A1 describes the workup of dilute aqueous solutions of acrylic and / or hydracrylic acid. 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, characterized in that in a first step i) from the aqueous 3-hydroxypropionic acid, a portion of the water is distilled off at a pressure of less than 900 mbar , Wherein the pH during the distillation is greater than 1.5, and part of the monomeric 3-hydroxypropionic acid undergoes dehydration to convert into oligomeric 3-hydroxypropionic acid during the distillation, and in a second step ii) the aqueous mixture obtained in step i) from monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid in the liquid phase at a pressure of less than 900 mbar to acrylic acid is reacted.

The pressure is the pressure in the reactor or in a distillation, the pressure in the distillation bottoms.

The pH during step i) is preferably at least 1.7, more preferably at least 1.9, 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 monomers

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 has been reduced is the difference between the water content of the aqueous 3-hydroxypropionic acid (starting material) used in step i) and the water content of the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid obtained in step i) (product ). 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 reduced is the difference between the content of monomeric 3-hydroxypropionic acid of the aqueous 3-hydroxypropionic acid (starting material) used in step i) and the content of monomeric 3-hydroxypropionic acid in step i) obtained 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 temperature during the reaction in step i) is preferably less than 100.degree. C., more preferably less than 90.degree. C., most preferably less than 80.degree. Too high temperatures in step i) favor the undesired dehydration of monomeric 3-hydroxypropionic acid to acrylic acid.

The pressure in the reaction in step i) is preferably from 10 to 300 mbar, particularly preferably from 20 to 200 mbar, very particularly preferably from 40 to 150 mbar. Lower pressures in step i) allow gentle removal of the water from the liquid phase. Too low pressures are uneconomical.

The distillative separation of water from aqueous acrylic acid is expensive or associated with high acrylic acid losses. The present invention is based on the finding that the water contained in the aqueous 3-hydroxypropionic acid is better separated off before dehydration. Due to the larger boiling point difference, water can be separated better from aqueous 3-hydroxypropionic acid than from aqueous acrylic acid.

In addition, monomeric 3-hydroxypropionic can react during the separation of water to oligomeric 3-hydroxypropionic acid, with additional water produced. This additional water can be separated off immediately in step i). The resulting oligomeric 3-hydroxypropionic acid can likewise be converted into acrylic acid in step ii) (retro-Michael addition). In contrast to the dehydration of monomeric 3-hydroxypropionic acid no more water is released in the cleavage of oligomeric 3-hydroxypropionic acid. A high proportion of oligomeric 3-hydroxypropionic acid in step ii) thus results in a lower water content of the aqueous acrylic acid obtained in step ii).

A relatively high proportion of oligomeric 3-hydroxypropionic acid and thus a low proportion of monomeric 3-hydroxypropionic acid are therefore desirable and can be achieved, for example, by a correspondingly long residence time in step i). Prevention of oligomeric 3-hydroxypropionic acid, as required in WO 2012/091 1 14 A1, is therefore detrimental to the process according to the invention. Particularly advantageously, the water is separated in step i) by means of a rectification column (rectification column 1).

The liquid phase in step ii) advantageously contains a high-boiling organic solvent. The high-boiling organic solvent dilutes the acrylic acid formed in the liquid phase and high-boiling by-products (high boilers).

The aqueous acrylic acid formed in the reaction in step ii) is preferably removed by distillation. Particularly suitable for this purpose are rectification columns (rectification column 2). By selecting the separation stages and the reflux ratio, the content of 3-hydroxypropionic acid in the distillate can be kept low.

The aqueous acrylic acid obtained in step ii) can be separated by distillation into a high-acrylic phase (crude acrylic acid) and a water-rich phase (acid water). To facilitate the separation, an entraining agent can be used. Suitable entraining agents are low-boiling hydrophobic organic solvents. Also suitable for the distillative separation are also rectification columns (rectification column 3).

Particularly advantageously, the rectification column 2 and the rectification column 3 are combined to form a single rectification column 4. The separation of the aqueous acrylic acid from the liquid phase in step ii) and the separation of the aqueous acrylic acid into a high-acrylic phase and a high-water phase in step iii) in the rectification column 4 are carried out, 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 is carried out above the side draw and the acrylic acid-rich phase (crude acrylic acid) is taken off liquid side draw.

The crude acrylic acid can be further purified by crystallization. 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 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 aqueous 3-hydroxypropionic acid produced by fermentation. Such a method is described, for example, in

WO 02/090312 A1 discloses.

Preparation of acrylic acid A portion of the water is distilled off from the aqueous 3-hydroxypropionic acid, part of the monomeric 3-hydroxypropionic acid reacting with elimination of water to give oligomeric 3-hydroxypropionic acid.

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 in step i) favor the undesired dehydration of monomeric 3-hydroxypropionic acid to acrylic acid.

The pressure in the reaction is preferably from 10 to 300 mbar, particularly preferably from 20 to 200 mbar, very particularly preferably from 40 to 150 mbar. Lower pressures in step i) allow gentle removal of the water from the liquid phase. Too low pressures are uneconomical.

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 resulting aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid preferably contains from 1 to 35% by weight of water, more preferably from 3 to 25% by weight of water, most preferably from 5 to 15% by weight of water , The water content is preferably lowered 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 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 reduced 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, i. to the pentamer, using the calibration factor of the monomeric 3-hydroxypropionic acid and the sum formed.

Monomeric acrylic acid and oligomeric acrylic acid can be determined analogously.

The water is advantageously separated off by means of a rectification column 1. The rectification column 1 is of a type known per se 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. Dual-flow trays below the feed (output part) and Thormann trays above the feed (enrichment part) are particularly preferred. 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 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 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 resulting aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is removed continuously from the bottom of the distillation and converted to acrylic acid.

The reaction of the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid to acrylic acid is carried out in the liquid phase at a temperature of preferably from 130 to 220 ° C, more preferably from 150 to 200 ° C, most preferably from 160 to 190 ° C, performed. 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 low pressure, the liquid phase contains less monomeric acrylic acid and in the case of distillative removal of the resulting acrylic acid, the undesirable formation of oligomeric acrylic acid in the condensate is suppressed.

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 liquid phase preferably contains a polymerization inhibitor 1. Suitable polymerization inhibitors 1 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 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 fed below the evaporator.

The liquid phase preferably contains from 20 to 95% by weight, more preferably from 40 to 85% by weight, most preferably from 50 to 80% by weight of the high-boiling organic solvent. The boiling point of the high-boiling organic solvent is 1013 mbar in the range of preferably 200 to 350 ° C, more preferably from 250 to 320 ° C, most preferably from 280 to 300 ° C. Suitable high-boiling organic solvents are, for example, dimethyl phthalate, diethyl phthalate, dimethyl isophthalate, diethyl isophthalate, dimethyl terephthalate, diethyl terephthalate, alkanoic acids, such as nonanoic acid and decanoic acid, biphenyl and / or diphenyl ether.

The reaction of the aqueous mixture of 3-hydroxypropionic acid and oligomeric

3-hydroxypropionic acid can be catalyzed base or acid. 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 resulting in the implementation of the aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid aqueous acrylic acid is preferably removed by distillation, more preferably by means of a rectification column (rectification column 2).

When using a rectification column 2, 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 2 and the aqueous mixture of monomeric 3 Hydroxypropionic acid and oligomeric 3-hydroxypropionic acid is the feed of the rectification column 2.

When using a rectification column 2, the polymerization inhibitor 1 is at least partially metered 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, more preferably from 200 to 600 mbar, most preferably from 300 to 400 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. The sump pressure results from the head pressure, the number and type of column internals as well as 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 above the take-off 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 aqueous acrylic acid is advantageously designed as a capture bottom. By incorporation, which improves 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 a high-boiling organic 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 inert organic solvent 1. The resulting aqueous acrylic acid can be separated by distillation into a high-acrylic phase (crude acrylic acid) and a water-rich phase (acid water).

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 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, hydroquinone and / or hydroquinone monomethyl ether. Very particular preference is given to phenothiazine and hydroquinone monomethyl ether.

To assist in separating the aqueous acrylic acid into a high-acrylic acid phase (crude acrylic acid) and a high-water phase (sour water), 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), 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 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. 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. 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 3 is usually made of austenitic steel, preferably of the material 1.4571 (according to DIN EN 10020). The cooling of the water-rich phase (acid water) separated off at the top of the rectification column 3 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 (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 nation 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) by means of a rectification column with a side draw (rectification column 4) carried out. The rectification column 4 combines the tasks of the rectification columns 2 and 3 in a single rectification column. In this case, the section below the side draw corresponds to the rectification column 2 and the section above the side draw corresponds to the rectification column 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 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 4th

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. The feed temperature is preferably from 25 to 150 ° C, more preferably from 40 to 100 ° C, most preferably from 50 to 70 ° C.

The heat supply 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. 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.

If an oxygen-containing gas is used for polymerization inhibition, this is preferably fed below the evaporator. 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 (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 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 withdrawn 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.

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 polymers, e.g. water-absorbing polymer particles, but also for the production of acrylic acid esters, e.g. Methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate.

Preparation of water-absorbing polymer particles

Water-absorbing polymer particles are obtained by polymerization of a monomer solution or suspension comprising 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 copolymerizable with the monomers mentioned under a) 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, ie 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 particularly preferred are Di- 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 before the polymerization by inerting, ie, flowing through with an inert gas, preferably nitrogen or carbon dioxide, are freed of dissolved oxygen. The oxygen content of the monomer solution before the polymerization is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is comminuted continuously 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 mechanical comminuted, for example by means of an extruder, wherein the neutralizing agent can be sprayed on, sprinkled or poured and then mixed thoroughly. 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. , more preferably from 35 to 70% by weight, most preferably from 40 to 60% by weight. Alternatively, a fluidized bed dryer or a paddle dryer can be used for drying.

The dried polymer gel is then ground and classified, wherein for grinding usually one- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills, can be used.

The average particle size of the polymer fraction separated as a 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 mean particle size here is the value of the mesh size, which results for accumulated 50 wt .-%. 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.

Too small polymer particles are therefore usually separated and recycled to the process. This preferably takes place before, during or immediately after the polymerization, ie before the drying of 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 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 described 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 Pat. No. 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, in EP 1 199 327 A2 oxetanes and cyclic ureas and in WO 2003/031482 A1 morpholine-2,3-dione and its derivatives are described 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 in addition to the surface postcrosslinkers before, during or after the surface postcrosslinking.

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 the 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 lent. 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 surface postcrosslinker-coated polymer Particles thermally dried, wherein 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 penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by the content of nonaqueous solvent or total solvent amount. 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 include 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 post-moistened for further improvement of 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 clump together and at higher temperatures water is already noticeably evaporating. 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 lymerpartikel. By rewetting the mechanical stability of the polymer particles is increased and their tendency to static charge reduced. Advantageously, the post-humidification is carried out in the cooler 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 reverse 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 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 start to 8 minutes, 100% by weight of eluent A, from 8 to 11.5 minutes, will be a mixture of 40% by weight of eluent A and 60% by weight of eluent B, from 11.5 to the end 100% by weight of eluent A 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, a bakerband SiOH 6 ml, 1000 mg SPE cartridge (JTBaker, 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 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 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 KK 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 1,500 g of an approximately 30% strength by weight aqueous 3-hydroxypropionic acid were initially introduced into a 21 three-necked flask with double jacket and distillation head, water was distilled off at 100 mbar for 3 hours and the remaining residue was distilled at 40 mbar. The double jacket was heated by means of heat transfer oil. The pH during the distillation was in the range of 2 to 3. The composition of distillate and distillation residue was analyzed. Tab. 1: Composition of the distillate

Tab. 2: Composition of the distillation residue

3HPA 3-hydroxypropionic acid

AS acrylic acid

Oligo-3HPA oligomeric 3-hydroxypropionic acid

Oligo-AS oligomeric acrylic acid

The results show that monomeric 3-hydroxypropionic acid is first converted into oligomeric 3-hydroxypropionic acid in the distillation residue. Only then does acrylic acid and oligomeric acrylic acid develop appreciably. Presumably, dehydration of 3-hydroxypropionic acid proceeds via oligomeric 3-hydroxypropionic acid as an intermediate. For high yield and high selectivity, therefore, monomeric 3-hydroxypropionic acid must be converted to oligomeric 3-hydroxypropionic acid.

Example 2

The reaction of monomeric 3-hydroxypropionic acid to oligomeric 3-hydroxypropionic acid is carried out in a forced circulation flash evaporator reactor and rectification column 1. The reactor used is a glass container with a double jacket. The amount of liquid in the reactor is about 2500 g. The reactor is at the same time the bottom of the rectification column. 1

The forced circulation expansion evaporator consists of a pump, a heat exchanger and a pressure maintenance valve. The contents of the reactor are circulated by means of the pump via the heat exchanger and the pressure maintenance valve.

The rectification column 1 used is a 10-bottomed bubble-cap colony with an internal diameter of 50 mm. The rectification column 1 is electrically accompanied by heating.

As feed, 1000 g / h of aqueous 3-hydroxypropionic acid are conveyed to the bottom of the bubble-cap colony. The aqueous 3-hydroxypropionic acid has the following composition:

40.2% by weight of water,

2.1% by weight of acrylic acid,

0.1% by weight of oligomeric acrylic acid,

52.6 wt .-% of 3-hydroxypropionic acid and

5.0% by weight of oligomeric 3-hydroxypropionic acid The forced circulation flash evaporator conveys the reactor contents in a circle. Before the pressure-maintaining valve, the pressure is 2.0 bar and the temperature is 175 ° C.

The pressure at the top of the rectification column 1 is 100 mbar. The vapor is condensed by means of a cooler and partially recycled as reflux into the rectification column 1 and partially discharged. There are discharged 298.8 g / h of condensate. The condensate has the following composition:

99.1% by weight of water,

0.9 wt .-% acrylic acid and

<0.0001% by weight of 3-hydroxypropionic acid

701.2 g / h of an aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid are discharged from the reactor. The mixture has the following composition:

20.1% by weight of water,

0.2% by weight of acrylic acid,

2.5% by weight of oligomeric acrylic acid,

32.4 wt .-% of 3-hydroxypropionic acid and

44.8% by weight of oligomeric 3-hydroxypropionic acid Example 3

The reaction of monomeric 3-hydroxypropionic acid to oligomeric 3-hydroxypropionic acid is carried out in a reactor with forced circulation flash evaporator and attached rectification column 1.

The reactor used is a glass container with a double jacket. The amount of liquid in the reactor is about 2500 g. The reactor is at the same time the bottom of the rectification column. 1 The forced circulation expansion evaporator consists of a pump, a heat exchanger and a pressure maintenance valve. The contents of the reactor are circulated by means of the pump via the heat exchanger and the pressure maintenance valve.

As feed, 1000 g / h of aqueous 3-hydroxypropionic acid are conveyed to the bottom of the bubble-cap colony. The aqueous 3-hydroxypropionic acid has the following composition: 40.2% by weight of water,

2.1% by weight of acrylic acid,

0.1% by weight of oligomeric acrylic acid,

52.6 wt .-% of 3-hydroxypropionic acid and

5.0% by weight of oligomeric 3-hydroxypropionic acid

The forced circulation evaporator evaporates the reactor contents in a circle. Before the pressure-maintaining valve, the pressure is 2.0 bar and the temperature is 175 ° C.

500 g / h of the reactor contents are removed, mixed with the feed and conveyed to the 5th floor of the bubble-cap colony.

The pressure at the top of the rectification column 1 is 100 mbar. The vapor is condensed by means of a cooler and partially recycled as reflux into the rectification column 1 and partially discharged. There are discharged 314.9 g / h of condensate. The condensate has the following composition:

99.35% by weight of water,

0.65 wt .-% acrylic acid and

<0.0001% by weight of 3-hydroxypropionic acid 685.1 g / h of an aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid are discharged from the reactor. The mixture has the following composition: 18.7% by weight of water,

<0.05% by weight of acrylic acid,

2.9% by weight of oligomeric acrylic acid,

28.4 wt .-% of 3-hydroxypropionic acid and

50.0% by weight of oligomeric 3-hydroxypropionic acid

Example 4

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.

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 174 ° C. The pressure in the reactor is 360 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 maintenance 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, 9 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 35 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: 29.5% by weight of water,

0.7% by weight of acrylic acid,

0.1% by weight of oligomeric acrylic acid

39.8% by weight of 3-hydroxypropionic acid.

27.3% by weight of oligomeric 3-hydroxypropionic acid,

0.9% 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

Diethyl phthalate was used as the high-boiling solvent. The high-boiling solvent additionally contained 0.1% by weight of phenothiazine and 0.5% by weight of hydroquinone monomethyl ether.

The reactor is removed 54 g / h residue.

The rectification column 4 has an inner diameter of 50 mm and is electrically accompanied by heating. The rectification column has a total of 80 trays and a side take-off, with 20 dual-flow trays below the side take-off and 60 bubble trays above the side take-off.

The side print between the 20th and 21st Soil is a catch bottom. There, the liquid is completely removed as crude acrylic acid and conveyed by a pump through a heat exchanger, while cooled to 15 ° C and transferred into a suspension crystallization. 294 g / h of crude acrylic acid having the following composition are obtained:

3.0% by weight of water,

97.0% by weight of acrylic acid,

<0.001% by weight of 3-hydroxypropionic acid.

<0.001% by weight of diethyl phthalate,

0.020% by weight of phenothiazine

0.040% by weight of hydroquinone monomethyl ether

<0.001% by weight of ethanol,

0.006% by weight of ethyl acrylate,

<0.001% by weight of 2-hydroxypropionic acid,

<0.001% by weight of formaldehyde,

<0.001% by weight of acetaldehyde and

<0.001% by weight of 2-hydroxy (iso) valeric acid

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 7.9 ° C. The crystal suspension produced in the crystallization is discontinuously separated by means of a centrifuge at 2000 U / min and a spin time of 1 min in crystals and mother liquor. The crystals are washed with pure acrylic acid and melted. The mother liquor is heated to 70 ° C and completely dosed to the 10th bottom of the rectification column 4.

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

10 g / h of a 0.5% by weight solution of phenothiazine in crude acrylic acid are metered into the 45th tray of the rectification column 4, and 5 g / h of a 1.0% by weight are added to the 80th tray of the rectification column 4. solution of hydroquinone monomethyl ether in acid water.

The vapor obtained at the top of the rectification column 4 is cooled to 20 ° C. in a heat exchanger and condensed (sour water). Of the sour water 7000 g / h are sprayed in the vapor upstream of the heat exchanger, metered 184 g / h to the 80th bottom of the rectification column 4 and discharged 83 g / h. The sour water has the following composition:

95.0% by weight of water,

5.0% by weight of acrylic acid,

0.025% by weight of ethanol,

0.015% by weight of ethyl acrylate,

<0.001 wt .-% formaldehyde and

<0.001% by weight of acetaldehyde

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

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 167 ° 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 maintenance 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 an aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3-hydroxypropionic acid and at high boiling point agent is metered into the circuit in front of 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 35 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:

29.5% by weight of water,

0.7% by weight of acrylic acid,

0.1% by weight of oligomeric acrylic acid

39.8% by weight of 3-hydroxypropionic acid.

27.3% by weight of oligomeric 3-hydroxypropionic acid,

0.9% 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 reactor is removed 53 g / h residue. The rectification column 4 has an inner diameter of 50 mm and is electrically accompanied by heating. The rectification column has a total of 60 trays and a side take-off, with 20 dual-flow trays below the side take-off and 40 bubble trays above the side take-off. The side print between the 20th and 21st Soil is a catch bottom. There, the liquid is completely removed as crude acrylic acid and conveyed by a pump through a heat exchanger, while cooled to 15 ° C and transferred into a suspension crystallization. There are obtained 304 g / h of crude acrylic acid having the following composition: 3.7% by weight of water,

96.3% by weight of acrylic acid, <0.001% by weight of 3-hydroxypropionic acid.

<0.001% by weight of diethyl phthalate,

0.020% by weight of phenothiazine

0.040% by weight of hydroquinone monomethyl ether

<0.001% by weight of ethanol,

<0.001% by weight of ethyl acrylate,

<0.001% by weight of 2-hydroxypropionic acid,

<0.001% by weight of formaldehyde,

<0.001% by weight of acetaldehyde,

<0.001% by weight of 2-hydroxy (iso) valeric acid and

<0.001% by weight of toluene

As a buffer between the continuous rectification and the discontinuous suspension crystallization, two 51 double-walled glass containers are used.

10 g / h of a 0.5% by weight solution of phenothiazine in crude acrylic acid are metered into the 45th tray of the rectification column 4, and 5 g / h of a 1.0% by weight are added to the 80th tray of the rectification column 4. solution of hydroquinone monomethyl ether in sour water.

The vapor obtained at the top of the rectification column 4 is cooled to 20 ° C. in a heat exchanger, condensed and collected in a phase separator. As entrainer toluene is used. 5 g / h of toluene are metered into the organic phase to compensate for losses of entrainer.

From the organic phase 7000 g / h are sprayed in the vapor upstream of the heat exchanger and metered 465 g / h to the 30th bottom of the rectification column 4. The aqueous phase (sour water) is discharged. The sour water has the following composition:

99.9% by weight of water,

0.022% by weight of acrylic acid,

0.014% by weight of ethanol,

0.0030% by weight of ethyl acrylate, 0.0013% by weight of formaldehyde,

<0.001% by weight of acetaldehyde and

0.068 wt .-% toluene The exhaust gas is passed through another heat exchanger (aftercooler).

Claims

claims

1 . A process for the continuous dehydration of aqueous 3-hydroxypropionic acid to acrylic acid, characterized in that in a first step i) from the aqueous 3-hydroxypropionic acid, a part of the water is distilled off at a pressure of less than 900 mbar, wherein the pH during the Distillation greater than 1, 5 and a part of the monomeric 3-hydroxypropionic acid during the distillation with elimination of water converts into oligomeric 3-hydroxypropionic acid, and in a second step ii) the obtained in step i) aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric 3rd -Hydroxypropionic acid in the liquid phase at a pressure of less than

900 mbar is converted to acrylic acid.

2. The method according to claim 1, characterized in that the obtained in step i) aqueous mixture of monomeric 3-hydroxypropionic acid and oligomeric

3-hydroxypropionic acid of 15 to 35 wt .-% water.

3. The method according to claim 1 or 2, characterized in that the water content in step i) is lowered by at least 15 wt .-%.

4. The method according to any one of claims 1 to 3, characterized in that the content of monomeric 3-hydroxypropionic acid in step i) by at least 25 wt .-% is lowered.

5. The method according to any one of claims 1 to 4, characterized in that the reaction in step i) is carried out at a temperature of less than 70 ° C.

6. The method according to any one of claims 1 to 5, characterized in that the reaction in step i) is carried out at a pressure of 40 to 150 mbar.

7. The method according to any one of claims 1 to 6, characterized in that in step i) the water is separated by means of a rectification column 1.

8. The method according to any one of claims 1 to 7, characterized in that in

Step ii) the liquid phase contains a high-boiling organic solvent.

9. The method according to any one of claims 1 to 8, characterized in that in

Step ii) aqueous acrylic acid is separated by distillation from the liquid phase.

10. The method according to claim 9, characterized in that in step ii) the aqueous acrylic acid is separated off by means of a rectification column 2.

1 1. Method according to one of claims 1 to 10, characterized in that the aqueous acrylic acid obtained in step ii) is separated by distillation in a step iii) in an acrylic acid-rich phase and a water-rich phase.

12. The method according to claim 1 1, characterized in that in step iii) an entraining agent is used.

A method according to claim 1 1 or 12, characterized in that in step iii), the aqueous acrylic acid is separated by means of a rectification column 3.

Process according to one of Claims 1 1 to 13, characterized in that the separation of the aqueous acrylic acid from the liquid phase in step ii) and the separation of the aqueous acrylic acid into a high-acrylic phase and a water-rich phase in step iii) are carried out in a rectification column 4 is, 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 is carried out above the side draw and the acrylic acid-rich phase is removed liquid at the side draw.

A method according to claim 14, 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 dividing wall. 16. The method according to any one of claims 1 to 15, characterized in that the acrylic acid-rich phase obtained in step iii) is purified by crystallization.