Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
PROCESS FOR PREPARING ACRYLIC ACID USING A HETEROGENEOUS ALUMINA CATALYST
Document Type and Number:
WIPO Patent Application WO/2016/026763
Kind Code:
A1
Abstract:
The invention relates to a process for preparing acrylic acid, which comprises contacting a reaction gas comprising 3-hydroxypropionic acid with a metal oxide-containing catalyst composition to obtain a gaseous reaction product containing acrylic acid, characterized in thatat least intermittently molecular oxygen is admixed to the reaction gas.

Inventors:
BRAND, Raphael Heinrich (Dackenheimer Weg 5, Weisenheim am Berg, 67273, DE)
ZAJACZKOWSKI-FISCHER, Marta (Otto-Dill-Straße 5, Neuhofen, 67141, DE)
BEBENSEE, Regine Helga (Ludwig-Börne-Straße 12, Ludwigshafen, 67061, DE)
HARTMANN, Marco (Breslauer Straße 1, Wörth, 76744, DE)
WÖRZ, Nicolai Tonio (Viktoriaplatz 1, Darmstadt, 64293, DE)
Application Number:
EP2015/068647
Publication Date:
February 25, 2016
Filing Date:
August 13, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
BASF SE (67056 Ludwigshafen, 67056, DE)
International Classes:
C07C51/377; C07C57/065
Domestic Patent References:
WO2013182818A12013-12-12
WO2012074818A22012-06-07
WO2011033689A12011-03-24
WO2006092271A22006-09-08
WO2008023039A12008-02-28
WO2002090312A12002-11-14
WO2003041832A12003-05-22
Foreign References:
EP0781743A11997-07-02
FR2882052A12006-08-18
FR2116003A51972-07-07
US20050222458A12005-10-06
US5250729A1993-10-05
DE102008038273A12010-03-04
EP2554258A12013-02-06
US20130274512A12013-10-17
JP2010180171A2010-08-19
EP2565211A12013-03-06
EP2565212A12013-03-06
EP13184086A2013-09-12
EP0854129A11998-07-22
EP0616998A11994-09-28
DE10039025A12002-02-21
Other References:
F. L. BUCHHOLZ; A. T. GRAHAM: "Modern Superabsorbent Polymer Technology", 1998, WILEY-VCH, pages: 71 - 103
Attorney, Agent or Firm:
REITSTÖTTER KINZEBACH (Sternwartstraße 4, München, 81679, DE)
Download PDF:
Claims:
A process for preparing acrylic acid, which comprises contacting a reaction gas comprising 3-hydroxypropionic acid with a metal oxide-containing catalyst composition to obtain a gaseous reaction product containing acrylic acid, characterized in that at least intermittently molecular oxygen is admixed to the reaction gas.

The process according to claim 1 , wherein the metal oxide-containing catalyst composition comprises at least one oxide of an element selected from the group consisting of aluminum, zirconium, titanium and silicon.

The process according to claim 1 or 2, wherein the reaction gas additionally comprises an inert diluent gas.

The process according to any of the preceding claims, wherein the reaction gas is contacted with the catalyst composition at a temperature of 200 to 400 °C.

The process according to any of the preceding claims, wherein the amount of molecular oxygen admixed to the reaction gas is from 0.1 to 10 % by volume, based on the total volume of the reaction gas.

The process according to any of the preceding claims, wherein the gaseous reaction product is treated with a quenching fluid to obtain a liquid mixture comprising acrylic acid and water.

The process according to claim 6, wherein the quenching fluid is an aqueous fluid.

The process according to claim 6, wherein the quenching fluid is an inert organic solvent.

The process according to any of the claims 6 to 8, wherein the liquid mixture is extracted with an extraction fluid to obtain an organic extract comprising the extraction fluid and acrylic acid.

The process according to claim 8, wherein the organic extract obtained is distilled to obtain crude acrylic acid. The process according to claims 6 to 10, wherein the acrylic acid is purified by crystallization.

12. The process according to claim 1 1 , wherein wherein the acrylic acid sent to the crystallization comprises less than 10% by weight of water. 13. The process according to claim 12, wherein the mother liquor from the crystallization is recycled into the rectification column.

14. The process according to claims 1 1 to 13, wherein the crystallization is a suspension crystallization.

15. The process according to any of the preceding claims, comprising vaporization of

aqueous 3-hydroxypropionic acid at a temperature of 120 to 250°C to provide the reaction gas. 16. The process according to claim 10, wherein the aqueous 3-hydroxypropionic acid has been prepared by fermentation.

Description:
Process for preparing acrylic acid using a heterogeneous alumina catalyst Description The invention relates to a process for preparing acrylic acid by contacting a reaction gas comprising 3-hydroxypropionic acid with a metal oxide-containing catalyst composition.

Because of its very reactive double bond and its carboxylic acid group, acrylic acid is a valuable monomer for preparation of polymers, for example water-absorbing polymer particles, binders for water-based emulsion paints, and adhesives dispersed in aqueous solvent.

Water-absorbing polymer particles are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening. The water-absorbing polymer particles are also referred to as superabsorbent polymers.

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

On the industrial scale, acrylic acid has been prepared almost exclusively from fossil raw materials. This is regarded as disadvantageous in view of the consumers' increas- ing awareness of environmental sustainability. There is therefore a need to produce water-absorbing polymer particles used in the hygiene articles from renewable raw materials.

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

US 2005/0222458 A1 is directed to a process for the preparation of α,β-unsaturated acids, esters and amides from α,β-hydroxycarboxylic acids, e.g. acrylic acid from 3- hydroxypropionic acid, in the presence of specific dehydration catalysts comprising metal oxides and/or alumosilicates. The specific metal oxides that are illustrated are γ- AI2O3 or S1O2. Determination of conversion and yields for said conversion are determined by GC-analysis. No suitable methods for the purification of acrylic acid have been disclosed.

US 5,250,729 discloses a process for the preparation of α,β-unsaturated acids starting from α,β-hydroxycarboxylic acid esters. The reaction is carried out in the presence of an acidic dehydration catalyst, for example crystalline alumosilicates, in the vapor- phase. Appropriate methods for work-up are not described.

Additional catalysts for dehydration reactions in gaseous phase are disclosed in DE 10 2008 038 273 A1 , WO 201 1/033689 A1 or EP 2 554 258 A1 . The exemplified catalysts are oxygen-containing AI-, Zr-, Ti-, Si-, S-, P- or W-compositions, compositions comprising an heteropolyacid deposited on a porous titania carrier, boron phosphates or rare-earth metal phosphates. A further method for catalytically dehydrating hydroxypropionic acid is described in US 2013/0274512 A1 . Catalysts employed here are mixed condensed phosphates.

The dehydration of 3-hydroxypropionic acid in the liquid phase is mentioned for example in WO 2006/092271 A2, WO 2008/023039 A1 , JP 2010-180171 , EP 2 565 21 1 A1 and EP 2 565 212 A1 .

According to WO 2006/092271 A2, the aqueous acrylic acid obtained is azeotropically dewatered. Because of the high water content of the aqueous acrylic acid, this process is energy-intensive.

In the process disclosed in WO 2008/023039 A1 , the water present in the reaction mixture is removed via the top of a distillation column and the acrylic acid formed is enriched at the bottom. The water removed still comprises a considerable amount of acrylic acid. This leads to corresponding acrylic acid losses.

The earlier patent application EP 13184086.0 relates to a process for preparing acrylic acid by dehydrating aqueous 3-hydroxypropionic acid to aqueous acrylic acid in the liquid phase, extracting the acrylic acid by means of an inert organic solvent and distilling the organic extract.

There is still need for a process that allows for an efficient preparation and sufficient purification of acrylic acid starting from 3-hydroxypropionic acid which is preferably derived from renewable resources. During dehydration of 3-hydroxypropionic acid in the presence of a solid dehydration catalyst in the vapor-phase, catalyst activity and the conversion generally decrease after prolonged reaction times. It is desirable to regenerate the catalyst composition to its initial activity, preferably without an interruption of the catalytic process.

The object was accomplished by a process for preparing acrylic acid, which comprises contacting a reaction gas comprising 3-hydroxypropionic acid with a heterogeneous aluminum oxide-containing catalyst composition to obtain a gaseous reaction product containing acrylic acid, characterized in that at least intermittently molecular oxygen is admixed to the reaction gas.

We have found that admixing molecular oxygen to the reaction gas comprising 3- hydroxypropionic acid leads to (re)activation of heterogeneous aluminum oxide- containing catalyst compositions and thereby improves conversion of 3- hydroxypropionic acid and selectivity towards acrylic acid, even after prolonged reaction times. We assume that polymeric deposits on the surface of the catalyst composition, which are probably generated by oligomerization or polymerization of acrylic acid, hinder the access of 3-hydroxypropionic acid to the active sites of the catalyst composition and thus decrease catalyst activity. The molecular oxygen may serve to oxidize and remove the polymeric deposits and to recover a high catalyst activity.

In the process of the present invention, dehydration of 3-hydroxypropionic acid is performed in the gas phase in a gas phase reactor. The reaction gas comprising 3- hydroxypropionic acid is generally provided by vaporization of aqueous 3- hydroxypropionic acid.

Vaporization is conveniently accomplished by means of an evaporator connected to the reactor. The evaporator converts liquid aqueous 3-hydroxypropionic acid into gaseous 3-hydroxypropionic acid. The now gaseous starting material is transferred into the gas phase reactor and converted in presence of a dehydrating catalyst composition.

Evaporators suitable for evaporating 3-hydroxypropionic acid are evaporators with limited thermal stress such as thin film evaporators, for example LUWA®, SAMBAY® or SAKO® evaporators (Buss-SMS-Canzler, Butzbach, Germany), falling film evaporators or coil tube evaporators. These evaporator types cause no or limited thermal decom- position under the evaporation conditions.

Vaporization of the liquid starting material in the evaporator preferably takes place at temperatures of 120 to 250 °C, more preferably of 150 to 240 °C, most preferably of 180 to 220 °C.

The aqueous 3-hydroxypropionic acid used in the dehydration reaction is preferably produced by a fermentative process, as for disclosed in WO 02/090312 A1. The aqueous 3-hydroxypropionic acid thus produced typically comprises, in addition to water, typically 35 to 70% by weight of 3-hydroxypropionic acid, 0 to 20% by weight of oligomeric 3-hydroxypropionic acid, 0 to 10% by weight of acrylic acid and may comprise one or more of the following minor constituents: oligomeric acrylic acid, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, methanol and ethanol where the total amount of minor constituents is usually less than 5% by weight (the percentages are based on the total weight of the aqueous 3-hydroxypropionic acid).

Preferably, the concentration of the aqueous 3-hydroxypropionic acid (including oligo- meric 3-hydroxypropionic acid) used in the inventive process is from 25 to 40% by weight, more preferably from 28 to 35% by weight, in particular about 30% by weight. Preferably, the amount of water of said aqueous solution is from 35 to 80% by weight.

Generally an inert diluent gas is used to transfer vaporized 3-hydroxypropionic acid through the gas phase reactor. As diluent gas any gas inert under the dehydration conditions is suitable, for example nitrogen or argon, preferably nitrogen. The total gaseous hourly space velocity (GHSV) is for example from 100 to 20000 l/l * h, preferably from 1000 to 10000 l/l * h, most preferably from 3000 to 7000 l/l * h. The GHSV is defined as volume of the total gaseous phase at standard temperature per bulk volume of catalyst composition per hour. The method of loading the gas with the vaporized 3-hydroxypropionic acid is not particularly restricted. For example the diluent gas streams through the evaporator and carries over the vaporized 3-hydroxypropionic acid.

As gas phase reactors, all reactors known to the skilled person can be considered, which are capable of converting 3-hydroxypropionic acid by gas phase dehydration to acrylic acid. Preferred in this context are multitube reactors, plate reactors or column reactors. These reactors accomodate a suitable catalyst and can be heated to a suitable reaction temperature. The reaction is conducted continuously in the vapor phase, generally at a temperature of 200 to 280 °C and a pressure of 50 mbar to 5 bar, more preferably 100 mbar to 2 bar, in particular about 1 bar. The catalyst is packed into the reactor in the conventional way, and the preheated reactants are passed through it, preferably downwardly. Residence times of the reactants in the reactor are determined according to well-known chemical engineering principles. Preferably, the residence time in the gas phase reactor is in a range of 0.1 to 1 .0 sec, more preferably 0.2 to 0.6 sec.

The reaction temperature for the gas phase reaction is preferably selected from a range of 200 to 400 °C, more preferably 250 to 350 °C.

The mass flow of aqueous 3-hydroxypropionic acid, calculated as weight of 3- hydroxypropionic acid based on the bulk volume of the catalyst composition, is prefer- ably from 0.1 to 1.5 kg/l * h, more preferably 0.5 to 1 .5 kg/l * h and most preferably 0.5 to 1 .0 kg/l * h, based on the bulk volume of the catalyst composition.

The concentration of 3-hydroxypropionic acid in the reaction gas is for example 0.5 to 5.0 % by volume, more particular 1 .0 to 3.0 % by volume, for example about 1 .3 to 2.0 % by volume.

At least intermittently, molecular oxygen is admixed to the reaction gas. Molecular oxygen can be admixed to the reaction gas during the entire reaction time. In another em- bodiment of the inventive process molecular oxygen is admixed to the reaction gas periodically for predefined periods of time. In yet another embodiment of the inventive process molecular oxygen is admixed to the reaction gas, only after the activity of the catalyst composition has dropped below a predefined level. In a further embodiment of the inventive process molecular oxygen is admixed to the reaction gas periodically, depending on the catalyst activity, for defined periods of time.

A preferred source of molecular oxygen is air. Preferably, air or another molecular oxygen-containing gas is admixed to the inert diluent gas after this is loaded with vaporized 3-hydroxypropionic acid. Any other method of admixing molecular oxygen to the reaction gas may be suitably adopted.

The amount of molecular oxygen admixed to the reaction gas should be below the lower explosion limit. Thus, a preferred amount of molecular oxygen admixed to the reaction gas is 0.1 to 7.0% by volume, based on the total volume of the reaction gas. Even more preferred is an amount of 0.5 to 5 % by volume, in particular 1 to 4 % by volume, for example about 3 % by volume.

The dehydrating catalyst used according to the inventive process is a metal oxide- containing catalyst composition. Preferably, the metal oxide-containing catalyst compo- sition comprises at least one oxide of an element selected from the group consisting of aluminum, zirconium, titanium and silicon.

Suitable aluminum oxide (alumina) catalysts for this application comprise essentially pure aluminas or doped alumina catalysts.

By the additional introduction of suitable dopants into the aluminum oxide catalyst composition defect electrons or additional electrons can be implemented. Preferably a certain number of acidic centers are introduced. The control of the number of acidic centers can be achieved based on the amount of the dopant. As suitable additional dopants the catalyst composition preferably contains elemental constituents such as silicon, titanium, tungsten or zirconium. In particular, the introduction of silicon dioxide into the aluminum oxide catalyst composition leads to the introduction of acidic centers. The number of acidic centers can be controlled by the amount of introduced silicon dioxide. The number of acidic centers increases with the amount of introduced silicon dioxide up to a maximum number of acidic centers, and decreases again with a further increasing amount of silicon dioxide after having reached the maximum number of acidic centers. Another option for the introduction of acidic centers is a surface treatment introducing sulfate or phosphate groups on the surface of the catalyst composition.

Acidic aluminas can be used at considerably lower temperatures and are preferred. The amount of acidic centers of the catalyst composition used in the inventive method is preferably from 0.3 to 1.5 mmol/g, more preferably 0.4 to 1 .2 mmol/g and most preferably 0.5 to 1 .0 mmol/g.

In a suitable determination method, the catalyst compositions are characterized by temperature programmed desorption of ammonia (TPAD) carried out on an apparatus constructed from Raczek analyzing technique GmbH, Hannover (Germany). The samples are conditioned at 400°C in helium flow. Afterwards, a mixture of 10% NHs He is passed over the sample at 70°C. The physisorbed ammonia is removed by flushing with helium at 120°C for 2 h. The chemisorbed ammonia is removed passing helium over the sample which was heated up to 400°C with a linear heating rate of 15°C/m The integration values of the peaks in the amount of ammonia that desorbs from the catalyst composition is reported as amount of acidic centers.

A large variety of acid alumina catalysts can be used for the dehydration step, but the most commonly used solid types include pure or doped gamma-alumina (y-A Os), tita- nia/y-alumina, silica-aluminate such zeolites (e.g., H-ZSM 5), silicoaluminophosphates ('SAPO' catalysts), and silica-alumina.

The γ-alumina is mostly comprised of the γ-phase, namely, more than 50 wt%, prefera- bly more than 90 wt%. In addition to all-y-phase alumina, catalyst blends composed of γ- and δ-phases may be used as well.

Aluminum oxide suitable for catalyst compositions is commercially available. Aluminum oxide catalyst compositions can also be made for example by precipitating or coprecipi- tating methods. Source materials are mainly aluminum salts, such as sulfates, chlorides and nitrates. Acetates, formates, or oxalates are used in some cases. In industrial practice nitrates or sulfates are preferred. Basic precipitation agents on an industrial scale are hydroxides, carbonates, and hydroxocarbonates of sodium, potassium, or ammonium. In case of Si02-containing catalyst compositions coprecipitating appropriate amounts of sodium silicate and aluminum nitrate from aqueous solution by bringing the solution to a pH of about 1 1 with sodium hydroxide or ammonium hydroxide, at a temperature of 50-70 °C, is the method of choice.

The resulting gels, for example mixtures of hydroxides, are then dried to a free-flowing powder, pelleted and calcined at 450-650°C to give the desired catalyst.

A desired silica content of the catalyst can be set by blending two or more S1O2- containing aluminas having different Si02-contents, or by blending a Si02-containing alumina with a Si02-free alumina such that the resulting blend has the desired S1O2- content. Blending can be accomplished by dry powder mixing. The dry powder blend can be converted into shaped bodies by pelleting, for example under addition of 3% graphite, or extrusion. Before the extruding or extrusion step the powder is preferably converted into a kneadable paste. Kneadable pastes can be made by treating suitable aluminas with an aqueous etching solution, for example aqueous nitric acid solutions or aqueous formic acid solutions, and/or water and kneading in a suitable kneader. Extrusion of the kneadable pastes can take place in an extruder, for example a screw extruder, at not particularly restricted speeds of revolution, optionally under addition of additives, for instance Walocel®. The extrudates can optionally be dried in an oven at a suitable temperature.

Examples of commercially available aluminas or silica/aluminas are Siral®, Pural®, Catapal®, Puralox® or Catalox® available from the Sasol Company, South Africa.

Siral® is based on orthorhombic aluminumoxide hydroxide (boehmite) and doped with S1O2. Various Siral® grades having different ratios of AI2O3 to S1O2 are available (Siral 1 (AI2O3/S1O2 = 99/1 ), Siral 5 (AI2O3/S1O2 = 95/5), Siral 10 (Al 2 0 3 /Si0 2 = 90/10), Siral 20 (AI2O3/S1O2 = 80/20), Siral 28M (Al 2 0 3 /Si0 2 = 72/28), Siral 30 (AI2O3/S1O2 = 70/30), Siral 40 (Al 2 0 3 /Si0 2 = 60/40)).

Pural® and Catapal® products are aluminum oxide compositions based on boehmite. Additionally Na20 is part of the composition in an amount of 0.002% by weight. Moreover carbon (up to 0.25%), S1O2 (0.01 -0.015%), Fe 2 0 3 (0.005-0.015%) and T1O2 (0.01 - 0.20%) may be part of Pural® and Catapal® compositions.

Pural® and Catapal® products may particularly be used as a starting material for preparing kneadable pastes as a basis for catalyst compositions according to the inventive process. Said aluminas may be chosen because of their excellent kneading and extru- sion properties. Puralox® and Catalox® are gamma-delta-theta phase aluminum oxide compositions of high purity, offering an amount of at least 95% of aluminum oxide in the composition.

The catalyst is customarily used in the form of cylindrical pellets. Pellet size is selected according to recognized chemical engineering principles, and usually ranges from 3-13 mm in all dimensions. The pore volume of the pellets and their specific surface area are likewise a matter of choice, and the pore volume will generally range from 0.2 to 0.8 cm 3 /g and the specific surface area determined by the BET-method will range from 100 to 500 m 2 /g, preferably from 200 to 450 m 2 /g, in particular 250 to 350 m 2 /g.

The specific surface area is determined by BET-measurements as follows: The specific surface area of a powder is determined by physical adsorption of a gas on the surface of the solid and by calculating the amount of adsorbate gas corresponding to a mono- molecular layer on the surface. Physical adsorption results from relatively weak forces (van der Waals forces) between the adsorbate gas molecules and the adsorbent surface area of the test powder. The determination is usually carried out at the temperature of liquid nitrogen. The amount of gas adsorbed can be measured by a volumetric or continuous flow procedure. The acrylic acid produced by dehydration essentially comprises acrylic acid, for example in an amount of 20 to 80% by weight (based on condensable constituents including water). Typically, the acrylic acid produced by dehydration further comprises one or more of the following minor constituents: 3-hydroxypropionic acid, oligomeric 3- hydroxypropionic acid, oligomeric acrylic acid, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, methanol, ethanol where the total amount of minor constituents is usually less than 5% by weight. The remainder to 100 % by weight is water.

After conversion to an acrylic acid containing reaction product the gaseous reaction product is suitably quenched by treating with a quenching fluid. The quenching fluid may be an aqueous fluid or an inert organic solvent, more preferably an aqueous fluid. Suitably, the quenching fluid is a bottom liquid withdrawn from a quenching tower described below and recycled to be brought in contact with the hot gaseous reaction product. The quenching of the gaseous reaction product results in an aqueous mixture Gaseous constituents of the gaseous reaction mixture which are not condensable (permanent gases) can leave the quenching tower. Non-condensable gases may for example be the carrier gas, e.g., nitrogen or argon, or molecular oxygen. These non- condensable gases maybe recycled to the dehydrogenation reaction. Preferably nitrogen is withdrawn after the quenching process and transferred back into the gas phase reactor. Additionally a vent stream can be discharged, to avoid the accumulation of unwanted by-products. As quenching units, common types in large scale gas phase dehydration are likewise preferably used. Such quench units are formed as columns or towers and can, exactly as for the reactors, be commercially obtained. The quenching fluid preferably contains one or more stabilizing polymerization inhibitors. Suitable polymerization inhibitors are for example N-oxyl compounds, phenol compounds, manganese salts such as manganese acetate, copper dialkyldithiocarba- mates such as copper dibutylthiocarbamate, nitroso compounds and amine compounds, and phenothiazine. The polymerization inhibitor which is contemplated by the present invention does not embrace such a substance as undergoes decomposition in the distilling column and gives such a product of decomposition as manifests an effect of inhibiting polymerization.

The N-oxyl compound is not particularly restricted. Any of the N-oxyl compounds which have been generally known heretofore as agents for inhibiting the polymerization of a vinyl compound may be used. Among other such N-oxyl compounds, 2,2,6,6- tetramethyl piperidinoxyls re ula (1 ):

(wherein R represents CH 2 , CHOH, CHCH 2 OH, CHCH 2 CH 2 OH, CHOCH 2 OH,

CHOCH 2 CH 2 OH, CHCOOH, or C=0 and R 2 represents a hydrogen atom or CH 2 OH) are used advantageously. It is preferred to use one or more compounds selected from the group 2,2,6,6-tetramethyl piperidinoxyl, 4-hydroxy-2 ,2,6,6-tetramethyl piperidinox- yl, and 4,4',4"-tris-(2,2,6,6-tetramethyl piperidinoxyl) phosphites which provide good effects in preventing polymerization, although any of N-oxyl compounds can be used without any limitation. Particularly when 2,2,6,6-tetramethyl piperidinoxyl or 4-hydroxy- 2,2,6,6-tetramethyl piperidinoxyl is used as the N-oxyl compound, since it forms a stabilizing agent without requiring to include any metal in the components, there is no possibility of corroding the metallic material of the equipment due to the presence of stabilizer and the waste liquid can be easily treated.

In this invention, the N-oxyl compound may be used in combination with an N-hydroxy- 2,2,6,6-tetramethyl piperidine compound and a 2,2,6,6-tetramethyl piperidine compound.

As representative examples of the N-hydroxy-2,2,6,6-tetramethyl piperidine compound, 1 ,4-dihydroxy-2,2,6,6-tetramethyl piperidine and 1-hydroxy-2,2,6,6-tetramethyl piperidine may be cited. These N-hydroxy-2,2,6,6-tetramethyl piperidine compounds may be used either singly or in the form of a mixture of two or more members. As typical examples of the 2,2,6,6-tetramethyl piperidine compound. 2,2,6,6- tetramethyl piperidine and 4-hydroxy-2,2,6,6-tetramethyl piperidine may be cited. These may be used either singly or in the form of a mixture of two or more members. Incidentally, N-hydroxy-2,2,6,6-tetramethyl piperidine compounds and 2,2,6,6- tetramethyl piperidine compounds are possibly contained as impurities in commercially available products of N-oxyl compounds. The use of such a commercially available product of N-oxyl compound equals the use in combination with N-hydroxy-2,2,6,6- tetramethyl piperidine compound and 2,2,6,6-tetramethyl piprdidine compound mentioned above.

As typical examples of the phenol compound, hydroquinone, methoquinone (p- methoxy-phenol) may be cited. The methoquinone proves favorable in respect that it excels the hydroquinone in the effect of preventing polymerization when it is used in combination with an N-oxyl compound and a phenothiazine compound. These phenol compounds may be used in the form of a mixture of two or more members.

As typical examples of the phenothiazine compound, phenothiazine, bis-(o

methylbenzyl)phenothiazine, 3,7-dioctylphenothiazine, and bis-(odimethylbenzyl)- phenothiazine may be cited.

The copper salt compound does not need to be particularly restricted. Either copper inorganic salts or copper organic salts can be used. As typical examples, copper dialkyldithiocarbamates, copper acetate, copper napthenate, copper acrylate, copper sulfate, copper nitrate, and copper chloride may be cited. These copper salt compounds are usable in the form of monovalent or divalent compounds. Among other copper salt compounds mentioned above, copper dialkyldithiocarbamates prove favorable from the viewpoint of effect.

As typical examples of the copper dialkyldithiocarbamate, copper dimethyldithiocarba- mate, copper diethyldithiocarbamate, copper dipropyldithiocarbamate, copper dibutyl- dithiocarbamate, copper dipentyldithiocarbamate, copper dihexyldithiocarbamate, copper diphenyldithiocarbamate, copper methylethyldithiocarbamate, copper

methylpropyldithiocarbamate, copper methylbutyldithiocarbamate, copper methylpen- tyldithiocarbamate, copper methylhexyldithiocarbamate, copper methylphenyldithiocar- bamate, copper ethylpropyl dithiocarbamate, copper ethylbutyldithiocarbamate, copper ethylpentyldithiocarbamate, copper ethylhexyldithiocarbamate, copper ethylphenyl- dithiocarbamate, copper propylbutyldithiocarbamate, copper propylpentyldithiocarba- mate, copper propylhexyldithiocarbamate, copper propylphenyldithio-carbamate, copper butylpentyldithiocarbamate, copper butylhexyldithiocarbamate, copper butylphenyl- dithiocarbamate, copper pentylhexyldithiocarbamate, copper pentylphenyldithiocarba- mate, and copper hexylphenyldithiocarbaniate may be cited. These copper dialkyldithiocarbamates may be a monovalent copper salt or a divalent copper salt. Among other copper dialkyldithiocarbamat.es cited above, copper dimethyldithiocarbamate, copper diethyldithiocarbamate, and copper dibutyldithiocarbamate prove favorable in respect of its effects and easy acquisition, and copper dibutyldithiocarbamate proves especially favorable.

As typical examples of the manganese salt compound, manganese dialkyldithiocarba- mates (wherein the two alkyl groups may be identical or different and each may be methyl, ethyl, propyl, or butyl), manganese diphenyldithicarbamate, manganese formate, manganese acetate, manganese octanoate, manganese naphthenate, manga- nese permanganate, and manganese salt compounds of ethylenediamine tetraacetic acid may be cited. These manganese salt compounds may be used either singly or in the form of a mixture of two or more members.

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

Typically, dehydration of 3-hydroxypropionic acid yields an aqueous acrylic acid for which the further purification is very difficult or is associated with high acrylic acid loss- es. The combination of extraction and distillation avoids these disadvantages.

In order to recover the bulk amount of acrylic acid essentially free of water, the mixture obtained after quenching is preferably extracted with an extraction fluid to obtain an organic extract comprising the extraction fluid and acrylic acid. In embodiments of the invention where the extraction fluid is an organic solvent, the extraction fluid and the quenching fluid may be identical, and the quenching and extraction steps can be accomplished at the same time.

Extraction of the mixture obtained after quenching can be accomplished in an extraction column. The design of the extraction column is known per se and may have the standard internals. Useful column internals include all customary internals.

Examples are trays, structured packings and/or random packings. Among the trays, preference is given to sieve trays and/or dual-flow trays. Among the random packings, preference is given to those comprising rings, helices, saddles, Raschig, Intos or Pall rings, barrel or Intalox saddles, or braids. Particular preference is given to dual-flow trays. In general, 10 to 25 theoretical plates are sufficient here.

The extraction column is operated at a temperature of preferably 30 to 70°C, more preferably 40 to 60°C, most preferably 45 to 55°C. In the extraction column, acrylic acid is extracted from the aqueous phase by means of an extraction fluid to obtain an organic extract comprising the extraction fluid and acrylic acid. The extraction fluid is incompletely miscible with water and, for example, has a solubility in water at 23 °C of preferably less than 5 g per 100 ml of water, more preferably less than 1 g per 100 ml of water and most preferably of less than 0.2 g per 100 ml of water. The boiling point of the extraction fluid at 1013 mbar is in the range from preferably 200 to 350°C, more preferably from 250 to 320°C, most preferably from 280 to 300°C. Suitable inert organic solvents used as extraction fluid are, for example, phthalic esters such as dimethyl phthalate and diethyl phthalate, isophthalic esters such as dimethyl isophthalate and diethyl isophthalate, terephthalic esters such as dimethyl tereph- thalate and diethyl terephthalate, alkanoic acids such as nonanoic acid and decanoic acid, biphenyl and/or diphenyl ether or mixtures thereof.

The ratio of aqueous phase and extraction fluid is preferably from 0.5:1 to 1 .5:1. To maintain the ratio, a portion of the aqueous extract can be recycled into the extraction column.

The aqueous extract removed at the top of the extraction column can be discarded and typically comprises, as well as water and traces of the inert organic solvent and of any catalyst used, for example one or more of the following minor constituents: 3-hydroxy- propionic acid, oligomeric 3-hydroxypropionic acid, acrylic acid, oligomeric acrylic acid, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, methanol and ethanol.

The organic extract removed at the base of the extraction column typically comprises, as well as the extraction fluid and any catalyst, acrylic acid in an amount of 10 to 35% by weight, water in an amount of 1 to 5% by weight and for example one or more of the following constituents in minor amounts: 3-hydroxypropionic acid, oligomeric 3-hydroxypropionic acid, oligomeric acrylic acid, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, metha- nol and ethanol.

The organic extract obtained after extraction according to the inventive process is then preferably distilled. The distillation is advantageously performed by means of a rectification column with the organic extract being introduced into the rectification column via a side feed. At the bottom of the rectification column, a stream rich in extraction fluid can be withdrawn and the stream rich in extraction fluid can be recycled into the extraction. In addition, an acrylic acid-rich stream can be withdrawn from the rectification col- umn via a side draw above the feed to the rectification column. At the top of the rectification column, a water-rich stream can be withdrawn and the water-rich stream can be recycled into the extraction. The design of the rectification column known per se and includes customary internals. The column internals used may in principle be all standard internals, for example trays, structured packings and/or random packings. Among the trays, preference is given to bubble-cap trays, sieve trays, valve trays, Thormann trays and/or dual-flow trays;

among the random packings, preference is given to those comprising rings, helices, saddles, Raschig, Intos or Pall rings, barrel or Intalox saddles, or braids. Particular preference is given to dual-flow trays.

In general, from 10 to 25 theoretical plates are sufficient in the rectification unit. The rectification is typically performed under reduced pressure, preferably at a top pressure of 70 to 140 mbar. The bottom pressure depends on the top pressure, the number and type of column internals and the fluid-dynamic requirements of the rectification, and is preferably 200 to 400 mbar.

The top condensate returned to the rectification column as reflux preferably comprises a polymerization inhibitor. Suitable polymerization inhibitors are phenothiazine, hydro- quinone and/or hydroquinone monomethyl ether. Very particular preference is given to phenothiazine. The reflux comprises preferably from 0.0005 to 1 % by weight, more preferably from 0.002 to 0.5% by weight and most preferably from 0.01 to 0.1 % by weight of the polymerization inhibitor. Advantageously, an oxygen-containing gas is additionally used to inhibit polymerization. Particularly suitable for this purpose are air/nitrogen mixtures having an oxygen content of 6% by volume (lean air).

The rectification column is typically manufactured from austenitic steel, preferably from material 1.4571 (to DIN EN 10020).

Appropriately, the organic extract is introduced into a rectification column via a side feed in the lower region thereof. It is preferably effected 2 to 5 theoretical plates above the bottom of the rectification column. The feed temperature is preferably from 20 to 200°C, more preferably from 50 to 180°C and most preferably from 80 to 160°C.

Suitably, the bottom of the rectification column is heated via internal and/or external heat exchangers, with the heat transfer medium preferably being steam, and/or via jacket heating. The heat is preferably supplied via external circulation evaporators with natural or forced circulation. Particular preference is given to external circulation evapo- rators with forced circulation. Evaporators of this kind are described in EP 0 854 129 A1. The use of a plurality of evaporators, connected in series or in parallel, is possible. Preference is given to operating 2 to 4 evaporators in parallel. The bottom temperature of the rectification column is typically 180 to 250°C, preferably 195 to 235°C. If an oxygen-containing gas is used to inhibit polymerization, this is preferably supplied below the evaporator.

The high boiler fraction obtained in the bottoms of the rectification column typically comprises, as well as the inert organic solvents and any catalyst, typically 0.01 to 1 % by weight of acrylic acid, 0.01 to 1 % by weight of oligomeric acrylic acid and one or more of the following minor constituents: 3-hydroxypropionic acid, oligomeric 3- hydroxypropionic acid, water, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, methanol and ethanol wherein the total weight of minor constituents is usually less than 2 % by weight.

The bottom liquid which comprises the inert organic solvent and is withdrawn from the rectification column is recycled via a heat exchanger into the top region of the extraction column. The bottoms liquid is preferably conducted via a solids separator (cyclone) and optionally supplemented with fresh inert organic solvent.

Above the side feed into the rectification column, a crude acrylic acid is withdrawn via a side draw, preferably 8 to 20 theoretical plates above the column bottom. The withdrawal of the crude acrylic acid is effected in a customary manner and is not subject to any restriction. A suitable removal method is via a collecting tray, in which case the entire reflux is collected and a portion is discharged and the other portion is used as reflux below the collecting tray, or via a tray with integrated removal means, preferably via a dual-flow tray with integrated removal means.

The crude acrylic acid withdrawn typically comprises, at least 95 % by weight of acrylic acid, 0 to 5% by weight of water, and may comprise one or more of the following minor constituents: 3-hydroxypropionic acid, oligomeric 3-hydroxypropionic acid, oligomeric acrylic acid, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, methanol and ethanol. The crude acrylic acid withdrawn is cooled by means of a heat exchanger (an example of a suitable coolant is surface water). The use of a plurality of heat exchangers, connected in series or in parallel, is possible. In the heat exchangers, which are known per se to those skilled in the art and are not subject to any particular restriction, the crude acrylic acid is preferably cooled to 40 to 90°C. A sub-stream of the crude acrylic acid withdrawn from the rectification column may be used as solvent for the polymerization inhibitor.

The low boiler stream removed at the top of the rectification column can be cooled indi- rectly, for example by means of heat exchangers (the coolant used may, for example, be surface water) which are known per se to those skilled in the art and are not subject to any particular restriction, or directly, for example by means of a quench. It is preferably removed by direct cooling. For this purpose, already condensed low boiler fraction is cooled by means of a suitable heat exchanger and the cooled liquid is sprayed in the vapor above the withdrawal point. This spraying can be effected in a separate apparatus or in the rectification column itself. In the case of spraying in the rectification column, the withdrawal point for the low boiler fraction advantageously takes the form of a collecting tray. Internals which improve the mixing of the cooled low boiler fraction with the vapor can enhance the effect of the direct cooling. All standard internals are useful in principle for this purpose, for example trays, structured packings and/or random packings. Among the trays, preference is given to bubble-cap trays, sieve trays, valve trays, Thormann trays and/or dual-flow trays. Among the random packings, preference is given to those comprising rings, helices, saddles, Raschig, Intos or Pall rings, barrel or Intalox saddles, or braids. Particular preference is given to dual-flow trays. In gen- eral, 2 to 5 theoretical plates are sufficient here. These trays are not included in the figures given so far for the number of theoretical plates in the rectification column. The direct condensation of the low boiler fraction can also be executed in more than one stage, with temperature decreasing in the upward direction. The low boiler stream removed via the top of the rectification column typically comprises 10 to 60% by weight of acrylic acid and up to 90 % by weight of water and may comprise one or more of the following constituents: 3-hydroxypropionic acid, oligomeric 3-hydroxypropionic acid, oligomeric acrylic acid, glycolic acid, 2-hydroxypropionic acid, formic acid, acetic acid, succinic acid, fumaric acid, formaldehyde, acetaldehyde, methanol and ethanol wherein the total weight of minor constituents is typically less than 5 % by weight.

A portion of the liquid withdrawn as low boiler fraction is recycled to the rectification column as reflux; the remainder of the low boiler fraction is discharged and recycled as aqueous phase into the extraction column.

The crude acrylic acid obtained in distillation can be further purified by crystallization, particularly advantageously by means of a suspension crystallization. The mother liquor obtained in the crystallization is advantageously recycled into the distillation. The acrylic acid sent to the crystallization provides a water content of preferably less than 10% by weight, more preferably less than 7% by weight, most preferably less than 5% by weight. The crude acrylic acid withdrawn from the rectification column can be used directly for production of water-absorbing polymer particles. Preference is given to further purifying the crude acrylic acid by crystallization. The mother liquor obtained in the crystallization can be recycled into the rectification column, preferably below the removal 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. 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 purified typically comprises at least 99 % by weight of acrylic acid.

The acrylic acid prepared by the process according to the invention can be used for preparation of acrylic esters such as methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate, and for preparation of polymers such as water-absorbing polymer particles.

The process according to the invention is illustrated in more detail by the examples which follow:

Examples

BET surface areas were determined according to DIN 66135 with a Tristar apparatus (Micrometrics).

Acidic centers were determined by an Ammonia-TPD (Temperature-programmed desorption) method employing a Autochemll2920 of micromeritics GmbH, Aachen. Conversions and selectivities were determined by HPLC-analysis:

Column: Prontosil 120-3-C18 AQ 3 micro meter, 150 * 4.6 mm (Fa. Bis- choff)

Temperature: 25 °C

Injection volume 50 micro liter

Flow: 1 ,5 mL/min

Detection: UV λ = 205 nm Eluent: A: 1000 mL H 2 0 + 1 mL 0.5 M H 2 S0 4

B: 1000 mL acetonitrile

In table 1 the eluent composition for different retention times is shown:

Table 1

Example 1

Dehydration of 3-hydroxypropionic acid was performed in a laboratory quartz glass gas phase reactor incorporating a reactor tube with a length of 100 mm and a diameter of 14 mm. The reactor tube is fitted with a concentric thermocouple of 2 mm diameter, reducing the effective cross section to 1 .51 cm 2 . The reactor tube is surrounded by a circulating bed of finely dispersed alumina, which is heated electrically, to ensure an isothermal profile inside the reactor. The product quench consists of a buffle plate, a pump (Gather pump NP22), a heat exchanger section and a liquid container. The solution in the container is circulated by the pump to the baffle plate. The baffle plate distributes the liquid evenly over the cross section of the reactor exit to ensure a highly effective gas adsorption. The product solution is then trickeld over the heat exchanger section and cooled to about 3 °C and then returned to the container.

The reactor was charged with dehydration catalyst ( 10 mL). The catalyst used was a Puralox SCF a 230/Pural 1 .0 mm composition (BET: 216 m 2 /g, acidic centers: 0,56 mmol/g).

The catalyst composition was prepared as follows:

700 g Puralox SCF a 230, 300g Pural SB, 20 g HNOs (65%) and 650 ml water were kneaded for 45 min. 20 g of polyethylene oxide (Alkox E160) and 20 g of Avicel pH 101 were added and kneaded for additional 45 min. A piston extruder was used to extrude strands with 1.5 mm diameter. These were firstly dried at 12 °C for 16 h in air, then calcined at 500 °C for 5 h in air.

The feed used was supplied as aqueous 3-hydroxypropionic acid from Cargill, with a concentration of about 60%. Nitrogen was used as diluent gas at a flow rate of 36.9 l/h.

The aqueous 3-hydroxypropionic acid comprised: 28.0% by weight of 3-hydroxypropionic acid,

2% by weight of oligomeric 3-hydroxypropionic acid,

68.8% by weight of water,

0.5% by weight of acrylic acid,

0.02% by weight of oligomeric acrylic acid,

0.31 % by weight of 2-hydroxyisovaleric acid,

0.12% by weight of succinic acid,

0.1 % by weight of 2-hydroxypropionic acid,

0.015% by weight of erythritol

0.007% by weight of arabitol

0.0015% by weight of formaldehyde,

0.001 % by weight of acetaldehyde,

0.1 % by weight of glucose, glucose esters and other high boiling substances

The liquid feed was vaporized by means of a SAMBAY ® evaporator (Buss-SMS- Canzler, Butzbach, Germany) operated at a temperature of 210 °C. The concentration of 3-hydroxypropionic acid in the reaction gas after vaporization was 2.9 % by volume, based on the total volume of reaction gas. The reaction temperature in the reactor was 252 °C. Catalyst loading was 0.51 kg/Ι· h, the residence time in the reactor was 0.44 sec. The conversion of 3-hydroxypropionic acid and the selectivity for acrylic acid were determined by HPLC after said reaction times (see table 2). After a reaction time of 270 h, molecular oxygen was admixed to the 3-hydroxypropionic acid-containing reaction gas at a concentration of 3 % by volume, based on the total volume of reaction gas. The dehydration reaction was run for 319 h in total.

Table 2: Conversion of 3-hydroxypropionic acid (HPA) and selectivity for acrylic acid.

From the table it can be seen that the conversion decreased continuously from 96.2 % after 14.5 h to 72.7% after 197.5 h. After 270 h of catalyst life time, 3% by volume of molecular oxygen was admixed to the reaction gas. This resulted in a significant increase of conversion and yield and selectivity. Selectivity appears to be largely independent of catalyst life time, treatment with oxygen however, leads also to a slight improvement of catalyst performance concerning selectivity. Example 2

Dehydration of 3-hydroxypropionic acid is performed in a pilot plant using catalyst composition A as follows: A falling film evaporator with a heating surface area of 0.16 m 2 (tube lenghth: 2 m; inner tube diameter: 30 mm) is fed with 3300 g/h of about 30 wt% HPA in water, 4825 NL/h N2 and 150 NL/h O2, resulting in a total gaseous stream of 8098 NL/h consisting of roughly 59.6 Vol% N 2 , 1.9 Vol% 0 2 , 3 Vol% HPA and 35.5 Vol% water. The aqueous 3-hydroxypropionic acid feed comprised:

28.0% by weight of 3-hydroxypropionic acid,

2% by weight of oligomeric 3-hydroxypropionic acid,

68.8% by weight of water,

0.5% by weight of acrylic acid,

0.02% by weight of oligomeric acrylic acid,

0.31 % by weight of 2-hydroxyisovaleric acid,

0.12% by weight of succinic acid,

0.1 % by weight of 2-hydroxypropionic acid,

0.015% by weight of erythritol

0.007% by weight of arabitol

0.0015% by weight of formaldehyde,

0.001 % by weight of acetaldehyde,

0.1 % by weight of glucose, glucose esters and other high boiling substances

2000 ml of catalyst A are placed in a reaction tube of 29 mm inner diameter and 3900 mm length, resulting in a fixed bed of 3000mm length. The reation tube is heated to 300°C and the gaseous reactant stream is led through the catalyst bed to dehydrate HPA to AA.

The gaseous reaction product is quenched with the product mixture in a recycle setup, to which 0.005% by weight of phenothiazine are added as stabilizer and the aqueous product mixture is transferred to the extraction:

The aqueous product comprised: 1 .5% by weight of 3-hydroxypropionic acid,

0.03% by weight of oligomeric 3-hydroxypropionic acid,

75.6% by weight of water,

22.5% by weight of acrylic acid,

0.03% by weight of oligomeric acrylic acid,

0.29% by weight of 2-hydroxyisovaleric acid,

0.0005% by weight of succinic acid,

0.1 % by weight of 2-hydroxypropionic acid,

0.0006% by weight of erythritol

<0.0001 % by weight of arabitol

0.0002% by weight of formaldehyde,

0.0002% by weight of acetaldehyde,

The extraction is performed in a 20-tray sieve tray column having an internal diameter of 50 mm. The aqueous phases from the dehydration are heated to 50°C and sent to the sieve tray column at the base. The feed of aqueous phase is in total 3060 g/h. At the top of the sieve tray column, 41 10 g/h of liquid from the bottom of the distillation and 33 g/h of diethyl phthalate with a temperature of 50°C as extractant are fed in.

2403 g/h of the aqueous extract withdrawn at the top of the sieve tray column are discarded.

The aqueous extract withdrawn at the top of the sieve tray column had the following composition:

1 .9% by weight of 3-hydroxypropionic acid,

0.04% by weight of oligomeric 3-hydroxypropionic acid,

95.8% by weight of water,

1 .4% by weight of acrylic acid,

0.04% by weight of oligomeric acrylic acid,

0.4% by weight of 2-hydroxyisovaleric acid,

0.0006% by weight of succinic acid,

0.1 % by weight of 2-hydroxypropionic acid,

0.0008% by weight of erythritol

<0.0001 % by weight of arabitol

0.0002% by weight of formaldehyde,

<0.0001 % by weight of acetaldehyde,

0.3% by weigth of diethyl phthalate At the base of the sieve tray column, 4925 g/h of organic extract are withdrawn and transferred into the distillation (3).

The organic extract withdrawn at the base of the sieve tray column had the following composition:

0.09% by weight of 3-hydroxypropionic acid,

0.01 % by weight of oligomeric 3-hydroxypropionic acid,

2% by weight of water,

14.8% by weight of acrylic acid,

0.04% by weight of oligomeric acrylic acid,

0.7% by weight of 2-hydroxyisovaleric acid,

<0.0001 % by weight of succinic acid,

0.02% by weight of 2-hydroxypropionic acid,

0.0001 % by weight of erythritol

<0.0001 % by weight of arabitol

<0.0001 % by weight of formaldehyde,

0.0001 % by weight of acetaldehyde,

82.3% by weight of diethyl phthalate

The distillation is performed in a 30-tray bubble-cap tray column having an internal diameter of 50 mm. The bubble-cap tray column has an electrical guard heater. The feed to the distillation is heated to 160°C and fed to the 5 th tray of the bubble-cap tray column.

The bottom liquid of the bubble-cap tray column is circulated by means of a pump through a shell-and-tube heat exchanger. Below the shell-and-tube heat exchanger, 2 l/h of air are metered into the circuit. The temperature and pressure in the bottom of the bubble-cap tray column are 220°C and 265 mbar.

41 10 g/h of liquid are withdrawn from the bottom of the bubble-cap tray column and recycled into the extraction. A further 25 g/h of liquid are discharged from the bottom of the bubble-cap tray column and discarded.

The liquid withdrawn from the bottom of the bubble-cap tray column had the following composition: 0.1 % by weight of 3-hydroxypropionic acid,

0.015% by weight of oligomeric 3-hydroxypropionic acid,

<0.0001 % by weight of water, 0.9% by weight of acrylic acid,

0.04% by weight of oligomeric acrylic acid,

0.9% by weight of 2-hydroxyisovaleric acid,

0.0001 % by weight of succinic acid,

0.02% by weight of 2-hydroxypropionic acid,

0.0001 % by weight of erythritol

<0.0001 % by weight of arabitol

<0.0001 % by weight of formaldehyde,

<0.0001 % by weight of acetaldehyde,

98.0% by weight of diethyl phthalate

Between the 15 th and 16 th trays of the bubble-cap tray column is installed a collecting tray. The liquid is withdrawn completely therefrom and conveyed by means of a pump through a heat exchanger, in the course of which it is cooled to 65°C, and is recycled to the 20 th tray of the bubble-cap tray column. 651 g/h of the cooled liquid are withdrawn as crude acrylic acid. 1 197 g/h of the cooled liquid are recycled to the 15 th tray of the bubble-cap tray column. The crude acrylic acid had the following composition:

<0.0001 % by weight of 3-hydroxypropionic acid,

<0.0001 % by weight of oligomeric 3-hydroxypropionic acid,

0.8% by weight of water,

99.2% by weight of acrylic acid,

<0.0001 % by weight of oligomeric acrylic acid,

<0.0001 % by weight of 2-hydroxyisovaleric acid,

<0.0001 % by weight of succinic acid,

<0.0001 % by weight of 2-hydroxypropionic acid,

<0.0001 % by weight of erythritol

<0.0001 % by weight of arabitol

<0.0001 % by weight of formaldehyde,

<0.0001 % by weight of acetaldehyde,

<0.0001 % by weight of diethyl phthalate

Between the 25 th and 26 th trays of the bubble-cap tray column is installed a collecting tray. The liquid is withdrawn completely therefrom and conveyed by means of a pump through a heat exchanger, in the course of which it is cooled to 25°C, and is recycled to the 30 th tray of the bubble-cap tray column. 126 g/h of the cooled liquid are recycled into the extraction. 407 g/h of the cooled liquid are recycled to the 25 th tray of the bubble-cap tray column. The reflux is stabilized with 0.005% by weight of

phenothiazine. The pressure at the top of the bubble-cap tray column is 100 mbar.

The liquid withdrawn from the 25 th tray of the bubble-cap tray column had the following composition:

<0.0001 % by weight of 3-hydroxypropionic acid,

<0.0001 % by weight of oligomeric 3-hydroxypropionic acid,

70% by weight of water,

30% by weight of acrylic acid,

<0.0001 % by weight of oligomeric acrylic acid,

<0.0001 % by weight of 2-hydroxyisovaleric acid,

<0.0001 % by weight of succinic acid,

<0.0001 % by weight of 2-hydroxypropionic acid,

<0.0001 % by weight of erythritol

<0.0001 % by weight of arabitol

0.0001 % by weight of formaldehyde,

0.0003% by weight of acetaldehyde,

<0.0001 % by weight of diethyl phthalate

The crude acrylic acid is optionally further purified by crystalliazion to obtain glacial acrylic acid with >99.5% by weight of acrylic acid and <0.5% by weight of water.