Title of Invention

METHOD FOR THE PURIFICATION OF (METH) ACRYLIC ACID

Abstract

The present invention relates to a process for production of (meth)acrylic acid, whereby first a crude (meth)acrylic acid is produced and this crude (meth)acrylic acid is then preferably continuously purified, whereby the preferably continuous purifica­tion of the crude (meth}acrylic acid comprises the following process steps; in a composition comprising (meth)acrylic acid and impurities, the impurities are precipitated in crystalline form from the composi­ tion; the crystalline impurities precipitated from the composition are separated. The invention also relates to the (meth)acrylic acid obtainable by this process, a device for production of (meth)acrylic acid, the use of the device or of a purifica­tion device for production of (meth)acrylie acid, (meth)acrylic acid, water-absorbing polymers, the use of water-absorbing polymers as well as fibres, sheets, foams and composites.

Full Text

The present invention relates to a process for production of (meth)acrylic add, the (me1h)acrylic acid obtainable by this process, a device for production of (meth)acrylic acid, the use of the device or of a purification device for the produc­tion of (meth)acrylic acid, (meth)acrylic acid, water-absorbing polymers, the use Of water-absorbing polymers and fibres, sheets, foams and composites. "(Meth)acrylic acid" is used in this text for the compounds with the nomenclature names "metfaacrylic acid" and "acrylic acid". Of the two compounds, acrylic acid is preferred both in connection with the process according to the invention as well as in connection with the chemical products according to the invention. Chemical starting products and intermediate products, in particular starting prod­ucts for polymer production, are currently produced in very large amounts, hi or­der to satisfy increasing demands on quality, these products may contain practi­cally no impurities. This is particularly the case for acrylic acid, if it is to be used for the production of water-absorbing polymers, which are, for example, used in hygiene articles. Impurities in the acrylic acid influence disadvantageously its polymerisation. Thus, with increasing amount of impurity in the acrylic acid, not only the portion of residual monomers in the acrylic acid polymers increases, but also the absorption behaviour of the water-absorbing polymers based on acrylic acid is disadvantageously influenced by impurities in the acrylic acid used. Acrylic acid is commonly obtained by catalytic gas phase oxidation of propylene with an oxygen-containing gas. In this way, in a two-step process, the propylene is first oxidised catalytically to acrolein, which is men converted to acrylic acid in a second process step, likewise using catalysts. The thus-obtained acrylic acid is removed in the form of an aqueous solution from the gaseous reaction mixture by absorption with water. The water in the thus-obtained aqueous acrylic acid is then separated with formation of a crude acrylic acid, by distillation, for example by means of an entrainer, whereby an acrylic acid-comprising bottom product (=crude acrylic acid) is obtained. Methacrylic acid can be produced analogously to acrylic acid, likewise by cata­lytic gas phase oxidation, whereby in this case, (^-starting compounds, such as, for example, isobutene, isobutane, tert-butanol or methacrolein are used. The puri­fication of the methacrylic acid obtained by the catalytic gas phase oxidation of C^starting compounds comprises, as for the purification of acrylic acid, the ab­sorption of the methacrylic acid in a suitable solvent and the subsequent separa­tion of the solvent whereby in this case, a crude methacrylic acid is obtained. Since both in the catalytic gas phase oxidation of the €3 or C4 starting compounds, besides the acrylic acid or the methacrylic acid* other oxidation products are also formed, such as, for example, rnaleic acid anhydride, which, hi the case of the production of water-absorbing polymers based on polyacrylates, can inhibit the polymerisation and have a disadvantageous affect on the absorption properties of the water-absorbing polymers, a further purification of the crude (meth)acrylic acid is necessary. The purification of the crude (meth)acrylic acid occurs by means of purification processes known from the prior art, for example, by means of distillation of crystallisation, whereby in the oase of crystallisation, it is techni­cally differentiated between the two processes of suspension crystallisation and layer crystallisation (Wintennantel et al., Chem. Ing. Tech. 1991, 63, 881-891; Sterner et al., Chem. Ing; Tech. 1985,57,91-102). Crystallisation has the advantage over distillation that impurities which cannot be removed by distillation can often be separated by means of crystallisation. Fur- thermore, crystallisation can be carried out at considerably lower temperatures in comparison to distillation, so mat the extent of formation of (meth)acrylic acid dimers or (meth)acrylic acid oligomers during the purification process can be re­duced. Crystallisation has, however, the disadvantage that impurities which may be present in the melt, in particular ftunaric acid, maleic acid and maleic acid an­hydride, also precipitate on crystallisation. Since the mother liquor or the melt is depleted in (meth)acrylic acid during the crystallisation, impurities hi the mother liquor are constantly enriched. As soon as the solubility limit is reached, these compounds precipitate from the mother liquor in crystalline form. In the case of a static layer crystallisation, these undesired crystals form on the floor or the walls of a crystalliser. Since the crystals stick to the surface, these impurities remain in the crystalliser when the uncrystallised melt is discharged. If the separated crystal layer is then melted, whereby commonly two or more fractions are formed, the precipitated crystalline impurities which are still present on the floor and on the walls of the crystalliser are again taken up by the warmed melt The result of this is that these impurities can never be completely removed and concentrate in the crystalliser. In a large scale purification plant, in the course of a day, considerable amounts of impurities, such as, for example, maleic acid, can accumulate, which can block the conduits and valves of the purification device for (meth)acrylic acid. Impurities can also condense as a solid coating on the crystalKser walls or on the floor. These deposits not only lead to blockages, but also affect a reduction of the capacity of a plant for purification of (meth)acrylie acid, since a considerable amount of the volume, for example of the tanks situated in the plant, is taken up by the precipitated crystals. It has thus become a necessity to separate the deposits. To this end, WO-A-OQ/45928 proposes to prevent the precipitation of impurities in the purification process for acrylic acid, by addition of a solvent or a solvent mixture, whereby the solvent or the solvent mixture is added in such an amount that the impurities are held in solution. Preferably, water is used as solvent The disadvantage of this process is, however, tibat maleic acid and maleic acid anhy­dride are not separated by this process from the mother liquor, but constantly en­rich in the mother liquor. The dilution of the mother liquor with solvent or the solvent mixture has, in addition, the result that in subsequently attached crystallis-ers, a crystallisation of the acrylic acid is made more difficult by the introduction of a solvent. The present invention thus had the object of providing a process with which the disadvantages described in the prior art could be overcome. The present invention had, in particular, the object of providing a process for pro­duction of (meth)acryUc acid, which comprises a purification process for (meth)acrylic acid, in which impurities, in particular tumaric acid, maleic acid or maleic acid anhydride, can be separated in a simple way, without making more difficult a subsequent crystallisation of the (meth)actylic acid. It is also an object of me present invention to provide a (meth)acryh'c acid, which, in comparison with the (meth)acrylic acid described in the prior art, is character­ised by a particularly low content in impurities, such as, for example, tumaric acid, maleic acid or maleic acid anhydride, A further object of the present invention was to provide water-absorbing poly­mers, which are characterised by a particularly low content in impurities, such as, for example, fumaric acid, maleic acid or maleic acid anhydride, and thus by a particularly advantageous skin tolerance. These objects were solved by a process for production of (meth)aerylie acid, whe­reby first a crude (meth)acrylie acid is prepared and tills erode (meth)acrylic acid is then preferably continuously purified, whejeby the preferably continues purifi­cation of the crude (meth)acryHc acid comprises the following process steps: a) in a composition comprising (meth)acrylic acid and impurities, the impuri­ties are precipitated in crystalline form from the composition; fa) the crystalline impurities precipitated from the composition are separated. In a preferred embodiment of the process according to the invention, the impurity is maleie acid anhydride, maleic acid, fomarie acid or mixtures of these com­pounds. In another preferred embodiment of the process according to the inven­tion, the impurities are polymerisation inhibitors such as, for example, phenothi-azine, hydroquinone monomemyl ether or hydrpquinone, which are added in the production of (meth)acrylic acid, in order to prevent a formation of (meth)acrylic acid polymers during the synthesis process and the thereupon-following purifica­tion process. If the impurity is maleic acid anhydride, the separation of the maleic acid anhy­dride from the composition preferably occurs in that in process step a) maleic acid anhydride is hydrolysed to at least 50 %, preferably to at least 75 %, particularly preferably to at least 96 % and yet mote preferably to at least 99.99 % to form maleic acid, whereby maleic acid and/or fumarie acid precipitate in crystalline form from the composition. Surprisingly, but nonetheless advantageously, it was noticed that from composi­tions comprising (meth)acrylic acid and maleic acid anhydride, the maleic acid anhydride can be separated hi a simple way by hydrolysis and the formation of maleic acid resulting therefrom (which depending on the circumstances can at least partially convert to fumaric acid in a slow isomerisation), since maleic acid (and also fumaric acid) are less soluble in (meth)acrylic acid compared to maleic acid •anhydride. It was also noted here, by means of intensive experiments on compositions with a different ratio of amounts of (meth)acrylic acid, maleic acid anhydride and water, that hours or even days can pass until a precipitation of maleic acid anhydride or of maleic acid and/or fumaric acid arising from the hy­drolysis of the maleic acid anhydride can be observed. In order to ensure as complete a hydrolysis of the anhydride as possible, for a suf­ficient separation of the maleic acid anhydride, it is preferred according to the invention that the hydrolysis of die maleic add anhydride in process step a) pref­erably occurs over a time period in a range from 1 to 250 hours, particularly pref­erably over time period from 2 to 100 hours and yet more preferably over a time period from S to 50 hours and preferably at a temperature in a range from -70 to 13 0°C, particularly preferably in a range from -3 0 to 1QOQC and yet more prefera­bly in a range from -10 to 20*C. The duration of the hydrolysis process is de­pendent on different factors, such as, for example, the temperature of the composi­tion, the relative amount ratios of the individual components (water, (meth)acrylic acid, other impurities) in the composition, as well as on the presence of catalysts which accelerate the hydrolysis of maleic acid anhydride. The skilled person will determine the necessary duration of the hydrolysis process by appropriate routine experiments, in which the progress of the hydrolysis in the composition is che­cked by sampling, for example by the determination of the water content (this decreases with increasing hydrolysis). Preferably, in the case of maleic acid anhydride as impurity to be separated, the controlled hydrolysis of the impurity occurs in that it is ensured that, depending on the pH-value of the composition and on the concentration of the maleic acid anhydride contained in the composition, an amount of water which is sufficient for the scale of the hydrolysis required is comprised in fhe composition. Thus, the composition comprises the water preferably in an amount in a range from 0.01 to SO wt%, particularly preferably in an amount in a range from 0.1 to 20 wt.%, yet more preferably in a range from 1 to 10 wt% and yet more preferably in a range from 2 to 5 wt.%, respectively based on the total weight of the composition. Pref­erably, in the composition, the amount of water is, however, limited in that at most 10 wt%, particularly preferably at most 5 wt%, yet more preferably at most 1 wt.% and even more preferably at most 0.1 wt% of the maleic acid, respectively based on the weight of the maleic acid in the composition, is present in the form dissolved in water, whereby these respective maximum amounts of water depend on the temperature of the composition. In a particularly preferred embodiment of the process according to the invention, the composition comprises water in such an amount that the maleic acid anhydride comprised in the composition is hydrolysed to at least 50 %, preferably to at least 75 %, particularly preferably to at least 96 % and yet more preferably to at least 99 % to form maleic acid and/or fumaric acid. Here, the skilled person will deter­mine the necessary amounts of water by means of simple trials, in which the a-mount of maleic acid anhydride in the composition and thus the progress of the hydrolysis is determined. If in the preferred process according to the invention, in, which maleic acid anhy­dride as impurity should be separated, in process step a) a composition is used which already has the necessary amount of water, in the equih'brium state, the necessary degree of hydrolysis will be reached and the maleic acid and/or fumaric acid crystallised out of the composition. If, however, the amount of water com­prised in a composition is not sufficient to enable the necessary degree of hy­drolysis after appropriate delays, it is preferred according to the invention to add to the composition an amount of water such that in the equilibrium state the de­sired degree of hydrolysis is reached and thus the maleic acid and/or fumaric acid are caused to precipitate. In addition to the addition of water, however, the neces­sary degree of hydrolysis can also be adjusted by addition of catalysts which ac­celerate fiie hydrolysis of maleic acid anhydride, such as, for example, inorganic or organic acids, or by a variation of other process parameters such as, for exam­ple, an increase in the temperature of the composition. In the case of a use of cata­lysts, these are preferably used in an amount in a range from 1 to 10,000 ppm, particularly preferably in a range from 10 to 5,000 ppm and yet more preferably hi a range from 100 to 1,000 ppm, respectively based on the total amount of maleic acid anhydride in the composition. If the purification of the composition comprising (meth)acrylic acid and maleic acid anhydride as impurity occurs discontinuously, in that, for example, the com­position is stored in a container, the composition is men preferably stored in this container for the above-mentioned time period and then conducted to an appropri­ate device for separation of the precipitated maleic acid and/or fumaric acid, for example a filter. If the purification of the composition comprising (meth)acrylic acid and maleic acid anhydride occurs continuously,, hi that, for example, the composition is transferred by means of suitable crystaltisers into a crystal suspen­sion, and this crystal suspension is then separated hi wash columns into (meth)acryUc acid crystals and a mother liquor, a sufficient hydrolysis of the maleic acid anhydride remaining in the mother liquor can be ensured, for exam­ple, in that the mother liquor is conducted in the cycle for a sufficiently long time by means of partial conducting back into the crystalliser. Thus according to the invention* impurities, such as, for example, maleic acid or maleic acid anhydride are not held hi solution, as described in WO-A-00/45928, but are precipitated from the composition, whereby hi the case of maleic acid an- hydride as impurity to be separated, this precipitation occurs by the controlled hydrolysis of the maleic acid anhydride. Since in the separation of maleic acid anhydride as impurity, compared to in the process described in WO-A-00/45928, at most small additions of water are necessary to this end, the process according to the invention enables in particular the targeted separation of maleic acid anhydride from a composition comprising (meth)acryUc acid, without decisively diluting this composition with water., and thus making more difficult a further crystallisation of (mem)acryUc acid from this composition carried out in subsequent purification steps. The separation of the maleic acid and/or of the fumaric acid in process step b) can be carried out by means of any process which allows a separation of solid and liquid materials. Preferably, the separation of the crystallised maleic acid and/or of the crystalline finnaric acid occurs by filtration, sedimentation or centrifuga-tidn. In a preferred embodiment of the process according to the invention, the composi­tion comprising (meth)acrylic acid and the impurities according to process step a) is the bottom product $2, which is obtained in that the aqueous (meth)acrylic acid Solution, preferably acrylic acid solution, obtained as quench phase is distilled in a first distillation step in the presence of an entrainer, preierably in the presence of toluene, in order to remove the water from the quench phase. The (meth)acrylic acid-comprising bottom product SI obtained in this first distillation step, which is water-poor compared to the quench phase, is distilled in a second distillation step, in order to separate low-boiling components, such as, for example, acetic acid. In this second distillation step, the (meth)acrylic acid-comprising bottom product S2 Hmpure crude (meth)acrylic acid stream) is obtained. The separation of maleic acid anhydride, maleic acid or finnaric acid from the bottom product S2 can thus occur evea before (meth)acrylic acid present in the bottom product S2 is sepa- rated, for example, by moans of crystallisation. It is also conceivable to first sepa­rate (meth)acrylic acid present in bottom product S2 by means of crystallisation, and to separate maleic acid, maleic acid anhydride or fumarlc acid from the thus retained mother liquor. The mother liquor can then be conducted to subsequent purification steps for the purpose of a tardier separation of (meth)acrylic acid which is still present m another preferred embodiment of the process according to the invention, the composition comprising (meth)acrylic acid and the impurities according to proc­ess step a) is L The fmemtocrvh'c .acid crystals, preferably me at least partially melted (meth)acrvlic acid crystals, or the mother liquor, which is obtained in Step B or in step Dor n. the crystal suspension, which is obtained step A or step C of a process, which comprises the following process steps: crystallisation of (meth)acrvlic add from an impure crude (meth)acrylic acid stream, preferably from the bottom product S2, from a process for pro­ duction of (meth)acrvUc acid, preferably with formation of a crystal suspen­ sion by means of a first suspension generator, separation of the (meth)acrylic acid crystals from the crystal suspension, preferably by means of a first separating devrce, preferably a first wash col­ umn, whereby a mother liquor is retained; as well as optionally renewed crystallisation of (mem)aerylic acid from the first mother liquor obtained from step B), preferably with fonnatibn of a second crystal suspen­ sion by means of a second suspension generator, separation of the (meth)acrylic acid crystals obtained in step C) from the second crystal suspension, preferably by means of a further separating de­ vice, preferably at a further wash column, whereby a second mother liquor is retained. According to the invention, by the term "mother liquor" in this text; all phases should be understood which are not present in crystalline form. In the case of a crystalline suspension by me term "mother liquor" is thus understood any liquid phase which is retained on separation of the crystals from the crystal suspension. The impure crude (meth)acryiic acid stream used in this particular embodiment of the process according to the invention in process A) is preferably based on (al) (meth)acrylic acid in an amount in a range from 50 to 99.9 wt%, particu­larly preferably in a range from $0 to 99.$ wt.% and yet more preferably in an amount in a range from 90 to 99.5 wt%, water, in an amount in a range from 0.01 to 50 wt%, particularly preferably in range from 0.03 to 10 wt.% and yet more preferably in an amount in a range from 0.05 to 1 wt.%, maleic acid anhydride in a total amount in a range from 0.01 to 5 wt.%, par­ ticularly preferably in a range from 0-05 to 1 wt% and yet more preferably in an amount in arange from 0.1 to 0.5 wt.%, as well as ( To the further impurities (oi4) belong, in the case of the production of acrylic a-cid, acrolein, acetic acid, propionie acid, aldehydes such as, for example, formal­dehyde, furfural arid benzaldehyde and protoanemonin, as well as polymerisation inhibitors such as, for example, HQME (hydroquinone monomeftiyl ether), HQ (hydroquinone) and phenothiazhie, which fire added in the further distillative puri­fication of the reaction products obtained in the catalytic gas phase oxidation of propylene, in order to prevent the polymerisation and the therefrom resulting for­mation of acrylic acid dimers or acrylic acid oligomers. In the case of the produc­tion of methacrylic acid, the further impurities (a4) are, for example, hydroxy-iso-butanoic acid, tso-bntanoic acid, methacrylamide, methyl methacrylate, methac-rolein, acetic acid,, acrylic acid, propionie acid, HQME and HQ. In a case of a two-step purification process (process step A to D), it is preferred according to a particular embodiment of mis process mat already in process step A a crude (meth)acrylic acid is used which has been previously conducted into the head of the separating device use in process step D, in order to guide in this way the product crystals abraded as crystal suspension in the second process step into the suspension generator of process step A. This variant has the energetic advan­tage of being able to dispense with a melting in the second step and of not having to freeze out again the crystals present now in the first step. In a preferred embodiment of the process according to the invention, in which one of the compositions I or n is used in process step a), the crystallisation is carried out as multi-step, preferably two-step crystallisation (process steps A to D). Ad­vantageously, multi-step processes are here operated according to the counter cur­rent flow principle, in which after the crystallisation, in every step, the crystal-lisate is separated from the mother liquor and the mother liquor is conducted into each step at lower temperature. The crystallisation process used for the crystallisation in step I and the separating process used for the separation in step H advantageously correspond to those crys­tallisation and separating processes which are described in WO 99/14181 Al, to whose content reference is hereby made as part of the disclosure. In particular in the case of the separation of maleic acid anhydride as impurity by means of the above-described controlled hydrolysis of the maleic acid anhydride to form maleic acid and/or fumaric add, preferably suspension crystallisation pro­cesses are Used and in particular those suspension crystallisation processes which allow a continuous design of the process according to the invention. The suspen­sion crystallisation can advantageously be carried out in a stirrer vessel crystal-User, in a scratch crystalliser, in a cooled plate crystalliser, in a crystallising screw, in a drum crystalliser, in a pipe bundle crystalliser or the like. Particularly advantageous here are, in turn, such crystallisers which can be operated continu­ously. These are preferably the cooled plate crystallisers or scratch coolers (see the dissertation of Poschmann on suspension crystallisation of organic melts and post-treatment of the crystals by sweating or washing, Diss. University of Bre­men, Shaker Verlag, Aachen 1996). Most particularly preferred, for crystallisation a scratch cooler is used. Advantageously, the temperature of the melt during the crystallisation lies be­tween -30 and +14°C, in particular between -15 and +14°C. The solid content hi the crystalliser advantageously lies between 0 and 85 g, preferably between 20 sad 40 g solid/100 g of the composition. The separation of the (meth)acryiic acid crystals from me crystal suspensions ob­tained in process steps A and G preferably occurs by means of wash columns. In a preferred embodiment of this process according to the invention, the suspension is supplied to a hydraulic wash columnin the upper part of the column. The mother liquor is discharged from the column by means of a filter, whereby a densely pa­cked crystal bed forms. The crystal bed and the mother liquor flow hi the direction of the floor of the wash column. At the floor of the column is located a moved, preferably rotating scratch device or scratoher, which generates a suspension again from the densely packed crystal bed. This suspension is preferably pumped and melted by means of amelter, pref­erably a heat exchanger. A part of the melt can, e.g. serve as wash melt; this is then pumped back into the column and preferably washes the crystal bed migrat­ing in opposing direction, i.e. the crystallised (meth)acryUc acid is washed in the counter-flow of the conducted-back (meth)acrylic acid. The wash melt effects on the one hand a washing of the crystals, on the other hand the melt at least partially crystallises on the crystals. The enthalpy of crystallisation being released warms the crystal bed in the washing region of the column. In this way, a purification effect analogous to the sweating of the crystals is achieved, hi this context, refer­ence is made to DEI0242746A1 as well as DE 101 49353 Al, which are hereby introduced as reference and whose disclosure with regard to the separation of (meth,)acrylic add crystals from crystal suspensions represents a part of the disclosure of the present invention. In a particular embodiment of this process according to the invention for produc­tion of (meth)acrylic acid, in which merely a one-step purification process is car­ried out (process steps A and B), at least a part of the mother liquor obtained hi step 3 is conducted back to step A. In this way the mother liquor can be con­ducted in a cycle. In an intended separation of maleic acid anhydride as impurity to be separated, thus, the extent of hydrolysis of maleic acid anhydride can be influenced, in addition to by the amount of water in the composition, also by the regulation of the amount of mother liquor which is conducted back to step A, and thus by the residence time of the mother liquor in the preferably continuous puri­fication process. It is also conceivable to conduct ,at least a part of the (meth)acrylie acid crystals obtained in process step B to process A for inoculation in crystalline form, as is likewise described in DE 102 11 686 Al. If ibis process according to the invention is operated with a two-step purification (process stages A to D), it is preferred according to the invention that the (meth)acrylic acid crystals obtained in process step B and/or in process step D in crystalline and/or at least partially melted form or the mother liquors obtained in process steps C and D are at least partially conducted back to one of process steps A to D, whereby the exact type of conducting bade of the crystalline or melted (meth)acrylic acid or of the mother liquors in the case of a two-step purification process is described in DE 102 11 686 Al. The particular embodiments described there in the case of a two-step purification process are also particularly preferred in the case of the process according to the invention for production of (meth)acryiic acid, which comprises a purification process with the process steps AtoD. If, in. the process according to the invention, the mother liquor separated in proc­ess step B or in process step D as composition comprising (meth)acrylic acid and imparities is used according to process step a), the mother liquor is at least par­tially directly conducted to the corresponding separating device for separation of the impurities according to the process b), for example for a filter. If, in the process according to the invention, the ftnetfa) acrvlic acid crystals ob­tained in process step B or in process step D as composition comprising (mem)acrylic acid and impurities is used according to process step a), it is pre- ferred according to the invention that the (meth)acrylic acid crystals are conducted at least partially melted and in melted form to the device for separation of the im­purities, in order to keep as low as possible a separation of (meth)acrylic acid crystals in the separating device for separation of impurities and thus a loss of yield. The separation of me impurities from a melted (meth)acrylic acid can be advantageously achieved in that in the case of a partial melting of (meth)acrylic acid crystals and conducting back of these melted (meth)acryiic acid crystals into one of the process steps A to D to achieve a good purity of the (meth)acrylic acid (see above details in connection with the process described in DE 102 11 686 Al) this stream of melted (meth)acrylic acid is conducted either directly or with help from a separate product cycle through the separating device for separation of the impurities. If, in the process according to the invention, the crystal suspension obtained in process step A or in process step C is used as composition comprising (meth)acrylie acid and impurities according to process step a), it is preferred ac­cording to the invention that the crystal suspension obtained in these process steps is conducted at least partially melted and in molten form through the separating device for separation of impurities. Then, me melted crystal suspension can be conducted again to the crystalliser. In the case of a one-step purification process (process steps A and B), as composi­tion comprising (meth)acrylic acid and impurities, preferably maleic acid anhy­dride, according to process step a), preferably a composition is used selected from: the mother liquor, which was obtained in process step B, in so far as this is at least partially conducted hack to process step A; the at least partially melted (meth)acrylic acid crystals, which were obtained in process step B; (3) the at least partially melted crystal suspension, which was obtained in step A. In addition to a separation of impurities from only one of the compositions (1), (2) or (3), in the case of a one-step purification process, a separation of the impurities at more than one position of the purification process or from more than one, dif­ferent, compositions can, of course, occur. It is thus conceivable to separate the impurities both from the mother liquor (composition (1)) as well as from the par­tially melted (meth)acryjic acid crystals (composition (2)). A separation from all compositions (1) to (3) is also possible. Most preferred is a separation from the mother liquor which was obtained in process step B, in so far as mis is at least partially conducted back into process A. m the case of a two-step purification process (process step A to D), as composi­tion comprising (meth)acrylic acid and impurities according to process step A, preferably a composition is used selected from: the mother liquor, which was obtained in process step B and conducted to process step C; the mother liquor, which was obtained in process step B and at least par­ tially conducted back into process step A; the mother liquor, which was obtained in process step D in so far as this mother liquor is at least partially conducted bade; preferably into process stepG; the at least partially melted (meth)acrylic acid crystals, which were obtained in process B; the preferably at least partially melted (meth)acrylic acid crystals, which were obtained in process step D and which are conducted back least par­ tially into process A; the crystal suspension, preferably the least partially melted crystal suspen­ sion, which was obtained in step A; the crystal suspension, preferably the at least partially melted crystal sus­ pension, which was obtained in step C; the crude (meth)acrylic add, which is used in process step A and which was previously conducted into the head of file second wash column. Also with regard to this two-step purification process, a separation of impurities can occur not only from one of the compositions (4) to (11), but also from at least two, preferably at least three and particularly preferably at least four of these compositions, whereby a separation from composition (8) is most preferred, fol­lowed by a separation from composition (10), in turn followed by a separation from composition (4), and in turn Mowed by a separation from composition (5) and in turn followed by a separation from composition (7). If the impurity, which is separated in process step b) from one or more of the compositions (1) to (11), is maleic acid anhydride, it should be ensured in the pro­cess according to the invention that the compositions comprise sufficient amounts pf water in order to ensure a sufficient hydrolysis of the maleic acid, in a continu­ous purification process, in which separated mother liquor and/or separated (meth)acrylic acid crystals are at least partially conducted back to one of the proc­ess steps A or B (or, in a process with two-step purification process, to one of the process steps A to D). If a composition does not comprise enough water for a sat­isfactory separation of the maleic acid anhydride, in the process according to the invention, water can additionally be introduced. The water can already be added before process step A to the impure crude (meth)acrylic acid stream from a proc­ess for production of (meth)acrylic acid and used in process step A. The increase of the amount of water in this composition has an effect, in a continuously operat­ing purification process, on the water content of all other compositions (1) to (6) and (8) to (11), so that the extent of crystallisation of maleic acid and/or fumaric add in these compositions can be regulated by means of the water content of the starting composition used in process step A. It is also conceivable to add the water directly to the individual compositions (1) to (11). In addition to influencing the extent of hydrolysis of the maleic acid anhydride by means of the amount of water in the composition, this can also be regulated over time, in that the maleio acid anhydride in a continuous purification process is con­ducted in the cycle by the conducting bade of mother liquor and/or (meth)acrylic acid. This duration is, in turn, dependent on the relative amounts in which the mother liquor or the optionally melted (meth)acrylic acid crystals are conducted back into me individual process steps. The skilled person will determine me indi­vidual process parameters which are necessary for a satisfactory precipitation of maleic acid and/or fumaric acid from the compositions, by means of simple rou­tine experiments. The present invention also relates to a device for production of acrylic acid, com­prising, as components connected together in fluid-conducting fashion, an acrylic acid reactor, a quench, tower, a distillation device and a purification device, which comprises a separation device for separation of impurities, which comprises the features (el) to (&4): (el) the device unit comprises a crystallisation region, preferably comprising a crystal suspension generator, a separating region, preferably comprising a wash column, a separating device, at least two guides as well as optionally at least one meiter, (e2) the crystallisation region comprises at least one inlet e2E_l and one outlet s2A_l, whereby the outlet e2A_l is connected by means of a first guide with an inlet e3E_I of the separating region ?3; (e3) the separating region comprises at least inlet s3B_l and at least one outlet 83A_I, whereby the inlet .«3B_1 of the separating region s3 is connected by means of the first guide with the outlet s2A_l of the crystallisation region and the outlet e3 A_l is connected With a second guide for the mother liquor separated in the separating region; (e4) the melter comprises an inlet &4E and an outlet e4A, whereby the inlet e4E of the melter is connected by means of a third guide with a further outlet e3 A_2 of the separating region for the removal of separated (meth)acrylic acid crystals and the outlet e4A by means of a fourth guide with a further inlet &3E_2 of the separating region for the conducting back of melted (meth)acrylic acid crystals info the separating region, or whereby the inlet e4E of the melter is connected by means of a fifth guide with a further outlet e3 A_2 of the crystallisation region for the removal of a crystal suspension and the outlet e4A by means of a sixth guide with a further inlet e2E_2 of the crystallisation region; (e5) through the separating device is conducted the second guide, the fourth guide or the sixth guide, whereby preferably the second, the fourth or the sixth guide is conducted through the separating device in such a way that the separation of crystallised impurities from the compositions conducted in the second, fourth or sixth guide is enabled. It is preferred according to the invention that the separating device (e5) is a filter, a centrifuge, a sedimentation device or hydrocycloae, whereby a filter is most particularly preferred. As filter, all filters can be used which make it possible to separate impurities precipitated from a composition discontinuously or continu­ously, preferably continuously. In this context, particularly preferred continuously working filters comprise in particular gravel or sand filters, suction filters, candle filters, leaf filters such as circular or axial leaf filters, centrifugal disc filters such as, for example, wash filters or residue filters, filter presses. Preferred continu­ously operating filters comprise in particular rotary filters, such as, for example, vacuum rotary filters, pressure rotary filters or drum filters, rotation pressure fil­ter^ in particular such which operate according to the principle of dynamic cross-flow filtration, belt filters such as, for example, vacuum belt filters, belt cell fil­ters, capillary belt filters or paper mat belt filters. The filters preferably used in the process according to the invention can comprise packed bed as well as layer filters. As materials for the packings or the layer filter, all materials known to the skilled person can be used which have a sufficient re­sistance to a chemical attack by, above all, (meth)acrylic acid. hi this context, it is further preferred that in the case of a use of a layer filter this is a filter comprising a sieve, preferably a sieve made of stainless steel, which has a mesh size in a range from 0.1 to 10,000 urn, preferably in a range from 10 to 1,000 urn and particularly preferably in a range from 100 to 500 jam. In a preferred embodiment of the device according to the invention, it comprises two of the above-mentioned purification device, whereby these two purification devices are connected with each other in fluid-conducting fashion in such a way that the second guide, which is connected with the outlet e3A_l of the separation region of the first purification device, leads into the inlet e2E_l of the crystallisa­tion device of the second purification device, whereby the separating region of the second purification device, by means of a seventh guide, with which in the sepa­rating region of the second purification device, separated (meth)acrylic acid crys­tals can be removed from the separating region of the second separation device and conducted into the crystallisation region of the first purification device. The present invention further relates to the use of the above-described device, of the above-mentioned purification device and of the above-mentioned process for production of (meth)aciyh'c acid, preferably for production of acrylic add, which has a purity of more than 90 wt.%, preferably more than 95 wt% and particularly more than 99.5 wt.%, respectively based on the acrylic add wiih impurities. The invention also relates to the (meth)acrylic add obtainable by the above-mentioned process. The present invention further relates to (meth)acrylic add, whereby the (meth)acrylic add has a maltic add anhydride content of less than SO ppm, pref­erably less man 10 ppm and yet more preferably less man 1 ppm. The present invention also relates to water-absorbing polymers which are obtain­able by polymerisation of the above-mentioned acrylic add. The present invention finally relates to the use of these water-absorbing polymers or of the above-mentioned (metb)acrylic add, in particular of the above-mentioned acrylic add, in fibres, sheets, foams and composites as well as fibres, sheets, foams and composites which comprise the above-mentioned water-absorbing polymers or the above-mentioned (meth)acrylic add, in particular the above-mentioned acrylic acid. The present invention is more closely illustrated by means of nqn-limtting figures. Fig. 1 shows the basis solution of the process according to the invention, in which crystallised impurities are separated continuously from a composition stored in a container. Fig. 2A shows the process course of a process according to the invention, in which, in a One-step purification process, impurities are separated from melted (meth)acrylic acid, which were obtained after separation from a crystal suspen- sion. Fig. 2B shows the process course depicted in fig. 2 A, whereby, however, the sepa­ration of the impurities does not occur directly within the product cycle, but by means of a separate cycle which is connected with the product cycle. Fig. 2C shows the process course of a process according to the invention, in which in a one-step purification process impurities are separated from the crystal suspen­sion, which was obtained in the crystalliser and which is, by means of a product cycle, at least partially melted and conducted back into the crystalliser, Fig. 2D shows the process course of a process according to the invention, in which, in a one-step purification process, impurities are separated from the mother liquor, which was obtained on separation of the crystallised (meth)acrylic acid in the wash column and which fa at least partially conducted back into the crystal-User. Fig. 3 shows at which positions in a two-step purification process a separation according to the invention of impurities can occur. According to the basis solution of the process according to the invention depicted in Fig. 1, a composition comprising (mefh)acrylic acid and impurities, in particu­lar maledc acid anhydride, and stored in a stock container 1, is stored for so long until a sufficient hydrolysis of the maleic acid anhydride has occurred In the case of a continuous crystallisation process for purification of (meth)acryu'c acid, the stock container 1 corresponds to a suspension generator, to a wash column or to a supply tine or discharge line for a crystal suspension, for crystallised (meth)acrylic acid separated from the crystal suspension, or for mother liquor, depending on from which composition the crystallised impurities, in particular the crystallised maleic acid and/or fumaric acid, arc separated. By means of a product cycle 2, the composition is removed continuously or dis-continuously from the stock container 1, whereby the product cycle is operated by means of a product cycle pump 3. From the product cycle 2, by means of a further product cycle 4, the composition is conducted through the separating device, in which crystallised impurities, in particular crystallised maleic add and/or fumaric acid are separated by means of discharge 8. The composition freed from the crys­tallised impurities is than conducted back into the product cycle 2 and, via this, then into stock container 1. Besides the procedure depicted in fig. 1, it s also pos­sible to conduct the composition conducted in product cycle 2 directly (and not by means of the separate product cyde 4), through the separating device 5. Fig. 2A shows the process course of a process according to the invention, in which, in a one-step purification process, impurities are separated from the at least partially melted (meth)acrylic acid, which was obtained after the separation in the wash column 10. The composition stored in stock container 1 and comprising (meth)acrylic acid and impurities is conducted to a crystal suspension generator 8. The formed crys­tal suspension is then conducted by means of a feed 9 into a separating device (e.g. a wash column) 10. In the separating device 10, the (meth)acrylic acid crys­tals are separated from the mother liquor. In addition, a part of the (meth)acrylic acid crystals are transferred again into a crystal suspension and this crystal sus­pension conducted in a product cycle 11 driven by means of a product cycle pump 12, in which product cycle 11 the (meth)acrylic add crystals are melted by means of a heat exchanger 13 and, for increase of the purity of the (meth)acrylic acid, at least partially conducted back as wash liquid to the counter-flow wash in the wash column 10. Thus, the wash column 10 serves to separate solid and liquid as well as to carry out a displacement washing, whereby the displacement washing is car­ried out without loss of wash fluid. The other part of the (meth)acrylic acid crys­tals leaves the system and flows into the product container 14 (if the purity of the (meth)acryiic acid obtained by means of the one-step purification process shown in diagram 2A is not sufficient, the (mem)acrylic acid crystals can be conducted to a farther purification step). The at least partially melted (meth)acrylic acid crystals are conducted through the separation device for separation of impurities 5. Ac­cording to fig. 2B, the at least partially melted (meth)acrylic acid crystals can also be conducted to the separating device by means of a separate product cycle. The separated impurities are removed via discharge line 6 and the composition freed from the impurities is conducted back into the separating device 10. In a preferred embodiment of this one-step purification process, at least a part of the mother liq­uor separated in wash column 10 is conducted back via feed 15 into the suspen­sion generator 8, the other part of the mother liquor is conducted as effluent into the mother liquor container 16. Fig. 2C and 2D show the one-step purification process corresponding to that of fig, 2A, whereby, however, not the at least partially melted (meth)acrylic acid crystals which were obtained after the separation hi the wash column 10, but ra­ther the crystal suspension (fig. 2D) obtained in the suspension generator 8 or the mother liquor of the separating device 5 conducted back into the suspension gen­erator 8 are supplied. Fig. 3 shows a two-step purification process, in which the mother liquor separated in wash column 8 is at least partially conducted into a further suspension genera­tor 17. The crystal suspension obtained in suspension generator 17 is conducted into a further wash column 18, in which .the (meth)acrylic acid crystals are sepa­rated from the mother liquor. The (meth)acrylic acid crystals separated in this sec­ond wash column 18 are, in crystalline form or after .they have been at least par­tially melted by means of a heat exchanger, conducted via feed line 20 to the first suspension generator. It is also preferred in mis two-step purification process that the crude (meth)acrylic acid is conducted out of the stock container 1 into the head of the second Wash column, in order, in this way, to conduct the product crystals scraped off in the second process step as crystal suspension into the sus­pension generator of process step A. This variant has the energetic advantage of being able to dispense with a melting in the second step and not having to freeze again the crystals now present in the first step. In fig. 3, it is illustrated at which positions a separation of crystallised impurities, in particular of crystallised maleic acid and/or fumaric acid is possible by means of separating device 22. The invention is now more closely illustrated by means of an example: EXAMPLE 113.5 g of a bottom product from a column with me composition given in table 1 was placed with 1.6 g water in a Erlenmeyer flask. The mixture was allowed to stand over a time period of 42 hours at room temperature and then filtered using a vacuum, filter. 108.5 g filtrate with the composition likewise given in table 1 were obtained. The solid remaining on the filter was then centrifuged at 4,000 rpm for 5 minutes, whereby 3.6 g of a solid was obtained. This solid was dissolved in 129.0 g highly pure acrylic acid, fbr the purpose of analysis. The composition of the solid is likewise given in table 1 (the components comprised in the composi- tions were analysed by means of respectively gas cbibmatography and Karl-Fischer-Titration; the relative amounts of die components in the bottom product, in the filtrate and in the solid do not add up to 10Q wi% exactly, because of the rounding up and rounding down of the analysis data obtained for these composi­tions. TABLE 1 15 Methyl hydrogoinone 2)Maleic acid anhydride/maleic acid We Claim: 1. A process for preparing (meth) acrylic acid, whereby first a crude (meth) acrylic acid is produced and this crude (meth) acrylic acid is then preferably continuously purified, wherein the preferably continuous purification of the crude (meth) acrylic acid comprises the following process steps: (a) hydrolyzing a composition comprising (meth) acrylic acid and impurities, wherein the impurity is fumaric acid, maleic acid or maleic acid anhydride , said hydrolysis reducing an amount of maleic acid anhydride by atleast 50 mole% based on an initial amount of maleic acid anhydride in the composition; (b) precipitating the impurities in crystal form from the composition of step (a), wherein maleic acid and/or fumaric acid are precipitated in crystal form; (c) separating the crystalline impurities from the composition. 2. The process as claimed in claim 1, wherein the hydrolysis of the maleic acid anhydride in process step (a) occurs over a time period in a range from 1 to 250 hrs at a temperature in a range from -70 to 130°C. 3. The process as claimed in claim 1, wherein the composition is (meth) acrylic acid crystals preferably in at least partially melted form or a mother liquor, which are obtained in step B or in step D of a process comprising steps of: (A) crystallisation of (meth) acrylic acid from an impure crude (meth) acrylic acid stream from a process for production of (meth) acrylic acid with formation of a crystal suspension by means of a first suspension generator; (B) separation of the (meth) acrylic acid crystals form the crystals suspension by means of a first separating device, whereby a mother liquor is retained; as well as optionally (C) renewed crystallization of (meth) acrylic acid from the first mother liquor obtained from step (B) with formation of a second crystal suspension by means of a second suspension generator; (D) separation of the (meth) acrylic acid crystals obtained in step (C) from the second crystal suspension by means of a further separating device, whereby a second mother liquor is retained. 4. The process as claimed in any one of the preceding claims, wherein the composition is the crystal suspension obtained in process step (A) or step (C) of a process which comprises the following process steps: (A) crystallisation of (meth) acrylic acid from an impure crude (meth) acrylic acid stream from a process for production of (meth) acrylic acid with formation of a crystal suspension by means of a first suspension generator; (B) separation of the (meth) acrylic acid crystals form the crystal suspension by means of a first separating device, whereby a mother liquor is retained; as well as optionally (C) renewed crystallization of (meth) acrylic acid from the first mother liquor obtained from step (B) with formation of a second crystal suspension by means of a second suspension generator; (D) separation of the (meth) acrylic acid crystals obtained in step (C) from the second crystal suspension by means of a further separating device, whereby a second mother liquor is retained. 5. A device for preparing of acrylic acid comprising, as components connected with each other in fluid-conducting fashion, an acrylic acid reactor, a quench tower, a distillation device and a purification device, which comprises a separating device for separation of impurities, which comprises the features (ε1) to (ε5): (ε1) the device unit comprises a stock container, a crystallisation region, a separating region, a separating device, at least two guides and optionally a melter; (ε2) the crystallisation region comprises at least one inlet ε2_l for an impure (meth) arylic acid stream and an outlet ε2A_l for a crystal suspension, wherein the outlet ε2A_l is connected by means of a first guide with an inlet ε3E_l of the separating region ε3; (ε3) the separating region comprises at least one inlet ε3E_l and at least one outlet ε3 A_l, whereby the inlet ε3E_l of the separating region ε3 is connected by means of the first guide with the outlet ε2A_l of the crystallisation region and the outlet ε3A_l is connected with a second guide for the mother liquor separated in the separating region; (ε4) the melter comprises an inlet s4E and an outlet ε4A, whereby the inlet ε4E of the melter is connected by means of a third guide with a further outlet ε3A_2 of the separating region for the removal of separated (meth) acrylic acid crystals and the outlet s4A is connected by means of a fourth guide with a further inlet ε3E_2 of the separating region or whereby the inlet ε4E of the melter is connected by means of a fifth guide with a further outlet ε3A_2 of the crystallisation region for the removal of a crystal suspension and the outlet ε4A is connected by means of a sixth guide with a further inlet ε2E_2 of the crystallisation region; (ε5) the second guide, the fourth guide or the sixth guide is conducted through the separating device; wherein the impure (meth) arylic acid stream is hydrolyzed in a stock container, said hydrolysis reducing an amount of maleic acid anhydride by atleast 50 mole% based on an initial amount of maleic acid anhydride in the impure (meth) arylic acid stream; wherein the impure (meth) arylic acid stream comprises (meth)acrylic acid and impurities, wherein the impurity is fumaric acid, maleic acid or maleic acid anhydride; wherein in the crystallisation region the impurities including maleic acid and/or fumaric acid are precipitated in crystal form from the impure (meth) arylic acid stream, and wherein in the separating device the crystalline impurities are separated. 6. The device as claimed in claim 5, wherein the separating device (s5) is a filter. 7. (Meth) acrylic acid obtained by a process as claimed in any one of the preceding claims has a maleic acid anhydride content of less than 50 ppm. 8. The (meth) acrylic acid as claimed in claim 7 for use in preparing water-absorbing polymers on polymerisation.

Documents:

193-delnp-2007-abstract.pdf

193-delnp-2007-Claims-(15-10-2012).pdf

193-delnp-2007-claims.pdf

193-DELNP-2007-Correspondence Others-(30-03-2012).pdf

193-DELNP-2007-Correspondence-Others-(1-1-2010).pdf

193-delnp-2007-Correspondence-Others-(15-10-2012).pdf

193-delnp-2007-correspondence-others-1.pdf

193-DELNP-2007-Correspondence-Others.pdf

193-delnp-2007-description (complete).pdf

193-DELNP-2007-Drawings.pdf

193-delnp-2007-Foprm-1-(15-10-2012).pdf

193-DELNP-2007-Form-1.pdf

193-delnp-2007-form-13.pdf

193-delnp-2007-form-18.pdf

193-delnp-2007-Form-2-(15-10-2012).pdf

193-delnp-2007-form-2.pdf

193-delnp-2007-form-26.pdf

193-DELNP-2007-Form-3-(30-03-2012).pdf

193-DELNP-2007-Form-3.pdf

193-DELNP-2007-Form-5.pdf

193-DELNP-2007-GPA (1-1-2010).pdf

193-delnp-2007-pct-306.pdf

193-delnp-2007-pct-409.pdf

193-delnp-2007-pct-notification.pdf

193-delnp-2007-pct-search report.pdf

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