A method for inhibiting the undesired free radical polymerization of acrylic acid present in liquid phase P.

Abstract

1. A method for inhibiting the undesired free radical polymerization of acrylic acid present in a liquid phase P, wherein the acrylic acid content of P is at least 10% by weight, and the liquid phase P contains glyoxal in the range of 25 to 1000 ppmw, relative to the weight of acrylic acid present in P, and the liquid phase P is mixed with protoanemonin in an amount such as to result in a protoanemonin content in the range of 0.5 to 100 ppmw, relative to the weight of acrylic acid present in P; and a liquid phase P having an acrylic acid content of at least 10% by weight and containing glyoxal in the range of 25 to 1000 ppmw and protoanemonin in the range of 0.5 to 100 ppmw, in each case relative to the weight of acrylic acid present in P.

Description

[Technical field]

[0001] The present invention relates to a method for inhibiting the undesired free radical polymerization of acrylic acid present in a liquid phase P, and to the liquid phases produced during the implementation of said method. [Background technology]

[0002] Acrylic acid, either as such, in the form of its salts and / or in the form of its esters (e.g. alkyl esters), is an important monomer used for the production of polymers which are used, for example, as adhesives or as water-absorbent materials (see, for example, WO 02 / 055469 and WO 03 / 078378).

[0003] The preparation of acrylic acid can be carried out, for example, by heterogeneously catalyzed partial oxidation of C3-precursor compounds (e.g. propylene, propane, acrolein, propionaldehyde, propionic acid, propanol and / or glycerol) in the gas phase (see, for example, WO 2010 / 012586, US 5,198,578, EP 1 710 227 A, EP 1 015 410 A, EP 1 484 303 A, EP 1 484 308 A, EP 1 484 309 A, US 2004 / 0242826, WO 2006 / 136336, DE 100 28 582 A and WO 2007 / 074044).

[0004] Such heterogeneously catalyzed partial gas phase oxidations generally do not give pure acrylic acid, but rather only give acrylic acid-containing product gas mixtures which contain not only acrylic acid but also components other than acrylic acid, from which the acrylic acid must be separated.

[0005] Both the type and the quantitative proportion of the components other than acrylic acid in the product gas mixture are influenced by the choice of C3-precursor compound, the catalyst employed, the reaction conditions under which the heterogeneously catalyzed partial gas-phase oxidation is carried out, the type and amount of impurity components other than the C3-precursor compound present in the C3-precursor compound employed as feedstock, and generally the choice of dilution gas for diluting the reactants in the reaction gas mixture (see for example DE 101 31 297 A, DE 10 2005 052917 A, WO 2007 / 074044 and DE 100 28 582 A).

[0006] The separation of acrylic acid from the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds typically employs a combination of various separation methods in order to achieve a purity suitable for the subsequent intended use of acrylic acid in the most economical way possible. The combination employed in a particular case therefore depends inter alia on the type and amount of components different from acrylic acid present in the product gas mixture.

[0007] A common feature of essentially all possible combinations of separation methods for separating acrylic acid from the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds is that the acrylic acid present in the product gas mixture is brought into a condensed, in particular liquid, phase in the main separation step, optionally after direct and / or indirect cooling of said product gas mixture.

[0008] This may be done, for example, by absorption in a suitable solvent (e.g. water, high-boiling organic solvent, aqueous solution) and / or by partial or substantially complete condensation (e.g. fractional condensation) (see, in this respect, for example, EP 1 388 533 A, EP 1 388 532 A, DE 102 35 847 A, EP 0 792 867 A, WO 98 / 01415, US 7,332,624, US 6,888,025, US 7,109,372, EP 1 015 411 A, EP 1 015 410 A, WO 99 / 50219, WO 00 / 53560, WO 02 / 09839, DE 102 35 847 A, WO 03 / 041832, DE 102 23 058 A, DE 102 43 625 A, DE 103 36 386 A, EP 0 854 129 A, US 7,319,167, US 4,317,926, DE 102 47 240 A, EP 0 695 736 A, EP 0 982 287 A, EP 1 041 062 A, EP 0 117 146 A, DE 43 08 087 A, DE 43 35 172 A, DE 44 36 243 A, DE 199 24 532 A, DE 103 32 758 A and DE 199 24 533 A). The separation of acrylic acid can also be carried out as described in EP 0 982 287 A, EP 0 982 289 A, DE 103 36 386 A, DE 101 15 277 A, DE 196 06 877 A, DE 197 40 252 A, DE 196 27 847 A, EP 0 920 408 A, EP 1 068 174 A, EP 1 066 239 A, EP 1 066 240 A, WO 00 / 53560, WO 00 / 53561, DE 100 53 086 A and EP 0 982 288 A. Advantageous embodiments of the separation also include the methods described in WO 2004 / 063138, WO 2008 / 090190, WO 2004 / 035514, DE 102 43 625 A and DE 102 35 847 A.

[0009] Components other than acrylic acid present in the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation are also usually in the condensed phase together with acrylic acid.

[0010] The documents DE 10 2009 027401 A, DE 10 2008 041573 A, DE 10 2008 040799 A, EP 1 298 120 A and EP 1 396 484 A disclose, in the context of heterogeneously catalyzed partial gas-phase oxidation, that if the reaction mixture from the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to acrylic acid contains C2-impurities, such as ethylene, then large amounts of the aldehyde (monomeric) glyoxal as a by-product generally occur in the product gas mixture, and that (monomeric) glyoxal is typically brought to a considerable extent into the condensed phase together with acrylic acid in the above-mentioned main separation of acrylic acid from the product gas mixture.

[0011] When the reaction gas mixture from the heterogeneously catalyzed partial gas-phase oxidation of a C3-precursor compound (e.g. propylene) contains both the above-mentioned C3- and C2-impurities, the primary separation of acrylic acid from the product gas mixture from the described heterogeneously catalyzed partial gas-phase oxidation usually produces a condensed phase containing not only acrylic acid but also glyoxal.

[0012] As known from EP 0 770 592 A, the aldehyde impurities present in acrylic acid, such as glyoxal, even in small amounts, significantly impair the properties of acrylic acid.Accordingly, according to the teaching of EP 0 770 592 A, in order to obtain optimal product quality, especially in the context of using such acrylic acid in free radical polymerization reactions for the production of polymers useful as superabsorbent polymers or dispersants or flocculants for oil drilling muds, the proportion of each aldehyde in acrylic acid should be less than 1 ppm.

[0013] The separation steps to be adopted for separating acrylic acid in the desired purity from the liquid phase obtained in the course of the described main separation and containing the desired product acrylic acid and the undesired by-product glyoxal can be a very wide combination of, for example, adsorption, extraction, desorption, distillation, stripping, rectification, azeotropic distillation, azeotropic rectification and crystallization methods, depending on the objective and the type and amount of other additionally present undesired secondary components.

[0014] The liquid phase comprising the desired product acrylic acid and the undesired by-product glyoxal can be obtained in the course of the above-mentioned separation processes in very diverse forms and in different quantitative proportions, which liquid phase therefore requires intermediate storage and / or thermal stress, e.g. by heating.

[0015] This is disadvantageous because long residence times and thermal stress increase the probability of undesirable free radical polymerization of the acrylic acid present in the liquid phase.

[0016] The latter is all the more true since, due to the physical similarity between acrylic acid and some secondary components, when using non-crystallizing thermal separation methods, long residence times in the separation equipment must be used to achieve a significant separation effect, and monomeric glyoxal promotes the tendency of acrylic acid to undergo undesirable free radical polymerization to a significantly higher extent than other possible impurities (see DE 10 2008 041573 A, DE 10 2008 040799 A and DE 10 2009 027401 A).

[0017] It is well known that the addition of inhibitors to acrylic acid present in the liquid phase can counteract the influence of residence time and thermal stresses that promote polymerization (see for example "Polymerisationsinhibierung von (Meth-)Acrylaten" [Inhibition of polymerization of (meth)acrylates], thesis of Dipl.-Ing. Holger Becker, Technische Universitat Darmstadt, 2003).

[0018] A large variety of inhibitors are recommended in the prior art in this regard (see for example EP 0 765 856 A which recognizes only a small part of these inhibitors).

[0019] However, according to EP 1 396 484 A (in particular column 2, lines 16 and 17), none of the known inhibitor systems give satisfactory results. Moreover, according to EP 1 396 484 A (e.g. column 7, paragraph

[0024] and column 1, lines 40-44), the variety of inhibitors recommended in the prior art does not include any particular preference.

[0020] In particular, EP 1 396 484 A notes in column 3, lines 5-10 that, although known inhibitors are capable of relatively effectively inhibiting the undesired free radical polymerization of acrylic acid due to thermal stress, the inhibitory action of said inhibitors is insufficient, particularly with regard to the cause and / or promotion of the undesired free radical polymerization of acrylic acid due to impurities such as glyoxal present therein.

[0021] One way to overcome the described difficulties is to avoid the formation of undesirable by-products such as glyoxal in the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds of acrylic acid (these are precursor compounds having 3 carbon atoms) to obtain acrylic acid (for example by choosing a suitable catalyst (see for example JP H11-35519) or by using a high purity C3-precursor feedstock (thus producing a reaction gas mixture which contains, for example, neither C2-impurities nor n-propane or cyclopropane; DE 35 21 458 A describes the possibility of purifying propylene produced, for example, from n-propane, and WO 2004 / 018089 A and WO 01 / 92190 A describe the production of propylene, for example, from methanol (modified feedstock basis)). However, this is disadvantageous, since the expenditure required in this respect undermines the economics of acrylic acid production.

[0022] WO 2012 / 045738 describes a method for inhibiting the undesired free radical polymerization of acrylic acid present in a liquid phase P having an acrylic acid content of at least 10% by weight and additionally comprising 25 to 1000 ppmw of glyoxal relative to the weight of acrylic acid present, the liquid phase P being mixed with 25 to 1000 ppmw of furfural.

[0023] WO 2020 / 020697 describes a method for inhibiting the undesired free radical polymerization of acrylic acid present in a liquid phase P having an acrylic acid content of at least 10% by weight and additionally comprising at least 100 ppmw of propionic acid and at least 100 ppmw of glyoxal relative to the weight of acrylic acid present, the liquid phase P being mixed with at least one compound of elemental copper.

[0024] WO 2018 / 185423, JP S47-17714, JP 2009-143875 and JP 2015-174851 refer to the inhibitory or retarding effect of protoanemonin on polymerization. [Prior art documents] [Patent documents]

[0025] [Patent Document 1] WO 02 / 055469 [Patent Document 2] WO 03 / 078378 [Patent Document 3] WO 2010 / 012586 [Patent Document 4] US 5,198,578 [Patent Document 5] EP 1 710 227 A [Patent Document 6] EP 1 015 410 A [Patent Document 7] EP 1 484 303 A [Patent Document 8] EP 1 484 308 A [Patent Document 9] EP 1 484 309 A [Patent Document 10] US 2004 / 0242826 [Patent Document 11] WO 2006 / 136336 [Patent Document 12] DE 100 28 582 A [Patent Document 13] WO 2007 / 074044 [Patent Document 14] DE 101 31 297 A [Patent Document 15] DE 10 2005 052917 A [Patent Document 16] EP 1 388 533 A [Patent Document 17] EP 1 388 532 A [Patent Document 18] DE 102 35 847 A [Patent Document 19] EP 0 792 867 A [Patent Document 20] WO 98 / 01415 [Patent Document 21] US 7,332,624 [Patent Document 22] US 6,888,025 [Patent Document 23] US 7,109,372 [Patent Document 24] EP 1 015 411 A [Patent Document 25] WO 99 / 50219 [Patent Document 26] WO 00 / 53560 [Patent Document 27] WO 02 / 09839 [Patent Document 28] WO 03 / 041832 [Patent Document 29] DE 102 23 058 A [Patent Document 30] DE 102 43 625 A [Patent Document 31] DE 103 36 386 A [Patent Document 32] EP 0 854 129 A [Patent Document 33] US 7,319,167 [Patent Document 34] US 4,317,926 [Patent Document 35] DE 102 47 240 A [Patent Document 36] EP 0 695 736 A [Patent Document 37] EP 0 982 287 A [Patent Document 38] EP 1 041 062 A [Patent Document 39] EP 0 117 146 A [Patent Document 40] DE 43 08 087 A [Patent Document 41] DE 43 35 172 A [Patent Document 42] DE 44 36 243 A [Patent Document 43] DE 199 24 532 A [Patent Document 44] DE 103 32 758 A [Patent Document 45] DE 199 24 533 A [Patent Document 46] EP 0 982 289 A [Patent Document 47] DE 101 15 277 A [Patent Document 48] DE 196 06 877 A [Patent Document 49] DE 197 40 252 A [Patent Document 50] DE 196 27 847 A [Patent Document 51] EP 0 920 408 A [Patent Document 52] EP 1 068 174 A [Patent Document 53] EP 1 066 239 A [Patent Document 54] EP 1 066 240 A [Patent Document 55] WO 00 / 53561 [Patent Document 56] DE 100 53 086 A [Patent Document 57] EP 0 982 288 A [Patent Document 58] WO 2004 / 063138 [Patent Document 59] WO 2008 / 090190 [Patent Document 60] WO 2004 / 035514 [Patent Document 61] DE 10 2009 027401 A [Patent Document 62] DE 10 2008 041573 A [Patent Document 63] DE 10 2008 040799 A [Patent Document 64] EP 1 298 120 A [Patent Document 65] EP 1 396 484 A [Patent Document 66] EP 0 770 592 A [Patent Document 67] EP 0 765 856 A [Patent Document 68] JP H11-35519 [Patent Document 69] DE 35 21 458 A [Patent Document 70] WO 2004 / 018089 A [Patent Document 71] WO 01 / 92190 A [Patent Document 72] WO 2012 / 045738 [Patent Document 73] WO 2020 / 020697 [Patent Document 74] WO 2018 / 185423 [Patent Document 75] JP S47-17714 [Patent Document 76] JP 2009-143875 [Patent Document 77] JP 2015-174851 [Patent Document 78] EP 0 253 409 A [Patent Document 79] WO 2007 / 074045 [Patent Document 80] DE 24 49 780 A [Patent Document 81] DE 196 27 850 A [Patent Document 82] DE 198 10 962 A [Patent Document 83] EP 0 722 926 A [Patent Document 84] WO 2006 / 002713 A [Patent Document 85] WO 2008 / 090190 A [Patent Document 86] DE 10 2007 004960 A [Patent Document 87] WO 98 / 01414 [Patent Document 88] WO 2005 / 035478 [Patent Document 89] DE 197 34 171 A [Patent Document 90] WO 2011 / 000808 [Patent Document 91] US 6,441,228 [Patent Document 92] US 6,966,973 [Patent Document 93] WO 2005 / 042459 [Patent Document 94] WO 2005 / 047224 [Patent Document 95] WO 2005 / 047226 [Patent Document 96] EP 0 608 838 A [Patent Document 97] DE 198 35 247 A [Patent Document 98] DE 102 45 585 A [Patent Document 99] DE 102 46 119 A [Patent Document 100] WO 2007 / 090991 [Patent Document 101] WO 2006 / 114506 [Patent Document 102] WO 2005 / 073160 [Patent Document 103] WO 2006 / 092272 [Patent Document 104] WO 076370 [Patent Document 105] WO 01 / 96271 [Patent Document 106] WO 03 / 011804 [Patent Document 107] WO 01 / 96270 [Patent Document 108] EP 0 778 255 A [Patent Document 109] DE 10 2007 029053 A [Non-patent literature]

[0026] [Non-Patent Document 1] "Polymerisationsinhibierung von (Meth-)Acrylaten" [Inhibition of polymerization of (meth)acrylates], thesis of Dipl.-Ing. Holger Becker, Technische Universitat Darmstadt, 2003 Summary of the Invention [Problem to be solved by the invention]

[0027] The object of the present invention is to provide an improved method for inhibiting the undesired free radical polymerization of acrylic acid present in the liquid phase P. The method should in particular be technically simple and economical to carry out and should not adversely affect the product quality, i.e. the quality of acrylic acid. [Means for solving the problem]

[0028] The inventors have therefore found a method for suppressing the undesired free radical polymerization of acrylic acid present in a liquid phase P, wherein the acrylic acid content of P is at least 10% by weight, the liquid phase P contains in each case relative to the weight of acrylic acid present in P a glyoxal content in the range of 25 to 1000 ppmw, and the liquid phase P is mixed with an amount of protoanemonin resulting in a protoanemonin content in the range of 0.5 to 100 ppmw.

[0029] The inventors have further discovered liquid-phase P, wherein the acrylic acid content of P is at least 10% by weight and which contains, in each case, relative to the weight of acrylic acid present in P, in the range of 25 to 1000 ppmw of glyoxal and in the range of 0.5 to 100 ppmw of protoanemonin. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The process has the particular feature that the liquid phase P contains glyoxal in the range of 50 to 500 ppmw and / or the liquid phase P is mixed with an amount of protoanemonin resulting in a protoanemonin content in the range of 1 to 50 ppmw, in each case relative to the mass of acrylic acid present in P.

[0031] The method according to the invention is based on the surprising experimental finding, compared to the current knowledge of the prior art, that protoanemonin effectively inhibits the undesired free radical polymerization of acrylic acid promoted by glyoxal.

[0032] For example, by reaction with secondary components containing hydroxyl groups (e.g. water, alcohols such as ethanol, etc.), monomeric glyoxal

[0033] [ka]

[0034] can form hemiacetals and / or acetals which generally no longer exhibit the polymerization-promoting properties typical of monomeric glyoxals, or in any case only to a substantially reduced extent compared thereto.

[0035] However, in the case of hemiacetals / acetals of glyoxal, the formation reactions are often easily reversible, which is why, under the influence of high temperatures or upon removal of glyoxal from the corresponding equilibrium, these hemiacetals / acetals reform monomeric glyoxal, which then has a similar effect on the undesired free radical polymerization.

[0036] In the case of water as the secondary constituent containing hydroxyl groups, the following readily reversible acetal-forming reaction is known (reference is also made in this case to the hydrates of glyoxal):

[0037] [ka]

[0038] Both of the above glyoxal hydrates are formed even under relatively mild conditions (low temperature, limited water content is sufficient).

[0039] The terms "monomeric" glyoxal monohydrate and "monomeric" glyoxal dihydrate are used herein to distinguish them from "polyglyoxal" and "oligoglyoxal" hydrates.

[0040] Examples of diglyoxal hydrate and triglyoxal hydrate are given below:

[0041] [ka]

[0042] [ka]

[0043] The formation of polyglyoxal hydrates is believed to proceed via monomeric glyoxal dihydrate as an intermediate (see also DE 10 2008 041573 A, DE 10 2008 040799 A and DE 10 2009 027401 A).

[0044] In contrast to the formation of monomeric glyoxal hydrates, the formation of polyglyoxal hydrates requires higher temperatures (generally only significant formation occurs at temperatures above 50° C.) and / or longer reaction times.

[0045] Therefore, for the reasons stated above, the term "glyoxal" in this document should be understood to include not only monomeric glyoxal, but also glyoxal that is reversibly chemically bound, for example in the form of acetals and / or hemiacetals of monomeric glyoxal (unless expressly stated otherwise and unless the term "glyoxal" explicitly includes at least one additional feature, such as, for example, "monomeric" glyoxal or "di" glyoxal "hydrate" or "monomeric" glyoxal "monohydrate").

[0046] Therefore, the general term "glyoxal" in this document should always be understood to mean the total amount of monomeric and reversibly bound glyoxal.

[0047] Thus, glyoxal contents reported in "mass %" and "ppmw" units should always be understood in this document to mean the total amount of monomeric and reversibly bound glyoxal present, e.g., in monomeric glyoxal monohydrate and monomeric glyoxal dihydrate, but are always calculated as "monomeric glyoxal" (i.e., they refer to the mass fraction of the total amount of H2C2O2 units present).

[0048] This is particularly relevant for the embodiment of the method according to the invention, since water is usually the main by-product of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to acrylic acid. Furthermore, water vapor, for example due to its relatively high molar heat capacity, is often used together as a diluent gas in the reaction gas mixture for the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to acrylic acid (see, for example, EP 0 253 409 A). The main separation of acrylic acid from the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to acrylic acid therefore often involves passing a liquid phase which contains not only acrylic acid and glyoxal, but also water. However, glyoxal hydrate can in principle also be formed in the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to acrylic acid.

[0049] Furthermore, the prior art often also recommends water or aqueous solutions as absorbents for the absorptive primary separation from the product gas mixture of the gas-phase partial oxidation of C3-precursor compounds (see, for example, EP 1 298 120 A and US 7,332,624).

[0050] In the context of the present invention, the content of glyoxal in the liquid phase P (or another liquid phase) treated according to the present invention (i.e. the total content in the liquid phase P of monomeric glyoxal and glyoxal reversibly bound in compounds such as monomeric glyoxal monohydrate and monomeric glyoxal dihydrate (e.g. monomeric glyoxal can also reversibly form hemiacetals and / or acetals with alcohols such as ethanol)) is determined as follows:

[0051] First, a derivatization solution D is produced. For this purpose, 2.0 g of a 50% by weight solution of 2,4-dinitrophenylhydrazine (manufacturer: Aldrich, purity: 97% or higher) is dissolved in 62 ml of a 37.0% by weight aqueous hydrochloric acid solution (manufacturer: Aldrich, purity: 99.999% or higher) at a temperature of 25° C. The resulting solution is then poured into 335 g of distilled water (also at a temperature of 25° C.) while stirring. After stirring for 1 hour at 25° C., it is filtered and the resulting filtrate is the derivatization solution D.

[0052] To determine the glyoxal content in liquid phase P, 1 g of derivatization solution D (this amount can be increased accordingly if necessary) is weighed into a screw-cap bottle of 10 ml capacity. A sample of liquid phase P is then weighed into the thus filled screw-cap bottle, the amount of the sample being in the range of 0.15-2.0 g.

[0053] The entire contents of the screw-cap bottle are then mixed by shaking and then allowed to stand for 10 minutes at a temperature of 25° C. During this period, the monomeric glyoxal present in the screw-cap bottle undergoes a chemical reaction with 2,4-dinitrophenylhydrazine to form the corresponding hydrazone H of the monomeric glyoxal. However, during this period, 2,4-dinitrophenylhydrazine also removes the monomeric glyoxal bound thereto in the form of hydrazone H from the monomeric glyoxal monohydrate and glyoxal dihydrate present in the screw-cap bottle (in contrast, a corresponding removal of monomeric glyoxal from the polyglyoxal hydrates present in the screw-cap bottle essentially does not occur).

[0054] The subsequent addition of 0.5 g of glacial acetic acid (manufacturer: Aldrich, purity: ≥ 99.8%) to the screw-cap bottle freezes the hydrazone formation already occurring. If the addition of acetic acid is accompanied by the formation of a solid precipitate, further acetic acid is added incrementally to redissolve the precipitate formed (however, the total amount of acetic acid added must not exceed 1.0 g). If the precipitate formed does not dissolve in the solution even after the upper limit of the permitted total acetic acid addition (1.0 g) has been reached, 0.5 g of dimethyl phthalate is measured in. If this addition also does not dissolve the precipitate formed, incrementally increasing amounts of dimethyl phthalate are added to bring about its dissolution (however, the total amount of dimethyl phthalate added must not exceed 1.0 g). If the precipitate formed does not dissolve in the solution even after the upper limit of the permitted total dimethyl phthalate addition (1.0 g) has been reached, 2 g of mixture G consisting of 9 g of acetonitrile and 1 g of dimethyl phthalate are added. If this addition fails to dissolve the precipitate formed, successively increasing amounts of Mixture G are added to cause this dissolution. The total amount of Mixture G added to cause the dissolution of the precipitate usually does not exceed 5 g (all the above dissolution tests are carried out at 25° C.).

[0055] The solution of hydrazone H produced in the screw-cap bottle as described is subsequently analyzed by HPLC (High Pressure Liquid Chromatography) for its hydrazone content (the molar amount of which directly defines the molar amount of glyoxal present in the liquid phase P) using the following operating conditions: Chromatographic columns used: Waters Symmetry C18, 150 × 4.6 mm, 5 μm (Waters Associates, Milford, Massachusetts, USA); Injection volume of the solution to be analyzed: 50 μl (time t=0) Temperature: 40℃ Eluent flow rate: 1.5ml / min; Analysis time: 17 minutes; Equilibration time: 8 minutes; Eluent: for the period t > 0 min to 15 min, a mixture of 30% by weight acetonitrile, 50% by weight water and 20% by weight tetrahydrofuran (all HPLC grade); >For the period from 15 minutes to 17 minutes, a mixture of 65% by weight acetonitrile, 30% by weight water and 5% by weight tetrahydrofuran; In the period from >17 min to 25 min, a mixture of 30% by weight acetonitrile, 50% by weight water and 20% by weight tetrahydrofuran (the column is then equilibrated again in preparation for the next analysis).

[0056] The retention time of glyoxal as hydrazone H is 7.613 min under the conditions described above.

[0057] The analysis is carried out using monochromatic radiation with a wavelength of 365 nm.

[0058] The analytical method employed is absorption spectroscopy.

[0059] By changing the eluent over the elution time, a high separation efficiency is ensured (liquid phase P generally contains not only glyoxal but also other by-product aldehydes and / or by-product ketones which form their corresponding hydrazones with 2,4-dinitrophenylhydrazine).

[0060] An advantageous application-specific calibration of the HPLC method is preferably carried out using a methanolic solution of monomeric glyoxal which contains 50 ppmw of monomeric glyoxal (see DE 10 2008 041573 A and DE 10 2008 040799 A).

[0061] For this purpose, it is treated with derivatization solution D as described hereinbefore and then subjected to the HPLC analysis described.

[0062] Protoanemonin, also known as 5-methylene-2(5H)-furanone (CAS number 108-28-1), can be dimerized to give anemonin:

[0063] [ka]

[0064] This dimerization is reversible: anemonin undergoes thermal decomposition to protoanemonin. Thus, protoanemonin present in the liquid phase P can be converted to anemonin and vice versa.

[0065] Therefore, for the reasons stated above, the term "protoanemonin" should be understood herein to include not only protoanemonin but also protoanemonin reversibly bound in the form of anemonin.

[0066] Therefore, the term "protoanemonin" itself should always be understood herein to mean the total amount of protoanemonin (monomeric protoanemonin) and anemonin (dimeric protoanemonin).

[0067] Thus, in this specification, the protoanemonin content reported in "mass %" or "ppmw" units should be understood to mean the total amount of monomeric protoanemonin and reversibly bound dimeric protoanemonin always present.

[0068] The liquid phase P (or another liquid phase) content of the protoanemonin treated according to the invention (i.e. the total content of liquid phase P of monomeric protoanemonin and reversibly bound dimeric protoanemonin) is, in the context of the present application, determined by GC (gas chromatography) as follows: Chromatographic columns used: Optima 35 MS 30 m × 0.25 mm × 0.25 μm (Agilent Technologies, Santa Clara, California, USA); Injection volume of the solution to be analyzed: 1 μl (time t = 0); Injector temperature: 280℃; Detector temperature: 320℃; Detector: FID; Split:1:50; Flow rate: 1ml / min; Pressure: 12.7psi / 80℃; Temperature program: 60℃~320℃ at 15℃ / min; 10 min at 320℃; Internal standard: γ-valerolactone

[0069] The retention times of protoanemonin and the internal standard are 4.5 and approximately 5 min, respectively, under the conditions chosen. Anemonin is completely converted to protoanemonin within the injector.

[0070] According to the invention, protoanemonin can be used in pure form or in solution in a suitable solvent, for example acrylic acid, For example, the concentration of protoanemonin in acrylic acid solvent may range from 0.1% to 10% by weight, in particular from 1% to 2% by weight.

[0071] However, it is also possible to increase the amount of protoanemonin in the liquid phase P by recycling the material stream containing the by-product protoanemonin using separation techniques such as adsorption, extraction, desorption, distillation, stripping, rectification, azeotropic distillation, azeotropic rectification and crystallization.

[0072] In the process according to the invention, the liquid phase P will often comprise at least 10% by weight, or at least 20% by weight, in particular at least 30% by weight, or at least 40% by weight, more particularly at least 50% by weight, or at least 60% by weight, or at least 70% by weight, or at least 80% by weight, more particularly at least 90% by weight, or at least 95% by weight, very particularly at least 98% by weight, or at least 99% by weight of acrylic acid (in each case relative to the weight of the liquid phase P).

[0073] The acrylic acid content is 1 It can be determined by H-NMR, gas chromatography, or HPLC.

[0074] In the process according to the invention, the liquid phase P will often also comprise water. In the process according to the invention, the water content of the liquid phase P may in principle be at least 1% by weight, or at least 5% by weight, or at least 10% by weight, or at least 20% by weight, or at least 30% by weight, or at least 40% by weight, or at least 60% by weight, or at least 80% by weight.

[0075] However, the method according to the invention is also particularly relevant when the liquid phase P treated according to the invention contains less than 30% by weight of water, for example 29% by weight or less, or 27% by weight or less, or 25% by weight or less, or 20% by weight or less, or 15% by weight or less, or 10% by weight or less, or 5% by weight or less (the lower the water content, the less glyoxal hydrates are formed).However, the water content of the liquid phase P is often ≧0.1% by weight or more, or 0.5% by weight or more, or 1% by weight or more (for example the water content of glyoxal hydrates is included in the amounts reported above).

[0076] The liquid phase P often comprises a high-boiling absorption medium in which, for example, acrylic acid from the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds is absorbed (see, for example, DE 10 2009 027401 A).

[0077] In the present specification, a high-boiling absorption medium should be understood to mean an absorption medium having a boiling point above the boiling point of acrylic acid at standard pressure. 5 The boiling point of the absorption medium at standard pressure is usually at least 20° C., preferably at least 50° C., particularly preferably at least 75° C., very particularly preferably at least 100° C. or at least 125° C. higher than the boiling point of acrylic acid at the same pressure (141° C. at 1 atm; whereas the boiling point of propionic acid is 141.35° C. at the same pressure; see WO 2007 / 074045). The boiling points of the aforementioned absorption media at standard pressure are often not higher than 400° C., frequently not higher than 350° C. and often not higher than 300° C. or not higher than 280° C. It is particularly advantageous if the boiling point of the absorption medium is in the range from 200° C. to 350° C. (based on standard pressure). Possible absorption media of this kind include, for example, all those recommended in DE 103 36 386 A, DE 24 49 780 A, DE 196 27 850 A, DE 198 10 962 A, DE 43 08 087 A, EP 0 722 926 A and DE 44 36 243 A, as well as DE 10 2009 027401 A.

[0078] High-boiling absorption media are generally organic liquids. Organic liquids often consist of organic molecules that do not contain externally active polar groups, at least to the extent of 70% by weight, and therefore cannot form, for example, hydrogen bonds. Particularly advantageous absorption media include, for example, diphenyl ether, diphenyl (biphenyl), the mixture of diphenyl ether (70% to 75% by weight) and diphenyl (25% to 30% by weight) known as Diphyl®, as well as dimethyl phthalate, diethyl phthalate, and mixtures of Diphyl and dimethyl phthalate, or mixtures of Diphyl and diethyl phthalate, or mixtures of Diphyl and dimethyl phthalate and diethyl phthalate. A group of mixtures very particularly suitable for absorption purposes is that which consists of 75% to 99.9% by weight of Diphyl and 0.1% to 25% by weight of dimethyl phthalate and / or diethyl phthalate.

[0079] The high-boiling absorbing medium in the context of this specification may also be an ionic liquid.

[0080] In the process according to the invention, the liquid phase P may comprise, for example, at least 1% by weight, or at least 5% by weight, or at least 10% by weight, or at least 20% by weight, or at least 30% by weight, or at least 40% by weight, or at least 60% by weight, or at least 80% by weight of the high-boiling absorbing medium.

[0081] This embodiment of the process according to the invention exerts its advantageous effect in particular when the liquid phase P contains glyoxal in the range from 25 to 1000 ppmw, in particular in the range from 50 to 500 ppmw, relative to the mass of acrylic acid present therein.

[0082] In all of the above cases, the content of propionic acid in the liquid phase P on a corresponding basis (relative to the mass of acrylic acid present) may simultaneously be 100 ppmw or more, or 150 ppmw or more, or 200 ppmw or more, or 250 ppmw or more, or 300 ppmw or more, or 400 ppmw or more, or 500 ppmw or more, or 600 ppmw or more, or 700 ppmw or more, or 800 ppmw or more, or 1000 ppmw or more, or 1500 ppmw or more, or 2000 ppmw or more, or 2500 ppmw or more.

[0083] In all of the above cases, the propionic acid content of the liquid phase P on the above basis is usually not more than 5% by weight, often not more than 4% by weight or not more than 3% by weight, often not more than 2% by weight or not more than 1% by weight.

[0084] The propionic acid content of liquid phase P is typically determined by gas chromatography.

[0085] Naturally, the liquid phase P may contain not only glyoxal and propionic acid, but also further secondary components and compounds such as formaldehyde, acrolein, furfural, crotonaldehyde, benzaldehyde, propionaldehyde, protoanemonin, allyl acrylate, formic acid, acetic acid, maleic acid, benzoic acid and / or maleic anhydride as typical by-products of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to give acrylic acid (for example in quantitative proportions as detailed in WO 2006 / 002713 A, WO 2008 / 090190 A, DE 10 2007 004960 A and DE 10 2009 027401 A, in particular in the various liquid substance mixtures of these examples).

[0086] As mentioned above, the liquid phase P treated according to the invention often needs to be stored for long periods of time, during which time the acrylic acid reacts to some extent with itself to form limited amounts of diacrylic acid by Michael addition (see, for example, WO 98 / 01414 and WO 2005 / 035478).

[0087] The process according to the invention is therefore also suitable for liquid phases P which contain not only the abovementioned amounts of glyoxal and acrylic acid, but also diacrylic acid in an amount of 100 ppmw or more, or 200 ppmw or more, or 300 ppmw or more, or 400 ppmw or more, or 500 ppmw or more, or 600 ppmw or more, or 800 ppmw or more, or 1000 ppmw or more, or 1500 ppmw or more, or 2000 ppmw or more, or 3000 ppmw or more, or 5000 ppmw or more, or 7500 ppmw or more, or 10000 ppmw or more.

[0088] The content of diacrylic acid in the liquid phase P treated according to the invention is generally less than or equal to 20% by weight, frequently less than or equal to 15% by weight or less than or equal to 10% by weight and often less than or equal to 5% by weight, relative to the weight of acrylic acid present therein.

[0089] The diacrylic acid content of liquid phase P was measured using a high resolution 1This can be determined in a simple manner by H-NMR (see "Polymerisationsinhibierung von (Meth-)Acrylaten" [Inhibition of polymerization of (meth)acrylates], thesis of Dipl.-Ing. Holger Becker, Technische Universitat Darmstadt, 2003). 1 The specific signal shape and signal position of the H resonance line, as well as the signal area, are evaluated.

[0090] The process according to the invention is suitable for suppressing the undesired free radical polymerization of acrylic acid present in the liquid phase P, both during its storage and during the handling associated with the process.

[0091] The latter case applies in particular when the liquid phase P is subjected to a thermal separation process (the relevant temperatures are generally above 50° C., usually above 60° C. or 70° C., or above 90° C. or 110° C., preferably below 150° C.). Said processes are generally thermal separation processes in which a gas (upward) flow and a liquid (downward) flow, i.e. two fluid streams, are countercurrently conducted in a separation column containing a separation interior, with the result that the gradient existing between the fluids results in a transfer of heat and mass, which ultimately results in the desired separation effect in the separation column. Examples of such non-crystallizing thermal separation processes are rectification, azeotropic rectification, extraction, desorption, stripping, distillation, azeotropic distillation and adsorption. Since the liquid phase P treated according to the invention is formed especially when the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to acrylic acid is subjected to absorption or fractional condensation or partial condensation for the elementary removal of acrylic acid from the product gas mixture, the process according to the invention is also suitable for the inhibition of the polymerization of the liquid phase P occurring in the course of such thermal separation processes. Of course, the method for inhibiting polymerization according to the invention is also suitable if the liquid phase P is subjected to a further separation process.

[0092] The term "thermal separation process" is intended to express that heat must be supplied to or removed from the system in order to achieve the desired separation effect (see DE 10 2008 041573 A and DE 10 2008 040799 A).

[0093] The liquid phase P to be treated by the treatment may contain protoanemonin added according to the invention at the start of the thermal separation process (i.e. protoanemonin may be fed to the thermal treatment already treated according to the invention), but it can of course also be added only in the course of the thermal separation process (for example dissolved in the reflux liquid during rectification, or dissolved in the absorption medium during absorption, or dissolved in the reflux liquid during fractional condensation, or dissolved in the quench liquid used for direct cooling during direct cooling of the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds).

[0094] Of course, the protoanemonin added to the liquid phase P according to the present invention does not have to be the only inhibitor system added to the liquid phase P. On the contrary, the liquid phase P may additionally contain nitroxyl radicals (also known as N-oxyl radicals) (for example those disclosed in DE 197 34 171 A, e.g. 4-hydroxy-2,2,6,6-tetramethyl-piperidine 1-oxyl or 1,4-dihydroxy-2,2,6,6-tetramethylpiperidine), phenothiazines, such as dibenzo-1,4-thiazine (phenothiazine), phenolic compounds, such as hydroquinone, 2,4-dimethyl-6-tert-butylphenol and hydroquinone monomethyl ether, molecular oxygen, cerium salts, such as cerium(III) salts, manganese salts (for example manganese(III) salts, e.g. manganese(III) acetate dihydrate and manganese(III) di-n-butyldithiocarbamate), p-phenylenediamines (for example those disclosed in DE 197 34 171 A), organic nitroso compounds, such as 4-nitrosophenol (and DE 197 34 171 A), methylene blue, as well as all other inhibitors disclosed, for example, in EP 0 765 856 A. The aforementioned inhibitors may be added to the liquid phase P in any suitable effective amount, i.e., for example, in the range of 5 to 1000 ppmw (based on the mass of acrylic acid present in P).

[0095] When carrying out a thermal separation process on the liquid phase P treated according to the invention in a separation column comprising attached separation internals (e.g. separation trays such as Dialflow trays), it is possible as an additional suppression measure to pass air or nitrogen-enriched air (lean air) as a source of molecular oxygen through the separation column (e.g. a rectification column or an absorption column), as is carried out, for example, in DE 10 2009 027401 A or DE 10 2007 004960 A.

[0096] Such thermal separation processes (e.g. all thermal separation processes described in WO 2011 / 000808, DE 103 36 386 A, DE 199 24 532 A, DE 199 24 533 A and DE 10 2007 004960 A) are preferably carried out according to the invention in apparatuses which meet the recommendations of US 6,441,228 and US 6,966,973.

[0097] The heterogeneously catalyzed partial gas phase oxidation to produce acrylic acid may employ, for example, a reaction gas starting mixture containing a total molar amount of C2-compounds (e.g. ethane, ethylene, acetylene, acetaldehyde, acetic acid and / or ethanol) of 100 molar ppm or more, or 200 molar ppm or more, or 300 molar ppm or more, or 400 molar ppm or more, or 500 molar ppm or more, or 750 molar ppm or more, or 1000 molar ppm or more, or 2000 molar ppm or more, or 3000 molar ppm or more relative to the molar amount of employed C3-precursor compounds (e.g. propane, propylene, acrolein, propionic acid, propionaldehyde, propanol and / or glycerol, preferably propylene and acrolein) present therein.

[0098] The total molar amount (on the same basis) of said C2-compounds in the starting mixture of reaction gases of the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to obtain acrylic acid is generally less than or equal to 10 000 molar ppm.

[0099] In addition, the reaction gas starting mixture used in the heterogeneously catalyzed partial gas-phase oxidation to produce acrylic acid may, for example, in the case of propylene or acrolein as C3-precursor compounds (but also in the case of other C3-precursor compounds different from n-propane), contain, relative to the mass of propylene / acrolein (C3-precursor compounds different from n-propane) present, 0.05% by mass or more of n-propane, or 0.2% by mass or more of n-propane, or 0.5% by mass or more of n-propane, or 1% by mass or more of n-propane, or 3% by mass or more of n-propane, or 5% by mass or more of n-propane, or 10% by mass or more of n-propane, or 20% by mass or more of n-propane. However, the reaction gas starting mixture of the heterogeneously catalyzed partial gas-phase oxidation of propylene and / or acrolein (C3-precursor compounds different from n-propane) to obtain acrylic acid typically contains 80% by volume or less, often 70% by volume or less, often 60% by volume or less (but usually 0.1% by volume or more) of n-propane.

[0100] The term "reactant gas starting mixture" should in all the above cases be understood to mean the gas mixture fed to the catalyst bed for the purpose of partial oxidation of the C3-precursor compounds present therein to obtain acrylic acid. In addition to the C3-precursor compounds, undesired impurities and molecular oxygen as oxidant, the reactant gas starting mixture generally also comprises an inert diluent gas, such as, for example, nitrogen, carbon dioxide, water, noble gases, molecular hydrogen, etc. Each of the inert diluent gases is usually configured such that at least 95 mol % of its starting amount remains unchanged in the course of the heterogeneously catalytic partial oxidation.

[0101] The proportion of C3-precursor compound in the reaction gas starting mixture may be in the range of, for example, 4 vol. % to 20 vol. %, or 5 vol. % to 15 vol. %, or 6 vol. % to 12 vol. %.

[0102] Based on the stoichiometry of the partial oxidation reaction of the C3-precursor compound to obtain acrylic acid, the reaction gas starting mixture usually contains an excess of molecular oxygen, generally for reoxidizing the oxidation catalyst again.

[0103] When applying the subsequent embodiments of the method according to the invention, this excess may be set particularly high, since an increase in the oxygen excess is generally also accompanied by an increase in the formation of the undesirable glyoxal secondary component.

[0104] Similarly, in the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds to obtain acrylic acid, when the method according to the invention is employed following partial oxidation, the maximum reaction temperature prevailing in the catalyst bed may be set at a relatively high level. One reason for this is that an increase in the maximum temperature is generally accompanied by an increase in the formation of the undesirable glyoxal secondary component. However, the use of an increased maximum temperature generally allows the use of catalysts with a relatively low activity, thus increasing the possibility of extending the catalyst's life. However, when using catalysts with a relatively low activity, the increase in the conversion of the C3-precursor compounds is often accompanied by their undesirable complete combustion. Glyoxal may also be formed as an intermediate in some cases.

[0105] In the context of the embodiment of the method according to the invention, it is likewise possible to proceed more slowly in the selection of the space velocity of the C3-precursor compound relative to the catalyst bed.

[0106] It has been found that the formation of glyoxal by-products is promoted by an increased water vapor content in the reaction gas mixture. The method according to the invention is therefore particularly relevant when the reaction gas starting mixture used for the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds contains 1% by weight or more, or 3% by weight or more, or 5% by weight or more, or 9% by weight or more, or 15% by weight or more, or 20% by weight or more of water vapor. However, the water vapor content of the reaction gas starting mixture is generally 40% by weight or less, and often 30% by weight or less.

[0107] The process for heterogeneously catalyzed partial gas phase oxidation to produce acrylic acid may also be carried out in a manner known per se, as otherwise described in the prior art.

[0108] When the C3-precursor compound is for example propylene and / or acrolein, the heterogeneously catalyzed partial gas phase oxidation may be carried out, for example, as described in WO 2005 / 042459, WO 2005 / 047224 and WO 2005 / 047226.

[0109] If the C3-precursor compound is, for example, propane, the heterogeneously catalyzed partial gas-phase oxidation to produce acrylic acid may be carried out, for example, as described in EP 0 608 838 A, DE 198 35 247 A, DE 102 45 585 A and DE 102 46 119 A.

[0110] When the C3-precursor compound is, for example, glycerol, the heterogeneously catalyzed partial gas phase oxidation to produce acrylic acid may be carried out, for example, as described in WO 2007 / 090991, WO 2006 / 114506, WO 2005 / 073160, WO 2006 / 114506, WO 2006 / 092272 or WO 2005 / 073160.

[0111] It has also been previously proposed to obtain propylene as a C3-precursor compound by partial dehydrogenation and / or oxydehydrogenation of propane arranged upstream of a partial gas-phase oxidation (e.g. WO 076370, WO 01 / 96271, EP 0 117 146 A, WO 03 / 011804 and WO 01 / 96270).

[0112] The process according to the invention can also be advantageously employed in particular when the glyoxal present in the liquid phase P is present in the form of monomeric glyoxal monohydrate and / or monomeric glyoxal dihydrate to an extent of at least 20 mol%, or at least 30 mol%, or at least 50 mol%, or at least 70 mol%, or at least 90 mol%, or at least 95 mol%.

[0113] The process according to the invention is particularly advantageous when the liquid phase P treated according to the invention originates from a product gas mixture from a heterogeneously catalyzed partial gas-phase oxidation of a C3-precursor of acrylic acid, which contains glyoxal in the range from 25 to 1000 ppmw, in particular in the range from 50 to 500 ppmw, relative to the molar amount of acrylic acid present in the product gas mixture (determination of the aforementioned glyoxal content of the product gas mixture relative to the molar amount of acrylic acid present may be achieved by cooling the latter, i.e. at least the acrylic acid present therein, the hemiacetals and / or acetals of glyoxal present therein and the monomeric glyoxals present therein, to effect a conversion into a condensed phase and analysing the latter for its glyoxal and acrylic acid content as soon as possible after its formation as described in this document for the liquid phase P).

[0114] The liquid phase P treated according to the invention is often also subjected to an azeotropic rectification to separate the water present therein. Particularly suitable entraining agents that come into consideration in this respect are heptane, dimethylcyclohexane, ethylcyclohexane, toluene, ethylbenzene, octane, chlorobenzene, xylene or mixtures thereof (for example 60% by weight of toluene and 40% by weight of heptane). However, methyl isobutyl ketone or isopropyl acetate can also be employed as alternative entraining agents. Alternatively, it is possible to proceed as described in EP 0 778 255 A, EP 0 695 736 A and US 2004 / 0242826. The liquid phase P treated according to the invention therefore also includes a liquid phase P which in particular comprises at least one of the aforementioned entraining agents and water. In general, the water content of such a liquid phase P is at least 10% by weight and the content of the azeotropic entraining agent is at least 1% by weight, often at least 2% by weight or at least 5% by weight.

[0115] The process according to the invention is also particularly relevant when the glyoxal present in the liquid phase P treated according to the invention is separated therefrom by crystallization, in which case glyoxal is enriched in the remaining mother liquor and acrylic acid is enriched in the crystallisate, which is recycled by the mother liquor to at least one of the process steps in which the liquid phase P treated according to the invention is obtained (produced) from the product gas mixture from the heterogeneously catalyzed partial gas-phase oxidation of C3-precursor compounds. The crystallization separation process can be carried out in a similar manner as described in DE 10 2008 041573 A, DE 10 2008 040799 A and WO 2007 / 074044, as well as in the DE 10 2007 029053 A documents.

[0116] Unless otherwise stated, ppm values ​​are by weight.

[0117] Unless otherwise stated, values ​​reported in % are by weight.

[0118] Unless otherwise stated, values ​​reported are based on absolute pressure.

[0119] Examples of the present invention and comparative examples General Experimental Procedure: 100.0 g of acrylic acid (with appropriate additives) is charged into a 250 ml glass bottle (magnetic stir bar). Nitrogen is then passed through for 30 minutes (approximately 70 l / h).

[0120] The amount of nitrogen is then reduced (14-18 l / h) and passes only above the acrylic acid. The glass bottle is then immersed in an oil bath previously heated to 103°C up to the filling level of the glass bottle (internal temperature 100°C). Stirring is carried out at 100°C (internal temperature) for 2 hours. The oil bath is then lowered. After an internal temperature of 50°C has been reached, the remaining acrylic acid is poured off.

[0121] The amount of polymerized acrylic acid is calculated based on the initial mass. EXAMPLES

[0122] The following examples using acrylic acid (>98% purity by weight) were carried out similarly to the experimental procedures described above using different amounts of protoanemonin / anemonin and glyoxal. Each experiment was repeated 2-6 times and the average values ​​obtained are reported.

[0123] [Table 1]

[0124] [Table 2]

[0125] [Table 3]

Claims

1. 1. A method for inhibiting undesired free radical polymerization of acrylic acid present in a liquid phase P, wherein the acrylic acid content of said liquid phase P is at least 10% by weight, said liquid phase P contains glyoxal in the range of 25 to 1000 ppmw, relative to the weight of acrylic acid present in P, and said liquid phase P is mixed with protoanemonin in an amount resulting in a protoanemonin content in the range of 0.5 to 100 ppmw, relative to the weight of acrylic acid present in P.

2. 2. The process according to claim 1, wherein the acrylic acid content of the liquid phase P is at least 30% by weight.

3. 3. The process according to claim 1 or 2, wherein the liquid phase P comprises at least 100 ppmw of propionic acid relative to the mass of acrylic acid present in P.

4. 4. The process according to claim 1, wherein the liquid phase P comprises, relative to the mass of acrylic acid present in P, in the range of 50 to 500 ppmw of glyoxal and / or in the range of 1 to 50 ppmw of protoanemonin.

5. 5. The process according to claim 1, wherein the liquid phase P comprises phenothiazine and / or methylhydroquinone in the range of 50 to 1000 ppmw relative to the mass of acrylic acid present in P.

6. 6. The process according to claim 1, wherein the liquid phase P has a temperature in the range of from 50° C. to 150° C.

7. 7. The process according to claim 1, which is carried out in a column for the distillative purification of acrylic acid.

8. The acrylic acid present in the liquid phase P is 3 - C, which is the product of the heterogeneously catalyzed partial oxidation of the precursor compound and is used in the partial oxidation 3 - The starting mixture containing the precursor compound is 3 - a molar total amount of C in the range of 100 to 10,000 molar ppm relative to the molar amount of the precursor compound 2 The method according to any one of claims 1 to 7, comprising the compound.

9. A liquid phase P for inhibiting undesirable free radical polymerization of acrylic acid, the liquid phase P having an acrylic acid content of at least 10% by weight, and comprising glyoxal in the range of 25 to 1000 ppmw, based on the weight of acrylic acid present in P, and protoanemonin in the range of 0.5 to 100 ppmw, based on the weight of acrylic acid present in P.

10. 10. The liquid phase P according to claim 9, wherein the acrylic acid content of said liquid phase P is at least 30% by weight.

11. 11. The liquid phase P according to claim 9 or 10, comprising at least 100 ppmw of propionic acid relative to the mass of acrylic acid present in P.

12. A liquid phase P according to any one of claims 9 to 11, comprising glyoxal in the range of 50 to 500 ppmw, based on the mass of acrylic acid present in P, and / or protoanemonin in the range of 1 to 50 ppmw, based on the mass of acrylic acid present in P.

13. A liquid phase P according to any one of claims 9 to 12, comprising phenothiazine and / or methylhydroquinone in the range of 50 to 1000 ppmw, based on the mass of acrylic acid present in P.

14. The liquid phase P according to any one of claims 9 to 13, having a temperature in the range of from 50°C to 150°C.

15. 15. The liquid phase P according to any one of claims 9 to 14, in a column for the distillative purification of acrylic acid.

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