Method for producing acrylic acid

Abstract

The invention relates to a method for producing acrylic acid, wherein: a product gas mixture containing acrylic acid, water vapor and secondary components is produced by means of a heterogeneously catalyzed gas-phase partial-oxidation of at least one C3 precursor of acrylic acid with molecular oxygen on catalysts in a solid aggregate state at an increased temperature; the product gas mixture is then conducted into a condensation column equipped with fittings which have a separating effect; the product gas mixture is allowed to rise within the condensation column and in the process fractionally condenses, wherein the product gas mixture is separated into: a sump liquid containing secondary products, which are heavier-boiling than acrylic acid, and secondary components, a raw acrylic acid, which contains water and secondary components in an overall depleted state, as the target product, an acidic water also containing acrylic acid and secondary components, and a residual gas mixture containing secondary components which are lower-boiling than water; the target product is conveyed out of the condensation column via a side draw, and the side draw is above the point where the product gas mixture is fed into the condensation column; and the fittings which have a separating effect consist of different stainless steels, and fittings which have a separating effect and consist of a first stainless steel comprising at least 18 wt.% nickel are used in first regions of the condensation column in which the product gas mixture has 5 to 70 wt.% water, 15 to 85 wt.% acrylic acid, at least 8 wt.% acetic acid and at least 2 wt.% formic acid.

Description

[0001] Process for the production of acrylic acid

[0002] A process for the production of acrylic acid, in which a product gas mixture containing acrylic acid, water vapor, and secondary components is produced by heterogeneously catalyzed gas-phase partial oxidation of at least one Ca precursor of acrylic acid with molecular oxygen over catalysts in the solid state at elevated temperature, the product gas mixture is then passed into a condensation column equipped with separating internals, the product gas mixture is allowed to rise within itself within the condensation column and is thereby fractionally condensed, the product gas mixture being separated into a bottom liquid containing subsequent products and secondary components with a higher boiling point than acrylic acid, a crude acrylic acid as the target product containing completely depleted water and secondary components, acidic water still containing acrylic acid and secondary components, and a residual gas mixture containing secondary components with a lower boiling point than water.the target product is led out of the condensation column via a side draw, and the side draw is located above the feed point of the product gas mixture into the condensation column, wherein the separating internals consist of various stainless steels and, in first regions within the condensation column with a gas mixture comprising 5 to 70 wt.% water, 15 to 85 wt.% acrylic acid, at least 8 wt.% acetic acid, and at least 2 wt.% formic acid, separating internals consisting of a first stainless steel with at least 18 wt.% nickel are used.

[0003] Acrylic acid is an important intermediate used, for example, in the production of polymer dispersions (possibly also in the form of their esters with alkanols) and water-superabsorbent polymers.

[0004] Acrylic acid is obtainable, inter alia, by heterogeneously catalyzed gas phase partial oxidation of Ca precursors (of C3 precursor compounds) of acrylic acid (this term is intended to cover in particular those chemical compounds which are formally obtainable by reduction of acrylic acid; known Cs precursors of acrylic acid are, for example, propane, propene, acrolein, propionaldehyde and propionic acid; however, the term is also intended to include precursor compounds of the aforementioned compounds, such as, for example, glycerol (starting from glycerol, acrylic acid can be produced, for example, by heterogeneously catalyzed oxidative dehydration in the gas phase; cf., for example, EP 1 710 227 A1, WO 06 / 114506 and WO 06 / 092272) with molecular oxygen on catalysts in the solid state at elevated temperature.

[0005] The starting gases mentioned, usually diluted with inert gases such as nitrogen, CO2, saturated hydrocarbons and / or steam, are passed in a mixture with molecular oxygen at elevated temperatures and optionally elevated pressure over (e.g. transition metal) mixed oxide catalysts and oxidatively converted into a product gas mixture containing acrylic acid, water and undesired by-products such as furfurals, benzaldehyde, acetone, formaldehyde and maleic anhydride etc., from which the acrylic acid must be separated (the by-products and the inert diluent gases other than steam are summarized in this document under the term "secondary components"; this term also includes the polymerization inhibitors usually added in acrylic acid separation processes).

[0006] From the documents DE 199 24533 A1, DE 199 24532 A1, WO 01 / 77056, DE 101 56 016 A1, DE 102 43 625 A1, DE 102 23058 A1, DE 102 35847 A1, WO 2004 / 035514, WO 00 / 53560, DE 10332 758 A1 and EP 2 114 852 A1, processes for the preparation of acrylic acid are known as described at the outset, in which a basic separation of a crude acrylic acid is carried out by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation. The term crude acrylic acid or crude acrylic acid expresses that the acrylic acid taken off via the first side draw is not a pure product, but a mixture which, in addition to acrylic acid (usually >50 or >60 wt.%, mostly >70 or >80 wt.%, often >90 wt.% and frequently >95 wt.% or more of the total weight), also contains water and secondary components such as lower aldehydes (e.g. furfural, acrolein, benzaldehyde), lower carboxylic acids (e.g. acetic acid, propionic acid, formic acid), etc.In any case, the total content of water and secondary components, relative to the acrylic acid content, in the crude acrylic acid is lower than in the product gas mixture of the gas-phase partial oxidation, which is why it is also said that the crude acrylic acid contains these components in a depleted state overall (individual components, however, may be present in a comparatively enriched state in the crude acrylic acid).

[0007] In some cases, the purity of the crude acrylic acid thus separated is already sufficient for the intended use of the acrylic acid (e.g., for the purpose of esterification or for the purpose of constructing polymers obtainable by radical polymerization). However, the separated crude acrylic acid is often subjected to at least one further thermal separation process in order to obtain a purer acrylic acid (one with a higher acrylic acid content in wt. % compared to the crude acrylic acid) from the crude acrylic acid, which has the degree of purity required for the respective intended use.

[0008] Thermal separation processes are understood to be those in which a physically at least two-phase system is created by adding or removing (usually thermal) energy, whereby the temperature and mass gradients existing between the phases lead to a heat and mass exchange, which ultimately causes the desired separation and extraction.

[0009] Thermal separation processes are frequently carried out in separation columns containing separation-effective internals, in which the aforementioned at least two material phases are generally conducted countercurrently to each other. Often, one of the two material phases is gaseous (it is generally conducted as the ascending phase in a separation column) and the other is liquid (it is generally conducted as the descending phase in a separation column). In principle, however, the at least two material phases can also be liquid (e.g., in the case of extraction), solid and liquid (e.g., in the case of crystallization), or solid and gaseous (e.g., in the case of adsorption).Examples of thermal separation processes in which one of the at least two material phases is liquid and one is gaseous, and thus a natural element of the term "thermal separation processes" used in this document, are rectification (an ascending vapor phase is passed in the separation column in countercurrent to a descending liquid phase), and desorption (the reverse process of absorption; the gas dissolved in a liquid phase is removed from the liquid phase by lowering the pressure above the liquid phase, by increasing the temperature of the liquid phase and / or by passing a gas phase through the liquid phase; if the passage of a gas phase is involved, desorption is also referred to as stripping).However, absorption (usually, a gas rising in a separation column is passed in countercurrent to at least one liquid absorbent descending in the separation column) and the fractional condensation of a gas mixture (gas / liquid phase example) are also part of the term thermal separation process. A particularly advantageous thermal separation process for the further purification of crude acrylic acid is crystallization.

[0010] Document WO 2023 / 006503 A1 describes that corrosion is observed during the fractional condensation of the product gas mixture of the heterogeneously catalyzed gas-phase partial oxidation in the condensation column used. Despite the removal of halide ions proposed to prevent corrosion, occasional and unexpected corrosion can occur in the condensation column.

[0011] The task was to improve corrosion prevention.

[0012] The object is achieved by a process for the production of acrylic acid, in which a product gas mixture containing acrylic acid, water vapor and secondary components is produced by heterogeneously catalyzed gas phase partial oxidation of at least one Ca precursor of acrylic acid with molecular oxygen over catalysts in the solid state at elevated temperature, the product gas mixture is then passed into a condensation column equipped with separating internals, the product gas mixture is allowed to rise within itself within the condensation column and is thereby fractionally condensed, the product gas mixture being converted into a bottoms liquid containing subsequent products and secondary components which have a higher boiling point than acrylic acid, a crude acrylic acid containing a total depletion of water and secondary components as the target product,an acidic water still containing acrylic acid and secondary components and a residual gas mixture containing secondary components boiling lower than water is separated, the target product is led out of the condensation column via a side draw, and the side draw is located above the feed point of the product gas mixture into the condensation column, characterized in that the separating internals consist of various stainless steels and in first regions within the condensation column with a gas mixture comprising 5 to 70 wt.% water, 15 to 85 wt.% acrylic acid, at least 8 wt.% acetic acid and at least 2 wt.% formic acid, separating internals consisting of a first stainless steel with at least 18 wt.% nickel are used. The present invention is based on the surprising finding that the corrosion tendency in the condensation column is different and that corrosion occurs particularly there,where the gas mixture in the condensation column has a specific composition. By using different stainless steels depending on the composition of the gas mixture, corrosion could be effectively and efficiently prevented.

[0013] The Ca precursor of acrylic acid is preferably propene and / or acrolein.

[0014] Preferably, at least one stream fed to the condensation column contains a source of halide ions. Fluoride ions and chloride ions are typically found as halide ions.

[0015] The halide ion-containing stream can be, for example, water, propene, sodium hydroxide solution, hydroquinone, hydroquinone monomethyl ether, diethyl phthalate, and / or phenothiazine. The halide ions can be present as impurities in these or other streams fed to the process.

[0016] The halide ions are not specifically removed from the condensation column.

[0017] The condensation column preferably contains dual-flow trays and crossflow trays as separating internals. The condensation column preferably has a crossflow tray region in which crossflow trays are arranged, the first regions within the condensation column containing a gas mixture comprising 5 to 70% by weight of water, 15 to 85% by weight of acrylic acid, at least 8% by weight of acetic acid, and at least 2% by weight of formic acid being preferably located in the crossflow tray region of the condensation column. The crossflow trays are preferably Thormann trays and / or Streuber trays. The number of crossflow trays in the condensation column is preferably 5 to 45, preferably 10 to 40, more preferably 15 to 35.

[0018] The number of separating internals consisting of the first stainless steel with at least 18 wt. % nickel in the condensation column is preferably 1 to 20, preferably 1 to 15, further preferably 1 to 10, particularly preferably 1 to 7. Further preferably, the number of separating internals consisting of the first stainless steel with at least 18 wt. % nickel in the condensation column is at most 20, preferably at most 18, further preferably at most 15, particularly preferably at most 12, particularly preferably at most 10, especially preferably at most 7. Likewise preferably, the number of separating internals consisting of the first stainless steel with at least 18 wt. % nickel in the condensation column is at least 1, preferably at least 2, further preferably at least 3.

[0019] Stainless steels within the meaning of this invention are steels containing at least 10.5 wt.% chromium. The preferred first stainless steel preferably contains at least 20.0 wt.%, more preferably at least 22.0 wt.%, and most preferably at least 24.0 wt.%, especially preferably at least 25 wt.% nickel. The first stainless steel preferably contains 24.0 to 26.0 wt.%, more preferably 24.5 to 25.5 wt.% nickel.

[0020] The preferred first stainless steel preferably contains 19.0 to 21.0 wt.%, particularly preferably 19.1 to 20.9 wt.%, most preferably 19.5 to 20.5 wt.%, of chromium and / or additionally preferably 3.0 to 5.0 wt.%, particularly preferably 3.5 to 4.8 wt.%, most preferably 4.0 to 4.5 wt.%, of molybdenum.

[0021] Furthermore, the first stainless steel may advantageously contain preferably 1.2 to 2.0 wt.%, particularly preferably 1.3 to 1.9 wt.%, most preferably 1.4 to 1.8 wt.%, of copper.

[0022] Preferably, the gas mixture of the first regions within the condensation column comprises 10 to 50 wt.% water, 20 to 70 wt.% acrylic acid, at least 10 wt.% acetic acid and at least 5 wt.% formic acid.

[0023] The gas mixture of the first regions within the condensation column preferably comprises 15 to 45% by weight of water. The gas mixture of the first regions within the condensation column preferably comprises 25 to 65% by weight of acrylic acid. The gas mixture of the first regions within the condensation column preferably comprises at least 12% by weight, preferably 8 to 18% by weight, more preferably 8.5 to 17.5% by weight, particularly preferably 9 to 17% by weight, of acetic acid. The gas mixture of the first regions within the condensation column preferably comprises at least 6% by weight, preferably 2 to 15% by weight, more preferably 2.5 to 14.5% by weight, particularly preferably 3 to 15% by weight, of formic acid.

[0024] Preferably, in second regions within the condensation column which differ from the first regions with regard to the composition of the gas mixture, separating internals consisting of a second stainless steel with less than 17 wt.% nickel are used.

[0025] The preferred second stainless steel preferably contains less than 16.0 wt.%, more preferably less than 15.0 wt.%, most preferably less than 14.0 wt.%, especially preferably less than 13.5 wt.%, and explicitly preferably less than 13.0 wt.% nickel. The first stainless steel preferably contains 10.0 to 15.0 wt.%, more preferably 12.5 to 13.0 wt.% nickel.

[0026] The preferred second stainless steel preferably contains 16.5 to 19.0 wt. %, particularly preferably 16.6 to 18.5 wt. %, very particularly preferably 17.0 to 18.0 wt. %, chromium and / or additionally preferably 2.0 to 2.5 wt. %, particularly preferably 2.1 to 2.4 wt. %, very particularly preferably 2.2 to 2.3 wt. %, molybdenum. The separating internals are preferably made essentially entirely from the respective steel. It is also conceivable that separating internals made from other stainless steels are used in addition to the first and second steel. Preferably, at least two different stainless steels are used for the separating internals.

[0027] A suitable first stainless steel, for example, is steel with the material number 1.4539 according to DIN EN 10088, also known as AISI 904L. A suitable second stainless steel, for example, is steel with the material number 1.4404 according to DIN EN 10088, also known as AISI 316L.

[0028] Preferably, the composition of the gas mixture is determined in the condensation column, preferably in the first regions. This can be done via at least one sampling point and offline analysis and / or via online analysis with at least one sensor device that is suitable and designed to determine the composition of the gas mixture, in particular the content of water, acetic acid, formic acid, and acrylic acid. The sampling points and / or the sensor devices are preferably arranged between a dual-flow tray and a cross-flow tray. Such a determination is advantageous because the operation of the condensation column can be adapted such that the first regions or the composition of the gas mixture are located there, near the separating internals made of the first steel.

[0029] The production of acrylic acid is described below:

[0030] Typically, the acrylic acid-containing product gas mixture of a heterogeneously catalyzed gas-phase partial oxidation of Ca precursors of acrylic acid with molecular oxygen on catalysts in the solid state can have, for example, the following contents (especially when propene is used as the Ca precursor):

[0031] 1 to 30 wt.% acrylic acid,

[0032] 0.05 to 10 wt.% molecular oxygen,

[0033] 1 to 30 wt% water,

[0034] > 0 to 5 wt% acetic acid,

[0035] > 0 to 3 wt% propionic acid,

[0036] > 0 to 1 wt.% maleic acid and / or maleic anhydride,

[0037] 0 to 2 wt% acrolein,

[0038] 0 to 1 wt% formaldehyde,

[0039] > 0 to 1 wt.% furfurals,

[0040] > 0 to 0.5 wt% benzaldehyde,

[0041] 0 to 1 wt.% propene, and the remainder consisting of essentially inert gases such as nitrogen, carbon monoxide, carbon dioxide, methane, and / or propane. The product gas mixture typically contains >0.005 mol%, frequently >0.03 mol%, of furfurals, based on the acrylic acid contained. However, the furfural content is generally <3 mol%.

[0042] The gas-phase partial oxidation itself can be carried out as described in the prior art. Starting from propene, the gas-phase partial oxidation can, for example, be carried out in two consecutive oxidation stages, as described in EP 0 700 714 A1 and EP 0 700 893 A1. Of course, the gas-phase partial oxidations cited in DE 197 40 253 A1 and DE 197 40 252 A1 can also be used.

[0043] To minimize the amount of secondary components formed, the propene gas-phase partial oxidation is preferably carried out as described in DE 101 48 566 A1. Polymer-grade propene or chemical-grade propene according to DE 102 32 748 A1 can be used as the propene source. If the Ca precursor used is propane, the partial oxidation can be carried out as described in DE 102 45585 A1.

[0044] In principle, however, gas phase partial oxidation can also be carried out as described in US 2006 / 0161019,

[0045] WO 2006 / 092410, WO 2006 / 002703, WO 2006 / 002713, WO 2005 / 113127, DE 10 2004 021 763 A1, EP 1 611 076 A1, WO 2005 / 108342, EP 1 656 335 A1, EP 1 682 478 A1, EP 1 682 477 A1, DE 10 2006 054214 A1, DE 10 2006 024 901 A1, EP 1 611 080 A2, EP 1 734030 A1, DE 10 2006 000 996 A1, DE 10 2005062 026 A1, DE 10 2005 062 010 A1, WO 2007 / 060036, WO 2007 / 051750 and WO 2007 / 042457.

[0046] The temperature of the product gas mixture leaving the gas phase partial oxidation is often 150 to 350°C, often 200 to 300°C, sometimes up to 500°C.

[0047] For application purposes, the hot product gas mixture is then cooled in a quench device 1 by direct cooling, generally to a temperature of 100 to 180°C, before it is passed, advantageously together with the quench liquid 1 used, for the purpose of fractional condensation, preferably into the lower section (preferably the lowest, e.g. the bottom space) of a condensation column containing separating internals.

[0048] In principle, all common internals can be considered as condensation column internals, in particular trays, structured packings, and / or random packing. Among the trays, bubble-cap trays, sieve trays, valve trays, and / or dual-flow trays are preferred. Typically, the total number of separating trays in a tray column is 20 to 100, frequently 20 to 80, and preferably 50 to 80. According to the invention, the condensation column is preferably one which, from bottom to top, contains, first, dual-flow trays and subsequently, hydraulically sealed crossflow trays (e.g., Thormann trays) as separating internals, as recommended in DE 102 43625 A1, DE 199 24532 A1, and DE 102 43 625 A1. The number of dual-flow trays can be 5 to 60, often 25 to 45, and the number of hydraulically sealed cross-flow trays can also be 5 to 60, often 30 to 50.For the sour water formation area (acrylic acid content of the reflux liquid viewed from bottom to top is generally <15 wt.%, or in some cases <10 wt.%), valve trays, as described in DE 199 24532 A1 and DE 102 43 625 A1, are preferred as separating internals. In principle, however, other common separating internals could also be used (the individual sections within the condensation column can, of course, also be designed in a completely equivalent manner (instead of one column above the other) as a series of correspondingly smaller columns).

[0049] As quenching device 1, all devices known in the prior art for this purpose (e.g. spray scrubbers, venturi scrubbers, bubble columns or other apparatus with sprinkled surfaces) can be used, with venturi scrubbers or spray coolers being used preferably.

[0050] For indirect cooling or heating of the quench fluid 1, it is preferably, but not necessarily, passed through a heat exchanger or heat transfer device, especially during start-up. All common heat exchangers or heat transfer devices are suitable for this purpose. Shell-and-tube heat exchangers, plate heat exchangers, and air coolers are preferred. Suitable cooling media are air for the corresponding air cooler and cooling liquids, especially water, for the other cooling devices.

[0051] As quench liquid 1, for example, bottoms liquid taken from the bottom of the condensation column (optionally combined with condensate taken out of the quench circuit 0), or high-boiling fraction or a mixture of such bottoms liquid and high-boiling fraction taken via a side takeoff near the bottom (particularly when the bottom space and the lowest (theoretical plate (the lowest separating internal) are separated by a chimney tray) can be used. If necessary, only the portion of the quench liquid 1 taken from the bottom of the condensation column is passed through the above-mentioned heat exchanger. The temperature of the quench liquid 1 on entry into the quench device 1 is generally advantageously 90°C to 120°C.

[0052] The point of introduction into the condensation column for the quenched (or otherwise cooled or uncooled) product gas mixture of the catalytic gas-phase partial oxidation (according to the invention, as described, preferably in a mixture with quench liquid 1 used for direct cooling) is advantageously located in the bottom space of this column, which advantageously contains an integrated centrifugal droplet separator and is generally separated from the lowest separating internal by a first chimney tray (in this case, it is expedient for application purposes to continuously feed high-boiling fraction into the bottom of the condensation column via a connecting line or overflow). In an exemplary and preferred embodiment (which is described exclusively below without limiting the general feasibility), this is the first dual-flow tray of a first series of expediently equidistantly arranged dual-flow trays.The chimney tray also functions as a collecting tray, from which condensate (high-boiling fraction) is continuously withdrawn and fed into the quench device 1 or the sump chamber as part of the quench liquid 1. The first series of dual-flow trays is terminated by a second chimney tray (collecting tray). From this second collecting tray, crude acrylic acid is continuously withdrawn as the medium-boiling fraction in the first side draw, preferably with a purity of >90 wt.% or >95 wt.%.

[0053] This crude acrylic acid is expediently fed to further distillative (rectification) and / or crystallization purification stages, and at least a portion of the bottom liquids and / or mother liquors obtained during this distillation (rectification) and / or crystallization is recycled to the condensation column below the first side draw but above the first collecting tray. This recycling is preferably heat-integrated. This means that cold, recycled mother liquor is passed through one or more series-connected indirect heat exchangers (e.g., spiral heat exchangers) in order to cool the crude acrylic acid withdrawn from the condensation column and passed through the heat exchanger on the opposite side, which is to be further purified by crystallization. At the same time, this heats the mother liquor. Two series-connected plate heat exchangers are preferably used for this purpose.

[0054] It is advisable to subject the crude acrylic acid extracted (as the medium-boiling fraction) to crystallization for further purification. In principle, there are no restrictions on the crystallization process used. Crystallization can be carried out continuously or batchwise, in one or more stages, to any desired degree of purity.

[0055] If necessary, water can advantageously be added to the crude acrylic acid to be purified prior to crystallization (this usually then contains, based on the amount of acrylic acid present, up to 20 wt.% or up to 10 wt.%, usually up to 5 wt.%, of water). If the aldehyde or other secondary component content is elevated, water can be omitted, since the aldehydes can then assume the function of water. According to the invention, the water is particularly advantageously added in the form of acid water. This leads to an increase in the yield of pure acrylic acid.

[0056] It is surprising that even with prior addition of acid water to the crude acrylic acid (this measure also increases the acrylic acid yield), acrylic acid (purity >98 wt. %) that meets the highest esterification requirements (e.g., for the production of n-butyl acrylate, 2-ethylhexyl acrylate, methyl acrylate, and ethyl acrylate) can be obtained in a single crystallization step. This crystallization step is expediently carried out as a suspension crystallization, as described in column 10 of DE 199 24532 A1 or in Example 1 of DE 102 23058 A1 (e.g., in a cooling disk crystallizer as described in WO 2006 / 111565). The acrylic acid crystals formed during suspension crystallization have a cubic to cuboid shape.The length (L) to thickness (D) ratio is usually in the range from L:D = 1:1 to L:D = 6:1, preferably in the range from 1:1 to 4:1, and particularly preferably in the range from 1.5:1 to 3.5:1. The thickness D of the crystals is usually in the range from 20 to 600 pm, often from 50 to 300 pm. The length L of the crystals is usually in the range from 50 to 1500 pm, often from 200 to 800 pm. In the case of acrylic acid suitable for esterification, the suspension crystals can be separated from the remaining mother liquor in a centrifuge (e.g. a 2- or 3-stage pusher centrifuge), the separated crystals advantageously being washed in the centrifuge using molten pure crystals. If the suspension crystals are separated from the remaining mother liquor by means of a washing column, e.g. a melt washing column (e.g.one according to WO 01 / 77056, or DE 101 56 016 A1, or DE 102 23 058 A1, or as described in WO 2006 / 111565, WO 04 / 35514, WO 03 / 41833, WO 02 / 09839, WO 03 / 41832, DE 100 36 881 A1, WO 02 / 55469 and WO 03 / 78378), even superabsorbent-grade acrylic acid (purity >99.7% by weight or >99.9% by weight), ie acrylic acid which is suitable for producing water-superabsorbent or other polyacrylates, can be achieved by means of a single crystallization stage. In this case, the total amount of separated mother liquor is conveniently returned to the condensation column.

[0057] However, the crystallization can also be carried out as fractional falling-film crystallization, as recommended in EP 0 616 998 A1. This can, for example, comprise two, three or more (e.g., 2 to 4) purification stages (suitable falling-film crystallizers can, for example, contain 1000 to 1400 crystallization tubes with a length of 10 to 15 m and an external diameter of 50 to 100 mm). The mother liquor separated in a higher purification stage can be recycled to one of the preceding purification stages. The mother liquor separated in the first purification stage is advantageously recycled in its entirety to the condensation column. As an alternative to recycling to one of the preceding purification stages, the mother liquors from the individual purification stages can also be recycled in their entirety to the condensation column. The pure product from the penultimate purification stage can be fed in whole or in part to the last purification stage.If only a partial feed is made, the remaining amount will usually be mixed with the pure product from the last purification stage to produce the final product suitable for consumption.

[0058] According to the invention, a portion of the crude acrylic acid withdrawn via the first side draw is expediently fed to the dual-flow tray located below the associated collecting tray. Mother liquor, which is optionally recycled to the condensation column, will generally also be fed to this tray. Prior to feeding, the mother liquor will generally be heated, as already described, to a temperature approximately corresponding to the withdrawal temperature of the crude acrylic acid.

[0059] Another portion of the crude acrylic acid withdrawn via the first side draw is advantageously heated by 10 to 15°C by indirect heat exchange and recycled to the condensation column above the withdrawal point, preferably immediately below the first downstream dual-flow tray. This measure has a beneficial effect on the acetic acid content of the crude acrylic acid withdrawn.

[0060] Above the second collecting tray, a second series of, preferably equidistant, dual-flow trays is arranged. These are then followed by hydraulically sealed crossflow mass transfer trays (e.g., Thormann trays or modified Thormann trays according to DE 102 43 625 A1), which are also preferably arranged equidistantly. The top dual-flow tray is optionally equipped as a distributor tray. This means, for example, that it has overflow channels with a serrated overflow.

[0061] The first of the Thormann trays from the bottom is, for technical reasons, one in which the liquid draining from the tray flows through six downcomers designed as pipes. These pipes are hydraulically sealed against the gas space of the underlying dual-flow tray. The weir heights of the six downcomers decrease in the flow direction of the cross-flow tray, for technical reasons. The hydraulic seal advantageously features idle openings with baffle plates. The downcomers are preferably evenly distributed in the second half, particularly preferably in the last third of the tray cross-section (opposite the inlet to the tray).

[0062] The hydraulic sealing takes place in a cup with an inclined overflow weir (45°C).

[0063] The cross-flow mass transfer trays are completed with a third chimney tray (collecting tray).

[0064] Above the third collecting tray there are preferably double-flow valve trays. The principle of valve trays and valve trays which can be used according to the invention can be found, for example, in Technical Progress Reports, Volume 61, Fundamentals of the Dimensioning of Column Trays, pages 96 to 138. They are essentially characterized in that they provide the steam flowing through with a flow opening corresponding to the respective load over a wide load range. According to the invention, ballast trays are preferably used. This means that the openings in the tray contain cages with openings closed by weights. W12 valves from Stahl, Viernheim, Germany, are particularly preferred according to the invention. Essentially water and constituents which are heavier than water volatile condense in the valve tray space. The condensate obtained is sour water.

[0065] The acid water is continuously withdrawn from the third collecting tray through the second side draw. A portion of the withdrawn acid water is recycled to the topmost crossflow mass transfer tray in the condensation column. Another portion of the withdrawn acid water is cooled by indirect heat exchange and, appropriately split, is also recycled to the condensation column. One cooled portion is recycled to the topmost valve tray (at a temperature of 15 to 25, preferably 20 to 25°C), and the other cooled portion is recycled to a valve tray located approximately centrally between the third collecting tray and the topmost valve tray in the condensation column (at a temperature of 20 to 35, preferably 25 to 30°C). According to the invention, the amount of acrylic acid present can be separated from the remaining amount of acid water withdrawn.

[0066] Part of the cooling (which can be carried out via one or more indirect heat exchangers connected in series) is achieved by passing the corresponding sour water portion over the evaporator of the Ca precursor (e.g. the propene evaporator) in order to convert liquid stored C3 precursor, e.g. propene, into the gas phase for the heterogeneously catalyzed gas phase oxidation.

[0067] The more volatile than water components are removed in gaseous form at the top of the condensation column as residual gas (or residual gas mixture) and are normally at least partially recycled to the gas-phase partial oxidation as diluent gas (cycle gas). To avoid condensation in the cycle gas compressor, the residual gas mixture is first superheated by indirect heat exchange. The non-circulated portion of the residual gas mixture is normally fed to combustion. As already described, a portion of the (preferably compressed) residual gas mixture is advantageously used as stripping gas to separate acrylic acid from the extract and from the bottom liquid of the condensation column. The gas-phase partial oxidation is advantageously carried out with an excess of molecular oxygen so that the residual gas mixture, and thus the first and second stripping gases, contain molecular oxygen if the residual gas mixture is used as such a stripping gas.

[0068] To inhibit polymerization, a solution of hydroquinone monomethyl ether (MEHQ) in acrylic acid or (preferably according to the invention) an MEHQ melt and (in both cases) optionally additionally a solution of phenothiazine in acrylic acid is fed to the uppermost of the hydraulically sealed crossflow mass transfer trays. The acrylic acid used is preferably pure acrylic acid, such as that produced during further purification of the extracted crude acrylic acid. For example, the pure acrylic acid (pure product) produced during further crystallization purification can be used. This solution is also expediently used for pure product stabilization.

[0069] In addition, a solution of phenothiazine (= PTZ) in pure product is fed in approximately in the middle of the column section with the hydraulically sealed cross-flow mass transfer trays.

[0070] In principle, the formation of sour water can also be practiced downstream of a first condensation column, for example (cf. DE 102 35 847 A1). In this case, the low-boiling gas stream escaping at the top of the first condensation column is expediently condensed to essentially water by direct cooling in a downstream space (second column) that is free of or contains internals. The condensate obtained in this way is in turn the sour water. A portion of the sour water is then expediently recycled to the top of the first condensation column to increase the separation efficiency. Another portion of the sour water is indirectly cooled in an external heat exchanger and used as the quench liquid 2, and the acrylic acid can in turn be extracted from the remaining amount of sour water according to the invention.Components of the low-boiling stream that are more volatile than water in turn form residual gas, which is normally at least partially recycled as cycle gas into the gas phase partial oxidation or used as stripping gas.

[0071] In the preferred variant of the process according to the invention, the dual-flow trays in the condensation column expediently extend approximately up to the cross section in the condensation column from which the acrylic acid contents of the reflux liquid, viewed towards the top of the column, are <90% by weight, based on the weight of the reflux liquid.

[0072] As already mentioned, the number of dual-flow trays for the preferred variant of fractional condensation described is generally 25 to 45. Their aperture ratio is suitably between 12 and 25%. The dual-flow trays preferably have circular holes with a uniform diameter as the passage points. The latter is suitably 10 to 20 mm. If necessary, the hole diameters in the condensation column can be tapered or enlarged from top to bottom and / or the number of holes can be reduced or increased (e.g., the hole diameter can be uniformly 14 mm and the aperture ratio can increase from top to bottom from 17.4% to 18.3%). However, the number of holes can also be constant across all dual-flow trays. Furthermore, the circular holes above the individual dual-flow trays are preferably evenly arranged in a strict triangular pattern (cf. DE 102 30 219 A1).

[0073] In addition, the punching burr of the through-holes punched out in the dual-flow trays in the condensation column preferably points downwards (thus reducing undesirable polymer formation).

[0074] According to the invention, it is useful if the number of dual-flow trays used in the condensation column corresponds to approximately 10 to 15 theoretical plates.

[0075] The number of hydraulically sealed crossflow mass transfer trays following the dual-flow trays in the condensation column preferred according to the invention will, as already mentioned, generally be 30 to 50. Their opening ratio will suitably be 5 to 25%, preferably 10 to 20% (the opening ratio generally represents the percentage of the passage cross sections in the total cross section; for the crossflow mass transfer trays preferably used, it is generally suitably in the aforementioned range).

[0076] Single-flow crossflow mass transfer trays are preferred according to the invention. As a rule, the number of hydraulically sealed crossflow trays for the preferred variant of fractional product gas mixture condensation is selected to correspond to approximately 10 to 30, often 25, theoretical plates.

[0077] Both the hydraulically sealed crossflow trays and any valve trays used with them have at least one downcomer. They can be single-flow or multi-flow, e.g., double-flow. Even in a single-flow design, they can have more than one downcomer. The inlet shafts of the valve trays are usually also hydraulically sealed.

[0078] The polymerization inhibition of the quench system 1 for the product gas mixture of the partial gas phase oxidation can be achieved both via polymerization inhibitors contained in the bottom liquid used for quenching (from the condensation column) and via polymerization inhibitors contained in the high boiler fraction used for quenching (from the condensation column).

[0079] The advantage of the process according to the invention lies in the fact that it enables an increased yield of crude acrylic acid with essentially the same purity. All statements made in this document apply in particular to a product gas mixture obtained by (preferably two-stage) heterogeneous partial oxidation of propene to acrylic acid. The preferred embodiment of the process according to the invention described above in no way limits its general feasibility.

[0080] Finally, it should be noted that both the first stripping gas and the second stripping gas advantageously contain molecular oxygen.

[0081] Examples

[0082] Example 1 (comparison example)

[0083] The procedure was as in Example 1 of EP 2 114 852 A1. All Thormann trays were made of the same (second) stainless steel (material 1.4571 according to DIN EN 10088: 16.5 to 18.5 wt.% chromium, 10.5 to 13.5 wt.% nickel, 2.0 to 2.5 wt.% molybdenum, up to 0.7 wt.% titanium). Corrosion was observed on Thormann trays 1 to 10 (counting from the lowest Thormann tray), in which the composition of the gas mixture included 5 to 70 wt.% water, 15 to 85 wt.% acrylic acid, at least 8 wt.% acetic acid, and at least 2 wt.% formic acid. Example 2 (according to the invention)

[0084] The procedure is as in Example 1. Thormann trays 1 to 10 (counting from the lowest Thormann tray) are made of (first) stainless steel (material 1.4539 according to DIN EN 10088: 19.0 to 21.0 wt% chromium, 24.0 to 26.0 wt% nickel, 4.0 to 5.0 wt% molybdenum, 1.2 to 2.0 wt% copper, up to 0.02 wt% carbon). The other Thormann trays were made of (second) stainless steel (material 1.4571 according to DIN EN 10088: 16.5 to 18.5 wt% chromium, 10.5 to 13.5 wt% nickel, 2.0 to 2.5 wt% molybdenum, up to 0.7 wt% titanium). No corrosion is observed on the Thormann floors.

Claims

Patent claims 1. A process for the production of acrylic acid, in which a product gas mixture containing acrylic acid, water vapor, and secondary components is produced by heterogeneously catalyzed gas-phase partial oxidation of at least one Ca precursor of acrylic acid with molecular oxygen over catalysts in the solid state at elevated temperature, the product gas mixture is then passed into a condensation column equipped with separating internals, the product gas mixture is allowed to rise within itself within the condensation column and is thereby fractionally condensed, the product gas mixture being separated into a bottom liquid containing subsequent products and secondary components with a higher boiling point than acrylic acid, a crude acrylic acid as the target product containing completely depleted water and secondary components, acidic water still containing acrylic acid and secondary components, and a residual gas mixture containing secondary components with a lower boiling point than water.the target product is led out of the condensation column via a side draw and the side draw is located above the feed point of the product gas mixture into the condensation column, characterized in that the separating internals consist of various stainless steels and in first regions within the condensation column with a gas mixture comprising 5 to 70 wt.% water, 15 to 85 wt.% acrylic acid, at least 8 wt.% acetic acid and at least 2 wt.% formic acid, separating internals consisting of a first stainless steel with at least 18 wt.% nickel are used.

2. Process according to claim 1, characterized in that the gas mixture of the first regions within the condensation column comprises 10 to 50 wt.% water, 20 to 70 wt.% acrylic acid, at least 10 wt.% acetic acid and at least 5 wt.% formic acid.

3. A method according to claim 1 or 2, characterized in that the first stainless steel contains at least 24 wt.% nickel.

4. Process according to one of claims 1 to 3, characterized in that in second regions within the condensation column which are different from the first regions with regard to the composition of the gas mixture, separating internals consisting of a second stainless steel with less than 17 wt.% nickel are used.

5. A method according to claim 4, characterized in that the second stainless steel contains less than 14 wt.% nickel.

6. Process according to one of claims 1 to 5, characterized in that at least one material stream fed to the condensation column contains a source of halide ions.

7. The method according to claim 6, characterized in that the halide ions are fluoride ions and / or chloride ions.

8. The process according to claim 6 or 7, characterized in that the stream containing a source of halide ions is water, propene, sodium hydroxide solution, hydroquinone, hydroquinone monomethyl ether, diethyl phthalate and / or phenothiazine.

9. Process according to one of claims 1 to 8, characterized in that the Ca precursor of acrylic acid is propene and / or acrolein.

10. Process according to one of claims 1 to 9, characterized in that the number of separating internals consisting of the first stainless steel with at least 18 wt.% nickel in the condensation column is 1 to 20.

11. Process according to one of claims 1 to 10, characterized in that dual-flow trays and cross-flow trays are used as separating internals of the condensation column.

12. Process according to claim 11, characterized in that the separating internals consisting of the first stainless steel with at least 18 wt.% nickel are cross-flow trays.

13. Process according to one of claims 10 or 11, characterized in that the number of separating internals designed as cross-flow trays in the condensation column is 5 to 45.

14. Process according to one of claims 1 to 13, characterized in that the composition of the gas mixture is determined in the condensation column.