Process for purifying acrylic acid by recycling parent acid

Abstract

The invention relates to a method for purifying acrylic acid by recycling parent acid, comprising steps a) to e). First, acrylic acid is a component of a product gas mixture (PGM), which also contains secondary components and optionally water. In step a), the product gas mixture (PGM) is separated in a separation column (T-Kol) into a crude acrylic acid stream (RAS) and a secondary component stream (NKS). Then, according to step b), a crude acrylic acid stream (RAS) is withdrawn from the separation column (T-Kol) and introduced into the device (V1). In the device (V1), the crude acrylic acid stream is further purified according to step c) to obtain purified acrylic acid (AAS) and parent acid (MS). The obtained mother acid (MS) is removed from the device (V1) according to step d) and optionally divided into two mother acid streams (MS-a and MS-b). In step e), the mother acid (MS) or optionally a portion thereof is recycled as a mother acid stream (MS-a) to the separation column (T-Kol), where the recycling of at least a portion of the mother acid (MS) or optionally 20 mother acid stream (MS-a) takes place above the position where the crude acrylic acid stream (RAS) is removed in the separation column (T-Kol).

Description

[0001] This invention relates to a method for purifying acrylic acid via mother acid recycling, comprising steps a) to e). First, acrylic acid is a component of a product gas mixture (PGM), which also contains minor components and optionally water. In step a), the product gas mixture (PGM) is separated into a crude acrylic acid stream (RAS) and a minor component stream (NKS) in a separation column (T-Kol). Then, according to step b), the crude acrylic acid stream (RAS) is removed from the separation column (T-Kol) and introduced into apparatus (V1). In apparatus (V1), the crude acrylic acid stream is further purified according to step c) to obtain purified acrylic acid (AAS) and mother acid (MS). According to step d), the obtained mother acid (MS) is removed from apparatus (V1) and optionally split into two mother acid streams (MS-a and MS-b). In step e), the mother acid (MS) or optionally a portion thereof is recycled as a mother acid stream (MS-a) to the separation tower (T-Kol), wherein at least a portion of the mother acid (MS) or optionally the mother acid stream (MS-a) is recycled above the location in the separation tower (T-Kol) where the crude acrylic acid stream (RAS) is taken out.

[0002] Acrylic acid is an important basic chemical. Due to its highly reactive double bonds and acidic functional groups, it is particularly suitable as a monomer for polymer preparation. For example, most of the acrylic acid monomers prepared are esterified prior to polymerization (e.g., to obtain adhesives, dispersions, or varnishes). Only a small portion is directly polymerized (e.g., to form "superabsorbents"). While direct polymerization of acrylic acid typically requires high-purity monomers, the purity requirement is less stringent if the acrylic acid is esterified prior to polymerization.

[0003] Acrylic acid can be obtained, in particular, by heterogeneous catalytic gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen over a solid catalyst at high temperatures. The term "C3 precursor" includes compounds that can be formally obtained by reduction with acrylic acid. In the preparation process, these C3 precursors are diluted in the gaseous state, typically with inert gases such as nitrogen, CO2, saturated hydrocarbons, and / or water vapor, mixed with molecular oxygen at high temperatures and optionally high pressures, and oxidized to a product gas mixture containing acrylic acid and minor components such as furfural, benzaldehyde, and maleic anhydride, from which acrylic acid must be separated.

[0004] Therefore, the resulting acrylic acid is not a pure product, but a mixture as described above, containing, in addition to acrylic acid (typically ≥90% or ≥95% of the total weight), typical byproducts of gas-phase oxidation, such as water, lower aldehydes (e.g., furfural, acrolein or methacrolein, benzaldehyde), lower carboxylic acids (e.g., acetic acid, propionic acid), and oligomers of acrylic acid.

[0005] The formation of acrylic acid oligomers occurs because acrylic acid, existing in the condensed phase, undergoes a reversible Michael addition reaction with itself and the dimer formed in this process to form acrylic acid oligomers (Michael adducts), and also through free radical polymerization. The presence of water (an unavoidable byproduct of the gas-phase catalytic oxidation of acrylic acid) and high temperatures promote the formation of acrylic acid oligomers.

[0006] Because the corresponding oligomers have higher boiling points than acrylic acid, they accumulate during the separation of acrylic acid by distillation and during the fractionation and condensation of product gas mixtures prepared by gas-phase catalytic oxidation in high-boiling-point ranges (e.g., in bottom liquids).

[0007] DE 10 2014 114 193 A1 and the granted EP patent EP-B 3 201 167 describe similar methods for preparing acrylic acid, wherein a temperature-controlled stream of mother acid is withdrawn from a crystallization apparatus for obtaining purified acrylic acid and directed to an absorption tower and a pyrolysis tower. Furthermore, a minor component stream containing oligoacrylic acid is supplied to the pyrolysis tower. As a result, the oligoacrylic acid can be pyrolyzed again, and overall, the method achieves better efficiency. However, DE 10 2014 114 193A1 does not disclose that the mother acid from crystallization is at least partially recycled as reflux to the separation tower, which occurs upstream of the crude acrylic acid discharged along the crystallization direction.

[0008] The methods employed under existing technology are advantageous in themselves and result in a corresponding yield of the valuable product, acrylic acid. However, these increased yields are only possible through high equipment costs and energy consumption.

[0009] In this context, the object of the present invention is to provide a method for purifying acrylic acid and a corresponding apparatus that, compared with the prior art, will further improve the purity of valuable products, while allowing the method to be carried out more efficiently in terms of equipment and energy consumption.

[0010] In a first aspect of the invention, the above objective is achieved by a method for purifying acrylic acid, the method comprising steps a) to e):

[0011] a) In a separation tower (T-Kol), the product gas mixture (PGM) containing acrylic acid and minor components is separated into a crude acrylic acid stream (RAS) and a minor component stream (NKS).

[0012] b) Take the crude acrylic acid stream (RAS) from the separation tower (T-Kol) and supply the crude acrylic acid stream to the unit (V1).

[0013] c) Further purify the crude acrylic acid stream (RAS) in the apparatus (V1) to obtain purified acrylic acid (AAS) and mother acid (MS).

[0014] d) Remove the mother acid (MS) from the apparatus (V1) and optionally separate the mother acid (MS) into two mother acid streams (MS-a and MS-b).

[0015] e) Recycle the mother acid (MS) or optional mother acid stream (MS-a) into the separation tower (T-Kol), wherein at least a portion of the mother acid (MS) or optional mother acid stream (MS-a) is fed into the separation tower (T-Kol) above the location where the crude acrylic acid stream (RAS) was taken out in step b).

[0016] In a second aspect of the invention, the above objective is achieved by an apparatus for purifying acrylic acid, the apparatus comprising:

[0017] -Separation Tower (T-Kol)

[0018] - Device (V1)

[0019] - The first pipeline (L1) connecting the separation tower (T-Kol) and the device (V1)

[0020] - A second pipeline (L2) is drawn from the device (V1) and splits into two pipelines (L2-a and L2-b), wherein pipeline (L2-a) is connected to the separation tower (T-Kol) and leads to the separation tower above pipeline (L1).

[0021] An advantage of the present invention is that the mother acid (MS) or optionally the mother acid stream (MS-a) is at least partially refluxed into the separation column (T-Kol), wherein at least the recirculation occurs in the separation column (T-Kol) above the position where the crude acrylic acid stream (RAS) is taken out in step b), which can significantly reduce the proportion of impurities of compounds with boiling points lower than acrylic acid, particularly the proportion of acetic acid in the crude acrylic acid stream (RAS) and the purified acrylic acid (AAS).

[0022] The advantage of the method according to the invention over the prior art is that, by recycling at least a portion of the mother acid (MS) or optionally the mother acid stream (MS-a) to the separation tower (T-Kol), minor components, such as low-boiling acetic acid or other low-boiling minor components, are more likely to escape to the top of the separation tower (T-Kol) above the point where the crude acrylic acid stream (RAS) is taken out. As a result, the crude acrylic acid stream (RAS) discharged along the direction of the device (V1) contains less acetic acid or other low-boiling minor components as impurities. This further leads to the fact that, compared to the method in which the mother acid (MS) is supplied to the separation tower (T-Kol) below the point where the crude acrylic acid stream (RAS) is taken out, the device (V1) and the purified acrylic acid (AAS) also contain less acetic acid and low-boiling minor components, as well as more acrylic acid.

[0023] Furthermore, within the scope of this invention, it is advantageous to guide the minor component stream (NKS) from the tray of the separation tower (T-Kol) to the cracking tower (S-Kol), where at least a portion of the minor component stream (NKS) is cracked to obtain monomeric acrylic acid. As a result, the implementation of the method becomes more efficient, and the method can be carried out more effectively.

[0024] Within the scope of this invention, it is also advantageous to recycle a second portion of the mother acid stream (MS-b) into the cracking tower (S-Kol) after separating the mother acid in step d), resulting in a more efficient method.

[0025] Therefore, another significant advantage of the present invention is that minor components, particularly oligomeric acrylic acid, obtained in the production of acrylic acid by the method and apparatus of the present invention can be decomposed and recycled back into the preparation process with increased efficiency, resulting in valuable products, namely, an increase in the yield and purity of acrylic acid.

[0026] The present invention will now be described in detail.

[0027] If method features related to the apparatus according to the invention are also mentioned in the following description, these features are preferably related to the method according to the invention. Similarly, the relevant features listed in conjunction with the method according to the invention preferably relate to the apparatus according to the invention.

[0028] In a first aspect of the invention, the above objective is achieved by a method for purifying acrylic acid, the method comprising steps a) to e):

[0029] a) In a separation tower (T-Kol), the product gas mixture (PGM) containing acrylic acid and minor components is separated into a crude acrylic acid stream (RAS) and a minor component stream (NKS).

[0030] b) Remove the crude acrylic acid stream (RAS) from the separation tower (T-Kol) and supply the crude acrylic acid stream to the device (V1).

[0031] c) Further purify the crude acrylic acid stream (RAS) in the apparatus (V1) to obtain purified acrylic acid (AAS) and mother acid (MS).

[0032] d) Remove the mother acid (MS) from the device (V1) and optionally separate the mother acid (MS) into two mother acid streams (MS-a and MS-b).

[0033] e) Recycle the mother acid (MS) or optional mother acid stream (MS-a) into the separation tower (T-Kol), wherein at least a portion of the mother acid (MS) or optional mother acid stream (MS-a) is fed into the separation tower (T-Kol) above the location where the crude acrylic acid stream (RAS) was taken out in step b).

[0034] The method according to the invention includes steps a) to e), and will be described below.

[0035] Figure 1 The method according to the invention is shown in its basic form, wherein, in step a), a product gas mixture (PGM) containing acrylic acid and minor components, and optionally water, is introduced into a separation tower (T-Kol) and separated into a crude acrylic acid stream (RAS) and a minor component stream (NKS). In this case, the crude acrylic acid stream (RAS) is completely free of minor components and optionally water, while the minor component stream (NKS) contains minor components with a boiling point higher than that of acrylic acid.

[0036] The product gas mixture from acrylic acid production is known to those skilled in the art. Typically, such product gas mixtures contain acrylic acid, diacrylic acid, and polyacrylic acid, as well as, for example, acetic acid, water, nitrogen, and oxygen.

[0037] Within the scope of this invention, the product gas mixture (PGM) preferably comprises acrylic acid and minor components, and optionally water, wherein the minor components are, for example, diacrylic acid, polyacrylic acid, nitrogen, maleic anhydride and oxygen, as well as formaldehyde, acrolein, formic acid, acetic acid, propionic acid, furfural, 2-furfural, benzaldehyde, 4-methoxyphenol, benzoic acid, phthalic anhydride, phenothiazine, propylene, propane, carbon dioxide and carbon monoxide.

[0038] If the product gas mixture (PGM) is treated before being introduced into the separation tower (T-Kol) (see also the combination device (V2) below), preferably by quenching with a portion of the minor component stream (NKS) and / or by mixing with at least a portion of the circulating gas stream (KGS), then after treatment (and before being introduced into the separation tower), the product gas mixture (PGM) preferably contains 35% to 50% by weight of acrylic acid, 10% to 25% by weight of diacrylic acid, 2% to 8% by weight of polyacrylic acid, and 20% to 40% by weight of [unclear - possibly referring to a specific component or component]. The mixture contains % nitrogen, 2% to 7% maleic anhydride, 0.2% to 2% benzoic acid, 1% to 5% water, 0.5% to 2% oxygen, 0.5% to 2% carbon dioxide, 0.2% to 2% 4-methoxyphenol, 0.2% to 2% acetic acid, and trace amounts (<0.5% in each case) of formaldehyde, acrolein, formic acid, propionic acid, furfural, 2-furfural, benzaldehyde, phthalic anhydride, phenothiazine, propylene, propane, and carbon monoxide.

[0039] If the product gas mixture (PGM) from acrylic acid preparation is introduced into a separation tower (T-Kol) without further processing, the product gas mixture (PGM) preferably contains 10% to 13% by weight of acrylic acid, 76% by weight of nitrogen, 4.5% by weight of water, 1.5% to 2.5% by weight of carbon dioxide, 2.5% to 3.5% by weight of oxygen, and 0.6% by weight of carbon monoxide. In addition, in each case, it also contains small amounts (in each case <0.5% by weight) of formaldehyde, acrolein, formic acid, acetic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, benzoic acid, phthalic anhydride, propylene, propane, and diacrylic acid.

[0040] The product gas mixture (PGM) is preferably supplied to the separation tower (T-Kol) at a temperature of 100 to 180°C.

[0041] Those skilled in the art are familiar with separation towers and the internal components that enable effective separation, and these can be found in textbooks on method engineering, particularly thermal separation (e.g., M. Baern et al., Technische Chemie, 2006, WILEY-VCH, Weinheim).

[0042] In the context of this invention, a separation column is preferably understood as a device for thermally separating mixtures. Suitable examples are distillation columns, condensation columns, and condensation columns. In this case, the column body is typically designed as a cylindrical tube that is insulated to prevent heat loss. To evaporate the mixture to be separated or a portion of the mixture to be separated, the lower end of the column may include an evaporator, or the evaporator may be designed as an external device adjacent to the column. Almost any conceivable heat source for evaporation can be used, such as hot water, electricity, microwaves, or waste heat from other processes or other process steps of this process. To condense the mixture to be separated or a portion of the mixture to be separated, the top of the column may include a condenser, or the condenser may be designed as an external device adjacent to the column. For cooling in the condenser, various operating media can be used, such as water, refrigerant, air, or a heating medium from other processes or other parts of this process. To improve heat and mass transfer in the column, the column body may include internal components such as sieves, bell-shaped or valve trays, packing materials such as Raschig rings or structured packing. Those skilled in the art will recognize that such separating internal components in a column are also referred to as trays. In principle, a tower with rotating internal components, known as a rotating tower, can also be used, which sprays the reflux liquid into droplets.

[0043] In the context of this invention, dual-flow trays and cross-flow trays are used in particular. Such dual-flow trays are known to those skilled in the art. In the context of this invention, these refer to horizontal internal members installed at intervals within a column and having openings through which vapor and liquid can flow counter-currently. These openings can be, for example, holes or slots, wherein the opening ratio can be set by the number of openings. Typically, dual-flow trays do not have drain pipes connecting them to the next tray. Of course, each dual-flow tray can be flush with the distillation column wall, but it can also be connected to the distillation column wall via a web.

[0044] Those skilled in the art are also familiar with crossflow trays themselves. In the context of this invention, these refer to horizontal internal components installed at a distance within a tower, where liquid is supplied to the tray on one side, flows across the tray, and comes into contact with rising gas. The liquid then reaches a drain well, where it is collected and supplied to the tray below. Crossflow trays may include, for example, perforated sieves, movable valves, bell elements, or tunnels.

[0045] According to the invention, dual-flow trays and / or cross-flow trays are preferably used as internal components for effective separation in the separation tower (T-Kol). Furthermore, the separation tower (T-Kol) may include further inlet and / or outlet points, for example at the top, to remove, for example, low-boiling-point substances and / or recycle components.

[0046] The operating pressure in the separation tower (T-Kol) is preferably 0 to 5 bar, more preferably 0 to 3 bar, and even more preferably 0 to 1.6 bar.

[0047] The crude acrylic acid stream obtained during the purification of acrylic acid is known to those skilled in the art. It is also known to those skilled in the art that this crude acrylic acid stream is typically not a pure product, but rather contains acrylic acid as the main component, but also contains small proportions of other components such as water, lower aldehydes (e.g., furfural, acrolein, benzaldehyde), lower carboxylic acids (e.g., acetic acid, propionic acid), and diacrylic acid.

[0048] In the context of this invention, the crude acrylic acid stream (RAS) comprises acrylic acid and minor components, and optionally water, wherein the minor components are understood to be, for example, acrolein, formic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, diacrylic acid, 4-methoxyphenol, and phenothiazine. Within the scope of this invention, the crude acrylic acid stream (RAS) particularly comprises 96% to 98% by weight of acrylic acid, 0.4% to 1.0% by weight of acetic acid, 0.2% to 2% by weight of water, and small amounts (<0.5% by weight in each case) of acrolein, formic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, diacrylic acid, 4-methoxyphenol, and phenothiazine.

[0049] Secondary components derived from acrylic acid preparation and / or acrylic acid purification are known to those skilled in the art. It is also known to those skilled in the art that such secondary component streams typically contain acrylic acid and oligoacrylic acid as well as polyacrylic acid as major components, but also contain high-boiling-point compounds such as benzaldehyde, furfural, and maleic acid.

[0050] In the context of this invention, the minor component stream (NKS) comprises acrylic acid and a minor component, and optionally water, wherein the minor component is, for example, diacrylic acid and polyacrylic acid, and in proportions of maleic acid, benzoic acid, benzaldehyde, and furfural. Within the scope of this invention, the minor component stream (NKS) particularly comprises 50% to 65% by weight of acrylic acid, 15% to 30% by weight of diacrylic acid, and 5% to 10% by weight of polyacrylic acid, 5% to 9% by weight of maleic anhydride, 0.5% to 2% by weight of benzoic acid, 0.5% to 1.5% by weight of water, and 0.5% to 1% by weight of 4-methoxyphenol, and further comprises small amounts (<0.5% by weight in each case) of acrolein, formic acid, acetic acid, propionic acid, furfural, benzaldehyde, phthalic anhydride, and phenothiazine.

[0051] Within the scope of this invention, the minor component stream (NKS) preferably has a temperature of 100°C to 130°C, particularly 105°C to 115°C, so as to achieve sufficient pre-concentration of the bottom liquid of the separation tower (T-Kol) on the one hand, before being transferred to the cracking tower (S-Kol), and on the other hand, to limit the formation of dimers in the bottom region of the separation tower (T-Kol).

[0052] Then, in step b), the crude acrylic acid stream (RAS) is removed from the separation tower (T-Kol) and introduced into the apparatus (V1). In the apparatus (V1), the crude acrylic acid stream (RAS) is further purified in step c) to obtain purified acrylic acid (AAS) and mother acid (MS).

[0053] Within the scope of this invention, the apparatus (V1) is preferably a crystallization apparatus. Crystallization apparatuses are known to those skilled in the art. In this case, the implementation of the method according to the invention is not limited to a specific crystallization method, wherein acrylic acid is purified by partial crystallization, separation of frozen acrylic acid from a liquid containing impurities (mother acid), and separation of pure acrylic acid crystals by melting. For example, falling film crystallization or suspension crystallization can be used as a combination of a cooling pan crystallizer and a washing tower, wherein the latter method variant is preferred.

[0054] If necessary, water (up to 10% by weight or more, preferably up to 5% by weight, based on the amount of acrylic acid contained) can be added to the crude acrylic acid stream (RAS) to be crystallized and purified before crystallization. This facilitates the separation of acetic acid contained in the crude acrylic acid and reduces the tendency for crust formation. In cases where the content of aldehydes or other minor components is increased, the addition of water can be omitted, as the aldehydes can take over the function of water in such situations.

[0055] After separation and washing of the crystals, they are typically placed in a container that already contains an advantageous amount of purified acrylic acid crystals. If necessary, a polymerization inhibitor, such as phenothiazine, may be added. However, usually, the residue of the polymerization inhibitor remaining in the crystals is sufficient, as it has already been added in the preceding method steps, which will be discussed later. A more precise explanation of the crystallization separation can be found, for example, in EP 0616998 A or DE 10223058 A.

[0056] Those skilled in the art are familiar with purified acrylic acid derived from acrylic acid preparation methods and / or acrylic acid purification methods. Typically, purified acrylic acid (AAS) contains acrylic acid and optionally small proportions of other components. Within the scope of this invention, purified acrylic acid (AAS) preferably contains >99.6% by weight of acrylic acid, in addition to small amounts (<0.3% by weight in each case) of acetic acid, water, propionic acid, and 4-methoxyphenol.

[0057] Within the scope of this invention, the term "mother acid" (also known as "mother liquor" in the prior art) refers to an acrylic acid solution which, after the pure product is separated in a crystallization apparatus, contains impurities separated in the crystallization apparatus, wherein the weight ratio of acrylic acid in the mother acid is preferably ≥80 by weight.

[0058] The mother acid (MS) obtained in this manner in step d) is then removed from the apparatus (V1) and optionally split into two mother acid streams. Within the scope of the invention, it is preferable to split the mother acid (MS) into two mother acid streams according to step d). The first mother acid stream thus obtained is referred to hereinafter as "mother acid stream (MS-a)" and the second mother acid stream obtained is referred to as "mother acid stream (MS-b)".

[0059] The mother acid (MS) removed from the apparatus (V1) typically comprises acrylic acid, and optionally water and acetic acid. Within the scope of the invention, the removed mother acid particularly comprises 90% to 95% by weight of acrylic acid, 3% to 6% by weight of water, 0.5% to 2% by weight of acetic acid, and 0.2% to 0.7% by weight of diacrylic acid, and small amounts (<0.5% by weight in each case) of formaldehyde, acrolein, formic acid, propionic acid, furfural, benzaldehyde, maleic anhydride, 4-methoxyphenol, and phenothiazine.

[0060] In step e), the mother acid (MS) or optional mother acid stream (MS-a) is recycled to the separation tower (T-Kol), wherein at least a portion of the mother acid (MS) or optional mother acid stream (MS-a) is fed into the separation tower (T-Kol) above the location where the crude acrylic acid stream (RAS) was taken out in step b).

[0061] If the mother acid (MS) is not separated in step d), according to the invention, it is preferable that the mother acid (MS) is fed completely into the separation tower (T-Kol) above the position where the crude acrylic acid stream (RAS) is taken out in step b).

[0062] The term "above the position" in step e) means that there is usually at least one (separated) tray between the take-out position in step b) and the feed position in step e), preferably at least five (separated) trays, and more preferably at least ten (separated) trays.

[0063] Furthermore, according to the invention, it is preferred that the mother acid (MS) be completely returned to the separation column (T-Kol), or if it is divided into two parts in step d), the mother acid stream (MS-a) is 10 to 50 trays above the crude acrylic acid stream (RAS) at the extraction position.

[0064] In one embodiment of the invention, it is also preferred that the mother acid (MS) is separated into two streams, and at least 80% by weight of the mother acid (MS) in the mother acid stream (MS-a) is recycled to the separation tower (T-Kol).

[0065] Furthermore, the mother acid (MS) is typically removed from the apparatus (V1) at a temperature slightly above the crystallization temperature of acrylic acid (15°C to 20°C), and, if appropriate, after being split into two mother acid streams (MS-a and MS-b), the mother acid stream (MS-a) is heated before being recycled to the separation tower (T-Kol). Preferably, the mother acid (MS) or optionally the mother acid stream (MS-a) is preheated at the feed location of the separation tower (T-Kol) to the corresponding thermodynamic equilibrium temperature to ensure the most efficient separation possible. In the context of this invention, "temperature control" refers to heating the mother acid or a portion of the mother acid stream to a temperature preferably 50°C to 100°C, more preferably 70°C to 100°C, and particularly 80°C to 95°C after discharge from the apparatus (V1).

[0066] Within the scope of this invention, it is conceivable that the mother acid (MS) or optional mother acid stream (MS-a) is directly or indirectly recycled to the separation tower (T-Kol). Here, "direct" should be understood as the device (V1) and the separation tower (T-Kol) being directly connected to each other via pipelines, while "indirect" should be understood as the mother acid (MS) or optional mother acid stream (MS-a) first passing through at least one other device, such as a device or tank for heating or cooling.

[0067] Furthermore, within the scope of this invention, the method according to the invention preferably further includes steps f) and g):

[0068] f) At least a portion of the minor component stream (NKS) is supplied from the trays of the separation column (T-Kol) to the cracking column (S-Kol).

[0069] g) At least a portion of the minor component stream (NKS) is cracked in a cracking tower (S-Kol) to obtain monomeric acrylic acid.

[0070] Figure 2 The above-described improvement to the method according to the invention is shown, wherein, firstly, an application already made to... Figure 1 The process involves, in step f), taking at least a portion of the minor component stream (NKS) from the bottom of the separation tower (T-Kol) and introducing it into the cracking tower (S-Kol). In this cracking tower, in step g), at least a portion of the minor component stream (NKS) is cracked to obtain the monomeric acrylic acid.

[0071] Separation towers are known to those skilled in the art. In the context of this invention, a pyrolysis tower refers to a tower in which oligomeric acrylic acid can be pyrolyzed into monomeric acrylic acid. According to the invention, pyrolysis towers are typically equipped with internal components that facilitate effective separation. In the context of this invention, the description already used in conjunction with the separation tower (T-Kol) is similarly applied to internal components that facilitate effective separation. Typically, the secondary component stream (NKS) is introduced into the central tray of the pyrolysis tower (S-Kol). The central tray is particularly the trays in tray regions 8 and 10.

[0072] If necessary, the minor component stream (NKS) can be completely introduced into the pyrolysis tower (S-Kol). However, within the scope of this invention, it is preferable to divide the minor component stream (NKS) into at least two parts. The separation of the minor component stream (NKS) can be carried out within the separation tower (T-Kol) or preferably outside the separation tower (T-Kol).

[0073] Preferably, according to step f), a first portion of the minor component stream (NKS) is introduced into the pyrolysis tower (S-Kol), and a second portion of the minor component stream (NKS) is returned directly and / or indirectly to the separation tower (T-Kol). In the last portion, the second portion of the minor component stream (NKS) is preferably used as a quenching liquid (see also below). The first and second portions of the minor component stream (NKS) can form any desired ratio (in volume %) relative to each other. The ratio of the first to the second portion of the minor component stream (NKS) is preferably in the range of 0.1 to 50 volume %, particularly in the range of 0.3 to 1.5 volume %.

[0074] In step g), at least a portion of the oligoacrylic acid contained in the minor component stream (NKS) is cracked in a cracking tower (S-Kol) to yield monomeric acrylic acid. This typically occurs thermally at a temperature >150°C. Cracking can be accelerated by adding a small amount of amine to the bottom of the cracking tower. Step g) is preferably performed as a countercurrent rectification.

[0075] As described above, those skilled in the art know that the minor component stream (NKS) typically contains oligoacrylic acid, such as diacrylic acid. To prevent the polymerization of the oligoacrylic acid after its degradation into the monomeric acrylic acid, the degradation tower (S-Kol) is preferably operated in a manner that inhibits polymerization. In principle, all polymerization inhibitors known to those skilled in the art can be used for this purpose. Phenothiazines, p-methoxyphenol, or mixtures of both compounds are commonly used as polymerization inhibitors.

[0076] Furthermore, according to the present invention, it is preferable to separate the mother acid (MS) into two mother acid streams (MS-a and MS-b) in step d), and the method further includes step h):

[0077] h) Feed the mother acid stream (MS-b) into the cracking tower (S-Kol).

[0078] therefore, Figure 2 Further, it is shown that, if necessary, in step h), after the mother acid (MS) separation preferably carried out in step d), a second portion of the mother acid stream (MS-b) is recycled to the cracking tower (S-Kol).

[0079] It has been demonstrated that if 60% to 95%, particularly 85% to 90%, of the pyrolyzable component stream is cleaved in step g), it is advantageous for the efficiency of the method according to the invention, especially for the yield of valuable products. Acrylic acid dimers and trimers are typically cleaved. A higher cleavage rate of 95% is advantageous regarding the yield of valuable products, but it is technically difficult to control because when the cleavage yield is >95%, the remaining product tends to form solids, making it difficult to handle.

[0080] In particular, high-boiling-point components are retained at the bottom of the cracking tower (S-Kol), and these components are extracted and sent to the treatment unit.

[0081] Furthermore, according to the present invention, dual-flow trays and / or cross-flow trays are preferred as effective internal components for separation in the separation tower (S-Kol), and dual-flow trays are more preferred.

[0082] Dual-flow trays and cross-flow trays are known to those skilled in the art and have been described in more detail in the context of this application.

[0083] The cracking tower (S-Kol) preferably includes trays 45 as internal components for effective separation, which are preferably designed as dual-flow trays. If the mother acid stream (MS-b) is fed into the cracking tower (S-Kol) according to step h), the feed preferably occurs at the top of the cracking tower (S-Kol), wherein the mother acid stream (MS-b) is preferably used as reflux within the cracking tower (S-Kol).

[0084] Furthermore, within the scope of this invention, step a) is preferably performed before another step a1):

[0085] a1) The product gas mixture (PGM) is supplied to unit (V2), where it is quenched. The quenched product gas mixture (PGM) is then removed from unit (V2) and supplied to the separation tower (T-Kol).

[0086] A portion of the minor component stream (NKS) from step a) is introduced into the quenching liquid device (V2) to obtain a quenching product gas mixture (PGM).

[0087] In this regard, we also refer to Figure 3 .

[0088] Figure 3 The above-described improvement to the method according to the invention is shown, wherein it is first applicable to Figure 1 and Figure 2 Furthermore, the product gas mixture (PGM) first passes through device (V2) before being introduced into the separation tower (T-Kol). In addition, a portion of the minor component stream (NKS) is separated and introduced into device (V2) as quenching liquid. Therefore, the quenching product mixture (PGM) discharged from device (V2) further includes a portion of the minor component stream (NKS) used as quenching liquid.

[0089] Typically, the temperature of the gas mixture obtained during the acrylic acid preparation process is between 150 and 350°C, usually between 200 and 300°C. The preparation of acrylic acid will be discussed later. Therefore, the obtained gas mixture must be cooled before entering the separation tower (T-Kol). Typically, it is cooled to 100 to 180°C.

[0090] Therefore, within the scope of this invention, the gas mixture obtained in the preparation of acrylic acid, referred to in this example as the "product gas mixture (PGM)," is preferably supplied first to the apparatus (V2). The apparatus (V2) is typically a quenching apparatus.

[0091] The quenching apparatus itself is known to those skilled in the art, as is the execution of the quenching step. Any liquid can be used as the quenching liquid; preferably, a portion of the secondary component stream (NKS) from step a) is used as the quenching liquid. If desired, at least a portion of the circulating gas stream (KGS) from step i) can also be used additionally or alternatively as the quenching liquid (see below). If the circulating gas stream (KGS) from step i) is used as the quenching liquid, then the complete circulating gas stream (KGS) from step i) is preferably used as the quenching liquid.

[0092] In the context of acrylic acid preparation, and particularly in the context of this invention, a quenching apparatus is understood as an apparatus for cooling a mixture of thermal product gases containing acrylic acid, thereby preventing possible reactions of its components, particularly preventing polymerization. For this purpose, all apparatuses known in the art (e.g., spray scrubbers, venturi scrubbers, bubble towers, or other devices with spray surfaces) can be used, wherein, according to the invention, a venturi scrubber or a spray cooler is preferred.

[0093] If desired, phenothiazine compounds can be added to the quenching device to stabilize or inhibit polymerization. Suitable phenothiazine compounds are, for example, phenothiazine itself, bis-(α-methylbenzyl)phenothiazine, 3,7-dioctylphenothiazine, and bis-(α-dimethylbenzyl)phenothiazine, with phenothiazine being preferred.

[0094] Furthermore, within the scope of this invention, the method according to the invention preferably includes another step i) performed simultaneously with or after step g).

[0095] i) A stripping stream (SGS) is supplied to the trays of the cracking tower (S-Kol), and a recirculating stream (KGS) containing the stripping stream (SGS) and the monomeric acrylic acid is taken out at the top of the cracking tower (S-Kol).

[0096] In this case, the stripping gas stream (SGS) is preferably directed toward the liquid surface of the bottom liquid. The stripping gas stream is specifically used as a circulating gas. In the context of this invention, "circulating gas" refers to a gas used in gas-phase oxidation to dilute the starting material and absorb the heat of reaction.

[0097] Within the scope of this invention, the circulating gas comprises nitrogen and oxygen, water vapor, carbon oxides and mixtures thereof in concentrations ranging from <5% by volume, as well as small amounts (<0.8% by volume) of ethylene, ethane, propylene, propane, acrolein, acrylic acid and acetic acid.

[0098] If steps g) and i) are performed simultaneously, this is preferably achieved by countercurrent distillation of the minor component stream (NKS) in a cracking column (S-Kol) to separate the minor components contained in the minor component stream (NKS). The obtained monomeric acrylic acid is typically discharged from the cracking column (S-Kol) as a gas mixture along with the supplied stripping gas stream (SGS) in the form of a recirculating gas stream (KGS) without condensation. The preferred selective stripping of the monomeric acrylic acid is based on its high vapor pressure.

[0099] In the context of this invention, when exiting the cracking tower (S-Kol), the recirculating gas stream (KGS) preferably contains nitrogen, acrylic acid, water, and oxygen, as well as a portion of carbon dioxide and acetic acid, particularly 50% to 65% by weight of nitrogen, 30% to 40% by weight of acrylic acid, 3% to 5% by weight of water, 2% to 3% by weight of oxygen, 1% to 2% by weight of carbon dioxide, and 1% to 2% by weight of acetic acid, and small amounts (in each case <0.7% by weight) of carbon monoxide, acrolein, formic acid, propionic acid, furfural, maleic anhydride, propylene, and propane. The recirculating gas stream (KGS) preferably has a temperature of 80°C to 100°C.

[0100] Furthermore, according to the present invention, preferably, after step i), the method according to the present invention includes another step j).

[0101] j) The circulating gas flow (KGS) taken out in step i) is supplied to the device (V2), wherein the circulating gas flow (KGS) is preferably combined with the quenched product gas mixture (PGM) in the device (V2), and the resulting mixture is then supplied to the separation tower (T-Kol).

[0102] If necessary, only a portion of the complete circulating gas flow (KGS) from step i) can be introduced into the device (V2) and / or a portion of the circulating gas flow (KGS) can be used as the quenching liquid. Preferably, according to step j), the complete circulating gas flow (KGS) from step i) is used.

[0103] In this regard, we also refer to Figure 4 .

[0104] Figure 4 A schematic diagram of the preferred embodiment of the invention described above is shown, wherein the stripping gas stream (SGS) is supplied on the trays of the cracking tower (S-Kol), and the recycle gas stream (KGS) comprising the stripping gas stream (SGS) and the monomeric acrylic acid is discharged at the top of the cracking tower (S-Kol). Otherwise, [the following has been addressed]. Figures 1 to 3 What is said also applies Figure 4 Furthermore, the circulating gas flow (KGS) is removed from the top of the separation tower (S-Kol) and introduced into the device (V2) according to step j), and then returned from there to the separation tower (T-Kol) along with the product gas mixture (PGM).

[0105] According to the present invention, the separation tower (T-Kol) is preferably designed as follows:

[0106] i) Distillation column,

[0107] ii) Distillation column, and / or

[0108] iii) Condensation tower.

[0109] Distillation columns, condensation columns, and condensation columns are known to those skilled in the art. First, generally refer to the description of separation columns for thermal separation given above. As mentioned above, those skilled in the art understand distillation as thermal separation carried out through multi-stage distillation. Those skilled in the art know that distillation is a thermal separation process, and a distillation column is a device used to perform this thermal separation process, in which evaporable liquids are recovered or evaporable solvents are separated from non-evaporable substances, which can then be collected by condensation. Accordingly, a condenser may be located at the top of such a separation column or as a separate device, in which substances in vapor form can be cooled to become liquid substances, thereby separating them from the mixture of substances.

[0110] Furthermore, according to the present invention, it is preferable that in the method according to the present invention,

[0111] i) in the separation tower (T-Kol) and / or

[0112] ii) In the pyrolysis tower (S-Kol),

[0113] Dual-flow trays and / or cross-flow trays are used as internal components for effective separation.

[0114] Furthermore, according to the invention, it is advantageous to prepare acrylic acid prior to the method for purifying acrylic acid, wherein a product gas mixture (PGM) containing acrylic acid, water vapor, and minor components is produced by heterogeneous catalytic gas-phase partial oxidation of at least one C3 precursor of acrylic acid with molecular oxygen on a solid catalyst at high temperature.

[0115] C3 precursors of acrylic acid are known to those skilled in the art. In principle, any C3 precursor of acrylic acid suitable for preparing acrylic acid by reaction with molecular oxygen on a solid catalyst can be used in this invention. Known C3 precursors of acrylic acid are, for example, propane, propylene, acrolein, propionaldehyde, and propionic acid.

[0116] According to the present invention, the preferred C3 precursor of acrylic acid is propylene and / or acrolein.

[0117] Partial gas-phase oxidation can be performed as described in the prior art. Starting with propylene, partial gas-phase oxidation can be carried out, for example, in two consecutive oxidation stages, as described in EP 0700714 A and EP 0700893 A. For example, partial gas-phase oxidation referenced in DE 197 40 253 A or DE 197 40 252 A can also be used. However, in principle, partial gas-phase oxidation can also be performed by other methods known to those skilled in the art from the prior art, which are not explicitly listed here.

[0118] Furthermore, it has been proven advantageous that

[0119] i) The crude acrylic acid stream (RAS) according to step b) contains at least 90% by weight of acrylic acid, as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural, and / or

[0120] ii) The mother acid (MS) according to step d) contains at least 90% by weight of acrylic acid, as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural.

[0121] In a second aspect of the invention, the method according to the invention can be advantageously performed by means of an apparatus according to the invention, the basic form of which will be described below.

[0122] Therefore, another subject of the present invention is an apparatus for purifying acrylic acid, comprising...

[0123] -Separation Tower (T-Kol)

[0124] - Device (V1)

[0125] - The first pipeline (L1) connecting the separation tower (T-Kol) and the device (V1)

[0126] - A second pipeline (L2) is drawn from the device (V1) and splits into two pipelines (L2-a and L2-b), wherein the pipeline (L2-a) is connected to the separation tower (T-Kol) and extends above the separation tower above the pipeline (L1).

[0127] Figure 6 A schematic diagram of an apparatus for the method of the present invention is shown in its basic form. The central elements are the separation tower (T-Kol) and the device (V1) shown therein. Figure 6 The first pipeline (L1) connecting the separation tower (T-Kol) and the device (V1) is shown. As part of the method according to the invention, the crude acrylic acid stream (RAS) is drawn from the separation tower (T-Kol) via this first pipeline (L1) and guided into the device (V1). Figure 6 A second pipeline (L2) leading out from the device (V1) is also shown. This second pipeline (L2) splits into two pipelines (L2-a and L2-b), wherein pipeline (L2-a) connects to the separation tower (T-Kol) and extends above the first pipeline (L1) to the separation tower (T-Kol). Those skilled in the art will know that at such locations where the pipeline is divided into two parts, there is typically a device, such as a valve, by which the flow of materials can be regulated.

[0128] The foregoing has provided corresponding definitions for the various components of the device and the terms used in relation to the method of the present invention, and these definitions also apply to this device.

[0129] The advantages of the apparatus of the present invention are essentially the same as those of the method of the present invention described above. This apparatus provides the equipment conditions for separating mother acid (MS), optionally obtaining two partial mother acid streams (MS-a and MSb), and at least partially resupplying them to the separation column (T-Kol), wherein the introduction occurs in the separation column (T-Kol) above the location where the crude acrylic acid stream (RAS) is taken out in step b).

[0130] It is particularly advantageous when the device according to the invention is integrated into a complete set of equipment for manufacturing acrylic acid. As described above, the device according to the invention can improve the overall efficiency of acrylic acid manufacturing while reducing equipment costs.

[0131] Furthermore, in the improved version, the method according to the invention can be advantageously performed by means of an improved device, which will be referred to below. Figure 7 A more detailed description is provided. This improved device also includes...

[0132] - Cracking tower (S-Kol)

[0133] - Connect the trays of the separation tower (T-Kol) to the third line (L3) of the cracking tower (S-Kol).

[0134] The pipeline (L2-b) is connected to the pyrolysis tower (S-Kol).

[0135] Figure 7 A schematic diagram of an improved device for the method of the present invention is shown. Firstly, it has been combined with... Figure 6 The above applies to Figure 7 . Figure 7 It also includes a pyrolysis tower (S-Kol). A pipeline (L2-b) that separates from the pipeline (L2) is connected to the pyrolysis tower (S-Kol). Figure 7 The third pipeline (L3) connecting the trays of the separation tower (T-Kol) to the pyrolysis tower (S-Kol) is also shown.

[0136] The advantages of the apparatus of the present invention are essentially the same as those of the method of the present invention described above. Furthermore, the apparatus provides conditions for extracting the minor component stream (NKS) from the bottom of the separation tower (T-Kol) and introducing it into the cracking tower (S-Kol), and after the mother acid (MS) is cracked, a second portion of the mother acid stream (MS-b) is recycled back to the cracking tower (S-Kol), where the oligomeric acrylic acid is cracked into monomeric acrylic acid.

[0137] The above explanation and reference Figure 4 The embodiments of the invention shown can preferably be executed on a device, and also include

[0138] - Device (V2)

[0139] Connect the device (V2) to the fourth line (L4) of the separation tower (T-Kol).

[0140] - The fifth pipeline (L5) is drawn from the cracking tower (S-Kol), and the fifth pipeline (L5) is connected to the device (V2).

[0141] - A branch of the first pipeline (L1) leading from the separation tower (T-Kol), wherein the branch leads to the device (V2).

[0142] Figure 8 A schematic diagram of an improved device for the method of the present invention is shown. Firstly, it has been combined with... Figure 6 and Figure 7 The above applies to Figure 8 . Figure 8 It also includes a device (V2), which is connected to the separation tower (T-Kol) via a pipeline (L4). Furthermore, Figure 8It includes a pipeline (L5) leading from the pyrolysis tower (S-Kol) and connecting the head of the pyrolysis tower (S-Kol) to the device (V2). It also includes a branch from the first pipeline (L1) to enable the introduction of a portion of the secondary component stream (NKS) as a quenching liquid into the device (V2).

[0143] The advantages of the apparatus of the present invention are essentially the same as those of the method of the present invention described above. Furthermore, this apparatus provides the conditions for extracting a circulating gas stream (KGS) containing stripping gas (SGS) and monomeric acrylic acid (KGS) from the top of the cracking tower (S-Kol). The latter is introduced into unit (V2) and from there returned to the separation tower (T-Kol) along with the product gas mixture (PGM).

[0144] Figures 1 to 8 List of reference numerals in the attached figures:

[0145]

[0146] The invention will now be described in detail by way of examples, but these examples are not intended to limit the invention. All features described and / or illustrated herein, whether individually or in any combination, constitute the subject matter of the invention and are not limited by their inductive or deductive relationships in the claims. The examples described below are simulations based on mass and energy balance and thermodynamic phase equilibrium models.

[0147] In principle, it should be mentioned that the measurements described in the examples are steady-state in each case. Those skilled in the art will know that, in steady state, the various steps described, such as the various supply and discharge lines for the product and minor components, occur simultaneously and continuously.

[0148] Example I1 according to the present invention

[0149] This example I1 according to the invention can be particularly aided by Figure 4 Let's visualize it. About Figure 4 For a description of the central element, please refer to the corresponding explanation above.

[0150] A hot product gas mixture (PGM) at approximately 270°C is supplied from a section of the equipment (not shown) where multiphase catalytic gas-phase partial oxidation is performed. This is then directed into unit (V2), which is designed as a quenching unit to prevent possible reactions between the components of the PGM and to lower its temperature. A portion of the minor component stream (NKS), described below, is used as the quenching liquid. Similarly, a gas mixture at approximately 95°C from the cracking tower (S-Kol) is supplied to unit (V2). This gas mixture will be discussed in more detail later.

[0151] The gas and liquid supplied to the unit (V2) are then fed to the bottom region of a separation tower (T-Kol) after leaving the unit. In this separation tower (T-Kol), the prepared acrylic acid is separated from the product mixture by thermal separation and withdrawn from the tower. The extracted acrylic acid stream, also referred to herein as crude acrylic acid stream (RAS), is of high concentration (approximately 97% by weight acrylic acid) and is at a temperature of approximately 100°C.

[0152] The crude acrylic acid stream (RAS) is supplied to unit (V1), which is designed as a crystallization unit where crystallization purification takes place. In addition to the crystallized high-purity acrylic acid, the so-called mother acid (MS) is retained in the crystallization unit and extracted from it.

[0153] Although, according to the prior art (see Comparative Example), the mother acid stream is directly supplied to the lower tray of the absorber, in the context of Example I1, the mother acid (MS) is first divided into first and second streams (MS-a and MS-b). When the second stream (Ms-b) is supplied to the cracking tower (S-Kol), the first stream of mother acid (MS-a) is recycled to the separation tower (T-Kol). Specifically, the tray recycled to the separation tower (T-Kol) is located above the tray from which the crude acrylic acid stream (RAS) is discharged along the direction of the unit (V1). Specifically, for Example I1, this means that the side outlet is located above tray number 15. The side outlet according to step b) is achieved by pulling in the chimney tray. The peripheral trays are dual-flow trays. The portion of mother acid added above the side outlet according to step e) is added to tray number 39 (top dual-flow tray) and discarded.

[0154] A stripping gas stream (SGS) is supplied as a recycle gas from the equipment section to the cracking tower (S-Kol) below the lowest tray. The temperature of this stripping gas stream (SGS) is approximately 85°C. A minor component stream (NKS) containing oligomeric acrylic acid at a temperature of approximately 109°C is supplied from the bottom of the separation tower (T-Kol) to the center tray of the cracking tower (S-Kol).

[0155] The minor component stream (NKS) also contains high-boiling-point substances such as benzaldehyde, furfural, and maleic anhydride. However, the main components of the minor component stream (NKS) are acrylic acid and its oligomers and polyacrylic acid.

[0156] In the cracking tower (S-Kol), these minor components, particularly acrylic oligomers, are further cracked and extracted as low-boiling fractions along with the stripping gas stream (SGS) as a recirculating gas stream (KGS) through the top of the cracking tower (S-Kol) and supplied to the unit (V2). High-boiling components, in particular, remain at the bottom of the cracking tower (S-Kol) and are extracted and sent to the disposal unit.

[0157] In this embodiment, a first portion of the mother acid (MS-a) is recycled to the tray of the separation tower (T-Kol), which is located above the tray. The crude acrylic acid stream (RAS) is discharged from it along the direction of the unit (V1). The yield of acrylic acid, as the valuable product, is significantly increased, while the content of acetic acid, as a byproduct, is reduced. Using the method and equipment according to the invention, 26.9 t / h of acrylic acid can be prepared, wherein the acetic acid content is <0.175 wt%.

[0158] The following example, I1, describes a specific implementation method for acrylic acid purification. Calculations were performed using BASF SE's "Chemasin" software / program.

[0159] (Describing steady state)

[0160] A product gas mixture (PGM) at 270°C was obtained from the heterogeneous catalytic gas-phase oxidation of propylene with a purity of "polymer grade". After flowing through the unit (V2) and cooling to 103.9°C, it had the following composition (693.5 t / h):

[0161] 41.65% by weight of acrylic acid,

[0162] 14.06% by weight of diacrylic acid,

[0163] 4.85% by weight of polyacrylic acid,

[0164] 0.37% by weight of acetic acid,

[0165] 2.35% by weight water,

[0166] 0.07% by weight of formaldehyde,

[0167] 0.04% by weight of acrolein,

[0168] 0.02% by weight of formic acid,

[0169] <0.001% by weight of allyl acrylate,

[0170] 0.03% by weight of propionic acid,

[0171] 0.11% by weight of furfural,

[0172] 0.09% by weight of benzaldehyde,

[0173] 3.56% by weight of maleic anhydride

[0174] 0.39% by weight of 4-methoxyphenol

[0175] 0.54% by weight of benzoic acid,

[0176] 0.20% by weight of phthalic anhydride,

[0177] 0.13% by weight of phenothiazine,

[0178] 0.10% by weight of propylene,

[0179] 0.06% by weight of propane,

[0180] 1.22% by weight of oxygen,

[0181] 0.84% ​​by weight of carbon dioxide,

[0182] 0.24% by weight of carbon monoxide

[0183] 29.09% by weight of nitrogen.

[0184] No other components were identified.

[0185] After the product gas mixture (PGM) is fed to the separation tower (T-Kol) and subjected to thermal separation, the obtained acrylic acid is withdrawn from the separation tower (T-Kol) through the first pipeline (L1). The temperature of this crude acrylic acid stream (RAS) (103.3 t / h) is 100.6°C, and its calculated composition is as follows:

[0186] 97.19% by weight acrylic acid,

[0187] 0.43% by weight of diacrylic acid,

[0188] <0.001% by weight of polyacrylic acid,

[0189] 0.59% by weight of acetic acid,

[0190] 1.29% by weight water,

[0191] Formaldehyde <0.001% by weight

[0192] 0.01% by weight of acrolein,

[0193] 0.02% by weight of formic acid,

[0194] <0.001% by weight of allyl acrylate,

[0195] 0.10% by weight of propionic acid,

[0196] 0.11% by weight of furfural,

[0197] 0.02% by weight of benzaldehyde,

[0198] 0.22% by weight of maleic anhydride,

[0199] 0.02% by weight of 4-methoxyphenol

[0200] Benzoic acid <0.001% by weight,

[0201] <0.001% by weight of phthalic anhydride,

[0202] 0.01% by weight of phenothiazine,

[0203] <0.001% by weight of propylene,

[0204] <0.001% by weight of propane.

[0205] No other components were identified.

[0206] After the crude acrylic acid stream (RAS) is supplied to a device (V1) designed as a crystallization apparatus and further purified by crystallization, high-purity acrylic acid (AAS) and mother acid (MS) are obtained, which are then recycled back to the separation process as described above.

[0207] The purified acrylic acid (AAS) (27.0 t / h) extracted from unit (V1) was produced at a temperature of 20.0 °C, and its calculated composition was as follows:

[0208] 99.78% by weight acrylic acid,

[0209] 0.1596% by weight of acetic acid,

[0210] 0.03% by weight of water,

[0211] 0.03% by weight of propionic acid,

[0212] 0.01% by weight of methoxyphenol

[0213] No other components were identified.

[0214] The mother acid (MS) discharged from unit (V1) is refluxed (78.7 t / h, consisting of MS-a: 70.7 t / h and MS-b: 8 t / h), with the temperature controlled at 93.0℃, and has the following calculated composition:

[0215] 92.92% by weight acrylic acid,

[0216] 0.56% by weight of diacrylic acid,

[0217] 0.9480% by weight of acetic acid

[0218] 4.72% by weight water,

[0219] 0.16% by weight of formaldehyde,

[0220] 0.01% by weight of acrolein,

[0221] 0.05% by weight formic acid,

[0222] <0.001% by weight of allyl acrylate,

[0223] 0.12% by weight of propionic acid,

[0224] 0.15% by weight furfural,

[0225] 0.03% by weight of benzaldehyde,

[0226] 0.29% by weight of maleic anhydride

[0227] 0.02% by weight of 4-methoxyphenol

[0228] Benzoic acid <0.001% by weight,

[0229] <0.001% by weight of phthalic anhydride,

[0230] 0.01% by weight of phenothiazine,

[0231] <0.001% by weight of propylene,

[0232] <0.001% by weight of propane.

[0233] No other components were identified.

[0234] A portion of the minor component stream (NKS) is introduced into the cracking tower (S-Kol), and a second portion of the minor component stream (NKS) is introduced into the device (V2) as quenching liquid.

[0235] In addition, a minor component stream (NKS) containing oligoacrylic acid is supplied from the bottom of the separation column (T-Kol) to the intermediate tray (tray number 8) of the cracking column (S-Kol) at a temperature of 108.3°C, and its composition is as follows:

[0236] 59.27% ​​by weight of acrylic acid,

[0237] 23.17% by weight of diacrylic acid,

[0238] 8.00% by weight of polyacrylic acid

[0239] 0.3697% by weight of acetic acid,

[0240] 0.94% by weight water,

[0241] Formaldehyde <0.001% by weight

[0242] 0.01% by weight of acrolein,

[0243] 0.01% by weight of formic acid,

[0244] <0.001% by weight of allyl acrylate,

[0245] 0.05% by weight of propionic acid,

[0246] 0.17% by weight furfural,

[0247] 0.14% by weight of benzaldehyde,

[0248] 5.80% by weight of maleic anhydride

[0249] 0.64% by weight of 4-methoxyphenol

[0250] 0.88% by weight of benzoic acid,

[0251] 0.33% by weight of phthalic anhydride,

[0252] 0.21% by weight of phenothiazine,

[0253] <0.001% by weight of propylene,

[0254] <0.001% by weight of propane.

[0255] No other components were identified.

[0256] After being re-pyrolyzed in the pyrolysis tower (S-Kol) and extracted from the top of the pyrolysis tower (S-Kol) along with the stripping gas stream (SGS) in the form of a recirculating gas stream (KGS), the recirculating gas stream (KGS) with the following calculated composition is supplied to the unit (V2) at a temperature of 95.6°C:

[0257] 33.18% by weight of acrylic acid,

[0258] <0.001% by weight of diacrylic acid,

[0259] 1.2578% by weight of acetic acid,

[0260] 4.26% by weight water,

[0261] 0.10% by weight of formaldehyde,

[0262] 0.07% by weight of acrolein,

[0263] 0.02% by weight of allyl formate,

[0264] 0.07% by weight formic acid,

[0265] <0.001% by weight of allyl acrylate,

[0266] 0.04% by weight of propionic acid,

[0267] 0.03% by weight of furfural,

[0268] <0.001% by weight of benzaldehyde,

[0269] 0.03% by weight of maleic anhydride,

[0270] <0.001% by weight of 4-methoxyphenol,

[0271] Benzoic acid <0.001% by weight,

[0272] <0.001% by weight of phthalic anhydride,

[0273] <0.001% by weight of phenothiazine,

[0274] 0.19% by weight of propylene,

[0275] 0.12% by weight of propane,

[0276] 2.36% by weight of oxygen,

[0277] 1.62% by weight of carbon dioxide,

[0278] 0.46% by weight of carbon monoxide

[0279] 56.17% nitrogen by weight.

[0280] No other components were identified.

[0281] For comparison, an embodiment of Comparative Example C1 of acrylic acid purification is described below. The comparative example was designed based on prior art (see, for example, DE-A 102 47 240), and... Figure 5 It is shown schematically in the diagram.

[0282] (Describing steady state)

[0283] Comparative Example C1

[0284] The comparative example is the same as Example I1 according to the invention, except that the first portion of the mother acid (MS-a) is recycled as reflux to the separation tower (T-Kol), wherein the reflux occurs on the tray of the separation tower (T-Kol), which is located below the tray, from which the crude acrylic acid stream (RAS) is discharged along the direction of the device (V1).

[0285] In this way, the crude acrylic acid stream (RAS) (103.3 t / h) has a temperature of 98.5°C and a calculated composition when it is taken out of the separation tower (T-Kol):

[0286] 96.83% by weight acrylic acid,

[0287] 0.27% by weight of diacrylic acid,

[0288] 0.7802% by weight of acetic acid

[0289] 1.66% by weight of water,

[0290] Formaldehyde <0.001% by weight

[0291] 0.01% by weight of acrolein,

[0292] 0.02% by weight of formic acid,

[0293] <0.001% by weight of allyl acrylate,

[0294] 0.10% by weight of propionic acid,

[0295] 0.09% by weight of furfural,

[0296] 0.02% by weight of benzaldehyde,

[0297] 0.17% by weight of maleic anhydride,

[0298] 0.02% by weight of 4-methoxyphenol

[0299] Benzoic acid <0.001% by weight,

[0300] <0.001% by weight of phthalic anhydride,

[0301] 0.01% by weight of phenothiazine,

[0302] <0.001% by weight of propylene,

[0303] <0.001% by weight of propane.

[0304] No other components were identified.

[0305] After the crude acrylic acid stream (RAS) is supplied to a device (V1) designed as a crystallization apparatus and further purified by crystallization, high-purity acrylic acid (AAS) and mother acid (MS) are obtained.

[0306] In this comparative example, the temperature for purifying acrylic acid (AAS) (27.0 t / h) was also 20°C, and the calculated composition was as follows:

[0307] 99.74% by weight acrylic acid,

[0308] 0.1950% by weight of acetic acid,

[0309] 0.03% by weight of water,

[0310] 0.03% by weight of propionic acid,

[0311] 0.01% by weight of 4-methoxyphenol

[0312] No other components were identified.

[0313] The reflux temperature of the mother acid (MS) (78.1 t / h) discharged from unit (V1) is controlled at 93.0℃, and it has the following calculated composition:

[0314] 93.01% by weight of acrylic acid,

[0315] 0.36% by weight of diacrylic acid,

[0316] 1.1576% by weight of acetic acid,

[0317] 4.73% by weight water,

[0318] 0.14% by weight of formaldehyde,

[0319] 0.01% by weight of acrolein,

[0320] 0.05% by weight formic acid,

[0321] <0.001% by weight of allyl acrylate,

[0322] 0.13% by weight of propionic acid,

[0323] 0.12% by weight of furfural,

[0324] 0.02% by weight of benzaldehyde,

[0325] 0.23% by weight of maleic anhydride,

[0326] 0.02% by weight of 4-methoxyphenol

[0327] Benzoic acid <0.001% by weight,

[0328] <0.001% by weight of phthalic anhydride,

[0329] 0.01% by weight of phenothiazine,

[0330] <0.001% by weight of propylene,

[0331] <0.001% by weight of propane.

[0332] No other components were identified.

[0333] In addition, a minor component stream (NKS) containing oligoacrylic acid at 107°C is supplied from the bottom of the separation tower (T-Kol) to intermediate tray number 8 of the cracking tower (S-Kol), with the following composition:

[0334] 63.55% by weight acrylic acid,

[0335] 18.67% by weight of diacrylic acid,

[0336] 8.00% by weight of polyacrylic acid

[0337] 0.3879% by weight of acetic acid

[0338] 0.99% by weight water,

[0339] Formaldehyde <0.001% by weight

[0340] 0.01% by weight of acrolein,

[0341] 0.01% by weight of formic acid,

[0342] <0.001% by weight of allyl acrylate,

[0343] 0.06% by weight of propionic acid,

[0344] 0.18% by weight furfural,

[0345] 0.14% by weight of benzaldehyde,

[0346] 5.87% by weight of maleic anhydride

[0347] 0.69% by weight of 4-methoxyphenol

[0348] 0.88% by weight of benzoic acid,

[0349] 0.33% by weight of phthalic anhydride,

[0350] 0.22% by weight of phenothiazine,

[0351] <0.001% by weight of propylene,

[0352] <0.001% by weight of propane.

[0353] No other components were identified.

[0354] After being re-pyrolyzed in the pyrolysis tower (S-Kol) and extracted from the top of the pyrolysis tower (S-Kol) along with the stripping gas stream (SGS) in the form of a recirculating gas stream (KGS), the recirculating gas stream (KGS) with the following calculated composition is supplied to the unit (V2) at a temperature of 94.8°C:

[0355] 32.65% by weight of acrylic acid,

[0356] <0.001% by weight of diacrylic acid,

[0357] 1.2013% acetic acid by weight

[0358] 3.98% by weight water,

[0359] 0.09% by weight of formaldehyde

[0360] 0.07% by weight of acrolein,

[0361] 0.02% by weight of allyl formate,

[0362] 0.07% by weight formic acid,

[0363] <0.001% by weight of allyl acrylate,

[0364] 0.04% by weight of propionic acid,

[0365] 0.03% by weight of furfural,

[0366] <0.001% by weight of benzaldehyde,

[0367] 0.02% by weight of maleic anhydride,

[0368] <0.001% by weight of methoxyphenol,

[0369] Benzoic acid <0.001% by weight,

[0370] <0.001% by weight of phthalic anhydride,

[0371] <0.001% by weight of phenothiazine,

[0372] 0.19% by weight of propylene,

[0373] 0.12% by weight of propane,

[0374] 2.39% by weight of oxygen

[0375] 1.65% by weight of carbon dioxide,

[0376] 0.47% by weight of carbon monoxide

[0377] 56.99% nitrogen by weight.

[0378] No other components were identified.

[0379] In this configuration, all other process parameters remain constant. This measure reduces the bottom temperature of the separation tower (T-Kol) to 107.0°C and the temperature of the crude acrylic acid stream (RAS) exiting the separation tower (T-Kol) to 98.5°C. The acrylic acid content of the crude acrylic acid stream (RAS) drawn from the separation tower (T-Kol) decreases to 96.83 wt%, and the acetic acid content increases to 0.7802 wt%. As a result, 99.74 wt% acrylic acid and 0.1950 wt% acetic acid of lower purity are obtained at the outlet of unit (V1).

[0380] To illustrate the positive technical effects of this invention, the contents of acrylic acid and acetic acid in purified acrylic acid (AAS) are compared below:

[0381]

[0382]

Claims

1. A method for purifying acrylic acid, comprising steps a) to e): a) In a separation tower (T-Kol), the product gas mixture (PGM) containing acrylic acid and minor components is separated into a crude acrylic acid stream (RAS) and a minor component stream (NKS). b) Remove the crude acrylic acid stream (RAS) from the separation tower (T-Kol) and supply the crude acrylic acid stream to the device (V1). c) Further purify the crude acrylic acid stream (RAS) in the apparatus (V1) to obtain purified acrylic acid (AAS) and mother acid (MS). d) Remove the mother acid (MS) from the device (V1) and optionally separate the mother acid (MS) into two mother acid streams (MS-a and MS-b). e) The mother acid (MS) or optionally the mother acid stream (MS-a) is recycled back to the separation column (T-Kol), wherein, At least a portion of the mother acid (MS) or optionally the mother acid stream (MS-a) is fed into the separation tower (T-Kol) above the location where the crude acrylic acid stream (RAS) is taken out in step b).

2. The method according to claim 1, further comprising steps f) and g). f) Supplying at least a portion of the minor component stream (NKS) from the trays of the separation column (T-Kol) to the cracking column (S-Kol), g) At least a portion of the minor component stream (NKS) is cracked in the cracking tower (S-Kol) to obtain monomeric acrylic acid.

3. The method according to claim 1 or 2, wherein, In step d), the mother acid (MS) is separated into two mother acid streams (MS-a and MS-b), and the method further includes step h): h) The mother acid stream (MS-b) is fed into the pyrolysis tower (S-Kol).

4. The method according to any one of claims 1 to 3, characterized in that, At least 80% by weight of the mother acid (MS) or optionally the mother acid stream (MS-a) is recycled to the separation column (T-Kol).

5. The method according to any one of claims 1 to 4, comprising another step a1) prior to step a). a1) The product gas mixture (PGM) is supplied to device (V2), the product gas mixture (PGM) is quenched in device (V2), the quenched product gas mixture (PGM) is removed from device (V2), and the quenched product gas mixture (PGM) is supplied to the separation tower (T-Kol). The secondary component stream (NKS) from step a) is introduced into the apparatus (V2) as a quenching liquid to obtain a quenching product gas mixture (PGM).

6. The method according to any one of claims 1 to 5, further comprising step i) performed after step g). i) A stripping gas stream (SGS) is supplied to the trays of the pyrolysis tower (S-Kol), and a circulating gas stream (KGS) containing the stripping gas stream (SGS) and monomeric acrylic acid is taken out at the top of the pyrolysis tower (S-Kol).

7. The method according to claim 6, further comprising step j): j) The recirculated airflow (KGS) extracted in step i) is supplied to the device (V2), wherein, In the device (V2), the circulating gas flow (KGS) is preferably combined with the quenching product gas mixture (PGM), and the resulting mixture is then supplied to the separation tower (T-Kol).

8. The method according to any one of claims 1 to 7, characterized in that, The separation tower (T-Kol) is designed as i) Distillation column, ii) Distillation column, and / or iii) Condensation tower.

9. The method according to any one of claims 1 to 8, characterized in that, i) in the separation tower (T-Kol) and / or ii) In the pyrolysis tower (S-Kol), Dual-flow trays and / or cross-flow trays are used as internal components for effective separation.

10. The method according to any one of claims 1 to 9, wherein, The method for purifying acrylic acid includes preparing acrylic acid, wherein at least one C3 precursor of acrylic acid is partially oxidized in the gas phase by molecular oxygen on a solid catalyst at high temperature to produce a product gaseous mixture (PGM) containing acrylic acid, water vapor and minor components.

11. The method according to claim 10, characterized in that, The C3 precursor of the acrylic acid is propylene and / or acrolein.

12. The method according to any one of claims 1 to 11, characterized in that, i) The crude acrylic acid stream (RAS) according to step b) contains at least 90% by weight of acrylic acid, as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural, and / or ii) The mother acid (MS) according to step d) comprises at least 90% by weight of acrylic acid, as well as water, acetic acid, diacrylic acid, propionic acid and 2-furfural.

13. The apparatus for purifying acrylic acid according to any one of claims 1 to 12, comprising: -Separation Tower (T-Kol) - Device (V1) - The first pipeline (L1) connecting the separation tower (T-Kol) and the device (V1) - A second pipeline (L2) is drawn from the device (V1) and splits into two pipelines (L2-a and L2-b), wherein, The pipeline (L2-a) is connected to the separation tower (T-Kol) and extends above the pipeline (L1) to the separation tower.

14. The device according to claim 13, further comprising: - Cracking tower (S-Kol) - Connect the trays of the separation tower (T-Kol) to the third pipeline (L3) of the pyrolysis tower (S-Kol). in, The pipeline (L2-b) is connected to the pyrolysis tower (S-Kol).

15. The device according to claim 14, further comprising: - Device (V2) - Connect the device (V2) to the fourth pipeline (L4) of the separation tower (T-Kol). -The fifth pipeline (L5) leading from the pyrolysis tower (S-Kol), wherein, The fifth pipeline (L5) is connected to the device (V2). - A branch of the first pipeline (L1) leading from the separation tower (T-Kol), wherein the branch leads to the device (V2).

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