Method and system comprising a temporary storage container for recovering acrylic acid

Abstract

The invention relates to a method for continuously recovering acrylic acid, wherein an auxiliary component stream, which comprises oligomeric acrylic acid, is supplied from a condensation column to a splitting column, at least one part of the oligomeric acrylic acid is split from the auxiliary component stream in the splitting column, thereby obtaining monomeric acrylic acid, and the monomeric acrylic acid is separated from the auxiliary components contained in the auxiliary component stream and is supplied to the condensation column.

Description

[0001] Process and plant with intermediate storage tank for the recovery of acrylic acid

[0002] The present invention relates to a process and a plant for the recovery of acrylic acid.

[0003] Acrylic acid is an important commodity chemical. Due to its highly reactive double bond and its acid function, it is particularly suitable as a monomer for the production of polymers. Of the acrylic acid monomers produced, for example, the majority is esterified prior to polymerization (e.g., to form adhesives, dispersions, or coatings). Only a small portion is polymerized directly (e.g., to form "superabsorbents"). While high-purity monomers are generally required for the direct polymerization of acrylic acid, the purity requirements for acrylic acid are not as high when it is esterified prior to polymerization.

[0004] Acrylic acid is obtainable, among other things, by heterogeneously catalyzed gas-phase partial oxidation of Ca precursors of acrylic acid with molecular oxygen over catalysts in the solid state at elevated temperatures. The term Ca precursor refers to chemical compounds that are formally obtainable by the reduction of acrylic acid. During production, these Ca precursors are diluted in the gaseous state, usually with inert gases such as nitrogen, CO2, saturated hydrocarbons and / or steam, mixed with molecular oxygen at elevated temperatures and, if appropriate, elevated pressure, over transition-metal mixed oxide catalysts and oxidatively converted into a product gas mixture containing acrylic acid and secondary components such as furfurals, benzaldehyde and maleic anhydride, from which the acrylic acid must be separated.

[0005] The acrylic acid obtained is not a pure product, but a mixture which, in addition to acrylic acid (usually > 90%, or > 95% of the total weight), also contains typical by-products of gas phase oxidation such as water, lower aldehydes (e.g. furfurals, acrolein or methacrolein, benzaldehyde), lower carboxylic acids (e.g. acetic acid, propionic acid) etc. as well as oligomers of acrylic acid.

[0006] The formation of acrylic acid oligomers is caused by acrylic acid in the condensed phase forming acrylic acid oligomers (Michael adducts) through reversible Michael addition to itself and to the resulting dimer, as well as oligomers formed by radical polymerization. The presence of water, the unavoidable byproduct of gas-phase catalytic oxidative production of acrylic acid, as well as elevated temperatures, promote the formation of acrylic acid oligomers.

[0007] Since the respective oligomers have a higher boiling point than acrylic acid, they accumulate in the high-boiling range (e.g. in the bottom liquid) both during distillative separation of acrylic acid and during fractional condensation of the product gas mixture of a gas-phase catalytic oxidative production.

[0008] DE 199 24 533 A1 discloses a process for producing acrylic acid as described above, in which a basic separation of crude acrylic acid is carried out by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas-phase partial oxidation. A recleavage of the acrylic acid oligomers present in the outlet of a quench liquid is to be integrated in such a way that the runtime of the process, in particular the fractional condensation, is not substantially reduced. The aim of the recleavage of the acrylic acid oligomers is to increase the yield of the desired product. DE 199 24 533 A1 provides a circulation reactor for the recleavage.

[0009] DE 102 47 240 A1 describes a very similar process for producing acrylic acid. A product gas mixture containing acrylic acid is first cooled by direct cooling with a quench liquid. The cooled product gas mixture is then passed into a condensation column equipped with separation internals. Crude acrylic acid is withdrawn from the condensation column and subjected to further crystallization purification. The mother acid obtained during this crystallization purification is completely recycled to the condensation column. A bottoms liquid containing acrylic acid oligomers is withdrawn from the bottom of the condensation column and used as a quench liquid.The portion of the quench liquid that does not evaporate during cooling of the product gas mixture is circulated via the sump and, if appropriate, via a heat exchanger. A portion of the quench liquid is discharged from this circuit as an outlet and fed to a cracking vessel for re-cleavage. The gaseous cracked gases containing acrylic acid are recirculated into the quench liquid circuit or into the condensation column, or into the quench liquid circuit and the condensation column. The cracked gases are subjected to countercurrent rectification and at least partial condensation before being recirculated. The resulting condensate quantity corresponds at least to the reflux quantity required for countercurrent rectification.

[0010] WO 2016 / 050582 A1 describes a process for recovering acrylic acid. Oligomeric acrylic acid from the bottom liquid of the condensation column is split into monomeric acrylic acid in a splitting column and recycled to the condensation column.

[0011] It was the object of the present invention to provide a process for the recovery of acrylic acid and a corresponding plant which further increases the yield of valuable product compared to the prior art, but at the same time enables a reduction of CO2 emissions and production downtimes.

[0012] The object is achieved by a process for the continuous recovery of acrylic acid, wherein a secondary component stream comprising oligomeric acrylic acid from a condensation column is fed to a cracking column, the oligomeric acrylic acid from the secondary component stream is at least partially cracked in the cracking column to obtain monomeric acrylic acid, the monomeric acrylic acid is separated from secondary components contained in the secondary component stream and is fed to the condensation column, characterized in that the secondary component stream from the condensation column is at least partially fed to an intermediate storage vessel instead of the cracking column and is temporarily stored therein when the cracking column is out of operation, and the temporarily stored secondary component stream from the intermediate storage vessel is fed to the cracking column together with the secondary component stream from the condensation column when the cracking column is back in operation.

[0013] The present invention has the significant advantage that, with the process according to the invention, secondary components arising during acrylic acid production, particularly oligomeric acrylic acid, can be split even if the splitting column fails, thereby increasing the yield of the desired product, i.e., acrylic acid, and its purity. The acrylic acid plant can continue to operate even if the splitting column fails. Furthermore, CO2 emissions are reduced because fewer secondary components need to be burned.

[0014] According to a preferred embodiment, the ratio of secondary component stream from the intermediate storage tank to secondary component stream from the condensation column, which are fed together to the cracking column, is between 1:5 and 2:1, preferably between 1:4 and 1:1, more preferably between 1:3 and 1:0.75. By adjusting the ratio, it is ensured that the secondary component stream from the condensation column can be removed in sufficient quantity and, at the same time, the intermediate storage tank is emptied again. The intermediately stored secondary component stream can be fed directly to the cracking column or indirectly as a mixture with the secondary component stream from the condensation column, with the intermediately stored secondary component stream in the second case preferably being fed into the feed of the secondary component stream from the condensation column to the cracking column.

[0015] It is also conceivable that part of the secondary component stream from the condensation column is fed to the intermediate storage vessel and the remainder of the secondary component stream from the condensation column is transferred to the combustion.

[0016] According to a preferred embodiment, the secondary component stream from the condensation column is cooled to 5 to 50 °C, preferably 10 to 40 °C, more preferably 15 to 30 °C, and particularly preferably below 18 to 22 °C, before being fed into the intermediate storage vessel. In this way, the viscosity can be adjusted, among other things.

[0017] According to a preferred embodiment, the intermediate storage container has a volume of 10 to 1,000 m 3 , preferably 150 to 850 m 3 , more preferably 250 to 700 m 3 , particularly preferably 400 to 600 m 3This volume ensures that sufficient by-component stream can be temporarily stored during the shutdown of the cracking column.

[0018] According to a preferred embodiment, at least the inner surface of the intermediate storage container is made of austenitic steel. The austenitic steel is preferably a chromium-nickel steel. The austenitic steel particularly preferably contains at least 8 wt.% nickel. The austenitic steel very particularly preferably contains from 16.0 to 20.5 wt.% chromium and from 7.0 to 13.0 wt.% nickel. By using austenitic steels as the container material, corrosion can be prevented.

[0019] According to a preferred embodiment, the temporarily stored secondary component stream is circulated in the intermediate storage tank. The circulation is preferably carried out by means of a pump that is connected in bypass mode via a bypass line with an inlet and outlet in the intermediate storage tank and continuously circulates the tank contents. Further preferably, the tank contents are passed through a heat exchanger in the bypass line during circulation, so that the temperature of the tank contents can be regulated.

[0020] According to a preferred embodiment, the intermediately stored by-component stream in the intermediate storage tank is blanketed with lean air. This ensures a sufficient oxygen content, which the inhibitor system (usually containing phenothiazine (PTZ) and / or 4-methoxyphenol (MEHQ)) requires to stabilize against polymerization. Furthermore, a maximum oxygen content is ensured to avoid an explosive atmosphere in the intermediate storage tank.

[0021] According to a preferred embodiment, the secondary component stream from the condensation column is fed to the intermediate storage tank below the liquid level of the already stored secondary component stream. This is preferably done via a submerged line. This ensures better mixing in the tank and prevents deposits.

[0022] Furthermore, it is possible to feed a solvent or a portion of the mother acid stream into the intermediate storage tank, preferably via a separate line. This can counteract excessive viscosity.

[0023] According to a preferred embodiment, the temperature of the intermediately stored secondary component stream is measured in the intermediate storage container. For this purpose, at least one temperature sensor is preferably arranged in and / or on the intermediate storage container. More preferably, six temperature sensors are arranged at different heights of the intermediate storage container, e.g., 2x at 5%; 2x at 30%; 2x at 65% of the height, in opposite directions around the circumference of the intermediate storage container. This provides an overview of the temperature distribution in the container.

[0024] According to a preferred embodiment, the viscosity of the intermediately stored secondary component stream is measured in the intermediate storage vessel. The measurement is preferably carried out using a viscometer. The viscosity should advantageously be low to ensure sufficient flowability of the intermediately stored secondary component stream.

[0025] According to a preferred embodiment, the secondary component stream essentially comprises acrylic acid, diacrylic acid and polyacrylic acid and proportions of maleic acid, benzoic acid, benzaldehyde, furfurals and water, in particular 50 wt.% to 60 wt.% acrylic acid or methacrylic acid, 20 wt.% to 30 wt.% diacrylic acid and 5 wt.% to 10 wt.% polyacrylic acid and 6 wt.% to 9 wt.% maleic acid, 1 wt.% to 2 wt.% benzoic acid, 0.5 wt.% to 1 wt.% water and 0.5 wt.% to 1 wt.% 4-methoxyphenol, in addition small proportions (each < 0.5 wt.%) of acetic acid, furfural, benzaldehyde, phthalic anhydride, phenothiazine and diacrylic acid. The secondary component stream preferably has a temperature of 100 °C to 130 °C, in particular 105 °C to 115 °C, in order to achieve, on the one hand, sufficient pre-thickening of the bottom liquid of the condensation column before transfer to the cracking column and, on the other hand, to limit the formation of dimers in the bottom region of the condensation column.

[0026] According to a preferred embodiment, 60% to 95%, preferably 85% to 90%, of the oligomeric acrylic acid from the secondary component stream is cleaved. Preferably, the dimers and trimers of acrylic acid are cleaved. Regarding the yield of the desired product, higher cleavage rates of 95% are advantageous, but are difficult to control industrially, since at cleavage yields > 95%, the remaining product tends to form massive solids and is therefore very difficult to handle.

[0027] It has surprisingly been found that the process according to the invention with the advantageous use of the intermediate storage tank has the advantage over the prior art that if the cracking column fails, the process can be continued without disadvantages. Solids continuously deposit in the cracking column and enter the cracking column as impurities in the secondary component stream or are formed during the cracking. The cracking column must therefore be regularly shut down and cleaned. Cleaning is preferably carried out with sodium hydroxide solution. During cleaning, either the entire acrylic acid production process must be stopped or the secondary component stream from the condensation column is fed directly to the incineration plant without cracking the oligomeric acrylic acid. Direct combustion of the secondary component stream comprising oligomeric acrylic acid produces more CO2 and the yield of acrylic acid is lower.

[0028] Further preferred details of the method are described below:

[0029] Preferably, a temperature-controlled mother acid stream from a crystallization device is divided into two substreams in the direction of the condensation column and in the direction of the splitting column. "Temperature-controlled" in the context of the present invention means that the mother acid stream or the mother acid substreams, after being discharged from the crystallization device, have been heated to a temperature of preferably 50°C to 100°C, preferably 60°C to 90°C, in particular 70°C to 80°C. The preferred embodiment of the process is not limited to a specific crystallization process, whereby acrylic acid is purified by partial crystallization, separation of the frozen acrylic acid from the impurity-containing liquid (mother acid), and melting of the separated pure acrylic acid crystals. For example,A falling film crystallization or a suspension crystallization as a combination of cooling disc crystallizers and washing columns can be used, the latter process variant being preferred.

[0030] In the present invention, "mother acid" (in comparable documents, possibly also called "mother liquor") refers to a solution of acrylic acid which, after separation of the pure product in a crystallization device, contains the impurities separated in the crystallization device, with acrylic acid accounting for a weight fraction of > 80 wt.% in the mother acid. Preferably, a first temperature-controlled mother acid substream is fed to the topmost tray of the 45-tray cracking column. In the present invention, the cracking column is preferably equipped with dual-flow trays as separating internals. A second temperature-controlled mother acid substream is preferably fed to tray 18 of the 75-tray condensation column.

[0031] Preferably, at least one stripping gas stream is fed below the lowest tray of the cracking column. The stripping gas stream is preferably directed toward the liquid surface of the bottom liquid. The stripping gas stream is used in particular as recycle gas. For the purposes of the present invention, "recycle gas" is understood to mean a gas which serves to dilute the reactants and absorb reaction heat in the gas-phase oxidation and which is essentially inert in the gas-phase reaction. The recycle gas essentially contains nitrogen and, in the concentration range <5 vol.%, oxygen, water vapor, carbon oxides and mixtures thereof, and very small amounts (<0.8 vol.%) of ethylene, ethane, propene, propane, acrolein, acrylic acid, and acetic acid.

[0032] Preferably, the secondary component stream comprising oligomeric acrylic acid from the condensation column is fed to a middle tray of the cracking column. This middle tray is, in particular, a tray in tray regions 8 and 10.

[0033] In the cleavage column, at least a portion of the oligomeric acrylic acid from the secondary component stream is cleaved to yield monomeric acrylic acid. This preferably occurs thermally at temperatures > 150 °C. The cleavage can be accelerated by adding small amounts of sodium hydroxide solution or amines to the bottom of the cleavage column.

[0034] Preferably, the secondary components contained in the secondary component stream are separated by countercurrent rectification in the attached cleavage column.

[0035] The monomeric acrylic acid obtained is preferably removed from the cleavage column as a gas mixture together with the supplied stripping cycle gas stream at the top of the cleavage column without condensation, and the gas mixture is then preferably fed below the lowest tray of the condensation column.

[0036] This advantageously recycles the monomeric acrylic acid back into the process. Since the acrylic acid is supplied in gaseous form and does not have to be evaporated there first, more energy is available in the condensation column to separate the acrylic acid from secondary components. The condensation column in the present invention is preferably designed as a tray column. It is preferably equipped with dual-flow trays in the lower section, Thormann trays in the middle section, and valve trays in the upper section.

[0037] The preferred process has the advantage over the prior art that partial condensation of the acrylic acid stripped in the cracking column to serve the reflux of the rectification column is dispensed with, and instead of the reflux generated by condensing the vapors at the top of the cracking column, a portion of the mother acid stream generated in the crystallization device is used. This eliminates the need for a condensation unit at the top of the cracking column, which reduces the equipment required. Furthermore, no cooling capacity is required for condensation. Furthermore, the energy introduced into the cracking column via a bottom heat exchanger is also available in the condensation column, which improves the separation of acrylic acid from secondary components.

[0038] A further advantage of the preferred process may be that, by conducting the process according to the invention, monomeric acrylic acid can be recovered from the respective oligomers in an improved yield and recycled to the overall process as low boilers. "Oligomeric acrylic acid" refers in particular to dimers and trimers of acrylic acid. Compared to the prior art, the processing loss of acrylic acid is reduced by 0.3%. In this application, "processing loss" is understood to mean the proportion of acrylic acid that, based on the acrylic acid supplied from the synthesis, cannot be separated from the secondary components and recovered as product.

[0039] In a further development of the preferred process, it has proven advantageous if the mother acid stream is switched against an acrylic acid stream from a condensation column for temperature control, in particular thermally. The acrylic acid stream is preferably withdrawn as the target product via a side draw of the condensation column and essentially fed to a crystallization device. This acrylic acid stream is hot (i.e., 95°C to 100°C) and highly concentrated (i.e., 95% to 98% by weight acrylic acid) and must be cooled before entering the crystallization device.

[0040] Preferably, the thermal energy present in the acrylic acid stream is transferred to the mother acid stream by means of a heat exchanger and thus coupled into the two mother acid substreams. This energy is thus available both in the condensation column for separating the acrylic acid and in the splitting column for splitting the dimeric acrylic acid, or less energy needs to be introduced into the splitting process via a bottom heat exchanger provided in the splitting column.

[0041] According to a preferred embodiment, the gas mixture is fed below the lowest tray of the condensation column indirectly by passing the gas mixture into a quenching device for quenching a product gas mixture containing acrylic acid.

[0042] The mother acid stream withdrawn from the crystallization device preferably comprises essentially acrylic acid and proportions of water and acetic acid, in particular 90 wt.% to 95 wt.% acrylic acid, 3 wt.% to 6 wt.% water and 1 wt.% to 2 wt.% acetic acid, as well as small proportions (each < 0.5 wt.%) of formaldehyde, propionic acid, furfural, maleic acid and diacrylic acid. The mother acid stream is withdrawn at a temperature just above the crystallization temperature of acrylic acid (15 °C to 20 °C) and preferably cooled to the corresponding thermodynamic equilibrium temperature at the feed point of the condensation column.

[0043] (201) of approximately 80 °C to ensure the most effective separation possible.

[0044] In addition, the stripping gas stream can essentially comprise nitrogen, acrylic acid, water, and oxygen, as well as proportions of carbon dioxide and acetic acid, in particular 80% to 85% by weight of nitrogen, 3% to 5% by weight of acrylic acid, 3% to 5% by weight of water, 3% to 4% by weight of oxygen, 2% to 3% by weight of carbon dioxide, and 1% to 2% by weight of acetic acid, as well as small proportions (each <0.7% by weight) of carbon monoxide, acrolein or methacrolein, formic acid, propene, and propane. The stripping gas stream preferably has a temperature of 80°C to 90°C, in particular approximately 85°C.

[0045] Another object of the present invention is a plant for the recovery of acrylic acid, comprising

[0046] - a condensation column,

[0047] - a splitting column,

[0048] - an intermediate storage container,

[0049] - a first line connecting the condensation column and the splitting column,

[0050] - a second line connecting the condensation column and the intermediate storage tank,

[0051] - a third line connecting the intermediate storage vessel and the cracking column, and

[0052] - a fourth line connecting the cracking column and the condensation column.

[0053] The method according to the invention can advantageously be carried out by means of the system according to the invention.

[0054] What is understood by the condensation column, the cracking column and the intermediate storage vessel in the sense of the present invention has already been described above with reference to the process according to the invention.

[0055] A first line connects the condensation column and the splitting column and feeds a secondary component stream from the condensation column to the splitting column. The splitting column and the condensation column are connected by a fourth line, through which the separated monomeric acrylic acid is fed to the condensation column.

[0056] A second line connects the condensation column and an intermediate storage vessel. The secondary component stream from the condensation column is fed to the intermediate storage vessel via the second line when the cracking column is out of operation. Consequently, the second line can be controllably closed / opened depending on the operating status of the cracking column. The intermediate storage vessel and the cracking column are connected via a third line, with the temporarily stored secondary component stream being fed into the cracking column via the third line when the column is back in operation. The third line can also be controllably closed / opened.

[0057] The plant preferably further comprises a fifth line connected to the cracking column, which feeds at least one gas stream as stripping cycle gas to the bottom region of the cracking column.

[0058] A sixth line preferably serves to feed a partial stream of the mother acid produced during crystallization to the cleavage column. A crystallization device and the condensation column are preferably connected by a seventh line.

[0059] Corresponding definitions of the elements of the system and the terms used have already been given above with regard to the method according to the invention and also apply to this system.

[0060] The advantages of the system according to the invention are essentially the same as those of the above-described process according to the invention. The present system provides the necessary equipment.

[0061] In a preferred embodiment of the plant according to the invention, it further comprises a quenching device for quenching a product gas mixture containing acrylic acid, which is arranged in the fourth line between the cracking column and the condensation column. This provides an apparatus for efficiently utilizing the gas mixture from the cracking column and its temperature for quenching the hot product gas mixture.

[0062] The plant according to the invention is particularly advantageous when integrated into an overall plant for acrylic acid production. As described above, the plant according to the invention can increase the overall efficiency of acrylic acid production.

[0063] If process features are mentioned in the description in connection with the system according to the invention, these preferably refer to the process according to the invention. Likewise, physical features listed in connection with the process according to the invention preferably refer to the system according to the invention.

[0064] Further objects, features, advantages, and possible applications will become apparent from the following description of exemplary embodiments of the invention with reference to the figure. All described and / or illustrated features, individually or in any combination, constitute the subject matter of the invention, regardless of their summary in the claims or their reference back to them. It shows:

[0065] Fig. 1 is a schematic representation of the system 1 according to the invention in one embodiment of the invention.

[0066] Figure 1 shows a schematic representation of the plant 1 according to the invention in one embodiment of the invention. Central elements are the condensation column 201, the intermediate storage vessel 203, and the cracking column 205.

[0067] A hot product gas stream at a temperature of approximately 270 °C is fed from a plant section (not shown), in which the heterogeneously catalyzed gas-phase partial oxidation is carried out. This gas stream is fed into a quench device 207 to prevent possible reactions between its components and to reduce its temperature. A gas mixture from the cracking column 205 at a temperature of approximately 95 °C is also fed to the quench device 207 via a fourth line 104. This gas mixture will be discussed in more detail later.

[0068] The gases and liquids fed to the quench device 207 are fed via line 110 into the bottom region of the condensation column 201 after exiting the quench device. In this condensation column 201, the acrylic acid produced is separated from the product mixture by absorption and removed from the condensation column 201 via a side draw via line 108. The withdrawn acrylic acid stream is highly concentrated (approx. 97% acrylic acid) and has a temperature of approximately 99 °C. This acrylic acid stream (also referred to as "crude acrylic acid") is fed via a series of devices not described further here to a crystallization device, in which the acrylic acid is purified by crystallization. In addition to crystalline, high-purity acrylic acid, so-called mother acid remains in the crystallization device and is removed via line 107 as a mother acid stream.

[0069] The mother acid stream is thermally coupled with the acrylic acid stream withdrawn from condensation column 201. The mother acid stream initially has a temperature of approximately 20 °C and is heated to approximately 93 °C by thermal coupling. In this way, the excess thermal energy of the acrylic acid stream can be transferred to the mother acid stream.

[0070] The temperature-controlled mother acid stream is divided into a first and a second mother acid substream. While the second mother acid substream is fed to the condensation column 201 via a line 109, the first mother acid substream is fed as reflux to the top tray of the cracking column 205 via a line 106, thus indirectly coupling the thermal energy of the acrylic acid stream into the cracking reaction.

[0071] A stripping gas stream is fed to the cracking column 205 via a line 105 as recycle gas from a plant section below the lowest tray (not to be described in detail). This stripping gas stream has a temperature of approximately 85 °C. A secondary component stream comprising oligomeric acrylic acid at a temperature of approximately 109 °C is fed to a middle tray of the cracking column 205 from the bottom of the condensation column 201 via a first line 101. The secondary component stream contains high boilers such as benzaldehyde, furfural, and maleic acid. However, the largest constituent of the secondary component stream is acrylic acid, its oligomers, and polyacrylic acid.

[0072] In the splitting column 205, these secondary components, particularly the acrylic acid oligomers, are split back and withdrawn via the top as the low-boiling fraction together with the recycle gas. This low-boiling fraction forms the gas mixture and is fed via line 104 to the quench device 207. The bottom of the splitting column 205 contains, in particular, the high-boiling fractions, which are withdrawn and disposed of.

[0073] In the present embodiment, the yield of acrylic acid as a valuable product is significantly increased by splitting the acrylic acid oligomers and recycling them to the condensation column 201. Using the process according to the invention and the plant 1 according to the invention, it is possible to produce 20.4 t / h of acrylic acid.

[0074] The secondary component stream from the condensation column 201 is fed to an intermediate storage tank 203 via a second line 102. This occurs when the cracking column 205 is shut down. When the cracking column 205 is restarted, the secondary component stream temporarily stored in the intermediate storage tank is fed into the cracking column 205 via a third line 103 together with the secondary component stream from the condensation column 201.

[0075] A concrete embodiment is described below using the example of the production of acrylic acid.

[0076] Example (the stationary state is described)

[0077] The procedure is as in the inventive example of WO2016 / 50582 A1. In the event of a shutdown or failure of the cracking column 205, the 2,460 kg / h of bottom liquid withdrawn from the bottom of the condensation column 201 are fed into an intermediate storage tank 203 instead of the cracking column 205 and temporarily stored there. When the cracking column 205 is back in operation, the bottom liquid temporarily stored in the intermediate storage tank 203 is fed into the cracking column 205 at approximately 650 to 1,350 kg / h, together with the bottom liquid from the condensation column 201.

[0078] Comparison example

[0079] The procedure is as in Example 1. There is no intermediate storage tank 203. In the event of a shutdown or failure of the cracking column 205, the 2,460 kg / h of bottom liquid taken from the bottom of the condensation column 201 is degassed and diluted with methanol and fed to the residue incineration.

[0080] The acrylic acid contained is lost and exhaust emissions are increased.

Claims

Patent claims 1. A process for the continuous recovery of acrylic acid, wherein a secondary component stream comprising oligomeric acrylic acid from a condensation column (201) is fed to a cracking column (205), the oligomeric acrylic acid from the secondary component stream is at least partially cracked in the cracking column (205) to obtain monomeric acrylic acid, the monomeric acrylic acid is separated from secondary components contained in the secondary component stream and fed to the condensation column (201), characterized in that the secondary component stream from the condensation column (201) is at least partially fed to an intermediate storage vessel (203) instead of the cracking column (205) and temporarily stored therein when the cracking column (205) is out of operation, and the temporarily stored secondary component stream from the intermediate storage vessel (203) is fed to the cracking column (205) together with the secondary component stream from the condensation column (201).when the cracking column (205) is back in operation., 2. Process according to claim 1, characterized in that the ratio of secondary component stream from the intermediate storage vessel (203) to secondary component stream from the condensation column (201), which are fed together to the cracking column (205), is between 1:5 and 2:

1.

3. Process according to claim 1 or 2, characterized in that the secondary component stream from the condensation column (201) is cooled to 5 to 50 °C before being fed into the intermediate storage vessel (203).

4. Method according to one of claims 1 to 3, characterized in that the intermediate storage container (203) has a volume of 10 to 1,000 m 3 has.

5. Method according to one of claims 1 to 4, characterized in that at least the inner surface of the intermediate storage container (203) is made of austenitic steel.

6. Method according to one of claims 1 to 5, characterized in that the intermediately stored secondary component stream is circulated in the intermediate storage container (203).

7. Method according to one of claims 1 to 6, characterized in that the intermediately stored secondary component stream in the intermediate storage container (203) is overlaid with lean air.

8. The method according to any one of claims 1 to 7, characterized in that the secondary component stream from the condensation column (201) is fed to the intermediate storage container (203) below the liquid level of the already intermediately stored secondary component stream.

9. Method according to one of claims 1 to 8, characterized in that the temperature of the intermediately stored secondary component stream is measured in the intermediate storage container (203).

10. Method according to one of claims 1 to 9, characterized in that the viscosity of the intermediately stored secondary component stream is measured in the intermediate storage container (203).

11. Process according to one of claims 1 to 10, characterized in that the secondary component stream essentially comprises acrylic acid, diacrylic acid and polyacrylic acid as well as proportions of maleic acid, benzoic acid, benzaldehyde, furfurals and water.

12. Process according to one of claims 1 to 11, characterized in that 60% to 95% of the oligomeric acrylic acid is split from the secondary component stream.

13. Plant (1) for the recovery of acrylic acid, comprising - a condensation column (201), - a splitting column (205), - an intermediate storage container (023), - a first line (101) connecting the condensation column (201) and the splitting column (205), - a second line (102) connecting the condensation column (201) and the intermediate storage tank (203), - a third line (103) connecting the intermediate storage vessel (203) and the cracking column (205), and - a fourth line (104) connecting the splitting column (205) and the condensation column (201).

14. Plant according to claim 13, characterized in that the plant (1) is integrated into an overall plant for the production of acrylic acid.

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