JP5104275B2 - Method for collecting polymerization inhibitor - Google Patents

Description

  The present invention relates to a method for recovering a water-insoluble polymerization inhibitor used in a separation step and purification step for acrylic acid obtained by air oxidation of propylene and the like, and a method for producing acrylic acid including this method.

  In the production of acrylic acid, usually, propylene, acrolein or the like is used as a raw material, and a reaction gas containing acrylic acid is obtained by performing catalytic gas phase oxidation with a solid catalyst. After the reaction gas containing acrylic acid is collected with water or an organic solvent to make an acrylic acid solution, it is separated from the produced acrylic acid by a series of separation and purification steps that perform at least one of extraction, distillation, and crystallization. Purified acrylic acid is obtained.

  In general, in order to prevent the generated acrylic acid from causing polymerization, a polymerization inhibitor is added at at least one of the separation step and the purification step. Examples of the polymerization inhibitor used include phenols such as hydroquinone and p-methoxyphenol, aromatic compounds containing heteroatoms such as phenothiazine, copper compounds such as copper dialkyldithiocarbamate and copper acetate, and manganese such as manganese acrylate. Compounds and the like. Since the boiling point of these compounds is higher than that of acrylic acid, the polymerization inhibitor should be contained in the bottoms together with other high-boiling impurities contained in the produced acrylic acid when distillation is performed in the purification process. When it is concentrated and crystallized in the purification step, it is concentrated in the crystallization residue together with other impurities.

  Usually, solutions containing polymerization inhibitors and impurities such as bottoms and residues are heat treated to recover acrylic acid by thermally decomposing the dimer of acrylic acid contained in the solution. . Acrylic acid produced by pyrolysis is separated by evaporation and recycled to the purification process. Moreover, since the collection | recovery residue produced by heat processing is discarded by an incineration process etc., the polymerization inhibitor contained in this has also been discarded simultaneously. However, in order to reduce the economic burden associated with the production of acrylic acid, it is desirable to recover and reuse the polymerization inhibitor rather than discarding it.

  As a method for reusing the above polymerization inhibitor, for example, the following methods are known. In Patent Document 1, a part of the effluent of the acrylic acid purification column is circulated as it is to the purification process, or the effluent is led to an evaporator and a fraction is circulated to the purification process of acrylic acid. A method is described. Also described is a method in which the bottoms are evaporated under low pressure and high temperature before circulation, and only the volatile components are circulated in the purification step.

  In Patent Document 2, the recovery residue after thermally decomposing the dimer of acrylic acid is extracted with water to recover hydroquinone, which is a highly polar polymerization inhibitor, in an aqueous solution. A method is described in which the obtained aqueous solution is circulated to a collection step of acrylic acid or the like.

  Patent Document 3 describes a method for extracting a polymerization inhibitor by performing extraction using an organic solvent such as isopropyl acetate, methyl acetate, and methyl isobutyl ketone on the bottoms produced by distillation of acrylic acid. Yes. This avoids the generation of an emulsion that may occur in the case of extraction using water by increasing the viscosity of the bottoms to be extracted and then performing extraction with an organic solvent. That is, entrainment at the time of recovery of the polymerization inhibitor is reduced by increasing the polymer in the liquid to be extracted.

  In Patent Document 4, the recovery residue after thermally decomposing the dimer of acrylic acid is extracted with water to recover the polymerization inhibitor in an aqueous solution, and volatilization is an impurity contained in the aqueous solution. A method is described in which, after the removal of the active substance, etc., it is recycled to the acrylic acid production process. By limiting the operating conditions during extraction with water, the operability is improved while keeping the entrainment of acrylic polymer and the like low, in contrast to reducing the recovery rate of the polymerization inhibitor. In addition, it is described that the accumulation of impurities in the purification system accompanying the reuse of the recovered polymerization inhibitor is reduced by adding a removal operation of the mixed impurities to the aqueous solution after extraction. Yes.

  Moreover, although it is different from the method in the production of these acrylic acids, Non-Patent Document 1 describes the production of acrylic acid ester purification tower cans in ester production in the step of producing esters by reacting acrylic acid and alcohol. It describes a method of circulating as it is in the process.

JP-A-51-91208 JP 54-52038 A JP 2002-161067 A JP 2003-221357 A

Ullmann's Encyclopedia of Chemical Industry 5th Edition (1985) Vol A1 P. et al. 168

  However, in Patent Documents 1 to 3 and Non-Patent Document 1, only the recovery of the polymerization inhibitor is studied, and the handling of impurities recovered together with the polymerization inhibitor is not considered. Among these, as in the methods described in Patent Document 1 and Non-Patent Document 1, in the method of circulating the bottoms of the purification distillation column to the purification process, heavy systems separated from the fraction product by distillation purification are used. The impurities are circulated again into the purification system, and the heavy impurities are concentrated in the purification system, which may cause the product quality to deteriorate.

  Further, among the methods described in Patent Document 1, even if an attempt is made to prevent the accumulation of impurities by circulating a volatile component obtained by evaporating the bottoms before circulation, it does not work if the polymerization inhibitor is hydroquinone. It was. This is because the boiling points of furfural, benzaldehyde, maleic anhydride, acrylic acid dimer, etc. that frequently appear as high-boiling impurities in acrylic acid are 162 ° C., 179 ° C., 202 ° C., and 250 ° C., respectively. Since the boiling point of hydroquinone, which is a polymerization inhibitor to be recovered, is 285 ° C., when it is volatilized to recover hydroquinone, all of these impurities are also recovered at the same time, resulting in the accumulation of impurities. It will be. In order to prevent this, even if another polymerization inhibitor is examined, for example, p-methoxyphenol having a low boiling point among polymerization inhibitors of acrylic acid has a temperature of 245 ° C., and this problem has not been solved. Even when phenothiazine was used as a polymerization inhibitor, the problem was not solved because the vapor pressure was smaller than that of hydroquinone. On the other hand, since metal complexes such as copper and manganese have substantially no vapor pressure, the polymerization inhibitor cannot be taken out by volatilization.

  Further, in the method described in Patent Document 2, even if extraction with water is performed, impurities in acrylic acid, including acrylic acid polymer, have sufficient solubility in water, so a polymerization inhibitor is selectively used. It could not be recovered.

  On the other hand, the method described in Patent Document 4 can be applied only to a water-soluble polymerization inhibitor, and has a problem that the application destination after recovery is limited because the recovery form is an aqueous solution. .

  Further, in the method described in Patent Document 3, recovery of impurities other than the polymer is not taken into consideration, and the polymerization inhibitor cannot be selectively recovered. In addition, the bottoms are made highly viscous to form a two-liquid phase during the extraction operation, but handling of the thickened liquid is difficult, especially when the scale is large, such as commercial operation. In some cases, the economic efficiency deteriorated.

  In the first place, acrylic acid can be mixed infinitely with any of polar solvents such as water, acids and alcohols, and nonpolar solvents such as aliphatic hydrocarbons, so that it does not form a two-component layer as it is. have. The bottomed liquid or crystallization residue generated in the purification process of acrylic acid contains insoluble substances such as acrylic acid polymer in addition to acrylic acid. Cannot be formed, and a tar-like deposited layer is formed by enclosing the solution. In addition, even when impurities composed of various compounds having a concentration equal to or higher than the solubility are contained, each component is not precipitated because it is added to the tar-like deposition layer made of an acrylic acid polymer or the like. For this reason, it was difficult to recover the active ingredient from the bottoms and the crystallization residue. This applies not only to the bottoms and the crystallization residue but also to the recovery residue after the acrylic acid dimer is recovered by heating to heat and contain the acrylic acid dimer.

  If the focus is only on the polarity of the substance to be collected, if the solvent used for extraction is made of a highly polar material such as water, a more polar substance will be selectively recovered. When used for extraction, it is thought that substances with lower polarity are selectively recovered. However, in reality, the tar-like deposited layer has a higher affinity with an organic solvent than water. Therefore, when an organic solvent is used for extraction, an acrylic layer that forms a tar-like deposited layer in the organic solvent is used. Incorporation of acid polymer was unavoidable.

  In addition, there is a problem even if it is attempted to separate the polymerization inhibitor and impurities due to the difference in polarity. Phenol compounds such as hydroquinone are classified as those having high polarity as polymerization inhibitors, but are more polar than aldehydes such as furfural and benzaldehyde compared to impurities contained in bottoms, but acrylics Compared with acid dimer and maleic acid, the polarity is low, and therefore, separation by simple difference in polarity was considered difficult.

  Therefore, in the production of acrylic acid by air oxidation of propylene, the present invention provides a separation step for obtaining crude acrylic acid from a gas of acrylic acid obtained in the production step for carrying out air oxidation of the raw material and a purification step for purifying the crude acrylic acid. In the above, while preventing the polymerization of acrylic acid using a polymerization inhibitor, while stably recovering the used polymerization inhibitor, when circulating the recovered polymerization inhibitor to the acrylic acid separation step and purification step, An object of the present invention is to recover the polymerization inhibitor so as to prevent the product quality from being deteriorated by impurities accompanying the polymerization inhibitor.

  This invention uses a water-insoluble polymerization inhibitor as a polymerization inhibitor used in a separation step for obtaining crude acrylic acid and a purification step for purifying crude acrylic acid, and then a bottom liquid obtained in the purification step, The water-insoluble solvent is extracted by adding water and a water-insoluble solvent to the residue after the crystallization residue or the acrylic acid dimer contained by heating them is recovered as acrylic acid. The above-mentioned problem has been solved by separating the water-soluble solvent layer and recovering the water-insoluble polymerization inhibitor from the water-insoluble solvent layer.

  First, even if a highly polar water-soluble compound such as hydroquinone is used as a polymerization inhibitor, some impurities have aldehydes that are less polar than such a water-soluble compound, whereas acrylic acid dimer Since a substance having a polarity higher than that of a water-soluble compound used for a general polymerization inhibitor is also contained, it is difficult to separate it from impurities by water or a water-insoluble solvent. However, when a water-insoluble compound having a low polarity is used as a polymerization inhibitor such as phenothiazine, the compound has a lower polarity than most impurities, so that it can be extracted with a water-insoluble solvent and separated from the impurities.

  However, water-insoluble polymerization with respect to the heavy liquid of the above-mentioned bottoms containing a lot of impurities, the above-mentioned crystallization residues, or the above-mentioned recovered residues (hereinafter collectively referred to as “residues”). To extract the inhibitor only with a non-water-soluble solvent, the interface between the water-insoluble solvent and the heavy liquid becomes unclear due to impurities, and the extraction does not work. In this invention, by using water and a water-insoluble solvent together, an aqueous layer containing impurities is formed between the heavy liquid layer containing most impurities and the water-insoluble solvent layer. Since a clear interface between the aqueous layer and the water-insoluble solvent layer is formed to obtain a transparent water-insoluble solvent layer, the polymerization inhibitor contained in the water-insoluble solvent layer is overlapped. The impurities remaining in the layer of the liquid can be separated and recovered.

  As the above water-insoluble solvent, a solvent having a solubility at 20 ° C. of 0.1% by weight or less is particularly preferable because an interface can be clearly formed. Specifically, a hydrocarbon which is liquid at normal temperature and has a boiling point lower than that of acrylic acid may be used.

  Moreover, the volume ratio of the water to be used is preferably 0.1 to 1 times that of the residue and the like, and the volume ratio of the water-insoluble solvent to be used is preferably 1/3 to 4 times. Furthermore, the volume ratio of the water-insoluble solvent to water is preferably 0.5 to 20 times. Both are values that can be suitably extracted and separated.

  Examples of the water-insoluble polymerization inhibitor that can be recovered in this invention include phenothiazine. Since phenothiazine is less polar than most impurities, it is likely that it can be separated from the impurities by the difference in polarity.

  In addition, since this invention collects a water-insoluble polymerization inhibitor, in the separation step of obtaining crude acrylic acid and the step of purifying crude acrylic acid, it is possible to use only a water-insoluble polymerization inhibitor. Most desirable from the viewpoint of the recovery efficiency of the polymerization inhibitor. However, since the effectiveness of the polymerization inhibitor may be further increased by using a combination of a plurality of types of chemicals, a crude acrylic acid is obtained from a water-soluble polymerization inhibitor that is not an object of recovery in this invention. It can also be used in combination in the separation step and the step of purifying the crude acrylic acid.

  Since the polymerization inhibitor recovered by this invention is clearly separated from impurities, the product quality of acrylic acid deteriorates due to impurities even if the recovered polymerization inhibitor is circulated in the separation process and purification process of acrylic acid. Can be prevented.

The present invention will be described in detail below.
The present invention relates to an acrylic acid having a production process for obtaining gas of acrylic acid by vapor-phase oxidation of propylene or acrolein, a separation process for obtaining crude acrylic acid from the gas of acrylic acid, and a purification process for purifying the crude acrylic acid. In the production of the above, a polymerization inhibitor added at least in one of the separation step and the purification step in order to prevent polymerization of the acrylic acid is recovered from the bottoms or crystallization residues generated in the purification step. These bottoms and crystallization residues contain the polymerization inhibitor to be added together with the acrylic acid multimers, by-products and impurities.

  In this invention, the polymerization inhibitor may be extracted directly from the bottoms or the crystallization residue, or the bottoms or the crystallization residue is heated to produce a large amount of acrylic acid contained in them. The polymerization inhibitor may be extracted from the recovered residue after the body is thermally decomposed and recovered as acrylic acid. Furthermore, before obtaining a polymer of acrylic acid, the polymerization inhibitor may be extracted by concentrating with a thin film evaporator and then performing thermal decomposition to obtain a recovered residue. These residues and the like contain impurities such as a polymer of acrylic acid and form a deposited layer of a tar-like substance.

  In recovering the polymerization inhibitor from the residue or the like, water and a water-insoluble solvent are added to the residue or the like. The water and the water-insoluble solvent may be added in order or at the same time. However, when the water-insoluble solvent is added first, the water-soluble polymer contained in the residue or the like may become sludge and easily precipitate. For this reason, it is more preferable to add water to the residue or the like in order so that water is added first, followed by the water-insoluble solvent. Water is more likely to mix with the tar-like heavy liquid contained in the residue, etc., and by adding water first, the viscosity of the heavy liquid having a high viscosity is lowered and easy to mix. As compared with the case where a water-insoluble solvent is added first, sludge deposition is reduced.

  As described above, when water and the water-insoluble solvent are added and mixed, the liquid phase is divided into two layers: a tar-like layer composed of the main component of the residue and an organic solvent layer of the water-insoluble solvent from the bottom. Or separated into three layers: the tar-like layer, the aqueous layer, and the organic solvent layer. In the case of two layers, most of the water is contained in the tar-like layer. In the organic solvent layer, the polymerization inhibitor contained in the residue or the like is extracted, and most of it is separated from impurities remaining in the tar-like layer. The polymerization inhibitor can be recovered in a state where the content of is low.

  Therefore, the water-insoluble solvent that forms the lightest organic solvent layer, the tar-like substance that forms the heaviest tar-like layer is the main component, and the residue containing the polymerization inhibitor to be recovered, It is necessary to sufficiently stir after the addition of water and the water-insoluble solvent so that the polymerization inhibitor is extracted in sufficient contact. If the stirring is insufficient, after simply extracting the residue and the like with water, the aqueous solution is then close to the state extracted with the water-insoluble solvent. Almost no organic solvent is recovered in the organic solvent layer.

  In the above stirring, the stirring time when mixing with the water and the water-insoluble solvent is preferably 3 minutes or more. If it is less than 3 minutes, the contact time between the water-insoluble solvent and the residue is too short, and there is a possibility that sufficient extraction cannot be performed. On the other hand, it is preferable that it is 30 minutes or less. If it exceeds 30 minutes, the equipment required for stirring becomes large, and there is a possibility that the liquid viscosity, which is considered to be derived from the polymer, may increase.

  If the amount of water added is small, the liquid phase will not be separated in the first place, or even if it is separated, it will be unavoidable that the tar-like substance constituting the heavy liquid is mixed into the organic solvent layer. Here, by adding an appropriate amount of water, the interface between the layer of the water-insoluble solvent and the other layer becomes clear and can be separated into two or more layers. Furthermore, if there is a certain amount of water, a water layer is formed between the organic solvent layer and the tar-like layer to form a three-component layer, so that tar-like substances are prevented from being mixed into the organic solvent layer. This makes it easy to separate the layers, increase the recovery rate of the polymerization inhibitor, and easily remove impurities that tend to concentrate between the water and the tar-like layer. On the other hand, even if there is a lot of water, there is no problem in the extraction operation itself if the residue and the water-insoluble solvent are sufficiently brought into contact by stirring or the like. However, the water layer also contains a part of the impurities contained in the residue, etc., and must be treated as waste liquid. If there is more water than necessary, it increases the burden of waste liquid treatment. Become. Moreover, the apparatus which performs collection | recovery must also be enlarged. Therefore, the amount of water added is preferably such that the liquid phase is barely separated into three layers. Specifically, the liquid phase can be separated by adding water at a volume ratio of 0.1 times or more and equal to or less than the above-mentioned residue etc. Addition of water that is equal to or less than the above is preferable because a three-component layer is easily formed.

  Furthermore, the ratio of the water-insoluble solvent to water is preferably 0.5 times or more by volume ratio. This is because if it is less than 0.5 times, sufficient polymerization inhibitor may not be recovered. On the other hand, it is preferable that it is 20 times or less. This is because if the ratio is too large, the layer heights of the three layers to be formed are greatly different, so that control of operation becomes difficult.

  Furthermore, the volume ratio of the water-insoluble solvent to the residue or the like is preferably 1/3 or more. This is because if the ratio is less than 1/3, there is a possibility that sufficient recovery of the polymerization inhibitor may not be performed. On the other hand, it is preferable that it is 4 times or less. Even if the amount of the solvent is increased excessively, the contribution to the recovery efficiency of the polymerization inhibitor is small, but there is a tendency for the equipment involved in a series of recovery to be large, which deteriorates the economy and the burden on the apparatus.

  Note that the main component of the tar-like layer is an acrylic acid polymer, which has a large number of carboxyl groups, and thus has high hydrophilicity and is hardly mixed into the organic solvent layer composed of the water-insoluble solvent, but is slightly included. It is. When the polymerization inhibitor extracted in the organic solvent layer is recovered and reused, it is preferable to remove the acrylic acid polymer as much as possible instead of circulating the organic solvent layer as it is. For removal, it is most convenient to filter with a microfilter. In aqueous solution systems, the acrylic polymer is highly tacky and filter clogging is likely to occur, but in non-water soluble solvent systems this clogging is relatively unlikely and the liquid phase is sufficiently separated. That's fine.

  In the recovery of the polymerization inhibitor carried out as described above, the temperature when mixing with the water and the water-insoluble solvent is preferably 20 ° C. or higher. This is because if it is lower than 20 ° C., the liquid viscosity becomes high and the extraction efficiency may be reduced. On the other hand, it is preferable that it is 50 degrees C or less. This is because when the temperature exceeds 50 ° C., the mutual solubility of the liquid layers increases, and there is a risk of forming two liquid layers or one liquid layer.

  In the recovery of the polymerization inhibitor carried out as described above, as a specific apparatus to be used, for example, a combination of a mixing tank having a stirring blade and a stationary separation tank, a line mixing by a static mixer, and a stationary separation tank The combination with is mentioned.

  The water-insoluble solvent used in the present invention is a liquid that has low solubility in water or is substantially insoluble and can dissolve the polymerization inhibitor. When the polarity of the solvent increases, the mutual solubility of the solvent and the residue, such as the solvent and the residue, increases, so that the proportion of the solvent taken into the aqueous layer and the residue in the extraction step increases. For this reason, in any case of discarding or recovering the solvent in the aqueous layer or the layer composed of residues or the like, deterioration in economic efficiency is inevitable. Moreover, the increase in solvent polarity makes it difficult to form a three-component layer, and tends to form a two-component layer or a one-component layer. Specifically, the solubility in water is preferably such that the solubility of the water-insoluble solvent in water at 20 ° C. is 1.5% by weight or less, and 0.1% by weight or less. It is more preferable because it is easily separated into three layers.

  If the water-insoluble solvent has a boiling point lower than that of acrylic acid and does not azeotrope with acrylic acid, even if it is introduced into the separation step and the purification step together with the recovered polymerization inhibitor, the water-insoluble solvent It is preferable because the organic solvent can avoid mixing into the product acrylic acid. Examples of such a water-insoluble solvent include aliphatic or aromatic hydrocarbons that are liquid at room temperature and have a boiling point lower than that of acrylic acid. Among these, in order to facilitate the depressurization operation for preventing polymerization, it is preferable that the boiling point is as high as possible in a range lower than that of acrylic acid, and since it is used in a large amount, it is an inexpensive compound that is widely used. Is more preferable. Specific examples include toluene, hexane, heptane and the like that have a solubility in water of 0.1 g or less.

  The polymerization inhibitor recovered in this invention is preferably as low in polarity as possible. If the polarity is high, among the various impurities contained in the above residues etc., the polarity is higher than that of aldehydes such as furfural and benzaldehyde, while the polarity is lower than that of acrylic acid dimer or maleic acid. This is because there is a high possibility that separation will not be possible depending on the polarity. For this reason, it is preferable that the polarity is lower than most of the impurities contained in the residue. Examples of such a low polarity polymerization inhibitor include phenothiazine. In the separation step and the purification step of the present invention, not only such a low-polarity polymerization inhibitor but also other polymerization inhibitors may be used in combination. However, such a highly polar polymerization inhibitor can hardly be recovered by this invention and cannot be efficiently reused.

  The polymerization inhibitor recovered as the organic solvent layer by the above procedure can be supplied to the separation step and the purification step and reused while being dissolved in the water-insoluble solvent. During extraction, impurities are mostly separated from the tar-like layer, so that it is possible to avoid accumulation of impurities during the separation step and the purification step.

  Next, the separation step and the purification step for obtaining the purified acrylic acid while discharging the residue and the like which are the targets for extracting the polymerization inhibitor according to the present invention will be described.

  The separation step includes a first separation step (collecting acrylic acid gas with water to obtain an acrylic acid aqueous solution, and then separating the water with azeotropic dehydration distillation to obtain crude acrylic acid); In step (1), acrylic acid is extracted from the aqueous acrylic acid solution, and then the solvent is separated in a solvent separation column to obtain crude acrylic acid. Moreover, the said refinement | purification process is a 1st refinement | purification process which obtains a bottom with a refined acrylic acid by performing distillation under reduced pressure in a refinement tower, and a 2nd refinement | purification process which obtains a crystallization residue while obtaining refined acrylic acid by crystallization. Any one of two types of steps can be selected and performed. These separation steps and purification steps will be described with reference to FIGS. In addition, the acrylic acid containing gas containing an impurity is obtained by performing vapor phase oxidation of propylene, acrolein, etc. in the manufacturing process prior to these processes.

  FIG. 1 shows a case where the first separation step and the first purification step are performed, and these steps will be described. The first separation step includes a collection step of collecting the acrylic acid gas with water in a collection tower to obtain an aqueous acrylic acid solution, and an organic solvent supplied with the azeotropic dehydration distillation tower and the acrylic acid. An azeotropic dehydration step of obtaining crude acrylic acid as dehydrated acrylic acid by dehydrating the aqueous acrylic acid solution by azeotropic distillation with water of an aqueous solution.

  First, the acrylic acid-containing gas A obtained in the production process is supplied to the bottom of the collection tower 11. On the other hand, water B is supplied from the upper part of the collection tower 11. In the lower part of the collection tower 11, while the supplied acrylic acid-containing gas A is cooled, a collection step is performed in which the acrylic acid is absorbed by the water B to obtain an aqueous acrylic acid solution. The acrylic acid-containing gas A is cooled by cooling the tower bottom liquid C made of an acrylic acid aqueous solution with the heat exchanger 12 and circulating a part thereof to the collection tower 11. The remainder of the column bottom liquid C is sent to the next azeotropic dehydration distillation column 15. Of the other components contained in the acrylic acid-containing gas A, those not absorbed by the water B are discharged as exhaust gas D from the tower top.

  In the collection tower 11, polymerization inhibitors E and E ′ are supplied in order to prevent acrylic acid from being polymerized. The polymerization inhibitors E and E ′ are supplied as an aqueous solution in accordance with the liquid composition in the collection tower 11. Therefore, the polymerization inhibitor used here needs to be water-soluble. In supply, it supplies so that it may add in the liquid of the supply line of said water B, or the circulation line of tower bottom liquid C. The polymerization inhibitor used in the subsequent steps includes at least a water-insoluble polymerization inhibitor other than those specified as being water-soluble, and may contain a water-soluble polymerization inhibitor.

  The column bottom liquid C is introduced into the middle of the azeotropic dehydration distillation column 15 to perform an azeotropic dehydration step. Here, the organic solvent circulating in the system selectively causes water and an azeotrope G with the organic solvent to flow out from the top of the tower, and the tower bottom liquid C, which is an aqueous acrylic acid solution, is dehydrated and dehydrated from the tower bottom. Crude acrylic acid J which is the dehydrated acrylic acid thus obtained is obtained. Part of the crude acrylic acid J is cooled by the heat exchanger 16 and circulated in the azeotropic dehydration distillation column 15, and the rest is sent to the next purification column 21. The azeotrope G is cooled by the heat exchanger 17 and then allowed to stand in the decanter 18. Since the water and the organic solvent are separated by standing, the upper layer liquid I mainly composed of the organic solvent is circulated to the azeotropic dehydration distillation column 15, and the lower layer liquid H mainly composed of the water is exposed to the outside. Discharge. The lower layer liquid H also contains impurities such as acetic acid having a boiling point lower than that of acrylic acid. The upper liquid I is supplied to the azeotropic dehydration distillation column 15.

  In the azeotropic dehydration distillation column 15, the operation temperature at which the distillation is performed is lowered by reducing the pressure in order to suppress the polymerization of acrylic acid. Furthermore, the polymerization inhibitory effect is enhanced by supplying the polymerization inhibitors K, K ′, and K ″. Here, the polymerization inhibitor is used as a solution of an organic solvent (corresponding to K in the figure) or acrylic. It is supplied by mixing with an acid aqueous solution (corresponding to K ′ in the figure) or by dissolving in crude acrylic acid J circulated through the azeotropic dehydration distillation column 15 (corresponding to K ″ in the figure). A method is mentioned. The recovered polymerization inhibitor K, K ′, K ″ used here can be the recovered polymerization inhibitor. In addition, the K ′ acrylic acid aqueous solution in the figure is an aqueous solution, but contains acrylic acid. Therefore, it may be mixed with a water-insoluble polymerization inhibitor.

  Although not shown, a light boiling separation column may be used for separation of acetic acid having a boiling point lower than that of acrylic acid and organic solvent contained in dehydrated acrylic acid. In this case, the liquid or gas separated from the top of the light boiling separation tower is circulated to the azeotropic dehydration distillation tower 15, and crude acrylic acid is obtained from the bottom of the light boiling separation tower.

  The steps up to here are the first separation step, and the obtained crude acrylic acid J is sent to the purification tower 21 serving as the first purification step. In the purification tower 21, while supplying the polymerization inhibitor L, the crude acrylic acid J is distilled under reduced pressure to obtain a purified acrylic acid gas M from the top of the tower. The purified acrylic acid gas M is cooled by the heat exchanger 24 to obtain purified acrylic acid N, and a part divided by the reflux drum 25 is circulated to the purification tower 21 to obtain the remaining purified acrylic acid N as a product. Here, before the cooling in the heat exchanger 24, the polymerization inhibitor L ′ is supplied, and the polymerization inhibitor L ″ is also supplied to the reflux liquid that circulates a part divided by the reflux drum 25. These polymerization inhibitors L , L ′ and L ″ are all water-insoluble polymerization inhibitors.

  On the other hand, a bottoms O is obtained from the bottom of the purification tower 21. In the bottoms O, heavy impurities such as acrylic acid dimer, furfural, benzaldehyde, maleic anhydride, acrylic acid oligomers and polymers, unrecovered acrylic acid, and the polymerization used so far Included with inhibitor. Some of them are cooled by the heat exchanger 22 and returned to the purification tower 21. The remainder of the bottoms O contains a polymerization inhibitor, and the bottoms O can be subjected to the method for recovering a polymerization inhibitor according to the present invention.

  In addition, instead of directly performing the polymerization inhibitor recovery method according to the present invention on the above-mentioned bottoms O, the bottoms O are once supplied to the thin film evaporator 23, Concentration may be performed. The light fraction P is returned to the purification tower 21, and the concentrated heavy fraction Q is supplied to the thermal decomposition recovery device 27. In the pyrolysis recovery device 27, the acrylic acid dimer contained in the heavy component Q is thermally decomposed to be recovered acrylic acid R, which is circulated to the purification tower 21 and recovered as purified acrylic acid N. The recovery residue S remaining after recovery by the pyrolysis recovery device 27 contains the polymerization inhibitor contained in the bottoms O, and the recovery inhibitor S recovers the polymerization inhibitor according to the present invention. The method can be done.

  As another procedure, even if the bottoms O is not directly passed through the thin film evaporator 23 but directly supplied to the pyrolysis recovery device 27, the recovered residue S obtained in the same manner is applied to the present invention. A method for recovering the polymerization inhibitor can be performed.

  The polymerization inhibitor recovered from the process of FIG. 1 can be used as the polymerization inhibitors K, K ′, and K ″ used in the azeotropic dehydration distillation column 15, and when a light boiling separation column is used. The light boiling separation tower may be used after being supplied.

  Next, FIG. 2 shows a case where the second separation step and the second purification step are performed, and these steps will be described. The second separation step is a collection step of collecting the acrylic acid gas with water in a collection tower to obtain an aqueous acrylic acid solution, and an acrylic solvent is prepared by adding an organic solvent to the aqueous acrylic acid solution in a solvent extraction tower. Solvent extraction step for obtaining an acrylic acid organic solution by extracting an acid, and a solvent separation step for obtaining crude acrylic acid as separated acrylic acid separated from the organic solvent by distillation under reduced pressure of the acrylic acid organic solution in a solvent separation tower including.

  First, the collection process which supplies acrylic acid containing gas AA obtained at the manufacturing process to the tower bottom of the collection tower 31 is performed. The contents performed in the collection tower 31 are the same as the processes performed in the collection tower 11 in the first separation step. That is, water AB is supplied from the upper part of the collection tower 31, and the acrylic acid-containing gas AA is cooled at the lower part of the collection tower 31, and the acrylic acid is absorbed by the water AB to obtain an aqueous acrylic acid solution. And Further, the tower bottom liquid AC is cooled by the heat exchanger 32 and a part thereof is circulated to the collection tower 31, and the remainder of the tower bottom liquid AC is sent to the next extraction tower 34. Further, among the other components contained in the acrylic acid-containing gas AA, those not absorbed by the water AB are discharged as exhaust gas AD from the top of the tower.

In the above collection tower 31, in order to prevent acrylic acid from polymerizing, the polymerization inhibitor AE,
AE 'is supplied. The polymerization inhibitors AE and AE ′ are supplied as an aqueous solution in accordance with the liquid composition in the collection tower 31. Therefore, the polymerization inhibitor used here needs to be water-soluble. In the supply, the water AB is supplied so as to be added to the liquid in the water AB supply line or the column bottom liquid AC circulation line.

  The above column bottom liquid AC is supplied to the top of the extraction column 34. A part of the organic solvent AJ separated by the solvent separation tower 36 described later is also supplied. A solvent extraction step is performed with the circulating organic solvent. Acrylic acid contained in the tower bottom liquid AC, which is an aqueous acrylic acid solution, is extracted into the supplied organic solvent, thereby obtaining a tower effluent AG, which is an organic solvent solution of acrylic acid. Further, from the bottom of the tower, water that was an acrylic acid aqueous solution is discharged as waste water AH in a state containing impurities.

  A column separation liquid AG, which is an organic solvent solution of acrylic acid, is supplied to the solvent separation column 36, and a solvent separation step of performing distillation under reduced pressure is performed. Separated organic solvent gas AI is obtained from the top of the column, and crude acrylic acid AL, which is separated acrylic acid separated from the organic solvent, is obtained from the bottom of the column. In the vacuum distillation, a polymerization inhibitor is supplied so that acrylic acid is not polymerized. Supplied together with the column effluent AG (corresponding to AK in the figure), supplied together with the circulated organic solvent AJ (corresponding to AK 'in the figure), or supplied together with the crude acrylic acid AL circulated at the bottom of the tower (Corresponding to AK "in the figure) and supplied to the solvent separation column 36. The recovered polymerization inhibitor is used for the polymerization inhibitors AK, AK ', AK" used here. be able to.

  The organic solvent gas AI obtained from the top of the solvent separation tower 36 is cooled by the heat exchanger 37 to form a liquid organic solvent AJ, and then partially returned to the solvent separation tower 36 through the reflux drum 38. The remainder is supplied to the extraction tower 34. On the other hand, the crude acrylic acid AL obtained from the bottom of the column is partially cooled by the heat exchanger 39 and circulated to the solvent separation column 36 together with the polymerization inhibitor AK ", and the rest is sent to the second purification step. Is the second separation step.

  In the crystallizer as the second purification step, a normal general crystallization process can be used, and a polymerization inhibitor is added to prevent polymerization of high-purity acrylic acid obtained in that step. . Accordingly, the method for recovering the polymerization inhibitor according to the present invention is performed from the residue obtained in the crystallization step.

  In the embodiment shown in FIG. 2, first, crude acrylic acid AL is supplied to the intermediate tank 41, and then supplied to a crystallizer main body 43 having a heating function and a cooling function for solidification and melting of acrylic acid. After stopping the supply of acrylic acid from the intermediate tank 41 to the crystallizer main body 43, a batch-type treatment method in which the crystallizer main body 43 performs crystallization of acrylic acid, and the crystallizer main body 43 and the intermediate tank 41 Any of the continuous processing methods in which the crystallization operation is performed in the crystallizer main body 43 while the crude acrylic acid is circulated between them is performed.

  In the batch type processing method, acrylic acid having a low purity that has not solidified inside the crystallizer main body 43 is first taken out. Next, the acrylic acid solidified inside the crystallizer main body 43 is melted by heating, whereby acrylic acid with an increased purity is taken out. Since the initial melt contains partially uncoagulated acrylic acid, the purity is lower than the melts thereafter. These are sent to a plurality of different intermediate tanks 45 according to the purity of acrylic acid.

  By performing the crystallization operation a plurality of times, acrylic acid with the highest purity is obtained as purified acrylic acid AQ. The acrylic acid having the lowest purity is sent to the thin film evaporator 42. Acrylic acid having intermediate purity is treated together with acrylic acid having the same degree of purity in a plurality of crystallization operations. For example, in a five-stage crystallization operation, the remaining liquid from the last crystallization operation is added to the stock solution of the second to fourth-stage crystallization operations.

  In the case of the continuous processing method, the residual liquid from the crystallization operation remains in the intermediate tank 41. Therefore, before performing the next crystallization operation, the liquid in the intermediate tank 41 is any of the intermediate tanks 45. It will be transferred to. Subsequently, the operation after the operation of heating and melting the liquid solidified inside the crystallizer main body 43 is the same as the batch type processing. In addition, for the purpose of simplifying complicated recycling, there are cases where other than acrylic acid having the lowest purity is sent to the thin film evaporator 42.

  Since the crystallized acrylic acid contains almost no polymerization inhibitor, the polymerization risk during the melting operation is high, and it is necessary to add a polymerization inhibitor. Specifically, the polymerization inhibitor is added to the supply liquid AV to the intermediate tank 41, the supply liquid AV ′ to the crystallization apparatus main body 43, the extraction liquid AV ″, the intermediate tanks 41 and 45, and the like. The remaining crystallization residue AP contains the polymerization inhibitor added so far, and the recovery method of the polymerization inhibitor according to the present invention can be performed on the crystallization residue AP.

  Further, the recovery method is not performed directly on the crystallization residue AP, but it is supplied to the thin film evaporator 42 as shown in FIG. 2 to remove the fraction AR and return it to the solvent separation column 36. After the heavy component is concentrated, the heavy component AS is supplied to the acrylic acid dimer thermal decomposition and recovery device 47. Here, the acrylic acid dimer is thermally decomposed into acrylic acid to obtain the recovered acrylic acid AT and the recovered residue AU. The recovery residue AU also contains a polymerization inhibitor, and the recovery method of the polymerization inhibitor according to the present invention can be performed on the recovery residue AU. Although not shown, the recovered acrylic acid AT is circulated to the extraction tower 34 and the solvent separation tower 36.

  As another procedure, even if the crystallization residue AP is not supplied to the thin film evaporator 42 but directly supplied to the thermal decomposition recovery device 47, the present invention is applied to the recovery residue AU similarly obtained. A method for recovering the polymerization inhibitor can be performed.

  The polymerization inhibitor recovered from the process of FIG. 2 can be used as the polymerization inhibitors AK, AK ′, and AK ″ used in the solvent separation tower 36.

  In addition, although said FIG.1 and FIG.2 is described with the combination of a 1st isolation | separation process and a 1st refinement | purification process, and the combination of a 2nd isolation | separation process and a 2nd refinement | purification process, The present invention can also be applied in combination with the purification step, or in combination with the second separation step and the first purification step. In the above description, the first purification step and the second purification step are different in the supply destination of the fraction P and the fraction AR obtained by the thin film evaporators 23 and 42. When performing at least one of the separation steps, these fractions may be returned to the purification tower 21 or returned to the solvent separation tower 36. Conversely, in the case of a combination of the first separation step and the second purification step, the fraction obtained in the thin film evaporator may be returned to the azeotropic dehydration distillation column 15.

<Process of FIG. 1>
Acrylic acid gas obtained by oxidizing propylene was supplied to the first separation step and the first purification step shown in FIG. In the first separation step and the first purification step, hydroquinone, p-methoxyphenol, and phenothiazine were used as polymerization inhibitors. In addition, mainly hydroquinone was supplied to the steps after the collection tower, phenothiazine was supplied to the steps after the dehydration distillation column, and p-methoxyphenol was supplied to the purification tower, and used in other steps as needed.

  The resulting bottoms was concentrated with a thin film evaporator, and the recovery of the polymerization inhibitor was performed on the recovery residue after recovering acrylic acid from the acrylic acid dimer with a thermal decomposition recovery device.

  The composition of the recovery residue was analyzed by gas chromatography (manufactured by Shimadzu Corporation: GC-8A). As a result, acrylic acid: 15.1% by mass, acetic acid: 0.05% by mass, acrylic acid dimer: 15.6% by mass, acrylic acid trimer: 7.3% by mass, total of maleic acid and maleic anhydride: 4.8% by mass, benzoic acid: 1.1% by mass, furfural: 0.1% by mass, Benzaldehyde: 0.3% by mass, other trace impurities and high boiling point components that cannot be detected by gas chromatography, and as a polymerization inhibitor, hydroquinone: 1.3% by mass, p-methoxyphenol: 1.8% by mass %, Phenothiazine: 1.2% by mass. The viscosity of this recovered residue was 1.6 Pa seconds at 20 ° C. and 453 mPa seconds at 40 ° C. when measured with a vibration viscometer (Yamaichi Electronics Co., Ltd .: VM-10A).

(Example 1, extraction with toluene: water = 1: 1)
100 ml of the collected residue is placed in a separatory funnel, 50 ml of water and 50 ml of toluene (manufactured by Wako Pure Chemical Industries, Ltd.) as a water-insoluble solvent are added, and the mixture is vigorously shaken up and down for 10 seconds 5 times, then 10 minutes Extraction was carried out by standing. In addition, toluene is substantially insoluble in water. The liquid temperature during operation was maintained between 35 and 40 ° C. After standing, the liquid phase was separated into three layers, and suspended matter was observed at the interface between the middle aqueous layer and the lowermost tar layer, but floating at the interface between the uppermost toluene layer and the aqueous layer. The thing was not confirmed. The toluene layer had a transparent and clean appearance. The toluene layer was taken out and analyzed for components by gas chromatography. Table 1 shows the recovery rate of each polymerization inhibitor and impurities as a percentage of the mass of each compound obtained from the toluene layer with respect to the mass of each compound contained in the original recovery residue. Phenothiazine, a polymerization inhibitor with low polarity, could be obtained with the highest recovery.

  In Table 1, “TOL” is toluene, “MIBK” is methyl isobutyl ketone, “PZ” is phenothiazine, “HQ” is hydroquinone, “Ma (H)” is maleic acid, and “Frfrl”. Represents furfural and "PhCHO" represents benzaldehyde. “Ma / PZ” is a value representing the ratio of the maleic acid recovery rate to the phenothiazine recovery rate, and indicates how much the maleic acid which is an impurity and the phenothiazine which is a polymerization inhibitor can be separated. The smaller this value, the better the recovery of phenothiazine.

  Further, when the filtered toluene layer was subjected to natural filtration using a filter paper processed with a fluororesin having a mesh size of 1 μm, the liquid quickly passed through the filter paper. The filter paper showed a brownish color, but no residue on the filter paper was confirmed.

(Extraction in Example 2, toluene: water = 5: 2)
In Example 1, water used for extraction was reduced to 20 ml, and the same operation was performed. No aqueous layer was formed and the liquid phase separated into two layers. When the upper toluene layer was similarly analyzed by gas chromatography, the recovery rate was as shown in Table 1. Phenothiazine, a polymerization inhibitor with low polarity, could be obtained with the highest recovery.

  When natural filtration was attempted for the toluene layer using the same filter paper as in Example 1, the liquid quickly passed through the filter paper, but the filter paper showed a brownish coloration throughout and was further derived from suspended matter on the interface. A small spot-like stain was observed.

(Comparative Example 1, extraction with methyl isobutyl ketone: water = 1: 1)
In Example 1, the same extraction operation was performed except that methyl isobutyl ketone (manufactured by Sigma-Aldrich Japan Co., Ltd.) was used as a water-insoluble solvent instead of toluene. The results are shown in Table 1. The solubility of methyl isobutyl ketone in water was 1.8% by weight at 20 ° C. Although phenothiazine, which is a polymerization inhibitor with low polarity, could be obtained with the highest recovery rate, the amount of maleic acid relative to phenothiazine was increased, and the rate of recovered polymerization inhibitor was lowered.

  When natural filtration was similarly performed on the methyl isobutyl ketone layer, the liquid quickly passed through the filter paper. The filter paper showed a brownish color, but no residue was observed on the filter paper.

(Comparative Example 2, extraction only with toluene)
In Example 1, the same operation was performed except that the same amount of toluene was added instead of water and extraction was performed with 100 ml of toluene. Although the liquid phase was separated into two layers, the toluene layer became opaque and a layer with a different color was formed below, but the interface with the top was unclear. Moreover, no suspended matter was confirmed. Table 1 shows the results obtained by leaving only the upper toluene layer while leaving the vicinity of the unclear interface, and analyzing by gas chromatography in the same manner. The recovery rate of phenothiazine decreased and the recovery rate of benzaldehyde exceeded.

  When the toluene layer was taken out and natural filtration was attempted in the same manner as in Example 1, clogging was confirmed. The obtained filtrate was transparent. When the outlet side of the filtration device was depressurized and suction filtration was performed, the liquid quickly passed through the filter paper, and an adhesive thin film was confirmed on the filter paper. Therefore, it was found that the separation from the tar-like substance in the residue was insufficient.

(Comparative Example 3, extraction with only methyl isobutyl ketone)
In Comparative Example 2, the same operation was performed using 100 ml of methyl isobutyl ketone instead of toluene. The liquid phase was entirely opaque without causing layer separation, and a part of the liquid phase could not be taken out, and the polymerization inhibitor could not be recovered.

(Comparative Example 4, extraction with water only)
In Comparative Example 2, the same operation was performed using 100 ml of water instead of toluene. Although the liquid phase was separated into two layers, suspended matter was confirmed in the aqueous layer, and a layer with a different color was formed below, but the interface with the upper part was unclear. Table 1 shows the results of gas chromatographic analysis in which only the upper aqueous layer was removed, leaving the vicinity of the unclear interface. As a result, almost no phenothiazine could be recovered.

<Step of FIG. 2>
Acrylic acid gas obtained by oxidizing propylene was supplied to the second separation step and the second purification step shown in FIG. 2 to obtain a crystallization residue. In the second separation step and the second purification step, hydroquinone, p-methoxyphenol, and phenothiazine were used as polymerization inhibitors. In addition, mainly hydroquinone was supplied to the steps after the collection tower, phenothiazine was supplied to the steps after the solvent separation tower, and p-methoxyphenol was supplied to the crystallization step.

  The resulting crystallization residue is concentrated in the same manner as the bottoms of Example 1 above, and the polymerization inhibitor is recovered with respect to the recovery residue obtained after recovering acrylic acid with a thermal decomposition recovery device. Went.

  The composition of the recovered residue was analyzed by the same gas chromatography as in Example 1. As a result, acrylic acid: 16.2% by mass, acetic acid: 0.2% by mass, acrylic acid dimer: 12.0% by mass, acrylic Acid trimer: 4.2% by mass, total of maleic acid and maleic anhydride: 1.4% by mass, benzoic acid: 0.2% by mass, benzaldehyde: 0.2% by mass, other trace impurities and gas chromatography It contained a high-boiling component that could not be detected by chromatography, and also contained hydroquinone: 0.6% by mass, p-methoxyphenol: 1.4% by mass, and phenothiazine: 1.8% by mass as polymerization inhibitors. The viscosity of the recovered residue was 908 mPa seconds at 20 ° C. and 326 mPa seconds at 40 ° C. as measured with a vibration viscometer (manufactured by Yamaichi Electronics Co., Ltd .: VM-10A).

(Example 3)
From this recovered residue, an attempt was made to recover the polymerization inhibitor by the same procedure as in Example 1. The results are shown in Table 1. Phenothiazine, which is a polymerization inhibitor having a low polarity, can be separated into three liquid layers, and can be obtained with the highest recovery rate.

Process diagram when performing the first separation step and the first purification step Process diagram when performing the second separation step and the second purification step

Explanation of symbols

11, 31 Collection tower 12, 16, 17, 22, 24, 32, 37, 39 Heat exchanger 15 Azeotropic dehydration distillation tower 18 Decanter 21 Purification tower 23, 42 Thin film evaporator 25, 38 Reflux drum 41, 45 Intermediate tanks 27, 47 Pyrolysis recovery device 34 Extraction tower 36 Solvent separation tower 43 Crystallizer body A, AA Acrylic acid-containing gas B, AB Water C, AC Tower bottom liquid D, AD Exhaust gas E, E ', K, K ', K ", L, L', L", AE, AE ', AK, AK', AK ", AV, AV ', AV" Polymerization inhibitor AJ Organic solvent G Azeotrope H Lower layer liquid I Upper layer Liquid J, AL Crude acrylic acid N, AQ Purified acrylic acid O Canned liquid Q, AS Heavy fraction R, AT Recovered acrylic acid S, AU Recovered residue AG Tower liquid AH Wastewater

Claims (7)

A production process for obtaining a gas of acrylic acid by gas phase oxidation of propylene or acrolein;
A collection step of collecting the acrylic acid gas with water in a collection tower to obtain an acrylic acid aqueous solution, and an azeotropic distillation of the organic solvent supplied in the azeotropic dehydration distillation tower and the water of the acrylic acid aqueous solution. A first separation step including an azeotropic dehydration step of obtaining crude acrylic acid as dehydrated acrylic acid by dehydrating the aqueous acrylic acid solution,
Alternatively, a collection step of collecting the acrylic acid gas with water in a collection tower to obtain an aqueous acrylic acid solution, and extracting an acrylic acid by adding an organic solvent to the aqueous acrylic acid solution in a solvent extraction tower Second separation including a solvent extraction step for obtaining an acrylic acid organic solution at a solvent, and a solvent separation step for obtaining crude acrylic acid as separated acrylic acid separated from the organic solvent by distillation under reduced pressure of the acrylic acid organic solution in a solvent separation tower Any one of the separation steps of the steps;
A first purification step comprising a vacuum distillation step of obtaining a purified acrylic acid by distillation of the crude acrylic acid under reduced pressure in a purification tower, and obtaining a bottoms from the tower bottom;
Or any one of the purification steps of the second purification step, including a crystallization step of obtaining a separated crystallization residue while obtaining purified acrylic acid by performing a crystallization operation on the crude acrylic acid in a crystallization apparatus; In the production of acrylic acid having
In order to prevent polymerization of the acrylic acid, a polymerization inhibitor added at least at one of the first separation step, the second separation step, the first purification step, and the second purification step is added to the can after the purification step. In the method of recovering from the effluent or the crystallization residue,
Using a water-insoluble polymerization inhibitor as the polymerization inhibitor,
Solubility in water and water at 20 ° C. for the recovered residue after pyrolyzing the effluent, the crystallization residue, or the acrylic acid dimer containing them by heating to recover acrylic acid A method for recovering a polymerization inhibitor, wherein the polymerization inhibitor is recovered by performing extraction with a water-insoluble solvent that is 0.1 wt% or less of the solvent, and separating the layer of the water-insoluble solvent. The bottom product, the crystallization析残residue, or to the recovered residue is extracted with 0.1 volumes of water by volume, the method of recovering the polymerization inhibitor according to claim 1. The polymerization inhibitor according to claim 1 or 2 , wherein the extraction liquid, the crystallization residue, or the recovery residue is extracted using the water-insoluble solvent having a volume ratio of 1 to 3 to 4 times. Recovery method. The polymerization prevention according to any one of claims 1 to 3 , wherein the amount of the water-insoluble solvent used for the extraction is 0.5 to 20 times by volume with respect to the amount of water used for the extraction. Agent recovery method. The method for recovering a polymerization inhibitor according to any one of claims 1 to 4 , wherein the water-insoluble solvent is a hydrocarbon that is liquid at room temperature and has a boiling point lower than that of acrylic acid. The method for recovering a polymerization inhibitor according to any one of claims 1 to 5 , wherein the polymerization inhibitor is phenothiazine. A method for producing acrylic acid, wherein the polymerization inhibitor used in the separation step or the purification step is collected by the method for collecting a polymerization inhibitor according to any one of claims 1 to 6 .

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