JPS6135977B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for recovering acrylic acid. More specifically, the present invention relates to a method for advantageously recovering acrylic acid from acrylic acid dimers or trimers produced in an acrylic acid manufacturing process.
More specifically, the present invention relates to an acrylic acid dimer that is produced in the apparatus and concentrated in the bottom liquid of an acrylic acid rectification column in the final step when producing acrylic acid by a catalytic gas phase oxidation reaction of propylene or acrolein. heating or higher polymers,
The present invention provides a method for carrying out a decomposition reaction and efficiently recovering acrylic acid as acrylic acid. When producing acrylic acid by catalytic gas phase oxidation of propylene or acrolein, the steps that constitute the process are generally a step of collecting the acrylic acid produced by the oxidation reaction as an aqueous solution, and a step of collecting acrylic acid from this collected liquid in an appropriate manner. The acrylic acid is extracted with an extraction solvent such as ethyl acetate or ethyl acrylate, and then passes through the solvent separation and acetic acid separation steps, and then the rectification step, and thus undergoes the acrylic acid manufacturing process where it is commercialized as purified acrylic acid. is normal. Acrylic acid is highly polymerizable, and it is well known that during the above steps, polymers are formed within the equipment, especially in each distillation column, often interfering with the operation of the equipment and resulting in a decrease in the yield of the product acrylic acid. There is. Therefore, as a conventional measure to prevent the polymerization of acrylic acid, a method of adding a polymerization inhibitor to these various steps, particularly a distillation column, has been widely adopted. Hydroquinone is well known as a typical polymerization inhibitor, and one or more other effective polymerization inhibitors such as molecular oxygen, phenols, amines, quinones, and inorganic salts are blended with it. It is common to use In addition to the above-mentioned polymers, acrylic acid also easily dimerizes and trimerizes. This multimerization reaction is considered to be a sequential reaction, and the production of dimer products, that is, dimers, is the main activity in the above steps. Furthermore, the formation of acrylic acid dimer due to dimerization of acrylic acid is significantly affected by temperature and residence time, and cannot be suppressed by adding the above-mentioned acrylic acid polymerization inhibitor. That is, the formation of acrylic acid dimer depends on the temperature and its residence time. Therefore,
It is said that even if efforts are made to lower the heating temperature or shorten the residence time in the usual acrylic acid purification process, especially in the distillation process, the production of 1 to 5% by weight of acrylic acid dimer is unavoidable. The acrylic acid dimer thus produced and the polymerization inhibitor added are sequentially concentrated in each step, and at the bottom of the rectification column in the final step, each is concentrated to 10 to 50 times its initial concentration. It is extremely disadvantageous economically to dispose of the tower bottom liquid containing such high concentrations of acrylic acid dimer and polymerization inhibitor as it is, and it is difficult to know how to use it, and especially how to decompose acrylic acid dimer into acrylic acid and recover it. Some suggestions have been made. For example, Japanese Patent Publication No. 45-19281 discloses that the residual liquid from the kettle is heat treated in the presence of a catalyst such as a secondary amine, tertiary amine or tertiary phosphine to decompose the polymer into acrylic acid monomers. U.S. Pat. No. 3,086,046 states that
A method is disclosed in which acrylic acid dimer-containing vapor obtained by evaporating the residual liquid in a thin film evaporator is introduced into a heated stainless steel tube, where it undergoes a decomposition reaction to recover acrylic acid. However, these methods have a maximum recovery of 60-80% by weight of acrylic acid on a laboratory scale.
However, the recovery rate is reduced due to the generation of a large amount of highly viscous residue, making it an extremely inefficient recovery method. Also, JP-A-51
Publication No. 91208 discloses that the bottom liquid is heated in an evaporator to
A method has been proposed in which the polymerization inhibitor in the distillate obtained by evaporating 80% is reused in the process.
It cannot be said that a sufficient effect has been obtained in terms of the recovery rate of the polymerization inhibitor. This is because, in the residue discharged from the evaporator, a considerable amount of polymerization inhibitors including hydroquinone, dimers and trimers of acrylic acid remained, and these had to be discarded. The present inventors have studied a method for simultaneously and almost completely recovering acrylic acid dimer concentrated in the bottom liquid of an acrylic acid rectification column and hydroquinone as a polymerization inhibitor, and have arrived at the present invention. That is, the present invention provides a process for collecting acrylic acid as an acrylic acid aqueous solution from a reaction product gas containing acrylic acid obtained by catalytic gas phase oxidation of propylene or acrolein, and separating and purifying acrylic acid from the aqueous solution. , the acrylic acid rectification column bottom liquid is introduced into the first acrylic acid dimer decomposition evaporator, and the acrylic acid is heated under conditions of a heating temperature in the range of 120 to 220°C and an operating pressure in the range of 20 to 500 mmHg. The acrylic acid dimer in the bottom liquid is decomposed and acrylic acid is evaporated as the main fraction, while the evaporation residue is introduced into a second acrylic acid dimer decomposition and evaporation device, and the heating temperature is 120-220°C.
℃ range and operating pressure in the range of 20 to 500 mmHg, the acrylic acid dimer in the evaporation residue is decomposed, and the acrylic acid dimer and hydroquinone used as a polymerization inhibitor in this process are mainly evaporated. On the other hand,
The evaporation residue is led to an acrylic acid dimer decomposition storage tank,
The composition of the liquid at the outlet of the storage tank is 1 to 25% by weight of acrylic acid, 9 to 49% by weight of acrylic acid dimer (however, the total of acrylic acid and acrylic acid dimer is 50% by weight or less), and 50 to 90% of other high boiling point substances. The evaporated content is circulated to the acrylic acid purification process, and the liquid at the outlet of the storage tank is extracted in the aqueous phase to dissolve and extract the acrylic acid content and acrylic acid dimer content. Disclosed is a method for recovering acrylic acid, characterized in that the aqueous solution is circulated through the acrylic acid collection process and/or the acrylic acid separation and purification process. The method of the present invention is applied to conventional acrylic acid purification processes. The acrylic acid purification process consists of a process of obtaining purified acrylic acid through an acrylic acid collection process, an extraction process, a solvent separation process, an acetic acid separation process, and an acrylic acid rectification process as described above. Depending on the acrylic acid collection conditions in the acrylic acid collection step, the step may simply constitute an acrylic acid separation step (such as a simple distillation operation or a method using an azeotrope agent). Acrylic acid dimer is already generated in the process of collecting the acrylic acid produced by the oxidation reaction as an aqueous solution, and is also extracted together with acrylic acid in the subsequent extraction operation, increases with each distillation operation, and continues to be used in the final refinement process. It is fed to a distillation column and eventually accumulates at the bottom of the column. Therefore, the bottom liquid of the acrylic acid rectification column contains acrylic acid, acrylic acid dimer, polymerization inhibitor, and other high-boiling substances. The composition of this bottom liquid can vary greatly depending on the operating conditions of each step, but it includes 20 to 65% by weight of acrylic acid,
It is present in the range of 30-60% by weight of acrylic acid dimer, 5-15% by weight of polymerization inhibitors such as hydroquinone, and 20% by weight or less as other high-boiling substances. According to the method of the present invention, as the evaporation operation in the first step, the acrylic acid dimer in the bottom liquid is decomposed into acrylic acid using an acrylic acid dimer decomposition evaporator, and the acrylic acid dimer is evaporated together with the acrylic acid contained in the bottom liquid to produce a distillate. Collected as On the other hand, the bottom extraction liquid is further subjected to a second step of evaporation operation, in which the acrylic acid dimer is not only decomposed and recovered as acrylic acid, but also acrylic acid dimer is distilled out. Surprisingly, this dimer acts as an entrainer, making it possible to simultaneously recover hydroquinone, which is concentrated in large quantities as a polymerization inhibitor. Furthermore, in the third step, undecomposed acrylic acid dimer and hydroquinone are countercurrently extracted from the liquid drawn from the bottom of the decomposition evaporator using an aqueous solvent in an extraction column, and recovered as an aqueous solution, while other products such as polymers are extracted. High boiling point substances are separated from the system as waste oil. According to the novel method of the present invention, the acrylic acid liquid recovered as the distillate of the first step acrylic acid dimer decomposition and evaporation device and the second step acrylic acid liquid recovered as the distillate of the acrylic acid dimer decomposition and evaporation device. The acrylic acid, hydroquinone and acrylic acid dimer solutions are recycled and recovered in the solvent separation or acetic acid separation process of the acrylic acid purification process. Furthermore, the aqueous solution of acrylic acid dimer, acrylic acid, and hydroquinone recovered as the extract from the third step extraction tower is recovered in a solvent separation step via an acrylic acid collection step and an acrylic acid extraction step. As mentioned above, the acrylic acid dimer generated in the system is completely decomposed and recovered into acrylic acid in the first step of the present invention, and the unrecovered portion is evaporated and recovered together with hydroquinone in the second step of evaporation operation, and acrylic acid and The hydroquinone is recycled into the purification system as an acrylic acid dimer solution. Furthermore, the unrecovered portions of acrylic acid, acrylic acid dimer and hydroquinone are recovered in the extraction column of the third step, and are recycled and reused in the above steps as an aqueous solution of acrylic acid, acrylic acid dimer and hydroquinone. Therefore, by adopting the process of the present invention, 80 to 98% of the generated acrylic acid dimer can be recovered as acrylic acid, and at the same time, 70 to 95% of the added polymerization inhibitor hydroquinone can be recovered and reused. It has become possible to greatly increase the purification yield and to significantly reduce the cost of the expensive polymerization inhibitor hydroquinone. The details of the present invention will be explained below with reference to FIG. Acrylic acid produced by catalytic gas phase oxidation of propylene or acrolein is collected in the form of an aqueous solution, extracted with an appropriate solvent such as ethyl acetate or ethyl acrylate, and then transferred to a solvent separation column and an acetic acid separation column (each of these steps (not shown) to supply line 1
It is then sent to rectification column 2. Acrylic acid is distilled from the top of the rectification column 2, passed through a condenser 3, and stored in a storage tank 4. A portion of it is returned to the rectification column as reflux, but the remainder is taken out as a product. At the bottom of the rectification column 2, acrylic acid containing other high-boiling substances such as concentrated acrylic acid dimers, polymerization inhibitors such as hydroquinone, and polymers is accumulated in addition to acrylic acid. This column bottom liquid is supplied to the first step acrylic acid dimer decomposition and evaporation device 7 by a pump 5 through a line 6, and the evaporated content is liquefied in a condenser 9 and stored in a storage tank 10. Collected as a solution. The liquid extracted from the acrylic acid dimer decomposition and evaporation equipment in the first step is stored in a storage tank 12 via line 8, and a portion is recycled to the decomposition and evaporation equipment via pump 13 and line 14, and the other part is sent through line 15. After that, the second
The acrylic acid dimer in the process is sent to the hydroquinone evaporator 16. The evaporated portion is liquefied in a condenser 18 and stored in a storage tank 19, and is recovered and reused in the above-mentioned processes as an acrylic acid dimer solution of acrylic acid and hydroquinone. On the other hand, the bottom draw liquid is stored in an acrylic acid dimer decomposition storage tank 21 via a line 17, a part is recycled to the device 16 via a pump 22 line 23, and the other part is sent via a line 24 to an extraction column 25. Sent. These decomposition evaporators 7 and 16
may be of any type, but preferably a thin film evaporator is used. This thin-film evaporator is equipped with a jacket and has a structure that allows the heat transfer surface to be heated by water vapor, etc., and it also has a structure that allows it to heat the heat transfer surface with water vapor. In order to carry out separation, it is necessary to withstand operation under reduced pressure. The operating conditions such as heating temperature and pressure of these first and second evaporators are usually heating temperature 120~220℃, operating pressure 20~
It is in the 500mmHg range. The composition of the rectification column bottom liquid supplied to the decomposition evaporator 7 through the line 6 can vary greatly depending on the operating conditions of various steps as described above, but it usually contains 20 to 65% by weight of acrylic acid and 30 to 60% by weight of acrylic acid dimer. It contains 5-15% by weight of the polymerization inhibitor hydroquinone and 20% by weight or less of other high-boiling substances mainly consisting of higher-order polymers of acrylic acid. The feed liquid is decomposed by 40 to 80% by weight, evaporated, condensed in a condenser 9, and then distilled into a storage tank 10. The distillate composition is 90-98% by weight of acrylic acid, 1-5% by weight of acrylic acid dimer, and 0.1-1% by weight of polymerization inhibitor, while the liquid composition entering the storage tank 12 via line 8 is 1% by weight of acrylic acid. ~15% by weight,
The content is 10 to 60% by weight of acrylic acid dimer, 5 to 30% by weight of hydroquinone as a polymerization inhibitor, and 40 to 60% by weight of other high-boiling substances such as polymers. The feed liquid is evaporated by 40 to 80% by weight in the evaporator 16, liquefied in the condenser 18, and stored in a storage tank 19. The distillate composition is 1 to 20% by weight of acrylic acid and acrylic acid dimer.
20-80% by weight, polymerization inhibitor hydroquinone 1-20
% by weight, while the liquid composition entering the acrylic acid dimer decomposition storage tank 21 via line 17 is acrylic acid
0.1 to 5% by weight, 5 to 30% by weight of acrylic acid dimer, 1 to 30% by weight of hydroquinone as a polymerization inhibitor, and 50 to 90% by weight of other high-boiling substances such as polymers.
In order to decompose the acrylic acid dimer in the storage tanks 12 and 21, their structure is such that they can be heated and maintained at 120 to 220°C, preferably 150 to 200°C, and their capacity is 1 to 8°C depending on the liquid residence time. It may be for a period of time, preferably from 3 to 8 hours. It is presumed that there is an equilibrium relationship in the decomposition reaction of acrylic acid dimer to acrylic acid. In reality, trimers and tetramers of acrylic acid are included among other high-boiling substances, and their equilibrium relationship exists in a multicomponent system, and their speed and equilibrium concentration are not well understood. However, the present inventors found that at temperatures below 120°C, no decomposition reaction occurs and conversely dimerization occurs to produce acrylic acid dimers, but at temperatures above 120°C, acrylic acid dimers (and trimers and tetramers) It was discovered that it decomposes. Therefore, the heating temperature of the decomposition and evaporation devices 7 and 16 and storage tanks 12 and 21 must be 120° C. or higher. The decomposition and evaporation device 7 evaporates acrylic acid but does not evaporate substances with a higher boiling point than acrylic acid dimer, and the decomposition and evaporation device 16 evaporates acrylic acid dimer but evaporates other high-boiling substances such as polymers. Adjust the temperature and pressure within the above operating range to prevent this from occurring. Therefore, in the device 7, acrylic acid dimer can be recovered with a high decomposition rate and as a highly purified product of acrylic acid. In addition, the device 16
In this case, the decomposition rate of acrylic acid dimer to acrylic acid may be reduced, and instead, the extraction of acrylic acid dimer acts as an entrainer of hydroquinone, which has the effect of making it possible to recover hydroquinone as an acrylic acid dimer solution. . In the decomposition evaporator 7, acrylic acid is directly distilled out of the system after the decomposition reaction, so the equilibrium is shifted to the acrylic acid side, the decomposition reaction progresses well, and the residence time is
It's small, about 10 minutes, which is good. However, since the acrylic acid decomposed in the storage tanks 12 and 21 is not distilled out of the system, a residence time of 1 to 8 hours is required as described above. Although the required residence time varies depending on the heating temperature and the liquid composition, a residence time of 3 to 8 hours is required under the temperature and composition conditions in the storage tanks 12 and 21 described above. A residence time of 10 hours or more is economically wasteful as there is no merit in decomposition considering the equilibrium relationship. The decomposition and evaporation devices 7 and 16 and storage tanks 12 and 21
If the decomposition of acrylic acid dimer is insufficient or the distillation of acrylic acid and dimer is insufficient and the total amount of acrylic acid and acrylic acid dimer in the composition of the liquid extracted from the acrylic acid dimer decomposition storage tank 21 is 50% by weight or more, the third The property of forming a homogeneous phase with water in the extraction tower during the process remains, making extraction impossible. In addition, if acrylic acid dimer decomposition progresses too much and acrylic acid and acrylic acid dimer are recovered from the system to reduce the total of acrylic acid and acrylic acid dimer in the liquid composition to 10% by weight or less, the extracted liquid becomes solid at room temperature and is extracted. Therefore, the operating conditions are selected in consideration of the feed liquid composition of each of the decomposition and evaporation devices 7 and 16, and the total amount of acrylic acid and acrylic acid dimer in the liquid composition extracted from the storage tank 21 is 50% by weight or less, usually 10~40
Must be adjusted to % by weight. The acrylic acid, dimer, and hydroquinone solutions of the distillate from the decomposition and evaporation units 7 and 16 are recycled to the solvent separation and acetic acid separation steps, and are recovered and reused as a polymerization inhibitor. The liquid extracted from the decomposition evaporator 16 from the storage tank 21 is passed through the pump 22 and line 24 to the upper part of the extraction tower 25, while water is supplied from the bottom of the tower through line 26 and extracted countercurrently, and the extracted liquid is passed through line 27 to the acrylic acid,
The acrylic acid dimer and hydroquinone aqueous solution are recovered, and as they are, they are recycled and recovered as a polymerization inhibitor to the solvent separation step via the step of collecting the acrylic acid as the aqueous solution and the extraction step. On the other hand, the raffinate of other high-boiling substances such as polymers is taken out of the system via line 28 and treated as waste oil. Although any type of extraction column 25 may be used, a rotating disk type is advantageously used because the raffinate has a high viscosity and a strong adhesive force. As extraction conditions, the temperature is 5 to 90°C, preferably
The temperature is 20 to 60℃, and the extractant ratio is the bottom extraction liquid: water =
1:1 to 10 (W/W), preferably 1:3 to 6, and the recovery rate of acrylic acid dimer and hydroquinone in the extraction column is 70 to 95% by weight and 50 to 90% by weight
It is. Even if the extractant ratio is increased to 1:6 or more, no increase in the recovery rate of acrylic acid dimer or hydroquinone can be expected; rather, the acrylic acid dimer or hydroquinone will be recovered as a dilute aqueous solution, which is economically disadvantageous. . Furthermore, if the extractant ratio is 1:3 or less and the amount of water is reduced, the recovery rate of acrylic acid dimer will drop sharply, and the recovery rate of hydroquinone will also inevitably drop. If the ratio is less than 1:1, a homogeneous phase is formed and extraction becomes impossible. As mentioned above, it is essential to add polymerization inhibitors such as hydroquinone in the acrylic acid purification process.
Although it is impossible to suppress the production of acrylic acid dimer, according to the present invention, the added hydroquinone can be reduced by 50% by recovery in three steps: the acrylic acid dimer decomposition and evaporation equipment 7, 16, the decomposition tanks 12, 21, and the extraction tower. ~90% is recovered, and 70-95% by weight of the produced acrylic acid dimer can be decomposed and recovered as acrylic acid. At the same time, more than 90% by weight of acrylic acid in the bottom liquid of the rectifying column can be recovered, so the purification yield (amount) can be greatly increased. In addition, in the method of the present invention, by changing the operating conditions in the second step and not using an extraction column in the third step, the acrylic acid dimer solution of hydroquinone is substantially recovered instead of an aqueous solution containing acrylic acid dimer and hydroquinone. It is also possible. However, in this case, a decrease in the recovery rate of acrylic acid dimer and hydroquinone is unavoidable. That is, specifically, by increasing the heating temperature or lowering the operating pressure in the second step, acrylic acid and acrylic acid dimer are evaporated as much as possible without evaporating other high-boiling substances such as polymers. At that time, hydroquinone will also be recovered. Naturally, it is unavoidable that the total amount of acrylic acid and acrylic acid dimer in the composition of the liquid extracted from the bottom of the device 16 is less than 10% by weight, which cannot be used for extraction operations and must be appropriately disposed of as waste oil. The recovery effect will be explained below using a specific example. Example: An acrylic acid production device with an annual production capacity of 25,000 〓 using propylene as a raw material using the catalytic gas phase oxidation method has a capacity of 1 m ³
Two thin-film evaporator-type acrylic acid dimer decomposition and evaporation devices with a heat transfer area of 3.0 m ² and an attached decomposition tank were installed in series, and a rotating disk-type extraction column with an inner diameter of 400 mm and 20 stages was also installed. The two decomposition and evaporation devices (front device and rear device)
The system was heated with steam at 180°C, and the pressure inside the system was reduced to 300 mmHg in the front unit and 80 mmHg in the rear unit. Additionally, each decomposition tank was heated to 180°C. When the bottom liquid of the rectification column of the acrylic acid production apparatus was supplied to the decomposition and evaporation apparatus under these operating conditions, the amounts of evaporation in the front and rear apparatuses were 63.6 and 63.0% by weight, respectively. The compositions of the feed liquid, distillate, and bottom extraction liquid of the front and rear units are as shown in the table, and the decomposition rate of acrylic acid dimer to acrylic acid and recovery of hydroquinone in the decomposition and evaporation unit and two decomposition tanks each are as follows. Rates totaled 55.2 each (97% vs. generated dimer)
and 53.1% by weight. Subsequently, the liquid extracted from the apparatus after decomposition and evaporation is supplied to the upper part of the extraction column, and on the other hand, 5 times (W/W) of water to the supplied liquid is supplied from the lower part of the extraction column at room temperature, and acrylic acid is added countercurrently. Dimer and hydroquinone were extracted, and 5 and 2% by weight of acrylic acid dimer and hydroquinone were added from the top of the extraction column, respectively.
It was recovered as an aqueous solution containing The recovery rate was 90% for acrylic acid dimer and 80% for hydroquinone. The raffinate liquid, which was about 50% of the feed liquid, was extracted from the bottom of the extraction column and treated as waste oil. The acrylic acid solution and hydroquinone acrylic acid dimer solution recovered from the front and rear devices are used in the solvent separation step and acetic acid separation step of the acrylic acid production device, and the acrylic acid dimer and hydroquinone aqueous solution recovered from the extraction tower are used in the acrylic acid collection step and After the acrylic acid extraction process, it was recycled as a polymerization inhibitor in the solvent separation process. Even after more than 6 months had elapsed, the acrylic acid production equipment was able to operate smoothly without any problems such as concentration or adhesion of high-boiling substances such as polymers in the above-mentioned processes. Additionally, during this period, the cost of hydroquinone was reduced to 1/5 compared to a process not employing the process of the present invention, and the purification yield of acrylic acid increased by about 3%. Example The extraction was carried out using the same equipment and the same conditions as in the example, but the extraction conditions (extraction agent ratio) were doubled (W/W) relative to the feed liquid.
When extracted with water, the recoveries of acrylic acid dimer and hydroquinone in the extraction column were 60.1% by weight and 70.3% by weight. Example The experiment was carried out using the same equipment and the same conditions as in the example, except that the operating conditions of the acrylic acid dimer decomposition and evaporation apparatus (previous unit) were a heating temperature of 180°C and a pressure of 200 mmHg (0.26 atm). was distilled out, and the distillation amount in the rear unit was 70%. The compositions of the feed liquid, distillate liquid and bottom extraction liquid at this time are as shown in the table, and the decomposition rate of acrylic acid dimer in the decomposition evaporator is 45
%, and the hydroquinone recovery rate was 58.5%.

【table】

[Table] Example The same equipment was used under the same conditions as in the example, but when the acrylic acid dimer decomposition and evaporation equipment (rear) was operated at a heating temperature of 180°C and a pressure of 30 mmHg, 80% of the feed liquid was distilled. . The compositions of the feed liquid, distillate and bottom extraction liquid at this time are as shown in the table. The liquid extracted from the bottom solidified at room temperature, making it impossible to extract and recover acrylic acid dimer and hydroquinone.
The decomposition rate of acrylic acid dimer in the decomposition and evaporation apparatus (two units) was 51.8% (the amount generated within the system was 95%), and the recovery rate of hydroquinone was 70.8%.

[Table] Comparative example The bottom liquid of the rectification column of the acrylic acid production apparatus was directly supplied to the extraction column without passing through the decomposition apparatus under the same conditions as in the example. However, it was not possible to recover acrylic acid dimer and hydroquinone. Comparative Example The same equipment was used under the same conditions as in the example, but when the extraction condition (extractant ratio) was 0.6 times (W/W) water to the feed liquid, a homogeneous phase formed in the column, making extraction impossible. Acrylic acid dimer and hydroquinone could not be recovered in the extraction step.

[Brief explanation of the drawing]

FIG. 1 shows an example of the process for recovering acrylic acid dimer and hydroquinone from the bottom liquid of an acrylic acid rectification column as defined by the present invention.

Claims (1)

[Claims] 1 In the process of collecting acrylic acid as an acrylic acid aqueous solution from a reaction product gas containing acrylic acid obtained by catalytic gas phase oxidation of propylene or acrolein, and separating and purifying acrylic acid from the aqueous solution, acrylic acid is purified. The bottom liquid of the distillation column is
The acrylic acid dimer in the bottom liquid is introduced into a first acrylic acid dimer decomposition and evaporation device, and the acrylic acid dimer in the bottom liquid is decomposed under conditions where the heating temperature is in the range of 120 to 220°C and the operating pressure is in the range of 20 to 500 mmHg. At the same time, acrylic acid is evaporated as the main fraction, while the evaporation residue is introduced into a second acrylic acid dimer decomposition evaporator, and the heating temperature is in the range of 120 to 220°C and the operating pressure is in the range of 20 to 500 mmHg. Below, the acrylic acid dimer in the evaporation residue is decomposed and the acrylic acid dimer and hydroquinone used as a polymerization inhibitor in this process are mainly evaporated, while the evaporation residue is led to an acrylic acid dimer decomposition storage tank, The composition of the liquid at the outlet of the storage tank is 1 to 25% by weight of acrylic acid, 9 to 49% by weight of acrylic acid dimer (however, the total of acrylic acid and acrylic acid dimer is 50% by weight or less), and 50 to 90% of other high boiling point substances. The evaporated content is circulated to the acrylic acid purification process, and the liquid at the outlet of the storage tank is extracted in the aqueous phase to dissolve and extract the acrylic acid content and acrylic acid dimer content. A method for recovering acrylic acid, characterized in that the aqueous solution is circulated through the acrylic acid collection process and/or the acrylic acid separation and purification process.