JPH10237012A - Recovery of acrylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain acrylic acid at a high yield from aqueous solution of acrylic acid obtained by contact of the gaseous product by catalytic gas-phase oxidation of propylene and/or acrolein, by introducing the solution into a distillation tower for azeotropic separation in the presence of a specific azeotropic solvent. SOLUTION: This recovery of acrylic acid comprises collecting, as an aqueous acylic acid solution, an acrylic acid-containing gas obtained by catalytic gas- phase oxidation of propylene and/or acrolein by its contact with water, introducing the aqueous acrylic acid solution into a distillation tower for azeotropic separation to distillate the solution in the presence of an azeotropic solvent [a solvent mixture of solvent A (ethyl acrylate, methyl methacrylate, vinyl acrylate, etc.) and solvent B (toluene, heptane, 1-heptene, methylcyclohexane, etc.) at a mixing ratio of solvent A to solvent B in the range of (10:90)-(75:25)].

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering acrylic acid. In more detail, the present invention contacts and collects an acrylic acid-containing gas obtained by subjecting propylene and / or acrolein to catalytic gas-phase oxidation using a molecular oxygen-containing gas with water and collects the obtained acrylic acid aqueous solution in a specific manner. The present invention relates to a method for efficiently separating and recovering acrylic acid by distillation in the presence of an azeotropic solvent.

[0002]

2. Description of the Related Art The production of acrylic acid by catalytic vapor phase oxidation of propylene and / or acrolein is widely practiced on an industrial scale. This method generally comprises an oxidation step of subjecting propylene and / or acrolein to catalytic gas phase oxidation using molecular oxygen, and a collecting step of bringing an acrylic acid-containing gas obtained by the catalytic gas phase oxidation into contact with water to collect the gas. And a collecting step of separating and collecting acrylic acid from the aqueous acrylic acid solution obtained in the collecting step.

Incidentally, the acrylic acid-containing gas contains by-products such as acetic acid, formic acid, acetaldehyde, and formaldehyde, and among these, the amount of acetic acid is relatively large. For this reason, in order to obtain high-purity acrylic acid, it is necessary to remove acetic acid. However, if acetic acid in acrylic acid is to be removed by distillation, the distillation temperature increases (the boiling point of acetic acid is 118.1 ° C.). ), The polymerization of acrylic acid is likely to occur. In addition, acrylic acid and acetic acid have a small specific volatility, and thus have a problem that it is difficult to simply separate them by distillation.

[0004] Therefore, in order to separate and recover high-purity acrylic acid from an aqueous solution of acrylic acid, that is, to separate acrylic acid from acetic acid and water, and recover high-purity acrylic acid containing substantially no acetic acid and water. For this purpose, an aqueous acrylic acid solution is led to an azeotropic separation tower, where distillation is performed in the presence of an azeotropic solvent, and azeotropic separation is performed using a ternary azeotropic distillation composed of acetic acid-water-solvent. The azeotropic dehydration method of distilling an azeotropic mixture of acetic acid, water and a solvent from the top of the column and obtaining acrylic acid from the bottom of the column is employed. Since impurities other than acetic acid have low boiling points, they can be easily removed without using azeotropic distillation.

For example, Japanese Patent Publication No. Sho 63-10691 describes a method of performing azeotropic distillation by adding a hydrocarbon such as toluene as an azeotropic solvent.

Japanese Patent Publication Nos. 46-4691 and 46-4691
-18967 discloses that ethyl acetate is used as an azeotropic solvent,
A method using butyl acetate, dibutyl ether, ethyl acetate, hexane, heptane, ethyl methacrylate, propyl acrylate and the like is described.

Japanese Patent Application Laid-Open No. Hei 5-246941 discloses that at least one obtained from diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl tert-butyl ketone and n-propyl acetate as an azeotropic solvent, toluene and heptane And a method of using in combination with at least one selected from methylcyclohexane.

[0008]

However, according to the study of the present inventors, it has been found that the conventional method has the following various problems.

That is, in the case of the method described in JP-B-63-10691, the separation of acetic acid is not sufficient.
Two distillation operations in the azeotropic separation tower and the acetic acid separation tower for separating acetic acid are required. Further, since a poorly water-soluble azeotropic solvent such as toluene is present near the bottom of the azeotropic separation tower, the liquid in the azeotropic separation tower is separated into two phases, an oil phase and an aqueous phase, and the polymerization is carried out. The inhibitor does not dissolve uniformly, and acrylic acid distributed at a high concentration in the aqueous phase is liable to undergo polymerization.

Japanese Patent Publication Nos. 46-4691 and 46-4691
In the case of using the azeotropic solvent described in JP-18967-A, even if an attempt is made to recover high-purity acrylic acid by a single distillation operation, the separation of any one of acetic acid, water and the solvent is insufficient, and a high boiling point Even if impurities are removed, a high-purity acrylic acid product cannot be obtained.

In the case of a method for recovering high-purity acrylic acid by one distillation operation as described in Japanese Patent Application Laid-Open No. Hei 5-246,941, acrylic acid is recovered in the azeotropic separation column, especially from the middle to the upper stage. Due to the high concentration, the polymerizability of acrylic acid in the tower may not be negligible. As described above, when the polymerizability of acrylic acid in the azeotropic separation tower is high, a polymer of acrylic acid is deposited in the tower, and it becomes difficult to operate the azeotropic separation tower continuously for a long time.

[0012] Thus, the present invention solves the above-mentioned problems of the prior art, and when acrylic acid aqueous solution is introduced into an azeotropic separation column and dehydrated by azeotropic distillation, polymerization of acrylic acid in the azeotropic separation column is carried out. Acetic acid is distilled off from the top of the azeotropic separation column as an azeotrope composed of acetic acid-water-solvent, whereby high-purity acrylic acid containing substantially no acetic acid is removed from the bottom of the column. It is intended to provide a method for efficiently recovering from the same.

[0013]

According to the study of the present inventors, ethyl acrylate, methyl methacrylate, vinyl acrylate, allyl acetate, isopropenyl acetate, vinyl propionate and methyl crotonate are used as azeotropic solvents. And at least one selected from toluene, heptane, 1-heptene, methylcyclohexane, cycloheptene, cycloheptadiene, cycloheptatriene, 2,4-dimethyl-1,3-pentadiene, methylcyclohexene and methylenecyclohexane It has been found that the above problem can be solved by using a mixed solvent containing, preferably, at least one selected from ethyl acrylate and methyl methacrylate and at least one selected from toluene and heptane. This has led to the completion of the present invention based on the findings.

That is, the present invention provides propylene and / or
Alternatively, an acrylic acid-containing gas obtained by subjecting acrolein to catalytic gas phase oxidation is brought into contact with water to be collected as an aqueous acrylic acid solution, and this aqueous acrylic acid solution is introduced into an azeotropic separation tower,
When the acrylic acid is separated and recovered by distillation in the presence of an azeotropic solvent, the following azeotropic solvents, solvent A and solvent B, are used.
A method for recovering acrylic acid, characterized by using a mixed solvent containing:

Solvent A: at least one selected from ethyl acrylate, methyl methacrylate, vinyl acrylate, allyl acetate, isopropenyl acetate, vinyl propionate and methyl crotonate.

Solvent B: at least one selected from toluene, heptane, 1-heptene, methylcyclohexane, cycloheptene, cycloheptadiene, cycloheptatriene, 2,4-dimethyl-1,3-pentadiene, methylcyclohexene and methylenecyclohexane.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The feature of the present invention is that an acrylic acid aqueous solution is introduced into an azeotropic separation column, where it is distilled in the presence of an azeotropic solvent to separate and recover acrylic acid. Is that a mixed solvent containing the solvent A and the solvent B is used.

By using the mixed solvent of the present invention,
The polymerization of acrylic acid in the azeotropic separation tower can be effectively prevented.

Further, by using the mixed solvent of the present invention, acetic acid, water and the solvent are distilled off from the top of the azeotropic separation column by one distillation operation in the azeotropic separation column, and High-purity acrylic acid containing substantially no acetic acid can be recovered. Therefore, according to the method of the present invention, it is possible to separate and recover high-purity acrylic acid containing substantially no acetic acid by distillation of an aqueous solution of acrylic acid in an azeotropic separation tower, and while preventing polymerization of acrylic acid. Can be.

In the mixed solvent of the present invention, crude acrylic acid containing acetic acid is recovered from the bottom of the azeotropic separation tower, and is introduced into an acetic acid separation tower to separate acetic acid, thereby substantially containing acetic acid. The present invention can also be applied to the case of producing high-purity acrylic acid by performing two distillation operations, ie, obtaining high-purity acrylic acid. Also in this case, the polymerization of acrylic acid in the azeotropic separation tower can be effectively prevented. Therefore,
According to the method of the present invention, high-purity substantially free of acetic acid is obtained by two distillations in an azeotropic separation column and an acetic acid separation column, and while preventing polymerization of acrylic acid in the azeotropic separation column. Acrylic acid can be separated and recovered.

The method of recovering acrylic acid by one distillation operation or the method of recovering acrylic acid by two distillation operations is determined by the necessity of acetic acid recovery, the wastewater treatment method, and the like. It can be appropriately determined in consideration of such factors.

Among the solvents A, ethyl acrylate and methyl methacrylate are preferably used. Solvent B
Among them, toluene and heptane are preferably used.

Accordingly, preferred examples of the mixed solvent of the present invention include ethyl acrylate + toluene, ethyl acrylate + heptane, methyl methacrylate + toluene, methyl methacrylate + heptane, ethyl acrylate + methyl methacrylate + toluene, Ethyl acrylate + methyl methacrylate + heptane and ethyl acrylate + methyl methacrylate + toluene + heptane can be mentioned.

As the above-mentioned ethyl acrylate, methyl methacrylate, toluene and heptane, those commercially available can be used as they are.

The mixing ratio of the solvent A and the solvent B is 10:90 to 75:25, preferably 20:80 to 5 by weight.
0:50. If the proportion of the solvent A is too large, the acetic acid concentration at the bottom of the azeotropic separation tower becomes high, and it becomes difficult to obtain high-purity acrylic acid containing substantially no acetic acid by a single distillation from an aqueous acrylic acid solution. . On the other hand, if the proportion of the solvent B is too large, the liquid separates into an oil phase and an aqueous phase in the azeotropic separation tower, and it becomes impossible to prevent the polymerization of acrylic acid.

FIG. 1 is a process diagram of a method for recovering high-purity acrylic acid by one distillation operation. Hereinafter, the present invention will be described with reference to FIG.

An acrylic acid-containing gas obtained by subjecting propylene and / or acrolein to catalytic gas-phase oxidation using molecular oxygen is introduced from a line 1 into an acrylic acid collecting tower 101, and brought into contact with water introduced from a line 2. Thus, an aqueous acrylic acid solution containing by-products such as acrylic acid and acetic acid is obtained from the line 4. As the water supplied from the line 2 to the acrylic acid collecting tower 101, water may be supplied from the line 13 and used. However, an aqueous acetic acid solution discharged from the bottom of the solvent recovery tower 103 is used as described later. Is preferred. Acrylic acid aqueous solution is directly introduced into the azeotropic distillation column 102,
If necessary, the acrolein dissolved in the aqueous acrylic acid solution may be introduced into an acrolein stripping tower (not shown), and then introduced into the azeotropic distillation tower 102. In this case, it is preferable to collect the released acrolein and circulate it to the reaction system.

In the azeotropic separation column 102, an acrylic acid aqueous solution is supplied from a line 4 and an azeotropic solvent is supplied from a line 5, and distillation is performed. An azeotropic mixture comprising acetic acid, water and an azeotropic solvent is distilled from the top of the column. And acrylic acid substantially free of acetic acid is obtained from the bottom of the column.

The composition of the aqueous acrylic acid solution supplied to the azeotropic separation tower 102 varies depending on the amount of water supplied from the line 2 to the acrylic acid collecting tower 101 and other operating conditions. Under the production conditions of 50 to 80% by weight of acrylic acid, 2 to 5% by weight of acetic acid and 20 to 4% of water
Those having a range of 0% by weight (total 100% by weight) are generally used.

The azeotropic mixture substantially consisting of acetic acid, water and the azeotropic solvent distilled from the top of the azeotropic separation column 102 is introduced into the storage tank 20, where the organic phase mainly consisting of the azeotropic solvent, Separate into an aqueous phase consisting of acetic acid and water.
The organic phase is circulated through line 5 to azeotropic separation column 102.
On the other hand, the aqueous phase is introduced into the solvent recovery tower 103 via the line 8 and distilled, and the azeotropic solvent is distilled off from the top of the solvent recovery tower, returned to the storage tank 20 via the line 9, and returned from the bottom of the tower. An aqueous acetic acid solution consisting essentially of acetic acid and water is withdrawn through line 14 and discharged out of the system. The aqueous acetic acid solution can be effectively utilized by being circulated from the line 10 to the acrylic acid collecting tower 101 and used as collected water to be brought into contact with the acrylic acid-containing gas from the line 1.

The acrylic acid extracted from the bottom of the azeotropic separation column 102 is a high-purity acrylic acid substantially free of acetic acid, sent to the esterification step via the line 15, and used as it is as a raw material for producing an acrylic ester. be able to.

In order to obtain a higher-purity acrylic acid product, acrylic acid extracted from the bottom of the azeotropic separation tower 102 may be introduced into the high-boiling matter separation tower 104 via the line 7 and distilled. . Line 1 from the bottom of the high boiling point separation tower
2, high-boiling substances such as polymerization inhibitors are extracted, and a high-purity acrylic acid product is obtained from the top of the column via line 11. The operation in each of the above steps can be performed under conditions generally used for this type of process. For example, in the case of the azeotropic separation column 102, high-purity acrylic acid having an acetic acid concentration of 0.05% by weight or less can be obtained by operating under the following conditions (at the time of steady operation).

Operating pressure: 100 to 200 mmHg, tower top temperature: 45 to 55 ° C., acrylic acid aqueous solution supply section (aqueous solution supply stage) temperature: 70 to
90 ° C., tower bottom temperature: 100 to 110 ° C., reflux ratio (total moles of reflux liquid per unit time / total moles of distillate per unit time): 1.1 to 1.6 In order to prevent the polymerization of acrylic acid in the separation tower, a polymerization inhibitor is usually added. In the present invention, it is preferable to add these generally used polymerization inhibitors.

The method for obtaining high-purity acrylic acid by one distillation operation does not require an acetic acid separation column, which simplifies the acrylic acid production process and significantly reduces the production cost of acrylic acid. There are advantages.

Next, the method of obtaining high-purity acrylic acid by two distillation operations is described in the step diagram of obtaining high-purity acrylic acid by one distillation operation shown in FIG. Except for providing an acetic acid separation column for separating acetic acid from crude acrylic acid containing acetic acid extracted from the bottom, the process can be basically performed according to the same steps as in FIG.

That is, the crude acrylic acid extracted from the bottom of the azeotropic separation tower 102 is introduced into the acetic acid separation tower, where acetic acid is separated, and high-purity acrylic acid containing substantially no acetic acid is recovered. This high-purity acrylic acid is sent to the esterification step and can be used as it is as a raw material for producing an acrylic ester. Of course, this high-purity acrylic acid can be further introduced into the high-boiling substance separation column 104 to separate the high-boiling substances to obtain a higher-purity acrylic acid product.

The operating conditions of the azeotropic separation column 102 for separating and recovering acrylic acid by the above-mentioned two distillation operations vary depending on the concentration of acetic acid in the crude acrylic acid extracted from the bottom of the column, but cannot be specified unconditionally. For example, it can be operated under the following conditions (at the time of steady operation).

Operating pressure: 100 to 200 mmHg, tower top temperature: 40 to 50 ° C., acrylic acid aqueous solution supply section (supply stage) temperature: 45 to 70
° C, tower bottom temperature: 95-105 ° C, reflux ratio (total number of moles of reflux liquid per unit time / total number of moles of distillate per unit time): 1.0-1.3 Distillation at the bottom gives crude acrylic acid with an acetic acid concentration of 1 to 9% by weight.

The above-mentioned crude acrylic acid is further introduced into an acetic acid separation tower, where acetic acid is separated to obtain high-purity acrylic acid. The distillation operation in the acetic acid separation column can be carried out by a conventional method under conditions generally used.

[0040]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1 Acrylic acid was recovered according to the process shown in FIG.

A mixed gas obtained by subjecting propylene to catalytic gas phase oxidation with a molecular oxygen-containing gas is supplied to an acrylic acid collecting tower 101.
And collected by contact with water. The collected aqueous solution of acrylic acid was led to an acrolein stripping tower (not shown) to emit acrolein, and 30% by weight of water and acetic acid 3.
An aqueous solution of acrylic acid containing 0% by weight was obtained. This aqueous solution of acrylic acid was introduced into an azeotropic separation column 102 equipped with a sieve tray having 50 stages and a 147 mm spacing between the stages, a distilling tube at the top of the column, a raw material supply tube at the center, and a bottom liquid extracting tube at the bottom of the column. The acrylic acid aqueous solution was distilled using a mixed solvent of methyl methacrylate and toluene (35:65 by weight) as an azeotropic solvent.

The amount of the polymerization inhibitor used was 15 parts by weight of copper dibutyldithiocarbamate based on the amount of vaporized acrylic acid vapor.
ppm, manganese acetate is 22.5 ppm, hydroquinone is 75 ppm, phenothiazine is 75 ppm,
Manganese acetate was supplied into the column in a form dissolved in the raw material from the raw material supply pipe, and the others were dissolved in the reflux liquid from the top of the column.
Further, 0.3% by volume of molecular oxygen with respect to the amount of vaporized acrylic acid was supplied to the bottom of the column. Here, the amount of evaporated vapor corresponds to the amount of heat added from the reboiler of the distillation column, and means the total amount of monomer vapor evaporated from the bottom of the column.

The operating conditions during the steady operation are as follows: the top temperature of the azeotropic separation column 102 is 50 ° C., the bottom temperature is 105 ° C., the top pressure is 160 mmHg, and the reflux ratio (total moles of reflux liquid per unit time / unit time) Total number of moles of distillate per unit) 1.32,
The supply amount of the aqueous acrylic acid solution from the line 4 was 9.3 liter / hour. The aqueous phase of line 8 contains 7.6% by weight of acetic acid and 0.6% by weight of acrylic acid, while the liquid withdrawn from the bottom of the azeotropic separation column 102 via line 15 is 96.9% by weight of acrylic acid, Acetic acid 0.03% by weight, other 3.10
% By weight, and the solvent was below the detection limit (1 ppm).

The aqueous phase obtained from the line 8 is supplied to the solvent recovery tower 1
The solvent is recovered from the top of the column via a line 9, while an aqueous acetic acid solution is removed from the bottom of the column via a line 14. The composition is 8.1% by weight of acetic acid, 0.64% by weight of acrylic acid, and the balance of water, which is recycled to the acrylic acid collecting tower 101 and brought into contact with the mixed gas after the catalytic gas phase oxidation. Used as

When the azeotropic separation column 102 was continuously operated for about 14 days under the above-mentioned conditions, a stable state was always obtained. After the operation was stopped, the inside of the distillation column was inspected. I was not able to admit.

Comparative Example 1 An azeotropic distillation operation of an aqueous acrylic acid solution was performed in the same manner as in Example 1 except that only methyl methacrylate was used as the azeotropic solvent and the reflux ratio was 1.24. .

The water phase in the line 8 during the steady operation contained 6.3% by weight of acetic acid and 4.5% by weight of acrylic acid, and the amount of acrylic acid was about 8 times as large as that of Example 1. On the other hand, the liquid withdrawn from the bottom of the azeotropic separation column 102 via the line 15 contains 96.2% by weight of acrylic acid, 0.3% by weight of acetic acid, and 3.5% by weight of acetic acid. Ten times more than one.

When the azeotropic separation column 102 was continuously operated under the above conditions, pressure loss in the column was recognized 5 days after the start of operation, and it was difficult to continue the operation. The operation was stopped, and a dismantling inspection was performed. As a result, generation of popcorn polymer was confirmed in the tower.

As described above, when only methyl methacrylate is used, it is impossible to prevent polymerization of acrylic acid in the azeotropic separation column 102 in order to recover high-purity acrylic acid in the column.

Comparative Example 2 The procedure of Example 1 was repeated, except that a mixed solvent of methyl isobutyl ketone and toluene (mixing weight ratio: 65:35) was used as the azeotropic solvent, and the reflux ratio was 1.42. The distillation operation of the acrylic acid aqueous solution was performed.

The water phase in line 8 during steady operation contains 6.9% by weight of acetic acid and 0.5% by weight of acrylic acid, while the liquid extracted from the bottom of azeotropic separation column 102 via line 15 is acrylic. It contained 96.9% by weight of acid, 0.08% by weight of acetic acid, 0.002% by weight of solvent and 3.02% by weight.

When the azeotropic separation column 102 was continuously operated for about 14 days under the above conditions, an almost stable state was obtained. However, after the operation was stopped, the distillation column was inspected for disassembly. The formation of popcorn polymer was observed.

Example 2 A distillation operation of an aqueous acrylic acid solution was carried out in the same manner as in Example 1 except that a mixed solvent of ethyl acrylate and toluene (mixing weight ratio: 35:65) was used as an azeotropic solvent. Was.

The aqueous phase of line 8 during steady operation contains 7.3% by weight of acetic acid and 0.5% by weight of acrylic acid, while the liquid withdrawn from the bottom of azeotropic separation column 102 via line 15 is acrylic. It contains 97.2% by weight of acid, 0.03% by weight of acetic acid, and 2.80% by weight of other components.
ppm) or less.

When the azeotropic separation column 102 was continuously operated for about 14 days under the above-mentioned conditions, a stable state was always obtained. After the operation was stopped, the inside of the distillation column was inspected. I was not able to admit.

Comparative Example 3 A distillation operation of an aqueous acrylic acid solution was carried out in the same manner as in Example 1, except that only ethyl acrylate was used as the azeotropic solvent, and the reflux ratio was 1.24.

The water phase in the line 8 during the steady operation contained 6.5% by weight of acetic acid and 4.9% by weight of acrylic acid, and the amount of acrylic acid was about 8 times as large as that of Example 1. On the other hand, the azeotropic separation column 1
The liquid extracted from the bottom of the column No. 02 via the line 15 is 96.5% by weight of acrylic acid, 0.3% by weight of acetic acid, and others.
It contained 2% by weight, and acetic acid was 10 times as large as that of Example 1. When the azeotropic separation column 102 was continuously operated under the above conditions, pressure loss in the column was recognized 6 days after the start of operation, and it was difficult to continue the operation. When the operation was stopped and the dismantling inspection was performed, generation of popcorn polymer was observed in the tower.

As described above, when only ethyl acrylate is used, it is impossible to prevent polymerization of acrylic acid in the azeotropic separation column 102 in order to recover high-purity acrylic acid in the column.

Example 3 The procedure of Example 1 was repeated, except that a mixed solvent of ethyl acrylate and heptane (35:65 by weight) was used as the azeotropic solvent, and the reflux ratio was 1.24. The distillation operation of the acrylic acid aqueous solution was performed.

The water phase of line 8 during steady operation contains 7.6% by weight of acetic acid and 0.5% by weight of acrylic acid, while the liquid withdrawn from the bottom of azeotropic separation column 102 via line 15 is acrylic. It contains 97.1% by weight of acid, 0.05% by weight of acetic acid and 2.85% by weight of other solvents.
ppm) or less.

When the azeotropic separation column 102 was continuously operated for about 14 days under the above conditions, a stable state was always obtained. After the operation was stopped, the inside of the distillation column was inspected. I was not able to admit.

Example 4 In Example 1, a mixed solvent of methyl methacrylate and heptane (mixing weight ratio 35:65) was used as an azeotropic solvent.
The distillation operation of the acrylic acid aqueous solution was performed in the same manner as in Example 1 except that the reflux ratio was 1.24.

The water phase in line 8 during steady operation contains 7.3% by weight of acetic acid and 0.6% by weight of acrylic acid, while the liquid withdrawn from the bottom of azeotropic separation column 102 via line 15 is acrylic. It contains 97.2% by weight of acid, 0.04% by weight of acetic acid, and 2.76% by weight of others.
ppm) or less.

When the azeotropic separation column 102 was operated continuously for about 14 days under the above conditions, a stable state was always obtained. After the operation was stopped, the inside of the distillation column was inspected, and no polymer was generated. I was not able to admit.

Example 5 A mixed gas obtained by subjecting propylene to catalytic gas-phase oxidation with a molecular oxygen-containing gas was introduced into an acrylic acid collecting tower, which was brought into contact with water, and the collected acrylic acid aqueous solution was introduced into an acrolein stripping tower. To disperse acrolein to obtain an aqueous acrylic acid solution containing 30% by weight of water and 3.0% by weight of acetic acid. Number of stages 50
An azeotropic separation column equipped with a sieve tray having a step and a step interval of 147 mm, a distillation pipe at the top of the tower, a raw material supply pipe at the center, and a bottom liquid withdrawal pipe at the bottom of the tower is used. As the azeotropic solvent, methyl methacrylate is used. Mixed solvent with toluene (mixed weight ratio 35:
65), the operation of distilling the aqueous acrylic acid solution was performed.

The amount of the polymerization inhibitor used was 15 parts by weight of copper dibutyldithiocarbamate based on the amount of vaporized acrylic acid vapor.
ppm and hydroquinone were 150 ppm, and both were supplied into the column in a form dissolved in the reflux liquid from the top of the column. Further, 0.3% by volume of molecular oxygen with respect to the amount of vaporized acrylic acid was supplied to the bottom of the column.

The operating conditions during steady-state operation were as follows: the top temperature of the azeotropic separation column was 47 ° C., the bottom temperature was 102 ° C., and the top pressure was 16 ° C.
0 mmHg, reflux ratio (total number of moles of reflux liquid per unit time / total number of moles of distillate per unit time) 1.17, and supply amount of acrylic acid aqueous solution was 10.7 liter / hour. The aqueous phase obtained from the top of this azeotropic separation column contains 1.5% by weight of acetic acid and 0.7% by weight of acrylic acid, while the liquid withdrawn from the bottom of the column is 95.1% by weight of acrylic acid, Acetic acid2.
2% by weight and 2.7% by weight.

When the azeotropic separation column was continuously operated for about 14 days under the above conditions, a stable state was always obtained. After the operation was stopped, the inside of the distillation column was inspected. I couldn't.

Comparative Example 4 An azeotropic distillation operation of an aqueous acrylic acid solution was carried out in the same manner as in Example 5, except that only toluene was used as the azeotropic solvent and the reflux ratio was 1.20. The aqueous phase obtained from the top of the azeotropic separation column during steady operation contains 4.2% by weight of acetic acid and 0.4% by weight of acrylic acid, and the liquid withdrawn from the bottom of the column is 94.1% by weight of acrylic acid. It contained 1.8% by weight of acetic acid and 4.1% by weight.

When the azeotropic separation column was continuously operated under the above conditions, pressure loss in the column was recognized four days after the start of operation, and it was difficult to continue the operation. When the operation was stopped and the disassembly inspection was performed, the formation of a viscous polymer in the tower was observed.

Comparative Example 5 The same procedure as in Example 1 was carried out except that a mixed solvent of ethyl methacrylate and toluene (mixing weight ratio: 50:50) was used as the azeotropic solvent and the reflux ratio was 1.01. The azeotropic distillation operation of the aqueous acrylic acid solution was carried out.

In the steady state, the aqueous phase of line 8 contained 7.0% by weight of acetic acid and 1.3% by weight of acrylic acid, and the amount of acrylic acid was twice that of Example 1. On the other hand, the azeotropic separation column 10
26.5% by weight of acrylic acid, 0.12% by weight of acetic acid, 0.02% by weight of solvent
It contained 0.5% by weight and 3.38% by weight. The amount of acetic acid was one digit higher than that of Example 1, and the amount of the solvent was 50 times or more that of Example 1.

When a mixed solvent of ethyl methacrylate having 6 carbon atoms and toluene was used, acrylic acid of sufficient quality could not be obtained by a single distillation operation. Comparative Example 6 An acrylic resin was prepared in the same manner as in Example 1 except that a mixed solvent of propyl acrylate and heptane (50:50 by weight) was used as the azeotropic solvent, and the reflux ratio was 1.08. An azeotropic distillation operation of the aqueous acid solution was performed.

In the steady state, the aqueous phase of line 8 contained 6.7% by weight of acetic acid and 1.5% by weight of acrylic acid, and the amount of acrylic acid was twice that of Example 1. On the other hand, the azeotropic separation column 10
The liquid withdrawn from the bottom of column 2 via line 15 was 96.3% by weight of acrylic acid, 0.15% by weight of acetic acid,
It contained 1% by weight and 3.54% by weight. The amount of acetic acid was one digit higher than that of Example 1, and the amount of solvent was 100 times or more that of Example 1.

When a mixed solvent of propyl acrylate having 6 carbon atoms and heptane was used, acrylic acid of sufficient quality could not be obtained by a single distillation operation.

[0077]

According to the present invention, the polymerization of acrylic acid in the azeotropic separation column can be effectively prevented by using a specific azeotropic solvent. For this reason, long-term continuous operation of the azeotropic separation column became possible.

Further, by using the azeotropic solvent of the present invention, an acrylic acid aqueous solution containing by-products such as acetic acid is distilled in an azeotropic separation column, and an azeotropic solvent comprising acetic acid, water and an azeotropic solvent is distilled from the top of the column. Distilling the mixture and separating and recovering high-purity acrylic acid substantially free of acetic acid from the bottom of the column.
By a single distillation operation, high-purity acrylic acid containing substantially no acetic acid can be obtained.

Further, an acrylic acid aqueous solution containing by-products such as acetic acid is distilled in an azeotropic separation column, and water and an azeotropic solvent or an azeotropic mixture composed of water, an azeotropic solvent and a part of acetic acid are distilled from the top of the column. Is separated from the bottom, crude acrylic acid containing residual acetic acid is separated from the bottom of the column, and this crude acrylic acid is led to an acetic acid separation tower, where acetic acid is separated, and high-purity acrylic acid substantially containing no acetic acid is separated. High-purity acrylic acid substantially free of acetic acid can be obtained by two distillation operations of recovering the acid.

In any of the above-described methods for recovering acrylic acid by one and two distillation operations, polymerization of acrylic acid in the azeotropic separation column can be effectively prevented.

[Brief description of the drawings]

FIG. 1 is a flowchart of one method of the present invention.

[Explanation of symbols]

 1 to 15 line 101 Acrylic acid collecting tower 102 Azeotropic separation tower 103 Solvent recovery tower 104 High boiling point separation tower 20 Storage tank

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takahiro Takeda 992, Nishioki, Okihama-shi, Aboshi-ku, Himeji-shi, Hyogo Nippon Shokuhin Co., Ltd. Nippon Shokubai

Claims (4)

[Claims] An acrylic acid-containing gas obtained by subjecting propylene and / or acrolein to catalytic gas phase oxidation is brought into contact with water to collect an aqueous acrylic acid solution, and the aqueous acrylic acid solution is introduced into an azeotropic separation column, A method for recovering acrylic acid, comprising using a mixed solvent containing the following solvent A and solvent B as the azeotropic solvent when separating and recovering acrylic acid by distillation in the presence of an azeotropic solvent. Solvent A: at least one selected from ethyl acrylate, methyl methacrylate, vinyl acrylate, allyl acetate, isopropenyl acetate, vinyl propionate and methyl crotonate. Solvent B: toluene, heptane, 1-heptene, methylcyclohexane, cycloheptene, cycloheptadiene,
At least one selected from cycloheptatriene, 2,4-dimethyl-1,3-pentadiene, methylcyclohexene and methylenecyclohexane. 2. The method according to claim 1, wherein the solvent A is at least one selected from ethyl acrylate and methyl methacrylate, and the solvent B is at least one selected from toluene and heptane. 3. The method according to claim 1, wherein the mixing ratio of the solvent A and the solvent B is 10:90 to 75:25 by weight. 4. Distilling acetic acid from the top of the azeotropic distillation column,
4. The method according to claim 1, wherein acrylic acid containing substantially no acetic acid is obtained from the bottom of the column.