JPH0710800B2 - Method for treating residual solution of methacrylic acid distillation still - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a method for treating a residual liquid of a distillation bottom of methacrylic acid. More specifically, it relates to a treatment method for effectively recovering methacrylic acid as methyl methacrylate from a distillation bottom liquid containing methacrylic acid as a main component.

[Problems to be Solved by Prior Art and Invention]

Methacrylic acid obtained by catalytically oxidizing isobutylene, tertiary butanol, methacrolein or isobutyraldehyde with molecular oxygen in the presence of water vapor in a one-step or two-step reaction is an ordinary purification means such as extraction and distillation. It can be a high-purity product. However, until methacrylic acid is finally obtained, light boiling point components such as aldehydes and ketones are removed from the aqueous methacrylic acid solution, the solvent is recovered after the extraction treatment, the acid having a lower boiling point than methacrylic acid is separated, and It is repeatedly exposed to heating conditions such as the step of separating the boiling point component. Therefore, the treatment of each step is carried out in the presence of a conventionally known polymerization inhibitor such as phenothiazine, benzophenothiazine, hydroquinone, hydroquinone monomethyl ether, methylene blue and molecular oxygen.
It is not possible to completely suppress the polymerization of methacrylic acid,
Polymers such as methacrylic acid and polymerization inhibitors are accumulated with each step, and the amount is not negligible in the final step. Therefore, in the bottom liquid after rectifying and distilling out methacrylic acid, a considerable amount of a polymer and a polymerization inhibitor are present in addition to the high-boiling point impurities generated by the reaction. If the distillate rate (distilled amount / supply amount) of methacrylic acid is too high, the purity of methacrylic acid will decrease and the liquid viscosity of the bottom liquid will increase, which will deteriorate the thermal efficiency of the reboiler and further the polymerization of methacrylic acid, etc. Will result in the promotion of Therefore, inevitably, the distillation rate of methacrylic acid will be suppressed below a certain range,
As a bottom liquid, a considerable amount of methacrylic acid is discharged together with a high boiling point component containing a polymer. Therefore, if the residual liquid in the can is discarded or incinerated, loss of methacrylic acid is caused, which leads to deterioration of the basic unit and increase in cost, and therefore recovery of methacrylic acid must be measured by some method.

Various methods have been proposed as a method for recovering methacrylic acid remaining in the final distillation bottom liquid in the process for producing methyl methacrylate by the acetone cyanohydrin method.

That is, the bottom liquid is cooled to crystallize methacrylic acid,
Method for obtaining high-purity methacrylic acid by centrifugation
42-1530), a method of treating the bottom liquid with a solution containing methanol and water or a solution containing methanol and water, precipitating and separating a polymerized product, and circulating the residual liquid in the esterification step (Japanese Patent Publication No. 47-35886). , And a method of steam distilling the bottom liquid in the presence of a nonvolatile inorganic acid (Japanese Patent Publication No. 46-35726).

However, in the case of methacrylic acid based on the above-mentioned oxidation reaction of hydrocarbons, which is the object of the present invention, the impurities produced are significantly different from those in the conventional method, and as a result, the composition of the final bottoms liquid is inevitably greatly different. come. Therefore, in the above crystallization method, many impurities are precipitated together with methacrylic acid, and even if it can be carried out, it requires recrystallization, which complicates the process and is not economical. Further, the method of adding water and methanol to precipitate and separate the polymer cannot be applied to the bottom liquid of the present invention because the polymer of methacrylic acid is soluble in methanol and cannot be separated from other impurities.
Further, mere steam distillation in the presence of an inorganic acid cannot be expected to distill methacrylic acid because methacrylic acid is the main component, unlike the case of the acetone cyanohydrin method in which methacrylic acid is esterified. As described above, the method effectively performed by the conventional method can hardly be adopted for the recovery of methacrylic acid by the oxidation reaction process.

On the other hand, as a general treatment method of the bottom liquid under the oxidation reaction process, there is a method of recovering methacrylic acid by re-evaporation of the bottom liquid in an evaporator or the like, or a method of recovering methacrylic acid by extraction with an organic solvent. Has been considered. However, the former has poor recovery and operational problems due to the polymerization of methacrylic acid, and the latter produces a large amount of slag due to the extraction solvent, which causes emulsification and flooding of the extraction column. None of them are effective methods.

[Means for solving problems]

The present inventors focused their attention on the above-mentioned problems, and as a result of diligent study on a method for effectively recovering methacrylic acid from a bottom liquid containing a polymer and a high-boiling component after distilling and separating methacrylic acid, the present invention The present invention has been completed by finding that the method for recovering methyl methacrylate as a residual solution containing a peculiar impurity related to (3) is excellent.

That is, the present invention relates to isobutylene, tertiary butanol,
Catalytic oxidation of methacrolein or isobutyraldehyde with a gas containing molecular oxygen in the presence of steam,
The methacrylic acid aqueous solution obtained by cooling this oxidation product is used as a bottom liquid obtained by distilling and separating methacrylic acid from crude methacrylic acid by extracting methacrylic acid with a solvent, recovering the solvent, and removing light boiling components. In recovering the residual methacrylic acid, the molar ratio of methanol is 1 to 5 times that of methacrylic acid and the amount of sulfuric acid is 2% by weight or more of the bottom liquid, and the bottom liquid is steam-distilled while esterifying,
Another object of the present invention is to provide a method for treating a residual liquid of a distillation bottom of methacrylic acid, which comprises distilling and recovering it as methyl methacrylate.

[Specific Description of the Invention]

The bottoms solution targeted by the method of the present invention is obtained by catalytically oxidizing isobutylene, tertiary butanol, methacrolein or isobutyraldehyde with a gas containing molecular oxygen in the presence of steam, and cooling the oxidation product. The methacrylic acid aqueous solution thus obtained is a bottom liquid obtained by distilling and separating methacrylic acid from crude methacrylic acid, in which the extraction of methacrylic acid with a solvent, recovery of the solvent, removal of light-boiling components, etc. have been carried out. . The composition of the bottom liquid is usually 60 to 95 parts by weight of methacrylic acid, 3 to 30 parts by weight of a high-boiling substance, and 1 to 10 parts by weight of a polymer. The high-boiling components in the bottom liquid are maleic acid, citraconic acid, benzoic acid, toluic acid, acids such as trimellitic acid, aldehydes such as benzaldehyde and tolualdehyde, and phenothiazine, hydroquinone, and hydroquinone monomethyl added as a polymerization inhibitor. Examples thereof include ethers, and the polymer is a polymer of methacrylic acid or methacrolein.

The method of the present invention can be applied to batch treatment of a small amount of bottom product, but in general, the bottom product discharged in a methacrylic acid production process which is industrially carried out on a large scale,
It is particularly excellent as a method for continuous treatment.

In the obtained reaction product, methacrylic acid and high boiling impurities such as maleic acid, citraconic acid, benzoic acid, toluic acid and trimellitic acid are not reacted in addition to the respective ester formed by the esterification reaction with methanol. Acids, methanol, and reaction product water coexist. Since the esterification reaction of acid is an equilibrium reaction, it is not possible to esterify most of methacrylic acid, which is an active ingredient, to methyl methacrylate by a simple esterification reaction, but steam distillation is performed during the esterification reaction. By doing so, only methyl methacrylate produced is distilled and recovered together with methanol and water. Therefore, since almost no methyl methacrylate remains in the system, the equilibrium reaction preferentially proceeds to the target side and the recovery rate of methacrylic acid as methyl methacrylate is improved. At this time, other acids which are originally high boiling components are partially esterified, but their vapor pressure is lower than that of methyl methacrylate, and only a slight amount is distilled out under the same conditions. Therefore, the equilibrium state in the reaction system remains almost unchanged, and methanol is effectively consumed in the desired reaction. By performing steam distillation while esterifying in this manner, methacrylic acid can be recovered as methyl methacrylate without distilling out the above-mentioned impurity components which were present in the bottom liquid and were difficult to separate from methacrylic acid. it can. Further, since methyl methacrylate is distilled out of the system at the same time as esterification, the residence time in the reaction system in which sulfuric acid is present is short, and the polymerization loss can be minimized.

The methanol used in the present invention is usually 1 to 5 times, preferably 1.2 to 2 times the molar amount of methacrylic acid in the bottom liquid, though it depends on the bottom liquid composition. A high esterification conversion can be achieved in a ratio. In this esterification reaction, the amount of methanol was 1
If it is less than twice the mole, a high esterification conversion cannot be expected, and if it exceeds 5 moles, the recovered methyl methacrylate is diluted, which is liable to cause a disadvantage.

The amount of sulfuric acid used in the present invention must be 2% by weight or more based on the bottoms of the bottom. When the amount of sulfuric acid used is 2% by weight or less, the amount of high-boiling components accumulated in the sulfuric acid layer increases, and the sulfuric acid concentration decreases, so the esterification reaction rate slows down, and solid deposition may occur in the sulfuric acid layer. is there.

In an industrial continuous operation, a predetermined amount of a sulfuric acid catalyst is continuously supplied and retained in the reaction system for 30 minutes to 2 hours, and then continuously extracted as waste sulfuric acid together with non-volatile impurities.

In the present invention, a large amount of water or steam necessary for performing steam distillation is supplied as reaction product water, but the composition of the bottom liquid, the amount of sulfuric acid used, the amount and rate of distillation of methyl methacrylate, the amount of waste sulfuric acid. It is appropriately selected depending on the composition and the like.
Generally, 20% by weight or more and 50% by weight of methacrylic acid in the bottom liquid
Up to weight% is used.

The method of the present invention can be carried out under any of normal pressure, reduced pressure and increased pressure, but it is usually carried out under normal pressure or slightly increased pressure at the boiling temperature of the reaction system.

The steam distillation distillate contains an excess amount of methanol and a small amount of methacrylic acid in addition to methyl methacrylate and water, but hardly distills out substances that are difficult to separate from methyl methacrylate.

This distillate is supplied to the product stream from an esterification column packed with a strongly acidic cation exchange resin as a catalyst in a suitable place in the process of producing methyl methacrylate from methacrylic acid, for example, in a process using isobutylene as a raw material. By doing so, methyl methacrylate, methanol and water can be separated and purified.

〔The invention's effect〕

According to the treatment method of the present invention, methacrylic acid, which was conventionally discarded or incinerated together with the high boiling point component and the polymer as a bottom liquid, can be easily and efficiently recovered as methyl methacrylate.

〔Example〕

The present invention will be specifically described below with reference to examples. The bottoms used in the examples are bottoms obtained by precision distillation of crude methacrylic acid obtained by the catalytic oxidation reaction of isobutylene, and the composition thereof is as follows.

% By weight methacrylic acid 94.34 benzoic acid 0.83 paratoluic acid 0.34 other volatile components 0.20 polymerization inhibitor 2.98 non-volatile components 1.31 Example 1 6 kg / hr of raw material containing 1.3 times the molar amount of methanol mixed with methacrylic acid in the bottom liquid Then, 2% by weight of sulfuric acid with respect to the bottom liquid was supplied to the reaction tank, and at the same time, 1500 g / hr of water was supplied. Reactive distillation was carried out at a reaction temperature of 106.5 ° C., during which the composition of the distillate was traced, and when the reactive distillation reached a stable state, the distillate was sampled for 1 hour, and the results are shown in Table 1. .

Table 1 shows that 93% of the methacrylic acid in the bottom liquid could be recovered as methyl methacrylate substantially free of harmful impurities.

Example 2 A 100-source reactive distillation apparatus equipped with a raw material supply port at the bottom of the reaction tank, a catalyst outlet at the middle side wall (40 parts of the volume of the expansion liquid), and a distillation column at the top was used for the methacrylic acid in the bottom liquid of the bottom. A 1.4-fold molar amount of methanol and 4% by weight of sulfuric acid in the bottom liquid were mixed and continuously fed at a rate of 61.2 kg / hr.
Reactive distillation was performed at a temperature of 115 ° C. while supplying kg / hr of steam. At the time when the sulfuric acid catalyst overflowed from the side wall outlet of the middle part of the reactor and was continuously withdrawn and the system became stable and steady, the distillate was sampled for 1 hour and 30 minutes.
Shown in the table.

The distillate in this experiment was separated into two layers.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Minoru Koshibe 1-3-16 Shinhiodai, Sakai City, Osaka Prefecture (72) Yoshihiro Sezaki 3-2-12-302 Yayoi Town, Izumi City, Osaka Prefecture

Claims (1)

[Claims] 1. Methacrylic acid obtained by catalytically oxidizing isobutylene, tertiary butanol, methacrolein or isobutyraldehyde with a gas containing molecular oxygen in the presence of steam, and cooling the oxidation product. Extraction of methacrylic acid with aqueous solution, recovery of solvent,
In recovering the methacrylic acid remaining in the bottom liquid obtained by distilling and separating methacrylic acid from the crude methacrylic acid from which the light-boiling components have been removed, the methanol molar ratio is set to 1 to 5 times the molar amount of methacrylic acid and the amount of sulfuric acid is A method for treating a distillation bottoms of methacrylic acid, which comprises steam-distilling the bottoms as 2% by weight or more of the bottoms while esterifying the bottoms and distilling and recovering the bottoms as methyl methacrylate.