JPS6215542B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improved continuous synthesis method for methyl methacrylate or ethyl methacrylate and a method for recovering unreacted methacrylic acid. In order to avoid the complexity of the description, the methyl esterification method of methacrylic acid will be described below, but the same can be said for the ethyl esterification method unless otherwise specified. Conventionally, methyl methacrylate has been produced by the acetone cyanohydrin method using acetone and hydrocyanic acid as raw materials, but this method is not satisfactory as an industrial production method for methyl methacrylate due to the use of highly toxic hydrocyanic acid and the production of large amounts of ammonium sulfate as by-products. It wasn't something that should have been done.
Recently, as an alternative to this production method, a method of producing methyl methacrylate by obtaining methacrylic acid by gas phase catalytic oxidation of isobutylene and esterifying it with methanol has been studied. isobutylene,
Methacrylic acid obtained by the oxidation reaction of tertiary butyl alcohol and/or methacrolein is obtained in the form of a 10 to 50% by weight aqueous solution containing acetic acid as a by-product and small amounts of other substances. A method is known in which methacrylic acid is extracted from this aqueous solution into an organic solvent, and then methacrylic acid is recovered from the solvent by distillation. Aromatic compounds, ketones, halogenated hydrocarbons, esters, etc. are known as extraction solvents for methacrylic acid. Particularly important in selecting this extraction solvent is the distribution coefficient of methacrylic acid. The distribution coefficient is expressed as follows: Distribution coefficient = (Concentration of methacrylic acid in the solvent (% by weight))/(Concentration of methacrylic acid in the aqueous layer (% by weight)). A large partition coefficient indicates that methacrylic acid can be extracted more efficiently. On the other hand, acetic acid entrained in the oxidation process is also an organic acid like methacrylic acid, and its extraction performance with respect to organic solvents is often similar to that of methacrylic acid. Therefore, if a solvent is selected by focusing only on the distribution relationship, acetic acid will also be extracted together with methacrylic acid, and in the esterification process after extraction, acetic acid and methanol will react to produce methyl acetate, resulting in the loss of methanol. Therefore, acetic acid must be separated and removed to a certain concentration or less before the esterification step. Traditionally, distillation has been the most common method for separating methacrylic acid and acetic acid.
This method is disadvantageous in terms of thermal efficiency and process simplification; therefore, the extraction solvent for methacrylic acid is appropriately selected, and in the methacrylic acid extraction step, methacrylic acid is added to the extract, and most of the acetic acid remains as a raffinate. Preference is given to methods of selective separation into the aqueous phase. Conventionally, it has been described in Japanese Patent Publication No. 52-3825 as a solvent that has a large partition coefficient for methacrylic acid and a partition coefficient of less than 1.0 for acetic acid, and is capable of separating acetic acid and methacrylic acid in the methacrylic acid extraction process. As shown, ethylbenzene or/and xylene are available. Furthermore, as described in Japanese Patent Publication No. 49-41413, it is known that the extraction efficiency of hydrocarbons such as heptane, toluene, and xylene decreases in a low concentration aqueous solution of methacrylic acid. The present inventors investigated this point in more detail regarding ethylbenzene and/or xylene, and found that these hydrocarbon solvents can be used as extraction solvents for methacrylic acid, and that the extraction rate of methacrylic acid does not rise above a certain limit. It was confirmed that it had a fatal defect and could not be used industrially, and in order to improve this defect, ethylbenzene,
Add 15% of methacrylic acid ester to a water-insoluble non-reactive organic solvent such as xylene, kyumene, cymene, etc. with a boiling point of 120°C or higher (hereinafter referred to as the first extractant component).
By adding in a range of ~30% by weight, it is possible to obtain an extraction solvent that can be used industrially, and when this extraction solvent is used, unreacted methacrylic acid in the esterification reaction process can be efficiently recovered. I found out. The present inventors conducted a detailed study on how the distribution coefficient of methacrylic acid changes with respect to the concentration of the raw material methacrylic acid aqueous solution, with and without adding methacrylic ester to a mixed solvent of ethylbenzene and/or xylene. The results shown in Figure 1 were obtained. Note that the results shown in the figure are hardly affected by the mixing ratio of ethylbenzene and/or xylene. Figure 1 shows that in the case of only xylene and/or ethylbenzene, the distribution coefficient of methacrylic acid decreases extremely when the concentration of methacrylic acid in the raw material aqueous solution becomes 3% by weight or less, and especially when the concentration of methacrylic acid in the aqueous solution decreases to 2% by weight. Below that, the partition coefficient approaches 1.0, and the solvent loses most of its ability to extract methacrylic acid, making it useless as an extraction solvent. This means that as long as the extraction operation is performed, there will always be a place inside the extractor where the concentration of methacrylic acid in the aqueous solution decreases, regardless of the concentration of raw material methacrylic acid. If used, there will be a limit to the extraction rate of methacrylic acid. For example, when the raw material methacrylic acid concentration is 20% by weight, if the same amount of extraction agent is used for the raw material aqueous solution, about 2% by weight of it will escape as raffinate water, so the upper limit of the extraction rate will be about 90%. Industrially, the loss of methacrylic acid is too large, and no matter how efficient the subsequent steps are, they lose their industrial meaning. The present inventors conducted studies to improve this point, and found that methyl methacrylate, which is an esterification reaction product of methacrylic acid and does not affect the esterification reaction, was added to the extraction agent in a range of 15 to 50% by weight. By adding it to the first component, it was possible to create an extraction solvent that exhibits sufficient extraction ability even in the low concentration range of methacrylic acid in an aqueous solution. An extraction solvent for methacrylic acid consisting of a combination of methyl methacrylate and ethylbenzene or/and xylene was already published in
As disclosed in Publication No. 41413, this invention shows that although methyl methacrylate alone has excellent static extraction power for methacrylic acid, it causes various troubles in dynamic conditions during actual operation. has been done. In order to improve these points, the inventors of Japanese Patent Publication No. 49-41413 improved the dynamic stability as an extraction solvent by combining a xylene isomer consisting of xylene, ethylbenzene or a mixture thereof with methacrylic acid ester. This is the result of the following. In this method, the mixing ratio of xylene isomers is limited to 50% by weight or less. However, as mentioned earlier, with a binary mixed extractant based mainly on methacrylic acid ester, the partition coefficient of acetic acid, which is a byproduct of the oxidation process, becomes large, making it difficult to efficiently separate methacrylic acid and acetic acid in the extraction process. This has the disadvantage that it is difficult to separate the two. As a result of examining this point, the present inventors have reversed the mixing ratio of this binary extractant mixture to a range of 15 to 50% by weight of the first extractant component, preferably in a range of 20 to 45% by weight. By adding methacrylic acid ester, the adverse effects of the methacrylic acid aqueous solution in the low concentration range when the first extractant component is used alone, as well as the selectivity of acetic acid separation of the binary mixed extractant mainly composed of methacrylic acid ester, can be improved. It was able to solve both of the negative effects associated with bad behavior all at once. Furthermore, the extract obtained by the above extraction operation is
It seems preferable in terms of the process to use methacrylic acid directly as a raw material in the esterification process, as described in Japanese Patent Publication No. 52-38535 and Japanese Patent Publication No. 49-41413, without separating it from the extraction solvent. is not that easy. The methacrylic acid aqueous solution obtained from the oxidation process of isobutylene, tertiary butyl alcohol or/and methacrolein is generally dilute at 20% by weight or less, and in order to recover methacrylic acid from this dilute aqueous solution with a high extraction rate, it is necessary to The amount of extraction medium to be used is usually between the same amount and about 2/5 of the amount of the raw material methacrylic acid aqueous solution. Since such a large amount of extraction solvent is used, the concentration of methacrylic acid in the extract is also dilute at about 15 to 30% by weight. If such a dilute extract is used directly as a raw material for esterification together with alcohol and an acid catalyst, a large amount of organic solvent will be present, so the esterification reaction tank will contain an organic solvent phase and a coordinating catalyst, and the esterification reaction will not be as large. This results in a mixed phase state consisting of two phases with the aqueous phase in which the part is carried out. Even if the methacrylic acid ester, which is the reaction product in the mixed phase, is effectively extracted into an organic solvent, or the reaction product water is distilled off by azeotropy with the organic solvent, organic compounds that have a strong affinity with methacrylic acid Since a large amount of solvent is contained, the concentration of methacrylic acid and alcohol in the aqueous phase, where an acid catalyst is present and where the esterification reaction mainly proceeds, is low, resulting in a significant decrease in the ester production reaction rate and unreacted In order to obtain a conversion rate that is high enough that the recovery of methacrylic acid is not a problem, a very long residence time is required, which, together with the large amount of feedstock due to the addition of a large amount of organic solvent as a raw material, is extremely This requires a large reactor volume and a powerful stirring power, making it almost impossible to realize unless special measures are taken, such as using a multi-vessel system, and even if it were possible, it would be extremely uneconomical. As stated in Japanese Patent Publication No. 52-38535, as long as a specific organic solvent is added, "the type of esterification reactor is not limited in any way, and depends on the catalyst used, reaction conditions, etc." It is by no means a simple matter of ``selecting the organic solvent as appropriate,'' and unless the reaction mode is determined, it is impossible to fully demonstrate the effect of adding an organic solvent. The present inventors investigated a method of significantly reducing the reaction volume while effectively utilizing the effect of adding an organic solvent, and as a result, they discovered a new method for effectively recovering unreacted methacrylic acid. . That is, the methacrylic acid ester and the first extractant component are recovered as a methacrylic acid extractant by distillation from the methacrylic acid extract, and recycled to the methacrylic acid extractor, and the methacrylic acid extractant first component solution is extracted from the bottom of the distillation column. is extracted and used as a raw material for esterification. This distillation method may include ordinary distillation or azeotropic distillation using an entrainer. The concentration of methacrylic acid in the extractant first component solution used as the raw material for the esterification reaction is 20
~80% by weight, preferably 30-60% by weight. The esterification method using a solution of the first extractant component of methacrylic acid can significantly reduce the reaction residence time by keeping the reaction rate in the esterification tank as low as 50 to 90%, preferably 70 to 90%. , becomes industrially feasible. The esterification reaction is carried out using a solution of the first extractant component of methacrylic acid in the presence of methanol and an acid catalyst. Examples of the acid catalyst used include phosphoric acid, sulfuric acid, benzenesulfonic acid, p-toluenesulfonic acid, etc. Alternatively, a cation exchange resin is used. During the esterification reaction, the water produced is distilled off by azeotropic distillation together with the methacrylic acid methyl ester that is also produced and the first extractant component contained in the raw materials, and the acid in the aqueous phase in the esterification reaction tank is removed. It is possible to keep the catalyst concentration constant. Specifically, this is carried out by adjusting the degree of heating or the pressure at which the esterification reaction is carried out. The esterification reaction is often carried out at normal pressure, but
Depending on the case, it may be carried out under reduced pressure or increased pressure. Therefore, the two liquid phases can be easily separated by decantation at the outlet of the esterification reaction tank, and the acid concentration on the separated aqueous phase side is kept constant, so that the acid concentration generated during the reaction can be reduced by filtration, etc. The aqueous phase can be recycled to the esterification reaction tank while replenishing the small amount of acid catalyst lost, and the acid catalyst, which has many pollution problems, can be disposed of outside the system. The esterification reaction can be carried out very effectively without any problems. The present inventors have already applied the same method to the esterification reaction of acrylic acid in JP-A-49-5916, and have confirmed that it is also extremely effective for methacrylic acid. Japanese Patent Publication No. 49-41413 also describes the use of recirculating the water phase, but this is a method of "extracting the produced water" from the water phase side, and the method of separating the produced water is different from the method of the present application. On the other hand, the method of the present application has disadvantages, such as the amount of acid catalyst discharged outside the system is significantly larger than that of the present invention, or that water separation must be performed separately by a method such as distillation. is more efficient than these. Although it is economical to carry out the esterification reaction in a single tank, it is of course possible to perform the esterification reaction in multiple tanks. The esterification reaction distillate vapor is condensed in a condenser and sent to the next methanol extraction step together with the oil phase separated by decantation at the outlet of the esterification reaction tank. Extract and separate with an aqueous solution of inorganic salts such as ammonium sulfate and common salt. The raffinate oil phase, which is mainly composed of the extractant first component, methyl methacrylate, and unreacted methacrylic acid, from which most of the unreacted methanol has been separated in the methanol extraction column, is then treated with distilled water to remove low-boiling substances and methacrylic acid ester. By separation, a low concentration extractant first component solution of unreacted methacrylic acid is recovered. The separation of low-boiling substances and methacrylic acid esters is generally carried out using two rectification columns in series, but other methods such as the use of a composite column, etc., depend on the method of assembling the distillation system or its operating conditions. do not have. Although it is possible to directly circulate the extractant first component solution of unreacted methacrylic acid obtained in this way to the esterification reaction tank, the concentration of methacrylic acid is also low and this is not advisable. As a result of various studies on this point, the inventors of the present application have found that the extractant first component solution of unreacted methacrylic acid is circulated to a middle position in the methacrylic acid extraction column where the methacrylic acid concentration in the extract liquid matches, and the methacrylic acid is distributed. Extracting methacrylic acid on the high methacrylic acid concentration side with a large coefficient increases the extractant ratio (extractant amount/
They found that it also leads to a reduction in the amount of raw material aqueous solution) and is therefore thermally advantageous in the next step of distillation, which separates the solvent. This extractant first component solution of unreacted methacrylic acid can be circulated before the extractant separation tower that performs extractant separation, but the effect is greater by the amount of the extraction step if it is circulated to the methacrylic acid extraction step. Next, the present invention will be explained in more detail with reference to the drawings, but FIG. 2 shows one embodiment of the present invention, and the present invention is not limited thereto. The crude methacrylic acid aqueous solution obtained by oxidation of isobutylene, tert-butyl alcohol or/and methacrolein is supplied to the top of the methacrylic acid extractor 2 through line 1 and then through line 35.
A mixed extractant comprising methyl methacrylate and a first extractant component is supplied. The methacrylic acid extract is extracted from line 5 and supplied to the middle stage of extractant separation column 7. On the other hand, the raffinate water is drawn out through line 6, and the extractant component dissolved in a small amount in the raffinate water is separated into the extracted component and waste water by the recovery tower 32. The waste water is discharged through line 34, and the extracted component is removed from line 33. more will be recovered. An extractant consisting of methyl methacrylate and a first extractant component is recovered from the top of the extractant separation column 7, and this extractant is recycled to the methacrylic acid extractor 2 through line 3. The extractant first component solution of methacrylic acid is supplied from the bottom of the extractant separation column 7 to the esterification reaction tank 18 through a line 8 . Make-up sulfuric acid and methanol are added to the esterification reactor through lines 9 and 12. Distilled vapor consisting of methacrylic acid, water, and the first extractant component from the esterification reaction tank 18 is condensed in a condenser 17 and extracted through a line 13. The esterification reactor bottoms are drawn out through line 14 and separated into an oil phase, water phase, and two liquid phases in a decanter 15. The aqueous phase is drawn out through line 16, and polymers and the like are removed in a strainer 17'. and 9
The recovered methanol and make-up sulfuric acid are recycled through lines 10 and 11 to the esterification reactor. The oil phase separated in the decanter 15 is supplied together with the esterification reaction tank distillate 13 to the bottom of the alcohol extractor 21 through a line 19, and unreacted methanol is extracted and separated using inorganic brine supplied from the top of the tower through a line 20. , is extracted from line 22, and is taken out from line 20, 2 in alcohol recovery tower 23.
Ammonium sulfate, recovered methanol, and waste water are separated from No. 4 and No. 25, respectively. The alcohol extractor raffinate oil phase is extracted from the top of the column through line 26 and supplied to the middle stage of low-boiling substance separation column 27, and low-boiling substances such as water, methyl acetate, methanol, etc. are separated from the top of the column through line 28, and then A bottoms liquid consisting of methyl methacrylate, extractant first component solution, and unreacted methacrylic acid is drawn out from the bottom through line 29. The bottoms are then supplied to the middle stage of the product conversion column 30, the product methyl methacrylate is separated from the top of the column through line 31, and the extractant first component solution of unreacted methacrylic acid is recovered from the bottom of the column and passed through line 4. It is circulated to the middle stage of methacrylic acid extraction 2 through. Although there is a risk that high-boiling substances may accumulate in the circulation system during the process of recycling, these high-boiling substances can be appropriately extracted from the lines 4, 8, etc. as necessary. Next, the method of the present invention will be further explained in Examples.
The present invention is not limited to these. Example 1 A methyl esterification reaction of methacrylic acid was carried out according to the same flow sheet as shown in FIG. 2 except that sulfuric acid from line 9 was not supplied. As a result, the compositions and flow rates of each part of the flow sheet shown in FIG. 2 were as shown in Table 2.

【table】

【table】

[Table] Example 2 Using Qyumene instead of xylene, the operating pressure conditions for each distillation column were 50 mmHg for distillation column 7 and 50 mmHg for distillation column 27.
Example 1 except that the temperature conditions of each distillation column were the same as in Example 1 by changing the temperature to 60 mmHg.
When operated in exactly the same manner as in Example 1, the composition of each part was almost the same as in Example 1. Example 3 Example 1 was performed using a mixed organic solvent of 57% by weight of xylene and 43% by weight of ethylbenzene instead of xylene.
When operated in exactly the same manner as in Example 1, the composition of each part was almost the same as in Example 1.

[Brief explanation of the drawing]

FIG. 1 shows the relationship between the concentration of methacrylic acid in the raw material aqueous solution and the distribution coefficient of methacrylic acid in the first extractant component or the first extractant component and the methacrylic acid ester solvent. FIG. 2 is a flow sheet of an example of implementing the method of the present invention.
3, 4, 5, 6, 8, 9, 10, 11, 12, 1
3, 14, 16, 19, 20, 22, 24, 2
5, 26, 28, 29, 31, 33, 34, 35
is a line, 2 is a methacrylic acid extraction column, 7 is an extractant separation column, 15 is a decanter, 17 is a condenser,
17' is a filter, 18 is an esterification reaction tank, 21
is an alcohol extractor, 23 is an alcohol recovery tower,
27 is a low boiling point separation column, 30 is a product conversion column, and 32 is a recovery column.

Claims (1)

[Claims] 1. From a dilute aqueous solution of methacrylic acid containing acetic acid etc. obtained by gas phase catalytic oxidation of isobutylene, tert-butyl alcohol or/and methacrolein, 15 to 50% by weight of methacrylic acid ester is used as an extraction solvent for methacrylic acid. After almost completely extracting methacrylic acid using a non-reactive organic solvent with a boiling point of 120°C or higher and insoluble in water, the extract is distilled to extract a mixed solvent of methacrylic acid ester and most of the organic solvent and methacrylic acid. Then, the obtained organic solvent solution containing methacrylic acid is subjected to an esterification reaction with alcohol in the presence of an acid catalyst, and at this time, the acid catalyst concentration in the reaction solution is kept constant. The water produced in the esterification reaction is removed by azeotroping with the organic solvent, and the methacrylic acid ester produced at this time is also distilled out. Meanwhile, the reaction liquid discharged from the bottom of the esterification reaction tank is separated from the organic solvent layer in a decanter. The aqueous layer containing the acid catalyst is recycled to the esterification reactor after separating the polymerization product, and the organic solvent layer is sent to the next alcohol extractor together with the esterification reaction distillate. The unreacted alcohol is extracted and recovered by contacting with an aqueous inorganic salt solution in the extractor, and the recovered alcohol is circulated to the esterification reactor.On the other hand, the raffinate organic solvent phase coming out from the upper part of the alcohol extractor is purified by distillation, etc. After separating the boiling matter, the product is separated into a high-purity methacrylic acid ester product and an organic solvent containing unreacted methacrylic acid, and the methacrylic acid-containing organic solvent is circulated to an appropriate location in an extractor for a dilute aqueous methacrylic acid solution. A method for recovering methacrylic acid, characterized by: