JPH0150687B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for effectively recovering acetone from an acetone-containing low-boiling mixture produced as a by-product in the process of synthesizing methyl methacrylate from acetone cyanohydrin. Methyl methacrylate is generally synthesized by synthesizing acetone cyanohydrin from acetone and hydrogen cyanide, converting it into methacrylamide using sulfuric acid, and then reacting it with methanol. However, in the process of purifying the methyl methacrylate synthesized in this way, various impurities must be removed, and the by-product acetone is particularly important as a main raw material in this process, so it must be removed first from other impurities. It is separated together with boiling point components as a low boiling point mixture, recovered in a recovery step, and used again for the synthesis of acetone cyanohydrin. The acetone recovered in this way contains excess methanol used in the esterification reaction,
Sulfur dioxide is caused by the sulfur content contained in the sulfuric acid and polymerization inhibitor used in the synthesis of acetone cyanhydrin, and acetic acid, which is sometimes used as a stabilizer for decomposition of acetone cyanhydrin and hydrogen cyanide, is esterified. Contains methyl acetate and methyl methacrylate. For this reason, in the conventional acetone recovery process, these impurities are removed with great care, but in particular the presence of methyl acetate and sulfur dioxide causes the following inconveniences in the process, so they must be thoroughly removed. is necessary. Methyl acetate is difficult to separate by distillation because its boiling point (57.5°C) is very close to that of acetone (56.13°C). Therefore, in the acetone cyanohydrin synthesis process, it is used in an acetone cyanohydrin purification tower or acetone cyanohydrin together with unreacted acetone. As it circulates through the hydrogen cyanide recovery distillation column, it accumulates, resulting in an excessive load on the distillation column, and eventually, methyl acetate still containing acetone and hydrogen cyanide has to be frequently discharged from the system. This results in an increase in waste liquid treatment costs and loss of raw materials acetone and hydrogen cyanide. On the other hand, sulfur dioxide
The alkaline catalyst commonly used in the reaction to synthesize acetone cyanhydrin from acetone and hydrogen cyanide is consumed and neutralized salts are produced, which can clog the acetone cyanhydrin purification process and increase the load on filters. cause problems. Conventionally, for example, (1) Methyl acetate was fed together with water to the top of the catalyst (H-type ion exchange resin) layer, an extractive distillation section was installed at the bottom to heat it from the bottom, and a reflux section was installed at the top to perform reactive distillation. A method for extracting acetic acid and methanol together with water from the lower part of the tower (Special Publication No. 43-6602
(2) Methyl lactate containing or not containing methanol, lactic acid, or water is continuously fed to a distillation column, and the number of plates and residence time are adjusted in the presence of an acidic catalyst or an ion exchange resin catalyst. A method of continuous hydrolysis with numerical limitations in view of the relationship with the decomposition rate constant (see Japanese Patent Publication No. 55-45055); (3) Purification of crude methyl methacrylate obtained using acetone cyanohydrin as a starting material The low boiling point distillate in the process is continuously distilled to separate impurities, and water is continuously supplied as a third component when recovering methacrylic acid methyl ester and methanol. Method of separating impurities (Tokukosho
39-24438)) have been proposed. However, these methods still have problems and are not fully satisfactory. In other words, in methods (1) and (2) above, since both are reactive distillation columns and the catalyst is packed in the packing, the reaction yield cannot be controlled within the limiting conditions of the distillation column (pressure, flow rate, and number of plates). The problem is that it is extremely difficult to maintain stable conditions at high temperatures, and there is no suggestion of a method for obtaining recovered acetone that does not contain sulfur dioxide and methyl acetate, which is our objective. In addition, the method (3) above is intended to recover methyl methacrylate and methanol from the bottom of the tower, and from the top of the tower,
Acetone, ether, methyl acetate and other impurities are removed. In other words, according to this prior art, sulfur dioxide is distilled out from the top of the column along with acetone, although this is not specified as methyl acetate and other low-boiling impurities. Therefore, the present inventors have overcome the problems of the prior art described above, and have developed a method for effectively recovering acetone that does not contain sulfur dioxide and that does not contain methyl acetate while effectively recovering methanol from methyl acetate. As a result of intensive research for development, we have arrived at the present invention. The acetone recovery method of the present invention recovers acetone from an acetone-containing low-boiling mixture that is produced as a by-product when synthesizing methyl methacrylate from acetone cyanohydrin, sulfuric acid, and methanol, while keeping the low-boiling mixture alkaline. It is characterized by recovering acetone by distillation. According to the present invention, for example, in the process of synthesizing acetone cyanohydrin from hydrogen cyanide and acetone in the presence of an alkaline catalyst, and then reacting sulfuric acid and methanol to synthesize methyl methacrylate, the produced methyl methacrylate is distilled. Acetone can be effectively distilled and recovered from a low boiling point mixture containing dimethyl ether, methanol, acetone, water and methyl acetate obtained from the top of the distillation column during purification. When acetone is recovered by distillation from such a low-boiling point mixture, it is necessary to distill the low-boiling point mixture while keeping it alkaline, otherwise the intended purpose cannot be achieved. The amount of alkali added to the low boiling point mixture is equal to the equivalent amount necessary to neutralize the acidic substances originally contained in the mixture and the equivalent amount necessary to neutralize the acetic acid generated by hydrolysis of methyl acetate. The amount is more than the total amount. The mixture to which alkali has been added is heated, if necessary, and allowed to stand for a suitable period of time, and then fed to an acetone recovery distillation column to recover acetone. In the acetone recovery tower, methanol-extracted water is introduced from the top of the column, and heating steam is introduced from the bottom of the column. If necessary, the pressure inside the tower can be reduced to keep the temperature inside the tower low to suppress polymerization of the polymerizable substance inside the tower. In this way, acetone substantially free of sulfur dioxide and methyl acetate can be recovered from the top of the column, and can be directly sent to the acetone cyanohydrin synthesis step for use without any hindrance. On the other hand, from the bottom of the acetone recovery tower, alkali acetate and methanol are extracted along with water and alkali sulfite. The liquid extracted from the bottom of the column is, if necessary, led to another distillation column to contain relatively low-boiling active components such as unrecovered acetone, methanol separated from acetone, and methanol derived by hydrolysis of methyl acetate. can be recovered from the top of the tower, and relatively high-boiling substances such as acetic acid and alkali salts of sulfite and water can be extracted from the bottom of the tower. By doing so, it is possible to effectively utilize the active ingredients without losing them, which is economically advantageous. Hereinafter, the present invention will be described in more detail with reference to the attached process diagrams showing an example of the implementation of the present invention. An alkali catalyst 1, acetone 2, and hydrogen cyanide 3 are fed into an acetone cyanohydrin synthesis reactor 4 for reaction, and the reaction mixture is led to a neutralization reactor 5 and made acidic with a neutralizing acid 6. The crude acetone cyanohydrin 7 thus obtained is sent to an acetone cyanohydrin purification tower 8, and unreacted hydrogen cyanide and acetone 9 are recovered from the top of the tower and returned to the synthesis reactor 4, and purified from the bottom of the tower. Acetone cyanohydrin 10 is obtained. Sulfuric acid 11 is added to the acetone cyanohydrin 10 thus obtained, and the mixture is fed into a methacrylamide reactor 12. The reaction solution is led to an esterification reactor 13, and a methanol solution 14 is added to the esterification reactor 13. The generated crude ester 18 is evaporated from the reactor 13 by heating, and after extracting and separating most of the excess methanol contained in the methanol extraction tower 16 using methanol extraction water 17, it is sent to the low boiling point separation tower 19. . Reactor 13
A waste liquid 15 is separated from the bottom of the methanol extraction column 16, and the extracted methanol liquid from the methanol extraction column 16 is supplied to the reactor 13 for reuse. In the low-boiling point separation column 19, water 20 is extracted from the center of the column, and other low-boiling point mixtures 21 (for example, dimethyl ether, sulfur dioxide, methanol,
acetone and methyl acetate) are distilled off from the top of the column. From the bottom of the column, crude ester 22 is obtained from which these low-boiling substances are separated, and then sent to a methyl methacrylate purification column 23 where it is separated into purified methyl methacrylate 34 and a high-boiling mixture 35. The low boiling point mixture 21 is adjusted to be alkaline by adding an alkali 24 to the alkali addition tank 25 if necessary, and then sent to the acetone recovery column 27 . In addition, in the alkali addition tank 25, the waste liquid 26 is separated and removed from the bottom of the tank if necessary (for example, when the liquid is separated into two phases due to the addition of the alkali). In the acetone recovery tower 27, water 2 is collected from near the top of the tower.
8 is added to recover the acetone in the low boiling point mixture from the top of the column, and the recovered acetone 29 can be directly sent to the acetone cyanohydrin synthesis reactor 4 for circulation and reuse. A waste liquid 30 is extracted from the bottom of the column, and if desired, in order to recover active ingredients such as methanol contained therein, it is sent to, for example, a methanol recovery column 31, where recovered methanol 32 and waste liquid 33 (water, alkali acetate, alkali sulfite, etc.) are collected. (contains salts, etc.) and can be separated into In this case, the recovered methanol 32 is transferred to the esterification reactor 13.
can be sent to and reused. Examples of the present invention will be shown below to further specifically explain the present invention. Example 1 Acetone cyanohydrin 566Kg/H and concentrated sulfuric acid
855Kg/H is mixed to produce methacrylamide sulfate at a temperature of 150℃ in a reactor, and 3 reactors with a capacity of 3000 equipped with a heating device and a condenser are connected in series together with 870Kg/H of a 39% methanol aqueous solution. The raw material was continuously fed to the esterification reaction and evaporation steps used in the above, and 753 kg/h of crude ester containing methyl methacrylate as the main component was obtained. Next, this crude ester was supplied to the lower part of a rotating disk type extraction tower (inner diameter 43 cm, 32 stages), and 195 kg/H of extraction water was added from the upper part of the tower to extract methanol contained in the crude ester.
The methanol aqueous solution obtained from the bottom of the column was reused for the esterification reaction after adjusting the methanol concentration again. Meanwhile, 636Kg/H of crude ester was obtained from the top of the column.
This is sent to the 11th stage from the top of a low-boiling point separation distillation column with a column diameter of 45 cm and 40 plates, and from the bottom of this distillation column, crude methacrylic acid that is substantially free of low-boiling substances is fed. 610Kg/H of methyl was obtained. This crude methyl methacrylate was further sent to a purification column, and purified methyl methacrylate (550 kg/h) was obtained from the top of the purification column. On the other hand, the liquid was extracted from near the center of the low-boiling point separation distillation column, left to stand, and then 50 kg/h of the water-rich lower phase was discharged, and the remainder was returned to the column. The top temperature of the low boiling point separation distillation column was 30℃ and the pressure was 260mmHg, and a fraction of 275Kg/H with the following composition was obtained from the top of the column, of which 11Kg/H
was extracted from the system, and the remainder was refluxed to the top of the column. Composition of the overhead fraction of the low-boiling point separation distillation column Weight % Acetone 68.8 Methanol 4.7 Methyl acetate 8.6 Dimethyl ether 5.0 Methyl methacrylate 9.5 Sulfur dioxide 2.4 Others 1.0 The fraction 11 kg/H extracted from the system was heated in a heater with a capacity of 100 25% by weight caustic soda aqueous solution 5Kg/H while maintaining the temperature at 60℃.
was added. 16 kg/h of a cloudy mixture with a pH of approximately 13.5 was extracted from this reactor and fed to the 14th stage from the top of an acetone recovery distillation column with an inner diameter of 12.6 cm and 28 plates. Add H,
The pressure at the top of the column was maintained at 250 torr, and a fraction with the following composition was obtained from the top and bottom of the column.

[Table] 7.9Kg/H of the above column top fraction was extracted outside the system (the rest was refluxed to the column), and along with 3.3Kg/H of liquefied hydrogen cyanide, acetone cyanide was produced using two reactors with a capacity of 650 in series. It was sent to the phosphorus synthesis reaction system.
Further, 379 kg/h of acetone and 176 kg/h of hydrogen cyanide were fed into this reaction system, and the temperature was adjusted to 15°C in the first tank.
The temperature of the second tank was kept at 5℃, and the pH was kept at 8.5.
% caustic potash aqueous solution was added to the first tank. Next, concentrated sulfuric acid was added and the mixture was distilled in a vacuum distillation column while maintaining acidity, and 566 kg/h of acetone cyanohydrin with a purity of 99.5% by weight was obtained from the bottom of the column. This vacuum distillation column overhead fraction was returned to the synthesis reactor. The acetone cyanohydrin thus obtained was used again for the production of methyl methacrylate, but there was no accumulation of methyl acetate or an increase in catalyst alkali consumption due to sulfur dioxide, and in the acetone cyanohydrin synthesis process, The problems encountered in the conventional acetone recovery method described above did not occur at all. The liquid extracted from the bottom of the acetone recovery tower has an inner diameter of 12
The mixture was fed into the top of a 3/8-inch Raschig ring packed column with a height of 6 m, and steam was sent from the bottom to obtain a fraction of 9.1 kg/h with the following composition. This fraction could be used as part of the methanol water for the esterification reaction. Composition Weight% Acetone 4.2 Methanol 10.2 Methyl methacrylate 11.3 Others (mainly water) 74.5

[Brief explanation of drawings]

The accompanying drawings are process diagrams showing an example of implementing the method of the present invention. 1... Alkali catalyst, 2... Acetone, 3...
Hydrogen cyanide, 4... Acetone cyanohydrin synthesis reactor, 5... Neutralization reactor, 8... Acetocyanhydrin purification tower, 13... Esterification reactor,
16... Methanol extraction column, 19... Low boiling point separation column, 21... Low boiling point mixture, 23... Methyl methacrylate purification column, 24... Alkali, 25...
Alkali addition tank, 27...Acetone recovery tower, 29
...Recovered acetone.

Claims (1)

[Claims] 1. To recover acetone from an acetone-containing low-boiling mixture that is produced as a by-product during the synthesis of methyl methacrylate from acetone cyanohydrin, sulfuric acid, and methanol, the low-boiling mixture is kept alkaline and distilled to recover acetone. An acetone recovery method characterized by the following.