JPS6158465B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for recovering and purifying methacrylonitrile,
More specifically, the present invention relates to a method for recovering and purifying methacrylonitrile by ammoxidation reaction of isobutylene or tert-butyl alcohol. It is known that methacrylonitrile is produced by a gas phase contact reaction between isobutylene and the like, ammonia and oxygen, that is, an ammoxide reaction. The products of this reaction include methacrolein, hydrocyanic acid, acrylonitrile, acetonitrile, isobutyronitrile, and the like as by-products in addition to methacrylonitrile. This by-produced methacrolein and hydrocyanic acid combine to form methacrolein cyanohydrin (boiling point: 95°C, 13 mmHg), which is an unstable high-boiling condensate. Therefore, when attempting to purify the reaction mixture by distillation, methacrolein cyanohydrin is produced in the separation and purification process, which is decomposed into methacrolein and hydrocyanic acid again during the subsequent distillation and mixed into the distillate. , the purity of methacrylonitrile was lowered, and a high purity product could not be obtained. The purification of methacrylonitrile is carried out according to a conventional process for acrylonitrile, an example of which is shown in FIG. The ammoxidation reaction product containing methacrylonitrile as a main component absorbed in the absorption water is subjected to extractive distillation together with a solvent in the recovery column 1, and the vapor at the top of the column is condensed in the condenser 2, and then transferred to a decanter (oil/water separator). ) The oil layer separated in step 3 becomes a liquid containing not only methacrylonitrile but also impurities such as methacrolein, hydrocyanic acid, and isobutyronitrile, and a saturated amount of water. This liquid is fed to the upper hydrocyanic acid removal section 4a of the hydrocyanic acid removal and dehydration tower 4, and hydrocyanic acid and some low-boiling substances are separated from the top of the tower.
The bottom liquid is separated into oil and water by a decanter 5, and the oil layer is fed to the upper stage of the lower dehydration section 4b. Dehydration section 4
The azeotrope mainly consisting of water and containing methacrolein distilled from the top of the column b is returned to the bottom of the prussic acid removal section 4a, and crude methacrylonitrile with reduced hydrocyanic acid and water is returned from the bottom of the dehydration section 4b. Take it out. This crude methacrylonitrile is passed through a low-boiling separation column 6 in which low-boiling substances are removed from the top of the column, the bottom liquid is fed to a product column 7, and trace amounts of low-boiling substances and high-boiling substances are removed through a line 8 at the top of the column.
and from line 9 at the bottom of the column, and the product methacrylonitrile is obtained from line 10. However, methacrolein and hydrocyanic acid partially react during the distillation process to produce methacrolein cyanohydrin. In particular, in the decanter section provided between the hydrocyanic acid removal step and the dehydration step, methacrolein and hydrocyanic acid are in high concentration and a large amount of water coexists, so the production of cyambidrin is significantly promoted.
This cyanohydrin is not removed in the bottom boiling separation column, but is fed into the product column, and during distillation in the product column, part of it decomposes into methacrolein and hydrocyanic acid again, and is mixed into the product methacrylonitrile, reducing its purity. Ta. The reason why methacrolein is particularly problematic during the production of methacrylonitrile compared to acrolein, which is produced as a by-product during the production of acrylonitrile, is that methacrolein produces a large amount of by-product compared to acrolein, and is also susceptible to hydration reactions. This is because the polymerization reaction is difficult to occur, so methacrolein does not disappear and remains at a high concentration in the procell. Several proposals have been made to solve this problem. For example, Japanese Patent Publication No. 50-23017 proposes removing hydrocyanic acid and carbonyl compounds as a side stream in the middle stage of the recovery column to prevent methacrolein cyanohydrin from being carried into the subsequent distillation process. but,
This is a process in which the entire amount of low-boiling hydrocyanic acid is extracted from the side of the recovery column, and in order to ensure product quality, a large amount of steam is consumed and a distillation column with a large diameter is required, increasing equipment costs. Furthermore, when hydrocyanic acid is used, it is necessary to increase the number of stripping pots or to further increase the amount of steam consumed in order to separate impurities. In addition, in order to stabilize cyanohydrin and prevent acrolein and hydrocyanic acid from being mixed into the product due to decomposition, oxalic acid (Special Publication
10112), sulfamic acid or acidic ammonium sulfate (Japanese Patent Publication No. 39-28316), sulfonic acid or aromatic sulfonic acid. However, this method is effective when the cyanohydrin content is low, but when the content is high, especially when continuous distillation is carried out, cyanohydrin accumulates and the content increases.
The effect of improving product purity is low. Japanese Patent Publication No. 43-18126 describes that in the first step, chemicals are added to decompose cyanohydrin, and acrolein and hydrocyanic acid are separated by distillation, and in the second step, chemicals are added to stabilize the remaining cyanohydrin, and cyanohydrin is separated by distillation. I have some suggestions. This method is effective when cyanohydrin is not concentrated during batch distillation, but when performing continuous distillation or when cyanohydrin has accumulated even during batch distillation, it is necessary to increase the amount of known inorganic or organic acids added. This creates problems with the processing of these acids and the materials of the equipment. According to studies conducted by the present inventors, in order to obtain an effect in continuous distillation, it is necessary to extract a large amount of the condensing part so as to prevent the concentration of cyanohydrin from increasing, which is not an economical method. As stated above, until now there has been no industrially completed technology for obtaining a highly purified product methacrylonitrile from crude methacrylonitrile containing methacrolein and hydrocyanic acid. This invention was achieved through extensive research to solve the above problems, and its gist is the ammoxidation reaction of isobutylene or tert-butyl alcohol containing methacrolein and hydrocyanic acid, with methacrylonitrile as the main component. When recovering and refining the product methacrylonitrile by distillation using water as a solvent, an acid is added to the dehydration distillation process liquid, and the water layer pH of the oil-water separator that separates the distilled concentrated water from the oil layer is set to 1.5. This is a method for recovering and purifying methacrylonitrile, which is characterized in that the concentration of methacrylonitrile is 4.0. In conventional production methods, there was no attempt to actively control the pH of the decanter between the prussic acid removal section and the dehydration section. Usually, the pH is around 4, the concentration of methacrolein and hydrocyanic acid is high, and a large amount of water coexists, promoting the production of methacrolein cyanohydrin. However, in this recovery and purification method,
By setting the pH of the decanter water layer to 1.5 to 4.0, the production of cyanohydrin can be significantly reduced. If the pH is 4.0 or higher, the effect will be poor, and if it is less than 1.5, the inhibitory effect will not improve compared to the amount of acid added. More preferably it is 2.0 to 3.5. Therefore, the concentration of cyanohydrin in the liquid fed to the subsequent distillation process is reduced, and the purity of the methacrylonitrile product can be improved. In order to obtain the product methacrylonitrile by this method, for example, when using the conventional process shown in FIG.
Adding an acid such as acetic acid or phosphoric acid to the prussic acid/dehydration tower 4 from the top of the tower or the top of the dehydration section 4b,
By adjusting the pH of the water layer in the decanter 5 to 1.5 to 4.0, the amount of cyanohydrin produced can be reduced and the amount of cyanohydrin carried into the subsequent distillation process can be reduced.
In addition, the added acid behaves as a high-boiling substance and is carried into the bottom layer of the subsequent low-boiling separation column and product column, exerting the effect of controlling the decomposition of cyanohydrin in the distillation process. Note that the acid to be added is not limited to acetic acid or phosphoric acid, but they are excellent in terms of ease of handling, cost, etc. A more suitable process to which this method is applied is the process shown in FIG. This process is the product tower 7
A high-boiling separation column 11 is installed in the front stage of the column 4, and high-boiling substances (isobutyronitrile, trace amounts of methacrolein, cyanvidrin, etc.) are extracted from the bottom of the column and removed from the liquid fed from the prussic acid removal/dehydration column 4. Crude methacrylonitrile is obtained from the top of the column. This crude methacrylonitrile is fed to the product column 7, and the line 12 at the top of the column
Distillation separation of low-boiling substances including methacrolein from
Product methacrylonitrile is obtained by removing gas from line 13 at the bottom of the column. In this process as well, acid is added to the dehydration distillation process liquid in the hydrocyanic acid removal/dehydration tower 4, and the pH of the decanter 5 is
is controlled to 1.5 to 4.0 to suppress the production of cyanohydrin, and high boiling substances are removed in the high boiling separation column and fed to the product column. Therefore, the amount of cyanohydrin mixed into the product column is extremely small, and there is virtually no isobutyronitrile that promotes the decomposition of cyanohydrin, so the decomposition of cyanohydrin hardly progresses and the purity of the methacrylonitrile product increases. will improve. According to studies conducted by the present inventors, the amount of water is greatly involved in the production of cyanohydrin, and methacrolein and hydrocyanic acid dissolved in the water layer are extremely likely to produce cyanohydrin. Therefore, in this method, if the water content of the crude methacrylonitrile liquid discharged from the hydrocyanic acid removal/dehydration tower is actively dehydrated in the hydrocyanic acid removal/dehydration step, for example, the water content is reduced to 0.2% or less, preferably 0.1% or less. For example, the condensate containing methacrolein at the top decanter 14 of the low boiling point separation column in the conventional process shown in Figure 1 or the top decanter 15 of the product column in the process shown in Figure 2 can be separated into two layers: oil and water. This is more preferable because it can suppress the production of cyanohydrin. In this method, the location of the oil-water separator that separates concentrated water from the oil layer is as shown in Figures 1 and 2 above.
As shown in the figure, it is not limited to the prussic acid removal section and the dehydration section. Depending on the process, the oil/water separator position may differ from that shown in FIG. 1 or 2. For example, as shown in FIG. 3, a liquid containing impurities such as methacrylonitrile, water, and methacrolein from which hydrocyanic acid has been removed in the prussic acid removal tower 16 is sent to the product tower 17, and the methacrylonitrile product is passed through the line 18 at the bottom of the tower. There is also a process of acquisition. In this case, water becomes an azeotrope with methacrylonitrile, and is distilled from the top of the product column 17 together with low-boiling substances such as methacrolein, mixed with the bottom of the hydrocyanic acid removal column 16, and mixed with the bottom of the hydrocyanic acid removal column 16.
The water layer is separated from the oil layer and dehydrated. Even in such a process, the object of the present invention can be achieved by setting the pH of the water layer in the decanter 19 to 1.5 to 4.0. Example 1 Unreacted ammonia is removed from a reaction product obtained by a catalytic reaction using isobutylene or tert-butyl alcohol, ammonia and oxygen as raw materials using a mineral acid, and the resulting gas is absorbed into an aqueous solution. Ta. An organic layer containing methacrylonitrile, methacrolein, and hydrocyanic acid as main components was recovered from this absorption liquid. This organic layer has an inner diameter of 25φ and a height of 400mm.
Feed at 450g/H into the middle stage of a hydrocyanic acid removal tower with a diameter of 450g/H to separate low-boiling components from the top of the tower, and after separating the aqueous layer from the bottom of the tower with a decanter, the organic layer is separated into an organic layer with an inner diameter of 25φ and a height of 350mm.
was fed to the upper stage of the dehydration tower. The top liquid of the dehydration tower was returned to the bottom pot of the hydrocyanic acid removal tower, and crude methacrylonitrile from which hydrocyanic acid and water had been removed was obtained from the bottom of the dehydration tower. At this time, a difference was observed in the cyanohydrin concentration in the bottom liquid of the dehydration tower depending on whether or not an acetic acid feed was applied to the top of the prussic acid removal tower. The results are shown in Tables 1 and 2.

【table】

[Table] Example 2 Even when using the same equipment as in Example 1 and simultaneously feeding hydrocyanic acid and methacrolein, the difference between the hydrocyanic acid removal tower and the dehydration tower depends on whether or not the acetic acid feed is carried out at the top of the hydrocyanic acid removal tower. Differences were observed in the cyanohydrin concentration in the bottom liquid. The results are shown in Tables 3 and 4.

【table】

[Table] Example 3 The hydrocyanic acid removal tower of the apparatus of Example 1 was enlarged to a height of 550 mm with an inner diameter of 25φ, and the dehydration tower was enlarged to an inner diameter of 30φ and a height of 450 mm, and the number of stages was increased. The effect was investigated by varying the amount of acetic acid fed to the top of the tower. Hydrocyanic acid-containing methacrylonitrile solution and methacrolein were simultaneously fed separately. The results are shown in Tables 5 and 6.

【table】

[Table] Example 4 Even when the same equipment as in Example 1 was used and phosphoric acid was fed as an acid to the top of the hydrocyanic acid removal tower, a difference was observed in the cyanohydrin concentration in the bottom liquid of the dehydration tower. . The results are shown in Tables 7 and 8.

【table】 [Brief explanation of the drawing]

FIGS. 1, 2, and 3 are flow sheets of methacrylonitrile production processes to which the recovery and purification method of the present invention can be applied. 1... Recovery tower, 2... Condenser, 3... Decanter, 4... Hydrocyanic acid removal/dehydration tower, 4a... Hydrocyanic acid removal section,
4b... Dehydration section, 5... Decanter, 6... Low boiling point separation column, 7... Product column, 8... Tower top line, 9...
... Tower bottom line, 10 ... Line, 11 ... High boiling separation column, 12 ... Tower top line, 13 ... Tower bottom line, 14 ... Low boiling separation column decanter, 15 ... Product column decanter, 16 ... ... Hydrocyanic acid removal tower, 17 ... Product tower, 18 ... Tower lower line, 19 ... Decanter.

Claims (1)

[Claims] 1 In recovering and purifying the product methacrylonitrile by distilling the ammoxidation reaction product of isobutylene or tertiary butyl alcohol containing methacrolein and hydrocyanic acid using water as a solvent, the dehydrating distillation step is carried out. A method for recovering and purifying methacrylonitrile, which comprises adding an acid to a liquid and adjusting the pH of the water layer of an oil-water separator to 1.5 to 4.0 to separate the distilled water from the oil layer.