JPH0333143B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention obtains methacrylic acid free of high-boiling point impurities from an aqueous methacrylic acid solution obtained by cooling a product gas containing methacrylic acid obtained by catalytic gas phase oxidation of isobutylene, tertiary butanol, or methacrolein. It's about process. More specifically, the present invention cools a reaction product gas containing methacrylic acid obtained by gas-phase oxidation of isobutylene, tertiary butanol or methacrolein using a multi-stage cooling tower of two or more stages. When obtaining an aqueous solution of methacrylic acid, a part of methacrylic acid, a small part of acetic acid, and substantially the entire amount of by-products having a boiling point higher than methacrylic acid are collected in the first column, and the remaining methacrylic acid is collected in the second and subsequent columns. The acid and most of the acetic acid are collected, and the condensate from the first column is extracted with aliphatic, alicyclic, or aromatic hydrocarbons having a boiling point of 150°C or less, either alone or in combination, and the condensate from the second column is extracted. relates to a method for recovering methacrylic acid, which is characterized by extracting with esters, ketones, and ethers having a boiling point of 150° C. or lower, singly or in combination, and distilling these extracts to recover methacrylic acid. In the process of producing methacrylic acid by gas-phase oxidation of _C4 compounds (isobutylene, tertiary butanol, methacrolein), the gas produced in the reaction is cooled and collected as an aqueous solution, from which it does not mix freely with water. A common method is to extract methacrylic acid using an organic solvent and subject it to subsequent purification steps. Although the method of producing methacrylic acid by gas phase oxidation of C ₄ compounds has reached the stage of industrialization, problems still remain in the oxidation reaction process. One of the problems is that many impurities other than methacrylic acid are present in the reaction product. That is,
Acetaldehyde, acetone, acrolein, methacrolein, acetic acid, acrylic acid, maleic acid,
The presence of many impurities such as citraconic acid and aromatic carboxylic acids has been recognized. Among these impurities, aromatic carboxylic acids are
They may adhere to the vessel walls or become suspended in the condensate, causing problems in the process. Furthermore, a portion of maleic acid migrates to the solvent phase during extraction, making it difficult to separate it from methacrylic acid in the subsequent purification step. In addition, many high-boiling point impurities cause problems, such as promoting the polymerization of methacrylic acid in the subsequent purification process. On the other hand, polymer substances that are thought to be generated when the reaction product gas is cooled are dissolved in the condensate, and this substance precipitates as scum at the interface of the two liquids during the extraction of methacrylic acid and gradually accumulates, impeding the operation of the extraction tower. causing problems such as making it difficult. Many of these problems are unique to the process of producing methacrylic acid using the _C4 acetylation method, and various proposals have been made to solve them. For example, JP-A No. 58-52239 proposes a method of selectively collecting only high-boiling point impurities before quenching the reaction product gas, and JP-A No. 58-99439
In JP-A-56-16438, a method of adding a basic substance to the condensate and subjecting it to extraction was proposed.
JP-A-56-79634 proposes a method in which the condensate is brought into contact with a hydrophobic solvent to form a scum, which is then removed and subjected to a post-extraction step. However, all of these methods are not necessarily satisfactory, as they are expensive or provide unsatisfactory results. As a result of various studies to solve this problem, the present inventors divided the cooling of the reaction product gas into two stages and in the first stage (first tower) substantially all of the high boiling point impurities were removed together with some methacrylic acid. After condensation, only methacrylic acid is selectively extracted with a hydrocarbon solvent independently of the methacrylic acid condensate in the latter stage (second column and later), and the methacrylic acid aqueous solution in the latter stage (second column and later) contains esters, ketones, The present invention was completed by discovering that the above-mentioned problems could be solved and substantially all of the methacrylic acid and most of the acetic acid could be recovered from an aqueous solution by extraction with a solvent consisting of ethers. That is, the method of the present invention is as follows. The methacrylic acid-containing gas obtained by oxidizing isobutyrelene, tertiary butanol or methacrolein is cooled in a multi-stage cooling tower of two or more stages. The type of the first column is preferably a direct cooling quench column type in consideration of the need for rapid cooling or the precipitation of high-boiling solid impurities. From the second tower onwards, indirect cooling type can be used. The first column collects substantially all by-products having a boiling point higher than methacrylic acid by circulating the condensate and bringing it into contact with the reaction product gas. A portion of methacrylic acid, acetic acid, etc. is also condensed here.
After removing low-boiling point components by distillation or bubbling inert gas under heating if necessary, the condensate from the first column is treated as a single aliphatic, alicyclic or aromatic hydrocarbon with a boiling point of 150°C or less. or extracted by a mixture. As described in JP-A No. 49-93307, hydrocarbon extraction solvents have a high affinity for methacrylic acid, but have a low affinity for maleic acid and citraconic acid, and most of these acids Not extracted.
Therefore, the amount of these acids remaining in methacrylic acid through the subsequent methacrylic acid purification step is substantially zero. Acetic acid also has a low affinity for this solvent and remains in the aqueous layer, but since the amount present in the condensate of the first column is originally small, it does not pose a particular problem in wastewater treatment. Furthermore, the amount of scum generated at the interface of the two liquid phases during extraction is small when the extractant is hydrocarbon-based, and it tends to collect at the interface, so if an extraction tower such as a rotating disk tower is used, the scum treatment will be large. Not a problem. By distilling off the extractant from the extract thus obtained, methacrylic acid substantially free of high-boiling impurities can be obtained. On the other hand, the uncondensed gas in the first tower is cooled by cooling towers in the second and subsequent stages to condense substantially all of the methacrylic acid and acetic acid. The aqueous methacrylic acid solution obtained by condensation in the second and subsequent cooling towers contains almost no high-boiling impurities. This condensate contains methacrolein,
After removing low-boiling substances such as acetone by distillation or bubbling inert gas under heating, esters, ketones, etc. with a boiling point of 150°C or less that are not mutually miscible with water,
Methacrylic acid and acetic acid are extracted using ethers alone or as a mixture. These solvents have a large ability to extract organic acids, and most of acetic acid can also be extracted. Furthermore, due to their nature, these solvents have the ability to extract a considerable amount of high-boiling point carboxylic acids, but as mentioned above, the condensate obtained here does not substantially contain these high-boiling point impurities. Therefore, the extraction operation is extremely easy, and the extract contains almost no impurities. Crude methacrylic acid is obtained by removing the solvent from the extract obtained here by distillation.
This is combined with methacrylic acid from the first cooling tower condensate and subjected to a subsequent purification process. An embodiment of the present invention will be described below with reference to FIG. R-1 and R-2 are respectively the first stage oxidation reactor,
In the second stage oxidation reactor, isobutylene or tertiary butanol, steam, and air are supplied as raw materials from line 1 to R-1. Methacrolein, steam, and air are supplied to R-2 from line 5 as raw materials. Although the reactors are shown in parallel in FIG. 1, it is also possible to directly connect the first and second stage reactors. The reaction product gas enters the first quencher T-1 from R-1 through line 2 and is partially condensed, so that almost the entire amount of high-boiling substances, methacrylic acid, acetic acid, and a portion of water are condensed. The reaction product gas from R-1 usually contains 2 to 10 vol% methacrolein, 0.1 to 1.0 vol% methacrylic acid, 10 to 30 vol% water vapor, and non-condensable gases (N ₂ , O ₂ , CO ₂
etc.) 60-85vol%, containing a small amount of high boiling point substances. R
In the quencher for -1, the gas is preferably 30
Cooled to ~70°C. Similarly, the reaction product gas from R-2 enters the first quencher T-2 for R-2 through line 6 and is condensed in the section. The reaction product gas from R-2 usually contains 1-6 vol% methacrylic acid, 0.2-2 vol% methacrolein, 10-30 vol% water vapor, 60-85% non-condensable gas, and a small amount of high-boiling substances (maleic acid, etc.) do. In the first quencher for R-2, the gas is preferably 30~
Cooled to 80℃. As a result, almost all of the by-product high-boiling impurities are collected. This first quencher for R-2 is preferably operated so that 30 to 70% of the methacrylic acid contained in the reaction product gas is condensed. Although separate quenchers are shown in Figure 1, R
There is no problem even if the generated gases of -1 and R-2 are combined and cooled. The operating conditions for the quencher in this case are:
It is preferable to select operating conditions similar to those of the first quencher for R-2 described above. Furthermore, when directly connecting the first and second stage reactors, the first
The operating conditions for the quencher are preferably the same as those described above. Uncondensed gas from T-1 and T-2 goes to line 4.8
The second
Enter Quencher T-3. At T-3, the gas is preferably cooled to 5-20 DEG C. and substantially all of the methacrylic acid, acetic acid and water are condensed. Most of the unreacted methacrolein in the reaction gas flows out through line 11 without being condensed, and is sent to the methacrolein recovery process (MAL recovery process) together with methacrolein from other processes, where it is taken out as recovered methacrolein through line 17. . The condensate of T-3 is removed in line 10. This condensate is removed through line 16 from the first low-boiling separation column T-5 to remove low-boiling components including methacrolein through line 15 and supplied to the first extraction column T-6. T-6 is supplied with extractant through line 18. As extractants, esters, ketones, and ethers that are immiscible with water and have a boiling point of 150° C. or lower and are capable of extracting acetic acid as well as methacrylic acid are used, and in some cases, a small amount of a hydrocarbon solvent may be added. Examples of esters, kettles, and ethers having a boiling point of 150° C. or lower include ethyl acetate, isopropyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, diisopropyl ether, and the like. This extraction produces almost no scum, so
Any type of extraction column can be freely applied. The extract is taken out through line 19 and the solvent is recovered in the first solvent separation column T-7. Recovered solvent from line 20 is recycled through line 18. The bottom liquid of T-7 is methacrylic acid containing almost no high-boiling impurities, and is taken out through line 21, led to a methacrylic acid purification step, and taken out through line 27 as purified methacrylic acid. On the other hand, by-product acetic acid is taken out from line 28. Further, the raffinate from extraction column T-6 is taken out through line 20 and sent to a solvent recovery step (not shown) as required to recover the dissolved solvent and combine it with the recovered solvent from T-7. and reuse. On the other hand, the condensates from the first quench towers T-1 and T-2 are taken out through lines 3 and 7, and are combined and supplied through line 12 to the second low-boiling separation tower T-4. Here, the low-boiling fraction is recovered via line 14 and the bottoms are taken out via line 13. This column may be omitted depending on the operating conditions of the quench column. The bottoms enters extraction column T-8 through line 13.
The condensate from the first quench tower (or from which low-boiling components have been removed as described above) supplied to extraction tower T-8 preferably contains 40 to 80% methacrylic acid.
Weight%, water 10-50% by weight, acetic acid 2-10% by weight, maleic acid 1-5% by weight, other high-boiling substances 0.2-50% by weight
Contains 3% by weight. Extractant is supplied through line 22 from the bottom of T-8. As extractants, aliphatic, alicyclic, and aromatic hydrocarbons with a boiling point of 150°C or less, which selectively extract only methacrylic acid and hardly extract high-boiling point carboxylic acids such as maleic acid, may be used alone or in combination. used as Examples of these aliphatic, alicyclic, and aromatic hydrocarbons include hexane, heptane, octane, ligroin, cyclohexane, benzene, toluene,
Examples include xylene. Since a small amount of scum is generated in T-8, a type of column such as a rotating disk extraction column is preferable. The extract is extracted through line 24 and supplied to the second solvent separation column T-9. Here, the solvent is distilled and recovered through line 26 and recycled to T-8. Methacrylic acid from the bottom of T-9 is taken out through line 25 and sent to the methacrylic acid purification process (MAA purification process). This methacrylic acid hardly contains high boiling point carboxylic acids such as maleic acid. Further, the raffinate of T-8 taken out from line 23 contains acetic acid and high-boiling point carboxylic acid, but usually contains almost no extraction solvent, so it is often sent as is to the wastewater treatment process. The present invention will be explained below with reference to Examples. Example 1 Using the apparatus shown in Table 1 according to the flow shown in Figure 1, a Bi-Mo based multicomponent catalyst was used as the first stage oxidation catalyst, and a Mo based heteropolyacid catalyst was used as the second stage catalyst. The oxidation reaction of isobutylene and methacrolein was carried out using the reactor, and the resulting gas was introduced through lines 2 and 6 to produce methacrylic acid in the following manner.
The operating conditions of each device were as shown in Table 1, and the flow rates of each component in the main lines were as shown in Table 2. The bottoms of the first quencher were drawn out through lines 3 and 7, and after removing low-boiling components such as methacrolein at T-4, they were sent through line 13 to the second extraction column. From line 22, n-octane was fed as an extractant to extract methacrylic acid. Scum was generated at the interface during extraction, but the amount generated was small, and long-term operation was possible by taking it out of the device intermittently. Also, in this extraction, almost all of the maleic acid and other high-boiling components were transferred to the raffinate. After n-octane was separated in the second solvent separation column T-9, the bottoms were extracted from line 25 and used for purification of methacrylic acid. On the other hand, the second quencher bottom liquid was sent to the perforated plate extraction tower T-6 after separating low-boiling components, and from the bottom, isopropyl acetate was sent as an extractant through line 18 to extract methacrylic acid and acetic acid. . Since the amount of scum that accumulated at the interface in extraction column T-6 was extremely small, continuous operation was possible without any special operation. The extract liquid was separated into an extractant using T-7, and the bottom liquid was sent to a methacrylic acid purification process via line 27 to obtain purified methacrylic acid.

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Claims (1)

[Claims] 1. When obtaining a methacrylic acid aqueous solution by cooling the reaction product gas containing methacrylic acid obtained by gas phase oxidation of isobutylene, tertiary butanol or methacrolein with a catalyst in a multi-stage cooling tower of two or more stages. In the first column, a portion of methacrylic acid, a small portion of acetic acid, and substantially the entire amount of by-products having a boiling point higher than methacrylic acid are collected, and the remaining methacrylic acid and most of the acetic acid are collected in the second and subsequent columns. The condensate from the first column is extracted with aliphatic, alicyclic, or aromatic hydrocarbons having a boiling point of 150°C or less, singly or in combination, and the condensate from the second and subsequent columns is extracted with an ester having a boiling point of 150°C or less. A method for recovering methacrylic acid, which comprises extracting with , ketone, or ether alone or in a mixture, and distilling these extracts to recover methacrylic acid.