JPS6312458B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an industrially advantageous method for producing methacrylic acid, and more specifically to isobutylene, t-
When methacrylic acid is produced from butanol, methacrolein, isobutyraldehyde, isobutyric acid, or a mixture thereof by a gas phase oxidation method, the by-product aqueous acetic acid solution is used in the production process.
The present invention relates to an industrially advantageous method for producing methacrylic acid. Isobutylene, t-butanol, methacrolein, isobutyraldehyde, isobutyric acid, or a mixture thereof (hereinafter abbreviated as isobutylene, etc.)
It is well known that methacrylic acid is produced by a gas phase catalytic oxidation method using methacrylic acid as a raw material, and it is well known that the method of supplying water to the reaction system is effective in improving reaction selectivity and catalyst life. Are known. In recent years, research on methods for producing methacrylic acid from isobutylene and the like has made great progress, and catalysts and production methods for obtaining methacrylic acid in fairly good yields have been proposed. However, even with these improved catalysts, the selectivity to methacrylic acid is still 100
%, many by-products are generated. Among the by-products, acetic acid is produced in relatively large amounts. The amount of acetic acid produced depends on the performance of the catalyst, but it is
It reaches ~30% by weight. However, the acetic acid produced as a by-product of the reaction is (a) water supplied to the reaction system together with the reaction raw materials, (b) water produced in the reaction, and (c) reaction product gas containing highly polymerizable methacrylic acid and methacrolein. It is directly quenched and is significantly diluted with water added to recover methacrylic acid, and (d) water added in the process of purifying the absorption liquid used to recover methacrolein. The concentration of acetic acid in the stream after recovery is generally very low, below 5% by weight, although it depends on the performance of the oxidation reaction catalyst. There are broadly two methods for treating an aqueous solution containing diluted acetic acid. One method is to dispose of the acetic acid as waste water, and the other is to recover acetic acid as a useful substance. Naturally, the method of disposing of wastewater requires treatment to avoid wastewater pollution. A combustion method is considered as a treatment method, but since the concentration of acetic acid is low, self-combustion is impossible, and therefore a large amount of auxiliary fuel is required, resulting in a large processing cost. Microbial treatment is also known as another treatment method, but in this case as well, in addition to the cost required for microbial treatment, the generated activated sludge must be separated,
A large amount of energy is required for drying and cooling. The next method of recovering acetic acid is preferable from the standpoint of effective resource utilization, but the low concentration of acetic acid in the aqueous solution to be recovered increases the recovery cost and poses a major obstacle. The following methods are known as typical methods for recovering acetic acid from an aqueous acetic acid solution. (1) Distillation method (2) Azeotropic distillation method (3) Solvent extraction method In the distillation method (1), water, which is present in large quantities, becomes a light boiling point component, and moreover, water has a low relative volatility relative to acetic acid, so a large amount of water is extracted. The number of stages in the distillation column and the reflux ratio are required, and it cannot be said to be a good method for recovering from a dilute aqueous solution. In the azeotropic distillation method (2), a component that can form an azeotrope with water is added as an entrainer to make it easier for water to rise to the top of the column, but the number of plates in the acetic acid separation column and the reflux ratio can be reduced. However, equipment for recovering the entrainer is required.
After all, this is not a good method for recovery from dilute aqueous solutions. The solvent extraction method (3) is a method in which acetic acid is selectively extracted using esters, ethers, ketones, etc. as an extraction solvent.
Vol.59(10), page 65 states that of the three methods above, the solvent extraction method is superior in terms of both capital investment and operating costs.
It is said that it is particularly excellent in the low concentration range. However, even in this solvent extraction method, the above literature shows that as the acetic acid concentration in the raw material decreases, both the invested capital and operating costs increase rapidly (for example, the acetic acid concentration in the raw material decreases from 10% to 5% by weight). As a result, both invested capital and operating costs increased by approximately 1.8 times). The same thing is stated in the same magazine, vol. 73(5), p. 55. For example, it is said that when the acetic acid concentration reaches 1% by weight, the energy cost required for separation alone will reach the estimated value of acetic acid. However, this trend is expected to become even stronger as energy costs soar. Acetic acid, which is produced as a by-product in the gas phase catalytic oxidation reaction process for producing methacrylic acid, requires a high recovery cost due to its low concentration, which increases the production cost of methacrylic acid. A solution is desired. As a result of intensive studies to solve these problems, the present inventors focused on using the dilute aqueous acetic acid solution, which is a by-product of the oxidation reaction, in the methacrylic acid manufacturing process in the following way. Even if the aqueous acetic acid solution produced as a by-product in the acid manufacturing process is circulated and supplied to the oxidation reaction process, the oxidation reaction of isobutylene, etc. is not inhibited at all, and most of the supplied acetic acid does not react on the catalyst and flows out as it is. For this reason, the acetic acid concentration in the reaction product is significantly increased. High-boiling compounds such as acid, acrylic acid, acetic acid, and water can be condensed from the produced gas. By supplying water to the methacrolein recovery process as an absorbent when absorbing and recovering methacrolein from the residual gas after condensing and separating high-boiling point compounds such as The present invention was completed based on the discovery that methacrolein can be absorbed and recovered much more efficiently than water while taking advantage of the simplicity and advantages of using it. In other words, the present invention makes it possible to obtain an acetic acid aqueous solution with a much higher concentration than conventional methods by implementing the above three findings separately or in combination of two or more of them, thereby reducing the cost of acetic acid recovery. As a result, it has become possible to greatly contribute to reducing the production cost of methacrylic acid. The present invention will be explained in detail below. The method for producing methacrylic acid by vapor phase catalytic oxidation of isobutylene and t-butanol is usually a two-stage oxidation reaction as follows. Isobutylene, t-butanol → methacrolein Methacrolein → methacrylic acid However, it is also possible to process from the raw materials to the final product, methacrylic acid, all at once without extracting the methacrolein produced in the middle. A catalyst for producing methacrolein and methacrylic acid by oxidizing isobutylene and/or t-butanol can be effectively applied to the present invention as long as it contains Mo, Bi, and Fe. The catalyst used for oxidizing methacrolein and/or isobutyraldehyde and/or isobutyric acid can be effectively applied to the present invention as long as it contains at least Mo and P. In particular, it can be effectively applied to the present invention if it contains at least Mo and P.
Those containing P, V and alkali metals are preferably used. It is also possible to directly convert isobutylene into methacrylic acid using a catalyst containing Mo and P, which is applicable to the present invention. The catalyst can be prepared by conventional methods, for example, as described in
4010, the method described in JP-A-52-46016 is preferably used. The amount of molecular oxygen used relative to isobutylene etc. varies depending on the raw material, but is usually preferably in the range of 0.5 to 20, particularly preferably in the range of 0.6 to 10. The feed gas may include other inert gases such as nitrogen, carbon dioxide,
It may also contain saturated hydrocarbons and the like. The oxidation reaction temperature varies depending on the catalyst, but is usually 200
~500°C, preferably 250-450°C is used. The supply amount of raw material gas is expressed as space velocity (SV).
Based on the NTP standard, 100 to 8000 hr ^-1 is used, more preferably 300 to 5000 hr ^-1 . The reaction pressure is under pressure,
It is possible to carry out the process under normal pressure or reduced pressure, but excessive pressurization or depressurization increases the power cost and is disadvantageous in terms of cost, so it is generally carried out in the range of 0 to 5 kg/cm ² G. be. The reaction may be carried out in a fixed bed, fluidized bed or moving bed. The acetic acid aqueous solution produced as a by-product in the oxidation reaction process as described above can be used in any one of the oxidation process, the quenching process of the gas produced in the oxidation process, and the process of recovering methacrolein from the residual gas after condensing and separating high-boiling compounds in the quenching process.
The acetic acid concentration in the acetic acid aqueous solution is not particularly limited, and the acetic acid aqueous solution produced as a by-product in the oxidation step can be used as it is. It can also be diluted with water if necessary within a range that does not impair economic efficiency. When an acetic acid aqueous solution is supplied to the oxidation process, it can be supplied to either of the two-stage oxidation reactions described above, and furthermore, it can be supplied only to the first-stage oxidation process, without separating and purifying the products of the first-stage oxidation, and the reaction product gas can be directly used in the second-stage oxidation process. It can also be supplied to the oxidation reaction process of isobutyraldehyde to methacrolein and methacrylic acid and the oxidation reaction process of isobutyric acid to methacrylic acid to perform the subsequent oxidation reaction. It is also possible to increase the concentration.
The amount of acetic acid aqueous solution supplied to isobutylene, etc. is preferably in the range of 1 to 30, particularly preferably in the range of 1 to 20, based on the molar ratio of acetic acid aqueous solution to isobutylene, etc. When an acetic acid aqueous solution is supplied to the quenching process, the amount supplied depends on the composition of the oxidation reaction product gas, but is usually the weight ratio of the acetic acid aqueous solution to the oxidation reaction product gas.
A range of 0.001 to 1.0 is preferable, and a range of 0.01 to 0.30 is particularly preferable. Excessive supply is undesirable because it reduces the concentration of methacrylic acid obtained from the quenching tower. When an acetic acid aqueous solution is supplied to the methacrolein recovery process, the amount supplied depends on the operating pressure and temperature of the absorption tower, but is usually in the range of 0.1 to 10 times the weight ratio of the methacrolein-containing gas, taking economic efficiency into consideration. to determine the pressure, temperature, and amount of absorbed liquid. Next, the present invention will be explained with reference to the drawings. FIG. 1 is a diagram showing the general steps of the present invention, that is, the steps of the first invention and the second invention. Compounds such as isobutylene, which are raw materials for methacrylic acid, and oxygen-containing gas are sent to the oxidation reaction step 2 through conduit 1.
The generated gas from the oxidation reaction step is sent to a quenching step 4 through a conduit 3 in order to quench easily polymerizable methacrylic acid and the like to minimize the production of polymers, and at the same time to recover it in liquid form. Here, methacrylic acid is recovered in the form of an aqueous solution, and a method generally employed is to cool and circulate the obtained methacrylic acid aqueous solution and bring it into direct contact with the reaction product gas. At this time, it is necessary to recover substantially all of the methacrylic acid in the reaction product gas in the quenching step, and in this case, it is not possible to substantially eliminate methacrylic acid from being contained in the exhaust gas by simply recycling the methacrylic acid aqueous solution. is not possible,
It is necessary to use more water that does not contain methacrylic acid.
For this reason, in the past, acetic acid produced as a by-product as a result of this was diluted. Furthermore, in order to facilitate the oxidation reaction, steam is usually supplied to the reaction system at the same time as the raw materials, but this also causes a decrease in the concentration of by-product acetic acid. However, in the present invention, the methacrylic acid aqueous solution is not used as a circulating solution, but the conduit 7 is used from the quenching step 4.
Since this is the remaining acetic acid aqueous solution after recovering and separating methacrylic acid from the product sent to the methacrylic acid recovery step 8, the acetic acid recovery step 13 is circulated through the conduit 11 to the oxidation step by circulating the acetic acid aqueous solution instead of steam. It became possible to supply a highly concentrated acetic acid aqueous solution to the acetic acid recovery process 13, and by circulating the acetic acid aqueous solution instead of the methacrylic acid aqueous solution to the quenching process through the conduit 5, it became possible to supply a highly concentrated acetic acid aqueous solution to the acetic acid recovery process 13. . Although a small amount of acetic acid is mixed into the off-gas discharged through the conduit 6, the amount is small;
After being treated in an exhaust gas treatment process (not shown), it is released into the atmosphere, but a portion can also be recycled untreated to the oxidation process. The aqueous methacrylic acid solution obtained through conduit 7 is sent to methacrylic acid recovery step 8 . Here, methacrylic acid is obtained from conduit 9, and an aqueous solution containing organic matter mainly consisting of by-product acetic acid is obtained from conduit 10. A portion of this is recycled via conduit 11 to the oxidation reaction step and/or via conduit 5 to the quenching step as described above, while the remainder is sent via conduit 12 to the acetic acid recovery step 13. Acetic acid is obtained from conduit 14;
Furthermore, if necessary, it is sent to a purification step to obtain glacial acetic acid. On the other hand, wastewater is obtained from the conduit 15,
Since this material contains a trace amount of organic matter, it is normally discharged from the system through a microbial treatment process (not shown). FIG. 2 is a diagram showing the case where methacrolein is contained in the reaction product gas, that is, the process of the third invention. A gas stream containing methacrolein is discharged from the quenching stage 4 via conduit 18 and sent to a methacrolein recovery stage 16. The methacrolein recovery process mainly consists of a methacrolein absorption tower using an absorption solvent and a stripper that separates the absorbed methacrolein from the absorption solvent. Methods using hydrocarbons such as light oil, kerosene, heavy oil, creosote oil, toluene, xylene, and alkylnaphthalene are known as absorption solvents for methacrolein, but in order to suppress the loss of the solvent into offgas, It is necessary to use high-boiling oils, in which case the temperature of the stripper separating methacrolein and solvent becomes too high, leading to operational difficulties due to polymerization of methacrolein. Also the second
When an acetic acid aqueous solution is supplied to the quenching step as in the invention described in 2007, acetic acid and the like are included in the gas passing through the conduit 18, so that they accumulate in the solvent and are difficult to remove. In addition to the above, water is also known as an absorption solvent for methacrolein. Although water is a preferable solvent in terms of simplifying the process, it has the disadvantage of poor absorption efficiency, and requires measures such as using a large amount of water or increasing the operating pressure, which is not necessarily economical. In the present invention, by supplying the acetic acid aqueous solution obtained from the methacrylic acid recovery process as an absorption solvent through the conduit 17, the absorption efficiency can be increased without sacrificing the process simplicity of water. In this case, the off-gas discharged from the methacrolein recovery step 16 is similar to the off-gas shown in FIG. 1, and only a small amount of acetic acid is contained therein and lost. After the methacrolein has been recovered, the off-gas passes through the conduit 6 and is further discharged into the atmosphere after residual organic matter is completely removed in an exhaust gas treatment step (not shown). On the other hand, the recovered methacrolein is circulated and supplied to the oxidation reaction step 2 through a conduit 19. This recovered methacrolein contains acetic acid and/or water, but
It is supplied to the oxidation reaction process without any problem. A portion of the absorption solvent can also be withdrawn via conduit 20 and sent to acetic acid recovery step 13. Figure 3 is a flow sheet illustrating in detail an example of producing methacrylic acid by a two-stage oxidation reaction using isobutylene as a raw material.For the sake of clarity, only the main equipment is shown, including the heat exchanger, pump, etc. , containers, etc. are omitted. The isobutylene oxidation reactor 101 is filled with a catalyst suitable for converting isobutylene to methacrolein, and the methacrolein oxidation reactor 1
02 is filled with a catalyst suitable for converting methacrolein to methacrylic acid. Isobutylene is passed through a conduit 21, oxygen-containing gas is passed through a conduit 22, and an acetic acid aqueous solution, which is the raffinate of the methacrylic acid extraction column 105, is supplied to the isobutylene oxidation reactor 101 through a conduit 23 after vaporization. The temperature of the reaction product gas exiting the reactor 101 is controlled to suit the subsequent reaction, and then oxidizes methacrolein together with additional oxygen-containing gas from conduit 26 and unreacted methacrolein recovered from conduit 27. Supplied to reactor 102. The reaction product gas leaving the reactor 102 is supplied to the quenching tower 103 through the conduit 29, but the quenching tower 10
The bottom liquid of the quenching tower is supplied after cooling through a conduit 30 to the upper part of 3 to rapidly cool the reaction product gas, condense most of the high-boiling substances such as methacrylic acid, acetic acid, and water, and further extract methacrylic acid. The raffinate of column 105, aqueous acetic acid, is fed via conduit 31 to the top of the column to absorb substantially no methacrylic acid in the gas stream exiting the top of the column. Quenching tower 1
In 03, high-boiling compounds such as methacrylic acid, acrylic acid, acetic acid, and water and a small amount of methacrolein are obtained as a bottom liquid, a part of which is circulated and supplied to the upper part of the quenching tower after cooling as described above. The other portion is supplied via conduit 32 to methacrolein recovery column 104 . Methacrolein is obtained from the top of the methacrolein recovery column 104 through a conduit 33, and normally it is recycled to the methacrolein oxidation reactor via a conduit 27, but depending on the purity of methacrolein, it can be transferred to an appropriate quenching column 103. It is also possible to circulate to different places. An aqueous solution of methacrylic acid or the like substantially free of methacrolein is obtained from the bottom of the methacrolein recovery column 104, and is supplied to the methacrylic acid extraction column 105 through a conduit 34. Various extractants can be used in the methacrylic acid extraction tower 105, but it is preferable to use one that can extract only methacrylic acid without extracting acetic acid.
Paraffins, olefins, alicyclic compounds of C ₄ or more, aromatic hydrocarbons of C ₆ or more, and halogen derivatives thereof are preferably used, and for example, pentane, hexane, heptane, octane, cyclohexane, etc. are particularly preferably used. The figure explains the case where pentane is used as the extractant, but the properties of the extractant (e.g. specific gravity, boiling point,
The operating conditions of the extraction column differ slightly depending on the freezing point, etc.), but this is obvious to those with general knowledge of chemical engineering. When pentane is used as the extraction agent, the methacrolein recovery tower bottom liquid, which is the extraction material, is supplied to the top of the extraction tower 105 through the conduit 34, and the pentane, which is the extraction agent, is supplied to the bottom of the extraction tower through the conduit 35. Ru. Further, in order to suppress the extraction of acetic acid in the methacrylic acid extraction tower 105, a small amount of water can be supplied to the top of the extraction tower via a conduit 38. The extraction column used may be a commonly used extraction column such as a packed column, perforated plate column, or rotating disk column, and a mixer-settler type extraction device may also be used. In the case shown in the figure, a pentane solution of methacrylic acid is obtained from the top of the extraction column 105, which is supplied to the next extractant separation column 106 through a conduit 36, where the extractant is separated from the methacrylic acid and extracted again with methacrylic acid. The methacrylic acid is recycled to column 105 and sent via conduit 39 to the next purification step, or, if the end product is a methacrylic acid ester, it can be sent unpurified to the next esterification step. An acetic acid aqueous solution is obtained as a raffinate from the bottom of the extraction column via a conduit 37, a part of which is circulated to the isobutylene oxidation reactor 101 via a conduit 23, and the other part is sent to a quenching tower 103 via a conduit 31. The other part is used for replenishment 51 of the absorption liquid in the methacrolein absorption tower 109, while the other part is supplied to the acetic acid extraction tower 107 via the conduit 40, where acetic acid is extracted and recovered. The extracting agent used at this time is preferably one having high extraction efficiency for acetic acid, such as esters, ethers, aromatic hydrocarbons, phosphoric acid esters, or mixtures thereof. The extraction column may be a commonly used extraction column such as a packed column, a perforated plate column, or a rotating disk column, and a mixer-settler type extraction device may also be used. The figure explains the case where ethyl acetate is used as the extracting agent, but the properties of the extracting agent (e.g. specific gravity, boiling point, freezing point, etc.)
The operating conditions of the extraction column will vary depending on the conditions, which is obvious to those with general knowledge of chemical engineering. When ethyl acetate is used as the extraction agent, the extraction agent is supplied from conduit 42 to acetic acid extraction column 107, and an ethyl acetate solution of acetic acid is obtained from the top of the column.
1 to the extraction column 108, where the extractant and acetic acid are separated, and the extractant passes through the conduit 42 to the extraction column 107.
The acetic acid is also sent via conduit 44 to an acetic acid purification process (not shown). Since a small amount of ethyl acetate is dissolved in the raffinate discharged from the acetic acid extraction tower through conduit 43, it undergoes an ethyl acetate recovery step (not shown) and is further subjected to microbial treatment, etc., as necessary. After proper wastewater treatment, it is disposed of as wastewater. The gas that has not been condensed and absorbed in the quenching tower 103 is discharged from the top of the tower through a conduit 45 and is supplied to the lower part of the methacrolein absorption tower 109, where methacrolein is absorbed and recovered by an absorption liquid. That is, if a methacrolein-containing gas is supplied from the conduit 45 to the methacrolein absorption tower 109 using the methacrylic acid raffinate as an absorbent, an acetic acid aqueous solution of methacrolein is obtained from the bottom of the tower. Ripper 110 is fed via conduit 48 and methacrolein is recovered overhead. The methacrolein recovered from the top of the column via conduit 49 is further circulated and supplied to oxidation reactor 102 via conduit 27. In this case, there is no problem even if methacrolein contains acetic acid aqueous solution, which is a component of the absorption solvent, and this means that the operating conditions of the stripper 110 for separating methacrolein, which is easy to polymerize, can be relaxed. It's advantageous. The aqueous acetic acid solution from which methacrolein has been separated is circulated to the methacrolein absorption tower through conduit 46, but in order to avoid accumulation of high-boiling point impurities, a portion is supplied to the acetic acid extraction tower through conduit 50, and a proportionate amount is also supplied to the acetic acid extraction tower through conduit 50. It is also possible to supply the raffinate from the methacrylic acid extraction column through the conduit 51. From the top of the methacrolein absorption tower, N ₂ , O ₂ ,
Exhaust gas containing CO and CO ₂ as main components is obtained, but
This is normally treated as exhaust gas and released into the atmosphere, but if necessary, it can be
1. It is possible to supply the methacrolein oxidation reactor 102 as a diluent gas. Examples will be described below. Example 1 The apparatus shown in FIG. 3 was used. (However, the acetic acid extraction column 107 and the extractant separation column 108 were not operated.) The reactor 101 consists of a SUS304 pipe with an inner diameter of 18 mm, and an isobutylene oxidation catalyst ( composition:
450 ml of Mo ₁₀ Bi ₁ Co ₇ Fe ₂₅ Sb ₁ Ox) was filled and heated to 350°C in a molten salt bath. The reactor 102 consists of a SUS304 pipe with an inner diameter of 23 mm, and a methacrolein oxidation catalyst (composition: Mo ₁₂ P _1.5
Zr ₁ V _0.25 Cs ₂ Mn _0.125 Ox) 1550 ml was filled and heated to 320°C in a molten salt bath. Isobutylene from conduit 21 at a rate of 0.135 Kg/H, air from conduit 22 at a rate of 0.834 Kg/H,
The bottom liquid of the methacrylic acid extraction column obtained from the conduit 37 after being vaporized through the conduit 23 was supplied as a reaction raw material at a rate of 0.334 kg/H. The composition of the reaction raw materials is in molar ratio: isobutylene: air: acetic acid: water = 5.0: 60.0:
1.5:33.5, and the feed rate was SV = 2400 hr ^-1 (NTP) for the isobutylene oxidation catalyst. The gas leaving the reactor 101 is pumped with air through the conduit 26 at a rate of 0.185
0.037 Kg/H of methacrolein recovered from the reaction product was added to the reactor 102 through the Kg/H conduit 27.
supplied. The reaction product gas exiting the reactor 102 was introduced into the lower part of a quenching tower 103, which had an inner diameter of 5 cm and a height of 120 cm and was filled with a Raschig ring. The bottom liquid of the methacrylic acid extraction tower is cooled to 5°C and supplied to the upper part of the quenching tower through a conduit 31 at a supply rate of 0.075 Kg/H.
It was cooled to ℃ and circulated. The inner diameter of the liquid at the bottom of the quenching tower is 4.
After removing methacrolein in a methacrolein recovery column 104 packed with a helipad and having a height of 100 cm, it is passed through a conduit 34 to a methacrylic acid extraction column (column diameter 4 cm,
It was supplied to the upper part of a rotating multi-blade tower (40 stages) 105 with a height of 150 cm. The extraction column is connected through conduit 35.
Methacrylic acid was extracted by feeding n-pentane at a feed rate of 0.721 Kg/H and H ₂ O at a feed rate of 0.013 Kg/H through conduit 38 (the extraction column was
℃ and an overhead pressure of 1 Kg/cm ² G). An acetic acid aqueous solution was obtained as the bottom liquid of the methacrylic acid extraction column from the conduit 37 at the bottom of the extraction column, and a portion of this was circulated to the reactor 101 and the quenching column 103. The gas discharged from the top of the quenching tower 103 was supplied through a conduit 45 to a methacrolein absorption tower 109, which had an inner diameter of 5 cm, a height of 120 cm, and was filled with a Raschig ring.
An acetic acid aqueous solution cooled to 3° C. was supplied to the upper part of the absorption tower through a conduit 46 at a rate of 2.89 kg/H to absorb methacrolein, and exhaust gas was discharged through a conduit 47. The bottom liquid of absorption tower 109 has an inner diameter of 5 cm and a height of
The methacrolein stripper 110 filled with a 120 cm helipack is fed to recover methacrolein, which is added with methacrolein from conduit 33 and circulated to the methacrolein oxidation reactor 102, where it is recycled to the methacrolein stripper bottom liquid. was circulated to the methacrolein absorption tower 109. Table 1 shows the composition and flow rate of each part when the entire system becomes steady. Comparative Example 1 The same catalyst and experimental equipment as in Example 1 were used. Water is vaporized from conduit 23 to 0.304Kg/H.
By supplying water at a rate of 0.072 kg/h and further supplying water to the quenching tower through conduit 31 at a rate of 0.072 kg/H,
The bottom liquid of the methacrylic acid extraction tower was not circulated to the reactor and the quenching tower, and the methacrolein absorption tower 109 was filled with xylene 2.89% instead of an acetic acid aqueous solution.
The same experiment as in Example 1 was conducted except that Kg/H was used. The xylene supplied to the methacrolein absorption tower 109 is obtained by redistilling the bottom liquid of the methacrolein stripper 110 (the apparatus is not shown in FIG. 1), and then extracting acetic acid,
The one from which methacrylic acid and the like were removed was used. Table 2 shows the material balance when operation reaches steady state.
It was shown to. The concentration of acetic acid in the outlet gas of reactor 102 (conduit 29) and the bottom liquid of the methacrylic acid extraction column (conduit 3)
7) and the feed liquid to the acetic acid extraction column 107 (tube 4
It can be seen that the acetic acid concentration of Example 0) is lower than that of Example 1. Furthermore, the exhaust gas discharged from the top conduit 47 of the methacrolein absorption tower 109 contained a large amount of xylene, which was lost from the methacrolein system. Example 2 The same catalyst and experimental equipment as in Example 1 were used. Water is supplied to the quenching tower 103 through the conduit 31 at a rate of 0.072 Kg/H, and the methacrylic acid extraction tower 10
5 The same experiment as in Example 1 was conducted except that the bottom liquid was not circulated to the quenching tower and xylene was supplied to the methacrolein absorption tower 109 at a rate of 2.89 Kg/H instead of an acetic acid aqueous solution. . The xylene supplied to the methacrolein absorption tower 109 is obtained by redistilling the bottom liquid of the methacrolein stripper 110 (the equipment is not shown in FIG. 3) and removing most of acetic acid, methacrylic acid, etc. Using. Table 3 shows the material balance when operation reaches steady state.
It was shown to. Compared to Comparative Example 1, the acetic acid concentration in the reactor 102 outlet gas concentration (conduit 29), the bottom liquid of the methacrylic acid extraction column 105 (conduit 37), and the liquid supplied to the acetic acid extraction column (conduit 40) is higher. Recognize. Example 3 The same catalyst and experimental equipment as in Example 1 were used. The same experiment as in Example 1 was conducted except that xylene was supplied to the methacrolein absorption tower 109 at a rate of 2.89 kg/H instead of an acetic acid aqueous solution. The xylene supplied to the methacrolein absorption tower 109 is obtained by redistilling the bottom liquid of the methacrolein stripper 110 (the apparatus is not shown in FIG. 3), and then converting it into acetic acid,
A product with most of the methacrylic acid etc. removed was used. Table 4 shows the material balance when operation reaches steady state.
It was shown to. Example 4 The same catalyst and experimental equipment as in Example 1 were used. 0.304Kg/H of vaporized water from conduit 23
2.89 xylene was supplied to the methacrolein absorption tower 109 instead of acetic acid aqueous solution.
The same procedure as in Example 1 was carried out except that the feed was carried out at a rate of Kg/H. The xylene supplied to the methacrolein absorption tower 109 is obtained by redistilling the tower bottom liquid of the methacrolein stripper 110 (the apparatus is not shown in FIG. 3), and converting it into acetic acid,
A product with most of the methacrylic acid etc. removed was used. Table 5 shows the composition and flow rate of each part when the operation reaches a nearly steady state.

【table】

【table】

【table】

【table】

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a schematic flow sheet of the first and second inventions of the present invention, and FIG. 2 is a flow sheet of the third invention of the present invention.
1 is a schematic flow sheet of the invention. FIG. 3 is a flow sheet for a two-stage oxidation reaction method that takes these into consideration. 2... Oxidation reaction step, 4... Rapid cooling step, 8...
Methacrylic acid recovery step, 13...acetic acid recovery step,
16... methacrolein recovery step, 1, 3, 5,
7, 9, 10, 11, 12, 14, 15, 17,
18, 20... Conduit, 101... Isobutylene oxidation reactor, 102... Methacrolein oxidation reactor, 103... Quenching tower, 104... Methacrolein recovery tower, 105... Methacrylic acid extraction tower, 10
6... Extractant separation column, 107... Acetic acid extraction column, 10
8... Extractant separation tower, 109... Methacrolein absorption tower, 110... Stripper.

Claims (1)

[Claims] 1. In producing methacrylic acid using isobutylene, t-butanol, methacrolein, isobutyraldehyde, isobutyric acid, or a mixture thereof as a raw material by a gas phase catalytic oxidation method using a gas containing molecular oxygen, A method for producing methacrylic acid, characterized in that a part of the acetic acid aqueous solution produced is recycled and supplied to the oxidation reaction step. 2. When producing methacrylic acid using a gas phase catalytic oxidation method using a gas containing molecular oxygen using isobutylene, t-butanol, methacrolein, isobutyraldehyde, isobutyric acid, or a mixture thereof as a raw material, one of the acetic acid aqueous solutions produced as a by-product. Methacrylic acid characterized in that part of the oxidation reaction product gas is circulated and supplied to a quenching step for rapidly cooling the product gas, and the product gas is separated into an aqueous solution part containing a high boiling point compound and a small amount of methacrolein and a gas part. manufacturing method. 3. When producing methacrylic acid using a gas phase catalytic oxidation method using a gas containing molecular oxygen using isobutylene, t-butanol, methacrolein, isobutyraldehyde, isobutyric acid, or a mixture thereof as a raw material, one of the acetic acid aqueous solutions produced as a by-product. Methacrolein is recycled from the residual gas obtained by rapidly cooling the produced gas and removing high-boiling point compounds to a methacrolein recovery process, absorbing and separating methacrolein, and returning at least a part of it to the oxidation process. Acid production method.