JP4091766B2 - Method for producing methacrolein - Google Patents

Description

【００８５】
このようなメタクロレインの製造プロセスを連続運転したところ、約３００日間の運転はきわめて安定しており、工業的に問題のないプロセスであった。
【００８６】
＜比較例１＞
実施例１で使用した図２に示されるプロセスのうち、メタクリル酸回収塔２４２を削除し、メタクロレイン吸収塔出ガス６はライン７の排出ガスとリサイクルガス３に分岐させて運転を行った。その結果、時間あたり約０．０５ｋｇのメタクリル酸が漏洩し、メタクリル酸とメタクロレインの総合収率が低下した。また、約１００日間の運転後、汚れ成分濃度の上昇によってメタクリル酸凝縮塔冷却器２２１の汚れが進行し、運転継続が不可能となった。
【００８７】
＜参考例１＞
実施例１で使用した図２に示されるプロセスのうち、メタクリル酸蒸留塔デカンター２５４の温度を約２０℃まで上げて運転を行った。その結果、デカンターで分離したメタクロレイン相中の水の濃度が約３質量％に増加した。
【００８８】
【発明の効果】
本発明によれば、工業的に有利な条件でメタクリル酸水溶液を連続的に製造しながら、同時に、農薬や医薬品等の貴重な出発原料となり得る高品質のメタクロレイン液を製造できるメタクロレインの製造方法を提供することができる。さらには、第１段酸化反応の転化率を比較的広い範囲で管理して運転することができ、触媒寿命の延長が可能である。
【図面の簡単な説明】
【図１】本発明のメタクロレインの製造方法を示すフローシートである。
【図２】実施例１のメタクロレインの製造方法を示すフローシートである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing methacrolein, more specifically, a method for producing methacrylic acid by catalytic gas phase oxidation reaction of isobutylene and / or tert-butyl alcohol. The present invention relates to a method for producing methacrolein that can be recycled and reused, obtained while controlling the quality of liquid methacrolein and used as a raw material for derivatives.
[0002]
[Prior art]
Conventionally, isobutylene, tert-butyl alcohol, or a mixture thereof is contact-oxidized with a molecular oxygen-containing gas in the first stage oxidation step, the reaction product gas is supplied to the second stage oxidation step, and molecular oxygen is again supplied. A number of industrial catalysts have been developed with respect to a method for producing methacrylic acid, which obtains methacrylic acid by catalytic oxidation with a contained gas. However, the performance of developed industrial catalysts is not always sufficient, and various processes have been proposed for producing methacrylic acid using these industrial catalysts efficiently for a long period of time.
[0003]
Usually, in order to use these industrial catalysts effectively for a long period of time, the first stage oxidation reaction is operated at a conversion rate of about 90 to 97%, and in the second stage oxidation reaction, the conversion rate is 50%. Drive at ~ 80%. As a result, unreacted isobutylene and tert-butyl alcohol are generated in the first stage oxidation reaction, and unreacted methacrolein is generated in the second stage oxidation reaction. Circulating and reusing gas containing unreacted substances and recovering unreacted substances are performed.
[0004]
As a method for producing methacrylic acid by a catalytic oxidation reaction of isobutylene or the like, for example, as disclosed in JP-B-6-84326 and JP-A-56-108735, there are the following methods.
[0005]
First, in the first stage oxidation reaction step, isobutylene or the like is subjected to catalytic gas phase oxidation to obtain methacrolein and a small amount of methacrylic acid in a gaseous state. Then, the obtained reaction product gas is supplied as it is to the second stage oxidation reaction step, and contact oxidation is performed to obtain a gas containing methacrylic acid. Next, this gas is sent to a methacrylic acid condensing tower where it is brought into contact with water or the like to obtain a methacrylic acid aqueous solution. Then, a gas containing unreacted methacrolein coming out of the methacrylic acid condensation tower is sent to the methacrolein absorption tower, where it is further brought into contact with water or the like to absorb and recover methacrolein. The obtained methacrolein aqueous solution is vaporized by contacting with the second oxidation process supply gas in the methacrolein diffusion tower, and supplied as a raw material gas to the second oxidation reaction process.
[0006]
By the way, in recent years, methacrolein has become important as a valuable starting material for agricultural chemicals and pharmaceuticals. However, when quality and purity are required as starting materials for agricultural chemicals, pharmaceuticals, etc., the conventional process of vaporizing methacrolein in a stripping tower can effectively acquire methacrolein without adding new steps. Have difficulty.
[0007]
On the other hand, the cost of the catalyst accounts for a large part of the production price of methacrylic acid, and extending the life of the catalyst is effective for reducing the cost. However, in the process of supplying the gas after the first stage oxidation reaction directly to the second stage oxidation reaction as described above, in order to prevent the performance deterioration of the second stage oxidation reaction catalyst, the conversion in the first stage oxidation reaction is performed. The rate must be kept high, and the first stage oxidation reaction must be operated at a high reaction temperature that tends to lead to catalyst degradation. This is not preferable from the viewpoint of extending the life of the first stage oxidation reaction catalyst.
[0008]
In addition, in the process made in consideration of the transition of the first stage oxidation reaction at a high conversion rate, most of the useful components (methacrylic acid, methacrolein) in the outgas of both oxidation reactions are recovered. Then, the recovered residual gas is led to the combustion reactor, and useful components in the gas (isobutylene, unrecovered methacrolein, methacrylic acid, etc.) are combusted and then circulated in the reaction process, and the composition of the reactor inlet gas is determined. Many have taken control. However, in such a process, it is impossible to recover and circulate gaseous isobutylene, which is an unreacted product of the first stage oxidation reaction, and it is an industrial practice to operate at a low conversion rate of the first stage oxidation reaction. It becomes disadvantageous.
[0009]
[Problems to be solved by the invention]
The present invention provides a method for producing methacrolein capable of producing a high-quality methacrolein solution that can be a valuable starting material for agricultural chemicals, pharmaceuticals, and the like while continuously producing an aqueous methacrylic acid solution under industrially advantageous conditions. The purpose is to provide. Furthermore, an object of the present invention is to provide a method for producing methacrolein that can be operated by controlling the conversion rate of the first stage oxidation reaction in a relatively wide range and can extend the catalyst life.
[0010]
[Means for Solving the Problems]
The above object of the present invention can be achieved by the following present invention.
(1) a reaction step (A) for obtaining a reaction gas containing methacrolein and methacrylic acid by catalytically oxidizing raw isobutylene and / or tert-butyl alcohol with a molecular oxygen-containing gas;
A methacrylic acid condensation step (B) in which the reaction gas obtained in the reaction step (A) is brought into contact with a methacrylic acid and acetic acid-containing aqueous phase, and methacrylic acid and water in the reaction gas are condensed to obtain a methacrylic acid aqueous solution;
The methacrolein and methacrylic acid-containing gas remaining after the methacrylic acid and water are removed by condensation in the methacrylic acid condensation step (B) are brought into contact with the aqueous phase containing methacrylic acid and acetic acid, and the methacrolein in the gas contains methacrylic acid and acetic acid. Methacrolein absorption step (C) to be absorbed in the aqueous phase;
In the methacrolein absorption step (C), the aqueous phase containing methacrylic acid and acetic acid that has absorbed methacrolein is sent to the distillation column, and the vapor containing methacrolein and water is distilled off from the top of the distillation column. A methacrolein distillation step (D) in which methacrolein is separated by removing the bottoms containing methacrylic acid, acetic acid and water to obtain methacrolein;
In the methacrolein absorption step (C), the methacrylic acid-containing gas remaining after removing the methacrolein is brought into contact with the acetic acid-containing aqueous phase, and the methacrylic acid in the gas is absorbed into the acetic acid-containing aqueous phase and recovered. )When
For producing methacrolein.
(2) The method for producing methacrolein according to (1), wherein the concentration of methacrylic acid in the aqueous phase containing methacrylic acid and acetic acid used in the methacrolein absorption step (C) is 15% by mass or more.
(3) In the methacrylic acid condensation step (B), the methacrylic acid concentration in the methacrolein and the methacrylic acid-containing gas remaining after removing methacrylic acid and water by condensation is 0.8% by volume or more (1) or ( 2) A method for producing methacrolein.
(4) The method for producing methacrolein according to any one of (1) to (3), wherein the acetic acid concentration in the acetic acid-containing aqueous phase used in the methacrylic acid recovery step (E) is 8% by mass or more.
(5) The methacrylic acid and acetic acid-containing aqueous phase used in the methacrolein absorption step (C) contains methacrylic acid, acetic acid and water taken out from the bottom of the distillation column in the methacrolein distillation step (D). The method for producing methacrolein according to any one of (1) to (4) above.
(6) The methacrylic acid and acetic acid-containing aqueous phase used in the methacrylic acid condensation step (B) is the acetic acid-containing aqueous phase that has absorbed methacrylic acid in the methacrylic acid recovery step (E). Any method for producing methacrolein.
(7) Part of the isobutylene-containing gas remaining after the recovery of methacrylic acid in the methacrolein recovery step (E) is returned to the reaction step (A), and in the reaction step (A), part of the isobutylene-containing gas and isobutylene as a raw material The method for producing methacrolein according to any one of the above (1) to (6), wherein a gas mixed with tert-butyl alcohol is contact-oxidized with a molecular oxygen-containing gas.
(8) The vapor containing methacrolein and water evaporated from the top of the distillation column in the methacrolein distillation step (D) is condensed, and this condensate is sent to a decanter, and is converted into a methacrolein phase and an aqueous phase by the decanter. The method for producing methacrolein according to any one of (1) to (7) above.
(9) The aqueous phase separated from the methacrolein phase by a decanter is mixed with the aqueous phase containing methacrylic acid and acetic acid that has absorbed the methacrolein sent to the methacrolein distillation step (D), and this mixed solution is mixed with the methacrolein distillation step ( The method for producing methacrolein according to (8) above, which is sent to D).
(10) The method for producing methacrolein according to (8) or (9), wherein the concentration of water contained in the methacrolein phase can be controlled by lowering the operating temperature of the decanter.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The method for producing methacrolein of the present invention includes the reaction step (A), the methacrylic acid condensation step (B), the methacrolein absorption step (C), the methacrolein distillation step (D), and the methacrylic acid recovery step (E In addition, it is possible to produce a high-quality methacrolein solution that can be a valuable starting material for agricultural chemicals and pharmaceuticals at the same time while continuously producing an aqueous methacrylic acid solution under industrially advantageous conditions. Furthermore, it is possible to operate by controlling the conversion rate of the first stage oxidation reaction in a relatively wide range, and it is possible to extend the catalyst life.
[0012]
The concentration of methacrylic acid in the methacrylic acid and acetic acid-containing aqueous phase used in the methacrolein absorption step (C) is preferably 15% by mass or more because a sufficiently high methacrolein absorption capability is obtained. By improving the methacrolein absorption efficiency, the leakage of methacrolein to the exhaust gas can be reduced. Further, in order to keep the methacrylic acid concentration in the aqueous phase containing methacrylic acid and acetic acid used in the methacrolein absorption step (C) within this range, methacrolein and methacrylic acid-containing gas (methacrylic acid) introduced into the methacrolein absorption step (C). The methacrylic acid concentration in the gas remaining after the methacrylic acid and water are removed by condensation in the acid condensation step (B) is preferably 0.8% by volume or more.
[0013]
The acetic acid concentration in the acetic acid-containing aqueous phase used in the methacrylic acid recovery step (E) is preferably 8% by mass or more because a sufficiently high methacrylic acid absorption capacity is obtained. Leakage of methacrylic acid into the exhaust gas can be reduced by leading the outgas from the methacrolein absorption step (C) to the methacrylic acid recovery step (E) and substantially removing methacrylic acid.
[0014]
As the methacrylic acid and acetic acid-containing aqueous phase used in the methacrolein absorption step (C), a bottom liquid containing methacrylic acid, acetic acid and water taken out from the bottom of the distillation column in the methacrolein distillation step (D) is used. It is preferable. Thereby, the methacrylic acid and acetic acid containing water phase used in the methacrolein absorption process (C) required in large quantities can be covered without draining.
[0015]
As the methacrylic acid and acetic acid-containing aqueous phase used in the methacrylic acid condensation step (B), it is preferable to use an acetic acid-containing aqueous phase in which methacrylic acid is absorbed in the methacrylic acid recovery step (E). Thereby, in addition to recovering methacrylic acid, the methacrylic acid and acetic acid-containing aqueous phase used in the methacrylic acid condensation step (B) can be covered without draining.
[0016]
A part of the isobutylene-containing gas that remains after the methacrylic acid is recovered in the methacrolein recovery step (E) is returned to the reaction step (A) and mixed with the raw material isobutylene and / or tert-butyl alcohol. Is preferably contact-oxidized with a molecular oxygen-containing gas. Part of the isobutylene-containing gas from which methacrolein and the methacrylic acid have been substantially removed from the methacrolein recovery step (E) is directly sent to the reaction step (A) without being burned, mixed with the raw material gas and recycled. By doing so, it is possible to adjust the gas (raw material gas) at the inlet of the reaction step (A) to an optimum composition for the first stage oxidation catalyst and to operate without lowering the utilization rate of isobutylene. This method is particularly effective under operating conditions where the conversion of isobutylene in the reaction step (A) is low.
[0017]
In the method for producing methacrolein of the present invention, the vapor containing methacrolein and water distilled from the top of the distillation column in the methacrolein distillation step (D) is usually condensed, and this condensate is decanted. And then separated into a methacrolein phase and an aqueous phase with a decanter. When the decanter operation temperature is set to be equal to or lower than the temperature at which methacrolein and water are separated, the methacrolein phase and the aqueous phase can be favorably separated.
[0018]
The aqueous phase separated from the methacrolein phase by the decanter is mixed with the aqueous phase containing methacrylic acid and acetic acid which has absorbed methacrolein sent to the distillation column in the methacrolein distillation step (D), and this mixed solution is sent to the distillation column. Is preferred. The water phase separated from the methacrolein phase by the decanter is mixed with the feed liquid of the methacrolein distillation step (D) and distilled again, so that the operation can be performed without discharging wastewater containing a small amount of methacrolein.
[0019]
Moreover, it is possible to control the water concentration contained in the methacrolein phase by lowering the operating temperature of the decanter. This makes it possible to produce high-purity methacrolein that is particularly useful as a raw material for agricultural chemicals and pharmaceuticals.
[0020]
Hereinafter, the present invention will be described with reference to the drawings.
[0021]
In FIG. 1, the flow sheet of an example of the manufacturing method of the methacrolein of this invention is shown. The present invention is not limited to the one shown in FIG.
[0022]
In the reaction step (A), a raw material gas containing isobutylene and / or tert-butyl alcohol and a molecular oxygen-containing gas is oxidized in the presence of a catalyst, and a reaction gas containing methacrolein and methacrylic acid (reaction step output gas) ) In the raw material gas, a part of the gas containing isobutylene after the recovery of methacrylic acid in the methacrylic acid recovery step (E) described later (the methacrylic acid recovery step output gas) is mixed.
[0023]
The catalytic oxidation reaction of isobutylene and / or tert-butyl alcohol may be performed according to a known method.
[0024]
As source gas (reactor-containing gas), isobutylene and / or tert-butyl alcohol, molecular oxygen-containing gas such as air, and the purpose of adjusting the concentration of isobutylene, tert-butyl alcohol and oxygen in the source gas A mixture with an inert gas (nitrogen, carbon dioxide, etc.) added in step (b) is used. In addition, the raw material gas may contain water, a small amount of hydrocarbons, and the like.
[0025]
As an oxygen source supplied for the oxidation reaction, it is preferable to use air from the viewpoint of economy, but it is also possible to use partially pure oxygen.
[0026]
The catalyst used in this reaction is not particularly limited, and known ones can be used. Examples of such a catalyst include a molybdenum-bismuth-iron composite oxide catalyst.
[0027]
The concentration of isobutylene and / or tert-butyl alcohol in the raw material gas can be varied within a wide range, but is usually 1 to 20% by volume, preferably 3 to 10% by volume. The amount of oxygen used with respect to isobutylene and / or tert-butyl alcohol is usually preferably 0.5 to 20 mole times, particularly preferably about 2 to 5 mole times.
[0028]
Usually, the reaction temperature is preferably 300 ° C. or higher because high activity is obtained, and 450 ° C. or lower is preferable in terms of catalyst life. The reaction pressure is from normal pressure to several atmospheres.
[0029]
The reaction may be a fixed bed or a fluidized bed.
[0030]
The conversion of isobutylene and / or tert-butyl alcohol in this reaction is preferably 80% or more, particularly preferably 90% or more, considering the recovery of isobutylene and the economics of the process. Further, the conversion of isobutylene and / or tert-butyl alcohol is preferably 99% or less, particularly preferably 97% or less, in order to extend the life of the catalyst.
[0031]
In FIG. 1, in order to avoid the raw material gas composition from entering the explosion range and to fully develop the performance of the catalyst, the gas remaining after the recovery of methacrylic acid from the methacrylic acid recovery step (E) (methacrylic acid recovery step) (Outgoing gas) is circulated. In the present invention, it is not necessary to remove organic components contained in a small amount in the methacrylic acid recovery step output gas by means such as combustion.
[0032]
In the methacrylic acid condensation step (B), the reaction step output gas obtained in the reaction step (A) is brought into contact with a methacrylic acid and acetic acid-containing aqueous phase, and methacrylic acid and water in the reaction step output gas are condensed to form methacrylic acid. An aqueous acid solution is obtained. In FIG. 1, as the methacrylic acid and acetic acid-containing aqueous phase, an acetic acid-containing aqueous phase in which methacrylic acid is absorbed in the methacrylic acid recovery step (E) described later is used. The methacrylic acid concentration and the acetic acid concentration in the methacrylic acid and acetic acid-containing aqueous phase are not particularly limited, but are preferably higher in consideration of extraction efficiency in the subsequent steps.
[0033]
In the methacrylic acid condensation step (B), the temperature of the reaction step gas obtained mainly in the reaction step (A) is lowered to condense methacrylic acid and water, which are by-products of the first stage oxidation reaction. .
[0034]
The gas remaining after the methacrylic acid and water are removed by condensation in the methacrylic acid condensation step (B) (the gas exiting the methacrylic acid condensation step) mainly contains methacrolein, and a small amount of methacrylic acid that has not been removed by the condensation. Is included. The concentration of methacrylic acid in the methacrolein and methacrylic acid-containing gas (methacrylic acid condensation process gas) is preferably 0.8% by volume or more.
[0035]
In order to bring the methacrylic acid concentration in the methacrylic acid condensation process output gas into the above range, the control temperature of the methacrylic acid condensation process (B) is, for example, a pressure of 0.35 kgf / cm. ² In the case of -G, it is preferably 50 ° C or higher.
[0036]
Further, in the methacrylic acid condensation step (B), it is preferable to circulate a large amount of the acetic acid-containing aqueous phase in which methacrylic acid has been absorbed in the methacrylic acid recovery step (E) to effectively condense methacrylic acid and water. .
[0037]
In the methacrylic acid condensation step (B), examples of the method of bringing the reaction step output gas into contact with the aqueous phase containing methacrylic acid and acetic acid include a method of using an absorption device such as a spray tower, a packed tower, and a plate tower. The operation conditions are not particularly limited as long as methacrylic acid and water in the reaction process gas can be condensed to obtain an aqueous methacrylic acid solution. However, methacrolein and methacrylic acid remaining after the methacrylic acid and water are removed by condensation are not limited. Operating conditions such that the methacrylic acid concentration in the acid-containing gas is 0.8% by volume or more are preferred.
[0038]
In the methacrolein absorption step (C), the methacrolein and the methacrylic acid-containing gas remaining after the methacrylic acid and water are removed by condensation in the methacrylic acid condensation step (B) (the methacrylic acid condensation step-out gas) contains methacrylic acid and acetic acid. The aqueous phase (absorbed water) is brought into contact, and the methacrolein in the methacrylic acid condensation process output gas is absorbed into the aqueous phase containing methacrylic acid and acetic acid. Thereby, most of the methacrolein produced | generated by the reaction process (A) contained in the methacrylic acid condensation process output gas is absorbed by absorption water. In FIG. 1, the methacrylic acid and acetic acid-containing aqueous phase is used by circulating the bottoms containing methacrylic acid, acetic acid and water taken out from the bottom of the distillation column in the methacrolein distillation step (D) described later. Yes.
[0039]
The concentration of methacrylic acid in the methacrylic acid and acetic acid-containing aqueous phase (absorbing water) used in the methacrolein absorption step (C) is preferably kept at 15% by mass or more because sufficiently high methacrolein absorption performance is obtained. Further, when the concentration of methacrylic acid in the absorption water is increased, the use performance can be reduced because a significant decrease in absorption performance is not observed even when the supply temperature of the absorption water is relatively high. For example, tower top pressure (top tower pressure) 0.3 kgf / cm ² In -G, it can drive | operate in the range of 5-25 degreeC (temperature of absorbed water).
[0040]
The concentration of acetic acid in the aqueous phase containing methacrylic acid and acetic acid is not particularly limited.
[0041]
In the methacrolein absorption step (C), an absorption tower using a normal tray or an absorption tower using various fillers can be used, but an absorption tower using various fillers is preferably used. In general, an absorption tower using various fillers has a lower pressure loss than an absorption tower using a tray having a number of stages having comparable absorption performance. If the pressure loss is low, the pressure loss of the gas flowing from the reaction step (A) to the methacrylic acid recovery step (E) can be reduced. Therefore, the gas remaining after the recovery of methacrylic acid in the methacrylic acid recovery step (E) ( This is effective in reducing the usefulness of the gas compressor used for increasing the pressure of the methacrylic acid recovery step output gas) and supplying it to the reaction step (A).
[0042]
In FIG. 1, the methacrylic acid and acetic acid-containing aqueous phase (absorbing liquid) that has absorbed methacrolein in the methacrolein absorption step (C) is led to the subsequent methacrolein distillation step (D). On the other hand, the bottoms containing methacrylic acid and acetic acid taken out from the bottom of the distillation column in the methacrolein distillation step (D) described later is returned to the methacrolein absorption step (C) and used as absorbed water. . In this way, by circulating the absorption water between the methacrolein absorption step (C) and the methacrolein distillation step (D), it is possible to supply a large amount of absorption water without discharging a large amount of waste water. it can.
[0043]
In the methacrolein absorption step (C), methacrylic acid and water other than methacrolein distilled and separated in the methacrolein distillation step (D) are also absorbed, so that the amount of absorbed water used in circulation gradually increases. For this reason, it is necessary to extract incremental increments from either process, and the extraction destination includes a method leading to the methacrylic acid condensation step (B) and a method leading to another step. It can be selected depending on the necessity of renewing the process liquid.
[0044]
Further, both the aqueous phase supplied from the methacrolein absorption step (C) to the methacrolein distillation step (D) and the aqueous phase supplied from the methacrolein distillation step (D) to the methacrolein absorption step (C) are both a large amount. Therefore, in order to adjust the supply temperature optimal for each process, it is preferable to use a liquid-liquid heat exchanger that exchanges heat with each other from the viewpoint of reducing the usage.
[0045]
In the methacrolein absorption step (C), the operation condition of the absorption tower is not particularly limited as long as it can absorb the methacrolein in the methacrylic acid condensation step output gas, and may be determined as appropriate. For example, tower top pressure (top tower pressure) 0.3 kgf / cm ² In -G, it can drive | operate in the range of 5-25 degreeC (temperature of absorbed water).
[0046]
In the methacrolein distillation step (D), the methacrylic acid and acetic acid-containing aqueous phase (absorbing liquid) that has absorbed methacrolein in the methacrolein absorption step (C) is sent to the distillation tower, and steam containing methacrolein and water is produced. Distillation and separation of methacrolein are carried out by distilling from the top of the column and taking out the bottoms containing methacrylic acid, acetic acid and water from the bottom of the distillation column to obtain liquid purified methacrolein. This purified methacrolein usually has a purity of 90% by mass or more.
[0047]
In the methacrolein distillation step (D), a distillation tower using a normal tray or a distillation tower using various fillers can be used. In particular, the lower part of the supply stage may show a reduction in methacrolein separation efficiency due to an increase in the concentration of methacrylic acid in the absorption water, so that a sufficient number of stages may be required. Is preferably a distillation column using various fillers.
[0048]
An azeotropic composition of water and methacrolein distilled from the top of the distillation column, and an aqueous phase containing methacrylic acid and acetic acid from which the methacrolein was substantially distilled and separated from the bottom of the distillation column as the bottoms. Taken out. This bottoms is returned to the methacrolein absorption step (C) and used as an aqueous phase (absorption water) containing methacrylic acid and acetic acid.
[0049]
The bottoms usually contain a small amount of methacrolein. Since the methacrolein in the bottoms largely affects the absorption performance of the methacrolein absorption step (C) used in circulation, the lower the better. Industrially, it is preferable to control the tower bottom temperature between 90 to 110 ° C. so as to be 150 ppm or less.
[0050]
The composition of methacrolein distilled from the top of the tower and water changes in composition depending on the tower top temperature, and the water tends to increase as the tower top temperature rises. Moreover, when the column top temperature is high, the concentration of methacrylic acid contained in a small amount in the vapor distilled from the column top tends to increase.
[0051]
Methacrylic acid contained in the vapor distilled from the top of the column will be contained in the methacrolein liquid obtained from the methacrolein distillation step (D). When the obtained methacrolein liquid is used as a raw material for the second stage oxidation process, methacrylic acid in the methacrolein may be oxidized in the second stage oxidation process, and the total of the first stage and second stage oxidation processes. The yield may be reduced. Therefore, it is preferable that the methacrylic acid concentration in the methacrolein liquid and the methacrylic acid concentration in the vapor distilled from the top of the tower be as low as possible. Industrially, a distillation column of a methacrolein liquid (a methacrolein phase separated from an aqueous phase by a decanter described later) so as to control the concentration of methacrylic acid in the methacrolein from the relationship with the utility load used. It is preferable to control the amount of reflux to the top.
[0052]
The operating conditions of the distillation column are not particularly limited, and may be determined as appropriate so that the composition of the vapor distilled from the top of the column becomes appropriate.
[0053]
Vapor containing methacrolein and water distilled from the top of the distillation column is led to a condenser and condensed. The condensing temperature affects the subsequent decanter operating temperature, and thus is appropriately selected depending on how the moisture contained in methacrolein is managed. It is preferable that the condensation temperature is usually selected so that the liquid temperature of the condensate led to the decanter is 4 to 10 ° C.
[0054]
The vapor condensate distilled from the top of the distillation column is sent to a decanter, where it is separated into a methacrolein phase and an aqueous phase. The decanter operating temperature may be appropriately determined below the temperature at which methacrolein and water are separated. In addition, since some of the impurities in the raw material isobutylene and tert-butyl alcohol affect the water concentration contained in methacrolein, the decanter operating temperature needs to take these effects into consideration. The residence time in the decanter may be appropriately determined within a range in which a desired degree of separation can be achieved.
[0055]
The aqueous phase separated from the methacrolein phase by the decanter contains methacrolein and methacrylic acid corresponding to the solubility at the decanter operating temperature. Therefore, the aqueous phase separated by the decanter may be drained, but it is circulated between the methacrolein absorption step (C) and the methacrolein distillation step (D) from the viewpoint of environmental and overall yield improvement. It is preferable to recover the methacrolein in the aqueous phase by distilling and separating again in the methacrolein distillation step (D) in a large amount of absorbed water. In particular, the aqueous phase separated by the decanter is put into an absorption liquid (methacrylic acid and acetic acid-containing aqueous phase in which methacrolein has been absorbed) sent from the methacrolein absorption step (C) to the methacrolein distillation step (D), It is preferable to send it to a distillation column and distill again. In addition, depending on the degree of renewal of the aqueous phase containing methacrylic acid and acetic acid used in the methacrylic acid condensation step (B), the aqueous phase separated by the decanter is changed to the aqueous phase containing methacrylic acid and acetic acid used in the methacrylic acid condensation step (B). The methacrolein in the aqueous phase is volatilized and supplied to the methacrolein absorption step (C).
[0056]
In the methacrylic acid recovery step (E), the methacrylic acid-containing gas that remains after the removal of methacrolein in the methacrolein absorption step (C) is brought into contact with an acetic acid-containing aqueous phase (absorbed water), and methacrylic acid in the gas is removed. It is absorbed in an acetic acid-containing aqueous phase and recovered to obtain a gas from which methacrolein and methacrylic acid have been substantially removed.
[0057]
In the methacrylic acid recovery step (E), the concentration of methacrylic acid in the absorbed water (methacrylic acid and acetic acid-containing aqueous phase) used in the methacrolein absorption step (C) is preferably as high as 15% by mass or more. A relatively large amount of methacrylic acid is contained in the gas from C) (methacrylic acid-containing gas), and this is carried out for the purpose of recovering this and suppressing a decrease in the overall yield. When the methacrylic acid in the methacrylic acid-containing gas is sent directly to the reaction step (A) without being recovered and used as a raw material gas, the methacrylic acid is oxidized again in the reaction step (A), and methacrolein or methacrylic acid, which is the target product. It may change to something other than acid. For this reason, it is preferable to perform the methacrylic acid recovery step (E) in order to prevent a decrease in yield.
[0058]
In the methacrylic acid recovery step (E), an absorption tower using a normal tray and an absorption tower using various fillers can be used as in the above-described methacrolein absorption step (C), but pressure loss is suppressed. Therefore, it is preferable to use an absorption tower using various fillers. Further, the methacrylic acid recovery step (E) can be provided at the upper part of the tower integrated with the methacrolein absorption step (C).
[0059]
The acetic acid concentration in the acetic acid-containing aqueous phase used in the methacrylic acid recovery step (E) is preferably 8% by mass or more. The acetic acid-containing aqueous phase is preferably substantially free of methacrylic acid. However, since waste water can be reduced, methacrylic acid and water in the reaction process output gas in the methacrylic acid condensation step (B) It is also preferable to use an aqueous phase obtained by substantially extracting and removing methacrylic acid from an aqueous methacrylic acid solution obtained by condensing the water.
[0060]
In FIG. 1, the acetic acid-containing aqueous phase in which methacrylic acid has been absorbed in the methacrylic acid recovery step (E) is sent to the methacrylic acid condensation step (B) and used as the methacrylic acid and acetic acid-containing aqueous phase. Thereby, the methacrylic acid absorbed in the acetic acid-containing aqueous phase can be taken out and recovered as an aqueous methacrylic acid solution.
[0061]
The operating temperature in the methacrylic acid recovery step (E) is preferably as low as possible, but is industrially preferably selected appropriately between 5 and 20 ° C.
[0062]
In the methacrylic acid recovery step (E), the method of contacting the methacrylic acid-containing gas with the acetic acid-containing aqueous phase (absorbed water) includes, for example, a method of using an absorption device such as a spray tower, a packed tower, or a plate tower. The operating conditions are not particularly limited as long as methacrylic acid in the gas is absorbed and recovered by the acetic acid-containing aqueous phase, and methacrolein and methacrylic acid can be substantially removed.
[0063]
In FIG. 1, the gas from which methacrylic acid and methacrolein are substantially removed from the methacrylic acid recovery step (E) (methacrylic acid recovery step output gas) is partially pressurized by a gas compressor, and the reaction step ( A is supplied to A), mixed with the raw material gas in the reaction step (A), and catalytically oxidized. The remaining methacrylic acid recovery process output gas is burned, detoxified and discharged in the subsequent process.
[0064]
A turbine or motor using high-pressure steam is used for driving the gas compressor, but it is preferable to effectively use the oxidation heat generated in the reaction step (A) as the energy source.
[0065]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0066]
<Example 1>
According to the flow sheet shown in FIG. 2, methacrolein was produced.
[0067]
Isobutylene was used as a raw material. In order to recover unreacted isobutylene, the pressure is increased by the circulating gas compressor 230 and is supplied from the line 2 to adjust the circulating gas introduced from the line 3 and the oxygen concentration at the inlet of the reactor 210 and the raw isobutylene concentration. Air was mixed, and isobutylene gas supplied from the line 1 was mixed therewith to obtain an inlet gas (raw material gas) of the reactor 210. The isobutylene concentration in the raw material gas was 6% by volume, and the oxygen concentration was 14% by volume.
[0068]
The reactor 210 was filled with a molybdenum-bismuth-iron composite oxide catalyst, and methacrolein was obtained by catalytic gas phase oxidation of isobutylene. The conversion of isobutylene was about 95%.
[0069]
The outgas (reactor outgas) from the reactor 210 containing methacrolein was supplied from the line 4 to the methacrylic acid condensation tower 220. Then, in the methacrylic acid condensing tower 220, a large amount of circulating liquid is sprayed on the high-temperature reactor outlet gas to quench it, and at the same time, methacrylic acid and water, which are by-products contained in the reactor outlet gas, are condensed to methacrylic acid. An acid aqueous solution was obtained. The pressure of the methacrylic acid condensation tower 220 is 0.35 kgf / cm. ² -G, gas temperature (control temperature) was about 55 ° C. Here, as the circulating liquid, an acetic acid aqueous solution that absorbed methacrylic acid in the methacrylic acid recovery tower 242 supplied from the line 9 and a part of the bottom liquid of the methacrolein distillation tower 250 supplied from the line 12 were used. . This circulating liquid was an aqueous phase containing methacrylic acid and acetic acid having a methacrylic acid concentration of 30% by mass and an acetic acid concentration of 6% by mass. The heat load necessary for the rapid cooling was provided by exchanging heat with cold water in the methacrylic acid condenser tower cooler 221.
[0070]
The aqueous methacrylic acid solution obtained by condensing methacrylic acid and water was led to the subsequent process from the line 17 and recovered by extracting and removing methacrylic acid. Further, a part of the liquid containing the remaining acetic acid from which methacrylic acid was extracted and removed was supplied from the line 8 to the methacrylic acid recovery tower 242.
[0071]
The output gas from the methacrylic acid condensation tower 220 (methacrylic acid condensation tower output gas) contained methacrolein and a small amount of methacrylic acid. The methacrylic acid concentration in the methacrylic acid condensation tower exit gas was 1% by volume.
[0072]
The methacrylic acid condensation tower output gas was supplied from line 5 to methacrolein absorption tower 240. In the methacrolein absorption tower 240, as the absorption water, the bottoms taken out from the bottom of the methacrolein distillation tower 250 is absorbed by an absorption liquid liquid heat exchanger 244 and an absorption liquid cooler 243 using cold water as a refrigerant. The temperature is adjusted to ° C. and supplied from the line 10. The methacrylic acid concentration in the absorption water was 19.6% by mass. Then, in the methacrolein absorption tower 240 (packed tower), the methacrylic acid condensation tower gas is supplied from the lower part of the tower, and a large amount of absorbed water (methacrylic acid and acetic acid-containing aqueous phase) is supplied from the top of the tower to make countercurrent contact. The methacrolein in the gas was absorbed into the absorption water.
[0073]
As the methacrolein absorption tower 240, an absorption tower filled with a regular packing, having a sufficient theoretical stage, and preventing liquid entrainment was used.
[0074]
The outgas from the methacrolein absorption tower 240 (methacrolein absorption tower outgas) was supplied from the line 6 to the methacrylic acid recovery tower 242 (packed tower). Then, in the methacrylic acid recovery tower 242, the gas discharged from the methacrolein absorption tower was brought into contact with the acetic acid aqueous solution supplied from the line 8, and the methacrylic acid in the gas was absorbed into the acetic acid aqueous solution and recovered. Here, the aqueous acetic acid solution supplied from the line 8 is a part of the liquid containing the remaining acetic acid obtained by substantially extracting and removing methacrylic acid from the aqueous methacrylic acid solution obtained in the methacrylic acid condensing tower 220. is there. The acetic acid concentration of this acetic acid aqueous solution was 8% by mass.
[0075]
As the methacrylic acid recovery tower 242, the one in which the regular packing is filled inside the recovery tower, which has a sufficient theoretical stage and prevents liquid entrainment is used. The temperature (operating temperature) of the gas output from the methacrylic acid recovery tower 242 was about 17 ° C.
[0076]
The acetic acid aqueous solution which absorbed methacrylic acid was added to the circulating liquid of the methacrylic acid condensation tower 220 through the line 9 and finally led to the subsequent process (extraction of methacrylic acid) from the line 17.
[0077]
In addition, a part of the methacrylic acid recovery tower 242 from which methacrylic acid and methacrolein have been substantially removed includes a part of recovery of unreacted isobutylene, adjustment of oxygen concentration in raw material gas and raw material isobutylene concentration, etc. Therefore, the pressure was raised by the circulating gas compressor 230, mixed with the raw material gas from the line 3, and supplied to the reactor 210, and the rest was discharged out of the system from the line 7. The discharged gas was burned in a combustion furnace and made harmless.
[0078]
On the other hand, the liquid discharged from the methacrolein absorption tower 240 (the liquid discharged from the methacrolein absorption tower) is taken out from the bottom of the methacrolein distillation tower 250 by the absorption liquid liquid heat exchanger 244 and the liquid liquid. The heat exchange was performed to a temperature (85 ° C.) suitable for the supply temperature of the methacrolein distillation column 250. Then, the methacrolein absorption tower effluent is supplied from line 11 to methacrolein distillation tower 250, vapor containing methacrolein and water is distilled off from the top of the tower, and methacrylic acid, acetic acid and water are distilled from the bottom of the distillation tower. The bottoms containing was taken out. The operating condition of the distillation column is a column top pressure of 0.033 kgf / cm. ² -G, tower top temperature 65 ° C., tower bottom pressure 0.068 kgf / cm ² -G, tower bottom temperature was 102 ° C. The concentration of methacrolein in the bottoms taken out from the bottom of the methacrolein distillation column 250 was about 100 ppm by mass.
[0079]
Most of the bottoms taken out from the bottom of the methacrolein distillation tower 250 is circulated from the line 10 to the methacrolein absorption tower 240 to be used as an absorption liquid, and a part of the bottom liquid is used for discharging excess liquid. To methacrylic acid condensation tower 220.
[0080]
The vapor distilled from the top of the methacrolein distillation column 250 was sent from the line 13 to the methacrolein distillation column condenser 252, condensed, and further cooled to about 6 ° C. The vapor condensate distilled from the top of the methacrolein distillation column 250 was sent to the methacrolein distillation column decanter 254 and separated into a methacrolein phase and an aqueous phase. The operating temperature of the methacrolein distillation column decanter 254 was about 6 ° C.
[0081]
A part of the methacrolein phase separated by the methacrolein distillation tower decanter 254 was refluxed from the line 14 to the top of the methacrolein distillation tower 250 and used for adjusting the top temperature. The remainder of the separated methacrolein phase was distilled from line 15 to obtain product methacrolein. The product methacrolein obtained had a purity of 97% by mass, and the concentration of water in methacrolein was 2.3% by mass.
[0082]
On the other hand, the aqueous phase separated by the methacrolein distillation column decanter 254 was extracted from the lower part of the methacrolein distillation column decanter 254 and returned from the line 16 to the supply stage of the methacrolein distillation column 250.
[0083]
Table 1 shows the average flow rate of each line. The unit is kg / H.
[0084]
[Table 1]

[0085]
When such a methacrolein production process was continuously operated, the operation for about 300 days was extremely stable and was an industrially problematic process.
[0086]
<Comparative Example 1>
In the process shown in FIG. 2 used in Example 1, the methacrylic acid recovery tower 242 was deleted, and the methacrolein absorption tower output gas 6 was branched into the exhaust gas of the line 7 and the recycle gas 3 for operation. As a result, about 0.05 kg of methacrylic acid leaked per hour, and the overall yield of methacrylic acid and methacrolein decreased. Further, after the operation for about 100 days, the methacrylic acid condenser tower cooler 221 became dirty due to the increase in the concentration of the dirty component, and the operation could not be continued.
[0087]
<Reference Example 1>
Among the processes shown in FIG. 2 used in Example 1, the operation was carried out by raising the temperature of the methacrylic acid distillation column decanter 254 to about 20 ° C. As a result, the concentration of water in the methacrolein phase separated by the decanter increased to about 3% by mass.
[0088]
【The invention's effect】
According to the present invention, production of methacrolein capable of producing a high-quality methacrolein solution that can be a valuable starting material for agricultural chemicals and pharmaceuticals at the same time while continuously producing an aqueous methacrylic acid solution under industrially advantageous conditions. A method can be provided. Furthermore, the first stage oxidation reaction conversion rate can be controlled and operated in a relatively wide range, and the catalyst life can be extended.
[Brief description of the drawings]
FIG. 1 is a flow sheet showing a method for producing methacrolein of the present invention.
2 is a flow sheet showing a method for producing methacrolein of Example 1. FIG.

Claims (10)

A reaction step (A) for obtaining a reaction gas containing methacrolein and methacrylic acid by catalytically oxidizing raw isobutylene and / or tert-butyl alcohol with a molecular oxygen-containing gas;
A methacrylic acid condensation step (B) in which the reaction gas obtained in the reaction step (A) is brought into contact with a methacrylic acid and acetic acid-containing aqueous phase, and methacrylic acid and water in the reaction gas are condensed to obtain a methacrylic acid aqueous solution;
The methacrolein and methacrylic acid-containing gas remaining after the methacrylic acid and water are removed by condensation in the methacrylic acid condensation step (B) are brought into contact with the aqueous phase containing methacrylic acid and acetic acid, and the methacrolein in the gas contains methacrylic acid and acetic acid. Methacrolein absorption step (C) to be absorbed in the aqueous phase;
In the methacrolein absorption step (C), the aqueous phase containing methacrylic acid and acetic acid that has absorbed methacrolein is sent to the distillation column, and the vapor containing methacrolein and water is distilled off from the top of the distillation column. A methacrolein distillation step (D) in which methacrolein is separated by removing the bottoms containing methacrylic acid, acetic acid and water to obtain methacrolein;
In the methacrolein absorption step (C), the methacrylic acid-containing gas remaining after removing the methacrolein is brought into contact with the acetic acid-containing aqueous phase, and the methacrylic acid in the gas is absorbed in the acetic acid-containing aqueous phase and recovered. And a process for producing methacrolein. The method for producing methacrolein according to claim 1, wherein the concentration of methacrylic acid in the aqueous phase containing methacrylic acid and acetic acid used in the methacrolein absorption step (C) is 15% by mass or more. The methacrolein according to claim 1 or 2, wherein the concentration of methacrylic acid in the methacrolein and the methacrylic acid-containing gas remaining after the methacrylic acid and water are removed by condensation in the methacrylic acid condensing step (B) is 0.8 vol% or more. Rain manufacturing method. The method for producing methacrolein according to any one of claims 1 to 3, wherein the acetic acid concentration in the acetic acid-containing aqueous phase used in the methacrylic acid recovery step (E) is 8% by mass or more. The aqueous phase containing methacrylic acid and acetic acid used in the methacrolein absorption step (C) is a bottoms containing methacrylic acid, acetic acid and water taken out from the bottom of the distillation column in the methacrolein distillation step (D). Item 5. A method for producing methacrolein according to any one of Items 1 to 4. The methacrochromic and acetic acid-containing aqueous phase used in the methacrylic acid condensation step (B) is an acetic acid-containing aqueous phase that has absorbed methacrylic acid in the methacrylic acid recovery step (E). Rain manufacturing method. Returning part of the isobutylene-containing gas remaining after the recovery of methacrylic acid in the methacrolein recovery step (E) to the reaction step (A),
7. The reaction step (A) according to any one of claims 1 to 6, wherein a gas obtained by mixing a part of the isobutylene-containing gas and the raw material isobutylene and / or tert-butyl alcohol is contact-oxidized with the molecular oxygen-containing gas. Method for producing methacrolein. A request to condense vapor containing methacrolein and water evaporated from the top of the distillation column in the methacrolein distillation step (D), send this condensate to a decanter, and separate it into a methacrolein phase and an aqueous phase by the decanter Item 8. A method for producing methacrolein according to any one of Items 1 to 7. The aqueous phase separated from the methacrolein phase in the decanter is mixed with the aqueous phase containing methacrylic acid and acetic acid that has absorbed methacrolein and sent to the methacrolein distillation step (D), and this mixed solution is added to the methacrolein distillation step (D). The method for producing methacrolein according to claim 8. The method for producing methacrolein according to claim 8 or 9, wherein the concentration of water contained in the methacrolein phase can be controlled by lowering the operating temperature of the decanter.

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