JPS6059891B2 - Method for separating methacrolein and methacrylic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for producing methacrylic acid by gas-phase oxidation of methacrolein and/or a compound that produces methacrolein under reaction conditions and molecular oxygen in the presence of an oxidation catalyst.
The present invention relates to a method for efficiently separating and recovering methacrylic acid and methacrolein.

Here, the compound that produces methacrolein under reaction conditions is, for example, isobutylene or -butanol, and will be abbreviated as isobutylene etc. hereinafter. When isobutylene, etc. and/or methacrolein is oxidized in the gas phase with water vapor and molecular oxygen (mostly air) in the presence of an oxidation catalyst, in addition to the target methacrylic acid, acetic acid, carbon monoxide, carbon dioxide, and A gaseous reaction mixture containing reacted isobutylene, etc. and/or unreacted methacrolein, oxygen, nitrogen, etc. is obtained.

It is necessary to efficiently separate and recover not only the target methacrylic acid but also methacrolein from these reaction mixtures. Many industrial oxidation catalysts for isobutylene, etc. and/or methacrolein to methacrylic acid have been developed, but their catalytic performance is still not industrially satisfactory, especially for low conversion rates of methacrolein. Currently, there are most catalysts that have a high methacrylic acid selectivity under the given reaction conditions, but the methacrylic acid selectivity drops significantly under the reaction conditions that give a high conversion rate of methacrolein. Developing a technology to effectively separate and recover unreacted methacrolein from an aqueous methacrylic acid solution and circulate it to an oxidation reactor with a relatively low This is an important factor that determines whether a manufacturing method can be established as an industrial manufacturing method or not. As a method for separating methacrolein from such a gaseous reaction mixture,
For example, JP-A-50-111017 discloses a method for separating and recovering methacrolein.

The outline is that a methacrolein-containing mixed gas is absorbed by 50 to 500 moles of water per mole of methacrolein, and then converted into an aqueous solution of methacrolein. There is a method of separating methacrolein by applying distillation or stripping after it is collected. In this method, when methacrolein is absorbed and separated from a gaseous reaction mixture, the absorption of methacrolein is carried out under high pressure or deep cooling, a large amount of water is used, or an absorption tower with an extremely large number of plates is used. will have to be used.

This is because methacrolein has a low solubility in water (6.1% by weight, 25°C) and is extremely difficult to absorb into water. If a large amount of water is used as an absorption solvent for methacrolein, not only the methacrolein absorption tower but also the methacrolein distillation tower and stripping tower following the absorption tower will be unnecessarily large. Absorbing methacrolein under high pressure means increasing the operating pressure of all equipment, including the reactor, before the methacrolein absorption tower by the same amount as the operating pressure of the methacrolein absorption tower. , it would be accompanied by great difficulties in terms of safety and economy. Furthermore, it is disadvantageous from an economic point of view to use deep cooling or an absorption tower with a large number of stages. Therefore, the use of an absorption solvent in which methacrolein has a low solubility, such as water, is clearly not a good idea as an industrial production method. Because of the low solubility of methacrolein in water, it was discovered that
-92007, it has been proposed to use alcohols instead of water as absorption solvents for methacrolein, but even if these solvents are effective as absorption solvents for methacrolein, the When methacrolein is recycled to the methacrolein oxidation reaction process, organic solvents such as these are recycled to the oxidation process together with the recovered methacrolein, even if only in small amounts, and the oxidation process is catalyzed by the organic solvent. Since there are risks of poisoning, inhibition of oxidation reactions, and abnormal reactions such as explosions due to contamination of the organic solvent vapor, organic solvents should not be used as methacrolein absorption solvents simply because they have good methacrolein absorption performance. It is not necessarily desirable to use it as

Under such circumstances, the present inventors have developed a methacrolein absorber that has good methacrolein absorption performance and that allows methacrolein after separation and recovery to be easily recycled to the oxidation reaction process without worrying about catalyst poisoning. As a result of intensive research into solvents and processes using them, it was discovered that an aqueous solution containing methacrylic acid has significantly better solubility of methacrolein than plain water, as shown in Figure 1. I've reached it.

Figure 1 shown in Figure 1 shows the Henry's constant of methacrolein in water or an aqueous methacrylic acid solution.
An object of the present invention is to completely separate methacrolein and methacrylic acid while keeping the concentration of the methacrylic acid aqueous solution as high as possible. The present invention provides a method for separating methacrolein and methacrylic acid from a reaction gas generated by gas phase oxidation of isobutylene etc. and/or methacrolein. Water and methacrylic acid in the reaction mixture are condensed by directly connecting a part of the condensate that has been condensed and accumulated in a reactor to the gaseous reaction mixture at a temperature of 100° C. or lower. When carrying out this condensation operation in one step, the operating temperature is 100'C or less, preferably 30 to 80°C.
It is ℃. In addition, when the condensation operation is carried out in two stages, the operating temperature in the first stage is 1000°C or less, preferably 30 to 80°C, and the operating temperature in the second stage is 1000°C or less, preferably 30 to 80°C. The operating temperature is 10 to 50°C, and it is desirable that the second stage temperature does not exceed the first stage temperature. When the operating temperature is higher than 100℃,
This is not preferred because the polymerization of methacrolein and methacrylic acid becomes significant. The method of direct countercurrent contact between the reaction mixture gas and the condensate is not limited to any conventional contact method such as a packed column, perforated plate column, bubble bell column, or spray column.

The methacrolein-containing gas exiting from the top of the condenser is then led to the bottom of a methacrolein absorption tower operated at a temperature of 30°C or lower, preferably from 0 to 15°C, and the 5 weight gas supplied from the top of the tower is Methacrolein is absorbed by countercurrent contact with an aqueous methacrylic acid solution containing methacrylic acid in an amount of 1% or more, preferably 1% or more by volume, and a gaseous body from which methacrolein has been almost completely separated from the top of the absorption column, An aqueous methacrylic acid solution that has absorbed methacrolein is obtained from the bottom of the tower.

If the operating temperature of the methacrolein absorption tower is 30°C or higher, it is not preferable because the absorption efficiency of methacrolein becomes poor.
Furthermore, if the methacrylic acid content of the absorption liquid used in the absorption tower is less than 5% by weight, the solubility will be low and the absorption efficiency will be poor, which is not preferable. The type of methacrolein absorption tower may be any commonly used type of absorption tower, such as a packed tower, perforated plate tower, bubble bell tower, or spray tower, and is not limited to the type.

The amount of the aqueous methacrylic acid solution used as an absorption solvent for methacrolein is preferably 8 or less, preferably 1 to 5 in molar ratio to the gas at the inlet of the absorption tower.

The methacrolein-absorbed aqueous methacrylic acid solution obtained from the bottom of the absorption tower in this way is transferred to the top of the next methacrolein stripping tower, which is operated at a temperature range of 30 to 100°C, preferably 50 to 80°C. In addition, the condensate obtained in the previous condenser is led to the middle stage or the top of the methacrolein stripping tower, respectively,
By supplying an inert gas such as nitrogen, air, carbon dioxide, isobutylene, etc. or/and methacrolein oxidation reaction exhaust gas, or combustion exhaust gas of the oxidation reaction exhaust gas from the bottom of the stripping tower, the methacrolein stripping tower feed liquid is The contained methacrolein is separated as a gaseous mixture at the top of the stripping column, and an aqueous methacrylic acid solution in which methacrolein has been almost completely separated and removed is obtained from the bottom of the stripping column. If the operating temperature of the methacrolein stripping tower is 30° C. or lower, the vapor pressure of methacrolein becomes low and the stripping effect becomes poor, which is not preferable.

Further, if the operating temperature is 1000C or higher, the polymerization of methacrolein and methacrylic acid becomes significant, which is not preferable. The type of the stripping tower may be any commonly used gas-liquid contact device such as a packed tower, perforated plate, bubble tower, or spray tower, and is not limited to its type. The amount of inert gas supplied to the bottom of the stripping tower is desirably in a molar ratio of 1 or less, preferably 0.3 or less, relative to the total amount of liquid supplied to the stripping tower.

Various methods can be considered for controlling the operating temperature in methacrolein absorption towers and methacrolein stripping towers, depending on the type of tower. Accordingly, the refrigerant or heat medium can be circulated and retained in the mantle tube or the coil.

The aqueous solution containing 5% by weight or more of methacrylic acid to be supplied to the top of the methacrolein absorption tower as a methacrolein absorption solvent may be an aqueous methacrylic acid solution obtained entirely elsewhere, or may be the bottoms of the methacrolein stripping tower. A part of it may be cooled and used for circulation.

In addition, the inert gas mixture containing methacrolein obtained from the top of the stripping tower may be used for the production of methacrolein or other purposes, or it may be used together with the inert gas to produce isobutylene, etc. or/and 7 methacrolein. It may be recirculated to the gas phase oxidation reactor.

The reaction mixture gas used in the present invention is a product gas obtained by oxidizing isobutylene, etc. and/or methacrolein, and includes methacrylic acid, acetic acid, mephthacrolein, unreacted isobutylene, etc., water vapor, carbon dioxide, carbon monoxide,
Consists of oxygen, nitrogen, etc.

The present invention will be explained in more detail below with reference to the drawings.
The figure is one embodiment of the present invention, and the present invention is not limited thereto.

Moreover, FIG. 2 is an example of a case where the condensation operation of the mixed gas at the reactor outlet is performed in two stages in separating methacrolein and methacrylic acid according to the present invention.

Isobutylene, etc. or/and methacrolein The reaction mixture gas discharged from the gas phase oxidation reactor 3 is supplied to the bottom of the first stage condenser 5 through line 4, and is recycled through line 6, cooler 8, and line 9 to produce methacrylate. Water vapor, methacrylic acid, etc. in the reaction mixture gas are condensed by direct countercurrent contact with the first stage condensate containing acid.

The first stage condensate containing methacrylic acid discharged from the bottom of the first stage condenser 5 is supplied to the middle stage of the methacrolein stripping tower 31 through the line 7 and the two heat exchanger heaters 25. The top gas of the first stage condenser 5 is passed through a line 10 to a second stage condenser 11 which is operated in a lower temperature range than the first stage condenser 5.
is supplied to the bottom of the tower, line 12, cooler 1 line 15
Most of the methacrylic acid and water vapor remaining in the top gas of the first stage condenser are condensed and separated by direct countercurrent contact with the second stage condensate containing methacrylic acid recycled through the first stage condenser. The second stage condensate containing methacrylic acid discharged from the bottom of the second stage condenser 11 is supplied to the middle stage 5 of the methacrolein stripping tower 31 through the line 13 and the heat exchanger 23. Here, the first stage condensate withdrawal line 7 and the second stage condensate withdrawal line 13 may be supplied separately to the methacrolein stripping tower 31 as shown in FIG. 2, or they may be combined into one and then the methacrolein It may also be supplied to the rain diffusion tower 31. The second stage condenser top gas from which most of the methacrylic acid and water vapor have been separated and removed in the first stage condenser 5 and the second stage condenser 11 is supplied to the bottom of the methacrolein absorption tower 17 through a line 16.

An aqueous methacrylic acid solution is supplied as a methacrolein absorption solvent to the top of the methacrolein absorption tower 17 through a line 26, and is brought into countercurrent contact with the methacrolein-containing gas rising inside the methacrolein absorption tower, thereby reducing the amount of gas in the gas. Absorbs methacrolein. The methacrolein absorption column top 4 gas from which methacrolein has been separated is discharged from line 20 and discharged into the atmosphere through exhaust gas processing equipment or the like.
The aqueous methacrylic acid solution that has absorbed methacrolein is extracted from line 19 and sent to the top of methacrolein stripping tower 31 through heat exchanger 21 and heater 22 . The methacrolein absorption tower 17 is desirably operated at as low a temperature as possible in order to increase absorption of methacrolein;
It is desirable to operate at a temperature range below 0°C, preferably from 0 to 15°C. Therefore, it is necessary to circulate the refrigerant in the absorption tower to maintain its temperature by inserting a cooling coil into the jacket tube tower. The aqueous methacrylic acid solution from the line 26 may be supplied by circulating a portion of the liquid extracted from the bottom of the methacrolein stripping tower 31, or may be an aqueous methacrylic acid solution obtained from a completely different source. In the methacrolein stripping tower 31, air, nitrogen, carbon dioxide, isobutylene, etc. or/and methacrylic oxidation reaction exhaust gas,
Alternatively, an inert gas such as combustion exhaust gas of the oxidation reaction exhaust gas is supplied, and methacrolein contained in the liquid supplied to the methacrolein stripping tower 31 from lines 19, 13, and 7 is diffused. A gaseous mixture containing methacrolein is withdrawn from the top through line 27.

This gaseous mixture may be used for the production of methacrolein or for other purposes, or may be circulated through the heater 28 to the inlet raw material gas line 1 of the gas phase oxidation reactor 3 for isobutylene and/or methacrolein. You can also do it.

The liquid extracted from the bottom of the methacrolein stripping tower 31 is passed through line 29, cooled by heat exchangers 24 and 23, and an aqueous methacrylic acid solution from which methacrolein has been almost completely separated is extracted from line 30, and purified to produce methacrylic acid. Sent to other processes. A part of the liquid extracted from the bottom of the methacrolein stripping tower 31 is
Heat exchanger 21. It may be used as a methacrolein absorption solvent in the methacrolein absorption tower 17 through the line 26 and the cooler 18. Since the operating temperature of the methacrolein stripping tower 31 decreases due to evaporation of most of the methacrolein and some of the water, the stripping tower is equipped with a mantle, a heating coil, etc., and the heat medium is circulated to reduce the temperature required. The operating temperature of the stripping tower must be maintained within the required temperature range.

The heat exchangers 21, 23, 24, the heaters 22, 25, the cooler 18 are incorporated into a system such as a methacrolein absorption tower, a methacrolein diffusion tower jacket or a heat transfer coil, etc.
It is sufficient to carry out the operation within a range that can maintain the necessary operating temperature range of the stripping tower, and there is no need to leave it as shown in the second figure, and it can be omitted or added as necessary. In addition, if the reaction mixture gas is condensed at one temperature level instead of at two high and low temperature levels as shown in Figure 2, the second stage of Figure 2 is used. The condenser 11, lines 12, 13, 15, 16, cooler 1 and heat exchanger 23 may be omitted, and the top gas line 10 of the first stage condenser 5 may be directly led to the methacrolein absorption tower 17.

Generally, when selecting the operating temperature of a condenser, it is desirable to select a temperature range in which normal cooling water can be used as a refrigerant in the coolers 8 and 14 attached to the condenser, and to operate the condenser in a temperature range as low as possible. This prevents polymerization of methacrylic acid in the condensate, and reduces the amount of water vapor entrained to the methacrolein absorption tower 17 through line 16 (or line 10) together with the second stage (or first stage) condenser top gas. However, the cooling load on the methacrolein absorption tower 17 and the cooler 18 is reduced accordingly, which is advantageous from an economic point of view, but by lowering the condenser operating temperature too much, the average temperature with the refrigerant used As the difference becomes smaller, the heat transfer area of the coolers 8 and 14 becomes significantly larger, resulting in a disadvantage.

For this reason, when using only a single-stage condenser, it is necessary to
The temperature is preferably 0°C or lower, and when using a two-stage condenser, the temperature of the first stage condenser is 30°C or higher and 80°C or lower, and the temperature of the second stage condenser is lower than the operating temperature range of the first stage condenser and 10°C. Above 5
It is desirable to select a temperature within a temperature range that does not exceed 0'C. Therefore, whether to use a single-stage condenser or a two-stage condenser is determined by comprehensively considering the required heat transfer area of the coolers 8 and 14 attached to the condenser, the cooling load on the methacrolein absorption tower, and the cooler 28. It is not possible to determine the superiority or inferiority of necessity. Condenser 5 of the present invention
and 11, the operating pressure of the methacrolein absorption tower 17 and the methacrolein stripping tower 31 is determined in relation to the operating pressure of the isophthylene etc. and/or methacrolein gas phase oxidation reactor 3, or the pressure boosting pressure is supplied to the line 16 or line 27. It is desirable to implement this to the extent that it is not necessary to install a machine.
The operating pressure is preferably 0 to 7 atg (gauge pressure), preferably 0.5 to 4 atg.

EXAMPLES The present invention will be explained below with reference to examples, but the present invention is not limited to these examples. Example 1 Methacrolein 1.6% by volume, methacrylic acid 1.57% by volume, acetic acid 0.52% by volume, steam 42.75% by volume
A reaction mixture gas of 4528 NeIhr at 310°C containing 53.5 volume % of non-condensable gases such as oxygen, nitrogen, carbon monoxide, and carbon dioxide is supplied to the first stage condenser 5 in the form of a porcelain Raschig ring packed column. .

When the temperature of the first stage condensate is 45-50℃, the first stage condensate is produced for 1698y1hr, the methacrylic acid concentration in the liquid is 15.24% by weight, and the methacrolein is 0.18%.
% by weight, acetic acid is 3.3% by weight. In addition, the gas exiting from the top of the first stage condenser 5 contains 2.6% by volume of methacrolein, 0.13% by volume of methacrylic acid, and 8% by volume of steam. A total of 2723 Ne1 hr of gas was supplied to the second stage condenser 11 in the form of a porcelain Raschig ring packed column.

When the second stage condensate temperature is 32-35℃, the second stage condensate is produced for 94.66y1hr, the methacrylic acid concentration in the liquid is 10.1% by weight, and methacrolein is 0.74wt. %, and acetic acid was 4.56% by heel weight. In addition, the gas coming out from the top of the second stage condenser 11 contains 2.7% methacrolein.
% melt, methacrylic acid 0.0 mol %, steam 4.6
A gas containing 2% by volume and a total amount of 2619 NeIhr is supplied to the bottom of the methacrolein absorption tower 17 in the form of a stainless steel Raschig ring packed column. From the top of the methacrolein absorption tower 17, a methacrolein stripping tower 3 containing 15.1% by weight of methacrylic acid, 3.4% by weight of acetic acid, and 0.0% by weight of methacrolein as an absorption solvent for methacrolein is passed.
A part of the bottom liquid from Step 1 was cooled to 8 to 10°C to produce 6378gIh.
It was supplied at a flow rate of r. From the bottom of the methacrolein absorption tower 17, 6689y1hr of methacrolein absorption liquid containing 3.55% by weight of methanochlorein, 14.46% by weight of methacrylic acid, and 3.4% by weight of acetic acid was obtained. From the top of the methacrolein absorption tower 17, 2451 Ne1 hr of gas containing 60 ppm of methacrolein was obtained. This methacrolein absorption tower 7 has a tower diameter of 55 Tfr! n, filling height 6TrL,
A mantle was applied and the operating temperature was maintained at 5-15°C. The first-stage condensate, second-stage condensate, and methacrolein absorption liquid obtained in this way are stored in the middle stage of the methacrolein stripping tower 31,
Each was supplied to the top of the tower. Nitrogen gas of 69△/hr was supplied to the bottom of the methacrolein stripping tower 31F, and from the top of the tower, 8.4% by volume of methacrolein and 10% of steam were supplied. Obtained 847 N'Ihr of methacrolein diffused gas containing % volume, 280
℃ and circulated to the inlet of the oxidation reactor 3. From the bottom of the methacrolein stripping tower 31, 8183 qIhr of bottoms containing 15.1% by weight of methacrylic acid, 3.4% by weight of acetic acid, and 0.02% by weight of methacrolein was obtained, and one portion was 6378y.
1 hr was recycled and used as a methacrolein absorption solvent, and the remaining 1805 y1 hr was extracted as a product. The methacrolein stripping tower was a stainless steel Raschig ring packed tower with a tower diameter of 55 mm and a filling height of 3 m, and was provided with a jacket tube, and was maintained at a temperature in the range of 60 to 70°C. Example 2 In Example 1, the second stage condenser 11 is omitted, and the top gas of the first stage condenser 5 is directly supplied to the lower part of the methacrolein absorption tower 17, so that the filling length of the methacrolein absorption tower 17 is reduced. As a result of operating in exactly the same manner as in Example 1 except for changing to 6.57n, 65 ppm of methacrolein was contained in the gas at the top of methacrolein absorption tower 17, and 3.5 ppm of methacrolein was collected from the bottom of the methacrolein absorption tower. Methacrolein absorption tower 6778y containing 14.41% by weight of methacrylic acid and 3.34% by weight of acetic acid
I got Ihr.

The composition and flow rate of other parts were almost the same as those of Example 1.

[Brief explanation of the drawing]

FIG. 1 shows the Henry's constant of methacrolein in an aqueous methacrylic acid solution.

Claims (1)

[Claims] 1. A gas containing a methacrylic acid monomer obtained by subjecting methacrolein and/or a compound that produces methacrolein under reaction conditions to a gas phase reaction at high temperature in the presence of an oxidation catalyst with molecular oxygen. In a method for separating methacrolein and methacrylic acid from a gaseous reaction mixture, the gaseous reaction mixture discharged from the reactor is first introduced into a condenser, and a part of the condensate that has been condensed and accumulated in the condenser is mixed with the gaseous reaction mixture. By direct countercurrent contact with the mixture at a temperature of 100°C or less,
The water and methacrylic acid in the reaction mixture are condensed, the majority of methacrolein and non-condensable gaseous bodies are separated from the reaction mixture, and this gas is then transferred to a methacrolein absorption tower operated at a temperature below 30°C. and supplying an aqueous solution containing 5% by weight or more of methacrylic acid from the top of the absorption tower,
The methacrolein-containing aqueous methacrylic acid solution obtained from the bottom of the methacrolein absorption tower after absorbing methacrolein in the gas is transferred to the top of the methacrolein stripping tower operated at a temperature of 30 to 1000C, and is then transferred to the gaseous reaction at the exit of the reactor. The condensate obtained by condensing the mixture is supplied to the middle stage or the top of the column, and by supplying an inert gas such as nitrogen or air from the bottom of the column, the methacrolein contained in each feed solution is removed. A method for separating methacrolein and methacrylic acid from the top of the column. 2. Claim 1, wherein the operation of bringing a part of the condensate pre-condensed and accumulated in a condenser into direct countercurrent contact with the gaseous reaction mixture at a temperature of 100° C. or less is one step or multiple steps. the method of. 3. The method according to claim 1, wherein a part of the methacrylic acid aqueous solution obtained from the bottom of the methacrolein stripping tower is used as an absorption solvent in the methacrolein absorption tower. 4. The method according to claim 1, wherein the gaseous mixture containing methacrolein obtained from the top of the methacrolein stripping column is circulated to the inlet of the oxidation reactor.