JPS6013739A - Purification of acrylic acid - Google Patents

Abstract

PURPOSE:To improve the purity of acrylic acid in the purification of crude acrylic acid containing acrolein as impurity, by taking out the crude acid from the system in a vapor-form, and condensing a part of the vapor thereby removing the slight amount of the acrolein existing in the crude acid. CONSTITUTION:A crude acrylic acid containing acrolein or its derivative as impurities is supplied to the evaporation kettle 1 to generate the vapor of acrylic acid containing impurities. The vapor is introduced into the condenser 3 to effect the condensation of a part thereof. Acrylic acid containing <=10ppm of low- boiling aldehydes is extracted from the system through the line 6. The vapor which is left uncondensed in the condenser 3 is introduced into the second condenser 4, and the acrylic acid having high content of the low-boiling aldehydes is taken out through the line 7. The evaporation of acrylic acid can be carried out under arbitrary pressure, i.e. atmospheric pressure, reduced pressure or positive pressure, and the temperature is also arbitrary provided that acrylic acid can be evaporated at the temperature. The condensation is carried out by measuring the acrolein concentration of the condensed liquid, and controlling the condensation ratio according to the concentration.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for purifying acrylic acid by removing trace amounts of aldehydes present as impurities in acrylic acid.

Conventionally, acrylic acid was produced using the Reppe method and the hydrolysis method of acrylonitrile.
In recent years, a method for producing acrylic acid using the direct oxidation method of propylene has been developed and has become the main method for producing acrylic acid.

When producing acrylic acid by the direct oxidation method of propylene, various low-boiling point and high-boiling point aldehydes are inevitably produced as by-products during the propylene oxidation process.

In order to produce the acrylic acid product from the oxidation product of propylene, several steps of purification such as extraction and distillation are required, but the acrylic acid product still contains about 50 to 500 ppm (by weight) of aldehydes. There is. If a small amount of aldehydes are present in the product acrylic acid, the induction period of the 1-polymer reaction tends to be longer when the acrylic acid is polymerized, and the degree of polymerization of the obtained polymer tends to be lower. .

In particular, low-boiling aldehydes such as acrolein have a strong effect of inhibiting the polymerization of acrylic acid, and it is known that even the presence of several 10,111) m of such aldehydes in the raw material acrylic acid significantly inhibits the polymerization of acrylic acid. There is. Therefore, the method of removing this trace amount of aldehydes is an important point when producing acrylic acid for polymer raw materials.
Although many proposals have been made, the main method is to convert aldehydes into high-boiling compounds and separate them by distillation or the like. For example, a method in which amines are added and then distilled (Japanese Patent Publication No. 48-81087, Japanese Patent Application Laid-Open No. 49-512
17), a method in which hydrazines are added and then distilled (Japanese Patent Publication No. 49-80812), and an amino acid is added and then distilled (Japanese Patent Publication No. 50-14, JP 57-88148). However, the substances to be added are expensive, and high-boiling compounds such as aldehydes are newly included in the residual liquid from which acrylic acid is extracted as a distillate through distillation. Since it is necessary to remove the high-boiling point compounds by some means when reusing the pot residue, it is difficult to say that this is an advantageous method as an industrial method.

By the way, these low-boiling aldehydes have a sufficiently higher specific volatility than acrylic acid, so it is thought that they can be easily separated using a conventional rectification method. However, as previously described, it is extremely difficult to separate the low-boiling aldehydes to 10 ppm or less using the rectification method alone.

The reason why it is difficult to separate these trace amounts of low-boiling aldehydes by distillation is that although low-boiling aldehydes such as acrolein are small in quantity, they easily dimerize or hydrate in the presence of water. This is to form dimers and hydrates,
These dimers and hydrates behave as high-boiling substances with respect to acrylic acid, and acrylic acid is Remains in the bottoms on the acid side. The remaining dimers and hydrates of low-boiling aldehydes are decomposed during storage or during the next step of rectifying and separating the bottoms, and the low-boiling aldehydes are regenerated into rectified acrylic acid. It has been found that the low-boiling aldehydes in the rectified acrylic acid cannot be separated to less than 1 Oppm by ordinary rectification methods alone. In order to overcome this difficulty, the present inventors
We investigated various methods of separating this trace amount of low-boiling aldehydes such as acrolein from acrylic acid, and found that after extracting the acrylic acid containing aldehydes as impurities in a gaseous state, It was discovered that acrylic acid containing almost no low-boiling aldehydes such as acrolein can be obtained by condensing a portion thereof, leading to the present invention. That is, the present invention relates to a method for producing M from acrylic acid, which comprises converting acrylic acid containing acrolein and its compounds as impurities into a gas state, and then condensing a portion of the gas to obtain purified acrylic acid.

In the present invention, acrylic acid is purified in the difi form of low-boiling aldehydes such as acrolein, and the hydrate is decomposed into low-boiling aldehydes, low-boiling aldehydes, and water in the gas state. This is thought to be because the low-boiling aldehyde exists in a gaseous state when a portion of acrylic acid is condensed, and is hardly contained in the condensed acrylic acid. The acrylic acid used in the present invention contains acrolein and its compounds as impurities.
The target is acrylic acid containing up to about 0 ppm, and the raw material source thereof is typically a direct oxidation product of propylene, but is not limited thereto. Examples of acrolein compounds include acrolein dimers and acrolein hydrates. In order to make such acrylic acid into a gas state, it is possible to use it under any pressure such as normal pressure, reduced pressure, and increased pressure.Therefore, the temperature should not only be the boiling point of 141'c at normal pressure, but also the temperature at which it will vaporize depending on the pressure. good. For partial condensation, it is possible to gradually change the condensation rate while analyzing the acrolein concentration in the condensate, but keep the acrolein concentration in the gaseous acrylic acid and the acrolein concentration in the condensate below t o ppm. The relationship between the condensation rate and the condensation rate is as shown in equation (1).
By controlling the condensation to achieve this condensation rate, the acrolein concentration in the condensate can be reduced to 1099 m or less.

Ordinary −1,08−0,008x x −−= ・−・tt
1 where R is the concentration of 7chlorein in the condensate, 1099
condensate (weight)
/ gaseous acrylic acid (weight including impurities).

X means the acrolein concentration in gaseous acrylic acid (including impurities) and is expressed in ppm (weight).

Condensation is generally performed by cooling the gas, and the cooling temperature is also necessarily determined by the pressure. However, in these operations, since the materials to be handled are highly polymerizable by heat, it is of course necessary to use an appropriate polymerization inhibitor in combination, and it is preferable to perform the procedures under reduced pressure and at as low a temperature as possible. An evaporator is usually used to bring the acrylic acid into a gaseous state, but steam from a rectifier or reboiler can also be used. The condensation operation can be performed using an indirect cooling type condenser, but it is also possible to employ a direct cooling type condenser or other condensation method. The remaining partially condensed acrylic acid vapor, which contains a large amount of low-boiling aldehydes, contains a considerable amount of acrylic acid and can be returned to the previous step of this method by separately cooling and condensing it. Alternatively, it can also be used as C, which is used as acrylic acid for producing acrylic esters.

A more detailed explanation will be given below with reference to the drawings, but FIG. 1 shows one embodiment of the present invention, and the present invention is not limited thereto. 1 # of acrylic acid containing low boiling point aldehydes is supplied from line 5 to the evaporator, and
acrylic acid vapor containing low-boiling aldehydes is generated through the process. This step generates the corresponding acrylic acid vapor, and can be omitted when using the corresponding acrylic acid vapor in a low-boiling substance separation column or a high-boiling substance separation column. The acrylic acid vapor containing low-boiling aldehydes is led to a condenser 3 where it is partially condensed, and acrylic acid containing 1 Q ppm or less of low-boiling aldehydes is taken out from a line 6. The condensation rate in the condenser 8 is controlled by changing the temperature and/or flow rate of the cooling medium flowing through the line 9. The remaining vapor that is not condensed in the condenser 8 is led to the condenser 4, and acrylic acid containing a large amount of low-boiling aldehyde is taken out from the line 7. The steam led to the condenser 4 can also be led in a vapor state to another distillation system for treatment.

When the method of this invention is applied to a method for producing acrylic acid by direct oxidation of propylene, the crude acrylic acid obtained by oxidation of propylene contains acetaldehyde and impurities.
It contains low-boiling substances such as acrolein, water, and acetic acid, and high-boiling substances such as maleic acid, phenol, benzaldehyde, and acrylic acid dimer. Although the content of these impurities varies depending on the propylene oxidation conditions and purification conditions and is not constant, the present invention is not affected by changes in the content of these impurities. Usually, rectification separation of low boiling point substances and/or high boiling point substances of acrylic acid is carried out by
The low-boiling substances are separated first and then the high-boiling substances, but the process of the invention can be carried out in either step. In addition, the method of rectification separation may be any method, such as the configuration of a rectification column such as a side cut or a composite column using a combination of multiple rectification columns, or the type of rectification column using a continuous method or a batch method. It can be implemented.

However, for the purpose of producing acrylic acid for use as a raw material for polymers, it is preferable to employ this method in a site with a low impurity content. For example, the bottom vapor of a low-boiling substance separation column, the top or bottom vapor of a high-boiling substance separation column, or the vapor from an intermediate stage of these columns can also be used. Next, the gaseous material extracted as vapor is condensed as described above.

Next, it will be explained in more detail with examples and comparative examples.
The present invention is not limited to these Examples and Comparative Examples.

Example 1 In the flow sheet shown in Fig. 1, using the equipment shown in Table 1,
Acrylic acid 98wt% (hereinafter referred to as %).

expressed in ppm t, t x ), moisture 0.5%
, acrolein 50 ppm, other low-boiling substances 20
Acrylic acid 8 consisting of 0 ppm, 1.5% of high boiling substances
After charging 00CC into evaporator 1, the same acrylic acid was supplied from line 5 at a rate of 20051/hr to purify acrylic acid. The operating pressure was 3 Q Torr, the water bath temperature was 70°C, the cooling water temperature in line 1O was 20°C, and the cooling water temperature in line 9 was 48°C.

164 El/br from line 6, 86 from line 7
A condensate of 9/br was obtained. After 5 hours of continuous operation under these conditions, the condensate from line 6 was sampled for 1 hour and its composition was found to be 0.1% water and 8 ppm acrolein.

Other low boiling point substances 150 ppm, high boiling point substances 0.2
It was acrylic acid with a purity of 99% or more.

Example 2 The acrylic acid supplied from line 5 was 98% acrylic acid.
, 0.4% water, 20 ppm of acrolein, 190 ppm of other low boiling point substances, and 1.7% of high lung point substances.The same as Example 1 except that the cooling water temperature in line 9 was 47°C. Acrylic acid was purified under the following conditions. 190 P/hr from line 6 1 from line 7
A condensate with a concentration of 109/br was obtained. 5 under this condition
After continuous operation for one hour, the condensate from line 6 was sampled for one hour and its composition was found to be 0.2% water.
, acrolein 91) I) ml Other low-boiling substances 1
80 pp111% purity 99 containing 0.2% high boiling substances
% or more of acrylic acid.

Comparative Example Acrylic acid was purified in the same manner as in Example 1 under the same conditions as in Example 1 except that the temperature of the cooling water in line 9 was 20°C. A condensate of 200 V/hr was obtained from line 6,
There was no condensate from line 7. After 5 hours of continuous operation under these conditions, the condensate from line 6 was sampled for 1 hour and its composition was found to be 0.5% water, 200 ppm of low-boiling substances other than water and acrolein, and 0 high-boiling substances. It was an acrylic acid with a purity of 99% or more containing .2%, but contained so ppm of acrolein.

[Brief explanation of the drawing]

FIG. 1 shows an example of an acrylic acid purification apparatus according to the present invention. In the figure, l is an evaporator for acrylic acid vapor generation 2 is a packed column for demister 8 is a purified acrylic acid condenser 4 is a condenser 5 for acrylic acid with a high acrolein content, and an acrylic acid supply line 6 is for taking out purified acrylic acid Line 7 is acrylic acid with a high acrolein content Takeout port 8 is the pot residual liquid takeout line 9 is the cooling medium line IO is 11 is the heat medium line for the evaporation pot Figure 1 Procedure amendment (voluntary) August 1981 λ Date 1, Indication of the case 1982 Patent Application No. 121955 2, Name of the invention Method for purifying acrylic acid 8, Person making the supplement Relationship with the case Patent applicant address 5-15 Kitahama, Higashi-ku, Osaka Name (209
) Sumitomo Chemical Co., Ltd. Representative Takeshi Hijikata 6, Detailed explanation of the invention in the specification subject to amendment. 6. Contents of amendment (1) On page 11 of the specification cylinder, line 10, “was/same as J 1
The following table is added between "Example 2J" in the first row of the month.

Claims (1)

[Claims] A method for purifying acrylic acid, which comprises extracting acrylic acid containing acrolein and its compounds as impurities in a gaseous state, and then condensing a portion of the gas to obtain purified acrylic acid.