JP2002145821A - Method for producing acrolein - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing acrolein by a catalytic reaction of a 3C hydrocarbon in gas phase, wherein the acrolein is collected from the reaction product gas. SOLUTION: This method comprises absorbing acrolein into an aqueous solution of acrylic acid by contacting the reaction product gas with the aqueous solution of >=5 wt.% acrylic acid concentration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering acrolein from a reaction product gas containing acrolein by absorbing it with a solvent. Acrolein is highly reactive and is an important intermediate in the chemical industry.

[0002]

2. Description of the Prior Art Several processes for producing acrolein are known, but the most advantageous method is the catalytic oxidation of propylene. Recently, it has been reported that acrolein is produced by catalytic oxidation of propane, which is cheaper than propylene. In the production of acrolein by catalytic oxidation of these hydrocarbons, air is usually used as an oxygen source. In addition, steam is supplied to a raw material gas to be subjected to catalytic oxidation.

As a method for recovering acrolein from a reaction product gas obtained by catalytic oxidation, there is known a method in which a reaction product gas is supplied to an absorption tower, and the acrolein is absorbed in water by countercurrent contact with water in the tower. Have been. As a method for separating acrolein from an acrolein aqueous solution obtained in an absorption tower, a method is known in which an acrolein aqueous solution is supplied to a stripping tower and distilled to separate acrolein and water. After cooling the water to a predetermined temperature, the water is circulated to the absorption tower and used for absorbing acrolein.

[0004]

The method of absorbing and recovering acrolein in a reaction product gas with water involves a method in which a large amount of water is transferred between the absorption tower and the stripping tower because the solubility of acrolein in water is not so large. There is a problem that it has to be circulated. In addition, the reaction product gas contains a large amount of acrylic acid and the like by-product other than acrolein, and when acrolein is absorbed by water, these are also absorbed by water at the same time. Therefore, when circulating water between the absorption tower and the stripping tower, to avoid accumulation of acrylic acid, etc.
Acrylic acid dissolved in water must also be separated. This separation can be effected by distillation or extraction, but both methods are very expensive. Accordingly, the present invention is intended to solve the above-mentioned problems by absorbing acrolein in the reaction product gas using a solvent instead of water.

[0005]

According to the present invention, in recovering acrolein from a reaction product gas containing acrolein obtained by catalytic oxidation of a hydrocarbon selected from the group consisting of propylene and propane, the reaction product By bringing the gas into contact with an aqueous solution containing 5% by weight or more of acrylic acid and absorbing acrolein in the reaction product gas into the aqueous acrylic acid solution, acrolein can be easily recovered from the reaction product gas.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the production of acrolein by catalytic oxidation of hydrocarbons can be carried out by any known method. Propylene is generally used as a raw material hydrocarbon, but propane with poor reactivity can be used as a raw material by selecting a catalyst. Air is usually used as the oxygen source. The reaction temperature differs between the case where propylene is used as a raw material and the case where propane is used as a raw material, but is usually 200 to 500 ° C. The reaction is carried out by a continuous method in which a raw material gas containing a raw material hydrocarbon and oxygen is continuously introduced into a reaction zone containing a catalyst. Preferably, the source gas contains water vapor.

The composition of the reaction product gas flowing out of the reaction zone varies greatly depending on the composition of the raw material gas, and is, for example, 5 to 40 mol% of acrolein, 30 to 80 mol% of nitrogen, and 5 to 40 mol% of steam. The main by-product is acrylic acid, the content of which is usually less than 20 mol%. Also,
In addition, carbon dioxide, oxygen, carbon monoxide, acetic acid, hydrocarbons and the like are included.

In the present invention, this reaction product gas is contained in an amount of 5% by weight.
The acrolein in the gas is absorbed into the aqueous acrylic acid solution by contact with the aqueous acrylic acid solution containing acrylic acid. The ratio of water and acrylic acid generated by the reaction varies greatly depending on the reaction conditions, but is, for example, about 7: 3 by weight. Therefore, when the ratio of water and acrylic acid in the reaction product gas is higher than the acrylic acid aqueous solution used for absorption, the amount of steam supplied to the reactor is adjusted to adjust the amount of water vapor supplied to the reactor. Preferably, the composition is close to the composition of the aqueous acrylic acid solution. That is, in the present invention, the composition of the reaction product gas is adjusted so that the aqueous solution of acrylic acid obtained by dispersing acrolein from the aqueous solution of acrylic acid that has absorbed acrolein has a concentration that can be circulated for absorption of acrolein as it is just cooled. Is preferred. When the concentration of the aqueous acrylic acid solution used for absorption is low and the amount of supplied steam is excessive, the amount of supplied steam is limited to a certain range, and the concentration of the circulated aqueous acrylic acid solution is adjusted directly to the aqueous solution. It is preferably carried out by supplying water.

The reaction product gas can be cooled by either an indirect cooling system or a direct cooling system. In the case of the direct cooling method, the condensate generated from the reaction product gas may be used as a refrigerant to make countercurrent contact with the reaction product gas. The reaction product gas is preferably cooled to 70 ° C. or lower, particularly to 60 ° C. or lower. Although the boiling point of acrolein is relatively low at 52 ° C., the reaction product gas usually contains a large amount of nitrogen and other non-condensable gases.
For example, even when cooled to about 40 ° C., most of acrolein is present in the gas without being condensed.

[0010] The reaction product gas cooled to a predetermined temperature is then brought into contact with an aqueous solution of acrylic acid containing 5% by weight or more of acrylic acid to absorb acrolein in the gas into the aqueous solution of acrylic acid. The absorption of acrolein by the aqueous solution of acrylic acid is usually performed at 50 ° C or lower, preferably at 40 ° C or lower. The lower the temperature of the aqueous acrylic acid solution, the more advantageous it is for the absorption of acrolein, but on the other hand, the cost of cooling the aqueous acrylic acid solution increases.
Therefore, the temperature of a practical acrylic acid aqueous solution is 0 at most.
Cooling to a temperature below 0 ° C is not advantageous for industrial operation. Although the solubility of acrolein in water is not very high, acrylic acid forms a homogeneous solution with acrolein in any ratio. Therefore, when an aqueous solution of acrylic acid is used, acrolein can be absorbed with a smaller amount of liquid than when water is used. The higher the concentration of acrylic acid in the aqueous acrylic acid solution, the more acrolein can be absorbed. However,
When the amount of the aqueous acrylic acid solution with respect to acrolein is small, the amount of acrolein that is not absorbed and is lost due to the non-condensed gas increases due to the vapor-liquid equilibrium. Therefore, in order to suppress the loss of acrolein, it is necessary to use a certain amount or more of an aqueous solution of acrylic acid with respect to acrolein. The amount of the aqueous solution of acrylic acid depends on the concentration of acrylic acid in the aqueous solution of acrylic acid. In a region where the concentration of the aqueous solution of acrylic acid is low, the required amount rapidly decreases as the concentration increases. However, when the concentration reaches about 20%, the degree of decrease in the required liquid volume due to the increase in concentration gradually decreases,
When the concentration becomes about 50%, the required liquid amount hardly decreases. Therefore, the concentration of the aqueous solution of acrylic acid used for absorbing acrolein is preferably 5% by weight or more, more preferably 10% by weight or more, in view of the required amount of the absorbing solution.

When an aqueous solution of acrylic acid is used to absorb acrolein, part of acrylic acid is lost due to entrained non-condensable gas due to the gas-liquid equilibrium. The loss amount naturally increases with an increase in the concentration of acrylic acid in the aqueous solution of acrylic acid.
Beyond, the amount of loss increases remarkably. Therefore, considering the amount of the aqueous solution of acrylic acid required to reduce the loss of acrolein to a certain amount or less, and reducing the amount of loss of acrylic acid from the aqueous solution of acrylic acid, the acrylic acid concentration of the aqueous solution of acrylic acid used to absorb acrolein is It is preferably at most 30% by weight, particularly preferably from 10 to 20% by weight.

For example, FIG. 1 shows a reaction product gas of 8 mol% of acrolein, 1 mol% of acrylic acid, 25 mol% of water and 60 mol% of nitrogen and an aqueous solution of acrylic acid at 30 ° C. in an absorption tower having 10 theoretical plates. It is the figure created based on the calculation value at the time of making a countercurrent contact and absorbing acrolein. The calculation is based on the assumption that the amount of acrolein flowing out of the tower without being absorbed is 1%. In FIG. 1, the horizontal axis is the weight ratio of acrylic acid in the aqueous acrylic acid solution, the left axis is the weight ratio of the aqueous acrylic acid solution to be supplied to the reaction product gas, and the right axis is the acrylic ratio in the reaction product gas. The weight ratio of acrylic acid flowing out from the top to acid.

The aqueous solution of acrylic acid that has absorbed acrolein is then heated to evaporate acrolein. Acrolein has a boiling point of 52 ° C., which is sufficiently lower than the boiling point of water 100 ° C. and the boiling point of acrylic acid 141 ° C., so that acrolein can be selectively diffused. The aqueous acrylic acid solution after the acrolein has been diffused is usually 50 ° C. or less,
Preferably, after cooling to 40 ° C. or less, it is used again for acrolein absorption. The aqueous solution of acrylic acid corresponding to the amount of acrylic acid absorbed from the reaction product gas is drawn out of the system to recover acrylic acid. When the concentration of the aqueous acrylic acid solution is low, the concentration is adjusted to a predetermined concentration by dehydration distillation. This dehydration distillation can also be performed simultaneously with the above-mentioned acrolein dissipation, that is, by distilling a part of water together with acrolein.

FIG. 2 shows an example of a flow sheet for producing acrolein from propylene using air as an oxygen source according to the method of the present invention. In FIG. 2, reference numeral 1 denotes a reactor containing an oxidation catalyst, and a mixed gas of propylene, air, and steam is continuously supplied to the reactor 1 to generate acrolein. One example of the composition of the reaction product gas is acrolein 8 mol%, acrylic acid 1 mol%, steam 25
Mol%, nitrogen is 60 mol%, and also contains oxygen, carbon monoxide, carbon dioxide, propylene, acetic acid, and the like. The reaction product gas is cooled to a predetermined temperature, for example, 40 ° C. in the cooler 2 and then introduced into the absorption tower 3. What is necessary is just to use about 10 theoretical plates as an absorption tower.
At the top of the absorption tower, for example, an acrylic acid aqueous solution having an acrylic acid concentration of 15% by weight is introduced in an amount of 1 to 10 times by weight, preferably 3 to 5 times by weight with respect to the reaction product gas, and allowed to flow down in the column. Absorb acrolein in it. Acrylic acid, water, acetic acid and the like in the reaction product gas are simultaneously absorbed by the acrylic acid aqueous solution. The aqueous acrylic acid solution flowing out of the bottom of the absorption tower is introduced into a stripping tower system including two distillation towers 4 and 5. Acrolein is distilled from the top of the distillation column 4. The aqueous solution of acrylic acid flowing out from the bottom of the column is distilled in the distillation column 5 to distill water from the top of the column. The acrylic acid aqueous solution flowing out of the bottom of the distillation column 5 is cooled to a predetermined temperature by the cooler 6 and then circulated to the absorption column 3. Part of the aqueous solution of acrylic acid is drawn out of the system to avoid accumulation of acrylic acid. Since the amount of acetic acid present in the reaction product gas is very small, the accumulation of acetic acid in the system can be avoided by extracting acetic acid by adding it to an aqueous solution of acrylic acid that is usually extracted outside the system. In addition, water can be distilled off together with acrolein in the distillation column 4. In this case, the aqueous acrylic acid solution flowing out of the bottom of the distillation column 4 is circulated to the absorption column 3 via the cooler 6 as it is, and the top distillate is further distilled in the distillation column 5 to remove acrolein from the top of the column. Distillation is performed and water is extracted from the bottom of the tower. When the ratio of water and acrylic acid of the reaction product gas introduced into the absorption tower is equal to that of the aqueous acrylic acid solution used for absorption, the aqueous acrylic acid solution can be circulated without being dehydrated.

[Brief description of the drawings]

FIG. 1 shows the relationship between the concentration of acrylic acid aqueous solution, the required amount of acrylic acid aqueous solution, and the amount of acrylic acid loss when absorbing the reaction product gas containing acrolein with the aqueous solution of acrylic acid.
It is a graph showing an example.

FIG. 2 is an example of a flow sheet when implementing the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reactor 2 Cooler of reaction product gas 3 Absorption tower 4 Distillation tower 5 Distillation tower 6 Cooler of acrylic acid aqueous solution

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) C07C 47/22 C07C 47/22 GH K

Claims (6)

[Claims] 1. A method for recovering acrolein from a reaction product gas containing acrolein obtained by catalytically oxidizing a hydrocarbon selected from the group consisting of propane and propylene, wherein said reaction product gas is contained in an amount of 5% by weight or more. A method comprising contacting with an aqueous solution of acrylic acid containing acrylic acid to absorb acrolein in the reaction product gas into the aqueous solution of acrylic acid, thereby separating the gas from the reaction product gas. 2. A method for recovering acrolein from a reaction product gas containing acrolein obtained by catalytically oxidizing a hydrocarbon selected from the group consisting of propane and propylene, wherein the reaction product gas is supplied to an absorption tower. An absorption step of bringing acrolein in a reaction product gas into an aqueous acrylic acid solution by bringing the aqueous solution of acrylic acid containing 5% by weight or more of acrylic acid into countercurrent contact in the column, an aqueous acrylic acid solution containing acrolein flowing out of the absorption tower Is supplied to a stripping tower system and heated to distill acrolein from the top of the tower, and an acrylic acid aqueous solution containing 5% by weight or more of acrylic acid is discharged from the bottom of the tower, and is discharged from the bottom of the stripping tower system A method comprising the steps of: circulating a cooled acrylic acid aqueous solution and then supplying the cooled acrylic acid aqueous solution to an absorption tower. 3. The method according to claim 2, wherein the acrylic acid concentration of the aqueous acrylic acid solution supplied to the absorption tower is 30% by weight or less. 4. The method according to claim 2, wherein the acrylic acid aqueous solution supplied to the absorption tower has an acrylic acid concentration of 10 to 20% by weight. 5. The reaction product gas is cooled to 60 ° C. or less,
5. A gas is supplied to an absorption tower after a part of the contained water vapor is condensed and removed.
The method according to any of the above. 6. The method according to claim 2, wherein the aqueous acrylic acid solution is supplied to the absorption tower at 40 ° C. or lower.