Method for purifying (meth)acrylic acid

Abstract

A method for purifying a crude (meth)acrylic acid obtained by a vapor phase catalytic oxidation method, characterized in that the crude (meth)acrylic acid having most parts of water and acetic acid removed therefrom, is fed to and distilled in a first distillation column of a purification system comprising first to third three distillation columns, the top fraction from the first distillation column is fed to and distilled in the second distillation column, the resulting top fraction is recovered as a high purity (meth)acrylic acid product, the bottoms from the first and second distillation columns are fed to and distilled in the third distillation column, and the resulting top fraction is fed to the first distillation column.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a method for purifying (meth)acrylic acid, particularly to a method for purifying a crude (meth)acrylic acid obtained by vapor phase catalytic oxidation, by distillation to obtain highly pure (meth)acrylic acid which is useful for the production of a highly water absorptive resin and for the production of a (meth)acrylic ester. In this specification, (meth)acrylic acid means acrylic acid or methacrylic acid. [0003] 2. Discussion of Background [0004] a. As a method for producing (meth)acrylic acid, a method of hydrolyzing the corresponding nitrile compound may, for example, be mentioned. However, at present, a vapor phase catalytic oxidation method of the corresponding hydrocarbon such as propylene or isobutylene, is mainly employed. Recently, a study has been made also on a vapor phase catalytic oxidation method using an inexpensive corresponding alkane as the starting material inste...

AI Technical Summary

Benefits of technology

"The present invention relates to a method for purifying crude (meth)acrylic acid obtained by vapor phase catalytic oxidation. The method involves distilling the crude acid in a purification system comprising three distillation columns. The top fraction from the first distillation column is fed to the second distillation column, resulting in a high purity (meth)acrylic acid product. The bottoms from the second and third columns are used as a material for producing a (meth)acrylic ester. The method also involves applying an aldehyde removal treatment to the top fraction from the first distillation column and using a thin film evaporator for the third distillation column. The purified (meth)acrylic acid obtained by this method has a low content of impurities and is suitable for use in the production of acrylic ester."

Problems solved by technology

Namely, if crude acrylic acid is used as a starting material for a polymer of acrylic acid without removing impurities, there will be a problem such as a delay in the reaction during the polymerization reaction, a decrease in the polymerization degree or coloring of the polymerized product.

However, it is not easy to remove by distillation impurities in crude acrylic acid obtained by vapor phase catalytic oxidation.

However, such a method is primarily intended to remove an aldehyde in the crude acrylic acid, whereby removal of maleic acid and / or maleic anhydride (these will be together hereinafter referred to as “maleic acids”) tends to be inadequate.

This method is effective for removal of maleic acids, but has a problem such that the added ammonia will be distilled from the top, such being not suitable for the production of highly pure acrylic acid.

Further, this application discloses batch treatment only, and discloses nothing about a method for continuously obtaining highly pure acrylic acid on a commercial scale.

Accordingly, it has been considered that with these techniques, it is not easy to continuously produce high purity acrylic acid by sufficiently removing impurities containing maleic acids from the crude acrylic acid.

However, in order to reduce the concentration of maleic acids in the starting material crude acrylic acid, it is necessary to remove maleic acids in the step of obtaining the crude acrylic acid, and such does not provide a substantial solution to the problem.

c. On the other hand, as a purification method for (meth)acrylic acid obtained by vapor phase catalytic oxidation of propylene or isobutylene, a distillation method is common, but (meth)acrylic acid is extremely susceptible to polymerization, and its handling was problematic.

However, if acrylic acid containing high boiling point impurities, is used as the starting material, there have been problems such that undesirable polymerization reactions or side reactions are likely to take place, thus leading to clogging of apparatus such as pipes by polymerized products, deterioration of unit consumption of main materials such as acrylic acid and an alcohol, and a decrease in the quality of the product.

The reason is that if an impurity, particularly furfural, is contained in the above-mentioned (meth)acrylic acid obtained by vapor phase catalytic oxidation, there will be a problem such as delay in the reaction, deterioration of the polymerization degree, coloration of the polymerized product, etc. at the time of the polymerization reaction for a water absorptive resin.

Crystallization usually requires a large initial investment, and from the economical viewpoint, a method by distillation is employed in many cases, but it is difficult to remove the above impurity, particularly furfural, by usual distillation.

This method is effective from the viewpoint of removal of the above-mentioned impurity, but has had a problem that it causes polymerization of (meth)acrylic acid during the rectification.

Formation of a polymer causes clogging in the distillation column, whereby the performance of the distillation column decreases, or it will be required to stop the operation.

In an operation for a short time, the effect of this method is confirmed, but in a continuous operation for a long time for a usual industrial operation, the effect has been still inadequate.

i. Further, in the purification of (meth)acrylic acid or its ester by distillation, if it is attempted to recover (meth)acrylic acid or its ester by means of a thin film evaporator from a heavy component containing (meth)acrylic acid or its ester discharged from the bottom of the distillation column, there is a problem such that clogging frequently occurs at a liquid-withdrawal tube or at an outlet portion of a liquid collection part of the thin film evaporator.

Accordingly, at such a portion, the liquid flowing down, has the majority of low boiling point components removed and thus essentially has bad fluidity, and besides, no stirring by stirring vanes takes place, whereby the liquid in contact with the inner wall surface tends to be hardly renewed by a fresh liquid.

Consequently, the retention time of the liquid in contact with the inner wall surface tends to be abnormally long, and (meth)acrylic acid or its ester remaining in the liquid, tends to gradually polymerize to change the liquid to be heavy, whereby the fluidity of this liquid further decreases, and a polymer tends to accumulate on the inner wall surface.

The accumulated polymer and precipitates not only hinder the flow of the liquid, but also clog the outlet portion of the liquid collection portion or the following liquid withdrawal tube, if they are peeled off from the inner wall surface.

k. Further, it is not desirable to obtain high purity acrylic acid from the top of the above-mentioned distillation column for purification while subjecting the bottom fraction of the distillation column for purification to thermal decomposition treatment to recover it for an acrylic acid purification step, because as the thermal decomposition treatment is carried out at a high temperature, many side-reactions or decomposition reactions will take place, whereby formation of undesirable by-products which cause to accelerate polymerization of acrylic acid, to contaminate an acrylic acid product or to present coloration to the product, or regeneration of aldehydes, takes place, and such compounds are likely to be recycled to the purification step of acrylic acid.

However, if the concentration of acrylic acid in the bottom fraction is lowered, the viscosity of the bottom fraction will increase, whereby precipitation of a polymer, etc. tends to readily take place, thus leading to a trouble of clogging at the withdrawal pipe.

Accordingly, the lowering of the acrylic acid concentration is limited, whereby it has been impossible to avoid a loss of acrylic acid in an amount corresponding to the one disposed as contained in the bottom fraction.

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