JPS5842179B2 - Koujiyund Telephthalsan Nobun Rihou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for separating high purity terephthalic acid.

More specifically, a mixture of p-xylene and p-toluic acid methyl ester is oxidized in the liquid phase with a molecular oxygen-containing gas in the presence of a heavy metal catalyst, and terephthalic acid is separated and recovered in high purity from the resulting oxidized mixture. It is about the method.

Conventionally, a mixture of p-xylene and p-)ruylic acid methyl ester is oxidized in the liquid phase with a molecular oxygen-containing gas in the presence of a heavy metal catalyst to obtain an oxidized mixture mainly containing p-toluic acid and terephthalic acid monomethyl ester. The method of obtaining a p-toluic acid methyl ester mixture by subsequently obtaining the mixture and then contacting the mixture with methanol is called the "Pitten method" or the "Vercules-Pitten method", in which dimethyl terephthalate is It has been implemented on a large scale industrially as a manufacturing method.

In the method, p-1 ruyl acid methyl ester and terephthalic acid dimethyl ester are separated from the esterification mixture by distillation, and the former is recycled to the oxidation step;
The latter is sent to a refining process and made into a product.

The Pitten method is an industrially excellent method because dimethyl terephthalate, which is a raw material for polyester, can be produced directly from p-xylene at a low cost.

On the other hand, terephthalic acid is also industrially useful as a raw material for polyester, and most of it is produced by oxidizing p-xylene with a molecular oxygen-containing gas in the presence of heavy metals and bromine in an aliphatic carboxylic acid solvent such as acetic acid. So-called “SD Law”)
and is produced by a method of hydrolyzing terephthalic acid dimethyl ester.

Both terephthalic acid dimethyl ester and terephthalic acid are useful as polyester raw materials, and it is obvious that it would be industrially advantageous if both could be produced at low cost by the Pitten method.

In the Pitten method described above, p-xylene and p-)
The oxidized mixture obtained by oxidizing a mixture of toluic acid methyl esters contains p-toluic acid and terephthalic acid monomethyl ester as main components, and also partially contains terephthalic acid.

The content of terephthalic acid in the oxidized mixture varies depending on the oxidation conditions, etc., but it is usually about 5 to 15% by weight based on the oxidized mixture, and this is not sufficient for the 5th esterification step with methanol. It is then converted to terephthalic acid dimethyl ester.

On the other hand, attempts have been made to separate and recover terephthalic acid in the oxidized mixture to advantageously produce both terephthalic acid dimethyl ester and terephthalic acid in the Pitten method.

However, the content of terephthalic acid in the oxidized mixture is relatively small, its particle size is small, and it is partially dissolved in the oxidized mixture, so it is extremely difficult to easily separate it with high purity and good recovery rate. It is.

Japanese Patent Publication No. 45-24136 discloses that one of p-xylene, water, and acetic acid is added to an oxidized mixture obtained by oxidizing p-xylene and p-toluic acid methyl ester in the Pitten method, and the mixture is heated and heated. A method is described in which free terephthalic acid is separated by heating and then cooling.

Although the purpose of this method is to separate high-purity terephthalic acid, there are disadvantages when it is actually intended to be implemented industrially.

That is, if p-xylene is used and the oxidized mixture is not removed before esterification in the next step, p-xylene will be present in the methanol recovered from the esterification unit.
Contamination with xylene makes purification of methanol extremely difficult.

Furthermore, if water or acetic acid is used, various problems will occur unless these are removed before esterifying the oxidation mixture, and especially when acetic acid is used, problems of corrosion of various equipment in the process cannot be avoided. .

The object of the present invention is to provide a method for separating high-purity terephthalic acid from the oxidized mixture of the Pitten process, which does not have the disadvantages of the conventional methods mentioned above.

Another object is to provide a method for recovering a large amount of high purity terephthalic acid from the oxidized mixture.

A further object of the present invention is to provide a method capable of separating high-purity terephthalic acid from the Pitten process using industrially easy operations.

According to the present invention, the objects and advantages are that a mixture of p-xylene and p-1 toluic acid methyl ester is oxidized in the liquid phase with a molecular oxygen-containing gas in the presence of a heavy metal catalyst, and p-toluic acid and terephthalic acid monomethyl ester are A method of obtaining an oxidation mixture mainly containing oxidation mixture and separating solid terephthalic acid from the oxidation mixture by solid-liquid separation at a temperature at which it can flow, the solid terephthalic acid separated from the solid-liquid being discharged from the top of the oxidation column of the oxidation. It has been found that this can be achieved by separating high purity terephthalic acid by washing with p-xylene containing p-h methyl ester.

According to the present invention, from the oxidation mixture of the Pitten process,
Very high purity terephthalic acid can be separated.

In addition, by using p-xylene containing p-toluic acid methyl ester discharged from the top of the oxidation tower as a washing liquid, not only an excellent washing effect is achieved, but also the mother liquor after washing can be used directly as S or A part of it can be sent to the oxidation tower and used as part of the oxidation raw material, making it an extremely advantageous flow from an industrial perspective.

Next, the method of the present invention will be explained in more detail.

The oxidation mixture from which terephthalic acid is separated in the present invention is an oxidation reaction mixture in the so-called Pitten method, in which a mixture of p-xylene and p-toluic acid methyl ester is oxidized into molecular form in the liquid phase in the presence of a heavy metal catalyst. It is an oxidized mixture obtained by oxidation with an oxygen-containing gas, and mainly contains p-toluic acid and terephthalic acid monomethyl ester.

In the Pitten method, p-xylene and p-toluic acid methyl ester are usually subjected to an oxidation reaction at a composition ratio of 4:1 to 1:4 by weight.

The oxidation is carried out in the presence of heavy metal catalysts; preferred catalysts are cobalt, manganese, mixtures of cobalt and manganese, and mixtures of manganese and nickel.

These heavy metals are used as metals or compounds soluble in the reaction system.

As such heavy metal compounds, salts of metals as described above and organic acids as described below are advantageously used.

(a) Aliphatic carboxylic acids having 1 to 20 carbon atoms, such as formic acid, acetic acid, propionic acid, butyric acid, stearic acid, and palmitic acid; (b) Aromatic carboxylic acids, such as benzoic acid and toluic acid; (C> Alicyclic carboxylic acids such as naphthenic acid;
Furthermore, complex compounds such as acetylacetonate complex salts, methyl acetoacetate, and ethyl acetoacetate of the above-mentioned metals, and inorganic compounds such as carbonates, oxides, and hydroxides of the above-mentioned metals can be similarly used.

Among these various metal compounds, the above (a) to
(c) Salts of organic acids, particularly preferred are acetates,
These are benzoate, toluate, and naphthenate, all of which are easily available and have good solubility in the reaction mixture.

The total weight of the heavy metal catalyst in terms of metal is 50 to 1500 ppm based on the total weight of the entire reaction mixture.

It is desirable to use it preferably in a range of 80 to 500 ppm.

In particular, as previously proposed by the present inventors, (1
) A mixture of manganese and cobalt is prepared such that the Durham atomic ratio of metallic manganese: metallic cobalt is 0.1:99.9 to 99:
1, preferably 1:99 to 90:10 (see Japanese Patent Publication No. 49-27574) or (2
) A mixture of nickel and manganese is prepared such that the Durham atomic ratio of metallic nickel to metallic manganese is 95=5 to 0.5:99.
5, preferably 90:10-2. :98 (see Japanese Unexamined Patent Publication No. 48-96542).

The oxidation reaction temperature depends on the oxidation catalyst, etc., but is generally in the range of 140 to 240°C, preferably 160 to 220°C.

Oxidation is carried out as described above by contacting a mixture of p-xylene and p-toluic acid methyl ester with a molecular oxygen-containing gas in the presence of a heavy metal catalyst as described above.

As the molecular oxygen-containing gas, various gases such as pure oxygen and a mixed gas of pure oxygen and an inert gas can be used, but air is the most preferred because it is easily available.

In the oxidation mixture obtained by the oxidation reaction as described above, in addition to the main products p-)luic acid and terephthalic acid monomethyl ester, about 5 to 1 terephthalic acid is present.
It is contained in an amount of about 5% by weight.

Since terephthalic acid mainly exists in the solid state in the oxidation mixture, terephthalic acid is separated by solid-liquid separation at a temperature at which the mixture can flow.

However, the purity of the separated solid terephthalic acid is poor.

According to the research of the present inventors, terephthalic acid purified to such an extent can be obtained by washing such solid terephthalic acid with p-xylene alone, but surprisingly it contains p-)ruylic acid methyl ester. It has been found that when washing with p-xylene, terephthalic acid of higher purity can be obtained than when p-xylene alone is used.

Since p-xylene containing p-)ruylic acid methyl ester can be easily obtained by condensing the exhaust gas discharged from the top of the oxidation tower in the Pitten method,
This may be used as a washing liquid, and by circulating the mother liquor after washing as an oxidation raw material again, the oxidation intermediate contained in the solid terephthalic acid can be effectively utilized.

High purity terephthalic acid can be obtained by washing the solid terephthalic acid after solid-liquid separation with p-xylene containing p-toluic acid methyl ester as described above, but according to the research of the present inventors, after the completion of the oxidation reaction. oxidation mixture to 200
When the washing operation of the present invention is performed on solid terephthalic acid which has been kept at a temperature of ~300°C, preferably 220~260°C for 15 minutes to 3 hours, preferably 30 minutes to 2 hours, and then separated into solid and liquid. It has been found that terephthalic acid of higher purity can be obtained in higher recoveries, with the particular advantage that the amount of terephthalic acid recovered is greater than the amount present in the oxidation mixture at the end of the oxidation. It was.

After heating and holding the oxidized mixture described above, the oxidized mixture is cooled to a temperature at which it can flow, that is, a temperature at which it is in a fluid state such that it can be supplied to a solid-liquid separator, for example, 140 to 200 ° C.
Preferably, the solid-liquid separation is carried out at 145 to 180°C, and the cooling is performed at a rate of 0.9°C/min to 5°C/min, preferably 1.6°C/min to 3°C/min. It's advantageous.

By cooling the oxidized mixture at such a cooling rate and performing solid-liquid separation at a temperature within the above range, the purity of the obtained terephthalic acid can be further improved, and the separation operation can be performed extremely easily, which is preferable.

In the present invention, the solid terephthalic acid separated as described above is washed with p-xylene containing p-toluic acid methyl ester discharged from the top of the oxidation tower.

Generally, the p-xylene-containing mixture of the condensed organic phase discharged from the oxidation tower contains 50 to 90% by weight of p-xylene.
, 10 to 40% by weight of p-toluic acid methyl ester, and 5 to 20% by weight of methylbenzoic acid, terephthalic acid dimethyl ester, and other acids and alcohols.
include.

In the present invention, it is industrially desirable to use such p-xylene containing p-toluic acid methyl ester.

Furthermore, there is no particular problem in adding a portion of p-toluic acid methyl ester and/or p-xylene to the mixture.

The washing temperature is 10 to 130℃, preferably 20 to 130℃.
A range of 20°C is suitable.

In the mother liquor after washing the solid terephthalic acid,
In addition to p-1-ruyl acid methyl ester and p-xylene, it contains various active ingredients that were contained in the separated terephthalic acid and can ultimately be converted into terephthalic acid dimethyl ester and terephthalic acid. This can be recycled as it is or in part to the oxidation tower and used as part of the raw material.

As described above, according to the present invention, high-purity terephthalic acid can be separated and recovered from the oxidized mixture in the Pitten method, and therefore has great industrial value.

Next, the method of the present invention will be described in detail with reference to Examples.

Example 1 Oxidation product 1 of Witten-Hercules method with acid value 218
00 parts by weight was heated to 240°C and held for 1 hour.

It was gradually cooled to 140° C. after one hour.

While maintaining this temperature, terephthalic acid is separated from door to door, and organic substances mainly consisting of p-xylene are distilled from the oxidation reactor.
50 parts by weight of a mixture of the distillate from the device for removing unreacted p-xylene from the oxidation product and the raw material p-xylene (this was used in this example because it was charged into the oxidation tower during steady operation in the Pitten process). 25:10:15) respectively).

The p-xylene content in the mixture is 50% by weight, p-
The toluic acid methyl ester content was 30% by weight.

The recovered terephthalic acid was 11.9 parts by weight, acid value 673,
Residual DMT 0.02%, p-toluic acid 0.16%, p
- formylbenzoic acid 0.12% and monomethyl terephthalate 0.03%.

In this case, the p-xylene-based organic matter (including 15 parts by weight of the raw material p-xylene) to be introduced into the oxidation reaction apparatus (
Therefore, in this example, the amount introduced into the oxidation reactor was 58
parts by weight (therefore, a portion was not used for washing but was introduced into the oxidation reactor).

) This value includes p-toluic acid methyl ester (MPT).
, dimethyl terephthalate (DMT), MPT obtained from the high-boiling by-product separation unit.

From the mother liquor during terephthalic acid separation, p-xylene, MPT
After distilling out 10 parts by weight mainly of the following, it was esterified.

In the case of an oxidation reaction product in which terephthalic acid was not separated, esterification was possible in the same manner, and the following treatments were carried out in the same manner.

Example 2 IOO parts by weight of the Pitten oxidation reaction product with an acid value of 220 were heated to 240°C, held for 0.5 hours, and then heated to 145°C.
The temperature was lowered to 1,000 yen, the terephthalic acid was separated from the furnace, and the obtained crude terephthalic acid was distilled from the oxidation reactor to form a mixture of organic substances mainly consisting of p-xylene and raw material p-xylene.
It was washed with 0 parts by weight.

The p-xylene content in the mixture is 50% by weight, and M
The PT content was 30% by weight.

The recovered terephthalic acid was 13.2 parts by weight, acid value 672,
Residual DMT 0.01%, p-toluic acid 0.14%, p
- formylbenzoic acid 0.12% and monomethyl terephthalate 0.03%.

This washing mother liquor was added when the oxidation product was cooled down.

p-xylene, MP from the mother liquor during filtration of terephthalic acid
5 parts by weight of isoelectric T, etc. were collected and esterified with less than 0.1% of residual p-xylene.The MPT separated from this ester was 4
This and the total amount of organic matter (45 parts by weight) recovered from the mother liquor during filtration of terephthalic acid were introduced into the oxidation reactor and oxidized, and the whole was balanced.

Comparative Example 100 parts by weight of the oxidation product of the Pitten method was heated to 240°C, held for 1 hour, and then gradually cooled to 140°C over 1 hour.

Terephthalic acid was separated from the furnace while maintaining this temperature.

The acid value of the obtained terephthalic acid was 630, and the residual DMT was 0.
05%, para-toluic acid 2.5%, trimethyl trimellitate 0.1%, p-formylbenzoic acid 0.3%, MM
T was 2.7%.

Example 3 100 parts by weight of a Pitten method oxidation reaction product with an acid value of 221 was heated to 240°C, held for 0.5 hours, and then heated to 145°C.
The crude terephthalic acid obtained was washed with 50 parts by weight of a mixture of reagent P-xylene and reagent MPT.

The P-xylene content in the mixture is 80% by weight, MPT
The content was 200 parts by weight.

The recovered terephthalic acid was 13.2 parts by weight, acid value 672,
Residual DMT 0.01μ, P-) Ruylic acid 0.13φ, P
- formylbenzoic acid o, 11%, monomethyl terephthalate 0.03%.

Example 4 100 parts by weight of the same oxidation reaction product as in Example 3 was heated to 240°C.
After raising the temperature to 145°C and holding it for 0.5 hours, the temperature was lowered to 145°C and the terephthalic acid was separated in a furnace. It was washed with 50 parts by weight of a mixture of xylene.

The P-xylene content in the mixture is 87.0% by weight, M
The PT content was 8.0 weight φ.

The recovered terephthalic acid was 13.2 parts by weight, acid value 672,
Residual DMT 0.01%, p-toluic acid 0.15%,
P-formylbenzoic acid Q, 13%, monomethyl terephthalate 0.07%.

Comparative Example 2 100 parts by weight of the same oxidation reaction product as in Example 3 was heated to 240°C.
The temperature was raised to 145° C. and held for 0.5 hours, and then the temperature was lowered to 145° C., and the terephthalic acid was removed from the furnace, and the obtained crude terephthalic acid was washed with 50 parts by weight of the reagent P-xylene.

The P-xylene had a purity of 99.8% by weight, and its impurities were mainly m-xylene and did not contain MPT.

The recovered terephthalic acid was 13.8 parts by weight, acid value 667,
Residual DMT 0.01%, P-toluic acid 0.35%, P
- formylbenzoic acid 0.20% and monomethyl terephthalate 0.38%.

Claims (1)

[Claims] 1 A mixture of p-xylene and p-toluic acid methyl ester is oxidized in the liquid phase with a molecular oxygen-containing gas in the presence of a heavy metal catalyst to obtain an oxidized mixture mainly containing p-toluic acid and terephthalic acid monomethyl ester, and the oxidation In a method of separating solid terephthalic acid from a mixture by solid-liquid separation at a temperature at which the solid terephthalic acid can flow, the solid terephthalic acid separated from the solid-liquid is separated from p-xylene containing methyl p-toluate discharged from the top of the oxidation tower. A method for separating high-purity terephthalic acid, which is characterized by separating high-purity terephthalic acid by washing with water.