JPS5845436B2 - Purification method of phthalic anhydride - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for purifying crude phthalic anhydride.

Many industrial processes for the production of phthalic anhydride are known, in particular the oxidation of hydrocarbons, i.e. 0-xylene or naphthalene, to phthalic anhydride with an oxidizing agent, in particular molecular oxygen, in the gas or liquid phase. ( Hydrocarbo
n Processingll November 971 issue, 1
89.190.218 and 219).

It is also known that industrially obtained crude phthalic anhydride contains impurities that cannot be removed or that cannot be easily removed by distillation.

A process for oxidizing naphthalene by gas phase oxidation to a reaction mixture consisting of naphthoquinone, phthalic anhydride and in some cases unreacted naphthalene has also been described, for example in DE-A-2
No. 245,555.

Such a reaction mixture is suitable for the production of anthraquinone by reacting the naphthoquinone contained in the crude reaction mixture with butadiene to produce tetrahydroanthraquinone, which is subsequently oxidized to anthraquinone.

This method is a method for simultaneously producing anthraquinone and phthalic anhydride from raw materials naphthalene, oxygen, and butadiene.

In the entire process, the phthalic anhydride formed as a coupling product in the oxidation of naphthalene can be separated off as crude phthalic anhydride, for example with a purity of 99%, according to DE 22 45 555, for example.

This phthalic anhydride contains small amounts of known impurities such as naphthalene, anthraquinone, phthalic acid, naphthoquinone and butenylnaphthalene, as well as unknown impurities.

These may be compounds that can be detected by gas chromatography.

They may also be impurities that are not detected by gas chromatography, but can be detected by other chemical and physical methods, such as trace sulfur determination or colorimetry.

The purpose of the process of the present invention is to substantially completely remove impurities in the crude phthalic anhydride.

The extent of impurity removal can be measured analytically by various methods.

For example, 5000 to 20000 ppm in crude phthalic anhydride
The content of impurities that are present in and can be determined by gas chromatography can generally be determined up to a detection limit of 10 ppm for each impurity, which can be identified by gas chromatography.

The vine sulfur content present in crude phthalic anhydride, for example from 20 to 200 ppm, can be determined analytically to about 1 ppm.

In the case of very low concentration ranges of impurities, the purity of phthalic anhydride is determined by the Hazen color number according to DIN No. 53409 of July 1976.

In this method, the discoloration of molten phthalic anhydride before and after heat treatment (250° C. for 90 minutes) is measured as a characteristic value of the purity of phthalic anhydride.

Often further chemical applications of phthalic anhydride require that the Hazen number of the molten phthalic anhydride after heat treatment (90 minutes and 250° C.) is less than 30.

This time, a method for purifying crude phthalic anhydride was discovered by crystallizing it from acetic acid.

This crystallization is accomplished essentially by preparing a homogeneous liquid mixture of impurities in phthalic anhydride acetic anhydride and crude phthalic anhydride at elevated temperature, e.g. 80-120°C, and then cooling from this to e.g. 20-50°C. It is carried out by obtaining a suspension of solid phthalic anhydride in a mother liquor consisting of acetic acid.

Generally, the crude phthalic anhydride is 1 to 3 times by weight, preferably 1
．． It is crystallized from 5 to 2.5 times the weight of acetic acid.

The solid phthalic anhydride is separated from this suspension mechanically, e.g. by suction filtration, p-filtration or centrifugation, and the adhering mother liquor is removed by washing with a pure solvent, e.g. acetic acid, and finally the solvent is removed. , for example acetic acid, is removed by distillation or drying.

This process provides phthalic anhydride that is substantially free of impurities identified by gas chromatography.

If the crude phthalic anhydride contains phthalic acid as an impurity, for example in an amount of 0.1 to 1%, a homogeneous liquid is obtained before crystallization due to the solubility of phthalic acid in acetic acid.

The obtained phthalic anhydride may contain phthalic acid contained in crude phthalic anhydride as an impurity.

The content of phthalic acid in phthalic anhydride can be determined by known analytical methods.

Crude phthalic anhydride contains sulfur compounds, such as thionaphthalene,
If sulfuric or sulfonic acids are present, these are substantially removed during crystallization from acetic acid.

In a particular embodiment of the process, a small amount of an oxygen compound of an element of the alkali metal group is added to a liquid mixture consisting of phthalic anhydride, acetic acid and impurities in the crude phthalic anhydride.

Suitable alkali metal oxygen compounds are, for example, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium acetate, potassium acetate, sodium phthalate and potassium phthalate.

Advantageously, 10 to 200 parts by weight of an alkali metal in the form of an oxygen compound, such as sodium, such as sodium hydroxide, are added per 100 parts by weight of sulfur in the phthalic anhydride used.

Since these alkali metal oxygen compounds are homogeneously dissolved in the liquid mixture consisting of phthalic anhydride, acetic acid, and impurities in the crude phthalic anhydride, they can cause problems as foreign nuclei prior to and during crystallization. It is no longer necessary to pre-treat the mixture to remove potentially insoluble components.

By adding small amounts of alkali metal oxygen compounds before crystallization, phthalic anhydride is obtained which is essentially completely free of sulfur compounds.

The phthalic anhydride obtained by the process of the invention is very pure and can generally be used for further chemical applications.

However, in some cases it is desirable to produce a very pure phthalic anhydride that does not contain residual amounts of phthalic acid that becomes colored in the molten state and that also does not change color even after relatively long heat treatments.

In order to produce phthalic anhydride that satisfies these very high purity requirements and color stability requirements, the present invention involves heat treating crystallized phthalic anhydride at a temperature of approximately 200 to 300°C. Special embodiments of the method are appropriate.

This heat treatment can be carried out, for example, by heating phthalic anhydride in a column to a reflux state under normal pressure or reduced pressure.

The heat treatment of phthalic anhydride is preferably carried out under reflux under normal pressure, and the bottom temperature is about 280-290°C.

The treatment can be carried out in the presence or absence of a gas, for example nitrogen or a mixture of nitrogen and oxygen or an inert gas.

During the heat treatment, trace amounts of thermally unstable compounds are converted into polymeric compounds.

Colorless phthalic anhydride, which does not change color further on exposure to heat, is obtained by heat treatment followed by distillation.

Furthermore, the phthalic acid moiety present in phthalic anhydride is decomposed into phthalic anhydride and water.

In a preferred embodiment, low-boiling compounds generated during the heat treatment, such as acetic acid and water, are removed by distillation.

In order to quantitatively separate low-boiling compounds, it is advantageous to separate a small proportion of phthalic anhydride as a first distillate during the heat treatment.

During the removal of low-boiling compounds, it is advantageous to add a small amount of inert gas, for example nitrogen, to the reactor for heat treatment.

It is also advantageous to use a nitrogen/oxygen mixture, for example with an oxygen content of 2 to 8% by weight, instead of the inert gas nitrogen.

The presence of this oxygen favors the conversion of trace amounts of unstable colored substances into high molecular weight compounds.

Discoloration of the column bottoms generally occurs due to heat treatment.

In a subsequent step, the phthalic anhydride is separated from the high-boiling color-changing components by distillation.

This distillation is advantageously carried out under reduced pressure, for example about 100 to 200 mm Hg.

In certain situations it may be advantageous to remove low-boiling compounds as a first distillation prior to distillation and in a further column.

The mother liquor obtained during crystallization can be reused for crystallization.

To offset the accumulation of impurities in the mother liquor, it is possible to remove some of the mother liquor from the circulation system.

The solvent acetic acid is removed by distillation from this mother liquor side stream and recycled to the crystallization stage.

The residue in this case consists of impurities separated during crystallization and phthalic anhydride.

This can be decomposed, for example, by combustion.

Acetic acid can be separated in pure form from the mother liquor by distillation and can be used to clean the mechanical separation of phthalic anhydride crystals when acetic acid is used as a cleaning agent.

Pure phthalic anhydride meeting the specifications for commercially available phthalic anhydride can be obtained from crude phthalic anhydride according to the method of the present invention.
It can be obtained with a yield of 0% or more, for example 95 to 98%.

Very high purity and color stable phthalic anhydride is required for the production of phthalic esters, which are used as plasticizers in the production of polyvinyl chloride, for example.

Example 1 Crude phthalic anhydride was prepared according to the method of DE 22 45 555 with the following composition: Phthalic anhydride Phthalic acid Naphthalene Anthraquinone Phthalic anhydride Impurities within the boiling range, especially butenylnaphthalene 99.0 Weight% 0.4 Weight% 0.3 Weight% 0.2 Weight% 0.09 Weight% Organic sulfur (total amount) 0.01 Weight% 100
．． 00% by weight 1kg of this crude phthalic anhydride was mixed with 0.2% sodium hydroxide.
It was dissolved in 2 kg of acetic acid at 100° C. with the addition of P to give a homogeneous liquid.

The liquid mixture was cooled to room temperature with stirring, and the crystals of phthalic anhydride were separated from each other.

The mother liquor was distilled under reduced pressure to recover acetic acid.

This distillate was used for cleaning phthalic anhydride crystals.

The phthalic anhydride crystals were freed from acetic acid by drying at 50° C. under reduced pressure.

The analysis values of the resulting crystals were as follows: Decaphthalic anhydride 99.997 wt.% Naphthalene rulphate (), 001 wt.% Anthraquinone o, ooi wt.% Phthalic anhydride boiling point range 0.001 wt. % impurities, especially butynylnaphthalene organic sulfur (total amount) <0.0001% by weight Example 2 The phthalic anhydride obtained in Example 1 was heated at normal pressure and
It was heated under reflux for 3 hours at a temperature of 0° C., and an initial fraction of 2% by weight, based on the phthalic anhydride used, was separated off.

The phthalic anhydride was then cooled to 200-220°C and heated to 10
Distilled again under 0.07Q Hg.

99% of the phthalic anhydride used was obtained as overhead product.

This distillate was subjected to a thermal test by heating to 250° C. for 90 minutes under normal pressure.

After this thermal test, the molten phthalic anhydride was clear and colorless.

The Hazen color number measured according to DIN No. 53409 is 1
It was 0.

Example 3 Phthalic anhydride obtained in Example 1 was melted in a container equipped with a reflux condenser and a gas inlet.

A slow stream of nitrogen containing approximately 4% by weight of oxygen was introduced into the molten phthalic anhydride while heating the phthalic anhydride to a temperature of 250°C.

This heating continued for 1 hour.

The phthalic anhydride was then charged into a distillation column and redistilled.

92.4% of the phthalic anhydride used was obtained as overhead product.

The measured color number of this redistilled phthalic anhydride according to DIN 53409 was 8.5.

Claims (1)

[Scope of Claims] 1 Naphthalene is oxidized in the gas phase to produce naphthoquinone and phthalic anhydride, and then the naphthoquinone in the crude oxidation mixture is reacted with butadiene to produce tetrahydroanthraquinone, and then the tetrahydroanthraquinone is oxidized to produce anthraquinone. A method for purifying crude phthalic anhydride, which comprises crystallizing the crude phthalic anhydride obtained as a coupling product from acetic acid. 2 A homogeneous liquid mixture of crude phthalic anhydride and acetic acid is prepared at 80-120°C, and pure phthalic anhydride is extracted from the solids in the mother liquor consisting essentially of acetic acid by cooling it to 20-50°C. Claim 1 obtained in the form of
The method described in section. 3. The method according to claim 1 or 2, wherein crude phthalic anhydride is crystallized from 1 to 3 times the weight of acetic acid. 4. The method according to any one of claims 1 to 3, wherein the crystallization is carried out in the presence of an oxygen compound of an alkali metal group element. 5. The method according to any one of claims 1 to 4, wherein the crystallization is carried out in the presence of 10 to 200 parts by weight of an alkali metal in the form of an oxygen compound based on 100 parts by weight of sulfur contained in the phthalic anhydride used. Method. 6. The method according to any one of claims 1 to 5, wherein the crystallization is carried out in the presence of IJium (hydroxidation). 7. After crystallization, the solid phthalic anhydride is mechanically separated from the mother liquor, washed with acetic acid as a solvent, and the remaining acetic acid is removed by drying or separated by distillation. The method described in. 8. The method according to any one of claims 1 to 7, wherein the crystallized phthalic anhydride is heat-treated at a temperature of 200 to 300°C. 9. The method according to any one of claims 1 to 8, wherein the heat treatment is performed in the presence of a small amount of nitrogen. 10. Crude phthalic anhydride is crystallized from acetic acid and the crystallized phthalic anhydride is heat treated at a temperature of 200-300°C in the presence of a small amount of nitrogen/oxygen mixture with an oxygen content of 2-8% by weight. A method for purifying crude phthalic anhydride. 11. The method according to any one of claims 1 to 10, wherein after the heat treatment, phthalic anhydride is separated from high-boiling discoloration components by distillation. 12 Separation by distillation from 100 to 20011XmHg
A method as claimed in claim 11 below.