DE2703161A1 - PROCESS FOR MANUFACTURING HIGHLY PURE TEREPHTHALIC ACID - Google Patents

Description

Process for the production of highly pure
Terephthalic acid

The present invention relates to a method for
Production of terephthalic acid, in particular a process for the production of high quality terephthalic acid, which is suitable as a starting material for polyethylene terephthalate by direct esterification, the oxidation of p-xylene in the liquid phase being carried out under specially selected conditions so that a p- Xylene conversion to terephthalic acid of
at least 95 % is achieved, whereupon the terephthalic acid slurry is subjected to a post-oxidation.

In the past, polyethylene terephthalate was made from dialkyl terephthalate and ethylene glycol. Recently, I turned my attention to a process for the polycondensation of
Terephthalic acid and ethylene glycol; the terephthalic acid starting material of this process must, however, be of high purity,
ie the content of 4-carboxybenzaldehyde (one of the impurities present in terephthalic acid, hereinafter referred to as 4-CBA

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must be below 500 ppm.

Usually contains the by conventional oxidation of p-dialkylbenzene Terephthalic acid produced in the liquid phase is a large one Amount of impurities including various reaction products, such as 4-CBA, p-toluic acid and other compounds with unknown chemical structure. To achieve a highly pure product suitable for polyethylene terephthalate The terephthalic acid slurry thus prepared must therefore be one Subsequent to oxidation by passing molecular oxygen through them (see e.g. GB PS 983 677) This post-oxidation removes the reaction admixtures as well a trace of other impurities, mainly of a coloring nature, which are not removed by oxidation in the liquid phase, these undesirable materials being present in the mother liquor of the slurry; through this post-oxidation the Improved quality of terephthalic acid. However, Cs was found that the amount of impurities removed by the post-oxidation is closely related to the conditions under which the oxidation of p-xylene takes place in the liquid phase, and the intended effect of the post-oxidation is often not achieved, which is evident from the conditions of the liquid phase oxidation depends.

Ee is already a process for the production of terephthalic acid has been proposed in which p-xylene catalytically in liquid Phase is oxidized to form a terephthalic acid slurry, whereby the 4-CBA content of the mother liquor of the slurry is below

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270316t

-JT-

800 ppm is maintained and the resulting slurry is post-oxidized by bubbling molecular oxygen-containing gas through them, creating highly pure terephthalic acid with a 4-CBA content below 500 ppm is used for the polycondensation with ethylene glycol is suitable. Of the various processes, the 4-CBA content in the mother liquor is below 800 ppm hold, from an economic point of view, the one that is most favorable in which the reaction time of the liquid phase oxidation is extended.

However, this method has the inherent disadvantage that when

large-scale work due to the longer response time

( "burned") is much acetic acid, which is the reaction solvent, is oxidized ^f, so that the process becomes uneconomical. Therefore, a method has now been sought by which the 4-CBA content in the mother liquor of the terephthalic acid slurry is reduced and the oxidation of the acetic acid solvent is prevented or substantially reduced. According to the invention it has now been found that in the oxidation of p-xylene in the liquid phase, a special catalyst system reduces the oxidation of the acetic acid solvent and shortens the reaction time, while at the same time improving the quality of the terephthalic acid product.

The aim of the present invention is therefore a method of production of terephthalic acid, which is characterized in that ■ an

(a) p-xylene in acetic acid as a solvent of a one-step oxidation with a gas containing molecular oxygen in presence of an oxide containing a heavy metal compound

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tion catalyst at a temperature of 100-250 ⁰ C. and a pressure of atmospheric pressure to 200 at to form a terephthalic acid slurry and subjected

(b) Post-oxidizing the teraphthalic acid slurry by bubbling a gas containing molecular oxygen therethrough subject,

wherein the oxidation of step (a) is carried out by keeping the A-CBA concentration in the mother liquor of the slurry below 800 ppm and the catalyst being comprised _{essentially of (1) a cobalt compound, expressed as Co 1,} of 200-600 ppm _t the solvent, (2) a manganese compound, expressed as Mn, from 0.1 to 1.5 times the amount of Co by weight; and (3) at least one bromine compound selected from the group consisting of hydrogen bromide, manganese bromide and cobalt bromide, expressed as Br, of 400 -2000 ppm of the base of the solvent.

In the first stage of the process according to the invention, p-xylene in acetic acid as a solvent is subjected to an oxidation in the presence of a catalyst containing a heavy metal compound to such an extent that a conversion of p-xylene into terephthalic acid of at least 95 %, preferably more than 98 % and in particular more than 99.5 % is achieved. -

The oxidation used according to the invention in the liquid phase, in which the p-xylene in acetic acid in the presence of a heavy metal compound-containing catalyst with a molecular Oxygen-containing gas is oxidized is known as the "SD" process (see e.g. US PSS 2,833,816, 2,962,361, 3 092 658 3 115 520 and 3 119 860). The procedures of these patents can be used with success as long as the

. 709832/1004

bonds of the catalyst and the oxidation conditions meet the requirements defined below. The inventiveGe Process can be carried out continuously or semi-continuously, with the continuous process the best results are achieved «

The amount of acetic acid is usually 0.5-20 parts, preferably 1-10 parts per part by weight of the p-xylene to be oxidized. The acetic acid should contain less than 30% water or other organic materials such as aldehydes or ketones.

The molecular oxygen-containing compound suitable according to the invention Gas contains 5-100 vol .- ^ oxygen, usually being of simplicity half air used. The amount of molecular oxygen-containing gas supplied to the reaction system is in terms of as 0.1-100 moles, preferably 3-100 moles, per mole of p-xylene.

The pressure to carry out the oxidation is not critical, as long as the reaction system is kept in a liquid state, and it is usually 10-50 kg / cm.

The catalyst used according to the invention is a Co-Mn-Br system. According to the invention, cobalt acetate, naphthenate or bromide, for example, can be used as the cobalt compound. The amount to use of the cobalt compound in terms of Co based of the solvent, 200-600 ppm, preferably 250-500 ppm and especially 300-400 ppm.

According to the invention, manganese acetate, naphthenate or bromide can be used. The amount of the manganese compound to be used is in terms of Mn dor base of Co, 0.5-1.7 times, preferably that

709832/1004

0.7 to 1.3 times the weight. Quantities outside of these specified Limits degrade the quality of the terephthalic acid produced.

According to the invention, hydrogen bromide, manganese or cobalt bromide can be used as the bromine compound. It has been found that other bromine compounds, such as sodium, potassium or ammonium bromide as well as tetrabromoethane, show only low activity and therefore do not achieve the intended effect.

The amount of the bromine compound to be used is, in terms of Br based on the solvent, 400-2000 ppm, preferably 700-1500 ppm. Too large an amount does not improve the quality of terephthalic acid and is uneconomical because it is makes the recovery of the bromine compound necessary for reuse. Too small an amount, however, leads to a worse one Quality of terephthalic acid. In order to produce high quality terephthalic acid, the amount of the bromine compound must be within of the above range can be selected so that, in terms of Br on the basis of Co, it is more than 2.0 times, preferably more than 2.5 times the weight. When using cobalt or manganese bromide, these act as both Bromine compound as well as cobalt or manganese compound.

The temperature for conducting the liquid phase oxidation _x usually is 100-250 ⁰ C, preferably 190-225 ⁰ C, in particular 2Q5-220 ° C This higher reaction temperature is critical in the present invention. In the known prior art, the oxidation of p-xylene in terephthalic acid in the liquid phase ^{should be carried out at a temperature below 200 °} C. in order to avoid unnecessary

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'40 - Avoid oxidation of the acetic acid solvent and deterioration in the quality of the terephthalic acid product. According to the invention, by using the catalyst system mentioned, the liquid phase oxidation can be carried out at a higher temperature, thus shortening the time required and noticeably reducing the oxidation of the solvent.

The liquid phase oxidation can be continuous or semi-continuous be performed. Γ j is critical to the 4-CBA Concentration in the mother liquor of the terephthalic acid slurry formed below 600 ppm, preferably below 700 ppm. If the concentration of 4-CBA is above 800 ppm, a large amount will be produced carried along by impurities in the precipitated terephthalic acid; therefore, it is subject to post-oxidation purification End product inferior and not for direct polycondensation suitable.

To reduce the 4-CBA concentration in the mother liquor to the desired value below 800 ppm, it is advisable to control the residence time of the starting material in the oxidation vessel; however, other measures can also be used, such as increasing the amount of solvent or reducing the water content in the reaction mixture, increase of the partial pressure of oxygen in the reaction vessel and improved dispersion of p-xylene and / or molecular oxygen-containing gas in the reaction mixture.

For example, the following measures can be recommended:

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(1) Bsi a continuous process with a weight

ratio of solvent to p-xylene of 3: 1 is the Evacuation time about 1.5-2.5 hours;

(2) In a continuous l / experience with a dwell time of one hour, the solvent / p-xylene ratio is about 6: 1; and

(3) In a continuous process, the water content becomes regulated to 5-14 wt .- ^ in the mother liquor of the reaction mixture.

The above numbers vary depending on other parameters the reaction conditions and cannot be standardized. The numbers given are therefore only examples and others Conditions for maintaining a particular A-CBA content can be determined through tests based on the above guidelines will.

Then the terephthalic acid slurry thus prepared becomes one Subject to post-oxidation. The main aim of these is oxidation various reaction intermediates, such as A-CBA, in the mother liquor. Therefore, the presence of a large amount of unreacted p-xylene starting material in the slurry should be avoided although some p-xylene is also removed during the post-oxidation.

The post-oxidation can be carried out continuously or semi-continuously. In the continuous process, the reaction mixture transferred to a post-oxidation vessel or crystallization tank, and before or during the cooling of the mixture to precipitate the terephthalic acid crystals, a molecular acid

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substance-containing gas passed through the slurry. In the semi-continuous process, a gas containing molecular oxygen is fed directly into the oxidation vessel upon completion introduced the oxidation in the liquid phase.

For as complete a post-oxidation as possible, it is advisable to introduce gas that no longer contains molecular oxygen, than is necessary to oxidize the reaction intermediates present in the slurry.

If too much gas is introduced, solvents will evaporate and water, which reduces the post-oxidation temperature leads; this in turn requires additional heating, making the process uneconomical. Through appropriate By regulating the flow rate of the oxygen-containing gas, the post-oxidation proceeds smoothly without further heating.

As the amount of impurities to be oxidized in the post-oxidation is small, the amount of molecular oxygen-containing gas to be introduced is between about 1/100 and 1/10 compared to liquid phase oxidation. Usually the flow rate of the gas is adjusted so that the exhaust gas is out after oxidation an oxygen concentration of 0.5-8 vol .- ^ S, preferably 1.5-4 vol .- ^. When the exhaust gas from the liquid Phase oxidation contains sufficient oxygen, this can be fed directly to the post-oxidation stage, or the feed can be done after it has been topped up with free gas containing molecular oxygen. The temperature to carry out the post-oxidation is 100-250 ° C, preferably 150 ° C. until the temperature at which the oxidation occurs in the liquid phase

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he follows. However, since the preferred post-oxidation temperature is lower than that of the liquid phase oxidation, the recommended temperature range is between the temperature of the liquid phase oxidation (referred to as T ⁰ C.) and (T-30) ^o C. Although If higher temperatures can be used, this requires further heating, which makes the process uneconomical. The oxidation catalyst used in the first stage can optionally be added.

According to the invention, terephthalic acid can be produced of a quality those with the best state-of-the-art product (e.g.

the product of Example 16) is comparable, the residence

and
shorter time / the oxidation of acetic acid is lower and

less catalyst is used.

The 4-CBA concentration in the alkaline solution and the permeability and oxidation of acetic acid mentioned in the examples are determined as follows:
4-CBA concentration in the mother liquor. The terephthalic acid slurry was removed from the reaction vessel for oxidation in liquid phase and recovery; the mother liquor subjected to a liquid / solid separation, whereupon the cutter liquor with ethylene glycol 'was esterified. Then the 4-CBA concentration was measured by gas chromatography. Permeability (Τ34θ)

The permeability was with a solution of 15 parts of terephthalic acid in 100 parts of 2N aqueous potassium hydroxide in one Spectrophotometer using; a quartz cell an inner one. Size of 1 cm and measured at a wavelength of 340 nyu.

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- VT -

Oxidation . * u

The oxidation was calculated on the basis of the amounts of £ q and CO ₂ (which are decomposition products of acetic acid) in the exhaust gas from the reaction vessel of each experiment, and the results were compared with those of Example 1 as 1.

The following examples illustrate the present invention, without restricting them. All parts are Goui parts.

Example 1 Continuous process

The experience took place in a device made from one made of titanium produced autoclave for oxidation in liquid phase, equipped with reflux condenser, stirrer, heating means, inlet for the starting material, Inlet for the solvent-based catalyst, inlet was provided for the molecular oxygen-containing gas, an outlet for the exhaust gas and an outlet for the reaction product; the The post-oxidation vessel was a titanium autoclave with a reflux cooler, Stirrer, gas inlet, inlet for the terephthalic acid slurry, Outlet for the exhaust gas and outlet for the reaction product was provided; the crystallization device is rare Provide coolants.

The following materials were continuously introduced into the liquid phase oxidation vessel in the specified amounts:

Parts

p-xylene acetic acid

Cobalt acetate tetrahydrate Manganese acetate tetrahydrate

47% aqueous hydrogen bromide

water

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The proportions of Co, fin and Br based on acetic acid were 330 ppm, 330 ppm and 1000 ppm, respectively. The reaction was carried out at 210 ⁰ C. and "obei air was passed with such a speed through the Reaktionsnischung that the exhaust gas containing an oxygen concentration of 4 V0I.-36 25 kg / cm ^2. The sealing time was set to 120 minutes.

The Terephthalsäureaufschlämmung thus prepared was transferred to the post-oxidation vessel and oxidized at 190 ⁰ C. and 17.5 kg / cm ² for a residence time of 85 Hinuten, wherein a mixture of air and exhaust gas from the oxidation vessel (O ₂ concentration of the mixture = 10 Uo 1.-55) was introduced at such a rate that the exhaust gas contained 4 Vol .- ^ oxygen.

Subsequently, the Terephthalsäureaufschläminung was transferred to the crystallizer for crystallization at 100 ⁰ C..

The separated crude terephthalic acid crystals were recovered by M Ischen with 4 times the amount by weight of acetic acid at 8O ⁰ C. for 20 minutes, washed, with stirring, and the washed terephthalic acid and dried.

The permeability of the terephthalic acid thus produced, the 4-CBA concentration in the mother liquor and the oxidation of acetic acid are shown in Table 1.
Example 2 to 5

Example 1 was with the reaction temperatures given in Table 1 and / or repeated residence times of the liquid phase oxidation, the results listed being achieved.

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Temp.
⁰ C. \ Jbt \ si.
Time
min 4-CBA
ppm Tebell e 1 *) oxidation Ex.
No. 210 120 310 W « 4-CBA _#
(ppm / TPA 1.0 1 • ι 95 500 91.5 270 0.8 2 220 85 280 90.0 400 1.0 3 210 45 1000 90.5 240 0.5 4 * Il 40 1300 78.0 800 0.4 5 * 68.0 1100

* Comparative examples ♦ * = terephthalic acid

Example 6 to 13

Modified Example 1 was made with the / active components shown in Table 2 repeated in the catalyst system, ujobei the results listed were achieved.

Example 14 to 16

changed

Example 1 was made with the / active components mentioned in Table 3 repeated in the catalyst system, the results listed were achieved.

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Table 2

Example,

Co Bin Br Mn / Co

Ve rui,

Time

min

4-CBA ppm

4-CBA
ppm / TPA

oxidation

-j
ο
co

10 »
11 *
12 ·
13 *

500

300

·· 250

"420

600

"100

"

n M

1500 1000

3000 1500

* Comparative example ** = terephthalic acid

1.0

0.91

0.758

1.27

1.82

0.33

0.1

120
120
120
120
120
120
120
120

300 300 310 290 290 900 450 500

91.0 91.0 91.0 91.5 84.0 55.0 91.0 85.0

270
270
280
260
260
700
400
450

1.1 1.0 1.0 1.0 1.1 0.6 1.5 1.1

«« ■ j ο co

Table 3

Ex • Co Mn Br Bromine-
connectq. Veriueil.
Time; min 4-CBA
ppm T ₃₄₀ W 4-CBA
ppm / TAP oxidation 14th
15th
16 * 330 330 900
330 330 960
330 600 1000 CoBr ₂
MnBr ₂
NaBr 120
120
175 350
340
300 91.0
91.0
93.0 300
29 0
300 1.0
1.0
1.6 709832 / * Comparative example
** = terephthalic acid
-ν 100 -

Claims (11)

Claims - Process for the production of highly pure terephthalals, uiobei man ', _; ^; (a) p-xylene in acetic acid as solvent a one-step oxidation with a molecular oxygen-containing gas in the presence of a Schuiermetallverbindung containing oxidation catalyst at a temperature of 100-250 ⁰ C. and a Oruck from atmospheric pressure to 200 atm for forming a Terephthalsäureaufschlämmung subjecting and (b) Post-oxidation the terephthalic acid slurry Passing a molecular oxygen-containing gas, characterized in that the liquid phase oxidation of step (a) occurs with the concentration of 4-carboxybenzaldehyde in the mother liquor of the slurry is kept below 800 ppm and the catalyst consists essentially of (i) a cobalt compound, expressed as Co, in an amount of 200-600 ppm of the solvent, (2) a manganese compound in terms of Mn, in an amount by weight of 0.5 to 1.5 times the weight of Co and (3) at least one A bromine compound from the group of hydrogen bromide or manganese or cobalt bromide, expressed as Br, in an amount of 400-2000 ppm, based on the solvent. 2.- The method according to claim 1, characterized in that the Oxi dation at a temperature of 190-225 ⁰ C, preferably 205-225 ⁰ C «is carried out. 709832/1004 ORIGINAL INSPECTED 3. - Method according to claim 1 and 2, characterized in that hydrogen bromide is used as the bromine compound. 4.- l / experience according to claim 1 to 3, characterized in that cobalt acetate as the cobalt compound and manganese acetate as the manganese compound is used. 5.- V / experience according to claim 1 to 4, characterized in that the temperature of stage (b) is between the oxidation ^{temperature T 0} C. of stage (a) and a temperature of (T-3O) ° C. is. 6.- The method according to claim 1 to 5, characterized in that the molecular oxygen-containing gas in step (b) is the exhaust gas from step (a) alone or in a mixture with fresh air. 7.- The method according to claim 1 to 6, characterized in that the content of less than 800 ppm 4-carboxybenzaldehyde in the Mother liquor of the terephthalic acid slurry is obtained by that he (1) in a continuous l / learn the residence time of the starting material controlled in the oxidation vessel; (2) the proportion of the solvent to the starting material to be oxidized elevated; (3) reduces the water content of the reaction system; (4) increases the partial pressure of oxygen in the oxidation vessel; and (5) the starting material p-xylene and the gas containing molecular oxygen are uniformly dispersed in the reaction medium. 8.- The method according to claim 1 to 6, characterized in that the oxidation stage (a) in a continuous process by regulating the residence time of the starting material in the reaction vessel 709832/1004 takes place, whereby the 4-carboxybenzaldehyde content in the mother liquor of the terephthalic acid slurry was reduced to below 800 ppm vuird. 9, - Method according to claim 1 to 6, characterized in that daQ steps (a) and (b) take place continuously. 10.- The method according to claim 1 to 9, characterized in that the slurry containing the solid terephthalic acid is subjected to crystallization and the precipitated terephthalic acid is separated and washed with acetic acid. 11.- The method according to claim 10, characterized in that the washing of the terephthalic acid is carried out with acetic acid by the solid terephthalic acid is suspended in acetic acid and the terephthalic acid slurry is subjected to a solid / liquid separation subject. The patent attorney: 709832/10 0