RU2467998C2 - Method of producing aromatic carboxylic acid - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method of producing aromatic carboxylic acid. Said method involves an oxidative step for oxidising an alkyl aromatic compound in the presence of a bromine compound to obtain an aromatic carboxylic acid; and a step for burning exhaust gas formed at the oxidation step in an incinerator. The gas obtained after burning the exhaust gas at burning temperature ranging from 450°C to 1000°C is cooled to 250°C or lower, and the time for cooling from 450°C to 250°C in the cooling process is not more than 1 second.

EFFECT: use of the present method enables to inhibit formation of bromine-containing dioxins.

11 cl, 1 tbl, 4 ex, 3 dwg

Description

FIELD OF THE INVENTION

The present invention relates to a method for the production of aromatic carboxylic acid.

BACKGROUND OF THE INVENTION

Aromatic carboxylic acids, such as terephthalic acid, are useful as raw materials for the synthesis of polymer esters or the like. For example, for the production of terephthalic acid, a typical representative of aromatic carboxylic acids, a method is used industrially in which a liquid-phase oxidation reaction of p-xylene with a gas containing molecular oxygen is carried out under elevated pressure. Typically, a liquid phase oxidation reaction is carried out in an oxidation reactor using a solvent containing an aliphatic carboxylic acid, such as acetic acid, in the presence of a heavy metal catalyst containing mainly cobalt or manganese and a bromine compound.

With this production method, high pressure off-gas is generated from the oxidation reactor. This exhaust gas contains, for example, residual oxygen; p-xylene as unreacted raw material; aliphatic carboxylic acid as a solvent; carbon monoxide, esters of aliphatic carboxylic acids and methyl bromide as by-products or the like. Usually the exhaust gas is released into the atmosphere, if necessary, removing useful components and energy, removing pollutants, etc.

Methyl bromide formed among the components of the exhaust gas from a bromine compound used as an auxiliary catalyst or the like, is one of the substances that contribute to the thinning of the ozone layer. Accordingly, it is desirable not to release methyl bromide into the atmosphere with the exhaust gas, preferably by removing it from the exhaust gas.

As a method of treating the exhaust gas containing methyl bromide, a combustion treatment is usually carried out. For example, it is proposed to burn off-gas by supplying a gas containing molecular oxygen, a combustion aid, and the like. in the presence of a catalyst (Patent Documents 1 and 2). However, the disadvantages of this method are the short life of the catalyst and the high cost of processing the exhaust gas.

Therefore, a method is proposed for treating by burning off an exhaust gas containing methyl bromide with fuel in a regenerative thermal oxidizing furnace (Patent Document 3). Patent Document 3 discloses that, according to this method, the exhaust gas can be efficiently processed by combustion without the use of a catalyst.

[Patent Document 1] JP-A-2000-189753

[Patent Document 2] JP-T-2001-515576 (The term “JP-T” as used herein means “published Japanese translation of the PCT patent application”)

[Patent Document 3] JP-A-2004-257606

Disclosure of invention

Technical challenge

In recent years, the harmfulness of chlorine-containing dioxins has become widely known; they also fear harm from bromine-containing dioxins, which can be formed during the combustion of bromine-containing materials, which makes it desirable to control their appearance. However, Patent Document 3 does not pay attention to the formation of bromine-containing dioxins during the combustion of methyl bromide, nor does the burning conditions necessary to slow the formation of bromine-containing dioxins, etc.

Therefore, the aim of the present invention is to carry out a combustion treatment while inhibiting the formation of bromine-containing dioxins in a method for the production of aromatic carboxylic acids, which includes the stage of processing by burning off-gas generated in the oxidation stage.

In addition, usually the lower the concentration of methyl bromide, the more difficult it is to achieve complete combustion and treatment. Patent Document 3 describes only the treatment by burning off an exhaust gas containing methyl bromide in an amount of about 30 ppm, and does not test the treatment on an off gas containing methyl bromide in a concentration of several ppm. up to 10 ppm

Therefore, another object of the present invention is to carry out a treatment for burning off-gas containing methyl bromide at a low concentration, difficult to process, in a method for producing aromatic carboxylic acids, which includes a step for treating burning off-gas generated in the oxidation step.

Technical solution

As a result of intensive research by the present inventors to achieve the above objectives, it was found that the above objective can be achieved by burning off-gas containing methyl bromide under special conditions, and thus the present invention was created.

The essence of the present invention is described as follows.

[1] A method for producing an aromatic carboxylic acid, which includes an oxidizing step for oxidizing an alkyl aromatic compound in the presence of a bromine compound to produce an aromatic carboxylic acid, and a step for burning the exhaust gas generated in the oxidation step in a combustion plant, where, after burning the exhaust gas at a temperature combustion of 450-1000 ° C, the gas after combustion is cooled to 250 ° C or lower and the cooling time from 450 to 250 ° C during cooling does not exceed 1 second.

[2] A method for producing aromatic carboxylic acid according to [1], wherein the cooling time from the combustion temperature to 250 ° C. does not exceed 1 second.

[3] A method for the production of aromatic carboxylic acid according to claims [1] or [2], where the gas after combustion is cooled by heat storage material.

[4] A method for producing an aromatic carboxylic acid according to any one of paragraphs. [1] - [3], where the combustion is carried out in the absence of a combustion catalyst.

[5] A method for producing an aromatic carboxylic acid according to any one of paragraphs. [1] to [4], where the combustion plant is a regenerative thermal oxidation plant.

[6] A method for producing an aromatic carboxylic acid according to any one of claims. [1] - [5], where the concentration of bromine-containing dioxins in the gas after cooling does not exceed 1 ng-TEK / m ³ in units of toxic equivalent.

[7] A method for producing an aromatic carboxylic acid according to any one of paragraphs. [1] - [6], where the stage of oxidation in a solvent based on an aliphatic carboxylic acid is carried out, and this method includes the step of separating a solid and a liquid, in which a suspension containing aromatic carboxylic acid obtained in the oxidation step and the solvent are separated into solid and liquid, obtaining a mass of aromatic carboxylic acid and a mother liquor, and a mother liquor recovery step, in which at least a portion of the mother liquor is reused in the oxidation step.

[8] The method for the production of aromatic carboxylic acid according to [7], wherein at the oxidation stage, at least 50% by weight of the mother liquor is reused.

[9] A method for producing an aromatic carboxylic acid according to any one of paragraphs. [1] - [8], where the concentration of methyl bromide in the exhaust gas does not exceed 100 ppm. by volume.

[10] A method for producing an aromatic carboxylic acid according to any one of paragraphs. [1] - [9], where the alkyl aromatic compound is p-xylene.

[11] A method for producing an aromatic carboxylic acid according to any one of paragraphs. [1] - [10], where aromatic carboxylic acid is terephthalic acid.

Beneficial effects

According to the present invention, in a method for producing aromatic carboxylic acid, which includes a step for treating the flue gas generated in the oxidation step by burning, the flue gas containing methyl bromide can be treated by burning while inhibiting the formation of bromine-containing dioxins, so the present invention is useful in that that the adverse environmental load can be reduced.

According to the present invention, the exhaust gas containing methyl bromide at a low concentration can also be effectively treated by burning.

Furthermore, according to the present invention, it is not necessary to supply a large amount of an oxygen-containing gas such as air, it is also not necessary to constantly supply combustion fuel and combustion is possible even in the absence of a catalyst, so that the exhaust gas containing methyl bromide can be efficiently processed by combustion . Therefore, the present invention is useful in that operating costs, installation costs and power consumption can be reduced.

In addition, according to the present invention, the formation of NOX (nitric oxide) can be inhibited.

According to this, the method of producing aromatic carboxylic acid according to the present invention is very safe, not creating a large adverse environmental burden, and its cost and energy consumption are low.

Brief Description of the Drawings

Figure 1 is a flowchart showing an example of an exhaust gas treatment step according to the present invention.

FIG. 2 is a flow diagram showing an example of a method for producing terephthalic acid according to the present invention.

FIG. 3 is a flowchart showing another example of a method for producing terephthalic acid according to the present invention.

Explication

1: Oxidization off-gas

2: solvent

3: Absorber exhaust gas

4: Liquid containing recoverable aliphatic carboxylic acid

5: Liquid containing recoverable esters of aliphatic carboxylic acid

6: exhaust gas

7: Combustion plant exhaust gas

8: Absorber exhaust gas

11: high pressure absorber

12: Heat exchanger

13: Gas expander (gas throttle)

14: Combustion plant

15: Absorber

21: Oxidation Reactor

22: Separator solids and liquids

23: Flushing unit

24: Solids and liquids separator and flushing unit

25: dryer

26: Fridge

31: Oxidation Reactor

32: Low temperature post-oxidation reactor

33: Pump

34: Heater

35: High temperature post-oxidation reactor

36: Instant evaporator

37: Separator solids and liquids

38: Flushing unit

39: Solids and liquids separator and flushing unit

40: dryer

41: Oxidation reactor refrigerator

42: Refrigerator low temperature oxidation reactor

43: Refrigerator of the high temperature oxidation reactor

44: Instant evaporator cooler

A: p-Xylene

B: Gas containing molecular oxygen

C: Suspension

D: Mass of terephthalic acid

E: stock solution

F: Mass of terephthalic acid (after washing)

G: Crude terephthalic acid crystals

H: Reusable (returnable) mother liquor

J: Drain the mother liquor

K: Detergent

L: Detergent waste

M: Waste gas from the oxidation reactor (before condensation)

N: Liquid Condensate

P: Oxidation reactor off-gas (after condensation)

Q: Reusable (Returnable) Aliphatic Carboxylic Acids and Aliphatic Carboxylic Acid Esters

a: p-Xylene

b: Gas containing molecular oxygen

c: Suspension

d: Suspension stage low-temperature additional oxidation

e: Suspension of the high temperature post-oxidation step

f: Mass of terephthalic acid

g: stock solution

h: Mass of terephthalic acid (after washing)

i: Terephthalic acid crystals

j: Reusable (returnable) mother liquor

k: Drainable waste mother liquor

l: Detergent

m: Waste detergent

n: Exhaust gas from the oxidation reactor (before condensation)

o: Liquid condensate

p: Oxidation reactor off-gas (after condensation)

q: Exhaust gas from the additional low temperature oxidation reactor (before condensation)

r: liquid condensate

s: Exhaust gas from the low temperature oxidation reactor (after condensation)

t: Exhaust gas from the high temperature oxidation reactor (before condensation)

u: liquid condensate

v: Exhaust gas from the high temperature oxidation reactor (after condensation)

w: Evaporator exhaust gas (before condensation)

x: Liquid Condensate

y: Evaporator exhaust gas (after condensation)

z: Reusable (Returnable) Aliphatic Carboxylic Acids and Aliphatic Carboxylic Acid Esters

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a method for the production of aromatic carboxylic acid, which includes an oxidation step in which the alkyl aromatic compound is oxidized in the presence of a bromine-containing compound to produce an aromatic carboxylic acid, and a step in which the exhaust gas generated in the oxidation step is burned in an incinerator where, after burning the exhaust gas at a temperature of 450-1000 ° С, the gas after combustion is cooled to 250 ° С or lower and the cooling time from 450 ° С to 250 ° С at the cooling stage does not exceed t 1 seconds.

That is, the present invention can inhibit the formation of bromine-containing dioxins by treating the exhaust gas containing methyl bromide under special combustion and cooling conditions.

The production method according to the present invention will be described in detail below. In this regard, it should be noted that in the following description the pressure value means absolute pressure and normal pressure is usually 0.101 MPa, unless otherwise indicated.

A method for producing an aromatic carboxylic acid according to the present invention includes at least an oxidation step in which the alkyl aromatic compound is oxidized to produce an aromatic carboxylic acid, and an exhaust gas treatment step in which the exhaust gas generated in the oxidation step is burned in an incinerator, and preferably further includes a step for separating the solid and the liquid and a step for recovering the mother liquor.

The type of aromatic carboxylic acid to which the present invention is applicable is not particularly limited, and for example, orthophthalic acid, isophthalic acid, terephthalic acid, trimellitic acid (benzenetricarboxylic acid), 2,6- or 2,7-naphthalenedicarboxylic acid, 4 , 4'-biphenyldicarboxylic acid and the like. In the group of these aromatic carboxylic acids, the present invention is preferably used for the production of phthalic acids (orthophthalic acid, isophthalic acid, terephthalic acid or the like), and it is particularly preferred to use it for the production of terephthalic acid.

Alkylaromatic compounds of the starting materials are also not particularly limited, and the aromatic ring may be monocyclic or polycyclic. As the alkyl groups, for example, a methyl group, an ethyl group, an n-propyl group, an isopropyl group and the like can be mentioned.

As specific examples of alkyl aromatic compounds, mention may be made, for example, of di- and trialkylbenzenes, di- and trialkylnaphthalenes and di- and trialkylbiphenyls. Preferred examples are m-diisopropylbenzene, p-diisopropylbenzene, o-diisopropylbenzene, o-cimen, m-cimen, p-cimen, o-xylene, m-xylene, p-xylene, trimethylbenzenes, 2,6- or 2,7- dimethylnaphthalene, 2,6-diisopropylnaphthalene, 4,4'-dimethylbiphenyl or the like. Due to its high reactivity, alkylbenzenes, alkylnaphthalenes, alkylbiphenyls and the like having from 2 to 4 alkyl groups having from 1 to 4 carbon atoms such as a methyl group, an ethyl group, an n-propyl group and an isopropyl group are most preferred.

Alkyl aromatic compounds may be partially oxidized. These are compounds in which the alkyl groups of the alkyl aromatic compounds are oxidized to an aldehyde group, an acyl group, a carboxyl group, a hydroxyalkyl group or the like, but they are not oxidized to the extent that they become a true aromatic carboxylic acid. Specifically, for example, mention may be made of 4-carboxybenzaldehyde (hereinafter abbreviated as “4CBA”), 3-methylbenzaldehyde, 4-methylbenzaldehyde, m-toluic acid, p-toluic acid, p-toluyl aldehyde, 3-formylbenzoic acid, 4-formylbenzoic acid, 2-methyl-6-formylnaphthalene and the like.

These starting materials may be used individually, or two or more materials may be used in combination.

In general, xylenes (o-xylene, m-xylene and p-xylene) are preferred as starting materials, and p-xylene is particularly preferred. When p-xylene is used as a starting material, aromatic compounds having partially oxidized alkyl groups are, for example, 4CBA, p-toluyl aldehyde, p-toluic acid and the like. and terephthalic acid is obtained as aromatic carboxylic acid.

As an example of a method for the production of terephthalic acid with p-xylene as the starting material, the oxidation stage (I), the stage of separation of solid and liquid (II), the stage of return of the mother liquor (III) and the stage of processing the exhaust gas (IV) will be explained below. but in the production of other aromatic carboxylic acids, a similar production is possible with arbitrary changes in these stages.

<Stage oxidation (I)>

In the oxidation step (I), aromatic carboxylic acid is obtained by oxidizing an alkyl aromatic compound in the presence of a bromine-containing compound. For example, it can be pointed out that a suspension consisting mainly of aromatic carboxylic acid and a solvent is obtained by oxidizing an alkyl aromatic compound used as a starting material in a solvent with a gas containing molecular oxygen in an oxidation reactor. In the above oxidation step (I), the above oxidation reaction (oxidation treatment) can be carried out only once or it can also be carried out two or more times. When the oxidation reaction (oxidation treatment) is carried out two or more times, its conditions may be the same as the conditions of the first oxidation reaction (oxidation treatment), or they may be different from the conditions of the first oxidation reaction (oxidation treatment).

When terephthalic acid is the target of producing aromatic carboxylic acid, p-xylene is usually used as the starting material, but as described above, in addition to p-xylene, the starting material may contain intermediates or impurities, such as p-toluic acid, p- toluyl aldehyde or 4CBA. Preferably, in the oxidation step (I), at least 90% by weight of p-xylene is oxidized to terephthalic acid, more preferably if at least 95% by weight is oxidized.

Solvents typically contain primarily aliphatic carboxylic acid. Aliphatic carboxylic acids are not particularly limited, but any acids from the group consisting of acetic acid, propionic acid, formic acid and butyric acid are preferred. In this regard, it should be noted that the terms “contain mainly” mean a content of at least 60% by weight of the total amount of solvent. Of these, preferred solvents are those containing mainly acetic acid due to its solubility, boiling point and ease of handling. Most preferred solvents are mixtures of acetic acid and water. With respect to mixing acetic acid and water, the amount of water is usually not less than 1 part by mass and preferably not less than 5 parts by mass and usually not more than 40 parts by mass, preferably not more than 25 parts by mass, and more preferably not more than 15 parts by weight based on 100 parts by weight of acetic acid. Better reaction efficiency is possible when the amount of water relative to the amount of acetic acid does not exceed the above upper limit; for the possibility of inhibiting the thermal decomposition of acetic acid during the oxidation reaction, it must be not lower than the above lower limit; due to this, costs can be reduced, which is preferable in terms of energy saving and economy.

The amount of solvent is usually not less than 100 parts by weight and preferably not less than 200 parts by weight and usually not more than 600 parts by weight and preferably not more than 400 parts by weight based on 100 parts by weight of the alkyl aromatic compound used in quality of the source material. When the amount of solvent is not lower than the aforementioned lower limit, the concentration of the suspension obtained in the oxidation reaction becomes satisfactory and complications such as blockage of pipelines are unlikely. In addition, if the amount of solvent does not exceed the above upper limit, miniaturization of the equipment is possible, which is preferable in the aspect of economy.

In addition, it is preferable to reuse the mother liquor returned from the solid-liquid separation step (II) described below as part of the solvents used in the oxidation step (I).

As a gas containing molecular oxygen, any gas may be used provided that the gas contains molecular oxygen; for example, air, oxygen enriched air, and oxygen diluted with an inert gas can be used. Of these gases, air is preferred due to its low cost and practicality. The rate of supply of a gas containing molecular oxygen is usually 3-100 times the number of moles of molecular oxygen per alkyl aromatic compound of the starting material.

The method for producing an aliphatic carboxylic acid according to the present invention includes an oxidation step in which the alkyl aromatic compound is oxidized in the presence of a bromine-containing compound to produce an aromatic carboxylic acid. The bromine-containing compound described herein is not particularly limited, but the latter can be mentioned, for example, heavy metal bromide used as a catalyst, a bromine-containing auxiliary catalyst or the like.

A catalyst is usually used in the oxidation step of the alkyl aromatic compound. The catalysts are not particularly limited, provided that they are capable of oxidizing the alkyl aromatic compound, converting it to aromatic carboxylic acid, but heavy metal compounds are usually used and a bromine-containing compound is used as an auxiliary catalyst. To accelerate the reaction, a combination with a co-oxidizing agent can be used.

As elements of heavy metals used in compounds of heavy metals, for example, cobalt, manganese, nickel, chromium, zirconium, copper, lead, hafnium, cerium, etc. can be mentioned. These elements can be used individually or in combination. The use of a combination of cobalt and manganese is particularly preferred. Such heavy metal compounds include, for example, acetate, nitrate, acetylacetonate, naphthenate, stearate, bromide, and the like. heavy metal, and heavy metal acetate or bromide is particularly preferred.

As bromine-containing compounds used as auxiliary catalytic agents, for example, molecular bromine, hydrogen bromide, sodium bromide, potassium bromide, cobalt bromide, manganese bromide or the like can be mentioned. and organic bromine compounds, for example, methyl bromide, methylene bromide, bromoform, benzyl bromide, bromomethyl toluene, dibromoethane, tribromoethane, tetrabromoethane or the like. These bromine-containing compounds can also be used individually or as a mixture of two or more kinds.

The concentration of the catalyst is not particularly limited, provided that it is in a range capable of accelerating the oxidation reaction, but as for the concentration of the heavy metal in the solvent, this concentration is usually 10 ppm. or more, preferably not less than 100 ppm. and more preferably 200 ppm or more and usually 10,000 ppm or less, preferably 5000 ppm or less and more preferably 3000 ppm or less. When the concentration of the heavy metal in the solvent is at or above the above lower limit, the reaction rate is preferably increased. In addition, if the concentration of heavy metal in the solvent does not exceed the above upper limit, production costs can be reduced and at the same time, the concentration of heavy metal and the concentration of bromine in liquid effluents and in the exhaust gas can be reduced, which is preferable in terms of safety and environmental protection Wednesday.

When a heavy metal compound catalyst is used in combination with a bromine-containing compound, the amount of bromine atom is usually not less than 0.05 mol, preferably not less than 0.1 mol and, more preferably not less than 0.5 mol and usually not more than 10 mol, preferably not more than 7 mol and, more preferably, not more than 5 mol per 1 mol of a heavy metal atom. When the molar ratio of the compound of the heavy metal to the bromine-containing compound is in this range, the catalytic activity advantageously increases.

When terephthalic acid is obtained using p-xylene as a starting material, preferably either a cobalt compound or a manganese compound or both, and the other compound are preferably used as catalyst, and a bromine-containing compound is preferably used as an auxiliary catalytic agent. Particularly preferred is a combination of cobalt acetate, manganese acetate and hydrogen bromide.

When a cobalt compound, a manganese compound, and a bromine-containing compound are used as a catalyst, the amounts of these compounds used are as follows. The amount of cobalt (based on metal) is usually at least 10 ppm. by weight, preferably not less than 100 ppm by weight and more preferably not less than 200 ppm by weight and usually not more than 2000 ppm by weight and preferably not more than 1000 ppm by weight, based on solvent. The amount of manganese (per metal) is usually at least 1 ppm. by weight and preferably not less than 5 ppm by weight and usually not more than 1000 ppm by weight and preferably not more than 500 ppm by weight, based on solvent. The amount of bromine (based on elemental bromine) is usually at least 10 ppm. by weight and preferably not less than 100 ppm by weight and usually not more than 3000 ppm by weight and preferably not more than 2000 ppm by weight, based on solvent.

To increase the reaction efficiency of the oxidation reaction, the pressure in the oxidation step is such that the mixture of the solvent and the starting material of the aromatic compound retains the liquid phase at the reaction temperature or at a higher pressure. As used herein, the term “retain a liquid phase” means “at least have a liquid phase”.

The pressure in the oxidation step is usually higher than normal pressure, preferably not less than 0.2 MPa and more preferably not less than 0.4 MPa and usually not more than 10 MPa, preferably not more than 7 MPa, more preferably not more than 5 MPa and even more preferably no more than 3 MPa. When the pressure in the oxidation step is in this range, the side reaction and decomposition of the compounds can be controlled, so that the reduction in yield can be usefully limited. In addition, by controlling the pressure of the oxidation step at the lowest possible level in the above range, a reactor having low pressure resistance can be used and cost can be reduced.

The reaction temperature in the oxidation step (I) (oxidation reaction temperature) is usually not less than 100 ° C, preferably not less than 140 ° C, more preferably not less than 150 ° C, and even more preferably not less than 170 ° C and usually not more than 300 ° C, preferably not more than 250 ° C, more preferably not more than 230 ° C, and even more preferably not more than 210 ° C. When the temperature of the oxidation reaction is in the above range, the reaction rate increases and the yield can be increased. When the temperature of the oxidation reaction is in the above range, the loss of solvent due to combustion can be reduced, and in addition, the side reaction and decomposition of the compounds can be controlled, so that a decrease in yield can be advantageously reduced.

When the oxidation reaction is carried out continuously, the productivity is preferably increased and the reaction time (average residence time) is preferably not less than 20 minutes, more preferably not less than 30 minutes, and even more preferably not less than 40 minutes and, preferably, no more 300 minutes, more preferably no more than 150 minutes, and even more preferably no more than 120 minutes. When the reaction time is at least the above lower limit, the reaction can be sufficiently advanced and aromatic carboxylic acid having high purity can be obtained. When the reaction time is not more than the above upper limit, it is possible to reduce the loss of solvent due to combustion, and to reduce the cost. In addition, it is preferable from the point of view of a possible reduction in reactor capacity.

The types of reactors that can be used in the oxidation step (I) are not particularly limited, and conventionally known reactors can be used. For example, any of such reactors may be used, such as a stirred reactor, a bubble tower reactor, a piston flow type reactor (continuous tubular type), or the like. To increase the efficiency of the reaction, full stirred tank reactors with a stirrer are preferred. The inlet of a gas containing molecular oxygen is usually carried out at the bottom of the reactor.

A gas containing molecular oxygen supplied from the inlet at the bottom of the reactor is used in the oxidation reaction of the alkyl aromatic compound of the starting material and then removed from the upper head of the reactor in the form of a gas containing a large amount of solvent vapor. Then the solvent is mainly separated from the exhaust gas in the refrigerator and the distillation tower, and the residue is discharged in the form of exhaust gas. It is advisable that the separated solvent be partially hydrolyzed by distillation or the like. and return to the oxidation reactor. By removing water in the solvent, the increase in the water content of the solvent formed as a by-product of the oxidation reaction is prevented.

The oxygen concentration in the exhaust gas is not specifically limited, but it is usually advisable to set this concentration to about 1.0% by volume or more, preferably 1.5% by volume or more, and more preferably 2.0% by volume or more and usually about 8.0% by volume or less, and preferably 7.0% by volume or less. The oxygen concentration in the exhaust gas, at a level not lower than the aforementioned lower limit, advantageously increases the reaction efficiency in the combustion of the exhaust gas described below (methyl bromide combustion efficiency). When the oxygen concentration in the exhaust gas does not exceed the above upper limit, safety is enhanced. In addition, if the oxygen concentration in the exhaust gas is in the range between the upper limit and the lower limit, this exhaust gas can be directly supplied to the regenerative thermal oxidation reactor described below, which is preferred.

If necessary, in the oxidation step (I) according to the present invention, after the above oxidation reaction, additional oxidation can be carried out. Additional oxidation should further oxidize the reaction mixture obtained in the above oxidation reaction by supplying a gas containing molecular oxygen without introducing additional starting material.

A preferred example of additional oxidation is to conduct additional oxidation of the reaction mixture at a lower temperature than the temperature of the first oxidation reaction (hereinafter referred to as “low temperature additional oxidation”). Preferably, the temperature of the low temperature post-oxidation is lower than the temperature of the first oxidation reaction by 1-20 ° C, more preferably lower by 5 ° C or more, and more preferably lower by 15 ° C or less. When the temperature of the low temperature post-oxidation is in the above range, the side reaction and decomposition of the compounds can be controlled. The pressure in the low temperature post-oxidation is such that the mixture in the reactor can retain the liquid phase at least at the reaction temperature or at a higher pressure, and this pressure is preferably from 0.2 to 10 MPa and, more preferably, not more than 5 MPa. If the pressure during low temperature oxidation does not exceed the above upper limit, the side reaction and decomposition of the compounds can be controlled, so that a decrease in yield can be advantageously inhibited. It is generally preferred that the pressure in the low temperature post-oxidation be lower than the pressure in the first oxidation reaction. Preferably, the low temperature additional oxidation is carried out continuously, and the reaction time is preferably from 5 to 150 minutes.

As the gas containing molecular oxygen supplied for low-temperature additional oxidation, the same gas can be used as in the first oxidation reaction. The molecular oxygen feed rate, based on the amount of the alkyl aromatic compound of the starting material, is preferably not less than 1/10000 (volume ratio, the same hereinafter), more preferably not less than 1/1000, and even more preferably not less than 1/100 and preferably not more than 1/5 and more preferably not more than 1/10, based on the molecular oxygen rate used in the first oxidation reaction.

In addition, the preferred range of oxygen concentration in the exhaust gas discharged from the low temperature post-oxidation and the corresponding justifications are the same as for the above oxidation reaction.

Another preferred example of additional oxidation is to conduct additional oxidation of the reaction mixture at a temperature higher than the temperature of the first oxidation reaction (hereinafter referred to as “high temperature additional oxidation”). In the case of high-temperature additional oxidation, part of the aromatic carboxylic acid particles in the suspension obtained by the first oxidation reaction are dissolved in a solvent. At the same time, intermediate compounds of the oxidizing agent are oxidized in these particles (contaminants that have not been oxidized to the target aromatic carboxylic acid) and the purity of the resulting aromatic carboxylic acid is increased. Preferably, the temperature of the high temperature additional oxidation is higher than the temperature of the first oxidation reaction by 1-150 ° C, more preferably 30 ° C or more, and even more preferably 50 ° C or more and more preferably 100 ° C or less, and even more preferably 80 ° C or less.

In the case where terephthalic acid is obtained with p-xylene as the starting material, the reaction temperature of the high temperature additional oxidation is specifically at least 235 ° C, more preferably at least 240 ° C and preferably not more than 290 ° C and more preferably not more than 280 ° C. When the reaction temperature of the high temperature additional oxidation is not lower than the aforementioned lower limit, the terephthalic acid particles readily dissolve and the purity should increase. On the other hand, when the reaction temperature of the high temperature additional oxidation does not exceed the aforementioned upper limit, the formation of coloring stains should be inhibited.

The pressure in the high temperature post-oxidation is such that the mixture in the reactor can retain the liquid phase at least at the reaction temperature or at a higher pressure, and this pressure is preferably not less than 0.5 MPa, more preferably not less than 1 MPa and, even more preferably, not less than 3 MPa and usually not more than 10 MPa. When the pressure during the high temperature post-oxidation does not exceed the above upper limit, the side reaction and decomposition of the compounds can be controlled, so that the reduction in yield can be reduced. It is generally preferred that the pressure in the high temperature post-oxidation is lower than the pressure in the first oxidation reaction. Preferably, the high temperature additional oxidation is carried out continuously, and the reaction time is preferably from 5 to 150 minutes.

As the gas containing molecular oxygen supplied for low-temperature additional oxidation, the same gas can be used as in the first oxidation reaction. The molecular oxygen supply rate, based on the amount of the alkyl aromatic compound of the starting material, is preferably not less than 0.003 times the number of moles, more preferably not less than 0.01 times the number of moles and preferably not more than 0.3 times the number of moles and more preferably not more than 0.1 times the number of moles, based on the amount of aromatic carboxylic acid in suspension. If the molecular oxygen supply is not lower than the aforementioned lower limit, the effect of additional oxidation tends to increase. In addition, if the molecular oxygen supply does not exceed the above upper limit, the burning of the solvent with oxygen should be prevented.

In addition, the oxygen concentration in the exhaust gas discharged from the high temperature post-oxidation is preferably low for controlling the combustion of the solvent, and a range of 0 to 0.5% by volume is desired.

Additional oxidation can be carried out two or more times. For example, low temperature additional oxidation can be carried out two or more times, or low temperature additional oxidation and high temperature additional oxidation can be performed one or more times each, or high temperature additional oxidation can be performed two or more times. According to the present invention, additional oxidation is carried out, preferably, one or more times.

A suspension containing aromatic carboxylic acid obtained in the oxidation step (I) is usually sent to the solid / liquid separation step (II) after passing through the intermediate processing step, if necessary. The intermediate processing step is not essential, and two or more intermediate processing steps are possible. As intermediate processing stages, single operations can be mentioned, for example, cooling, heating, increasing pressure, reducing pressure, concentrating, diluting, precipitating, adding, and the like; crystallization or dissolution is usually carried out. For example, when it is desired to increase the degree of extraction of the target aromatic carboxylic acid, crystallization is carried out, and if it is desired to increase purity, dissolution is carried out. These intermediate processing steps are randomly selected depending on the type of target aromatic carboxylic acid, the quality of the target product, and the like.

When crystallization is carried out as an intermediate processing step, crystals containing aromatic carboxylic acid as the main component are usually recovered by cooling the suspension in the crystallizer under reduced pressure. The crystallizer may be the only one, but it is preferable to carry out crystallization in the form of a multi-stage process, providing a sequence of several crystallizers. However, if the subsequent stage of separation of solid and liquid (II) is carried out at elevated pressure, there are cases when crystallization accompanied by a drop in pressure is preferred not to be carried out. In this regard, it should be noted that, according to the present invention, the expressions "at elevated pressure" or "state of elevated pressure" mean a pressure in excess of normal pressure.

<Stage separation of solid and liquid (II)>

The suspension obtained by the oxidation reaction is then preferably sent to the stage of separation of solid and liquid (II) and separated into a mass of aromatic carboxylic acid and mother liquor. The stage of separation of solid and liquid (II) can be carried out either at normal pressure, or at elevated pressure, or at reduced pressure, but it is preferable to carry out it at normal pressure or in a state of high pressure and, more preferably, in a state of high pressure.

When the separation of the solid and the liquid is carried out in a state of elevated pressure, it is possible to obtain a separated mass having a large internal energy, and it is easy to carry out the drying of this mass. The pressure from the suspension in the separator of solid and liquid is preferably not less than 0.2 MPa and more preferably not less than 0.3 MPa and usually not more than 5 MPa, preferably not more than 3 MPa, more preferably not more than 2 MPa, even more preferably not more than 1.5 MPa, and particularly preferably not more than 1.2 MPa. When the pressure from the slurry side in the solid and liquid separator is lower than the pressure in the above range, the pressure resistance of the solid and liquid separator can be made lower, thereby reducing the installation cost of the equipment.

The mother liquor obtained by the separation of solid and liquid contains mainly a solvent; contains dissolved aromatic carboxylic acid, unreacted alkyl aromatic compound, catalyst, by-products, water, etc. In this regard, it should be noted that the terms “contains mainly” mean “not less than 60% by weight of the total amount”. In addition, the separated mass obtained by separating the solid and the liquid also retains the mother liquor. Therefore, to prevent a decrease in the purity of aromatic carboxylic acid, it is advisable to rinse this separated mass. Detergents are not particularly limited if, under pressure and temperature conditions in the solid and liquid separator, they are in a liquid state, but the same compound with a solvent is usually used, for example, acetic acid or water is used. Since aromatic carboxylic acid is dissolved in liquid waste after washing, it is advisable to recover or reuse this aromatic carboxylic acid to prevent loss. Reuse of liquid waste after washing can be carried out either at the oxidation stage (I), or at intermediate technological stages, or at the stage of separation of solid and liquid (II).

The amount of washing liquid is preferably not less than 0.03 times the amount, more preferably not less than 0.05 times the amount, and even more preferably not less than 0.1 times the amount, and preferably not more than 5.0 a multiple of, more preferably, not more than 4.0 times the amount, and even more preferably not more than 3.0 times, based on the weight ratio of the solids content of the separated mass. In order to prevent boiling up of the washing liquid, the pressure in the solid-liquid separator at which the washing liquid is introduced is preferably higher than the vapor pressure of the washing liquid. The difference between the vapor pressure of the washing liquid and the pressure in the separator of the solid substance and the liquid during washing is preferably not less than 0.01 MPa, more preferably not less than 0.02 MPa and preferably not more than 2.0 MPa, more preferably not more than 1.0 MPa and, even more preferably, not more than 0.5 MPa.

Well-known solid and liquid separators may be used, but when a preferred apparatus is used that is capable of both separating solid and liquid and washing, these steps are simplified. Such installations include, for example, a sump with a mesh basket, a sump with a solid basket, a rotary vacuum filter, a horizontal belt filter, a rotary filter press, etc. Of these apparatuses, those that are capable of separating solid and liquid under high pressure are preferred, and in particular, a sump with a mesh basket, a separator with a solid basket and a rotary filter press.

The liquid content in the mass, divided into solid and liquid and, if necessary, washed, is usually not more than 50% by mass, preferably not more than 30% by mass, more preferably not more than 20% by mass, and particularly preferably not more than 15% by weight and usually not less than 0.5% by weight and preferably not less than 1% by weight. The separated mass is sent to the drying stage and get crystals of aromatic carboxylic acid. The resulting crystals of aromatic carboxylic acid, if necessary, are subjected to a further purification process, including a hydrogenation step, a crystallization step, a solid-liquid separation step, a drying step and the like, and an aromatic carboxylic acid having a higher purity is obtained.

<Stage of the return of the mother liquor (III)>

The production method according to the present invention preferably has a step of recovering the mother liquor (III). The mother liquor (III) recovery step is a step of reusing part of the mother liquor obtained in the solid-liquid separation step (II) by returning to the oxidation step (I). The mother liquor obtained at the stage of separation of solid substance and liquid (II) can be reused at intermediate processing stages or at the above stage of separation of solid substance and liquid (II), but these stages should not be included in the stage of return of mother liquor (III) .

From a study conducted by the inventors of the present invention, it was found that when the mother liquor (III) recovery step is carried out, the concentration of methyl bromide in the exhaust gas of the oxidation step (I) decreases to values in the range of several ppm. up to ten or more ppm The reason for this fact is not clear enough, but it is believed that the increase in catalytic activity in the reaction system due to the return of the catalytic component contained in the mother liquor to the oxidation stage (I) is likely to decrease as a result of decomposition of the bromine compound and the formation of methyl bromide. Furthermore, by carrying out the mother liquor recovery step, the overall process can increase the yield of aromatic carboxylic acid, which is preferred.

When the production method according to the present invention includes the step of recovering the mother liquor (III), the degree of return of the mother liquor [(mass of the returned mother liquor) / (total mass of the mother liquor) × 100] is preferably at least 50% by mass and, more preferably at least 70% by weight. The higher the degree of return, the lower the concentration of methyl bromide in the exhaust gas can be advantageously made. The degree of return may be 100% by mass, but preferably it is not more than 95% by mass and, more preferably, not more than 90% by mass. When the degree of return is in the above range, it is possible to slow down the accumulation of contaminants in the reaction system and to advantageously improve the quality of the aromatic carboxylic acid product.

The mother liquor obtained at the stage of separation of solid and liquid (II), if necessary, accumulate in containers for mother liquor or the like. and then separated into the returned mother liquor and the mother liquor sent to waste, and the returned mother liquor is introduced into the oxidation step (I). The degree of return can be controlled by controlling the amount of mother liquor sent to waste. The mother liquor sent to the waste is usually discarded after the recovery of useful components such as solvents, catalysts, etc.

The reusable mother liquor is preferably returned to the oxidation step (I) under high pressure. Specifically, for example, both the oxidation step (I) and the step of separating the solid and liquid (II) and the like. carried out at elevated pressure, and the resulting mother liquor is returned to the oxidation stage (I), maintaining the state of elevated pressure. If necessary, at intermediate stages, you can repeatedly increase the pressure of the mother liquor. As a result, the reused mother liquor is returned to the oxidation step (I), maintaining elevated pressure, high temperature and high catalytic activity, so that the concentration of methyl bromide in the exhaust gas can be made lower. In addition, it is possible to save the energy necessary for re-heating the mother liquor returned to the oxidation step (I), which is preferred.

Accordingly, the pressure of the reused mother liquor is the same as the pressure on the side of the slurry in the solid and liquid separator, and is preferably not less than 0.2 MPa and, more preferably, not less than 0.3 MPa. By keeping the pressure of the returned mother liquor high, the temperature of the returned mother liquor can also be kept high. On the other hand, the pressure of the returned mother liquor is usually not more than 5 MPa, preferably not more than 3 MPa, more preferably not more than 2 MPa, even more preferably not more than 1.5 MPa, and particularly preferably not more than 1.2 MPa Reducing the pressure of the returned mother liquor makes it possible to economically use the installation with a slightly lower bar resistance.

<Stage exhaust gas treatment (IV)>

Next, with reference to FIG. 1, an explanation will be given of a preferred embodiment of an exhaust gas (IV) treatment step according to the present invention.

In the oxidation step (I), the gas discharged from the oxidation reactor is normally cooled in a refrigerator attached to the oxidation reactor. Condensates, such as solvents, flow back to the oxidation reactor, and the remaining gas is discharged as off-gas of the 1st oxidation stage. The refrigerator may be replaced by a distillation tower or the like. When the production method includes the stages of low-temperature additional oxidation and high-temperature additional oxidation, the gases from the reactors of the additional oxidation are also treated in a similar manner, and they become exhaust gases 1 of the oxidation stage. These exhaust gases can be processed individually or can be processed together.

The components included in the exhaust gas are usually solvents, unreacted starting materials, oxygen, by-products, and the like. The flue gas is usually under high pressure, and its pressure is preferably not less than 0.2 MPa and also preferably not less than 0.4 MPa and usually not more than 5 MPa, preferably not more than 3 MPa and more preferably no more than 2 MPa. To remove contaminants, the exhaust gas is usually treated by burning in an incinerator and released into the atmosphere after separation and return of various useful components contained therein and energy recovery, such as pressure and heat.

When terephthalic acid is obtained with p-xylene as a starting material, the components included in the exhaust gas are aliphatic carboxylic acid, such as acetic acid, as a solvent; p-xylene as unreacted starting material; an aliphatic carboxylic acid ester such as methyl acetate, methyl bromide, carbon monoxide, carbon dioxide and the like. as by-products. The content of these components in the exhaust gas is, for example, not more than 10% by volume of aliphatic carboxylic acid or aliphatic carboxylic esters, not more than 8% by volume of oxygen, not more than 6% by volume of the sum of carbon monoxide and carbon dioxide, not more than 25 ppm by volume of methyl bromide, not more than 20 ppm by volume of p-xylene, etc.

First, the exhaust gas of oxidation stage 1 is introduced into the lower part of the high pressure absorber 11, at the same time, solvent 2 is supplied from the upper part, and aliphatic carboxylic acid and its esters are dissolved and absorbed by solvent 2 and removed from the exhaust gas. The design of the high pressure absorber 11 is not particularly limited, but a packing column, a spray column, a wall column, and the like are used, and a packing column is preferred. As solvent 2, water is usually used.

Aliphatic carboxylic acid is absorbed in the upper part of the high pressure absorber 11, and esters are dissolved and absorbed by the absorbed aliphatic carboxylic acid in the lower part. The liquid containing the recovered aliphatic carboxylic acid esters 5 is discharged from the bottom of the column, and the liquid containing the recovered aliphatic carboxylic acid 4 is discharged from the middle of the column, and the exhaust gas of the absorber 3 is discharged from the upper head.

When the liquid containing the recovered carboxylic acid esters 5 is returned to the oxidation step (I) and reused, an equilibrium amount is maintained to preferably inhibit further side formation of the esters.

When the liquid containing the recovered aliphatic carboxylic acid 4 is returned to the oxidation step (I) and reused, solvent losses in the oxidation step can preferably be inhibited. In addition, when water is contained in solvent 2, it is advisable to dehydrate solvent 2 in a dehydration tower before returning to the oxidation step (I).

The exhaust gas of the absorber 3 still maintains a high pressure state, and it is advisable to recover and reuse the energy of this pressure. As a recovery method, a method of energy recovery with various types of turbines or the like can be mentioned. after processing the exhaust gas by burning, and a method of processing the exhaust gas by burning after recovering energy with a gas choke (gas expander) or the like. Given the energy loss in the processing stage by incineration, the latter method is preferred.

Specifically, after heating the exhaust gas at about 150-160 ° C. in the heat exchanger 12, the energy is recovered with a gas choke (gas expander) 13 and the exhaust gas of the gas choke (gas expander) 6 released is processed by burning in the combustion plant 14. The energy recovered with the gas choke ( gas expander) 13, is used as a power source of the air compressor used in the oxidation stage (I), or, if necessary, the energy can be used in the generator, so that the efficient use of energy is possible.

Since harmful gases, such as carbon monoxide, methyl bromide, are contained in the exhaust gas of the gas throttle (gas expander) 6, the exhaust gas is discharged after treatment to remove contaminants. Of the components contained in this gas, it is most difficult to process methyl bromide, and usually a combustion process is used in the presence or absence of a catalyst. However, the cost of the combustion process in the presence of a catalyst is high, while combustion in the absence of a catalyst requires a large amount of energy. In addition, the lower the concentration of methyl bromide in the exhaust gas, the more difficult the complete combustion process is. In addition, methyl bromide, when subjected to a combustion process, becomes bromine and hydrogen bromide, but, depending on the combustion conditions, it further reacts with carbon monoxide and aromatic compounds remaining after combustion, and the formation of bromine-containing dioxins is possible.

In addition, it is believed that methyl bromide in the exhaust gas is related to the fact that heavy metal bromide as a catalyst and a bromine-containing compound as an auxiliary catalytic agent caused a side reaction with a solvent and the like. at the stage of oxidation (I).

Due to these circumstances, the exhaust gas is processed by burning in a combustion apparatus according to the present invention, and the combustion process is carried out under specific conditions.

The concentration of methyl bromide in the exhaust gas supplied to the combustion unit 14 is not limited, but for the exhaust gas at the oxidation stage in the method for producing aromatic carboxylic acid, this concentration is usually not more than 100 ppm. by volume, preferably not more than 50 ppm by volume and, more preferably, not more than 29 ppm by volume and usually not less than 1 ppm by volume, preferably at least 2 ppm by volume and more preferably not less than 3 ppm by volume. When the concentration of methyl bromide in the exhaust gas is lower than the above lower limit, the combustion treatment becomes more difficult, but the need for the treatment required to remove contaminants is reduced.

In order to reduce the concentration of methyl bromide in the exhaust gas, it is desirable to increase the catalytic activity in the oxidation step (I). Means for increasing the catalytic activity in the oxidation stage (I) are not limited, and specifically, a means for increasing the concentration of cobalt in the reaction solution, a means for decreasing the concentration of bromine in the reaction solution so as not to deactivate the catalyst, and a means for increasing the degree of reuse stock solution at the stage of return of the stock solution (III).

Of these agents, it is preferable to increase the degree of reuse of the mother liquor in the mother liquor (III) recovery step to 50% by mass or more, and preferably 70% by mass or more. When the degree of reuse of the mother liquor is in the above range, it becomes possible to reduce the concentration of methyl bromide in the exhaust gas to 29 ppm. by volume or less and also up to 25 ppm by volume or less. In addition, by optimizing the concentration of cobalt in the reaction solution, the concentration of bromine in the reaction solution, and the degree of reuse of the mother liquor, it is possible to increase the methyl bromide concentration to 20 ppm. by volume or less and also up to 15 ppm by volume or less.

The oxygen concentration in the exhaust gas supplied to the combustion unit 14 is not limited and is usually not less than 1.0% by volume, preferably not less than 1.5% by volume and more preferably not less than 2.0% by volume and usually not more than 8.0% by volume and preferably not more than 7.0% by volume. When the concentration of oxygen in the exhaust gas is at or above the above lower limit, the combustion of methyl bromide becomes easy. But when it does not exceed the above upper limit, the exhaust gas at the oxidation stage can be used directly, which is very beneficial. In particular, when the regenerative thermal oxidation apparatus described below is used as the incinerator, good combustion can be carried out even with such off-gas that has a low oxygen concentration.

Exhaust gas can be used directly, but by conducting the necessary dilution and concentration, the methyl bromide concentration and oxygen concentration can be controlled.

According to the present invention, the formation of bromine-containing dioxins is inhibited by specific combustion conditions and cooling conditions of the exhaust gas.

The temperature of the off-gas combustion in the combustion plant 14 is usually not less than 450 ° C, preferably not less than 550 ° C and more preferably not less than 600 ° C and usually not more than 1000 ° C, preferably not more than 900 ° C and more preferably not more than 850 ° C. When the temperature of the flue gas combustion is at or above the lower limit above, the complete combustion of methyl bromide can be easily carried out and the formation of bromine-containing dioxins can also be inhibited. When the temperature of the combustion of the exhaust gas does not exceed the above upper limit, energy costs can be reduced.

The gas after combustion is cooled to 250 ° C or lower, preferably to 200 ° C or lower, more preferably to 170 ° C or lower, and even more preferably to 150 ° C or lower. Accordingly, the temperature of the exhaust gas of the incinerator 7 is not more than 250 ° C, preferably not more than 200 ° C, more preferably not more than 170 ° C, and even more preferably not more than 150 ° C. When the temperature of the gas after combustion is in the above range, it is possible to slow the formation of bromine-containing dioxins in the gas after combustion. The lower limit of the cooling temperature of the gas after combustion (the temperature of the exhaust gas of the combustion unit 7 is also the same) is not limited, but preferably it is at least 70 ° C and, more preferably, at least 80 ° C, thereby reducing the cooling time and save energy.

According to the present invention, in the process of cooling the gas after burning, the cooling time from 450 ° C to 250 ° C does not exceed 1 second and the cooling time from 450 ° C to 200 ° C does not exceed 1 second (according to the present invention, the time required for cooling the gas after burning, sometimes called simply “cooling time”). In addition, it is preferable that the cooling time from the combustion temperature to 250 ° C. does not exceed 1 second and the cooling time from the combustion temperature to 200 ° C. does not exceed 1 second. As much as possible, reducing the residence time in the temperature range of about 300 ° C during gas cooling after combustion, it is possible to effectively inhibit the formation of bromine-containing dioxins.

Methods for rapidly cooling gas after combustion are not limited, and for example, a method of bringing gas after combustion into contact with a cooling agent, a method of bringing gas after combustion into contact with a heat absorber or heat storage material, a method of mixing this gas with an inert gas having a low temperature, a method of irrigation of gas after burning in water in a scrubber, or the like, a method of introducing gas after burning in water, and the like.

According to the present invention, the concentration of bromine-containing dioxins in the gas after cooling can be reduced to such an extremely low level as 1 ng-TEK / m ³ or less, preferably 0.1 ng-TEK / m ³ or less, more preferably 0, 01 ng-TEK / m ³ or less, and even more preferably 0.007 ng-TEK / m ³ or less. Bromine-containing dioxins include isomers having a dioxin ring and isomers having a dibenzofuran ring. Examples include polybrominated dibenzo-para-dioxins (PBDDs), polybrominated dibenzofurans (PBDFs), or the like. In addition, according to the present invention, it is possible to control the concentration of bromine-containing dioxins in the exhaust gas with a low concentration of methyl bromide in the above manner, the complete combustion of which is traditionally considered difficult and in which the appearance of bromine-containing dioxins is likely.

According to the present invention, as a method for analyzing and evaluating bromine-containing dioxins, after determining the amount of each isomer of bromine-containing dioxins according to the Preliminary Research Method for Polybromodibenzo-para-dioxin and Polybromodibenzofuran of the Ministry of Environmental Protection (2002), bromine-containing dioxins are calculated as the value of the toxic equivalent (Toxic Equivalent, hereinafter referred to as FEC), based on the concept described in Reference Data 3 in the “Report e on the results of a study of current conditions for emissions of bromine-containing dioxins ”(2002). That is, when the toxicity of 2,3,7,8-TCDD (tetrachlorodibenzo-p-dioxin) is taken as 1, the toxicity values of other dioxins are calculated strictly in terms of toxic equivalent factor (Toxic Equivalent Factor, hereinafter referred to as “TEF”) and find the total, which is the fuel and energy complex. In addition, you can also refer to "Environmental Health Criteria No. 205, polybrominated dibenzo-para-dioxins and dibenzofurans" published by the Center for Environmental Information Science (October 2000).

The types of combustion plants for use in accordance with the present invention are not limited, and examples include a direct combustion plant in which a combustion catalyst is not used, a catalytic combustion plant in which a combustion catalyst is used, a regenerative thermal oxidation reactor in which heat storage material is used, and so on. .p., and any of these incinerators can be used. A regenerative thermal oxidizing reactor and a direct combustion unit suitable for high temperature combustion are preferred.

As combustion plants usable in accordance with the present invention, plants having a combustion chamber and a refrigerator in the combustion plant are preferred. Since combustion plants having such a structure can cool the gas immediately after combustion, the formation of bromine-containing dioxins can be prevented. Also from this point of view, a regenerative thermal oxidizing reactor or direct combustion plant is preferred.

Furthermore, as installations for burning according to the present invention, installations having heat storage material in a combustion installation are preferred. By using combustion plants having such a structure, the formation of bromine-containing dioxins can be prevented, since such plants have excellent combustion efficiency and good stability at the combustion temperature, and quick cooling can be easily performed with them.

In this regard, it should be pointed out that the above combustion catalyst is not particularly limited, but specifically, manganese oxide, copper oxide, chromium oxide, a palladium-based catalyst, platinum-based, and the like, another halogen-resistant catalyst, etc.

A direct combustion unit is usually equipped with a flue gas preheater, a furnace (combustion chamber) in which flue gas can be burned in the absence of a combustion catalyst, and a refrigerator in which heat from the flue gas is recovered after combustion. The exhaust gas preheated in the heat exchanger of the preheating chamber is mixed either with fuel or with air, or both of them in the combustion chamber, heated for a certain period of time at a temperature higher than the ignition temperature, and then oxidized and decomposed and released after cooling in the fridge. It is advisable to use the heat recovered in the refrigerator for preheating the exhaust gas.

The regenerative thermal oxidizing reactor includes a pre-heating chamber equipped with heat-accumulating material filled with ceramic filler, almost eliminating heat loss, a combustion chamber for carrying out the process of burning off exhaust gas pre-heated by heat-accumulating material, and a heat recovery chamber (refrigerator) for recovering gas heat after burning; The regenerative thermal oxidation reactor has a structure capable of maintaining the temperature in the combustion chamber even when the burner is off. In addition, the regenerative thermal oxidation reactor has a structure in which the preheating chamber and the heat recovery chamber can be alternately reversibly switched by varying the exhaust gas channel. For example, the installation disclosed in JP-A-2002-303415 may be used.

In addition, a model suitable for applications in which combustion and cooling is possible can be selected from commercially available regenerative thermal oxidation reactors such as Regenerative Thermal Oxidizer (Dürr Company, Germany), Rotary Regenerative Thermal Oxidizer (Chugai Ro. Co., Ltd .), Twin-tower Regenerative Thermal Oxidizer (Chugai Ro. Co., Ltd.) and Single-tower, Multi-room Regenerative Thermal Oxidizer (Chugai Ro. Co., Ltd.).

In a regenerative thermal oxidation reactor, the off-gas is usually heated to the designated temperature by heat storage material in the preheating chamber and then burned in the combustion chamber. The combustion temperature can be controlled by the amount of fuel and the amount of combustible substances in the exhaust gas. After combustion, the gas is sent to the heat recovery chamber, the heat of this gas is absorbed, it is cooled by heat storage material for a short period of time and the gas is released into the external space of the combustion plant.

In a regenerative thermal oxidizing reactor, the cooling rate of the gas after combustion can be controlled by optimizing the amount of flowing off-gas and the amount and external shape of the packaged heat storage material or the like. So, the gas cooling time from 450 ° C to 250 ° C or lower in the heat recovery chamber can be brought up to 1 second or less, and in addition, the cooling time from 450 ° C to 200 ° C or lower can be brought up to 1 second or less . In addition, the cooling time from the combustion temperature to 250 ° or lower can be brought to 1 second or less, in addition, the cooling time from the combustion temperature to 200 ° C or lower can be brought to 1 second or less. It is preferable to use the heat recovered in the refrigerator for preheating the exhaust gas. As described above, the supply of exhaust gas and the discharge of exhaust gas after combustion can be switched by valves of the switchgear alternately between the chambers. In recent years, regenerative thermal oxidation reactors are mainly rotary valve systems, but they are not particularly limited, provided that they are consistent with the essence of the present invention.

Regenerative thermal oxidation reactors are advantageous in that the combustion process can be carried out without oxygen from the outside even when the oxygen concentration in the exhaust gas is not more than 8.0% by volume and not more than 4.0% by volume or the concentration of methyl bromide in the exhaust gas is not more than 25 ppm by volume or less and not more than 15 ppm in volume and especially not more than 10 ppm by volume. In addition, in a regenerative thermal oxidizing reactor, a high degree of heat recovery can be obtained, the cost of combustion can be significantly reduced, and energy can be saved.

When burning using regenerative thermal oxidizing reactors, the above-mentioned combustion catalysts or the like can be used to accelerate combustion, but given the high combustion temperature and lower emissions and costs, it is preferable to burn off-gas in the absence of combustion catalysts.

Compared to direct incineration plants, regenerative thermal oxidation reactors have a high stability of the combustion temperature, and the combustion temperature itself is controlled to a low temperature, and rapid cooling can be easily carried out so that the formation of NOX (nitric oxide) can be advantageously inhibited. Regenerative thermal oxidation reactors have high technological efficiency, and the number of such plants can be reduced, as a result of which equipment costs can be reduced.

Components such as bromine and hydrogen bromide usually remain in the exhaust gas of the combustion unit 7, so it is advisable to absorb these residues through gas-liquid contact with alkalis and reducing agents in the absorber 15 and then release the gas into the atmosphere. As alkalis, sodium hydroxide, potassium hydroxide, and the like are preferably used. and sodium sulfite, urea and the like are preferably used as reducing agents, but are not limited to these substances. A scrubber or the like can be used as an absorber, along with a tower with a nozzle, a tower with spray irrigation and a column with irrigated walls. The exhaust gas of the absorber 8 is discharged after receiving confirmation that this gas is in a state that allows release into the atmosphere.

When terephthalic acid is obtained with p-xylene as a starting material, this terephthalic acid is industrially produced in two ways — the PTA method and the QTA method, taking into account differences in the required quality and the like, and the present invention is effectively applicable to both methods. Examples of applying the present invention to both processes are described below.

An explanation of the PTA method is given first with reference to FIG. 2. Unless otherwise specified, the process conditions are the same as in the explanation above.

In the first oxidation step (I), p-xylene A is oxidized with a molecular oxygen containing gas B in an oxidation reactor 21 in a solvent in the presence of a catalyst, to obtain a suspension C containing terephthalic acid and a solvent. The reaction temperature is the same as above, but in the PTA method, a temperature of from 170 to 200 ° C is most preferred.

The exhaust gas M of the oxidation reactor from the oxidation reactor 21, after condensation and separation in the refrigerator 26 of the condensed liquid N containing mainly the solvent, is discharged from the oxidation reactor 21 as the exhaust gas P of the oxidation reactor and sent to the exhaust gas (IV) processing step. The refrigerator 26 can be single-stage or multi-stage, and a similar separation is possible even when a distillation column is used instead of the refrigerator 26. The water content in the reaction solution is controlled by dropping part of the condensed liquid N obtained by condensing the exhaust gas M of the oxidation reactor from the reaction system. The oxygen concentration in the exhaust gas P is usually from 1.0 to 8.0% by volume, but in the PTA method, this concentration should be slightly lower. For example, this concentration is not more than 4% by volume.

If necessary, after the oxidation reaction in the reactor 21, it is possible to carry out low-temperature additional oxidation. The off-gas emitted by the low temperature post-oxidation can also be sent to the off-gas (IV) treatment step. In the PTA method, the oxygen concentration in the exhaust gas produced by the low temperature post-oxidation should also be slightly lower, similar to the main oxidation reaction.

The resulting suspension C is separated into a mass of terephthalic acid D and the mother liquor E in a separator 22 for separating a solid and a liquid in the separation of the solid and the liquid (II), if necessary, through an intermediate processing stage. If necessary, the mass of terephthalic acid D is washed with washing liquid K in the washing unit 23 and a mass of terephthalic acid (after washing) F and liquid waste of the washing liquid L are obtained. As shown by the dashed line in part of FIG. 2, a solid separation unit 22 can be used and liquids and a flushing unit 23 in one unit 24 capable of separating a solid and a liquid and conducting flushing.

The resulting mass F is dried in dryer 25 and crude crystals of terephthalic acid G are obtained. After this, in hydrogenation refining processes (dissolution stage, hydrogenation step, crystallization step, solid-liquid separation step, washing step, drying step or the like), not shown, high purity terephthalic acid crystals are obtained from crude crystals of terephthalic acid G.

On the other hand, a part of the mother liquor E obtained in the solid-liquid separation step (II) is preferably sent to the oxidation step (I) through the mother liquor (III) recovery step. That is, the mother liquor E, after accumulation in the mother liquor vessel or the like, is separated, if necessary, into the reusable mother liquor H and the waste waste of the mother liquor J, and the reused mother liquor H is returned to the oxidation reactor 21 to the oxidation step (I). If necessary, the liquid waste of the washing liquid L can also be returned to the oxidation reactor 21 to the oxidation step (I).

The off-gas obtained in the above steps is sent to the off-gas (IV) treatment step and processed.

The liquid containing the recovered aliphatic carboxylic acid 4 and the liquid containing the recovered aliphatic carboxylic acid esters 5 obtained in the exhaust gas (IV) treatment step can be returned to the oxidation reactor 21, if necessary after treatment in a dehydration column or the like ., as reusable aliphatic carboxylic acids and esters of aliphatic carboxylic acids Q.

The following is an explanation of the QTA method with reference to FIG. 3. The process conditions are the same as in the explanation above unless otherwise indicated.

In the oxidation step (I), p-xylene is oxidized with molecular oxygen-containing gas b in an oxidation reactor 31 in a solvent in the presence of a catalyst, to obtain a suspension of c containing terephthalic acid and a solvent. The reaction temperature and pressure are the same as above, but in the QTA method, the most preferred conditions are 180-230 ° C and 1-3 MPa. In the oxidation step, it is more preferable to oxidize at least 98% p-xylene by weight.

The exhaust gas of the oxidation reactor n from the oxidation reactor 31, after condensation and separation in the refrigerator 41 of the condensed liquid o, containing mainly the solvent, is discharged as exhaust gas p of the oxidation reactor and sent to the exhaust gas (IV) processing step. The refrigerator 41 can be single-stage or multi-stage, and a similar separation is possible even when a distillation column is used instead of the refrigerator 41. The water content in the reaction solution is controlled by dropping part of the condensed liquid o obtained by condensing the exhaust gas n from the reaction system. The oxygen concentration in the exhaust gas p is usually from 1.0 to 8.0% by volume, but in the QTA method this concentration should be slightly higher. For example, this concentration is at least 4% by volume.

In the QTA method, high temperature additional oxidation is carried out after the oxidation reaction in the oxidation reactor 31 in the oxidation step (I). At this time, it is preferable to carry out low-temperature additional oxidation between the oxidation reaction and high-temperature additional oxidation. The reason for this is that terephthalic acid having a high purity can be obtained by conducting preliminary low-temperature additional oxidation to high-temperature additional oxidation.

The suspension c obtained in the oxidation reactor 31 is sent to the low-temperature additional oxidation reactor 32 and the low-temperature additional oxidation is carried out, additionally supplying gas b containing molecular oxygen. Preferably, the low-temperature additional oxidation is carried out continuously and the reaction time is slightly shorter than the main oxidation reaction, preferably from 5 to 90 minutes.

The off-gas emitted by the low temperature post-oxidation can also be sent to the off-gas (IV) treatment step. That is, the exhaust gas q of the oxidation reactor from the oxidation reactor 32, after condensation and separation in the refrigerator 42 of the condensed liquid r containing mainly the solvent, is discharged as the exhaust gas s of the oxidation reactor and sent to the exhaust gas (IV) processing step. The refrigerator 42 may be single-stage or multi-stage, and a similar separation is possible even when a distillation column is used instead of the refrigerator 42. In the QTA method, the oxygen concentration in the exhaust gas produced by the low temperature post-oxidation should also be slightly higher, similar to the main oxidation reaction.

In the resulting low temperature additional oxidation slurry d, the pressure is usually increased by means of a pump 33 and then it is heated to the designated temperature by means of a heater 34, transferred to the high temperature additional oxidation reactor 35 and further oxidized to supply gas b containing molecular oxygen. It is desirable that this high-temperature additional oxidation is carried out continuously and the reaction temperature is preferably from 5 to 120 minutes.

The off-gas emitted by the high temperature post-oxidation can also be sent to the off-gas (IV) treatment step. That is, the exhaust gas t of the oxidation reactor from the oxidation reactor 35, after condensation and separation in the refrigerator 43 of the condensed liquid u containing mainly the solvent, is discharged as the exhaust gas v of the oxidation reactor and sent to the exhaust gas (IV) processing step. The refrigerator 43 may be single-stage or multi-stage, and a similar separation is possible even when a distillation column is used instead of the refrigerator 43. The oxygen concentration in the off-gas emitted by the high temperature post-oxidation is low, but this off-gas can be processed in the off-gas (IV) treatment step by combining with the off-gas p and s having a high oxygen concentration.

The resulting suspension of high-temperature additional oxidation is sent to the stage of separation of solid and liquid (II) after it is, if necessary, subjected to intermediate processing steps, and usually it is subjected to pressure reduction and cooling to the prescribed pressure and temperature in an instantaneous evaporator 36 and then divided into a mass of terephthalic acid f and the mother liquor g in the solid-liquid separator 37. Suspension e can also be subjected to separation of the solid and liquid and without reducing pressure and cooling, but to inhibit the decomposition of a certain amount of solvent, it is nevertheless desirable to lower the temperature to some extent, reducing the pressure and conducting cooling. In addition, when the mother liquor (III) recovery step described below is carried out, it is preferable that the temperature of the mother liquor to be returned is not much higher than the pressure and temperature of the oxidation reactor 31, so it is advisable to reduce the pressure and conduct cooling. The preferred pressure after depressurization is close to the preferred pressure of the solid-liquid (II) separation step, as described above. In addition, the gas w of the instant evaporator from the instant evaporator 36 is discharged as exhaust gas from the instant evaporator after condensation and separation in the refrigerator 44 of the condensed liquid x containing mainly solvent. If necessary, the off-gas evaporator off-gas can also be processed at the off-gas (IV) treatment stage.

If necessary, the resulting mass of terephthalic acid f is washed with washing liquid l in the washing unit 38 and a mass of terephthalic acid is obtained after washing h and the liquid waste of the washing liquid m. As shown by the dashed line in part of FIG. 3, a solid-liquid separation unit 37 and a washing unit 38 can be used in one unit 39 capable of separating a solid and a liquid and conducting washing.

If necessary, the resulting mass h is further dried in the dryer 40 and obtained in the form of terephthalic acid crystals i.

On the other hand, a part of the mother liquor g obtained in the solid-liquid (II) separation step is preferably sent to the oxidation step (I) through the mother liquor (III) recovery step. That is, the mother liquor g, after accumulation in the mother liquor vessel or the like, is separated, if necessary, into the reused mother liquor j and the waste waste of the mother liquor k and the reused mother liquor j are returned to the oxidation reactor 31 to the oxidation step ( I). If necessary, the liquid waste m washing liquid can also be returned to the oxidation reactor 31 to the oxidation stage (I).

The off-gas obtained in the above steps is sent to the off-gas (IV) treatment step and processed.

The liquid containing the recovered aliphatic carboxylic acid 4 and the liquid containing the recovered aliphatic carboxylic acid esters 5 obtained in the exhaust gas (IV) treatment step can be returned to the oxidation reactor 31, if necessary after treatment in a dehydration column or the like ., as reusable aliphatic carboxylic acids and esters of aliphatic carboxylic acids z.

EXAMPLE

The present invention will now be described specifically with reference to examples, but they do not limit the present invention in any way.

Measuring method

(1) Methyl bromide concentration

The methyl bromide concentration was measured by gas chromatographic analysis under the following conditions. A gas chromatograph GC-14B and chromatopack C-R3A (both manufactured by Shimadzu Corporation) were used. A glass column having a diameter of 3 mm and a length of 5 m was used, and DC-550 silicone (manufactured by SHINWA CHEMICAL INDUSTRIES, LTD.) Was used as a filler. Nitrogen was used as the carrier gas, and the flow rate was 30 ml / min.

The column temperature during the analysis was 45 ° C, and the temperature of the detector was 100 ° C.

(2) The concentration of bromine-containing dioxins

After determining the amount of each isomer of bromine-containing dioxins according to the Preliminary Method for the Study of Polybromodibenzo-p-dioxin and Polybromodibenzofuran of the Ministry of the Environment (2002), the concentrations of bromine-containing dioxins were calculated as the value of the toxic equivalent (TEC), based on the concept described in the Reference Data 3 in the “Report on the results of a study of current conditions for emissions of bromine-containing dioxins”, (2002).

Bromine-containing dioxins included isomers having a dioxin ring and isomers having a dibenzofuran ring.

EXAMPLES AND COMPARATIVE EXAMPLES

In the following Examples and Comparative Examples, reactors in the oxidation reaction, including low-temperature and high-temperature additional oxidation, were used full stirred tank reactors with a stirrer. The pressure value was absolute pressure.

EXAMPLE 1

1 part by weight of p-xylene, 3.35 parts by weight of a solution containing a catalyst was added to the reactor (cobalt acetate, manganese acetate and hydrogen bromide were dissolved in acetic acid containing 14% by weight of water), and 6.24 parts by weight reused mother liquor in a state that substantially retained such pressure and temperature as they were during its separation from the solid-liquid separation unit described below, air was introduced, creating a 20-fold excess of molecular oxygen with respect to p-xylene, and an oxidation reaction was carried out at a temperature of 195 ° C., a pressure of 1.34 MPa (absolute pressure) and a reaction time (average residence time) of 60 minutes. The concentration of cobalt / manganese / bromine in the reaction solution was 300/300/1000 ppm. by weight, respectively. The oxygen concentration in the exhaust gas was set at 5.0-7.0% by volume.

The exhaust gas from the reactor was finally cooled to a temperature of 45 ° C and a pressure of 1.16 MPa in a multi-stage refrigerator, the condensates obtained from each refrigerator were combined, some of them were recovered, and the residue was returned to the reactor. The recoverable amount of condensate was set so that the concentration of water in the mother liquor of the suspension was 10% by weight.

A suspension containing mainly crude terephthalic acid and a solvent was recovered from the oxidation reaction reactor. In 5.84 parts by weight of the suspension, the crude terephthalic acid was 2.05 parts by weight, the mother liquor was 3.79 parts by weight, the concentration of the suspension was 35% by weight.

This suspension (5.84 parts by weight) was continuously introduced into the low-temperature additional oxidation reactor and low-temperature additional oxidation was carried out, setting the temperature to 183 ° C, a pressure of 1.04 MPa and a reaction time of 30 minutes. The amount of air introduced was 1 / 44.6 of the amount introduced in the preceding oxidation reaction. The oxygen concentration in the exhaust gas was regulated by air in the range from 5.0 to 7.0% by volume.

The exhaust gas from the low-temperature additional oxidation reactor was finally cooled to a temperature of 45 ° C and a pressure of 1.16 MPa in a multi-stage refrigerator, the condensates obtained from each refrigerator were combined and returned to the low-temperature additional oxidation reactor.

Then, the suspension recovered from the low-temperature additional oxidation reactor was heated and its pressure was increased; the suspension was continuously introduced into the high-temperature additional oxidation reactor and high-temperature additional oxidation was carried out by setting the temperature to 260 ° C, a pressure of 5.5 MPa and a reaction time of 50 minutes. Air was introduced so that the molar ratio of molecular oxygen to terephthalic acid was 0.12. The oxygen concentration in the exhaust gas was set in the range from 0 to 0.5% by volume.

The exhaust gas from the high-temperature additional oxidation reactor was finally cooled to a temperature of 45 ° С and a pressure of 1.16 MPa in a multi-stage refrigerator, the condensates obtained from each refrigerator were combined and returned to the high-temperature additional oxidation reactor.

The suspension extracted from the high-temperature additional oxidation reactor was decompressed and cooled in an instant-action evaporator to reach the temperature and pressure range of the first reactor, then it was sent to a sump with a mesh basket and solid and liquid were separated, maintaining temperature and pressure, and the terephthalic mass was separated acids and mother liquor. This mass was washed with acetic acid in the mesh of the sump and was discharged from the exhaust valve; the pressure was continuously reduced to atmospheric pressure to evaporate the trapped liquid, then drying was carried out and 1.56 parts by weight of terephthalic acid crystals were obtained.

On the other hand, the mother liquor obtained by separating the solid and the liquid was separated by 35% by weight of the mother liquor sent to the waste waste and 65% by weight of the mother liquor for reuse, maintaining temperature and pressure, and the reused mother liquor was transferred into the first reactor. After the recovery of useful components, such as terephthalic acid, acetic acid and a catalyst, the waste waste of the mother liquor that was drained was sent for reuse, and the residue was purified from harmful impurities and released into the outer space.

The exhaust gases from the reactor, the low-temperature additional oxidation reactor and the high-temperature additional oxidation reactor were combined through the above refrigerator and processed as exhaust gas having a temperature of 45 ° C. and a pressure of 1.16 MPa, in the exhaust gas treatment stage shown in FIG. 1.

That is, by supplying the exhaust gas 1 to the high pressure absorber 11 from the bottom of this absorber, the water used as solvent 2 was sprayed from the top of the same absorber. The pulverized water absorbed acetic acid or the like. in the exhaust gas. In addition, at the bottom of the absorber, water in which acetic acid was absorbed dissolved and absorbed methyl acetate or the like. in the exhaust gas. The absorbed methyl acetate was recovered as a liquid containing the recovered aliphatic carboxylic acid esters 5 from the bottom of the absorber, returned to the reactor and reused. Acetic acid or the like extracted from the middle stage of the absorber in the form of a liquid containing the recovered aliphatic carboxylic acid 4, was dehydrated in a dehydration column and after dehydration was also returned to the reactor and reused.

On the other hand, gas 3, which is the remainder of the exhaust gas, exited the upper head of the absorber and was heated at 155 ° C in the heat exchanger 12. After that, gas 3 was sent to the gas choke (gas expander) 13 and used as part of the air compressor’s power source the form of exhaust gas 6, having a pressure of 0.1 MPa. The oxygen concentration in the exhaust gas 6 was 6.4% by volume and the methyl bromide concentration was 13.7 ppm. by volume.

The exhaust gas 6 was introduced into the regenerative thermal oxidizing reactor used as the combustion plant 14, and combustion was carried out at a combustion temperature (temperature in the combustion chamber) of 820 ° C, a cooling time in the cooling chamber not exceeding 1 second, and exhaust temperature (temperature exhaust gas) 80-90 ° C (in this document, the cooling time means the cooling time of the gas after combustion to 200 ° C or lower, the same goes on). Namely, the cooling time required to cool the gas at 820 ° C after burning in a regenerative thermal oxidation reactor to 200 ° C or lower does not exceed 1 second. In this regard, it should be noted that after raising the temperature to the combustion temperature, air and fuel were not provided.

When measuring the concentration of methyl bromide in the exhaust gas of the incinerator, its value was at the detection limit or lower, i.e. was less than 0.1 ppm by volume. The measured concentration of bromine-containing dioxins in the exhaust gas of the combustion plant was at or below the detection limit, i.e. was less than 0.007 ng-TEK / m ³ . According to this, it is believed that complete combustion of methyl bromide, which was almost completely converted to bromine, was achieved. The results are shown in Table 1.

Then, the components of the exhaust gas of the combustion plant, such as bromine and hydrogen bromide, were absorbed by gas-liquid contact with alkali and a reducing agent in the absorber 15 and the gas, after receiving confirmation of its harmless state, was released into the atmosphere.

EXAMPLE 2

Exhaust gas 6 was obtained from the gas throttle (gas expander) 13 in the same manner as in Example 1. The oxygen concentration in the exhaust gas 6 was 6.6% by volume, and the concentration of methyl bromide was 13.9 ppm. by volume (oxygen concentration and methyl bromide concentration were slightly different from the concentrations of Example 1 due to operational variations). The exhaust gas 6 was introduced into the same regenerative thermal oxidation reactor as in Example 1, and was subjected to a combustion process at a combustion temperature of 600 ° C, a cooling time of not more than 1 second, and an exhaust temperature of 80-90 ° C. The time required to cool a gas with a temperature of 600 ° C after burning in a regenerative thermal oxidizing reactor to 200 ° C or lower did not exceed 1 second. In this regard, it should be noted that after raising the temperature to the combustion temperature, air and fuel were not provided.

As a result, the concentration of methyl bromide in the exhaust gas of the incinerator was 1.1 ppm. by volume. The concentration of bromine-containing dioxins in the exhaust gas of the combustion plant was at or below the detection limit, i.e. was less than 0.007 ng-TEK / m ³ . According to this, it is believed that complete combustion of methyl bromide, which was almost completely converted to bromine, was achieved. The results are shown in Table 1.

EXAMPLE 3

The exhaust gas 6 obtained from the gas throttle (gas expander) 13 in the same manner as in Example 1 was diluted with nitrogen gas, resulting in an oxygen concentration of 2.0% by volume and a methyl bromide concentration of 4.1 ppm. by volume (the ratio of oxygen concentration to methyl bromide concentration was slightly different from this ratio in Example 1 due to operational variations). The exhaust gas 6 was introduced into the same regenerative thermal oxidation reactor as in Example 1, and was subjected to a combustion process at a combustion temperature of 820 ° C, a cooling time of not more than 1 second, and an exhaust temperature of 80-90 ° C. The time required for cooling a gas with a temperature of 820 ° C after burning in a regenerative thermal oxidation reactor to 200 ° C or lower did not exceed 1 second. In this regard, it should be noted that after raising the temperature to the combustion temperature, air and fuel were not provided.

As a result, the concentration of methyl bromide in the exhaust gas of the incinerator was at or below the detection limit, i.e. was less than 0.1 ppm by volume. The concentration of bromine-containing dioxins in the exhaust gas of the combustion plant was at or below the detection limit, i.e. was less than 0.007 ng-TEK / m ³ . According to this, it is believed that complete combustion of methyl bromide, which was almost completely converted to bromine, was achieved. The results are shown in Table 1.

EXAMPLE 4

Exhaust gas 6 was obtained from a gas throttle (gas expander) 13 in the same manner as in Example 1. The oxygen concentration and the concentration of methyl bromide in the exhaust gas 6 were 6.4% by volume and 13.7 ppm. by volume, respectively. The exhaust gas 6 was introduced into the direct combustion apparatus used as the combustion apparatus 14, and combustion was carried out at a combustion temperature of 1000 ° C, a cooling time of not more than 1 second, and an exhaust temperature of 180 ° C. That is, the cooling time required for cooling the gas at a temperature of 1000 ° C. after combustion in a direct combustion installation to 200 ° C. or lower did not exceed 1 second. In this regard, it should be pointed out that in order to maintain the combustion temperature at 1000 ° C, air and fuel (heavy diesel fuel) were continued to be provided.

As a result, the concentration of methyl bromide in the exhaust gas of the incinerator was at or below the detection limit, i.e. was less than 0.1 ppm by volume. The measured concentration of bromine-containing dioxins in the exhaust gas of the combustion plant was at or below the detection limit, i.e. was less than 0.007 ng-TEK / m ³ . According to this, it is believed that complete combustion of methyl bromide, which was almost completely converted to bromine, was achieved. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

Exhaust gas 6 was obtained from the gas throttle (gas expander) 13 in the same manner as in Example 1. The oxygen concentration in the exhaust gas 6 was from 5.0 to 7.0% by volume, and the concentration of methyl bromide was 13.2 ppm . by volume. The exhaust gas 6 was introduced into the catalytic combustion unit used as the combustion unit 14, and the combustion process was carried out using a palladium-based oxidation catalyst at a combustion temperature of 352 ° C. and an outlet temperature of 352 ° C.

As a result, the measured concentration of methyl bromide in the exhaust gas of the incinerator was 0.8 ppm. by volume. The measured concentration of bromine-containing dioxins in the exhaust gas of the incinerator was 120 ng-TEK / m ³ . The results are shown in Table 1.

After that, the exhaust gas of the catalytic combustion unit was absorbed by gas-liquid contact with alkali and a reducing agent in the absorber 15.

COMPARATIVE EXAMPLE 2

Exhaust gas 6 was obtained from the gas throttle (gas expander) 13 in the same manner as in Example 1. The oxygen concentration in the exhaust gas 6 was from 5.0 to 7.0% by volume, and the concentration of methyl bromide was 13.0 ppm . by volume. The exhaust gas 6 was introduced into the catalytic combustion unit used as the combustion unit 14, as in Comparative Example 1, except that the combustion temperature was 401 ° C and the exhaust temperature was 401 ° C.

As a result, the measured concentration of methyl bromide in the exhaust gas of the combustion plant was at or below the detection limit, i.e. was less than 0.1 ppm by volume. The concentration of bromine-containing dioxins in the exhaust gas of the incinerator was 31 ng-TEK / m ³ . The results are shown in Table 1.

Industrial Applicability

According to the present invention, the exhaust gas can be treated by burning, inhibiting the formation of bromine-containing dioxins, so that this method is applicable in the production environment as a method for the production of aromatic carboxylic acid, which reduces the adverse environmental load.

In addition, according to the present invention, the methyl bromide in the exhaust gas can be efficiently treated by burning, so that this method is applicable in a production environment as a method for the production of aromatic carboxylic acid, in which operating costs, installation costs and energy consumption can be reduced.

In addition, according to the present invention, the exhaust gas can be treated by burning, inhibiting the formation of not only bromine-containing dioxins, but also aromatic compounds such as benzenes and NOX (nitric oxide), so this method is applicable in the production environment as a method for producing aromatic carboxylic acid, which reduces the environmental burden.

The present invention is based on Japanese Patent Application No. 2007-278939, filed October 26, 2007, and Japanese Patent Application No. 2007-278940, filed October 26, 2007, descriptions, claims, drawings and abstracts of which are included in this document by reference.

Claims (11)

1. A method for producing an aromatic carboxylic acid, comprising: an oxidative stage of oxidation of an alkyl aromatic compound in the presence of a bromine compound to produce an aromatic carboxylic acid and a stage for burning off-gas generated in the oxidation stage in an incinerator,
where after combustion of the exhaust gas at a combustion temperature of from 450 to 1000 ° C, the gas after combustion is cooled to 250 ° C or lower and the cooling time from 450 to 250 ° C during cooling does not exceed 1 s. 2. The method of producing aromatic carboxylic acid according to claim 1, where the cooling time from the combustion temperature to 250 ° C does not exceed 1 s. 3. The method of obtaining aromatic carbon. acid according to claim 1 or 2, where the gas after combustion is cooled by heat-accumulating material. 4. The method of producing aromatic carboxylic acid according to claim 1 or 2, where the combustion is carried out in the absence of a combustion catalyst. 5. The method of producing aromatic carboxylic acid according to claim 1 or 2, where the installation for combustion is a regenerative thermal oxidative installation. 6. The method of producing aromatic carboxylic acid according to claim 1 or 2, where the concentration of bromine-containing dioxins in the gas after cooling is not more than 1 ng-TEK / m ³ in units of toxic equivalent. 7. The method for producing an aromatic carboxylic acid according to claim 1 or 2, wherein the oxidation step is carried out in a solvent based on an aliphatic carboxylic acid, and the method includes the step of separating a solid and a liquid, in which a suspension containing aromatic carboxylic acid obtained in the oxidation stages, and the solvent, are separated into a solid and a liquid, obtaining a mass of aromatic carboxylic acid and a mother liquor, and the stage of returning the mother liquor, in which at least part of the mother the solution is reused in the oxidation step. 8. The method of producing aromatic carboxylic acid according to claim 7, where at the oxidation stage, at least 50% by weight of the mother liquor is reused. 9. A process for producing an aromatic carboxylic acid according to claim 1 or 2, wherein the concentration of methyl bromide in the effluent gas does not exceed 100 million by volume ^-1. 10. The method of producing aromatic carboxylic acid according to claim 1 or 2, where the alkyl aromatic compound is p-xylene. 11. The method of producing aromatic carboxylic acid according to claim 1 or 2, where the aromatic carboxylic acid is terephthalic acid.

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