JPH10291958A - Production process of 2,6-naphthalene dicarboxylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous production process excellent in production efficiency, having sufficient crystal isolating performance even raising recycling rate of the reaction base solution in the production of 2,6- naphthalenedicarboxylic acid from 2,6-dialkylnaphthalene as a raw material by liquid phase oxidation. SOLUTION: In the continuous production process of 2,6-naphthalene dicarboxylic acid in which 2,6-dialkylnaphthalene is oxidized using molecular oxygen gas in solvent containing lower aliphatic carboxylic acid in the presence of a catalyst consisting of heavy metal compound and bromine compound, the reaction base solution separated from the resulting oxidation reaction product slurry by a solid-liquid separation process, is circulated into the oxidation process after heat treatment at >=150 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing 2,6-naphthalenedicarboxylic acid, and more particularly, to a process for producing 2,6-naphthalenedicarboxylic acid by liquid phase oxidation of 2,6-dialkylnaphthalene. The present invention relates to a method for industrially advantageously producing 2,6-naphthalenedicarboxylic acid by recycling a mother liquor separated from 6-naphthalenedicarboxylic acid to an oxidation reaction step again.

[0002]

2. Description of the Related Art 2,6-Naphthalenedicarboxylic acid and its esters are useful substances as raw materials for high-performance polyesters. Conventionally, a method of oxidizing 2,6-dialkylnaphthalenes in a solvent containing a lower aliphatic carboxylic acid using a catalyst containing cobalt, manganese and bromine to obtain 2,6-naphthalenedicarboxylic acid is disclosed in No. 2,666 and Japanese Patent Publication No. 56-3337. Generally, in the production of aromatic carboxylic acids by the liquid-phase oxidation of aromatic hydrocarbons, relatively expensive catalysts such as cobalt, manganese, and bromine are used. is required.

[0003] Among the many aromatic acids, in the method for producing terephthalic acid, which has maintained the longest history and the huge production amount, the recycling of metal catalysts mainly composed of manganese and cobalt has been carried out for several decades. Has a commercially successful track record over the years. The circulating reuse of the catalyst in the terephthalic acid production equipment is based on the fact that the reaction mother liquor obtained by separating from the terephthalic acid generated by the oxidation reaction of para-xylene is reused as a solvent for the oxidation reaction through a process for re-preparing catalyst components and the like. (Japanese Patent Publication No. 46-14339) and a method of recovering and reusing a metal catalyst from the reaction mother liquor by various physical or chemical methods (Japanese Patent Publication No. 41-1).
No. 8577).

[0004] Comparing these two methods, it is natural that the method of circulating and using the reaction mother liquor as it is as the solvent for the oxidation reaction is simple, and is excellent in terms of capital investment and operation cost. . However, when the reaction mother liquor is directly recycled to the oxidation reactor as a solvent for the oxidation reaction, among the various impurities generated by the oxidation reaction, substances that are harmful to the oxidation reaction, that is, reaction inhibitory substances, accumulate in the solvent and increase. This may hinder the rapid progress of the oxidation reaction. In addition, impurities that deteriorate terephthalic acid quality and impurities that can hinder the smooth operation of manufacturing equipment accumulate,
There is a possibility that the number will increase, and thus the method must be naturally restricted. Due to these factors,
In many cases, the terephthalic acid producing apparatus employs a method combining the above methods.

[0005]

In a production apparatus for 2,6-naphthalenedicarboxylic acid, which has recently been produced on a commercial scale, the technical skeleton is the same as that of terephthalic acid as an aromatic carboxylic acid. The technology has been followed and improved. The same applies to the recycle of metal catalysts and bromine catalysts.The 2,6-dialkylnaphthalene is oxidized in a solvent containing a lower aliphatic carboxylic acid, and the resulting 2,6-naphthalenedicarboxylic acid crystals are separated. A method of reusing the recycled mother liquor as a solvent for the oxidation reaction is being pursued.

However, when the mother liquor is circulated and used as an oxidation reaction solvent in a 2,6-naphthalenedicarboxylic acid production apparatus, unlike the terephthalic acid production apparatus, the 2,6-naphthalenedicarboxylic acid produced in the oxidation reaction step is different from the terephthalic acid production apparatus. Since the acid crystal particles are very small and the solubility of 2,6-naphthalenedicarboxylic acid in the reaction solvent is very small, unlike the case of terephthalic acid production equipment, the crystal particles can be passed through several crystallization tanks. The diameter is not easily increased, and it is very difficult to exhibit complete separation performance in the solid-liquid separation step. As a result, part of the 2,6-naphthalenedicarboxylic acid crystals, which are finer than the average crystal particles generated in the oxidation reaction step, are contained in the mother liquor. When the reaction mother liquor containing the fine crystals is repeatedly circulated and used as the solvent for the oxidation reaction in the oxidation reactor, the ratio of the fine crystals contained in the slurry supplied to the solid-liquid separation step is increased. Cannot exert its solid-liquid separation ability. The purpose of the present invention is to
2,6-Dialkylnaphthalene as raw material by liquid phase oxidation
When producing naphthalenedicarboxylic acid, even if the recycling rate of the reaction mother liquor is increased, sufficient crystal separation ability is obtained,
An object of the present invention is to provide a continuous production method with excellent production efficiency.

[0007]

The present inventors have conducted intensive studies on the above-mentioned problems in the production of 2,6-naphthalenedicarboxylic acid. As a result, the mother liquor recycled to the oxidation reactor was heated at a required temperature in advance. By,
The present inventors have found that a sufficient solid-liquid separation ability can be maintained for a long period of time even when the mother liquor recycling rate is increased. That is, the present invention oxidizes 2,6-dialkylnaphthalene in a solvent containing a lower aliphatic carboxylic acid in the presence of a catalyst consisting of a heavy metal compound and a bromine compound, using molecular oxygen gas,
When continuously producing 6-naphthalenedicarboxylic acid, after heating the mother liquor separated from the oxidation reaction product slurry through the solid-liquid separation step at a temperature of 150 ° C. or higher,
This is a method for producing 2,6-naphthalenedicarboxylic acid, which is circulated to an oxidation reaction step.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram illustrating a method for producing 2,6-naphthalenedicarboxylic acid according to the present invention. In FIG. 1, the raw material 2,6-dialkylnaphthalene is introduced into an oxidation reaction step 2 from a flow path 1 and is subjected to molecular oxygen in a solvent containing a lower aliphatic carboxylic acid in the presence of a catalyst comprising a heavy metal compound and a bromine compound. An oxidation reaction is performed using the gas. The oxidation reaction product slurry from the oxidation reaction step 2 is crystallized in the crystallization step 3.
And then sent to the solid-liquid separation step 4. In the solid-liquid separation step, crystals are separated, and a crude 2,6-naphthalenedicarboxylic acid 6 is obtained through a drying step 5. The mother liquor separated in the solid-liquid separation step is introduced into a mother liquor tank 7. Part of mother liquor is recovered
8 to collect the catalyst components and solvent. The remainder of the mother liquor is mixed in the mixing tank 9 by adding a catalyst component from the flow path 11, and then circulated to the oxidation reaction step 2 through the heat treatment tank 10. In FIG. 1, the mother liquor recycling rate is defined by the following equation. Mother liquor recycling rate = A / (A + B) x 100 (%) where A is the amount of mother liquor sent from the mother liquor tank 7 to the preparation tank 9 (k
g / h) B is the amount (kg / h) of mother liquor sent from the mother liquor tank 7 to the recovery step 8.
h)

The 2,6-dialkylnaphthalene used as an oxidizing material in the present invention includes 2,6-dimethylnaphthalene, 2,6-diethylnaphthalene, 2,6-diisopropylnaphthalene and the like. As the lower aliphatic carboxylic acid used as a solvent in the oxidation reaction step, formic acid, acetic acid, propionic acid, butyric acid and the like or a mixture thereof can be mentioned, and acetic acid is most preferable. The solvent may contain water, but the content is preferably 30% by weight or less. The amount of the solvent used is 1 to 20 times by weight, preferably 2 to 10 times the weight of the oxidation raw material 2,6-dialkylnaphthalene.
Weight times.

A catalyst comprising a heavy metal compound such as a cobalt compound or a manganese compound and a bromine compound is used as a catalyst in the oxidation reaction step. If necessary, heavy metal compounds such as iron, cerium and nickel may be added. Examples of the cobalt, manganese and other heavy metal compounds include organic acid salts, hydroxides, halides, carbonates and the like, and particularly preferred are acetates and bromides. The bromine compound may be any compound that dissolves in the reaction system and generates bromine ions, such as hydrogen bromide, inorganic bromide such as sodium bromide and cobalt bromide, and bromoacetic acid and tetrabromoethane. Organic bromides are exemplified, and hydrogen bromide, cobalt bromide and manganese bromide are particularly preferred.

The amount of the catalyst used is such that the total amount of cobalt, manganese and other heavy metal components is from 0.03 to 0.1 in terms of atomic ratio to 2,6-dialkylnaphthalene as the oxidizing raw material.
3, preferably 0.04 to 0.2. In addition, bromine is added so that the atomic ratio to the oxidation raw material of 2,6-dialkylnaphthalene is 0.015 to 0.15, preferably 0.02 to 0.1. When the amount of the catalyst used is smaller than the above range, the yield of 2,6-naphthalenedicarboxylic acid in the reaction decreases. On the other hand, when the amount of the catalyst used is larger than the above range, the amount of the catalyst accompanying the generated 2,6-naphthalenedicarboxylic acid crystals increases, and the catalyst cost increases, which is industrially disadvantageous. .

The atomic ratio of manganese to cobalt of the heavy metal component is 4 to 15, preferably 6 to 10, and the atomic ratio of cobalt to 2,6-dialkylnaphthalene is 0.006 to 0.04. Increasing the atomic ratio of manganese to cobalt increases the catalytic activity.However, if the atomic ratio of manganese to cobalt is too high, trimellitic acid and manganese by-product form a complex salt to form 2,6-naphthalenedicarboxylic acid. Since the complex salt adheres to the crystal, the purity of the crystal decreases.

The reaction temperature in the oxidation reaction step is from 170 to
It is in the range of 250 ° C, preferably 180-240 ° C.
If the reaction temperature is too low, a large amount of reaction intermediates such as 6-formyl-2-naphthoic acid remain in the product, and the obtained crystals of 2,6-naphthalenedicarboxylic acid become small,
The efficiency of solid-liquid separation may be affected. If the reaction temperature is too high, it is not preferable because by-products such as naphthalenetricarboxylic acid increase and the combustion loss of the solvent increases. The pressure of the oxidation reactor may be any pressure at which the reaction system can maintain a liquid phase at the above-mentioned temperature.
40 kg / cm ² G, preferably 10 to 30 kg / cm
Is a ² G.

The gas containing molecular oxygen used in the oxidation reaction includes oxygen gas or a gas obtained by mixing oxygen with an inert gas such as nitrogen or argon, and air is the most common. In the present invention, the reaction is carried out in a continuous production apparatus. A batch or semi-continuous reaction system has low efficiency, and a continuous production apparatus is suitable for efficiently producing 2,6-naphthalenedicarboxylic acid on an industrial scale. The reaction may be completed by a single oxidation reaction,
In order to increase the reaction yield, the reaction is preferably performed by connecting a plurality of reactors in series. Although a stirring tank or a bubble column is used for the oxidation reactor, a stirring tank is preferably used in order to sufficiently stir the inside of the reactor.

A gas containing molecular oxygen is continuously supplied to the oxidation reactor, and the gas after the reaction is continuously withdrawn outside the reactor as an off-gas. A reflux condenser is provided in the reactor,
A large amount of solvent entrained in the off-gas and water generated by the oxidation reaction are condensed. The condensed solvent and water are usually refluxed to the reactor, but a part of the solvent and water may be drawn out of the reaction system in order to adjust the water concentration in the reactor.
The molecular oxygen-containing gas content is adjusted so that the oxygen concentration in the off-gas discharged from the oxidation reactor is in the range of 0.5 to 5% based on the dry gas after condensing water or the solvent. . When the oxygen concentration in the off-gas is lower than the above range, undesired phenomena such as an increase in the amount of the reaction intermediate and an inferior color of the obtained 2,6-naphthalenedicarboxylic acid occur. On the other hand, if the oxygen concentration is higher than the above range, the cost required for the gas compressor or the like is unnecessarily increased.

The effluent containing the 2,6-naphthalenedicarboxylic acid crystals produced in the oxidation reactor is sent to the next oxidation reactor connected in series, where the final oxidation reaction is carried out again with the molecular oxygen-containing gas. Is preferred. The oxidation reaction product slurry is preferably depressurized and cooled through one or more crystallization tanks connected in series in the next crystallization step, and sent to the solid-liquid separation step. In the solid-liquid separation step, a slurry containing 2,6-naphthalenedicarboxylic acid generated by the oxidation reaction is separated into crystals and a mother liquor by a solid-liquid separator. Solid-liquid separation is usually performed under atmospheric pressure. There is no particular restriction on the separation temperature, but it is usually lower than the boiling point of the solvent at atmospheric pressure, about 50 ° C.
It is performed in the range of ~ 110 ° C. Examples of the type of solid-liquid separator include a centrifugal sedimentator, a centrifugal filter, and a vacuum filter.However, since the crystal grain size of 2,6-naphthalenedicarboxylic acid is extremely small, usually a crystal having a small grain size is used. However, a decanter-type centrifuge capable of collecting efficiently is adopted.

The present invention relates to a method for handling a mother liquor containing fine crystals leaking in the solid-liquid separation step. The crystal particle size distribution in the slurry supplied to the solid-liquid separation step is determined by the conditions of the oxidation reaction step and the presence or absence of a crystallizer,
It cannot be said unconditionally because it is determined by various factors in the operation of the solid-liquid separator. An example of the case where the mother liquor is not recycled to the oxidation reactor is as follows. Measured by a measuring device).

[0018]

Table 1 1 μm or less 1.4% 1 to 5 μm 14.4% 5 to 10 μm 11.7% 10 to 20 μm 21.3% 20 to 30 μm 20.1% 30 to 50 μm 22.4% 50 μm or more 8.7%

In a decanter type centrifuge, a few μm is generally used.
Although it is said that the above particles can be collected, it can be seen from the above table that the slurry supplied to the solid-liquid separator contains a lot of particles smaller than this level. Therefore, in reality, it is inevitable that some fine crystals leak into the mother liquor that has exited the solid-liquid separator. The amount of leaked fine crystals varies depending on the basic performance and operating conditions of the solid-liquid separator, the amount of slurry supplied to the solid-liquid separator, etc., in addition to the crystal particle size distribution in the slurry, An example in which the mother liquor is not recycled to the oxidation reactor is about 1% by weight. A part of the mother liquor from the solid-liquid separation step is sent to a recovery step for recovering the solvent and the catalyst,
The remaining part is sent to a preparation tank for preparing a solvent for an oxidation reaction for recycling. In the blending tank, the insufficient solvent and catalyst are added and re-prepared, and then supplied again to the oxidation reactor as a solvent for the oxidation reaction. In addition, there is a method in which the raw material 2,6-dialkylnaphthalene is supplied to the preparation tank and mixed with the solvent in advance, and then supplied to the oxidation reactor. However, undesired deterioration in the process of being sent to the preparation tank or the oxidation reactor is also available. May be a concern,
More preferably, the 6-dialkylnaphthalene alone is directly supplied to the oxidation reactor.

According to the method of the present invention, the remaining portion of the mother liquor sent to the recovery step is supplied to the oxidation reactor after being subjected to a heat treatment. The heat treatment may be performed on the mother liquor supplied to the preparation tank upstream of the preparation tank, that is, on the solvent for the oxidation reaction supplied to the oxidation reactor downstream of the preparation tank, that is, after preparation in the preparation tank. Is more preferred. In the heat treatment step, a method of staying at a predetermined temperature in a heat treatment tank is generally used, but the heat treatment step may be held at a predetermined temperature in a liquid feed pipe. In any method, the heat treatment temperature must be 150 ° C. or higher, preferably 1 ° C.
60-240 ° C. The heat treatment time is desirably 3 minutes or more, and a 100% mother liquor recycling rate can be achieved even with about 5 minutes as is apparent from the following examples. If the mother liquor to be recycled is not subjected to heat treatment, or if supplied to an oxidation reactor while the heat treatment temperature is too low, as will be apparent from a comparative example described later, the mother liquor leaks into the separated mother liquor as the continuous operation time elapses. Since the amount of fine crystals gradually increases, which seriously hinders long-term production, the mother liquor recycling rate was at most about 30% in the conventional 2,6-naphthalenedicarboxylic acid production apparatus. However, according to the present invention, the mother liquor recycling rate can be 40% or more.

JP-A-6-293697 states that when fine crystals leaking into the mother liquor are supplied as they are to the oxidation reactor, the resulting crystals of 2,6-naphthalenedicarboxylic acid become larger. . However, according to the results of continuous experiments by the present inventors, such a phenomenon was not observed at all. This difference is probably due to
No. 293697 is considered to be derived from the fact that it is based on the experimental results of a batch type experimental apparatus. At least in the continuous experimental apparatus, if the mother liquor containing the fine crystals is supplied to the oxidation reactor as it is, a serious obstacle occurs in the solid-liquid separation step, and further in the oxidation reaction step, in the smooth continuation of the operation as the operation time elapses. It will be obvious.

[0022]

Next, the present invention will be described in detail with reference to examples. However, the present invention is not limited by the following examples.

Production Example A 60-liter internal volume in which an oxidation reactor is provided with a stirrer, a reflux condenser, a raw material 2,6-dimethylnaphthalene supply pipe, a solvent supply pipe, an air supply pipe, a reaction product extraction pipe, and the like. 2,6-naphthalenedicarboxylic acid was continuously produced using an autoclave with an external heater made of titanium.
The reaction liquid level is always controlled and almost 5
The operation was performed so as to be 0%. The 2,6-dimethylnaphthalene raw material was heated to about 130 ° C. in order to keep it in a liquid state, and supplied to the oxidation reactor using a piston type pump. The solvent was prepared by adding a catalyst component in advance in a preparation tank, and supplied to the oxidation reactor using a piston type pump. The mixing tank was heated and controlled by an external heating device. The raw material 2,6-dimethylnaphthalene supply amount, solvent supply amount, and solvent component were as follows.

[0024]

[Table 2] 2,6-Dimethylnaphthalene supply amount 100 parts by weight Solvent supply amount 500 parts by weight Solvent component Water 5% Manganese 8000 ppm Cobalt 1200 ppm Br 5000 ppm Residual acetic acid

Here, manganese is manganese acetate tetrahydrate,
Cobalt was added as cobalt acetate tetrahydrate, and bromine was added as hydrobromic acid. The supply rate of the compressed air was adjusted so that the oxygen concentration in the vent gas of the oxidation reactor was in the range of 1.8 to 2.2%. The temperature of the oxidation reaction was controlled by an external heater so that the temperature of the thermometer inserted in the lower part of the middle of the oxidation reactor became 210 ° C. The reaction pressure was adjusted with a downstream pressure regulating valve such that a pressure gauge installed downstream of the reflux condenser had a pressure of 20 atm.
Part of the condensate condensed in the reflux condenser was discharged out of the system, and the remaining part was refluxed to the oxidation reactor.

First, only a solvent containing a predetermined catalyst component and the like is charged into a reactor, the temperature is raised to a predetermined temperature while stirring, and then 2,6-dimethylnaphthalene and air are supplied as raw materials. The reaction product (slurry) was continuously withdrawn from the reaction product withdrawal tube so that the liquid level reached a predetermined value. The extracted reaction product was received in a storage tank equipped with a stirrer and heated to 80 ° C. The storage tank was released to atmospheric pressure. The slurry extracted from the storage tank was supplied to a decanter-type centrifugal separator, where the slurry was separated into wet crystals partially containing mother liquor and mother liquor. The mother liquor was once stored in a mother liquor tank heated to 80 ° C., and samples in the following Examples and Comparative Examples were collected.

A part of the mother liquor extracted from the mother liquor tank was sent to the preparation tank, and the remaining part was discarded. This mother liquor recycling rate is defined as follows. Mother liquor recycling rate = A / (A + B) x 100 (%) where A is the amount of mother liquor sent to the mother liquor tank (kg / h) B is the amount of mother liquor discarded (kg / h) The catalyst was added, mixed with the solvent components shown in Table 2, and sent to the heat treatment tank. The heat treatment tank was made of titanium with a stirrer, and was heated and maintained at a predetermined temperature by an external heating device. The treatment time in the heat treatment tank was adjusted by controlling the liquid level. The mother liquor that exited the heat treatment tank was completely circulated to the oxidation reactor.

Examples 1-3, Comparative Examples 1-2 In the above production examples, 2,6-naphthalenedicarboxylic acid was produced by changing the mother liquor recycling rate, the temperature of the heat treatment tank and the average residence time, and Each time, the amount of crystals contained in the mother liquor collected from the mother liquor tank was measured by a filtration method. Table 3 shows the measurement results of the amount of crystals contained in the mother liquor.

[0029]

[Table 3] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Mother liquor recycling rate (%) 100 100 100 100 100 50 Heat treatment tank Temperature (° C) 200 220 160 160 80 130 Residence time ( min) 20 20 20 5 20 20 Elapsed time (hr) Crystal content in mother liquor (%) 24 1.6 1.9 2.4 1.7 2.1 1.7 48 1.7 2.3 2.6 2.3 5.4 3.5 72 2.0 1.4 1.8 2.5 8.2 5.2 96 1.8 1.9 1.8 1.6 13.0 8.1 120 2.1 2.2 2.0 1.9 18.9 12.9

The following can be understood from the above measurement results of the crystal content. 1) When the heat treatment is performed at 130 ° C. (Comparative Example 2)
Although the mother liquor recycling rate is as low as 50%, the crystal content in the mother liquor still shows a tendency to increase at the end of the experiment, whereas at a processing temperature of 160 ° C. or higher (Examples 1 to 4) It showed a substantially constant value. Therefore, if the heat treatment is performed at 160 ° C. or more, long-time continuous operation can be performed without any trouble even if the mother liquor recycling rate is 100%. 2) Lower the liquid level in the heat treatment tank to reduce the average residence time of the solvent to 5.
Even in the case where the setting was made to be minutes (Example 4), the crystal content in the mother liquor showed a substantially constant value, regardless of the operation elapsed time. Therefore, even if the mother liquor recycling rate is 100%, a long-time continuous operation can be performed without any trouble if the heat treatment time is 5 minutes or more.

[0031]

The mother liquor separated from the oxidation reaction product slurry through the solid-liquid separation step by the method of the present invention is heated and circulated to the oxidation reaction step, thereby increasing the recycling rate of the reaction mother liquor. Since sufficient crystal separation capacity can be obtained, the load on the recovery step is significantly reduced,
Solvent and catalyst losses can be reduced. Therefore, the method of the present invention provides a continuous production method of 2,6-naphthalenedicarboxylic acid with extremely excellent production efficiency, and the present invention has great industrial significance.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a method for producing 2,6-naphthalenedicarboxylic acid of the present invention.

[Explanation of symbols]

 2: Oxidation reaction step 3: Crystallization step 4: Solid-liquid separation step 5: Drying step 7: Mother liquor tank 8: Recovery step 9: Mixing tank 10: Heat treatment tank

Claims (2)

[Claims] 1. A method of oxidizing 2,6-dialkylnaphthalene using a molecular oxygen gas in a solvent containing a lower aliphatic carboxylic acid in the presence of a catalyst comprising a heavy metal compound and a bromine compound.
In the continuous production of 2,6-naphthalenedicarboxylic acid, after heating the mother liquor separated from the oxidation reaction product slurry through the solid-liquid separation step at a temperature of 150 ° C. or higher,
A method for producing 2,6-naphthalenedicarboxylic acid, wherein the method is circulated to an oxidation reaction step. 2. The method for producing 2,6-naphthalenedicarboxylic acid according to claim 1, wherein the heat treatment is performed for 3 minutes or more, and at least 40% of the mother liquor separated in the solid-liquid separation step is circulated.