Field of the Invention
The present invention relates to a method of reusing a water medium used when terephthalic acid is produced.
Background Art
Purified terephthalic acid is produced in the following manner: crude terephthalic acid is heated under pressure to dissolve it in water and an oxide intermediate 4-carboxybenzaldehyde which is a major impurity in the crude terephthalic acid is reduced to paratoluic acid which is easily removed by crystallization by bringing the crude terephthalic acid into contact with a catalyst, for example, palladium in the presence of hydrogen, followed by solid-liquid separation. At this time, the water used as the solvent contains a lot of impurities and is dumped as wastewater, requiring large equipment for treating wastewater.
A method of removing impurities from this wastewater is shown in, for example, Japanese Patent No. 2899927. This is a method in which the water after the crystallized terephthalic acid is collected by solid-liquid separation in the above process is extracted with paraxylene. In this method, however, impurities cannot be removed to the extent that the water has the purity required to reuse the water
for purifying crude terephthalic acid.
Also, a means of flashing water under reduced pressure to thereby drop the temperature may be adopted to crystallize terephthalic acid. Because high-boiling point impurities like those contained in the wastewater from solid-liquid separation are not present in the vaporized water generated in this process, it is considered that the condensed water is not returned to a crystallization vessel but is withdrawn and used again as water for purifying crude terephthalic acid directly. However, a large amount of paratoluic acid is contained in the vaporized water and the concentration of paratoluic acid in a final product is increased in this method. Therefore, it is necessary to reduce the concentration of 4-carboxybenzaldehyde in the crude terephthalic acid in advance to control the concentration of paratoluic acid in a final purified terephthalic acid, giving rise to the problem that the production cost of the crude terephthalic acid is increased.
SUMMARY OF THE INVENTION
In the conventional method of extracting impurities with paraxylene from wastewater after solid-liquid separation, a lot of impurities are contained in the wastewater after the extraction with paraxylene and the wastewater cannot be reused as water for purifying crude terephthalic acid. Also, the method in which the vaporized
water generated in a process of crystallizing crude terephthalic acid is condensed and then directly recycled has the problem that the concentration of 4-carboxybenzaldehyde in crude terephthalic acid must be reduced to maintain the purity of purified terephthalic acid and the production cost of the crude terephthalic acid is increased.
Moreover, there is the problem that not only paratoluic acid which can be made into final product terephthalic acid by oxidation is thrown away together with wastewater but also a large scale wastewater treatment system must be installed.
The present invention provides a method which makes it possible to reuse wastewater discharged from a process for purifying crude terephthalic acid as the water, for example, used to purify crude terephthalic acid, and which can raise the yield of terephthalic acid.
The present invention relates to a process of producing terephthalic acid in which the crude terephthalic acid obtained by oxidizing paraxylene is heated under pressure to dissolve it in water, terephthalic acid is crystallized by vaporizing water under reduced pressure from the solution, the condensed water obtained by condensing the vaporized water produced at the vaporization step is brought into contact with paraxylene, and then the condensed water thus separated is reused at any of the steps of the terephthalic acid manufacturing process.
The present invention relates to the process in which the condensed water is reused as the water for dissolving crude terephthalic acid, to the process in which the crude terephthalic acid is dissolved by heating at 225°C or higher under 5MPa-G or higher and then the pressure is reduced in one stage or in a multistage to vaporize 30 to 70% of the water used to dissolve crude terephthalic acid. The present invention also relates to the process in which the condensation temperature is 70°C or higher in the process of condensing the vaporized water.
The condensed water obtained by condensing the vaporized water is preferably brought into contact with paraxylene at 40°C to 200°C.
Also, the present invention provides the process in which the condensed water separated after contacted with paraxylene is bubbled using a gaseous material. As the gaseous material, nitrogen or steam is preferable.
Also, the present invention provides the process in which paraxylene obtained after contacted with the condensed water and separated is fed to an oxidation process and used as a raw material for producing crude terephthalic acid and the process in which paraxylene contained in the bubbled gaseous material is supplied to an oxidation step and used as the raw material for producing crude terephthalic acid.
Paraxylene which is the raw material of terephthalic acid is used as an extracting agent in the present
invention, whereby wastewater which is discharged from a process of purifying crude terephthalic acid and currently dumped as wastewater, which needs a large scale wastewater treatment system, can be reused as the water for purifying the crude terephthalic acid without using a process of reproducing an extracting agent which process usually needs large equipment and energy.
Further, paratoluic acid contained in the wastewater and currently dumped can be recovered as terephthalic acid which is a target product.
Accordingly the present invention relates to a process for producing terephthalic acid in which crude terephthalic acid obtained by oxidizing paraxylene is dissolved in water by heating at 225°C or higher under a pressure of 5Mpa-G or higher and terephthalic acid is crystallized by vaporizing 30 to 70% of the water used to dissolve said crude terephthalic acid under reduced pressure, wherein
a) the water vaporized in the crystallizing process is condensed and is
brought into contact with paraxylene and separated and
b) said condensed separated water is reused for dissolving said crude
terephthalic acid
c) and paraxylene obtained after it is brought into contact with the
condensed water and separated is fed to an oxidation process and used as a
raw material for producing crude terephthalic acid.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS
Fig. 1 is an explanatory view illustrating a practical process for purifying wastewater according to the present invention.
PREFERRED EMBODIMENT OF THE INVENTION Paraxylene is oxidized to crude terephthalic acid using air by using a cobalt compound, manganese compound and bromine compound as a catalyst in the presence of an acetic acid solvent under the reaction pressure of 0.4 to 5 MPa-G and reaction temperature of 160 to 260°C. Major impurities in this crude terephthalic acid are oxide intermediates of 500 to 5000 ppm by weight of 4-carboxybenzaldehyde and 100 to 1000 ppm by weight of paratoluic acid. Other impurities are benzoic acid, hydroxylmethylbenzoic acid, isophthalic acid and the like, high-boiling point impurities produced from these impurities by combining into various forms and low-boiling impurities produced resulting from the decomposition of these impurities.
The crude terephthalic acid obtained in this manner is heated to 225°C or higher and preferably 250 to 310°C under a pressure of 5 MPa-G or higher and. preferably 7 to 9 MPa-G to dissolve it in water. At this time, the concentration of terephthalic acid in water is adjusted to 10 to 40% by weight and preferably 20 to 35% by weight. When the concentration of terephthalic acid is low, this is undesirable because the amount of water is increased and the equipment which will be explained later is large-sized. Also, when the concentration of terephthalic acid is high, this is undesirable because in the case of vaporizing 30 to 70% by weight of water as will be described later, the concentration of impurities in the non-vaporized mother liquor is increased and impurities in purified terephthalic acid is thereby increased.
In succession, the above terephthalic acid is brought into contact with a catalyst, for example, palladium in the presence of hydrogen to reduce the oxide intermediate 4-carboxybenzaldehyde, which is a major impurity in crude terephthalic acid, to paratoluic acid which is easily removed by crystallization. Then, water is vaporized under reduced pressure in one stage or a multistage and preferably 3 stages to 7 stages.
In the present invention, it is desirable that the amount of vaporization be larger since the vaporized water is purified and reused. However, if the amount of vaporization is large, this is undesirable because the concentration of impurities in the non-vaporized mother liquor is increased and impurities in purified terephthalic acid is thereby increased. Therefore, the reduction in pressure is controlled such that 30 to 70% by weight and preferably 35 to 60% by weight of water is vaporized. The temperature of the system is dropped to 100 to 175°C by the heat of vaporization to precipitate terephthalic acid crystals. Although 5 to 50% by weight of paratoluic acid is vaporized and carried to the vaporized water side, almost all other impurities are left in the mother liquor. Also, low-boiling point impurities present in a minute amount are considered to be vaporized and carried to the vaporized water side. However, these impurities can be removed by condensing the vaporized water at a temperature of 70°C or higher and preferably 80 to 100°C. In order to remove low-boiling point impurities, the condensation temperature is preferably higher. If the extraction and bubbling operation which will be described later are carried out under atmospheric pressure, inexpensive apparatuses can be used to carry out these operations and the upper limit of the temperature is therefore 100°C.
Since the high-boiling point impurities and the low-boiling point impurities are removed in this manner, the
major impurity in the condensed water is paratoluic acid and the concentration of paratoluic acid is 100 to 5000 ppm by weight. This condensed water is brought into contact with paraxylene having a concentration of 3 to 200 parts by weight, preferably 5 to 100 parts by weight and more preferably 10 to 35 parts by weight based on 100 parts by weight of the condensed water to remove paratoluic acid from the condensed water. It is more desirable that the amount of paraxylene be larger because the removal rate of paratoluic acid is increased. However, because paraxylene is used as the raw material of crude terephthalic acid after extracted, the upper limit of paraxylene is determined inevitably from the total balance.
The temperature at which the condensed water is brought into contact with paraxylene is usually 40 to 200°C, preferably 60 to 150°C and more preferably 80 to 100°C.

The removal rate of paratoluic acid can be raised by carrying out the extraction in a counter current multistage operation and the extraction is preferably carried out in a multistage (2 to 10 stages and preferably 2 to 4 stages) counter-current operation. In the present invention, it is unnecessary to remove paratoluic acid completely and for example, a removal rate of 70% or higher is satisfactory. Therefore, 10 or less stages are enough for the extraction. Also, the number of extraction stages is increased to reduce the amount of an extracting agent in
usual. However, paraxylene which is an extracting agent in the present invention is used as the raw material of the crude terephthalic acid and therefore extracting agent regenerating unit which is necessary in usual extraction operation is omitted. Since 10 parts by weight or more of paraxylene can be used as the extracting agent based on 100 parts by weight of the condensed water, sufficient removal rate can be achieved even if the number of extraction stages is 4 or less. For example 90% by weight or more of paratoluic acid in the condensed water can be removed by counter current two-stages extraction using 25 parts by weight of paraxylene based on 100% by weight of the condensed water.
In a large scale terephthalic acid plant, the amount of condensed water is 50 t/hr or more, and thus it is better that the extraction tower is not constituted of one large tower but of a multi-tower system with a parallel operation to avoid mal-distribution of flow. Therefore, in the case where the amount of condensed water is 50 t/hr or more in this large terephthalic acid plant, mixer-settler type which is combinations of a stirrer and a settler and connected in series to make a multi-stage extraction system is preferable. The speed of the blade top of the stirrer is 0.25 m/sec or more and preferably 0.5 to 5 m/sec. Higher speed stirring raises the efficiency of extraction, enabling the equipment to be small-sized and is therefore preferable. If the stirring speed is too high, paraxylene
is made into microparticles and the settling time required to separate the solution into two layers is longer. Therefore, the stirring speed is preferably designed to be 5 m/sec or less. When the speed of the blade top of the stirrer is 0.5 to 5 m/sec, the settling time is 1 to 30 minutes, preferably 2 to 5 minutes, in the middle stages of the counter-current multistage extraction and 2 to 60 minutes, and preferably 5 to 15 minutes, in both terminal stages of the counter-current multistage extraction.
The water after extraction is bubbled using a gaseous material to recover paraxylene left in the water discharged from the last stage of the counter-current multistage extraction. At this time, nitrogen may be used as the gaseous material. However, it is difficult to recover paraxylene from nitrogen. Therefore, steam is preferable to recover paraxylene by condensing it. The amount of steam at this time is 0.05 to 10 parts by weight, preferably 0.5 to 5.0 parts by weight and more preferably 1.0 to 2.0 parts by weight based on 100 parts by weight of the condensed water. A larger amount of steam can raise the removal rate of paraxylene. However, even if a small amount of paraxylene is left, this does not affect the quality of a purified terephthalic acid. Since 95% or more of paraxylene left in the water is vaporized in a crystallization vessel and returned to the extractor, it is only necessary to use steam in an amount of 0.5 parts by weight or more based on 100 parts by weight of the
condensed water. Also, when the amount of steam is 1.0 parts by weight or more based on 100 parts by weight of the condensed water, the odor of paraxylene is not left, which precludes the necessity of odor ventilation measures and the amount falling in the above range is therefore desirable.
Also, a coalescer may be used to remove paraxylene. As the coalescer, there is, for example, a fluorine type resin (trade name: Phase Sep Coalescer, manufactured by Nihon Pall Ltd.). In this case, there is the case where entrained terephthalic acid is contained and it is therefore necessary that the rate of vaporization of crystals on a vaporizing plate is made to be 3000 kg/hr or less and preferably 2000 kg/hr or less per unit square meter to thereby reduce the amount of the entrained terephthalic acid and at the same time, the temperature of the water before treated by the coalescer is set to 70°C or higher and preferably 90°C to 100°C so as not to precipitate terephthalic acid.
Paraxylene from which impurities are extracted contains no high-boiling point impurities and therefore has no adverse influence on the quality of a product. It is therefore possible to use the obtained paraxylene as the raw material to produce the crude terephthalic acid without changing any condition.
EXAMPLES
The present invention will be described hereinafter by way of examples. However, these examples are not construed as limiting the present invention.
Example 1
Fig. 1 is an example of an apparatus used in the present invention. A crude terephthalic acid 1 was heated together with water 14 and purified water 15 to 300°C under 10 MPa-G in a heating dissolution step 8 to dissolve the crude terephthalic acid. The total amount of these waters was 3.0 parts by weight based on 1.0 parts by weight of the crude terephthalic acid. After the terephthalic acid was dissolved, 4-carboxybenzaldehyde in the crude terephthalic acid was reduced to paratoluic acid in a hydrogenating step 9 and terephthalic acid was precipitated in a crystallization step 10. At this time, the pressure in a final crystallization vessel was 0.5 MPa-G to vaporize 50%
by weight of the water. The precipitated terephthalic acid
*
was separated in a solid-liquid separation step 11, dried in a drying step 12 and taken out as a purified terephthalic acid 2 product.
The water 5 vaporized in the crystallization step was fed to a condenser 3 where the water was condensed at 95°C. At this time, the concentration of paratoluic acid in the condensed water was 700 ppm by weight. The vaporized water was then fed to an extraction step 13 to carry out a two-stage counter current operation. Specifically, the
condensed water was brought into contact with 25 parts by weight of paraxylene based on 100 parts by weight of the condensed water for two minutes in a first stage stirrer 13-1. Then, the condensed water was settled for three minutes in a first settler 13-2, followed by separating. Then, the resulting condensed water was brought into contact again with 25 parts by weight of paraxylene based on 100 parts by weight of the condensed water for two minutes in a second stage stirrer 13-3. Then, the condensed water was settled for 5 minutes in a second settler 13-4, followed by separating. At this time, paraxylene was flowed counter to the current of the condensed water. The stirrer had a four-slant paddle blade and rotated at a blade top speed of 3.0 m/sec.
The concentration of paratoluic acid was 30 ppm by weight and the concentration of paraxylene was 500 ppm by weight in the water thus obtained. The resulting water was fed to a stripping tower 16 and bubbled with 1.0 parts by weight of steam based on 100 parts by weight of the condensed water. The concentration of paratoluic acid was 30 ppm by weight and the concentration of paraxylene was 1 ppm or less by weight in the bubbled water.
The above extracted paraxylene was fed to an oxidation step 7 together with oxidation raw material paraxylene supplied from a paraxylene supply path 18 and used as the raw material for the production of crude terephthalic acid.
This operation ensured that 50% of the water used for purifying crude terephthalic acid could be reused without dropping the concentration of impurities in the crude terephthalic acid.
The present invention makes it possible to purify wastewater into a reusable state without consuming a lot of energy and to recover paratoluic acid, which is currently dumped, as a final product terephthalic acid.

WE CLAIM:
1. A process for producing terephthalic acid in which crude
terephthalic acid obtained by oxidizing paraxylene is dissolved in water
by heating at 225°C or higher under a pressure of 5Mpa-G or higher and
terephthalic acid is crystallized by vaporizing 30 to 70% of the water
used to dissolve said crude terephthalic acid under reduced pressure,
wherein
a) the water vaporized in the crystallizing process is condensed and is
brought into contact with paraxylene and separated and
b) said condensed separated water is reused for dissolving said crude
terephthalic acid
c) and paraxylene obtained after it is brought into contact with the
condensed water and separated is fed to an oxidation process and used
as a raw material for producing crude terephthalic acid.

2. A process of producing terephthalic acid as claimed in claim 1,
wherein the condensation temperature is 70°C or higher in the process of
condensing the vaporized water.
3. A process of producing terephthalic acid as claimed in claims 1 or
2, wherein the condensed water obtained by condensing the vaporized
water is brought into contact with paraxylene at 40°C to 200°C.
4. A process of producing terephthalic acid as claimed in any one of
claims 1 to 3, wherein the condensed water separated after it is brought
into contact with paraxylene is bubbled using a gaseous material.
5. A process of producing terephthalic acid as claimed in claim 4,
wherein the gaseous material used for bubbling is nitrogen or steam.
6. A process for producing terephthalic acid substantially as
hereinbefore described with reference to the accompanying drawings.