Description
PROCESS FOR PRODUCING CRUDE AROMATIC DICARBOXYLIC ACID TO BE FED TO HYDROGENATION PURIFICATION
Technical Field [0001]
The present invention is concerned in process for producing a crude aromatic dicarboxylic acid which is preferred for hydrogenation purification upon producing a high purified aromatic dicarboxylic acid by conducting hydrogenation purification after producing a crude aromatic dicarboxylic acid by liquid phase oxidation of a dialkyl aromatic hydrocarbon with an oxygen-containing gas.
Particularly, in the process for producing the aromatic dicarboxylic acid by liquid phase oxidation, it relates to a composition of a liquid phase oxidation catalyst and reaction conditions capable of producing a crude aromatic dicarboxylic acid efficiently by liquid phase oxidation while bringing out the performance of a hydrogenation catalyst upon conducting hydrogenation purification in the form of an aqueous solution of crude aromatic dicarboxylic acid produced more economically.
Background Art [0002]

A process for producing an aromatic dicarboxylic acid from a dialkyl aromatic hydrocarbon as the starting material, such as p-xylene or m-xylene, by conducting liquid phase oxidation with an oxygen-containing gas under the presence of a catalyst comprising cobalt (Co), manganese (Mn), and bromine (Br) in an acetic acid solvent has been practiced industrially in a large scale along with increase in the application use. [0003]
Since the aromatic dicarboxylic acid by the said process includes intermediate oxidation reaction products such as 4-carboxy-benzaldehyde (4-CBA) and 3-carboxy-benzaldehyde (3-CBA) and colored materials such as diphenyls and f luorenones, the aromatic dicarboxylic acid is dissolved in water, caused to take place hydrogenating reaction through a noble metal catalyst supported on carbon, to remove or decrease the content of impurities, formed into a purified aromatic dicarboxylic acid and provided as the starting material of aromatic polyesters.
This process is a method of converting the intermediate product and the colored material which are considered to be disadvantageous for the production of aromatic polyesters such as for use in fibers into easily soluble material in water by hydrogenating reaction and then removing hydrogenated impurities while dissolving them as they are in aqueous solvent upon crystallization and recovery of the aromatic dicarboxylic acid.
[0004]
On the other hand, the studies for the reaction conditions and the catalyst composition in the liquid phase oxidation reaction with an oxygen containing gas in the acetic acid solvent have been conducted so far for providing the highly pure aromatic dicarboxylic acid capable of direct polymerization with glycols without a purifying step by the hydrogenation reaction in the subsequent stage.
As shown in Patent Document 1 (JP-B No. Sho 45-36732), Patent Document 2 (JP-B No. Sho 53-30700), Patent Document 3 (JP-B No. Sho 56-21015), they are methods conducted so far of improving the reaction temperature and the catalyst composition in the oxidation reaction and producing a highly pure aromatic dicarboxylic acid with less content of aromatic monocarboxylic acid aldehyde such as (4-CBA) and colored materials as the impurity,having purity sufficient for direct polymerization only by the oxidation reaction step. [0005]
Accordingly, for the catalyst composition in the production of a crude aromatic dicarboxylic acid (aromatic dicarboxylic acid not at high purity) by the oxidation reaction using a dialkyl aromatic hydrocarbon as the starting material, because the removal of impurities have greatly depended on the hydrogenation purification step in the subsequent stage, there have been no proposals as the production process for crude
aromatic dicarboxylic acids suitable to hydrogenation
purification in view of the improvement in the composition of
the oxidation catalyst, since the proposal by Patent Document
4 (JP-B No. Sho 34-2666) of "catalyst comprising heavy metal
and bromine".
[0006]
Patent Document 1: JP-B No. Sho 45-36732
Patent Document 2: JP-B No. Sho 53-30700
Patent Document 3: JP-B No. Sho 56-21015
Patent Document 4: JP-B No. Sho 34-2666
Patent Document 5: JP-B No. Sho 53-24057
Patent Document 6: JP-A No. Hei 11-228492
Disclosure of the Invention
Problem to be Solved by the Invention
[0007]
A flow for the production of a purified aromatic dicarboxylic acid industrially practiced at present is shown by the flow for the production of purified terephthalic acid as in Fig. 1, which is divided into two steps of a production step for crude terephthalic acid (CTA) and a purification step for purified terephthalic acid (PTA).
That is, in the process for producing a crude terephthalic acid (CTA), p-xylene as starting material, acetic acid solvent, and catalyst (Co, Mn, Br) are introduced into an oxidation
reaction vessel 1 stirring at a high temperature (180 to 210°C) and at a high pressure (10 to 20 kg/cm2G), and feeding air from the lower part of the reaction vessel 1 to continuously conduct oxidation reaction. An exhaust gas after consuming oxygen by the conduction of the oxidation reaction is discharged together with solvent vapors from the upper portion of the reaction vessel 1. The vapor-mixed exhaust gas is passed through a condensed-cooler 2 in which a condensable ingredient is condensed and separated into a liquid condensate and a reaction exhaust gas by a gas-liquid separator 3. While most of the liquid condensate is recycled to the reaction vessel 1, the portion is extracted for the control of water concentration of the solvent in the reaction vessel. [0008]
The reaction product is withdrawn from the lower portion of the reaction vessel by a liquid level control and a predetermined reaction volume to an additional oxidation reaction vessel (not illustrated in Fig. 1 but provided with a stirrer, a condensed-cooler, a gas-liquid separator, like the reaction vessel), and an oxygen-containing gas such as air is fed to complete the oxidation reaction. The reaction product completed with the oxidation reaction is transferred to a crystallizer 4 at a lower pressure and cooled by pressure flashing evaporation and the resultant terephthalic acid is precipitated and formed the slurry with the solvent. The
obtained resultant slurry is separated and washed by a solid-liquid separator 6 such as a filter device and terephthalic acid crystals are recovered. Then, they are dried by a drier to produce a crude terephthalic acid (CTA). [0009]
Then, in the production step for purified terephthalic acid (PTA), the said dried crude terephthalic acid (CTA) and water are introduced quantitatively into a slurry preparation vessel 10 to prepare an aqueous slurry at a predetermined concentration (20 to 33 wt%). The slurry is also sent quantitatively to a heater 12 and a dissolving vessel 11 at a high pressure (60 to 80 kg/cm2G) and terephthalic acid is dissolved along with heating into an aqueous solution and then sent to a hydrogenation purification vessel 13. In a hydrogenation purification vessel 13 at a high temperature (260
to 290°C) and a high pressure (60 to 80 kg/cm2G) filled with an activated carbon catalyst put a noble metal such as Pd, the aqueous solution of terephthalic acid containing impurities and hydrogen gas are supplied simultaneously and passed through the catalyst bed, and the hydrogenation reaction of the impurities are conducted. [0010]
The aqueous solution of terephthalic acid put to the hydrogenation reaction treatment as described above is cooled by flash evaporation with the pressure being lowered stepwise
in a plurality of crystallizers connected in series (3 to 6 stages) (the plural crystallizers are not illustrated) to precipitate crystals of terephthalic acid. The purified slurry in which terephthalic acid is crystallized is separated and washed by a solid-liquid separator such as centrifugal separator to recover crystals of terephthalic acid. Then, the purified terephthalic acid is dried through a dryer and produced as the product (PTA) in this step.
Accordingly, since the purifying effect have functioned effectively in the hydrogenation purification step, there has been less requirement for the production process of the crude aromatic dicarboxylic acid on the premise of hydrogenation purification, and there have been no proposals for the oxidation reaction step suitable to hydrogenation purification including both the steps of oxidation reaction and hydrogenation purification. [0011]
Particularly, for the catalyst for oxidation reaction in the liquid phase in the production of the crude aromatic dicarboxylic acid, there have been no proposals for the improvement of the catalyst composition comprising Co, Mn and Br since the invention of Patent Document 4 described above for "conducting liquid phase oxidation with molecular oxygen under the presence of heavy metal and bromine".
Further, the patents having feature in the control ratio
for the catalyst ingredients in a quantitative range such as an Mn/Co ratio or a Br/Co ratio to the Co concentration in the oxidation reaction solvent being as a reference as described above (Patent Documents 1 to 3 described above) are proposals disclosing that high purity terephthalic acid could be formed while decreasing the impurity to about 500 ppm or less of 4-CBA as the intermediate product. [0012]
However, while the improvements can provide high purity aromatic dicarboxylic acids as the starting material for aromatic polyesters not by way of the hydrogenation purification step in the subsequent stage, the combustion loss of the acetic acid solvent put to oxidation combustion along with severe oxidation reactions is described to be about 0.2 as the acetic acid base unit (weight ratio of combustion acetic acid based on weight of the formed terephthalic acid) as shown in the example of the Patent Document 3 described above and this is not an economically excellent method.
Then, also in other proposals described above (Patent Documents 1 and 2 described above), while the combustion amount of acetic acid is not specifically described, since this is a method of obtaining terephthalic acid with an equivalent 4-CBA content of the proposal described above, it is considered that the combustion amount of acetic acid is not different greatly therein.
[0013]
On the other hand, in the hydrogenation purification step, impurities such as intermediate reaction products contained in the crude aromatic dicarboxylic acids are hydrogenated into highly water soluble products.
For example, the hydrogenation purification step has been practiced based on that intermediate reaction products of the aromatic monocarboxylic acid aldehyde such as 4-CBA are converted into aromatic monocarboxylic acid methyl such as p-toluic acid and colored impurities such as fluorenone are hydrocracked and the content of them as the impurities are greatly decreased in the purified aromatic dicarboxylic acid crystals along with crystallization and recovery of the aromatic dicarboxylic acid. [0014]
Among them, in the hydrogenation purification of the aromatic monocarboxylic acid aldehyde (4-CBA), the hydrogenation reaction from the aromatic monocarboxylic acid aldehyde (4-CBA, M.W. 150) to aromatic monocarboxylic acid methyl (p-toluic acid M.W. 136) is a substantially quantitative reaction (91wt% on M.W.) and, while aromatic monocarboxylic acid methyl (p-toluic acid) is contained also after the hydrogenation reaction substantially in the same amount (about 90 wt%) as that of the aromatic monocarboxylic acid aldehyde (4-CBA) contained in the crude aromatic dicarboxylic acid, it has been conducted
as a method of obtaining a purified terephthalic acid with p-toluic acid in the crystals not exceeding 150 ppm by the improvement for the crystallizing method from an aqueous solution of hydrogenated terephthalic acid as shown in Patent Document 4 (JP-B No. Sho 53-24057) and Patent Document 5 (JP-A No. Hei 11-228492). Since other impurities become easily removable by crystallization after the reaction due to the hydrocracking, etc., improvement for the crystallization method has been conducted considering to a critical step for the decrease in the content of the aromatic monocarboxylic acid methyl (p-toluic acid) in the purification. [0015]
The improvement method is a method of reducing the aromatic monocarboxylic acid aldehyde (4-CBA) contained in the crude aromatic dicarboxylic acid to aromatic monocarboxylic acid methyl (p-toluic acid) by hydrogenation reaction, and then conducting crystallization successively by multi-stage flash evaporative cooling (pressure releasing evaporative cooling) system, so that the aromatic monocarboxylic acid methyl (p-toluic acid) is not taken into the recovered crystals. [0016]
Accordingly, JP-B No. Sho 53-24057 describes that it is sufficient to obtain the crystallized terephthalic acid not exceeding 150ppm by providing the multi-stage flash evaporation in the steps of 5 to 8 from terephthalic acid containing 2000
to 6000 ppm of p-toluic acid content based on terephthalic acid. Then, in the example, terephthalic acid with 2500 ppm of p-toluic acid content is dissolved in water and multi-stage flash evaporation is conducted to obtain 150 ppm or less of crystallized terephthalic acid.
Further in the example of JP-ANo. 11-228492, terephthalic acid containing 3000 ppm of p-toluic acid is dissolved in water to obtain crystallized terephthalic acid containing 150 ppm or less of p-toluic acid. [0017]
Accordingly, the content of the aromatic monocarboxylic aldehyde (4-CBA) in the aromatic dicarboxylic acid (CTA) produced by the oxidation reaction as the aromatic dicarboxylic acid (CTA) suitable to hydrogenation purification greatly dominates the amount of aromatic monocarboxylic acid methyl (p-toluic acid) contained the purified aromatic dicarboxylic acid. However, this does not require an aromatic dicarboxylic acid (CTA) of such a high purity that the content of the aromatic monocarboxylic acid aldehyde (4-CBA) is 500 ppm or less as proposed in Patent Documents 1 to 3 described above but this is to conduct such oxidation reaction as forming a crude aromatic dicarboxylic acid (CTA) containing about 3000 ppm of the aromatic monocarboxylic acid aldehyde (4-CBA).
For this purpose, it is considered that a mild liquid phase oxidation method for dialkyl aromatic hydrocarbon suitable to
the premise of the hydrogenation purification step must be in existence separately from the proposals described above (Patent Documents 1 to 3). [0018]
For the crude aromatic dicarboxylic acid (CTA) for use in hydrogenation purification, it is considered that the content of the aromatic monocarboxylic acid aldehyde (4-CBA) represented as the impurity is preferably within a range from 2000 to 3500 ppm, and the catalyst ingredient (Co, Mn, and Br) and reaction conditions to provide a mild oxidation reaction capable of producing a crude aromatic dicarboxylic acid (CTA) containing 2000 to 3500 ppm of the aromatic monocarboxylic acid aldehyde (4-CBA) are the subject of the present invention.
Means for the Solution of the Problem [0019]
The present inventors have made overall investigations on the reaction temperature and the oxidation catalyst (Co, Mn, Br) for the mild oxidation reaction for forming a crude aromatic dicarboxylic acid containing 2000 to 3500 ppm of an aromatic monocarboxylic aldehyde. Particularly, they have found that the lowering of reaction temperature and the decrease in the content of bromine (Br) which are most sensitive to the reactivity in the oxidation reaction give an effect on the hydrogenation purification catalyst in the subsequent stage and
have made a study on the oxidation catalyst composition and the reaction temperature as a method of producing a crude aromatic dicarboxylic acid for use in hydrogenation purification. [0020]
As a result, the present inventors have found that the amount of the acetic acid solvent lost (acetic acid base unit) by combustion along with formation of an aromatic dicarboxylic acid in the oxidation reaction is in a trade-off correlation with the amount of an aromatic monocarboxylic acid aldehyde (4-CBA) contained in the resultant aromatic dicarboxylic acid, and the combustion amount of acetic acid (acetic acid base unit) is effected lower as the reaction temperature is lower for an identical content of the aromatic monocarboxylic acid aldehyde (4-CBA) contained in the resultant aromatic dicarboxylic acid, and the combustion amount of the acetic acid is not decreased
even when the temperature is lowered to 185°C or lower. Then, it has been confirmed that the combustion amount of acetic acid (acetic acid base unit) is at a level of 37 to 45 kg/ton per amount of the resultant dicarboxylic acid which is at a level remarkably lower compared with that of the example of the patent (refer to Patent Document 3) (0.2 = about 200 kg/ton). [0021]
Accordingly, for forming an aromatic dicarboxylic acid containing 2000 to 3500 ppm of an aromatic monocarboxylic acid aldehyde (4-CBA) for use in hydrogenation purification, it has
been found that the reaction temperature is conducted preferably
within a range of from 197 to 185°C.
However, while it is considered necessary to increase the amount of the catalyst (Co, Mn, Br) for forming an aromatic dicarboxylic acid of an identical content of aromatic monocarboxylic acid aldehyde (4-CBA) for lowering the reaction temperature, when the Br content ratio in the oxidation catalyst was lowered in view of requirement, for example, for the mild reaction activity and suppression of apparatus corrosion, it was found that this promoted the clogging of the hydrogenation catalyst bed in the subsequent stage to force the operation to be interrupted and gave an effect on the hydrogen activity in the hydrogenation purification step (Reference Example 1) . At the same time, it has been found that this is due to the ash content in the resultant aromatic dicarboxylic acid, an Mn content is abnormally high in the ash content and this is caused due to the lowering of the ratio of the amount of Br to the amount of Mn (Br/Mn) . Then, it has been found that the Br/Mn ratio as the limit for forming the ash content is depended on the reaction temperature, in a correlation on the contrary to the reaction temperature in the following relation equation (2). Accordingly, it has been found that it is necessary to increase the amount of Br and decrease the amount of Mn along with lowering of the reaction temperature. [0022]
This is estimated to be attributable to that the effect of Co, Mn catalysts and Br as the promoting and the regenerating agent of the reaction in the liquid phase auto-oxidation reaction is lowered, and the automatic regeneration of Mn can not proceed to form precipitation of Mn oxide. Further, while it is generally considered that the activity of the hydrogen purification catalyst is lowered by cohesion and dropout of an active metal and adhesion of obstacles on it, since precipitation of the Mn oxide causes lowering of the hydrogenation activity before clogging of the catalyst bed, the formation of the ash content has to be avoided in view of the life of the hydrogenation activity.
Then, control for the amount of the catalyst (Co, Mn, Br) which is required along with lowering of the reaction temperature is increased by the Br/(Co+Mn) weight ratio, it has also been found that when the Br/(Co+Mn) weight ratio is
increased to 1.7 or more at a reaction temperature of 185°C, this gives no effect on the content of the aromatic monocarboxylic acid aldehyde (4-CBA) in the aromatic dicarboxylic acid (CTA) . Then, it has been found that at a
reaction temperature exceeding 185°C, Br used in an amount of 1.7 as the Br/(Co+Mn) weight ratio is a sufficient amount.
Accordingly, it is preferred that the limit amount of Br is controlled between the minimum amount of Br/Mn ratio calculated by the relation equation (2) correlated with the each
reaction temperature and the maximum amount of 1. 7 in Br/ (Co+Mn)
ratio as the catalyst composition.
[0023]
Based on the findings described above, for producing a crude aromatic dicarboxylic acid for use in hydrogenation purification with the content of the aromatic monocarboxylic acid aldehyde of 2000 to 3500 ppm, it is possible to produce a most economical crude aromatic dicarboxylic acid and provide a process for producing a crude aromatic dicarboxylic acid capable of bringing out the performance of the hydrogenation purification catalyst for a long time by preparing the catalyst composition of the oxidation catalyst such that it contains:
(1) the catalyst metal (Co+Mn) content in an amount of 2,650 ppm
or less and in an amount or more of the content represented by a
relation equation :
(Equation Removed)
(in which
(Co+Mn) is (Co+Mn) content (ppm),
t is reaction temperature (°C) (temperature range: 185 to 200°C)),
(2) Mn/Co weight ratio is controlled within a range from 0.2 to 1.5, preferably, from 0.2 to 1,
(3) Br content is contained in an amount of 1.7 or less as a Br/(Co+Mn) by weight ratio and at least in an amount or more represented by a relation equation (2):
(Equation Removed)
(in which
Br/Mn is Br/Mn weight ratio (wt/wt), and
t is reaction temperature (°C), and
conducting oxidation reaction at a reaction temperature for the liquid phase oxidation within a range from 185°C to 197°C, preferably, within arrange from 185°C to 195°C.
Effect of the Invention [0024]
The present invention can provide a catalyst ingredient (Co, Mn, and Br), and reaction condition to provide a mild oxidation reaction capable of producing a crude aromatic dicarboxylic acid (CTA) containing an aromatic monocarboxylic aldehyde (4-CBA) within a range of 2000 to 3500 ppm of the content of the aromatic monocarboxylic acid aldehyde (4-CBA) represented typically as impurities in the crude aromatic dicarboxylic acid (CTA) most suitable for hydrogenation purification.
Best Mode for Practicing the Invention [0025]
In the present invention, an aromatic dicarboxylic acid containing from 2,000 to 3,500 ppm of an aromatic monocarboxylic
acid aldehyde for use in hydrogenation purification is produced by oxidizing a dialkyl aromatic hydrocarbon as a starting material in a liquid phase with an oxygen-containing gas in an acetic acid solvent under the presence of a catalyst comprising Co, Mn, and Br.
As the starting dialkyl aromatic hydrocarbon, p-xylene, or m-xylene is used and acetic acid is used for the reaction solvent. In the oxidation catalyst prepared in the acetic acid solvent, the amount of Co+Mn contained is at the content of 2,650 ppm at the
maximum, the content is at least 2,065 ppm at 185°C, and controlled within the range of the calculation amount by the relation equation (l)decreasing the content along with increase in the reaction temperature.
The amount of Co+Mn of 2650 ppm is a catalyst content of forming an aromatic dicarboxylic acid with the content of an aromatic monocarboxylic acid aldehyde of about 2,000 ppm at the
amount of Br/(Co+Mn) =1.7 (wt/wt) at a reaction temperature of 185°C, which is a catalyst content with less effect of Br to the increase of Br from (Br/(Co+Mn)=l. 7. Further, this is also the catalyst content with no requirement for increasing the reaction activity by increasing the amount of Co+Mn (amount without need to lower the aromatic monocarboxylic acid aldehyde content to below 2,000 ppm). [0026]
Further, the amount of Co+Mn calculated by the relation (1)

is the catalyst content for forming the aromatic dicarboxylic acid containing about 3,500 ppm of the aromatic monocarboxylic acid aldehyde at the maximum by the oxidation reaction, and the amount of Co+Mn represents the minimum amount of catalyst controlled as the control amount increased from the calculated amount based on the relation equation (1) at each reaction temperature for the requirement to decrease the content of the aromatic monocarboxylic acid aldehyde or the amount of Br. Accordingly, it is preferred that the amount of Co+Mn in the acetic acid solvent is controlled within the range of content between both of the said amounts of Co+Mn.
Further, in this oxidation reaction, like proposals in the Patent Document 1 to 3 described above, the Mn/Co ratio (weight ratio) is controlled within a range from 0.2 to 1.5, preferably, from 0.2 to 1 as the cocatalyst effect of Mn to the Co catalyst. It is preferred that the Mn/Co ratio (weight ratio) is lowered along with lowering of the reaction temperature in view of the restriction on the reaction temperature and the Br/Mn ratio (weight ratio), and the Mn/Co ratio is preferably controlled to an amount of 0.49 or less in view of the restriction on the Br/Mn ratio at
a reaction temperature of 185°C. [0027]
Then, for the amount of Br required to be increased along with lowering of the reaction temperature, to increase the amount to over Br/(Co+Mn) =1.7 at a reaction temperature of 185°C becomes

less on the effect to the content of the aromatic monocarboxylic acid aldehyde (4-CBA) as a reaction product. Along with the increase of the temperature, to control at Br/(Co+Mn)=1.7 or less provides a sufficient control amount. Accordingly, the catalyst is prepared to such a content in the control range of at least an amount calculated by the relation equation (2) or more to the Mn content in view of the limit content for the ash formation in the resultant aromatic dicarboxylic acid with Br/(Co+Mn)=1.7 as the maximum amount of Br control. [0028]
That is, Br is prepared in an amount to at least the amount calculated by the relational (2) or more in view of the formation of the ash content which has to be avoided for the aromatic dicarboxylic acid for use in hydrogenation purification, and the
limit amount of Br at the reaction temperature of 185°C becomes 5.18 at Br/Mn ratio. This is an amount corresponding to the control amount of 0.49 as the Mn/Co ratio for preparing the catalyst at Br/(Co+Mn) ratio=1.7. [0029]
The acetic acid solvent prepared as described above is supplied in an amount of 2.5 to 4 times by weight to the dialkyl aromatic hydrocarbon as the supplied starting material to the oxidation reaction vessel, and an oxygen-containing gas such as air is blown into the reaction vessel to conduct oxidation reaction. An exhaust gas after consuming oxygen by the oxidation reaction
is exhausted together with generation of solvent vapors from the upper portion of the reaction vessel and the solvent vapors are condensed through a condensed-cooler 2, and then liquid condensates are separated by a gas/liquid separator 3 and recycled to the reaction vessel. The oxygen concentration in the reaction exhaust gas separated with the liquid condensates is measured and it is discharged out of the reaction system while controlling the reaction pressure. The temperature of the reaction vessel, that is, the reaction temperature is controlled within a range from 185
to 197°C, preferably, from 185 to 195°C by the control of the vapor generation by controlling the pressure (11 to 18 Kg/cm2G) . [0030]
Further, the supplied amount of the oxygen-containing gas to be blown is adjusted and controlled such that the oxygen concentration in the reaction exhaust gas is from 2.5 to 4 vol%.
Further, a portion of the recycled liquid condensatee is withdrawn to control the water content of the solvent during reaction and it is controlled to about 8 to 15 wt%, preferably, 10 to 13 wt%. It is controlled by supplying the acetic acid solvent (not containing catalyst) corresponding to the withdrawal amount of the liquid condensate so as to maintain the concentration of the oxidation catalyst in the reaction solvent. [0031]
The reaction product formed by the accomplishment of smooth oxidation reaction should be withdrawn from the lower portion by
controlling the liquid level or the like and the reaction volume should be kept at constant. The reaction volume corresponds to the residence time of the supplied reaction mixture, in which is preferably controlled within a range from 0.7 to 1.5 hours and, more preferably, from 1 to 1.3 hours in this reaction. [0032]
In the oxidation reaction conducted as described above, the acetic acid solvent is consumed by being decomposed into gaseous carbon dioxide (including carbon monoxide) and water by co-oxidation reaction with the reaction of the starting dialkyl aromatic hydrocarbon. The co-oxidation reaction of acetic acid occurring in this case is defined as combustion loss of acetic acid and industrially calculated and evaluated as acetic acid combustion amount per aromatic dicarboxylic acid to be produced (acetic acid base unit kg/ton).
It is considered that combustion amount depends on the severeness of the oxidation reaction activity and is in a trade-off relation with the content of the aromatic monocarboxylic acid aldehyde remaining in the resultant aromatic dicarboxylic acid as an index. Accordingly, the level of content (2,000 to 3,500 ppm) of the aromatic monocarboxylic acid aldehyde (4-CBA) aimed in the process of the present invention is different from the level of content of the aromatic monocarboxylic acid aldehyde (4-CBA) (500 ppm or less) aimed in the Patent Document 3 described above, so the combustion amount of acetic acid is greatly different from the
oxidation process for producing aromatic dicarboxylic acid of the present invention. The area of oxidation reaction aimed at the present invention is different from the area aimed at the Patent Document 3 described above. Further, the combustion amount of acetic acid is greatly decreased in the economical production of the aromatic dicarboxylic acid aimed at in the process of the invention. [0033]
However, even in the oxidation reaction forming an identical content of the aromatic monocarboxylic acid aldehyde (4-CBA), the combustion amount of acetic acid (acetic acid base unit) is lowered in the oxidation reaction at a lower reaction temperature and, further, the extent of the lowering thereof relative to the temperature tends to be decreased as it approaches 180°C. Then, at the reaction temperature of 180°C, the combustion amount of acetic acid (acetic acid base unit) is substantially identical with
that at the reaction temperature of 185°C.
Accordingly, when the oxidation reaction is conducted at
the reaction temperature of 185°C or higher and 197°C or lower in the process of the invention, the combustion amount of acetic acid (acetic acid base unit) is substantially 45 kg/ton or less, to produce an aromatic dicarboxylic acid with the aromatic monocarboxylic acid aldehyde (4-CBA) within the content range of from 2,500 to 3,500 ppm. For producing a purified aromatic dicarboxylic acid of further favorable quality, it is a preferred
reaction condition to conduct oxidation reaction within a range
from 185 to 195°C. [0034]
Then, combustion of the acetic acid solvent is accompanied with co-oxidation reaction of acetic acid with the reactant material in the reaction vessel. The content of the aromatic monocarboxylic acid aldehyde (4-CBA) is measured and, so long as the content of the aromatic monocarboxylic acid aldehyde (4-CBA) is permitted, the residence time in the reaction vessel is preferably shorter as possible in which is from 0.7 to 1.5 hours. For producing the aromatic dicarboxylic acid with the content of the aromatic monocarboxylic acid aldehyde (4-CBA) within a range from 2,000 to 3,500 ppm and stabilizing the combustion amount of acetic acid, it is further preferred to conduct oxidation reaction within a range of the residence time from 1 to 1.3 hours. [0035]
The reaction product mixture formed as described above is withdrawn from the reaction vessel to an additional oxidation reaction vessel, the reaction of the reaction active component and the reaction intermediate product remaining in the reaction product is completed to terminate the oxidation reaction by the supply of the oxygen-containing gas in the additional oxidation reaction vessel. Then, the reaction product mixture is transferred to and cooled in crystallizer4, and the resultant aromatic dicarboxylic acid is crystallized and in a slurry, which
is then passed through steps of solid-liquid separation,acetic acid washing and drying to produce a powdery crude aromatic dicarboxylic acid containing 2,000 to 3,500 ppm of an aromatic monocarboxylic acid aldehyde. [0036]
Then, the crude aromatic dicarboxylic acid produced as described above is supplied to a purifying step and, after converting the crude aromatic dicarboxylic acid into an aqueous solution as described above, it is put to hydrogenation purification on the catalyst of Pd supported on active carbon at a high temperature and high pressure state to maintain hydrogenation activity for a long time (1 to 3 years). Then, it is crystallized by multi-stage flash (evaporation) cooling, and put to solid/liquid separation, water washing and drying to produce crystal powder of a purified aromatic dicarboxylic acid, in which the aromatic monocarboxylic acid methyl content is 150 ppm or less in the produced purified aromatic dicarboxylic acid, and it becomes a purified aromatic dicarboxylic acid capable of use as a starting material in aromatic polyesters.
Example [0037]
The process of the present invention is to be further described in details for specific embodiments with reference to examples, comparative examples, and reference examples.
[0038] Examples 1 to 4, and Comparative Examples 1-2
Along with the flow of a crude terephthalic acid (CTA) production step shown in Fig. 1, oxidation reaction was conducted using p-xylene as a starting material to produce a crude terephthalic acid.
As an oxidation reaction equipment, a high pressure oxidation reaction vessel (inner volume of about 48 m3) equipped with a rotary stirrer was used and, while conducting oxidation reaction by supplying a starting material p-xylene, a reaction solvent containing a catalyst, and pressurized air, acetic acid (not illustrated) for controlling the water content in the reaction solvent was supplied separately.
Then, a vapor mixed reaction exhaust gas generated by oxidation reaction was withdrawn from the upper portion of the oxidation reaction vessel, passed through a condensed-cooler 2 and the condensation ingredient in the mixed exhaust gas was condensed and cooled to separate liquid condensate, which were recycled to the oxidation reaction vessel.
A portion of the liquid condensate to be recycled is branched from a recycling pipe and withdrawn for controlling the water content in the solvent during reaction. [0039]
Further, the reaction exhaust gas separated with the liquid condensate was introduced into a gas absorber (not illustrated) at a high pressure and discharged through washing treatment with
acetic acid and water.
On the other hand, the reacted and formed resultant slurry was withdrawn to an additional oxidation reaction vessel (not illustrated) and, after conducting additional oxidation reaction, it was treated by a crystallizer 4 and a solid/liquid separator 6 (solid-liquid separation and washing of cake) to obtain a wet cake of the resultant terephthalic acid. Then, after drying by a dryer 7, a crude terephthalic acid (CTA) was obtained.
The oxidation reaction was conducted by supplying acetic acid containing a catalyst in an amount of three times by weight based on 100 parts by weight/hr of the starting p-xylene, blowing a pressurized air, controlling the exit pressure of a reaction exhaust gas so as to reach respective reaction temperatures and blowing air such that the oxygen gas concentration in the reaction exhaust gas was about 3.5 vol% to conduct oxidation reaction. Then, the reaction product was withdrawn to an additional oxidation
reaction vessel by y ray liquid level control such that the residence time in the reaction vessel was about 70 min. The concentration of water during reaction was controlled to about 11.5 wt% (water in reaction mother liquid).
In this case, the reaction temperature, the content of the catalyst (Co, Mn, Br) in the supplied acetic acid solvent, and the 4-CBA content of the crude terephthalic acid were as shown in Table 1 to Table 6. At the same time, gaseous carbon dioxide (CO2) and carbon monoxide (CO) in the exhaust gas were measured, and the
combustion amount of acetic acid (acetic acid base unit) was converted per resultant terephthalic acid to the supplied p-xylene (kg/ton TPA) and were also shown together in Table 1 to Table 6.
[0040]
Table 1 Example 1 Reaction temperature 195°C

(Table Removed)
[0041]
Table 2 Example 2 Reaction temperature 197°C

(Table Removed)
[0042]
Table 3 Example 3 Reaction temperature 190°C

[0043]
Table 4 Example 4 Reaction temperature 185°C

(Table Removed)
[0044]
Table 5 Comparative Example 1 Reaction temperature 200°C

(Table Removed)
[0045]
Table 6 Comparative Example 2 Reaction temperature 180°C

(Table Removed)
[0046]
Relations between 4-CBA content in the resultant terephthalic acid and the combustion amount of acetic acid are plotted in Fig. 2.
It was found that the combustion amount of acetic acid was in a trade-off relation with the 4-CBA content and that the combustion amount of acetic acid of the lower reaction temperature was decreased for forming a terephthalic acid of an identical 4-CBA content in this oxidation reaction, and the combustion amount of acetic acid did not decrease even when the temperature was lowered
to below 185°C.
Further, also for the relation between the content of the metal catalyst (Co+Mn) and the reaction temperature, it is necessary to decrease the amount of Co+Mn along with increase of the temperature, and the content of Co+Mn for forming terephthalic
acid containing about 3,500 ppm (3450 to 3510 ppm) and about 2,000 ppm (1,900 to 2,100 ppm) at each reaction temperature is plotted on the reaction temperature (185 to 200°C) in Fig. 3 (O and • ).
In this reaction, for producing CTA for use in hydrogenation purification, Co+Mn in the solvent is prepared to the amount between each line of the 4-CBA contents of 3,500 ppm and 2,000 ppm at each reaction temperature in view of Fig. 3.
Then, while this reaction was conducted at Br/(Co+Mn) ratio of 1.7, when it is conducted with the reaction temperature in a
higher temperature region (190°C, 195°C), CTA for use in hydrogenation purification can be produced by increasing the amount of Co+Mn along with decreasing the Br/(Co+Mn) ratio. For this purpose, symbol o in Fig. 3 is the control amount of catalyst with the minimum amount of Co+Mn at each temperature in Fig. 3. [0047]
Then, the relation of symbols o in Fig. 3 is represented by the formula as below:
(Co+Mn)ppm = -0 . 460(t-185)3+18. 4(t-185)2-277 . 5(t-185)+2065 (where t: reaction temperature (°C) (temperature range: 185 to 200°C))
Further, since decrease in the amount of Br at the reaction
temperature of 185°C causes formation of the ash content and it is difficult to decrease the amount of Br to below Br/(Co+Mn) =1.7, the catalyst control amount (Co+Mn amount=2,650 ppm) at 185°C in the reaction condition for forming terephthalic acid containing
2,000 ppm of 4-CBA (symbol # in Fig. 3) is the maximum amount for the catalyst preparation amount in this reaction.
Accordingly, the amount of the catalyst metal (Co+Mn) in the oxidation reaction solvent for producing CTA (4-CBA content: 2,000 to 3,500 ppm) for use in hydrogenation purification is prepared as the catalyst amount for the amount of Co+Mn calculated by the relation equation described above or more and the catalyst amount for the amount of Co+Mn within the control range 2,650 ppm or less (within a range of fat dotted line shown in Fig. 3). [0048] Reference Example 1
According to the production flow of purified terephthalic acid shown in Fig. 1, production of crude terephthalic acid (CTA) and production of purified terephthalic acid (PTA) by hydrogenation purification were conducted using p-xylene as the starting material.
The oxidation reaction conditions in the production of CTA were identical with those in Examples 1, 2, 3, and 4 in which a reaction solvent was supplied in an amount of three times by weight of the starting p-xylene, and air was supplied such that the concentration of oxygen (O2) in the exhaust gas was about 3.5 vol%, and the reaction was conducted continuously at a reaction
temperature of 193°C. Withdrawing of the formed reaction product and liquid condensates, post treatment for the withdrawn products, etc. were conducted in the same way as in the examples. [0049]
In order to prepare the catalyst composition of Co, Mn and Br in the reaction solvent to be supplied, a solution of catalyst metal (Co, Mn), hydrobromic acid, and acetic acid were supplemented by using a separated mother liquor of the reaction product as the starting preparation material. Since the reaction solvent was prepared by refraining from the supplement to the Br content in the separated mother liquor consumed in the reaction, mainly supplementing Co, Mn, measuring the 4-CBA content and supplementing the catalyst metal (Co, Mn) such that the 4-CBA content of the resultant CTA was about 2500 ppm, Co, Mn, Br in the solvent were traced on the respective processes as plotted in Fig. 4 (increase of Co, Mn ingredient and decrease of Br ingredient).
CTA was produced in this course with the content of 4-CBA in a range from 1,800 to 3,300 ppm. [0050]
Then, for conducting hydrogenation purification on the thus produced CTA in accordance with the flow in Fig. 1, it was mixed with water to prepare an aqueous slurry at about 27 wt% CTA and
then heated and dissolved (285°C) and supplied to a hydrogenation purification vessel filled with a solid catalyst (0.5% Pd active carbon catalyst) to conduct hydrogenation purification of CTA. [0051]
In the hydrogenation purification vessel, pressure is kept at 70 Kg/cm2G along with the supply of the hydrogen gas and CTA in an amount at about 3 (w/w hr) ratio to the amount of the
purification catalyst was supplied as an aqueous solution from the portion above the catalyst layer to conduct purification reaction. Then, it was supplied to the crystallizer 14, and evaporative cooling due to stepwise pressure release was conducted at five steps in the crystallizer 14 to precipitate crystals of purified terephthalic acid (PTA).
The obtained PTA crystal slurry was supplied to a solid-liquid separator 15 to recover PTA crystals, which were dried to obtain PTA as products. [0052]
Meanwhile, the pressure difference before and after the hydrogen purification bed (differential pressure) increased only in one-half month (14 days), the pressure on the side of supplying the aqueous solution increased to a pressure of 75 Kg/cm2G or higher and pressure fluctuation was also increased, so that supply of the aqueous solution was interrupted and hydrogenation purification was stopped. After water washing the hydrogenation purification vessel 13, when the temperature was lowered and the pressure was lowered to open the vessel, black fine tar-like substance was clogged in the catalyst bed and especially the upper portion. The results of measurement for the pressure difference at the
exit/inlet of the solid catalyst bed (differential pressure (AP)
were plotted as shown in Fig. 5. At the same time, the measured
values for the ash content in the CTA supplied was also plotted.
From the result, it was suggested that abrupt increase of
the differential pressure (AP) in the catalyst bed was increased in co-relation with the ash content of the supplied CTA. [0053]
Taking up the ash content measured as described above (at 1.25 day, 13.25 day), the result of measurement for the breakdown of the contained metals are as shown in the following Table 7.
The results suggested that the ash content is attributable to the catalyst ingredient and, for the increase of the ash content, the amount of Mn in the ash content increased abnormally (13.25 day: Mn/Co=8.3) compared with the Co, Mn composition in the CTA catalyst ingredient (Mn/Co=0.49) so as to suggest most of them due to precipitation of Mn. It was also suggested that this was caused by the lowering of the Br concentration in the CTA catalyst ingredient. [0054]
Table 7 Ash content and contained metal

(Table Removed)
[0055]
Reference Examples 2 to 4, Examples 5 to 14, and Comparative
Examples 3 to 11
An oxidation reaction apparatus having a high pressure reactor (inner volume: 20L) equipped with a stirrer, a heater and a reflux condenser and provided a starting material introduction port, an oxygen-containing gas feed port, and a reaction product withdrawing port was used, 8 kg of a reaction solvent in which cobalt acetate, manganese acetate and hydrobromic acid were dissolved in acetic acid was charged therein and, after introduction of a nitrogen gas, temperature and pressure were increased to a
temperature of 200°C and a pressure of 19 Kg/cm2G while stirring.
Successively, 2 kg/hr of p-xylene was supplied from the starting material introduction port, air was fed from the oxygen-containing gas feed port, the feed amount and the pressure of air were controlled such that the oxygen concentration in the exhaust gas from the upper exhaust gas exit of the reflux condenser was about 4 vol%, the temperature was set to predetermined levels (200°C, 195°C, 185°C) and then reaction was conducted for 1 hour. [0056]
Then, while keeping the respective reaction temperatures described above and continuing the supply of p-xylene and air, the reaction solvent (solvent in which catalyst was prepared) was started to supply at a ratio of 7.5 kg/hr from the starting material introduction port, and the reaction product was withdrawn from the reaction product withdrawing port to a product receiver under liquid level control such that the inner volume was 9L. At the same time, the recycling liquid was withdrawn from the branch pipe
for recycling liquid condensate at a ratio of 1.5 kg/hr to conduct reaction continuously. The average staying time of the starting reaction mixture was therefore controlled to about 1.1 hours. [0057]
Then, a continuous reaction was started and, after lapse of 3 hours, supply of p-xylene and the reaction solvent and withdrawal of the products and the recycling liquid condensates were stopped and then supply of air was conducted as it was for one min and then stopped.
After the end of the reaction and after cooling the reaction
product remaining in the rector to about 70°C, they were taken out and filtered, further washed with 6 kg of acetic acid, and dried by a dryer to obtain resultant terephthalic acid (about 3 kg).
The content of the catalyst (Co, Mn, Br) prepared in the solvent and the prepared compositional ratio (Mn/Co ratio and Br/Mn ratio) at each of the reaction temperatures are shown in Table 8 to Table 11. The content of the catalyst (Co, Mn, Br) in the reaction solvent correspond to about 1.25 times the amount in Table 8 to Table 11 (since recycling liquid is withdrawn at 1.5 kg/hr).
The 4-CBA content of terephthalic acid and ash content obtained by the respective reaction conditions were measured, which are shown together in Table 8 to Table 11.
For the catalyst control in the reaction solvent, examples prepared such that the 4-CBA content in the resultant telephthalic acid were within a range from 2,300 to 2,800 ppm are shown.
[0058]
Table 8 Reaction temperature 200°C

[0059]
Table 9 Reaction temperature 195°C

(Table Removed)
[0060]
Table 10 Reaction temperature 190°C
(Table Removed)
[0061]
Table 11 Reaction temperature 185°C

(Table Removed)
[0062]
From the results, it can be seen that the ash content increases abruptly when the Br/Mn ratio in the catalyst ingredient decreases at each of the reaction temperatures. And it was found that the Br/Mn ratio forming the limit thereof (Reference Example 4, Example 8, Example 10, Example 14) is present inherently at each
of the reaction temperatures.
Accordingly, it can be said that also the ratio of Mn (Mn/Co) in the catalyst metal (Co, Mn) is tended to be decreased preferably
(Mn/Co=1.5 → 0.2) along with lowering of the reaction temperature
in view of the restriction of the Br/Mn ratio.
[0063]
Further, it was also found that they substantially
correspond to the value for Br/Mn ratio = 2.5 → 2.2 at 7 day → 8 day where the ash content in the terephthalic acid increased in
Reference Example 1 (Reaction temperature 193°C) and the value for Br/Mn ratio as the limit in the examples (Example 8 :limit
Br/Mn=1.82 at 195°C; Example 10 :limit Br/Mn=3.04 at 1900C).
Then, the limit Br/Mn ratio was plotted at each of the reaction temperatures as shown in Fig. 6 and co-relation exists between the reaction temperatures and the limit Br/Mn ratio according to the following relation.
Br/Mn=-0.00115(t-185)3+0.0362(t-185)2-0.5803(t-185)+5.18 (where t: reaction temperature (°C) (temperature range: 185 to 200°C))
Further, in the examples (including reference examples and comparative examples), the Br content (Br/(Co+Mn) ratio) to the amount of catalyst metal (Co+Mn) and the catalyst metal (Co+Mn) are plotted as shown in Fig. 7. From Fig. 7, it can be seen that the co-relation between the amount of the catalyst metal (Co+Mn) and the Br/(Co+Mn) ratio is different at each of the reaction
temperatures, and the effect of increasing the amount of Br is different respectively. Then, at the reaction temperature of
185°C that requires the greatest amount of Br, the effect of increasing the amount as the Br/(Co+Mn) ratio appears large, but the effect does not appear even when Br/(Co+Mn) ratio is increased to about 1.7 or more. Accordingly, in the oxidation reaction at
185°C, it is considered that the Br/(Co+Mn) ratio of 1.7 is the maximum preparation amount (the amount of Br for Br/(Co+Mn)=l.7
or less is a sufficient amount at a reaction temperature of 185°C or higher).
Accordingly, at each of the reaction temperatures in this reaction, it is preferred to use Br controlled to the amount of Br at least for the Br/Mn ratio or more calculated from the relation described above and prepared within the range for the amount of Br as the Br/(Co+Mn) ratio of 1.7 or less. [0064] Example 15
In accordance with the flow of the pure terephthalic acid production in Fig. 1, production for a crude terephthalic acid and purified terephthalic acid were conducted continuously using p-xylene as the starring material by the method described in Reference Example 1.
As the oxidation reaction solvent, a metal catalyst (Co, Mn) solution and hydrobromic acid were supplemented and prepared so as to provide the catalyst content shown in Table 12 and oxidation
reaction was conducted at a temperature of 193 ± 1°C.
[0065] Table 12

(Table Removed)
[0066]
As a result, for terephthalic acid produced from the crude terephthalic acid production step, those with the 4-CBA content of about 2,500 ppm was obtained, and the combustion amount of acetic acid in the oxidation reaction (acetic acid base unit) was about 42 kg/ton. Further, the purified terephthalic acid produced by the purified terephthalic acid production step contained p-toluic acid by 120 ppm or less, and the pressure in the hydrogenation purification bed 13 progressed at about 70 Kg/cm2G and smooth operation was being continued for about 1 year with no appearance of pressure fluctuation, with the pressure difference (differential pressure) of 1 Kg/cm2 or less.
Brief Description of Drawings [0067]
[Fig. 1] is a view showing the flow of steps for producing a crude terephthalic acid using p-xylene as a starting material
and steps for producing a purified terephthalic acid by dissolving the crude terephthalic acid into water and applying hydrogenation purification.
[Fig. 2] is a view showing the amount of an acetic acid solvent lost by combustion upon oxidation reaction (acetic acid base unit) to 4-CBA(4-carboxybenzaldehyde) contained in terephthalic acid formed by liquid phase oxidation of starting p-xylene with air under the presence of a catalyst using acetic acid as the solvent (the combustion amount of acetic acid (acetic acid base unit) represents the amount of acetic acid lost by combustion as the amount to the amount of resultant terephthalic acid (kg/ton TPA)).
[Fig. 3 ] is a view illustrating the amounts of Co+Mn prepared upon forming terephthalic acid with the content of 4-CBA (4-carboxybenzaldehyde) of 2,000 ppm and 3,500 ppm at each of the reaction temperatures, in the liquid phase oxidation of starting material p-xylene by air under the presence of a catalyst using acetic acid as a solvent (symbol O: 3,500 ppm of 4-CBA content,
symbol •: 2,000 ppm content of 4-CBA) (reaction temperature is shown by index (x) for (reaction temperature - 185)). As a result,
the relational formula between the reaction temperature (reaction
temperature-185) (x) upon forming terephthalic acid with 3,500 ppm
of 4-CBA content and the concentration of a prepared metal catalyst
(amount of Co+ Mn amount) (y) is shown in the drawing. Each being
prepared at Br/(Co+Mn)=1.7).
[Fig. 4] illustrates the progress for the concentration of the oxidation catalyst (Co, Mn, Br) in a supplied solvent prepared upon continuous production of terephthalic acid by liquid phase air oxidation of p-xylene (at 6 hr interval)(4-CBA content in terephthalic acid was 1,800 to 3,300 in the course of process).
[Fig. 5] shows a pressure difference (differential pressure) generated in a hydrogenation catalyst bed upon conducting hydrogenation purification after continuous production of terephthalic acid by liquid phase air oxidation of p-xylene (identical with Fig. 4), and then conducting hydrogen purification after converting terephthalic acid into an aqueous solution. The view also shows the ash content of terephthalic acid supplied to the hydrogenation purification vessel.
[Fig. 6] is a view showing the Br/Mn ratio (by symbol o), in the examples of conducting preparation of catalyst (Co, Mn, Br) for forming terephthalic acid with the content of 4-CBA of about 2,500 ppm (within a range from 2300 to 2800ppm)at each of the temperatures by liquid phase air oxidation using p-xylene as the starting material, those examples before abrupt increase in the formation of the ash content (Reference Example 4, Example 8, Example 10, Example 14) (as a result, the Br/Mn ratio (y) is correlated by the relation with the reaction temperature (reaction temperature -185)(x) and a equation of the correlating relation is shown in the graph). Accordingly, the Br/Mn region in the lower side of the relation equation in the drawing at each of the reaction
temperature can be defined as a high ash content region).
[ Fig. 7 ] is a view showing the Br/ (Co+Mn) ratio and the amount of Co+Mn for the prepared catalyst composition (Co, Mn, Br) upon forming terephthalic acid with the content of 4-CBA of about 2,500 ppm (2,330 to 2,720 ppm) by liquid phase air oxidation of p-xylene as the starting material, with the reaction temperature being as a parameter.
Description of Reference Numerals [0068]
1 ... oxidation reaction vessel,
2 ... condensed-cooler,
3 ... gas-liquid separator,
4 ... crystallizer,
5 ... condenser,
6 ... solid-liquid separator,
7 ... dryer,
8 ... product hopper,
9 ... powder supply device,

10 ... slurry preparation vessel,
11 ... dissolving vessel,
12 ... heater,
13 ... hydrogenation purification vessel,
14 ... crystallizer (plural crystallizers),
15 ... solid-liquid separator.
16 ... dryer,
17 ... product hopper.

Claims
[1] A process for producing a crude aromatic carboxylic acid for use in the process stage of hydrogenation purification by conducting liquid phase oxidation with an oxygen-containing gas using a dialkyl aromatic hydrocarbon as a starting material in an acetic acid solvent under the presence of a metal catalyst comprising cobalt (Co) and manganese (Mn), and bromine (Br) as an oxidation promoter, and conducting hydrogenation purification by a catalyst of noble metal supported on carbon to produce a purified aromatic dicarboxylic acid, including:
1) controlling the catalyst composition for the liquid phase oxidation for producing the crude aromatic dicarboxylic acid such that it contains:
(1) the catalyst metal (Co+Mn) content in an amount of 2,650 ppm
or less and in an amount or more of the content represented by a
relation:
(Co+Mn) = -0.460(t-185)3+18.4(t-185)2-277.5(t-185)+2065 (in which
(Co+Mn) is (Co+Mn) content (ppm), t is reaction temperature (°C) (temperature range: 185 to 200°C)),
(2) Mn/Co weight ratio is controlled within a range from 0.2 to 1.5, preferably, from 0.2 to 1,
(3) Br content is contained in an amount of 1.7 or less as a Br/(Co+Mn) by weight ratio and in an amount or more represented
by a relation:
Br/Mn=-0.00115(t-185)J+0.0362(t-185)"-0.5803(t-185)+5.18
(where
Br/Mn is Br/Mn weight ratio (wt/wt), and
t is reaction temperature (°C) (temperature range: 185 to
200°C)),
2) conducting oxidation reaction at a reaction temperature for the
liquid phase oxidation within a range from 185 to 197°C, and
3) producing a crude aromatic dicarboxylic acid with the content
from 2,000 to 3,500 ppm of an aromatic monocarboxylic aldehyde as
the intermediate product of the liquid phase oxidation reaction.
[2] The process for producing a crude aromatic dicarboxylic acid for use in hydrogenation purification according to claim 1, wherein the dialkyl aromatic hydrocarbon is p-xylene, the aromatic dicarboxylic acid is terephthalic acid, and the aromatic monocarboxylic acid is 4-carboxybenzaldehyde.
[3] The process for producing a crude aromatic dicarboxylic acid for use in hydrogenation purification according to claim 1 or 2, wherein the average residence time of the starting mixture for the liquid phase oxidation reaction is from 0.7 to 1.5 hours and, preferably, from 1 to 1.3 hours, and the water content in the reaction solvent is controlled to 8 to 15% by weight and, preferably, from 10 to 13% by weight.