JPS6050775B2 - Continuous production method of terephthalic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for oxidizing p-xylene to terephthalic acid.

The production of terephthalic acid by oxidizing p-xylene in the liquid phase with an oxygen-containing gas is described, for example, in British Patent No. 807.
, No. 091, No. 833,438, No. 841,42
No. 5 and No. 1,062,48? It is known as described in the specification.

The catalysts used are heavy metals or heavy metal mixtures in combination with bromide or bromine. Preferred heavy metals are cobalt and manganese, used alone or in combination with other heavy metals or with each other. Particularly preferred is the combination of cobalt and manganese. This oxidation process is carried out in a solution of an aliphatic monocarboxylic acid having up to about 8 carbon atoms (acetic acid is usually used). The primary use of terephthalic acid is in the production of polyethylene terephthalate. Polyethylene is a polymer useful for melt spinning into synthetic fibers or making films. Initially, terephthalic acid was converted to its dimethyl ester by esterification with methanol, this dimethyl ester was then transesterified with ethylene glycol to form a glycol ester, and this glycol ester was then polymerized to form polyethylene tereph-5. It was a tally. Recently, a method of directly esterifying terephthalic acid with ethylene glycol has been established, and this direct esterification method offers cost advantages over the indirect method via dimethyl ester. But about the direct method! Therefore, it is necessary to use high purity terephthalic acid as a starting material. The known p-xylene oxidation processes described above do not, during normal operation, provide terephthalic acid of sufficient purity for the direct process. It is therefore usually necessary to subject the terephthalic acid so obtained to one or more purification steps to bring it to a purity suitable for the direct esterification process. (An example of a purification step is the method described in British Patent No. 994,7). Such purification steps are expensive and can offset the benefits generated by direct esterification methods.
Therefore, it would be highly desirable to oxidize p-xylene to terephthalic acid of sufficient purity for use in direct esterification with ethylene glycol without separate purification operations. There are two main types of impurities present in terephthalic acid obtained by oxidation of p-xylene, the first being incomplete oxidation products, especially 4-carboxybenzaldehyde, and the second being It is a by-product resulting from side reactions and generally having a higher molecular weight than terephthalic acid.

Significant amounts of 4-carboxybenzaldehyde can lead to inferior quality polyesters, so it is desirable to keep its concentration as low as possible. Although the nature of the above-mentioned by-products has not been completely identified, even trace amounts can have a negative effect on the derived polyester, e.g. imparting color and/or reducing the resulting molecular weight (and thus the molecular weight obtained). melting point) may be lower than the desired molecular weight. In particular, since the color of the derived polyester is imparted by bromine-containing organic compounds as impurities in the terephthalic acid, it is desirable to keep the bromine concentration of the terephthalic acid as low as possible. The by-products in terephthalic acid resulting from side reactions during the oxidation reaction are numerous and have different effects on the quality of derived polyethylene terephthalate; It is convenient to generally assess the effect of variations in process parameters on by-products by measuring the proportions of products and expressing the results as total measured by-products in terephthalic acid. Japanese Patent Publication No. 45-3673 describes a method for producing terephthalic acid suitable for direct polymerization with glycols without any additional purification steps.

In this method, the heavy metal compound is a catalyst comprising a cobalt compound and a manganese compound, and the weight is 1 to 20 W calculated based on the weight of cobalt metal in which manganese metal is present.
t% of the lower aliphatic carboxylic acid in which the amount of cobalt metal is used as a solvent.
It is proposed to use a catalyst corresponding to %. British Patent No. 1,334,551 proposes in a similar process the use of a catalyst comprising a cobalt source, a manganese source and a bromine source consisting of cobalt bromide or elemental bromine; In the catalyst, (a) the amount of cobalt is 0.05 to 0.50 wt% of the C1-C8 aliphatic carboxylic acid used as a solvent, and (b) the amount of manganese is 1 to 10% of the cobalt. , and (c
) bromine to cobalt weight ratio from 1.5:1 to 6.0:1
It is.

Although both of the above-mentioned publications suggest carrying out the process at temperatures between 80 and 230°C, all data disclosed in both publications were obtained at a temperature of 190°C. .

British Patent No. 1,249,198 also describes using a catalyst consisting of cobalt, manganese and bromine to
The production of high purity and low color terephthalic acid carried out in acetic acid solvent at a temperature of 000 C and a pressure of 3 to 15 k9/d (gauge) has been proposed, in which the catalyst (
1) The amount of manganese is from 2 x 10-7 to less than 1 x 10-5 gram atoms per y of acetic acid,
) the amount of bromine is from 5 x 10-6 to 5 x 10-5 gram atoms per q of acetic acid, and (111)
The atomic ratio of cobalt to manganese is between 1 and 100.

Although the catalyst composition is defined somewhat differently in GB 1,249,198 than in GB 1,334,551, there is considerable overlap between the two.

Furthermore, British Patent No. 1,249,198 states that the upper limit of cobalt is 5.5 wt% (5500 ppp) of acetic acid.
m), but all specific examples conducted within the specified temperature range have a value as high as 1.m) per q of acetic acid.
Cobalt concentration higher than 5 x 10-5 gram atom cobalt (4). 0885 Wt% or 885 ppm) was not used. Producing terephthalic acid of sufficient purity to directly esterify ethylene glycol in producing polyethylene terephthalate without additional complex purification steps reduces the cost of terephthalic acid suitable for this purpose. Although the main factor is
That's not the only factor involved.

Air oxidation of p-xylene in acetic acid solvent in the presence of a catalyst consisting of cobalt, manganese and bromide oxidizes a proportion of the acetic acid to carbon monoxide and carbon dioxide, among others, which are present in the exhaust gas. be lost. Under some conditions, losses of acetic acid by such oxidation (hereinafter sometimes referred to as "combustion" for convenience) can be as high as 40 parts for each part (1) part of terephthalic acid produced, and are thus significant. This may also lead to increased costs. Furthermore,
As mentioned above, although significant amounts of 4-carboxybenzaldehyde in terephthalic acid contribute to inferior quality polyesters, certain conditions used to reduce this impurity to very low levels can make it acceptable. Combustion of acetic acid to a high degree may occur. 4- in terephthalic acid acceptable for use in the direct esterification of ethylene glycol in the production of polyethylene terephthalate.
The proportion of carboxybenzaldehyde will, of course, depend on the quality of the product polyester with respect to what it is then intended to convert into fibers or other objects.

However, in general, to produce polyethylene terephthalate of acceptable quality, the 4-
The proportion of carboxybenzaldehyde is approximately 1000 ppm
It was found that the value should not be exceeded. As an evaluation of acetic acid lost through combustion, the concentrations of carbon monoxide and carbon dioxide in the exhaust gas are measured, and the burnup is determined by supplying p-xylene 1
It is expressed as the number of moles of [CO+CO2] produced per mole.

The burn-up value is kept below 0.65, preferably below 0.5. In this case, by carefully controlling the parameters of the oxidation reaction, excess acetic acid can be used to produce polyethylene terephthalate by esterifying it directly with ethylene glycol without burning it off, and often after a simple washing process. We have devised a method that allows us to obtain terephthalic acid suitable for

According to this invention, an acetic acid solvent consisting of acetic acid and water and p
- introducing a liquid phase mixture of xylene containing a catalyst into the oxidation reaction zone, and adjusting the ratio of acetic acid solvent in the oxidation reaction zone to p-xylene fed to the oxidation reaction zone from 4:1 to 7:1; (by weight), bubble oxygen-containing gas into the mixture in the oxidation reaction zone, maintain the temperature of the mixture in the oxidation reaction zone within the range of 160-180°C, The proportion of water in the acid solvent is maintained within the range of 1-7 Wt%, and the proportion of cobalt is between 1200 and 120% of the acetic acid solvent in the reaction mixture of the oxidation reaction zone when the catalyst is a mixture of cobalt compounds, manganese compounds and bromides. 2500 weight P
pm, and the proportion of manganese is 1 to 3 of the weight of cobalt.
0 wt%, the proportion of bromide is at least 15 times the total weight of cobalt and manganese, and the continuous terephthalic acid production process is characterized in that the average residence time of the mixture in the oxidation reaction zone is between 1 and 2 hours. provided.

The acetic acid solvent consists of acetic acid and a small amount of water, and the amount of water is controlled within the range of 1-7 wt% in the reaction mixture in the reaction zone.

The oxygen-containing gas has a relatively low oxygen content, e.g. 3% by weight of oxygen.
The following mixture of oxygen and an inert gas (eg nitrogen) is preferred.

Air or a mixture of air and nitrogen or oxygen with an oxygen content of 10 to 30% is particularly suitable. The temperature at which the oxidation reaction is carried out in the method of this invention is 160-180°C.
It is within the range of At temperatures below 160'C, the proportion of 4-carboxybenzaldehyde in terephthalic acid increases to unacceptably high levels, while at temperatures above 180'C the burnup increases disproportionately and by-products The proportion of objects also increases beyond expectations.

As far as the catalyst used in the process of the invention is concerned, the proportion of cobalt is 1% of the acetic acid present in the reaction mixture of the oxidation reaction zone.
It should be in the range of 200-2500 ppm.

At lower concentrations the proportion of 4-carboxybenzaldehyde in terephthalic acid rises to unacceptable levels, while at higher concentrations the burn-up of acetic acid exceeds what is acceptable for the economic production of terephthalic acid. and increase. The proportion of manganese in the catalyst used in the process of this invention is 1 to 30 wt%, preferably 1 to 20 wt% of the weight of cobalt.
should be within the range of

At manganese proportions exceeding 30 wt% of cobalt, the proportion of 4-carboxybenzaldehyde in terephthalic acid begins to rise to unacceptable levels, and the proportion of p-toluic acid, another product of incomplete oxidation, begins to rise to unacceptable levels. The proportion also begins to rise, while the proportion of both impurities mentioned above also rises very rapidly when the proportion of manganese falls below 1 Wt% of cobalt. The proportion of bromide in the catalyst used in the process of this invention should be at least one times the combined weight of cobalt and manganese.

Below this value, the proportion of 4-carboxybenzaldehyde in terephthalic acid begins to rise to an unacceptable level, and the proportion of p-toluic acid also begins to rise. Changing the bromide concentration has no effect on acetic acid burn-up. Therefore, in this invention, there is no upper limit on the bromide content with respect to the 4-carboxybenzaldehyde content in the produced terephthalic acid or the burnup of acetic acid. On the other hand, if the proportion of bromide is too high, bromide-containing impurities may be introduced into the terephthalic acid and cause excessive corrosion of metal equipment and increased costs due to bromide loss. Therefore, the level of bromide is preferably less than 5.0 times, more preferably less than 2 times, the total a weight of cobalt and manganese.
The ratio of the acetic acid solvent in the reaction mixture in the oxidation reaction zone to the p-xylene fed to the oxidation reaction zone in the method of this invention is 4.
:1 to 7:1 (by weight).

Ratios below 4:1 increase the proportion of 4-carboxybenzaldehyde in the terephthalic acid to unacceptably high levels, while ratios above 7:1 tend to increase the burn-up of acetic acid.

Furthermore, the use of larger quantities of acetic acid, in addition to its own high costs, requires the use of larger equipment for the production of terephthalic acid as well as for the recovery of acetic acid, thereby reducing the investment cost of the process. increases. The proportion of water in the reaction mixture in the oxidation reaction zone of the inventive process is maintained within the limits of 1 to 7% by weight of acetic acid solvent in the mixture.

Above 7 wt%, the proportion of 4-carboxybenzaldehyde in the resulting terephthalic acid increases to an unacceptable level, and the proportion of p-toluic acid also begins to rise, while below 1 wt%, the acetic acid burn-up increases beyond what is acceptable for economical production of terephthalic acid.

The average residence time of the oxidation reaction mixture in the oxidation reaction zone of the process of this invention is between 112 and 2 hours. Below 112 hours, the proportion of 4-carboxybenzaldehyde in the resulting terephthalic acid rises to an unacceptable level, and the level of p-toluic acid also begins to rise, while above 2 hours, the acetic acid burns off. The concentration increases beyond what is acceptable for economical production of terephthalic acid.

The average residence time is preferably 40 to 10 minutes. In the process of this invention, the p-xylene and acetic acid solvents in the oxidation reaction zone are substantially in liquid phase and are therefore maintained at a pressure sufficient to maintain this liquid state at the reaction temperature.

Although higher pressures can of course be used, there is generally no advantage to using significantly higher pressures. Generally, pressures in the range of 5 to 2 bars per bar are suitable, although higher pressures can be used, for example up to (4) bars absolute. The terephthalic acid present as a solid in the liquid reaction mixture can be separated from the catalyst and the liquid consisting mainly of acetic acid solvent by conventional methods, for example by filtration, in particular by centrifugation.

It is advantageous to carry out the separation at elevated temperatures, ie above 90° C., and when the separation is carried out at temperatures above the boiling point of the liquid reaction mixture at normal pressure, the separation must be carried out under pressure. At least a portion of the mother liquor recovered from the separation of terephthalic acid can be recycled to the oxidation reaction zone along with fresh p-xylene. However, to maintain acceptable levels of byproducts in the reaction mixture, it is common to add a percentage of fresh acetic acid to the recycled mother liquor. Fresh acetic acid can be recovered by distillation of the mother liquor that is not recycled. The catalyst can also be recovered in any suitable manner from the portion of the mother liquor that is not recycled. The level of water (product of the oxidation reaction) in the reaction mixture is
By distilling off water together with acetic acid during the reaction,
Can be controlled. The rate at which the oxygen-containing gas is passed through the liquid reaction mixture in the oxidation reaction zone is such that the oxygen concentration in the exhaust gas is below a concentration that could result in a hazardous situation, especially one due to the formation of an explosive mixture. speed. Generally, the upper limit of the oxygen concentration in the exhaust gas from the oxidation reaction zone is a value not exceeding about 8V01%. However, it is preferred that a significant concentration of oxygen be present in the exhaust gas. The reason for this is that if little or no oxygen is present in the exhaust gas, insufficient oxidation results in too high a proportion of impurities. Therefore, it is preferable that the oxygen concentration in the exhaust gas is at least 3v01%. 3-5
An exhaust gas oxygen concentration of v01% is extremely favorable.

The cobalt and manganese compounds used in the catalyst are preferably organic or inorganic salts of these metals.

Metal salts should be soluble in acetic acid. Particularly suitable cobalt salts are bromides and salts of lower (C1-C8) aliphatic carboxylic acids, especially acetic acid, and of aromatic carboxylic acids (eg toluic acid, terephthalic acid). Cobalt naphthenate may also be used. Particularly suitable manganese salts are the salts of lower (C, to C8) aliphatic carboxylic acids, especially acetic acid, and the salts of aromatic carboxylic acids (eg toluic acid, terephthalic acid). The bromine used in the catalyst is preferably elemental bromine, cobalt bromide or especially hydrobromic acid.

In general, the use of alkali metal bromides, such as sodium bromide, is not preferred because it increases the proportion of 4-carboxybenzaldehyde compared to other bromides. The terephthalic acid produced according to the oxidation conditions of the inventive process is of high purity and therefore requires complex and expensive chemical treatments (e.g. British Patent No. 99
The catalytic hydrogenation method described in the Specification of No. 47 is not required.

Instead, relatively simple and low cost processing is sufficient to produce terephthalic acid of the purity desired for the production of polyethylene terephthalate polymers from the oxidation reaction of the process of this invention. A particularly suitable treatment method is described in British Patent Application No. 149.
64/75, in which, after stopping the passage of oxygen-containing gas into the oxidation reaction mixture, the mixture is heated to the average temperature of the reaction mixture during the passage of the oxygen-containing gas. and 25° C. above for at least 112 hours, and then the solid terephthalic acid is separated from the mother liquor. Thus, according to a particularly preferred embodiment of the invention, the acetic acid A liquid phase mixture consisting of a solvent and p-xylene and containing a catalyst is introduced into the oxidation reaction zone, and the ratio of the acetic acid solvent in the reaction mixture in the reaction zone to the p-xylene supplied to the reaction zone is set to 4 by weight.
:1 to 7:1, oxygen-containing gas is bubbled through the mixture in the oxidation reaction zone, and the mixture in the oxidation reaction zone is heated to 160-180°C for an average residence time of 112-2 hours.
, remove exhaust gas from the reaction zone, and transfer the L reaction mixture to a holding zone where the oxidation reaction mixture is heated at a temperature of 50° C. below and 25° C. above the average temperature of the oxidation reaction zone. 1. A continuous process for the preparation of terephthalic acid comprising: maintaining the inlet mixture for an average holding time of at least 11 days at a temperature between and then transferring the reaction mixture to a separation zone to separate solid terephthalic acid: an oxidation reaction. The reaction mixture in the zone is maintained such that the proportion of water is within the range of 1 to 7 wt% of the acetic acid solvent in the reaction mixture, the catalyst is a mixture of cobalt compounds, manganese compounds and bromides, and the proportion of cobalt is 1200-2500 weight P of acetic acid solvent present in the reaction mixture in the oxidation reaction zone
pm, and the proportion of manganese is 1 to 1 of the weight of the cobalt.
30 wt% and the proportion of bromide is at least 1 times the combined weight of cobalt and manganese.

Because of the temperature at which the holding treatment is carried out, the holding treatment is usually carried out at a pressure sufficient to maintain the reaction mixture in the liquid phase, for example at a pressure below 50 bar.

If desired, the reaction mixture can be kept stirred by any suitable means during this holding process. Since the oxidation reaction is usually carried out substantially isothermally, the holding treatment is carried out on the reaction mixture while it is still at the oxidation reaction temperature or slightly below (e.g. less than 20° C.) the oxidation reaction temperature. is generally preferred. (The temperature drop mentioned above occurs due to heat loss to external environmental objects during the holding time or during transfer to the holding zone, for example). However, within the limits mentioned above, it is within the scope of this invention to heat the reaction mixture to a temperature above that of the oxidation reaction or to actively cool it to a somewhat lower temperature. The holding time is at least 1 h hour, suitably 112-6 hours, preferably 112-3 hours. Separation of terephthalic acid after the holding treatment is carried out by the method described above. After separation, the terephthalic acid can be washed in a filter or centrifuge with mother liquor, fresh solvent or water. The separated terephthalic acid (with or without holding treatment) is reslurried with mother liquor or fresh acetic acid solvent, and the resulting slurry is optionally heated for a period of time at elevated temperature. It may be advantageous to retain and re-isolate the terephthalic acid.

The temperature at which the reslurried terephthalic acid is held is suitably in the same range as, but not limited to, the temperature used for the holding operation prior to separation, and is approximately 100
Temperatures of 0.degree. C. or higher may be used. The holding operation of reslurried terephthalic acid is usually carried out at a pressure at least sufficient to maintain the slurry in the liquid phase, for example up to 50 bar. The retention time is usually within 6 hours, for example 112-6
time, preferably 1 to 3 hours. The acetic acid solvent used in the filter washing operation or the washing operation by reslurrying with fresh solvent may contain an amount of water, such as less than 10 wt% water.

According to the method of this invention, it is possible to obtain high quality terephthalic acid without excessive and expensive consumption of acetic acid due to combustion.

After relatively simple and inexpensive processing, the terephthalic acid becomes suitable for immediate esterification of ethylene glycol in the production of polyethylene terephthalate. For this purpose, the terephthalic acid thus obtained by the process of the invention is disclosed in British Patent No. 1,062,482
It has the same compatibility as "terephthalic acid" made by the air oxidation process of p-xylene at temperatures of 200° C. or higher and then purified by the catalytic hydrogenation process of British Patent No. 994,7. Furthermore, the method of the present invention has the following advantages:
There is less burning of acetic acid than in the process carried out in 'C. Furthermore, the proportion of 4-carboxybenzaldehyde in terephthalic acid obtained by the method of this invention is generally
lower than that obtained in an example of the method described in GB 1,249,198. The invention will be illustrated by the following examples.

Examples Tables 1, 10 and 11 show the oxidation conditions for the individual examples.

The oxidation reaction was generally carried out in an electrically heated titanium pressurized vessel equipped with a reflux condenser and a stirrer. A catalyst-containing mixture of p-xylene and acetic acid solvent was continuously fed to the reaction vessel at a constant rate. The contents of the vessel were maintained at the indicated temperature and pressure, and the volume of the reactants was kept constant by continuously withdrawing portions of the contents. The aqueous concentration of acetic acid solvent in the reaction mixture was controlled by adding acetic anhydride as needed to reduce the feed rate of acetic acid solvent. In all other examples except Example 14, the catalyst was cobalt acetate,
It was made by mixing manganese acetate and hydrochloric acid aqueous solution in appropriate proportions. The combustion of acetic acid during the oxidation reaction is
The concentration of carbon monoxide and carbon dioxide in the exhaust gas from the reactor is measured and calculated as the number of moles of [CO+CO2] produced/
Evaluation was made by expressing the numerical value of supplied [p-xylene] moles. Examples 1 to 14 and Comparative Examples A to R Oxidation conditions are shown in Table 1.

After steady state was achieved in the reactor, the liquid slurry effluent from the reactor was cooled to 20-25°C and the solid terephthalic acid was filtered off, washed with water and dried. Isolated terephthalic acid was analyzed for 4-carboxybenzaldehyde and p-toluic acid.

The results are incorporated in Tables 2-6. Table 2 shows temperature 170
The effect of cobalt concentration on °C, Table 3 shows the effect of the ratio of bromide to the sum of cobalt and manganese, Table 4 shows the effect of 17
Table 5 shows the effect of water concentration in the acetic acid solvent at 0°C, Table 6 shows the effect of temperature, and Table 6 shows the effect of the ratio of the acetic acid solvent to p-xylene (solvent ratio) to be supplied. These effects are further explained in FIGS. 1-5, respectively. The following abbreviations are used in the figures.

4CBA=4-carboxybenzaldehyde PTA=p
-Toluic acid PX = p-xylene Further results are shown in Tables 7-9.

Table 7 shows the effect of residence time, Table 8 shows the effect of manganese concentration, and Table 9 shows the effect of bromide type. Example 15 Oxidation conditions are shown in Table 10.

After steady state has been achieved in the reactor, the reactor. The liquid slurry effluent was transferred to a separate container where it was held at 160'C for 2 hours.

After cooling to 100°C, solid terephthalic acid was filtered off and reslurried with 3 times its weight of 95% acetic acid aqueous solution.
℃ and held at this temperature for 2 hours.

After cooling to 100'C, tele. Phthalic acid was filtered off and analyzed.

This terephthalic acid was then used to produce polyethylene terephthalate by direct esterification with ethylene glycol using standard procedures. The properties of this polyethylene terephthalate were then evaluated. The results are shown in Table 10. Comparative Example S The oxidation conditions are as shown in Table 10 and are substantially the same as those disclosed in GB 1,062,48.

After the initial isolation of terephthalic acid, a portion (S1) was analyzed,
Then, the properties of the polymer were evaluated by using it in the production of polyethylene terephthalate (according to the procedure of Example 15).
.

The remaining part (S2) is dissolved in water at 280°C and hydrogenated at 280°C in the presence of a hydrogenation catalyst (see GB 994,769) to crystallize terephthalic acid,
Filtered off at 00°C, washed by reslurriing with 100°C water, and refiltered. The product was analyzed as described above and used in the production of polyethylene terephthalate. The results are shown in Table 10. These results indicate that the polymers obtained from terephthalic acid produced by the method of the present invention have very similar properties compared to the polymers made from terephthalic acid that has been purified by a separate hydrogenation step. It shows that it has. Examples 16-17 Oxidation conditions are shown in Table 11.

After steady state was achieved in the reactor, the reaction mixture effluent from the reactor was held in a separate vessel at the temperature and time specified in Table 11.

This was then cooled to 20°C, washed with 20°C water to remove the mother liquor, dried, and analyzed. The terephthalic acid thus isolated was reslurried with 95 wt% acetic acid aqueous solution (ratios, temperatures and times are shown in Table 11).
. It was then filtered and analyzed at the temperatures shown in Table 11.

[Brief explanation of the drawing]

The attached figure is a graph curve showing the effect of each parameter of the oxidation reaction conditions in the method of this invention. Figure 1 shows the effect of catalyst concentration at 170°C, Figure 2 shows the effect of catalyst concentration at 170°C.
FIG. 3 shows the effect of bromide/(cobalt+manganese) ratio at 70° C., FIG. 4 shows the effect of moisture at 170° C., FIG. 4 shows the effect of temperature, and FIG. 5 shows the effect of solvent ratio. 4CBA=4-carboxybenzaldehyde, P
TA=p-toluic acid, PX: p-xylene.

Claims (1)

[Scope of Claims] 1. A liquid phase mixture of an acetic acid solvent consisting of acetic acid and water and p-xylene, which contains a mixture of a cobalt compound, a manganese compound and a bromide as a catalyst, is introduced into an oxidation reaction zone, and an oxidation reaction is carried out. In a continuous process for the production of terephthalic acid consisting of bubbling an oxygen-containing gas through the mixture in the zone and separating solid terephthalic acid from the mixture;
- maintain the ratio of water to xylene in the range 4:1 to 7:1 by weight, maintain the temperature of the mixture in the oxidation reaction zone between 160°C and 180°C, and maintain the proportion of water in the acetic acid solvent in the mixture. The proportion of cobalt was maintained between 1 wt% and 7 wt% and the proportion of cobalt was between 1200 and 1200% of the acetic acid solvent present in the reaction mixture.
2500 ppm by weight, the proportion of manganese is between 1 wt% and 30 wt% of the weight of cobalt, the proportion of bromide is at least 1.5 times the combined weight of cobalt and manganese, and the average residence time of the mixture in the oxidation reaction zone. A continuous method for producing terephthalic acid, characterized in that the production time is 1/2 hour to 2 hours. 2 Claim 1 characterized in that the proportion of bromide is 5.0 times or less the total weight of cobalt and manganese
the method of. 3. Process according to claim 1 or 2, characterized in that the bromide is elemental bromine, cobalt bromide or hydrobromic acid. 4. The method according to any one of claims 1 to 3, characterized in that the oxygen concentration in the exhaust gas from the oxidation reaction zone is at least 3 vol%. 5 After stopping the supply of oxygen-containing gas into the oxidation reaction mixture,
The mixture is maintained at a temperature between 50° C. below and 25° C. above the average temperature of the reaction mixture during the oxygen-containing gas bubbling period for at least 1/2 hour, after which solid terephthalic acid is separated from the mother liquor. The method according to any one of claims 1 to 4, characterized in that: 6. The separated terephthalic acid was reslurried with the mother liquor of the reaction mixture or fresh acetic acid solvent, and the slurry was
The method according to any one of claims 1 to 5, characterized in that terephthalic acid is re-isolated after being maintained at a temperature of 0° C. or higher for a certain period of time.