| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | January 19, 1993 |
| PATENT TITLE |
Process for producing purified terephthalic acid |
| PATENT ABSTRACT | In a process for producing purified terephthalic acid which comprises catalytically oxidizing p-xylene in liquid phase to produce crude terephthalic acid containing 4-carboxybenzaldehyde as a main impurity therein, and treating the crude terephthalic acid with hydrogen in the presence of hydrogenation catalyst in a reaction vessel, thereby to produce purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed range in a stationary manner, there is provided an improvement which makes it possible for the treatment to reach the stationary state promptly after partial exchange of deactivated catalyst for a new one. The improvement comprises feeding into the reactor crude terephthalic acid which contains 4-carboxybenzaldehyde in an amount larger than that in the stationary state of the treatment, and treating the terephthalic acid until the treatment reaches the stationay state |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | July 3, 1991 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. In a process for producing purified terephthalic acid which comprises catalytically oxidizing p-xylene in liquid phase to produce crude terephthalic acid containing 4-carboxybenzaldehyde as a main impurity therein, and treating the crude terephthalic acid with hydrogen in the presence of a palladium, ruthenium, rhodium, osmium, iridium, platinum, platinum black, palladium black, iron, or cobalt-nickel hydrogenation catalyst in a reaction vessel, thereby to produce purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed range in a stationary manner at a temperature of 255.degree.-300.degree. C. and under a pressure of 10-110 Kg/cm.sup.2 with a partial pressure of hydrogen being 0.5-20 Kg/cm.sup.2, the improvement comprising exchanging the hydrogenation catalyst in the reaction vessel in part for a new one when the catalyst is deactivated, and thereafter feeding into the reactor crude terephthalic acid which contains 4-carboxybenzaldehyde in an amount larger than that in the stationary state of the treatment for restarting the treatment of the crude terephthalic acid, and treating the terephthalic acid until the treatment reaches the stationary state. 2. The improvement as claimed in claim 1 wherein 20-70% of the deactivated catalyst is exchanged for a new one. 3. The improvement as claimed in claim 1 wherein the hydrogenation catalyst is granulated palladium supported on activated carbon. 4. In a process for producing purified terephthalic acid which comprises catalytically oxidizing p-xylene in liquid phase to produce crude terephthalic acid containing 4-carboxybenzaldehyde as a main impurity therein, and treating the crude terephthalic acid with hydrogen in the presence of a palladium, ruthenium, rhodium, osmium, iridium, platinum, platinum black, palladium black, iron, or cobalt-nickel hydrogenation catalyst in a reaction vessel, thereby to produce purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed range in a stationary manner at a temperature of 255.degree.-300.degree. C. and under a pressure of 10-110 Kg/cm.sup.2 with a partial pressure of hydrogen being 0.5-20 Kg/cm.sup.2, the improvement comprising exchanging the hydrogenation catalyst in the reaction vessel in part for a new one when the catalyst is deactivated, and thereafter treating the crude terephthalic acid under a partial pressure of hydrogen smaller than that in the stationary state. 5. The improvement as claimed in claim 4 wherein 20-70% of the deactivated catalyst is exchanged for a new one. 6. The improvement as claimed in claim 4 wherein the hydrogenation catalyst is granulated palladium supported on activated carbon. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention relates to a process for producing purified terephthalic acid. More particularly, it relates to an improvement in the process for producing purified terephthalic acid containing 4-carboxybenzaldehyde as a main impurity in an amount of fixed small range in a stationary manner. DESCRIPTION OF THE PRIOR ART Purified terephthalic acid is needed for the production of polyester fibers. Such purified terephthalic acid has been heretofor produced by, for example, such a process as described in Japanese Patent Publication No. 41-16860. According to the process, p-xylene is catalytically oxidized in liquid phase to produce crude terephthalic acid usually containing 4-carboxybenzaldehyde as a main impurity in an amount of 0.1-0.4% by weight, the crude terephthalic acid is fed as an aqueous slurry into a reaction vessel and made into an aqueous solution under a high temperature and pressure, and is treated with a hydrogenation catalyst exemplified by a granulated palladium catalyst supported on activated carbon. When the treatment of the crude terephthalic acid is carried out in a stationary manner, the resultant purified terephthalic acid usually contains 1-25 ppm of 4-carboxybenzaldehyde. However, the efficiency of treating the crude terephthalic acid to hydrogenate the 4-carboxybenzaldehyde to p-methylbenzoic acid is reduced with time in the above process for the production of purified terephthalic acid by various reasons, mainly by reason of deactivation of hydrogenation catalyst. Accordingly, the catalyst is usually exchanged at regular intervals in the process. In general, a hydrogenation catalyst has a high initial activity. Thus, in working the above process for the purified terephthalic acid, when a catalyst is deactivated and such a deactivated catalyst is entirely exchanged for a new one, undesirable by-products are produced by excessive hydrogenation reaction, but also an excessive amount of 4-carboxybenzaldehyde is reduced, thereby to cause a decrease in stability of purity of the produced terephthalic acid. Moreover, at the restart of the treatment, the crude terephthalic acid is adsorbed on the activated carbon of the catalyst to generate heat, so that the catalyst is liable to break to pieces. The broken catalyst may mix with the resultant terephthalic acid. Therefore, for the purpose of lengthening the life of hydrogenation catalyst used in the process, there is disclosed in East German Patent No. 212,162 that the deactivated catalyst in a reaction vessel is not entirely exchanged for a new one, but only a part of the catalyst is exchanged for a new one while the deactivated catalyst is again used as it is together with the exchanged new one. According to this method, the life of catalyst may be lengthened, but there is needed much time for the stationary state to be reached in which the treatment of the crude terephthalic acid provides in a stationary manner purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed small range after the partial exchange of catalyst. Such a prolonged time to reach the stationary state brings about a great deal of loss in raw materials as well as undesirable production of terephthalic acid outside the standard. BRIEF SUMMARY OF THE INVENTION Therefore, it is an object of the invention to provide an improvement in a process for the production of purified terephthalic acid which comprises treating crude terephthalic acid with hydrogen in the presence of hydrogenation catalyst, the improvement making it possible for the treatment to reach the stationary state promptly after partial exchange of the catalyst. In accordance with the invention, there is provided an improvement in a process for producing purified terephthalic acid which comprises catalytically oxidizing p-xylene in liquid phase to produce crude terephthalic acid containing 4-carboxybenzaldehyde as a main impurity therein, and treating the crude terephthalic acid with hydrogen in the presence of a hydrogenation catalyst in a reaction vessel, thereby to produce purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed range in a stationary manner at a temperature of 255.degree.-300.degree. C. and under a pressure of 10-110 Kg/cm.sup.2 with a partial pressure of hydrogen being 0.5-20 Kg/cm.sup.2, the improvement comprising exchanging the catalyst in the reaction vessel in part for a new one when the catalyst is deactivated, and thereafter feeding into the reactor crude terephthalic acid which contains 4-carboxybenzaldehyde in an amount larger than that in the stationary state of the treatment for restarting the treatment of the crude terephthalic acid, and treating the terephthalic acid until the treatment reaches the stationary state. There is also provided a further improvement of the invention in the process, which comprises exchanging the catalyst in the reaction vessel in part for a new one when the catalyst is deactivated, and thereafter treating the crude terephthalic acid under a partial pressure of hydrogen smaller than that in the stationary state. DETAILED DESCRIPTION OF THE INVENTION Herein the specification, by the stationary state of treatment is meant the state of treatment which provides in a stable and stationary manner purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed small range in the treatment of crude terephthalic acid containing 4-carboxybenzaldehyde as a main impurity with hydrogen in the presence of hydrogenation catalyst to produce the purified terephthalic acid. The crude terephthalic acid is obtained by the catalytic oxidation of p-xylene in liquid phase. The deactivation of hydrogenation catalyst may be detected by an increase of content of 4-carboxybenzaldehyde in the resultant purified terephthalic acid. Furthermore, the quality of terephthalic acid produced is usually related with transmittance at 340 nm. Thus, in the industrial production of purified terephthalic acid, the purity of terephthalic acid as the product is determined by content of 4-carboxybenzaldehyde and transmittance at 340 nm. These control values are maintained within a fixed range when the treatment is carried out in a stationary manner. When the control values have come outside the fixed range, it is necessary that the catalyst in the reaction vessel be partly exchanged for a new one to increase the efficiency of the treatment. After the partial exchange of catalyst and when the hydrogenation of crude terephthalic acid has been restarted, the resultant purified terephthalic acid usually has a very small content of 4-carboxybenzaldehyde on account of high activity of catalyst, and accordingly the resultant purified terephthalic acid has a high transmittance at 340 nm. Although depending upon the degree of deactivation of catalyst, usually 20-70% of the deactivated catalyst in a reaction vessel is exchanged for a new one. After the deactivated catalyst in the reaction vessel has been in part exchanged with a new one in a manner as above, an aqueous slurry of crude terephthalic acid is fed into the reaction vessel and made into an aqueous solution under a high temperature and pressure to restart the treatment. According to the invention, crude terephthalic acid containing 4-carboxybenzaldehyde in a larger amount than that in the stationary state is fed into the reaction vessel after the reaction is stabilized until the stationary state is reached. Alternatively, the treatment is carried out under a partial pressure of hydrogen smaller than that in the stationary state. Of course, crude terephthalic acid containing 4-carboxybenzaldehyde in a larger amount than that in the stationary state may be fed into the reaction vessel and treated under a partial pressure of hydrogen smaller than that in the stationary state until the stationary state of the reaction is reached. This operation reduces the time in which the reaction reaches the stationary state. In the stationary state of the treatment, the crude terephthalic acid is usually fed into the reaction vessel as an aqueous solution containing 24-30% by weight of terephthalic acid. The hydrogenation treatment is carried out usually at temperatures of 255.degree.-300.degree. C. under pressures of 10-110 Kg/cm.sup.2 and a partial pressure of hydrogen of 0.5-20 Kg/cm.sup.2. The hydrogenation catalyst used includes, for example, palladium, ruthenium, rhodium, osmium, iridium, platinum, platinum black, palladium black, iron, or cobalt-nickel, each supported on activated carbon |
| PATENT EXAMPLES | available on request |
| PATENT PHOTOCOPY | available on request |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |