JPH0525867B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention involves oxidizing P-xylol and/or P-toluic acid methyl ester (PTE) with an oxygen-containing gas at elevated temperature and pressure in the presence of an oxidation catalyst containing heavy metals, and converting the oxidized mixture into Esterification with methanol at elevated temperature and elevated pressure, fractionating the crude ester into PTE-enriched fraction, residual fraction and crude DMT, which are returned to the oxidation step, and converting the crude DMT into crude DMT:water Amount ratio 3:1~
0.1 to 1, continuously hydrolyzed with water at a temperature of 350 to 180 °C and the pressure required to maintain the liquid phase,
The present invention relates to a process for producing terephthalic acid from dimethyl terephthalate as an intermediate by crystallizing TPS at a temperature of 300-100° C., replacing the mother liquor with demineralized water and obtaining TPS from the reaction mixture. A process for producing TPS from DMT as an intermediate, in which continuous hydrolysis is carried out in two stages starting from crude DMT with a limited content of oxidized intermediates and other by-products. is publicly known (West German Patent Application No. 2916197). When this method is preferably carried out,
DMT first in downstream along with water for hydrolysis
flows through the reactor. For example 1:3 crude DMT:
With the proportion of water, a conversion of about 70% is set at the pressure and temperature conditions mentioned above. produced as the main product
TPS must be separated from the impurity-containing mother liquor by crystallization, washing, and solid-liquid separation operations from monomethyl terephthalate (MMT). Dissolution occurring in the mother liquor with additional crystallization
A portion of MMT is obtained and returned to the method.
The other part is lost upon removal of the isomer of terephthalic acid. Methanol-water containing MMT. The separation of methanol by pressure distillation applied to a mixture is known from the above-mentioned publication, but it is obtained for a slightly different purpose, namely an insufficient conversion rate.
This is to make the amount of MMT available by returning the bottom fraction, which consists mainly of water and TPS, to hydrolysis. In another embodiment of this known process, a part of the reaction solution is removed in the downflow hydrolysis step of the two-stage hydrolysis process, so that methanol is removed from the equilibrium reaction and the reaction solution is returned again to the downflow hydrolysis step. be done. In the specification of West German Patent Application No. 1618503,
A method for producing TPS from alkyl terephthalates is described, in which aqueous solutions of alkyl terephthalates are heated under autogenous pressure to
The reaction is carried out with water at a temperature in the range of 0.degree. C. until the hydrolysis is essentially complete, and the terephthalic acid is separated from the resulting hydrolyzate. For industrial implementation of the hydrolysis reaction, cascade stirred kettles, ie vibrating sections under extremely high pressure, are required. This method is only suitable for the hydrolysis of pure DMT. For example, when there is about 2-3% by weight methanol in the liquid at 270°C, the conversion is 95%. Cooling of this reaction mixture to a temperature of 140° C. or below produces TPS significantly containing MMT. In order to discharge the methanol produced, a very large amount of water must also be removed. The invention is based on Witten's DMT oxidation process, which is distinguished by low energy requirements, mild reaction conditions and relatively low requirements on the strength of the reactor and vessel materials. We start from the challenge of manufacturing TPS as a. In this case, the methanol must only act as a process auxiliary that remains during the process and only losses must be replenished. A conversion rate of 90% to TPS can be achieved by forming the hydrolysis reaction as a reactive distillation.
Or it must be raised even higher. The resulting methanol is removed at high pressure and temperature levels and the energy transferred via a heat exchanger should be used, for example, for fractionating the crude ester. Furthermore, according to the prior art, mother liquors, suspensions and revertants, in particular MMT revertant fluids (West German patent application no.
2916197) should also be saved. The steps of the process involving methanol distillation operated under pressure require an additional amount of energy for reheating the mother liquor (Pos. 89 or 120) or crystal slurry into the downflow reactor. Significant energy savings can be achieved if the upstream water produced in the countercurrent reactor (Pos. 27) no longer needs to be distilled. This problem involves introducing heated steam at 350 to 180°C from the bottom of a multi-stage hydrolysis reactor, introducing heated crude DMT at 350 to 140°C from the top of the same reactor, and introducing 90% or more of heated steam from the bottom to the reactor. removing the reaction mixture as a liquid phase or a solid phase suspended in a liquid phase at an exchange rate of
Methanol-water comes from the head. Heat the mixture to a temperature of 300~
According to the method mentioned at the outset, the mixture is removed at 150° C., the mixture is condensed, a part of the condensate is fed as reflux through the head of the hydrolysis reactor, and the other part is returned to the process. It will be resolved. This allows the hydrolysis reaction to be carried out for the first time under similar process-technical conditions and in a device similar in principle to the reverse reaction of the esterification reaction applied in Witten's DTM method, thereby allowing This allows for simplification and standardization of the entire method. According to the invention, terephthalic acid was used from 90 to
Produced depending on the by-product content of crude DMT of 99.9%, the terephthalic acid is processed into fibers by direct esterification with ethylene glycol, followed by polycondensation.
It can be converted into a polyester suitable for the production of endless systems, sheets and molded articles. It is particularly advantageous that no reversion of the mother liquor containing MMT and other by-products to the hydrolysis is contemplated. Preferably, methanol-water removed from the top of the hydrolysis reactor. The mixture is used to heat the reboiler for crude ester distillation. This makes it possible to carry out the distillation of the crude ester in an energetically advantageous manner. The reaction mixture taken off from the hydrolysis reaction contains terephthalic acid completely dissolved in 1.5 to 5 parts of water in the preferred method of operation. Methanol-water obtained from the head of the hydrolysis reactor. The mixture contains 10-90% by weight, preferably 30-90% by weight.
The extremely high methanol content of 60% by weight allows energetically advantageous separation and reuse. By forming a hydrolysis reaction of 2 to 50 stages, preferably 10 to 40 stages, the hydrolysis can be carried out in a single reactor. However, instead of a multistage reactor it is also possible to carry out the hydrolysis reaction in a cascade reactor. By forming a washing stage for the reaction mixture leaving the hydrolysis reactor, exchange of the mother liquor after single or multi-stage crystallization in 1 to 10 hydrocyclone stations is achieved with simultaneous concentration. An even stronger concentration effect is achieved by using a centrifuge station in the washing step. A countercurrent washer, which can also be applied in the washing step to the same effect, replaces the mother liquor quantitatively by the washing liquid. The amount of methanol used per kg of terephthalic acid is less than 0.05 kg in the process according to the invention, the methanol being used in the esterification of the oxide and acting only as a process aid which undergoes the reaction to form the crude ester. After separation of the crude DMT from the crude ester, the methanol is separated again by hydrolysis to form methanol-water. This is produced as a mixture, which is used in the preferred mode of operation to operate the crude ester distillation evaporator. From there, a portion of the mixture is transferred to a methanol rectification step and pure methanol is returned to esterification (Figure 3). Another portion of the mixture is used as reflux to the hydrolysis reactor. Acetic acid is neither used as a reaction medium nor as a washing medium. Next, the method according to the present invention will be described with reference to FIGS.
This will be explained with reference to the figures and Examples 1a, 1b and 2. The water supplied as material stream 102 is heated to the evaporation temperature in the heat exchanger 2. Steam heated to approximately 260-290 °C is supplied from the bottom of reactor 3 at a pressure of approximately 45-80 bar, and conversely from the top at a pressure of 140-350 bar.
Crude DMT (stream 101) heated to 0.degree. C. is fed. The methanol produced from the hydrolysis reactor 3 is methanol-water. It is removed as a mixture (material stream 103) from the condenser 4 at a temperature of, for example, 240°C.
The reaction mixture forming at the bottom is partly conducted via the reboiler 1 and the other part is transferred as a material stream 104 via a throttle valve to two crystallizers 5 and 7 with a stirring device and condensers 6 and 8. be done. A portion of the condensate produced in heat exchangers 6 and 8 is removed, while the remainder is returned to crystallizers 5 and 7. The resulting suspension is transferred as wash water to a wash stage comprising three hydrocyclones 9 to 11 connected in series with a supply of heated water (material stream 108). After passing through the hydrocyclone, the purified suspension is transferred to a receiving vessel 12 equipped with a stirring device and further to a condenser 14.
and 16 are transferred to a continuously connected crystallization apparatus and cooled to 100°C. The suspension is then passed through a centrifuge 17 to produce terephthalic acid (material stream 107).
and filtrate (material stream 106). The mother liquor obtained in the washing step (stream 105) is removed. Terephthalic acid leaves the process via dryer 18. Steam from the dryer is condensed in heat exchanger 19 and collected together with the filtrate from centrifuge 17. The method implementation of FIG. 2 is also similar to that of FIG. Countercurrent cleaning tower 10 instead of hydrocyclone 10
A is provided. Liquid cyclones 9a and 1
1a is used only for concentration of solids in suspensions. FIG. 3 is a preferred embodiment of the method according to the invention for methanol circulation with energy recovery. Example 1a shows the procedure starting from about 99% crude DMT with a TAE content of 0.1% by weight and when the TPS is completely dissolved in the reactor stage. Purification takes place in a cyclone washing step. The following table shows the composition (Kg) of the very important product streams and some temperatures (0
°C) is listed:

[Table] Example 1b is about 99% with TAE content of 0.01% by weight
Starting from crude DMT. Other conditions are equal to those of Example 1a. In this case, the table below shows:

【table】

[Table] Example 2 is approximately 96% with TAE content of approximately 0.01% by weight.
% crude DMT and the procedure is shown when TPS is dissolved in the reactor step. The purification is designed as a countercurrent washer. You will get numbers like the following table:

【table】 [Brief explanation of the drawing]

1 and 2 are flow sheets for carrying out the method according to the invention, and FIG. 3 is a flow sheet for methanol circulation including energy recovery according to the method according to the invention. 3,3a... Hydrolysis reactor, 4,4a... Condenser, 5,5a; 7,7a... Crystallization device, 9,
9a,; 10; 11, 11a...liquid cyclone,
10a... Countercurrent cleaning tower.

Claims (1)

[Claims] 1 P-xylol and/or P-toluic acid methyl ester (PTE) is oxidized with an oxygen-containing gas at elevated temperature and pressure in the presence of an oxidation catalyst containing a heavy metal; The oxidation mixture is esterified at elevated temperature and pressure, and the crude ester is fractionated into a PTE-enriched fraction, a residual fraction, and crude DMT (dimethyl terephthalate), which is returned to the oxidation step. The crude DMT:water is continuously hydrolyzed with water at a ratio of 3:1 to 0.1:1, a temperature of 350 to 180℃, and the pressure required to maintain a liquid phase, and TPS (terephthalic acid) is hydrolyzed at a temperature of 300℃. In a process for producing terephthalic acid from dimethyl terephthalate as an intermediate by crystallizing at ~100°C, replacing the mother liquor with demineralized water, and obtaining TPS from the reaction mixture, heating steam at 350-180°C is added in multiple stages. Introduced from the bottom of the decomposition reactor, 350~140
Introducing heated crude DMT at °C from the top of the reactor,
and from the bottom the reaction mixture is taken out as a liquid phase or a solid phase suspended in the liquid phase at a conversion of 90% or more, and from the top the methanol-water mixture is taken out at a temperature of 300 to 150 °C, A process as described above, characterized in that the mixture is condensed, a part of the condensate is fed as reflux from the head of the hydrolysis reactor, and the other part is returned to the process. 2. The method according to claim 1, wherein the heat liberated by condensation is used for fractional distillation of the crude ester. 3. The method according to claim 1 or 2, using 99.9 to 90% crude DMT. 4. Process according to claim 3, wherein the reaction mixture withdrawn from the bottom of the hydrolysis reactor contains 1.5 to 5 parts, preferably 2 to 3 parts, of TPS completely dissolved in water. 5 The methanol-water mixture removed from the head of the hydrolysis reactor is 10 to 90% by weight, preferably 30% by weight.
5. The method of claim 4 having a methanol concentration of ~60% by weight. 6 Hydrolysis reaction in 2 to 50 steps, preferably 10 to 50 steps
6. A process according to claim 1, which is carried out in 40 stages and in a single reactor. 7. Process according to any one of claims 1 to 5, in which the hydrolysis reaction is carried out in a cascade of hydrolysis reactors, preferably 3 to 5 stages of hydrolysis reactors. 8. Claims 1 to 7, in which after crystallization of TPS, the mother liquor is replaced with demineralized water in 1 to 10 stages, preferably 3 to 6 stages of hydrocyclone stations.
The method described in any one of the preceding paragraphs. 9. Claims 1 to 7, in which after crystallization of TPS, the mother liquor is replaced with demineralized water in a countercurrent washing step.
The method described in any one of the preceding paragraphs. 10. Process according to any one of claims 1 to 7, characterized in that after crystallization of the TPS, the mother liquor is replaced with demineralized water in one or multiple centrifuge stations.