DE1212063B - Process for the continuous production of dimethyl terephthalate - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Tnt. CL:

C07c

German KL: 12 ο-14

Number: 1212063

File number: B 76411IV b / 12 ο

Filing date: April 20, 1964

Opening day: March 10, 1966

There are known processes for the continuous production of dimethyl terephthalate in which on an esterification column given terephthalic acid as a suspension and in countercurrent to it from below superheated methanol vapor is passed, whereby the terephthalic acid esterifies to dimethyl terephthalate and liquid dimethyl terephthalate is drawn off at the foot of the column (cf. USA. Patent 2 813 891). The water formed during the esterification becomes vaporous together with excess Methanol drawn off at the top of the column. Any high-boiling components carried along are separated from the steam and returned to the column. There is a major disadvantage in that, together with the dimethyl terephthalate, the esterification catalyst is not or not completely esterified terephthalic acid, impurities and optionally part of the suspending agent subtracted from. In order to obtain pure dimethyl terephthalate, one must then still have that at the foot of the Further purify impure dimethyl terephthalate accumulating in the esterification column.

It has now been found that dimethyl terephthalate is obtained by esterifying terephthalic acid with methanol, optionally in the presence of customary esterification catalysts, at elevated temperature and optionally under elevated pressure, if terephthalic acid is introduced into a dimethyl terephthalate melt containing monomethyl terephthalate and optionally customary esterification catalysts, with excess methanol vapor at the same time Temperature between 150 and 340 ⁰ C, preferably between 220 and 300 ⁰ C, and pressures between 1 and 100 ata, preferably between 1 and 20 ata, initiates the dimethyl terephthalate together with unreacted methanol and formed water in vapor form from the vapor mixture 10 to 90%, especially 30 to 80%, of the dimethyl terephthalate contained therein is ^{condensed at the top of a rectification column at a temperature between 140 and 300 0} C and the condensate is returned to the rectification column, then the rest of the column The vapor mixture is condensed outside the rectification column, this condensate is cooled to temperatures between -10 and 40 ° C. and the dimethyl terephthalate which has crystallized out is separated off.

A portion of the mother liquor obtained during the separation of the crystallized dimethyl terephthalate and / or the washing methanol which may be obtained, or a portion thereof, are expediently added to the solution or suspension of dimethyl terephthalate processes for the continuous production of
Dimethyl terephthalate

Applicant:

Aniline & Soda Factory in Baden

Aktiengesellschaft, Ludwigshafen / Rhein

Named as inventor:

Dr. Richard Sinn,

Dr. Hubert Kindler,

Dr. Hans Nienburg, Ludwigshafen / Rhine;

Dipl.-Ing. Ludwig Vogel, Frankenthal (Palatinate);

Dr. Albrecht Wallis, Ludwigshafen / Rhine

lat in methanol, which is obtained from the condensation of the vapors leaving the column, is recycled.

Particular advantages are obtained if the mother liquor obtained in the separation of the crystallized dimethyl terephthalate at a pressure that is about as high or z. B. 0.1 to 2 ata higher than the pressure in the esterification vessel, evaporates, passes the vapors containing essentially methanol and water at approximately the same pressure in a separating column, at the lower end of which withdraws water and at the upper end vaporous methanol methanol vapors optionally superheated to a temperature up to 34O ⁰ C and supplies it to the esterification vessel. Part of the concentrated mother liquor from the evaporator for the mother liquor is drawn off and either returned to the dimethyl terephthalate suspension or worked up separately to remove by-products.

In order to avoid the accumulation of parts from the esterification container, the higher boiling as the dimethyl terephthalate, continuously or intermittently small amounts of product removed and fed to a separate container. This product releases superheated methanol vapor directed, and the outflowing vapors, which contain most of the dimethyl terephthalate, are fed into the melt in the esterification tank or directly into the rectification column. The non-evaporable residue is discharged and discarded.

609 537/421

3 4

Another important advantage is that there is more methanol in the melt, which also has the effect of largely accelerating the reaction in one stage.
pure ester obtained, as all high-boiling impurities i _m esterification reactor 3 accumulate over Cleaners, the catalyst and not or only partly time heavier than the dimethyl terephthalate esterified as starting material in the esterification 5 boiling and not esterifiable impurities are retained container. The ester is therefore to the z. B. be introduced selectively with the terephthalic acid in contrast to the known processes. To remove them, it is continuously removed from the reaction mixture, so that a relatively high concentration of melt is maintained in or discontinuously in the esterification container through line 9 into container 8, overlying non-esterified products. There, by introducing overheated the can and thus the use of a Ge-methanol vapor through pipe 10, the volatile gas flow column is superfluous. The risk of clogging parts - essentially dimethyl terephthalate - from such a column, which also contains solids, vapors and is returned to the esterification vessel 3, is thus excluded. Another demonstration. What remains is the non-distillable part of the process can be seen in the fact that the 15 parts. A stirrer built into the container keeps dimethyl terephthalate from a methanolic solution in granular form and allows a solution to be obtained by crystallization. Use this to steam to dryness. The residue is then connected to an additional cleaning effect. practically free from dimethyl terephthalate.

The process is closer with reference to the drawing. A vapor mixture is generated from the return cooler 6

described. ao deducted, which consists essentially of dimethyl

The terephthalic acid is added at 1 and consists of terephthalate, methanol and water. It will Condensed through the lock 2 in the heatable esterification by cooling, for example by container 3 fed. When commissioning the man it has a cooled heat exchange surface liquid dimethyl is carried in this container. Condenterephthalate can be used particularly advantageously submitted in which the continuous or 25 perform sation by changing the steam mixture discontinuously added terephthalic acid sus- through line 12 in already formed condensate in is pending. If necessary, a container 13 is also introduced. The contents of this container Esterification catalyst, such as sulfuric acid, titanium dioxide, is thereby kept at the boiling point. The fumes Zinc oxide, aluminum oxide or calcium oxide, tensile (predominantly methanol) are deposited in the cooler 14. An agitator in this container is not beaten and returned in liquid form. One can required, since the container 13 introduced through the tube 4 is also at a lower temperature Overheated methanol vapor will keep enough stirring action than boiling temperature and keep the heat over possesses to dissipate the suspended terephthalic acid to cooling surfaces.

keep. However, the temperature at which the vapors condense

ren. The amount of methanol vapor introduced from dimethyl terephthalate, methanol and water

is adjusted so that the esterification tank is appropriately chosen so high that the ester in

leaving, excess methanol and the resulting liquid is completely dissolved,

Water vapor forming during the esterification is only possible, for example between 90 and 140 ^° C. One can

entrain a lot of dimethyl terephthalate, as caused by the vapors but also in a suspension of

the esterification reaction is newly formed, already crystallized dimethyl terephthalate occurs in the

Introduce more by the amount of the methanol-water mixture from the rectification column 5,

Dime flowing back to the esterification tank For extensive separation of the dimethyl

ethyl terephthalate. terephthalate from the condensate is cooled

The vapors leaving the esterification vessel, continuously or discontinuously in a known manner, are passed through the rectification column 5 to the return flow cooler 6 in a crystallizer 15 at temperatures. The steam mixture is there between -10 and +40 ⁰ C from. This cooling to a temperature which is above the melting point can be appropriately cooled by cooling surfaces or by partial melting of dimethyl terephthalate, i.e. above 141 ° C., by evaporating the solvent below. In this cooler, a reduced! Pressure can be made.
Part (advantageously 10 to 90%, in particular 30 to 50 of the crystallized dimethyl ester is finally 80%) of the dimethyl entrained in the vapor mixture borrowed with the aid of a separation device 16, e.g. terephthalate and flows as a return through the filter or a centrifuge, from the mother lonne 5 to the esterification tank 3 back. The residual liquor is separated off, if necessary with methanol, the borrowed vapors can flow to the condensation container via line 17, washed 13 and are condensed there. The rectifier 55 and removed at the exit point 30,
The purpose of column 5 is that the heavier than the esterification is generally attributed to compounds boiling from dimethyl terephthalate for the sake of energy saving with an esterification container if possible. This small excess of methanol, for example the 1,7- bis compounds, includes above all the monomethyl ester. 4.0 times, especially 1.5 to 3 times the

The esterification process can shot at normal pressure 60 the theoretical for the esterification of the

operate. However, it is advisable to work under the required amount of terephthalic acid,

increased pressure, e.g. B. up to 100 ata, preferably up to

to 20 ata. As a result, the partial sate usually sinks a suspension with a high concentration

pressure of the dimethyl terephthalate, resulting in less tration of solids. Such suspensions are

Ester is discharged. One can then find it difficult to promote without 65, as they tend to wean.

Difficulties temperatures up to 340 ^° C. In addition, they can crystallize out quickly

turn, whereby high reaction rates of the dimethyl terephthalate due to a supersaturation

be achieved. As a result of the increased pressure, the solution dissolves any impurities that

were distilled over together with the dimethyl terephthalate and methanol, crystallize out at the same time. In such cases you can find the solution - preferably before inserting the Crystallization in container 13, but possibly also in crystallization stage 15 itself - Dilute with more solvent. Fresh methanol can be used for this, but is preferred part of the mother liquor obtained in the subsequent separation of the crystals, provided that in this the impurities have not yet reached a too high concentration, or possibly that accruing washing methanol. These solutions flow from the separation device 16 via a pump 18 and line 19 to container 13 or 15.

The methanol contained in the mother liquor and the washing methanol are used insofar as these are used Products are not returned to the crystallization tank, expediently again for the esterification after separating the water and dissolved products. The separation of the water rectification is carried out in a manner known per se in a separating column 20, and with advantage under about the pressure at which the esterification is carried out. then the methanol leaving in vapor form from the top of the separating column 20 via line 21 can be used without Feed intermediate condensation to the esterification container 3 via the superheater 11. The water will taken at the foot of the column 20. The mother liquor contains parts of heavier, more volatile, partly solids dissolved, for example small amounts of dimethyl terephthalate and volatile esters of others Carboxylic acids, if they were added to terephthalic acid. Therefore it can be done by pump 18 brought to the required pressure mother liquor not introduced directly into the separating column 20 but is expediently in an evaporator 22 approximately at the pressure of the Separating column 2 (f largely evaporated; the vapors are then passed into the separating column via line 24 20 headed. In order to retain volatile solids, such as dimethyl terephthalate, these can be used Vapors before entering the separating column through a small, with little reflux from the reflux condensate 31 operated column 23 which is placed on the evaporator 22 lead.

The dissolved solids accumulate in the solution located in the evaporator 22. To a To maintain steady concentration, a certain amount of the solution must be continuous or can be removed intermittently via line 25. You can directly into the container 13 or in part Crystallizer 15 are recycled, provided the dissolved solid consists essentially of dimethyl terephthalate consists. The other part of the solution - with a higher content of contaminating solids the entire amount - can be worked up, for example, by having them all in one The crystallizer 26 cools, the precipitated solid is separated off on a filter 27 and the mother liquor brings it back into the evaporator 22 via pump 28.

If the mother liquor of the dimethyl terephthalate contains higher-boiling components, these will also be held back in column 23. If they are present in larger quantities, such as p-toluic acid ester, for example in the air oxidation of p-xylene, the feed to the column is advantageously carried out 23 in its middle part and pulls a methanol-water mixture from the evaporator at the top 22, via line 29 from the higher-boiling components. If necessary, they can be further processed immediately, for example to be further oxidized to monomethyl terephthalate.

Dimethyl terephthalate is a valuable intermediate for the manufacture of polyesters.

The parts given in the example are parts by weight.

example

In the system described, in the esterification vessel 3, which is equipped with a stirrer and which contains a mixture of 64% dimethyl terephthalate, 30% monomethyl terephthalate, 5% terephthalic acid and 1% zinc oxide as a catalyst, 8.3 parts per hour are added with a screw Terephthalic acid, which contains 0.05% zinc oxide, and 8.0 parts of methanol are pressed in. The methanol was heated in a preheater to 300 ⁰ C. The temperature in the esterification vessel is 300 ° C. and at the top of the attached column 255 ° C., the pressure is 3.1 atm a portion of the filtrate from the separation of the dimethyl terephthalate from the aqueous methanol-dimethyl terephthalate suspension in the centrifuge 16 is presented. The other part of the filtrate and the methanol with which the dimethyl terephthalate had been washed on the centrifuge are worked up by distillation. In the column 23, dimethyl terephthalate is obtained as the bottom product and a methanol / water mixture is obtained as the top product, which is separated in the downstream column 20.

From the esterification boiler 5% of the terephthalic acid fed in are semi-continuously relaxed in the boiler 8. The dimethyl terephthalate is driven off by injected methanol vapor.

The yield of dimethyl terephthalate, based on the terephthalic acid used, is 98.5%, the purity 99.6%.

Claims (1)

Claim: Process for the continuous production of dimethyl terephthalate by esterification of terephthalic acid with methanol, optionally in the presence of customary esterification catalysts, at elevated temperature and optionally elevated pressure, characterized in that terephthalic acid is introduced into a monomethyl terephthalate and optionally customary esterification catalysts over a temperature of excess methylene vapor, and at the same time with excess heat of dimethyl terephthalate , preferably ata 150-340 ° C 220-300 ⁰ C and pressures between 1 and 100, preferably ata 1-20, initiates the dimethyl terephthalate together with unreacted methanol and formed water in the form of vapor is withdrawn from the vapor mixture 10 up to 90%, especially 30 up to 80% of the dimethyl terephthalate contained therein is ^{condensed at the top of a rectification column at a temperature between 140 and 300 0} C and the condensate is returned to the rectification column, then the remaining Liehe vapor mixture out of the rectification column condenses, cools this condensate to temperatures between -10 and +40 ⁰ C and separating the crystallized dimethyl terephthalate. 1 sheet of drawings 609 537/421 3.66 © Bundesdruckerei Berlin