Title of Invention

"A PROCESS OF PRODUCING POLYESTERS AND COPOLYESTERS BY CONTINUOUS ESTERIFICATION"

Abstract In a process of producing polyesters and copolyesters, a continuous esterification of dicarboxylic esters and diols or a continuous transesterification of dicarboxylic esters with diols is effected in n reaction pressure stages connected in series, wherein the pressure existing in the first reaction pressure stage, which is higher as compared to the remaining reaction pressure stages, successively decreases from reaction pressure stage to reaction pressure stage, the combined vapor flows of the individual reaction pressure stages are rectified in a rectifying column, and the diol component obtained at the bottom of the rectifying column is recirculated at least to the first reaction pressure stage. To reduce the technical effort it is provided that at least the last reaction pressure stage has a vacuum, the pressure in the rectifying column is larger than in the last reaction pressure stage, and the vapors discharged from the last reaction pressure stage are condensed to the pressure existing in the rectifying column.
Full Text This invention relates to a process of producing polyesters and copolyestets by continuous esterification.
Description:
This invention relates to a process of producing polyesters or copolyesters by continuous esterification of dicarboxylic esters and diols or by continuous transesterification of di-carboxylic esters with diols in n reaction pressure stages connected in series, wherein the pressure existing in the first reaction pressure stage, which is higher as compared to the other reaction pressure stages, successively decreases from reaction pressure stage to reaction pressure stage, the combined vapor flows of the individual reaction pressure stages are introduced into a rectifying column and rectified, and the diol component obtained at the bottom of the rectifying column is recirculated at least to the first reaction pressure stage.
It is known that the continuous production of polyethylene terephthalate (PET) from terephthalic acid (TPA) and ethane diol (EG) is effected by esterification in a first reaction stage under an elevated pressure and in a further reaction stage under normal pressure. To further oligomerize the es-

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terification products, there are subsequently provided two prepolycondensation stages operating under a vacuum and a final reactor for producing the finished polyester melt in a fine vacuum.
For performing such 5-stage process, EP-B-0 244 546 describes apparatuses for vapor condensation and vacuum generation by means of multistage steam jet pumps each including a preceding spray condenser and succeeding direct-contact condensers. The steam jet pumps are operated with purified process water vapor from the rectifying column above the first reaction pressure stage, which process water vapor has an excess pressure of 2 to 3 bar, a recirculation of cooling water being effected from the direct-contact condensers towards the rectifying column. The disadvantages of this process substantially consist in a high organic contamination of the waste water in open process circuits and in a large and cost-intensive amount of apparatus on the whole, as each reaction stage, also the second reaction stage of the esterification, has a separate condensation system. Moreover, the formation of diethylene glycol (DEG) is increased as a result of the elevated pressure in the first reaction pressure stage and at the bottom of the rectifying column.
In general it should be noted that when rectifying the original superheated vapor stream with a weight content of about 50 % EG in the rectifying column, a considerable part of the steam gets lost both materially and energetically as operating steam for operating steam jet pumps.
An improvement of the above-described process is achieved in that the vapors of the first and second reaction pressure stages of the esterification are supplied together to a rectifying column, and two vacuum stages are combined in a single prepolycondensation stage.

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The disadvantage of a single prepolycondensation stage, however, is a size-related limitation of the capacity as a result of an increased generation of gas and the increasing risk of an entrainment of droplets as well as a generally undesired increase of the content of carboxylic groups in the polyester or copolyester as a result of a faster evaporation of EG during the prepolycondensation without pressure differentiation.
The subject-matter of US-A-4,670,580 is a 4-stage total process of producing PET, wherein in the second reaction stage an esterification under a vacuum is provided, and wherein a typical acid conversion of 97 % is effected. This conversion appears to be too low for a lasting gas relief in the prepolycondensation stage, i.e. an increased conversion requires longer dwell times, i.e. larger and more expensive reaction apparatuses. In any case, an additional rectifying column with recirculation of EG or some other, separate condensation system with subsequent recovery of EG is necessary.
In the esterification under a vacuum in the second reaction stage, there are also considerable reservations as regards the quality and flexibility of the plant with respect to the addition of additives; as without normal pressure and without stirring the melt, in particular in the textile PET production, admixing additives free from agglomerate and without evaporation losses is quite difficult.
It is the object of the present invention to improve the above-described process such that the amount of apparatus required and the operating costs for cooling water and for energy are decreased without impairing the quality of the polyesters or copolyesters produced and the flexibility of the process.

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This object is solved in that proceeding from the above-described process at least the last reaction pressure stage is operated at subatmospheric pressure of 100 to 900 mbar (absolute), the pressure in the rectifying column is larger than in the last reaction pressure stage, and the vapors discharged from the last reaction pressure stage are condensed to the pressure existing in the rectifying column.
The various processes for esterification or transesterifica-tion are performed in n reaction pressure stages connected in series, the pressure decreasing successively with increasing monomer conversions or advancing sequence of reaction pressure stages.
In the esterification process for producing polybutylene terepththalate (PBT), the reaction pressure stages are exclusively operated under a vacuum, whereas in the esterification process for producing PET and polytrimethyl terephthalate (PTT) the first reaction pressure stage is operated under an excess pressure and the last reaction pressure stage is operated under a vacuum. In the transesterification process, proceeding from dimethyl terephthalate (DMT) and 1,4-butanediol (BDO), the first reaction pressure stage is substantially operated under normal pressure.
For the technical effect of the inventive process it is irrelevant whether the individual reaction pressure stages are disposed in a single apparatus or constitute separate reactors.
For performing the inventive process, at least two reaction pressure stages are required. For a particularly advantageous procedure it is expedient to utilize three reaction pressure stages. The application of a process with four reaction pressure stages likewise is possible, wherein the vapors of the last reaction pressure stage are condensed to the pressure of

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the penultimate reaction pressure stage or the pressure of the rectifying column.
A condensation of the vapors of the last reaction pressure stage and a correspondingly larger rectifying column are avoided by condensing the vapors to a comparatively higher pressure. For the case that the process is performed by using two reaction pressure stages, the height of the pressure in the rectifying column lies between the pressure in the first reaction pressure stage and that of the second reaction pressure stage. When using more than two reaction pressure stages, the pressure in the rectifying column is not larger than the pressure existing in the penultimate reaction pressure stage.
The condensation of the vapors is effected by means of a compressor, blower or ventilator. However, it is particularly advantageous to perform the condensation of the vapors by means of a gas jet pump by using superheated vapors from the first reaction pressure stage as operating steam, since vapors of the first reaction pressure stage are available at no cost in a sufficient quantity and thus the condensation of vapors by using compressors, blowers or ventilators is only considered in the second place.
A particular aspect of the inventive process should be seen in that the pressure existing in the rectifying column can be controlled by an exhaustion of residual gas, which is effected behind the condenser mounted at the head of the rectifying column, for instance by means of a liguid jet pump, a liquid ring pump or also by means of a blower.
Especially in PET esterification, a further aspect of the inventive process is the pressure control in the vacuum esteri-fication stage under partial load in consideration of a pressure decreasing with smaller flow rate, for instance from






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To the first reaction pressure stage (I) operated under excess pressure, EG and TPA are simultaneously introduced as paste in a molar ratio 97 to 99 %, preferably to 97.9 to 98.5 %.
By means of a gas jet pump, the vapors from the third reaction pressure stage (III) are condensed to the pressure of the rectifying column. The operating steam is withdrawn from the vapors of the first reaction pressure stage (I). Under nominal load, the pressure of the vapors is increased by a factor of 1.6 to 2.6 during the condensation of the vapors from the last stage.
The continuous production of PTT esterification product is expediently effected by means of a two-stage process with the parameters indicated in Table 2 by way of example, using a paste prepared from 1,3-propanediol (PDO) and TPA in a molar ratio of 1.15 to 2.20, preferably Table 2


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When producing PBT transesterification product by means of a two-stage process, butanediol (BDO) and liguid DMT are separately introduced into the first reaction pressure stage with a molar ratio of 1.2 to 1.4. The process parameters are indicated in Table 3 by way of example.

The inventive process is schematically represented in the drawing and will subsequently be explained in detail by way of example. In the accompanying drawing.
Fig. 1 shows a process diagram for producing esterification/transesterification products by means of two reaction pressure stages for PPT and PBT;
Fig. 2 shows an aspect of the process diagram in accordance with Fig. 1; and
Fig. 3 shows a process diagram for producing PET esterifi-cation product by means of three reaction stages.
As shown in Fig. 1, a prefabricated diol/dicarboxylic acid paste is introduced into the first reactor (2) via line (1). The product stream discharged from the first reactor (2) via line (3) is supplied to the second reactor (4). Via line (5), the finished esterification product is discharged from the second reactor (4). The vapors discharged from the first reactor (2), minus a small bypass stream conducted via line (7)

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and provided for pressure regulation in the first reaction stage, are supplied to a heated gas jet pump (8) via line (6), by means of which gas jet pump the gas stream introduced via line (9), which is composed of the vapors of the second reactor (4) discharged via line (10) and a ballast stream to be used for controlling the pressure in the second reactor (4) and flowing through line (11), is condensed to the pressure of the rectifying column (12). The vapors discharged from the first reactor via line (6) and the vapors discharged from the second reactor via lines (10, 9) are charged into the gas jet pump (8). In line (14), the vapor stream discharged from the gas jet pump (8) via line (13) is combined with the vapor bypass stream supplied via line (7) and charged into the inlet of the rectifying column (12), in which the low-boiling components are separated from the high-boiling diol component. The low-boiling components discharged at the head of the rectifying column (12) via line (15) are condensed in the condenser (16). The condensate flowing off via line (17) is divided into a reflux stream and a product stream via the reflux tank (18). The reflux stream is supplied to the head of the rectifying column (12) via line (19), and the product stream is withdrawn via line (20). The diol enriched at the bottom of the rectifying column (12) is for the most part supplied to the first reactor (2) via lines (21, 22), and only a small part branched off via line (23) is supplied to the second reactor (4). It is possible to feed additives and/or comonomers into the second reactor (4) via line (24). In addition, an aliquot of the diol flowing through line (22) can be branched off via line (22a) and be used for preparing the paste.
For the case that the process diagram represented in Fig. 1 is used for the purpose of transesterification, diol is fed into the first reactor (2) via line (1) and DMT is fed into the first reactor (2) via line (25).

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To be able to operate the process diagram represented in Fig.
1 also under a vacuum, it is provided in accordance with Fig.
2 to effect a residual gas compression behind the condenser
(16) by means of a pump (27) exhausting the residual gas from
the reflux tank (18) via line (26). The condensate flowing
out of the reflux tank (18) via line (28) is brought to nor
mal pressure by means of a dipping tank (29), and the conden
sate is then removed from the process via line (30).
In the process diagram as shown in Fig. 3, EG/TPA paste is introduced into the first reactor (32) via line (31), the outlet of said reactor being fed into the second reactor (34) via line (33). The product stream leaving the second reactor (34) via line (35) is supplied to the third reactor (36), from which the finished esterification product is discharged via line (37). Via line (38), the vapors discharged from the first reactor (32), minus a small bypass stream flowing through line (39) and used for pressure control in the first reaction stage (32), are supplied to the heated gas jet pump (40). Via line (43), an amount of ballast stream from the vapors of the second reactor (34) flowing through line (42) is added to the vapors discharged from the third reactor (36) via line (41), in order to condense the vapors of the third reactor (36) to the pressure of the rectifying column by means of the gas jet pump (40). If necessary, the vapors discharged from the second reactor (34) via line (42) are combined with the vapors discharged from the gas jet pump (40) via line (45) and with the bypass stream supplied via line (39), upon flowing through the pressure maintaining valve (44), so that the pressure of the second reactor (34) is larger than or equal to the pressure in the rectifying column. Via line (46), the combined vapors are fed into the rectifying column (47), where the low-boiling components are separated from the high-boiling diol component. The low-boiling components discharged at the head of the rectifying column (47) via line (48) are condensed in the condenser (49). Via

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line (50), the condensate flows into the reflux tank (51), from which part is again charged into the head of the rectifying column (47) via line (52). For vacuum operation of the rectifying column (47) under partial load, the reflux tank (51) is connected with a suction pump (54) via line (53). The condensate discharged from the reflux tank (51) via line (55) is brought to normal pressure in the dipping tank (56) and removed via line (57) for further processing. The ethane diol containing a small amount of high-boiling components at the bottom of the rectifying column (47) is recirculated for the larger part to the first reactor (32) via lines (58, 59) and for the smaller part to the second reactor (34) via line (60). From the diol flowing through line (59) an aliquot can be branched off via line (59a) and be used for preparing the paste.

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Claims
1. A process of producing polyesters and copolyesters by continuous esterification of dicarboxylic esters and diols or by continuous transesterification of dicarboxylic esters with diols in n reaction pressure stages (2, 4, 32, 34, 36) connected in series, wherein the pressure existing in the first reaction pressure stage (2, 32), which is higher as compared to the remaining reaction pressure stages (4, 34, 36), successively decreases from reaction pressure stage to reaction pressure stage, the combined vapor flows of the individual reaction pressure stages are introduced into a rectifying column (12, 47) and rectified, and the diol component obtained at the bottom of the rectifying column is recirculated at least to the first reaction pressure stage, characterized in that at least the last reaction pressure stage has a vacuum, the pressure in the rectifying column (12, 47) is larger than in the last reaction pressure stage, and the vapors discharged from the last reaction pressure stage are condensed to the pressure existing in the rectifying column.
2. The process as claimed in claim 1, characterized by 2 to
4 reaction pressure stages (2, 4, 32, 34, 36) connected
in series.
3. The process as claimed in any of claims 1 and 2, charac
terized in that for producing PET7esterification prod
uct, the feed mixture is supplied to a first reaction
pressure stage (32) operated with a pressure of 1.1 to
5.0 bar, which is followed by a second reaction pressure
stage (34) operated with a pressure of 0.5 to 1.5 bar
and then by a third reaction pressure stage (36) oper
ated with a vacuum of 0.1 to 0.9 bar.
* Polyethylere terephthalate

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4. The process as claimed in any of claims 1 and 2, charac
terized in that for producing PTT yfesterification prod
uct, the feed mixture is supplied to a first reaction
pressure stage (2) operated with a pressure of 1.0 to
5.0 bar, which is followed by a second reaction pressure
stage operated with a pressure of 0.5 to 1.3 bar.
5. The process as claimed in any of claims 1 and 2, charac
terized in that for producing PBT/transesterification
product, the starting substances are supplied to a reac
tion pressure stage (2) operated with a pressure of 0.8
to 1.2 bar, which is followed by a second reaction pres
sure stage (4) operated with a pressure of 0.2 to 0.8
bar.
6. The process as claimed in any of claims 1 to 5, charac
terized in that the vapors discharged from the last re
action pressure stage (4, 36) are condensed by means of
a compressor, blower or ventilator.
7. The process as claimed in any of claims 1 to 5, charac
terized in that the vapors discharged from the last re
action pressure stage (4, 36) are condensed by means of
a gas jet pump (8, 40).
8. The process as claimed in claim 7, characterized in that
as operating fluid for the gas jet pump (8, 40) vapors
of the first reaction pressure stage (2, 32) are used.
9. The process as claimed in any of claims 1 to 8, charac
terized in that the pressure existing in the rectifying
column (12, 47) is controlled by a residual gas exhaus
tion provided behind the condenser (16, 49) disposed at
the head of the rectifying column.
** Polytrlmethylene terephthalate *** Polybutylene terephthalate

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10. The process as claimed in claim 9, characterized in that
the residual gas exhaustion (27, 54) is performed by-
means of a liquid jet pump, a liquid ring pump or a
blower.
11. The process as claimed in any of claims 1 to 10, charac
terized in that the esterification products or trans-
esterification products are subjected to a prepolycon-
densation and polycondensation.
12. The process as claimed in any of claims 1 to 11, charac
terized in that an aliquot of the diol recirculated to
the first reaction pressure stage via line (22, 59) is
branched off via line (22a, 59a) and used for preparing
the paste.

In a process of producing polyesters and copolyesters, a continuous esterification of dicarboxylic esters and diols or a continuous transesterification of dicarboxylic esters with diols is effected in n reaction pressure stages connected in series, wherein the pressure existing in the first reaction pressure stage, which is higher as compared to the remaining reaction pressure stages, successively decreases from reaction pressure stage to reaction pressure stage, the combined vapor flows of the individual reaction pressure stages are rectified in a rectifying column, and the diol component obtained at the bottom of the rectifying column is recirculated at least to the first reaction pressure stage. To reduce the technical effort it is provided that at least the last reaction pressure stage has a vacuum, the pressure in the rectifying column is larger than in the last reaction pressure stage, and the vapors discharged from the last reaction pressure stage are condensed to the pressure existing in the rectifying column.


Documents:


Patent Number 210097
Indian Patent Application Number 00549/KOLNP/2003
PG Journal Number 38/2007
Publication Date 21-Sep-2007
Grant Date 18-Sep-2007
Date of Filing 30-Apr-2003
Name of Patentee ZIMMER AG
Applicant Address BORSIGALLEE 1,60388 FRANKFURT AM MAIN,GERMAN
Inventors:
# Inventor's Name Inventor's Address
1 REISEN MICHAEL MERIANPLATZ 1,60316 FRANKFURT AM MAIN,GERMANY
2 WILHELM FRITZ RENDELER STRASSE 79A,61184 KARBEN,GERMANY
PCT International Classification Number C O 8 G 63/78
PCT International Application Number PCT/EP01/13776
PCT International Filing date 2001-11-27
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 10127147.6 2001-06-02 Germany
2 10064361.2 2000-12-21 Germany