Process for preparing residues containing dihydroxy compounds

Details Process for preparing residues containing dihydroxy compounds Process for preparing residues containing dihydroxy compounds

1997-09-04

1998-08-04

Acquah, Samuel A.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Removing and recycling removed material from an ongoing...

528272, 528501, 528503, 526 65, 526 66, 526 68, C08F 200, C08G 6500

Patent

active

057895007

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to processes for working up dihydroxy compound residues from the preparation of polyesters by ester-forming derivatives with a molar excess of a dihydroxy compound, a), and recovering the starting materials.
Polyesters, especially polyalkylene terephthalates, are widely prepared using (trans)esterification/polycondensation processes in which a first stage involves an esterification or transesterification and the actual polycondensastion is carried out in one or more further stages (cf. Chemiefasern/Textilindustrie 40 (1992), 1058-1062, and Ullmann's Enzyklopadie der technischen Chemie, 4th edition, vol. 19, pages 61-88).


BRIEF DESCRIPTION

This process will now be briefly explained by way of example with reference to the preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol.
In a first reaction space, terephthalic acid is esterified with a molar excess, preferably 50-120 mol %, in particular 70-100 mol %, of 1,4-butanediol, and the esterified compound is subjected to the actual polycondensation in further steps. The vapors from the esterification are transferred into a column in which the low boiling THF/water are removed overhead to leave a bottom product which, as well as excess 1,4-butanediol, additionally contains small amounts of oligomers, polymers and terephthalic acid.
The esterification product is subsequently polycondensed, advantageously in the case of continuous processes in at least two stages, a precondensation and a postcondensation.
For economic reasons it is desirable in this connection for as many as possible of the resulting reaction products and dihydroxy compounds present in excess to be further treated, for example in order to recover the 1,4-butanediol and to minimize waste.
In the preparation of polyesters, it is customary to work up the vapors from the polycondensation separately or not at all. It is not desirable to recycle the vapors into the esterification even after removal of the low boilers, since bottom products of this type have an excessive solids content. This solids content (oligomeric and polymeric constituents of the vapors) leads to a reduction in product quality due to fisheyes in the polyester. Fisheyes are higher melting, gellike particles which, for example, are the cause of nodules or breakages in fibermaking or of visible defects in film making.
With terephthalic acid as one of the starting materials there is the additional problem that the esterification requires a significantly larger excess of butanediol in order to shift the equilibrium of the ester reaction to the desired extent. The vapors from the polycondensation, especially the precondensation, contain a significantly larger proportion of the excess diol than the vapors from the esterification.
It is an object of the present invention to remedy the above-described disadvantages and to improve the workup of the vapors to the effect that most of the dihydroxy compounds present therein can be recycled into the esterification and that, based on the total amount of diol used, the loss of diol is minimized. At the same time, the product quality of the polyester shall be preserved.
We have found that this object is achieved, surprisingly, by
1) combining the vapors of step a) and some of the vapors of step b) of the process in at least one column A) and removing the low boiling constituents of the vapors overhead and recycling the bottom product, which predominantly contains the excess dihydroxy compounds and also oligomeric and polymeric reaction products, into step a), and
2) transferring the other part of the vapors of step b) of the process into at least one column B) and removing the low boiling constituents of the vapors overhead and discharging the bottom product from the column and subsequently subjecting it to a further treatment for recovering the dihydroxy compound.
The splitting of the vapors from the polycondensation is a technically simple way of achieving a recycle of the excess diol into the esterification. The process is more econom

REFERENCES:
patent: 4110316 (1978-08-01), Edging et al.
patent: 4239882 (1980-12-01), Kimura et al.
patent: 4499261 (1985-02-01), Heinze et al.
patent: 5236558 (1993-08-01), Buyalos et al.
Ulmanns Encyklopadie . . . Band 19,61-88.
Chemiefasern/Textilindustrie 40 (1992), 1058-1062.

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