Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Patent
1991-08-15
1994-02-15
Dees, Jose' G.
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
562483, C07C 6739
Patent
active
052868966
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for the production of DMT-intermediate product according to the preamble of claim 1 as well as its working up to medium pure or fibregrade dimethylterephthalate (DMT-p) and/or medium pure or pure terephthalic acid (MTA/PTA).
Dimethylterephthalate (DMT) is produced in numerous large scale plants according to the so-called Witten-DMT-Process (cf. German Patent Specification 10 41 945). DMT is subsequently worked up to polyesters by reaction with multifunctional alcohols. Those high molecular compounds--also known as saturated polyesters--are worked up inter alia to form fibres, filaments, films or moulded parts.
STATE OF THE ART
According to the Witten-DMT-Process, a mixture of para-xylene (p-X) and para-toluic acid methylester (p-TE) is oxidised in the liquid phase in the absence of solvents and of halogen compounds at a pressure of about 4 to 8 bar and a temperature of about 140.degree. to 180.degree. C. with atmospheric oxygen in the presence of dissolved heavy metal oxidation catalysts, e.g. in the presence of a mixture of cobalt and manganese compounds (cf. German Patent Specification 20 10 137).
Subsequently to the oxidation step, the reaction mixture obtained, which consists predominantly of monomethyl terephthalate (MMT), p-toluic acid (p-TA) and terephthalic acid (TA), dissolved or suspended, resp., in para-toluic acid methyl ester (pT-ester) and dimethyl terephthalate (DMT), is esterified with methanol at a pressure of 20 to 25 bar and a temperature of about 250.degree. to 280.degree. C. Higher pressures are technically possible but for cost reasons are not used. The esterification product is distillatively separated into a p-TE-fraction, a crude DMT fraction and a high boiling, tar-like residue. The p-TE-fraction is fed back into the oxidation. The high boiling, tar-like distillation residue contains, inter alia, all the constituents of the catalyst system, which according to EP-B-0 053 241 can be recovered and fed back into the oxidation.
The crude DMT originating from the distillation and having a typical purity of 97 to 99.9% contains, besides about 0.05 to 2% of isomers of DMT (dimethylortho- and dimethylisophthalate [DMO, DMI]), still partly interfering amounts of terephthalaldehydic acid-methylester (TAE), monomethylterephthalate (MMT), para-toluic acid (p-TA) and other impurities resulting from the p-xylene employed or from side reactions.
For purification of the crude DMT to pure fibregrade) DMT (DMT-p), with it being necessary in particular, for TAE and the isomers to be removed, it is known to treat the crude DMT by methanolic recrystallisation (German Offenlegungsschrift 20 14 012, Hydrocarbon Processing, Nov. 1983, P. 91). For attaining a purity of the DMT (sum of impurities inclusive of the isomers DMO and DMI less than 100 ppm) sufficient for fibre production (fibregrade DMT), it has been to some extent usual to carry out the recrystallisation twice with intermediate washing with methanol, with the methanol being fed in countercurrent. In addition a final distillation of the DMT has still been necessary hitherto with a crude DMT of low purity.
The filtrate residue obtained in the methanolic recrystallisation by filtration from the mother liquor still contains, in addition to the isomers DMO and DMI, large amounts of DMT and other valuable products (intermediate products in the DMT-process), so that the filtration residue was hitherto as a general rule for the largest part fed back into the oxidation. As a result of this feeding back of filtration residue, the isomers accumulate up to values of 8 to 12% by weight so that the antecedent reactors and especially the distillation and recrystallisation must have additional capacities. The feeding back of the filtration residue was, according to the state of the art, necessary for obtaining sufficiently "neutral prcducts" for the adjustment of the melting point and the acid number in the oxidation and the flowability of the oxidation product to the esterification.
From German Pat
REFERENCES:
patent: 3914287 (1975-10-01), Takeda et al.
patent: 4092481 (1978-05-01), Bunger
patent: 4096340 (1978-06-01), Fujii et al.
patent: 4126755 (1978-11-01), Bunger et al.
patent: 4342876 (1982-08-01), Klingmann
Korte Hermann-Josef
Miletic Anton
Neutzler Hans U.
Schoengen Anton
Schroeder Johann H.
Clarke Vera C.
Dees Jos,e G.
Huls Aktiengesellschaft
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