July 19, 1960 A. WATZL ETAL 2,945,788 PROCESS FOR THE PURIFICATION OF DIMETHYLTEREPHTHALATE Filed Nov. 19, l956

AZEOTROPE

DISTILLATE

CONDENSER CONDENSED

WACUUM DISTILLATE DRY DISTILLATION MXTURE N2 COLUMN

MPURE - SOD WACUUMFILTER ETHYLENE DMETHYL DMETHYL GLYCOL TEREPHTHALATE TEREPHTHALATE ETHYLENE GLYCOL

INVENTORS: ANTON WATZL by aERHARD 965% SIGGEL ATT'YS 2,945,788 Patented July 19, 1960

2 phthalate is immediately suitable for reesterification or Subsequent polycondensation to polyethylene terephthal 2,945,788 ate. There are gained polycondensates of high degree of PROCESS FOR THE PURIFICATION OF viscosity with K values from 50 to 57. DMETHYLTEREPHTHALATE The best mode contemplated for practicing the inven Anton Watz, Kleinwallstadt (Ufr), and Erhard Siggel, tion involves the use of ethylene glycol as the aliphatic Laudenbach (Main), Germany, assignors to Vereinigte glycol. The following is a specific illustration thereof. Glanzstoff-Fabriken A.G., Wuppertal-Elberfeld, Ger Example many Thirty grams of crude dimethylterephthalate are mixed Filed Nov.19, 1956, Ser. No. 622,778 in a flask with 270 grams of ethylene glycol and azeo tropically distilled with the introduction of dry nitrogen 4 Claims. (C. 202-42) in a column of 30 cm. at about 44 Torr (1 Torr equals 1 mm. Hg). The azeotrope goes over at about 120° C. i This invention, in general, relates to production of into a cooled condenser. The dimethylterephthalate dimethylterephthalate and more particularly to the puri 5 separated from the glycol by vacuum filtering can be fication thereof. used immediately for reesterification or for polyconden The purification of dimethylterephthalate can be car sation. This process is illustrated in the flow sheet of ried out either by distillation or by recrystallization from the accompanying drawing. solvents such as methanol or carbon tetrachloride. With 20 The invention is hereby claimed as follows: the distillation under atmospheric pressure, an excellent 1. A process for purification of dimethylterephthalate purification cannot be attained because, at the relatively characterized by azeotropic, vacuum distillation at about high temperatures, decomposition phenomena are un 44 mm. Hg and about 120° C. of dimethylterephthalate avoidable, which phenomena, in turn, produce new in in the presence of ethylene glycol. purities. The disadvantage with the recrystallization lies 25 2. A process for purification of dimethylterephthalate in the fact that large amounts of mother liquor accrue which comprises distilling under vacuum of about 44 mm. the separation of which is unwieldy and results in consid Hg and at about 120° C. from a body of impure dimeth erable losses. It has already been suggested to carry out ylterephthalate and ethylene glycol an azeotrope of di the distillation of dimethylterephthalate in a continuous methylterephthalate and ethylene glycol; condensing the process, the material passing through three distillation 30 azeotrope; removing the ethylene glycol from the con zones. Also this method of purification is unwieldy on densed azeotrope. account of the extensive equipment and does not admit 3. A process for purification of dimethylterephthalate an extreme purification, as this is absolutely necessary which comprises distilling under vacuum of about 44 for the further processing of dimethylterephthalate into mm. Hg and at about 120° C. from a body of impure corresponding polycondensates. 35 dimethylterephthalate and ethylene glycol an azeotrope It is, therefore, an object of the present invention to of dimethylterephthalate and ethylene glycol; condensing provide an improved process for purification of dimethyl the azeotrope; and removing the ethylene glycol from terephthalate. the condensed azeotrope; and recovering the purified di Another object is to provide a process for purifying methylterephthalate. crude dimethylterephthalate in a single distillation step 40 4. A process for purification of impure dimethyltereph to a purity sufficient for utilization thereof in reesterifica thalate selected from the group consisting of impure tion or in production of polycondensates. dimethylterephthalate derived by depolymerization of In accordance with the invention, dimethylterephthal polyethyleneterephthalate and impure dimethyltereph ate is subjected to extreme purification when the crude thalate derived by esterification by terephthalic acid dimethylterephthalate in the presence, preferably at least 45 with methanol comprising distilling under vacuum from an equal amount by weight, of aliphatic glycols, prefer a body of said impure dimethylterephthalate and ethylene ably ethylene glycol, is subjected to a distillation under glycol an azeotrope of dimethylterephthalate and eth vacuum. This discovery is surprising because by a dis ylene glycol; condensing the distillate; and separating the tillation with methanol, for instance, no purification in ethylene glycol from the dimethylterephthalate in the the desired sense is possible. The present process is 50 condensed distillate to obtain substantially pure dimeth characterized by the fact that the crude or impure di ylterephthalate. methyl ester, which was gained either by depolymeriza tion of polyethyleneterephthalate or by esterification of References Cited in the file of this patent terephthalic acid, is purified by vacuum distillation in a single step in the overall process. The glycol used for 55 UNITED STATES PATENTS the distillation can be used over again immediately for 2,773,090 Leibu ------Dec. 4, 1956 further distillation, without having it first redistilled. 2,825,737 Safer et al. ------Mar. 4, 1958 The ethylene glycol is separated from the dimethyltereph 2,829,155 Muench et al. ------Apr. 1, 1958 thalate by sucking off, e.g., by vacuum filtration, the excess glycol from the condensed ethylene glycol-di 60 OTHER REFERENCES methylterephthalate mixture making an intermediate "Azeotropic Data' (Horsley), published by American desiccation superfluous and providing a considerable ac Chemical Society (Washington, D.C.), 1952 (page 131 celeration of the overall process. The dimethyltere relied on).