Improvements in the purification of crude dimethyl terephthalate by distillation

摘要

Pure dimethyl terephthalate is recovered from mixtures of the ester with methanol, water and other impurities by a process in which a distillation column 1 is maintained at a pressure at which the condensation temperature of the vaporized methanol-water mixture is above the solidification temperature of the ester, i.e. at least 8 atmospheres, an overhead fraction of methanol, water and low-boiling impurities is led off by line 5 to a second, dehydrating, distillation column 10 from which pure methanol is obtained overhead by line 11 and water and impurities are drained off by line 35, the ester mixed with high-boiling impurities is led by line 9 into a third distillation column 14 maintained at atmospheric or slightly elevated pressure up to 1 atmosphere gauge, to which column methanol vapour is also fed by line 13, and the pure ester is separated from methanol after the fraction consisting of ester and methanol distilled off by line 18 is condensed. High-boiling impurities are discharged by line 16. Crude mixtures preferably contain 1.5 to 8 parts of methanol per part of ester and the water content is between 1 and 30% of the methanol content. Two columns instead of column 1 may be used. The vapours in line 18 are condensed at atmospheric or reduced pressure in a vessel 20 and the condensate is cooled further, e.g. to between 40 DEG C. and -10 DEG C., in a container 21 to crystallize the ester, which is then separated from methanol in a centrifuge 24. The ester, after washing, if desired, with methanol from line 26, is passed to a drier 28 while excess methanol is passed to a reservoir 25 for use in the esterification of terephthalic acid or for circulation to the second column 10 by line 33.