Terephthalic acid prepn. from di:methyl terephthalate - by hydrolysis with hot steam and recovery by crystallisation from reaction mixt.

摘要

Terephthalic acid, (I), is prepd. from dimethyl terephthalate, (II), as intermediate prod., by (a) p-xylene and/or p-Me toluate (III) oxidn. with O2-contg. gases in the presence of heavy metal-contg. oxidn. catalysts at increased temp. and pressure; (b) oxidn. mixt. esterification with MeOH at increased temp. and pressure; (c) crude ester sepn. by distn. into a (III)-rich fraction, which is returned to oxidn., a residue fraction and crude (II). (d) The crude-(II) is hydrolysed continuously with water, at crude-(II): water wt. ratio 3:1 to 0.1:1, at 350 to 180 deg.C and under a pressure sufficient to maintain the liq. phase. (e) (I) is crystallised out at 300 to 100 deg.C. (f) The mother liquor is replaced by desalted water and (I) is recovered from the reaction mixt. Novelty comprises (i) introducing hot steam, at 300 to 180 deg.C, at the bottom of a multi-step hydrolysis reactor, and hot crude-(II), at 350 to 140 deg.C, to the head of the reactor. (ii) At conversion 90% or over, a reaction mixt. is drawn off at bottom as liq. phase or as solid phases suspended in the liq. phase, while an MeOH-water mixt. is drawn off at head, at 300 to 150 deg.C. (iii) The mixt. is condensed. (iv) A condensate pt. is recycled to hydrolysis reactor head, and the other pt. is returned to the process. (I) obtd. can be directly esterified with ethylene glycol and converted to fibre- and film-forming polyesters. Recycling of mono-methyl terephthalate-contg. mother liquor is unnecessary.