process for the production of terephthalsäure from conclusion as intermediate

摘要

Parent patent disclosed prepn. of terephthalic acid, (I), from dimethyl terephthalate, (II), as intermediate, by first oxidn. of p-xylene and p-Me toluate, (III), (1:1-1:3 ratio) with O2-contg. gases in presence of heavy metal-contg. catalysts at 140-170 deg.C and 4-8 bar. The oxidn. mixt. is then esterified with MeOH at 230-280 deg.C and 20-30 bar. The crude ester is sepd. by distn. into a (III)-rich fraction, which is recycled to oxidn., and a residue and into crude-(II). Crude-(II) is hydrolysed with water at (II) to water ratio 3:1 and 0.1:1, at 350 to 140 deg.C, under pressure to maintain liq. phase. (I) is crystallised at 300-150 deg.C, the mother liquor is replaced by desalinated water and (I) is recovered from the reaction. Crude-(II) has a limited oxidn. intermediate- and other by-prod. content. Hydrolysis takes place in 2 steps. After the 1st hydrolysis step reaction mixt. is expanded and sepd. into conc. suspension of solid (I) and mother liquor and into residual mother liquor- contg. liq. phase. The mother liquor is then continuously diluted and replaced, under hydrolysis conditions, with desalinated water passed countercurrently to the crystallising (I). (e) The combined mother liquors are cooled so that only dissolved (I) and by-prods. are pptd. and sepd. The liq. phase is sepd. by distn. into an MeOH-water mixt., sepg. overhead, and aq. phase as sump prod. Part of the aq. phase is discharged. The residue is combined with the sepd. solids, reheated and returned to 1st hydrolysis step. Improvement comprises adding crude-(II), or (III), as auxiliary for sepg. useful prods., mono-Me terephthalate, (IV) in addn. to (I) and (II) as solids, by cooling, from the combined mother liquors. The useful prods. are then sepd. with the auxiliary as organic phase. Auxiliary addn. improves useful prod. (IV), (I) and (II) sepn. and the filterability of (IV).