Ammonium → 4-(Hydroxymethyl)benzoic acid

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FIG. 1 is a diagram of the invented process using sodium hydroxide and ammonium hydroxide. Terephthalic acid, sodium hydroxide and ammonium hydroxide are metered into reservoir 1 by lines 2, 3 and 4 in ratios of one mole per mole at the beginning of the electrochemical reduction process to form sodium ammonium terephthalate. Added ammonium hydroxide is used as a buffer to raise the pH from approximately 7 to within the range of from about 8.0 to about 11.0. The sodium ammonium salt solution is thereupon pumped by suitable means by line 5 to electrolysis cell 6 wherein the salt of terephthalic acid is reduced to sodium p-hydroxymethylbenzoate (Na p-HMB). The Na p-HMB is thereupon pumped by line 7 back to the reservoir 1 which contains Na p-HMB, NH4OH, and monosodium terephthalate. After optimum levels of Na p-HMB have been reached in the reservoir 1, a portion of the electrolysis cell output of line 7 is pumped by line 8 to reactor 9 to which make-up terephthalic acid and make-up sodium hydroxide are added by lines 10 and 11. Make-up terephthalic acid and sodium hydroxide are required because of the reduction of the sodium ammonium terephthalate to Na p-HMB and to salt out the Na p-HMB. Recycle ammonium salt from reactor 9 is pumped by line 25 to line 18 and thence to reservoir 1. The disodium terephthalate in reactor 9 is pumped with the solution removed from reservoir 1 by line 12 to chill unit 13 wherein the Na p-HMB is salted out by the presence of the disodium terephthalate. The resulting slurry in chill unit 13 is pumped by line 14 to filter 15 wherein solids comprising Na p-HMB are filtered and forwarded by line 16 to be recrystallized in crystallizer 17. Mother liquor from filter 15 is recycled to reservoir 1 by line 18. Mother liquor after dewatering from crystallizer 17 is recycled to reservoir 1 by line 19, which joins line 18. Recycle slurry from filter 15 is returned to chill unit 13 by line 22 if recycle is necessary to obtain sufficient cooling. Make-up terephthalic acid and ammonium hydroxide are added to line 18 by lines 20 and 21 to restore the sodium:ammonium:terephthalate ratio and are reacted in reservoir 1. Sodium p-hydroxymethylbenzoate is removed from crystallizer 17 by line 25 to reactor 23 where it is acidified to obtain p-hydroxymethylbenzoic acid.

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