Dilute acetic acid recovery by extractive distillation

Recovering acetic acid from a dilute aqueous stream (e.g. cellulose acetate or terephthalic acid plant wastewater) using diisobutyl ketone (DIBK) as an extractive entrainer. Because acetic acid and water form no azeotrope but have a relative volatility that flattens toward 1 as the mixture concentrates, straight distillation needs an impractically tall column and heavy reboil duty; the entrainer raises water's relative volatility so the extractive column takes water overhead in far fewer stages, and a second column then strips the entrainer from the acid bottoms for recycle.

Modeled with NRTL, and the property method is the whole point here rather than a detail: an entrainer works by changing liquid non-ideality, and a cubic equation of state with van der Waals mixing has no term for it — no binary interaction parameter ships for acetic-acid/water or DIBK/water, so Peng-Robinson runs this flowsheet at kij = 0 and represents none of the effect the column depends on. It does not converge either. NRTL carries the DECHEMA-fitted acetic-acid/water binary and reaches the DIBK pairs through UNIFAC.

Honesty note: the extractive column does its job — the water distillate comes off essentially acid-free — and the recovery column returns the entrainer at >99.9% DIBK for recycle, but the acid product is a concentrate, not glacial acid: it leaves at ~64 mol% acetic acid, up from 20 mol% in the feed, carrying the water the extractive column did not take overhead. Acetic acid and water are close-boiling with no azeotrope, so a finishing column (or the vapor-association model the acetic-acid-recovery-nrtl example uses) is what takes it the rest of the way. The water distillate is a real DIBK/water heteroazeotrope at ~12 mol% DIBK; a plant decants that overhead and returns the organic layer, which this screening flowsheet does not model.