Acetic acid recovery — NRTL with vapor-phase dimerization

Concentrating dilute aqueous acetic acid (30 mol%) to a 99.8% acetic-acid bottoms product by distillation, modeled with the NRTL activity package and chemical-theory vapor-phase association (acetic acid dimerizes, 2 A -> A2, in the vapor). This is the physics a cubic equation of state gets wrong: the pinned NRTL acetic-acid/water binary (DECHEMA) plus the cited dimerization constants (Nagy et al., Molecules 2020) give the real bubble curve and latent heat, so the close-boiling acid/water pair actually separates and converges where Peng-Robinson does not. Honesty note: acetic acid and water are close-boiling with no azeotrope, so the water distillate is only ~87% water (the acid product is the concentrated, high-purity stream); a sharper water cut needs more stages / reflux than this screening case uses.