Acetic acid recovery — NRTL with vapor-phase dimerization

Concentrating dilute aqueous acetic acid (30 mol%) 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 neither product is sharp at this screening duty — the bottoms comes out ~79 mol% acid and the distillate ~91 mol% water. Both are the solver's own converged, mass-closing answer; a sharper cut needs a taller column at higher reflux than 20 stages / R=1.6, and at that duty this column stops closing its component balance (see the CLAUDE.md note), so the honest result is reported rather than the target quoted.