Toluene removal from 1-butanol solvent (modified UNIFAC, Dortmund)

A recovered 1-butanol solvent contaminated with 5 mol% toluene, redistilled to send the toluene overhead: 24 stages, reflux 3, an 8% distillate. There is no fitted toluene/1-butanol binary, so this runs on modified UNIFAC (Dortmund) — the variant that cuts infinite-dilution error from 42% to 16% in this tree, and a dilute contaminant in a solvent is exactly the infinite-dilution regime. Dortmund puts the toluene/butanol azeotrope at 67 mol% toluene (original UNIFAC: 70%), which is why the overhead comes out at 43% toluene rather than pure, and the bottoms at 1.7% toluene — two-thirds of the contaminant removed per pass. The 3-point azeotrope difference between the two variants is the size of the model choice on this pair.