How a Column Still Works
A column still achieves high-ABV distillation in a single run by stacking multiple vapour-liquid equilibrium stages vertically. Vapour rises from the boiler, makes contact with descending liquid on each plate or packing stage, and transfers ethanol upward through repeated partial condensation and re-evaporation. The more stages the column contains — and the higher the reflux ratio — the closer the distillate approaches the ethanol-water azeotrope at about 97% ABV (95.6% by mass).
This simulator models each stage using Van Laar activity coefficients to calculate the vapour composition in equilibrium with the liquid at that stage. Murphree plate efficiency scales the theoretical enrichment to account for real-world mass transfer limitations in packed columns and tray columns.
Theoretical Plates and Packing Height
A theoretical plate (or ideal stage) is the unit of separation in column distillation. At each theoretical plate, the vapour and liquid are in perfect equilibrium — the vapour is as rich in ethanol as thermodynamics allows given the liquid composition on that stage. Real column packing (copper mesh, SPP, Raschig rings) and real trays approach but never reach this ideal. The ratio of real height required to achieve one theoretical plate is called HETP — Height Equivalent to a Theoretical Plate.
For typical copper mesh packing, HETP ranges from 10 to 30 cm depending on vapour rate, packing density and column diameter. A 120 cm packed column running efficiently might achieve 4–8 theoretical plates. Use the plate slider to model your column's separation capability.
Reflux Ratio and Distillate Rate
Reflux ratio is the amount of condensed vapour returned to the column divided by the amount taken as distillate. A reflux ratio of 4:1 means four parts are returned for every one part collected. Higher reflux enriches the column but reduces the rate at which distillate is collected — there is always a trade-off between distillate ABV and throughput.
At total reflux (no distillate taken) the column achieves maximum separation — this is the theoretical minimum number of plates required for a given separation (the Fenske equation). In practice, home distillers run reflux ratios of 2:1 to 8:1 for neutral spirits, and lower ratios for flavoured spirit where some congener carry-through is desirable.
The Ethanol-Water Azeotrope
Ethanol and water form a minimum-boiling azeotrope at approximately 97% ABV (95.6% by mass, ~89.4 mol%). At this composition, the vapour and liquid have identical compositions — the VLE equilibrium curve meets the diagonal on the x-y diagram. No column still, regardless of plate count or reflux ratio, can exceed this limit through distillation alone.
The azeotrope is shown as a dashed line on the ABV by Plate chart and the VLE diagram. Watch how adding plates pushes the distillate ABV toward that line but never past it.