Rectification

Still distillation, only repeated many times over — not by running the same liquid through again and again, but in one pass, inside a column.

The column is packed with something of large surface area: plates, spirals, mesh, rings. Vapour goes up, condensed liquid runs down, and at every contact between them part of the vapour condenses and part of the liquid evaporates. Each time, the vapour comes out a little richer in ethanol.

One such contact is worth roughly one distillation and is called a theoretical plate. A column with a dozen or so plates does in a single pass what a dozen separate runs would — which is the entire point of the design.

The liquid running back down is reflux, returned by the dephlegmator. The more vapour goes back, the cleaner the product and the slower it comes. That trade — purity costs output — cannot be dodged in rectification. Nor can the azeotrope, which stops everything at 95.6% however tall you build the column.