Still

A still is an apparatus used to distill liquid mixtures by heating to selectively boil and then cooling to condense the vapor. A still uses the same concepts as a basic distillation apparatus, but on a much larger scale. Stills have been used to produce perfume and medicine, Water for Injection (WFI) for pharmaceutical use, generally to separate and purify different chemicals, and to produce distilled beverages containing Ethanol.

Since ethyl alcohol boils at a much lower temperature than water, simple distillation can easily separate highly concentrated alcohol from a mixture (although ethanol and water become azeotropic once ethanol reaches concentration above 95%). Usually a still used for this purpose is made of copper, since it removes sulfur-based compounds from the alcohol that would make it unpleasant to drink. Modern stills are made of stainless steel with copper innards (piping, for example, will be lined with copper along with copper plate inlays along still walls). Using this combination of metals is much cheaper as it prevents erosion of the entire vessel and lowers copper levels in the waste product (which in large distilleries is processed to become animal feed). All copper stills will require repairs about every eight years because of copper erosion from the compounds it is designed to remove; this erosion is therefore unavoidable. The alcohol industry was the first to use anything close to a modern distillation apparatus and led the way in developing what is now a large part of the chemical industry.

There is also an increasing usage of distillation of gin under glass and PTFE, and even at reduced pressures, to facilitate a fresher product. This is irrelevant to alcohol quality, because the process starts with triple distilled grain alcohol, and the distillation is used solely to harvest botanical flavors such as limonene and other terpene like compounds. The ethyl alcohol is relatively unchanged.

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Wikipedia