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Fractional distillation for the complete purification of alcohol (e.g. for vodka) requires a tall, narrow column mounted above the boiler packed with small pieces of some inert material such as glass, ceramics, or stainless steel. The purpose of the packing is to provide multiple surfaces where the vapour rising from the boiler meets the condensed vapour cascading down towards the boiler. At each interface in the column there is a mini-redistillation, resulting in pure alcohol emerging from the top of the column while the impurities flow back down towards the boiler.
To achieve the above, the packing material should have the following characteristics: i) the individual pieces should not pack tightly together, but be of such a shape that they leave plenty of space for the vapour to rise against the falling liquid; ii) the pieces should have a large surface area and crevices where liquid can be trapped. The two most commonly used packings today are Raschig rings and stainless steel filaments.
Raschig rings. Described by Dr. Raschig over 100 years ago, they are widely used in small laboratory stills and consist of short lengths of tubing, usually glass, the length and diameter being the same, e.g. 6 mm x 6 mm.
Stainless filaments. Commercial distilleries which use packed columns frequently employ a product made from s/s filaments woven into a blanket, the blanket then being rolled into a cylinder. It is so much more efficient than Raschig rings that the column height can be reduced for the same level of purification.
The Carriage Still also uses stainless steel filaments, employing a simple household article to be found in any supermarket. The reasons for making this choice were that, compared to Raschig rings, they are: a) readily available; b) one-tenth the cost of Raschig rings; c) occupy just 2% of the space in the column compared to 26% for Raschig rings; and d) have a very much larger surface area available for vapour/liquid interchange. Because of the much greater open space in the column, alcohol production can be doubled without danger of flooding