The Fractionating Still provides a rich product, but one that is a mixture of volatile components. It does a better job than the pot still, if it is not driven too hard. The best product is obtained at the slowest operation.
The Compound Still offers a way to obtain almost perfect separation of each volatile component, in order of increasing boiling points. A compound still has a "reflux" condenser on top of a fractionating column, which returns most or all of the condensate to the column.
This is a natural extension of the simple fractionating still, and the addition of an additional component to the top of a fractionating column is why we call it a "compound still". The process of condensation and re-evaporation from the packing is exactly the same - except at the top of the column. In the compound still, the rich vapor at the top is condensed and returned, rather than being removed. When this cooled reflux is re-evaporated in the topmost zone of the column, the vapor is enriched again. This vapor is then condensed, and returned for another cycle of purification. With each cycle the product gets closer and closer to the best separation possible.
Over time, the most volatile component will find its way to the top of the column, and the less volatile components will work their way downward, establishing a state of equilibrium. The top zone of the column will have a constant temperature, equal to the boiling point of the most volatile component concentrated there, and it becomes a constant composition "reservoir zone". Below this zone, temperature increases as you move toward the boiler as the normal interchange between vapor and reflux condensing in the packing re-asserts itself. The net effect is to shorten the fractionating column as space is needed at the top for this "reservoir".
When you begin to remove some of the top condensate, rather than return it to the column, the equilibrium is disturbed. If you draw it off very slowly, the column can adjust and form a new equilibrium as the fractions lower down slowly move up the column. If you take it too quickly, equilibrium is lost and the whole device reverts to behaving like a simple fractionating still.
One way to think of this is as if the column is a high wire artist carrying a sack of potatoes. If you suddenly took a sack of potatoes from him, he would lose his balance. Take one potato at a time and you won't affect his equilibrium very much. Take potatoes at a steady rate and he will adjust to this steady routine, establishing a new equilibrium. A compound still works in precisely the same way. If a small proportion (say 10%) is removed, vapor further down the column moves up slowly, maintaining its equilibrium as it goes. If you get greedy, and take too much too quickly, the upward rush disturbs the equilibrium and contaminates the product.
The compound still introduces a major new factor to the distillation process: CONTROL.
The compound still enables you to take a mix of volatile liquids and to separate and collect the constituents one at a time in order of their volatility. The lightest fractions, such as ethyl acetate, are the first to be isolated, which means that you can remove them entirely. After the lightest fractions, you are then able to collect the purest ethanol achievable by distillation: the azeotropic mixture of 96% ethanol and 4% water.
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