Alcoholic distillation is basically the process of separation the more volatile component, alcohol, from the less volatile component, water, from a water/alcohol solution, by heating the solution and condensing and collecting the alcohol rich vapours released, as a high alcohol strength liquid (spirit).
The starting material for most distillations, such as wine for brandy and beer for whisky, are usually weak alcoholic solutions (5%-7% v/v ethanol)
This seems counter intuitive, as it would seem that if the desired product is alcohol, the starting solution should have a high alcohol concentration.
The reason for starting with a relatively low alcohol starting material is that alcohol molecules (A) and water molecules (W) interact with themselves (A-A, W-W, the reasons why they form liquids) and between each other (A-W, the reason they form solutions).
The concentration of the species A-A, W-W and A-W depends on the alcohol (A) concentration.
Like most substances, like attracts like, in the same way that alcohol molecules will have a stronger affinity for other alcohol molecules (A-A) and water having a strong affinity for itself (W-W).
In a weak alcoholic solution, fewer molecules of alcohol will have an opportunity to interact with each other, being separated by many water molecules.
This results in fewer, stronger A-A formations, and more of the weaker A-W formations, making it easier for the more volatile alcohol molecules (A) to be liberated from the solution on heating.
On the other hand the collected distillate, from a batch/pot distillation, will be high in alcohol, and form many A-As, making it much harder to concentrate the alcohol even further through a second distillation.
In fact, a water/alcohol solution with a alcohol concentration of 95.6% alcohol sees the number of A-A molecules becoming so high that the alcohol becomes as reluctant as water molecules to vaporize of, in fact they will have the same volatility and no more separation or concentration of the alcohol becomes possible by standard distillation.
Solutions of this nature are called azeotropic solutions.
The phenomenon, where volatilities of substances deviate from that predicted by Raoult's law, is refered to as non-ideal behaviour, or deviation from ideality, which happens to be a positive deviation with regards to a water/alcohol solution.
So far we have assumed that the only alcohol present in wine is ethanol, however there many other important, if minor quantities of other alcohols present. (see other alcohol)
In a batch/pot distillation, the alcoholic vapours coming of the bulk of the liquid are condensed above the liquid on the still outlet, on the way to the condenser.
Equilibriums with complex and interesting volatility deviations from the ideal are established there between the many alcohols and other components involved, all involving the phenomenon described above.
Batch distillation and fractional distillation
Pot or batch distillation
A pot still consists of a pot and an outlet to a condenser. The outlet itself also serves as a condenser and to some extent as a fractionating column (see fractional distillation).
Batch distillation as the name suggest starts with a single batch of low alcoholic starting material (5-7%v/v alc.), that is heated in a pot still and a low wine distillate of between 26-50% v/v alcohol is collected.
This may then be further concentrated by a second distillation to a higher strength brandy of up to 80%v/v alcohol.
(Brandy is derived from the dutch word brandewijn, meaning burned wine).
The pre condensation of minor but important flavour components along the pot still's outlet, is a place where complex equilibriums are established.
These equilibriums involve radical deviations from the ideal, predicted behaviour of the volatile compounds' volatilities (see distillation theory).
The result is that the product fraction make...