Distillation
As mentioned above, the difference in the boiling points of alcohol and water is utilized in
distillation to separate these liquids from each other. Basic distillation apparatus consists of
three parts: the still or retort, for heating the liquid; the condenser, for cooling the vapours; and
the receiver, for collecting the distillate.
The pot still
A pot still is a type of still used in distilling spirits such as whisky or brandy. Heat is applied
directly to the pot containing the wash (e.g. for whisky) or wine (for brandy). This is called a
batch distillation (as opposed to a continuous distillation).
At sea level, water boils at 100 degrees Celsius (212 °F) but alcohol boils at 78 degrees Celsius
(172 °F). During distillation, the vapour is richer in alcohol than the liquid. When this vapour is
condensed, the resulting liquid contains a higher concentration of alcohol. In the pot still, the
alcohol and water vapour, combined with vapours of the multitude of aroma components such
as esters, alcohols that give the mash or wine its aroma, evaporate and flow from the still
through the condensing coil. There they condense to the first distillation liquid, the so-called
'low wines', with a strength of about 25-35% alcohol by volume, which then flows into a second
still below. It is then distilled a second time to produce the colourless spirit, collected at about
70% alcohol by volume. Maturation in an oak aging barrel typically causes the brown color to
develop over time.
The modern pot still is a descendant of the alembic, an earlier distillation device.
The pot still, used primarily in Scotland and Ireland for whiskey production and in France for
brandies, has had only brief use in distilled spirits production elsewhere and is gradually
becoming obsolete. Even in countries in which the pot still has long been used, it has been
replaced by continuous distillation for the major portion of alcoholic-liquor production, and its
current use is limited to production of flavouring whiskeys and other flavouring ingredients.
The flavour profile of a pot-still product is more complex than that of a continuous-still product
of the same alcohol content. This is a result of the different distillation methods. At a given
temperature and pressure, vapours over a boiling mixture have a composition that is a function
of the vapour pressures of the components of the mixture. In a pot still, the temperature of the
fermentation mixture rises as the lower-boiling-temperature alcohol vaporizes. Meanwhile, the
alcohol content of the distillate drops as the rising temperature vaporizes more water along
with the alcohol. Distillation is allowed to continue until the alcohol content of the distillate falls
to a predetemined level. Because of the rising temperature encountered in distilling a single
batch, the composition of the first part of the condensate to leave the pot is different from that
of the last part. The composition of the final product is the average of the composition of the
vapours condensed during the entire run. By contrast, the temperature of the continuous still is
held approximately constant throughout the run. This results in a flavour profile that is more
uniform.
Continuous still
A column still, also called a continuous still, patent still or Coffey still, is a variety of still
consisting of two columns invented in 1826 by Robert Stein, a Clackmannanshire distiller, and it
was first used at the Cameron Bridge Grain Distillery in Fife, Scotland. The design was enhanced
and patented in 1831 by an Irishman, Aeneas Coffey. The first column (called the analyzer) has
steam rising and wash descending through several levels. The second column (called the
rectifier) carries the alcohol from the wash where it circulates until it can condense at the
required strength.
Column stills behave like a series of single pot stills, formed in a long vertical tube. The tube is
filled with either porous packing or bubble plates. The rising vapour, which is low in alcohol,
starts to condense in the cooler, higher level of the column. The temperature of each
successively higher stage is slightly lower than the previous stage, so the vapour in equilibrium
with the liquid at each stage is progressively more enriched with alcohol. Whereas a single pot
still charged with wine might yield a vapour enriched to 40-50% alcohol, a column still can
achieve a vapour alcohol content of 96%; an azeotropic mixture of alcohol and water. Further
enrichment is only possible by absorbing the remaining water using other means, such as
hydrophilic chemicals or azeotropic distillation.
A column still is an example of a fractional distillation, in that it yields a narrow fraction of the
distillable components. This technique is frequently employed in chemical synthesis; in this
case, the component of the still responsible for the separation is a fractionating column.
A continuous still can, as its name suggests, sustain a constant process of distillation. This, along
with the ability to produce a higher concentration of alcohol in the final distillate, is its main
advantage over a pot still, which can only work in batches. Continuous stills are charged with
pre-heated feed liquor at some point in the column. Heat (usually in the form of steam) is
supplied to the base of the column. Stripped (approximately alcohol-free) liquid is drawn off at
the base, while alcoholic spirits are condensed after migrating to the top of the column.
Column stills are frequently used in the production of grain whisky and is the most commonly
used type of still in the production of Bourbon Whiskey and other American whiskeys.
Distillation by column still is the traditional method for production of Armagnac although
distillation by pot still is allowed. The use of column stills for the distillation of Cognac is
forbidden. Distillation by column stills are permitted for Calvados A.O.C. and Calvados
Domfrontais. Calvados Pays d'Auge A.O.C. is required to be distilled by pot still.
EXPLANATION OF THE PROCESS:
The still has two interconnected, copper- lined, vertical columns, one called the analyser, the
other rectifier. Each column is subdivided horizontally by a number of perforated plates. Both
columns are pre heated by steam and the cool wash is pumped into the top of the rectifier
where it descends down the column by way of a long, serpent like pipe or coil. The heated wash
passes out of the rectifier and is taken into the top of the analyser. The wash is freed from the
coil pipes and as it descends the column it is met by a current of raw steam which has been
introduced into the bottom of the analyser.
The steam strips the wash of it’s alcohol, and as the alcoholic vapours rise they are led by a pipe
into the bottom of the rectifier. The spent or used wash is removed from the bottom of the
analyser. Meanwhile the alcoholic vapours and steam start to rise in the rectifier. They meet at
certain points the cold wash which is being carried down by the wash coil in the rectifier. This
causes partial condensation, with the vapours getting cooler and the wash, on it’s way to the
analyser, getting warmer. When the vapours reach the top of the rectifier they are condensed
on a old radiator or water frame before being drawn off as alcohol. The foreshots and the feints
- the first and last part of the distillate – are full of impurities and are sent to be redistilled. The
middle cut, the portable fraction, is very pure but not completely so. It retains some congeners
and has alcoholic strength of 95%. This is reduced by water to a maturing strength depending
upon the alcoholic beverage being distilled. Depending upon the spirit being made it will be
decided as to whether the spirit is going to be aged or not.
Spirits produced from the column still at 95% alcohol is referred to as Neutral Spirit.