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| Multi Plate Column |
Plates or
trays as they are sometimes called, are used in distillation to enhance the
mixing of liquid and vapor in the column of the still, whether it is a pot
still with a column or a column still. While their function is the same in both batch, post stills and continuous distillation, column
stills, but their actual function is very different.
Richard Seale, one of the most knowledgeable
distillers anywhere has taken some time to explain what they really do and
how. Basic batch distillation does not employ plates, but
it is important to understand that the lyne arm is a basic type of plate. The
cooling of the atmosphere causes liquid, known as reflux to form on the sides
of the lyne arm and that mixes with the rising vapor. The lyne arm then becomes the most vital part
of the batch still and its shape affects its function as a plate and directly
affects the performance of a still.
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| Lyne Arm at Top of Pot Still |
In batch
(pot) distillation the wine is distilled till all the alcoholic spirit is
removed in as the name implies, a batch process. Our lyne arm, affects the
level of reflux) which in turn affects two fundamental things: the timing of
the arrival of the various components of the wine and the contact time with
copper. The nature of our rum will be
decided by the cuts. But affecting the timing of the components will affect
quality of the cuts. Basically, having a good lyne arm developing good reflux
will help ensure all of the undesirable heads arrive early and all of the
undesirable tails arrive late so we can make our ideal cuts. Poor reflux means
poor separation by timing yields poor quality hearts. Low contact time with copper also leads to
poor quality hearts.
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| Double Retort Pot Still |
When we
add plates to a pot still stem, we are basically “supercharging” our lyne arm.
If we add a condenser we are adding even more to our lyne arm. We are using additional cooling over and
above what the atmosphere gives and this together with the additional surface of
the plates will improve the reflux. It is the equivalent of using a giant lyne
arm. The plates in the pot still don't
remove any congeners. They don't "purify" anything. They don't make a
"lighter rum". The plates just affect the timing and contact time.
Our cuts determine the congener profile but the plates by controlling the
timing give us far greater control over what we “select” with our cuts. Copper is thought to play a role in ester
formation so enhanced time will increase congeners. The cuts, the copper
contact and fermentation, determine how "heavy" we make the rum. Not
the plates. Our double retort still at Foursquare has
plates above the pot and a cooling condenser. The retorts also function as a kind of plate. Many Caribbean pot stills were modified in the
19th century to add plates.
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| Large Column Still |
Now in
continuous distillation plates likewise enhance liquid/vapor contact. The same
way you have batch distillation without plates, you could in theory build a
continuous column system without plates. But it would be impractically tall.
The enhanced liquid/vapor contact with external condenser cooling, makes the
continuous column system practical. And with enough plates and reflux we can
separate pure ethanol, but we are talking over 75 plates in the last column
alone from the other congeners. In a steady state continuous column still the
plates enhance the physical separation in the column and we draw off heads and
tails from physical points where these congeners are concentrated. We also draw ethanol or light spirit, from the
physical point where the ethanol is positioned in the column. The net effect of plates is to assist in
“purification” or making lighter spirits or indeed making pure alcohol.
Plates
have very different roles depending on whether it is continuous or batch
distillation. The function is the same, the role is different. While on the topic, there are no hybrid
stills. Still are either batch or continuous. With or without plates. I hope that Richard’s explanation has helped
understand the function and the use of plates in the process of distillation.
