Monday, December 4, 2017

What are the Plates in a Still All About?

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.
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.

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.
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.