The Principle of Distillation
The following chart illustrates the primary configuration of a Scottish Malt Whisky distillery with two pot stills. Distilleries with three pot stills and triple distillation add one more step accordingly. Many large distilleries have four, six, or more pot stills, which are not operated in series but in parallel.
The connection of the pot stills can be even more complex. For example, the first distillate from several wash stills or several production cycles can be led into a single spirit still. A ratio of 3:2 or 4:3 of wash stills to spirit stills is also common. It is also possible to triple distill with two pot stills by distilling the final product of the second distillation again in the emptied spirit still.
The chart represents a simple distillery with a wash still and a spirit still.
Functional Chart of a Pot Still Distillery
The wash still has a simple task: it is used for the first distillation of the wash, or in simpler terms, the beer. The capacity of the stills and the wash backs is usually coordinated.
When hot steam is introduced into the heating cylinders, the wash still begins to heat the wash. Through the convection inside the still, the wash is turned. It rises along the warm areas of the cylinders and sinks back down along the cooler areas.
After approximately 30 minutes, the interesting part begins. The liquid above the heating cylinders starts to boil, and light substances (mostly flavor-carrying esters) rise into the air above the liquid level.
The constant supply of gaseous substances results in a slight overpressure in the still, and the gases rise into the neck of the still. However, they don’t go far. The still’s wall is still too cold, causing the evaporated substances to condense on the wall. Over time, droplets accumulate and form larger drops that flow back into the pot.
With increased energy input, the entire wash starts boiling at a temperature of approximately 173°F (78°C). The still heats up, and the liquid surface becomes turbulent. The high heat input creates powerful bubbles, causing the liquid to foam and splash up to the upper part of the neck.
That’s why wash stills have small windows in the neck, allowing observation of the bubbling wash. If the boiling temperature of the wash is too high, liquid can enter the condenser through the lyne arm. This would not be a problem if the wash did not contain solid parts of the barley grains, which can clog the thin pipes of the condensers.
Therefore, the stillman must remain attentive. Distilleries that do not have time to monitor the boiling process add soap to the wash. This destroys the surface tension and prevents it from boiling over. Since soap liquefies at 122-140°F (50-60°C) and only boils at temperatures well above 212°F (100°C), no soap molecules can enter the distillate.
ACE Copper Pot Still
The first distillation in the wash stills takes approximately 4 to 7 hours. The wash still reaches a temperature of approximately 173°F (78°C), the boiling point of ethanol. The entire heat input is used for the evaporation of alcohol.
The distillation usually concludes after 4 hours. Apart from 1% abv, all the alcohol has evaporated and accumulated in the low wines receiver.
However, the low wines receiver does not only contain alcohol but also substances with a lower boiling point than alcohol, as well as some substances with a higher boiling point. These molecules are pulled into the low wines receiver along with the light alcohol molecules due to the bubbling liquid. Among these molecules, there is also a significant amount of water, which forms an azeotrope with the alcohol. After the first distillation, the low wines typically have an alcohol content of 20% to 25%.
After distillation, the pot ale (also known as spent wash) remains in the wash still. It has a residual alcohol content of approximately 1%. However, valuable proteins and minerals from the barley grains are also retained in the pot ale. Therefore, after emptying the still, the pot ale is concentrated through evaporation and sold as high-quality animal feed.
Since large pot stills have a wall thickness of only a few millimeters (about 3/16″), they are very sensitive to both overpressure and negative pressure. The worst-case scenario is the collapse of a still due to negative pressure. When the distillation is stopped, and the pot ale is drained, and the pot still cools down, negative pressure is created inside. If the pressure becomes too high, the pot still implodes with a loud bang. To prevent this, every pot still now has an automated pressure relief valve that maintains the pressure balance with the environment. Another vent valve is used for filling and draining the stills, which is usually operated simultaneously with the pumps.