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Theory of vapour pressure vs. atmospheric pressure

Under thermodynamic equilibrium the vapour pressure is the pressure exerted by a vapour with its condensed phase in liquid phase.

If the temperature of the liquid is increased the kinetic energy of the molecules increase. The result of the increased kinetic energy is that an increased number of molecules transition from the liquid to the vapour and thereby increasing the vapour pressure.

The image to the right, figure 1, shows the particles in equilibrium between liquid form and vapour form.

The amount of kinetic energy and therefore temperature required for the particles to transition from liquid to vapour (evaporation) is reduced as the pressure above the fluid is reduced. Figure 2 below shows the relation between boiling temperature and pressure for four common liquids.


Figure 1

Vapor pressure of four liquids

Figure 2

Azeotropic Point

The azeotropic point for a water-ethanol mixture is 95.63%. This is the highest theoretically purity that can be achieved, through distillation under atmospheric pressure. Azeotrope or “constant boiling mixture” is the point at which a mixture of two or more components are boiled and the resulting vapour is made up of the same proportions as the base fluid.

The azeotrope changes as the atmospheric pressure changes. The theoretical maximum purity that can be obtained in the still “Eve” is 100% ethanol due the low pressure environment which the mixture is being boiled in.

The graph to the right, figure 3, shows the vapour to liquid relation for an ethanol-water mixture under atmospheric pressure.

During distillation a theoretical model is used to calculate the boiling point of the individual components consisting mainly of water and ethanol. This model is then compared to the actual values being measured in the still. The difference between theoretical model and actual readings “delta” are the set parameters of the still.

The Antoine equation is the theoretical model used to predict the boiling point of the various fluids.

vapor-liquid equilibrium

Figure 3

Antoine equation for the prediction of boiling point for water and ethanol

Variables for different fluids. Figure 5

Figure 5


The below example of the still set point is on the principle of the difference between theoretical and actual measurements.


Figure 6


Figure 7

The graphs above show a still operating pressure of 65 mbar absolute. Under this pressure the boiling point of ethanol is 21 degC and the boiling point of water is 38 degC.

If the still delta set point is set to 0 degC, the still control system will manage the cooling to the deflagmator to ensure the still head temperature remains at 21 degC. This will theoretically result in only ethanol vapour to pass through the deflagmator and all water vapour to be condensed and returned as reflux to the column below.

If the delta set point of the still is set to 2 degC, this controller would maintain a still head temperature of 23 degC. This would result in a vapour mixture comprising of mainly ethanol with a small component being water.

The set point can therefore control the desired ABV% during the distillation process.

The below figure 8, is actual data taken during a second distillation

Typical Distillation Chart

Figure 8

Copper contact and importance in sulphury aroma removal

“Eve” contains a total of 19m2 of copper contact. This is housed inside the column in the form of small pipes with a diameter of 15mm. Copper is well known to remove the sulphury taste in distilled spirits. Till this day this phenomenon is not clearly understood, however the below study shows the comparison between a stainless steel still and a copper still. A clear reduction in dimethyl trisuphide (DMTS) is observed. This is the component promoting the sulphury aroma in spirits. The human perception threshold for DMTS is approximately 0,1µg/litre with typical values in spirits being between 1µg/litre to 6µg/litre.

Compound level

Figure 9

Sulphur dioxide (SO2) reduction in low wine prior to distillation

There are three common methods for the reduction of Sulphur dioxide (SO2) in low wine prior to distillation. Each of these methods has pros and cons and we highly encourage reading the research article published by Institute of Brewing and Distilling (IBD). “SO2 reduction in distilled grape spirits by three methods” [Qingxuan Zhang, Jinhua Du,* Yuhong Jin, Zhiyun Zhao and Yingya Li] Published online in Wiley Online Library: 24 October 2013

    The methods listed in the article are:
  1. Calcium oxide
  2. Powdered activated charcoal
  3. Hydrogen peroxide

The method chosen for this specific line of brandy is the addition of hydrogen peroxide to the base wine 12 hours prior to distilling. Hydrogen peroxide was chosen as it performed best at retaining the lighter components. This in conjunction with the cold distillation ensures a crisp and fresh spirit.

Aldehydes formation and prevention

A large proportion of the aldehydes formed during distillation is acetal and it contributes to a pungent and unpleasant taste in the distilled spirit. During distillation acetal favours high ethanol percentage and high temperatures. Due to the low distillation temperature, the formation of a acetal is reduced resulting in a much “softer” spirit being produced.

“SO2 reduction in distilled grape spirits by three methods” [Qingxuan Zhang, Jinhua Du,* Yuhong Jin, Zhiyun Zhao and Yingya Li] Published online in Wiley Online Library: 24 October 2013; Page 318