ABV dependent volatility of fusel oils

[Edwin Croissant]

On first sight one would expect that methanol with its lower boiling point would concentrate in the heads and that isoamyl alcohol with its boiling point of 130 degrees Celcius would not show up in the distillate at all. Reality is not that simple. The volatility of fusel oils and methanol in a water-ethanol mixture are dependent on the ethanol concentration (ABV) as reflected in the following graph:

Figure 8. Relative volatility of congeners and ethanol.PNG

For methanol this means that, contrary common believe, methanol is released rather late in the distillation process and isoamyl alcohol (sharp and burning odor) is carried over rather early during a simple distillation and spread out over the distillate during a second distillation:

volatility.png

It looks to me that making a neutral with a pot still means making cuts in the stripping run. (Isoamyl concentration is shown in ml/100ml pure alcohol and in mg/l distillate )

When a packed column or a column with many plates is used, the fusel oils concentrate in that region of the column where the volatility is the same as the ethanol water mixture and will therefore not reach the top of the column as shown in the following drawing:

Figure 3.Typical Concentrations of Compounds in the Rectifier.PNG

While searching the internet I found two things in relation to fusel oils that might be of interest:

When distilling is done with a packed column the fusel oils do not reach the top of the column and remain therefore in the boiler after the run. In some recipes, part of the backset is recycled to form the base of the next wash. Fusel oils are according to the article Fusel Oil Recycle—A Silent, Odorous Killer 10 to 15 times more toxic to yeast then ethanol. Maybe this is the reason that the fermentation process stalls after many generations. The fusel oils can perhaps be removed at the end of the distillation run by going from fractional distillation mode to pot still mode.

When the fusel oils are not removed in a column, the fusel oils concentration will rise to a level that demixing take place. This instantly lowers the boiling point of the ethanol-water mixture, causing a uncontrolled evaporation which forces the fusel oils out of the top and bottom of the column. The heat required for the evaporation is retrieved from the supplied vapor and the resulting condensation result in flooding. After the hiccup the column stabilize again until the next disruption (fusel oils induced column hiccups). It seems to me that the more liquid the column contains the more fusel oils can be tolerated, the least sensitive a column is for these hiccups. A column with plates or a packed column with a bubble ball or torpedo underneath should be therefore be more stable to run.

Happy distilling,

Edwin