I'm exploring the feasibility to design a low power (500W electrical powered and air cooled) continuous still and was looking for a way to remove the unwanted higher alcohols like methanol as you cannot make cuts with this type of distilling. During my research I stumbled upon a 1996 study called: A study on the possibilities to lower the content of methyl-alcohol in eaux-de-vie de fruits. This is a free publication of the European Commission and can be downloaded here.
I was expecting that due to its lower boiling point methanol concentrate in the heads. Surprisingly this study found the opposite: methanol concentrate in the tails due to molecular polarity (I'm not a chemical engineer, so don't ask me what this this means
). It seems that more factors then boiling point alone determine this separation process. Sometimes one picture can say more then a thousand words so I made a graph in Veusz based on the data of distillation number 5 of the above mentioned study:
I found a similar graph on the dutch forum which I presume is from a Bosnian or Serbian report about plum brandy as I traced it origins back to a Serbian forum about distilling. The black dots are alcohol strength in ABV the bars are the methanol concentration in mg per liter absolute alcohol, destilacija means distillation and frakcije means fractions in Bosnian according to Google translate. Another graph surfaced on that Serbian forum showing the relative volatility of congeners and ethanol from the chapter “Chemical Aspects of Distilling Wines into Brandy” in issue 137 of “Chemistry of Winemaking”. Below 40% ABV methanol is less volatile then ethanol! A similar graph can be found in the chapter “From pot still to continuous stills: flavor modification by distillation” in “The Alcohol Textbook”, page 263 figure 8.
Based on the data in the study I think that an economical distiller reusing the tails from the previous run in the next run will end up with a leveled out 50% higher methanol concentration compared to the initial run after three or four runs. In that case 1 liter of pear brandy (40% ABV) would contain more then 6 ml of methanol. Ingestion of 10 ml of methanol can make you blind
. I think that people making pear brandy, grappa etc. should think twice about reusing their tails. Saving these tails for a separate run is, I think, a really bad idea. Methanol can be removed with a dedicated column (30 to 50 plates) fed with at least 70% ABV. The few hobby build continuous stills I found incorporate such dedicated column.
Please note that I am not (yet?) a distiller so everything I write in this context is from a theoretical point of view only.
Happy distilling