Evaporation during open fermentation

[Lassiter Distilling Co]

We recently modified some tanks to make them easier to get into and out of for cleaning, essentially removing the entire top of the tank. We've always done open fermentation, but recently I've noticed that the liquid level in our fermenter seems to be lower than where it started. 

Do those of you who do open fermentation experience significant loss in yield as a result? I haven't seen a dramatic decrease in yield yet, but I just noticed the lower liquid level today. we'll see if a significant amount of the alcohol had evaporated when I run it off on Saturday. 

In any case, if you run open ferments and experience evaporative loss, what do you do to combat this?

[meerkat]

The volume will decrease, and part of it is due to the evaporation of the ethanol, but the main reason for the volume decrease is the loss of CO2.

If we take the hypothetical example of 1000 kg of water with 200 kg of sugar added we would have a volume of about 1120 litres, since 1 kg of sugar increases the volume by about 0.6 litre. Roughly 100 kg of the sugar is converted to CO2 and is evolved. The other 100 kg will be converted to alcohol with an SG of about 0.8 and ignoring shrinkage we would have a volume of 1080 litres, or a loss of 40 litres.

In large commercial distilleries the CO2 that is evolved is taken via a scrubber to recover the alcohol that is carried off. A very rough calculation indicates around 3 or 4% of the ethanol is carried off with the CO2 and in large distilleries this is worth recovering. If you lost 3% of your 80 litres that is another 2.4 litres. So the evaporation losses are real (2.4L) but small compared with the CO2 loss (40L).

[nabtastic]

not trying to be a smart*** but check for leaks too.  you'll lose some liquid that would have formed condensate on a cover as well

[Distiller Mike]

On 2/17/2017 at 6:09 AM, meerkat said:

The volume will decrease, and part of it is due to the evaporation of the ethanol, but the main reason for the volume decrease is the loss of CO2.

If we take the hypothetical example of 1000 kg of water with 200 kg of sugar added we would have a volume of about 1120 litres, since 1 kg of sugar increases the volume by about 0.6 litre. Roughly 100 kg of the sugar is converted to CO2 and is evolved. The other 100 kg will be converted to alcohol with an SG of about 0.8 and ignoring shrinkage we would have a volume of 1080 litres, or a loss of 40 litres.

In large commercial distilleries the CO2 that is evolved is taken via a scrubber to recover the alcohol that is carried off. A very rough calculation indicates around 3 or 4% of the ethanol is carried off with the CO2 and in large distilleries this is worth recovering. If you lost 3% of your 80 litres that is another 2.4 litres. So the evaporation losses are real (2.4L) but small compared with the CO2 loss (40L).

I know this is an old post. Good info here. A healthy ferment will off gas 8Cf. per 1000L per 24 hours on average with a distillers yeast and good Brix. There will obviously be settlement as fermentation takes process. You should see condensation on the vessel walls during this process which leads to a loss in volume. One aspect that I have not seen in this thread is, do or are you controlling the temp during the first 72 hours (minimum) after inoculation? The fermentation process generates it own convection and can/will speed up evaporation. Lastly, 99% of distilleries open ferment.