The final temperature will depend on how good your still is at
fractionating off the various components in the distillate.
If its just a pot still, this is poor, and you'll need to run up to about
94-96C to get any decent yield. In taking it this high you'll probably
segregate it, and keep that below about 92C for drinking, and re-run the
92-96C stuff with the next batch of wash.
If you've got a typical reflux still however, the ethanol will be coming
off in a more distinct band. If you watch the temperatures as it heats up
(without using a thermostat etc), it will get up to the 80's and then just
sit there for quite a while as the ethanol comes off. Only towards the end
of the run, as the pot starts to get low in ethanol will the temperature
start to slowly increase again. I find that by the time its showing 94C,
its only dripping out slowly and has dropped in purity down to about 40% -
so not worth any more collection.
I suspect that with the really good fractionating columns, tall & with
heaps of reflux, that this plateau is even lower in temperature, and more
It all comes down to the performance of your still as to where it will run.
I don't believe that you need to control the power input to the pot,
provided that the maximum power you're giving it is correctly matching the
column diameter, type of packing, and reflux ratio. If all this is in
balance, you should be able to just leave it turned on, and not mess around
with trying to adjust power input to deliver a particular temperature.
The temperature of the vapour is determined by % alcohol in it - nothing
else (quiet from those of you running vacuum stills - shhhh!!). If you
have a wash at 10% alcohol, it will be boiling at 93C in the pot. Any
further heat you apply to it will just go into making alcohol vapour (at
55% purity), not heating the liquid any more. The only time the liquid
(and hence vapour above it) will get hotter than that is when it gets lower
in alcohol content (eg inc to 96C at 5% in the wash). Same sorta thing is
happening at the top of the reflux column ... the alcohol coming off the
top of it will be in equilibrium between its temperature and its purity...
so if it coming out at 80% purity the vapour temperature at the top there
will be about 80C. The only way this will change is if the purity changes.
So go deal to the column design (height, diameter, packing & reflux
ratio), not the amount of heat you're putting in the base of it
for a graph of the
relationship between temperature and purity.
I think that the guidelines to "not go above 80" are somewhat wrong.
Some of it is based on the misconception that because the boiling point of
pure (100%) ethanol is 78C, and that for pure (100%) 2-propanol is 82C,
that by getting somewhere just above that for ethanol, but below propanol,
you're only going to get pure ethanol coming off is just plain wrong. It
doesn't work like that - get a mixture of different alcohols & water
together and the whole relationship changes - they will all be coming off
in some proportion, over the whole range of temperatures. For ethanol &
water this can be anywhere between 78 & 100 C
The other part to it has some merit, but needs to be tempered by knowledge
of your still's performance. Sure, if you have a really good reflux still,
the vapour temperature at the top of it should be in the low 80's for most
of the run. Once it starts increasing, then its indicating that you're
running out of ethanol in the pot. But if your still design is only
mediocre (as most are), and not pushing the 94%+ purity as a norm, then
this could be happening while theres still a lot of decent ethanol to be
collected. Gees- you'd probably give up on pot stills.
There is no great risk in pushing your still up to the 90's (vapour
temperature measured at top of column). The biggest problem is that those
tails will start to taint the flavour of your good distillate. So collect
them into a seperate container, and jugde for yourself by nose & taste
which you want to use. Those that aren't suitable for drinking will still
have worthwhile alcohol in them, so collect them until you have enough to
do a seperate redistillation (water them down with a little water first),
or simply add them to the next wash that you distill.
for guidelines on doing