Cooling the vapour
>My question is this, On the reflux tower is it going toThe answer is the one that will give the most contact between the vapour and
>be best to wrap with the cooling water line or run the pipes through the
>column? The column is about 1 meter high by about 63 MM (2½ inches).
the coolant. If possible, see about running a cooling coil inside the top
of the column, as this will give you more surface area for the vapour to
condense on; failing that, running pipes through the column might be the
next alternative (even consider packing some scrubbers around them so as to
increase their amount of cold surface area available). Wrapping plastic
hoses around the head of the column is a very poor way of conducting heat
in/out of the area. If you must do this, at least use metal (copper) pipes,
possibly soldered/braised in place to as to maximise the heat transfer.
Rember, you need to be supplying the same amount of cooling at the top of
the column as what you put in to heat the pot. Quite a chalange if you
start using big kW to heat the pot.
What ever you do, only do the cooling at the very top of the column, ABOVE
the packing. Don't put the cooling pipes through the column at various
heights. When you do this, you upset the equilibrium balance between the
temperature gradient and the liquid and vapour flows up the column.
Picture a portion of the column - it has hot vapour rising through it, and
cooler liquid dripping down into it. The vapour will release a little
energy as some of its water content condenses and as the vapour cools a
little (2-3 C), however this is the heat being used to vapourise a little of
the alcohol content in the liquid, and to also heat up the liquid a bit.
Heat in = Heat out.
If you put a cooling source midway through the column you disrupt this; the
liquid dripping down will be quite a bit colder, and the rising vapour will
no longer have enough available heat content to get the alcohol out of it.
So this portion of the column doesn't do anything. No gain in alcohol % .
No point having it there. the liquid is going to have to drip down quite a
bit further, and pass quite a bit more vapour before it gets back up to
working temperature. The rising vapour is going to be cooled too - so it
won't be as effective on its upward path either. Same reason why you should
insulate the column - avoid unnessesary heat losses sapping the energy from
the vapour. What you are after is a nice steady transition in temperature
up the column, from its base (96C ish) to the top (around 78-79C).
Do all the condensing/ reflux at the top, so that it can quickly come to the
right temperature before it starts it path down the column. Don't be a
skimp on the amount of reflux - the more liquid you return down the column,
the higher % alcohol you will get out of the column. Try to keep the reflux
ratio above 2-3 as a minimum (eg return 2-3 mL/s back to the column for
every 1 mL you keep as distillate), and more like 8-10 for good results.
Without enough liquid being refluxed, the vapour will only be losing a
little water content but not gaining any additional alcohol. Likewise,
don't over-cool the reflux either. Sure you want to condense it, but don't
supercool it by more than a couple of degrees. If you cool it by more than
about 20C, the rising vapour will never have a chance (sufficient energy) to
heat it & vapourise its alcohol content.
I'm trying to put together a wee interactive page where you can play with
these ratios, cooling rates etc yourself and judge the effect it has on the
column performance, but it will be a few weeks away yet.