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Cooling the vapour

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  • Ackland, Tony (CALNZAS)
    Jim ... The answer is the one that will give the most contact between the vapour and the coolant. If possible, see about running a cooling coil inside the top
    Message 1 of 1 , Jun 8 1:20 PM

      >My question is this, On the reflux tower is it going to
      >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 answer is the one that will give the most contact between the vapour and
      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.


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