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47852Re: Vacuum Distillation Column

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  • tgfoitwoods
    Apr 4 9:06 PM
      bjpow,

      This is a very interesting restatement of the oldest misconception in
      spirit distillation, that by holding the boiling point (at any pressure)
      lower than the boiling point of water but greater than the boiling point
      of ethanol, only ethanol will vaporize from the wash. While it sounds ok
      at first blush, it's just not true.

      The proportions of water and ethanol in tha vapor are controlled by
      their respective vapor pressure as a fraction of the ambient pressure,
      even when that pressure has been lowered from atmospheric nominal 760mm
      Hg. The individual vapor pressures are determined by both the vapor
      pressure of the pure liquid (water or EtOH) and the mole fraction of
      that compound, that is, what fraction of the total molarity of the
      solution is that concentration. None of this changes with pressure.

      An unhappy side effect of keeping a depressed wash temperature is that a
      water-ethanol mixture has a very sepcific boiling point (at any given
      pressure) and until the wash is heated to that temperature, vaporization
      is painfully slow.

      In summary, if you are able to hold a 10% wash, which boils at 90C, at a
      steady temp of 80C, just a bit over the boiling point of ethanol, you'd
      get a vapor with about a 60% ethanol content, but you'd get it so slowly
      that you'd never do it again.

      Note that I'm speaking wash and vapor temperature here, and NOT the
      temperature that reflux 'stillers measure at the tops of their columns;
      that's something entirely different.

      Zymurgy Bob, a simple potstiller

      --- In Distillers@yahoogroups.com, "bjpow42o" <bjpow42o@...> wrote:
      >
      >
      >
      > Thanks for the information Sid.
      > Right now I have a small scale experimental setup without a column.
      The vacuum pump pulls down to 40 torr, that's what I meant "near"
      vacuum... why does that scare you? I read somewhere that at pressures
      of 70 torr or less the water/alcohol azeotrope does not exist. That
      made me think that I could get nearly pure alcohol through a single run
      simple distillation. It sounds like this is not the case. If I set the
      temperature no higher than the boiling point of alcohol at 40 torr could
      I not get pure alcohol off?
      > I am looking to scale up the design to about 50 liter capacity. I was
      planning on using SS scrubbers in the column if I can, but I am unsure
      of the size of the column and reflux ratio (if needed?). I also read
      somewhere that it is better to have a wider column when distilling under
      a vacuum for some reason. Have you heard this before? Any ideas on the
      height of the column (didn't understand "by as high as particle")? I'm
      trying to avoid getting another engineering degree if possibile :)
      > Any further information from your past research would be greatly
      appreciated.
      > Thanks again!
      >
      ----snip----
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