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Collecting impurities

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  • Tony & Elle Ackland
    A question for the techo s (preferably some serious training in organic chemistry), or anyone with a GLC at their disposal .... Background 1)I know that the
    Message 1 of 1 , Jun 2, 2000
      A question for the techo's (preferably some serious training in organic
      chemistry), or anyone with a GLC at their disposal ....

      Background

      1)I know that the wash is quite a mixture of water and different organic
      compounds, eg all the alcohols (like methanol, ethanol, propanol (1&2),
      butanol ...and so on), esters, etc.
      2) I also know that although these each have their own specific boiling
      point, that this point is only valid for these various compounds in their
      pure state.

      Mix them together and the whole game changes, eg if it were only water &
      ethanol & methanol you CAN'T simply heat to between 64C & 78C and only get
      pure meths, then lift to between 78C & 100C and only get pure ethanol. No
      sir.

      Rather, you get a mixture of various concentrations, over a range of
      temperatures. For binary mixtures you can sorta predict this using
      activity coefficients & partial pressures (though it didn't give me an
      azeotrope for water/ethanol), but things get a little messy trying to do
      this for really complex mixtures (such as we have).

      Questions

      1) Has anyone done a series of sampling of the distillate over a run,
      seeing if there any basic (or complex) rules of thumb of when each compound
      occurs....

      eg methanol... does it all basically come off at the lower temps, or does
      it hang around with the water/ethanol and keep coming off in equal
      proportions right up to the big temps ?

      also the cogeners/fusels ... do these wait right until the end to show up,
      or are we actually collecting some proportions of these right through the
      run as well ?

      2) How does this change with the move from simple pot stills collecting at
      40-60%, through the medium designs doing 80-90%, and then the fractionating
      columns at 95%. I'd presume that the various distributions tighten up, but
      by what amount ?

      3)How much does the presence of the other high&low alcohols change the
      standard "ethanol-water" binary mix prediction ?

      4)Does anyone have a accurate way of predicting either the binary (or
      higher !) equilibrium such as to get the azeotrope occuring (and then
      shifting depending on the pressure/vacuum) (I've had little sucess using
      Margules or van Laars activity coefficients - give the basic shape but can
      keep stepping up to 100% purity)

      hmmmm....I warned ya ....

      Has anyone got any data, or is a wizz at the theory, or handed down bits of
      lab lore ?

      Tony
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