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Re: [Distillers] Vacuum

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  • Mike Nixon
    George wrote: Subject: [Distillers] Vacuum This is probably aimed more towards Mike Nixon but anyone can answer if you can help. I am hung up on a problem that
    Message 1 of 2 , Nov 5 2:31 PM
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      George wrote:
      Subject: [Distillers] Vacuum

      This is probably aimed more towards Mike Nixon but anyone can answer if you
      can help.

      I am hung up on a problem that I know cannot be right but I do not know the

      If you have a tank that is kept warm with ethanol in it that is connected to
      a empty tank that is kept cold. You apply vacuum to the whole thing until
      the ethanol starts to boil. The vapor would move across to the cold tank and
      condenses. Wouldn't the condensing vapor create as much vacuum as it

      If the vacuum pump was shut off after this starts, what causes the process
      to stop. If you create a cubit meter of vapor on one side (taking heat from
      the tank, any applied heat and the air around it) and you condense a cubit
      meter of vapor on the other (transferring heat to the cold air around the
      tank), would your vacuum remain the same. I know their has to be something
      that will stop this process without any additional vacuum being applied. I
      know it can't, but Why.

      Thanks in Advance
      Nice reasoning George! The answer to this is that there is nothing to stop
      the process if you supply energy to keep it going.

      Lowering the pressure in the system so that it is lower than the vapor
      pressure of mix in the boiler will cause it to boil, and the composition of
      the vapor would be higher in volatiles than the original mix. If you then
      shut off the vacuum pump, the mix would continue to boil until the system is
      saturated with vapor, then stop. The heat to boil the mix would come from
      the mix itself, so that would cool if you provided no additional heat. If
      you provided no more heat, then this cooling would lower the vapor pressure
      of the mix and it would stop boiling fairly soon, but even if you maintained
      the temperature at its original level, it would eventually stop anyway, but
      the concentration of the saturated vapor would be higher.

      Vapor condensing in the receiving vessel gives out heat, so the receiving
      vessel would warm up, and this too would stop collection fairly soon if that
      heat were not removed.

      So, what you have is a heat engine! Lower the pressure in the system so it
      is lower than the vapor pressure in the boiler and you start boiling. Add
      heat to the boiler to keep this going. Remove heat from the receiving
      vessel and vapor will continue to condense. Net result is transfer of heat
      energy from boiler to receiving vessel, heat that is carried by the vapor
      molecules. So you start the system going by lowering the pressure, then you
      can turn off the vacuum pump and keep the process going by doing work,
      pushing heat into the boiler and extracting it from the receiving vessel.
      The situation is no different to distilling under ambient pressure, whatever
      that might be on a particular day. Take your still up a high mountain, and
      it will work just as before. The only difference being the temperatures at
      which it works.

      In a perfect world, you could have a perfectly heat insulated system that
      lost no heat to the outside world, and you could heat the boiler with the
      heat given off in the receiving vessel. This is the basis of 'heat
      management' in large distilleries, where the heating bill can be very high.
      Unfortunately, entropy being what it is, you can never get perfect
      insulation nor perfect heat transfer, so the dream of a perpetual motion
      machine is still beyond our reach!

      Mike N

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