Re: [Distillers] Vacuum
- George wrote:
Subject: [Distillers] Vacuum
This is probably aimed more towards Mike Nixon but anyone can answer if you
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!