Posted by nospam on November 5, 2003, 12:58 am
It would seem that water distilled using passive solar could still contain
liquid contaminants that boiled at temperatures around or below that of
water -- certain alcohols coming to mind.
Is there a relatively simple post-distillation treatment people tend to
use for such things, or is it simply assumed that most of us are going
to draw water from a non-liquid-polluted source?
Posted by TheDoc on November 5, 2003, 1:45 am
Post distillation treatment of alcohol ... hmmm how about drinking it !!!!
Posted by nospam on November 5, 2003, 3:22 am
Can't say the though failed to cross my mind. But let's say it was
methanol, just hypothetically?
Some sort of really complex thermostatic system to determine exactly what's
boiling would be horrendously against the simplicity of a basic solar still
design. A mechanism could be created to throw away the first x% of a batch
which, though wasteful, would eliminate alcohol from the equation.
Are there any toxic, common liquids that boil at just over 100C? Someone
tell me I'm being excessively paranoid?
Posted by Duane C. Johnson on November 5, 2003, 4:21 am
The very best stills have a kind of pre heater to boil off
anything with even a slightly lower vapor pressure.
See the Gilmont laboratory water still:
And other advanced concept stills.
Well if it boiled over 100C it wouldn't boil.
You would worry about things that boil under 100C.
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Posted by Ecnerwal on November 5, 2003, 12:18 pm
Two comments - the typical solar still (that I'm familiar with) doesn't
actually boil the water. While the water does go through vapor phase,
it's more a dance of relative humidity than a boiling process, and is
more akin to condensate on a windowpane than a typical boiling still.
There's no practical way to prevent this type of still from collecting
low-boiling fractions. They work quite well for desalinization, and
deminralization of typical tap water, but not so well for water polluted
with benzine, methanol, etc...
For a boiling still (quite reasonable when driven by a solar
concentrator, say), contaminants with higher boiling points are not that
much of an issue, as they won't boil off until the water is gone, and if
you keep refilling the water to keep the still from boiling dry, they
won't ever boil off. Draining the boiler from time to time is probably a
good idea, more to prevent excessive mineral build-up, but it would also
remove any concentration of higher boiling point contaminants.
Lower boiling point fractions can be removed by using a temperature
sensor to divert collection when the boiler is below 100C/212F (adjust
for altitude, or you'll never collect any water at high altitude). It
need not be "very complex" - the sensing end is a few bucks, an
automatic diversion valve would be the expensive part, but that might be
found in surplus for cheap; or you might just involve an operator.
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