Dear Glen Walpert:
On Sep 28, 7:09 am, Glen Walpert wrote:
On Thu, 27 Sep 2007 18:06:47 -0700, "N:dlzc D:aol T:com \(dlzc\)"
....
Gravity.
Wishful thinking. Where are you going to get the
feedwater containing no noncondensible gasses in
solution? In all real distillation plants a continuosly
operating vacuum pump is required to maintain
vacuum and prevent the condensers from filling with
noncondensible gasses. There is no way you are
going to eliminate the vacuum pumps with any kind of
inverted tube arrangement.
But they don't have to be large, and they don't
even have to run continuously (just frequently).
There are also going to be controls...
The vacuum pumps need to be sized to the load,
and it is not a foregone conclusion that a larger
pump running intermittently would be more efficient
than a smaller one running continuosly. Consider
also that the vacuum pump cannot pump out just
the noncondensible gasses, it must pump out the
gas mix in the condenser which will be mostly
water vapor -
So you can get some condensate here, but it will likely have "vacuum
pump oil" in it...
the pumping rate establishes the percentage
noncondensible gasses in the condenser, amd
the optimum rate needs to be established as part
of a distillation plant design.
You could even run it without a vacuum pump
until it shut itself down, drop and purge the gas
bubble, then "forklift" your pipes back up.
Does this use less energy per gallon produced?
Available on a desert island. Simple block and tackle would do.
Since the (de)compression rate is likely low, and the condensation
production rate is necessarily low, if you were not using animal
power, it could be *more* efficient. But you still have to supply or
waste a good deal of heat.
....
For reasonable efficiency real distillation plants
are multi-stage, where the latent heat of
condensation from one stage is used to boil
feedwater in the next stage, with up to 5 stages
being used in larger plants (in the days before
reverse osmosis made them uneconomical by
comparison).
Scaling is real problem too...
True, but one which can be solved by limiting brine
concentration and with chemical treatment and/or
periodic cleaning.
In the case of the marine vacuum distillation unit, they simply have a
constant flow of brine. Probably need to have a "tube within a tube"
to refresh the fluid near the boiling interface.
Sucessive stages operate at lower pressures, and
corresponding lower temperatures. The 1100 or
so BTU required to boil one pound of water can
thus boil up to 5 pounds of water instead.
You still need enough thermal gradient to get the
heat to flow through all those heat exchangers.
By using low thermal differentials between the hot
and cold ends you either reduce capacity to a
pittance or require huge and expensive heat
exchangers, in either case not competitive.
TANSTAAFL.
... a characteristic article ...
http://www.hcn.org/servlets/hcn.Arti...ticle_id=17136
This was not proposed to be a source of free
energy, violate the second law of thermodynamics,
or poke fingers in anyone's eyes.
As usual with this sort of article there are no
meaningful numbers included, perhaps because
a complete design analysis has not been
done.
More than likely omitted because:
- the reporter's eyes were glazing over, or
- they are working on a patent (since you can probably even patent
cheese now).
I think it was something that someone could do
fairly cheaply, to get drinkable water from salt
water. In other words "a graduate or
undergraduate college project".
Doing an analysis of this approach would be a
good student exercise. Not much point building
one without doing the anylysis first - a complete
engineering analysis including the selection or
design of all heat exchangers, mist eliminators,
pumps, piping etc., including both performance
and cost calculations. It is always cheaper to
optimize a pencil and paper or computer model
than hardware, especially for something so well
understood as heat transfer and fluid flow.
I just wonder if you get any improvement in
what is left in the brine, vs. what also
evaporates at the lower temperatures...
I doubt if that would be much of a factor. What
contaminants would be in the feedwater which
would evaporate less compared to water as
boiling point is reduced by low pressure?
Water does get involved in some azeotropes (some alcohols), so
depression of boiling point would not help there. And
thermodynamically, if one of the things you were trying to remove
became a solid at high vacuum (NaOH maybe?) it might help.
The biggest issue with distillate quality is
carryover; a fine mist of unevaporated water
droplets are inevitably produced by boiling
regardless of temperature, and while most
of these can be separated out, some always
make it through to the condenser. This is a
big issue where biological contamination
exists in the feedwater,
Such as natural brines...
requiring chlorination of the distillate to
make it potable. It might be possible to
eliminate this factor by eliminating the
boiling of bulk liquid, and instead
evaporating from a thin film of water flowing
over the heat exchanger surfaces, but I
doubt if it would be cost effective. Perhaps
it would be another good student exercise.
I wonder if the increased viscosity of droplets at lower temperature
would assist in more efficient removal?
Thanks for the discussion...
David A. Smith