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Mark Borgerson

In article ,
says...
Mark Borgerson wrote:
In article ,
says...
SNIP
You need to get back to the gas law to see where this error lies. You
have to *create* the vacuum. That requires a HUGE increase in volume
for whatever the initial headspace is. For this to happen you need a
much longer tube to start with.

You seem to have missed the fact that I proposed filling the tubes
completely with water so that the initial head space would be zero.

No, it won't be zero. It can't be. If it is, then you have a solid
liquid stream, and it's just a siphon. You have to have headspace. And
it has to be sufficient to maintain separation of the seawater and
freshwater to prevent contamination when filling the tubes. And it has
to be large enough to prevent percolation carryover when boiling is
initiated.

At that point you release the pressure on the water and it falls
to the point where water weight plus vapor pressure equals 1ATm.

A solid liquid loop will not separate into two separate columns. They
have to be separated by a headspace. You can heat the seawater side and
create a headspace by liberating dissolved gases, then let the columns
drop to create vacuum, but you will have contaminated the freshwater side.

The head space is generated by the evaporation (or boiling) of some of
the water in a column. It's exactly the same principle that you get it
you fill a closed tube full of mercury and then invert it, placing the
end in a reservoir of mercury. (We call these things barometers.)
You start with no head space, but when you invert it, VOILA!
head space appears as the mercury sinks to a level where the weight
of the mercury equals the atmospheric pressure. You get a much
better vacuum with mercury, since it has a much lower vapor pressure
at room temperature.

A column of water will behave the same way. The column just has
to be much taller.

Some of the historical references on water barometers mention that,
despite precautions, the water in the barometer eventually got
contaminated with dissolved gases and they lost their accuracy.

At that point, you essentially have two water barometers,
interconnected at the top. One is salty and warm, and
one is fresh and cold. Neither need be too much longer
than 33 feet. The actual height of the water will be
less than 32 feet by a factor dependent on the temperature
of the water in the warm side.

The real practical problem lies in the addition of the dissolved
gases in the seawater to the water vapor in the headspace.
What we have here is a rather inefficient degassing column.
I spent a lot of time degassing seawater while working on
my MS in chemical oceanography. I was trying to measure
the dissolved hydrogen in seawater, and the oxygen, nitrogen,
methane, and other gases kept getting in the way!

Getting rid of the disssolved gases in the headspace and
as bubbles forming on the sides of the tube is going to
be a major headache.

Not a headache, an impossibility (they're not really dissolved at that
point though) :-) That, and the increase in pressure due to water vapor
will make this an oscillating, self-quenching system. It'll require
more and more heat as the partial pressures of the non-condensables
increases, and the column heights will drop as the pressure goes up,
with the diffusion path increasing the whole time.\

I agree with that part---except for the oscillation part. I think
the processes are slow enough and the thermal and physical masses
are high enough that the oscillations will be damped out and you
will see a slow change to equilibrium with little or no overshoot.

Mark Borgerson

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