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Some of the historical references on water barometers mention that,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

N:dlzc D:aol T:com (dlzc) brought forth on stone tablets:
Dear Richard Casady:

"Richard Casady" wrote in message
...

On Sat, 29 Sep 2007 19:44:35 -0700, Keith Hughes
wrote:


Sounds like perpetual motion to me, but I'm
having a hard time envisioning what you're
describing above.

Of course you are, since it is basically nonsense.
No mention of where the energy comes from.


Links were provided. "waste heat" (from what process?) and / or
"solar heat" have been cited so far. All the vacuum does is move
boiling temperature closer to ambient. Making more common
materials suitable for this application.

David A. Smith




.... and much more importantly, making the process viable with far
lower quality heat.

bob
s/v Eolian
Seattle

On Mon, 08 Oct 2007 10:13:17 -0700, Keith Hughes
wrote:

...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.
....
A solid liquid loop will not separate into two separate columns.
....
Keith Hughes

Perhaps it would be better for you to check what is the maximal rise
(head) of a syphon. Can you guess what it might be?

Brian Whatcott Altus OK

On Mon, 08 Oct 2007 05:33:39 -0700, Keith
wrote:

Boiling Point Elevation
The boiling point of a solution is higher than that of the pure
solvent. Accordingly, the use of a solution, rather than a pure
liquid, in antifreeze serves to keep the mixture from boiling in a hot
automobile engine.....
From:
http://www.chemistryexplained.com/Ce...roperties.html


Actually, no. Ethylene glycol in its pure liquid state boils near
200 degC
http://www.dow.com/ethyleneglycol/about/properties.htm

It is usually cut to 50% dilution for use as an antifreeze.

Brian Whatcott Altus OK

On 2007-10-08 18:17:58 -0400, Mark Borgerson said:

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.

Though I consider this whole discussion impractical, I haven't seen
anyone mention that the fresh-water side will be drawn down fairly
regularly. And, of course, the sea water side will be replenished from
time to time.

Suck hard enough on the fresh-water side and you get even better
"vacuum" at the top. (Dissolved gasses are likely to be a problem,
though.) Cool the fresh-water side and water vapor will condense there
-- the whole point of the exercise.

Thinking only momentarily on a problem that I have little interest
in... if the fresh-water side is evacuated to the point that the
salt-water side is slightly below the top, every once in a while (or
perhaps often), the fresh-water side will be empty and only the
previously-dissolved gasses evacuated.

The required evacuation pumps and one-way valves sound like the problem
at the moment.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's new pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI pages: http://homepage.mac.com/jerelull/BVI/

Brian Whatcott wrote:
On Mon, 08 Oct 2007 10:13:17 -0700, Keith Hughes
wrote:

...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.
...
A solid liquid loop will not separate into two separate columns.
...
Keith Hughes

Perhaps it would be better for you to check what is the maximal rise
(head) of a syphon. Can you guess what it might be?

Uhmmm, maybe you should follow the thread. If you *pump* the water up
the columns, evacuating the headspace as you go, until the columns meet,
you can siphon pretty much any height you want. Until outgassing
creates a headspace. If the bases of the columns are at differing
heights, you have a siphon until that happens.

Keith Hughes

Mark Borgerson wrote:
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.

You seem to be forgetting that the whole purpose is to Purify/desalinate
the water. No initial headspace = single process stream = contamination
on the distillate side.

(We call these things barometers.)

Except when we call them Mcleod gauges...

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.

True, but you need to keep the context - water purification. The
contamination control features are as crucial to the operational
constraints as are the physical parameters. Thus, you have to *Start*
with headspace. Sure, you could purge the freshwater side until the
contaminants are removed, but by then most, if not all, of your
production will be wasted.


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.

Yes, you can only deaerate so far prior to filling. Personally, I've
never seen an absolute pressure water barometer. IME they are primarily
used in an inverted u-tube configuration for DP measurements. BTW,
mercury barometers suffer the same fate, primarily through oxidation of
the mercury, changing the density. Just look at that almost black film
layer on any old barometer.


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.

You may be right, but I doubt it. Unless you control the temperature
versus pressure relationship, which is virtually impossible with any
passive heating process, then I'd expect self quenching would result in
an oscillating system.

Keith Hughes

On Mon, 08 Oct 2007 17:36:59 -0700, Keith Hughes
wrote:


... If you *pump* the water up
the columns, evacuating the headspace as you go, until the columns meet,
you can siphon pretty much any height you want.
...
Keith Hughes


No, you cannot syphon to any height.
Yes, you can pump to any height, within reason.
There is a difference.
If you pump to 40 feet head, quite filling an inverted u-tube, for
instance, when the pump is halted, the water falls until a particular
maximum height is no longer exceeded, if the foot of the u-tube is
opened to water whose surface is at atmospheric pressure..

Brian W

Brian Whatcott wrote:
On Mon, 08 Oct 2007 05:33:39 -0700, Keith
wrote:

Boiling Point Elevation
The boiling point of a solution is higher than that of the pure
solvent. Accordingly, the use of a solution, rather than a pure
liquid, in antifreeze serves to keep the mixture from boiling in a hot
automobile engine.....
From:
http://www.chemistryexplained.com/Ce...roperties.html


Actually, no. Ethylene glycol in its pure liquid state boils near
200 degC
http://www.dow.com/ethyleneglycol/about/properties.htm

It is usually cut to 50% dilution for use as an antifreeze.

Brian Whatcott Altus OK

Since he used the term "solvent", the assumption, at least on my end, is
that he's talking about a solvent/solute system, not a solution of
miscible liquids.

Keith Hughes

Jere Lull wrote:
On 2007-10-08 18:17:58 -0400, Mark Borgerson said:

snip

Though I consider this whole discussion impractical,

I don't know that I'd call the *discussion* impractical; the device
certainly. Kind of the point of the discussion.

I haven't seen
anyone mention that the fresh-water side will be drawn down fairly
regularly. And, of course, the sea water side will be replenished from
time to time.

Suck hard enough on the fresh-water side and you get even better
"vacuum" at the top. (Dissolved gasses are likely to be a problem,
though.) Cool the fresh-water side and water vapor will condense there
-- the whole point of the exercise.

Thinking only momentarily on a problem that I have little interest in...
if the fresh-water side is evacuated to the point that the salt-water
side is slightly below the top, every once in a while (or perhaps
often), the fresh-water side will be empty and only the
previously-dissolved gasses evacuated.

The required evacuation pumps and one-way valves sound like the problem
at the moment.

The whole exercise was to get a passive system. If you're going to add
a vacuum pump, then you just provide continuous evacuation on the
freshwater side, using a demister that drains into the freshwater pool,
to separate the water vapor from the non-condensables. But if you
accept the need for a pump, why use this rather byzantine approach at all?


Keith Hughes