Re: Potable Water - The Third Way.

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.

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Some of the historical references on water barometers mention that,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.

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

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.

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.

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.

Keith Hughes

<<SNIP>>

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

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.  As soon as you release the pressure
and start warming the seawater side, bubbles are going
to form all along the tube as the temperature rises and
the pressure is less than 1ATM except at the bottom
of the tube.

Mark Borgerson

<snip>

Uhmm, a manual valve is a manual valve.  A "one-way" valve is a
checkvalve, and you wouldn't need to close it.

Ahh, no. See below...

It is an example, for illustration purposes.  It's the headspace (i.e.
the amount of volume *not* filled with water, prior to closing the valve
and letting the water columns 'fall').

The point is, whatever your starting headspace volume is, to get
anywhere near a vacuum, the *VOLUME* of the headspace must increase 100
fold. That is the relevance of the ideal gas law. If you start with a
100ml headspace, then to get a decent vacuum, the water columns have to
drop to a point where the headspace is 10L. *AT THAT POINT* you have
sufficient vacuum to support a water column of around 30'.  But the
columns have dropped significantly to achieve that vacuum, and thus the
columns must be much higher, as must the initial water column height.

The *only* way the headspace volume increases is if the water columns
drop significantly, and the only way significant vacuum is created is if
the sealed volume increases tremendously.

No, a lot higher.  You're confusing vapor pressure with the "steam"
pressure during distillation. Huge difference.  Vapor pressure is the
countervailing force (fighting condensation as it were) on the FRESH
water side of the system (and the seawater side).  Vapor pressure in
both columns will be about the same, so you have to boil the seawater
column to get any significant vapor transfer.  This results in *much*
higher pressure, which lowers the columns and increases the vapor path
and....

Yes, because you're mistaking the amount of "vacuum" you'll have
available when the system reaches equilibrium.

Then you are assuming an almost perfect vacuum, which can't happen since
the boiling Must significantly raise the headspace pressure.

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.

Keith Hughes

I see the problem.  I am assuming that you completely fill a 40 foot
tube with water using a pump capable of providing about 16-20 PSIG.
That fills the tube completely with water--at which point you
close the tube (with a one-way valve).  When you release the
pressure at the bottom end, the water falls to the point where the
weight of the water column  is one atm (about 14.7PSIA) minus the
vapor pressure of water at 20deg C.    The vapor pressure of water
at 20C is about 17.5mmHg, or about 2.3% of the 760mmHg standard
atmosphere.

Since a mercury has a density 13.6, the column of water will
be 13.6 * (760- 17.6)mm high.  That's  10.1m high, or
about 33.12 feet high.   In a 40-foot tube, that would leave
about 7 feet of water vapor at the top of the tube and 33 feet
of water below the vapor.

What is the 1 liter to which you refer?

This is not a closed system---the tube is open to a reservoir at
atmospheric pressure at the bottom.

I'm assuming that you start with a head space (or initial volume)
of zero.  You then simply have to evaporate enough water to fill
the top of the tube with water vapor to the point where vapor
pressure + water weight = 1ATM.

Well,  not much higher----only about 17.5 mmHG higher.  But that IS
a lot higher than zero!  ;-)

AHA!,  you're assuming a much higher operating temperature than me.
I was assuming something on the order of 20 to 25C.   You're going
to have to add to your energy budget the heat necessary to raise
the water temperature from 20C to 60C, then.

If the equilibrium pressure is really 1/3ATM,  then there will be
about 20 feet of water in the 40-foot tube and 20 feet of vapor.
If you're going to work at those temperatures and pressures, you
probably need only a  22-foot tube.

Mark Borgerson

<snip>

A quick thumbnail guesstimation at where equilibrium would likely be
reached.  I didn't take the time to calculate the exact heights.

Then I think you would be wrong, unless your columns are significantly
longer than that, probably more like 50+ feet.

There is no vacuum to hold the water up - the vacuum is what you are
trying to *create*.  The water columns will drop until there is an
equilibrium point reached between the external atmospheric pressure, the
height (weight as you state) of the water column, and the pressure in
the headspace (the U-tube).  The water columns *must* retreat, or the
headspace stays at atmospheric pressure.  If the tubes are long enough,
and the initial column heights are high enough, then when you reach
equilibrium, you'll have close to a vacuum and close to 33' water column
heights.  And a lot more empty headspace than you started with.

Use the ideal gas law: PV=nRT

For our evacuation purposes, nRT is a constant (#moles is constant, R
doesn't change, and assume constant temperature), so if you start with a
volume of 1 liter, and a pressure of 14.7 psia, and you want to reduce
that pressure to 1.47psia, then you need a 10-fold volume increase. You
want to reduce it to 0.147psia? then you need a 100-fold initial-volume
increase.

Well, diffusion is the primary mechanism.  What happens when your 'heat
engine' creates water vapor?  It doesn't just immediately condense on
the other side. It creates pressure on the heating side, which does two
things. One, it drives both the water columns *downward*, and it raises
the boiling point on the seawater side (it does, however, make
condensation on the fresh side more efficient as well).  You can't look
at this as a static system where the pressure stays the same or the
column heights stay the same.  It's a dynamic system, and will reach an
equilibrium point with the columns much lower than the initial starting
point, and the headspace pressure much higher.

And don't forget, there will also be significant evaporation (due to low
partial pressures) on the freshwater side that will be in equilibrium
with (and in opposition to) the condensation process. It's not as simple
a system as it seems.

That's why this system *will* work, but it must work very slowly.

My 'guess' would be that the system would end up operating around
4-5psia when equilibrium is reached, which would require a temp of about
60°C (140°F) to maintain boiling.

Here in my neck of the woods, our energy from the sun ranges from about
220-360 BTU/ft^2/Hr measured at normal incidence, depending on the time
of year. A couple of decades ago I worked at a solar test lab and we
tested all kinds of collectors, including swimming pool collectors which
are unglazed (i.e. no cover over them to exclude wind).  Bare copper
tubes, painted black, with no wind, are about 15% efficient at solar
absorption (#'s are from my old memory, so...) when the tubings'
longitudinal surface is perpendicular to the incident angle. However,
with a 3 mph wind (per ASHRAE 95-1981 which we used for indoor system
simulations) that efficiency drops to the low single digits.  When you
factor in off-angle response (i.e. since the tubes won't be on a
tracking mount to keep them 'aimed at the sun") the basic efficiency
drops from ~15% to probably ~8%, and with the wind, between -3% to 3%.
So, using only the tube as a collector is a real challenge. Probably be
better using a flat-plate collector as the primary heater, but that's
another major addition to the complexity.

Of course, too much heat would kill the system with over pressurization.

No, in fact the lower pressure raises it a bit. Latent Heat of
Vaporization for water is inversely proportional to the pressure, albeit
the change is less than 10% IIRC.

Non-condensables are a rate limiter for the process, unless you want to
spend more energy for vacuum deaeration.

Certainly possible, but not easily doable.

Well, if you added a convection cell as above (another system that
requires time to reach an equilibrium condition to work), then the
periodic headspace purging would quench both the distillation and the
seawater convection systems. In reality, the purging would be likely be
very frequent given the size of tubes that would be practical.

Yep, sure does.

Keith Hughes

How do you get 33' as 1/2 of the diffusion path.   I think there will be
about 33 feet of water in the column on each side---to provide the
weigth that pulls the pressure down.   That would leave only  about
7 feet of water vapor path on each side of the column.

I'm not sure that 'diffusion' is the proper term for the motion
of the water vapor.  After all,  the heat engine is providing
water vapor on one side and condensing it on the other---so there
is a net mass flow and probably a small pressure differential to
move the vapor.

Still (pun intended),  you need  a lot of heat to provide the energy
to evaporate the water or it will soon cool to the point where
its vapor pressure is reduced and the process slows drastically.
The fact that the water 'boils' near room temperature does not
reduce the amount of heat required to change the water from
liquid to vapor.

As has been discussed,  the simple idea does not address the problems
of salt buildup in the seawater side, or the addition of dissolved
gasses to the vacuum part of the loop.  

With a large enough (or double) saltwater tube you might get a
convection cell going with the cold, saltier water sinking and
pulling up warmer seawater to the top.

You could solve the dissolved gas problem by periodically pumping
both tubes up enough to displace the accumulated gases.

Now the project is getting complex enough that an RO system
starts to look attractive!

Mark Borgerson

This is just plain wrong.  As a *unit of measure* 32 feet of water
column equals about 13.9 psi.  Meaning, if you pumped a 40' column up to
a 39' height with water, equalized the headspace to atmospheric pressure
(assuming 14.7psia), sealed it, then allowed gravity to *drain* the
water column to a height of 2', the resulting pressure in the headspace
will be about 0.8psia. Now you also have 33' of empty evacuated column.

Sorry, this makes no sense. Putting water in does not cause it to "pull
down". Yes, you have supply makeup water to maintain column height lost
to evaporation.

Yes, and this "migration" is simple diffusion.  *And* you have (in the
example above) 33' of column it has to diffuse through on the seawater
side, and however many feet of column on the freshwater side it has to
traverse prior to condensation. If both columns (fresh and sea) are
referenced to the same height, then the evacuated column height on both
sides will be the same, and that diffusion path will be up to 66'.  That
does not happen quickly.

In reality, though, the columns won't be referenced to the same level,
with the freshwater column being referenced (i.e. the bottom is opened
to) the deck height on the boat. So the freshwater column will be, say
8' higher than the seawater column. The diffusion path is still the
same, but the evacuated seawater column would then be 37', with 29' on
the freshwater side.

No, this does *not* create a vacuum in the sense you seem to mean. It
maintains an equilibrium pressure by lowering the partial pressure of
water vapor generated by the 'boiling' process on the seawater side.

This relates to the critical rate-limiting feature of the system -
maintaining pressure.  When you evaporate, or sublime, water into the
headspace, the pressure in the headspace increases.  Condensation on the
other side lowers the pressure, and an equilibrium pressure will
eventually be established. For any given temperature, the evaporation
rate is going to be limited by the partial pressures at the
headspace/water-surface interface. It's a feedback loop, More
evaporation -> more water vapor molecules liberated to the headspace ->
more pressure in the headspace -> slower evaporation until the pressure
is reduced.  And to reduce the pressure, those molecules have to diffuse
up to 66'.

Yes, VERY slowly. You can increase *throughput* by increasing the column
diameters, but how practical is that on a boat?

Not quite true...you can seal the 'freshwater' column, using only the
column walls for condensation surfaces, until you have sufficient
condensate collected to allow the freshwater column to be opened.

It doesn't *have* to be that way, BUT.... :-)

And what's 'practical' for useability, is impractical for functionality.
There are no 'soft tubing' materials I'm aware of that have anything
approaching decent heat absorbance, conduction, or emissivity
properties, so that will be another very significant rate limiter in the
system.

You actually *can't* pump faster than gravity, unless you want to suck
seawater up the column on the other side.

Doubtful that you'd ever get a solid chunk of salt (and short of having
a bypass circulation loop - cooling the column and further reducing
efficiency - I don't see how a pump could even help the situation), but
of course as the salinity increases, the boiling point increases, and at
some point the process will just stall.  The heat input won't be
sufficient to boil the brine solution.  Then you have to stop, drain,
clean, and start over.  How quickly this happens will depend on column
heights and diameters, but it'll happen at some point. Just another
rate-limiting feature.

All these rate limiters are natures way of saying that there is no
thermodynamic free lunch.  A low energy input system will have a low
output (in terms of whatever work you want the system to do).

Keith Hughes

It works but does it work as well as other methods that are simpler and
easier to implement. Also if you have no fresh water on hand to start
with there is no way to make it work. I can see someone getting a
"Darwin Award" by accidentally spilling all there existing freshwater
supply in a failed attempt to get this contraption going.

That makes no sense. You are going to have a hard time pumping water out
of the fresh water side any faster than gravity can deliver it. The
salty side OTOH,  if you rely only on gravity to feed it, will become a
solid block of salt once you have evaporated enough water from it.

-jim

A 32' column of water is a continuous vacuum pump.  As long as you put
water (salt water) into the column it will pull down and keep a vacuum
in the top of the column.  The fresh water distills off the top of the
saltwater column then migrates as steam to the other side and distills
in the fresh water side....also creating a vacuum.  You draw off the
fresh water on one side and pump salt water into the other side.  The
salt water side is painted black to absorb sun heat and the fresh
water side is painted white to reflect the suns heat.  You only need a
few degrees difference for distillation and the vacuum creates the
boiling at low temperatures...even ice will change state to steam in a
vacuum.  The idea works.

In a practical sense, I would use soft tubing for the sides and a
solid "U" shaped piece of copper tubing for the top center with a ring
soldered to it so it could be hoisted up the mast of a sailboat.  It
would take a 30 to 40 foot mast to do the job.  The bottom end of the
salt water tube could go to a through hull for a continuous supply of
salt water and the bottom end of the fresh water tube could go to a
small pump to remove the water without breaking the vacuum.

Ah well, another great idea skuppered by dat old devil science :-)

Bruce in Bangkok
(brucepaigeATgmailDOTcom)

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.

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).  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.

Gravity.

On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
<betwys1@sbcglobal.net> wrote stuff
and I replied:

But what is the cheap source of getting the vacuum? I figured there
had to be a vacuum, although it was not said. But how do you get it?

Human bevaviour: Bestiality with a brain

Well, at least this respondent Jim, is operating at shall we say the
7th grade level of science/engineering insight.    Like so many other
products of the domestic school system, he seems to have a severe
case of self-esteem syndrome.  

 Still, he may be retrievable, starting with a science demonstration
he may have missed.   Place a beaker of water in a bell-jar and pump
the air out.

When 99% of the air has been pumped out, the water in the beaker is
boiling vigorously, until, in the usual way, the beaker boils dry.
The beaker feels cool to the touch, naturally.

To quote him: "unless I have a cheap source of heating this won't
work..."

    For the $64 prize:   NOW do you get it?

Brian Whatcott    Altus OK

Well no, he obviously hadn't figured that out. Nor can anybody figure
out what is going to hold a column of water 40 ft high as was stated in
the original post. The tubes may be 40 feet but the column of water will
be considerably less. How much less will depend on how much energy is
heating on the hot side and how much energy is cooling on the cool side.
The total amount of energy needed is not going to be any different than
any other distilling method.
    Unless you have the free or cheap sources of cooling and heating at
specific temperatures this isn't going to work any better either.

-jim

Dear Larry:

What Brian left to the reader's imagination, is that the head
space of the tubes is at a near perfect vacuum, flooded only with
water vapor.  You might recall that a perfect vacuum will lift a
column of water about 32 feet, on a high pressure day.  Or had
you not figured that out?

David A. Smith

dlzc <dlzc1@cox.net> wrote in news:1190415672.506271.93890
@k79g2000hse.googlegroups.com:

http://en.wikipedia.org/wiki/Boiling_point

"The boiling point of water is 100 °C (212 °F) at standard pressure. On
top of Mount Everest the pressure is about 260 mbar (26 kPa) so the
boiling point of water is 69 °C. (156.2 °F)."

AT 40' ASL, the boiling point must be down to...to....211.95F!

Larry

Dear Brian Whatcott:

There are ship-board distiller units that use an engine to pull a
vacuum, and the engine's waste heat to boil that water, to generate
drinking water.  A little shorter...

David A. Smith

You've heard all about distilling water, and you've heard all about
reverse osmosis,  but you haven't heard about low-cost, low energy
stills: they are brand new.

Briefly:
Take one forty ft vertical tube filled with saline.
Take one forty ft vertical tube filled with fresh water.
Connect them with a little engineering help - at the top.

The boiling point of water at sea level pressure is about 100 deg C

The boiling point of water at the top of a sealed 40 ft column of
water is near ambient.
So, it doesn't take much heat to boil the brackish water, and have it
pass to the fresh column where it is slightly cooled to hold the near
vacuum conditions at the boiling level.

[An engineering effort of a U of Utah group I think]

Brian Whatcott   Altus OK