🚀 go-pugleaf

remove-this wrote:
> 
> >>
> >> PS of course you could put the membrane only 500 or 1000 ft down and Pump the
> >> water up.
> >> Not quite so fancy (I love physics that looks like magic) But it would use
> >> only around 30% of the energy of conventional reverse osmosis plants (no need
> >> to compress the flow-by water) not to mention no containment pressure vessel
> >> and hence the membranes don't have to be so compact to be cramed into pressure
> >> vessels so they would be cheaper etc etc etc.
> >
> >John & Rhea Cook
> >'Words don't grow on trees, you know' Me
> >
> There's just one snag to all of this.  The water around the base of
> the tube will become increasingly salt and, unless there is a good
> current down there to replace it with new sea water, the system will
> stop working.  Sorry about that.  Ah well, back to the drawing board.
> 
> Rusty

The real snag is that the fresh water in the tube would NOT rise above
the level of the sea water around it. There can be absolutely no pumping
action! The reason I claim that is that if it did, you would be able to
create a perpetual motion machine, or, even better, a device that
creates energy. Think about it, if the fresh water level gets above the
level of the sea, we can attach a generator to it and get electricity.
If the water at the base of the tube becomes more salty, we can get even
more energy since the mixing of fresh water and salt water is exothermic
(other wise the salt would percipitate out). If the water around the
tube cooled (as someone suggested) then we could use the temperature
difference to get even more energy. The laws of Thermodynamics are
absolute, Newt can't repeal them, Bill can't violate them (I've been
waiting a week to use that line).

I know that it is counter-intuitive to think that the water would not
move from a higher pressure to a lower pressure, but what you have to
consider is the partial pressure of water on each side of the membrane.
(I am borrowing the concepts from gasses, a chemist would probably be
able to come up with a more accurate analysis.) Think about what would
happen if you had low pressure water on one side of the membrane and
high pressure methanol on the other, and the membrane is impervious to
the methanol. Water inside the membrane can diffuse in both directions,
and would equilibrate only when as much is diffusing in one direction as
the other. This means that the water would diffuse from the low pressure
side into the high pressure side (and dissolve into the methanol). It
would only stop when as much water is coming out of the methanol/water
solution as is going in.

John.