🚀 go-pugleaf

"John M. Cavallo" <john.cavallo@tcs.wap.org> writes: 
> 
> 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. 

[brine and fresh water mixing]

> 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.) 

The word you're looking for, John, is fugacity.  There are several 
fugacities being ignored here, including the pressure drop across
the membrane, which rises as the water about the membrane becomes 
increasingly saline.  With a deep enough "well", yes, the fresh
water should rise above the level of the brine.  However, this
will be temporary, as the deeper water accumulates salt.

No, returning the fresh water to the ocean will not automatically
mix it back in; you'd have to pump it down to depth to return
the system to its original state.  The steady state in this case is
a very long salinity gradient with very little flow up the tube.

One way to think of this is that the salt is falling down to the 
ocean floor.

Unless there's something I've missed?

Regards,

--Tom