From: tceisele@mtu.edu (Eisele)
Subject: Re: Request: info on desalination and solar energy
Organization: Michigan Technological University
Distribution: Inet
Date: Sun, 15 Nov 1992 17:19:03 GMT

In article <X17SBR95@cc.swarthmore.edu> dpeders1@cc.swarthmore.edu (Daniel Pedersen) writes:
>
>Actually, you remove the water from the sea-water.  You've probably done
>this yourself - take the saltwater, boil it, condense the vapor and - BAM -
>pure H2O.  All you're left with is the salt sediment in the seawater tank.
>
>So all you would need is to raise the seawater temperature above 100 C,
>which is well within the range of currently available, mass produced thermal 
>solar collectors.  You're right, it does take a lot of energy, but then
>again the sun puts out a lot of energy.  The limiting factor is the
>temperature needed:  If your collector could only achieve 60 C (like some
>hot air based solar collectors)  then you couldn't boil the water.
>
>So, how about it?  Anyone out there with sources/information for me about
>desalination?
>
Actually, as someone else mentioned, reverse osmosis is a much more
energy-efficient method for desalinating seawater, and is (I believe) the
method that the Middle East countries use ("Perry's Chemical Engineering
Handbook", published by McGraw-Hill, has a pretty good introduction to
reverse osmosis in it).  The basic process is to pass the seawater through
a series of narrow, parallel channels that are separated by semi-permeable
membranes, half of which have been treated to only let positive ions
through, and the other half of which only let negative ions through (the
two types alternate).  A DC electric potential is then applied across the
whole assembly.  Because of the way the membranes are arranged, half of the
channels become more concetrated with salt, while the other half become
more nearly pure water, so the products are (nearly) fresh water, and a
reasonably concentrated brine (which can have some industrial value for
salt production.  I believe Dow Chemical also uses this as the first step
of making magnesium metal from seawater).  The problem with this process is
that it needs electricity as the energy input, so it isn't really all that
suitable for solar applications unless you have some really good solar
cells. It may be possible to do something similar with temperature
differences instead of electrical potentials, using changes in the
diffusion coefficients of various ions across the membranes with
temperature, but I've never actually heard of anyone doing this.
	Reverse osmosis does have a significant cost item, the membranes
have to be thin in order to work, and are both kind of fragile and subject
to fouling, so they have to be replaced regularly.  Distillation is going
to be a lot easier to maintain, which may make up for the higher energy
consumption.  Incidentally, one can distill water without going all the way
to boiling, just so long as you have a condenser that is cooler than the
evaporator (although the throughput will be lower).  For seawater, this
might even be a good idea, because there are a lot of chemicals dissolved
in seawater that are less volatile than water, but will still be picked up
if you go all the way to boiling.

Tim Eisele
Metallurgical Engineering
Michigan Tech University
tceisele@mtu.edu
 


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