Pat, I think you have a far greater understanding of such matters than I do.
The Sahara Forest Project seems to use the “seawater greenhouse” method of desalination.
bellona.org/filearchive/fil_SFP_Folder2.0_convert.pdf
Based on a demonstration project in Oman, it estimates that it can desalinate 50 cubic meters of water per day per hectare. The electrical usage to pump the water into the greenhouse, and to run the ventilation fans to keep the plants cool, is said to be “modest”
But I have no way of evaluating their claims.
Thank you for the link to the pdf.
I took a look at it and, as I suspected, they are probably giving away area (and thus manufactured materials) in exchange for simplicity.
I don’t have huge experience, but I did operate a brine concentrator for a while as a lab technician. I could tell several stories about weird goings on there from various sorts of technologies being evaluated, from accidental releases of hydrogen cyanide:
One chemist to another: “What’s that smell?”
Second chemist: “I dunno . . . smells like almonds . . .”
First chemist: “Why do we have an almond smell? The only thing I know of that smells like almonds is hydrogen cyanide . . .”
Second chemist: “Do you think? . . . No! It couldn’t be!
How could the concentrator being emitting hydrogen cyanide?!
”
First: “I dunno, but I think we probably ought to evacuate the lab and then send someone back in with a gas mask to find out what in the heck is going on!
”
Turns out it was an accidental chemical waste dump into the tanker truck sized lab sample. When they acidified the lab sample to lower the pH, they were well on their way to killing themselves.
to people getting their pupils fogged over (fortunately the fogging was reversible, and within a day or two was gone) with funny chemicals but, ultimately all of the attempts to substitute something for vapor compression or similar processes went awry.
I’ve also read quite a bit about solar technologies and about desalinization in the past, and even attempted to come up with inventions and original designs. Still, after everything I researched, I came away with the impression that things like vapor compression are the best.
If I were doing it just for me, I would be doing it on a small scale and I’d probably use water and steam on the actual desalinization side and a low boiling point fluid like propane, butane or
methyl acetate* on the vapor compression side in the interest of having vapor boiling and condensation at reasonable densities and pressures. I’d have a
countercurrent heat exchanger to reversibly warm up and cool down the sea water and use the high pressures of the low boiling point fluid to allow for a piston compressor rather than a turbine or centrifugal pump so as to keep both the size and design complexity of the apparatus to a minimum.
In big industrial outfits, they do a lot less counter-flow heat exchanging and do away with a secondary fluid altogether by using a specially designed vapor turbine to compress the steam and use it to boil the brine.
And that’s where the brine concentrator I used to take data from came in, so we could see whether or not the pipes would get encrusted with salt precipitates and the like, and if they did, to experiment with salt nucleation chemicals to force the salt to preferentially precipitate out on the ‘nucleation dust’ or powder rather than to scale the inside of the stainless tubes.
This last, the powder precipitation, is one of the things that made it so robust, because the number of cheap chemicals that you can use to attempt that is huge, and you can almost always find something. Also, stainless steel resists corrosion of these mildly corrosive brines and other solutions (like black liquor from
pulp mills – concentrating the
black liquor actually results in a fuel you can use to help power the wood processing).
Once you find the right combination of chemicals and pipe materials and sizes, the only thing that really gets chewed up are the brine pumps, because they’ve always got a lot of chemical ‘precipitation grit’ in the fluid they pump; but mechanical pumps are cheap, even when the amount of water distilled is large. And when they wear out, they just get replaced and either recycled for their metal or thrown away.
Nothing else seems as simple, versatile, efficient, thermodynamically and chemically perfectible, and compact, or at least that’s how it seems to me.
:twocents:
*Of all the ones I researched, I liked acetone and methyl acetate the best, for their reasonable atmospheric boiling points. If you pushed the pressure up on them a bit, you could operate the brine side at very close to atmospheric pressure and 212 degrees Fahrenheit.
