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Solute-based desalination

Salt out the salt
  [vote for,

The dissolving capacity of water depends on what else is dissolved in water, I here assert. H20 only has so many bonds it can make. I propose that by adding a different solute to seawater it would be possible to get unwanted solutes (i.e. NaCl) to precipitate out as easily removed crystals.

Whatever is added must be easy to remove. Reacting away salts is not easy and if it is possible, those substance will be expensive. Plus you usually get rid of only one ion and the other remains in the water.

Whatever is added must be evaporated out. I am pretty sure I proposed this scheme here using CO2 but I think I deleted it from the HB, hoping to win an Innocentive prize with it (nope). So here it is back It is back because it occurred to me that one could also use fluorocarbons or chlorofluorocarbon in the same role: extremely polar, extremely soluble in water, also very volatile and so easy to remove with a pressure change. Also I still wonder if it would work.

1. Seawater is admitted to treatment chamber.

2. CO2 (or fluorocarbon) is added under pressure. As CO2 dissolved in water increases, salts precipitate out.

3. Sweep away salts from bottom of chamber.

4. Reduce pressure and CO2 / CH2F2 outgasses and can be reclaimed.

5. Treated water has markedly less solute. It will still need refining if it is for drinking but it may be adequate for irrigation / industry. CO2 is nice in that some remaining in the water will make it bubbly which everyone likes.

bungston, Sep 23 2016

Vacuum desaltinig Vacuum_20Desalting
Per an anno by [8th of 7], linking this old Idea may be appropriate --it saves energy by recycling heat. [Vernon, Sep 25 2016]

Vacuum salt process http://www.saltasso...ion/vacuum-process/
The method currently used to produce most food grade salt [AusCan531, Sep 25 2016]

Patterned energy https://www.science...09/160921164147.htm
Using sound to draw peaceful pictures. [wjt, Sep 29 2016]


       I think Richard Feynman had an interesting comment once, which is appropriate here. He said (in some context that I now forget) "Hmmm."   

       I like the idea of displacing salt with a soluble gas under pressure, that can then itself be removed under lower pressure. However, I am not sure if CO2 (or any gas) will displace metal and halogen ions from solution.   

       You could sort of test this, if you have one of those drink carbonators. Just make a saturated solution of salt at room temperature (make sure it's saturated), then decant it (minus excess salt) into the soda machine. Then add CO2 and leave it to stand for a day or so. Then squirt out the carbonated water and see if you have a significant amount of solid salt left behind.   

       This might work better with HCl - a highly soluble gas that is fairly easy to remove afterwards.   

       [+] for a cool idea, even though it may not work.
MaxwellBuchanan, Sep 23 2016

       Existing designs of RO columns work well enough to make this distinctly halfbaked.
8th of 7, Sep 23 2016

       I'm tending to think this won't work well because if you add something ionic to water, you are not changing the aspect of water that lets existing salt in it be dissolved. I do see that the goal is to overload water's ability to dissolve something, but for ionic substances that ability is quite significant.   

       You might need a specially-designed molecule that can work in a semi-biological way. Think of the polar water molecule in terms of key-in-lock molecular biology. You want that water molecule to fit in your special molecule, such that the water can no longer attract ions. But once you have that, then you might need more energy to separate the water from the special compound than you need to separate water from salt!   

       One other possibility involves alcohols (of which there is a wide variety). Many are "miscible" with water (mixable in any proportion). So if you took one liter of salty water and added 100 liters of methanol to it, would the salt stay dissolved? The idea here is the the alcohol molecules would so outnumber the water that no two water molecules would be near enough to each other that they could support an ion in-between them. And since alcohols mostly don't dissolve salts....   

       If the salt falls out of solution, then separating methanol from water is relatively easy (methanol has a low boiling point).
Vernon, Sep 23 2016

       In the lab I work in, we used to need, and occasionally prepare isopropanol saturated with caesium chloride and water.   

       I once saw someone panic after first preparing a saturated caesium chloride (CsCl) saturated water, then adding isopropanol. Pure water dissolves CsCl really well, so a lot went in. Then when the isopropanol is added the water dissolves in it much better than the CsCl, and/or the isopropanol displaces CsCl from the water. So lots of CsCl came out of solution and formed a thick layer at the bottom of the flask. And it's really expensive - not something you want to have just sitting around.
The solution solution (heh) was to add more water to get it to dissolve again, then transfer a bit to every other such bottle in the room, so the boss was none the wiser.

       Anyway, if you can add something you actually want dissolved and enough of the salt just drops out for free then you're golden. Provided you don't mind some of the salt being left behind, anyway.
Loris, Sep 23 2016

       That is a good story, Loris. It is making me think...   

       Both your story and Vernon's methanol proposition involve changing the solvent such that its ability to dissolve salts is less. I despair of throwing around volume of alcohol comparable to the volumes of water I hope to treat.   

       I want to outcompete the salt and displace it. The problem, as Vernon points out, is that NaCl is really soluble. Which is why the ocean is full of it, I suppose. My thought is that even if CO2 is less soluble (is it? I feel like there should be a value that describes solubility, like specific heat), CO2 can also be pressurized considerably, a factor which will not affect NaCl. There must be some point where the pressure is such that CO2 exceeds the solubility of the NaCl and can displace most of it.
bungston, Sep 23 2016

       Alternatively, if scCO2 dissolves salt better than water, it could be added to salt water, the desalted water removed then the CO2 evaporated.
FlyingToaster, Sep 23 2016

       It's all about energy.   

       You have to find an energetically better way than the existing ones of removing salts from water - specifically, sea water.   

       Messing around with solvents and dissolved gasses, preferential solubility and solvent extraction, is all very well; but you've got to do it more cheaply than it's done at the moment.
8th of 7, Sep 23 2016

       " You have to find an energetically better way than the existing ones of removing salts from water - speifically, sea water. "   

       The Society of Petroleum Engineers are attempting to contact you, but I told them I didn't know where you lived.
normzone, Sep 23 2016

       On the table linked by [LimpNotes], the most soluble ionic compound at 20°C is antimony trichloride, at 910 grams per 100 grams of water.
MaxwellBuchanan, Sep 23 2016

       Actually, ice is probably the best ionic solid to displace salt.
MaxwellBuchanan, Sep 23 2016

       Pondering phase changes and solubility: of course one can obtain distilled water by boiling it. It takes a fair amount of heat to boil water. I wonder if, to claim x amount of distilled water from n amount of seawater, one requires the same energy whether boiling is achieved by heat or by atmospheric pressure reduction.   

       Also I think antimony has an unfair advantage because of its massiveness. The table should really be according to moles of solute, not by weight. Hopefully someone will edit that Wikipedia table and set it right.
bungston, Sep 23 2016

       //I wonder if, to claim x amount of distilled water from n amount of seawater, one requires the same energy //   

       The minimum energy requirement will be the enthalpy of solution of the salt (or I may mean entropy, or perhaps Gibbs Free Energy - one of those, anyway). In any event, you can't get away with less than that.   

       Of course, if you can find a solute that displaces NaCl from solution, it will be because the energy change of dissolving the new solute exceeds that of de-solving the NaCl. But then you've still got to get your new solute back out.   

       I think the minimum energy requirement will apply whatever method you use, but it's unlikely that distillation or other methods operate at or even close to that limit.
MaxwellBuchanan, Sep 23 2016

       I find this discussion very interesting although I admit not being able to follow all of the chemistry, even though I have an involvement with the salt production industry. A process in common use is to simply subject the saline brine to a vacuum thus lowering the energy required to evaporate off the water although the initial infrastructure is pricey..   

       Economics will be the achilles heel of this idea even if you found a solute which neatly displaces the salt. My business partner often tells me that salt is literally worth less than dirt: if you had a 10,000 tonne pile of salt stacked up on the side of a salt lake and a 10,000 tonne pile of reasonable quality topsoil, the dirt would be more valuable than the salt.   

       The second major hurdle is that you need to deal with all the other salts besides halite which are mixed into your starting brine. You're going to have a range of 'salts' drop out of your treated solution. First there will be calcium sulphate (gypsum) then sodium sulphate, then magnesium sulphate then your sodium chloride (halite) then calcium and magnesium chloride etc. All these other precipitates have some value in their own rights but don't drop out in nice clean stratifications. They will be contaminated by each other (forming double salts) and by other contaminants ranging from iron to organics. Even if you have a market for several of these other salts you're going to be stuck with an ever growing stockpile of the salts for which you don't have a market. And don't think you'll 'just dump them back into the sea' in any developed country with modern environmental regulations.   

       This all comes down to what you are trying to achieve here bungston? If you want small quantities of high purity salts (of whichever composition) this can already be done by existing processes. If you want vast quantiiies of common NaCl then solar evaporation is the way to go. You can produce millions of tonnes for $10-12 per tonne or much less. As these production sites tend to be on the coast, shipping to other parts of the world is fairly cheap and easy.
AusCan531, Sep 24 2016

       // subject the saline brine to a vacuum thus lowering the energy required to evaporate off the water //   

       No, no, no.   

       Dropping the pressure lowers the temperature at which the water boils off from the solution. That appears to be a "saving" of energy, and the water is the most volatile (lowest boiling) component - except for trace amounts of alcohols, hydrocarbons etc.   

       But you then have to put energy into the pumps that create the vacuum. The energy required to separate the water from the ionic dissolved solids is pretty much independant of pressure and temperature.   

       There are indeed efficiency gains from being able to run the process at much lower overall temperatures - less heat is lost from the process to the environment than with distilliation at normal atmospheric pressure, fewer issues with feed heating and condenser-side energy recovery. But if you do the math for a "perfectly" insulated atmospheric still relying only on heating, and an uninsulated vacuum still, you'll find that the energy demand to produce one unit of product is the same. That's because the energy to break one unit's worth of ionic bonds is the same. You either put it in as heat at the start of the process, or pumping power at the far end.   

       Atmospheric distillation has the advantage over vacuum distillation for food products in that little or no posttreatment is needed. With a vacuum plant, the water has been through the pumps, often contaminating it with lubricants, which have to be removed.   

       // And don't think you'll 'just dump them back into the sea' in any developed country with modern environmental regulations. //   

       ... unless Trump gets elected, in which case it will be just fine - as long as you do the dumping in some non-American bit of the ocean ...   

       // This all comes down to what you are trying to achieve here bungston? //   

       We read it as desalinated water - producing salt as the byproduct.
8th of 7, Sep 24 2016

       //You can produce millions of tonnes [of salt] for $10-12 per tonne//   

       That's impressive - equivalent to the solids from about 35 tonnes of seawater, and should contain about 0.5 grams of gold, worth about $20. (I'm assuming that gold is present at 13ppb in seawater, and that most or all of it will drop out with the salts.) Of course you've still got to extract the gold from all that salt...
MaxwellBuchanan, Sep 25 2016

MaxwellBuchanan, Sep 25 2016

       Most energy calcs are enveloping because in reality energy measures are all over the place when looking at the one h20 molecular level.   

       It would be nice to energize the h20 in a patterned 3D way as to make it less soluble but not enough to boil. Various salts should drop out.   

       Make that water dance, make it dance like a crazy person so everyone steps away.   

       Disclaimer: A couple of dances might be necessary for the different salts in solution.
wjt, Sep 29 2016

       //I am pretty sure I proposed this scheme here using CO2 but I think I deleted it from the HB...//   

       I looked for it on the Wayback Machine and found the idea title "High pressure CO2 desalination" and the approximate date, 1 August 2014. Unfortunately the idea itself was not archived.
xaviergisz, Sep 29 2016

       Someone offered an innocentive prize for a desalination method much cheaper than current methods. As if anyone with such a method that actually worked would give it away for their chump change.   

       But a harebrained method that conceivably could work but had never been tested - sure you can have that for $10K. And now I have given it up to this crowd for nothing but love.
bungston, Sep 29 2016

       Only the person that has the idea is emotionally invested in that idea. No one is going to run with someone else's idea unless they see a lower activation energy pathway, or more assured returns.
wjt, Sep 30 2016

       One should probably not run with any idea unless you see /a lower activation energy pathway, or more assured returns/.
bungston, Sep 30 2016

       I mentioned it recently, and found it's been suggested here before, but what about electrolysis of water, then running a (PEM) fuel cell to reclaim some of the power plus produce pure water?   

       Also, there are methods for containing most of the heat in a closed cycle for distilling water very cheaply. DEKA has been working on it for over a decade.
TIB, Sep 30 2016


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