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New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 3:53 AM

We know from CSP industry that liquidising salt requires temperatures of 200°C to 800°C (depending on salt's mineral composition). If a closed container of sea water is subjected to such temperatures, will salt in sea water liquidise and separate from water? Could we skim such liquid salt from the top of the water (flotation techniques)? Ignoring financial implications.

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#1

Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 5:07 AM

If you heat salt water to 200ºC to 800ºC, what makes you think you will have water?

Just collect and condense the steam.

Distillation.

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#2
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Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 6:11 AM

Closed container! no possibility to evaporate. Once liquid salt is removed and pressure is released, superheated steam can be used to run turbine....

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#3
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Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 6:34 AM

How can you use the steam after the pressure is released?

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#10
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Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 12:31 PM

Good luck, but I suspect any vessel you come up with that can withstand the pressure/temperature with this corrosive factor, you will find other reactions, possibly including the carry-over in the SC water (which is a solvent in its own right, just as is SCC (super-critical carbon dioxide), and possible decomposition of some of the dissolved salts to other things resembling acids, especially at the upper limit of temperature.

True, IF salt did not dissolve in SC water, and did melt, it would probably have a higher density than the SC phase. But we know already this is not going to work.

Clean up the water first if you want a steam cycle. See my earlier post. You will recovery far more energy that way.

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#13
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Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 4:50 PM

That would be called a bomb!

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#4

Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 8:00 AM

Your question has so very many inconsistencies with reality I suspect you maybe trolling for trouble.

First, salt (NaCl) has a higher density than water. This is why salt crystals sink before they dissolve. If one could float on the other (like oil on water) water would be on top.

Second, salt dissolves in water. There is no boundary between these two molecular types.

Third, for water to remain a liquid and not a gas at these temperatures the pressure must be from 10 to 1000+ atmospheres of pressure. Since salt will not be a gas at these temperatures and one atmosphere of pressure then simple distillation will easily separate these two.

You are not even consistent within your question. Salt dissolved in water is already liquid.

Just because you heard something from a California Staffing Professional (CSP) doesn't mean you understand anything they were trying to convey.

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#18
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Re: New method of sea water desalination by flotation of liquid salt

09/12/2017 12:18 AM

My apologies, CSP stands for Concentrated Solar Power.

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#5

Re: New method of sea water desalination by flotation of liquid salt

09/08/2017 9:14 AM

Having design a brining operation distillation unit. Your questions sounds like a layman with a dream.

To answer your questions,

If a closed container of sea water is subjected to such temperatures, will salt in sea water liquidise and separate from water?

no

Could we skim such liquid salt from the top of the water (flotation techniques)?

no

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#6

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 10:22 AM

We know from CSP industry that liquidising salt requires temperatures of 200°C to 800°C (depending on salt's mineral composition).

How do "we" know this? Give references please, because I have never heard the phrase "liquidizing salt".

There are only two ways of getting salt into liquid form; melting or dissolving in a liquid. NaCl melting point is 801ºC.

If a closed container of sea water is subjected to such temperatures, will salt in sea water liquidise and separate from water?

In your crazy scenario, you would need to have a pressure vessel. Once the temperature is enough to melt (a much better word than liquidise don't you think?) the salt, you are hoping to have two immiscible liquids with the molten salt as the top layer.

In this phase diagram, notice that at 1000 K (727ºC, sorry - couldn't find a diagram that covered temps to 800ºC), your pressure would need to be over 25 MPa (that's 3626 psi) to keep the water from boiling. This actually puts it in the supercritical regime (the olive green area in the upper right portion of the diagram).

Supercritical fluids usually increase solubility, so I doubt very much that they would be immiscible, and even if they were, I doubt that the salt would be the top layer.

I hope you didn't spend too much time on this! Your time would be better spent in studying chemistry and physics.

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#7
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 10:37 AM

I know they use Molten salt as both a heat transfer fluid (HTF), as well as a thermal energy storage medium. The 'liquidized salt' is a term I never heard of before.

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#19
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 12:34 AM

Great graph, thanks.

GUYS!!!! feel free to pile on me with cynical comments, just provide some reasoning, however remote it may be. Any background to your thinking is appreciated. The point of my question is not related to desalination of sea water only. Concentrated Solar Power is currently using molten salts (at 400°C-600°C) as heat transfer medium and steam generated in such plants is at +-550°C. Newest coal plant boilers work with supercritical water steam of +-700°C. Gas turbines work with temperatures of +-1000°C. My thought (however vague) - or brain fart if you prefer, was meant to combine seawater desalination with generation of superheated team to run turbines and produce electricity. Combining these two processes should be economically beneficial????

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#20
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 10:31 AM

You would be better served spending some thought on:

(1) the stack gas heat content in a power plant

(2) how to use the stack gas to heat something without corroding the dickens out of it

(3) up-scaling your idea of a desalination process to utilize low temperature heat for (a) district heating, (b) water purification, (c) chemical processing and food processing.

I understand that dry commodities are worth far more than soggy ones, and drying of food commodities is a very useful means of long term storage, increasing the transportability of the food, and eases distribution to affected zones in times of crisis.

(4) how to possibly harness low-grade heat sources to "third-pass" generation by thermally affected electrical devices. (these do exist, but are not the best known), generally exhibit fairly low operating efficiency, as expected...

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#22
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 11:31 AM

What stack?

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#23
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 1:54 PM

Every fossil fueled power plant has an exhaust stack. Coal fired boilers have particularly high stacks, as compared to the shorter ones used by gas turbine plants, even the combined cycle ones.

The exhaust gas temperature lower limit is determined by such things as carbon dioxide and NOx and SOx (primarily SOx) content, the lower the SOx, the lower the final stack temperature can be made (by heat extraction processes) without condensing out sulfuric acid, etc.

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#25
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 2:20 PM

Yes. The OP is talking about a Concentrated Solar Powered operation. No exhaust stack. Now a yet to be introduced at the time of my earlier reply cooling stack is a whole other possibility.

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#21
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 10:49 AM

The confusion comes from the differing goals of the two processes.

Desalinating seawater by distillation requires adding energy to the solution(to evaporate the water) then removing energy from the system (to condense the water into a recoverable liquid). Superheating the steam not only slows down the condensation step (as there is more energy that needs to be removed) but taking the temperature that high can allow 'heavier' compounds to evaporate from the brine, thus 'contaminating' the steam and requiring further processing to make the water potable.

Superheated stem may have more energy than regular steam, but can that additional energy be efficiently tapped by a turbine in an open-loop setting? I could see the energy being harvested in a closed loop, where the pressure at both ends is controlled, and the resulting 'nearly boiling' water rains back into the supply basin to be superheated once again. In the open loop turbine + desalination system, there would also be questions about the potability of the water after it passes through the turbine machinery, as well as concerns about potential exposure of the turbine to corrosive salt.

While the combines system might work, it seems like it would be better to set up two plants, one for desalination, one for power generation, and eliminate the maintenance headaches of the de-power-salin-gener-ation plant.

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#24
In reply to #21

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 2:17 PM

Fair enough.

Desalination by distillation is common process that has been well researched. My intention was to introduce new process by combining two seemingly unrelated processes; desalination (by liquid separation) and power generation (Steam production).

You are correct that open-loop systems for superheated steam are not common and contamination of potable water that goes through the turbine should be considered.

As one participant has mentioned previously, there should be a sufficient amount of waste heat available for distillation of sea water through the use of cooling towers/stack.

Unfortunately, you won't find many coal thermal plants close to the sea. Most of the nuclear plants are located at the sea and utilization of nuclear heat for desalination has been considered.

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#26
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 2:50 PM

And, as I stated earlier, places where desalination is considered a feasible method of getting potable water are places that do not experience much rainfall, which indicates very few cloudy days. A Solar Concentration Array could provide the head to run the distilling process from sunup to sundown, with no additional energy required from oil, coal or nuclear.

With proper protection for the heat-exchanger coils, I could also see a nuclear reactor using a closed-loop water cooling system that dumps its heat into a salt-water basin, but I see that as a delicate balancing act, as the water that boils away needs to be replaced to the reactor won't overheat, but adding cold sea water to the basin will drop the temperature of the mix, 'shutting down' the desalination system until it heats back up. And again, you will need to drain the basin to remove the collected sea salt, which means another 'heat sink' will be needed for the reactor.You COULD have two desalinating cooling basins so the heat can be switched from dumping into A to dumping into B to allow A to be cleaned. But again, the almost Seusian combination of two different plants would make it a nightmare for the Maintenance staff.

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#27
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/12/2017 3:47 PM

I don't know which comic books you got that out of. There are plenty of coal burners near the ocean in Japan, Korea, China, Florida, and other places.

Nukes also.

How about you dump this idea, and figure out how to make a very high current, low potential generator (about 0.5-2.0 volts DC, but with over 1000 ampere output), then connect this to a bank of membrane capacitive deionization stacks?

Tunable output water quality, very high water recovery ratio, seawater capable, low pressure (minimal pump energy or even gravity flow under some conditions), and extremely low fouling/scaling potential, with very high chlorine tolerance.

The energy requirement even for seawater is no more than 2 kW-hr/m3, usually right at 1 kW-hr/m3, so it is almost like common thievery. This is the lowest specific energy requirement for any desalination process.

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#28
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/25/2017 9:57 AM

Over this past weekend, I also think I got the definitive answer for this 'combining a power plant and a desalination plant' project.

The TV Show How Its Made (Shown on the Science Chanel in the US, also shown in Canada, where it is produced), Season 15, Episode 3, Segment B: "Desalinated Water" provided information that basically mooted this entire thread.

First off, commercial Desalination is not done by distilling, but by filtering, so the power requirements are much less than this thread estimated.

Secondly, the desalination plant shown in the segment was located next to an electrical power plant that used seawater for cooling, and sent the used seawater (with its added heat load, but not enough to even boil the water) to the desalination plant for conversion into drinking water.

Finally, the concentrated brine that is left over after the desalination filtering was piped back to the power plant, where it is diluted so it will not upset the local salinity levels and returned to the sea.

In conclusion, the current methods of desalinating water are already very energy-efficient, and are designed to minimize their impact on the local environment. A 'new method' that requires more energy and more time to process a set volume of water is not likely to be the groundbreaking 'disruptive' technology it may appear to be at first.

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#29
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/25/2017 4:45 PM

So they would lead you to believe. You cannot remove salt from water with a "filter".

A reverse osmosis membrane is not a filter at the level where salt ions are rejected.

It may be configured similarly to a cross-flow filter, but that is where the similarity ends.

Water is transported by the actual chemical structure of the membrane itself, such that there are large rings in the chemical backbone (relative to the molecular size of water molecule, but the ions are rejected by electrostatic repulsion (for one thing), and the size of the ions (hydrated with many water molecules) for another.

SWRO - seawater RO is only efficient (and not all that efficient) when the brine (reject, also called concentrate) not recovered (could be less than half of the incoming pressurized water (up to 800-900 psig), is subsequently let down through an energy recovery turbine, and that can obviously only recover a small fraction of the energy input, albeit energy input is below that of simple or multiple effect distillation.

Membrane capacitive deionization will far outperform SWRO in purifying seawater with respect to the energy input per cubic meter of recovered potable water.

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#30
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/26/2017 8:47 AM

I accept that my understanding based on that segment is limited, after all they tend to oversimplify things to bring the vocabulary down around High School Sophomore level, so the canister with all the layers when shown in cross section may not be a 'filter' in technical terms, it's a description that all the viewing audience will understand.

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#31
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/26/2017 8:58 AM

Good technical explanation. Basically what you are referring to is Molecular weight cut-off (MWCO)

Molecular weight cut-off (MWCO) is a term used to describe a pore size of a membrane. The smaller the MWCO, the tighter the membrane pore size.

as an example;
•RO elements have ~100 amu (Dalton) molecular weight cut-off.
•NF has 200-400 amu (Dalton) molecular weight cut-off.

A membrane with MWCO of 100 means it can reject molecules with the molecular weight of 100 well (i.e. >90%).

Reverse osmosis (RO) is the finest level of filtration available.
•Barrier to all dissolved salts and inorganic molecules
•Organic molecules with a molecular weight greater than approximately 100 are rejected.
•Water molecules pass freely through the membrane creating a purified product stream.
•Rejection of dissolved salts is typically 95% to greater than 99%.

when I designed UF's and RO's, I used AMT (Advanced Membrane Technologies), then it became PTI I have no idea what it's called now. They had an excellent website that was packed with useful information. (ΔP, flow rate, ect...) of their membranes. Then it became useless because they wanted you to call them.

I have use Synder since, they have some useful info

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#32
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/26/2017 9:20 AM

Check out the link on Synder's web site,.. they have some interesting stuff.

one is why people say its a filter,.. where the ones in industry call it fracturization. Like this one or this.. and when you put your mouse over some of the graphics offered, its animated.

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#33
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/26/2017 9:35 AM

"one is why people say its a filter,.. where the ones in industry call it fracturization."

It's all about how much detail is needed, like how the General Public talks about "Plastic" when the technical terms are "Resin" "Thermoplastic Resin" and "Thermoset Resin". People call Bakelite the "first commercial plastic" when it's a thermoset material, not thermoplastic.

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#34
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Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/26/2017 10:23 AM

When one works in a area specific, that area specific terminology can really be devastating.

My post point though was the information.

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#8

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 11:19 AM

Use the heat to remove the water from the salt, per #1. Then scrape the remains of the minerals from the bottom of the pot. (Let the pot cool first, of course)

Anything else is folly.

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#9

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 12:26 PM

the liquid-vapor boundary terminates in an endpoint at some critical temperature Tc and critical pressure pc. This is the critical point. In water, the critical point occurs at around 647 K (374 °C; 705 °F) and 22.064 MPa (3200 psia or 218 atm

(source Wikipedia)

Thus, for most of the temperature range you specified, there is no such thing as liquid water existing. There are much less energy intensive means of desalinating brackish and/or seawater. Look at Voltea's information on membrane capacitive deionization. Their company headquarters is in the Netherlands, but manufacturing is in Dallas, TX. This is by far, the lowest net energy input required for desalination, and I will praise them on this accomplishment every chance I get.

When I saw the number on this, I was flabbergasted it could be this low.

We are talking order of magnitude 1 KWh/m3 if my memory serves me correctly. This is far below the energy requirement for removing salt from seawater by RO. Unfortunately, on initial search, I cannot locate the precise energy figures.

compare to traditional desalination methods

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#11

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 1:28 PM

Water is a solvent. It has broken down the bonds that would hold NaCl into solid state. So you no longer have a solid to change into a liquid state. All increasing the temperature would do is change the saturation point allowing the water to disolve and hold more NaCl.

How do you ignor the financial implications? Allot more costly to heat water beyond its vaporation point needed to distill it at 100ºC.

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#12

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 4:39 PM

Nothing wrong except:

Water boils at a much lower temperature than salt melts (except at very extreme pressures)

Salt remains dissolved in the water

Salt is heavier than water and sinks, not floats.

Maybe you can ignore the financial implications, but you can't ignore the physics.

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#14

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 5:07 PM

It would take far less energy to vaporize the water, leaving the salt behind, than it would take to turn the salt molten while still in the seawater.

Am I missing something here?

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#15
In reply to #14

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/08/2017 5:24 PM

Instead of pressure, I'd use a vacuum.. about 29".. at 29" water will boil at about 113-115 degrees F.

the what I'd did when I design a brining distillation unit. Far less energy input.

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#16

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/11/2017 4:02 AM

No, but one can separate fresh water from salt water at temperatures much lower than that using other techniques, such as evaporation and reverse osmosis [RO], which have found their way into the mainstream. There are techniques available in RO that can bring the cost below 3.5kWh/tonne, which makes heating saltwater to 800degC a hideously uneconomic proposition.

In any case, in any separation process involving the application of heat, it is the most volatile component, in this case water, that comes off first.

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#17
In reply to #16

Re: New Method of Sea Water Desalination by Flotation of Liquid Salt

09/11/2017 12:47 PM

"No, but one can separate fresh water from salt water at temperatures much lower than that using other techniques, such as evaporation and reverse osmosis [RO], which have found their way into the mainstream. There are techniques available in RO that can bring the cost below 3.5kWh/tonne, which makes heating saltwater to 800degC a hideously uneconomic proposition."

And if the desalination plant is being set up in an arid country (as most plants of that type are) and there is plenty of clear sunny days (which most arid countries have) one could also build a desalination plant that relied on concave mirrors to focus the sunlight to boil the water, thus reducing the energy 'cost' of that most important and most power-hungry step to 'free.' If you were to have the collection/condensation tank underground, below the plant, the relative coolness of the earth there would allow for easy gathering of the water from the steam.

Set the shift work up so the evaporation tanks are cleaned of brine and refilled with seawater at night, and the only thing left to do is find a market for all that Gourmet Sea Salt you are collecting.

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