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Making Saltwater Drinkable With Graphene
An anonymous reader writes "Graphene once again proves that it is quite possibly the most miraculous material known to man, this time by making saltwater drinkable. The process was developed by a group of MIT researchers who realized that graphene allowed for the creation of an incredibly precise sieve. Basically, the regular atomic structure of graphene means that you can create holes of any size, for example the size of a single molecule of water. Using this process scientist can desalinate saltwater 1,000 times faster than the Reverse Osmosis technique."
So how durable is this membrane when it comes to dealing with impurities?
Using this process scientist can desalinate saltwater 1,000 times faster than the Reverse Osmosis technique.
Well isn't that swell for 'scientist', but does scientist plan to share?
what about the holes getting blocked by minerals and impurities? seems high maintenance job.
How does this filter work on bacteria and viruses? The standard of living in the 3rd world would go up dramatically with free access to clean water.
Basically, the regular atomic structure of graphene means that you can create holes of any size, for example the size of a single molecule of water. Using this process scientist can desalinate saltwater 1,000 times faster than the Reverse Osmosis technique.
It is a RO membrane, just a really good one? They've described exactly how a RO membrane works. Of course this may have more "holes per sq inch" or whatever, maybe even 1000 times as many.
"Science flies us to the moon. Religion flies us into buildings." - Victor Stenger
If they've found a way to desalinate water with much less energy, practically, that's huge.
Bruce Perens.
God I loved "Top Secret"
I love Jesus, except for his foreign policy.
I just wonder if a graphene membrane could filter out the words "awesomely", "incredibly" and "super" from awesomely incredibly super texts, leaving only texts. *That* would be quite useful.
Ezekiel 23:20
The TFA is just a BS article that says nothing.
A better link (and is in the TFA) is Nanoporous Graphene Could Outperform Best Commercial Water Desalination Techniques
However that references Nanoporous graphene could outperform best commercial water desalination techniques
Now we finally we get to the actual link Water Desalination across Nanoporous Graphene (which unfortunately you need to have the right credentials to see - which I don't)
How come I can follow those links and the TFS can't?
I am Slashdot. Are you Slashdot as well?
Once we figure out how to make nanobots out of stem cells and graphene, every problem known to humanity will be solved!
http://alternatives.rzero.com/
Here's a link to the original paper on Grossman's website.
I think they're abusing the terminology a bit, using "RO" to refer to reverse osmosis conducted with existing membrane technologies. The point at issue is that thermodynamics demands that a certain amount of energy be expended in order to reduce the entropy of a homogeneous salt solution by separating it into pure (or at least low-salinity) water and high-salinity leftovers. This is totally independent of the means by which the molecules are separated. In reverse osmosis, that manifests as a minimum pressure necessary to force salt water through any selectively permeable membrane.
Practical RO systems operate with a pressure drop (and therefore energy consumption per unit volume) that's double or triple the osmotic pressure, in order to achieve useful flow rates across thick membranes with relatively low pore densities. A better filter would allow that excess pressure to be reduced, but can't do anything about the cost of reducing the entropy.
Bugrit! Millenium hand and shrimp!
Water molecule size, roughly 0.340 nm
Salt molecule size, roughly 0.500 nm
Graphene molecule size, roughly 0.142 nm
Difference in size between water and salt molecule, roughly 0.160 nm
The difference in size between water and salt is just barely more than the size of a single graphene molecule, so that leaves absolutely *NO* margin for error when designing the graphene sheet with those holes.
This might very well have already been proven to really work... but I expect it would be extremely cost ineffective at larger scales owing to the consistent and extremely accurate precision that would be needed when trying to do this at a macroscopic scale.
File under 'M' for 'Manic ranting'
Figure 8 on Page 6 of the actual paper shows what they're measuring. They're comparing filter materials by Salt rejection % vs Water permeability measured in L/cm2/day/MPa. That unit incorporates all the energy-efficeny goodness you want in a filter without looking at what pump technology is actually used to provide the energy input. It says that more filtered water (L) per square centimeter of filter (/cm2) per day (/day) per MegaPascal of pressure (/MPa, the energy input) is more good. Assuming any particular pump technology would give you a number for MPa/MJ that you could apply, but it doesn't help you understadn the performance of the filter itself. The figure for improvement vs existing technology they actually give is 2-3 orders of magnitude (100-1000x) so TFS is taking the optimistic side.
The bottom line is that this has a huge potential but is still a ways from practical application.
You can purify water with activated carbon ("purify" is highly subjective, unless a governmental authority has taken the time to define it; otherwise, it's up to the marketing department). If you want to remove chlorine and objectionable tastes and odors, a simple activated carbon cartridge works great. If you want to remove heavier VOC's (volatile organic compounds) and THM's (trihalomethanes), you can use a compressed carbon block. And you can use a 1 micron absolute carbon block if you want to do all of the above, as well as achieve five log reduction (99.999%) in Giardia and Cryptosporidium cysts, as well as removing 95% of lead in water (most lead found in water is particulate and not ionic).
Desalinating is a little more complicated than this. Currently, there are three (fairly simple) methods of desalinating water: reverse osmosis, steam (or vapor compression) distillation, and de-ionization. RO is usually the preferred method, because a commercial RO unit can purify a high volume of sea water at around 70-90% efficiency.
Steam or vapor compression distillation requires a lot of energy, leaves a massive amount of residue, and depending on mineral concentrations of the feed water, requires constant cleaning to prevent the equipment breaking down.
De-ionization requires no energy, but depending on the type of DI resins used, can quickly exhaust the filter bed, requiring regeneration, which again, doesn't require a lot of energy, but it does have a chemical cost to strip and regenerate the Cation/Anion resins.
Regardless of which method of desalination is being used, the feed water should be filtered to remove sediment and volatile organics (or post-filtration, in the case of DI).
The graphene method is essentially creating a thin film membrane like RO. If you jump past the original article, and go to Water Online, the method proposed would be actually be using a thin film scaffolding to support the nano layer of graphene. At that point, you might as well use RO, unless the actual production models (the graphene method proposed is still highly theoretical as the authors admit that consistently producing graphene with a uniform pore diameter is not practical yet) would allow greater pure water production at higher efficiencies than currently available with RO.
If you want to make ultra-pure water (say USP water-for-injection grade) you need to use a combination of all the above. What results you want will determine the method or number of steps required.
The most useful substance never mass-produced.
I once took an excursion to Reddit, and later HN. Unlimited up/down voting sucks when dealing with a hive-mind.
RO is not like using a traditional filter. I'll see if I can explain it quickly without the explanation getting too muddy. The last RO project I worked on was in 1990 (and wasn't for salt, but same principles apply), but I doubt the basic structure of the equipment has changed much. Probably more changes are in the actual membranes.
On an industrial scale membranes are placed in canisters and usually in large banks of them. The way the canister is built is usually a couple of sheets of membrane, sandwiching a substrate that allows a reasonable liquid flow rate through it, the whole is then spiral wound (like a roll of paper towel), or better yet, like film on a film winder that goes into a film development tank for those who remember film cameras and how to develop negatives :). The edges of the substrate and membranes are attached to a framework such that the purified liquid can be collected and channelled out either one or both ends of the spiral assembly when the assembly is inserted into a properly designed tube/canister. You put the wound membrane assembly in the tube that has one inlet and two or three outlets (depending on whether you want the purified liquid outlets at either end or just one). So say we have one feed outlet and one purified outlet. On the inlet side you flow your feed liquid at high pressure. One of the two outlets is your "purified" liquid and the other is an outlet for feed liquid.
Because of the pressure differential between the feed side of the membrane and the substrate side of it, the "pure" liquid will be forced through and then flow through the substrate and the pure liquid outlet (at a much, much lower flow rate than the pressurized feed liquid). On the feed side of the membrane, this results in a slightly higher concentration as it passes the membrane and thus, the feed outlet side has a higher concentration of solute than the inlet. But you are always maintaining a flow across the membrane at high pressure and what you end up with is the slightly higher concentration liquid flowing out the far end from the inlet. Note that the downstream line from the canister is still under pressure.
So you don't really need to backflush to clean it, or not as often as you might think. You always have a flow of material over the surface in low enough concentration to keep the salt in solution. Granted that sometimes they will chain membrane canisters, the outlet from one going into the next. Or they may have a feedback loop that keeps a set (higher) concentration on the outlet. This reduces the inlet flow and increases the concentration of the output, but it also increases the pressure required. Regardless, the membrane is usually kept from clogging from the movement of the feed.
FWIW, in some systems you might want a certain concentration on the outlet to use as feed for another process. You might be able to use it to concentrate sugars, or even the salt we're talking about. The more water you squeeze out, the less you need to evaporate. But in the case of desalination, I can think of cheaper ways to get salt (like mining), but this serves as an example of what can be done.
For maintenance in some operations (like for example, in the food industry), once the system is shut down, they will run cleaners through the system and if it needs to stay shut down for a period, they'll fill the system with purified water (if water is the output they can use that). They might add a bacterial inhibitor so that nothing could possibly grow and build up in the system. If they don't keep the canisters full of liquid they will dry out and usually become useless. And they are quite expensive.
Pure water is not always what is sought after. Lower pressure RO, usually called ultra filtration has various uses. For instance, I saw one project using it in making raspberry juice. Don't ask me what they were doing with it, I just saw it in passing at a food research place. I was seconded to a research institute in a past life to study using RO to purify waste
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When thinking of water filtration, a lot of you automatically conjure up a mental picture of a conventional water filter -- ie, dirty water poured from the top, and impurities get trapped in between, and clean clear water drips out from the bottom
In large scale water filtration operation, that traditional top-down model does not work
Instead, raw water is pumped into the inner tube of a double-layered pipe, which is slanted upwards, at a 30-60 degree angle
Sections of wall of the inner tube are made up of filtering membrane - such as Graphene
As the raw water flows upstream , and because of the smaller diameter of the inner tube , pressure building up inside the inner tube of the double layered pipe.
Because of the higher pressure inside the inner tube, molecules of clean water flows out of the inner tube, through Graphene (or other filtration membrane), into the larger pipe on the outer layer of the double-layered pipe
And because the pipe is slanting upward, gravity causes the filtered (clean) water in the outer pipe to flow down and eventually it gathers at a collecting point (usually a tank, or a pool) at the bottom
At the top of the double-layered pipe, there is an opening for the inner-pipe for the impure-water to exit
Because of the outlet, there is no need to do any "back flushing" since impurities, including salt, are continuously being flushed away
Hope this helps
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