New Nano Desalinization Method

lbmouse writes "The Technology Review is reporting that researchers at the Lawrence Livermore National Laboratory have announced a way to use carbon nano-tube technology to reduce the cost of desalination of ocean water by 75 percent over current methods of reverse osmosis. From the article: 'The technology could potentially provide a solution to water shortages both in the United States, where populations are expected to soar in areas with few freshwater sources, and worldwide, where a lack of clean water is a major cause of disease.' The technology may also lead to new ways of eliminating carbon dioxide emitted from power plants."

Comments

Perfect

[Who235]

Now, as sea levels rise, we can just drink it up.

Woo-Hoo!

If it involved boiling the water...

[i kan reed]

We'd probably call it vaporware

Mandatory "Top Secret" reference

[CrystalFalcon]

- Do you realize what this would mean to the starving nations of the planet?

- WOW! They'd have enough salt to last forever!

Small pore, more flow ?

[karvind]

Does anyone have any idea why the small pores have higher flow rate through them ? My classical fluid dynamics class beats me here. Should be something to do with quantum effects at that scale, but can't guess it. Quantization in electronic states makes sense to me, but don't know what it is doing to 'flow dynamics'.

Cool work nevertheless. I wish they could do something with silicon nanowires as silicon is the second most abundant element on earth.

Re:Small pore, more flow ?

[kebes]

I'm reading the original Science article now (sorry, only available to subscribers, although the Science summary may be available to the general public).

The reason that the gas and liquid transport through nanotubes is so much higher than you might expect is due to the smoothness of the inside walls. The classic hydrodynamic equations have some amount of surface roughness inherently built into them. If you just naively scale them down to nano-dimensions, you'll predict very high resistance to fluid flow. However carbon nanotubes have "perfect" inside walls, that are atomically flat. This allows the water molecules (or gas, or whatever travelling inside them) to travel without "getting caught" or "bumping" into defects. In essence the atomic smoothness of the walls brings us into a whole new (nano) hydrodynamic regime.

This effect was predicted by computer simulations previously, but now has been actually observed in real samples. Very impressive.

Orchid fractals

[LiquidCoooled]

I once read something about a class of fractals called >orchids.
They are the result of monitoring crowd flow dynamics and producing the formulas.

They too noticed that for a large crowd (concert, football match) crowd flow speed INCREASES with a number of small gates rather than one large gate, hence one by one through the turnstyles actually makes the process quicker.

This appears to be a similar unintuitive process.

Anyway, I know it wasn't totally on topic I just thought I would share.

I like the other method...

[PatTheGreat]

This article raises two thoughts in my wonderful little head.

1) Why do they bother calling it "reverse osmosis?" From a quick review of high school biology, I have come to realize "reverse osmosis" really means "pumping through a filter."

2) I saw this other method in Discover that I really liked. Basically, it proposes using deep water and methane to flash-freeze water. All you need to do is to pump methane into water of the right depth, and it instantly freezes into that flammable ice mining rigs love to dig up and play with, without like, refrigerating it. Anyways, as it freezes, all the salt gets pushed out and it floats to the top, so all you have to do is melt the ice and reuse the methane. It appealed to the recycler in me, and it seems to me some tubes and plumbing would be easier than nanotubes, eh?

Re:stop watering your lawn

[BigCheese]

That's easy. Zoysia. That's what I've had for years and I never water. Rain is pretty irregular here in Kansas too.
Plant a lawn that works with your local climate. It's better for the environment and better for the household budget.

Where does the lawn water go?

[Latent Heat]

I think the argument the lawn-waterers are making is that if they pump water out of the ground and sprinkle that water back on the lawn, most of that water will percolate back through the soil back into the ground water. Whether that argument holds up or not depends on such factors as the rate of transpiration and evaporation off the grass, whether the runoff water percolates back into the ground water reservoir or runs off somewhere else. That paper mill may be sucking the water out of the ground and then discharging it in polluted form in a stream, thus depleting the water table.

I am hard pressed that anyone living where there is normal rainfall for growing grass (i.e. Georgia) and has a water table high enough to tap with a private well isn't simply recycling the water by pumping it from below and discharging it on the surface. In fact, ground-source heat pumps are the next big thing in saving energy resources -- some of the systems are closed loop with a coil to pipe in the ground, other systems are open loop, lifting water from a well and discharging it on the surface. The various state DNR's that issue permits for such open loop systems want you to discharge on the surface -- they certainly don't want you pumping water that you have handled directly back into the aquifer without being filtered through the ground.

I agree that lawn watering is a serious use of resources in the desert Southwest U.S. You can be Fremen in your view of lawns on Arrakis, but to argue the same point on Caladan is stretching matters a bit far.

Re:Where does the lawn water go?

[SatanicPuppy]

If that was actually the case, then no one in georgia would need to water their lawns. Unfortunately Georgia has been teetering on drought conditions (pdf warning) for years, and lawn watering actually is a big issue in a hot state like Georgia, because it doesn't soak right back down into the watershed, and it sure as hell doesn't replenish the aquifers. It evaporates. Poof. Gone. Georgia soil is mostly clay, and as you probably know, water doesn't travel through clay very well at all.

Georgia, btw, happens to be where I live. One of the main "crops" here is slash pine, which is what most paper is made from. TONS of papermills. Papermills use tons of water. They don't use crap water either, they pump the good stuff out of deep aquifers. We've got salt intrusion all down the damn coast, up into S. Carolina, and down into Florida. What does that mean? It means your magic well in a coastal county is full of salt, and the salt is moving inland. Why?

Ground water takes a while to replenish, and aquifers take, literally, centuries. When you pump water out of the ground, it doesn't come right back, and when it does come back, it moves in from the surrounding area and the ground water levels everywhere go down. That's the whole idea of a watershed, and there are 52 watersheds in georgia. Sounds like a lot doesn't it? Well there are 5 around atlanta, and they're all laughably overutilized. Pull that water out of the ground and dump it in a river, and some evaporates, and the rest of it flows on out to sea. Only the tiniest fraction of that water makes it back into the ground. So when you have low ground water on the coast, the ocean moves in to fill the lack.

A hundred years ago you could drill a hole in the ground, and you'd get a spring, water bubbling out on it's own. Now you drill a hole 5 times as deep, and put a big pump on it to get the same amount. We're running it down, and running it down quick, and, thanks to the attitude that we live in a land of inexaustable water, it's only getting worse.

I'm not that much of an environmentalist. I'm really not. But water is a big deal, a HUGE deal, and people who think that the supply is inexhaustable anywhere are living in a dreamworld. In the Southeast, it's a problem. In the midwest, it's a crisis, we're talking 10 years at best. It's no better in the west. We need a way to create cheap, clean water, and we need it BAD and we need it NOW. Failing that, we need people to stop blowing water on crap that doesn't matter.

Re:Wow, 75% cheaper

[wealthychef]

LLNL is a national laboratory. This technology will probably be available more broadly than if it was developed by a private company. This sounds like really good news for the world, especially e.g. African nations where potable water is a huge issue.

1000 BTU/pound of water

[Latent Heat]

It takes about 1000 BTU's to evaporate a pound of water, and about 1000 BTU's are given up when that water condenses. Assuming 140,000 BTU (don't know if it is the high or low heating value, which also depends on water condensation from the H2O of combustion) for a gallon of Diesel fuel, a gallon of Diesel can evaporate 140 pounds or about 18 gallons of water. For people making maple syrup by direct evaporation (requires 30-40 to 1 concentration), it takes about two gallons of Diesel to make a gallon of maple syrup (an appetizing thought when you pour that syrup on your pancakes).

That ocean water scheme is taking much lower grade heat, thermodynamically, than the energy in Diesel fuel, but it still requires 1000 BTU's of heat per pound of water (8000 BTU's per gallon). That is a lot of heat to take out of the environment, and a lot of heat to transfer.

Another way for more efficient desalination is to recycle that 1000 BTU/lb -- use 1000 BTU to evaporate a pound of water to purify it and then condense that water vapor to get back that heat to evaporate more water. Trouble is that water condenses at the same temperature it evaporates, and you need at least a small temperature differential to get heat to flow downhill.

There are two approaches to recycling the heat. One approach is multi-effect distillation. You evaporate at a higher temperature and pressure, and then condense at that same temperature, which you use to evaporate other water at a lower temperature and pressure in a vacuum chamber. You have a cascade of evaporators at successively lower pressures and keep reusing the same heat. This method was developed by Norbert Rillieux, the Louisiana son of a French engineer and an American former slave, and is widely used in food preparation -- sugar from cane or beets, orange juice concentrate, and so on.

The second approach is vapor compression. You boil at one temperature, but you condense at a higher temperature by compressing the vapor to a higher pressure using something akin to an automotive supercharger driven by an electric motor, and that way the heat from condensing at a slightly higher temperature and pressure is recovered by the evaporator. This requires only a single "effect" on account of the vapor pump instead of the multi-effect cascade into successively lower pressure chambers, but it needs the electric motor and vapor pump, and you need to move a lot of heat at low temperature differentials across large surface area plate heat exchangers.

Reverse osmosis is a pure mechanical process that doesn't require exchange of the 1000 BTUs per pound of water, but the osmosis membrane offers resistance to pumping in excess of the natural osmotic pressure (the pressure differential required to overcome the salinity differential, the PV work representing the true thermodynamic cost of desalinating the water, which is much less than the 1000 BTU's per pound). By the way, it is always more cost effective to desalinate slightly-salty (brackish) water from marshes or irrigation runoff or other sources than going for the highly-salty sea water on account of the energy inherent in the dissolved salt as reflected in the higher osmotic pressure).

Re:could be important for a hydrogen economy

[rossifer]

Out of curiosity, why would it be important to purify the water before separation into hydrogen/oxygen?

Well, if there's salt in the water and you attempt electrolysis, you'll get chlorine gas and NaOH in solution. It's actually the modern process for producing sodium lye (aptly named the chlor-alkali process). Once you run out of chloride ions to convert to chlorine, then you start to produce hydrogen gas, but now you've got some high pH liquid in your reaction vessel, and you probably have other reactions going on that you didn't intend...

Regards,
Ross