

Pentagon weapons-maker finds method for cheap, clean water (Perforene) - drucken
http://uk.reuters.com/article/2013/03/13/us-usa-desalination-idUKBRE92C05720130313

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ajb
I'm suspicious of this. I thought I read somewhere that the force required for
reverse osmosis is mostly due to osmotic pressure, and a more permeable
membrane won't change that at all. It just reduces friction. Here's a cite:
[http://arstechnica.com/science/2011/08/desalinization-is-
thi...](http://arstechnica.com/science/2011/08/desalinization-is-this-as-good-
as-it-gets/) " A state-of-the-art facility is now within a factor of two of
the theoretical energy minimum,".

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eldr
Thanks for the insight. Some quick (wikipedia) research would seem to indicate
that osmotic pressure (27atm for seawater) is independent of membrane
construction, at least under ideal conditions. Supporting the membrane during
operation would seem to be a huge challenge, especially as they indicate that
it has a tendency to tear during handling.

This article claims a 100x reduction in required pressure for the graphene
membranes, while the Science article claims that state-of-the-art plants are
already operating at 2x the theoretical minimum. These links would seem to
support the claims of the Science article, that RO plants currently run at
about 54atm (i.e 2x the theoretical minimum): [1]
[http://www.lenntech.com/processes/desalination/reverse-
osmos...](http://www.lenntech.com/processes/desalination/reverse-
osmosis/general/reverse-osmosis-desalination-process.htm) [2]
<http://www.remco.com/ro_quest.htm>

I do hope that I'm missing something here though, and that fresh water for all
is around the corner. I think it would go a long way towards improving the
lives of many millions of people.

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Someone
Theoretically, I think the only energy spent is on the resistance inside the
membrane.

Maintaining vessel pressure is not work and hence, not energy spent (in the
same way as lifting a heavy weight and putting it down again does not do work)

Practically, maintaining pressure will take energy, but that may be
recoverable (that first link talks about a Energy Recovery Device that helps
with that)

So, I guess the article is wrong about the '99% less pressure needed' claim
and is theoretically right about the '99% less energy' claim. I also guess
that, in practice, that 99% will be a lot lower.

Corrections welcome.

~~~
ajb
But it's not just about maintaining pressure. Osmotic pressure can do work,
and pushing against it requires work.

Here is an experiment you can do in your kitchen to demonstrate this: Take an
egg, and peel a bit of shell off one end so the membrane is visible but
unbroken. Push a straw into the other end , and put the egg (straw upwards) so
the membrane is in contact with water. Osmotic pressure will force the
contents of the egg upwards _against gravity_ out through the straw.

The osmotic pressure has done work (lifted the contents of the egg) and water
has become mixed with the egg contents. To obtain the water back, you would
need to put in at least the same energy.

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timedoctor
It's amazing the types of advances that are still posible in material
sciences. Not sure that I'm skeptical, I tend to trust that Lockheed would not
come up with a press release unless they were fairly sure about the
technology.

I wonder if there are a bunch of advances such as this that are developed but
not commercialised?

