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Unimpeded permeation of water through helium-leak-tight graphene-based membranes

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Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#21

Post by twawaaay »

igravious wrote: post #18
twawaaay wrote: post #7

Earlier quoted context omitted.

No, it can't. Desalination by reverse osmosis requires certain force to push water through the filters which is dictated by osmotic pressure. It is unavoidable. To imagine it, think about it this way: to overcome osmotic pressure you only need to pump the water above the filter to a certain height. The height you need is only dictated by the magnitude of the pressure and not the filter. And as the water flows, you on…

“We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.” I'm having trouble parsing this.

It seems to suggest the filter is composed of layers spaced exactly to match water molecule size ("two-dimensional" -- meaning space between two layers, "monolayer of water" -- meaning only single layer of molecules of water fits between the sheets).

It seems other molecules can't pass because there is already water blocking passage or the moment the molecule finds itself into filter the channel narrows down ("reversible narrowing of the capillaries in low humidity").

I honestly don't like the terminology used and the way some people try to make themselves sound more intelligent by making sentences more complex than necessary.

Also I am not sure "low humidity" is a good term to use when we are talking monolayers. Humidity is a statistical term (and also assumes presence of air) and what happens in this filter has nothing to do with statistics. Is the space around a molecule other than water "low humidity" region? Dunno... seems a bit forced to me.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#22

Post by jojobas »

twawaaay wrote: post #19
jojobas wrote: post #15

Earlier quoted context omitted.

Current seawater desalination technology runs at about 2x the osmotic pressure power consumption, the current membranes require quite some pressure to push even pure water through.

Of course. You want a high enough water flow to get the installation to filter lots of water. You have to have pressure above osmotic pressure. You can increase the flow of water through filter either by increasing pressure or by adding more filters.

No, it's just that there is a friction-like loss for getting any amount of water through the membrane. See here. https://sci-hub.ru/10.1016/j.desal.2015.01.005

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#23

Post by twawaaay »

jojobas wrote: post #22
twawaaay wrote: post #19

Earlier quoted context omitted.

Of course. You want a high enough water flow to get the installation to filter lots of water. You have to have pressure above osmotic pressure. You can increase the flow of water through filter either by increasing pressure or by adding more filters.

No, it's just that there is a friction-like loss for getting any amount of water through the membrane. See here. https://sci-hub.ru/10.1016/j.desal.2015.01.005

"The trends from the analysis presented in this paper show that in both seawater and brackish desalination we are approaching thermodynamic limitations."

That's excerpt from the article you linked.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#24

Post by jhoechtl »

mensetmanusman wrote: post #16
pmayrgundter wrote: post #4

Earlier quoted context omitted.

Helium is very hard to store since it's a noble gas and so monatomic; I think it's smaller even than H2. So storing it (and H2 for that matter) is hard. Strangely, their material is able to do that while also passing water, H20, which is a much larger molecule. They suggest in the summary that there's more going on than just the size of the porosity. Haven' read more. Maybe someone else will know how useful this is f…

The kinetic diameter of water is smaller than helium because its polar nature allows it to adsorb and diffuse across solid surfaces. This behavior helps it fit through small holes better than helium can.

That sounds counterintuitive. I have a leak-free water supply yet bet it wouldn't be Helium thight.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#25

Post by jojobas »

twawaaay wrote: post #23
jojobas wrote: post #22

Earlier quoted context omitted.

No, it's just that there is a friction-like loss for getting any amount of water through the membrane. See here. https://sci-hub.ru/10.1016/j.desal.2015.01.005

"The trends from the analysis presented in this paper show that in both seawater and brackish desalination we are approaching thermodynamic limitations." That's excerpt from the article you linked.

No, it's not quite like that.

With pipes there's a linear (or quadratic for turbulent flow) resistance, low pressure still results in some flow.

With membranes you have a threshold pressure below which you get no flow at all.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#26

Post by wswope »

netr0ute wrote: post #17
twawaaay wrote: post #13

Earlier quoted context omitted.

This "new science" would have to disprove everything we know about thermodynamics. No, not possible in the regular sense of the world. Sure, we might find everything we know about physics is wrong but if this happens it will most likely serve to explain why things behave certain way rather than give us superpowers to ignore thermodynamics.

> No, not possible in the regular sense of the world. How do you know that?

Enthalpy of solution + first law of thermodynamics.

Salt dissolved in water is in a very "stable", low-energy state, and getting it out necessarily requires a lot of energy input.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#27

Post by rkagerer »

mensetmanusman wrote: post #16
pmayrgundter wrote: post #4

Earlier quoted context omitted.

Helium is very hard to store since it's a noble gas and so monatomic; I think it's smaller even than H2. So storing it (and H2 for that matter) is hard. Strangely, their material is able to do that while also passing water, H20, which is a much larger molecule. They suggest in the summary that there's more going on than just the size of the porosity. Haven' read more. Maybe someone else will know how useful this is f…

The kinetic diameter of water is smaller than helium because its polar nature allows it to adsorb and diffuse across solid surfaces. This behavior helps it fit through small holes better than helium can.

What is kinetic diameter?

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#28

Post by mensetmanusman »

rkagerer wrote: post #27
mensetmanusman wrote: post #16

Earlier quoted context omitted.

The kinetic diameter of water is smaller than helium because its polar nature allows it to adsorb and diffuse across solid surfaces. This behavior helps it fit through small holes better than helium can.

What is kinetic diameter?

https://pubs.acs.org/doi/10.1021/jp412588f

Has to do with how to define the diameter of a molecule/atom when you consider how it interacts with other matter.

Atoms are fuzzy since they are mostly electrons moving about, thus there are a few different ways to define diameter depending on how this cloud behaves.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#29

Post by andrewflnr »

MisterTea wrote: post #11
hexator wrote: post #2

Can we get a simpler overview of what this means for a layperson?

At my work we use a gizmo called a helium mass spectrometer leak detector. We use it to test the hermetic seal of a welded part. It pulls a vacuum on a part or chamber and we spray helium at it and the machine looks for helium leaking through. The units are weird: atm-cc/sec or torr liters/sec. 1x10-6 torr/liters sec is the limit for a crack large enough to let water through. Most customers require 1x10-7 or better d…

Do you have a good reference or search terms for that reverse flow thing? It sounds crazy. Searching ["reverse flow" of helium vacuum pump] doesn't seem to be doing anything for me, at least as a layman.

Re: Unimpeded permeation of water through helium-leak-tight graphene-based membranes

#30

Post by klodolph »

netr0ute wrote: post #17
twawaaay wrote: post #13

Earlier quoted context omitted.

This "new science" would have to disprove everything we know about thermodynamics. No, not possible in the regular sense of the world. Sure, we might find everything we know about physics is wrong but if this happens it will most likely serve to explain why things behave certain way rather than give us superpowers to ignore thermodynamics.

> No, not possible in the regular sense of the world. How do you know that?

Imagine two columns of water in a single U-shaped pipe, with a selectively permeable membrane at the bottom. One column contains salt water. The columns will have unequal levels, because of the osmotic pressure.

If you had a different type of membrane that had a different pressure, you could swap it out and see the water level change.

Repeatedly swapping the two membranes would allow you to pump water up the column, and you could siphon off this energy to create a perpetual motion machine, with some extra energy to spare.

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