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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
by twawaaay 4y ago
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 only need to top up the water column which is your only energy expenditure. The only thing that the filter dictates is how quickly the water will flow -- which is how much filter area you need to filter a certain amount of water in a unit of time. But pumping the water to certain height requires always exactly the same amount of energy regardless of the properties of the filter.
For sea water the pressure is 25-33 bar (at 25C) which translates to column of water 250-330 m high.
In practice building a column 1/3km in height would not be practical so you can start imagining other configurations. For example, you could come up with a piston that is pushed by a spring to maintain roughly constant pressure and you inject water from time to time to maintain piston position.
But you will always be working against this pressure you have to overcome for the sea water to start getting separated from salt in one go, or you could use multiple stages to reduce pressures at the cost of more complex processing. Whatever you do there is minimum cost in energy you have to expend that does not depend on the filter.
But there is still a possibility for cheaper/lower maintenance filters and so studying materials that can pass water and stop everything else can still provide a lot of value.
- msandford 4y agoYou'll note, I hope, that I said "new science" which might mean such a thing is possible. Yes I agree that osmotic pressure is the thing you have to overcome. Unless somehow there's some new understanding of the world that supersedes our knowledge of osmotic pressure.
- twawaaay 4y agoThis "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.
- netr0ute 4y ago> No, not possible in the regular sense of the world. How do you know that?
- twawaaay 4y agoBecause osmotic pressure is a result of thermodynamics.
- wswope 4y agoEnthalpy 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.
- klodolph 4y agoImagine 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.
- twawaaay 4y agoThanks. That's very good example I have not thought of.
- jojobas 4y agoCurrent 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.
- twawaaay 4y agoOf 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.
- jojobas 4y agoNo, 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 https://sci-hub.ru/10.1016/j.desal.2015.01.005
- twawaaay 4y ago"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.
- jojobas 4y agoNo, 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.
- igravious 4y ago“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.
- twawaaay 4y agoIt 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.