[HN Gopher] Pressure-driven distillation for fast and selective ...
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Pressure-driven distillation for fast and selective water
purification
Author : wglb
Score : 40 points
Date : 2023-07-29 14:41 UTC (1 days ago)
(HTM) web link (techxplore.com)
(TXT) w3m dump (techxplore.com)
| credit_guy wrote:
| Unfortunately, at least in the US, desalination cost is dominated
| by the high cost of capital, not by the cost of the energy.
|
| For example, the largest desal plant in the US is the Carlsbad
| plant in California [1]. For our purposes, it's a very nice
| plant, because the numbers are very round: it cost $1 BN to
| build. It has an output of 190k cubic meters per day, or 5.7
| million m3 per month. The plant was funded with bonds. Let's say
| you want to build it today. A private person has access to 30y
| mortgage rates of about 7.7%. But this being a large corporation,
| and using municipal bonds that are tax exempt, let's say you can
| borrow at 5.5%. If you plug in a mortgage calculator, that comes
| to $5.7 MM per month, which is very fortunate, or a very round
| $1.00 per m3.
|
| The energy use of desalination is in the range of 2.5 to 4.0 kWh
| per m3 ([2], page 91). Let's say it is 4.0 for the Carlsbad plant
| (to be conservative; most likely it is lower). The average cost
| of electricity for US industrial users is $0.0775 [4], so that's
| about 19 cents per m3.
|
| Recap: $1.00 capex cost per m3, $0.19 electricity cost per m3.
| Increasing the efficiency of desalination by a factor of 2 would
| not make desalinated water twice as cheap, but only about 10%
| cheaper.
|
| There is an asterisk here though: I don't know why, but the cost
| of electricity in California is much higher than the average cost
| in the US. It costs 18 cents vs 7.75 cents. So, for the
| particular case of California, the capex cost still dominates,
| but not by a factor of 5, only by a factor of 2.
|
| [1]
| https://en.wikipedia.org/wiki/Claude_%22Bud%22_Lewis_Carlsba...
|
| [2]
| https://www.energy.gov/sites/default/files/2019/09/f66/73355...
|
| [3] https://www.amtaorg.com/wp-
| content/uploads/07_Membrane_Desal...
|
| [4]
| https://www.eia.gov/electricity/monthly/epm_table_grapher.ph...
| nine_k wrote:
| The cool trick is to use a nano-porous hydrophobic material with
| some air trapped in the pores. Water evaporates into the air,
| then re-condenses on the other side, while salt can't pass. The
| water does not stick inside, because the material (based on
| silane) is hydrophobic.
|
| The practically important part is that the membrane does not
| degrade from the chlorine and ozone, the typical water
| disinfectants.
| mdp2021 wrote:
| > _while salt can 't pass_
|
| But will not the separated salt clog the membrane? Separated
| salt is an issue, e.g. in distillation - you have to remove it
| to remain decently efficient.
|
| And speaking of efficiency: do energy costs make mass
| desalinization with this technology feasible?
| freshpots wrote:
| Salt doesn't pass through RO membranes and these appear to
| have the same pore size. However, they all foul and you
| perform maintenance like backwashing and chemical cleaning as
| the flow rate goes down.
|
| Here is an overview: https://www.suezwaterhandbook.com/water-
| and-generalities/fun...
|
| In terms of efficiency, it is cost dependent. Making drinking
| water is not cost prohibitive at this time but for large
| scale agriculture it would typically be considered too
| expensive to irrigate with RO desalinated water. This
| technology appears to lower the energy requirements but more
| research and scaling it up is needed.
| Gibbon1 wrote:
| When I've looked into RO for irrigation it seems that cost
| is a problem but it's tantalizingly close to economic. I
| think you need to get the cost down by 2/3rds.
|
| However boron is a problem, most RO water has more boron
| than is healthy for plants. Though I saw one press release
| talking about a membrane that unexpectedly produces water
| with low levels of boron.
| freshpots wrote:
| Interesting, TIL. I wasn't aware of boron issues with RO.
| Is this for seawater RO to irrigation water?
|
| This link is interesting:
| https://www.globalwaterintel.com/sponsored-content/boron-
| a-k...
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(page generated 2023-07-30 23:01 UTC)