4 ms·

Not the full column. You pump out the top x feet of water from the pipe. The equalizing levels of inside and outside the pipe is what drives the upward flow th

Not the full column. You pump out the top x feet of water from the pipe. The equalizing levels of inside and outside the pipe is what drives the upward flow through the membrane.

p1mrx 3y ago
Pumping x feet of water creates x feet of pressure difference. How is that better than doing reverse osmosis at the surface? Perhaps the advantage is that you can lift in multiple stages, so each pump doesn't bear the full column pressure. Also the pumps aren't exposed to salt.
- jethro_tell 3y ago
  Why would you need to lift in different stages? If you take the top 3 feet out of the pipe, the water pressure will push three more feet into the pipe. Am I missing something here?
  - lazide 3y ago
    The membranes require hundreds of feet of head worth of pressure, not 3 ft. The pressure at the membrane will only ever be the difference. It has nothing to do with the depth the membrane is actually under the water.
    - jethro_tell 3y ago
      That makes sense, but then, why put it on a pipe at all?
      - lazide 3y ago
        Near as I can tell, it offers no actual advantage to do so, and creates several serious disadvantages.
- amscanne 3y ago
  In theory, wouldn’t you be able to just keep pumping from the surface of the pipe (increasing x) until x provided a sufficient differential pass through the membrane? It would then stay at x even while you continue pumping. The pump would merely need to lift the water from x below sea level.
  - p1mrx 3y ago
    Pumping from the surface, like with a vacuum? The maximum height of a drinking straw is about 10 meters. It's generally easier to lift water by pushing from the bottom than pulling from the top.
    - jychang 3y ago
      10 meters at sea level. A lot more when you have an ocean’s weight of water pushing on the other end.
      - p1mrx 3y ago
        That's irrelevant, because the pressure at the water surface is ~1 atm regardless of its depth below sea level. Air pressure increases slightly as you descend a well, but not enough to change the outcome.
        jychang 3y ago
        The site pressure at a bottom of a well is irrelevant. You do realize air pressure is not what matters, but pressure as a whole? That includes water pressure. The bottom of the straw is not in a well; it’s more akin to a cassion, so you need to factor in the water pressure as well. What you’re saying would imply that the Titan submersible would be at the same pressure whether under an ocean of water, or in a well the same depth. No. The former is 375atm, the latter is a bit more than 1atm. You only need to lift the top layer of water a little bit. The rest of the water column would come up after it. Of course, you cannot lift the top of the siphon more than 10m above sea level.
- JoshuaDavid 3y ago
  Maybe the thought is that if you drop a 4000m-deep-to-surface pipe into Monterey Canyon, there will be a pressure differential at the base of the pipe (since sea water is denser than fresh water). I don't think the math works out all that spectacularly though. The pressure difference would be a about 1.1 MPa, and desalination of seawater takes ~6MPa, so by running a pipe down to the bottom of Monterey Canyon you could... save almost 20% of the energy costs. Which makes sense. You can't build a perpetual motion machine by sticking a pipe with a membrane at the bottom and a turbine at the top into the ocean. Even if it's a very long pipe.

Not the full column. You pump out the top x feet of water from the pipe. The equalizing levels of inside and outside the pipe is what drives the upward flow through the membrane.

p1mrx 3y ago

Pumping x feet of water creates x feet of pressure difference. How is that better than doing reverse osmosis at the surface? Perhaps the advantage is that you can lift in multiple stages, so each pump doesn't bear the full column pressure. Also the pumps aren't exposed to salt.

jethro_tell 3y ago
Why would you need to lift in different stages? If you take the top 3 feet out of the pipe, the water pressure will push three more feet into the pipe. Am I missing something here?
- lazide 3y ago
  The membranes require hundreds of feet of head worth of pressure, not 3 ft. The pressure at the membrane will only ever be the difference. It has nothing to do with the depth the membrane is actually under the water.
  - jethro_tell 3y ago
    That makes sense, but then, why put it on a pipe at all?
    - lazide 3y ago
      Near as I can tell, it offers no actual advantage to do so, and creates several serious disadvantages.
amscanne 3y ago
In theory, wouldn’t you be able to just keep pumping from the surface of the pipe (increasing x) until x provided a sufficient differential pass through the membrane? It would then stay at x even while you continue pumping. The pump would merely need to lift the water from x below sea level.
- p1mrx 3y ago
  Pumping from the surface, like with a vacuum? The maximum height of a drinking straw is about 10 meters. It's generally easier to lift water by pushing from the bottom than pulling from the top.
  - jychang 3y ago
    10 meters at sea level. A lot more when you have an ocean’s weight of water pushing on the other end.
    - p1mrx 3y ago
      That's irrelevant, because the pressure at the water surface is ~1 atm regardless of its depth below sea level. Air pressure increases slightly as you descend a well, but not enough to change the outcome.
      - jychang 3y ago
        The site pressure at a bottom of a well is irrelevant. You do realize air pressure is not what matters, but pressure as a whole? That includes water pressure. The bottom of the straw is not in a well; it’s more akin to a cassion, so you need to factor in the water pressure as well. What you’re saying would imply that the Titan submersible would be at the same pressure whether under an ocean of water, or in a well the same depth. No. The former is 375atm, the latter is a bit more than 1atm. You only need to lift the top layer of water a little bit. The rest of the water column would come up after it. Of course, you cannot lift the top of the siphon more than 10m above sea level.
JoshuaDavid 3y ago
Maybe the thought is that if you drop a 4000m-deep-to-surface pipe into Monterey Canyon, there will be a pressure differential at the base of the pipe (since sea water is denser than fresh water). I don't think the math works out all that spectacularly though. The pressure difference would be a about 1.1 MPa, and desalination of seawater takes ~6MPa, so by running a pipe down to the bottom of Monterey Canyon you could... save almost 20% of the energy costs. Which makes sense. You can't build a perpetual motion machine by sticking a pipe with a membrane at the bottom and a turbine at the top into the ocean. Even if it's a very long pipe.