Thought experiment - Using Water column to desalinate

[Rmay635703]

In a variety of areas the ocean and desalination equipment are on the wrong side of a mountain (aka Israel and their Dead Sea that should get desalination process water for mining)

It’s universally accepted that it only takes a water column in vacuum of 33.9 feet (-14.xx psi) (or less for contaminated water) to boil.

Many projects trying to get sea or fresh water over a high elevation are finding that on ground pipe is the cheapest for in some cases a couple hundred feet up and potentially thousand plus down in elevation change (Sahara, Salton and Israel all examples)

Why don’t they build a continuous pipe with a section “over the hill” in suction rated for high vacuum with both ends covered in water.

The water would flash under its own weight making it trivial to separate salt and flashed water. (Run either intermittently or via continuous means if you don’t mind vacuum losses increasing energy use)

The energy requirements of such a system are much lower than RO (1000psi vrs 14.xx) and could be net zero if the downward elevation is high enough to also generate electricity.

Based on the complexity of traditional desalination plants this seems to be several orders of magnitude lower.

 

[PandaBear]

Because 1) it cost a lot of money to build a bunch of pipes even if it doesn't cost energy to move elevations as a net. and 2) Friction of moving water on no elevation change is still a lot of energy. 3) If you are going to do that why not just pump and release separate, and use the time difference on pump vs release to store electricity between peak and low demand hours to balance out the grid's load? 4) Land still cost money and acquiring land for this is both costly ($) and risky (someone may sabotage it since it is Middle East and you know what between different people). We have a hard enough time in the relatively peaceful Asia acquiring land to build high speed rail in India and Indonesia and Japan still couldn't get Shinkansen to go from Tokyo to Narita airport after 25 years and gave up.

It is much easier to just vacuum distill locally or recirculate the heat back to input water for boiling in general, or pump fresh water from further away. Probably even easier to vacuum distill your local grey water (non toilet, but shower and sink water) since it has lower sodium content than sea water.

 

[Rmay635703]

Because 1) it cost a lot of money to build a bunch of pipes even if it doesn't cost energy to move elevations as a net. and 2) Friction of moving water on no elevation change is still a lot of energy. 3) If you are going to do that why not just pump and release separate, and use the time difference on pump vs release to store electricity between peak and low demand hours to balance out the grid's load? 4) Land still cost money and acquiring land for this is both costly ($) and risky (someone may sabotage it since it is Middle East and you know what between different people). We have a hard enough time in the relatively peaceful Asia acquiring land to build high speed rail in India and Indonesia and Japan still couldn't get Shinkansen to go from Tokyo to Narita airport after 25 years and gave up.

It is much easier to just vacuum distill locally or recirculate the heat back to input water for boiling in general, or pump fresh water from further away. Probably even easier to vacuum distill your local grey water (non toilet, but shower and sink water) since it has lower sodium content than

But vacuum distillation is rare in general, even in the stationary sense, because of the nature of water you can pull a vacuum using relatively small amounts of energy compared to RO .

You comment about pipes being expensive, they are considering ocean floor RO because pumping water up with it’s friction is still infinitely cheaper and more efficient than generating 1000psi against the filter, where the ocean generates it statically.

 

[Astro14]

What do you think the production rate is going to be from the water boiling off the surface area of the column in the pipe?

How much energy will you spend to condense it on the other side? Because unless you get a temperature change, it’s just going to stay as vapor.

What’s you suggest will work, but it won’t make very much.

In order to increase the productivity, you’re going to need much bigger pumping capacity to have it vaporize in a large chamber, and a very big set of condensation coils.

Both are going to take a considerable amount of energy.

If it were that simple, expect it would’ve already been done.

You’ve encountered the difference between physics - describing how things behave, and engineering - deriving useful performance out of that behavior

 

[Rmay635703]

it doesn’t require any of that, a completely passive solution would be possible by varying the pumping rate up and closing off the flow going down.
You would pulse vaporization and condensation in cycles repeatedly

Segregatation of force condensed vapor and salt rich affluent is a function of gravity and piping design at the vaporization point.

We are only talking 14psi afterall in a desert environment, the 3 way point is such that a small pressure change moves you between boiling and condensation.

The “engineering “ could be handled by something extraordinarily low tech and obsolete, potentially without moving parts in the active zone.

 

[kschachn]

Did you think of this yourself or see one of the many technical papers and articles discussing the merits and limitations?

 

[JimPghPA]

When the water boils off in the vacuum of such a system, it is endothermic, ie the evaporator will require an input of heat energy ( no problem in your desert enviroment ), however, when the purified water leaves the remaining water will soon become saturated with enough salt that it holds onto moisture vapors and they will no longer leave the water at any decent rate ( big problem ) so now you have to remove this brine solution without destroying the vacuum ( another problem ) ya gots to get it to flow fast enough so new sea water can take its place ( doable but somewhat complicated ), but we're not done yet, on the condensation side, when water vapor condenses all of the heat within the vapor is concentrated into one much smaller area, the temperature goes up, and as it does the vapor pressure of the condensate water goes up and soon it will not allow any more water vapor to condense at its surface. So, basicly, you need a really good sink of heat on the pure water condensate side. This may or may not be doable.

Such a project always has many hidden engineering problems. Someplace or something to dump condensate heat into has to be colder than the section you are trying to cool inside this condensing system. Usually at least 15 degree F if there is some type of circulating system such as a fan moving air.

So, is there a significant section of material that can be used as a heat dump, on the clean water condensate side?

The pipe section up and over and down the mountain should be very well thermaly insulated so the water vapors dont get desert sun hot.

There will be a lot of heat that needs dumped at the clean water condensate side for this to have any chance of working.

So, if there was a giant cool heat dump on the clean water side, that would be a good start to such a project.

 

[Gyro Gearloose]

There is no free lunch in thermodynamics. If you use water going downhill to pull a vacuum on a vessel of water you are going to have to pump a lot of water over the hill to boil off a small amount of water from the vessel. Energy required for work on gases is P X V. The V is what kills you.

This is the same reason that power plants have to condense turbine exhaust steam and pump water back into the boilers as liquid rather than try to recompress the spent steam.

 

[PandaBear]

Maybe I should be more specific on vacuum and distillation. The current

it doesn’t require any of that, a completely passive solution would be possible by varying the pumping rate up and closing off the flow going down.
You would pulse vaporization and condensation in cycles repeatedly

Segregatation of force condensed vapor and salt rich affluent is a function of gravity and piping design at the vaporization point.

We are only talking 14psi afterall in a desert environment, the 3 way point is such that a small pressure change moves you between boiling and condensation.

The “engineering “ could be handled by something extraordinarily low tech and obsolete, potentially without moving parts in the active zone.

You probably can generate a much bigger pressure and temperature delta with sunlight beaming into a focus to boil water during day time, than the land required to put up such pipe to siphon (I think siphon is the word you want if you want it to be free of energy input) water to a place with easier evaporation.

Still even using solar water heater is not enough energy to evaporate water for desalination. If you want you probably can do it much cheaper on the rooftop of a beach house, heat up water to evap them in a tank that would naturally cool down at night or underground. I can guarantee you it will use less energy than moving water as far as you suggest, and it will still not be economical.

 

[Astro14]

it doesn’t require any of that, a completely passive solution would be possible by varying the pumping rate up and closing off the flow going down.
You would pulse vaporization and condensation in cycles repeatedly

Segregatation of force condensed vapor and salt rich affluent is a function of gravity and piping design at the vaporization point.

We are only talking 14psi afterall in a desert environment, the 3 way point is such that a small pressure change moves you between boiling and condensation.

The “engineering “ could be handled by something extraordinarily low tech and obsolete, potentially without moving parts in the active zone.

So you’re just going to let the water evaporate?

Again - cross section of the pipe is your surface area for the evaporation. That evaporation takes place - and where do you think the condensation will take place? Much of it will be in the supply pipe - nothing flowing down hill. What’s flowing down hill isn’t pure, it’s the leftover salt water from the most recent filling. To flush that out with fresh water condensate will take a very, very long time, as there is little net condensate yield on that side of the path.

No moving parts, OK, if you say so - also - little to no yield.

If you think it will work - then build one. Fresh water is in short supply in many parts of the world - your free pump would sell out in a heartbeat.

If it actually worked.

 

[mk378]

It takes a lot of heat to evaporate water, at any pressure. If the only source of heat is the incoming water, it would take about 50 gallons dropping 20 F in temperature to evaporate one gallon. That cold, moderately concentrated salt water is of no further use.

The trick is to recover heat from the condenser and return it to the evaporator. This will require compressing the vapor to make it condense at a higher temperature that can drive heat across a heat exchanger. At low pressure the volume of water vapor to condense a gallon would be enormous.

 

[PandaBear]

There is no free lunch in thermodynamics. If you use water going downhill to pull a vacuum on a vessel of water you are going to have to pump a lot of water over the hill to boil off a small amount of water from the vessel. Energy required for work on gases is P X V. The V is what kills you.

This is the same reason that power plants have to condense turbine exhaust steam and pump water back into the boilers as liquid rather than try to recompress the spent steam.

I don't think that's his point. I think what he meant was moving water pass a mountain to another place in the same elevation that has higher heat or lower pressure to get "free" distillation.

I already mentioned that friction to move water even assuming same elevation, and that likely would be easier to process at the source of water than pumping them around. Also the land and construction cost and the "free energy" is probably easier to get using solar water heater people mount on rooftop instead of pumping all over the place. Still not cheap enough.

 

[Rmay635703]

Did you think of this yourself or see one of the many technical papers and articles discussing the merits and limitations?

Cavitation and Rapid freezing is the limitation .

We demonstrated this concept in school but not with water.

It takes a lot of heat to evaporate water, at any pressure. If the only source of heat is the incoming water.

Local conditions are 100-130f, paint the pipe black and it will be warm enough at the top.

I don't think that's his point. I think what he meant was moving water pass a mountain to another place in the same elevation that has higher heat or lower pressure to get "free"
I already mentioned that friction to move water even assuming same elevation, and that likely would be easier to process at the source of water than pumping them around.

Yes if you have the elevation change anyway because you are going to want to move ocean water anyway you may as well use the elevation change for energy. There are 3 projects that are in very hot places with those conditions that have a lot of “desire” to transport ocean water to a hot dry below sea level spot but ocean level canal isn’t feasible.


Also Other pilots are running a half to a full mile of pipe to reduce energy use of Reverse osmosis by locating the filters deep enough in the ocean to overcome the pressure required.

“Lifting “ water can be done much much more efficiently than generating high pressure or vacuum by traditional ground level means.

 

[Nessism1]

My local golf course struggles mightily to grow grass, because the "recycled" water being pumped in is contaminated, likely with salt. Many other golf courses use reclaimed water and seem to do okay, but not this one. We are having a wet winter, and there is more grass growing than I've seen in years. It's just nuts.

 

[crashz]

I like what you're thinking but you have a bit of a dilemma in the siphon part. You'll have the greatest vacuum potential just after the crest of the siphon, but by inserting a vapor collection device there, you will create a vacuum break. The vapor must remain at the same negative pressure as the pipe. But if this remains, you will not get any meaningful volume of vapor to travel out of the pipe on its own. Typically boiling by temperature increase allows vapor to travel under pressure to wherever you want it to go. By boiling in vacuum, you lose those forces. Additionally you will lose vapor to the siphon itself, as the the pumping rate must be enough to continue to provide the vacuum without stalling and therefore the flow will be enough to drag the vapor with it. The vapor has a very low mass, but it still has some and associated velocity. But you have no force counteracting it.

 

[JimPghPA]

The diameter of the vapor pipe would have to be huge.

But thats only one of many problems.