gas (DO) saturation under pressure in water

[shane21]

I understand that pure water that's being aerated will only take up just so much dissolved oxygen (DO) and that that depends on the temperature of the water. I see ranges of 14 ppm at close to freezing and around 6 ppm at 50 C as normal maximums.

I understand, too, that you can temporarily super saturate the water, like via electrolysis, to well above those levels, then over x number of hours, depending on whether it's being agitated or not, it'll inevitably drop back down to normal saturation levels for its given temperature.

I see tons of, what I'd call, nano bubbles, almost like smoke in the water, from electrolysis rising up and accumulating near and spreading out across just under the water surface, which I assume are the O, and larger bubbles, I assume are H, breaking the surface directly above emitter.

Please correct me if any of my assumptions above are incorrect or incomplete to fully understanding.

My question is; If I have a pump inlet pipe near the surface in the thick of those nano bubbles of oxygen and suck them into an accumulator or expansion tank that's being pressured up to 80PSI, that water then would stay, for as long as it's there under pressure, at that higher super saturated DO level, yes?

And, when I later spray it out through a nozzle into air directly from that pressurized tank to a surface a foot away, what would you guess of how much of that higher saturation DO % in the water would be lost to the air and not still retained in the water spray mist hitting surface a foot away?

IOW's, lets say we'd sucked in and started at 30ppm DO super saturated water under pressure and normal saturation for water was 10ppm for our temperature, how much, would you guess, would still be in the water that hit the surface a foot away and not already lost in the foot of air after it came out of the nozzle? Assume, too, that nozzle, if it makes any difference, was emitting that DO super saturated water at 50-100 microns.

Thank you for any thoughts.

- Shane

 

[MortenA]

Are you trying to construct a de-aerator tower?
I think you should consider - where would you oxygen go? There would need to be some other gas flowing through the system where the O2 could mix with and leave the system? Generally i think de-aerator towers works by contacting water with fairly pure N2 in counter flow. Also, not being an expert, but to me spraying does not seem like a super way of getting rid of small entrained bubble? Wouldnt they just become smaller?
Easiest way to get rid of O2 is by adding an O2 scavenger like Sodium Sulfite - common in low and medium pressure systems, Hydrazine - common in high pressure systems, Sodium erythorbate (depending on what you are using your water for)?

 

[moltenmetal]

I doubt that the bubbles generated by electrolysis, irrespective how tiny they might be, will dissolve in water to a concentration higher than saturation at whatever pressure the electrolysis is happening at. There is no driving force for super-saturation- once the gas and liquid phases are in equilibrium, they're in equilibrium. If you want to super-saturate water with oxygen, you need to dissolve the oxygen into the water at a pressure higher than ambient, then gently reduce the liquid pressure. Gas will start coming back out of solution immediately- persistent supersaturation like you see with CO2 in water won't happen with oxygen because there is no reaction between O2 and water- it's pure physical dissolution.

From a safety perspective I hope you're talking about a membrane or salt bridge-separated cell, so the oxygen is completely separated from the hydrogen being generated at the other electrode...

If your goal is to dissolve oxygen in water at a high concentration, you can carry out the electrolysis at a higher pressure and/or a lower temperature, or you can compress the oxygen and feed it into water at a higher pressure and give it time to dissolve. Trying to pump gaseous oxygen and water together in a pump to a higher pressure will be either a success or failure depending on the type of pump and the volumetric proportions of gas and liquid fed. A better way to achieve what you want, assuming you want the electrolysis to happen at a low pressure, would be to pump water through a venturi eductor which is used to draw in the oxygen at its suction nozzle. That will keep oxygen out of the pump, reducing the risk that it accumulates there and causes you to lose prime. Compression ratios attainable with a venturi are limited though.

 

[shane21]

MortenA & moltenmetal,

Thank you for your comments, got in a DO meter so I can see better what happens when I create and then pump the DO enriched water and hold it under pressure.

This is for aeroponically and intermittently (5 seconds on, 5 minutes off) misting of roots in a mostly sealed up enclosed grow barrel.

I'd figured if I captured and then later delivered to the nozzle super saturated DO water, whatever does not then get to the roots in the water, escaping in the spray right then, will, at least, raise O2 air levels within the closed barrel. That's also good for the roots and for any later fluid spray cycles that have below saturated DO levels to pick up some additional DO themselves then, too, with it being sprayed into and through that now O2 enriched air accumulating in there.

That's the plan anyways, and initial tests, showing elevated DO levels are holding under pressure well beyond the reserve tank water that had had the same treatment at the same time but not been picked up by the pump and pressurized. So far, so good, testing continues.

- Shane

 

[moltenmetal]

Physical equilibrium is achieved very fast. If you were to make water saturated with 02 at say 1 or 2 bar(g) pressure, and then spray it into an atmosphere near 0 bar(g) (atmospheric pressure), you should expect the excess O2 to flash off at the nozzle pretty much instantly. Any water droplets from the spray might be elevated in O2 concentration for a few seconds but not longer than that.

If you want to raise the oxygen content of a closed barrel, the best way to do that is to add excess oxygen to the barrel. But why you would want to do that to grow plants is puzzling to me.

 

[bimr]

Don't believe it is energy efficient. See the attached paper for the calculation.

Oxygen is not very soluble as the amount of oxygen is just 3.1 % gas volume/water volume at 20⁰ C. The amount of oxygen in the pressurized liquid is proportional to the pressure. If you pressurized the liquid to twice the atmospheric pressure, the amount of oxygen when it is released to the atmosphere is only 50%

 

[shane21]

moltenmetal,

Ideally, at a minimum, I'd like that the sprayed water, when it hits the roots to be DO saturated. If it came out at something higher and then lost it to the air there, that's OK, too, as the roots and the water on them are still in contact with it and any later sprayings with less than saturated levels might pick up some more DO from that O2 rich environment then.

Most water is not anywhere close to being DO saturated and is actually quite deficient and could stand to be enhanced, even if it's not brought up and over into super saturated levels.

As it turns out, same as the fish like well oxygenated water and will die off when deprived of it, so, too, do the roots of plants. In fact, O2 is as essential for the vitality and health of the roots as is CO2 to the leaves during photosynthesis.

http://www.hortidaily.com/article/25147/The-importance-of-dissolved-oxygen-in-irrigation-water

- Shane

 

[shane21]

bimr,

Digging into that research report now, thanks for that!

If any elevated levels of DO stays in the water after pressure is released as it's sprayed, that's actually preferred.

My intent is to, at a minimum, assure the water sprayed onto plant roots has as much DO as the temperature of that water will permit. That it is, at least, saturated. Pressurizing the water was already going on as a function of the system using an expansion tank so the pump did not have to run continuously. (I pressure up to 145PSI, then spray roots at 80PSI intermittently for 5 seconds on, 5 minutes off, pump only comes on twice an hour.) Creating O2 via electrolysis was means to raise DO to saturation and then by going past and above those levels I was seeing a 'smoke' of small bubbles accumulating first just under the surface and building in downward, that would quickly dissipate and the water clear after electrolysis shut off. (The hydrogen component was, I believe, the bigger bubbles that were breaking the surface and promptly being released.) So, grabbing the O2 bubble packed top layer first with pump inlet near surface, and then putting that water under pressure, forced more of them into being DO and not just bubbles, which made the pressurized solution then super saturated. Which, when later sprayed assured even higher O2 expelled into the enclosed root barrel, even if water from being sprayed dropped back down to only being just saturated then.

That's been the plan, anyways, seems to be working out according to my DO meter, still testing...