Removing oxygen from water using nitrogen sparging

[RDS12]

Hi,
I am having a difficult time understanding why sparging water with nitrogen removes the dissolved oxygen in it.
Could someone please help me understand the concept behind it?

 

[georgeverghese]

The Henry constant for O2 solubility in water is based on pure O2 mixed with water vapor in the vapor, so if you have an N2-O2-H2O mix in the vapor, the partial pressure of O2 in the vapor space is reduced correspondingly. Does this help clear things?
Using N2 gas as a stripping medium also help to increase the interfacial mass transfer coeff and speed up the transfer.
Using N2 as a stripping medium in this application is the basis for a process license for better large volume sea water de aeration for a large petroleum EXPRO company.

 

[RDS12]

When oxygen is dissolved in water is it still considered to be in the vapor phase?
I guess I don't understand the O2-H2O vapor phase aspect. If I have no oxygen in the vapor phase, how can adding N2 lead to a reduction in oxygen's partial pressure?

 

[mbt22]

Pure nitrogen bubbles going in have zero partial pressure O2, and ideally going out are in equilibrium with the water, containing O2/H2O/N2. Oxygen is more volatile than water(!), you reduce the concentration of oxygen in the water.

Matt

 

[moltenmetal]

There is oxygen dissolved in the water, and no oxygen in the nitrogen. Oxygen diffuses down the oxygen concentration gradient into the nitrogen, and leaves with the nitrogen.

If you start with a batch of water, and sparge with a large amount of completely oxygen-free nitrogen, eventually the water will be scrupulously free of dissolved oxygen. You'll also lose some water to evaporation into the nitrogen.

In a continuous contactor, the ratio of gas to liquid flow, the number and arrangement of contact stages, the effective area of the gas/liquid interface, and fluid properties affecting the mass transfer coefficient, are all important to determine how much stripping of the target molecule occurs.

It's important to realize that concentration gradients are the driver for mass flow by diffusion even if there is a pressure gradient which you might think would drive the flow in the other direction. While it is possible to make diffusion "stand still" or even to reverse it by means of a large advective (pressure difference-driven) flow of something else, diffusion under static conditions doesn't care about pressure-only about concentration. Helium-neon laser tubes contain mixtures of helium and neon in a sealed glass assembly, but over a period of years, helium diffuses straight through the glass boretube and through the glass/mirror seals and glass-electrode seals to leave the tube, because the concentration of helium in the tube is much higher than the concentration of helium in the atmosphere. The diffusivity of helium is high, and as good a job as you do to make that glass assembly impermeable, it isn't absolutely impermeable to something as small as helium. Another example is oxygen diffusing through teflon tubing into pressurized hydrogen- it can be very surprising just how much, and how fast, even those big oxygen molecules diffuse through the teflon.

 

[jari001]

Related follow-up question: If I have a batch of water in a tank and some appreciable amount of the tank is empty, can I affect O[sub]2[/sub] removal by supplying and removing N[sub]2[/sub] from the head space? My thought is that by removing the equilibrated vapor phase, I will drive O[sub]2[/sub] diffusion out of the liquid phase.

 

[Latexman]

Yes, lowering the O2 concentration of the vapor phase by N2 purging the vapor space will speed O2 diffusion out of the liquid due to an increased concentration differential. That's how Fick's law works.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RDS12]

Hi thanks for all the Input so far.

I understand that Oxygen would diffuse to Nitrogen.

I am just still confused about the partial pressure aspect. How can the partial pressure of Oxygen decrease if there is not Oxygen in the vapor Phase to begin with?
All of the Oxygen is dissolved in the my water...

 

[jari001]

@RDS12 - The oxygen dissolved into your water is in the form it would be in the air, it hasn't dissolved in the sense an ionic compound would. I picture the O[sub]2[/sub] solvation as O[sub]2[/sub] flanked by water molecules with the hydrogens facing the oxygen molecule. So the oxygen in the water is analyzed like a gas because it is still a gas, therefore the partial pressure of the oxygen will dictate its ability to participate in mass transfer and/or chemical rxns.

@Latexman - Thanks for the answer, I was refreshing myself on Fick's Law just as I came to check this thread.

 

[Latexman]

If you have oxygen dissolved in the water, per Henry's Law, p = Hx, you will have oxygen in the vapor phase at equilibrium.

p = partial pressure of oxygen in the vapor phase
H = Henry's constant
x = mole fraction of oxygen in the liquid phase.

During the sparging, you may not be at equilibrium, but the system will move towards equilibrium, and the oxygen will move into the vapor phase.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RDS12]

Ok it finally makes sense now!
Thank you so much for the detailed explainations!

 

[RDS12]

Now an Extension to the original question...

While my dissolved Oxygen Needs to Transfer to the Nitrogen to be removed from the water...

Can I just consider the Diffusion of Oxygen in Nitrogen to determine my mass Transfer coefficient or would it be better to consider the Diffusion of Oxygen in water?
I guess it would be best to the use the one that is the smallest since this will be the one limiting the rate of my mass Transfer.
Does this seem Logical?

 

[Latexman]

It's best to evaluate both and get the overall rate of mass transfer. It may turn out there is a significant difference in the liquid and vapor mass transfer rates, then the smallest rate will be about the same as the overall rate. But, unless you have some experience in these things, or until you crunch the numbers, you need to look at both. Otherwise, you don't know.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[moltenmetal]

If you replace the air in the headspace of an unmixed water tank with nitrogen, you will reduce the dissolved oxygen concentration- a little- but only if you were to leave that water sitting for a comparatively long time. The interfacial area is low and the mixing in the water phase is poor, meaning that the resulting mass transfer will be slow.

Sparging the tank with nitrogen will increase both interfacial area and liquid mixing GREATLY, relative to just replacing the headspace gas.

Pulling a vacuum on the headspace is a different matter. Vacuum can be a very effective de-gassing measure if it is done properly.

 

[EdStainless]

When people do this they often combine techniques. They warm the water (to reduce gas solubility), pull a light vacuum (to speed the gas evolution), and sparge with very coarse bubbles of high purity nitrogen. If you bubble too aggressively the surface of the water will be turbulent and some of the oxygen will be reabsorbed.
This method will remove all gasses other then nitrogen (such as CO2).

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P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

Of course boiling the water does the same thing faster. A heat exchanger between the feed water and treated outlet water makes it energy efficient.

 

[25362]

From Wikipedia:

The solubility of gases in water is usually thought to decrease with temperature, but this only occurs to a certain temperature, before increasing again. For nitrogen, this minimum is 74 °C and for oxygen it is 94 °C

 

[Compositepro]

The point about solubility versus temperature is interesting but not very relevant. The purpose of boiling is not to reduce solubility but to accomplish steam stripping. The water vapor carries away any dissolved gasses very efficiently and is cheaper than nitrogen.

 

[EdStainless]

Unless you can warm the water and nitrogen sparge. It speeds the process and greatly reduces the nitrogen demand.
Boiling is often undesirable because of the greatly increased water loss.
Though if you have live steam available you can just build a DA like power plants use.
Basically sparging with steam and using and eductor to pull exhaust the gasses.


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P.E. Metallurgy, Plymouth Tube