How to remove hydrogen gas from water

[jimhokie]

We have a system that generates chlorine from the salt in sea water as a biofouling control in a condenser. We have a problem with hydrogen gas by-product collecting in un-ventable high points in the system, such as inside a plate-and-frame heat exchanger. Is there a chemical that could be injected into the system that could bind to the hydrogen gas to scavenge it, preventing a dangerous build up of hydrogen? Or any other thoughts for how to deal with the issue? Currently, our best guess is to ensure high enough flowrate to prevent gas from collecting, but it's a significant challenge to know what that flowrate would have to be.

Thanks for any suggestions!
Jim

 

[Compositepro]

Well, there really should not be any unventable pockets is any piping system, or air will also be a problem. Some sealed flashlights contain a pellet of catalyst material to remove hydrogen that the batteries might vent. I do not see that as a practical solution to your problem.

 

[jimhokie]

Don't get me started on the system piping design with unventable pockets! At least the other high points in piping can be dealt with by adding vents, but the high points within the plate-and-frame heat exchanger represent the challenge.

As a mechanical guy with low aptitude in chemistry, I could only envision some benign chemical that could be injected into the system that would bind with the hydrogen, allowing it to go into solution with the seawater and pass safely through.

 

[moltenmetal]

Something readily reduced by hydrogen at room temperature, with no catalyst, in an aqueous environment? Doubtful. You've already got a really reactive oxidant present (chlorine) and the hydrogen isn't reacting with it, is it? Not without a catalyst, heat or light.

Think about it: if you're OK with an additive, why not shut off the electricity and just add bleach instead of making it in situ by electrolysis?

Failing to see the problem with the hydrogen accumulation, unless the exchanger is mounted with all the connections pointing downward or the exit line goes downhill without a vent- then you're losing heat exchange area gradually over time, but the solution is obvious- add a vent. Same with the rest of the high points: what's the issue, unless there's a tank somewhere with a headspace full of AIR that you're gradually turning into an air-hydrogen mixture... Are the velocities really that low that gases don't reach an equilibrium where they're swept out as fast as they're added?

 

[jimhokie]

Moltenmetal...thanks for your comments. "Think about it: if you're OK with an additive, why not shut off the electricity and just add bleach instead of making it in situ by electrolysis?" I think direct injection of chlorine in either liquid or gas form would be problematic due to the quantity required, safety reasons in the event of a release of the chlorine, and other issues specific to the installation. I was hoping for an additive that would deal with the relatively minute quantity of hydrogen being produced as a by-product of the process that could more easily and safely stored for use

"Failing to see the problem with the hydrogen accumulation...". The concern is the potential for an explosion. I know pure hydrogen without oxygen present should not be a hazard, but an explosion in a similar plant is what sparked the issue for us, so somehow an explosive combination was able to occur. And yes, the obvious solution is to provide a vent, which can be done easily in piping high points, but not within the flow channels of the plate-and-frame heat exchanger.

As far as the flow velocity being sufficient to continually flush the gases out, that's what we just don't know, and the cooler vendor will not provide us the necessary details claiming proprietary info.

Based on your first paragraph, it sounds like a magic additive to remove the hydrogen that I had envisioned does not exist. Thank you for your response!

 

[StoneCold]

The hydrogen would be produced right at the electrolysis point. Just add a high point, and vent or use the first high point to add a vent. Most of your hydrogen should be vented out in the first vent. This assumes that your velocities are not that high. Maybe 10ft/sec or less.

My two cents

Regards
StoneCold

 

[moltenmetal]

You wouldn't add chlorine gas for the reasons you've given- you'd add bleach solution instead. Bleach isn't benign, but people handle things much more hazardous than bleach every day without incident. Bleach costs more than chlorine, so if you need a massive amount, you'd be stuck with chlorine.

You make a mole of hydrogen for every mole of chlorine when you electrolyze water, and if you do the math, you'll know that you would need to add roughly as much of whatever hydrogen-destroying additive someone could imagine for you, as you would add in bleach if you went the non-electrolysis route. So even if the idea wasn't dead from a chemical reactivity basis, you wouldn't be adding "a tiny amount".

Any separator you add to remove H2 would need to be downstream of the electrolysis point a bit, to ensure that you don't also vent chlorine. It does take at least some mixing and RT for the chlorine to dissolve, even though in a mixed cell you're making OH- ions right next door to dissolve some of it as hypochlorite (Cl0-).

You're not making enough hydrogen to affect the heat transfer between the plates of the exchanger as it flows by, unless you're circulating in a closed system without any vents at all... Unless you're massively over-chlorinating, the volume fraction of gas generated in a single pass in order to do adequate breakthrough chlorination is tiny. The hydrogen only accumulates at high spots because it can do so over long periods of time. If your exchanger is designed such that it has a dead zone at the top which can fill with gas, it's been designed wrong. And if it is being operated at velocities low enough to be unable to sweep out gas bubbles, it's been designed wrong.

 

[EdStainless]

The water should be de-gassed between the generator and the injection point.
This could be done with a small flash tank and an extraction pump.

But you really should press the system builder on why you are getting hydrogen.
There should be very little generated.

It could be that the hydrogen is very soluble in the cold water where it is generated but as you heat the water the solubility decreases rapidly and you exceed saturation.

These generation systems are used so that you don't have to handle chemicals. The volumes of Cl or bleach required at a power plant are huge and the safety issues are also large.

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