Chemistry and Physics in sealed part-used scrubbers

Sutty

The reactions in soda-lime which absorb CO2 are fairly well known:

CO2 + 2NaOH > Na2Co3 + H2O + heat

Na2Co3 + Ca(OH)2 > 2NaOH + CaCO3

The direct reaction of CO2 and Ca(OH)2 also occurs, but the reaction with NaOH is much faster and is why it (or KOH) is added to Ca(OH)2 in soda-lime to speed up the whole process.

Now assuming that we have a part-used scrubber, the 1st part of the scrubber bed will have had more of the reaction occur, and the last part very little or none.
Now we seal the scrubber up and leave it for a while (days/weeks, whatever).
The reactions will work in either direction and will have an equilibrium, now to my thinking that potentially means that the used scrubber material will release CO2 and if only used scrubber was present would acheive an equilibrium with a very small amount of CO2 in the gas surrounding the granules. However gas will diffuse freely through the scrubber bed and any CO2 released will get bound by unused lime elsewhere in the scrubber. Eventually over time it will change from a scrubber with exhausted soda-lime at one end and "fresh" lime at the other, to a scrubber with evenly used soda-lime throughout.

My questions are:

1. How long will this process take? -is it too slow to matter?

2. Will an evenly part-used scrubber behave differently from a scrubber which has normal uneven use?

Anyone got any answers?

Thanks,
Neil

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Rickoz

IF Marek wasn't banned he could answer this one, He's a Chemist.
PM him, that part still works.
Rickoz

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Duncan Price

1. The kinetics are unfavourable - there will possibly be some redistribution on a local scale around particles of sorb but not throughout the whole bed. The equilibrium position is pretty much in favour of CaCO3 hanging onto the CO2 as well.

2. The effective contact times will be different. An unevenly used scrubber can be modelled as a smaller scrubber whereas the other case is basically a the same sized scrubber that you started out with but with a less efficient absorbant.

nigelh

Basicaly no.

Things trundle down the energy slope and the reaction that releases energy, and this one's a biggie, will sulk and reject undoing itself. This is sad as otherwize we could recycle our scrubber material by blowing CO2 free gas through it.

Entropy will increase and my CaCO3 will never be CO2 again just as I will never again be 18.

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nigelh

York

Scutty,
You find the second reaction in many documents, however in the 80s it was demonstrated that this reaction does not occur. The regeneration of sodium hydroxide never exceeds 29% of its original concentration and would take up to two days. It is therefore a good assumption that Na2Co3 + Ca(OH)2 > 2NaOH + CaCO3 is not a part of your scrubbing process. Sodium hydroxide and calcium hydroxide react with CO2 in water in a similar pathway, however sodium hydroxide yields more enthaply and a higher entropy, hence will be preferred as long as it is present. In effect, sodium hydroxide "jump-starts" your scrubber in the first use - which is why pre-breathing is even more important on the subsequent dives, as sodium hydroxide is no longer available.

That aside, you are very correct in that we are dealing with conceptually reversible reactions. The equilibrium however favors mostly the right side of your chemical equation (the product), and the Arrhenius terms make for a very slow reversion rate, effectively 0 at room temperature. You are most likely to have some reactions reverse while the scrubber is still hot. In that case, even very little CO2 released creates a high concentration directly surrounding your pellets and will hence counterbalance the equilibrium, hindering the release of further CO2. I agree with you and suspect that overall this effect may be bigger than we always assume, however I have not seen experimental data to suppoprt this.
It is technically possible to "bake" the used scrubber material to increase the reverse reaction rate and encourage the release of CO2, but before people start shoving their old scrubber into the oven, let me warn you that quality control is hardly possible at home.

Joerg

Molecular Products Ltd

The reaction sequence shown in your note is an oversimplification of the actual chemical reaction sequence that occurs. Sodium carbonate is not formed at the pH of the reactions, it is sodium bicarbonate that acts as the carrier molecule. A better description is:

Overall: CO2 + Ca(OH)2 => Ca(CO3) + H2O made up of the individual stages:

CO2 + H2O <=> CO2 in solution (it just dissolves - it does not form carbonic acid)

CO2 + NaOH => NaHCO2

NaHCO3 + Ca(OH)2 => Ca(CO3)2 + NaOH + H2O

The NaOH in stage 3 is recycled to stage 2 and behaves as a catalyst. It should also be noted that there is a net gain in water.
The overall reaction is not reversible under the conditions you describe and will not increase the capacity of the soda lime or allow it to be safely reused. Once soda lime has been left open or part used the water balance in the material is changed and the performance will be affected. Soda lime that is too dry or too wet will reduce the CO2 removal rate and place the diver at risk. It is not safe to keep part used soda lime for reuse for more than a few hours in your rebreather if it is to be again. This is because the water balance will start to redistribute and will adversely effect performance.

The thinking on redistribution is also not useful as even if the unit did become homogenous, (which it won’t) the capacity would still be less than if the unit was used all at one time. This is because the reaction zone would be larger and the effective capacity of the remaining unreacted material will therefore be reduced.

Ben Field

Hi, Any chance you could further quantify that?

I only ask as everyone I've ever met leaves partially used lime in their unit between dives, 2-3hours on a day, overnight on weekends and even 1-2weeks between short trips. Most often the unit is sealed after a brief hose and lung emptying/cleaning/drying period of less than an hour.

thanks,

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mattmexico

Quote ...... Once soda lime has been left open or part used the water balance in the material is changed and the performance will be affected. Soda lime that is too dry or too wet will reduce the CO2 removal rate and place the diver at risk. .... Quote end.


yo


whow ... so what I have been doing for years ( and showing others too ) is wrong and could put me at risk ... and others too.


thanx
Matt

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nigelh

Me too.

Looking at the way the whole system is wringing wet even at the end of a short dive I suspect the 'lime is at the humidity it 'usually' works at after 10 minutes or so. That may not be optimum but there's not much I can do to change it.

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nigelh

Molecular Products Ltd

Ben when we say it is not safe I must clarify that we mean it is compromised and any further use of the material could have very umpredictable performance, splashing round in the shallows is probably OK but pushing it, taking it deep or expecting it to perform to manufacturers guidlines is not advisable.