Quote: (Originally Posted by
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
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