Re: [PSUBS-MAILIST] open source scrubber design

["Sean T. Stevenson" <cast55@telus.net>]

More accurately, the reaction occurs in stages:

1) Gaseous carbon dioxide (CO2) dissolves in water, creating a weak CO2 
solution. The scrubber media itself is not anhydrous.  Typically 15-20% of the 
media weight is water content.  In addition to that, there is additional 
moisture in the cabin air, so between both sources, there is ideally 
sufficient solvent to dissolve most of the CO2 in the passed air:

CO2 (g) + H2O (l) -----> CO2 (aq) + H2O (l)

2) Sodium hydroxide (NaOH), which is also in aqueous solution (as Na+ and OH- 
ions), in the water content in the scrubber media, reacts with the aqueous CO2 
as the solutions mix, to form sodium carbonate and water:

CO2 (aq) + 2NaOH (aq)  -----> Na2CO3 (aq) + H2O (l)

3) These products are short lived, as CO2 continues to react with them to form 
sodium bicarbonate:

Na2CO3 (aq) + H2O (l) + CO2 (aq) -----> 2NaHCO3 (aq)

4) This sodium bicarbonate product immediately reacts with the calcium 
hydroxide in the scrubber media to form calcium carbonate.  The calcium 
carbonate is not very soluble in water, and so precipitates out to solid form.  
This reaction regenerates the sodium hydroxide (so NaOH is being created at 
the same rate as it is being destroyed in step 2 above), and also produces 
water:

NaHCO3 (aq) + Ca(OH)2 (s) -----> CaCO3 (s) + NaOH (aq) + H2O (l)


Excess water in the scrubber is not as serious a safety concern in a sub as it 
is in a rebreather, since the rebreather diver breathes directly from the 
scrubber output.  In the event of a flooded scrubber in a rebreather, the 
sodium hydroxide and calcium hydroxide in the scrubber media rapidly dissolve 
in the flood water, producing a highly alkaline solution which is both 
corrosive and toxic if ingested.  As well, the scrubbing capacity is 
eliminated or significantly reduced, requiring immediate bailout to an 
alternative gas supply.  In a submersible, flooding a scrubber completely 
implies that you have flooded the cabin, so CO2 removal is not the most 
pressing concern.  Partial flooding or inadvertently getting the scrubber bed 
wet will produce these same caustic products and reduce the scrubber capacity, 
but as you are not breathing directly from the scrubber outlet, you need not 
worry about aspirating these products.  From a design perspective, you would 
probably just want to ensure that a scrubber which accidentally got wet is not 
going to drip corrosive liquids on you, and of course be aware of the expected 
CO2 level rise in your cabin with the scrubber failed.

As for moisture entering the scrubber, proximity to the operator's face is of 
little importance.  Local condensation from humid exhalation will rapidly 
evaporate again in the presence of drier air.  Unless you constrain the path 
of your breath to flow through the scrubber, by exhaling directly into it or 
inhaling directly from its output (as with a diver's rebreather), the inlet 
air will be of the average relative humidity given the net moisture production 
and removal rates.  In a larger gas space, gas mixing and diffusion make a 
steady-state approximation more accurate, in which case your scrubber must be 
designed to circulate the cabin air and remove CO2 at a rate at least equal to 
the rate of production.  In a closed environment, you're not going to deplete 
your scrubber media any faster by running it longer.  It's the net reaction 
rate that matters.

-Sean


On Tuesday 15 March 2011 11:35:46 you wrote:
> Thanks, Sean,
> 
> I had meant to ask if the scrubber reaction produces water, but then
> completely forgot to.  Thanks for addressing the possibility of sodium 
> hydroxide (NaOH) or lye being in the outflow from the scrubber as well as 
> residue from the Sodasorb or other material.
> 
> Would you consider it to be better to not have the scrubber intake too
> close to the crew member's face in order to reduce the  concentration of
> the moisture entering the scrubber?  Obviously that  would mean the CO2
> would be more dilute as well since it would mix more with  cabin air prior
> to the scrubber intake.  Would that make the scrubber's  operation more
> "evened out" with less chance of the undesirable products?
> 
> Thanks,
> Jim
> 
> 
> In a message dated 3/15/2011 12:25:50 P.M. Central Daylight Time,
> cast55@telus.net writes:
> 
> Remember  the scrubber reaction:
> 
> CO2 (g) + Ca(OH)2 (s) -----> CaCO3 (s) + H2O  (l)
> 
> in the presence of NaOH as a (net non-consumed) catalyst.
> 
> Not only is water evolved from the reaction, but excess water in the  form
> of
> condensation will prompt some of the NaOH to go into solution, so  any
> condensate drippings you get from the scrubber bed itself are  caustic.
> Dealing with this in a rebreather is problematic  because the diver's
> exhaled
> gas is at 100% relative humidity to begin  with, so rebreather designers
> play
> tricks with counterlung and scrubber  water traps / condensing surfaces,
> hydrophobic membranes, etc.  The  latter ensures that at worst, the air
> passing
> through the scrubber is at  100% relative humidity non-condensing, but then
> since the scrubber  reaction is exothermic, the additional heat pushes the
> RH
> down  (sufficiently to account for the additional water produced by the
> reaction).  Where it does not, you get condensation, although this  can
> happen
> on a very small scale where the condensate is not large enough  to coalesce
> 
> into drips and cause a problem, but merely "dampens" the  scrubber bed and
> reduces the effective surface area for scrubbing, so that  the scrubber
> doesn't
> last as long as it could under ideal  conditions.
> 
> In a submersible, the problem is more easily avoided, since  it takes some
> time
> for the cabin air to get close to 100% RH, and there  are ways a designer
> can
> condense out or otherwise remove moisture from the  cabin air to push it
> down.
> ABS standard is a relative humidity  between 30 and 70% RH, and this is
> probably what you should shoot for.  (50% RH is often quoted as a desirable
> target for human comfort in  terrestrial buildings).  One thing to note is
> that
> a scrubber will  not function effectively in completely dry air (0% RH),
> since
> the first  stage of the reaction requires that the CO2 in the air first
> dissolve into  aqueous solution before it can react with the NaOH.  Thus,
> the
> higher  end of the humidity range actually provides more opportunity for
> scrubbing, provided you keep the air below the condensation  threshold.
> 
> -Sean



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