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Sean,
Thanks for the additional detail. Understanding the mechanics of the
process is very helpful and clarifies a lot in my mind. I
appreciate it.
Jim
In a message dated 3/15/2011 4:38:26 P.M. Central Daylight Time,
cast55@telus.net writes:
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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