some basics about scrubber chemicals

tecdivertraining

One of my Mod 1 Inspiration students took some time ot to put together some basic information on scrubber chemicals maybe some of you out thier will find it useful.


Matt,

Here's the basics about the chemistry that takes place in scrubber
chemicals. This isn't my primary area of expertise, but these are really
simple reactions and respond well to a simple analysis using basic chemical
ideas. The thing to remember is that the way the scrubber is packed (even if
packed correctly) will make things less clear cut than in the simplified
scenarios that I present here, but the ideas here should still stand up well
in the real world. If you want to get a clearer picture of the exact effects
of cramming lots of odd shaped particles of varying sizes into a cylinder
then your best bet is to speak to a chemical engineer or a physicist with a
thing about fluid flow. But here goes...

Carbon dioxide is a weak acid and, like most non-metal oxides, forms a
mineral acid when dissolved in water. As simple gases go it is readily
soluble and so the moisture in exhaled air is enough to convert a fair
amount of exhaled carbon dioxide to carbonic acid. The rest will dissolve in
moisture deposited on the scrubber particles from previous exhalations
provided the scrubber has been adequately pre-breathed.

Scrubbers are metal oxides or hydroxides (or a mixture), which in most cases
are alkalis. An acid-base neutralization reaction takes place when the
carbonic acid hits the scrubber. This leads to the formation of one or more
carbonate salts and water. Assuming no significant channelling, the rate at
which this reaction occurs depends on several factors: the strength and
solubility of the alkali used as a scrubber, the temperature, scrubber
particle surface area and carbon dioxide partial pressure. Given the
weakness of carbonic acid it is not a particularly efficient reaction.

The alkalis used tend be based on the alkali metals (group I in the periodic
table, leftmost column) and the alkaline earth metals (group II, column
right next to group I). So what's the difference?

Group I (which includes sodium and lithium) forms very strongly alkaline
hydroxides, which are very soluble. Their strength promotes a more efficient
reaction with the carbon dioxide (the reaction has a low activation energy).
This liberates the heat of reaction a little more rapidly than group II
hydroxides and so will cause the scrubber to warm more effectively in the
early stages of a dive. The solubility might also help the reaction start as
the moisture covering the scrubber will contain a fair amount of dissolved
scrubber particles from the start. The drawback here is that a major flood
has greater potential for causing injury with group I based scrubbers,
although mixing them with lime can greatly reduce their ability to dissolve
catastrophically and the lime itself is a group II scrubber.

Group II (which includes calcium and magnesium) forms weakly alkaline
hydroxides, which are far less soluble than group I. To give you some idea,
about 1.5 grams of calcium hydroxide will dissolve in a litre of water
whereas several hundred grams of sodium hydroxide will dissolve in the same
volume. The difference in strength would make group II considerably less
efficient early in the dive and marginally less so once the scrubber is
warm. The reaction is pretty much the same as group I in terms of the energy
it releases per unit mass of scrubber used, but it is harder to start.

The effect of temperature is something of a big deal in all chemical
reactions. I don't have the exact numbers for scrubber chemicals, but
typically a rise of just ten celsius will DOUBLE the rate of a reaction. The
implications of this if a diver fails to pre-breathe adequately, especially
in cold water or on previously used scrubber (even if it has a quite a bit
of lifetime left) are potentially massive. The reaction may never quite
start well enough to release the heat needed to warm the scrubber to an
acceptable working temperature and so hypercapnia may gradually creep in as
the dive goes on despite having lots of unused chemical. Ambient temperature
scrubber in the tropics is working about eight times better than the same
scrubber in near freezing conditions, so carbon dioxide is likely to travel
eight times further up the container before being completely removed in cold
conditions with no pre-breathing.

Particle surface area is one we talked about during the theory section of
the course. The relationship is entirely linear (double the surface area and
you expect the rate of reaction to double). The smaller particles will also
be penetrated more thoroughly by the gas leading to longer lifetime for a
given chemical, so it simply becomes a case of seeing how small you can go
whilst maintaining acceptable ease of breathing and low dust levels. In most
chemicals, at low particle sizes there does tend to be a lot more
variability in the range of particle sizes, which may have implications for
channelling.

As carbon dioxide partial pressure rises the scrubber will work more
efficiently. This relationship is not linear; assuming perfect scrubber
packing the efficiency should rise faster than the partial pressure rises
because increased rate of reaction will lead to a temperature rise which in
turn makes the reaction faster. This has two implications - work harder and
the scrubber will work harder, and go deeper and the scrubber will work
harder (or ascend and the scrubber doesn't work so well, which means that if
it's on the edge of breakthrough close to the shallow stop on a deco
dive...go figure).

Scrubber LIFETIME is primarily a consequence of the chemical you use (both
the amount of carbon dioxide it can react with per unit mass of scrubber and
how efficiently it is consumed). Particle size is also a factor and the
solubility of the PRODUCT of the extraction.

Let's start with mass of carbon dioxide removed per unit mass of scrubber.
If 100 percent of the scrubber reacts:

Calcium hydroxide would remove 1.19 grams of carbon dioxide per gram of
scrubber or 2.67 grams per cubic centimetre of scrubber
Magnesium hydroxide would remove 1.51 grams of carbon dioxide per gram of
scrubber or 3.56 grams per cubic centimetre of scrubber
Sodium hydroxide would remove 1.10 grams of carbon dioxide per gram of
scrubber or 2.31 grams per cubic centimetre of scrubber
Lithium hydroxide would remove 1.83 grams of carbon dioxide per gram of
scrubber or 2.67 grams per cubic centimetre of scrubber

Needless to say the reaction doesn't go to 100% use, or even close. Being
strong alkalis, the group I compounds will continue grabbing all the carbon
dioxide out of circulation when only a fairly thin layer of absorbant
remains, meaning you get better use out of the top half of the canister
whereas the weaker bases will begin to let carbon dioxide through earlier.

The gas has to penetrate the absorber particles to react once the surface is
used. The lower density of the group I compunds makes this easier, as does
the high solubility of the carbonate formed on the surface of the scrubber
particles. Group II carbonates are not soluble. As well as being more
efficient than sodium hydroxide per unit mass used, lithium hydroxide is
less dense (so more porous) and a canister of the stuff would be a fair bit
lighter. The very high charge density on the lithium ion also gives it a
greater ability to polarize the carbon dioxide molecules (metal ions are
positive and the electrons in chemical bonds are negative, so when molecules
approach one another the attraction between these oppositely charged
particles becomes significant). Polarization leads to distortion of chemical
bonds away from ideal geometries, weakening the bonds and increasing the
likelihood of a successful reaction. Lithium hydroxide would appear to be at
least twice as effective as sodium hydroxide overall for a given canister
volume.

As far as price goes it depends where you look, but the supply situation
looks good for the time being, especially given the recycling of batteries.
Lithium hydroxide, on average, would appear to be 'a few times' more
expensive than sodium hydroxide. Demand is high due to its use in degreasers
and batteries, but, whilst lithium is very scarce, it is being produced in
respectable quantities, a new plant having opened in Chile late last year
which has significantly increased global output of the refined hydroxide.

From a biology viewpoint the group I hydroxides are bad news, causing
serious burns quite rapidly in the solid state and in solution, although
with chemical stabilisers this problem is better controlled than it was, at
least at body temperature. Group II hydroxides tend to cause burns more
slowly, although the calcium oxide that is used to supplement some scrubbers
is very damaging (it was in Dragersorb a few years ago and might still be).
Calcium oxide is probably the original chemical weapon - chalk, limestone or
even oyster shells were baked in kilns by the Romans and the resulting
'quick lime' powdered and put in bags. It was then chucked over battlements
or launched from catapults in a favourable wind and would cause severe burns
to the skin and blindness that was often permanent. The message to take away
from this is take care around the dust that these things produce in the
bottom of the bulk container, if you spill a lot of it then wear goggles and
gloves when you clear up. Alkalis are far more damaging to the body than
acids, especially to the mucus membranes. If you start operating a business
involving rebreathers in Europe or the States I'd suggest issuing goggles
during packing (or use a dive mask) and maybe issuing gloves (Marigolds
would do) and dust masks (like those you'd use for sanding paint) to cover
your backside in the event of an accident - it's up to customers whether
they use the safety gear or not. Also it might be worth keeping a couple of
bottles of sterile saline at an eyewash station as burns are very prone to
infection if tap water is used to clean them and rapid treatment makes a
huge difference to the level of recovery. Don't touch this stuff with damp
or sweaty hands. The effects of the other scrubber alkalis are similar to
quick lime.

Other metal hydroxides exist, but aren't really suitable for use as
scrubbers, mostly due to bulk per unit carbon dioxide removed, but in a few
cases the issue is toxicity.

To my mind it would perhaps make sense to think of lithium hydroxide in
scrubbers the way we think of helium in trimix - you add to the blend what
you know you need, but no more, and so create a mix that is specific to the
job in hand. Since scrubber can be changed at the end of a dive lithium
hydroxide would be added simply to extend the maximum duration of a specific
single dive; using it on short dives would be an unnecessary cost.

Storage of partly used material seems to be a source of debate. Here there
are two things to consider. Firstly, if the scrubber is kept out of contact
with atmospheric carbon dioxide it can be stored with confidence. Put it in
an airtight container and seal it with tape for extra security, having
removed it from the unit to protect the unit from its caustic effects.
Secondly, if the scrubber is allowed to dry out then precipitation of
dissolved material will occur and may cause the particles of scrubber to
stick together, kind of like cement. Whilst unlikely to have much impact, on
a bad day this might have implications for channeling. Store the canister
upright and seal while still moist, so any significant pockets of liquid
rapidly drain out of the bottom of the canister before precipitation occurs.
Before putting it back in the unit give the canister a few sharp raps to
break up any 'stickiness' between particles. This is probably being
overcautious, but so what?

Here's a thought on scrubber lifetime. If you know how much oxygen you've
used then you know how much carbon dioxide you produced. Be conservative and
assume a 1:1 ratio to allow for anaerobic respiration during periods of
considerable stress on the dive (even if there weren't any). You know your
scrubber is good for three hours at Lance Armstrong rates of metabolism in
the coldest conditions. This converts to an actual volume of carbon dioxide
that the scrubber will accept and you know how much you produced, so you
know your remaining scrubber lifetime in poor conditions. Just a thought.

Hope this is interesting. Any questions, just ask.

Have fun,

Hugh

__________________
Mathew Partridge
Technical Director
Pro-Tech Dive College
www.protechdivers.com
www.tech-ccr.com

DanDunfee

[quote=This has two implications - work harder and
the scrubber will work harder, and go deeper and the scrubber will work
harder (or ascend and the scrubber doesn't work so well, which means that if
it's on the edge of breakthrough close to the shallow stop on a deco
dive...go figure).
=====================================
Matt

This statement doesn't square with the empirical data. This shows that over a pretty wide range of scrubber types, sizes, and configurations, all other things being equal, the scrubber duration decreases about 14 % for each ata of depth.... and recovers some 'efficiency' as you come up from a deeper dive. How do you explain ??

Regards........Dan

__________________
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