Re: [PSUBS-MAILIST] CO2 Scrubbers

["Paul Suds" <paul_suds@hotmail.com>]

While doing my research on scrubbers, I came across this from a website and 
saved it. I used to do some training (volunteer) of firefighters, who used 
Bio-Marine Bio-Packs. I don't know whether they are still in business. One 
day while preparing for the class, I read the tech manual, and tore a unit 
down. I noticed that the absorbent material was called Sodasorb. I believe 
if you type "rebreathers" into your search engine, you will find more than 
enough information for a good design. As a long time scuba diver (since 
1974, double hose regulator days), I always keep in the back of my head a 
term that came out during my initial training "hypercapnia." Remember, It's 
not just the oxygen that you have to contend with, in addition to a good 
oxygen sensor, you better have a very good and sensitive CO2 sensor as well 
(maybe two of each). Usually, the CO2 won't build up very fast in a sub, but 
that depends on free air space, metabolism, and time under water. Unless you 
are staying down for a long time, it probably will not be a problem. For my 
sub design, I have a lot less air space, and calculated a little over 11.8 
hrs for it to get to .5% at 1 atm (.005 ATA). This is based on 60 ft3. With 
a safety margin of 4 on this, I would not stay down more than 3 hours 
without a good scrubber.  Here's the article. For you rebreather types, 
please correct me if I am wrong. And please, always do your own research. 
Get advise from other people, but never bet your life on it unless you have 
thoroughly researched it first and have talked to the experts (not me :). 
Oh, I think they are confused between partial pressures and percents. They 
tend to use them interchangably. They use .005% instead of .005 ATA (partial 
pressure decimal units).
.005 ATA would be .5% at 1 atmosphere.
Suds
A. Scrubbers
We live in the best scrubber ever designed. It took nature several million 
years to develop the optimal mechanisms that remove the carbon dioxide from 
our planets atmosphere. So far it has worked well, even with the increased 
human population and activities. Animals are a major source of CO2. Average 
human activity produces 500 ml of CO2 per minute which represents 4% of the 
exhaled gas.. That adds up to a real problem if you put us in a place with 
no ventilation, such as underwater.

The "optimal" scrubber.
Two things are most important for optimal operation of a scrubber. One; is 
that the gas must come in contact with the absorbent material and two; the 
gas must be near the absorbent material long enough for a chemical action to 
occur. Traditional design has been to place the absorbent materials in a 
cylindrical housing "can" and pass the gas from one side to the other. If it 
did not work well you got a bigger can until it removed a sufficient amount 
to make the gas breathable, less than .5% by volume. This worked if you did 
not mind having to deal with 8 to 10 pounds of absorbent. for a two to four 
dive. Most of these "coffee" can devices are less than 30% efficient as one 
pound of absorbent will reduce the CO2 produced by a swimming diver for 
forty minutes. A two hour dive should require only three pounds of absorbent 
if the device is optimal.


B. How do scrubbers work?
Carbon dioxide is an acid gas. Acid gases can be reduced by oxides, 
peroxides and hydroxides. Hydroxides are used in most scrubbers because of 
their stability and ease of handling.
The hydroxides most commonly used are:

Sodium.
Potassium.
Calcium.
The Dewey Alma division of Grace Chemical Company makes a trade name 
product"Sodasorb" which is a mixture of all three hydroxides with the major 
component being calcium. Other companies make similar compounds with other 
additives such as barium and lithium.

The process. The reaction occurs in three phases; gas, liquid and solid.

Gas; the CO2 must come in contact with the pours of the granules.
Liquid; The pours of the granule hold water containing dissolved hydroxides. 
Optimal moisture content of the granule is 12 to 19%. More or less moisture 
will slow the reaction.
Solid; The undissolved hydroxides structures the matrices.
The absorption of CO2 is a chemical process. The CO2 comes in contact with 
the moist film containing the dissolved hydroxides and is taken into 
solution forming carbonic acid which reacts with the hydroxides to form 
sodium carbonate. This process regenerates the water and the sodium 
carbonate reacts with the hydrated lime to form calcium carbonate, caustic 
soda and caustic potash. The process produces excess water and heat. The 
process works best when the three conditions of gas, liquid and solid are 
maintained, the liquid being the major concern. To little liquid will not 
allow the first step to occur. Too much liquid will dissolve the granules 
and form a paste like composition which closes the pores in the granules.


C. Problem conditions.

High gas velocities. The gas will not be in the granule bed long enough for 
the reaction to occur.
Cold temperature; The water in the granules will freeze into a solid state 
at low temperatures, below 29 d.F.
Moisture retention on the canister surfaces: The hot gasses within the 
scrubber will transport moisture to any cooler surface within the canister. 
If moisture condenses on these surfaces they can dissolve the granules into 
a paste and restrict the gas passage.
Heat; The granule bed will generate temperatures up to 130.d.F. This is good 
for underwater use but bad for surface use.
Channeling; This is a condition in which the gas flowing through the 
scrubber bed finds a hole in the bed or flows along the wall of the 
canister. The bypassing gas is not scrubbed and the CO2 is passed through 
the scrubber to the user.
Back pressure: The granules are packed so tight that they are crushed into 
small sand like bits and restrict the passage of the gas.
Water entering the scrubber: The granules will melt together into a slush 
like paste which will create back pressure, loss of reaction and CO2 passing 
through the system to the user.
Retention of water in the scrubber canister: The process of reducing 5% CO2 
from the breathing mixture for two hours will generate app., eight ounces of 
water. This is excess water that must be conducted out of the granule 
canister. A properly designed canister will allow the hot gas to exit the 
canister before condensation occurs. Water traps are used to reservoir the 
excess water or drains can be installed to "bleed" the system.
Contamination; The scrubber canister must be thoroughly cleaned after each 
use. Biological contaminants can crosscommunicate infectious agents from one 
person to another. The use of "disposable" scrubbers and counter lungs help 
reduce this problem.
The "TEN COMMANDMENTS" of scrubbers:
Removing carbon dioxide from the breathing gas is a "chemical reaction". The 
canister is not a "filter".
Heat from the canister is a positive indication that the absorber is 
working. A cold canister is a bad indication.
Do not reuse materials as the chemical action depletes their effectiveness.
Excessive water will "melt" the absorbent materials. Remove all excess water 
from the canister.
Quality of the absorbent materials can not be determined in the field. 
Depleted materials may give an immediate reaction to carbon dioxide. But 
depleted materials will stop reacting within minutes of use. Insure a 
quality material by knowing your supplier and keeping the materials sealed 
air tight prior to use. Pre packaged absorbers in a disposable cartridge is 
the best way to insure quality and quantity.
Using rebreathing equipment with scrubber monitoring systems is desirable if 
available.
"Old hands", at rebreather use may tell you that you can smell and taste the 
buildup of carbon dioxide in the breathing gas. This is true. But, the 
content must be above 0.08% by volume to do this.

(**********Hmmm... this is not right, please check...Suds**********)

The maximum allowed to be safe by the US Navy is 0.05% by volume. 0.1% by 
volume will render you unconscious within seconds.
Do not use a canister that is below 20 deg, Fahrenheit temperature. For cold 
water operation the canister must be pre heated to above freezing to insure 
the chemical process will start.
Never use a rebreather with a used canister as:
You do not know how long the unit will last.
The previous user may be infectious
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