ADVENTURES IN SCRUBBERLAND

WHAT ARE SCRUBBERS AND WHAT DO THEY DO?

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 0.5% by volume. This worked if you did not mind having to deal with 8 to 10 pounds of absorbent. For a two to four hours 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 sixty minutes. A two hour dive should require only two 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:

1. Sodium.
2. Potassium.
3. 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.

1. Gas; the CO2 must come in contact with the pours of the granules.
2. 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.
3. 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:

1. Removing carbon dioxide from the breathing gas is a "chemical reaction". The canister is not a "filter".

2. Heat from the canister is a positive indication that the absorber is working. A cold canister is a bad indication.

3. Do not reuse materials as the chemical action depletes their effectiveness.

4. Excessive water will "melt" the absorbent materials. Remove all excess water from the canister.

5. 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.

6. Using rebreathing equipment with scrubber monitoring systems is desirable if available.

7. "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. The maximum allowed to be safe by the US Navy is 0.05% by volume.

8. 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.

9. Never use a rebreather with a used canister as:
    a. You do not know how long the unit will last.
    b. The previous user may be infectious.

10. Be aware of the "9" and stay safe