How a diving carbon dioxide absorber works article question

sensor330

I noticed that this article was closed for discussion but I had one question.

What is the color indicator in the lime indicating. The reaction zone or the used lime?

Scott

Beanie

The colours of the diagrams are explained in the key

The article How a diving carbon dioxide absorber works can be found in the library

And discussion or questions can be raised in this thread


Article: http://www.rebreatherworld.com/rebre...tml#post153718

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netmage

As I understand it, both..., any material having been exposed to and reacted w/ CO2.

zzzzzzzz

Sent PM to Molecolar products, and here is the similar text for interest:

The article states that low flow is favorable to the reaction by maintaining a compact reaction front.

The question relates to geometry rather than "flow" since it is the combination of flow rate and geometry that determines gas velocity through the scrubber.

In two identical rebreathers, say one has an axial scrubber, the other has a radial scrubber. For simplicity, let us assume they have the same amount of scrubber material and a similar geometry, i.e. a long-ish cylindrical scrubber. This means that both machines have the same mass flow rate assuming identical divers.

Which is likely to have the highest scrubbing efficiency?

The radial machine is likely to have the lower gas velocity profile within the scrubber material, but has more surface area and therefore an inherently less compact reaction zone because of the gas distribution. The radial machine has a lower beathing resistance given the cross-sectional area advantage over the axial machine.

The axial machine has the lower cross-sectional area to the flow, and therefore higher gas velocity through the scrubber material. Given the lower cross sectional area, this design is susceptible to have a more compact reaction area, however higher gas velocities would tend to impede the absorption process.

This assumes that concentration gradient driven diffusion processes are the dominant means by which the CO2 is absorbed.

The question therefore is whether scrubbing efficiency is the same in both these scenarions, or whether a lower gas velocity is the distinct advantage for scrubbing efficiency.

best,

z

tibby

I would think purple = used. Otherwise it would mean we would see cases where the purple zone would "move" along an axial scrubber. I haven't seen any scrubbers like that. Anyone?
Tibby

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Does the voice of reason change when you add helium to the mix?...hmm

Freef

So is there an ideal 'scrubber volume to loop volume' ratio, or are divers so different that any 'ideal' would only be of use to a small number of us?

With regard to the velocity, wouldn't the density of the gas be a factor as well as the CO2 is more diluted the higher the pressure [deeper you are]?

__________________
David.

Currently owner of two differently sized ankles.

onetime

Don't take these questions for me wanting to extend the life of absorbent in terms of reusing just trying to get a handle on efficiency/process of scrubbing.

When the absorbent in a scrubber canister is "spent", do you know how much of the absorbent material might still remain viable if the gas was flowing say in the opposite direction? (I may be assuming incorrectly here but I'm thinking that the gas passing through the scrubber will predominantly follow the same path on every pass through the scrubber with some faces of the sorb being hit with high CO2 regularly and some maybe not getting hit much at all).

Or does the absorbent react to the CO2 all the way through rather than just the surface area of the pellets?

I guess my thinking is that maybe reversing the flow of gas through the scrubber canister at some point in the dive might yield more "useable" sorb.

zzzzzzzz

With regard to gas density:

Gas density should be a factor in scrubber efficiency, mainly because it affects the diffusion process, usually by impeding the molecules from diffusing to the active chemicals. It would likely slow down the CO2 molecules, thereby requiring lower velocities to allow the diffusion process to occur, or larger scrubber volume.

Gas density would therefore work in a way similar to increased gas velocity to impede diffusion.

The CO2 molecules are typically absorbed within the granules because there is a rarity of these molecules within the granules owing to the fact that they are decomposed chemically.

Scrubber geometry has a similar effect as velocity in that it affects the mean distance a molecule must travel to attain the active chemicals, but is less sensitive assuming an even CO2 distribution at the inlet.

So, the question remains the same, is gas velocity within the scrubber bed the main driver wrt scrubber efficiency?

z

aknelson

The blue coloring is very good visible and measurable especially just after usage. You are able to estimate how much of the absorbent is used.
Some folks use the term "burn zone"
I will trow the stuff out if 60% and more is used or the absorbent got wet. Time counts for me most beside checking the absorbent.

The best way to test the Effect:
I did some Experiments with the Absorbent by placing Absorbent and CO2 from our Welding shop into a clear plastic bag. The bag will deflate complete
Pay attention the blue coloring will fade off over time and it turns grayish. Therefore you can not trust what you see....Temperature is a good indicator for Absorbent usage.


Andreas

Freef

I think that the absorbsion process takes place throughout the whole of the scrubber 'grain', not just on the surface. You must also consider that the dwell time when the diver pauses between breathing in and out has the effect of stopping the flow through the scrubber.

A change from a small bore such as the loop hose to the CL and scrubber assembly will also drastically reduce the velocity of the gas allowing for a longer time in the scrubber than you may think when you suck test the loop on assembly and evacuate the air quite quickly.

__________________
David.

Currently owner of two differently sized ankles.