Why depth has an effect on sorb capability to scrub CO2

[fsardone]

Stuartv,
please note that all discourse here are approximations and qualitative ways to try and explain a phenomena that people in the NEDU or rebreather designers and constructors failed to quantify due to the fact the phenomena is complex and depends on many variables. Please refer to one of the links @1c3d1v3r has provided through the page he mentioned in the post above:
Design Guidelines for Carbon Dioxide Scrubbers. Revision. A

In such cases data is collected and assembled in graph which are then used to interpolate between know points.

Yes, there is 10 X mass going through the scrubber, but it is 10 X as dense, so the dwell time is the same. I.e. A CO2 molecule going through the loop is going to spend just as much time in the scrubber (unless shortened by reacting and being "scrubbed"), whether you're at 100m or 10m. Right?

To answer you question
Yes, time it is going to stay the same. Also concentration (of CO2) it is. In fact we express concentration in term of mol/lt and therefore will stay constant please refer to
The effect of pressure on rates of reaction

On the other end since reaction depends on collition between molecules the presence of other molecules, which are not part of the reaction, will be in the way. You can explain it in physical terms (perturbation of a flow from laminar, or a little turbulent, in more turbulence due to increased Re number) or in chemical terms (probability of a useful collision between a CO2 and CaOH molecule il lower due to the "diluition" of CO2 by He N2 and O2. But this as 1c3d1v3r has pointed out is counterbalanced by the fact gas has higher energy due to increased pressure and therefore there are more collisions (between gas molecules and gas and the surface of the scrubber) but also diffusion and effusion rates have effects here and keep in mind that CO2 has the highest molar mass of all gases here (H2O, He, O2,N2) and therefore the slowest diffusion and effusion rates. Please refer to:
Kinetic Theory of Gases
for a quick primer.

Finally if we think to the gas equation of state we see that the term P*V is pretty much the energy of the gas, in such case we need to thing in term of partial pressure of each gas and if we keep nRT constant (R is a constant n is the number of molecules of CO2 and T we well assume constant) the energy of the CO2 will stay constant while the energy of all other gases will increase with the increase n and therefore the increased P*V factor.

How all of this affects the reaction? Well TESTING has shown that you need more length of scrubber and depth affects negatively scrubber efficiency. This is the important bit for the average scubadiver in CCR.

Hope it helps although I know it murks things up!

Cheers

 

[rsingler]

A CO2 molecule can collision and change path from or to soda lime. Extra N2 or He molecules do not interfere with the scrubbing chemical reaction or the reaction in O2 sensors other than by thermal transfer.

While temperature plays a critical role in scrubber efficiency, this statement is very misleading.
No one is saying that other gas molecules interfere with the reaction. And I'll accept your argument that there is a minor decrease in granule heat retention at higher gas density.
But with short dwell time and high gas density, there is a finite distance that must be traveled that is interfered with by competing physical molecules, just as poor packing contributes to channeling and breakthru. To suggest that gas density plays no physical role and it's all just thermal effects of the extra molecules is being a POV warrior for temperature, to the exclusion of significant other factors.

 

[Alaskan Scuba Dude]

Would it be for the same reason you breathe more at depth when using OC? Gas laws.