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CO2:O2 degassing and Flux....some thought experiments and theory

Discussion in 'CO2 Enrichment' started by Tom Barr, Oct 8, 2013.

  1. Tom Barr

    Tom Barr Founder
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    One of the benefits of speaking to hobbyists at clubs is coming up up with new ways to explain concepts.
    Lately, I spoke at 2 clubs and used an analogy for the degassing and CO2 addition theory.
    I was trying to explain Flux, or Fick's 1st law of diffusion.

    J = -D (Change in Concentration C / Change in the distance X)


    Sometimes it is useful to think about this as a Science experiment where you want to compare treatments and you need an upper reference and then a lower reference.
    This way ALL your treatments will fall somewhere in between those two.
    These are often extreme references, say like 1000 ppm of NO3 and then 0.00 ppm NO3/DI water.

    High/low references:
    For high Flux(HF), I used a pizza pan comparison as an aquarium, a very shallow huge surface area and virtually no distance for the gas to diffuse in/out of water.
    For low Flux(LF), I used a water bottle sealed with a cap.

    Now we know if we kept adding CO2 to a sealed water bottle, we end up with carbonated water and the CO2 concentration will continue to build proportional to the amount of CO2 added. It has not flux and nowhere to go except into solution.
    One of the larger issues with hobbyist is that they often assume this is what their aquarium is like, that the CO2 will simple keep building. They do not factor in Fick's law of diffusion. As the concentration in the water increases, so does the flux from the high concentration in the water, to the air above. The flux in not linear.
    Aquariums with higher current/surface rippling, less scum layer, reduced degassing at the skimmer/prefilter section/wet/dry sections, better mixing etc, will have a higher but more stable flux.

    Aquariums with less current/mixing/surface scum layer/canister filtration/less rippling will have a lower flux and less stable flux. Since the concentration changes a great deal AND the distance is increased, we get LESS flux out of the system. So the concentration just keeps increasing.
    This is NOT true for the O2, the O2 drops and really is mostly well under 100% with the air above. O2 is consumed at a higher rate than the flux from the air above can replenish it. If you have high flux, then O2 will be at near 100% the concentration in the air above.


    Let's look at the pizza pan aquarium. Say it's an Extra large pan, and the water is only 1/4" deep.
    Even if I added a lot of CO2 at one edge of the pan, by the time the CO2 rich water made it just a little ways, maybe 1/4 slice, the CO2 would be greatly reduced, it would NEVER mix evenly across the entire pan, the surface area is huge relative to the volume. As soon as the CO2 is added in the thin layer, the gas diffuses out.
    This is the other extreme, the aquarium degases TOO MUCH. You can do this with aeration, spray bars, over flows, wet/drys etc also. All the CO2 you add is lost to the air above too fast to match the flux.

    This is a decent math page on the concept of flux:

    http://mathbench.umd.edu/modules/cell-processes_diffusion/page08.htm

    Does Temperature change the flux?
    What do you think?

    What happens when you add surface scum to the coefficient D? Do all gases have the same coefficient?
    Does O2 dissolve easier than CO2 in water?
    Does your brain hurt?
     
  2. UDGags

    UDGags Lifetime Charter Member
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    I'll go with this answer. Yes, I had to get out my transport phenomena book from college :)

    [​IMG]
     
  3. Tom Barr

    Tom Barr Founder
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    Haha, that's the long version.

    I think the simpler version above works well for our purposes with scaring people too much.
     
  4. Tom Barr

    Tom Barr Founder
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    Haha, that's the long version.

    I think the simpler version above works well for our purposes with scaring people too much.
     
  5. Tom Barr

    Tom Barr Founder
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    So does the Flux of CO2 rich water increase with higher temperatures or decrease???
    And..........Why?
     
  6. Tom Barr

    Tom Barr Founder
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    So does the Flux of CO2 rich water increase with higher temperatures or decrease???
    And..........Why?
     
  7. Jonathan

    Jonathan Junior Poster

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    Any water soluble gas becomes more soluble as temperature decreases because, in and of themselves, gas molecules have little affinity for each other -- after all, that is why they are gases. When dissolved in water, CO2 forms weak bonds with the water molecules. The lower the temperature, the stronger the bonds, the greater the amount of CO2 that can be dissolved.
     
    #7 Jonathan, Oct 8, 2013
    Last edited by a moderator: Oct 8, 2013
  8. UDGags

    UDGags Lifetime Charter Member
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    Yep, kinetic energy increases with temperature for the gas molecules.

    Another easy example that goes with Tom's is pop/soda being left out at room temperature becomes flat. The temperature of the pop increases so the CO2 leaves the system. Same analogy with boiling water and oxygen can be used in terms of taste.
     
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