Pulling air through saturated kalk

[Doctorgori]

I have no chemistry skills past HS, so what would be in the gas? edited to add; appreciate any thoughts

 

[Dkeller_nc]

Nitrogen and oxygen. The kalkwasser, depending on how efficient the air/kalkwasser mixing is, will pull all of the CO2 out of the air that's bubbled through the solution. That's why kalkwasser solutions that are left to stand for a period of time will develop a thin skin of precipitated calcium carbonate on the surface.

 

[JimWelsh]

Nitrogen and oxygen. The kalkwasser, depending on how efficient the air/kalkwasser mixing is, will pull all of the CO2 out of the air that's bubbled through the solution. That's why kalkwasser solutions that are left to stand for a period of time will develop a thin skin of precipitated calcium carbonate on the surface.

Argon be all, "What am I -- chopped liver!?!?"

 

[Dkeller_nc]

Argon be all, "What am I -- chopped liver!?!?"

Ha! Well, true, I didn't count the noble gases nor the trace amounts of radon from radioactive decay, but I figured we could ignore those since they're such a small percentage.

 

[JimWelsh]

But, argon is in greater concentration than CO2, which you did decide to mention.

 

[Doctorgori]

Nitrogen and oxygen. The kalkwasser, depending on how efficient the air/kalkwasser mixing is, will pull all of the CO2 out of the air that's bubbled through the solution. That's why kalkwasser solutions that are left to stand for a period of time will develop a thin skin of precipitated calcium carbonate on the surface.

What are those rigs called where a jar has a lid and a pipe that extends into the liquid and you draw the air from the top? (the air gap between the surface and the lid)? So whatever that thing is called, I was thinking I could feed my skimmer that air from the top of my semi hermetically sealed Kalk res.....assuming you could even rig a skimmer to have enough draw to pull some gas through a liquid ...
Anyway, just scatter brain storming some way of getting cheap and easy CO2 depleted air.....

 

[Dkeller_nc]

But, argon is in greater concentration than CO2, which you did decide to mention.

True; I didn't mention water vapor either.

 

[Doctorgori]

I’m not sure to post this here or over in a similar existing thread in the DIY forum.
I made a simple reactor of sorts....my skimmer pulls air through a container filled with saturated Kalk. I was just wondering if anything in the air would be toxic or problematic and if the rate of co2 consumption in a 5 gal bucket is practical ... I was thinking of using a pH probe but also wonder how to even interpolate the data ...(attach=full?) pardon the large duplicate pictures, won't edit ???

 

[Dkeller_nc]

If I'm interpreting correctly, you're trying to use a kalkwasser solution to absorb CO2 from your incoming air to the skimmer to raise the pH of your aquarium. While this will indeed work to some extent, the amount of carbon dioxide that you take out will be fairly limited.

You can calculate this; the solubility of calcium hydroxide in water is 1.7 g/L at 20 deg C. That yields a molar concentration of 1.7 g/L / 74.1 g/mol = 0.023 moles Ca(OH)2/L. Because the solubility of calcium carbonate is negligible in water, all of the calcium in the solution will eventually be reacted with the CO2 in the air to form calcium carbonate. That means that 0.023 moles of CO2 will be taken up per liter of kalkwasser solution, which is 44 g/mol * 0.023 mol/L = 1 gram of CO2 per liter of solution.

Here's where things get a bit complicated. The CO2 concentration in air is about 400 ppm, depending on the season and one's location. Inside a tightly sealed and energy efficient home, it can be a good bit higher - perhaps 600 - 800 ppm. At any rate, we can use the atmospheric concentration to figure out the maximum amount of air that can be "cleaned" by bubbling through the kalk solution. Since 400 ppm is 400 mg/L, we arrive at 1000 mg CO2/L kalkwasser/ 400 mg CO2/L of air = 2.5 L of air with CO2 removed per liter of kalkwasser. Since 5 gallons = 18.9 liters, a total of 47.25 Liters of air can be "scrubbed" of CO2 if the bucket's filled all the way to the brim.

Using the specifications for a Tunze 9004 DC skimmer, we see that it will pull 40 gallons per hour of air when run wide-open. 40 gallons is 151 liters, so you can see that the kalkwasser solution will be exhausted extremely quickly. There are lots of caveats to this calculation; you're probably not going to be running a skimmer wide-open since it will likely overflow, and the mass transfer between the air bubbled through the kalkwasser solution won't be 100% efficient. However, if you use reasonable numbers for the air draw through your skimmer, and a reasonable estimate of the efficiency of the mass transfer between the air bubbles and the solution, you still arrive at the conclusion that you'll have exhausted the capacity of the kalkwasser to absorb CO2 from the air in considerably less than a day. This is why most that decide to scrub CO2 from the air going to their skimmer opt for solid CO2 adsorption media, which has a far, far higher capacity for CO2 adsorption than the relatively weak strength of a saturated kalkwasser solution.

 

[Doctorgori]

That means that 0.023 moles of CO2 will be taken up per liter of kalkwasser solution, which is 44 g/mol * 0.023 mol/L = 1 gram of CO2 per liter of solution.

Exactly what I was looking for,,, appreciate it.....yeah I strongly suspected 10tsp of kalk would be limited in CO2 absorption capacity but wasn’t sure how to figure it out.....otoh lime & water is also very cheap and easy to source and implement,,, but again had no clue if practical
edited to add; if I'm figuring this right then $3 worth of Ms.Wages (450grams) makes about 300 liters of Kalk and removes about 300 grams of CO2
...I went ahead and scaled it up to 5 gal just to see ... also since it’s bubbling I’m wondering if it works exceeding 2tsp /gal ...

it’s Sat morn nothing else to do

 

[JimWelsh]

You're not necessarily limited to just how much kalk you can get to dissolve. If you add a large excess of kalk to the bucket, then as CaCO3 precipitates, that should allow more Ca(OH)2 to dissolve, and giving the bucket much more CO2-adsorbing capacity than just plain saturated kalk, especially if the bubbling helps keep it stirred. Just a thought. With this approach, you could probably use the pH of the solution in the bucket to determine when the bucket is getting depleted; once the pH starts to drop below 12.4 or something like that, the excess kalk is finally getting depleted. https://nvlpubs.nist.gov/nistpubs/jres/56/jresv56n6p305_A1b.pdf

 

[Randy Holmes-Farley Verified Community Expert]

This will work to some extent, but I might just use a sodium hydroxide solution. Far more concentrated (so faster acting on CO2 entering it), needs no stirring, and probably cheaper.

I did this in the lab to absorb hydrogen sulfide gas, which gets trapped in the high pH water as sulfide the same way carbonate will (from CO2).

https://www.newscientist.com/article/dn12727-chemical-sponge-could-filter-co2-from-the-air/

 

[JimWelsh]

Since 400 ppm is 400 mg/L

Slow down a minute, there, sparky! In aqueous solution, that is correct, but we're talking about CO2 concentration in air, where "PPM" has a different meaning. When describing gasses, "PPM" is referring to a mole fraction (PPMV), not mg/L. The average molar mass of air is 28.96 g/mol, and the density of air can be taken as 1.292 g/L, so each liter of air contains only 1.292 / 28.96 = 0.0446 moles of air, of which only 400 / 1,000,000 is CO2, so that's 0.0446 * 0.0004 = 0.00001784 moles of CO2 per liter of air. The weight of that CO2 is 0.00001784 * 44 = 0.000785 grams, or 0.785 mg. The assumption you made put that value at 400 mg, so the actual adsorbing capacity of the kalk (in terms of the number of liters of air that can be "scrubbed") is around 500 times your estimate, I believe. Reference for the air molar mass and density numbers is here: https://www.engineeringtoolbox.com/molecular-mass-air-d_679.html

 

[PaulPerger]

What are those rigs called where a jar has a lid and a pipe that extends into the liquid and you draw the air from the top?

We used to call those "Bongs".

 

[JimWelsh]

This will work to some extent, but I might just use a sodium hydroxide solution. Far more concentrated (so faster acting on CO2 entering it), needs no stirring, and probably cheaper.

So, if someone is using your more concentrated 2-part based on sodium hydroxide instead of sodium carbonate for dosing, then they could get the benefit of CO2 scrubbing of their skimmer intake, without adversely affecting the alkalinity of the 2-part (since we know that adding CO2 to a solution affects pH, but not carbonate alkalinity), unless the Na2CO3 concentration reaches saturation, yes?

EDIT: I did the math, and even when using the higher-pH 2-part recipe at 1.5 times, or B-Ionic, strength, and even if all of the hydroxide got converted to carbonate, the Na2CO3 concentration would still be only around 50% of a saturated solution at room temperature.

EDIT2: But, then it dawns on me that a tank running like this would see a nice increase in pH whenever the alk portion of the 2-part was refilled, that would then drop continuously as both the CO2 adsorbing capacity of the solution decreased, and also as the OH / CO3 ratio of the solution being dosed drops. Probably not a good idea to do this. I'll show myself out now.

 

[Dkeller_nc]

Slow down a minute, there, sparky! In aqueous solution, that is correct, but we're talking about CO2 concentration in air, where "PPM" has a different meaning. When describing gasses, "PPM" is referring to a mole fraction (PPMV), not mg/L. The average molar mass of air is 28.96 g/mol, and the density of air can be taken as 1.292 g/L, so each liter of air contains only 1.292 / 28.96 = 0.0446 moles of air, of which only 400 / 1,000,000 is CO2, so that's 0.0446 * 0.0004 = 0.00001784 moles of CO2 per liter of air. The weight of that CO2 is 0.00001784 * 44 = 0.000785 grams, or 0.785 mg. The assumption you made put that value at 400 mg, so the actual adsorbing capacity of the kalk (in terms of the number of liters of air that can be "scrubbed") is around 500 times your estimate, I believe. Reference for the air molar mass and density numbers is here: https://www.engineeringtoolbox.com/molecular-mass-air-d_679.html

Oops - Not enough coffee this morning.

You're correct, although technically it's the volume of the substance per liter of air. Of course, since the ideal gas law applies in this case, you're still right. It turns out, though, that the conversion works out to 0.732 mg of CO2/liter of air for 400 ppm of CO2.

At any rate, correcting my erroneous assumption above yields 1366 Liters of Air scrubbed to 0% CO2 per liter of kalkwasser. We then arrive at 25,820 Liters of scrubbed air for 5 gallons of kalkwasser solution. Much better. Except, unfortunately, I used a very wimpy skimmer for the analysis. A more realistic skimmer would be something like the Vertex Omega 150, which draws 650 - 900 liters per hour of air. So the solution will be exhausted in 29 - 40 hours. Derp. So the bottom line is that the conclusion remains the same - kalkwasser just isn't very efficient at scrubbing CO2 because very little of it dissolves in solution, and especially since it's rather unlikely that one's indoor CO2 concentration matches the outdoor air. And unfortunately, while in theory more calcium hydroxide that's on the bottom of the container (or more likely, stirred up from the bubbles) will dissolve as the calcium hydroxide precipitates out of solution, in practice the particles of calcium hydroxide tend to get coated with precipitated calcium carbonate, interfering with additional dissolution.

As always, Randy's solution is considerably better.

One in theory could get quite a large capacity for CO2 from a NaOH solution, since the theoretical solubility limit of NaOH in water at 20 deg C is a little more than a kilogram. However, since most people would regard a large bucket of extremely concentrated sodium hydroxide setting around to be quite dangerous, we arrive back at the solid soda-lime absorbent solution to the problem with a convenient indicator dye to tell you when it's exhausted.

 

[Randy Holmes-Farley Verified Community Expert]

So, if someone is using your more concentrated 2-part based on sodium hydroxide instead of sodium carbonate for dosing, then they could get the benefit of CO2 scrubbing of their skimmer intake, without adversely affecting the alkalinity of the 2-part (since we know that adding CO2 to a solution affects pH, but not carbonate alkalinity), unless the Na2CO3 concentration reaches saturation, yes?

EDIT: I did the math, and even when using the higher-pH 2-part recipe at 1.5 times, or B-Ionic, strength, and even if all of the hydroxide got converted to carbonate, the Na2CO3 concentration would still be only around 50% of a saturated solution at room temperature.

EDIT2: But, then it dawns on me that a tank running like this would see a nice increase in pH whenever the alk portion of the 2-part was refilled, that would then drop continuously as both the CO2 adsorbing capacity of the solution decreased, and also as the OH / CO3 ratio of the solution being dosed drops. Probably not a good idea to do this. I'll show myself out now.

lol

 

[Dkeller_nc]

That means that 0.023 moles of CO2 will be taken up per liter of kalkwasser solution, which is 44 g/mol * 0.023 mol/L = 1 gram of CO2 per liter of solution.

Exactly what I was looking for,,, appreciate it.....yeah I strongly suspected 10tsp of kalk would be limited in CO2 absorption capacity but wasn’t sure how to figure it out.....otoh lime & water is also very cheap and easy to source and implement,,, but again had no clue if practical
edited to add; if I'm figuring this right then $3 worth of Ms.Wages (450grams) makes about 300 liters of Kalk and removes about 300 grams of CO2
...I went ahead and scaled it up to 5 gal just to see ... also since it’s bubbling I’m wondering if it works exceeding 2tsp /gal ...

it’s Sat morn nothing else to do

Yeah, it's fun to play around with chemical engineering - I turned it into a career.

The calculation for how much CO2 a given quantity of calcium hydroxide will absorb is a good deal simpler, btw. In this case, there's 450 g Ca(OH)2/74.1 g Ca(OH)2/mole = 6.1 moles. Since there's one mole of Calcium per mole of Ca(OH)2, you'd be able to capture 6.1 moles of CO2, or 268.4 grams of CO2. Based on the 400 ppm CO2 concentration assumption, and using Jim's correction for ppm as PPMV (thanks, Jim, I really shouldn't do calculations before coffee!), you could "clean" 366,666 liters of air. Or 407 hours of operation of that Vertex 150i running wide-open. Unfortunately, however, you'd need a swimming pool's worth of solution, since calcium hydroxide has such a low solubility (1.73 g/L at 20 deg C).

 

[Doctorgori]

Appreciate the thoughts and replies everyone...truly most experiment fail but I did learn something...thanks

So the solution will be exhausted in 29 - 40 hours. Derp. So the bottom line is that the conclusion remains the same - kalkwasser just isn't very efficient at scrubbing CO2 because very little of it dissolves in solution, and especially since it's rather unlikely that one's indoor CO2 concentration matches the outdoor air. And unfortunately, while in theory more calcium hydroxide that's on the bottom of the container (or more likely, stirred up from the bubbles) will dissolve as the calcium hydroxide precipitates out of solution, in practice the particles of calcium hydroxide tend to get coated with precipitated calcium carbonate, interfering with additional dissolution.

As always, Randy's solution is considerably better.

One in theory could get quite a large capacity for CO2 from a NaOH solution, since the theoretical solubility limit of NaOH in water at 20 deg C is a little more than a kilogram. However, since most people would regard a large bucket of extremely concentrated sodium hydroxide setting around to be quite dangerous, we arrive back at the solid soda-lime absorbentsolution to the problem with a convenient indicator dye to tell you when it's exhausted.​

....jumping to the end, I saw 30 sumthin lbs of the dry/air use stuff on eBay for like $100 ... not sure if at that price it is cheaper than aqueous solution of sodium hydroxide but probably easier....
...Anytime there is a chance we can skip the lab/pharmaceutical grade stuff and use cheap off the shelf solution bears a lil exploration
-- So where does cost vs. practical intersect? ... I was sorta hoping for chemical nirvana; cheap and easy; dump half a bag of pickling lime in a 5g bucket, hook a skimmer inlet to the top and voila, no more CO2...come back every 48-72 hrs and recharge for $1.50

I did read the link Randy offered

"The process involves pumping air from the atmosphere through a chamber containing sodium hydroxide, which reacts with the CO2 to form sodium carbonate. "
​

...this ain't over lol (I can't see or spell so pardon the multiple edits ...lol)

 

[JimWelsh]

...we arrive back at the solid soda-lime absorbent solution to the problem....

I guess sometimes there are very good reasons why professionals do the things that they do the way that they do them.