Sciencemadness Discussion Board

Looking for NO2 scrubber feedback

javert - 19-8-2017 at 09:52

Hi guys. I've read several posts here about scrubbing NO2 gas. I have a process which generates some nasty fumes of nitrogen dioxide and I'm looking for a way to scrub them. Some fiddling with lab material and I have the following setup:

To explain, the first stage is the one of the right in the blue carboy. Fumes are meant to be bubbled into the tipo of the fritted tube. Solution is 10 % HNO3 and 0,2 % H2O2 as suggested in one page in the "Kanto Denka process". Nitrogen dioxide is expected to be oxidized to more nitric acid.

The second stage in the left has a saturated solution with sodium carbonate. Assuming the first stage is successful, this one intends to neutralize any nitric acid which passes to the solution in order to protect the vacuum pump.

I'm regulating the flow so passage to the first stage is slow. Due to the pressure drop bubbling in the second stage seems rather high but bubbling in the first stage is actually quite slow intentionally to help with the scrubbing.

Haven't tested it yet. I plan to connect this scrubber to the glove box chamber where the fumes are going to be released. Assuming that the original nitric acid is fully converted to nitrogen dioxide, the most nitrogen dioxide generated is about to 3,75 mol or 172 g NO2 since I'm using about 240 mL of 70 % HNO3 initially.

Anyone with experience can provide me some feedback about the likeliness of this concoction to work?

Scrubber.jpg - 281kB

javert - 19-8-2017 at 10:03

Yeah, don't mind the yellow coloration in the sodium carbonate carboy (left). Some vacuum pump oil spilled into it initially but will be cleaned.

Sulaiman - 19-8-2017 at 10:52

NO2 does not dissolve very quickly,
vortex-stirring in the first scrubber would probably help allow a significant gas flow rate,
based on minimal experience.

unionised - 19-8-2017 at 11:33

What sort of vac pump are you using?
Some don't like air that's saturated with water.

Lots of small bubbles are better than a few big ones so something like the "air stones " they use in Aquaria might help.
Vortex mixing -as Sulaiman has said is also a good thing.

physics inclination - 20-8-2017 at 03:42

Perhaps in addition to a gas dispersion tube/air stone, you could also add a layer of sand or glass beads or something the bottom of the HNO3 container, which might keep the bubbles in contact with the water longer and force them to break up.
Of course, the sand would have to be acid-washed pure kind, or whatever other bubbling medium would have to be acid-resistant and cleaned prior, to prevent contamination of the HNO3 if you intend to reuse it. Alternatively though, you could let it get dirty in the collector and re-purify it later which you may do anyways so I guess it wouldn't make a difference in that case :P

In any case it's a nice setup :)

Melgar - 20-8-2017 at 05:32

If you're getting these fumes from nitric acid, you can mix H2O2 with the nitric acid to oxidize it back to HNO3 before it even leaves the container. But that's too obvious, and you're probably doing something with nitrous acid or alkyl nitrites or something.

If you just want to neutralize it, ammonia solution would do it, since it reacts quite readily in the gas phase. A primary amine or an amino acid would probably give you something more interesting though.

unionised - 20-8-2017 at 07:25

If you pick the wrong amino acid you may get a nitrosamine.
In any event, since the scrubber isn't likely to be perfect, vent the vac pump to the outside world.

javert - 21-8-2017 at 06:50

Thanks for input guys!
Quote: Originally posted by unionised  
What sort of vac pump are you using?
Some don't like air that's saturated with water.

Lots of small bubbles are better than a few big ones so something like the "air stones " they use in Aquaria might help.
Vortex mixing -as Sulaiman has said is also a good thing.


The diffuser tubes I'm using have a fritted end, so that is covered.

I'm using a small Mastercool vacuum pump. Didn't found in the booklet a warning about moist air.

I doubt I can find a magnetic stirring tray able to handle the weight of a full carboy so I'll have to pass on that one.
Quote:

Perhaps in addition to a gas dispersion tube/air stone, you could also add a layer of sand or glass beads or something the bottom of the HNO3 container, which might keep the bubbles in contact with the water longer and force them to break up.
Of course, the sand would have to be acid-washed pure kind, or whatever other bubbling medium would have to be acid-resistant and cleaned prior, to prevent contamination of the HNO3 if you intend to reuse it. Alternatively though, you could let it get dirty in the collector and re-purify it later which you may do anyways so I guess it wouldn't make a difference in that case


I like the idea of a packing medium to increase the contact time. How porous is the sand, in the sense of how much scrubbing solution I could have left in the interstices? I could try acid washing and using filtering gravel too.
Quote:
If you just want to neutralize it, ammonia solution would do it, since it reacts quite readily in the gas phase. A primary amine or an amino acid would probably give you something more interesting though.


Yeah I just want to neutralize the bulk of the NO2 before the air escapes through the duct. My place is located in a urban area and visibly reddish gases coming out of the building that happen to be corrosive and noxious could give us some bad press.

If ammonia reacts faster than hydrogen peroxide I'm willing to try. What is the indicated concentration of ammonia and the pH in the scrubbing solution? If it's acid so ammonia fumes are prevented and the concentration is low enough so I could add the ammonia in each scrubbing session I'm sold.


Fritted tube.jpg - 197kB

[Edited on 21-8-2017 by javert]

javert - 22-8-2017 at 08:38

Another question:

Looking for ideas and info, I found out Buchi Labortehcnik AG, a manufacturer of laboratory equipment, offers a commercial scrubber for lab scale. When looking into the manual, I found out they use a 20 % NaOH solution for acid scrubbing then followed by bubbling into a saturated FeSO4 specific for NOx.

So far I haven't found info about the reaction between NOx and ferrous sulfate. Assuming that the commercial scrubber does it job, I assume the reaction must be fast enough to be able to happen in what is a quite small reaction vessel (about 1 L).

Do you know what the reaction between FeSO4 and NO2 is? I know by experience that ferrous sulfate solutions are unstable and start generating Fe3+ and precipitating the hydroxide unless pH is low. Can the reaction with the NOx happen at low pHs?

physics inclination - 22-8-2017 at 09:49

Wait before you go with the ammonia scrubbing route, does anyone think it might produce the hazardous amounts of ammonium nitrite?

I'm wondering specifically if the undissolved NO2 could bind to the ammonium ions in solution (because ammonia gas makes some ammonium and hydroxide in aqueous solution) and maybe form the dangerous nitrite instead of the relatively more safe nitrate. And if you use the big containers a lot without emptying any of the solid ammonium salt precipitates (NO3 and/or NO2), then you would definitely have a dangerous situation with lots of ammonium nitrite at once :o
edit: Also the ammonium nitrate would add to the danger then, because the nitrite detonating would set off the nitrate detonating too!

Again, I'm not sure that this would happen, I just wanted to check with others.

[Edited on 8-22-2017 by physics inclination]

physics inclination - 23-8-2017 at 10:11

Sorry to reply without content, I just need to bump this to the top of the threads list because no one has answered the previous question I asked, and I don't want javert to get hurt if my worry was correct about using ammonia to scrub NOx. Sorry I worry a lot sometimes.

Melgar - 25-8-2017 at 20:25

Quote: Originally posted by physics inclination  
Wait before you go with the ammonia scrubbing route, does anyone think it might produce the hazardous amounts of ammonium nitrite?

Ammonium nitrite doesn't exist, actually. The two oxygen atoms in nitrite react with the four hydrogens in ammonium producing two water molecules, and two nitrogen atoms. Of course, the two nitrogen atoms combine into N2, and bubble away.

I'm pretty sure the only solid you'd be left with would be ammonium nitrate. If you had 10%+ ammonia, you could just send your gas stream directly above the liquid surface, so as to eliminate the need for suckback traps,

unionised - 26-8-2017 at 04:33

Quote: Originally posted by Melgar  
Quote: Originally posted by physics inclination  
Wait before you go with the ammonia scrubbing route, does anyone think it might produce the hazardous amounts of ammonium nitrite?

Ammonium nitrite doesn't exist, actually.


Other opinions are available
https://en.wikipedia.org/wiki/Ammonium_nitrite

Melgar - 27-8-2017 at 04:15

Quote: Originally posted by unionised  
Other opinions are available
https://en.wikipedia.org/wiki/Ammonium_nitrite

Ok, so qualify that as "Ammonium nitrite doesn't exist in significant amounts for long enough to cause problems, since water catalyzes its decomposition and is also its main decomposition product." That better?

[Edited on 8/27/17 by Melgar]

physics inclination - 27-8-2017 at 13:56

ok thanks for the info, I guess ammonium nitrite wouldn't be a problem then; at first I didn't know it decomposed so quickly and safely.

AJKOER - 29-8-2017 at 11:07

"Reaction between ammonia and nitrogen dioxide"
G. Bedford and J. H. Thomas
J. Chem. Soc., Faraday Trans. 1, 1972,68, 2163-2170
DOI: 10.1039/F19726802163, Paper
Link: http://pubs.rsc.org/-/content/articlelanding/1972/f1/f197268...

"Abstract:
The reaction between ammonia and nitrogen dioxide has been studied in the temperature range 615–660 K. The products of reaction are N2, N2O, NO and H2O. A mechanism is proposed which leads to the following parameters: For NH3+ NO2[graphic omitted]NH2+ HNO2 k1= 10(12.6 ± 0.3) exp (–115.69 ± 0.42/RT)cm3 mol–1 s–1(activation energies are expressed in kJ mol–1) and for NH2+ NO2[graphic omitted] NH + HNO2, NH2+ NO [graphic omitted] N2+ H2O k5/k2= 10–2.30 ± 0.3 exp (26.78 ± 4.18/RT)"

This makes sense as nitrogen dioxide is a stable free radical, so the reaction of an excess .NO2 with NH3 apparently proceeds as follows:

NH3 + .NO2 = .NH2 + HNO2
.NH2 + .NO2 = NH + HNO2

Also, some .NO presence (from the decomposition of 2 HNO2 = H2O + NO + NO2, see https://www.google.com/url?sa=t&source=web&rct=j&... ):

.NH2 + .NO = N2 + H2O
NH3 + .NO = .NH2 + HNO (see "Theoretical Rate Constants for the NH3 + NOx = NH2 + HNOx (x ) 1, 2) Reactions by ab Initio MO/VTST Calculations - Emory Chemistry - Emory University" at https://www.google.com/url?sa=t&source=web&rct=j&... )
NH + .NO → N2 + .OH (See https://en.m.wikipedia.org/wiki/Imidogen )

Note, 2 HNO = H2N2O2 (see https://en.m.wikipedia.org/wiki/Nitroxyl ), which is unstable and can decompose into water with the liberation of the reported N2O.

Also, with more ammonia, in my opinion, so additional possible products including the liberation of nitrogen:

NH3 + HNO2 = NH4NO2 --> N2 + 2 H2O
.......

Bottom line, a heated mix of NH3/air or O2/NO2 may be a method to test for scrubbing NO2. However, the introduction of O2 to convert NO to NO2 for furthering processing may be effective, but I believe the surrounding chemistry with other compounds interacting also with O2 should be considered as well. I would recommend employing transition metals salts, to activate the oxygen with the formation of the superoxide radical anion, and with a pH under 4.8, or a gas phase medium, introducing .HO2 as the active species. Note, this makes the system more complex to theoretically assess efficiency, in my opinion. Some examples of suggested and also problematic reactions:

Half cell reactions (electrochemical cell):
Fe(ll) ---Copper ions--> Fe(lll) + e-
e- + O2 --> .O2-
Net reaction resulting in superoxide/.HO2 formation:
Fe(ll) + O2 ---Copper ions--> Fe(lll) + .O2- (reference, see for example, http://pubs.acs.org/doi/abs/10.1021/ja01600a004 )

Gas phase, or pH under 4.8, as H+ + .O2- = .HO2, sample reactions:
.NO + .HO2 = .NO2 + .OH (p. 396, source below)
NH3 + .HO2 --> .NH2 + H2O2 (see, for example, Table 21, at https://books.google.com/books?id=VF4FCAAAQBAJ&pg=PA416&... )
Fe(ll)/Cu(l) + H2O2 --> Fe(lll)/Cu(l) + .OH + OH-
H2O2 + .OH --> H2O + .HO2 (see, for example, reference below)
.NO2 + .OH = HNO3 (see https://www.shodor.org/os411/courses/411d/module01/unit05/ra... and the presence of HNO3 has been observed to enhance H2O2 based scrubbing of NOx, see https://www.researchgate.net/publication/229204100_The_absor...)
.NH2 + .HO2 --> NH3 + O2 (or, NH + H2O2, see Table 21 reference above)

[Edited on 30-8-2017 by AJKOER]

[Edited on 30-8-2017 by AJKOER]