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Author: Subject: Ostwald style nitric production
vanBassum
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[*] posted on 30-1-2020 at 13:00


Its been a while since the last post but, I am happy to report that I have re-created your process with success. I had a bit of difficulty with the adhesion of the NiO to the support (a broken ceramic plate) so I used a bit of Mg(OH)2 witch seems to work well. My setup isn't quite as fancy as yours, just some pieces thrown together. I ran the setup for about 30 min and I could clearly smell the dioxide. There was about 2 drops of acid witch turned a piece of PH paper red and fizzed when dropped on some carbonate. Now I know that I am able to get this working I will try to build a better setup. I may switch to platinum since only a very little amount is needed to create a catalyst so its possible to do for a few bucks. The most difficult part would be the steady generation of ammonia, I have been playing around with the electrolytic process posted previously on this topic but had little success. The smell of ammonia was somewhat present after a while but the main problem for me would be the production of hydrogen. This is something I rather not have mixed with air flowing over a hot catalyst...

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Alkoholvergiftung
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[*] posted on 30-1-2020 at 14:07


Try ammoniumdichromat on Zeolithe or Chromchloride and waterglass. After few secounds you see an Brown Cloud.

[Edited on 30-1-2020 by Alkoholvergiftung]
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MarkRob
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[*] posted on 20-4-2020 at 05:20


I do quite a lot of work with diesel emissions control hardware and might be able to help out - I think possibly diesel SCR hardware might be useful. The SCR catalyst pack can be salvaged from a euro 6 and some euro 5 diesels, so they might be a few in scrap yards at this point. Adblue is also a very widely available urea source.

I've been trying to design the ideal system... I'm thinking an atmospheric pressure system, something like:

adblue fluid -> controlled flow via drip -> one end of tube heated to 250C

then inside the tube, from one end there is:

adblue inlet -> some unreactive granules to ensure stable evaporation/decomposition -> crushed SCR catalyst -> air inlet via flow control system -> some objects to create turbulence and ensure mixing (e.g. some blocks of smashed china clay pottery?) -> second catalyst -> outlet to air cooling tube coil -> cooling/condenser coil in fridge -> water drain and gas outlet

The idea would be for the first catalyst to break down the urea into ammonia, SCR catalyst is designed to do this using the water vapour from the adblue. The the oxidation takes place on the second catalyst as normal.

As use of air results in lots of excess nitrogen, and the urea decomposition produces CO2, there is going to be a large buildup of unreactive gasses in the system. I was trying to come up with a way to avoid silica gel packs, but they do seem to solve a lot of problems.

Attachment: Chemical_and_mechanistic_aspects_of_the-SCR-catalysts.pdf (675kB)
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MarkRob
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[*] posted on 21-4-2020 at 06:56


Okay after a bit more of a think... It looks to me like fairly high conversion efficiency of around 30% of max theoretical could be achieved using only SCR catalyst in two stages followed by a one through system - very simple and easy to construct.

If I've done the maths right then only about 1 minute gas residence time is required for 90% NO->NO2 oxidation in the product gas from the dual catalyst design I described in the last post. There would be some acid in the condensed water, so the condenser output could be used to feed the absorption column, giving a maximum ~30% concentration product assuming the second catalyst is 100% efficient, or ~15% if its 50% efficient like SCR catalyst would appear to be. There is then some NO loss in the absorber off-gas. Overall conversion of adblue to acid by mass would be something around 20%.

So, whole system would be:

adblue tank -> controlled flow via drip system -> reaction vessel heated to 250C, and comprising of:

adblue inlet -> vapourisation region (inert granules) ->crushed SCR catalyst -> carefully metered air inlet -> turbulator/mixer region -> second catalyst pack (probably finely crushed and thin, to make the exchange with the surface diffusion limited and the flow laminar) -> exhaust gas to condenser, comprising of:

air cooled coil, then fridge cooled coil at 4C, then condensate collector.

The gas then passes through a further coil designed to give a 1 minute residence time for oxidation of the NO.

Then a once through absorption column fed by the gas at bottom and the collected condensate at top. Acid product collected from a tray at the bottom.
From scaling the sizes of industrical atmospheric pressure acid plants, the absorption column needs to be quite large, perhaps 100l per kg/day of product.

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[Edited on 21-4-2020 by MarkRob]
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MarkRob
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[*] posted on 22-4-2020 at 14:06


Okay after a bit more research, maybe a single stage catalyst using Al2O3 powder (as a support/packing) with added Mn2O3 (from exhausted alkaline cells) and TiO2 (from pigment suppliers) would work fine with Urea (no SCR salvage required!).

I'd be a little worried that the TiO2 might cause N2 formation, but it seems undoped TiO2 has little effect on NH3, but does decompose and hydrolise Urea to form ammonia. Mn2O3 looks to have a ~55% conversion efficiency for NO at 550K, and likely higher at higher temperatures, 60 or 70% might be practically feasible.

Some useful links here (I can't download the full papers)

https://www.cheric.org/research/tech/periodicals/view.php?se...

https://www.cheric.org/research/tech/periodicals/view.php?se...

So the system would then look like:

Urea tank ->
Drip system (for highly controlled flow) ->
Tube heated to 250C at inlet end, possible with Al2O3 granules or similar to aid stable boiling/decomposition of the adblue (sudden bursts in the flow would be bad).->
Then an air inlet from the metered air supply (possibly from air compressor to allow unattended operation?). ->
Catalyst. It might also be a good idea to have some length of tube before the catalyst to minimize flow variations from drips of adblue entering the tube. ->
Air cooled tube, spiralling down to allow condensate to flow ->
Minifridge cooled tube at ~5C, spiralling down through holes in top and bottom of the minifridge->
Condensate/gas separator, this might be really easily made via some sort of wick going from tube into top of absorption chamber.->
Extra tube to ensure ~1 minute or more of gas residence time->
Gas flows into bottom of absorption chamber.
Then the gas flows upwards in a serpentine path through the absorption chamber whilst the condensate flows downwards, before dripping out of the bottom as ~5molar acid (hopefully)...

Couple of useful papers attached, from the Lee et al publication, it looks to me like aqueous phase reactions forming HNO3 are very rapid (seconds), so the rate of acid formation in the absorption chamber is limited by dissolution of the NO2 into the acid solution. Therefore a high surface area is important.
Commercial systems tend to use trays and dripping showers, these result in high uptake of NO2 due to constant droplet/tray surface renewal. In the case of a small scale system, this is probably impractical, but a very thin layer of fluid with high surface area can be created by wicking it into an acid resistant porous polymer fabric. Uptake will then be limited by the aqueous diffusivity of NO2, which is really low, of order 10^-5cm^2/s. The good news is a really thin cloth (<1mm) makes the timescales manageable.
Assuming a serpentine cloth with 10mm air cavity between each layer of fabric, I calculated of order 1L absorption chamber volume per L/day of produced product. This should be manageable!





Attachment: LeeJGR81EvaluationNO2.pdf (1.6MB)
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Attachment: Mn2O3_ammonia_catalyst.pdf (2.4MB)
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MarkRob
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[*] posted on 25-4-2020 at 05:40


I'm talking to myself here but...
I was wondering if N2O4 production would be much more useful.
This allows concentrated nitric to be produced and is a useful reagent itself.
The problem is that producing dehydrated NO2 is hard without a large drop in yield. I think it could be done with very rapid cooling following the catalyst chamber (gas stream cooled to ~30C or lower in a few seconds, to prevent significant oxidation and acidification of the condensate).
This should be possible using a ~8m long coil of ~3mm ID stainless steel tube in a ~5C water bath inside a fridge. Problem is the fridge will be taking up all the heat from the condensing water, so its going to be hard to stop it overheating.
The condensate from the tube will be mildly acidic and could be harvested as a dilute acid product.
Due to the rapid cooling, there will be considerable suspended condensate mist, so a non woven filter will be needed to clean that up.
Finally a standard freezer at -15C could be used to clean out the residual water as ice, the inside of the tube will be coated with ice, so it'd be best to remove as much water as liquid before this stage or it will ice up rapidly.
At standard domestic freezer temperatures, partial pressure of water is under 200Pa, so the gas stream will be quite well dehydrated after the freezer stage.
The final stage to get solid N2O4 would use an industrial chest freezer, these are widely available with -45C setpoint temp, which would freeze out almost all of the NO2 as solid N2O4.
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[*] posted on 28-4-2020 at 14:00


Some more interesting papers attached
It looks like (from Sadykov) Fe2O3 is a very effective catalyst, with >90% selectivity, but it needs to be formed into ~1mm size "prills" to enable gas flow. Also it's not clear how important the Fe2O3 preparation is, and how much preparation from solution of Fe nitrate or chloride increases the conversion efficiency. There are lots of papers on Fe2O3 nanoparticle preparation from FeCl3, but the particles tend to have very regular sides, and Sadykov seems to be implying that irregular atomic layers at the surface of the particles improves performance, so a spray pyrolysis of FeCl3 solution is best

A method is described here:
https://www.researchgate.net/profile/Burcak_Ebin/publication...

I'm wondering if it could be as simple as FeCl3 solution sprayed into a flame.


Attachment: Structure_and_Morphology_Evolution_of_Fe.pdf (1.7MB)
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Attachment: Oxide_catalysts_for_ammonia_oxidation_in.pdf (1011kB)
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[*] posted on 21-10-2020 at 14:37


I hope I condensed this info correctly


Calculating ratio of ammonia to air
Slowly draw back 10ml, disconnect the syringe and put a finger over the end quickly, then put the tip into some water. The ammonia dissolves into the water and the water fills the syringe to replace the volume. You now know what ratio of ammonia to air is being fed to the reactor..

Observation
The pulsing glow happens due to the concentrated ammonia solution in the condenser falling back into the flask as drops, the cold concentrated solution emits gas as it hits the hot solution of urea.

means you should need to add air as well. The ammonia air mixture was quite oxygen rich and so the unreacted O2 formed the needed O2 in the sep funnel. diluting the reaction with more air and slowing the next reaction down

Optimization
did the NiO work, it worked well. The reaction zone glowed much hotter and pulsed hotter with the higher flow rates from the air/ ammonia generator. I'd bet that this would self sustain once it has got to this temperature. Will try next run.


I'd say if you get fairly anhydrous NH3 without the CO2 at an optimum air mix, the reaction might just self sustain the heating. Nickle oxide I strongly suspect to be more active than the cobalt, but that's a hunch at this stage.

side note about the air/ammonia ratio: In my experimentation and research I found out that if too much ammonia is present one would only get nitrogen and water because ammonium nitrite would be formed _in situe_. On the other side, if too much oxygen is present, the nitrogen oxides tend to decompose back into oxygen and nitrogen. One should try to maintain a slight oxygen excess for better yield of the nitrogen oxides and put another air inlet after the catalyst tube.

ammonia generator could have a few improvements made, I did a literature search for anything that can catalyze the urea decomposition reaction

For optimal absorption in an amateur setting, 3 column in a row can be used. A strong cooling of the exit gases and the tower is recommended for superior efficiency

maintain a slight oxygen excess for better yield of the nitrogen oxides and put another air inlet after the catalyst tube.
For optimal absorption in an amateur setting, 3 column in a row can be used. A strong cooling of the exit gases and the tower is recommended for superior efficiency

last few runs the catalyst was heated, then the air was started and at the same time the ammonia generator switched on. The rate of ammonia slowly increasing up to about 6% the total volume. The slow introduction of ammonia be critical to how the oxide performs as a catalyst.


The tube - how many other options are there?
I'd say the reaction can be lowered to 500C, which is borosilicate range of working temps, just pack more catalyst into a longer tube to allow for the slower rate of reaction. I have eyed off a piece of tubing used for thermocouples, a pyro-ceramic of some sorts. A suitable alternative would be something like a copper tube with some glass tape wound around it( automotive exhaust shop), make a paint with sodium silicate and some silica flour(inhalation hazard) from a ceramics supply. Then some nichrome wire around that and more insulation over the top.

The dried air/ammonia "burns" hotter than with the water rich vapour I was using. 1- it meant the high temperatures could have caused contaminants in the expanded clay balls to react with the catalyst or fuse with the catalyst, killing it.
2- it makes more concentrated acid without the introduction of water into the stream.




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