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vanBassum
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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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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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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)
This file has been downloaded 936 times
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MarkRob
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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.
Attachment: kineticsofnitricoxideozonation.pdf (307kB)
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[Edited on 21-4-2020 by MarkRob]
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MarkRob
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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)
This file has been downloaded 775 times
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MarkRob
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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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MarkRob
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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)
This file has been downloaded 686 times
Attachment: Oxide_catalysts_for_ammonia_oxidation_in.pdf (1011kB)
This file has been downloaded 1015 times
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symboom
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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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mysteriusbhoice
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I finally got around to doing this and yes it is Mn2O3-CuO onto sand then packed into the quartz tube then the manganese acetate - copper acetate mix
is blasted to infinity and beyond with a torch to bake the mixed metal oxide catalyst in the tube itself which also binds the sand together to form a
porous catalyst block.
https://www.youtube.com/watch?v=mnItD3juQyE
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BauArf56
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Quote: Originally posted by Chemetix  | Here is the pilot plant.
It's a photo shoot of the unit in operation.
In a converted TV stand you can see the urea decomposer in the mantle front left. The black lump in the middle is the air compressor. The reactor tube
runs into the converted 2L sep. funnel which admits air via the custom condenser fitting. The condensate runs into a reservoir where excess gasses run
to an absorption tower.
This shot shows where the ammonia air mixture is fed into the reaction zone, the nice red glow is transmitted up the quartz.
Red fumes; condensation forms on the walls and the condenser. That's nitric, baby! You can see the tower behind it and the take off tap to collect the
tower absorption.
The condensate ends up in this collection flask and you can see the splash head type design to let the outlet gasses pass into the tower. There is
still a lot of fumes, enough to be visibly red, but once they go to the tower there is no evidence of anything leaving the tower. Kind of important
not filling the room with NOx.
Does it fizz with bicarb? You betcha!
This Christmas. Give the gift of nitric!
[Edited on 24-12-2016 by Chemetix]
[Edited on 24-12-2016 by Chemetix] |
urea decomposer? Urea should decompose into cyanuric acid, right? Does it give off NOx when heated?
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Belowzero
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It produces NH3 which is then catalysed to produce NOx.
Urea being a relatively cheap bulk source.
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mysteriusbhoice
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New video on this setup.
https://www.youtube.com/watch?v=7OAMSBFL36s
The ammonia is regulated by solution and raising and lifting a straw to regulate the ammonia level with a NaOH trap which also prevents ammonia from
being absorbed then the actual reactor with Mn2O3-CuO catalyst on sand.
below is a pic of my new venturi absorber.
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symboom
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Materials
quarts tube
Fish bubbler
Ammonia solution
Copper acetate, manganese acetate
Catalysis bed (sand) maybe activated silica or silica gel works
Sodium hydroxide
Would an aluminum tube work due to nitric acid passivation effect also instead of the quarts tube but the catalysis would have to be unable to attack
the aluminum.
As pipes go
Copper would be attacked
And so would stainless steel.
[Edited on 12-5-2021 by symboom]
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Chemetix
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| Quote: Originally posted by symboom | Materials
Would an aluminum tube work due to nitric acid passivation effect also instead of the quarts tube but the catalysis would have to be unable to attack
the aluminum.
As pipes go
Copper would be attacked
And so would stainless steel.
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Are you asking if aluminium can be used for the catalyst chamber? If so then, no, the temperatures that are reached will melt aluminium. Borosilicate
will melt which is around the 600-700 degree mark.
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mysteriusbhoice
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I have finally made a NO2 generator thats better than a birkeland eiyde using the ostwald process scaled down.
It uses urea and NaOH ammonia generator controlled by stirring rate and air inlet in the generator stage itself.
https://www.youtube.com/watch?v=b9mjbTU7sFc
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mysteriusbhoice
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I used a nichrome preheater in my lastest version so the catalyst doesnt wink out because getting it to run self sustaining is annoying...
I then also added a 1st ammonia cleanup stage because excess ammonia is mixed with NO2 and this stage contains oxalic acid or citric acid just to pull
ammonia out.
ultimately you lose some HNO3 but the 1st stage is a simple bubbler which is shallow since NH3 is super soluble.
The 2nd stage is where the air diffuser which absorbs majority of the nitric and after a 3rd recovery stage then 4th venturi scrubber with alkaline
solution to ease presure drop and scrub any remaining gases.
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Texium
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Thread Moved
19-11-2023 at 09:51 |
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